Poly(vinylpyridine) Segments in Block Copolymers: Synthesis, Self

Perspective Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Poly(vinylpyridine) Segments in Block Copolymers: Synthesis, SelfAssembly, and Versatility Justin G. Kennemur*

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Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States

ABSTRACT: This Perspective provides a current survey on the synthesis, self-assembly, and vast variety of applications possible from block copolymer (BCP) systems containing at least one segment of either poly(2-vinylpyridine), P2VP, or poly(4vinylpyridine), P4VP. Strategies and insight toward P2VP or P4VP as the segment of choice and their influence on material properties, effective interaction parameters, and hosting a variety chemistries from small molecules to nanoparticles are provided. It is with hope that this work serves as a modern field guide to the versatility and utility of these chemistries for those new to or continuing research with these BCP systems.

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INTRODUCTION Four years after the seminal publication by Leibler1 describing the self-consistent mean-field theory of block copolymer (BCP) microphase separation, Fréchet and de Meftahi published a small review entitled “Poly(Vinylpyridine)s: Simple Reactive Polymers with Multiple Applications”.2 In the 35 years since, synergy of these two promising fields has produced a complex array of materials that combine unique function and autonomous self-assembly.3 Poly(vinylpyridine)s (PVP)s, which encapsulate the isomers of poly(2-vinylpyridine) (P2VP) and poly(4-vinylpyridine) (P4VP), are opportunistic segments for use in BCPs having a structural similarity to polystyrene (PS) but with the unique functionality of the nitrogen within the aromatic pendants (Scheme 1). Poly(3-vinylpyridine) (P3VP) is another possible isomer although rarely seen in the literature,4 possibly due to the high monomer cost. No BCPs with P3VP as a segment were discovered in literature and therefore this isomer will not be discussed in this Perspective. The basicity and coordinative

ability of the pyridinyl nitrogen provide a landscape for chemical modification to PVP and to the nanoscale BCP domains they populate in bulk or solution. Pyridines are also well-established chelating agents for a variety of inorganic species. This allows PVP domains to act as polydentate ligands for variety of metal ions or nanoparticles toward a divergent set of applications in optical, magnetic, electronic, catalytic, conductive, and cargo-releasing materials. Many well-written reviews showcase BCPs with P2VP and/or P4VP as one of a variety of material possibilities for specific applications.5−22 Upon surveying the literature, it became apparent that a current perspective encapsulating modern synthetic strategies, properties, self-assembly behavior, and a sampling of the rich variety of material possibilities specific to P2VP and P4VP in BCPs would be beneficial to those beginning or continuing to utilize these unique segments. Such an analysis is provided herein; however, a comprehensive survey over the past several decades would be extensive and too much for one perspective. Therefore, highlights and discussion are focused on reports over the past few years that exemplify the continuing frontier of these materials in a wide variety of areas. Readers are encouraged to reference the reviews mentioned throughout for more depth on specific applications. This work is concluded

Scheme 1. Monomer Isomers 2-Vinylpyridine (2VP) and 4Vinylpyridine (4VP) and Their Respective Polymers

Received: August 3, 2018 Revised: December 31, 2018

© XXXX American Chemical Society

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DOI: 10.1021/acs.macromol.8b01661 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Recent examples of the use of flow chemistry in the carbanionic synthesis of P2VP and P2VP-containing BCPs. (A) A tangential four-way micromixer (purple inset), which combines P2VP monomer and s-BuLi initiator at speeds governed by the total flow, allows for targeted molar mass of P4VP at varying degrees of molar mass distribution. Adapted with permission from ref 46. (B) Metered addition of s-BuLi initiator to styrene followed by sequential addition of 2VP creates PS-b-P2VP with skewed molar mass distribution of the PS segments and narrow distribution of the P2VP segments. Shown are three plots of mass fraction (y-axis) versus molar mass (x-axis) with skewed distributions correlating to the BCP cartoons behind them. These systems, with nearly the same number-average molar mass and volume fraction, self-assemble into different morphologies (cylindrical, perforated lamellar, and lamellar) depending on the skewed dispersity of the PS segments. Adapted with permission from ref 47.

circumvent these issues include those mentioned above in addition to highly diluted polymerization conditions (1−2% final polymer in solvent) and the use of cosolvents, such as DMF, to retain solubility.35,36 Jérôme and co-workers have also reported a method to make P4VP in living fashion at 0 °C using pyridine as a cosolvent.37 For sequential block polymerization, the general order of addition follows the trend styrenes, dienes, vinylpyridines, and then [meth]acrylates. However, elegant examples that circumvent this order can be found.38,39 Lee and co-workers have shown the active P2VP anion is capable of initiating the anionic polymerization of n-hexyl isocyanate to produce unique coil−rod BCPs.40 Aside from one-pot sequential anionic polymerizations, P2VP and P4VP can be terminated with a variety of agents that are commonplace for chain-end functionalization, such as substitution reactions on alkyl halides or ring-opening of epoxides.41 Termination with ethylene oxide is a popular choice to produce an alcohol end functionality with the ability to subsequently produce poly(ethylene oxide) (PEO)42 through ring-opening polymerization (ROP) techniques. Advanced difunctional terminating agents and macro-terminating agents have allowed design of advanced structure such as star polymers43,44 and even undecablock copolymers where P2VP is the 1st and 11th block.45 This subsection will conclude with two recent reports that utilize the burgeoning field of flow chemistry to anionically synthesize P2VP and BCPs with control over both the numberaverage molar mass (Mn) and the molar mass distribution or dispersity, Đ. Frey and co-workers reported control over P2VP anionic polymerizations using a continuous flow reactor containing a tangential four-way mixing device.28,46 With judicious control over the total flow, this setup allows adjusting the flow rate of diluted sec-butyllithium (s-BuLi) initiator and of diluted monomer within the mixer. This is followed by a second mixing device that introduces a terminating agent into the active polymerization stream. The end result is a series of P2VP samples containing nearly the same Mn but with

with a prospective on the exciting areas in need of continued exploration.

