Process for the preparation of L-lactide of high chemical yield and optical purity

A process for the preparation of 100% optically pure L(+)-lactide, said process comprising the steps of:

• a. charging L(+)-lactic acid in a reactor, subjecting it to melt polymerization in consecutive steps, initially at 150° C. to 160° C. for 2 hours to 3 hours under a nitrogen atmosphere, then at a temperature of 150° C. to 160° C. for 1.5 hours to 2 hours at 100-110 mm Hg, then at 50° C. to 160° C. for 1.5 hours to 2 hours at 30-40 mm Hg, and finally at 150° C. to 160° C. for 1 hour to 1.5 hours at 0.01 to 0.1 mm Hg with continuous stirring at 60-70 RPM to obtain a viscous oligomer;
• b. cooling the viscous oligomer obtained in step (a) to a temperature of between 25° C. to 30° C. under nitrogen flow;
• c. adding a tin metal catalyst having a particle size of less than 150 microns or a zinc metal catalyst having a particle size of less than 10 microns to the viscous oligomer obtained in step (b), in an amount which is 0.1 wt % to 0.5 wt % of the weight of the viscous oligomer in the reactor to form a mixture;
• d. deploymerizing the mixture of step (c) in consecutive steps, initially at 160° C. to 170° C. for 1 hour to 1.5 hours under nitrogen atmosphere, then at 170° C. to 180° C. for 1 hour to 1.5 hours at 100-110 mm Hg, then at 180° C. to 190° C. for 1 hour to 1.5 hours at 10 to 20 mm Hg, and finally at 190° C. to 200° C. for 2 hours to 2.5 hours at 0.01 to 0.1 mm Hg to obtain a crude lactide;
• e. purifying the crude lactide obtained in step (d) by crystallization from a solvent to obtain pure L(+) lactide.