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 (Tp) of 150–250 °C. The maximum PLA molecular weight obtained was 90 kDa at 200 °C after 89 h under vacuum. Above 200 °C, PLA is thermally degraded by specific scission. The DP activation energy (Ea) was also investigated and was found to be larger than that required by catalyzed ring opening polymerization (ROP). Furthermore, the Ea was higher than the deactivation energy (Ed) for both DP and ROP. PLA yields were higher at lower Tp, while the yield of lactide increased with Tp. 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.