Metalloblock Copolymers - American Chemical Society

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Metalloblock Copolymers: New Functional Nanomaterials Jiawen Zhou, George R. Whittell, and Ian Manners* School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom ABSTRACT: Block copolymers containing metal centers and related segmented architectures are attracting attention as a class of materials with properties and functions that complement those of their all-organic analogues. These materials phase-separate into nanoscopic, metal-rich domains either in bulk or in thin films and form core−shell nanoparticles in selective solvents. Moreover, the linkages binding the metal centers can be varied from covalent bonds that lead to “static” bonding through to noncovalent, kinetically labile, or “dynamic” interactions, which affords remarkable flexibility in terms of materials design. In this Perspective we provide an overview of recent progress and future challenges in terms of applications in nanopatterning and sensing, and as stimuli-responsive and self-healing materials.

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INTRODUCTION The self-assembly of organic building blocks represents a promising alternative to “top-down” processing routes1 in the preparation nanoscale functional materials.2,3 The size of the components, however, may vary from small monodisperse molecules4 to larger more polydisperse macromolecules5 with concomitant changes in the factors that control the assembly process. In this regard the use of block copolymers has attracted growing interest, as these materials self-assemble in either solution,6,7 thin films,8,9 or the bulk9,10 to yield particles or domains of sizes appropriate for nanoscale applications.11 Furthermore, for coil−coil diblock copolymers, the thermodynamics that govern these processes are generally well understood,12 leading to the rational design of macromolecules for use in self-assembly. This approach has now afforded targeted materials with applications in solar energy conversion,13,14 photonics,15 nanolithography,16−19 and medicine.20−22 The function of a nanostructured material prepared by block copolymer self-assembly is dependent upon its size, morphology, and also the chemical composition of the blocks. With regards to the latter, the incorporation of metals into polymer structures has expanded the range of properties exhibited,23−30 and thus serves to complement the impressive accomplishments achieved with all-organic macromolecules. Research in this area began in 1955 with the synthesis of poly(vinylferrocene),31 which exhibits a side-chain location of the metal centers. Nevertheless, this material was not convincingly characterized until 1970,32 and it was not until 1977, with the report of platinum-containing polyynes, that a well-characterized, high molecular weight mainchain metallopolymer was realized.33 Since these pioneering breakthroughs, the range of polymer synthetic methods that are compatible with either the presence of metal centers or which enable the postpolymerization incorporation of metal ions has expanded rapidly. For instance, polycondensation,34−37 ringopening polymerization,38,39 electropolymerization,40,41 and living anionic42−44 as well as controlled radical polymerization45−52 have © XXXX American Chemical Society

all now been used to form metallopolymers with either side- or main-chain metal centers. The realization of controlled polymerization methods, which are tolerant of metal centers or ligands capable of metal binding, has enabled the formation of metal-containing block copolymers. The first examples were prepared in 1992 by the ring-opening metathesis polymerization (ROMP) of a ferrocenyl-substituted norbornene, which placed the iron centers in the side-chain structure (Figure 1a).53 Key to the success of this reaction was the judicious choice of a molybdenum−imido initiator that was compatible with the oxidation potential of ferrocene. Shortly thereafter, in 1994, the first metalloblock copolymer with the main-chain inclusion of metal centers was reported.43 This was achieved by the living, anionic ring-opening polymerization (ROP) of a strained silicon-bridged [1]ferrocenophanea reaction that has since been modified to enhance compatibility with various comonomers.55−58 In addition to anionic means, this class of cyclic monomer was amenable to transition metal catalyzed59,60 and photolytic ROP procedures.61,62 Controlled radical polymerizations have not only been used in the syntheses of metal-containing block copolymers but also to prepare copolymers capable of ligating metal centers. For instance, poly(styrene-block-4-(phenylethynyl)styrene) has been synthesized using 2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO) as the initiator and subsequently derivatized with Co2(CO)8 to yield the metallopolymer (Figure 1b).54 Pendent terpyridine groups have also been introduced into block copolymer structures by both nitroxide-mediated radical polymerization (NMP) and reversible addition−fragmentation chain transfer polymerization (RAFT),63 which can then be used to form metal complexes. Atom transfer radical polymerization (ATRP) was unsuccessful in this reaction due to competing complexation of Received: January 14, 2014 Revised: April 4, 2014

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dx.doi.org/10.1021/ma500106x | Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Block copolymers containing (a) Fe53 and (b) Co.54

