Supramolecular Polymers based on Hydrogen Bonding Essay

Supramolecular Polymers based on Hydrogen Bonding - Essay Example The paper tells that a supramolecular polymer refers to any kind of self-assembly that results in the creation of polymer-like aggregates occurring through reversible interactions between one or more kinds of components. Because of the reversible interactions, these polymers can thermally equilibrate with their monomers, unlike conventional polymers. These polymers are responsive to external stimuli. The mechanical properties of these polymers respond strongly to changes in solvent or temperature because of their reversible interactions, and so, they are in continuous equilibrium with their environment. Therefore, the functional properties of these polymers are highly useful. Supramolecular polymers show polymer like rheological properties because of their macromolecular structure and can also form gels if the self-assembled chains are long enough. Supramolecular polymers comprise of non-covalently bonded monomers and are of various types that include crystals, colloids, gels, liquid crystals, and hydrogen bonded polymers. Hydrogen bonded supramolecular polymers are those polymers in which the monomers are held together only by hydrogen bonds. The utility of hydrogen bonds in bringing polymers together was first shown by Stadler and coworkers. Supramolecular polymers can generally be classified into main-chain and side-chain polymers. The main-chain polymers are further divided into linear main-chain polymers, networks and linear polymers based on their bidirectional units. Side-chain polymers are further divided into two classes. The first one includes polymers with binding motifs in the side-chain, and the second one includes polymers with binding motifs in the main chain (Ligthart 2006). Supramolecular polymers of the linear main-chain type can be formed via the assembly of bifunctional or multifunctional monomers or planar structures that can assemble on both sides of a plane (Ligthart 2006). Figure 1 — Two classes of supramolecular polymers (a) Main -chain polymers, (b) Side-chain polymers (Source: Ligthart 2006, p. 3) According to Greef et al. (2009), supramolecular polymers can be classified based on the type of interactions that lead to their formation. Accordingly, the different types of supramolecular polymers that can be classified based on the interactions include those that are formed by hydrogen bonds, hydrophobic interactions, ?-? interactions, and metal-ligand binding. However, Greef et al argue that this scheme of classification, although useful, ignores mechanistic details that have been revealed as research in the field progresses. They thus propose another system of classification, which includes two groups. The first group of polymers are those that have monomers of single type, which undergo complementary end-group or self-complementary interactions. With the help of complementary couples that are directional (A-B), and self-complementary binding motifs (A-A), all kinds of polymeric structures such as cross-lin ked networks, linear homo-polymers and copolymers, and branched structures can be synthesized (Ligthart 2006). For instance, polymerization of an A2 monomer which results from the reversible A: A self-complementary interaction, comes under the first group. The polymerization of A-B monomer through a reversible A: B complementary end-group interaction is another example of the first group of supramolecular