50th Anniversary Perspective: Polymers with Complex Architectures

Perspective pubs.acs.org/Macromolecules

50th Anniversary Perspective: Polymers with Complex Architectures George Polymeropoulos,† George Zapsas,† Konstantinos Ntetsikas,† Panayiotis Bilalis,† Yves Gnanou,‡ and Nikos Hadjichristidis*,† †

Division of Physical Sciences & Engineering, KAUST Catalysis Center, Polymer Synthesis Laboratory, and ‡Division of Physical Sciences & Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ABSTRACT: The scope of this Perspective is to highlight innovative contributions in the synthesis of well-defined complex macromolecular architectures and to emphasize the importance of these materials to polymer physical chemistry, physics, theory, and applications. In addition, this Perspective tries to enlighten the past and show possible pathways for the future. Among the plethora of polymerization methods, we briefly report the impact of the truly living and controlled/ living polymerization techniques focusing mainly on anionic polymerization, the mother of all living and controlled/living polymerizations. Through anionic polymerization well-defined model polymers with complex macromolecular architectures having the highest molecular weight, structural and compositional homogeneity can be achieved. The synthesized structures include star, comb/graft, cyclic, branched and hyberbranched, dendritic, and multiblock multicomponent polymers. In our opinion, in addition to the work needed on the synthesis, properties, and application of copolymers with more than three chemically different blocks and complex architecture, the polymer chemists in the future should follow closer the approaches Nature, the perfect chemist, uses to make functional complex macromolecular structures by noncovalent chemistry. Moreover, development of new analytical methods for the characterization/purification of polymers with complex macromolecular architectures is essential for the synthesis and properties study of this family of polymeric materials. controlled/living polymerizations”.1 Almost all complex macromolecular architectures (star, comb, graft, cyclic, dendritic, etc.) were first synthesized by anionic polymerization and appropriate linking chemistry. Of course, the complex structures need a few linking steps, and consequently the main product, even after careful fractionation, is contaminated by several precursors in minute amounts, as advanced analytical techniques (SEC under critical conditions, temperature gradient interaction chromatography, and others) have shown. The contamination of the desirable polymer by side products is unavoidable, since it is impossible to achieve exact stoichiometry in linking reactions. Nevertheless, this is not a drawback of anionic polymerization but of the linking reactions. The broad portfolio of anionically synthesized model polymers gave rise to many studies in polymer physical chemistry, physics, theory, and applications of polymers. For example, connecting two immiscible blocks to a single junction point results in four different morphologies (lamellae, double gyroid, cylinders, and spheres) as expected from theory and observed by transmission electron microscopy and small-angle X-ray scattering. The addition of a third block or the change from linear to nonlinear architectures leads to a plethora of complex morphologies.

1. INTRODUCTION Polymer chemists, in their efforts to mimic Natural polymeric products, have developed several polymerization methods to synthesize macromolecules with high degree of structural, compositional, and molecular homogeneity. Alongside, by understanding the kinetics and thermodynamic of polymerization, polymer chemists are encouraged to get inspiration from Nature and the way it creates complex macromolecules. The majority of biological macromolecules (DNA, proteins, etc.) are characterized by long-range order, precise sequence of repeating units, monodispersity, and ability to form high order 3D structures in aqueous environment via noncovalent chemistry. From a point of view of structural and compositional simplicity, linear homopolymers are the most representative examples of synthetic macromolecules. Even in this simple case, the man-made polymers are characterized by polydispersity index higher than one (Đ > 1). Among the numerous polymerization techniques, anionic polymerization, the mother of all living polymerizations, is the only method providing polymers with extremely low polydispersity (Đ ≤ 1.05) and excellent control over the molecular weight, structure, composition, and functionality. Anionic polymerization was the springboard for the advent of other controlled/living polymerizations, which have been developed quickly thanks to their simplicity. We can easily say that “what is the past for anionic polymerization is the present and the future of © XXXX American Chemical Society

Received: November 28, 2016 Revised: January 22, 2017

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DOI: 10.1021/acs.macromol.6b02569 Macromolecules XXXX, XXX, XXX−XXX

Perspective

Macromolecules Scheme 1. Synthesis of Decaoctachlorosilane

stars via anionic polymerization, using tetra- and hexafuntional chlorosilanes. Through viscosity measurements in θ solvents, they found that g′ = {[η]star/[η]linear}M values are equal to 0.773 (4-arm) and 0.625 (6-arm), while in a good solvent the g′ values are higher, where [η]star and [η]lin are the intrinsic viscosity of the star and the corresponding (same molecular weight) linear polymer. Graessley et al.8 by studying the effect of branching on the shear-rate viscosity, using the same samples, concluded that star-like polymers exhibit higher viscosity at low shear rates compared to the corresponding linear but remain below the linear curve when high shear is applied. Expanding the same methodology, 8-, 12-, and 18-arm PI stars were synthesized using specially prepared chlorosilanes with 8, 12, and 18 SiCl groups;9−12 an example is given in Scheme 1. The high homogeneity of the synthesized samples was confirmed by ultracentrifugation and size exclusion chromatography (SEC) coupled with low-angle laser light scattering. Additionally, using anionic polymerization and chlorosilane chemistry, regular star polymers bearing 64 up to 270 arms have been synthesized.13,14 The majority of published work on symmetric stars was dealing with styrenic and dienic monomers; however, Hadjichristidis and Mays et al.15 reported the synthesis of star homopolymers of poly(methyl methacrylate) (PMMA) via anionic copolymerization of the living arms with ethylene glycol dimethacrylate (difunctional monomer). Furthermore, Hadjichristidis’ group16 utilizing divinylbenzene (DVB) was able to synthesize star homopolymers of (PHIC)n and star diblock copolymers of (PHIC-bPI)n, where PHIC is poly(n-hexyl isocyanate). Fetters et al.17 reported the light scattering studies of high molecular weight 18-arm star diblock copolymers of (PS-bPI)18, using isorefractive solvents for PS and PI. The authors proved the existence of a core−shell structure with the PI as the core. Another efficient pathway for the synthesis of star polymers involves the utilization of multifunctional initiators. Quirk et al.18 reported the synthesis synthesis of a hydrocarbon-soluble trifunctional initiator by reacting sec-butyllithium (s-BuLi) with 1,3,5-tris(1-phenylethenyl)benzene (tri-DPE). It was found that the initiator was efficient for the polymerization of styrene in the presence of tetrahydrofuran (THF) in a molar ratio [THF]/[s-BuLi] = 20, but not for butadiene in the nonpolar benzene, due to strong association effects. To overcome this problem, without changing the high 1,4 content of the PB chains, the authors proposed the addition of s-BuOLi in a molar

Considering that Nature has been perfecting macromolecular chemistry for million years, we believe that the progress made on synthetic polymers is still in its infancy. Only the synthesis and the properties of linear homopolymers and diblock copolymers are very well documented, so more work has to be carried out on more complex structures. The question of Bates and co-workers,2 if multiblock polymers consist of Panacea or Pandora’s box, is still unanswered (see Multicomponent− Multiblock section). Since this Perspective is not a comprehensive review, we attempted to include only the most important scientific reports that advanced polymer chemistry along with the evolution caused in polymer physics. We consider anionic polymerization as the most representative method for the synthesis of welldefined polymers with complex macromolecular architecture as well as 100% predictable structure and composition and therefore is the main method discussed in this Perspective. However, we report a few representative examples of complex macromolecules synthesized by other controlled/living polymerization methods. Finally, through this Perspective, we draw attention on unresolved problems regarding polymers with complex macromolecular architectures, proposing Nature’s perfection as the optimum example. Each section of this Perspective has its own conclusion/ perspective and future remarks.

2. STAR POLYMERS 2.1. Symmetric Star Polymers. One of the early examples of macromolecules with complex architecture is the star-like polymers. These polymers consist of several (>2) linear chains connected to a central junction point.3 Due to the more compact structure and the high segment density, the star polymers reveal major differences, compared to their linear counterparts, in their mechanical, viscoelastic, and self-assembly properties.4 The first synthesis of star polymers was demonstrated by Flory5 via step-growth polymerization of caprolactam using cyclohexanonetetrapropionic and/or dicyclohexanone octacarboxylic acid as multifunctional initiators. Later, Morton et al.6 reported the synthesis of polystyrene (PS) stars with three and four arms by anionic polymerization using trichloromethylsilane (CH3SiCl3) and tetrachlorosilane (SiCl4) as linking agents. This seminal work was the key to synthesize well-defined stars and understand the influence of the star structure on the physical properties. On the basis of Morton’s work, Hadjichristidis and Roovers7 reported the synthesis of 4- and 6-arm poly(isoprene) (PI) B


Perspective

Macromolecules ratio of [s-BuLi]/[s-BuOLi] = 2. In a similar way Sakellariou et al.19 reported the synthesis of a novel hydrocarbon-soluble trifunctional organolithium initiator, with no polar-additive requirements, based on a tridiphenylethylene compound, 4,4,4(ethane-1,1,1-triyl)tris(((3-(1-phenylvinyl)benzyl)oxy)benzene). Recently, Frey et al.20 achieved the synthesis of poly(ethylene oxide) (PEO) stars by ring-opening polymerization (ROP) of ethylene oxide, in the presence of diphenylmethylpotassium (DPMK) and a modified polyglycerol as multifunctional initiator. The final stars exhibited rather broad molecular weight distribution and functionality between 26 and 55. Along with the ongoing research on how the complex macromolecular architecture affects the hydrodynamic and bulk properties of the obtained polymers, there is an extensive interest of the scientific community on their mechanical properties. Among the plethora of polymers tested for mechanical performance, polyolefin-based materials lie on the core of this research. It is well-known that the highly branched polyethylene (low density PE, LDPE) exhibits high processability, while the linear version (high density PE, HDPE, and linear low density PE, LLDPE) possesses excellent mechanical properties.21,22 The synergy of anionic polymerization, chlorosilane chemistry, and hydrogenation constituted the main method for the synthesis of precisely controlled linear/ branched PE structures.23 Following this strategy, Hadjichristidis’ group24,25 synthesized linear, symmetric, and asymmetric stars, combs, and α,ωPBs via anionic polymerization high-vacuum techniques and appropriate selective chlorosilane chemistry. The subsequent hydrogenation led to formation of the corresponding branched PE samples bearing 7−11 wt % unavoidable butene monomeric units. On the other hand, since the discovery of a new polymerization method by Shea and co-workers,26−28 many linear and nonlinear PE-based materials have been synthesized.29,30 Characteristic examples will be discussed later on. Besides the extensive study of the dilute solution properties of symmetric stars, there was a growing interest concerning their morphological behavior. The first self consistent field theory (SCFT) on symmetric star block copolymers was elaborated by Matsen and Schick31,32 using the standard Gaussian chain model. It was indicated that the inner block (A) is preferentially located inside the microdomain, while the augmentation of the number of arms (n) shifts the phase boundaries toward higher volume fraction of the A block. This behavior results in decreasing the critical point (χN)c and increasing the asymmetry of the curves33 (Figure 1). The first exploratory results on the microphase separation of (PI-b-PS)8 with PS as the outer block were investigated by Thomas and co-workers.34 Increasing the volume fraction ( f) of the PS block from 0.09 to 0.91%, the authors reported the formation of PS spheres, cylinders, double gyroid, and lamellae in PI matrix and the reverse phases of cylindrical and spherical structures, only, of PI domains in PS matrix. The main discrepancy of the observed morphologies, compared to the linear analogues, was the existence of the ordered bicontinuous double gyroid (OBDG) morphology when the minority component was the outer segment at a f PS = 0.27%.35 It was concluded that due to the star-shaped architecture and the overcrowding effect, the augmentation of the number of arms causes higher conformational restriction at the inner block.

Figure 1. Variation of (χN)c with volume fraction of inner A block ( f) for (A-b-B)n star block copolymers with various n. Reproduced with permission from ref 33.

Additionally to transmission electron microscopy (TEM) measurements, dielectric spectroscopy along with rheology and small-angle X-ray scattering (SAXS) were employed to study the local and global dynamics in microphase separated star diblock copolymers of (PI-b-PS)n. The dielectric measurements were conducted at temperatures well below the order−disorder transition (ODT) while the ordered state morphology, studied by SAXS, revealed the formation of PS spheres at f PS = 0.25, PI cylinders at f PS= 0.68, and lamellar structure at f PS = 0.41.36 The synthetic procedures established by anionic polymerization high-vacuum techniques were the keystone toward the synthesis of complex macromolecular architectures by other controlled/living polymerization techniques, such as atom transfer radical polymerization (ATRP), reversible addition− fragmentation chain-transfer polymerization (RAFT), ringopening polymerization (ROP), or a combination of the above-mentioned techniques. Since the scope of this Perspective is to concisely highlight the progress on complex macromolecular architectures and propose new strategies toward this direction, the above-mentioned techniques will briefly examined. One of the first publications describing the synthesis of starlike polymers via ATRP was on 3-arm star homopolymers of PMMA, using a trifunctional dichloroacetate initiator.37 Employing the same methodology and utilizing tetra-, hexa-, and octa-functionalized initiators, the synthesis of the corresponding symmetric star consisting of styrene, methyl, n-butyl, and n-hexyl acrylates, and/or methacrylates was feasible.38,39 The yield of the resulting polymers was high with low polydispersity index and in good agreement of the experimental molecular weight with the theoretical one. Moreover, in the case of star diblock copolymers of (PS-bP2VP)3, [P2VP: poly(2-vinylpyridine)], the study of the bulk morphology along with viscosity measurements revealed the simultaneous growing of three blocks from all 3-CH2Br endgroups of the trifunctional initiator, verifying the efficiency of the adopted synthetic procedure.40 In the case of RAFT polymerization, the control is strongly affected by the direction of fragmentation of the thiocarbonyl agent and more specifically is influenced by the leaving ability of the groups attached to the two sulfur atoms.41−43 Using multifunctional RAFT agents bearing 6 and/or 12 external 3benzylsulfanylthiocarbonylsulfanylpropionic acid groups, Kowollik and co-workers44 reported the synthesis of poly(n-butyl acrylate) (PnBuA) and PS stars. The polymerization was C


Perspective

Macromolecules performed at 60 °C using azobisisobutyronitrile (AIBN) as thermal initiator while the star-like structures were confirmed by molecular characterization of the arms after cleavage from the central core. Furthermore, using a difunctional monomer (DVB), Pan et al.45 reported the synthesis of PS star homopolymers and star diblock copolymers of PS and poly(N-isopropylacrylamide) (PNIPAM) (Scheme 2). The linear macro-RAFT PS agents were synthesized in bulk, from benzyl dithiobenzoate and AIBN.

Scheme 3. Schematic Illustration of the Differences in the Hydrodynamic Diameters of Amphiphilic Miktoarm and Star Block Copolyethers

Scheme 2. Synthesis of PS Star Homopolymers and Star Diblock Copolymers of PS and Poly(N-isopropylacrylamide)

reported by Hadjichristidis’ and Avgeropoulos’ research groups.48 Besides the biodegradability/biocompatibility of polyesters/ polyethers and the many biomedical applications, the major impact of polymer chemists, in their effort to mimic Nature, is the synthesis of polypeptides. Usually the polymerization is carried out via the ring opening of α-amino acid Ncarboxyanhydrides (NCAs) using mainly primary amines as initiators, under inert or high-vacuum conditions.49−52 The intensive interest in peptide-based polymers steams from their ability to bear several functionalities and to form high order structures, mimicking the Natural relative proteins.53 Since the synthesis of linear polypeptides is reported by comprehensive reviews and books,54−57 in this Perspective we will only focus on the synthesis of polypeptides via ROP of NCAs with complex macromolecular architectures. A typical example for this category of polymeric materials is the published work by Müllen et al.58 describing the formation of 4-arm star γ-benzyl-L-glutamate (PBLG) and ε-benzyloxycarbonyl-L-lysine (PZLL) along with that of Hadjichristidis’ group59 succeeding in the synthesis of 3-arm homo- and 3-arm star diblock copolypeptides [(PBLG)3, (PZLL)3, (PBLG-bPZLL)3, and (PZLL-b-PBLG)3]. The adopted synthetic procedure, using high-vacuum techniques and n-hexylamine as initiator,59 led to well-defined, narrow polydispersity polypeptides. The results from morphological studies60 showed a strong influence of chain topology on the copolypeptide miscibility as well as the existence of α-helical secondary structure (Scheme 4). For linear copolymers, lamellae morphology with the corresponding helicoidal structure in each lamella was evident, whereas in the star topology mixing of PBLG and PZLL blocks occurs with smaller persistence length α-helices packed in a pseudohexagonal lattice. The exciting properties of synthetic polypeptides and their combination with conventional polymers gave rise to a new category of materials, the so-called hybrid polymeric or macromolecular chimeras.61−63 These hybrid polymeric materials are usually synthesized utilizing the amino end group of a

Coming back to anionic polymerization, Hadjichristidis et al.46 reported the synthesis of 4-arm star tetrablock quarterpolymers with each arm constituting of four different heterocyclic monomers, two epoxides and two lactones, by using a “catalyst switch” strategy. The epoxides (1,2-butylene oxide and ethylene oxide) were polymerized sequentially using a trihydroxy initiator in the presence of phosphazene base (tBuP4). After the polymerization of epoxides, diphenyl phosphate was added in order to neutralize the phosphazenium alkoxide and promote the polymerization of δ-valerolactone and ε-caprolactone (ε-CL). In a similar way, Satoh et al.47 reported the synthesis of amphiphilic star block copolyethers. The ROP of 2-(2-(2methoxyethoxy)ethoxy)ethyl glycidyl ether (hydrophilic monomer) and decyl glycidyl ether (hydrophobic monomer) afforded the synthesis of star diblock copolymers [(AB)3, (BA)3, (AB)4, and (BA)4] and miktoarm star copolymers [A2B2, AB2, and A2B]. The self-assembly in water, studied via dynamic light scattering (DLS) and TEM, revealed that the branched architecture affects the structure and size of the resultant micelles but do not affect the critical micelle concentration (CMC) (Scheme 3). Similar results, demonstrating the influence of the architecture on the self-assembly of 3-arm star triblock terpolymers consisting of PS, P2VP, and PEO, have also been D


Perspective

Macromolecules Scheme 4. Schematic Illustration of the Self-Assembly in Diblock and Star Block Copolypeptides (Reproduced with Permission from Ref 60)

Scheme 5. Three Different Asymmetric Star Polymers

2.2.1. Molecular Weight and Topology Asymmetry. The synthesis of asymmetric stars was initially reported by Fetters72 utilizing the anionic polymerization method combined with chlorosilane chemistry in order to synthesize PS or PB stars composed of two arms of equal molecular weight, PSB or PBB, and a third one, PSA or PBA, with either half or twice the molecular weight of the identical arms (Scheme 6). Later, this Scheme 6. Synthesis of PSA(PSB)2 Asymmetric (Molecular Weight) Stars

method was extensively developed by Mays’ and Hadjichristidis’ groups.73,74 Chlorosilanes were used as linking agents for the selective stepwise replacement of the chlorine atoms by the living polymer chains. The different reactivity of living polymer ends, toward the Si−Cl bond, plays a catalytic role due to steric hindrance effects, charge localization on the terminal carbon atom, and the excluded volume of the living polymer chain which is affected by the reaction solvent.75 The reactivity is also influenced by the molecular weight of the living chain, the solvent (polar or nonpolar), and the temperature. Therefore, well-defined star polymers can be obtained when all these parameters are improved. A serious disadvantage of this method is that is time-consuming requiring high-vacuum techniques. A methodology used for synthesis of asymmetric stars of the AnA′n type via divinylbenzene (DVB) led to formation of star polymers consisting of highly living cross-linked polydivinylbenzene core from which the arms of a different/same are emanated. The addition of a new monomer results in the formation of the final asymmetric star (Scheme 7). By this

first polymeric block, which serves as macroinitiator for the ROP of the corresponding NCAs. Following this approach, Iatrou et al.64 synthesized 4-arm star diblock copolymers of (PEO-b-PBLG)4, while adopting the vice versa procedure Dong et al.65 transformed the terminal amine groups of a 4-arm star (PBLG)4 into −Br for the ATRP of D-gluconamidoethyl methacrylate. 2.2. Asymmetric Star Polymers. An important and special family of star polymers is the asymmetric stars, which are characterized by an asymmetry factor compared to the symmetric ones described previously. Three parameters play a significant role: (a) molecular weight asymmetry (whose arms are identical in chemical structure, but differ in molecular weight), (b) topological asymmetry (the arms correspond to block copolymers, but differ to the block sequence connected to the branch point), and (c) chemical structure asymmetry (chemically different arms, generally called miktoarm stars, from the Greek word μικτός meaning mixed), as illustrated in Scheme 5. Many reviews and chapters have been dedicated to the first two categories of asymmetric stars; thus, a few representative examples are presented here.66−68 Miktoarm stars (abbreviated as μ-star polymers) have attracted much attention recently; therefore, a more detailed description will be provided in this Perspective. These type of stars exhibit unique and novel selfassembly properties in bulk and selective solvents, by extending the limits of the typical morphology map of linear block copolymers.69−71

Scheme 7. Synthesis of AnA′n Type Asymmetric Star Polymers

methodology employing anionic polymerization, asymmetric stars of the (PSA)n(PSB)n were synthesized. SEC analysis revealed the presence of high molecular weight species, probably due to formation of linked stars.76 Serious drawbacks associated with this method are the architectural limitation (only asymmetric stars of the AnA′n type can be synthesized) and the steric hindrance due to the fact that a fraction of the E


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Macromolecules

different end-functional groups also belong to this category (Scheme 8). These μ-star polymers are classified as nonlinear

living arms A are not included in the star structure and the initiation rate is not the same for all active sites. This approach was also used in combination with ATRP, to afford the formation of PEOnPSn asymmetric stars.77 Quirk and collaborators78,79 managed to prepare several types of asymmetric stars by reaction of living PS (only low molecular weight) with either 1,3-bis(1-phenylethenyl)benzene (MDDPE) or 1,4-bis(1-phenylethenyl)benzene (PDDPE). Great caution should be adopted in this method concerning the stoichiometry of the reaction and the isolation/characterization of the final products. Nevertheless, it constitutes a valuable method in order to functionalize the final arms via a reaction with a suitable electrophilic agent. Hirao’s group80,81 developed a general method utilizing DPE derivatives with protected chloromethyl groups to synthesize well-defined PS asymmetric stars of the AA′2, AA′2A″2, AA′3, AA′A″2, and AA′4A″4 types. This strategy includes difficult multistep procedures, where living PS chains react with 1,1bis(3-methoxymethylphenyl)ethylene followed by transformation. of the methoxymethyl groups to chloromethyl groups using BCl3 at 0 °C. These DPE-functionalized polymers were used for linking reactions with living polymer chains, followed by the coupling with chloromethyl groups. Recently, the same group using this method synthesized a series of novel welldefined asymmetric ABC, ABCD, and ABCDE star polymers containing conductive polyacetylene segments and its soluble precursors, by living anionic polymerization of 4-methylphenyl vinyl sulfoxide (4MPVO).82 Hadjichristidis’ group83 reported the inverse star block copolymer architecture, with four PS-b-PI copolymers as arms. Two of the arms were connected to the star center by the PS block while the other two by the PI block. Anionic polymerization and chlorosilane chemistry (SiCl4) were employed also in this procedure in order to synthesize the final well-defined star copolymer as resulted by SEC, membrane osmometry (MO), LS, and nuclear magnetic resonance (NMR) spectroscopy. Recently, Avgeropoulos et al.84,85 managed to synthesize asymmetric star copolymers of the A(BA′)n=2,3 through anionic polymerization and chlorosilane chemistry. The morphological studies showed that these complex copolymers have great impact on the microdomain morphology of block copolymers and remarkable mechanical properties. Rheological studies were also performed on various types of asymmetric stars, where the experimental data showed good agreement with the tube model, concerning linear viscoelastic properties, at polymer concentrations above 40−50%. At lower polymer concentrations, the tube model predictions systematically deviate and overpredict the effect of solvent dilution on the magnitude of the dynamic moduli.86−88 ATRP was also employed for the successful synthesis of asymmetric stars89−91 with molecular weight, topology, and functional group asymmetry, but anionic polymerization still remains the most successful and reliable technique for this type of complex macromolecular architecture. 2.2.2. Chemical Asymmetry (Miktoarm Star Polymers, μStars). The term miktoarm stars, μ-stars, was adopted by Hadjichristidis’ group referring to stars with chemical asymmetry. A different name is also coined for this type of star polymer, such as heteroarm stars (hetero coming from the Greek word έτερος, meaning the other), but is not appropriate since it doesn’t attribute the concept of a star composed of different arms. Stars bearing arms of similar chemical nature but

