Step-growth or condensation polymerization is a stepwise reaction of bi or multifunctional monomers to form long-chain polymers. As all the monomers are reactive, most of the monomers are consumed at the early stages of the reaction to form small chains of reactive oligomers, which then combine to form long polymer chains in the late stages. Hence, the reaction has to proceed for a long time to achieve high molecular weight polymers.

Many natural and synthetic polymers are produced by step-growth polymerization, including polyesters, polyethers, urethanes, and polyamides.

Based on the type of monomers involved, there are AABB type or ABAB type polymers. The monomers AA and BB with the same functional groups at both ends are involved in forming AABB type polymers. For example, poly(ethylene terephthalate) or PET is formed by the reaction between monomers glycol and terephthalic acid.

Whereas ABAB type polymers are formed from AB monomers with two different functional groups at both ends. 6-Aminohexanoic acid is an AB-type monomer with an amine and an acid functional group at each end. It undergoes self-condensation to form nylon 6, an AB-type polymer. In both cases, A’s functional group exclusively reacts with B’s and vice-versa to form a new covalent bond.

Step-growth polymerization offers several advantages over chain-growth polymerization; for example, no initiator is required to start the polymerization, and termination reactions are absent. However, long reaction times are typically required to achieve high conversion degrees and high molecular weights.