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SYNTHESIS AND ADVANCEMENTS Industrial production utilizing 2VP or 4VP has been largely focused on their use as one of several components in traditional radical copolymerizations (typically in suspension or emulsion) to produce modified rubbers23 or cross-linked ion-exchange resins.24 However, focus herein will be placed on living-like strategies which are amenable to block polymerizations and, to the best of my knowledge, not yet prevalent within industrial production.25 Carbanionic Polymerizations. These polymerizations are a classic and effective method to produce very well-defined BCPs containing P2VP or P4VP segments. Once highly purified, 2-vinylpyridine (2VP) and 4-vinylpyridine (4VP) monomers will begin to autopolymerize within a week even under cold storage (−20 °C),26 and during polymerization the reactivity of the PVP anion propagates to completion almost instantaneously (kp ≈ 2500−3500 L mol−1 s−1, depending on conditions).27,28 Under many conditions, particularly at warm temperatures and in less polar solvents, the propagating PVP anion can undergo side reactions through the −NC−H moiety of the pyridine ring which ultimately results in branching and cross-linking.29 This is especially true for P4VP since the ring has two of these moieties.29 Methods to obtain well-defined, linear polymerizations were pioneered by Fontanille and Sigwalt beginning in the 1960s30,31 and are still commonly used today.32,33 These include the use of cold temperatures (5 kDa) and at the cold temperatures typically used (∼−78 °C).35,36 Strategies to B


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Macromolecules predictably increasing dispersity dependent on the flow rate (Figure 1a). Very recently, Gentekos and Fors47 reported BCP synthesis of PS-b-P2VP with which Đ of the initially synthesized PS segment could be broadened and skewed in a positive or negative molar mass distribution through metered addition48 of the s-BuLi initiation solution. It was determined that PS-b-P2VP systems containing the same Mn and PS volume fraction ( f PS) would self-assemble into a variety of different morphologies (cylindrical, perforated lamellar, lamellar) depending on the shape, breadth, and skew of Đ in the PS segment (Figure 1b).47 One of the challenges with respect to controlling polymerization of PVP through anionic methods in flow chemistry is the very fast rate of propagation of 2VP and 4VP as mentioned earlier. Further development on sophisticated flow systems and utilization of kinetic reduction strategies, such as choice of counter cations, may serve to improve this promising field. Controlled Radical Polymerization (CRP). CRP has presented new opportunities in the architectures, block combinations, and block sequences for BCPs containing both P2VP and P4VP without the stringent purifications and glassware methods required for anionic polymerization but usually at the expense of slower reaction times and diverse chain-end functionalities that may need to be considered. The late 1990s produced the first attempts at CRP focused on 4VP using nitroxide-mediated polymerization (NMP)49,50 and atom transfer radical polymerization (ATRP)51 although some complications were evident. For NMP, the prototypical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) initiator showed a linear trend of molar mass as a function of conversion. However, conversions were often modest (∼50%), and Đ exceeded 1.2 at these conversions.50,52 The use of 2,2,5Trimethyl-4-phenyl-3-azahexane-N-oxyl (TIPNO) initiator showed slightly improved results under similar conditions.53 Later this was also found to be the case for 2VP using TEMPO.54 Over the years, improved methods for NMP of 4VP have been reported. The use of N-tert-butyl-N-(1diethylphosphono-2,2-dimethylpropyl) nitroxide (DEPN or SG1) improved the rate of 4VP polymerization and reduced Đ < 1.2, albeit still to moderate conversions ∼50%.52 This was also used to create P4VP-b-poly(N,N-dimethylacrylamide) (PDMAA) successfully.52 Well-defined P4VP-b-poly(methyl methacrylate) (PMMA) was later produced through the use of BlocBuilder or MAMA-SG1. This nitroxide appears superior for P4VP as high conversions (99%) and low Đ (1.1) were reported at both higher (∼18 kg mol−1) and lower (∼4.1 kg mol−1) targeted molar mass.55 ATRP comprises a bulk amount of the CRP literature pertaining to PVPs, and many are based around circumventing coordination of the pyridinyl nitrogen of 2VP and 4VP to the copper catalysts used.51 This coordination effect does not necessarily prevent control over the polymerization but does significantly reduce the rate. Dampening of this coordination can be performed by using an auxiliary ligand that binds more strongly to copper; for example tris[(2-dimethylamino)ethyl]amine (Me6TREN).56 It is also noteworthy that copper chloride catalysts and alkyl chloride initiators were found to be necessary, instead of their bromide counterparts, to maintain good control.51 Tsarevsky, Matyjaszewski, and co-workers mechanistically explored this phenomenon further and also developed a method to produce well-defined P4VP in protic media.57 Alternative ATRP strategies, such as reverse ATRP, have also been used to create P4VP with relatively good

control.58−60 ATRP of 2VP has been largely unsuccessful due to an exacerbated deactivation effect hypothesized from the ortho positioning of the pyridinyl nitrogen. Recently, Zhang and co-workers successfully reported the ATRP of 2VP by “activating” the monomer through complexation with a hydrogen bonding donor 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), which also potentially deters this deactivation effect.61 Although successful, these polymerizations exhibited moderate to low control over theoretical molar mass and often resulted in higher dispersities. Some reports marry together two or more CRP techniques to best suit the monomers used in the BCP system. For example, Long, Nishide, and co-workers presented a methodology to obtain P4VP-b-PMMA through a bifunctional CRP initiator capable of performing NMP on 4VP and ATRP on MMA in a stepwise fashion.62 Wang, Luo, and co-workers have reported a tandem ATRP, NMP, and singleelectron transfer nitroxide-radical coupling (SET-NRC) to produce BCPs of P4VP and PMMA, PS, or poly(ethylene glycol) (PEG).63 Reversible addition−fragmentation transfer (RAFT) polymerization is an attractive strategy for CRP of 2VP and 4VP due to the absence of metal catalysts and the variety of chain transfer agents (CTA)s that can be potentially explored. McCormick and co-workers reported successful RAFT of both 2VP and 4VP in DMF using a singular CTA, cumyl dithiobenzoate (CDB), which produced molar mass near theoretical up to 30 kg mol−1 in addition to high conversion (>80%) and narrow Đ (≤1.2).64 A polystyrene dithiobenzoate CTA also successfully produced PS-b-P4VP in dispersion.65 Although RAFT is used frequently to produce a variety of PVP-based BCPs discussed later, very few papers exist which focus on the screening, kinetics, and potential optimization of RAFT on 2VP and 4VP using the various CTAs (trithiocarbonates, xanthates, dithiocarbamates, etc.) available. Evidence suggests that trithiocarbonates, such as S-dodecyl-S′-(α,α′dimethyl-α″-acetic acid) trithiocarbonate (aka DDMAT or DIBTTC), in DMF at 70 °C are also very effective CTAs for the control of P4VP in addition to acrylates for BCP synthesis.66,67 The use of 1:1 benzoyl peroxide:CTA was reported to yield narrow Đ (1.11) P2VP after 24 h at 25 °C at ∼40% conversion, providing a lower temperature preparation alternative to these materials.68 An area of research gaining continuous traction is visible light driven CRP, and recently Xin and co-workers described RAFT of P2VP through bluelight initiation of the DDMAT.69 Although this presents an exciting new direction to obtain PVP segments through facile methods, more research is needed to reduce the amount of DDMAT required to control these polymerizations. Finally, when using RAFT, consideration of the remaining CTA end functionality should be taken: Active research continues to evolve for removing or transforming the CTA end groups through a variety of methods.70,71

