This paper describes the synthesis and the biodegradation of optically active copolymers, poly[(R)-1-methyltrimethylene carbonate-co-epsilon-caprolactone] and poly[(S)-1-methyltrimethylene carbonate-co-epsilon-caprolactone] and poly[(RP)-1,3-dimethyltrimethylene carbonate-co-epsilon-caprolactone] and poly[(S,S)-1,3-dimethyltrimethylene carbonate-co-epsilon-caprolactone]. The copolymers were prepared using a novel organolanthanide or AlEt3-H2O as the polymerization initiators, and their biodegradation by various; enzymes and acclimated activated sludges was studied as a function of their composition, stereochemistry, crystallinity, T-g (glass transition temperature), T-m (melting point), molecular weight, and polydispersity. The copolymers＇ synthesized exhibited high molecular weights with rather narrow molecular weight distributions and produced thermoplastic films when (R)-1-MTC (1-MTC = 1-methyltrimethylene carbonate) or (R,R)-1,3-DTC (1,3-DTC = 1,3-dimethyltrimethylene carbonate) content is less than 50 mol %. Optically active and racemic copolymers prepared using 1-MTC/CL (CL = epsilon-caprolactone) ratios of 17/83 to 19/81 were effectively biodegraded by lipoprotein lipase, cholesterol esterase and activated sludge. For the copolymers prepared using various 1,3-DTC/CL ratios, the poly(rac-1,3-DTC-co-CL) was biodegraded faster than the poly[(R,R)-1,3-DTC-co-CL] and poly[(S,S)-1,3-DTC-co-CL] copolymers regardless of the 1,3-DTC/CL ratio. Biodegradation of all of these copolymers generated numerous cavities on the outermost surface of polymer films or solid masses without changing their molecular weight and polydispersity.