Scheme 8. Different Types of Miktoarm Star (μ-Star) Polymers

branched block copolymers and have recently attracted much attention due to their morphological nanoscale structures and supramolecular assemblies, formed in bulk as well as in selective solvents. In order to synthesize well-defined μ-arm star polymers, the use of living polymerization techniques is essential and particularly anionic polymerization. First, molecular weight can precisely predicted in a wide range from 103 up to 106 g/ mol, and second, molecular weight distribution is narrow (Đ < 1.10). Additionally, the produced living polymers bear highly reactive chain-end anions, which are stable under appropriate conditions. During the past 25 years, major progress in controlled/living radical polymerization as well as the introduction of highly efficient chemical reactions (“click” chemistry) enabled the synthesis of a variety of new μ-star polymers. In this Perspective we will focus on some examples of synthetic procedures through anionic polymerization techniques. We also present some unique properties of these systems in bulk and in solution, which render these star polymers important for future applications. Two general strategies have been employed for the synthesis of μ-star polymers via anionic polymerization. The first is based on divinyl compounds, either homopolymerizable (e.g., divinylbenzene) or non-homopolyF


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Macromolecules

Scheme 9. Synthesis of 3-Arm ABC μ-Star Terpolymer by Anionic Polymerization and Selective Chlorosilane Chemistry

merizable (e.g., DDPE), and the second on multifunctional linking agents, which will be described more extensively later on. A variety of linking agents have been used for the synthesis of star polymers,75 with chlorosilanes92 as the most commonly used for nonpolar chains and chloro/bromomethylbenzenes for the polar ones.93,94 The usage of DVB for synthesizing miktoarm stars was first introduced by Eschwey and Burchard95 and mainly developed by Rempp and colleagues.96−99 The mechanism was already described concerning the homopolymer asymmetric stars. The DVB method100−102 has been employed for synthesis of μ-star copolymers of the AnBn type, with A = PS and B = poly(tertbutyl methacrylate) (PtBuMA), poly(tert-butyl acrylate) (PtBuA), PEO, or P2VP. Key advantages of this method are its simplicity and that it can be performed under an inert atmosphere, avoiding high-vacuum techniques. The main drawbacks are architectural limitations and the formation of loops or networks due to intramolecular and intermolecular reactions, respectively. Quirk and co-workers102 developed an approach utilizing double diphenylethylenes for synthesizing 3 and 4 μ-stars. PDDPE is frequently used for synthesis of 3 μ-A2B and 3 μABC, whereas the MDDPE for A2B2 4 μ-stars. The crucial point of the linking procedure is the control of stoichiometry between the living A chains and the DDPE; otherwise, a mixture of star and linear polymers occurs. A major restriction is the difference in the rate constants of initiation for the two new active sites, resulting in a bimodal molecular weight distribution. To overcome this issue, polar compounds should be added. However, the addition of s-BuOLi to the living coupled DDPE derivative, prior to diene monomer insertion, was found to produce well-defined μ-stars with high 1,4 content. Similar to the DVB method, B arms cannot be isolated from the reaction mixture and molecularly characterized. Nevertheless, this method is valuable for the synthesis of ωfunctionalized μ-stars. As mentioned, multifunctional chlorosilane compounds are effective coupling agents for synthesizing multiarm star polymers, since they react readily and quantitatively with the living polymers. During the synthesis of regular stars using multifunctional chlorosilanes, the reactivity of the living polymer toward the Si−Cl bond was found to follow the sequence of PB−Li+ > PI−Li+ > PS−Li+ due to steric hindrance. The reactivity of the Si−Cl bond decreases during the linking of the chlorine atom by the living polymer chain due to electron donating effect and steric hindrance. All these characteristics rendered chlorosilane chemistry as an effective procedure for the synthesis of a variety of μ-star polymers. Mays73 was the first to successfully synthesize one sample of well-defined AB2 type 3-arm, consisting of PS (A) and PI (B). Living PS first reacted with a large excess of CH3SiCl3 to produce a chain-end (−SiCl2) functionalized PS. After removal of unreacted CH3SiCl3, living PI was coupled with PS-SiCl2, resulting AB2 3-arm star. Hadjichristidis’ group extended this methodology to almost all possible combinations of AB2 and AB3 μ-star polymers using tetrachlorosilane (SiCl4) instead of CH3SiCl3.103−105 High degree of molecular and compositional homogeneity was indicated by various molecular characterization methods. Additionally, 3-arm ABC μ-stars were synthesized for the first time as illustrated in Scheme 9.106 For the synthesis of these 3 μ-ABC terpolymers, three chlorine atoms were consecutively and selectively replaced. The first chlorine atom replaced by PI−Li+ using excess of CH3SiCl3, the

second one by titration with PS−Li+ and the third by excess of PB−Li+. Using the same procedure, asymmetric AA′B miktoarm stars were also synthesized, where A = PI and B = PS.107 This study reports stars bearing two chemically identical PI with different molecular weights. Synthesis of 4-arm μ-star copolymer (PS)2(PI)2 as well as 4-arm ABCD μ-star quarterpolymer was accomplished through a similar procedure, where PS−Li+, poly(4-methylstyryl)lithium (PMS−Li+), PI−Li+, and PB−Li+ sequentially added to SiCl4.108 The obtained polymer was the first successful four-component μ-star polymer. In the case of (PS)2(PI)2, the PS−Li+ was added in two steps (excess and titration) in order to succeed maximum control over the polymer architecture. Novel ABC μ-star containing poly(dimethylsiloxane) (PDMS) have also been synthesized via reaction of the above prepared in-chain(-SiCl) functionalized PS-b-PI with the living PDMS.109 The adopted methodology further demonstrated the effective synthesis of various well-defined μ-star polymers, such as AB3, A2B2, AB5, and A8B8 types, where A and B arms are PS, PI, PB, 2-methyl-1,3-pentadiene, 1,3-cyclohexadiene, and their block copolymer segments.110−117 Living PMMA and other related living polymers derived from alkyl methacrylate monomers cannot be directly used in this methodology, since the silyl enol ether linkage is readily hydrolyzed upon exposure to air. To introduce PMMA segments into μ-star polymers, the reaction sites were converted from the Si−Cl to DPE anion, as illustrated in Scheme 10. After the synthesis of in-chain (−SiCl) functionScheme 10. Synthesis of 3-Arm (PS) (PI) (PMMA) μ-Star Polymer Using Selective Chlorosilane Chemistry and Double DPE

alized PS-b-PI, the Si−Cl was reacted with dilithium compound, Li(CPh2CH2CH2CPh2)Li, to convert it to DPE anion. The formed anion was used as initiator for the polymerization of MMA, resulting in 3-arm ABC μ-star polymer of PS, PI, and PMMA.118 This modification of the reaction site is advantageous since any monomer besides MMA that undergoes the living anionic polymerization with DPE anion can be employed. Hadjichristidis et al.119 successfully synthesized 4-arm ABCD μ-star quarterpolymer by adding the dual functionality of DPE (-SiCl and double bond) in the aforementioned methodology. As shown in Scheme 11, 1-(4-dichloromethylsilylphenyl)-1G


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Macromolecules Scheme 11. Synthesis of 4-Arm (PS)(PI)(PDMS)(P2VP) μStar Quarterpolymer Using Selective Chlorosilane Chemistry and DPE

solvent for PI. Aggregation numbers were found to increase in the order I2S < S2I < SI.131,132 Gido’s and Hadjichristidis’ groups133 demonstrated the discrepancies between theoretical predictions and experimental results of AnB miktoarm stars as the value of n increases. They reported even more significant deviations from theory for (PI)5PS type miktoarm stars eliminating the formation of spherical and cylindrical morphologies in such highly asymmetric structures. On the basis of the initial phase diagram introduced by Milner,130 Gido’s and Hadjichristidis’ groups concluded a new phase diagram shown in Figure 2, where the discrepancies with the theoretical predictions are indicated with dark shaded symbols.

phenylethylene was used as the linking agent, where PI living chains and living PDMS were selectively coupled with the two chlorine atoms. The target 4-arm μ-star quarterpolymer was synthesized by addition of PS−Li+ to the in-chain DPE function and the subsequent polymerization of 2VP. A series of 4-arm A2B2, 4-arm A2BC, and 5-arm A2B2C μ-star polymers were also synthesized by similar methodologies. The methodologies described above cannot be used for the synthesis of multicomponent/multiarm μ-star polymers, since the reaction site always disappears after the introduction of each arm. In order to overcome this difficulty, Hirao et al.120−123 recently proposed a conceptual “iterative strategy”. In this methodology, the reaction system is designed in such way that the reaction site is regenerated after the insertion of arms at each reaction sequence. Accordingly, the same or different arms can successively introduced by repeating the reaction sequence, producing a series of many component and armed μstar polymers. Since a new reaction site is always regenerated for the next step, several arms can be added. The resultant μ-arm star polymers described above consist a very important category of polymers due to their interesting morphological properties. As in the case of linear copolymers, the miktoarm stars can microphase separate due to the existence of incompatible blocks. Altering the linear to nonlinear architecture, morphological studies are very important, since these materials shift the boundaries between different morphologies.124 Some examples mainly in bulk characterization are given below. The first microphase separation study on PS(PI)2 miktoarm stars with 37 vol % PS, synthesized by Hadjichristidis’ group,125 was found to self-assemble into hexagonally close-packed cylinders of PS in the PI matrix as revealed by TEM studies. This observation is in contrast to alternating lamellae structure expected for the linear diblock copolymer with the same volume fraction. Subsequently, more studies104,126−129 with larger number of samples, covering a wider range of compositions, showed major discrepancies in the phase diagram for miktoarm star copolymers in comparison to the corresponding linear diblock copolymers. Analogous phase boundaries shifts were also observed in the case of A3B (A:PI and B:PS) miktoarm stars.125 These findings are in qualitative agreement with the theoretical predictions reported by Milner.130 Light scattering and viscosity measurements were also performed for these miktoarm stars, where the influence of architecture on the micellization properties was investigated by keeping the overall molecular weight and composition constant. All samples formed spherical micelles in n-decane, a selective

Figure 2. Phase diagram in which the morphology is given for the volume fraction of the B component, φB, and molecular asymmetry, ε. Shaded symbols indicate samples whose morphology disagrees with that predicted by theory. Reproduced with permission from ref 133.

More interesting results were extracted through morphological experiments on μ-ABC star terpolymers. Herein, the block sequence does not affect the morphology, in contrast to linear ABC materials, due to the existence of only one junction point from which all blocks emanate. In most cases, the junction points are located onto lines, instead of interfaces (Scheme 12). In ABC miktoarm star terpolymers, the microphase separation is directed by two independent composition variables φA, φB (φC = 1 − φA− φB) and by three interaction parameters (χAB, χAC, and χBC).134 Theoretical approximations and morphology predictions were carried out by Dotera et al. and by Pan et al.135,136 using Monte Carlo simulations and dynamic density functional theory (DDFT) calculation methods, respectively. More studies are essential in this field in order to completely understand the complex mechanism of microphase separation via SCFT or other methods. Moving to experimental results, Thomas’ and Hadjichristidis’ research groups118,137 investigated the phase behavior of 3 μarm star terpolymers of PS, PI, and PMMA. PS and PMMA blocks showed high incompatibility toward PI, whereas incompatibility between PS and PMMA is weak. All samples showed three microphases and 2D periodic microstructure of an inner PI column, surrounded by PS corona in the matrix of PMMA. No interface between PI and PMMA was observed, meaning that PMMA blocks passed through the PS domains to reduce unfavorable contact between PI and PMMA. A new morphology was also discovered, wherein the PMMA domain H


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Recently, Hadjichristidis’ group reported the synthesis of a novel 3-miktoarm star quarterpolymer P2VP(PDMS-b-PI-bPS)2.138 The synthesis involved (a) the sequential anionic terpolymerization to afford PS-b-PI-b-PDMSO−Li+, (b) selective coupling of the living polymer with the two −Cl groups connected to the −Si atom of the heterofunctional linking agent (chloromethylphenylethenyl)dichloromethylsilane (CMPMDS),139 and (c) coupling of P2VP−Li+ with phenyl chlorine atom. The melt morphology of this miktoarm star quarterpolymer determined using rheology, small-angle neutron, and X-ray scattering data in conjunction with dissipative particle dynamics simulations of model molecule matches the volume fractions of the experimental system. The resulting new morphology showed perforated lamellae structure with hexagonally perforated lamellae of P2VP containing closepacked protrusions of PS connecting adjacent lamellae layers of PS. The PI blocks form lining between the P2VP and PS domains, constrained by the molecular architecture. Lately, great interest has been expressed for the synthesis and physical properties of well-defined polypeptides and polypeptide hybrid materials with different macromolecular architectures.140,141 Utilizing anionic polymerization in combination with ROP, Hadjichristidis’ and Hirao’s group63 reported the synthesis of 3 μ-stars (PS)2(PBLG or PBLL), (PS)(PI)(PBLG or PBLL) and 4 μ-stars (PS)2[P(α-MeS)](PBLG or PBLL), (PS)2(PBLG or PBLL)2 [P(α-MeS): poly(α-methylstyrene). Moreover, in collaboration with Ikkala’s group, the first hierarchical smectic self-assembly of miktoarm macromolecular chimeras composed of two coil-like arms (PS and PI) and mesogenic α-helical polypeptide arm (PBLL) was reported.142,143 The PBLL α-helices were packed within lamellar nanodomains leading to an overall smectic variation of rod- and coil-containing layers. Furthermore, the coil-containing lamellae

Scheme 12. Schematic Illustrations of the Arrangement of Copolymer Chains: (a) AB Diblock Copolymer: the Chemical Junction Points Are Confined on the Interface; (b) Linear ABC Triblock Copolymer: the Chemical Junction Points Are Confined on the Interfaces, (c) ABC star: the Chemical Junction Points Are Confined on a Line

was surrounded by alternating PI and PS microdomains, forming hexagonal-shaped cylindrical columns with p6mm symmetry (Figure 3).

Figure 3. (A) Schematic presentation of the two-dimensional microdomain morphology exhibiting hexagonally packed cylinders of PI with a concentric PS annulus in a matrix of PMMA. The PS and PMMA arms are partially mixed within the PS domain. (B) Two-dimensional microdomain morphology exhibiting the PI arm not forming hexagonally packed cylinders and the PI/PS and PS/PMMA surfaces have a rhombohedral shape. (C) Representative chain conformation of the miktoarm star terpolymer, indicating three-dimensional microdomain morphology and the location of the junction point is exhibited. (D) Schematic showing the junction points residing at the vertexes where the three types of microdomains intersect. Reproduced with permission from refs 118 and 137. I


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Macromolecules

and PLLA-b-PM, followed by “click” reaction with two-azidoend-functionalized PEG to afford 5-arm stars (PEG)(PCL)(PS)(PLLA)(PM) [PEG: poly(ethylene glycol), PLLA: poly(Llactide), and PM: poly(meth)acrylate-type segments] as well as 5-arm stars of (PEG)(PCL)(PS)(PLLA)(PAA) [PAA: poly(acrylic acid)]. The ABCDE star quintopolymers selfassembled into vesicles with potential application as controlled delivery carriers due to their excellent stability, satisfactory drug loading efficiency, and pH responsiveness. Advancements in synthetic polymer chemistry allowed the synthesis of a wide range of star polymers, symmetric and asymmetric, by several polymerization methods, and their combinations. Nowadays, it is feasible the synthesis of star polymers with more than three incompatible arms. Symmetric stars due to the single connecting point and the same number of blocks adopt characteristic features, such as lower viscosity, smaller hydrodynamic volume, better solubility, lower glass transition temperature, etc., in comparison to linear derivatives. On the other hand, among asymmetric, μ-arm star polymers have received great attention in recent years, since exhibit unique morphologies in bulk and selective solvents, enriching the variety of new morphological microstructures. Therefore, μarm stars are recognized as promising materials with many potential applications in the field of nanotechnology such as electronic and optical devices, nanomaterials for lithography, drug delivery etc. In order to use these polymers, which adopt fascinating morphologies, the self-assembled structures on thin films should be investigated. Fundamental questions concerning the understanding of selfassembly and the parameters controlling the phase behavior in macromolecular level are under continuous examination. At the moment, the synthetic abilities are more dominant than the theoretical ones for the complex macromolecular systems. It is expected that a better elucidation of the structure−property relationship will lead to remarkable new macromolecules with numerous applications. It is thus time to accomplish further collaborations among the polymer chemists and physicists in order to achieve new advances in polymer chemistry.

has an inner structure composed of PS and PI rectangular cylinders, leading to hierarchical smectic phase (Figure 4).

Figure 4. Schematic illustration of the self-assembly in miktoarm macromolecular chimera composed of two coil-like arms (PS and PI) and a mesogenic α-helical polypeptide arm (poly(ε-tert-butyloxycarbonyl-L-lysine), PBLL). Reproduced with permission from ref 142.

PE-based symmetric 4-arm star block copolymers and 3miktoarm stars have been synthesized recently via polyhomologation, based on the in situ generated boron−thexylsilaboracyclic initiating sites for the polymerization of dimethylsulfoxonium methylide.144 Moreover, A2B, (AB)2B copolymers, and (AC)2(BC), (A: PE; B, C: PS or PMMA) terpolymers were synthesized combining boron chemistry, polyhomologation, and ATRP. 1,4-Pentadiene-3-yl 2-bromo-2-methylpropanoate was first synthesized, followed hydroboration with thexylborane to afford B-thexylboracyclanes, a multi-heterofunctional initiator with two initiating sites for polyhomologation and one for ATRP. After polyhomologation of dimethylsulfoxonium methylide the α,ω-dihydroxyl (PE-OH)2-Br served as macroinitiator for the ATRP of styrene. Both (PE-OH)2-Br and (PEOH)2-(PS-Br) were transformed into trifunctional macroinitiators (PE-Br)2-Br and (PE-Br)2-(PS-Br), through esterification reaction/ATRP yielding (AB)2B and (AC)2(BC) 3miktoarm star co/terpolymers.145 Additionally, using bis-Bthexyl-silaboracycles as initiator for polyhomologation, welldefined 4-arm hydroxy-terminated PE stars were synthesized. PE stars successfully transformed to (PE-b-PCL)4, or (PE-bPMMA2)4 through ROP of ε-caprolactone or ATRP of MMA.30 Combination of this general strategy with other polymerization methods may lead to novel PE-based complex macromolecular architectures such as multiarm stars (8-, 12-, and 16-arm), H-shaped, molecular brush copolymers, etc. Besides anionic polymerization, there has been a rapid growth of controlled/living radical polymerization techniques over the past decade. These polymerization methods involve atom transfer radical polymerization (ATRP),146,147 transition metal-mediated living radical polymerization,148 nitroxidemediated radical polymerization (NMRP),149 and reversible addition−fragmentation transfer radical polymerization (RAFT).150 The development of such controlled/living polymerization systems has opened the way for the synthesis of regular and μ-star polymers included monomers that cannot be polymerized through anionic polymerization (DMAEMA, NiPAAM, etc.). Zhao and co-workers151,152 reported the synthesis of welldefined amphiphilic ABCDE star quintopolymers combining ROP, RAFT, and azide−alkyne cycloaddition reaction (CuAAC). The trifunctional agent, propargyl 5-cyano-5phenylthiocarbonylsulfanyl pentanoyloxy-2-hydroxymethyl-2methylpropanoate (PCP), was used for the synthesis of alkyne-in-chain-functionalized diblock copolymers PCL-b-PS

3. CYCLIC POLYMERS On a molecular level, the diffusion of the chain ends is the crucial parameter influencing many properties of linear polymers, including glass transition temperature,153 melt-flow dynamics,154 and crystallinity.155 On the contrary, the lack of free chain ends on ring polymers, along with topological restrictions caused by the cyclic architecture, has motivated polymer physicists to deeply explore and understand the fascinating properties of this class of polymeric materials. To this direction, the synthetic polymer chemists should overcome a major challenge: the intramolecular coupling of polymeric chain ends along with the simultaneous elimination of unwanted byproducts (uncoupled linear precursors and/or polycondensation derivatives). Among the living and controlled/living polymerization techniques, anionic polymerization,156 in combination with high-vacuum techniques,157 is a powerful tool toward complex macromolecular architectures including cyclic.68 It was the pioneering work of Roovers et al.158 on the synthesis of high molecular weight ring PS, using sodium naphthalene as difunctional initiator and dichlorodimethylsilane (CH3)2SiCl2 as the linking agent in concentration below the critical equilibrium concentration159 given by the equation J


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Macromolecules ⎛ 3 ⎞3/2 M ceq = ⎜ ⎟ ⎝ 2π ⟨r 2⟩ ⎠ NA

presence of s-BuOLi (to afford high 1,4-content), followed by polymerization of St (or St-d8), while the cyclization performed using bis(dimethylchlorosilyl)ethane (BDCSE) as the coupling agent. It was found that using BDCSE, the coupling reaction is faster and in a more controllable manner. Moreover, in order to enlighten the self-assembly behavior of cyclic structures in selective solvents and compare the aggregation phenomena with the corresponding linear diblock and triblock copolymers, micellization studies were conducted using small-angle neutron scattering (SANS) and DLS. Surprisingly, the aggregation number of the ring structures was found to be the smallest among the different macromolecular architectures, and most importantly SANS measurements revealed 37% of dangling chains in the case of the triblock precursors that do not appear in the corresponding cyclic. Bearing in mind that only 5% of dangling chains can be detected, it was proven that the synthesized cyclic structures were at least 87% pure. Since the research focusing on the domain dspacing was limited only to lamellae morphology,162 Hadjichristidis, Gido, and coworkers166 studied the microphase separation of a series of five cyclic block copolymers of styrene and butadiene, with the same molecular weight though altering the PS volume fraction. From the TEM exploratory results it was evident that when the cyclic and the corresponding linear copolymers adopt the same morphology, the domain dspacing of the cyclic was 84%−89% of the corresponding linear. Besides, another milestone toward understanding the morphological properties of the ring block copolymers was the evidence that connecting the end blocks together, the interface tends to curve away, leading eventually to different morphologies. As briefly reported above, the initial research on ring polymers involved the homodifunctional ring closure of living dianionic polymers. The major drawback of this method is the usage of exact stoichiometric quantities of reagents,167 leading eventually to a mixture of linear and ring polymers; thus, tedious purification techniques must be employed to isolate the crude cyclic polymers. An alternative approach to overcome the linear byproducts, caused by the inexact stoichiometries, is the direct cyclization between two different polymer end groups. The earliest work utilizing α,ω-heterodifunctional polymers was reported by Deffieux et al.168,169 employing cationic polymerization to synthesize well-defined poly(vinyl ether) macrocycles. The thermal properties of the synthesized ring polymers revealed the higher glass transition temperature for the cyclic structures compared to the corresponding linear. Employing the same methodology via anionic sequential terpolymerization, Hadjichristidis’ group170 reported for the first time the synthesis of a cyclic triblock terpolymer of PS, PI, and PMMA. The synthetic procedure involved the intraamidation of an α,ω-amino acid precursor synthesized by sequential terpolymerization using 2,2,5,5-tetramethyl-1-(3lithiopropyl)-1-aza-2,5-disilacyclopentane as initiator and subsequently as amine generator as well as 4-bromo-1,1,1trimethoxybutane as terminator and carboxylic acid generator. The cyclic triblock terpolymer exhibited narrow polydispersity but low molecular weight; thus, no microphase separation studies were feasible (χN < 10.4). Recently, “click” chemistry was employed by Hadjichristidis et al.171 to synthesize well-defined cyclic diblock and multiblock copolymers of PS and PI via anionic polymerization, using a protected acetylene functionalized initiator (5-triethylsilyl-4pentynyllithium). The synthetic procedure is depicted in

(1)

where c is the concentration, M the molar mass, NA Avogadro’s number, and ⟨r2⟩ the mean-square end-to-end distance of the linear precursor. In a similar way, Hogen-Esch et al.160,161 reported the synthesis of well-defined cyclic diblock copolymers of PS-bPDMS. First, the polymerization of styrene was carried out at −78 °C in THF, using lithium naphthalenide as initiator, followed by addition of hexamethylcyclotrisiloxane (D3) and (CH3)2SiCl2 as coupling agent. The morphology of these ring diblock copolymers, studied by Thomas’ group,162 revealed alternating lamellae morphology. The lower domain dspacing for the cyclic structures, compared to the corresponding linear, attributed to the double looped chain conformation of cyclic versus a mixture of single looped and bridged chain conformation in the corresponding A-b-B-b-A triblock copolymers. Schematic illustration depicting the chain conformation of cyclic and the corresponding linear samples is given in Scheme 13. Scheme 13. Schematic Illustration Showing the Chain Conformation of Cyclic and the Corresponding Linear Copolymers

Hemery et al.163 reported the anionic polymerization for the synthesis of cyclic PS utilizing DDPE. The ring homopolymers were obtained with yield of approximately 50%. By using a combination of different analytical techniques, these PS rings were accurately characterized. For example, the dilute viscosity measurements revealed that the ratio g′ = [η]cyclic/[η]linear is equal to 0.67, in good agreement with theoretical calculations, where [η]cyclic and [η]linear are the intrinsic viscosity of the cyclic and the corresponding linear.164 Moving a step forward, Hadjichristidis et al.165 reported the synthesis of cyclic PS-b-PB diblock copolymers. The synthetic approach involved the reaction of (1,3-phenylene)bis(3-methyl1-phenylpentylidene) dilithium initiator with butadiene in the K