Figure 2. Examples of “static” bonded (a) side-chain66 and (b) main-chain metalloblock copolymers67 and “dynamic” linked suprametallo (c) block copolymer68 and (d) star-shaped polymer.69

structural diversity when compared to carbon. This arises from the variations in coordination number and geometry for that which are observed for complexes of transition metals and which relate, in part, to the oxidation state and electronic configuration of the central element. These latter parameters also dictate the rate of ligand exchange at the metal center, leading to the “static” and “dynamic” extremes of bonding, and which have fundamental implications for the applicability of the material.65 The spectrum of structure and bonding evident in metallopolymers is clearly demonstrated by the examples in Figure 2.

the catalyst by the terpyridine groups. This method was successfully employed, however, for the synthesis of both linear and star diblock copolymers containing pendent bis(terpyridine)ruthenium(II) complexes.64 Key to success in this instance was the postpolymerization attachment of a preformed metal complex, thus excluding free terpyridine from the polymerization reaction. Besides the synthetic issues that are raised by the requirement to tolerate/incorporate metal centers in polymerization reactions/polymer structures, metals also offer enhanced B


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Figure 3. Examples of ligand moieties employed to obtain (a) side-chain70 and (b, c) main-chain supramolecular metalloblock or segmented copolymers.71,72

Figure 4. Synthesis of the Co-containing block copolymer employing Grubbs third-generation catalyst and standard ring-opening metathesis polymerization conditions. Polymerization of the first block is followed by block copolymer chain extension utilizing the Co-containing monomer.

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SYNTHETIC STRATEGIES An impressive diversity of metalloblock copolymer structures has been realized over the past two decades, which encompasses linear chains (Figure 2a−c) as well as dendrimeric structures (Figure 2d). Metalloblock copolymers may contain linkages either within the metal-containing segment or between the blocks, which range from the covalent to ionic extremes of bonding. This variation can impart “static” or “dynamic” character, respectively, to the interaction, which in the latter case may enable reversible ligand binding. These interactions are key to the properties of metallosupramolecular polymers (Figure 2c).

There are two possible approaches to metal-containing block copolymers. The first route involves polymerization of monomers in which the metal center is already present, while the second involves a postpolymerization metalation step. There is, however, an extension of the latter where the formation of metal−ligand bonds is used to establish the polymer backbone. The materials are classed as metallosupramolecular polymers, and a number of such systems have been prepared that are based on different ligands and metal−ligand interactions (Figure 3). Although these polymers do not possess a block architecture, they are segmented and therefore also prone to phase segregation (vide supra). C


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Figure 5. (a) TEM image of an unstained, microtomed cross section of a thermally annealed, Co-containing block copolymer (scale bar: 100 nm). Insets in (a) magnification of the cylindrical domains (both white and black scale bars in insets are 20 nm) with the highest magnification and thinnest microtomed section showing small metal nanoparticles inside the cylinders. Room temperature response of the thermally annealed block copolymer to (b) a permanent magnet along with (c) superconducting quantum interference device magnetometer measurements to quantify the magnetic properties. Reproduced with permission from ref 66. Copyright 2011 Nature Publishing Group.

Figure 6. Schematic representation of the preparation of a functionalized nanoporous PS thin film from a PS−[NiII]−PEO block copolymer. Reproduced with permission from ref 80.

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THIN FILM SELF-ASSEMBLY: APPLICATIONS OF NANOPATTERNING The ability to pattern on the nanoscale, with control over periodicity and long-range order, is of fundamental importance to the development of electronic components with reduced feature size. The commensurate dimensions of macromolecular chains with the target sizes, therefore, make the self-assembly of block copolymers an attractive approach to nanopatterning.12 Access to metalloblock copolymers structures that contain elements that may serves as, or precursors to, magnetically or electronically useful materials would therefore greatly simplify this approach. In addition, the presence of metal centers in one of the blocks can enhance etch contrast on exposure to plasmas, which enables efficient selective removal of the complementary block.73−75 Tew and co-workers have prepared a Co-containing block copolymer by the sequential ring-opening methathesis polymerization of two strained, cyclic olefins, one of which contained a pendent Co−carbonyl fragment (Figure 4).66 This material selfassembled in the bulk to afford Co-containing cylinders of ca. 20 nm in diameter separated by ca. 40 nm of organic matrix, which upon thermal treatment yielded ca. 5 nm Co nanoparticles confined within the parent cylindrical domain (Figure 5). This assembly was shown to be ferromagnetic at ambient temperature with saturation magnetization, remanent magnetization, and coercivity of 3.5 emu g−1, 0.61 emu g−1 and 200 Oe, respectively. Cobalt nanoparticles of similar size and prepared by thermal

