Porcine pancreas lipase (PPL) immobilized on silica particles with different size were prepared and employed successfully for ring-opening co-polymerization of 5-benzyloxy-trimethylene carbonate (BTMC) with 5,5-dimethyl-trimethylene carbonate (DTC) for the first time. Three kinds of silica particles with different sizes (150-250, 75-150 and 1 mum) were selected as carriers for enzyme immobilization. The structure of copolymers were confirmed by H-1 and C-13 NMR which showed no decarboxylation occurrence during the polymerization. The (M-n) of poly (BTMC-co-DTC) decreased rapidly with the increasing of immobilized PPL concentration. The carrier size of immobilized PPL affected both the catalytic activity and the polymer yield. The highest molecular weight (M-n = 26,400) of poly(BTMC-co-DTC) was obtained at around 0.1% concentration of immobilized PPL on silica particles with size of 75-150 mum. (C) 2003 Elsevier Science Ltd. All fights reserved.