Method of preparing glycolide

A method, comprising:

• a1) adding a 70 wt. % glycolic acid aqueous solution to a reactor, heating the glycolic acid aqueous solution to a temperature of between 100 and 110° C. at a heating rate of 1.0-1.5° C./min, and maintaining the temperature under atmosphere for 1.0-2.0 hours;
• b1) heating the glycolic acid aqueous solution from the temperature of between 100 and 110° C. to a temperature of between 140 and 150° C. at a heating rate of 1.0-1.5° C./min, and maintaining the temperature under atmosphere for 1.0-2.0 hours;
• c1) heating the glycolic acid aqueous solution from the temperature of between 140 and 150° C. to a temperature of between 155 and 165° C. at a heating rate of 1.0-1.5° C./min, and maintaining the temperature under atmosphere for 1.0-2.0 hours;
• d1) heating the glycolic acid aqueous solution from the temperature of between 155 and 165° C. to a temperature of between 200 and 210° C. at a heating rate of 1.0-1.5° C./min, decreasing a pressure of the glycolic acid aqueous solution from the atmosphere to an absolute pressure of between 90.0 and 95.0 torr at a decompression rate of 1.0-1.5 torr/min, and maintaining the temperature and pressure for 1.0-1.5 hours;
• e1) under the temperature of between 200 and 210° C., decreasing the pressure of the glycolic acid aqueous solution from the absolute pressure of between 90.0 and 95.0 torr to an absolute pressure of between 10.0 and 20.0 torr at a decompression rate of 1.0-1.5 torr/min, and maintaining the temperature and pressure for 1.0-2.0 hours, to yield an oligoglycolic acid;
• a2) adding a binary complex catalytic system to the oligoglycolic acid obtained in e1), to yield a reactant mixture;
• b2) reducing an absolute pressure of the reactant mixture from atmosphere to between 3.0 and 5.0 torr at a decompression rate of 1.0-1.5 torr/min;
• c2) heating the reactant mixture to a temperature of between 250 and 260° C. at a heating rate of 10-15° C./min, and maintaining the temperature for 20-30 minutes;
• d2) heating the reactant mixture from the temperature of between 250 and 260° C. to a temperature of between 270 and 275° C. at a heating rate of 1.0-1.5° C./min, and maintaining the temperature for 20-30 minutes;
• and
• e2) heating the reactant mixture from the temperature of between 270 and 275° C. to a temperature of between 280 and 285° C. at a heating rate of 1.0-1.5° C./min, and maintaining the temperature for 60-70 minutes, to yield a crude glycolide.