Summary

This chapter covered polymers -- the giant, repeating-unit macromolecules central to modern materials -- along two classification threads. By source: natural polymers such as starch, cellulose, natural rubber, silk and wool occur ready-made in living organisms, while synthetic polymers such as polythene, nylon-6,6, terylene and bakelite are manufactured from small organic monomers. By linking mechanism: addition (chain-growth) polymerisation joins unsaturated monomers one at a time onto a single growing chain via a free-radical initiation/propagation/termination sequence, with no atom lost (polythene from ethene, in its LDPE branched/free-radical and HDPE linear/Ziegler-Natta forms); condensation (step-growth) polymerisation instead joins bifunctional monomers together at many independent points across the reaction mixture, eliminating a small molecule (usually water) at every linking step (nylon-6,6's amide linkages from a diamine and a diacid, terylene's ester linkages from a diol and a diacid, and bakelite's cross-linked methylene-bridged network from phenol and formaldehyde). Copolymerisation extends the addition mechanism to two different monomers polymerised together, as in the synthetic rubbers Buna-S (with styrene) and Buna-N (with acrylonitrile). Natural rubber, itself an addition polymer of isoprene (cis-1,4-polyisoprene), is transformed by vulcanisation -- sulfur cross-linking between neighbouring chains -- from a soft, tacky, weak raw material into a strong, elastic, temperature-stable product. Finally, whether a polymer's backbone contains readily hydrolysable ester/amide linkages (as in the deliberately designed biodegradable polymer PHBV) or an inert, fully saturated carbon backbone (as in polythene and PVC) determines whether it is biodegradable or non-biodegradable -- a distinction with real and growing environmental consequences. Across every example in this chapter, the same two questions are worth asking of any newly-met polymer: where does it come from (natural or synthetic), and how are its mo …