Transition metal ions: weak links for strong polymers - CiteSeerX

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Transition metal ions: weak links for strong polymers Dirk G. Kurth*ab and Masayoshi Higuchib Received 26th May 2006, Accepted 16th August 2006 First published as an Advance Article on the web 31st August 2006 DOI: 10.1039/b607485e Extending macromolecular chemistry beyond carbon-based polymers offers fascinating perspectives and an enormous potential to improve the capacity of macromolecular materials with new dynamic properties.

Introduction The discovery of transition metal catalysis in polymerization reactions more than 50 years ago has lead to modern organometallic chemistry and has revolutionized the world of catalysis and polymers.1 Since then catalyst improvement and functional group tolerant catalysts paved the way for new applications and commercialization of polymers. Due to the commercial success of polymers macromolecular chemistry is a highly developed field. The creation of macromolecular structures utilizing inorganic elements adds a new dimension to the field of polymer materials. Coordination numbers and geometries become additional variables and metal ions give access to redox, magnetic, optical or reactive properties that are not accessible through carbon based polymers.2,3 The potential for these

a

Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany. E-mail: [email protected]; Fax: ++49(0)331-567 9202; Tel: ++49(0)331-567 9211 b National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Dr Dirk G. Kurth is a Project Leader at the Max Planck Institute of Colloids and Interfaces (Germany) and a Director at the National Institute for Materials Science (Japan). He studied chemistry at the University of Cologne and the RWTH Aachen before he moved to the USA to work with T. Bein. Following his PhD, which he received from Purdue University in Indiana, he moved to Strasburg to work with J.-M. Lehn. In 1996 he Dirk G. Kurth moved back to Germany to join the Interface Department of H. Mo¨hwald at the Max Planck Institute of Colloids and Interfaces. Since 2003 he is also a lecturer at the University of Potsdam. His research interests include the structure–property relationships of supramolecular materials with hierarchical architecture including nano-structures, thin films, clusters and mesophases.

This journal is ß The Royal Society of Chemistry 2006

materials can be envisioned if we consider the impact of inorganic solid-state materials on technology. The importance of metal ions in biological systems as cofactors in enzymes and metalloproteins for oxygen transport in respiration or for electron-transfer in photosynthesis may give a hint at the potential and the impact on technology that inorganic–organic composite materials may develop. Over 40 elements including main group metal elements (Si, Ge), transition metals or rare earth elements in addition to the 10 elements found in organic polymers (C, H, N, O, B, P, halides) are available for organometallic polymers. Therefore, the variations of organometallic polymers seem endless. Several types of organometallic polymers can be distinguished based on how the metal is integrated in the polymer as outlined by Rehahn (Fig. 1).4 Metals can be pendent to or they can be present in the polymer backbone. In coordination polymers the coordinative bond between the metal and the ligand is an integral part of the backbone. The polymer backbone can act as polymeric chelate, which embeds the metal ion; breaking the coordinative bond does not affect the polymer backbone. Finally, metals can be part of a network structure with a continuous 3-D topology.

Dr Masayoshi Higuchi is a Deputy Head of the Functional Modules group at the National Institute for Materials Science (NIMS) and a Guest Associate Professor at Keio University. He was born in 1969 in Niigata, Japan. He received his degree of Dr Eng. from Osaka University in 1998. He was appointed as a Research Associate of Keio University in 1998, and promoted to an Assistant Professor in 2003. He moved to NIMS in 2004. Masayoshi Higuchi He was also a research fellow of the Japan Society for the Promotion of Science for Young Scientists (JSPS, 1995–1998), a project researcher at the Kanagawa Academy of Science and Technology (KAST, 2001– 2004) and Core Research for Evolutional Science and Technology (CREST, since 2003). He received the Chemical Society of Japan Award for Young Chemists in 2002. His current research is focused on organic–metallic hybrid nanomaterials using novel topological p-conjugated supramolecules. Soft Matter, 2006, 2, 915–927 | 915

Fig. 1 Metals can be pendant to (left) or they can be present (center) in the polymer backbone or the metal is embedded in a polymeric chelate (right). In the coordination polymer (center) the coordinative bond is an integral part of the polymer backbone.

In this review we will focus on soluble coordination or metallo-supramolecular polymers, as they are referred to, that emerge spontaneously through metal ion induced selfassembly of ligands, that is, the metal–ligand interaction can be considered kinetically labile at room temperature. So far, we can distinguish two different basic strategies: conventional (polydisperse) oligomers or polymers are functionalized with suitable metal ion receptors, e.g. 2,29:69,20 terpyridines, and are assembled through metal ions.5–8 The good solubility of these polymers makes them attractive for research and technology alike, but by design the concentration of metal complexes is low. Alternatively, molecular polytopic ligands can be assembled to various polymeric architectures with a high concentration of metal complexes, and depending on the ligand design, a high charge density. Here, we will take a closer look at the fundamental aspects of the latter systems.

Metallo-supramolecular polymers Coordination polymers are assembled directly from metal ions and ligands. While polymers based on kinetically inert transition-metal complexes are readily characterized in solution by standard analytical means, polymers formed by kinetically labile transition-metal complexes have until very recently successfully evaded detailed characterization in solution. The overwhelming majority of the resulting networks or metal–organic frameworks (MOFs) are isolated and characterized as crystalline solids.9,10 Here, the binding constants are generally so small, that in solution no polymeric assemblies are formed. The coordination network exists only in the solid state. The diversity of the resulting framework architectures is remarkable and has been extensively discussed in previous reviews.11–13 Due to the large apparent surface area, perhaps the most promising application of MOFs is the specific uptake of gases through the control of functional group chemistry and the windows, pores and channels of the architecture.14 The genuine technological needs in fuel gases such as methane and hydrogen foster research towards this direction.15,16 The ability to characterize these crystalline materials to a high degree by crystallography provides a good perspective to establish accurate structure–property relationships. The above-mentioned examples underline the power of using metal–ligand interactions to construct metal–organic materials in terms of possible structures, properties, and applications. Recent developments indicate another new trend 916 | Soft Matter, 2006, 2, 915–927

in which kinetically labile metal–ligand interactions are employed to form macromolecular assemblies in solution. Kinetically labile interactions offer the ability to construct materials with unprecedented properties: materials built up through weak interactions can assemble, disassemble and reconstruct in a dynamic fashion under ambient conditions, thus establishing optimization routines! Since the interactions are often on the order of kT and thus compare to entropic forces, such materials can be adaptive and responsive. Moreover, different interactions, like hydrogen bonding, electrostatics or coordinative bonds, can compete within a complex molecular architecture so that structure and property are dynamic, that is, they depend on external parameters, such as temperature, pH, solvent, ionic strength or external fields, and in addition such materials have the ability to self-repair, self-anneal and self-correct under ambient conditions (on account of weak interactions). In the seminal work on metallohelicates Lehn et al. have established the proof of principle.17 Thermodynamics The rapid growth of metallo-supramolecular chemistry, which aims at preparing sophisticated polymetallic architectures based on various intercomponent interactions, has considerably transformed the field of inorganic coordination chemistry. The resulting complicated and aesthetically appealing assemblies provided a driving force to rationalize their formation, culminating in ‘‘the principle of maximum site occupancy’’.18,19 Here we discuss polymers that are formed by the interaction of metal ions and ditopic ligands.20 The ligand or monomer consists of two identical metal ion receptors linked through an arbitrary spacer (Fig. 2). We will consider only the case of extended linear macromolecular assemblies, e.g. as they would occur in case of a rigid ditopic ligand. Discrete ring type structures with small molecular weight are not expected to exhibit interesting non-linear polymer properties. We assume that two species can exist in solution. A complex ML in which a metal ion is bound to one receptor, and a ML2 complex in which a metal ion is coordinated to two receptors. The ML2 complex constitutes the link in the polymer backbone. The end groups are either free receptor sites or ML complexes. In a first approximation, we assume that the metal ions bind independently of each other and that the equilibrium

Fig. 2 Metal induced self-assembly of ditopic ligands results in macromolecular assemblies. In solution these polymers form dynamic equilibrium structures due to the weak interactions of ligand and metal ion. The ditopic ligand consists of two metal ion receptors linked by a spacer. Here only linear, extended structures are considered, e.g. through rigid ligands. In a first approximation the equilibrium structure is determined by the complex formation constants.


concentrations of all species can be described by the stability constants of the two types of complexes: K1 ~ K2 ~

½ML ½M½L

½ML2 ½ML½L

where [L], [M], [ML] and [ML2] are the concentrations of free receptors, free metal ions, and metal ions coordinated by one and two receptors, respectively. Following the law of mass conversation the concentrations of the various species are given by:

Fig. 4 Average number of monomers in an assembly ,n. as a function of stoichiometry, y, for different monomer concentrations (a: 1023 mol L21, b: 1024 mol L21, c: 1025 mol L21).

2[Mon] = [L] + [ML] + 2[ML2], [L] = 2[Mon](1 2 p), [ML] = 2[Mon]p(1 2 q), [ML2] = [Mon]pq, and [M] = y[Mon], where p is the fraction of receptors involved in ML complexes and q is the fraction of receptors involved in ML2 complexes, respectively. The concentration of the ditopic ligand is given by [Mon] and the ratio of the concentrations of metal ion to monomer is given by the coefficient y. These six equations completely describe the equilibrium concentration of each species. The following graph depicts the dependence of the average number of monomers in an assembly ,n. as a function of concentration for different values of y (Fig. 3). For this example the stability constants of the Fe(II) terpyridine complexes are used (log [K1] = 7, log [K2] = 14).21 We note that the molecular weight depends on the concentration and the stoichiometry of the constituents. In case of an exact stoichiometry of metal ion to ligand we observe an exponential polymer growth with increasing concentration. It can be seen that terpyridine ligands and Fe(II) form assemblies with very high molecular weight. In case of a different stoichiometry (y ? 1) polymer growth stops if the deficient component is consumed, which corresponds to the plateau region at higher concentrations.

Fig. 3 Average number of monomers (ditopic ligands) per assembly ,n. as a function of concentration for different stoichiometries. Here, the stability constants of Fe(II) and terpyridine are used (log [K1] ,, log [K2]).


The dependence of the average number of monomers in an assembly as a function of stoichiometry, y, for different concentrations is shown in Fig. 4. The average assembly length depends strongly on the stoichiometry. We note that the curves are asymmetric with respect to y = 1. An excess of metal ions results in longer assemblies than in case of a monomer excess. Experimentally it is generally difficult to achieve an exact 1 : 1 stoichiometry. It is, therefore, advised to use a slight excess of metal ions in the synthesis. For the synthesis of metallo-supramolecular coordination polymers the ditopic terpyridine–Fe(II) system is, therefore, interesting because it forms large macromolecular assemblies. Also, as a first row transition element Fe(II) is kinetically labile so the assembly process can be carried out at room temperature in water. Equally interesting with terpyridine ligands are the other first row transition elements, such as Co(II) and Ni(II) due to their high stability constants and even faster ligand exchange kinetics.22 Several important conclusions can be drawn from this analysis. Metallo-supramolecular coordination polymers can form very high molar masses if the metal ion, the ligand, and the concentration are chosen appropriately. For the present case it is advantageous if the second binding constant is larger than the first because the second reaction constitutes the growth step. In case of rigid structures we can anticipate additional effects that will affect the assembly process, such as accumulation of charge or formation of lyotropic phases.23 In case of charged polymers additional effects may arise through the polyelectrolyte effect such as ionic strength dependent properties. It is evident that the length depends critically on concentration, which adds a challenge in characterization. It is not possible to change the concentration in order to optimize signal intensity without affecting the structure. Also, any method that interferes with the equilibrium, such as chromatographic methods or viscosity commonly used in polymer analysis, cannot be applied to these systems. If an excess of one component is used, which is preferentially the metal ion, the length of the assembly becomes independent of concentration for higher concentration. This effect can be useful in analytical problems, for instance to maximize signal intensity, e.g. in scattering experiments, by raising the concentration without affecting the length distribution. A constant length is also feasible by introducing a second monotopic ligand, which acts as a capping unit. Soft Matter, 2006, 2, 915–927 | 917

We can anticipate these materials to respond to shear because the interactions are weak and labile. Rupturing the metal ion ligand interactions will produce smaller polymer segments, which is associated with a reduction in viscosity (vide infra). The use of metal ions and ligands provides additional variables to tailor the properties of the resulting assemblies. The ligands are generally Lewis bases and, therefore, protons and metal ions compete for the ligands so the equilibrium is affected by the pH of the solution. The pH can be used to shift the equilibrium, e.g. to move the length into a specific range independently of ligand and metal ion concentration, or the polymer can be considered to be pH responsive. The same is true for coordinating solvent molecules. The simultaneous equilibria of protons, metal ions and ligands are temperature dependent as well.24 In summary, there are plenty of variables that affect the architecture and the polymer properties of these systems, which add to the richness of these systems. Systematic studies on how molecular weight correlates with polymer-like properties (enhanced viscosity, glassy solid state etc.) for such systems have yet to be established.

Examples Perhaps the first example of well-defined coordination polymers based on kinetically labile complexes goes back to the work of Rehahn and coworkers. Using phenanthroline-based bis-dendate ligands, this group prepared a variety of polymers with Ag(I) and Cu(I) (Fig. 5).25 Due to the ligand design the formation of discrete species in favor of polymers is largely excluded. Rehahn and coworkers assumed that the coordination polymer decomposed exclusively via displacement of metal ions by coordinating solvent molecules. To avoid decomposition, the polymer must be soluble in (innocent) non-polar solvents. In a suitable solvent, ligand-exchange should be reduced and hence the polymer can be characterized. The use of metal ions renders the polymers generally highly charged calling for polar solvents. Attachment of alkyl chains to the ligand was assumed to be a means to tailor solubility. The authors demonstrate that in non-polar solvents like 1,1,2,2tetrachloroethane (TCE), the resulting polymers behave like

well-defined macromolecules. In the presence of coordinating solvents, like acetonitrile or pyridine, the species behave more like low molecular weight aggregates. NMR titrations indicate a degree of polymerization of 6–8 if 0.8 equivalents of Cu(I) are used. At a 1 : 1 stoichiometry, the degree of polymerization increases to above 20. In contrast to Cu(I), the Ag(I) polymer shows a fast ligand exchange in a TCE–CH3CN mixture (4 : 1). In the absence of coordinating species there is practically no ligand exchange as documented by mixing Cu(I) and Ag(I) polymers. For large-scale synthesis capillary viscosimetry is used to adjust the 1 : 1 stoichiometry in order to obtain high molar mass polymers. Analysis of the intrinsic viscosity yield an apparent intrinsic viscosity of [g] # 30 mL g21 for a high molar mass polymer based on Cu(I) in 0.01 M NH4PF6– acetone. The salt is necessary to suppress polyelectrolyte effects. A similar value is found for a stable, random coil Ru(II) coordination polymer of molar mass Mn # 40 000 g mol21.26 However, the Huggins constant (kH # 9) of the kinetically labile polymer is very large, which is a result of the equilibration. A lowering of the concentration causes depolymerization of the coordination polymer thus enhancing the reduction of the viscosity. Time dependent measurements support this hypothesis. Addition of solvent causes reequilibration of the system, manifested in a time dependent decay of the viscosity to a final value. The authors point out that even a small amount of coordinating species in solution, e.g. water, can cause depolymerization. The concept of using polytopic ligands based on terpyridines for the construction of metallo-supramolecular polymers was introduced by Constable and Thompson (Fig. 6).27 This study focused on the synthesis of kinetically inert Ru(II) complexes that were susceptible to characterization. Since then 2,29:69,20terpyridine has attracted a great interest for the construction of metallo-supramolecular polymers. This ligand binds many transition metal ions, generally possesses high binding constants and forms stereochemically well-defined complexes. With most transition metal ions terpyridine forms octahedral complexes, so it is ideally suited to build up linear supramolecular polymers through substitution in the 4-position of the central pyridine ring. The high binding constants also allow studying metal ion induced self-assembly in aqueous media, which is most attractive for putting such systems into

Fig. 5 Kinetically labile coordination polymers based on the di-topic phenanthroline ligand and Cu(I) and Ag(I) ions.

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Fig. 6 Structures of different polytopic ligands introduced by Constable and Thompson for the construction of metallo-supramolecular coordination polymers.27,28

practical use. In addition, terpyridine transition metal complexes possess a variety of attractive properties that make them interesting in photophysical, electrochemical, and magnetic studies. Due to the high charge density the solubility of these so-called metallo-supramolecular polyelectrolytes (MEPEs) in aqueous solutions critically depends on the counter ions, in particular since most ligands studied so far are not very water-soluble. The first water soluble MEPEs based on 1,4bis(2,29:69,20-terpyridin-4-yl)benzene and Fe(II)OAc2 were reported more than five years after the work of Constable and Thompson and opened an avenue to study and employ these systems in many ways.29 Transition metal acetates are often used now in the synthesis of MEPEs. While the rheological and polyelectrolyte properties of MEPEs await further studies the solid-state structure was recently solved using electron diffraction.30 As pointed out above it is generally not possible to grow crystals of such equilibrium systems in particular under supersaturation, commonly employed for crystal growth. At such high concentrations the assemblies become very large and form amorphous precipitates. However, it is possible to grow nanoscopic crystals of MEPEs directly on a surface at dilute conditions. In the absence of strong polymer–surface interactions, long chains experience a larger entropy loss than shorter ones, so a surface region is predominantly occupied by short chains. The formation of extended assemblies is further reduced by This journal is ß The Royal Society of Chemistry 2006

working under dilute conditions. This approach may be of general utility in order to characterize the first, nanocrystalline, particulates in coordination polymers and frameworks to address fundamental issues on how such systems grow. Due to the low concentration only nanoscopic crystals are obtained which are not suitable for X-ray diffraction. The analysis by electron diffraction reveals a primitive monoclinic unit cell, in which the MEPE forms linear rods, which are organized into sheets (Fig. 7). Four sheets intersect the unit cell, while adjacent sheets are rotated by 90u with respect to each other. Mo¨ssbauer spectroscopy of bulk samples confirms the pseudooctahedral coordination geometry and indicates an average length of approximately 8 repeat units in the solid state. The positive charge can be used to deposit the MEPEs on oppositely charged surfaces. Alternating deposition of MEPEs and suitable polyelectrotytes, such as poly(styrene-sulfonate) gives rise to well-defined layered films on flat substrates as well as colloidal templates (Fig. 8).31,32 An alternative route consists of immobilizing discrete metallo-units. Pyrenyl tails attached to the terpyridine moiety cause assembly of the discrete units into linear arrays in particular in water where p–p interactions become strongest. The formation of p-stacking is evident from the crystal structure and from the fluorescence properties of the thin films.33 Applications of such systems currently include electrochromic devices.34–36 Coordination polymers based on Schiff-base rare earth complexes were reported in 1994 by Archer et al. (Fig. 9).37 In polar solvents such as DMSO or NMP molar masses of 30 000 were achieved. These polymers exhibit good stability and high glass transition temperatures. The concept was extended to other well-characterized rare earth coordination polymers.38,39 The incorporation of rare earth metal ions makes these systems interesting for novel photophysical studies and applications. Embedding metal ions in liquid crystal polymers is a route towards mesophases that combine the properties of metal ions and mesophases that are easily oriented. For instance, Serrano et al. explored this concept to synthesize nematic polyesters based on metallomesogenic metal moieties (Fig. 10).40,41 This polymer displays an enantiotropic nematic mesophase. Cooling the melt results in a nematic glass. Macroscopic ordering is induced by drawing fibers from the nematic melt. X-Ray scattering indicates an orientation of the mesogenic cores along the stretching direction, which demonstrates that such polymers are readily processable. The Cu(I) centers make these polymers paramagnetic and a weak exchange interaction of antiferromagnetic character between the copper centers was detected. While many other such systems have been reported in the literature,43 strong coupling between metal centers and therefore cooperative phenomena are generally not observed because the distance between metal centers is too large.44 Another route towards mesophases of metallosupramolecular polyelectrolytes is based on the exchange of the acetate counter ions by suitably charged amphiphilic molecules (Fig. 11). Amphiphilic self-assembly of MEPE and negatively charged surfactants, such as dehexadecyl phosphate (DHP), affords the corresponding polyelectrolyte–amphiphile complex (PAC).45 The PAC, which readily dissolves in organic solvents, spreads at the air–water interface and the resulting Langmuir Soft Matter, 2006, 2, 915–927 | 919

