Condensation polymerisation occurs between monomers with two functional groups. During the reaction a small molecule is given off as the monomers bond covalently to produce a polymer.The polymer formed usually has other atoms in addition to carbon in the main chain. The formation of polyesters is an example of condensation polymerisation. Condensation polymers can also occur naturally and some examples include proteins, starch, gycogen and cellulose.

The formation of polyester is an example of a condensation polymerisation reaction. At the formation of each ester link a water molecule is given off.

Monomers that form polyester have two functional groups each.

Pictured on the left is an animation of ethylene glycol and terephthalic acid polymerising. Notice that the alcohol has two hydroxy groups as does the acid have two carboxy groups.

The "R" in terephthalic acid represents a benzene ring.