Chain extension of poly(L-lactic acid) (PLLA) with unsaturated groups (PLBM) was attempted using benzoyl peroxide (BPO) and the resulting variation in molecular weight and mechanical properties was explored. Bulk copolymerization Of L-lactic acid (LA)/1,4-butanediol (BD)/maleic acid (MA) (100/1/1) isomerized some of the cis-structured maleate units into trans-structured fumarate units. The optically active LA promoted isomerization during the condensation polymerization. Chain extension of PLBM with BPO did not bring about a discernible increase in the molecular weight when the chain extension was carried out in various solvents with different radical abstraction abilities. In contrast, the hot pressing of PLBM containing BPO in-creased the molecular weight and sometimes produced chloroform-insoluble gels depending on the BPO concentration and temperature. The chain extension at low temperatures increased the flexibility of PLBM considerably. However, PLBM lost the flexibility precipitously as the chain-extension temperature increased above 120degreesC. The biodegradation rate of PLBM was much slower than that of PLLA. The biodegradation rate was further lowered by the chain extension. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1802-1807, 2003.