Influence of decomposition temperature of aromatic sulfonic acid catalysts on the molecular weight and thermal stability of poly(l-lactic acid) prepared by melt/solid state polycondenstaion

Abstract

Melt/solid polycondensation (MP/SSP) of poly(l-lactic acid) (PLLA) was conducted by using several aromatic sulfonic acids as the catalysts. It was clarified that the increase in molecular weight and the thermal stability of the obtained polymer are correlated with the 5% weight loss temperatures (Td,5%) of the sulfonic acids determined by thermal gravimetric analysis. When 2,5-dimethyl-, 2,4-dimethyl- and 4-methyl-benzenesulfonic acids showing Td,5% around 150 °C were used as the catalysts, PLLA polymers with high molecular weight and excellent thermal stability could be obtained. On the other hand, 2,4,6-trimethylbenzenesulfonic acid showing Td,5% around 100 °C and 4-dodecylbenzenesulfonic acid with Td,5% above 200 °C gave PLLA polymers with remarkably low thermal stability. The residual catalyst involved in the product polymer or SO3 generated during the polymerization decreased the thermal stability of the polymer. The oligomeric fractions of the PLLA products were extracted with acetone and analyzed by 1H NMR and mass spectra. It was verified that the catalyst had efficiently been removed by thermal annealing while it remained active during the SSP to produce a thermally stable PLLA with high molecular weight.