Improved preparation of D,L-lactide from D,L-lactic acid using microwave irradiation

D,L-Lactide was effectively synthesized by a two-step reaction under microwave irradiation, namely, the polycondensation of D,L-lactic acid into oligo(D,L-lactic acid) (ODLLA) and the unzipping depolymerization of the ODLLA into D,L-lactide. The influences of microwave irradiation power, irradiation time, catalyst type, and catalyst concentration on the polycondensation and the unzipping depolymerization were investigated, respectively. The results revealed that both polycondensation and unzipping reaction were remarkably accelerated and improved by microwave energy, and the reaction time for preparation of D,L-lactide catalyzed by zinc powder was shortened by at least 50% compared with conventional heating method. The yield of D,L-lactide by microwave method was 40.3% starting from aqueous D,L-lactic acid when 1.2% (w/w) of zinc powder was applied as a catalyst and the microwave time was 3.2 hours. An alternative protocol with advantages of fast reaction and a high yield was developed for the preparation of D,L-lactide from aqueous D,L-lactic acid.