Chapter 28: Polymerisation

Condensation polymerisation is detailed as a reaction where monomers bond together while simultaneously eliminating a small molecule, typically water or hydrogen chloride. This process requires monomers containing at least two reactive functional groups, such as an amine reacting with a carboxylic acid or an acyl chloride. Polyamides, including synthetic fibers like nylon and Kevlar, are created through this linkage, forming the strong amide bond. Nylon, made from diamine and dicarboxylic acid monomers, gains its high tensile strength from cold drawing, which aligns the polymer chains, fostering extensive hydrogen bonding between them. Kevlar, a polyamide containing benzene rings, is exceptionally strong and abrasion-resistant due to its long, regular chains that facilitate substantial intermolecular hydrogen bonding. Biochemically, proteins and polypeptides are classified as polyamides, formed from alpha-amino acids linking via peptide bonds (amide bonds). Polyesters form similarly, produced by the reaction of dicarboxylic acids with diols, or from hydroxycarboxylic acids, resulting in the characteristic ester linkage. Examples include Terylene and the plant-derived Poly(lactic acid) or PLA. Crucially, the chapter addresses environmental issues, noting that unlike chemically inert, nonbiodegradable polyalkenes (addition polymers), both polyamides and polyesters are degradable. Their inherent ester and amide links are susceptible to breakage via hydrolysis under acidic or alkaline conditions. The text also covers conducting polymers, exemplified by poly(ethyne), whose electrical conduction is enabled by an extended pi bonding system that allows delocalized electrons to move freely along the chain. Finally, systematic approaches for polymer deduction are outlined, enabling students to predict the type of polymerisation, deduce repeat units, and identify monomer structure based on the presence or absence of functional groups in the polymer backbone.