Enzyme-Catalyzed Stereoelective Ring-Opening Polymerization of α-Methyl-β-propiolactone

The lipase-catalyzed stereoelective ring-opening polymerization of racemic α-methyl-β-propiolactone (MPL) was investigated. Using the lipase PS-30 from Pseudomonas fluorescens, a direct route to optically active (S)-enriched poly(α-methyl-β-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 Mn values from 2600 to 2900 g/mol and [α]25D +12.2° to +19.0° (c 0.9 g/dL, CHCl3). Analysis of the polymer chain end structure by 1H and 13C NMR showed that these products have hydroxyl and carboxylic acid termini. Based on the analysis of chain stereosequence distributions by 13C 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 a broad region from ∼25 to 100 °C where the total ΔHf is 12.7 cal/g.