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Macromolecules

Scheme 14. General Reactions for the Synthesis of α-Acetylene-ω-azido-PS-b-PI Cyclic Diblock and the Corresponding Multiblock Copolymers

Scheme 14. The reported work consists one of the few examples of complex macromolecular architectures via combination of anionic polymerization and “click” chemistry. Another efficient route toward cyclic structures is the direct coupling reaction between similar end groups of homodifunctional polymers. Employing this methodology, Huang et al.172 synthesized first α,ω-propargyl-telechelic PS and PEO and then the corresponding cyclic copolymers via the Glaser coupling reaction. The intramolecular cyclization occurred in the presence of Cu(I)Br and PMDETA, with yield nearly 100%. The high efficiency of the Glaser coupling reaction and the fact that it could be performed in the presence of oxygen opens new pathways for a wide range of cycle-based polymers, such as tadpole and cyclic−linear−cyclic. Hadjichristidis et al.,173 employing the Glaser coupling reaction in combination with anionic polymerization, succeeded in the synthesis of cyclic triblock terpolymers of PI, PS, and P2VP. First, α-tert-butyldimethylsilyloxy-ω-hydroxy triblock terpolymer was synthesized by sequential terpolymerization using 3-(tert-butyldimethylsilyloxy)-1-propyllithium as initiator, followed by end-capping the living ends with ethylene oxide. After deprotection of the tert-butyldimethylsilyl groups and esterification with 4-pentynoic acid, the produced α,ω-dialkyne triblock terpolymer was submitted to Glaser coupling for the intramolecular ring closure at room temperature in the presence of Cu(I)Br/PMDETA to afford the cyclic terpolymer. The first exploratory/preliminary TEM results (Figure 5) showed major differences between the morphologies of the cyclic and the corresponding linear, revealing the tremendous influence of the cyclic structure on the microphase separation of triblock terpolymers. Despite the fact that anionic polymerization is the main method for the synthesis of well-defined polymers, with high molecular and compositional homogeneity, the early discovery of controlled/living radical polymerization in combination with “click” reaction174−176 allowed a wide range of polymers to be cyclized. Some typical examples are given in Scheme 15.167 Lately, Hawker et al.177 reported the synthesis of amphiphilic PS-b-PEO cyclic diblock copolymers via ATRP and Huisgen 1,3-dipolar cycloaddition. The synthesized cyclic samples were used for lithography applications and revealed decrease of 30%

Figure 5. TEM image of the (A) linear triblock terpolymer exhibiting the morphology of hexagonally close-packed core−shell cylinders, consisting of PS (white), which are surrounded by PI (black), in the matrix of P2VP (gray). (B) Cyclic triblock terpolymer exhibiting a worm-like morphology formed by the P2VP blocks (gray areas), surrounded by PS (white) in the matrix of the PI domains (black). Reproduced with permission from ref 173.

in domain dspacing, when compared to the corresponding linear diblock copolymers. Moreover, Grayson’s research group178 utilizing RAFT polymerization in combination with “click” chemistry reported the synthesis of biocompatible amphiphilic cyclic diblock copolymer consisting of PEG and PCL. The self-assembly behavior of the cyclic polymers in water, studied by DLS, revealed that the size of the micelles, formed by the ring polymer, consists of more compact self-assembled units. The smaller hydrodynamic diameter of the ring polymers (Scheme 16) is attributed to the requirement of the core-forming block L


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Macromolecules Scheme 15. Synthesis of Cyclic Polymers by Intra Cu(I) Catalyzed Azide−Alkyne Cycloaddition

the critical condition183 as well as temperature gradient interaction chromatography.184 In the case of cyclic polymers the zero-shear viscosity is approximately half compared to the corresponding linear with the same molecular weight.185 Moreover, upon small-amplitude oscillatory shear measurements it was found that the major difference of the stress relaxation modulus [G(t)] between ring and linear samples occurred in the intermediate regime, where the entangled cyclic structures surprisingly do not exhibit the entanglement plateau but instead are characterized by an extended relaxation regime.186 Additionally, the ring polymers relax significantly faster compared to the linear ones of the same molar mass, an observation that has been previously reported.187 To summarize the rheological properties of ring polymers, more studies are essential to fully understand their melt dynamics. Furthermore, few other types of ring-based architectures have been developed where the rings are connected to either linear or star chains. One of the early examples of complex cyclic architectures is the tadpoles, as described in the pioneering research of Quirk and Ma.188 Applying anionic polymerization living polystyrene was synthesized, which subsequently coupled with double diphenylethylene. The obtained difunctional living PS bearing two carbanionic sites was used as difunctional macroinitiator for the anionic polymerization of butadiene. The two poly(butadiene) living ends were coupled under high dilution with dichlorodimethylsilane to form the double tadpole. Another fascinating example of complex cyclic architecture was recently reported by Matsushita’s group189 describing the synthesis of a series of comb-shaped ring PS, composed of a ring backbone and multiple linear branches with different molecular weight. The synthetic procedure involved the anionic polymerization of a vinyl-functionalized ring backbone, while in a second step the backbone reacted with an excess of living linear chains. The outcome of this research is that the intramolecular excluded volume effect is weakened when the

to form loops and stretch with restrictions, as imposed by the cyclic architecture. Scheme 16. Self-Assembly of Linear and Cyclic PEG-b-PCL Diblock Copolymers in Water, Revealing Different Hydrodynamic Diameters Due to Different Architecture

As previously mentioned, the major drawback of the synthesized ring polymers is the contamination with the corresponding linear due to incomplete cyclization. Since cyclic polymers cannot reptate as the linear counterparts, there is a considerable interest in their melt dynamics.179,180 Over 30 years of extensive research,181 demonstrated that the ring polymers exhibit a universal behavior clearly distinct from their linear counterparts.182 In order to study ring polymers with less than 0.1% of contaminants, state of the art purification methods were employed, such as liquid interaction chromatography at M


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Macromolecules Scheme 17. Synthesis of 3-Miktoarm Star Cyclic Triblock Terpolymer

The backbone and the branches may exist as homopolymers or copolymers.192

branch molecular weight was increasing and subsequently led to the conclusion that the comb-shaped ring samples adopt the behavior of star-like polymers. Monteiro et al.,190 using controlled/living radical polymerizations and selective linking chemistry, were able to achieve the one-pot synthesis of a 3-arm star structure bearing on each chain a chemically different ring (Scheme 17). Following this methodology Monteiro et al.191 were able to synthesize more ring-based polymers with complex architectures. This section summarizes the up to date research on cyclic polymers, highlighting a few characteristic examples with interesting properties. Though many efforts have been made toward well-defined, pure, and high molecular weight ring polymers, a lot of work is needed in order to extend the frontier of polymer chemistry and physical chemistry/physics toward this direction.

Scheme 18. Structure of Graft PA-g-PB Copolymers

Graft copolymers were intensively studied in bulk to elucidate the equilibrium morphologies via TEM. Compared with the linear analogues, the branching points induce additional constraints on the thermodynamically driven process of microphase separation leading to the reduction of the longrange order; the higher the number of branching points, the higher the reduction. Mechanical and viscoelastic properties of graft copolymers are influenced by their bulk morphology, which are strongly related to the chemical composition of the backbone and branches as well as their mutual immiscibility. For instance, their use has been extensively studied as a potential compatibilizing agent of immiscible blends. Because of entropy, most of the polymer pairs are incompatible. Mixing polymer blends with graft copolymers consisting of the same blocks is a powerful process to improve the mechanical performance.195 Anionic polymerization constitutes the most valuable synthetic tool to yield model topologies, desired functional groups, control over the length of side chains and backbone, and narrow polydispersity. For both combs and grafts, synthetic routes can be generalized into three categories: (a) “graftingonto” method, (b) “grafting-from” or macroinitiator method, and (c) “grafting-through” or macromonomer method (Scheme 19).67,74 The “grafting-onto” methodology requires the attachment of side chains onto a linear backbone by coupling reaction. Hereby, preformed living chains attack the electrophilic sites along the backbone such as chlorosilanes, esters, anhydrides, epoxides, and benzyl halides. Adopting appropriate reaction conditions, the backbone and the side chains are covalently bonded in order to give the desired topologies of comb or graft architectures.67,192 One of the first and very common synthetic pathways to synthesize graft copolymers is chloro- or bromomethylation of the phenyl group of the PS backbone and the subsequent

4. COMB AND GRAFT POLYMERS Well-defined branched structures, labeled by the term comb and graft polymers, have steadily gained ground over the past 30 years. Enhanced physical properties are distinctly different compared to their linear counterparts due to unique chemical design. The structure−properties correlation associates them with plethora of applications, ranging from high impact plastics and pressure-sensitive adhesives to thermoplastic elastomers (TPEs). The past 15 years the production of model branched polymeric materials has expanded from the conventional industrial field (mechanical and viscoelastic properties) to more spreading high-tech applications such as electronics, stimuli-responsive surfaces, and biomedical materials.192,193 Anionic polymerization enables the synthesis of model comb and graft polymers with precisely controlled architecture, functionalities, and molecular weight. However, the advent of ATRP, ring-opening metathesis polymerization (ROMP), and NMRP helped to expand the range of monomeric units in the graft structure. Grafting is recognized as a well-established method for enhancing mechanical properties. For this reason, graft-shaped polymers are considered the most important polymer structure among the whole class of branched architectures. For instance, ABS (acrylonitrile−styrene−polybutadiene), high impact PS (butadiene-g-styrene), and cellulose-g-acrylonitrile are highly commercialized grafted materials.194 The main structural difference between comb and graft polymers is that in combs the backbone and branches are constituted by the same polymer chain whereas in graft copolymers backbone and branches are different (Scheme 18). N


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Macromolecules

Scheme 19. General Categories of Graft Copolymer Synthesis: (a) “Grafting Onto”, (b) “Grafting From” (Macroinitiator Method), and (c) “Grafting Through” (Macromonomer Method)

protocol, the same group copolymerized anionically the difunctional monomer 4-(vinylphenyl)-1-butene with styrene to form the backbone. The next step was the partial hydrosilylation of butene to afford chlorodimethylbutylsilyl active sites which were finally coupled with different polyanions, yielding comb (PS-g-PS) and graft [(PS-g-PI), (PS-g-PMMA)] topologies.201 The hydrogenation of PB backbone was another approach toward the synthesis of PE-based graft copolymers as has been reported by Hadjichristidis’ group.24 Polymerization of butadiene in benzene resulted linear PB with 92% 1,4-content. The 1,2-PB (8%) was hydrosilylated by reaction with (CH3)2SiHCl in the presence of Pt catalyst, in order to generate −SiCl groups (along the backbone) followed by reaction with PB or PS anions to obtain either PB-g-PB or PBg-PS. Block−graft copolymers constitute another promising category of graft copolymers where both “grafting-onto” and “grafting-from” methods can be used (Scheme 21).

coupling reaction with another living polymer chain (Scheme 20). Scheme 20. Chloromethylation of PS

The chloromethylation of preformed PS backbone is conducted in carbon tetrachloride (CCl4) with chloromethyl methyl ether in the presence of SnCl4 as catalyst. However, the direct reaction of the backbone chloromethyl groups with living anions is accompanied by backbone−backbone coupling as a result of metal−halogen exchange. The conversion of −CH2Cl groups to −CH2OSi(CH3)2Cl surpassed the side reactions concerning lithium−chlorine exchange as reported by Rahlwes et al. for the synthesis of PS-g-PI graft copolymers.196 The partial chloromethylation and bromomethylation were also applied in graft copolymers containing polar blocks as side chains followed by the coupling reaction with the backbone. Hadjichristidis’ group exploited the bromomethylation method toward the synthesis of graft copolymers with polyelectrolyte branches (PS-g-PMA) after the hydrolysis of the corresponding PS-g-PtBu.197,198 The low reactivity of polyanions during coupling reactions was always on demand for the synthesis of comb/graft polymers via the “grafting-onto” strategy. Recently, Fernyhough et al.,199 used PSLi end-capped with DPE (to avoid side reactions) and reacted with partly chloromethylated PS backbone to afford PS comb, followed by sulfonation to produce comb shaped sodium poly(styrenesulfonate) (NaPSS). The “grafting onto” strategy was also employed by Zhang and Ruckenstein200 to prepare graft copolymers using the epoxy groups of poly(glycidyl methacrylate) followed by reaction with living chains of PS or PI. On the basis of this

Scheme 21. Block−Graft Structure

The first synthesis of block−graft copolymers was reported by Se and co-workers.202 The anionic sequential copolymerization of styrene and (4-vinylphenyl)dimethylvinylsiloxane conducted in THF at −78 °C aiming to backbone formation. To afford the final block−graft structure, PI−Li+ was covalently attached to the side reactive groups of PVS segments yielding the PS-b-(PVS-g-PI). In a similar fashion, Xu and co-workers203 reported the synthesis of (PS-g-PI)-b-PI. The sample was synthesized by NMRP and “grafting-onto” strategy. Using the same monomers, Hadjichristidis et al.204 accomplished the synthesis of O


Perspective

Macromolecules Scheme 22. Hydrosilylation of 1,2-PB toward Grafted Polymers

“grafting-onto” since it overcomes the “steric hindrance” barrier by the simultaneous growth of all side chains from initiating sites along the backbone. In 1978, Hadjichristidis and Roovers208 reported the synthesis of well-defined PI-g-PS copolymers by adopting the lithiation of the allylic hydrogen of the polyisoprene backbone. The lithiation was conducted in the presence of strong chelating agent, i.e., N,N,N′N′-tetramethylethylenediamine (TMEDA) and s-BuLi.209 This synthetic route followed also by other research groups to synthesize well-defined poly(dienes-g-styrene) polymers.210,211 The successful synthesis of graft and dendritic grafts of PS-gPMMA by two consecutive controlled polymerization methods (NMRP/ATRP) was reported by Grubbs et al.212 The NMRP copolymerization of styrene and p-(4′-chloromethylbenzyloxymethyl)styrene incorporates chloromethyl groups onto the backbone. The chloride sites used for the ATRP of styrene, MMA, and n-butyl methacrylate yielding PS-g-PS, PS-g-PMMA, and PS-g-PnBMA. In a similar way, Hadjichristidis et al.39 reported the synthesis of PS-g-PtBMA and PS-b-(PS-g-PtBMA) by copolymerization of styrene and 4-methylstyrene along with ATRP of tBMA. Well-defined graft copolymers consisting of PB backbone and PHIC or PCL grafts are also reported by the “grafting from” method. The hydroboration−oxidation of anionically synthesized PB homopolymers introduced −OH groups along the 1,2-PB segments of the backbone. The obtained backbone reacted with cyclopentadienyltitanium(IV) trichloride (CpTiCl3) followed by addition of n-hexyl isocyanate (HIC) or ε-caprolactone. Adopting the same methodology, the double bonds of 3,4-PI units of PS-b-PI3,4 copolymer modified to −OH groups which were then employed for subsequent polymerization of HIC or ε-CL leading to PS-b-(PI-g-PHIC) or PS-b(PI-g-PCL), respectively.213 According to TEM studies, morphological behavior of these block−graft and double graft materials is affected by the behavior of smaller architectural units (constituting block copolymers). The role of branch point location along the backbone can be examined by comparing the behavior of regular with random multigrafts. Moreover, the increased branch points per molecule showed to suppress the long-range order, as revealed from TEM measurements.214−216 Inspired by Ziegler−Natta catalysts, Li and co-workers217 successfully combined coordination polymerization and anionic graft polymerization. A series of PP-g-PS copolymers were prepared via Ziegler−Natta polymerization of propylene and pallyltoluene (p-AT). In a next step, metalation of benzyllic sites with n-BuLi/t-BuOK and subsequent grafting of PS chains from the backbone took place. The obtained PP-g-PS when mixed

more complex (PS-g-PI)-b-PS and (PS-g-PS)-b-(PS-g-PI) graft−block copolymers. An important extension of block−grafts is the block−double graft (BDG) copolymers, a term coined by Hadjichristidis’ group. Through the corresponding nucleophilic reactions, PB-gPS graft and PB-g-(PS)2 graft copolymers were synthesized via hydrosilylation of the backbone with the corresponding chlorosilanes [HSi(CH3)2Cl and HSi(CH3)Cl2] followed by coupling with PS−Li+ anions. As an example the synthesis of PB-g-PS is given in Scheme 22.205 In another contribution, Hadjichristidis’ group prepared copolymers and terpolymers such as PS-b-[1,2-PB-g-(X)2] where X is PS, PB, PI, or PS-b-PI (Scheme 23).206 The PS-bScheme 23. Synthetic Procedure for the Synthesis of PS-b[1,2-PB-g-(X)2]

1,2-PB backbone was synthesized via anionic polymerization, followed by hydrosilylation of the 1,2-PB block and subsequent attachment of the X groups into the backbone. The obtained BDG copolymers showed enhanced elastomeric properties after analysis of the strain−stress behavior due to unique chain conformation in the microphase-separated state.207 Key parameters including molecular weight of the branches as well as the number and the type of functionalities along the 1,2-PB addition strongly affect the mechanical strength antagonizing commercial thermoplastic elastomers (TPEs). Beyond the steric hindrance problem of linking high molecular weight side chains with the backbone the “graftingonto” strategy still remains an efficient and accurate methodology. The main advantage of this approach is the predetermined molecular weight of backbone/side chains and therefore calculation of the grafting density. The “graftingfrom” method appeared to gain ground compared with the P


Perspective

Macromolecules

Scheme 24. Representative Examples of Block Double Graft (a−d), Double Comb, and Exact Comb Architectures

Scheme 25. Synthesis of Functionalized Comb Polystyrenes: (a) Macromonomer A, (b) Macromonomer B, and (c) Synthesis of Comb−Vinyl Copolymer

reactivity ratios between them, compositional and molecular weight heterogeneity occur during polymerization. The formation of PS or PMMA macromonomers has been exemplified by many research groups.218−221 The most common procedure to obtain methacrylate or styrenic macromonomer is the polymerization of the corresponding monomers, followed by end-capping with ethylene oxide and subsequent reaction with methacryloyl chloride. Alternatively, PS−Li+ chains have been end- capped with 1,1-diphenylethylene and subsequently reacted with vinylbenzene bromide or chloride. PS or PMMA macromonomers were copolymer-

with PP/PS blends indicated enhancement of thermal properties (crystallinity and glass transition temperature, Tg). In the third method, “grafting-through” or macromonomer (MM) strategy, the grafting can be accomplished by endfunctionalized or side-functionalized macromonomers. The macromonomers possess oligo- or polymeric chains with polymerizable headgroup. One of the advantage for “graftingthrough” method is that macromonomers can be separately prepared and fully characterized. The number of branches per backbone is governed by the molar concentration of macromonomer to comonomer. Nevertheless, due to different Q


Perspective

Macromolecules Scheme 26. Two Different Synthetic Pathways for the Synthesis of PE-Based Homobrushes and Cobrushes

macromonomers resulted in combs with various functionalities (−OH, −COOH, and −C8F17). Controlled/living radical polymerization methods provide a versatile pathway for the synthesis of macromonomers, but the grafting yield still remains an issue. Well-defined grafted topologies consisting of hydrophobic polymethacrylate backbone and hydrophilic poly(2-ethyl-2-oxazoline) side chains were synthesized by Weber et al.234 The end-capping of living oligo(2-ethyl-2-oxazoline) after transformation to a macromonomer submitted to RAFT polymerization using 2cyanobutan-2-yl-dithiobenzoate as chain transfer agent (CTA). The study of low critical solution temperature (LCST) behavior in aqueous solution for a series of comb polymers revealed cloud points that can be tuned from 35 to 80 °C by varying the content of MMA. A subcategory of graft copolymers is the exact graft copolymers, a term given by Hadjichristidis’ group. An example of exact PI-g-PS graft copolymer synthesized by Hadjichristidis et al.235 using 1,4-bis(1-phenylethenyl)benzene (PEB), is reported. Exact control of all structural factors of graft copolymers (main and side grafted chains, number and position of grafted chains along the main one) has been achieved by combining living anionic polymerization and specially designed linking reactions.236−242 Lately, an exact comb of 1,4-PI with three branches, where the middle branch had twice the molecular weight of the other two branches, was synthesized through anionic polymerization and chlorosilane chemistry (DCMSDPE).243 Over the past decade, several comb topologies have been studied in order to elucidate the effect of architecture on the rheological properties related to industrial applicability. In general, for a molten polymer, the molecular structure dictates the physical properties. In graft topologies, the presence of long chain branching results in dramatic effect on dynamical and rheological behavior.244 The determination of effect of branching is difficult for commercial branched polymers. An exact comb with defined molecular weight for the backbone and the branches is an excellent model for branched polymers. Linear rheology of comb and star comb of PS, PB, and PI has been studied by Vlassopoulos’ group245−247 on samples with different chemistry, branch molecular weight, and number of branches. The motif of macromomomer methods has also been used for the synthesis of polymeric brushes. These tethered chains are potentially attached onto a substrate or a backbone. Hereby,

ized with several vinyl monomers to yield comb and graft copolymers.222 Upon using highly reactive metallocene catalysts and the above-mentioned route, Hadjichristidis and co-workers synthesized methacryloyl macromonomers of PS, PI, and PDMS and copolymerized them with methyl methacrylate toward the synthesis of PMMA-g-PS, PMMA-gPI, and PMMA-g-PDMS graft copolymers.223 Also, Hadjichristidis et al.224,225 demonstrated an in situ approach toward the synthesis of comb and graft copolymers avoiding the isolation of macromonomer. A wide range of possible architectures consisting of PS-b-(PI-g-PI)-b-PS, PS-b[PI-g-(PI-b-PS)]-b-PS, and (PS-g-PS)-b-(PI-g-PI)-b-(PS-g-PS) were synthesized by anionic polymerization and 4-(chlorodimethylsilyl)styrene (CDMSS). The key point of the substitution reaction is located on the selectivity of organolithium with the silyl chloride group rather than the styryl-tipped double bond. By the addition of a few butadiene units to the living macroanions better control toward its reaction with CDMSS is accomplished.226−228 The expansion of in situ macromonomer methodology led to the synthesis of a wide variety of grafted polymer topologies including comb, star−comb, and comb-on-comb as well as block−graft copolymers as illustrated in Scheme 24.229,230 Utilizing static and DLS, comb-like complex architectures consisting of PS, PB, and PI were subjected to micellization studies. For instance, n-decane was used as selective solvent for the PI or PB blocks while N,N-dimethylacetamide (DMA) for PS, indicating that the complexity of the structure does not favor large micellar structures.231 Emanation of two chains from each branch point along the chemically identical backbone generates additional comb topologies.226 Double tailed comb or double comb PB are two of the well-suited terms for their description. Beyond the synthetic interest, graft copolymers gained attention due to their increased tensile characteristics. For instance, strain-tension measurements in double graft PS-g-PI interestingly showed high strain at break far exceeded that of commercial TPEs product.232 In a more recent work, Quirk et al.233 prepared functionalized comb of PS by anionically synthesizing two individual macromonomers such as ω-(p-vinylbenzyl)PS and α-4pentenyl-ω-(p-vinylbenzyl)PS (Scheme 25). The first MM was obtained by end-capping PS−LI+ with 4-vinylbenzyl chloride (VBC) and the second by polymerizing St with 4pentenyllithium. Finally, anionic copolymerization of the above R


Perspective

Macromolecules

of linear macromolecules with a number of covalently attached chains at both chain ends. Typical examples are H-type, superH-type, and pom-pom-like polymers. Regarding ω-branched polymers, both divergent and convergent methods can be employed for the successful synthesis. The first one involves the coupling of preformed chains to suitable linking sites located at the main chain end, while in the second, linear polymers have a number of anionic initiating sites placed at the chain end. Thus, new polymer chains can originate, leading to the formation of the desired structure. Roovers et al.252,253 utilizing the convergent method synthesized umbrella-type copolymers of PS-b-PB. The 1,2groups of the PB block were hydrosilylated to introduce Si−Cl groups, followed by coupling with living PB to afford the final structure (Scheme 27).

the transition from graft copolymers to polymer brushes is defined by the critical point where the density of the attached side chains begins to affect the ability of the backbone to adopt a random coil configuration. High grafting density PE-based brushes synthesized by Hadjichristidis’ group employing polyhomologation (Scheme 26).248,249 Novel well-defined bilayered molecular cobrushes with internal PE blocks and OH- functionalized polyethylene homobrushes have been synthesized through two different routes. Three different polymerization methods (anionic polymerization, polyhomologation, and ROMP) have been successively combined in order to obtain the cobrushes. In the second route, polyhomologation via an OH-protected allyl compound and ROMP resulted in homobrushes. PE-branched double graft copolymers were prepared by ATRP of a novel monomer [2-((methyldivinylsilyl)oxy)ethyl methacrylate (DVSiOMA)] and subsequent chemical modification of divinylsilyl groups to B-thexylsilaboracycles by hydroboration along with polyhomologation to afford PS-coPDVSiOMA-g-(PE-OH)2.144 Notably, the past 15 years the existed polymerization methods remained immutable. Thus, polymer chemists explored the polymerization of novel monomers that can be interpreted as potential side chains. To that end, the existing synthetic methodologies are maybe insufficient. Currently, the noncovalent “grafting onto” approach exploiting hydrogen bonds, host−guest modulation, electrostatic and π−π interactions has emerged as a promising tool. Amphiphilic graft copolymers can also be used as drug and gene carriers. In this direction, the control of grafting density can readily handle their hydrophilic/hydrophobic balance. Graft copolymers may also constitute promising soft materials in tissue engineering as super soft elastomers. At the end, the continuation of rheological studies into branched polymers will always offer impetus over the control and optimization of industrial procedures (film blowing, blow molding, and finer spinning).

