Efficient Synthesis of Lactide with Low Racemization Catalyzed by Sodium Bicarbonate and Zinc Lactate

Abstract

Lactide is an important monomer in the industrial production of PLA, which is one of the materials with the greatest potential for replacing petroleum-based polymers. In view of the low yields of lactide when synthesized by metal catalysts, as well as the low purity and racemization seen in recently reported catalytic systems, we have developed a new catalytic method—a complex system of Zn(La)2 and NaHCO3 that offers both high yield and high purity. The yield of lactide reached above 95.63% with a purity up to 97.86% in only 3.25 h. Furthermore, the effects of the catalyst loading and the addition time of the catalyst were studied to optimize the yield of lactide and deduce a reasonable reaction mechanism in conjunction with NMR analysis. The reaction of NaHCO3 and the oligomer is thought to generate carboxylate anions, which would dramatically inhibit further polymerization. Thus, the bond between ester oxygen and methyne carbon was broken due to the attack of the carboxylate anion on the asymmetrical carbon atom, and the ester bond was broken due to the attack of the hydroxyl group on the carbonyl carbon, which results in the decrease of the molecular weight of the residual oligomers. As a result, the rate of lactide production increased rapidly as the molecular weight of the polymer decreased.