The lipase-catalyzed stereoelective ring-opening polymerization of racemic alpha-methyl-beta-propiolactone (MPL) was investigated. Using the lipase PS-30 from Pseudomonas fluorescens, a direct route to optically active (S)-enriched poly(alpha-methyl-beta-propiolactone), PMPL, was demonstrated. From a comparative study of different organic media, polymerizations conducted in toluene and heptane proceeded more rapidly than those carried out in dioxane. The enantiomeric ratios E in toluene, heptane, and dioxane were 4.1 +/- 0.2, 0.9, and 2.0, respectively. Thus, from the point of view of reaction rates and enantioselectivity, toluene was found to be the preferred solvent. PMPL products prepared in toluene by PS-30 catalysis had M(n) values from 2600 to 2900 g/mol and [alpha]D-25 + 12.2 degrees to + 19.0 degrees (c 0.9 g/dL, CHCl3). Analysis of the polymer chain end structure by H-1 and C-13 NMR showed that these products have hydroxyl and carboxylic acid termini. Based on the analysis of chain stereosequence distributions by C-13 NMR, it was concluded that stereoselectivity during propagation results from catalyst enantiomorphic-site control. Investigation of the thermal behavior of PMPL (75% (S)) by DSC showed that melting occurs over abroad region from similar to 25 to 100 degrees C where the total Delta H-f is 12.7 cal/g.