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CHOOSING P2VP OR P4VP AS THE SEGMENT Basic Properties. The positioning of the pyridinyl nitrogen of P2VP and P4VP presents different material and chemical properties that are worthy of consideration depending on the purpose of the BCP system. As will be discussed, the choice of isomer leads to different association constants and apparent solubility parameters; the latter of these differences is exemplified by a substantial interaction parameter (χ) between these segments in the bulk state. The glass transition temperatures (Tg) of P2VP (∼104 °C) and P4VP (∼142

C


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Figure 2. Structures of the P2VP and P4VP repeating unit highlight the increased steric hindrance of the P2VP nitrogen lone pair due to its proximity with the backbone. A sampling of potential associated species that are predicted to associate more strongly toward P4VP as the species becomes larger.

°C) are notably different.72 Therefore, P4VP segments will require higher temperatures to thermally process and also to investigate microphase separation phenomena of BCPs in the melt state. Differences in solvation are evident for each polymer. At the segment molar masses typically used in block polymer research (5−100 kg mol−1), P2VP is soluble in THF and CHCl3 while P4VP often requires addition of polar cosolvents (DMF, DMSO, and NMP).36 Common alcohols tend to dissolve both isomers quite well due to favorable hydrogen-bonding interactions. Early reports on the Hansen and Hildebrand solubility parameter for P2VP (19.8 and 21.3 MPa−1/2, respectively)73,74 can be found. However, experimentally determined values for P4VP were not found to the best of my efforts. Although the group contribution method suggests P2VP and P4VP isomers have the same solubility parameters,75 P4VP is likely to have a higher solubility parameter than P2VP based on its improved miscibility in higher polarity solvents. This difference in solubility parameter stems from the positioning of the pyridinyl nitrogen and is noteworthy with regards to finding appropriate solvents for solution characterizations, selective domain swelling, or film casting. Because of the basicity of the pyridine ring (pKa of pyridinium ion = 5.23),76 the hydrophilicity of both systems will increase significantly upon lowering pH, and both P2VP and P4VP will become water-soluble under acidic conditions. As will be discussed later, this presents opportunistic pHresponsive behavior for both P2VP and P4VP segments in aqueous media. Hosting Acids/Metals/Nanoparticles. Although P2VP can be readily protonated, the positioning of the pyridinyl nitrogen in close proximity to the polymer backbone can deter binding of large organic molecules, metal salts, or nanoparticles due to steric repulsion. Figure 2 displays a relative overview of the many forms of chemical association possible for the pyridinyl nitrogen. When moving from small (hydrogen halides) to large (nanoparticles), the larger objects will have more difficulty associating with constrained geometry of the P2VP repeating unit. Some of these chemistries can have a large variety of size options such as the organic guest molecules and the choice of metal salts. P4VP is much more accessible to binding of large molecular guests and nanoparticles which also results in improved binding stability.77 This phenomenon has been described a number of times within the literature. For example, Aizawa and Buriak described complications with the deposition of silver cations within a PS−P2VP system and ascribed it to steric restrictions of P2VP in addition to competition of easily accessible protons to the P2VP sites.78

A large body of research has been aimed at loading nanoparticles within the domains of PVP or PVP-containing BCPs and have been the subject of several recent reviews.79−83 Many examples have shown that the choice of the PVP isomer plays an important role. In a pertinent example, Kim and coworkers showed that microphase-separated domains of P2VPb-P4VP preferentially absorb gold nanoparticles into the P4VP matrix at lower concentrations and then eventually populated both domains, albeit to a much lesser extent in P2VP domains, at increasing nanoparticle concentration (Figure 3).84 The

Figure 3. Transmission electron microscopy (TEM) images of P4VPb-P2VP self-assembled into a lamellar morphology (left). At lower particle loading, P4VP exhibits preferential absorption of gold nanoparticles (blue squares) due to the improved coordinative ability of the pyridinyl nitrogen in the para position (middle). Upon increasing the gold nanoparticle content, some begin to absorb into the P2VP domains (green squares) but to a lesser extent than P4VP (right). Reproduced with permission from ref 84.

same concept extends to large organic guests as well. Hadjichristidis, Ruokolainen, and co-workers showed through infrared (IR) spectroscopy that bulky cholesteryl hemisuccinate (CholHS) can only partially hydrogen bond to P2VP segments (∼30%) regardless of excess CholHS loading.85 Ikkala, Rannou, and co-workers showed that P4VP segments were able to host up to a stoichiometric equivalent of CholHS although higher loadings resulted in competing crystallization of the mesogen.86 Therefore, when designing PVP-containing BCP systems to host sterically demanding guests or nanoparticles, considerations should be taken toward D


