Addition polymerisation

Addition polymerisation is polymerisation in which unsaturated monomers, each carrying a carbon–carbon double bond (C=C), add directly to one another so that no other molecule is released. Malay: pempolimeran penambahan · Chinese: 加成聚合.

The key feature is that every atom of every monomer stays in the polymer. Each double bond opens into a single bond, freeing a bond on each carbon that links to the next unit, and the chain grows without losing anything. Because nothing is given off, the empirical formula of the repeating unit is the same as that of the monomer. This makes addition polymers straightforward to work out: identify the C=C, open it, and repeat. The common addition polymers are polythene from ethene, polypropene from propene, PVC from chloroethene and polystyrene from phenylethene.

Example. Thousands of ethene molecules, CH₂=CH₂, undergo addition polymerisation to form polythene, (–CH₂–CH₂–)ₙ. No water or other by-product appears, so all the mass of the ethene ends up in the polymer.

Confusion to avoid. Do not confuse addition polymerisation with condensation polymerisation. Addition uses unsaturated monomers and releases nothing; condensation uses monomers with two functional groups and eliminates a small molecule such as water at every link. If a question mentions water being given off, it cannot be addition.

When you draw an addition polymer, show the double bond replaced by a single bond, put the repeating unit in brackets, and write n outside; the examiner looks for continuation bonds sticking out of each side of the brackets. A missing continuation bond, or a double bond left inside the brackets, is a frequent way marks slip away here.

This everyday process makes most of the plastics around us, from shopping bags to bottles, which is why examiners return to it often. Being able to state the definition, name the monomer and draw the repeating unit protects several marks in SPM Chemistry.