Fig. 7 Structure of MEPE based on FeOAc2 and 1,4-bis(2,29:69,20-terpyridin-4-yl)benzene. The MEPE forms linear rods that are organized into sheets. The unit cell consists of four sheets, while each sheet is rotated by 90u with respect to each other. The coordination geometry is pseudooctahedral.30

monolayer can be transferred on solid supports by means of the Langmuir–Blodgett (LB) technique.46 Co-adsorbing PACs and long chain alkanes on graphite affords perfectly straight nanostructures (Fig. 12).47 This multi-component self-assembly process consists of several steps. First, the alkanes adsorb in an epitaxial way on the

Fig. 8 Electrostatic layer-by-layer self-assembly of MEPEs affords well-defined multilayers. The method can be applied to any type of surface, provides excellent thickness control, is readily automated and a large number of different components can be assembled into the layers.

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lattice of the basal plane of graphite, a long-known phenomenon, giving rise to a lamellar monolayer. The alkane wets the graphite preferentially because it is used in excess and because of the commensurability of the alkane and graphite lattice. This monolayer represents a corrugated surface potential and, therefore, acts as a template for the adsorption of the PACs. The entire process evolves through multi-step self-assembly with interactions at several levels and length scales including metal ion coordination, electrostatics, and van der Waals forces. The combination of rigid-rod polymers and flexible surfactants gives rise to thermotropic polymorphism. The structure of the room temperature phase of the PAC based on DHP, 1,4bis(2,29:69,20-terpyridin-4-yl)benzene, and Fe(II) is shown in Fig. 13.48 A combination of X-ray scattering and molecular modeling was used to reveal details of the architecture. Notably, DHP forms an interdigitated layer in this structure in contrast to the solid-state structure of DHP and the typical packing motives encountered in amphiphilic architectures.49 The PAC structure is a nice example of a multi-component hierarchical architecture: At the molecular level the structure is determined by the design of the ligands and the metal coordination algorithm. At the mesoscopic length scale structure arises through the interaction of the MEPE rods and the amphiphilic molecules. And finally at the macroscopic level, structure arises through the packing of the PAC rods into the final architecture. The phase transition in the amphiphilic mesophase is explored to deliberately induce mechanical strain in an This journal is ß The Royal Society of Chemistry 2006

Fig. 9 Example of a linear cerium(IV) Schiff-base coordination polymer.37

Fig. 10 Coordination polymers as part of a mesophase described by Serrano et al.42

assembly of tightly coupled metal ions to coordination centers. Melting of the alkyl chains in the amphiphilic mesophase induces mechanical strain thus in turn distorting the coordination geometry around the central metal ions. As a result, the crystal field splitting of the d-orbital subsets decreases resulting in a spin transition from a low-spin to a high-spin state (Fig. 14). The diamagnetic–paramagnetic transition is reversible. Liquid crystalline materials are readily processed into various device architectures, and the concept can be expanded to virtually all metallo-supramolecule polymers with suitable electronic configurations.51 A rare system showing ferromagnetic exchange interactions was reported by Haase et al.52 Here, the Schiff-base mesogenic units containing Ni(II) metal ions are located in the side chain, however, the units form a ladder-like structure (Fig. 15). Presumably, the metal centers and the counter ions form a linear chain so that the metal centers come in close contact

Fig. 11 Exchange of the counter ions with amphiphilic molecules results in polyelectrolyte–amphiphile complexes (PACs). The amphiphiles render the MEPEs soluble in organic solvents.45

Fig. 12 Sequential multi-component, multi-step self assembly of long chain alkanes and PACs results in perfectly straight rods on the basal plane of graphite. The alkanes wet the graphite surface preferentially forming a monolayer that acts as a template for the adsorption of PACs. Like this, the straight structure that is preprogrammed in the components is amplified from molecular to mesoscopic length scales.47 Left: AFM images of spin-coated PACs on graphite. Right: schematic structure of graphite lattice (grey), alkane monolayers (red) and PAC (not to scale, simplified).


Fig. 13 The room temperature structure of a PAC. Notably the amphiphiles (dihexadecyl phosphate) form an interdigitated layer. The interstitial space is occupied by MEPE rods. The structure was obtained from X-ray scattering and molecular modeling.50

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Fig. 14 Melting of DHP in a PAC results in a distortion of the coordination geometry (top), giving rise to a reversible spin transition from a low- to a high-spin state (bottom).51

enabling cooperativity. In that respect, we can also consider this material a polymer with labile metal ligand interactions. However, the material does not show liquid-crystalline properties presumably due to the crosslinking of polymers through the metallomesogenic units. In fact, the metallopolymers are not soluble; during the synthesis the metallopolymer forms a colloidal solution and is precipitated as a solid. The ferromagnetic interaction can be described by either ferromagnetically coupled dimers or linear chains. So far, no detailed structural data are available for this material due to the insoluble nature. The presence of cooperative magnetic properties like remnant magnetization and coercivity indicate a strong superparamagnetism. Introducing Co(II) into these methacrylate polymers results in mesophases leading to 1-D Heisenberg antiferromagnetic chain structures.53 However, the presence of monomeric Co-centers that obey a Curie–Weiss behavior and dimeric Co-ions, which are best described by an isotropic Heisenberg exchange operator, constitute limiting factors towards a true 1-D magnetic arrangement. The question, which type of magnetic ordering can be achieved in mesophases, therefore, remains an objective of strong interest.54 The rich chemistry of terpyridines has afforded a series of other interesting ligands with photoluminescent properties reported by Wu¨rthner et al.55,56 as well as chiral coordination polymers.57,58 The rich photo- and electrochemical properties of porphyrins make the corresponding polymers interesting materials from the point of view of charge storage and transport, solar

Fig. 15 Structural model for the nickel metallopolymer. Right side: Schiff-base Ni complex (only mesogenic side chains present). Left side: ladder structure showing the stacking of the side chains in a linear chain. (R indicates the polymer backbone.)52

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Fig. 16 Self-assembly of a Co-porphyrin polymer as described by Michelsen and Hunter.59

energy conversion and nonlinear optics. Michelsen and Hunter reported an approach to make soluble porphyrin polymers through metal ion coordination of the central metal ion and pendant covalently attached pyridine rings (Fig. 16).59 Solubility is ensured by introducing 2-ethylhexyl side chains to the porphyrin core. Metallation with Co provided the monomer that self-assembles to a porphyrin polymer. At concentrations greater than 10 mM the polymer precipitates and can only be redissolved in coordinating solvents such as pyridine. This observation indicates the formation of high molar weight polymers. Unfortunately, the authors have not investigated the resulting precipitates, which may also be interesting colloids (vide supra). In solution, the occurrence of polymerization was shown by pulsed-gradient spin-echo NMR diffusion experiments, which provides insight into the relative size of the assemblies. By adding monomer to the polymer solution, which induces a chain shortening, the authors further demonstrated with size exclusion chromatography the polymeric nature of the assembly. The association constant was determined to be approx. 106 M21 and the polydispersity index is between 1.5 and 2.5 as expected for equilibrium polymers. The maximum molar weight observed was 136 kDa corresponding to approx. 100 repeat units. At higher concentrations precipitation occurred indicating even higher molar masses. Another area of interest for which supramolecular chemistry holds great promises is stimuli-responsive materials because the environmental variables can have a large effect on the degree of interactions between individual components of the materials. This is true in particular for metallo-supramolecular polymers. An alteration of the strength of the noncovalent part of the intermolecular bonds can result in dramatic modification of the supramolecular structure and thus drive significant changes in the properties, such as the binding strength and exchange kinetics.3,60 Rowan and Beck have introduced This journal is ß The Royal Society of Chemistry 2006

Fig. 17 Representative scheme of the formation of metallo-supramolecular gels by assembling the 2,6-bis(benzimidazolyl)-pyridine ligand, transition metal ions and lanthanides in the appropriate ratio.61,62

multistimuli, multiresponsive metallo-supramolecular polymers on the basis of lanthanides and 2,6-bis(benzimidazolyl)pyridine (BIP) ligands (Fig. 17).61,62 This ligand can bind transition metal ions in a 2 : 1 ratio as well as lanthanides in a 3 : 1 ratio. Therefore, assembling ligands, transition metal ions and lanthanides in the appropriate stoichiometry results in metallo-supramolecular gels. Transition metals such as Zn(II) or Co(II) give rise to linear polymers due to the preferred octahedral coordination geometry of the ligand and these metal ions. Adding 3 mol% of the lanthanide (Eu(III) or La(III)) with respect to the ligand, spontaneously results in gel-formation in a CHCl3–CH3CN mixture. A total of four different gels were prepared and characterized (Co–La, Zn–La, Co–Eu and Zn–Eu). All four gels are thermoresponsive showing a reversible gel–sol transition upon heating (Fig. 18). The optical changes indicate that upon heating the La–ligand interaction is thermally broken. It is not surprising that these gels are also thixotropic, that is they exhibit a shear-thinning under stress. Shaking the Zn–La gel results in a free-flowing liquid, which upon standing thickens again to a the gel-like material. The Eu(III)-containing polymer shows an intense metal centered luminescence, which is facilitated by the antenna effect of the ligand. This is in effect a light conversion process, which occurs by absorption of radiation by the ligand, followed by a ligand-to-metal energy transfer process finally resulting in a metal centered emission. The photochemical properties make these materials interesting for various applications but they also offer a probe to investigate the assembly–disassembly process. Heating the Zn–Eu polymer shows a substantial reduction in the lanthanide-based emission providing further evidence that the lanthanide-ligand bond is thermally broken. Lanthanides are oxophilic and bind well to carboxylic acids. Therefore, it is not surprising that the This journal is ß The Royal Society of Chemistry 2006

polymers are also chemoresponsive. Addition of 0.85 wt% formic acid to the Zn–Eu polymer results in the loss of the mechanical stability and quenches the Eu(III) emission. This result is consistent with the formic acid displacing the ligand on the Eu(III) cation resulting in a switching off of the antenna effect. The process is reversed upon drying the material in vacuum to remove the formic acid and resewelling in acetonitrile. The rheological behavior has been studied in more detail by Rowan et al.63,64 The susceptibility to sheer has nicely been demonstrated by Sijbesma and Paulusse in coordination polymers of diphosphanes and Pd(II). In toluene the polymer is strong enough that an analysis of the molecular weight is possible by standard chromatographic techniques. Also, the assembly kinetics are slow enough not to interfere with chromatography; equilibration requires several days. Exposing the solution to ultrasound results in significant

Fig. 18 a) The thermoresponsive nature of the Co–La and b) the thixotropic response of the Zn–La system (figure reproduced with permission from ref. 61).

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reduction of the molecular weight. Re-equilibration restored the original molecular weight. Notably, ultrasonic chain scission is a nonrandom process that acts preferentially on longer chains. A possible application of ultrasonic chain scission could lie in the area of transition-metal catalysis by creating highly reactive species under controlled conditions.65 In a quest to develop organic–inorganic hybrid materials, which are stable at high temperatures and yet processable, Rowan et al. have employed the ditopic ligand shown in Fig. 19, which combines the BIP unit (vide supra) for dynamic polymerization and 1,4-diethynylbenzene as the functional unit. Due to the conjugation this new ligand is more emissive than similar BIP ligands that lack conjugation.66–68 The optical properties of this compound allow following the self-assembly process in solution. Upon addition of Zn(II) three isosbestic points are observed suggesting the equilibrium of a finite number of spectroscopically distinct species. The authors assign the UV–vis spectra of a free ligand, and 1 : 1 Zn-ligand and 1 : 2 Zn-ligand complexes. Metal-binding also shows a pronounced effect on the emission characteristics. The formation of polymers is further demonstrated by an increase in the viscocity of the solutions at higher concentrations. This approach of assembling conjugated polymers from smaller building blocks has a high potential for utilizing these materials because generally high molecular weight conjugated polymers are hard to process due to their high transition temperatures, limited solubility and high solution viscosities. In addition, structural defects and impurities in such polymers are not uncommon, often making the processing intricate and time-consuming.68 Using Zn(II) and Fe(II) the authors readily prepared fibers and thin films amendable to further characterization. The nature of the core in the ditopic BIP ligand has a profound impact on the material’s properties. While rigid ligands are expected to form rigid-rod type polymers, the introduction of flexible linkers allows formation of rings.69 A short core, e.g. penta(ethylene glycol), in the ligands encourages the formation of macrocyclic species and the resulting material does not show polymer-like properties. On the other hand, macromonomers based on poly(tetrahydrofuran) telechelic units give rise to polymeric materials with improved mechanical strength. This work also hints to another problem in the formation of high molecular weight materials. If polydisperse macromonomers are used, it is intrinsically not possible to achieve the desired 1 : 1 ratio of ligand to metal by simply weighing the components. The authors used NMR titration to assess the maximum degree of polymerization.

Fig. 19 Chemical structure of the ditopic ligand based on the BIP unit and 1,4-diethynylbenzene.66

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Here it becomes clear that (simple) standard procedures have to be developed to control the stoichiometry precisely in order to achieve comparable and reproducible results. Craig et al. have demonstrated an elegant way to manipulate the dynamics of metal–ligand interactions by employing the equivalent of a macromolecular ‘‘kinetic isotope effect’’ (Fig. 20).70 The independent control of the dynamic versus the thermodynamic properties is achieved by simple steric effects at the metal center of square planar Pd(II) and Pt(II) complexes. Ligand exchange at this metal center generally occurs through an associative mechanism, in which the attacking nucleophile associates to the metal center prior to the departure of the original ligand. The pentacoordinate transition state is sterically crowded relative to the reactant and product complex. Steric effects in the spectator ligands of the complex, therefore, profoundly affect the rate of ligand exchange while having little effect on the stability of the complex. Pincer motifs have proven to be attractive in this regard due to the ease of steric manipulation of the spectator ligands and the added advantage of increased stability give the moderate stability of Pd(II) complexes.71,72 Using this approach the authors studied supramolecular networks with a continuous three-dimensional topology based on polypyridine and a ditopic pincer ligand (cross-linker) in DMSO. The authors show that the mechanical properties of the material are linked to solvent mediated ligand displacement reactions and that the dynamical properties are governed by the dissociation of the cross-links in the network. The mechanical properties are, therefore, determined by the relaxations that occur when the cross-links are dissociated from the polymer backbone. The dynamic properties are scaled through the dissociation rates of the cross-links independently of their rate of formation and so dissociation is effectively equivalent to equilibration. Noteworthy, this work shows how the molecular view of the dynamics and mechanism of metal–ligand exchange reactions is not only a qualitative but also a quantitative representation of the viscoelastic properties of bulk materials.73 While thermodynamics are a primary design consideration in supramolecular chemistry, the dynamics of the interactions are particularly important under nonequilibrium conditions, e.g. imposed under mechanical stress.71,72,74 Recently, Craig et al. used single-molecule force spectroscopy to study the mechanical activation of the ligand substitution reaction. Measuring the rupture force as a function of loading rate the authors could determine the dissociation rate constants in these systems.75 The concept of dynamic 3-D topologies has been taken to a higher degree of complexity by Weck et al. by combining metal ion coordination and hydrogen bonding as cross-links (Fig. 21).76 By balancing the thermodynamic and kinetic variables of the constituents the authors envision systems for rapid prototyping and facile modification of the physical properties of the cross-linked materials. This type of orthogonal functionalization and cross-linking offers new options to the functional group incompatibility of many polymerization techniques.77 A system of great practical use is presented by Cohen Stuart and coworkers.78 They employ water-soluble 2,6-dicarboxypyridine ligands that bind transition metal ions and lanthanides. This journal is ß The Royal Society of Chemistry 2006