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P2VP and P4VP remain the more hydrophilic block versus PMMA since P4VP retains its larger χ over P2VP. Other notable systems investigated include P2VP-b-poly(4-tertbutylstyrene) (PtBS), χPtBS−P2VP = 0.11 (T = 150 °C, Vref = 118 Å3),33 P4VP-b-poly(4-tert-butoxystyrene) (PtBOS), χP4VP−PtBOS = 0.40 ± 0.02 (T, Vref not reported),66,96 and P4VP-b-poly(N-acryloylpiperidine) (PAPI), χP4VP−PAPI ≈ 0.03 (T = 150−200 °C, Vref = 166 Å3).97 Jung, Ross, and co-workers reported well-segregated films of P2VP-b-polydimethylsiloxane (PDMS) that had dynamic dimensional tunability. Although χP2VP−PDMS was not quantified, its declared large value is consistent with the fact that PDMS is very hydrophobic and has a lower solubility parameter (15.5 MPa1/2) than PS (18.5 MPa1/2).98 Because of the hydrophilicity of the PVP segment, it is highly miscible with segments of PEO in the bulk state.99,100 Interestingly, P2VP-b-PEO (Mn = 80 kg mol−1, f PEO ≈ 0.5) exhibits a lower critical ordering transition from a disordered to lamellar morphology at ∼160 °C upon heating, suggesting a favorable interaction between PVP and PEO segments aids in miscibility at lower temperatures.100 Recently, Olsen and co-workers investigated a series of P2VP-bpoly(oligoethylene glycol)methacrylate (POEGMA) and discovered that selectively increasing amounts of acidification within the P2VP phase caused these systems to microphase separate in bulk.101,102 It was also determined that the type and valency of counteranion species can affect the differences in χ.101 An interestingly high incompatibility was discovered for diblock systems composed of P2VP-b-P4VP.103 These materials exemplify the potential difference in solubility parameter and hydrophilicity between the PVP isomers. As mentioned earlier, Kim and co-workers showed selective gold nanoparticle absorption into the P4VP domains of a lamellar P2VP-b-P4VP system.84 It was determined that χP2VP−P4VP ≈ 0.26 at T = 150 °C and Vref = 118 Å3, which falls within the values for χPS−P2VP and χPS−P4VP (Table 1).103 Continuing knowledge of the microphase separation tendencies between P2VP, P4VP, and other select blocks is of importance for rational design not just in the bulk state. For example, polymerization-induced self-assembly (PISA) relies on polymerization and microphase separation to occur in situ and can result in a well-defined BCP nano-objects containing PVP.104−106 With reliable χ parameters, the choice of cosegments and the length of the PVP block needed to achieve microphase separation with said segments may be targeted by rational design prior to PISA. The interaction parameters in Table 1 apply to microphase separation in the bulk state. As will be discussed, solution selfassembly is also a prominent area of PVP BCP research. Assembly within a solution adds a third and complex component to the incompatibility behavior. This is best exemplified by PVP-b-PEO systems in aqueous media. While both P2VP and P4VP segments are determined to be miscible with PEO in the bulk state, these segments microphase separate easily to form micellar structures in aqueous solution. This is due to insolubility of PVP in (nonacidic) water solutions while PEO is miscible. When PVP-b-PEO is added to an aqueous environment, the combination of PEO + H2O now has enough incompatibility with PVP to form core−shell structures. As can be imagined, the association of a guest selectively into the PVP core continues to proliferate an even more complex χ parameter for the (PVP + guest)-b-(PEO + H2O) system. Continued exploration and quantification of χ in

the use of P2VP or P4VP depending upon the amount of loading and the binding strength desired. Microphase Separation. Developments in understanding the microphase separation tendencies between P2VP or P4VP with select segments have been ongoing. The interaction parameter, χ, is used to quantify the degree of miscibility (or immiscibility) between block segments as a function of temperature (T) and the sum of the overall degree of polymerization (N) of each segment.1 PS-b-P2VP and PS-bP4VP were the materials of choice for initial investigations of χ for PVP-containing systems.87−91 Although there are some challenges to compare resulting χPS−P2VP values determined through different methods, temperatures, and reference volumes (Vref), there is a general agreement for χPS−P2VP ≈ 0.1 at temperatures well above Tg (∼150 °C) and a segment averaged Vref ≈ 99.5 Å3 corrected for thermal expansion at 150 °C.87−91 Literature focused on PS-b-P4VP systems are in partial agreement with χPS−P4VP ≈ 0.33 under similar T and Vref described above.89,90 However, χPS−P2VP and χPS−P4VP increase to approximately 0.11 and 0.40, respectively, when using Vref = 118 Å3 and T = 150 °C (Table 1).92 Here it should be noted Table 1. Effective Interaction Parameters for a Variety of PVP-Containing Block Copolymer Systems BCPa

χ (150 °C)b

method of determinationc

ref

P2VP−P4VP P2VP−PS P4VP−PS P2VP−PMMA P4VP−PMMA P2VP−PtBS P4VP−PtBOS P4VP−PAPI

0.26 0.11 ± 0.01 0.40 ± 0.02 0.05 0.08 0.11 0.41 ± 0.02d 0.03e

ODT, RPA FRES, ODT, RCB, RPA RCB, ODT RPA RPA ODT RCB RCB

103 87−91 89, 90 95 95 33 96 97

a

PS = polystyrene, PMMA = poly(methyl methacrylate), PtBS = poly(4-tert-butylstyrene), PtBOS = poly(4-tert-butoxystyrene), PAPI = poly(N-acryloylpiperidine). bV ref = 118 Å3 unless otherwise reported. cODT = order-to-disorder transition analysis, RPA = random phase approximation SAXS analysis, FRES = forward recoil spectrometry, RCB = random copolymer blend analysis. dT and Vref not reported. eT = 150−200 °C, Vref = 166 Å3.

that Vref is simply a parameter that encapsulates a certain volume of the BCP of interest and therefore a certain amount of demixing energy (positive χ) for a BCP within Vref. It can be chosen arbitrarily or can be an average molar volume of the two BCP segments. When comparing χ values between different BCP systems, as is done in Table 1, consistency in Vref provides a more rigorous comparison.93,94 Nevertheless, the 3-fold increase in χPS−P4VP over χPS−P2VP suggests P4VP is significantly more hydrophilic than P2VP (note: PS is generally accepted as the hydrophobic segment). This is consistent with the trends in solubility described earlier and may be rationalized by a much higher degree of polarizability due to the para positioning of the pyridinyl nitrogen on P4VP (Figure 2).90 Han and Kim95 performed a thorough study to compare χ between P2VP-b-PMMA and P4VP-b-PMMA using smallangle X-ray scattering (SAXS) in the disordered, nonfluctuating regime and the random phase approximation (RPA) theory developed by Leibler.1 They determined χP2VP−PMMA = 0.05 and χP4VP−PMMA = 0.08 (adjusted to T = 150 °C and Vref = 118 Å3).95 Although these parameters are significantly smaller than those with PS, this study suggests that E


