polycondensation

Polycondensation, also called condensation polymerization, is a type of polymerization in which growth occurs by the stepwise reaction of monomer units bearing at least two functional groups, with the elimination of small molecules as byproducts such as water, methanol, or hydrogen chloride.

In contrast to chain-growth polymerization, each coupling reduces the number of reactive groups in the system; molecular weight increases as the extent of reaction rises. The reaction typically proceeds through successive condensations and is driven by removing the byproduct, often by heating, vacuum, or azeotropic distillation.

Common families include polyesters formed from diols and dicarboxylic acids (for example polyethylene terephthalate), polyamides such as nylons, and polycarbonates obtained from diols and phosgene or derivatives, with the elimination of a small molecule like HCl. The process yields linear, branched, or crosslinked polymers depending on the functionality of the monomers and reaction conditions.

Properties and processing of polycondensation polymers depend on structure and degree of polymerization. High molecular weight requires careful control of monomer purity and efficient removal of byproducts; polymers are often melt-processed, solution-processed, or used in fibers and coatings. They can be more hydrolytically sensitive than certain vinyl polymers, affecting processing and end-use performance.

Applications are broad, including fibers, bottles and packaging, coatings, and engineering plastics. Environmental considerations focus on byproduct management during synthesis and on strategies for recycling, including depolymerization back to monomers or other value-added products.