反应详情

The PLA resin can be formed by polymerizing lactide. Lactide is a dimeric form of lactic acid, in which two lactic acid molecules are condensed to form a cyclic diester. Like lactic acid, lactide exists in a variety of enantiomeric forms, i.e., “L-lactide”, which is formed from two L-lactic acid molecules, “D-lactide”, which is formed from two D-lactic acid molecules, and “meso-lactide”, which is formed from one L-lactic acid molecule and one D-lactic acid molecule. In addition, 50/50 mixtures of L-lactide and D-lactide that have a melting temperature of about 126° C. are often referred to as “D,L-lactide”. Any of these forms of lactide, or mixtures thereof, can be polymerized to form a PLA resin for use in this invention. The L/D ratio in the PLA resin is controlled through the ratio of these enantiomeric forms of lactide that are used in the polymerization. In an especially preferred process, mixtures of L-lactide and meso-lactide are polymerized to form a polymer having a controlled level of D-lactic acid enantiomeric units. Suitable processes for polymerizing lactide to form PLA having controlled L/D ratios are described, for example, in U.S. Pat. Nos. 5,142,023 and 5,247,059, both incorporated herein by reference.