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Figure 4. Recent representative examples of selective hosting of organic guests within the PVP phase of PS-b-PVP systems. (A) Selective swelling of the P2VP phase of a gyroid forming PS-b-P2VP system with ethanol causes the retainment of the gyroid morphology by the PS phase while the increase of the P2VP domain size causes Bragg reflection of visible light. Upon initial swelling, the colorless material (left) turns red (middle) and then green (right) after partial evaporation of the ethanol. Vitrification of the P2VP phase on the exterior of the monolith causes the green phase to become locked in place but can be returned to the bulk state (left) through heating and full removal of the EtOH. Reproduced with permission from ref 114. (B) Thin-film casting and directed self-assembly of a PS-b-P2VP thin film orients the lamellar morphology perpendicular to the surface. The P2VP phases can be quaternized by exposure to methyl iodide (MeI) vapor, which turns the thin film from a nanopatterned BCP to a block copolymer electrolyte (BCE) without loss of morphology. TEM images show the BCP film (left) and BCE film (right) has nearly identical morphology while the BCE film has slightly improved contrast contributed by the iodide anions. Adapted with permission from ref 129. Copyright 2017 The Royal Society of Chemistry. (C) Halogen bonding of a low-surface energy guest, diiodoperfluorooctane (DIPFO), into the P4VP phase of PS-b-P4VP results in hierarchical self-assembly. A biphasic columnar structure of P4VP and DIPFO exist within a matrix of PS while DIPFO also migrates to the surface due to its low surface energy. Adapted with permission from ref 130. Copyright 2017 Elsevier.

to help mitigate the differences in surface energy of block segments and help align morphologies perpendicular to the substrate with long-range order.109 In nearly all cases, the solvents are volatile and can be effectively removed after treatment to return a bulk patterned film. For PS-b-P2VP, acetone has been shown to effectively solvent anneal both of these domains.110,111 For PS-b-P4VP, THF has been shown a suitable solvent108 in addition to mixed solvent systems.112

complex solutions will certainly benefit the rational design and understanding of such systems. Another prominent area of research with regards to induced microphase separation of PVP BCPs is the use of solvent vapor annealing to assist in the mobility, orientation, and alignment of thin film morphologies. The advancements and strategies involved in these techniques are vast and have been reviewed recently.107 For brevity, some important concepts as they pertain to PVP containing BCPs will be discussed. The high Tg values for P2VP and P4VP result in less polymer melt diffusivity below the thermal limit of decomposition, and therefore vapor annealing is advantageous for fast selfassembly.108 As shown in Table 1, P2VP and especially P4VP can be considered “high-χ” BCPs when segmented with low-polarity counterparts such as PS.92 Because a high χ can be correlated to a high difference in solubility parameters between the segments, finding a solvent that can nonselectively swell both domains is a challenge. Another benefit of the solvent vapor is that it alters the surface energy of the air/film interface

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PS-b-PVP SYSTEMS AND PVP COORDINATION PS-b-P2VP and PS-b-P4VP have been the workhorse materials for investigations into the selective binding of guest molecules or particles into the PVP domains of bulk nanostructured BCPs. This is exemplified by the fact that self-assembly of PSb-P2VP alone has received its own recent review.22 The prominence of these studies is due, in part, to a longstanding commercial availability of these BCPs with a variety of N and f PS and a variety of self-assembled morphologies in the bulk or thin film state. The appearances of PVP-containing BCPs in F


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Figure 5. Fabrication scheme to produce hierarchical multiscale hyperporous structured block copolymer (HMH-BCP) through chemical modification of PS-b-P4VP BCPs with a siloxy-functionalized epoxide (GPTMS). The reduced surface energy of the GPTMS-modified P4VP causes a disparity in solubility while solvent/nonsolvent exchange within the structure leads to varying degrees of macro- and nanophase separation. Reproduced with permission from ref 155. Copyright 2015 American Association for the Advancement of Science.

reversibly transformed from colorless to red to green in appearance through the swelling and trapping of the domain at various stages, which mimic the photonic crystal phenomenologies of butterfly wings (Figure 4a). When conditions allow both PS and PVP to rearrange morphology to accommodate guests within the PVP phase, it is possible to observe formation of new morphologies not commonplace for diblock systems which often depends highly on the interplay between microphase separation and ordering of the guest molecules.85,124 Quaternization of PVP using alkyl halides provides permanent ionization to the segment and is a methodology to produce block copolymer electrolytes (BCEs) for ionic conduction in electrochemical technologies. Arges, Nealey, and co-workers have been exploring the effect of methyl iodide vapor annealing and quaternization of P2VP after directed selfassembly (DSA) and perpendicular alignment of PS-b-P2VP thin films.128 It was discovered that this quaternization does not disrupt the aligned nanostructure, even after anion exchange strategies, and this can provide direct insight into how defects in the DSA alter the ion conductivity for these systems (Figure 4b).129 Finally, a new concept utilizing halogen bonding of 1,8-diiodoperfluorooctane (DIPFO) additives was recently reported by Metrangolo, Ikkala, and co-workers.130 Halogen bonding of iodo groups to P4VP, coupled with the low surface energy of the fluorinated DIPNO, leads to an increase in χ and aids in DSA of aligned cylindrical PS−P2VP domains partitioned between lamellar ordering of the DIPFO guests (Figure 4c).130 Inorganic Guests. The combination of directing the selfassembly of BCP thin films and the ability to include a variety of inorganic precursors selectively into PVP domains has resulted in a plethora of research aimed at utilizing PS−PVP systems for the development of nanostructured optical devices,131,132 semicondutors,119,133,134 catalysis,135,136 plasmonics,137,138 multimetal composities,139,140 ion conduction

many reviews have also been largely focused on studies on PSb-P2VP and, to a slightly lesser extent, PS-bP4VP.6,8,12−17,19−22 For this reason, only a general perspective of their divergent utility is provided along with highlights of some recent developments. Organic Guests. The basicity and hydrogen-bonding capability of the pyridinyl nitrogen allow for selective adsorption of small molecules into the PVP phase. Alcohols,113,114 phenols,115−121 carboxylic acids,85,86,122,123 and sulfonic acids124,125 are effective guests as they either protonate or hydrogen bond with PVP and tend to have low miscibility within the hydrophobic PS domains. Two main effects can be predicted when selectively adding small molecules into only the PVP domains of the bulk BCP. First, protonation or enhanced polarity and hydrophilicity from the pyridine−guest complex often leads to an increase in χ between the PS and PVP−guest phase. Sometimes, depending on the counterion, a decrease in χ has been discovered. An increase in χ allows for sharper domain interfaces and access to ordered microstructures at smaller domain scales.92 Second, the overall volume fraction of the PVP−guest domains increases with increasing guest content. This can be expected to crossover into different morphologies (order−order transitions) while traversing horizontally along the BCP phase diagram.126 Because of the hydrophobicity and Tg ≈ 100 °C of PS, swelling of PVP domains with polar solvents and at lower temperatures often leads to swelling of the PVP domains selectively while the morphology of vitrified PS is maintained. Utilization of this concept was pioneered by work from Thomas and co-workers.113,127 Recently, Chiang and coworkers utilized selective adsorption of ethanol into the P2VP phase of gyroid-forming PS-b-P2VP thin films.114 Upon swelling, the colorless thin films retained the gyroid morphology but increased in domain size to allow Bragg reflection of visible light.114 The photonic films could be G


