Method for preparing block copolymer

A method for preparing a block copolymer comprising the steps of:

• (a) biosynthesizing a poly (3-hydroxypropionate) initiator;
• and
• (b) subjecting a lactide monomer to a ring-opening polymerization in the presence of the poly (3-hydroxypropionate) initiator prepared in step (a) to prepare a polylactide-poly (3-hydroxypropionate) block copolymer, wherein the ring-opening polymerization is performed at a temperature of 150° C. to 180° C. for 5 minutes to 20 minutes;
• wherein the method further comprises a step of hydrolyzing the poly (3-hydroxypropionate) initiator prepared in the step (a), before performing the step (b), wherein the hydrolysis is performed for 4 to 80 hours at room temperature to 200° C.,
• wherein the poly (3-hydroxypropionate) initiator has a weight average molecular weight of 1,500 to 200,000 g/mol.