Condensation Polymers

Condensation polymerisation is a process in which monomers join together with the loss of a small molecule — usually water — for each new bond formed. This is fundamentally different from addition polymerisation, where monomers simply add together without losing anything. Condensation polymers include some of the most commercially important materials: polyesters and polyamides.

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Polyesters

Formation

A polyester is formed when a diol (a molecule with two –OH groups) reacts with a dicarboxylic acid (a molecule with two –COOH groups). Each ester bond formed releases one molecule of water.

The monomers must be bifunctional — they must have a reactive functional group at each end of the molecule so that the chain can continue growing in both directions.

Example: PET (polyethylene terephthalate) / Terylene

Monomers:

• Ethane-1,2-diol (HOCH₂CH₂OH)
• Benzene-1,4-dicarboxylic acid (terephthalic acid, HOOC–C₆H₄–COOH)

The –OH from the diol reacts with the –COOH from the dicarboxylic acid to form an ester bond (–COO–) and release water. This repeats at both ends of each monomer, building a long chain.