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Macromolecules

Figure 6. Example of the unique self-assembly possible from diverse BCP segment sequencing. By having the hydrophobic PS core segment (black) in the middle of a PEO-b-PS-b-P2VP system, the pH-responsive behavior of P2VP allows for precursor micelles with a mixed PEO (blue) and P2VP (green) corona at low pH (∼2). Upon raising the pH, the P2VP becomes insoluble, changing the micelle into a “patchy” version with contracted P2VP segments. These patches are then shown to aggregate into supracolloidal chains following thermal treatment. Reproduced with permission from ref 173. Copyright 2018 The Royal Society of Chemistry.

membranes,141 antimicrobial coatings,142 and nanoelectronic devices. For the latter, recent reviews by Cummins and Morris,19 Osuji,12,21 Nealey,17 and their co-workers comprehensively address these techniques, offer insight into future challenges, and circumvent the need to go into great detail on DSA and nanoelectronic device manufacture within this Perspective. In brief, the techniques of sequential infiltration synthesis (SIS),111,143,144 metal salt inclusion (MSI),145−147 and aqueous metal reduction (AMR)148−154 are all suitable for PS-b-PVP systems and provide a large playground upon which new developments in lithographic patterning of metal and metal oxide structures can be explored with increasing pattern fidelity, minimized defects, and smaller features. An interesting alternative strategy utilizing PS-b-P4VP was recently reported by Lee, Park, and co-workers where chemical modification to P4VP domains was performed using (3-glycidoxypropyl)trimethoxysilane (GPTMS) and resulted in covalent attachment to the pyridine ring.155 This unique reaction removes the pyridine’s aromaticity, thereby greatly reducing the hydrophilicity of the PVP domains. The introduction of GPTMS, dubbed a “surface energy modifying agent”, in addition to judicious solvation control within the PS-b-P4VP matrix produced hierarchical multiscale (nano and macro) hyperporous structures that are laced with inorganic silica following O2 plasma treatment (Figure 5).

introduced by Jérô me and co-workers and provide an alternative to P2VP core systems.163 The PS segment is insoluble in water at all pH values, and therefore the combination of these three segments presents a dynamic core−shell−corona solution assembly where the P2VP shell can traverse aqueous solubility at varying pH without loss of the micelle framework due to the fixed PS core. Soon after, Aizawa and Buriak capitalized on this morphology to selectively bind different metals salts, HAuCl4 and then AgNO3, into the P2VP and PEO phase, respectively, to produce dual-metal core−shell nanostructures on silicon surfaces.78 The dynamic playground of these systems ranges from self-assembly of unique architectures164 to templating of single- or dual-component nanoparticles and continues to be a prominent area of interest.165−168 Jérôme and co-workers have reported pH-responsive flower-type micelles that were able to selectively compartmentalize or expose biotin guests bound onto the P2VP phase of P2VP-b-PEO-b-poly(ε-caprolactone) (PCL) triblock terpolymers.169 In this case, PCL serves as a biocompatible and biodegradable core segment. Tsitsilianis and co-workers have also extended these chemistries to CBABC terpolymers as potential hydrophilic drug nanocarriers170 and pH-sensitive assemblies that deliver gold nanoparticles.171 As mentioned earlier, CRP techniques have allowed facile design of block sequencing atypical of the traditional anionic techniques. Eisenberg and co-workers first reported a sequential ATRP synthesis to produce PEO-b-PS-bP4VP, which places the hydrophobic PS segment within the middle of the triblock sequencing.172 The importance of this block sequencing was recently exemplified by Gröschel and coworkers where they report the solution assembly of PEO-b-PSb-P2VP produced by sequential RAFT polymerization from a PEO macro-CTA.173 The evolution of unique self-assemblies of supracolloidal chains from patchy micelles was realized as the P2VP segment was subjected to varying pH (Figure 6).

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PVP WITH PEO IN DIBLOCK OR TRIBLOCK SYSTEMS While χP2VP−PEO has been shown to be considerably small in the bulk state,99,100 complexation of PVP with amphiphiles has been shown to promote microphase separation between these segments.156 Furthermore, P2VP and P4VP are largely insoluble in water at neutral to high pH and water-soluble at pH < ∼5.157,158 PEO, on the other hand, retains solubility in water at all pH levels. Therefore, aqueous solution assembly of PVP−PEO containing BCPs has been an area of great interest due to the aforementioned ability for the PVP phase to host a variety of chemistries in addition to its pH-responsive behavior.10 Several decades of advancements have utilized micellization of PVP-b-PEO with organic or metallic species to produce a variety of micelles or vesicles with control over size and distribution.159−161 For example, Förster and co-workers showed the pH-responsive behavior of P2VP allows for the release of cargo, such as hydrophilic dyes, from BCP vesicles upon lowering pH.162 Some concerns exist that the high Tg and brittle nature of PVP could be nonideal as a stand-alone micellar core for cargo delivery due to buckling and leakage.5 Linear triblock systems composed of PS-b-P2VP-b-PEO were

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PVP WITH POLY(tert-BUTYL [METH]ACRYLATE): TOWARD BLOCK POLYAMPHOLYTES Segments of poly(tert-butyl acrylate) (PtBA) and poly(tertbutyl methacrylate) (PtBMA) can be synthesized through anionic and CRP techniques and are therefore able to be introduced into BCPs containing PVPs through synthetic strategies described earlier. A unique aspect of these segments is their ability to be converted into poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMAA), respectively, through hydrolysis of the ester groups. Therefore, BCPs containing a combination of PVP segments and PAA segments have the ability to be selectively ionized. The PVP segments will be H


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Macromolecules

assemblies change from a positive to negative polyelectrolytes at low and high pH, respectively, while assembling into more of a electrostatically neutralized coacervate at pH near the iep. Triblock terpolymers containing P2VP, PtBMA, and a third segment chemistry (typically PS or polybutadiene (PB)) have also been explored. Collective works of Müller, Schacher, and co-workers have explored linear ABC triblock terpolymers of PB-b-P2VP-b-PtBMA182 and the hydrolyzed version PB-bP2VP-b-PMAA183 in addition to unique ABC miktoarm star terpolymers which each arm being either PB, P2VP, or PtBMA.184 For the latter, the P2VP arms were quarternized with methyl iodide, and subsequently the I− counteranions were exchanged with less hydrophilic triiodide (I3−) as a trigger to induce hierarchical changes in the self-assembly of the star terpolymers (Figure 8). Tsukruk and co-workers have utilized advanced star architectures containing various combinations of PS, P2VP, PAA, and poly(N-isopropylacrylamide (PNIPAM) from which multiresponsive monolayers185 and multicompartmental microcapsules186,187 can release cargo through a variety of stimuli. Loos, ten Brinke, and co-workers have investigated PtBMA-b-PS-b-P4VP synthesized by ATRP.188 For this material, the PtBMA and P4VP segments flank the central PS segment, allowing for microphaseseparated regions of PtBMA and P4VP within a matrix of inert and glassy PS. Following chemical modification and ionization of the PtBMA and P4VP domains, the concept of “charge mosaic” membranes are possible. Grafting of PAA-bP4VP from glass surfaces using ATRP, such as reported by Brittain and co-workers,189 also presents opportunities to utilize stimuli-responsive behavior of functional brush surfaces. It was shown that the thickness of the brush surface expands and contracts considerably under varying pH conditions.

protonated at low pH and the PAA segments will be deprotonated at high pH (>8), while both segments will exhibit a much more hydrophobic coacervate character in the mid-pH range (5−7) with an isoelectric point (iep) at pH ≈ 5.5.174 The iep is the pH value where the positive and negative charges on the block polymer are theoretically equal. Furthermore, because of the ability to quaternize PVP, one can “lock in” the ionic character of this block at all pH ranges while still allowing for dynamic ionization of PAA. An early study of P2VP-b-PAA can be traced back to the work of Stille and co-workers in 1971,175 while the concept of ampholytic block polymer systems containing PVP and PAA was reintroduced by Rempp and co-workers in 1991.34 Over the past couple decades, Tsitsilianis174,176−178 and more recently Papadakis179,180 and co-workers have collaborated to bring understanding of the self-assembly, structural, and rheological properties of ABA triblock copolymers of PtBA−P2VP−PtBA (a telechelic polyelectrolyte) or PAA-b-P2VP-b-PAA (a block polyampholyte) under a variety of pH conditions. Their collective findings in additional to other phenomenology associated with block polyampholyte assembly were recently reviewed.181 A great example of the dynamic reassembly possible for these systems as a function of pH can be highlighted by a multisegmented, multiarm star terpolymer architecture of exactly nine PS arms and nine (P2VP-b-PSS) arms emanating from a cross-linked poly(divinylbenzene) core (Figure 7).176 When traversing the iep for these systems, the

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OTHER SEGMENTS WITH PVP A variety of styrenic alternatives have been explored in combination with PVP systems. They include PtBS,33,190,191 PtBOS,66,96 poly(trimethylsilylstyrene) (PTMSS),191 and styrene−acrylonitrile (SAN).67 The purpose for many of these styrenic alternatives was to modify or increase the χ parameter discussed earlier and allow access to smaller domain periodicities in self-assembled thin films. Zhang and coworkers recently used thiol-terminated P4VP chains as nucleophiles to substitute onto (or “grafting to”) the backbone of poly(p-chloromethylstyrene) (PCMS) to produce PCMS-g-

Figure 7. Dynamic pH-responsive solution assembly of a multisegmented, multiarm star terpolymer architecture of exactly nine PS arms and nine (P2VP-b-PSS) arms emanating from a cross-linked poly(divinylbenzene) core. The block polyampholyte P2VP-b-PSS arms rearrange in solution as the isoelectric point (iep) is traversed. Adapted with permission from ref 176. Copyright 2011 The Royal Society of Chemistry.

Figure 8. Hierarchical self-assembly of an ABC miktoarm star terpolymer composed of PB (blue arm), PtBMA (red arm), and quaternized P2VP (qP2VP, gray arm). In water the PB and PtBMA join to form a self-assembled core (purple) with a soluble qP2VP shell. Counterion exchange of the iodide (I−) with more hydrophobic triiodide (I3−) results in the assembly of superstructures up to micrometers in size. Reproduced with permission from ref 184. I


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Macromolecules P4VP.192 Because RAFT was used to produce the PCMS backbone, the grafted macro-CTA could then be used to grow a linear PS segment using emulsion techniques to produce (PCMS-g-P4VP)-b-PS as multicompartment nanoparticles that bind gold nanoparticles and show high catalytic efficiency toward alcohol oxidation.192 Dienes are another oft-used segment with PVP systems with polyisoprene (PI)193,194 or polybutadiene (PB)195−197 being the most common. One of the main advantages to these segments is their ability to be cross-linked through facile methods, such as UV light/ photoinitiator systems, 197 and therefore create robust aggregate assemblies that persist in a variety of solvent conditions. Poly(ethyl acrylate) (PEA) has been used as a middle segment in P4VP-b-PEA-b-P4VP systems, synthesized through RAFT polymerization, due to the high solubility of PEA in ionic liquids.198−200 The ionic liquid dissolved triblocks, in combination with thermoreversible cross-linking agents such as poly(hydroxystyrene)199,200 or ZnCl2,198 present supramolecular ion gels with dynamic viscoelastic properties over a wide range of temperatures. Ionic liquids have also been used to completely dissolve P2VP-b-PMMA, and upon quaternization of P2VP using ethyl bromide, the P2VP domains microphase separate into periodic structures within the ionic liquid.201 This phenomenon was used to create magnetic thermoset monoliths that operate without the use of metal oxides or magnetic nanoparticles.202 Other notable segments that have been reported with PVP BCPs include polymethacrylates bearing liquid crystalline stilbene mesogens,203 segments bearing semiconducting tetraphenylbenzidine units53 or poly(3-hexylthiophene) oligomers,204 hydrophilic poly(glycidol),205 rodlike micelle forming poly(ferrocenyldimethylsilane),206 and helical microphase forming poly(L-lactic acid).207

reported through the use of BCP blends containing PVP and PSOH chemistries. Matsushita and co-workers have investigated the synthesis and self-assembly of PI-b-PS-b-P2VP at segmental values of N in which χPS−PI, χPS−P2VP, and χPI−P2VP are large enough to create three distinct microphases of each segment within the bulk state.39,214 More recently,215,216 and in collaboration with Sinturel and co-workers,217 this system has been shown to self-assemble into tetragonally packed ensembles of rectangular cylinders216 and rectangular rods217 through blending of these linear triblock systems containing different block compositions.

