Lipase-Catalyzed Ring-Opening Polymerization of Trimethylene Carbonate^†

This work was directed at extending the use of lipase-catalyzed ring-opening polymerizations to cyclic carbonate monomers. Of the seven lipases screened for bulk trimethylene carbonate (TMC) polymerization (70 °C, 120 h), Novozym-435 from Candida antarctica gave almost quantitative monomer conversion (97%) and poly(TMC) with a M_n = 15 000 (M_w/M_n = 2.2) with no apparent decarboxylation during propagation. The lipases from Pseudomonas species (AK and PS-30) and porcine pancreas (PPL) also exhibited high monomer conversions (>80%, 120 h) but gave lower molecular weight polymers with broad polydispersity. Analyses by ¹H-NMR spectroscopy suggested that poly(TMC) prepared by Novozym-435-catalyzed polymerization had terminal −CH₂OH functionalities at both chain ends. A monotonic increase in monomer conversion with time and the rapid increase in M_n as a function of monomer conversion for Novozym-435-catalyzed TMC bulk polymerization at 70 °C suggest that the polymerization has chain-type propagation kinetics. An increase in conversion above 66% did not substantially change M_n. The percent conversion was larger when the reaction temperature was increased from 45 to 55 °C. Further increase in the reaction temperature from 55 to 85 °C did not give higher percent conversion values. The molecular weight decreased substantially as the reaction temperature was increased from 55 to 85 °C (M_n from 24 400 to 5 900). The highest poly(TMC) molecular weight (M_n = 24 400) was obtained by conducting the polymerization at 55 °C. Monomer conversion and molecular weight as a function of the percent reaction water content (w/w) were investigated. Increasing the water content resulted in enhanced polymerization rates and decreased molecular weights. Separation of the oligomeric products from polymerizations of TMC in dried dioxane and toluene catalyzed by porcine pancreatic lipase led to the isolation of di- and triadducts of trimethylene carbonate. Based on the symmetrical structure of these products and the end-group structure of high molecular weight chains, a mechanism for chain initiation and propagation for lipase-catalyzed TMC polymerization was proposed.