Poly(lactic acid) Degradation into Methyl Lactate Catalyzed by a Well-Defined Zn(II) Complex

Poly(lactic acid) (PLA) was degraded to methyl lactate (Me-La) by an imino monophenolate Zn(1)2 catalyst in the presence of tetrahydrofuran, as the solvent, and methanol, as the protic source. As well as solution-based polymerization and degradation, catalyst stability was assessed and discussed. The chemical degradation of four different commercial samples of PLA, varying in molecular weight, was studied. The effect of PLA concentration (0.05–0.2 g mL–1), reaction temperature (40–130 °C), and catalyst concentration (4–16 wt %) on conversion, yield, and selectivity were studied and results statistically analyzed. Mass-transfer limitations were assessed by utilizing two different PLA particle sizes and altering the stirring speed. Results revealed that the main variables affecting PLA degradation are temperature and catalyst concentration. It was possible to observe Me-La formation even at 40 °C, although the reaction times were significantly longer when compared to the highest temperatures. Conversions of 100%, as determined by 1H NMR spectroscopy and gel permeation chromatography, were possible in short times (<15 min) depending on temperature and catalyst concentration. A reaction mechanism for the production of Me-La from PLA, which considers the formation of chain-end groups as intermediates is presented and values for the kinetic constants are determined from the model. The activation energy for the initial degradation step was in the range 39–65 kJ mol–1, decreasing with increasing catalyst loading.