treatment of the corresponding homopolymer, however, were demonstrated to be paramagnetic at room temperature. It was therefore concluded that confinement within the 1-D cylindrical structure caused increased magnetic coupling and resulted in the observed difference in magnetic behavior between the Co nanoparticles within the two different environments. Well-ordered nanoporous thin films can be generated through the selective removal of one of the blocks from a self-assembled block copolymer thin film. The exact method employed depends on the chemical composition of the block: plasma76 or UV etching77 are effective for poly(methyl methacrylate), hydrolysis for polyesters,78 and treatment with HF is suitable for the removal of polysiloxanes.79 Metallosupramolecular diblock copolymers are different in that the blocks are linked together in a metal−ligand complex, which can dissociate under the action of certain stimuli. An advantage of such an approach is that cleavage is dependent on the chemistry of the linking complex rather of the nature of the blocks. Recently, Fustin, Gohy, and coworkers prepared a nanoporous thin films from a PS−[NiII]− PEO (PS = polystyrene, PEO = poly(ethylene oxide)) metallosupramolecular block copolymer, where both organic blocks were end-functionalized with a terpyridine ligand (Figure 6).80 This nickel-containing system had several advantages over similar block copolymers containing bis(terpyridine)ruthenium complexes because selective removal of the PEO block from the latter was complicated by the high stability constant of the linker.65 While bis(terpyridine)nickel D


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Figure 7. (a) Height-mode atomic force microscopy image of an annealed PS-b-PFEMS (φPFEMS = 0.25) thin film and (b) a depiction of the thin film structure. (c) Phase-mode magnetic force and (d) scanning electron microscopy images of pyrolyzed, UV cross-linked PS-b-PFEMS (inset scale bars = 50 nm). (e) Proposed structure of the resulting nanostrcutures. Reproduced with permission from ref 87.

Figure 8. (a) Schematic illustration of the in situ potential-triggered, selective adsorption of ferritin molecules onto a self-assembled diblock copolymer thin film (MSE: mercury sulfate reference electrode). (b) Structure of PS-b-PFEMS diblock copolymer employed in this study. (c) Schematic of a ferritin molecule with the peptide shell (12 nm diameter) around the iron-containing core (8 nm diameter). Reproduced with permission from ref 88. Copyright 2012 Wiley.

blocks were decomplexed, and free PEO block was rinsed away by a selective solvent. Fluorescence spectroscopy after addition of a europium salt confirmed the presence of the PS-bound terpyridines on the pore walls. Block copolymers containing polyferrocenylsilane (PFS) represent a class of material for which self-assembly in thin films has been well studied.39,73−75,81 These systems microphase

complexes are thermodynamically stable, they are also kinetically labile, which enables substitution of a block upon application of an adequate stimulus. PS−[NiII]−PEO, synthesized via a twostep assembly procedure, self-assembled in a thin film forming PEO cylinders oriented perpendicular to the substrate. After cross-linking the PS matrix with UV light and exposing the film to an excess of cyanide, a strong competing ligand for Ni(II), the E


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Figure 9. Schematic diagram illustrating ferrocene-mediated catalytic oxidation of glucose and AFM images in tapping mode of electrodes coated with nanostructured PFS-b-PI electron mediators (drawn as a green layer in the scheme). Reproduced with permission from ref 89.

was found that by varying the block ratio of the copolymer and the composition of the casting solvent, the morphology of the latter could be controlled (Figure 9). Furthermore, these thinfilm and micellar structures could be made permanent, and thus stable in physiological environments, by cross-linking the organic block with OsO4. When the composite electrode was held at a negative potential, the observed current increased with the concentration of glucose, and the absolute value for current was largest for the bicontinuous PFDMS-b-PI morphology. Of particular relevance to practical applications, the system was demonstrated to be sensitive to changes in glucose concentration similar to that found in blood. The self-assembly of ABC triblock copolymers offers a wider range of geometries for nanolithography than the line and dot patterns available from the use of diblock structures. Manners, Ross, and co-workers have described the synthesis of polystyrene-block-poly(ferrocenylethylmethylsilane)-block-poly(2-vinylpyridine) (PS-b-PFEMS-b-P2VP), in which the block volume fractions were 16, 28, and 56%, respectively. This material self-assembled as a thin film to afford a core/shell cylindrical morphology containing a PS core with PFS shell, all within a P2VP matrix.90 After selectively removing the PS and P2VP blocks by oxygen reactive ion etching, hollow cylinders were obtained from which the ring pattern could be transferred to another polymeric layer by imprinting. This final step depicts a further approach for the use of block copolymer films in nanolithography. Square symmetric arrays of cylinders represent another morphology that cannot readily be obtained from the self-assembly of AB diblock copolymers, but which would be essential for compatibility with the rectangular Cartesian coordinate system used in integrated circuits. The triblock copolymer PI-b-PS-b-PFEMS with respective volume fractions of 25, 65, and 10%, however, was shown to afford this structure when self-assembled in the bulk.91 Nonetheless, to obtain squarepacked patterns with long-range order in thin films, it was found necessary to blend the terpolymer with PS homopolymers (Figure 10). The self-assembly of these blends in templates

