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Polyester synthesis is the construction of macromolecules whose repeating backbone linkage is the ester group, –CO2–, by step-growth polycondensation of diols with dicarboxylic acids (or their derivatives) or by ring-opening polymerization of lactones and related cyclic esters ^[1][2]. In polymer synthesis and properties the concept is bounded by that ester repeat connection as the chain-forming event, and it excludes polyamides, polycarbonates (unless an ester is also present as the defining link), and biological polyhydroxyalkanoates assembled solely by microbial synthases when the object of definition is the chemical polymerization. Essential features are a bifunctional or cyclic monomer set, an esterification or ring-opening mechanism, and a degree of polymerization controlled by stoichiometry, conversion, water (or alcohol) removal, or initiator concentration. Lipase B from Candida antarctica catalyzes both polycondensation and lactone ring-opening to polyesters under mild conditions, providing an enzymatic realization of the same ester-forming elementary step ^[1][2][3]. The concept is distinguished from polyurethane synthesis by the absence of a carbamate linkage and from polyesterification used only to make oligomeric polyols by the intention to form a high polymer. Conceptual limits exclude transesterification of a preformed polyester when no new chain is grown from monomers.