Polymers

Polymers are very large molecules built from repeating units called monomers. In addition polymerisation, unsaturated monomers join without losing any atoms, usually by a free-radical chain mechanism: ethene gives polythene, tetrafluoroethene gives Teflon and acrylonitrile gives polyacrylonitrile (orlon). In condensation polymerisation, monomers with two functional groups join while losing small molecules such as water, giving polyamides (nylons), polyesters (terylene) and resins (Bakelite).

Polymers are also classified by their forces. Elastomers such as rubber have weak forces and stretch; fibres such as nylon have strong hydrogen bonds and high tensile strength; thermoplastics such as polythene soften on heating and can be remoulded; thermosetting polymers such as Bakelite form cross-links and set permanently. Low-density polythene is branched and made by free-radical polymerisation at high pressure, while high-density polythene is linear and made with a Ziegler–Natta catalyst.

Natural rubber is cis-1,4-polyisoprene; heating it with sulphur (vulcanisation) adds cross-links that make it harder and less sticky. Synthetic rubbers include neoprene (from chloroprene), Buna-S (buta-1,3-diene and styrene) and Buna-N (buta-1,3-diene and acrylonitrile). Biodegradable polymers such as PHBV and nylon-2–nylon-6 break down in the environment. EAPCET questions almost always ask for monomers or for the type of polymer.

A single monomer table covers almost every question from this chapter.

Once the formulas feel familiar, test them the way the exam does: practise Polymers questions chapter-wise, then attempt a full previous year paper to see whether it holds up under a 3-hour timer.