Scheme 27. Synthesis of Umbrella Copolymer

5. BRANCHED POLYMERS In general, branched polymers possess distinct physical properties from their linear analogues, but due to their complex structure, the development of techniques for the synthesis of branched polymers with well-defined molecular characteristics and their characterization is still a challenge. Nowadays, all recent developments in synthetic procedures offer the opportunity to obtain even more complex architectures. Branched and hyperbranched polymers with their unique properties are potential candidates for a variety of applications, including drug delivery and viscosity modifiers. Many synthetic procedures have been presented for the synthesis of hyperbranched polymers, mostly by polycondensation reactions.250,251 As a result of the high molecular and compositional heterogeneity, their structures could not be easily defined. In this Perspective, examples of several synthetic approaches will be demonstrated, mainly by anionic polymerization. This method led to hyperbranched polymers with welldefined characteristics and helped polymer scientists to deeply explore their properties. ω-branched, α,ω-branched, and other more complex architectures will be presented. The term ω-branched polymers refers to linear chains with a number of branches covalently attached at one chain end, depicting a variety of structures such as umbrella and palm trees. The α,ω-branched polymers consist

The macromonomer strategy has been used for the synthesis of highly branched polymers through the convergent approach. Bifunctional compounds that have been extensively used are 4(chlorodimethylsilyl)styrene (CDMSS), vinylbenzyl chloride (VBC), and 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE). Depending on the degree of branching and the molecular weight of the living polymers, the final product may be a hyperbranched or a star polymer. In the case of VBC there is limited control during the synthesis and the products are rather polydispersed.254 Pom-pom-type PS homopolymers were synthesized by Knauss et al.255 through a synthetic route involving anionic polymerization of styrene followed by slow addition of CDMSS in the first step. Briefly, a living PS star with a hyperbranched core via continuous addition of the coupling agent to living PS chains was obtained. The next step was the addition of styrene to the living PS star anions to produce “star-block-linear diblock” PS−Li+ followed by coupling with CH3SiCl2 in order to synthesize the desired architecture. Through a similar S


Perspective

Macromolecules

Employing the same method, (PI)5PS(PI)5 copolymers (pom-pom shaped) were synthesized through the reaction of living PI chains with the hexafunctional chlorosilane agent Cl3SiCH2CH2SiCl3 (SiCl:Li molar ratio of 6:5), followed by reaction of the remaining −SiCl group with a difunctional PS.267 Pom-pom or dumbbell copolymers with a high functionality at the end-grafted chains were synthesized according to the divergent methodology. First a 1,2-PB-b-PS-b-1,2-PB triblock copolymer was prepared by anionic polymerization bearing short PB blocks, using naphthalene potassium as difunctional initiator.268 The pendant double bonds on the 1,2-PB blocks were exposed to hydroboration−oxidation reaction, generating −OH groups. These groups were transformed into alkoxides, by reacting with cumylpotassium and were used as initiating sites for the polymerization of EO. Following this method, polymers with low degree of polymerization and compositional heterogeneity were obtained. Macromonomers (“grafting through” method) can also be used for the synthesis of α,ω-branched polymers. A difunctional living polymer chain, acting as a macroinitiator, can polymerize only few macromonomers, thus producing a pom-pom (or dumbbell) copolymer.224,269 A2BC2 Janus H-shaped terpolymers, where A is PDMS, B is PB, and C is PS, were synthesized by Hadjichristidis’ group,270 using anionic polymerization and two linking agents, [(chloromethylphenyl)ethyl]methyldichlorosilane (CMPEMDS) and DCMSDPE. This methodology involves the linking reaction of BzCl-(PDMS)2, prepared from PDMSO−Li+ and CMPEMDS, with the living miktoarm star copolymer [(PS)2PB-DPH−Li+]. The synthesis of the living star was achieved by (i) the selective reaction of living PS with the two chlorines of DCMSDPE, (ii) the addition of s-BuLi to the vinyl bond of the DPE, and (iii) polymerization of 1,3-butadiene (Scheme 30). Furthermore, the aggregation properties of the synthesized Janus H-shaped terpolymers were studied by DLS in methyl ethyl ketone, a selective solvent of PDMS and PS. The complexity of the architecture along with “microdomain separation” phenomena led to formation of unimolecular or dimeric aggregates (Figure 6). In an extension of the methodology involving DPE derivatives, the preparation of chain-end-functionalized polymers with a definite number of bromomethylphenyl (−BnBr) end groups and their utilization in the synthesis of dumbbell polymers have also been reported by Hirao’s group.271 Through the macromonomer strategy more complex architectures than the α,ω-branched polymers have also reported by Hadjichristidis’ group.226,272,273 Styrenic single and double star-tailed macromonomers were first obtained by reaction of living homo/miktoarm stars with the −Cl groups of 4-(chlorodimethylsilyl)- and 4-(dichloromethylsilyl)styrene, respectively. The in situ anionic homopolymerization of macromonomers with s-BuLi or copolymerization with butadiene or styrene led to single/double homo/miktoarm star-tailed molecular brushes and combs as well as block copolymer consisting of linear PS chain and double miktoarm (PB/PS) star-tailed brush-like polymers (Scheme 31). As previously highlighted, the above-mentioned synthetic strategies led to well-defined branched architectures with interesting results concerning their physical properties. Anionic polymerization high-vacuum techniques still remain in a leading role for the successful synthesis of these polymers with complex architectures. Although the development of controlled/living

synthetic route, ((PS)nPS)m star-shaped homopolymers were also synthesized by the same group.256 Recently, following a similar synthetic procedure and using CDMSS and (CH3)2SiCl2, Avgeropoulos’ group257−260 synthesized well-defined H-type homopolymers of 3,4-PI (high 3,4 content, ∼55−60%) and through collaboration with Wang’s group rheological measurements were conducted. Interesting results were reported, leading to new theoretical models for the nonlinear dynamic behavior of long chain branched (LCB) polymers and their mixtures with the corresponding linear. The measurements performed in the presence of large external deformations under either simple shear or uniaxial extension. A slightly different synthetic approach was reported for the synthesis of well-defined symmetric and asymmetric H-type PB by Mays’ group.261−264 This synthetic strategy involves anionic polymerization techniques in combination with the linking agent, DCMSDPE, and chlorosilane chemistry (Scheme 28). Also, TGIC and melt dynamics experiments were performed, where the hierarchical model can calculate the rheological behavior accurately. Scheme 28. Synthetic Route for the Synthesis of Symmetric H-Shaped PB

Furthermore, anionic polymerization and chlorosilane chemistry were used for the preparation of α,ω-branched polymers. The employed procedures are similar to that described above, except that the reactions are performed at both chain ends. Super-H-type, pom-pom, or dumbbell copolymers (BnABn) are some examples that will be described. Roovers et al.265 first synthesized H-type PS homopolymers using the difunctional sodium naphthalene as initiator and CH3SiCl3 as coupling agent. Hadjichristidis’ group266 using a convergent approach and anionic polymerization associated with SiCl4 prepared (PI)3PS(PI)3 super-H-shaped copolymers. A difunctional PS chain with −SiCl3 groups derived from the polymerization of styrene in THF using sodium naphthalene as initiator and reacted with large excess of SiCl4. After elimination of SiCl4 excess and addition of living PI arms, the desired superH-type structure was achieved (Scheme 29). Scheme 29. Synthesis of Super-H-Type Copolymer by Anionic Polymerization and Selective Chlorosilane Methodology

T


Perspective

Macromolecules Scheme 30. General Reactions for the Synthesis of Janus H-Shaped Terpolymers

solution, should be performed. We strongly believe that these materials can be used extensively in applications such as gene and drug delivery, as dispersion stabilizers, and generally in the control of interfacial properties of various industrially significant systems like coatings, adhesives, films, and others.

6. DENDRITIC POLYMERS In recent years scientists attempt to synthesize novel materials with complex structures due to the increased demand for sophisticated properties from current society. An endless source of inspiration toward structure complexity is Nature. One of the most widespread configuration in Nature is the dendritic pattern, not only in living organisms (dendritic cells, neurons, bronchioles, and tree branching) but also in abiotic phenomena (lightning pattern, rivers deltas, and snowflakes).280 Hyperbranched organic molecules similar to the above motif were first synthesized by Vögtle et al.,281 but the term “dendrimer” was introduced by Tomalia and co-workers282,283 to describe a family of branched poly(amidoamines) (PAMAM), having between branched points only 1−3 atoms. Dendrimers have attracted great attention due to their unique properties, such as high density, low intrinsic viscosity, intramolecular topological cavities, and ability for multifunctionalization.284−286 In this Perspective, examples of several synthetic approaches concerning dendritic-like polymers will be presented. Dendrimer-like polymers are comprised of polymeric chains between their branching points and are divided into four subcategories: (a) hyperbranched polymers, (b) dendrigrafts, (c) dendronized polymers, and (d) dendrimer-like polymers, depending on the distribution of the branches throughout the macromolecule (Scheme 32).287

Figure 6. Graphical representation of the hydrodynamic volume of (a) Janus H-shaped copolymer with separated PS and PDMS blocks in THF, (b) Janus H-shaped copolymer with mixed PS and PDMS blocks in THF, and (c) star-shaped precursors (PS)2PB in THF. Reproduced with permission from ref 270.

polymerizations274−276 in combination with “click” chemistry277,278 and other techniques279 managed the past decade to compete with anionic polymerization through the synthesis of more complex macromolecular architectures and the utilization of a wide variety of monomers, still this technique has more to offer in this field. As perspective, numerous actions should take place in this area due to the tremendous opportunities that these materials can offer in various applications. First, new initiators and more specifically, multi-initiators should be discovered and through combination with the aforementioned techniques (i.e., macromonomer strategy) may lead to new complex architectures with controllable molecular characteristics (composition, topology, and functionality) and new properties. Finally, novel analytical methods should be developed in order to characterize these complex architectures, as well as more studies, especially on the self-assembly of the complex copolymers in either bulk or U


Perspective

Macromolecules

Scheme 31. Macromolecular Architectures Produced by Homopolymerization and Copolymerization of Single/Double Homo/ Mikto Star-Tailed Styrenic Macromonomers with Butadiene (Blue Chains) or Styrene (Red Chains)

convergent synthesis render the manufacturing of large scale quantity of dendrimers difficult.290 The synthesis of dendritic-like polymers was first outlined by Gnanou et al.291 combining anionic ring-opening polymerization with reactions that generate branch points. In this work the hydroxyl groups of commercially available multifunctional initiator were activated through deprotonation, for polymerization of EO with diphenylmethylpotassium (DPMK). Transformation of the terminal hydroxyl into dihydroxyl groups was achieved through formation of ether linkage with chloromethyl groups bearing two −OH’s protected in acetal form. After deprotection, two −OH’s were formed in every branch for subsequent polymerization of EO (Scheme 34). The same group reported the synthesis of dendrimer-like PEOs up to eight generations (8G).292 The first synthesis of biodegradable dendrimer-like polymers was reported by Hedrick and co-worker293 in 1998. A hexahydroxy-functional 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) derivative was used as initiator for ROP of εcaprolactone catalyzed by stannous-2-ethylhexanoate [Sn(Oct)2]. The branching reaction was achieved by esterification of hydroxyl end groups with bis-MPA having two protected hydroxyl groups. After deprotection and subsequent ROP of CL second generation (2G) dendrimer-like PCL was formed. The same methodology was followed for synthesis of 3G dendrimer-like PCLs and poly(L-lactide)s.294−296 These reports highlight also the strong dependence between architecture and thermal properties (melting point, degree of crystallinity) of the semicrystalline PCL. ATRP was also utilized for synthesis of well-defined macromolecules with dendritic structure. The concept of changing the terminal groups to branch points can find applications similarly in the case of vinyl monomers. Multifunctional initiators bearing 4-, 6-, and 8-initiating sites were used for ATRP of styrene by Gnanou et al.297 After completion of the first generation, the chain ends modification with two ATRP initiators per arm was achieved in two steps. First, nucleophilic substitution of the ω-bromo end groups using 2amino-1,3-propanediol followed by esterification with bromoisobutyryl bromide. Finally, 3G of PS dendrimer-like molecules were produced by subsequent ATRP. By combining different polymerization methods, water-soluble dendrimer-like block copolymers were obtained by ROP of EO, followed by ATRP of tert-butyl acrylate and deprotection to afford poly(acrylic

Scheme 32. Schematic Illustration of Subclasses in the Dendritic Family (Reproduced with Permission from Ref 287. Copyright 2009 The Royal Society of Chemistry)

Two fundamentally different synthetic routes have been employed to synthesize dendrimers or dendritic-like polymers (Scheme 33).288 The first route is to grow the dendrimer Scheme 33. Synthesis of Dendrimer through Divergent and Convergent Methodologies (Reproduced with Permission from Ref 288. Copyright 2006 The Royal Society of Chemistry)

divergently from the core outward.283 The second approach, the so-called convergent method introduced by Fréchet and Hawker, enables the synthesis of dendrimers radially from the surface to the focal point.289 Although the final products exhibit similar structure, there are pros and cons for each method. Dendrimers synthesized by divergent approach are often characterized by defects (missing arms). This is due to the steric hindrance effect especially for dendrimers with many generations. On the contrary, in convergent approach, imperfections can be minimized since dendrons are synthesized, characterized, and purified thoroughly before connection to a multifunctional core. However, additional steps required in V


Perspective

Macromolecules Scheme 34. Synthesis of Dendrimer-like PEO

Scheme 35. Synthesis of 7G Dendrimer-like PMMA (Reproduced with Permission from Ref 305)

acid) segments.298 The reverse ATRP of styrene followed by branch modification and ROP of EO led to synthesis of amphiphilic dendrimers.299 A novel iterative divergent method for the synthesis of dendritic polymers was introduced by Percec and co-workers.300,301 This new synthetic concept termed TERMINI (TERminator Multifunctional INItiator) uses protected multifunctional compounds designed to react fully and irreversibly with the terminal groups of a living polymer and having a latent difunctional initiator. Well-defined dendritic PMMA by Cumediated controlled radical polymerization with high molecular weight (up to 460 kg/mol) were synthesized by this methodology. An anionic version of the TERMINI synthetic approach was reported by Gnanou et al.302 In this work, 1,1bis(4-bromophenyl)ethylene functions as terminating agent and multifunctional initiator. The first generation was created by anionic polymerization of St from a tetrafunctional initiator (core). After the polymerization the living chain ends were transformed into oxygen anion through end-capping with EO, followed by addition of the TERMINI agent. The resulting phenyl bromides moieties were activated by s-BuLi and anionic polymerization of St or 1,3-butadiene took place. The transformation reaction followed by living anionic polymerization was repeated several times yielding 3G and 7G dendritic

polymers with narrow polydispersity (Đ = 1.04) and controlled molecular weight. Hirao and co-workers303 developed a novel stepwise iterative methodology based on the “arm first” approach. Herein, living polymers bearing two α,ω-protected functional groups were used as precursors for the arm synthesis. After the attachment of these polymers to the core, the protective groups were removed and active branch points were formed for further linking reactions. On the basis of that, Hirao’s group304,305 synthesized 4G dendrimer-like PMMA. First, α-functionalized living PMMA with two tert-butyldimethylsilyloxymethylphenyl (SMP) groups was reacted with α,α′-dibromoxylene as the core agent. After completion of the reaction, the two PMMA chains were treated with a 1:1 mixture of (CH3)3SiCl and LiBr to convert the four SMP termini to BnBr groups. The resulting dendritic PMMA was characterized by branched structural homogeneity. The same process was used for synthesis of 3G PMMA with four branches at each generation and 7G with 508 PMMA segments (Scheme 35) by incorporating a core compound substituted with four −BnBr functions. Furthermore, different monomers like St or t-BuA were utilized to obtain the corresponding dendrimers by the above-mentioned procedure.306,307 This methodology combines the benefits of anionic polymerization (low polydispersity and precise control of molecular weight) along with the quantitative deprotection W


Perspective

Macromolecules Scheme 36. Synthesis of 2G and 3G Dendrimer-like PB

the double bond with s-BuLi took place in order to initiate the polymerization of butadiene from the available anionic site. The as-synthesized living star reacted with DCMSDPE to introduce DPE moiety between both stars. The synthesis continued with the addition of s-BuLi and polymerization of butadiene to afford a living 2G dendron. By coupling the formed dendron with CH3SiCl3, 3G dendritic-like PB occurred (Scheme 36). The number-average molecular weights of the resulting polymers by this method were in a good agreement with the theoretical ones, and the polydispersities were narrow (PDI ∼ 1.1). Hutchings et al.312,313 developed another approach for the convergent synthesis of dendritic-like star-branched PS. In this synthetic procedure α,ω-AB2 PS macromonomer (A, B different functionalities) was synthesized by anionic polymerization initiated by 3-(tert-butyldimethylsilyloxy)-1-propyllithium and end-capped with 1,1-bis(4-tert-butyldimethylsiloxyphenyl)ethylene followed by deprotection. The macromonomers functionalized with one primary alcohol and two phenol groups were assembled into a dendritic structure by a series of Williamson coupling reactions and subsequent endgroup modification. “Click” chemistry was also employed for the synthesis of dendrimer-like polymers with the convergent method. Monteiro and co-workers314 used ATRP to synthesize well-defined polymers with precise end-group functionality and coupling them quantitatively by “click” reaction to form 2G and 3G dendritic polymers. Despite the fact that the molecular weight of these polymers was not high (∼50 kg/mol), the generations were connected with biodegradable linkers, rendering these polymeric materials potential candidates for biomedical applications. One of the most interesting properties of dendrimer-like polymers is the significant difference in the intrinsic viscosity ([η]dendrimer‑like) in comparison with their linear analogues. Experimental results from Hirao’s305,315 and Gnanou’s302 groups showed that dendritic polymers reach a maximum of intrinsic viscosity as a function of molecular weight, indicating the absence of entanglements by the increment of the molecular weight. On the contrary, for linear polymers the viscosity increases linearly with the molecular weight due to the numerous entanglements of the polymer chains (Figure 7).316 The uniqueness of dendrimer-like polymers rendered them ideal models for rheological studies.317 Although there are

and transformation of the end groups for chain branching reactions. Recently, He and co-workers308 reported a novel method for the synthesis of dendrimer-like star polymers by continuous anionic polymerization. This approach based on the selective addition of s-BuLi toward 1,3-bis(1-phenylethenyl)benzene results in stoichiometric monoadduct when THF is used as solvent. The product, an anionic inimer, is used as the branching agent. Second generation is formed after the equivalent addition of PS−Li+ followed by further polymerization of styrene. Repeating addition/polymerization in an alternative way led to formation of 5G dendritic polymers. The facile functionalization of the anion groups at the periphery, without the need of precipitation/purification for the intermediate generations, is the major advantage of this approach. In convergent approach for the synthesis of dendrimer-like polymers, the exterior dendritic polymeric fragments, so-called “dendrons”, were first synthesized followed by attachment to a multifunctional core. Toward this direction, Knauss et al.309 reported the usage of CDMSS as the coupling agent, containing a polymerizable vinyl group and −Cl moiety for the quantitative reaction with polymeric anions. After the slow addition of CDMSS to a solution of PS−Li+ the central living anion was used to initiate polymerization of styrene to form a living star, which was coupled by further addition of stoichiometric amount of CDMSS. This process is efficient but the number of arms is variable. Hadjichristidis et al.310 used the same asymmetric coupling agent to synthesize 2G dendritic polymers of PS and PI. The first step involves the coupling of the PI−Li+ with the chlorosilane group, which is much more reactive than the vinyl group toward anionic species, forming a PI macromonomer. The second step was further addition of PI−Li+ yielding an in-chain functionalized PI with an anion at the linking point. The resulting anion was used to initiate polymerization of isoprene to generate the living star. Finally, the living PI star was coupled with either CH3SiCl3 or SiCl4 to obtain 2G dendritic-like polymers. This methodology was extended to the synthesis of 2G or 3G dendrimer-like PB.311 The main difference was the use of DCMSDPE as coupling agent. First, PB with in-chain double bond was synthesized by coupling living PB with the dichlorosilane group of DCMSDPE. Afterward, activation of X


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ities for large-scale production. Novel organic reactions and one-pot synthesis with high conversion are essential in order for dendritic polymers to be used in real-life applications.

7. MULTICOMPONENT−MULTIBLOCK LINEAR POLYMERS Although linearity is far from being considered as a complex structure, due to challenging synthetic procedures and the unusual/unique properties, some linear quarter/quinto polymers are deservedly classified as complex macromolecules. Synthesis of these multicomponent−multiblock polymers can be accomplished by sequential monomer addition or by synthesis of two different diblocks (e.g., AB and CD) and their subsequent connection using a coupling agent to afford ABCD. In both approaches special attention should be paid to the reagent purification and to the stoichiometry of the coupling reaction, respectively. Hadjichristidis and co-workers324,325 synthesized linear tetrablock quarterpolymer of PS-b-PI-b-PDMS-b-P2VP. Living PS-b-PI-b-PDMS was first synthesized by sequential anionic polymerization of the corresponding monomers and subsequently reacted with 2-(4-chloromethylphenyl)ethyldimethylchlorosilane as heterofunctional linking agent due to the higher reactivity of the silyl chloride than the benzyl chloride (−BnCl) group toward the silanolate anion. Living P2VP block was reacted with the ω-BnCl-functionalized triblock terpolymer in the presence of CsI catalyst at −78 °C to afford the tetrablock quarterpolymer. This polymer cannot be synthesized only by sequential addition due to low nucleophilicity of living PDMS to initiate polymerization of 2VP and vice versa. Four phases of the microphase-separated structure were observed for the first time by TEM. The PI, PDMS, and P2VP microstructures formed triple coaxial cylinders with a hexagonal shape packed in a hexagonal array in the PS honeycomb shaped matrix (Figure 8).

Figure 7. Mark−Houwink−Sakurada plots showing the dependence of the intrinsic viscosity (log[η]) on the molecular weight (log[M]) for various topological polymers. Reproduced with permission from ref 316. Copyright 2006 The Royal Society of Chemistry.

extensive studies for linear polymers in this field, there are limited models predicting the rheological behavior of branched polymers due to the structure complexity. In 2007, Vlassopoulos et al.318 reported the rheological behavior of dendrimer-like 3G PB, where for the first time three distinct plateau modulus were observed, corresponding to the three layers of the dendrimer-like polymer. This result is in good agreement with the principle of hierarchical motion of the branched structures from the outside inward.319,320 Only a few groups studied the morphology of dendrimer-like polymers in solution or in bulk. The first attempt was from Deffieux and co-workers321 in order to visualize well-defined 3G dendrimer-like PS (Mn = 1.43 × 107 g/mol, Đ = 1.08) by AFM tapping mode. Even though isolated polymeric structures were observed on the AFM images, the size was different from that calculated due to strong interactions between the polymer and the surface. Avgeropoulos et al.322 synthesized 2G dendritic polymers by anionic polymerization procedures in combination with chlorosilane chemistry, consisting of PB and PI for morphological study in bulk. The observed morphology was hexagonally close-packed cylinders of PI (minority component) in the PB matrix. Fan, Shen, and co-workers323 synthesized dendritic-linear block copolymer consisting of a dendron with PEG tails and PS linear chain. The effect of dendron generation and salt concentration on the morphology was also studied. The results showed that 1G or 2G dendritic-like polymers undergo a phase transition from the morphology of hexagonally packed cylinders to lamellae structure upon increasing the salt concentration, while the 3G always displays a lamellae phase. Nature-mimicing dendritic architecture enriched polymers with extraordinary and sometimes unexpected properties. A multivalent surface with high number of functionalities, intramolecular cavities, high density, low intrinsic viscosity, and size comparable to proteins along with the physicochemical properties from polymer itself rendered these materials powerful candidates for future applications. For the past four decades scientists focused mainly on synthesis of well-defined dendrimer-like polymers by combining different synthetic strategies with molecular parameters like chemical structure, molecular weight, polydispersity, and branching density. The results confirmed that the synthesis of dendritic polymers with up to seven generations or with 508 segments is feasible. These polymers are of major importance for extracting new model theories in order to understand better the structure−properties relationship. However, high reaction steps, large excess of reagents, extensive purification, and sometimes very demanding techniques decrease the possibil-

Figure 8. Schematic illustration of the model for the hexagonal triple coaxial cylinder structure observed in PS-b-PI-b-PDMS-b-P2VP quarterpolymer and the corresponding TEM image. Reproduced with permission from ref 324.

