The polymerization of epsilon-caprolactone, (epsilon-CL) using porcine pancreatic lipase (PPL) as the catalyst was studied. Polymerization reactions (4 days, 65 degrees C) of E-CL at similar to 10% (w/v) concentrations in dioxane, toluene, and heptane using butanol as an initiating species (monomer/butanol ratio = 14.7) gave poly(epsilon-caprolactone) (PCL) with M(n),values (by GPC) of 313, 753, and 1600, respectively. Monomer conversion to PCL for these polymerizations was 33, 55, and 100%, respectively. M(n) measurements of PCL products by H-1 NMR end group analyses were slightly lower (by a factor of similar to 0.9) t;han the values obtained by GPC. Polymerizations conducted in heptane at 37, 45, 55, and 65 degrees C showed the highest extent of monomer conversion at 65 degrees C. Therefore, subsequent studies were conducted at 65 degrees C in heptane. For a polymerization carried out with a 15/1 monomer/butanol ratio and similar to 0.29 mmol of water, similar to 70 and similar to 100% of the monomer had been converted to PCL by reaction times of 24 and 96 h, respectively. Polymer molecular weight increased slowly with conversion, suggesting that this is a chain polymerization with rapid initiation and slow propagation. Increases in the epsilon-CL/butanol ratio from 15/1 up to where no butanol was added showed only a modest increase in product molecular weight from 1600 to 2700. This was explained by the fact that the water present in polymerizations was active in chain initiation. Variation in the monomer/butanol ratio at constant water concentration resulted in PCL chains with 0-0.65 mol fraction of butyl ester and 0.33-0.86 mol fraction of carboxylic acid chain end groups (by H-1 NMR analyses). The presence of water concentrations in polymerization reactions above that which is strongly enzyme bound is believed to be an important factor which limited the formation of PCL chains of significantly higher molecular weight.