Method for preparing high molecular weight poly-L-lactic acid

A method for preparing poly(L-lactic acid), comprising:

• a) providing a biogenic guanidine (BG) as a catalyst and an acid salt of a trace metal as an activator (Act), and adding the catalyst, the activator, and L-lactide monomer (LLA) to a polymerization reactor;
• b) evacuating and purging the polymerization reactor with nitrogen three times to remove air, evacuating the polymerization reactor and conducting bulk polymerization of the L-lactide monomer under vacuum;
• wherein:
• the trace metal is selected from a group consisting of potassium (K), iron (Fe), zinc (Zn), and calcium (Ca);
• the bulk polymerization comprises a first reaction stage and a second reaction stage;
• in the first reaction stage, a reaction temperature is between 125 and 140° C., a reaction pressure is between 0.4 and 0.6 torr, a reaction time is between 16 and 24 hours, and a resulting product is a medium molecular weight poly-L-lactic acid (m-PLLA) having a weight average molecular weight (Mw) of between 4.0×104 and 5.0×104;
• in the second reaction stage, a reaction temperature is between 140 and 160° C., a reaction pressure is between 0.1 and 0.3 torr, a reaction time is between 25 and 60 hours, and a final product is a high molecular weight poly-L-lactic acid (h-PLLA) having a weight average molecular weight of between 4.0×105 and 5.5×105;
• the first reaction stage of the bulk polymerization is represented by equation I:
• the second reaction stage of bulk polymerization is represented by equation II: