Supramolecular Polymers and Assemblies. Andreas Winter
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Metal‐to‐ligand coordination
As a result, from their comparably low strength and non‐covalent character, most of these interactions feature a certain degree of reversibility. In other words, the monomeric units of a supramolecular polymer can readily be assembled and disassembled resulting in new, interesting properties and potential applications that are not (or only hardly) accessible for covalent polymers. Utilizing all of these different types of non‐covalent interactions allows one to assemble “tailor‐made” supramolecular polymers, which reveal the key property, i.e. reversibility of binding [29].
Moreover, with respect to the synthesis of any type of supramolecular polymer, the directionality of the utilized non‐covalent bond is of utmost relevance. In classical polymers, the covalent bonds with their perfect directionality represent the linkage of two monomeric units. On the contrary, supramolecular polymers often rely on interactions, which feature a much lower degree of directionality (e.g. ion‐pairing or charge–transfer interactions). Only hydrogen‐bonding interactions, certain types of metal‐to‐ligand coordination, and host–guest inclusion complexes offer very high directionalities, which are comparable to those of conventional covalent bonds. In summary, the directionality and strength of the non‐covalent interactions represent crucial parameters when dealing with (linear) supramolecular polymers.
Taking the aforementioned “structural monomer classification” scheme, one can distinguish between two cases: first, a single molecular component is equipped with supramolecular binding site of either a self‐complementary or a complementary nature. For instance, the reversible association into an A2‐system in solution is based on the self‐complementary A:A interactions (accordingly, an A–B‐type system is formed by reversible complementary A:B interactions). Second, the two different homoditopic monomers A–A and B–B may exhibit only one type of interaction; in this case, the supramolecular polymerization process is driven solely by the complementary interactions between A and B.
Finally, the classification of supramolecular polymerizations according to their thermodynamics has to be considered: for supramolecular polymers, ΔG0 can be expressed as a function of the monomer conversion from zero (p = 0) to full conversion (p = 1). Here, the particular mechanism by which a supramolecular polymer is formed from monomeric components has to be considered (this process is dependent on various parameters, e.g. concentration, solvent, and temperature).
1.3 Supramolecular Polymerization Mechanisms
The conventional polycondensation and polyaddition reactions require monomers with (at least) two functional groups by which the covalent bonds are formed. Considering linear polymers, either a single type of monomer bearing two different functionalities (the so‐called AB‐type monomer) or two different monomers each equipped with two functionalities of the same kind (the so‐called homoditopic AA‐ and BB‐type monomers) can basically be employed in this context. Accordingly, supramolecular polymers can be formed by the polyassociation of monomers, which possess (at least) two appropriate binding sites to undergo non‐covalent interactions (Figure 1.2) [39]. As for their covalent counterparts, AB‐type monomers (e.g. Ic) as well as pairs of AA‐ and BB‐type monomers (Ib) can be utilized for this purpose. In principle, one can further distinguish between two different monomer classes: directly interacting monomers (Ia–c) and those that are connected via metal ions (IIa–c). Supramolecular polymerizations according to type‐I typically involve hydrogen‐bonding or ionic interactions as well as host–guest complexation (however, ionic interactions can also be involved in type‐II polymers where coordinative interactions are involved). The self‐complementary monomers (e.g. Ia and IIa) give supramolecular polymers with only a single type of monomeric unit. Accordingly, complementary binding units can be employed to assemble homopolymers (i.e. Ic and IIc) as well as alternating copolymers (i.e. IIb and IIc).
Figure 1.2 Schematic representation of the different types of supramolecular polymerization.
Source: Winter et al. [39]. © 2012 Elsevier B.V.
In general, the formation of polymers, via a polycondensation or polyaddition, represents a kinetically controlled process [26]: the potential barrier for the forward reaction (i.e. the polymerization) is much lower than for the backward reaction (i.e. depolymerization). As a consequence, neither changes in concentration (i.e. dilution) nor temperature (i.e. heating) will affect the molar mass or the degree of conversion. In supramolecular polymerizations, the situation is much more complicated – due to the (partially) reversible character, depolymerization must be considered. Thus, heating or dilution of a kinetically unstable supramolecular polymer will have a tremendous effect with respect to the molar mass – unlike for a common macromolecule, a decrease of Mn will be the result. According to Carothers' equation (Eq. (1.1)), the molar mass of polymers, which have been prepared by either a polyaddition or a polycondensation, can be correlated to the monomer conversion. However, for supramolecular polymers, featuring a certain degree of reversibility of the non‐covalent interactions, the molar mass will mainly depend on the strength of these interactions, i.e. their association constants (Ka values). As depicted in Figure 1.3, both high Ka values and high monomer concentrations are required to obtain the supramolecular polymers with high DPs. Besides these considerations, other parameters, such as concentration, temperature, and the particular reaction conditions (i.e. presence of additives or reagents, etc.), will also have an impact on the entire assembly process.
where DP: degree of polymerization, p: monomer conversion.
Figure 1.3 Representation of the theoretical DP as a function of the association constant (Ka in M−1) for a typical supramolecular polymerization according to an isodesmic model at two different concentrations.
Source: Brunsveld et al. [27]. © 2001 American Chemical Society.
This assumption, however, represents only a simplification of the actual mechanism of a supramolecular polymerization – even when only considering the thermodynamic equilibrium state, where assemblies are formed that, ideally, retain their polymeric structure over time. In this respect, three main mechanisms can be listed (Figure 1.4) [26]:
1 The isodesmic supramolecular polymerization (IDP) is comparable to the step‐growth polymerization of esters (i.e. showing a broad dispersity, Đ) and the DP is strongly