Bulk polytransesterification of L-lactic acid esters: An alternative route to synthesize poly(lactic acid)

The synthesis of poly(lactic acid) (PLA) by bulk polytransesterification of different L-lactic acid esters was successfully carried out. The experiments were performed in two steps: a first one at 170 degrees C, maximum temperature, and atmospheric pressure, to obtain a viscous oligomer; and a final step at 180 degrees C and under reduced pressure to improve the byproduct elimination and increase the polymerization degree. The ethyl L-lactate polytransesterification in the presence of the binary catalytic system Sb2O/Zn(O2CCH3)(2)center dot 2H2O allowed achieving the highest average molecular weight, 22,300 g mol-1. The yellowing of the melt polymer samples was recorded. The resulting materials were subjected to several characterization techniques: the chemical structure of PLA was confirmed by 1H-NMR, the molecular weight was determined by size exclusion chromatography, thermal behavior measured by differential scanning calorimetry and racemization extent by specific rotation measurements. A special attention was paid to the polymer optical purity, as it is of major importance to assess its quality and the significance of the synthesis route. To the best of our knowledge, there is no previous report dealing with the synthesis of PLA by bulk polytransesterification. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012