Step-growth polymerization refers to a type of
polymerization mechanism in which bi-functional or multifunctional
monomers react to form first
dimers, then
trimers, longer
oligomers and eventually long chain
polymers. Many naturally occurring and some synthetic polymers are produced by step-growth polymerization, e.g.
polyesters,
polyamides,
polyurethanes, etc. Due to the nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight. The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain — each person has two hands (= reactive sites). There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers are produced.