Following the same synthetic approach, well-defined linear pentablock quintopolymers were synthesized by the same group.326 Linear tetrablock quarterpolymer PS-b-PI-b-P2VP-bPEO and pentablock quintopolymers PS-b-PI-b-P2VP-bPtBMA-b-PEO were synthesized by anionic sequential addition using benzylpotassium as initiator. Since the reactivity of each monomer increases in this order S = I < 2VP < tBMA < EO, the sequence of monomer addition is reasonable.327 Avgeropoulos et al.328 combined anionic polymerization with hydrosilylation/chlorosilane chemistry to synthesize wellY


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Natural structures highlights the wide gap between the two families and clearly indicates that particular features in Natural macromolecules are absent in those produced synthetically. Nature has spent millions of years developing a hierarchical strategy to construct complex macromolecules. Thus, 50 years of intense research seems an undoubtedly short time period in comparison. The perpetual effort of the scientific community to mimic Nature’s complexity gave birth to polymeric materials with sophisticated structures like star, cyclic, graft, dendrimer, and multiblock polymers. On the contrary, biological systems constitute mainly from linear macromolecules (proteins, DNA), and the structural diversity arises from the high order conformation through noncovalent interactions (hydrogen bonds, etc.). It is obvious that combination of covalent (polymer chemistry) and intermolecular noncovalent (supramolecular chemistry) interactions is perfectly suited for the synthesis of Natural-like macromolecules. Unfortunately, only a handful of research groups focus on structural macromolecular formation using concepts from supramolecular chemistry. There is no doubt that progress should be made in this emerging field to aid the transition from classical polymer chemistry to that observed in Nature. Another characteristic of Natural macromolecules is the monodispersity. Unquestionably, in our opinion this is the “Holy Grail” for polymer chemists. Until now synthetic monodispersed polymers is something unattainable, even for linear homopolymers. Despite the fact that anionic polymerization is considered as the most efficient polymerization technique, limitations concerning the polydispersity, especially for complex macromolecular architectures, remain. The answer to this problem may be found in biosynthetic pathways like DNA replication or proteins formation. Nature uses unfolding DNA as template to synthesize complementary DNA chains. In similar way, specially designed templates can be used for fabrication of monodispersed functional polymers with high sequence variation. Furthermore, advancements in solid-phase synthesis and design of more efficient molecular catalysts could be contributory toward this direction. Besides polydispersity, there are some common drawbacks regarding the synthesis of complex macromolecular architectures. The high number of synthetic steps along with the usage of highly demanding synthetic techniques lead to limited scale production. To overcome this problem, one-pot synthetic procedure and highly efficient coupling methodologies along with more sophisticated initiators need to be implemented. To conclude, the toolbox that polymer chemists possess is very limited to reach the high level of complexity found in biomaterials, but no one can predict the course and the final destination of the journey called “synthesis of polymers with complex architecture”. To our opinion, in addition to the work needed on synthesis, properties, and applications of copolymers with more than three chemically different blocks and complex architecture, polymer chemists should follow closer the approaches that Nature, the perfect chemist, uses to make functional complex macromolecular structures by noncovalent chemistry. Moreover, development of new analytical methods for the characterization/purification of complex macromolecular architectures is essential for the synthesis and properties study of these polymers.

defined heptablock quartepolymers of ABCDCBA type (A: PS, B: PB, C: 3,4-PI, D: PDMS). The self-assembly in bulk of these quartepolymers, studied by TEM, revealed 3-phase 4-layer alternating lamellae morphology. Through differential scanning calorimetry (DSC) along with TEM observations the authors claimed that 3,4-PI and PDMS are miscible due to low χ Flory−Huggins parameter (Figure 9). Therefore, the morphology is comparable with the ABCBA of linear pentablock terpolymers.

Figure 9. Schematic illustration explaining the heptablock quartepolymers morphology and a TEM image for comparison. Reproduced with permission from ref 328. Copyright 2016 Wiley Periodicals, Inc.

Besides anionic polymerization, other methods along with high efficiency chemical reactions were employed for the synthesis of multicomponent−multiblock polymers. Tunca and co-workers241 combined in one-pot reaction maleimideterminated PEG anthracene- and azide-terminated PS, alkyneand bromide-terminated PtBuA, and tetramethylpiperidine-1oxyl (TEMPO)-terminated PCL to generate PEG-b-PS-bPtBA-b-PCL using Cu(0), CuBr, and PMDETA as catalyst. The obtained quarterpolymers showed narrow polydispersity (Đ = 1.12) and M̅ n ∼ 13 kg/mol. Recently, Zhang, Liu et al.330 reported the synthesis of amphiphilic linear tetrablock quarterpolymer of PEG-b-PS-b-PNIPAM-b-PDMAEMA through the combination of ATRP, RAFT, and “click” chemistry. The first step was to synthesize ω-azide PEG-b-PS and transform the terminal group to a RAFT agent via “click” chemistry, followed by sequential RAFT polymerization of NIPAM and DMAEMA to afford the quartepolymer. Even though multicomponent−multiblock linear polymers consist of many blocks, the one-pot synthetic procedure is feasible. These polymeric materials demonstrate unique multiphase-separated structures, ideal for applications in lithography and membrane separation.

8. CONCLUSIONS: FUTURE PERSPECTIVE Over the past 50 years, there has been tremendous progress in the synthesis of macromolecules with complex architecture. The “catalyst” to initiate this journey was the discovery of living anionic polymerization by Szwarc and co-workers156 in 1956. After this ignition point, only few groups were involved in synthesis of complex macromolecules, establishing the pillars for macromolecular architecture. Resurgence in this field occurred two decades ago as new polymerization techniques arose from organic chemistry (ATRP) along with some efficient chemical reactions (“click” chemistry). Nowadays polymer chemists possess the know how to synthesize imaginative and tailor-made macromolecules with well-defined molecular structure and fascinating properties. Nevertheless, the comparison of synthetic macromolecules with Z


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AUTHOR INFORMATION

interests include polyhomologation, synthesis, and self-assembly of block copolymers.

Corresponding Author

*E-mail: [email protected] (N.H.). ORCID

Konstantinos Ntetsikas: 0000-0002-9236-931X Panayiotis Bilalis: 0000-0002-5809-9643 Yves Gnanou: 0000-0001-6253-7856 Nikos Hadjichristidis: 0000-0003-1442-1714 Notes

The authors declare no competing financial interest. Biographies

Konstantinos Ntetsikas graduated from the Department of Materials Science Engineering (DMSE) of University of Ioannina, Greece, in 2008. He obtained his MSc in 2010 from the Interdepartmental Postgraduate Study Program ‘Chemistry and Materials Technology’ in collaboration of DMSE and Chemistry Department also at University of Ioannina. Furthermore, he received his PhD (2015) in Polymer Chemistry from the same Department under the guidance of Prof. Apostolos Avgeropoulos. Since 2016, he joined Prof. Nikos Hadjichristidis’ research group as postdoctoral fellow at King Abdullah George Polymeropoulos received his BSc in Materials Science Engineering from the University of Ioannina in Greece in 2009. He completed his MSc (2011) and PhD (2015) in Polymer Chemistry at the University of Ioannina, under the supervision of Prof. A. Avgeropoulos. He is currently a postdoctoral fellow in Catalysis Center, at King Abdullah University of Science and Technology (KAUST), working at the Polymer Synthesis Laboratory under the guidance of Prof. N. Hadjichristidis. His research interests include synthesis of well-defined block copolymers through different polymerization methods and the self-assembly behavior in selective solvents or in bulk.

University of Science and Technology (KAUST). His scientific interests include polymer synthesis by anionic polymerization techniques, morphological and thermal characterization of linear and nonlinear block co/terpolymers, nanoimprint lithography, and direct self-assembly (DSA) techniques.

Panayiotis Bilalis received his BSc in Chemistry from the University of Patras in 2003. He completed his MSc (2005) and PhD (2008) degree from the University of Athens in Polymer Chemistry under the supervision of Prof. N. Hadjichristidis. Since then he has been a

George Zapsas was born in Athens and attended Department of Materials Science Engineering (DMSE), University of Ioannina, completing a BSc degree in 2007 and MSc (Polymer Chemistry) under the guidance of Prof. A. Avgeropoulos (2009). In 2010, he earned a PhD position in the same department where he worked under the supervision of Prof. N. Zafeiropoulos on the synthesis and characterization of block copolymer based nanocomposites. In 2015, after completion of his PhD, he moved to KAUST as a postdoctoral fellow in Prof. N. Hadjichristidis’ research group. His current research

postdoctoral researcher at the National Center for Scientific Research “Demokritos” (2009−2012) and at University of Athens (2013− 2015). He is currently a Research Scientist at King Abdullah University of Science and Technology (KAUST), working at the Research Group of Prof. N. Hadjichristidis. His research interests are in the field of biomaterials where many interdisciplinary fields such as polymer chemistry, drug delivery, and nanotechnology intersect. AA


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ACKNOWLEDGMENTS

The research reported in this publication was supported by the King Abdullah University of Science and Technology (KAUST).

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ABBREVIATIONS PS polystyrene CH3SiCl3 trichloromethylsilane tetrachlorosilane SiCl4 dichlorodimethylsilane CH3SiCl2 (CH3)3SiCl chlorotrimethylsilane PI polyisoprene [η] intrinsic viscosity SEC size exclusion chromatography PMMA poly(methyl methacrylate) DVB divinylbenzene PHIC poly(n-hexyl isocyanate) refractive index increment dn/dc s-BuLi sec-butyllithium tri-DPE 1,3,5-tris(1-phenylethenyl)benzene THF tetrahydrofuran PB polybutadiene PEO poly(ethylene oxide) ROP ring-opening polymerization DPMK diphenylmethylpotassium PE polyethylene SCFT self-consistent field theory n number of arms f volume fraction OBDG ordered bicontinuous double gyroid TEM transmission electron microscopy SAXS small-angle X-ray scattering ODT order−disorder transition ATRP atom transfer radical polymerization RAFT reversible addition−fragmentation chain-transfer polymerization P2VP poly(2-vinylpyridine) PnBuA poly(n-butyl acrylate) AIBN azoisobutylnitrile PNIPAM poly(N-isopropylacrylamide) t-BuP4 phosphazene base ε-CL ε-caprolactone DLS dynamic light scattering CMC critical micelle concentration NCAs N-carboxyanhydrides PBLG γ-benzyl-L-glutamate PZLL ε-benzyloxycarbonyl-L-lysine DPE diphenylethylene DDPE double diphenylethylene MDDPE 1,3-bis(1-phenylethenyl)benzene PDDPE 1,4-bis(1-phenylethenyl)benzene 4MPVO 4-methylphenylvinyl sulfoxide MO membrane osmometry NMR nuclear magnetic resonance Đ polydispersity index PtBuMA poly(tert-butyl methacrylate) PtBuA poly(tert-butyl acrylate) PMS−Li+ poly(4-methylstyryl)lithium DDFT dynamic density functional theory E molecular asymmetry CMPMDS chloromethylphenylethenyldichloromethylsilane P(α-MeS) poly(α-methylstyrene)

Yves Gnanou is Distinguished Professor and Dean of Physical Science and Engineering Division at King Abdullah University of Science and Technology (KAUST), Saudi Arabia. He joined KAUST from École Polytechnique in Paris, where he held the title of vice president of academic affairs and of research. Previously, he held the position of professor and director of the Laboratoire de Chimie des Polymères Organiques at Bordeaux University, France, from 1999 to 2007. During his tenure in Bordeaux, he was also an adjunct professor at the University of Florida from 2002 to 2007 and a visiting professor at MIT from 1989 to 1990. Gnanou’s current research interests include the use of “green” organic catalysts in polymerization and the synthesis of functional architectures based on natural polymers and on CO2. He received the Langevin Prize and the Berthelot Medal from the French Academy of Sciences, and in 2009 he was elected as a member of the French Academy of Agriculture.

Nikos Hadjichristidis is Distinguished Professor of Chemical Sciences at King Abdullah University of Science and Technology, Saudi Arabia, and Emeritus Professor at the University of Athens, Greece. His research focuses mainly on the synthesis of novel homopolymers and copolymers with well-defined complex macromolecular architectures by using anionic polymerization (AP) high vacuum techniques as well as combination of AP with other polymerization methodologies (C1 and C3 polymerizations, ROP, ROMP, ATRP, etc.). These polymers are ideal models for checking the theory, understanding, and improving the performance of industrial polymers and are potential candidates for high-tech applications. He has received several awards including The Macro Group United Kingdom Medal for Outstanding Achievements (2016), the ACS National Award for Polymer Chemistry (2015), the ACS, Rubber Division Chemistry of Thermoplastic Elastomers Award (2011), The ACS, Polymeric Materials Science and Engineering (PMSE) Division Cooperative Research Award (2010), and The International Award of the Society of Polymer Science, Japan (SPSJ, 2007). AB


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(5) Schaefgen, J. R.; Flory, P. J. Synthesis of Multichain Polymers and Investigation of their Viscosities. J. Am. Chem. Soc. 1948, 70, 2709− 2718. (6) Morton, M.; Helminiak, T. E.; Gadkary, S. D.; Bueche, F. Preparation and Properties of Monodisperse Branched Polystyrene. J. Polym. Sci. 1962, 57, 471−482. (7) Hadjichristidis, N.; Roovers, J. Synthesis and Solution Properties of Linear, Four-Branched, and Six-Branched Star Polyisoprenes. J. Polym. Sci., Polym. Phys. Ed. 1974, 12, 2521−2533. (8) Graessley, W. W.; Masuda, T.; Roovers, J.; Hadjichristidis, N. Rheological Properties of Linear and Branched Polyisoprene. Macromolecules 1976, 9, 127−141. (9) Hadjichristidis, N.; Guyot, A.; Fetters, L. J. Star-branched Polymers. 1. The Synthesis of Star Polyisoprenes Using Octa-and Dodecachlorosilanes as Linking Agents. Macromolecules 1978, 11, 668−672. (10) Hadjichristidis, N.; Fetters, L. J. Star-branched Polymers. 4. Synthesis of 18-arm Polyisoprenes. Macromolecules 1980, 13, 191− 193. (11) Bauer, B. J.; Hadjichristidis, N.; Fetters, L. J.; Roovers, J. Starbranched Polymers. 5. The Theta Temperature Depression for 8-and 12-arm Polyisoprenes in Dioxane. J. Am. Chem. Soc. 1980, 102, 2410− 2413. (12) Roovers, J.; Hadjichristidis, N.; Fetters, L. J. Analysis and Dilute Solution Properties of 12-and 18-arm-star Polystyrenes. Macromolecules 1983, 16, 214−220. (13) Roovers, J.; Zhou, L. L.; Toporowski, P. M.; van der Zwan, M.; Iatrou, H.; Hadjichristidis, N. Regular Star Polymers with 64 and 128 Arms. Models for Polymeric Micelles. Macromolecules 1993, 26, 4324− 4331. (14) Roovers, J.; Toporowski, P. M.; Martin, J. Synthesis and Characterization of Multiarm Star Polybutadienes. Macromolecules 1989, 22, 1897−1903. (15) Efstratiadis, V.; Tselikas, G.; Hadjichristidis, N.; Li, J.; Yunan, W.; Mays, J. W. Synthesis and Characterization of Poly(methyl methacrylate) Star Polymers. Polym. Int. 1994, 33, 171−179. (16) Zorba, G.; Pitsikalis, M.; Hadjichristidis, N. Novel Well-Defined Star Homopolymers and Star-Block Copolymers of Poly(n-hexyl isocyanate) by Anionic Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 2387−2399. (17) Nguyen, A. B.; Hadjichristidis, N.; Fetters, L. J. Static Light Scattering Study of High-Molecular Weight 18-arm Star Block Copolymers. Macromolecules 1986, 19, 768−773. (18) Quirk, R. P.; Tsai, Y. Trifunctional Organolithium Initiator Based on 1, 3, 5-tris (1-phenylethenyl) benzene. Synthesis of Functionalized, Three-Armed Star-Branched Polystyrenes. Macromolecules 1998, 31, 8016−8025. (19) Theodosopoulos, G. V.; Hurley, C. M.; Mays, J. W.; Sakellariou, G.; Baskaran, D. Trifunctional Organolithium Initiator for Living Anionic Polymerization in Hydrocarbon Solvents in the Absence of Polar Additives. Polym. Chem. 2016, 7, 4090−4099. (20) Knischka, R.; Lutz, P. J.; Sunder, A.; Mulhaupt, R.; Frey, H. Functional Poly(ethylene oxide) Multiarm Star Polymers: Core-First Synthesis Using Hyperbranched Polyglycerol Initiators. Macromolecules 2000, 33, 315−320. (21) Mendelson, R. A. Polyethylene Melt Viscosity: Shear RateTemperature Superposition. Trans. Soc. Rheol. 1965, 9, 53−63. (22) Mendelson, R. A.; Bowles, W. A.; Finger, F. L. Effect of Molecular Structure on Polyethylene Melt Rheology. I. Low-Shear Behavior. J. Polym. Sci., Polym. Phys. Ed. 1970, 8, 105−126. (23) Rachapudy, H.; Smith, G. G.; Raju, V. R.; Graessley, W. W. Properties of Amorphous and Crystallizable Hydrocarbon Polymers. III. Studies of the Hydrogenation of Polybutadiene. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 1211−1222. (24) Hadjichristidis, N.; Xenidou, M.; Iatrou, H.; Pitsikalis, M.; Poulos, Y.; Avgeropoulos, A.; Sioula, S.; Paraskeva, S.; Velis, G.; Lohse, D.; Schulz, D. N.; Fetters, L. J.; Wright, P. J.; Mendelson, R. A.; Garcıa-Franco, C. A.; Sun, T.; Ruff, C.s J. Well-Defined, Model Long

NMRP DMAEMA PCP

nitroxide-mediated radical polymerization 2-(dimethylamino)ethyl methacrylate 5-cyano-5-phenylthiocarbonylsulfanyl pentanoyloxy-2-hydroxy-methyl-2-methylpropanoate PEG poly(ethylene glycol) PLLA poly(L-lactide) PAA poly(acrylic acid) CDMSS 4-(chlorodimethylsilyl)styrene DCMSDPE 4-(dichloromethylsilyl)diphenylethylene LCB long-chain branched TGIC temperature gradient interaction chromatography CMPEMDS [ ( c h l o r o m e t h y l p h e n y l ) e t h y l ] m e t h y l dichlorosilane BnBr bromomethylphenyl TPEs thermoplastic elastomers ABS acrylonitrile−styrene−polybutadiene CCl4 carbon tetrachloride NaPSS poly(styrenesulfonate) DMP dimethyl phthalate BDG block-double graft TMEDA tetramethylethylenediamine CpTiCl3 cyclopentadienyltitanium(IV) trichloride p-allyltoluene p-AT BuOK potassium tert-butoxide MMs macromonomer strategy N,N-dimethylacetamide DMA VBC 4-vinylbenzyl chloride CTA chain transfer agent LCST low critical solution temperature PEB 4-bis(1-phenylethenyl)benzene DVSiOMA 2-((methyldivinylsilyl)oxy)ethyl methacrylate ROMP ring-opening metathesis polymerization NA Avogadro’s number the mean-square end-to-end distance ⟨r2⟩ PDMS polydimethylsiloxane D3 hexamethylcyclotrisiloxane BDCSE bis(dimethylchlorosilyl)ethane SANS small-angle neutron scattering Cu(I)Br copper bromide PMDETA N,N,N′,N″,N″-pentamethyldiethylenetriamine stress relaxation modulus G(t) PAMAM poly(amidoamine) bis-MPA 2,2-bis(hydroxymethyl)propionic acid Sn(Oct)2 stannous-2-ethylhexanoate tert-butyldimethylsilyloxymethylphenyl SMP AFM atomic force microscopy BnCl benzyl chloride DSC differential scanning calorimetry TEMPO tetramethylpiperidine-1-oxyl

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REFERENCES

(1) Hadjichristidis, N.; Hirao, A. In Anionic Polymerization: Principles, Practice, Strength, Consequences and Applications; Springer: Japan, 2015; Preface. (2) Bates, F. S.; Hillmyer, M. A.; Lodge, T. P.; Bates, C. M.; Delaney, K. T.; Fredrickson, G. H. Multiblock Polymers: Panacea or Pandora’s Box? Science 2012, 336, 434−440. (3) Mishra, M. K.; Kobayashi, S. Stars and Hyperbranched Polymers; Marcel Dekker: New York, 1999. (4) Roovers, J. In Encyclopedia of Polymer Science and Technology, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; John Wiley & Sons, Inc.: 1985; Vol. 2, p 478. AC


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Macromolecules Chain Branched Polyethylene. 1. Synthesis and Characterization. Macromolecules 2000, 33, 2424−2436. (25) Carella, J. M.; Gotro, J. T.; Graessley, W. W. Thermorheological Effects of Long-Chain Branching in Entangled Polymer Melts. Macromolecules 1986, 19, 659−667. (26) Luo, J.; Shea, K. J. Polyhomologation. A Living C1 Polymerization. Acc. Chem. Res. 2010, 43, 1420−1433. (27) Shea, K. J.; Walker, J. W.; Zhu, H.; Paz, M.; Greaves, J. Polyhomologation. A Living Polymethylene Synthesis. J. Am. Chem. Soc. 1997, 119, 9049−9050. (28) Wagner, C. E.; Rodriguez, A. A.; Shea, K. J. Synthesis of Linear α-Olefins via Polyhomologation. Macromolecules 2005, 38, 7286− 7291. (29) Zhang, H.; Alkayal, N.; Gnanou, Y.; Hadjichristidis, N. Polymethylene-Based Copolymers by Polyhomologation or by Its Combination with Controlled/Living and Living Polymerizations. Macromol. Rapid Commun. 2014, 35, 378−390. (30) Zhang, Z.; Gnanou, Y.; Hadjichristidis, N. Well-Defined 4-arm Stars with Hydroxy-Terminated Polyethylene, Polyethylene-b-Polycaprolactone and Polyethylene-b-(Polymethyl methacrylate)2 Arms. Polym. Chem. 2016, 7, 5507−5511. (31) Matsen, M. W.; Schick, M. Microphase Separation in Starblock Copolymer Melts. Macromolecules 1994, 27, 6761−6767. (32) Matsen, M. W. Effect of Architecture on the Phase Behavior of AB-type Block Copolymer Melts. Macromolecules 2012, 45, 2161− 2165. (33) Olvera de la Cruz, M.; Sanchez, I. C. Theory of Microphase Separation in Graft and Star Copolymers. Macromolecules 1986, 19, 2501−2508. (34) Herman, D. S.; Kinning, D. J.; Thomas, E. L.; Fetters, L. J. A Compositional Study of the Morphology of 18-armed Poly(styreneisoprene) Star Block Copolymers. Macromolecules 1987, 20, 2940− 2942. (35) Floudas, G.; Pispas, S.; Hadjichristidis, N.; Pakula, T.; Erukhimovich, I. Microphase Separation in Star Block Copolymers of Styrene and Isoprene. Theory, Experiment, and Simulation. Macromolecules 1996, 29, 4142−4154. (36) Floudas, G.; Paraskeva, S.; Hadjichristidis, N.; Fytas, G.; Chu, B.; Semenov, A. N. Dynamics of Polyisoprene in Star Block Copolymers Confined in Microstructures: A Dielectric Spectroscopy Study. J. Chem. Phys. 1997, 107, 5502−5509. (37) Ueda, J.; Matsuyama, M.; Kamigaito, M.; Sawamoto, M. Multifunctional Initiators for the Ruthenium-Mediated Living Radical Polymerization of Methyl Methacrylate: Di- and Trifunctional Dichloroacetates for Synthesis of Multiarmed Polymers. Macromolecules 1998, 31, 557−562. (38) Matyjaszewski, K.; Miller, P.; Pyun, J.; Kickelbick, J.; Diamanti, S. Synthesis and Characterization of Star Polymers with Varying Arm Number, Length, and Composition from Organic and Hybrid Inorganic/Organic Multifunctional Initiators. Macromolecules 1999, 32, 6526−6535. (39) Moschogianni, P.; Pispas, S.; Hadjichristidis, N. Multifunctional ATRP Initiators: Synthesis of Four-Arm Star Homopolymers of Methyl Methacrylate and Graft Copolymers of Polystyrene and Poly(tbutyl methacrylate). J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 650−655. (40) Polymeropoulos, G.; Moschovas, D.; Kati, A.; Karanastasis, A.; Pelekanou, S.; Christakopoulos, P.; Sakellariou, G.; Avgeropoulos, A. Synthesis via ATRP, Kinetics Study and Characterization (MolecularMorphological) of 3-Arm Star Diblock Copolymers of the (PS-bP2VP)3 Type. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 23−33. (41) Zheng, Q.; Pan, C. Synthesis and Characterization of Dendrimer-Star Polymer Using Dithiobenzoate-Terminated Poly(propylene imine) Dendrimer via Reversible Addition-Fragmentation Transfer Polymerization. Macromolecules 2005, 38, 6841−6848. (42) Darcos, V.; Duréault, A.; Taton, D.; Gnanou, Y.; Marchand, P.; Caminade, A.; Majoral, J.; Destarac, M.; Leising, F. Synthesis of Hybrid Dendrimer-Star Polymers by the RAFT Process. Chem. Commun. 2004, 35, 2110−2111.

