Lipase-catalyzed ring-opening polymerization of medium-size lactones to polyesters

Abstract

Enzymatic ring-opening polymerization of medium-size lactones, ϵ-caprolactone (ϵ-CL) and δ-valerolactone (δ-VL), was performed by using a lipase as a catalyst. For the polymerization of ϵ-CL in bulk, a lipase derived from Pseudomonas fluorescens (lipase PF) showed the highest catalytic activity among the powdery lipases examined, i.e., the highest molecular weight was achieved by using lipase PF. The polymerization behavior depends on the lipase origin as well as on the polymerization condition. From 1H and 13C NMR analyses the polymer was found to possess one terminal carboxylic acid group and one hydroxyl group. δ-VL was enzymatically polymerized, yielding the corresponding polyester. The polymerization rate of δ-VL catalyzed by lipase PF is larger than that of ϵ-CL, whereas the molecular weight of poly(δ-VL) is lower than that of poly(ϵ-CL) obtained under similar reaction conditions. The lactones were also enzymatically polymerized in organic solvents. The relationship between the type of the solvent and the polymerization behavior was investigated. An immobilized lipase derived from Candida antarctica causes a much faster polymerization of the lactones than the powdery lipases. The immobilized lipase can be reused as a catalyst for the polymerization of ϵ-CL.