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Polykondensation is a type of step-growth polymerization in which monomers react to form a polymer, releasing a small molecule as a byproduct. This small molecule, often water, is eliminated from the reacting functional groups of the monomers. Unlike chain-growth polymerization, where monomers add sequentially to a growing chain, polycondensation involves the reaction of difunctional or polyfunctional monomers. Each reaction step forms a new bond and a byproduct, increasing the molecular weight of the polymer gradually.

Common examples of polymers formed through polycondensation include polyesters, polyamides, and polycarbonates. For instance, the formation of polyester involves the reaction between a dicarboxylic acid and a diol, with the elimination of water. Polyamides are formed from the reaction of a dicarboxylic acid and a diamine, also releasing water. Polycarbonates can be synthesized from bisphenols and phosgene. The rate of polycondensation is generally slower than chain-growth polymerization, and achieving high molecular weights can require careful control of reaction conditions, such as high temperatures and efficient removal of the byproduct to drive the equilibrium towards polymer formation. The mechanism typically involves the formation of dimers, trimers, and eventually long polymer chains.