(43) Bernard, J.; Favier, A.; Zhang, L.; Nilasaroya, A.; Davis, T.; Barner-Kowollik, C.; Stenzel, M. Poly(vinyl ester) Star Polymers via Xanthate-Mediated Living Radical Polymerization: From Poly(vinyl alcohol) to Glycopolymer Stars. Macromolecules 2005, 38, 5475−5484. (44) Hao, X.; Nilsson, C.; Jesberger, M.; Stenzel, M.; Malmström, E.; Davis, T.; Ö stmark, E.; Barner-Kowollik, C. Dendrimers as Scaffolds for Multifunctional Reversible Addition-Fragmentation Chain Transfer Agents: Syntheses and Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5877−5890. (45) Zheng, G.; Pan, C. Preparation of Star Polymers Based on Polystyrene or Poly(styrene-b-N-isopropyl acrylamide) and Divinylbenzene via Reversible Addition-Fragmentation Chain Transfer Polymerization. Polymer 2005, 46, 2802−2810. (46) Zhao, J.; Pahovnik, D.; Gnanou, Y.; Hadjichristidis, N. One-Pot Synthesis of Linear- and Three-arm Star-Tetrablock Quarterpolymers via Sequential metal-Free Ring-Opening Polymerization Using a “Catalyst Switch” Strategy. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 304−312. (47) Satoh, Y.; Miyachi, K.; Matsuno, H.; Isono, T.; Tajima, K.; Kakuchi, T. Synthesis of Well-Defined Amphiphilic Star-Block and Miktoarm Star Copolyethers via t-Bu-P4 Catalyzed Ring-Opening Polymerization of Glycidyl Ethers. Macromolecules 2016, 49, 499−509. (48) Polymeropoulos, G.; Zapsas, G.; Hadjichristidis, N.; Avgeropoulos, A. Synthesis and Self-Assembly of Amphiphilic Triblock Terpolymers with Complex Macromolecular Architecture. ACS Macro Lett. 2015, 4, 1392−1397. (49) Leuchs, H.; Manasse, W. Ü ber die Isomerie der Carbäthoxylglycyl glycinester. Ber. Dtsch. Chem. Ges. 1907, 40, 3235−3249. (50) Leuchs, H.; Geiger, W. Ü ber die Anhydride von α-Amino-Ncarbonsäuren und die von α-Aminosäuren. Ber. Dtsch. Chem. Ges. 1908, 41, 1721−1726. (51) Szwarc, M. The Kinetics and Mechanism of N-carboxy, aAmino-Acid Anhydride (NCA) Polymerisation to Poly-amino Acids. Ad. Polym. Sci. 1965, 4, 1−65. (52) Sekiguchi, H. Mechanism of N-carboxy-α-amino acid anhydride (NCA) polymerization. Pure Appl. Chem. 1981, 53, 1689−1714. (53) Pahovnik, D.; Hadjichristidis, N. Ring Opening Polymerization of N-CarboxyAnhydrides for Preparation of Polypeptides and Polypeptide-Based Hybrid Materials with Various Molecular Architectures. Anionic Polymerization; Springer: 2015; pp 307−337. (54) Hadjichristidis, N.; Iatrou, H.; Pitsikalis, M.; Sakellariou, G. Synthesis of Well-Defined Polypeptide-Based Materials via the RingOpening Polymerization of α-Amino Acid N-Carboxyanhydrides. Chem. Rev. 2009, 109, 5528−5578. (55) Aliferis, T.; Iatrou, H.; Hadjichristidis, N. Living Polypeptides. Biomacromolecules 2004, 5, 1653−1656. (56) Hanski, S.; Houbenov, N.; Ruokolainen, J.; Chondronicola, D.; Iatrou, H.; Hadjichristidis, N.; Ikkala, O. Hierarchical Ionic SelfAssembly of Rod-Comb Block Copolypeptide-Surfactant Complexes. Biomacromolecules 2006, 7, 3379−3384. (57) Klok, H. A.; Deming, T. J. Macromolecular Engineering of Polypeptides Using the Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydrides. Macromol. Eng. 2011, 1, 519−540. (58) Klok, H.; Hernandez, J.; Becker, S.; Müllen, K. Star-Shaped Fluorescent Polypeptides. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1572−1583. (59) Aliferis, A.; Iatrou, H.; Hadjichristidis, N. Well-Defined Linear Multiblock and Branched Polypeptides by Linking Chemistry. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4670−4673. (60) Gitsas, A.; Floudas, G.; Mondeshki, M.; Butt, H. J.; Spiess, H. W.; Iatrou, H.; Hadjichristidis, N. Effect of Chain Topology on the Self-Organization and Dynamics of Block Copolypeptides: From Diblock Copolymers to Stars. Biomacromolecules 2008, 9, 1959−1966. (61) Schlaad, H.; Antonietti, M. Block Copolymers with Amino Acid Sequences: Molecular Chimeras of Polypeptides and Synthetic Polymers. Eur. Phys. J. E: Soft Matter Biol. Phys. 2003, 10, 17−23. (62) Klok, H.-A.; Lecommandoux, S. Solid State Structure, Organisation and Properties of Peptide-Synthetic Hybrid Block Copolymers. Ad. Polym. Sci. 2006, 202, 75−111. AD


Perspective

Macromolecules (63) Karatzas, A.; Iatrou, H.; Hadjichristidis, N.; Inoue, K.; Sugiyama, K.; Hirao, A. Complex Macromolecular Chimeras. Biomacromolecules 2008, 9, 2072−2080. (64) Karatzas, A.; Bilalis, P.; Iatrou, H.; Pitsikalis, M.; Hadjichristidis, N. Synthesis of Well-Defined Functional Macromolecular Chimeras Based on Poly(ethylene oxide) or Poly(N-vinyl pyrrolidone). React. Funct. Polym. 2009, 69, 435−440. (65) Qiu, S.; Huang, X.; Dai, X.-H.; Zhou, W.; Dong, C. M. StarShaped Polypeptide/Glycopolymer Biohybrids: Synthesis, Self-Assembly, Biomolecular Recognition, and Controlled Drug Release Behavior. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 2009−2023. (66) Hadjichristidis, N.; Pispas, S.; Pitsikalis, M.; Iatrou, H.; Vlahos, C. Asymmetric Star Polymers: Synthesis and Properties. Adv. Polym. Sci. 1999, 142, 71−127. (67) Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H. Polymers with Complex Architecture by Living Anionic Polymerization. Chem. Rev. 2001, 101, 3747−3792. (68) Hadjichristidis, N.; Iatrou, H.; Pitsikalis, M.; Mays, J. W. Macromolecular Architectures by Living and Controlled/Living Polymerizations. Prog. Polym. Sci. 2006, 31, 1068−1132. (69) Hückstädt, K.; Göpfert, A.; Abetz, V. Synthesis and Morphology of ABC Heteroarm Star Terpolymers of Polystyrene, Polybutadiene and Poly(2-vinylpyridine). Macromol. Chem. Phys. 2000, 201, 296− 307. (70) Hayashida, K.; Arai, S.; Shinohara, Y.; Amemiya, Y.; Matsushita, Y. Systematic Transitions of Tiling Patterns Formed by ABC StarShaped Terpolymers. Macromolecules 2006, 39, 9402−9408. (71) Li, Z.; Hillmyer, M. C.; Lodge, T. P. Morphologies of Multicompartment Micelles Formed by ABC Miktoarm Star Terpolymers. Langmuir 2006, 22, 9409−9417. (72) Pennisi, R. W.; Fetters, L. J. Preparation of Asymmetric ThreeArm Polybutadiene and Polystyrene Stars. Macromolecules 1988, 21, 1094−1099. (73) Mays, J. W. Synthesis of ″Simple Graft″ Poly(isoprene-gstyrene) by Anionic Polymerization. Polym. Bull. 1990, 23, 247−250. (74) Pitsikalis, M.; Pispas, S.; Mays, J. W.; Hadjichristidis, N. Nonlinear Block Copolymer Architectures. Adv. Polym. Sci. 1998, 135, 1−137. (75) Hsieh, H. L.; Quirk, R. P. In Anionic Polymerization: Principles and Practical Applications; Marcel Dekker: New York, 1996. (76) Frater, D. J.; Mays, J. W.; Jackson, C. J. Synthesis and Dilute Solution Properties of Divinylbenzene-Linked Polystyrene Stars with Mixed Arm Lengths: Evidence for Coupled Stars. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 141−151. (77) Du, J.; Chen, Y. Preparation of Poly(ethylene oxide) Star Polymers and Poly(ethylene oxide)-Polystyrene Heteroarm Star Polymers by Atom Transfer Radical Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2263−2271. (78) Quirk, R. P.; Lee, B. Anionic Synthesis of Polystyrene and Polybutadiene Heteroarm Star Polymers. Makromol. Chem., Macromol. Symp. 1992, 53, 201−210. (79) Quirk, R. P.; Yoo, T. Anionic Synthesis of ω-l,l-Diphenylethylene-Terminated Polystyrene Macromonomers. Rational Synthesis of ABC Hetero Three-Armed-Star-Branched Polymers. Polym. Bull. 1993, 31, 29−36. (80) Hirao, A.; Hayashi, M.; Haraguchi, N. Synthesis of Well-Defined Functionalized Polymers and Star Branched Polymers by Means of Living Anionic Polymerization Using Specially Designed 1,1-Diphenylethylene Derivatives. Macromol. Rapid Commun. 2000, 21, 1171−1184. (81) Hirao, A.; Hayashi, M. Synthesis of Well-Defined Functionalized Polystyrenes with a Definite Number of Chloromethylphenyl Groups at Chain Ends or in Chains by Means of Anionic Living Polymerization in Conjunction with Functional Group Transformation. Macromolecules 1999, 32, 6450−6460. (82) Zhao, Y.; Higashihara, T.; Sugiyama, K.; Hirao, A. Synthesis of Functionalized Asymmetric Star Polymers Containing Conductive Polyacetylene Segments by Living Anionic Polymerization. J. Am. Chem. Soc. 2005, 127, 14158−14159.

(83) Tselikas, Y.; Hadjichristidis, N.; Lescanec, R. L.; Honeker, C. C.; Wohlgemuth, M.; Thomas, E. L. Architecturally-Induced Tricontinuous Cubic Morphology in Compositionally Symmetric Miktoarm Starblock Copolymers. Macromolecules 1996, 29, 3390−3396. (84) Shi, W.; Lynd, N. A.; Montarnal, D.; Luo, Y.; Fredrickson, G. H.; Kramer, E. J.; Ntaras, C.; Avgeropoulos, A.; Hexemer, A. Toward Strong Thermoplastic Elastomers with Asymmetric Miktoarm Block Copolymer Architectures. Macromolecules 2014, 47, 2037−2043. (85) Shi, W.; Fredrickson, G. H.; Kramer, E. J.; Ntaras, C.; Avgeropoulos, A.; Demassieux, Q.; Creton, C. Mechanics of an Asymmetric Hard−Soft Lamellar Nanomaterial. ACS Nano 2016, 10, 2054−2062. (86) Frischknecht, A. L.; Milner, S. T.; Pryke, A.; Young, R. N.; Hawkins, R.; McLeish, T. C. B. Rheology of Three-Arm Asymmetric Star Polymer Melts. Macromolecules 2002, 35, 4801−4820. (87) Lee, J. H.; Fetters, L. J.; Archer, L. A. Branch-Point Motion in Asymmetric Star Polymers. Macromolecules 2005, 38, 4484−4494. (88) Lee, J. H.; Fetters, L. J.; Archer, L. A. Stress Relaxation of Branched Polymers. Macromolecules 2005, 38, 10763−10771. (89) Francis, R.; Lepoittevin, B.; Taton, D.; Gnanou, Y. Toward an Easy Access to Asymmetric Stars and Miktoarm Stars by Atom Transfer Radical Polymerization. Macromolecules 2002, 35, 9001− 9008. (90) Blencowe, A.; Tan, J. F.; Goh, T. K.; Qiao, G. G. Core CrossLinked Star Polymers via Controlled Radical Polymerisation. Polymer 2009, 50, 5−32. (91) Matyjaszewski, K. Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives. Macromolecules 2012, 45, 4015−4039. (92) Morton, M.; Helminiak, T. E.; Gadkary, S. D.; Bueche, F. Preparation and Properties of Monodisperse Branched Polystyrene. J. Polym. Sci. 1962, 57, 471−482. (93) Orofino, T. A.; Wenger, F. Dilute Solution Properties of Branched Polymers. Polystyrene Trifunctional Star Molecules. J. Phys. Chem. 1963, 67, 566−575. (94) Mayer, R. Organized Structures in Amorphous Styrene/cis-l,4Isoprene Block Copolymers: Low Angle X-ray Scattering and Electron Microscopy. Polymer 1974, 15, 137−145. (95) Eschwey, H.; Burchard, W. Star Polymers from Styrene and Divinylbenzene. Polymer 1975, 16, 180−184. (96) Tsitsilianis, C.; Chaumont, P.; Rempp, P. Synthesis and Characterization of Hetero-Arm Star Copolymers. Makromol. Chem. 1990, 191, 2319−2328. (97) Tsitsilianis, C.; Graff, S.; Rempp, P. Hetero-Arm Star Copolymers with Potentially Ionogenic Branches. Eur. Polym. J. 1991, 27, 243−246. (98) Tsitsilianis, C.; Lutz, P. J.; Graff, S.; Lamps, J. P.; Rempp, P. Core-First Synthesis of Star Polymers With Potentially Ionogenic Branches. Macromolecules 1991, 24, 5897−5902. (99) Rein, D.; Rempp, P.; Lutz, P. J. Recent Developments in the Field of Star-Shaped Polymers. Makromol. Chem., Macromol. Symp. 1993, 67, 237−249. (100) Tsitsilianis, C.; Papanagopoulos, D.; Lutz, P. J. Amphiphilic Heteroarm Star Copolymers of Polystyrene and Poly(ethylene oxide). Polymer 1995, 36, 3745−3752. (101) Tsitsilianis, C.; Voulgaris, D. Poly(2-vinylpyridine)-Based StarShaped Polymers. Synthesis of Heteroarm Star (AnBn) and Star-Block (AB)n Copolymers. Macromol. Chem. Phys. 1997, 198, 997−1007. (102) Quirk, R. P.; Yoo, T.; Lee, B. Anionic Synthesis of Heteroarm, Star-Branched Polymers. Scope and Limitations. J. Macromol. Sci., Part A: Pure Appl.Chem. 1994, 31 (8), 911−926. (103) Iatrou, H.; Siakali-Kioulafa, E.; Hadjichristidis, N.; Roovers, J.; Mays, J. W. Hydrodynamic Properties of Model 3-Miktoarm Star Copolymers. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1925−1932. (104) Tselikas, Y.; Hadjichristidis, N.; Iatrou, H.; Liang, K. S.; Lohse, D. J. Morphology of Miktoarm Star Block Copolymers of Styrene and Isoprene. J. Chem. Phys. 1996, 105, 2456−2462. AE


Perspective

Macromolecules

to Macromolecular Architectures. Macromolecules 2014, 47, 1883− 1905. (124) Lohse, D. J.; Hadjichristidis, N. Microphase separation in block copolymers. Curr. Opin. Colloid Interface Sci. 1997, 2, 171−176. (125) Hadjichristidis, N.; Iatrou, H.; Behal, S. K.; Chludzinski, J. J.; Disko, M. M.; Garner, R. T.; Liang, K. S.; Lohse, D. J.; Milner, S. T. Morphology and Miscibility of Miktoarm Styrene-Diene Copolymers and Terpolymers. Macromolecules 1993, 26, 5812−5815. (126) Pochan, D. J.; Gido, S. P.; Pispas, S.; Mays, J. W.; Ryan, A. J.; Fairclough, J. P. A.; Hamley, I. W.; Terrill, N. J. Morphologies of Microphase-Separated A2B Simple Graft Copolymers. Macromolecules 1996, 29, 5091−5098. (127) Pochan, D. J.; Gido, S. P.; Pispas, S.; Mays, J. W. Morphological Transitions in an I2S Simple Graft Block Copolymer: From Folded Sheets to Folded Lace to Randomly Oriented Worms at Equilibrium. Macromolecules 1996, 29, 5099−5105. (128) Avgeropoulos, A.; Dair, B. J.; Thomas, E. L.; Hadjichristidis, N. Swelling Behavior of Ordered Miktoarm Star Block Copolymer− Homopolymer Blends. Polymer 2002, 43, 3257−3266. (129) Floudas, G.; Hadjichristidis, N.; Iatrou, H.; Pakula, T.; Fischer, E. W. Microphase Separation in Model 3-Miktoarm Star Copolymers (Simple Graft and Terpolymers). 1. Statics and Kinetics. Macromolecules 1994, 27, 7735−7746. (130) Milner, S. T. Chain Architecture and Asymmetry in Copolymer Microphases. Macromolecules 1994, 27, 2333−2335. (131) Ramzi, A.; Prager, M.; Richter, D.; Efstratiadis, V.; Hadjichristidis, N.; Young, R. N.; Allgaier, J. B. Influence of Polymer Architecture on the Formation of Micelles of Miktoarm Star Copolymers Polyethylene/Poly(ethylenepropylene) in the Selective Solvent Decane. Macromolecules 1997, 30, 7171−7182. (132) Pispas, S.; Hadjichristidis, N.; Potemkin, I.; Khokhlov, A. Effect of Architecture on the Micellization Properties of Block Copolymers: A2B Miktoarm Stars vs AB Diblocks. Macromolecules 2000, 33, 1741− 1746. (133) Yang, L.; Hong, S.; Gido, S. P.; Velis, G. Hadjichristidis, N. I5S Miktoarm Star Block Copolymers: Packing Constraints on Morphology and Discontinuous Chevron Tilt Grain Boundaries. Macromolecules 2001, 34, 9069−9073. (134) Okamoto, S.; Hasegawa, H.; Hashimoto, T.; Fujimoto, T.; Zhang, H.; Kazama, T.; Takano, A.; Isono, Y. Morphology of Model Three-Component Three-Arm Star-Shaped Copolymers. Polymer 1997, 38, 5275−5281. (135) Gemma, T.; Hatano, A.; Dotera, T. Monte Carlo Simulations of the Morphology of ABC Star Polymers Using the Diagonal Bond Method. Macromolecules 2002, 35, 3225−3237. (136) He, H.; Huang, L.; Pan, C. Localizations of Junction Points of ABC 3-Miktoarm Star Terpolymers. J. Chem. Phys. 2003, 118, 9861− 9863. (137) Sioula, S.; Hadjichristidis, N.; Thomas, E. L. Direct Evidence for Confinement of Junctions to Lines in an 3-Miktoarm Star Terpolymer Microdomain Structure. Macromolecules 1998, 31, 8429− 8432. (138) Kirkensgaard, J. J. K.; Fragouli, P.; Hadjichristidis, N.; Mortensen, K. Perforated Lamellae Morphology in Novel P2VP(PDMS-b-PI-b-PS)2 3-Miktoarm Star Quarterpolymer. Macromolecules 2011, 44, 575−582. (139) Bellas, V.; Iatrou, H.; Pitsinos, N. E.; Hadjichristidis, N. Heterofunctional Linking Agents for the Synthesis of Well-Defined Block Copolymers of Dimethylsiloxane and tert-Butyl Methacrylate or 2-Vinylpyridine. Macromolecules 2001, 34, 5376−5378. (140) Deming, T. Facile Synthesis of Block Copolypeptides of Defined Architecture. Nature 1997, 390, 386−389. (141) Hadjichristidis, N.; Iatrou, H.; Pitsikalis, M.; Sakellariou, G. Synthesis of Well-Defined Polypeptide-Based Materials via the RingOpening Polymerization of α-Amino Acid N-Carboxyanhydrides. Chem. Rev. 2009, 109, 5528−5578. (142) Junnila, S.; Houbenov, N.; Hanski, S.; Iatrou, H.; Hirao, A.; Hadjichristidis, N.; Ikkala, O. Hierarchical Smectic Self-Assembly of an

(105) Avgeropoulos, A.; Hadjichristidis, N. Synthesis of Model Nonlinear Block Copolymers of A(BA)2, A(BA)3, and (AB)3A(BA)3 type. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 813−816. (106) Iatrou, H.; Hadjichristidis, N. Synthesis of a Model 3-Miktoarm Star Terpolymer. Macromolecules 1992, 25, 4649−4651. (107) Lee, C.; Gido, S. P.; Pitsikalis, M.; Mays, J. W.; Beck Tan, N.; Trevino, S. F.; Hadjichristidis, N. Asymmetric Single Graft Block Copolymers: Effect of Molecular Architecture on Morphology. Macromolecules 1997, 30, 3732−3738. (108) Iatrou, H.; Hadjichristidis, N. Synthesis and Characterization of Model 4-Miktoarm Star Co-and Quarterpolymers. Macromolecules 1993, 26, 2479−2484. (109) Bellas, V.; Iatrou, H.; Hadjichristidis, N. Controlled Anionic Polymerization of Hexamethylcyclotrisiloxane. Model Linear and Miktoarm Star Co-and Terpolymers of Dimethylsiloxane with Styrene and Isoprene. Macromolecules 2000, 33, 6993−6997. (110) Tsoukatos, T.; Hadjichristidis, N. Synthesis of Model Polycyclohexylene/Polyethylene Miktoarm Star Copolymers with Three and Four Arms. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2575−2582. (111) Mavroudis, A.; Avgeropoulos, A.; Hadjichristidis, N.; Thomas, E. L.; Lohse, D. J. Synthesis and Morphological Behavior of Model Linear and Miktoarm Star Copolymers of 2-Methyl-1, 3-Pentadiene and Styrene. Chem. Mater. 2003, 15, 1976−1983. (112) Zioga, A.; Sioula, S.; Hadjichristidis, N. Synthesis and Morphology of Model 3-Miktoarm Star Terpolymers of Styrene, Isoprene and 2-Vinyl Pyridine. Macromol. Symp. 2000, 157, 239−249. (113) Cho, D.; Park, S.; Chang, T.; Avgeropoulos, A.; Hadjichristidis, N. Characterization of a 4-Miktoarm Star Copolymer of the (PS-b-PI)3 PS Type by Temperature Gradient Interaction Chromatography. Eur. Polym. J. 2003, 39, 2155−2160. (114) Allgaier, J.; Young, R. N.; Efstratiadis, V.; Hadjichristidis, N. Synthesis and Characterization of Polyisoprene/Polybutadiene A2B2 Star Copolymers. Macromolecules 1996, 29, 1794−1797. (115) Velis, G.; Hadjichristidis, N. Synthesis of Model PS(PI)5 and (PI)5PS(PI)5 Nonlinear Block Copolymers of Styrene (S) and Isoprene (I). Macromolecules 1999, 32, 534−536. (116) Mavroudis, A.; Avgeropoulos, A.; Hadjichristidis, N.; Thomas, E. L.; Lohse, D. J. Synthesis and Morphological Behavior of Model 6Miktoarm Star Copolymers, PS(P2MP)5, of Styrene (S) and 2-Methyl1,3-Pentadiene (P2MP). Chem. Mater. 2006, 18, 2164−2168. (117) Avgeropoulos, A.; Poulos, Y.; Hadjichristidis, N.; Roovers, J. Synthesis of Model 16-Miktoarm (Vergina) Star Copolymers of the A8B8 Type. Macromolecules 1996, 29, 6076−6078. (118) Sioula, S.; Hadjichristidis, N.; Thomas, E. L. Novel 2Dimensionally Periodic Non-Constant Mean Curvature Morphologies of 3-Miktoarm Star Terpolymers of Styrene, Isoprene, and Methyl Methacrylate. Macromolecules 1998, 31, 5272−5277. (119) Mavroudis, A.; Hadjichristidis, N. Synthesis of Well-Defined 4Miktoarm Star Quarterpolymers (4μ-SIDV) with Four Incompatible Arms: Polystyrene (S), Polyisoprene-1,4 (I), Poly(dimethylsiloxane) (D), and Poly(2-vinylpyridine) (V). Macromolecules 2006, 39, 535− 540. (120) Hirao, A.; Hayashi, M.; Loykulnat, S.; Ryu, S. W.; Haragushi, N.; Matsuo, A.; Higashihara, T. Precise Syntheses of Chain-MultiFunctionalized Polymers, Star-Branched Polymers, Star-Linear Block Polymers, Densely Branched Polymers, and Dendritic Branched Polymers Based on Iterative Approach Using Functionalized 1,1Diphenylethylene Derivatives. Prog. Polym. Sci. 2005, 30, 111−182. (121) Ito, S.; Goseki, R.; Ishizone, T.; Hirao, A. Successive Synthesis of Well-Defined Multiarmed Miktoarm Star Polymers by Iterative Methodology Using Living Anionic Polymerization. Eur. Polym. J. 2013, 49, 2545−2566. (122) Higashihara, T.; Yoo, H. S.; Hayashi, M.; Hirao, A. Combining Living Anionic Polymerization with Branching Reactions in an Iterative Fashion to Design Branched Polymers. Macromol. Rapid Commun. 2010, 31, 1031−1059. (123) Hirao, A.; Goseki, R.; Ishizone, T. Advances in Living Anionic Polymerization: From Functional Monomers, Polymerization Systems, AF


