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In addition polymerisation, the monomers must have a C=C double bond. However, in condensation polymerisation, the monomers do not need a C=C double bond but they do need two functional groups.

When two monomers react in a condensation reaction, a small molecule (usually water) is produced as a by-product.

Making polyesters

Polyesters occur naturally as biological polymers, but can also be made artificially. Man-made polymers are often used in clothing. Polyesters are an example of a condensation polymer. A simple polyester can be made from a monomer which has two hydroxyl groups (-OH) and another monomer which has two carboxylic acid groups (-COOH).

For example, ethane diol (HO-CH₂-CH₂-OH) can react with butanedioic acid (HOOC-CH₂-CH₂-COOH):

n HO-CH₂-CH₂-OH + n HOOC-CH₂-CH₂-COOH → (-CH₂-CH₂-OOC-CH₂-CH₂-COO-)_n + 2n H₂O

We can simplify the equation by using symbols to represent the carbon chain in the middle of each of the two monomers.

n HO-■-OH + n HOOC-♦-COOH → (-■-OOC-♦-COO-)_n + 2n H₂O