Condensation polymerisation

Condensation polymerisation is polymerisation in which monomers each carrying two functional groups join together, and a small molecule, most often water, is eliminated every time a new bond forms. Malay: pempolimeran kondensasi · Chinese: 缩合聚合.

The two functional groups are what let a monomer bond on both sides and so build a chain. When two groups react, they release a small molecule between them, so unlike addition polymerisation the polymer does not contain all the atoms of its monomers. The lost molecule is usually water, but can be another small molecule such as hydrogen chloride. Two families of condensation polymer appear in the syllabus: polyesters, whose chains are held by ester links, and polyamides, whose chains are held by amide links. Proteins are natural condensation polymers of amino acids.

Example. Nylon is a polyamide formed by condensation polymerisation: a diamine and a dicarboxylic acid join through amide links, releasing a water molecule at each join. Terylene, a polyester, forms in the same way through ester links.

Confusion to avoid. Do not confuse condensation with addition. Condensation needs monomers with two functional groups and always eliminates a small molecule; addition needs unsaturated monomers with a C=C and eliminates nothing. The presence or absence of a by-product is the quickest way to tell the two apart in an exam.

A helpful memory aid is the word itself: condensation, as in water vapour condensing, reminds you that a small molecule is lost. Working out how many water molecules are eliminated when a given number of monomers join is a calculation the paper sometimes sets, so keep the rule of one small molecule lost per link firmly in mind.

Condensation polymerisation links this chapter to proteins from biology and to the manufactured fibres nylon and terylene, so a precise definition, including the eliminated molecule, secures marks across SPM Chemistry.