Fig. 20 Exchange mechanism for pincer-pyridine complexes. The dynamics of ligand exchange can be controlled through steric manipulation of the spectator ligand around the metal center. a) Direct displacement of one pyridine by another pyridine. b) Solvent assisted ligand exchange. Bottom: ditopic pincer cross-linkers.70

As mentioned above, some metallo-supramolecular polymers tend to precipitate from solution if the concentration of the constituents exceeds a critical limit. Due to the use of

Fig. 21 Scheme illustrating the approach of Weck et al. using metal ion coordination and hydrogen bonding to generate reversible crosslinks for new materials with tailored thermodynamic and kinetic properties (figure reproduced with permission from ref. 76).


organic ligands, solubility is an issue that needs careful attention in metallo-supramolecular polymer research. Mirkin and Oh have used this property deliberately to form colloidal particles from coordination polymers (Fig. 22).79 A homochiral carboxylate-functionalized binaphthyl bismetallo-tridentate Schiff base (BMSB) is used as a building block. The particles form via coordination to the carboxylate groups on the BMSB periphery. The process of particle formation is completely reversible as evidenced by the formation of the starting materials upon addition of excess pyridine. The choice of the BMSB ligand, the type of metallation, and the ancillary ligands make it possible to tailor the chemical and physical properties of the resulting polymers and particles. It is interesting to note that the particles are stable in water and also common organic solvents. Inclusion of binaphthyl in the ligand affords fluorescence in the complex as well as the particles. The ancillary ligands (L in Fig. 22) further allow manipulation of the electronic nature of the metal ions. For instance, increasing the s-donor capability of the ancillary ligand induces a red-shift of the absorption maximum. The ability to produce these materials in entantiopure form makes them attractive in catalysis and separation. This concept has recently been used by Nakanishi et al. to obtain nanoscale architectures from dipyrrin ligands.80 Soft Matter, 2006, 2, 915–927 | 925

Fig. 22 Preparation of colloidal particles from metal ions and BMSB. Adding diethyl ether to the reaction mixture results in particle formation and precipitation. A coordinating solvent like pyridine dissolves the particles.79

Summary Once, the introduction of transition metal ions in polymer synthesis revolutionized polymer chemistry. Now it seems that carrying metal ions into macromolecular assemblies may provide an equally strong impact on polymer chemistry and materials science. It is safe to predict that in the future polymer research will exploit the elements of the entire periodic table in systematic ways as weak or strong chain or networking forming units. The extension of macromolecular chemistry beyond carbon-based polymers offers unlimited structural possibilities and provides an enormous potential to improve the capacity of macromolecular materials with many new dynamic, thermal, electronic, electrical, photo-electrical, static, mechanical etc. properties.81 If we look at the development of organometallic polymer chemistry as briefly outlined above it becomes clear that the chemistry and physics of dynamic metallo-supramolecular polymers is little understood. Much work needs to be done in order to come to an understanding of these materials as it is self evident in other areas of supramolecular coordination and macromolecular chemistry. As Rehahn and Lahn pointed out in 2001, well-defined polymers from kinetically labile metal complexes are still nearly unknown, however, by now several different systems are available that allow characterization of polymer properties in solution.82 The analysis and characterization of thermodynamic and kinetic fundamentals of these dynamic materials at molecular levels remains a central challenge. In particular, methods have to be improved and innovated that allow studying these materials without perturbing the equilibrium and its equilibrium structure, be it by increasing the concentration to enhance signal intensity or to add electrolyte to avoid ordering through long range electrostatic interactions. Finally, understanding the nonlinear properties down to the molecular level will be a rewarding exercise because such an understanding is required to bring these materials into engineering science and to applications. However, the prospects of realizing the rational design of smart materials may well be worth the effort.

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