Perspective

Macromolecules

mers and Their Linear Precursors. Macromolecules 1995, 28, 3485− 3489. (163) Lepoittevin, B.; Dourges, M. A.; Masure, M.; Hemery, P.; Baran, K.; Cramail, H. Synthesis and Characterization of Ring-Shaped Polystyrenes. Macromolecules 2000, 33, 8218−8224. (164) Fukatsu, M.; Kurata, M. Hydrodynamic Properties of FlexibleRing Macromolecules. J. Chem. Phys. 1966, 44, 4539−4545. (165) Iatrou, H.; Hadjichristidis, N.; Meier, G.; Frielinghaus, H.; Monkenbusch, M. Synthesis and Characterization of Model Cyclic Block Copolymers of Styrene and Butadiene. Comparison of the Aggregation Phenomena in Selective Solvents with Linear Diblock and Triblock Analogues. Macromolecules 2002, 35, 5426−5437. (166) Zhu, Y.; Gido, S. P.; Iatrou, H.; Hadjichristidis, N.; Mays, J. W. Microphase Separation of Cyclic Block Copolymers of Styrene and Butadiene and of Their Corresponding Linear Triblock Copolymers. Macromolecules 2003, 36, 148−152. (167) Jia, Z.; Monteiro, M. J. Cyclic Polymers: Method and Strategies. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 2085−2097. (168) Schappacher, M.; Deffieux, A. Synthesis of Macrocyclic Poly(2chloroethyl vinyl ether)s. Makromol. Chem., Rapid Commun. 1991, 12, 447−453. (169) Rique-Lurbet, L.; Schappacher, M.; Deffieux, A. A New Strategy for the Synthesis of Cyclic Polystyrenes: Principle and Application. Macromolecules 1994, 27, 6318−6324. (170) Pantazis, D.; Schulz, D.; Hadjichristidis, N. Synthesis of a Model Cyclic Triblock Terpolymer of Styrene, Isoprene, and Methyl Methacrylate. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1476− 1483. (171) Touris, A.; Hadjichristidis, N. Cyclic and Multiblock Polystyrene-block-polyisoprene Copolymers by Combining Anionic Polymerization and Azide/Alkyne “Click” Chemistry. Macromolecules 2011, 44, 1969−1976. (172) Zhang, Y.; Wang, G.; Huang, J. Synthesis of Macrocyclic Poly(ethylene oxide) and Polystyrene via Glaser Coupling Reaction. Macromolecules 2010, 43, 10343−10347. (173) Polymeropoulos, G.; Bilalis, P.; Hadjichristidis, N. WellDefined Cyclic Triblock Terpolymers: A Missing Piece of the Morphology Puzzle. ACS Macro Lett. 2016, 5, 1242−1246. (174) Lonsdale, D. E.; Bell, C. A.; Monteiro, M. J. Strategy for Rapid and High-Purity Monocyclic Polymers by CuAAC “Click” Reactions. Macromolecules 2010, 43, 3331−3339. (175) Laurent, B. A.; Grayson, S. M. An Efficient Route to WellDefined Macrocyclic Polymers via “Click” Cyclization. J. Am. Chem. Soc. 2006, 128, 4238−4239. (176) Xu, J.; Ye, J.; Liu, S. Synthesis of Well-Defined Cyclic Poly(Nisopropylacrylamide) via Click Chemistry and Its Unique Thermal Phase Transition Behavior. Macromolecules 2007, 40, 9103−9110. (177) Poelma, J. E.; Ono, K.; Miyajima, D.; Aida, T.; Satoh, K.; Hawker, C. J. Cyclic Block Copolymers for Controlling Feature Sizes in Block Copolymer Lithography. ACS Nano 2012, 6, 10845−10854. (178) Zhang, B.; Zhang, H.; Li, Y.; Hoskins, J. N.; Grayson, S. M. Exploring the Effect of Amphiphilic Polymer Architecture: Synthesis, Characterization, and Self-Assembly of Both Cyclic and Linear Poly(ethylene gylcol)-b-Polycaprolactone. ACS Macro Lett. 2013, 2, 845−848. (179) De Gennes, P. G. Reptation of a Polymer Chain in the Presence of Fixed Obstacles. J. Chem. Phys. 1971, 55, 572−579. (180) Doi, M.; Edwards, S. F. Dynamics of Concentrated Polymer Systems. Part 1. Brownian Motion in the Equilibrium State. J. Chem. Soc., Faraday Trans. 2 1978, 74, 1789−1801. (181) McKenna, G. B.; Hostetter, B. J.; Hadjichristidis, N.; Fetters, L. J.; Plazek, D. J. A Study of the Linear Viscoelastic Properties of Cyclic Polystyrenes Using Creep and Recovery Measurements. Macromolecules 1989, 22, 1834−1852. (182) Pasquino, R.; Vasilakopoulos, T. C.; Jeong, Y. C.; Lee, H.; Rogers, S.; Sakellariou, G.; Allgaier, J.; Takano, A.; Bras, A. R.; Chang, T.; Gooben, S.; Pyckhout-Hintzen, W.; Wischnewski, A.; Hadjichristidis, N.; Richter, D.; Rubinstein, M.; Vlassopoulos, D. Viscosity of Ring Polymer Melts. ACS Macro Lett. 2013, 2, 874−878.

ABC Miktoarm Star Terpolymer with a Helical Polypeptide Arm. Macromolecules 2010, 43, 9071−9076. (143) Junnila, S.; Houbenov, N.; Karatzas, A.; Hadjichristidis, N.; Hirao, A.; Iatrou, H.; Ikkala, O. Side-Chain-Controlled Self-Assembly of Polystyrene−Polypeptide Miktoarm Star Copolymers. Macromolecules 2012, 45, 2850−2856. (144) Zhang, Z.; Zhang, H.; Gnanou, Y.; Hadjichristidis, N. Polyhomologation Based on in situ Generated Boron-ThexylSilaboracyclic Initiating Sites: A Novel Strategy Towards the Synthesis of Polyethylene-Based Complex Architectures. Chem. Commun. 2015, 51, 9936−9938. (145) Zhang, Z.; Altaher, M.; Zhang, H.; Wang, D.; Hadjichristidis, N. Synthesis of Well-Defined Polyethylene-Based 3-Miktoarm Star Copolymers and Terpolymers. Macromolecules 2016, 49, 2630−2638. (146) Matyjaszewski, K.; Xia, J. Atom Transfer Radical Polymerization. Chem. Rev. 2001, 101, 2921−2990. (147) Wang, J.-S.; Matyjaszewski, K. Controlled/″Living″ Radical Polymerization. Atom Transfer Radical Polymerization in the Presence of Transition-Metal Complexes. J. Am. Chem. Soc. 1995, 117, 5614− 5615. (148) Ouchi, M.; Terashima, T.; Sawamoto, T. Transition MetalCatalyzed Living Radical Polymerization: Toward Perfection in Catalysis and Precision Polymer Synthesis. Chem. Rev. 2009, 109, 4963−5050. (149) Hawker, C. J.; Bosman, A. W.; Harth, E. New Polymer Synthesis by Nitroxide Mediated Living Radical Polymerizations. Chem. Rev. 2001, 101, 3661−3688. (150) Chiefari, F. J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Living Free-Radical Polymerization by Reversible Addition-Fragmentation Chain Transfer: The RAFT Process. Macromolecules 1998, 31, 5559−5562. (151) Ye, C.; Zhao, G.; Zhang, M.; Du, J.; Zhao, Y. Precise Synthesis of ABCDE Star Quintopolymers by Combination of Controlled Polymerization and Azide−Alkyne Cycloaddition Reaction. Macromolecules 2012, 45, 7429−7439. (152) Liu, H.; Miao, K.; Zhao, G.; Li, C.; Zhao, Y. Synthesis of an Amphiphilic PEG-PCL-PSt-PLLA-PAA Star Quintopolymer and its Self-Assembly for pH-Sensitive Drug Delivery. Polym. Chem. 2014, 5, 3071−3080. (153) Clarson, S. J.; Semlyen, J. A. Cyclic Polysiloxanes: 1. Preparation and Characterization of Poly(phenylmethylsiloxane). Polymer 1986, 27, 1633−1636. (154) McLeish, T. Polymers Without Beginning or End. Science 2002, 297, 2005−2006. (155) Shin, E. J.; Jeong, W.; Brown, H. A.; Koo, B. J.; Hedrick, J. L.; Waymouth, R. M. Crystallization of Cyclic Polymers: Synthesis and Crystallization Behavior of High Molecular Weight Cyclic Poly(εcaprolactone)s. Macromolecules 2011, 44, 2773−2779. (156) Szwarc, M. Living” Polymers. Nature 1956, 178, 1168−1169. (157) Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. Anionic Polymerization: High Vacuum Techniques. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3211−3234. (158) Roovers, J. E. L.; Toporowski, P. M. Synthesis of High Molecular Weight Ring Polystyrenes. Macromolecules 1983, 16, 843− 849. (159) Jacobson, H.; Stockmayer, W. H. Intramolecular Reaction in Polycondensations. I. The Theory of Linear Systems. J. Chem. Phys. 1950, 18, 1600−1606. (160) Yin, R.; Hogen-Esch, T. E. Synthesis and Characterization of Narrow-Molecular Weight Distribution Polystyrene-Poly(dimethylsiloxane) Macrocyclic Block Copolymers and Their Isobaric Precursors. Macromolecules 1993, 26, 6952−6957. (161) Yin, R.; Amis, E. J.; Hogen-Esch, T. E. Synthesis of Macrocyclic Polystyrene-Polydimethylsiloxane Block Copolymers. Macromol. Symp. 1994, 85, 217−238. (162) Lescanec, R. L.; Hajduk, D. A.; Kim, G. Y.; Gan, Y.; Yin, R.; Gruner, S. M.; Hogen-Esch, T. E.; Thomas, E. L. Comparison of the Lamellar Morphology of Microphase-Separated Cyclic Block CopolyAG


Perspective

Macromolecules (183) Cho, D.; Park, S.; Kwon, K.; Chang, T.; Roovers, J. Structural Characterization of Ring Polystyrene by Liquid Chromatography at the Critical Condition and MALDI-TOF Mass Spectrometry. Macromolecules 2001, 34, 7570−7572. (184) Lee, H. C.; Lee, H.; Lee, W.; Chang, T.; Roovers, J. Fractionation of Cyclic Polystyrene from Linear Precursor by HPLC at the Chromatographic Critical Condition. Macromolecules 2000, 33, 8119−8121. (185) Roovers, J. The Melt Properties of Ring Polystyrenes. Macromolecules 1985, 18, 1359−1361. (186) Kapnistos, M.; Lang, M.; Vlassopoulos, D.; Pyckhout-Hintzen, W.; Richter, D.; Cho, D.; Chang, T.; Rubinstein, M. Unexpected Power-Law Stress Relaxation of Entangled Ring Polymers. Nat. Mater. 2008, 7, 997−1002. (187) Roovers, J. Viscoelastic Properties of Polybutadiene Rings. Macromolecules 1988, 21, 1517−1521. (188) Ma, J. Synthesis of Well-Defined Macrocyclic Block Copolymers Using Living Coupling Agent Method. Macromol. Symp. 1995, 91, 41−49. (189) Doi, Y.; Iwasa, Y.; Watanabe, K.; Nakamura, M.; Takano, A.; Takahashi, Y.; Matsushita, Y. Synthesis and Characterization of CombShaped Ring Polystyrenes. Macromolecules 2016, 49, 3109−3115. (190) Jia, Z.; Lonsdale, D. E.; Kulis, J.; Monteiro, M. J. Construction of a 3-Miktoarm Star from Cyclic Polymers. ACS Macro Lett. 2012, 1, 780−783. (191) Amir, F.; Hossain, M. D.; Jia, Z.; Monteiro, M. J. Precise Grafting of Macrocyclics and Dendrons to a Linear Polymer Chain. Polym. Chem. 2016, 7, 6598−6607. (192) Goodwin, A.; Kang, N.; Mays, J. W. Graft and Comblike Polymers. In Anionic Polymerization: Principles, Practice, Strength, Consequences and Applications; Hadjichristidis, N., Hirao, A., Eds.; Springer: Japan, 2015; pp 625−658. (193) Feng, C.; Li, Y.; Yang, D.; Hu, J.; Zhang, X.; Huang, X. WellDefined Graft Copolymers: from Controlled Synthesis to Multipurpose Applications. Chem. Soc. Rev. 2011, 40, 1282−1295. (194) Cerrada, M. L.; De la Fuente, J. L.; Fernandez-Garcia, M.; Madruga, E. L. Viscoelastic and Mechanical Properties of Poly(butyl acrylate-g-styrene) Copolymers. Polymer 2001, 42, 4647−4655. (195) Tang, T.; Huang, B. Interfacial Behavior of Compatibilizers in Polymer Blends. Polymer 1994, 35, 281−285. (196) Rahlwes, D.; Roovers, J. L.; Bywater, S. Synthesis and Characterization of Poly(styrene-g-isoprene) Copolymers. Macromolecules 1977, 10, 604−609. (197) Pitsikalis, M.; Woodward, J.; Mays, J. W.; Hadjichristidis, N. Micellization of Model Graft Copolymers in Dilute Solution. Macromolecules 1997, 30, 5384−5389. (198) Pitsikalis, M.; Sioula, S.; Pispas, S.; Hadjichristidis, N.; Cook, D. C.; Li, J. B.; Mays, J. W. Linking Reactions of Living Polymers with Bromomethylbenzene Derivatives: Synthesis and Characterization of Star Homopolymers and Graft Copolymers with Polyelectrolyte Branches. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 4337−4350. (199) Fernyhough, C. M.; Young, R. N.; Ryan, A. J.; Hutchings, L. R. Synthesis and Characterisation of Poly(sodium 4-styrenesulfonate) Combs. Polymer 2006, 47, 3455−3463. (200) Zhang, H.; Ruckenstein, E. Selective Living Anionic Polymerization of a Novel Bifunctional Monomer 4-(Vinylphenyl)-1-butene and the Preparation of Uniform Size Functional Polymers and Amphiphilic Block Copolymers. Macromolecules 1999, 32, 5495−5500. (201) Zhang, H.; Ruckenstein, E. Well-Defined Graft Copolymers Based on the Selective Living Anionic Polymerization of the Bifunctional Monomer 4-(Vinylphenyl)-1-butene. Macromolecules 1999, 32, 6082−6087. (202) Se, K.; Yamazaki, H.; Shibamoto, T.; Takano, A.; Fujimoto, T. Model Block−Graft Copolymer via Anionic Living Polymerization: Preparation and Characterization of [Poly((4vinylphenyl)dimethylvinylsilane)-graf t-polyisoprene]-block-polystyrene. Macromolecules 1997, 30, 1570−1576.

(203) Xu, F.; Li, T.; Xia, J.; Qiu, F.; Yang, Y. Polystyrene-gpolyisoprene)-b-polystyrene Comb-coil Block copolymer in Selective Solvent. Polymer 2007, 48, 1428−1434. (204) Tsoukatos, T.; Pispas, S.; Hadjichristidis, N. Complex Macromolecular Architectures by Combining TEMPO Living Free Radical and Anionic Polymerization. Macromolecules 2000, 33, 9504− 9511. (205) Xenidou, M.; Hadjichristidis, N. Synthesis of Model Multigraft Copolymers of Butadiene with Randomly Placed Single and Double Polystyrene Branches. Macromolecules 1998, 31, 5690−5694. (206) Velis, G.; Hadjichristidis, N. Synthesis of Model Block− Double-Graft Copolymers and Terpolymers of Styrene (S), Butadiene (Bd), and Isoprene (I): Poly[S-b-(1,2Bd-g-X2)] (X: S, Bd, I, S-b-I). J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 1136−1138. (207) Zhu, Y.; Weidisch, R.; Gido, S. P.; Velis, G.; Hadjichristidis, N. Morphologies and Mechanical Properties of a Series of Block-DoubleGraft Copolymers and Terpolymers. Macromolecules 2002, 35, 5903− 5909. (208) Hadjichristidis, N.; Roovers, J. Conformation of Poly(isopreneg-styrene) in Dilute Solution. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 851−858. (209) Falk, J. C.; Schlott, R. J. Anionic Graft Copolymers. II. Styrene Grafts on EPT Rubbers. J. Macromol. Sci., Chem. 1973, 7, 1663−1668. (210) Al-Jarrah, M. F.; Al-Kafaji, J. K.; Apikian, R. L. Anionic Graft Polymerisation of Styrene from Polyisoprene. Dependence of Grafting Efficiency on Polyisoprene Structure. Br. Polym. J. 1986, 18, 256−258. (211) Uhrig, D. Mays, Synthesis of Well-Defined Multigraft Copolymers. Polym. Chem. 2011, 2, 69−76. (212) Grubbs, R. B.; Hawker, C. J.; Dao, J. L.; Frechet, J. M. A Tandem Approach to Graft and Dendritic Graft Copolymers Based on “Living” Free Radical Polymerization. Angew. Chem., Int. Ed. Engl. 1997, 36, 270−272. (213) Touris, A.; Kostakis, K.; Mourmouris, S.; Kotzabasakis, V.; Pitsikalis, M.; Hadjichristidis, N. Complex Macromolecular Architectures Based on n-Hexyl Isocyanate and ε-Caprolactone Using Titanium-Mediated Coordination Polymerization. Macromolecules 2008, 41, 2426−2438. (214) Xenidou, M.; Beyer, F. L.; Hadjichristidis, N.; Gido, S. P.; Beck Tan, N. Morphology of Model Graft Copolymers with Randomly Placed Trifunctional and Tetrafunctional Branch Points. Macromolecules 1998, 31, 7659−7667. (215) Zhu, Y.; Burgaz, E.; Gido, S. P.; Staudinger, U.; Weidisch, R.; Uhrig, D.; Mays, J. W. Morphology and Tensile Properties of Multigraft Copolymers with Regularly Spaced Tri-, Tetra-, and Hexafunctional Junction Points. Macromolecules 2006, 39, 4428−443. (216) Beyer, L. F.; Gido, S. P.; Buschl, C.; Iatrou, H.; Uhrig, D.; Mays, J. W.; Chang, M. Y.; Garetz, B. A.; Balsara, N. P.; Beck Tan, N.; Hadjichristidis, N. Graft Copolymers with Regularly Spaced, Tetrafunctional Branch Points: Morphology and Grain Structure. Macromolecules 2000, 33, 2039−2048. (217) Li, J.; Li, H.; Wu, C.; Ke, Y.; Wang, D.; Li, Q.; Zhang, L.; Hu, Y. Morphologies, Crystallinity and Dynamic Mechanical Characterizations of Polypropylene/Polystyrene Blends Compatibilized with PP-g-PS Copolymer: Effect of the Side Chain Length. Eur. Polym. J. 2009, 45, 2619−2628. (218) Chung, T.; Lu, H.; Ding, R. Synthesis of Polyethylene-gpolystyrene and Polyethylene-g-poly(p-methylstyrene) Graft Copolymers. Macromolecules 1997, 30, 1272−1278. (219) Kowalczuk, M.; Adamus, G.; Jedlinski, Z. Synthesis of New Graft Polymers via Anionic Grafting of.Beta.-butyrolactone on Poly(methyl methacrylate). Macromolecules 1994, 27, 572−575. (220) Schulz, G. O.; Milkovich, R. Graft Polymers with Macromonomers. I. Synthesis from Methacrylate-Terminated Polystyrene. J. Appl. Polym. Sci. 1982, 27, 4773−4786. (221) Tanaka, S.; Uno, M.; Teramachi, S.; Tsukahara, Y. Determination of Chemical Composition Distribution of Poly(methyl methacrylate)-graf t-Polystyrene Prepared from ω-p-Vinylbenzyl Polystyrene Macromonomer by Adsorption High-Performance Liquid Chromatography. Polymer 1995, 36, 2219−2225. AH


Perspective

Macromolecules

Specially Designed In-Chain Multifunctionalized Polystyrenes. Macromolecules 2011, 44, 5638−5649. (240) Ito, S.; Ishizone, T.; Hirao, A. Precise Synthesis of New Exactly Defined Graft Copolymers Made up of Poly(alkyl methacrylate)s by Iterative Methodology Using Living Anionic Polymerization. Macromolecules 2015, 48, 8307−8314. (241) Ito, S.; Senda, S.; Ishizone, T.; Hirao, A. Syntheses of ExactlyDefined Multi-Graft Polymers with Two or More Graft Chains per Branch Point by a New Iterative Methodology. Polym. Chem. 2016, 7, 2078−2086. (242) Ito, S.; Goseki, R.; Ishizone, T.; Hirao, A. Synthesis of WellControlled Graft Polymers by Living Anionic Polymerization Towards Exact Graft Polymers. Polym. Chem. 2014, 5, 5523−5534. (243) Ratkanthwar, K.; Hadjichristidis, N.; Lee, S.; Chang, T.; Pudukulathan, Z.; Vlassopoulos, D. Synthesis and Characterization of an Exact Comb Polyisoprene with Three Branches Having the Middle Branch Twice the Molecular Weight of the Other Two Identical External Branches. Polym. Chem. 2013, 4, 5645−5665. (244) Daniels, D. R.; McLeish, T. C. B.; Crosby, B. J.; Young, R. N.; Fernyhough, C. M. Molecular Rheology of Comb Polymer Melts. 1. Linear Viscoelastic Response. Macromolecules 2001, 34, 7025−7033. (245) Kapnistos, M.; Koutalas, G.; Hadjichristidis, N.; Roovers, J.; Lohse, D. J.; Vlassopoulos, D. Linear Rheology of Comb Polymers with Star-Like Backbones: Melts and Solution. Rheol. Acta 2006, 46, 273−286. (246) Lentzakis, H.; Vlassopoulos, D.; Read, D.; Lee, H.; Chang, T.; Driva, P.; Hadjichristidis, N. Uniaxial Extensional Rheology of WellCharacterized Comb Polymers. J. Rheol. 2013, 57, 605−625. (247) van Ruymbeke, E.; Lee, E.; Chang, T.; Nikopoulou, A.; Hadjichristidis, N.; Snijkers, F.; Vlassopoulos, D. Molecular rheology of Branched Polymers: Decoding and Exploring the Role of Architectural Dispersity Through a Synergy of Anionic Synthesis, Interaction Chromatography, Rheometry and Modeling. Soft Matter 2014, 10, 4762−4777. (248) Zhang, Z.; Hadjichristidis, N. Well-Defined Bilayered Molecular Cobrushes with Internal Polyethylene Blocks and ωHydroxyl-Functionalized Polyethylene Homobrushes. Macromolecules 2016, 49, 1590−1596. (249) Zhang, H.; Gnanou, Y.; Hadjichristidis, N. Well-Defined Polyethylene Molecular Brushes by Polyhomologation and Ring Opening Metathesis Polymerization. Polym. Chem. 2014, 5, 6431− 6434. (250) Tomalia, D. A.; Naylor, R.; Goddard, W. A., III Starburst Dendrimers: Molecular-Level Control of Size, Shape, Surface Chemistry, Topology, and Flexibility from Atoms to Macroscopic Matter. Angew. Chem., Int. Ed. Engl. 1990, 29, 138−175. (251) Fréchet, J.; Hawker, C.; Wooley, L. The Convergent Route to Globular Dendritic Macromolecules: A Versatile Approach to Precisely Functionalized Three-Dimensional Polymers and Novel Block Copolymers. J. Macromol. Sci., Part A: Pure Appl.Chem. 1994, 31, 1627−1645. (252) Wang, F.; Roovers, J.; Toporowski, P. M. Synthesis and Characterization of “Umbrella” Shaped Polymers. Macromol. Symp. 1995, 95, 255−263. (253) Wang, F.; Roovers, J.; Toporowski, P. M. Umbrella-Star Block Copolymers. J. Macromol. Sci., Part A: Pure Appl.Chem. 1995, 32, 951− 958. (254) Knauss, D. M.; Al-Muallem, H. A. Polystyrene with Dendritic Branching by Convergent Living Anionic Polymerization. II. Approach Using Vinylbenzyl Chloride. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4289−4298. (255) Knauss, D. M.; Huang, T. Star-block-Linear-block-Star Triblock (Pom-Pom) Polystyrene by Convergent Living Anionic Polymerization. Macromolecules 2002, 35, 2055−2062. (256) Knauss, D. M.; Huang, T. ((PS)nPS)m Star-Shaped Polystyrene with Star-Shaped Branches at the Terminal Chain Ends by Convergent Living Anionic Polymerization. Macromolecules 2003, 36, 6036−6042. (257) Liu, G.; Sun, H.; Rangou, S.; Ntetsikas, K.; Avgeropoulos, A.; Wang, S.-Q. Studying the Origin of “Strain Hardening”: Basic

