AbstractWith the object to synthesize polyesters by enzymatic catalysis in organic media, two directions have been investigated: (1) the condensation polymerization of linear ω‐hydroxyesters and (2) the ring‐opening polymerization of lactones. The commercially‐available crude porcine pancreatic lipase (PPL), suspended in organic solvents, was the preferred enzyme for the reactions. In order to determine the optimal conditions for the condensation polymerization, the bifunctional methyl 6‐hydroxyhexanoate was used as a model compound to study the influence of the following parameters: type of the enzymecatalyst, kind of solvent, concentration, temperature, duration, size of the reaction mixture, and stirring. Film‐forming polyesters with a degree of polymerization (DP) up to about 100 were obtained from linear aliphatic hydroxyesters inn‐hexane at reflux temperature (69°C). Yet concurrently with the intermolecular condensation polymerization, macrolactones were also formed by intramolecular reaction. Two aromatic hydroxyesters did not react under these conditions. For the ring‐opening polymerization of lactones the reaction of ϵ‐caprolactone with methanol as the preferred nucleophile, was studied. Polyesters with a DP of up to 35 were obtained inn‐hexane at temperatures between 25 and 40°C. The degrees of polymerization of the polyesters were determined by comparative analyses of the end groups in the1H‐NMR spectra and by determination of molecular weights either by vapor phase osmometry, gel permeation chromatography, or intrinsic viscosity. © 1