separate into ordered arrays in which the domains derived from PFS are rich in both iron and silicon. The presence of these elements facilitates the use of these assemblies as nanolithographic masks due to the resistance that they provide to oxygen reactive ion etching.73,74,81−86 Anionic polymerization of styrene and subsequent addition of ethylmethylsila[1]ferrocenophane resulted in a polystyrene-block-poly(ferrocenylethylmethylsilane) (PS-b-PFEMS) block copolymers, which although differing in both molecular weight and PFEMS volume fraction, both afforded cylindrical morphologies in the bulk. 87 Thin films were obtained by spin-coating a toluene solution onto silicon and, after annealing, were found to comprise of PFS cylinders oriented perpendicular to the substrate. Cross-linking the PS matrix by treatment with UV radiation resulted in excellent retention of the parent morphology upon subsequent pyrolysis at 600 °C and the formation of a ca. 6 nm Fe nanoparticle at the surface of each cylinder (Figure 7a,b). The pyrolyzed film was studied by magnetic force microscopy (MFM), which indicated the presence of superparamagnetic, single domain structures that would be expected on the basis of particle size (Figure 7c−e). Schwarzacher and co-workers applied a similar PS-b-PFEMS (φPFEMS = 0.23) diblock copolymer to prepare a redox-active nanopatterned surface that can both direct and trigger ferritin adsorption.88 When cast as a thin film on gold, the PFEMS formed hexagonally packed cylindrical domains that lie perpendicular to the substrate, as was observed on silicon. These can then act as charge carrying channels between the substrate and the electrolyte, thus enabling ferritin adsorption to be initiated by the application of a positive bias (Figure 8). Through studies in glucose sensing, Park and co-workers have investigated the role of block copolymer morphology on the mediation of electron transport between an electrode and reaction center.89 The sensor comprised a porous carbon electrode, coated with a layer of glucose oxidase and then covered with a film of polyisoprene-b-poly(ferrocenyldimethylsilane) (PI-b-PFDMS). It F


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sizes (Figure 11).70 By changing the relative molecular weights of the two blocks, both spherical and wormlike micelles were obtained in heptanes and larger aggregates in a mixture of heptanes and THF. O’Reilly and co-workers have synthesized metallosupramolecular triblock terpolymers and obtained interesting hollow nanostructures by their self-assembly in solution.72 Employing controlled radical polymerization techniques, such as RAFT and NMP, amphiphilic diblock copolymers poly(tert-butyl acrylate)block-poly(methyl acrylate) (PtBA-b-PMA) were synthesized and then end-capped with a pincer ligand for Pd complexation. After removal of the butyl groups, the third block, a similarly endcapped PS, was attached to the now poly(acrylic acid)-blockpoly(methyl acrylate) (PAA-b-PMMA) through supramolecular complexation via the palladium center (Figure 12). Spherical micelles were formed by slowly adding deionized water to a polymer solution in DMF and then exhaustively dialysing against nanopure water. The resulting micelles had a narrow size distribution (Dh = 74 nm), as analyzed by DLS, and a PAA shell layer. The latter could be selectively cross-linked to around 25% with a diamine linker to ultimately yield shell cross-linked nanoparticles. In a subsequent step, the nanoparticle core could be removed by selective cleavage of the noncovalent bond at the PS−Pd−PMMA inner core−shell interface. The bilayer shell structure of the resulting nanocages was confirmed by AFM and TEM analysis, with uranyl acetate staining in the latter enabling the hydrophilic PAA shell (Figure 12d, dark rim) to be distinguished from the PMMA layer. These novel hollow phase-separated materials are being studied as nanoreactors for catalytic processes and for drug delivery applications. The groups of Schubert, Gohy, and co-workers have studied a range of amphiphilic AB block copolymers, where the blocks were linked together through supramolecular complexation.101,102 The bis(terpyridine)Ru(II) linked block copolymer PS20−[RuII]−PEO70 was found to self-assemble in solution to afford spherical micelles, which was similar to the behavior observed for covalently linked PS20-b-PEO70.103 Nonetheless, the metalloblock copolymer exhibited some interesting differences to the organic analogue, such as a change in micellar hydrodynamic size on the addition of salt. This effect was due to the

Figure 10. (a) Scheme of the bulk morphology of the triblock terpolymer showing PFS (red) and PI (light gray) parallel cylinders forming a checkerboard pattern in a matrix of PS. (b) SEM image of the thin film of the terpolymer blended with 17.9 wt % of PS homopolymer (Mn = 27 kg mol−1) on oxidized Si after spin-coating and annealing in chloroform vapor for 2.5 h at ambient temperature, followed by etching with oxygen RIE to remove both PI and PS blocks. Reproduced with permission from ref 91.

consisting of shallow grooves of different widths etched into Si was also studied. It was demonstrated that for square arrays the 90° or 45° orientation of the lattice vector of the PFS array with respect to the trench edge and the spacing between the trench edge and the PFS microdomains were controlled by the surface chemistry of the substrate. This work illustrates that control over geometry of the final self-assembled pattern can be not entirely be achieved by the design of the polymer but also requires templated substrates with chemically functionalized topography.

