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 M n values from 2600 to 2900 g/mol and [α] 25 D +12.2° to +19.0° ( c 0.9 g/dL, CHCl 3 ). Analysis of the polymer chain end structure by 1 H and 13 C NMR showed that these products have hydroxyl and carboxylic acid termini. Based on the analysis of chain stereosequence distributions by 13 C 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 Δ H f is 12.7 cal/g.