An amphiphilic block copolymer, poly(ethylene glycol)-block-poly(L-lactideco-2-methyl-2-benzoxycarbonyl-propylene carbonate) [PEG-b-P(LA-co-MBC)], was synthesized in bulk by the ring-opening polymerization Of L-lactide with 2-methyl-2-benzoxycarbonyl-propylene carbonate (MBC) in the presence of poly(ethylene glycol) as a macroinitiator with diethyl zinc as a catalyst. The subsequent catalytic hydrogenation of PEG-b-P(LA-co-MBC) with palladium hydroxide on activated charcoal (20%) as a catalyst was carried out to obtain the corresponding linear copolymer poly(ethyleneglycol)-block-poly(L-lactide-co-2-methyl-2-carboxyl-propyle necarbonate) [PEG-b-P(LA-co-MCC)] with pendant carboxyl groups. DSC analysis indicated that the glass-transition temperature (T-g) of PEG-b-MA-co-MBQ decreased with increasing MBC content in the copolymer, and T-g of PEG-b-P(LA-co-MCC) was higher than that of the corresponding PEG-b-P(LA-co-MBC). The in vitro degradation rate of PEG-b-P(LA-co-MCC) in the presence of proteinase K was faster than that of PEG-b-P(LA-co-MBC), and the cytotoxicity of PEG-b-P(LA-co-MCC) to chondrocytes from human fetal arthrosis was lower than that of poly (L-lactide). (c) 2005 Wiley Periodicals, Inc.