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FUNCTIONAL CORE−SHELL NANOPARTICLES VIA SOLUTION SELF-ASSEMBLY The self-assembly of amphiphilic block copolymers in solution is a powerful tool to obtain ordered structures in both the nano- and microdomains.92 In a selective solvent for one of the blocks, core−corona micelle structures with a wide range of morphologies were formed.93−95 The employment of metalloblock copolymers in this process is particularly attractive due to the additional electronic, magnetic, and optical properties that these materials may provide.96−100 Gates and co-workers recently applied the solution self-assembly of well-defined gold(I) complexes of isoprene−phosphaalkene block copolymers to access gold(I) nanostructures with tunable shapes and

Figure 11. Synthesis of an organic−phosphalkene block copolymer (PI-b-PMP) [PI = polyisoprene, PMP = poly(mesitylphosphaalkene), THT = tetrahydrothiophene] to generate gold(I) nanostructures. Reproduced with permission from ref 70. G


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Figure 12. (a) Synthetic pathway to a supramolecular triblock copolymer and (b-i) its solution self-assembly to obtain noncovalently connected micelles. (b-ii) Selective cross-linking of the PAA shell to afford noncovalently connected nanoparticles. (b-iii) Removal of the polystyrene core to generate a hollow PAA-b-PMA nanocage. (c) TEM image of nanocage obtained by drop-casting on a carbon grid and staining with uranyl acetate (image shown has been inverted for clarity). Reproduced with permission from ref 72. Copyright 2009 Royal Society of Chemistry.

the metal centers, leading to interesting optical, electronic, and magnetic properties.107−112 MacLachlan and co-workers described the synthesis and characterization of a novel comb-type metal−organic block ionomer and showed that it assembles into colloidally stable wormlike PB nanostructures.113 Their material was based on a poly(styrene)-block-poly(hydroxylethyl methacrylate) copolymer (PS270-b-PHEMA30), which was then functionalized with monoquaternized 4,4′-bipyridine. The resulting ionomer was complexed in THF solvent by slow addition of an aqueous solution of Na3[Fe(CN)5NH3] and 18-crown-6 (Figure 13a). Evaporation of the solvent then afforded two-dimensional arrays (Figure 13b), which were used as precursors to metal oxide nanostructures. Wang and co-workers reported a conceptually new method for the synthesis of PB nanoshells with tunable size by employing

Debye screening of the charged metal complexes, thus reducing the repulsive interactions between neighboring chains. The self-assembly of other supramolecular diblock copolymers containing nickel(II) or cobalt(III),104 and that of bis(terpyridine)containing metallotriblock terpolymers, such as A-b-B-[Ru]-C105 and A-[Ru]-B-[Ru]-A,106 has also been investigated. For instance, PS32-b-P2VP13-[Ru]-PEO70, which contains the pH-responsive P2VP block, formed spherical micelles with a PS core, a P2VP inner shell, and a PEO outer shell,105 and these structures underwent predictable size changes in response to variations in pH. Prussian blue (PB)a metal−organic coordination framework of Fe4[Fe(CN)6]3·nH2O constructed from Fe(II) and Fe(III) vertices bridged by cyanide ligandscan exhibit electronic delocalization and magnetic communication between H


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symmetrical copolymers. Moreover, it was demonstrated that the termini of the cylindrical micelles remain active to the addition of further PFS block copolymer after all of the initial chains have been incorporated.117 By analogy to living polymerizations of molecular monomers, which result in well-defined covalent polymers with active chain ends, living crystallization-driven selfassembly afforded structures with lengths that were proportional to the amount of block copolymer added. This technique proved to be a powerful tool to create a variety of monodisperse cylinders (Figure 15a), symmetric segmented block comicelles (Figure 15b),118 complex architectures (Figure 15c), and more recently noncentrosymmetric block comicelles (Figure 15d).119 Monodisperse cylindrical micelles have also been demonstrated to form lyotropic liquid crystal phases, the directors of which can be aligned by the application of an electric field.120

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STIMULI-RESPONSIVE MATERIALS

