Method for producing polylactic acid block copolymer

A method of producing a polylactic acid block copolymer comprising a segment(s) composed of L-lactic acid units and a segment(s) composed of D-lactic acid units, the method comprising:

• obtaining a mixture by mixing poly-L-lactic acid and poly-D-lactic acid one of which has a weight average molecular weight of 60,000 to 300,000 and the other of which has a weight average molecular weight of 10,000 to 50,000, said mixture having a weight average molecular weight of not less than 90,000 and a degree of stereocomplexation (Sc) satisfying Inequality (1):
• Sc=ΔHh/(ΔHl+ΔHh)×100>60 (1)
• (wherein ΔHh represents heat of fusion of stereocomplex crystals (J/g), and ΔHl represents heat of fusion of crystals of poly-L-lactic acid alone and crystals of poly-D-lactic acid alone (J/g));
• and
• subsequently subjecting said mixture to solid-phase polymerization at a temperature lower than the melting point of said mixture.

A method of producing a polylactic acid block copolymer, the method comprising:

• obtaining a mixture by mixing poly-L-lactic acid and poly-D-lactic acid in which a ratio between weight average molecular weight of the polylactic acid having a higher weight average molecular weight and weight average molecular weight of the polylactic acid having a lower weight average molecular weight is not less than 2 and less than 30, said mixture having a weight average molecular weight of not less than 90,000 and a degree of stereocomplexation (Sc) satisfying Inequality (1):
• Sc=ΔHh/(ΔHl+ΔHh)×100>60 (1)
• (wherein ΔHh represents heat of fusion of stereocomplex crystals (J/g), and ΔHl represents heat of fusion of crystals of poly-L-lactic acid alone and crystals of poly-D-lactic acid alone (J/g));
• and
• subsequently subjecting said mixture to solid-phase polymerization at a temperature lower than the melting point of said mixture.