Lipase-catalyzed ring-opening bulk polymerizations of 6(S)-methyl-morpholine-2,5-dione (MMD) were investigated. Selected commercial lipases were screened as catalysts for MMD polymerization at 100 degrees C. Polymerizations catalyzed with 10 wt % porcine pancreatic lipase type H crude (PPL), lipase from Pseudomonas cepacia, and lipase type VII from Candida rugosa resulted in MMD conversions of about 75% in 3 days and in molecular weights ranging from 8200 to 12,100. Poly(6-methyl-morpholine-2,5-dione) [poly(MMD)] had a carboxylic acid group at one end and a hydroxyl group at the other end. However, lipase from Mucor javanicus showed lower catalytic activity for the polymerization. During the polymerization, racemization of the lactate residue took place. PPL was selected for further studies. The rate of polymerization increased with increasing PPL concentration under otherwise identical conditions. When the PPL concentration was 5 or 10 wt % with respect to MMD, a conversion of about 70% was reached after 6 days or I day, respectively, whereas for a PPL concentration of I wt %, the conversion was less than 20% even after 6 days. High concentrations of PPL (10 wt %) resulted in high number-average molecular weights (< 3 days); with a lower concentration of PPL, lower molecular weight poly(MMD) was obtained. The concentration of water was an important factor that controlled not only the conversion but also the molecular weight. With increasing water content, enhanced polymerization rates were achieved, whereas the molecular weight of poly(MMD) decreased. (c) 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3030-3039, 2005.