the block copolymer of polyesters and polycarbonates comprises at least one of each of the constituting units of and having the general formulae : ## str1 ## where the sign * indicates an asymmetric carbon atom ; r 1 , r 2 , r 3 and r 4 are either a hydrogen atom or a methyl group ; m and n are respective a natural numbers ranging from 300 to 5000 : the block copolymer is obtained by successive ring - opening and copolymerization of optically active β - butyrolactones and cyclic carbonates in the presence of a catalyst . the block polymer obtained has high molecular weight and high melting temperature , and can thus be used as a high strength material for practical use and which has bio - degradable and hydrolyzable nature .