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In step-growth condensation polymerization, reactive groups on bifunctional monomers form covalent links while releasing small molecules such as water or HCl. Polyesters contain ester linkages and can be made from a diol paired with a dicarboxylic acid or diacyl chloride; PET, made from ethylene glycol and terephthalic acid, is a key example used in fibers and bottles. Ester linkages lack the strong hydrogen-bond donors found in polyamides, contributing to differences in polarity, melting behavior, and hydrolytic stability. The lesson also introduces transesterification catalysts and ring-opening routes to biodegradable polyesters such as PLA and PCL. High chain length depends on suitable monomer ratios and purity, limiting moisture and removing by-products.