(722be) A Two-Step Process for the Synthesis of Poly(L-lactic acid)

Poly(L-lactic acid) (PLLA) is a biodegradable polymer that can be synthesized by ring-opening polymerization of lactide of by direct polycondensation of L-lactic acid. High molecular weight PLLA can be obtained by ring-opening polymerization; however, the process requires a production of high purity lactide and the overall process is complex and costly. Direct polycondensation converts L-lactic acid to PLLA via step-growth polymerization but high molecular weight PLLA is difficult to obtain. In this work, we investigate a two-step direct polycondensation of lactic acid. The first step consists of dehydration of aqueous lactic acid and the prepolymerization at low pressure or by gas purging. The prepolymer is then solid-state polymerized to increase it molecular weight. The polymerization conditions that affect the performance of the polymerization include reaction temperature, catalyst concentration, and water removal efficiency. Discoloration of the reaction mixture at high catalyst concentration is also a problem to be resolved. The polymerization kinetics and the effect of condensate removal by diffusional mass transfer are investigated to develop a quantitative understanding of the polymerization of lactic acid.