Polymeric materials that exhibit a response in the presence of external stimuli are currently an area of intense research, which is focused on the synthesis of new smart materials.121 One subclass comprises self-healing materials, which have the ability to repair themselves after being damaged. When present in combination with other properties, this phenomenon offers improved lifetimes and wider applicability.122 The dynamic noncovalent interactions that constitute metal−ligand interactions are ideally suited to self-healing materials and provide strong motivation to incorporate these linkages into polymeric structures. Self-healing materials based on segmented metallosupramolecular polymers copolymers were developed by Beyer, Rowan, Weder, and coworkers in 2011.71 The polymers were based on a macromonomer comprising a rubbery, amorphous poly(ethylene-cobutylene) core with 2,6-bis(1′-methylbenzimidazolyl)pyridine (Mebip) ligands at the termini (Figure 16). Combining equimolar amounts of Zn(NTf2)2 (Tf = triflate, CF3SO2−) and the polymer in solution (polymer·[Zn(NTf2)2]1.0) resulted in a rapid increase in viscosity, which was attributed to the formation of a supramolecular assembly. Employing the same procedure, supramolecular assemblies with Zn2+: polymer ratios of 0.9−0.7 were synthesized (polymer·[Zn(NTf2) 2]0.9, polymer·[Zn(NTf2)2]0.8, and polymer·[Zn(NTf2)2]0.7). TEM and SAXS analysis of the films prepared from the materials revealed microphase-separated lamellar morphologies, where the metal− ligand complexes form a “hard phase”, which physically crosslinks the poly(ethylene-co-butylene) “soft” domains. The optical absorption spectrum of the macromonomer has a band with a maximum at 313 nm, which is characteristic of the Mebip ligand. The absorption spectra of the supramolecular assemblies, however, show a weakening of this band and a new peak at 341 nm. Low-molecular-mass Zn2+−Mebip complexes fluoresce weakly, suggesting that a considerable portion of absorbed light is converted into heat. The proposed optical healing process was based on the presumption that this energy could be utilized to melt the hard phase and locally dissociate the supramolecular motif, resulting in a decrease in viscosity and liquefaction of the material. To study the optical healing process, 350−400 μm thick films of the materials polymer·[Zn(NTf2)2]0.7−1.0 were cut with a depth of ∼50−70% of the film thickness. These samples were subsequently exposed to ultraviolet radiation with a wavelength of 320−390 nm and an intensity of 950 mW cm−2. Figure 17c shows that under these conditions two consecutive exposures of 30 s were sufficient to heal the cuts completely.

Figure 13. (a) Structure of the metal−organic block ionomer (BI) containing [FeII(CN)5] and (b) TEM image of sample prepared by drop-casting a THF solution of the polymer with 4% (v/v) H2O onto a carbon-coated copper grid. Reproduced with permission from ref 113. Copyright 2011 Wiley.

miniemulsion periphery polymerization.114 Typically, miniemulsion droplets with a pentacyanoferrate periphery were obtained from a mixture of water, poly(ethylene glycol)-b-poly(propylene glycol)-b-poly(ethylene glycol) terminated with pentacyano (4-(dimethylamino)pyridine)ferrate, toluene, and hexadecane. After adding Fe3+, the solution turned to dark blue, indicating the formation of PB coordination polymers. PB nanoshells with a diameter of ca. 65 nm were obtained as a blue powder after filtration and drying. It was shown that the size of the nanoshells was tunable by varying the amount of toluene in the reaction mixture (Figure 14). Manners, Winnik, and co-workers have shown that diblock copolymers with a short crystalline core-forming poly(ferrocenyldimethylsilane) block yield cylindrical micelles in a selective solvent for the longer coblock.115 Well-defined polyisoprene-block-poly(ferrocenyldimethylsilane) (PI-bPFDMS) synthesized by sequential anionic polymerization was self-assembled in a selective solvent for the PI block yielding micelles with a crystalline PFDMS core and a PI corona.116 Depending on the degree of polymerization of the two blocks, different micellar morphologies were formed ranging from cylinders with PI rich materials to lamellar structures for more I


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Figure 14. (a) Copolymer structure and (b) SEM micrographs of the PB nanoshells. The inset in the SEM micrograph is at higher magnification and arrows denote the collapse of deformed nanoshells. Reproduced with permission from ref 114.

