Hydrogen Bonding Modules for Use in Supramolecular Polymers

Nature utilizes a wide range of noncovalent interactions to maintain the well-defined, three-dimensional structure of key biomacromolecules, such as proteins, DNA, and RNA. Because their discrete structures are the key to their biomacromolecular function, noncovalent interactions truly underpin all biological complexity and function and, thus, life itself. Supramolecular polymer chemistry takes its inspiration from nature and has emerged as a major research focus in the past two decades. The incorporation of supramolecular interactions into polymeric systems can alter or improve the material properties of conventional polymers. Long before chemists began engineering these interactions into macromolecules, it was recognized that hydrogen bonding between polymer chains could have a profound effect on their properties. For example, the fact that Kevlar has a tensile strength that is 7-fold higher than steel, on an equal weight basis, has been partly attributed to the formation of hydrogen bonds between amide NH and carbonyl groups. Likewise, the strength of Nylon 6,6 originates in the same type of sheet-like hydrogen bonding networks formed between polymer chains, whereas the elasticity derives from amorphous regions. Given the remarkable properties provided by the noncovalent contacts in Kevlar and Nylon 6,6, chemists saw a wide range of possibilities for new properties in supramolecular polymeric systems. For example, the loss of hydrogen bonding at higher temperature could lead to more easily processable polymeric materials, associated with relatively low melt viscosities. More generally, the overriding idea was that the reversible character of noncovalent interactions could lead to stimuli-responsive polymers, whose properties could be tuned by external stimuli, such as solvent, temperature, and light. In this context, a considerable effort has been focused on hydrogen bonding, because there is a lot known about how to tune its strength, and it is very responsive to environmental effects, especially solvent and pH. There are two distinct and common ways to incorporate supramolecular motifs into polymers. When noncovalent interactions are employed to hold either monomeric or polymeric units together, main-chain supramolecular polymers are obtained. In contrast, noncovalent functionalization on the side-chains of covalent polymer backbones affords side-chain supramolecular polymers that may assemble into networks. In recent years, these two self-assembly approaches have been developed extensive-