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PROSPECTIVE It is with hope that this Perspective has convinced the reader on the promise that PVP BCPs have for a multitude of complex material discoveries in polymer science. The discussions above merely penetrate the surface of the substantial work that has been done in this field over the past three decades. Improvements to anionic polymerization and the discoveries with CRP over this time now provide us with a seemingly limitless number of block chemistries and architectures with which to include PVP as a dynamic addition. While 2VP and 4VP monomers are relatively cheap and widely available for purchase, the availability of PVP containing BCPs are significantly less widespread and come at a hefty price. To the best of my knowledge, industrial scaling of PVP-based BCPs has not come to fruition and perhaps continuing to discover even higher levels of versatility for these systems, spanning a variety of niche applications, may warrant so in the future. Nevertheless, PVP-containing BCPs stand to be a playground upon which research can explore and develop our current understanding of complex and dynamic functional systems. Considering the large variety of organic and inorganic guests that can be associated within PVP domains, it is predicted that these materials will continue to be of high focus for the design of nanostructured assemblies for many applications. Polymer chemistry will continue to have an integral role in the future development of PVP-containing BCP systems as the diversification of these segment chemistries, topologies, and architectures is destined to produce increasingly advanced functional materials. Even still, some very fundamental concepts still remain in need of attention. For example, accurate prediction and quantitative analysis on the degree of quaternization of P2VP, given its aforementioned steric limitations, remain somewhat elusive. In the literature it is common to find that a quaternization reagent, such as methyl iodide, is used while infrared spectroscopy and titration with AgNO3 are the most common methods for determining the degree of quaternization. Significant variability in the degree of quaternization is often found when using similar procedures (i.e., stoichiometric equivalents of MeI, solvent, and temperature used). In concert with synthetic advancements, new segment combinations and BCP architectures will require further, in-depth, understanding of microphase separation tendencies and χ parameters, especially those in complex (e.g., solvated, doped, and ionized) systems. The peculiar LCST behavior of PVP−PEO systems in the bulk state is one notable example.86 The past decade has witnessed remarkable advancements of morphological predictions through theory and computation, with some of them turning attention toward dynamic hydrogen-bonding systems, such as PVP BCPs in the solution,18 bulk-blended,211,218,219 or even brush state.220

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PVP BCPs AND BLENDS The rich self-assembly of systems containing PVP extends even further through the use of homopolymer and/or BCP blends. One can imagine these systems as an extension of the organic additives section described earlier, with the difference being that the blends are macromolecular in nature. Much like phenols are popular additives for selective hydrogen bonding within the PVP phases of self-assembled BCPs, the associative interactions between PVP and poly(p-hydroxystyrene) (PSOH) have also been a choice system for experimental and theoretical understanding BCP/blend behavior.75,116,208−213 Addition of PVP homopolymer to PS-bPSOH systems (or the addition of PSOH homopolymer to PSb-PVP systems) operates in a manner similar to small molecule phenols; the selective association of PVP and PSOH makes them miscible and begins to increase the volume fraction of the PVP/PSOH phase. Accompanying this increase, order−order transitions are realized when traversing horizontally along the BCP/blend phase diagram.211 For these blends, the choice of P2VP or P4VP remains important. Recently, Kuo and coworkers investigated the hydrogen bonding strength between PS-b-PSOH/P4VP and PS-b-PSOH/P2VP systems in the “wet-brush”209 homopolymer loading regime.75 Based on the higher steric demand for PSOH to hydrogen bond with P2VP over P4VP, these systems displayed order−order transitions at different homopolymer compositions, and only the PS-bPSOH/P4VP system was found to produce a gyroid morphology.75 Even more complex hierarchical structures, such as square arrays116 and core−shell cylinders,213 have been J


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ACKNOWLEDGMENTS Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund (55378-DNI7) and the FSU Energy and Materials Hiring Initiative for support during writing of this perspective and Brandon Fultz for assistance in the literature discovery.

Continued computational predictions on the rich variety of self-assembly possibilities as the experimental systems become more complex (for example, linear ABCD BCPs or blends coupled with varying size and electrostatic binding strength of guests) will blueprint synthetic and experimental efforts. In many nanotechnology applications, such as lithography, defectfree DSA will continue to be a forefront endeavor for advancements utilizing PVP BCPs in thin films. Advancements in this regard continue at a rapid pace over the past several years.108,143,221,222 With the rich variety of organic and inorganic additives that can selectively migrate within the PVP domains of self-assembled BCPs, engineering principles to guide self-assembly in a scalable fashion with minimal defects may warrant their use in the next generation of semiconductors with sub-10 nm features due to their high χ parameters. Predictable understanding of solution assembly, particularly in dynamic environments (pH, temperature, and light), will promote next-generation micellar or polymersome assemblies with promise toward applications ranging from drug delivery11 to photonic fluids.223 Collaborative efforts from all areas of expertise in polymer science will ensure PVPcontaining BCPs will have a potential role in solving some of the grand challenges ahead.224

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Justin G. Kennemur: 0000-0002-2322-0386 Notes

The author declares no competing financial interest. Biography

Justin G. Kennemur grew up in Virginia and earned a B.S. degree in chemistry at Radford University. After working for three years as an analytical polymer chemist at Polymer Solutions Incorporated in Blacksburg, VA, he attended graduate school at North Carolina State University and earned a Ph.D. in polymer chemistry under the guidance of Prof. Bruce M. Novak. After a postdoctoral appointment coadvised by Prof. Marc A. Hillmyer and Prof. Frank S. Bates at the University of Minnesota, he began his appointment as an Assistant Professor in the Department of Chemistry and Biochemistry at Florida State University in the Fall of 2014. His research interests revolve around a continued fascination with autonomous selfassembly of polymeric systems and leveraging organic chemistry to afford complex yet precise macromolecules containing dynamic functionality and varying degrees of stereochemistry. K


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Poly(vinylpyridine) Segments in Block Copolymers: Synthesis, Self

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