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 °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 ( 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.