(222) Ederle, Y.; Isel, F.; Grutke, S.; Lutz, P. Anionic Polymerization and Copolymerization of Macromonomers: Kinetics, Structure Control. Macromol. Symp. 1998, 132, 197−206. (223) Batis, C.; Karanikolopoulos, G.; Pitsikalis, M.; Hadjichristidis, N. Metallocene-Catalyzed Copolymerization of MMA with Anionically Synthesized Methacryloyl Macromonomers. Macromolecules 2000, 33, 8925−8930. (224) Pantazis, D.; Chalari, I.; Hadjichristidis, N. Anionic Polymerization of Styrenic Macromonomers. Macromolecules 2003, 36, 3783− 3785. (225) Vazaios, A.; Hadjichristidis, N. Anionic Polymerization of Styrenic Macromonomers of Polyisoprene, Polybutadiene, and Polystyrene. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1038−1048. (226) Driva, P.; Iatrou, H.; Lohse, D. J.; Hadjichristidis, N. Anionic Homo- and Copolymerization of Double-Tailed Macromonomers: A Route to Novel Macromolecular Architectures. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4070−4078. (227) Nikopoulou, A.; Iatrou, H.; Lohse, D. J.; Hadjichristidis, N. Anionic homo- and Copolymerization of Styrenic Triple-Tailed Polybutadiene Macromonomers. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3513−3523. (228) Christodoulou, S.; Iatrou, H.; Lohse, D. J.; Hadjichristidis, N. Anionic Copolymerization of Styrenic-Tipped Macromonomers: A Route to Novel Triblock−Comb Copolymers of Styrene and Isoprene. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4030−403. (229) Koutalas, G.; Iatrou, H.; Lohse, D. J.; Hadjichristidis, N. WellDefined Comb, Star−Comb, and Comb-on-Comb Polybutadienes by Anionic Polymerization and the Macromonomer Strategy. Macromolecules 2005, 38, 4996−5001. (230) Koutalas, G.; Lohse, D. J.; Hadjichristidis, N. Novel Block− Comb/Graft Copolymers with the Macromonomer Strategy and Anionic Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4040−4049. (231) Zamurovic, M.; Christodoulou, S.; Vazaios, A.; Iatrou, H.; Pitsikalis, M.; Hadjichristidis, N. Micellization Behavior of Complex Comblike Block Copolymer Architecture. Macromolecules 2007, 40, 5835−5849. (232) Weidisch, R.; Gido, S. P.; Uhrig, D.; Iatrou, H.; Mays, J.; Hadjichristidis, N. Tetrafunctional Multigraft Copolymers as Novel Thermoplastic Elastomers. Macromolecules 2001, 34, 6333−6337. (233) Liu, B.; Quirk, R. P.; Wesdemiotis, C.; Yol, A. M.; Foster, M. D. Precision Synthesis of ω-Branch, End-Functionalized Comb Polystyrenes Using Living Anionic Polymerization and Thiol−Ene “Click” Chemistry. Macromolecules 2012, 45, 9233−9242. (234) Weber, C.; Becer, C. R.; Hoogenboom, R.; Schubert, U. S. Lower Critical Solution Temperature Behavior of Comb and Graft Shaped Poly[oligo(2-ethyl-2-oxazoline)methacrylate]s. Macromolecules 2009, 42, 2965−2971. (235) Paraskeva, S.; Hadjichristidis, N. Synthesis of an Exact Graft Copolymer of Isoprene and Styrene with Two Branches. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 931−935. (236) Hirao, A.; Watanabe, T.; Kurokawa, R. Precise Synthesis of Exact Graft Polystyrenes with Branches from Two to Five in Number by Iterative Methodology Based on Living Anionic Polymerization. Macromolecules 2009, 42, 3973−398. (237) Hirao, A.; Murano, K.; Kurokawa, R.; Watanabe, T.; Sugiyama, K. Precise Synthesis of Exact Graft Copolymers, Poly(methyl methacrylate)-exact graft-Polystyrene, by Iterative Methodology Using a Specially Designed In-Chain-Functionalized AB Diblock Copolymer Anion. Macromolecules 2009, 42, 7820−7827. (238) Hirao, A.; Murao, K.; Abouelmagd, A.; Uematsu, M.; Ito, S.; Goseki, R.; Ishizone, T. General and Facile Approach to Exact Graft Copolymers by Iterative Methodology Using Living Anionic In-ChainFunctionalized AB Diblock Copolymers as Key Building Blocks. Macromolecules 2011, 44, 3302−3311. (239) Hirao, A.; Uematsu, M.; Kurokawa, R.; Ishizone, T. Facile Synthetic Approach to Exact Graft (Co)polymers and Double-Tailed Polystyrene: Linking Reaction of Living Anionic Polymers with AI


Perspective

Macromolecules Difference between Extension and Shear. J. Rheol. 2013, 57 (1), 89− 104. (258) Sun, H.; Ntetsikas, K.; Avgeropoulos, A.; Wang, S.-Q. Breakdown of Time−Temperature Equivalence in Startup Uniaxial Extension of Entangled Polymer Melts. Macromolecules 2013, 46, 4151−4159. (259) Sun, H.; Liu, G.; Ntetsikas, K.; Avgeropoulos, A.; Wang, S.-Q. Rheology of Entangled Polymers Not Far above Glass Transition Temperature: Transient Elasticity and Intersegmental Viscous Stress. Macromolecules 2014, 47, 5839−5850. (260) Sun, H.; Lin, P.; Liu, G.; Ntetsikas, K.; Misichronis, K.; Kang, N.; Liu, J.; Avgeropoulos, A.; Mays, J.; Wang, S.-Q. Failure Behavior After Stepwise Uniaxial Extension of Entangled Polymer Melts. J. Rheol. 2015, 59 (3), 751−767. (261) Rahman, M. S.; Aggarwal, R.; Larson, R. G.; Dealy, J. M.; Mays, J. W. Synthesis and Dilute Solution Properties of Well-Defined HShaped Polybutadienes. Macromolecules 2008, 41, 8225−8230. (262) Chen, X.; Rahman, M. S.; Lee, H.; Mays, J. W.; Chang, T.; Larson, R. Combined Synthesis, TGIC Characterization, and Rheological Measurement and Prediction of Symmetric H Polybutadienes and Their Blends with Linear and Star-Shaped Polybutadienes. Macromolecules 2011, 44, 7799−7809. (263) Rahman, M. S.; Lee, H.; Chen, X.; Chang, T.; Larson, R.; Mays, J. W. Model Branched Polymers: Synthesis and Characterization of Asymmetric H-Shaped Polybutadienes. ACS Macro Lett. 2012, 1, 537−540. (264) Chen, X.; Lee, H.; Rahman, M. S.; Chang, T.; Mays, J. W.; Larson, R. Analytical Rheology of Asymmetric H-Shaped Model Polybutadiene Melts. Macromolecules 2012, 45, 5744−5756. (265) Roovers, J.; Toporowski, P. M. Preparation and Characterization of H-Shaped Polystyrene. Macromolecules 1981, 14, 1174− 1178. (266) Iatrou, H.; Avgeropoulos, A.; Hadjichristidis, N. Synthesis of Model Super H-Shaped Block Copolymers. Macromolecules 1994, 27, 6232−6233. (267) Velis, G.; Hadjichristidis, N. Synthesis of Model PS(PI)5 and (PI)5PS(PI)5 Nonlinear Block Copolymers of Styrene (S) and Isoprene (I). Macromolecules 1999, 32, 534−536. (268) Bayer, V.; Stadler, R. Synthesis and Properties of Amphiphilic “Dumbbell”-Shaped Grafted Block Copolymers, 1. Anionic Synthesis via a Polyfunctional Initiator. Macromol. Chem. Phys. 1994, 195, 2709− 2722. (269) Hadjichristidis, N.; Pitsikalis, M.; Iatrou, H.; Pispas, S. The Strength of the Macromonomer Strategy for Complex Macromolecular Architecture: Molecular Characterization, Properties and Applications of Polymacromonomers. Macromol. Rapid Commun. 2003, 24, 979−1013. (270) Christodoulou, S.; Driva, P.; Iatrou, H.; Hadjichristidis, N. Synthesis and Micellization Behavior of Janus H-Shaped A2BC2 Terpolymers. Macromolecules 2008, 41, 2607−2615. (271) Haraguchi, N.; Hirao, A. Synthesis of Well-Defined Star− Linear Block Polystyrenes by Coupling Reaction of Chain-Functionalized Polystyrenes with a Definite Number of Benzyl Bromide Moieties with Polystyryllithiums. Macromolecules 2003, 36, 9364− 9372. (272) Vazaios, A.; Lohse, D. J.; Hadjichristidis, N. Linear and Star Block Copolymers of Styrenic Macromonomers by Anionic Polymerization. Macromolecules 2005, 38, 5468−5474. (273) Driva, P.; Lohse, D. J.; Hadjichristidis, N. Well-defined Complex Macromolecular Architectures by Anionic Polymerization of Styrenic Single and Double Homo/Miktoarm Star-Tailed Macromonomers. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 1826−1842. (274) Matyjaszewski, K.; Tsarevsky, N. V. Macromolecular Engineering by Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 2014, 136, 6513−6533. (275) Hill, M. R.; Carmean, N.; Sumerlin, B. S. Expanding the Scope of RAFT Polymerization: Recent Advances and New Horizons. Macromolecules 2015, 48, 5459−5469.

(276) Alfurhood, A. J.; Bachler, P. R.; Sumerlin, B. S. Hyperbranched Polymers via RAFT Self-Condensing Vinyl Polymerization. Polym. Chem. 2016, 7, 3361−3364. (277) Espeel, P.; Du Prez, F. E. Click”-Inspired Chemistry in Macromolecular Science: Matching Recent Progress and User Expectations. Macromolecules 2015, 48, 2−14. (278) Cook, A. B.; Barbey, R.; Burns, J. A.; Perrier, S. Hyperbranched Polymers with High Degrees of Branching and Low Dispersity Values: Pushing the Limits of Thiol−Yne Chemistry. Macromolecules 2016, 49, 1296−1304. (279) Zheng, Y.; Li, S.; Weng, Z.; Gao, C. Hyperbranched Polymers: Advances from Synthesis to Applications. Chem. Soc. Rev. 2015, 44, 4091−4130. (280) Miguel, A. F. Dendritic Design as an Archetype for Growth Patterns in Nature: Fractal and Constructal Views. Front. Phys. 2014, 2, 1−7. (281) Buhleier, E.; Wehner, W.; Vögtle, F. Cascade”- and “NonskidChain-Like” Synthesis of Molecular Cavity Topologies. Synthesis 1978, 1978, 155−158. (282) Tomalia, D. A.; Dewald, J. R.; Hall, M. R.; Martin, S. J.; Smith, P. B. Preprints 1st SPSJ. Polym. Conf., Soc. Polym. Sci. Jpn., Kyoto 1984, 65. (283) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. A New Class of Polymers: Starburst-Dendritic Macromolecules. Polym. J. 1985, 17, 117−132. (284) Konkolewicz, D.; Monteiro, M. J.; Perrier, S. Dendritic and Hyperbranched Polymers from Macromolecular Units: Elegant Approaches to the Synthesis of Functional Polymers. Macromolecules 2011, 44, 7067−7087. (285) Tomalia, D. A.; Fréchet, J. M. J. Discovery of Dendrimers and Dendritic Polymers: a Brief Historical Perspective. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2719−2728. (286) Taton, D.; Feng, X.; Gnanou, Y. Dendrimer-Like Polymers: a New Class of Structurally Precise Dendrimers with Macromolecular Generations. New J. Chem. 2007, 31, 1097−1110. (287) Carlmark, A.; Hawker, C.; Hult, A.; Malkoch, M. New Methodologies in the Construction of Dendritic Materials. Chem. Soc. Rev. 2009, 38, 352−362. (288) Boas, U.; Christensen, J. B.; Heegaard, P. M. H. Dendrimers in Medicine and Biotechnology: New Molecular Tools. J. Mater. Chem. 2006, 16, 3785−3797. (289) Hawker, C.; Fréchet, J. M. J. Preparation of Polymers with Controlled Molecular Architecture. A New Convergent Approach to Dendritic Macromolecules. J. Am. Chem. Soc. 1990, 112, 7638−7647. (290) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. About Dendrimers: Structure, Physical Properties, and Applications. Chem. Rev. 1999, 99, 1665−1688. (291) Six, J. L.; Gnanou, Y. From Star-Shaped to Dendritic Poly(ethylene oxide)s: Toward Increasingly Branched Architectures by Anionic Polymerization. Macromol. Symp. 1995, 95, 137−150. (292) Feng, X.; Taton, D.; Chaikof, E. L.; Gnanou, Y. Toward an Easy Access to Dendrimer-Like Poly(ethylene oxide)s. J. Am. Chem. Soc. 2005, 127, 10956−10966. (293) Trollsås, M.; Hedrick, J. L. Dendrimer-Like Star Polymers. J. Am. Chem. Soc. 1998, 120, 4644−4651. (294) Trollsås, M.; Hawker, C. J.; Remenar, J. F.; Hedrick, J. L.; Johansson, M.; Ihre, H.; Hult, A. Highly Branched Radial Block Copolymers via Dendritic Initiation of Aliphatic Polyesters. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2793−2798. (295) Trollsås, M.; Atthof, B.; Wursch, A.; Hedrick, J. L.; Pople, J. A.; Gast, A. P. Constitutional Isomers of Dendrimer-like Star Polymers: Design, Synthesis, and Conformational and Structural Properties. Macromolecules 2000, 33, 6423−6438. (296) Trollsås, M.; Claesson, B.; Atthoff, B.; Hedrick, J. L.; et al. Conformational and Structural Properties of High Functionality Dendrimer-Like Star Polymers Synthesized from Living Polymerization Techniques. Macromol. Symp. 2000, 153, 87−108. (297) Lepoittevin, B.; Matmour, R.; Francis, R.; Taton, D.; Gnanou, Y. Synthesis of Dendrimer-Like Polystyrene by Atom Transfer Radical AJ


Perspective

Macromolecules Polymerization and Investigation of their Viscosity Behavior. Macromolecules 2005, 38, 3120−3128. (298) Hou, S.; Chaikof, E. L.; Taton, D.; Gnanou, Y. Synthesis of Water-Soluble Star-Block and Dendrimer-Like Copolymers Based on Poly(ethylene oxide) and Poly(acrylic acid). Macromolecules 2003, 36, 3874−3881. (299) Angot, S.; Taton, D.; Gnanou, Y. Amphiphilic Stars and Dendrimer-Like Architectures Based on Poly (ethylene oxide) and Polystyrene. Macromolecules 2000, 33, 5418−5426. (300) Percec, V.; Barboiu, B.; Grigoras, C.; Bera, T. K. Universal Iterative Strategy for the Divergent Synthesis of Dendritic Macromolecules from Conventional Monomers by a Combination of Living Radical Polymerization and Irreversible TERminator Multifunctional INItiator (TERMINI). J. Am. Chem. Soc. 2003, 125, 6503−6516. (301) Percec, V.; Grigoras, C.; Kim, H. J. Toward Self-Assembling Dendritic Macromolecules from Conventional Monomers by a Combination of Living Radical Polymerization and Irreversible Terminator Multifunctional Initiator. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 505−513. (302) Matmour, R.; Gnanou, Y. Combination of an Anionic Terminator Multifunctional Initiator and Divergent Carbanionic Polymerization: Application to the Synthesis of Dendrimer-Like Polymers and of Asymmetric and Miktoarm Stars. J. Am. Chem. Soc. 2008, 130, 1350−1361. (303) Matsuo, A.; Watanabe, T.; Hirao, A. Synthesis of Well-Defined Dendrimer-like Branched Polymers and Block Copolymer by the Iterative Approach Involving Coupling Reaction of Living Anionic Polymer and Functionalization. Macromolecules 2004, 37, 6283−6290. (304) Watanabe, T.; Tsunoda, Y.; Matsuo, A.; Sugiyama, K.; Hirao, A. Synthesis of Dendrimer-Like Star-Branched Poly(methyl methacrylate)s of Generations Consisting of Four Branched Polymer Chains at Each Junction by Iterative Methodology Involving Coupling and Transformation Reactions. Macromol. Symp. 2006, 240, 23−30. (305) Hirao, A.; Matsuo, A.; Watanabe, T. Precise Synthesis of Dendrimer-like Star-Branched Poly(methyl methacrylate)s up to Seventh Generation by an Iterative Divergent Approach Involving Coupling and Transformation Reactions. Macromolecules 2005, 38, 8701−8711. (306) Yoo, H. S.; Watanabe, T.; Hirao, A. Precise Synthesis of Dendrimer-Like Star-Branched Polystyrenes and Block Copolymers Composed of Polystyrene and Poly(methyl methacrylate) Segments by an Iterative Methodology Using Living Anionic Polymerization. Macromolecules 2009, 42, 4558−4570. (307) Yoo, H. S.; Watanabe, T.; Matsunaga, Y.; Hirao, A. Precise Synthesis of Dendrimer-like Star-Branched Poly(tert-butyl methacrylate)s and Their Block Copolymers by a Methodology Combining α-Terminal-Functionalized Living Anionic Polymers with a Specially Designed Linking Reaction in an Iterative Fashion. Macromolecules 2012, 45, 100−112. (308) Zhang, H.; He, J.; Zhang, C.; Ju, Z.; Li, J.; Yang, Y. Continuous Process for the Synthesis of Dendrimer-Like Star Polymers by Anionic Polymerization. Macromolecules 2012, 45, 828−841. (309) Knauss, D. M.; Al-Muallem, H. A.; Huang, T.; Wu, D. T. Polystyrene with Dendritic Branching by Convergent Living Anionic Polymerization. Macromolecules 2000, 33, 3557−3568. (310) Chalari, I.; Hadjichristidis, N. Synthesis of Well-Defined Second-Generation Dendritic Polymers of Isoprene (I) and Styrene (S): (S2I)3, (SI′I)3, (I″I′I)3, and (I′2I)4. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1519−1526. (311) Orfanou, K.; Iatrou, H.; Lohse, D. J.; Hadjichristidis, N. Synthesis of Well-Defined Second (G-2) and Third (G-3) Generation Dendritic Polybutadienes. Macromolecules 2006, 39, 4361−4365. (312) Hutchings, L. R.; Roberts-Bleming, S. J. DendriMacs. WellDefined Dendritically Branched Polymers Synthesized by an Iterative Convergent Strategy Involving the Coupling Reaction of AB2 Macromonomers. Macromolecules 2006, 39, 2144−2152. (313) Kimani, S. M.; Hutchings, L. R. A Facile Route to Synthesize Well-Defined Polybutadiene DendriMacs. Macromol. Rapid Commun. 2008, 29, 633−637.

(314) Urbani, C. N.; Bell, C. A.; Lonsdale, D.; Whittaker, M. R.; Monteiro, M. J. Self-Assembly of Amphiphilic Polymeric Dendrimers Synthesized with Selective Degradable Linkages. Macromolecules 2008, 41, 76−86. (315) Hirao, A.; Watanabe, T.; Ishizu, K.; Ree, M.; Jin, S.; Jin, K. S.; Deffieux, A.; Schappacher, M.; Carlotti, S. Precise Synthesis and Characterization of Fourth-Generation Dendrimer-like Star-Branched Poly(methyl methacrylate)s and Block Copolymers by Iterative Methodology Based on Living Anionic Polymerization. Macromolecules 2009, 42, 682−693. (316) Chen, H.; Kong, J. Hyperbranched Polymers from A2 + B3 Strategy: Recent Advances in Description and Control of fine Topology. Polym. Chem. 2016, 7, 3643−3663. (317) van Ruymbeke, E.; Muliawan, E. B.; Hatzikiriakos, S. G.; Watanabe, T.; Hirao, A.; Vlassopoulos, D. Viscoelasticity and Extensional Rheology of Model Cayley-Tree Polymers of Different Generations. J. Rheol. 2010, 54, 643−662. (318) van Ruymbeke, E.; Orfanou, K.; Kapnistos, M.; Iatrou, H.; Pitsikalis, M.; Hadjichristidis, N.; Lohse, D. J.; Vlassopoulos, D. Entangled Dendritic Polymers and Beyond: Rheology of Symmetric Cayley-Tree Polymers and Macromolecular Self-Assemblies. Macromolecules 2007, 40, 5941−5952. (319) Lee, J. H.; Orfanou, K.; Driva, P.; Iatrou, H.; Hadjichristidis, N.; Lohse, D. J. Linear and Nonlinear Rheology of Dendritic Star Polymers: Experiment. Macromolecules 2008, 41, 9165−9178. (320) Watanabe, H.; Matsumiya, Y.; van Ruymbeke, E.; Vlassopoulos, D.; Hadjichristidis, N. Viscoelastic and Dielectric Relaxation of a Cayley-Tree-Type Polyisoprene: Test of Molecular Picture of Dynamic Tube Dilation. Macromolecules 2008, 41, 6110− 6124. (321) Deffieux, A.; Schappacher, M.; Hirao, A.; Watanabe, T. Synthesis and AFM Structural Imaging of Dendrimer-Like StarBranched Polystyrenes. J. Am. Chem. Soc. 2008, 130, 5670−5672. (322) Avgeropoulos, A.; Rangou, S.; Krikorian, V.; Thomas, E. L. Synthesis and Self-Assembly of 2nd Generation Dendritic Homopolymers and Copolymers of Polydienes with Different Isomeric Microstructures. Macromol. Symp. 2008, 267, 16−20. (323) Cai, H.; Jiang, G.; Shen, Z.; Fan, X. Effects of Dendron Generation and Salt Concentration on Phase Structures of Dendritic− Linear Block Copolymers with a Semirigid Dendron Containing PEG Tails. Macromolecules 2012, 45, 6176−6184. (324) Takahashi, K.; Hasegawa, H.; Hashimoto, T.; Bellas, V.; Iatrou, H.; Hadjichristidis, N. Four-Phase Triple Coaxial Cylindrical Microdomain Morphology in a Linear Tetrablock Quarterpolymer of Styrene, Isoprene, Dimethylsiloxane, and 2-Vinylpyridine. Macromolecules 2002, 35, 4859−4861. (325) Fragouli, P. G.; Iatrou, H.; Hadjichristidis, N. Synthesis and Characterization of Linear Tetrablock Quarterpolymers of Styrene, Isoprene, Dimethylsiloxane, and 2-Vinylpyridine. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 514−519. (326) Fragouli, P.; Iatrou, H.; Lohse, D. J.; Hadjichristidis, N. Linear Pentablock Quintopolymers (l-SIDMV) with Five Incompatible Blocks: Polystyrene, Polyisoprene-1,4, Poly(dimethylsiloxane), Poly(tert-butyl methacrylate), and Poly(2-vinylpyridine). J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3938−3946. (327) Ekizoglou, N.; Hadjichristidis, N. Synthesis of Model Linear Tetrablock Quarterpolymers and Pentablock Quintopolymers of Ethylene Oxide. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2166−2170. (328) Ntaras, C.; Polymeropoulos, G.; Zapsas, G.; Ntetsikas, K.; Liontos, G.; Karanastasis, A.; Moschovas, D.; Rangou, S.; StewartSloan, C.; Hadjichristidis, N.; Thomas, E. L.; Avgeropoulos, A. Synthesis, Characterization and Self-Assembly of Well-Defined Linear Heptablock Quarterpolymers. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1443−1449. (329) Durmaz, H.; Hizal, G.; Tunca, U. Linear Tetrablock Quarterpolymers via Triple Click Reactions, Azide-Alkyne, Diels− Alder, and Nitroxide Radical Coupling in a One-Pot Fashion. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1962−1968. AK


Perspective

Macromolecules (330) Chen, J.; Liu, M.; Gong, H.; Cui, G.; Lü, S.; Gao, C.; Huang, F.; Chen, T.; Zhang, X.; Liu, Z. Synthesis of Linear Amphiphilic Tetrablock Quarterpolymers with Dual Stimulus Response Through the Combination of ATRP and RAFT by a Click Chemistry Site Transformation Approach. Polym. Chem. 2013, 4, 1815−1825.

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