Synthesis of polylactic acid by direct polycondensation under vacuum without catalysts, solvents and initiators

In order to reduce the production cost of polylactic acid (PLA), L-lactic acid was polymerized by direct polycondensation (DP) under vacuum without catalysts, solvents and initiators. Experiments were conducted at polymerization temperatures (T-p) of 150-250 degrees C. The maximum PLA molecular weight obtained was 90 kDa at 200 degrees C after 89 h under vacuum. Above 200 degrees C, PLA is thermally degraded by specific scission. The DP activation energy (E-a) was also investigated and was found to be larger than that required by catalyzed ring opening polymerization (ROP). Furthermore, the E-a was higher than the deactivation energy (E-d) for both DP and ROP. PLA yields were higher at lower T-p, while the yield of lactide increased with T-p. The total yield of PLA and lactide was approximately 52-75 wt.%. The uncatalyzed DP method may have applications in on-site cell plant production, where compactness is required, as well as safe and simple operating procedures. (C) 2009 Elsevier B.V. All rights reserved.