gelation properties of pyridine-end-functionalized poly(ethylene glycol)-poly(L-lactide) [PEG-(PLLA) 8-py] star block copolymers in the presence of transition metal ions (Figure 18).123 Oscillatory rheology measurements showed that the transition metal ions interact with the pyridine ligands, forming complexes with 5−10 times higher storage modulus than in NaCl solutions at similar concentrations. It was found that the nature of the metal and counterion also influenced the properties of the hydrogels. For instance, the presence of Mn(II) ions afforded an increase in the aggregate dimensions, a larger gel window, and an increase in the (thermal) stability due to the formation of additional cross-links arising from coordination of the pyridine ligands to the metal centers. Tew and co-workers have very recently expanded the range of macromolecular topologies that have been realized for metalcontaining copolymers.124 Utilizing ring-expansion metathesis polymerization (REMP), they prepared a high molecular weight, cyclic poly(norbornene)-based scaffold that could be quantitatively functionalized with terpyridine ligands. The reaction of these moieties with the (terpyridine)ruthenium end-groups of linear polymers then afforded novel metallosupramolecular cyclic brush systems (Figure 19). The authors subsequently extended this work with the formation of cyclic polymer-based gels, prepared by cross-linking the terpyridine-functionalized cycles with divalent metal ions, such as iron(II) and nickel(II). In the latter case, the reversible nature of the metallosupramolecular linker was utilized for the demonstration of stimuli-responsive behavior. Thus, the addition of excess terpyridine results in competition of the nickel(II) centers, breaking the cross-links and affording a gel-to-fluid transition. Gohy and co-workers designed a supramolecular network based on the hierarchical self-assembly of diblock copolymer micelles bearing terpyridine ligands at the periphery of the

Figure 15. TEM micrographs of (a) monodisperse micelles of PI550-bPFS50, (b) A−B−A triblock comicelles of XLM(PI1424-b-PFS63)-bM(PFS60-b-PDMS660)-b-XLM(PI1424-b-PFS63) in which the PI coronae have been cross-linked, (c) scarflike comicelles obtained by growing PI342-b-PFS57 cylinders off PI76-b-PFS76 crystals, and (d) A−B diblock comicelles of XLM(PI1424-b-PFS63)-b-M(PFS60-b-PDMS660) formed by selective dissolution and regrowth of the B-block in (b). Reproduced with permission from references 118, 119, and 120. Copyright 2009, 2010 Nature Publishing Group; Copyright 2012 AAAS.

Metallohydrogels are prepared by cross-linking hydrophilic macromonomers with metal ions. Recently, Feijen and co-workers reported the aqueous solution behavior and thermoreversible J


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Figure 16. (a) Proposed mechanism of the photohealable metallosupramolecular, phase-separated network. (b) Synthesis of the macromonomer and polymerization by addition of Zn(NTf2)2. DEAD = diethyl azodicarboxylate. Reproduced with permission from ref 71. Copyright 2011 Nature Publishing Group.

corona.125 The addition of metal ions to a solution of these micelles then induced the formation of a network based on intermicellar bis(terpyridine)−metal complexes, resulting in a network (Figure 20). Initially, poly(styrene)-block-poly(tertbutyl acrylate) (PS80-b-PtBA200) end-functionalized with a terpyridine at the PtBA block was self-assembled into micelles with a PS core and PtBA corona (Figure 19). Three metal ions, Zn2+, Fe2+, and Ni2+ were then selected to trigger the second level of assembly. As the stability constants of complexes formed with terpyridine are different for each metal ion, the characteristic properties of the networks could be tuned by the choice of the metal. The nonlinear viscoelastic response of the gels was investigated as well as the stimuli responsive character by the addition of strong competing ligands. On exposing the iron and nickel based gels to an excess of KCN, the micellar network underwent a gel-to-sol transition. Mechanical forces could also be employed to break the micellar network, as it was shown that at high deformation, during the rheology experiment, the material showed liquid-like characteristics and at low deformation the network was recovered. In a subsequent study, it was reported that the properties of the micellar gel depend on the micellar shape (spherical or cylindrical), solvent, and temperature.126 The stimuli-responsive self-assembly behavior of redox-active PFS-containing block copolymers has been explored.96 Solutions

of polystyrene-block-poly(ferrocenylmethylphenylsilane) (PS-bPFMPS) block copolymers in dichloromethane, a common solvent for both blocks, yielded well-defined spherical micelles after oxidation with hexahaloantimonate salts of the tris(4bromophenyl)ammoniumyl cation. These species comprised a fully solvated PFS core, and after reduction with decamethylcobaltocene, micelle disassembly to unimers was observed. It was shown that the process had excellent reversibility when the innocent [SbF6]− counterion was used. Employing a PFDMS block afforded a change in the morphology of the oxidized micelles from spheres to ribbons. This was due to the semicrystalline nature of the oxidized PFS core promoting the formation of micelles with lower interfacial curvature. Vancso and co-workers synthesized a dual-responsive hydrogel composed of poly(N-isopropylacrylamide)-block-polyferrocenylsilane, which was both thermo- and redox-responsive. Additionally, they showed that reduction of silver nitrate by PFS chains led to hydrogel−silver nanoparticles composites, which had strong antimicrobial activity while maintaining a biocompatibility with cells.127

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SUMMARY AND FUTURE OUTLOOK As illustrated in this Perspective, the presence of metal centers in a block copolymer can introduce a wide range of useful properties to this fascinating class of soft materials. Although this paper K


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Figure 17. Characterization of Zn-based metallosupramolecular polymers. (a) SAXS data for the polymers and varying amounts of Zn(NTf)2, shifted vertically for clarity. (b) A representative TEM image depicting the lamellar morphology of polymer·[Zn(NTf2)2]1.0. (c) Demonstrating the optical healing of polymer·[Zn(NTf2)2]0.7 on exposure to light in the wavelength range 320−390 nm for 30 s twice at an intensity of 950 mW cm−2. (d) Surface temperature of polymer·[Zn(NTf2)2]0.7 on irradiation under the same conditions, as a function of time. (e) Optical healing of polymer·[Zn(NTf2)2]0.7 while under a load of ca. 8 kPa (width, 21 mm; thickness, 0.31 mm; mass, 5.25 g). Reproduced with permission from ref 71. Copyright 2011 Nature Publishing Group.

mainly focuses on application-oriented research on metalloblock copolymers and metal-containing segmented structures, there exist many recent significant examples of synthetic breakthroughs and detailed fundamental studies that may underpin important future advances. Disadvantages of these materials are their significant complexity in terms of synthesis and their relatively high cost compared to commodity polymers. However, for the vast majority of emerging applications, only a very small amount of material is required, making such issues much less significant. An important goal for the future is to take the current knowhow through to real-life applications; therefore, simplifying the synthetic approaches, for example by using “click” methods to tether blocks together, will likely be of key importance. Involving metals as an integral part of polymer structures offers many new opportunities for both fundamental and applied research. Block copolymers containing a metal centers represent some of the most intriguing examples of this broad class of materials, and we hope that this Perspective will help inspire future efforts in the polymeric materials community.

Figure 18. Illustration of a PEG-(PLLA)-py star block copolymer hydrogel in the presence of metal ions. The figure depicts hydrophobic aggregates as well as metal−ligand coordination complexes, which may be part of a micellar structure. Reproduced with permission from ref 123. Copyright 2012 Wiley. L


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Figure 19. (a) Structure and (b) pictorial representation of a metallosupramolecular cyclic brush polymer (polymer = poly(ethylene glycol) or polystyrene). Reproduced with permission from ref 124.

Figure 20. Hierarchical self-assembly of a PS-b-PtBA-terpyridine copolymer leading to the formation of a supramolecular network of micelles. Reproduced from ref 125 with permission. Copyright 2009 Royal Society of Chemistry.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I.M.). Notes

The authors declare no competing financial interest. Biographies

George R. Whittell was born in Ipswich, England, in 1975. He completed both his B.Sc. and Ph.D. degrees at the University of Bristol, the latter working with N. C. Norman in the area of transition metal−boryl chemistry. In 2000, he took up a Royal Society Postdoctoral Fellowship with W. R. Roper, F. R. S., working on the synthesis and reactivity of osmium−stannyl complexes. After a subsequent position with H. Braunschweig, synthesizing transition metal−borylene compounds, he joined the research group of Ian Manners as a senior research associate. Jiawen Zhou was born in Zhejiang, China, and then moved to Germany as a child and grew up in the western part of Germany. She studied chemistry at the Heinrich-Heine-Universität Düsseldorf where she obtained her diploma degree. Her doctoral work was supervised by Prof. Helmut Ritter and focused on modified lactones for ring-opening polymerizations, synthesis of polyester diols, and materials with shapememory effects. In 2010, she moved to Karlsruhe for a postdoctoral stay in Prof. Christopher Barner-Kowollik’s group at Karlsruher Institut für Technologie and worked on reversibly cross-linked networks via RAFT-HDA chemistry. In the summer of 2011, she moved to Bristol and began a postdoctoral research in Prof. Manners’ group. Her current research interests include synthesis and self-assembly of organometallic block copolymers. In her spare time she enjoys sports, wandering the

Ian Manners was born in London, England, in 1961. After receiving his Ph.D. from the University of Bristol in 1985 in the area of transition-metal

locality, and reading books. M


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chemistry, he conducted postdoctoral work in Germany in main-group chemistry and in the USA on polymeric materials. He joined the University of Toronto, Canada, in 1990, and after 15 years returned to his Alma Mater to take up a Chair in Inorganic, Macromolecular and Materials Chemistry. His research interests focus on the development of new synthetic reactions and self-assembly protocols and their applications in molecular synthesis, polymer and materials science, and nanoscience.

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ACKNOWLEDGMENTS J.Z. acknowledges the Deutscher Akademischer Austauschdienst (DAAD) for a postdoctoral fellowship. I.M. thanks the Engineering and Physical Sciences Research Council (EPSRC) for funding this research.

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