Synthesis and properties of high-molecular-weight poly(L-lactic acid) by melt/solid polycondensation under different reaction conditions

The melt/solid polycondensation of L-lactic acid (LA) was successfully conducted using a binary catalyst system comprising tin dichloride hydrate and p-toluenesulfonic acid (TSA). First, a thermal oligocondensate of LA with a degree of polymerization of eight was mixed with the binary catalyst and subjected to melt polycondensation at 180 degreesC for 5 h to obtain a melt polycondensate with an average molecular weight of 13 000 Da. This melt polycondensate was then heat treated at 105 degreesC for 2 h and subjected to solid-state polycondensation under various conditions to obtain a high-molecular-weight polymer of poly(L-lactic acid) (PLLA). In a typical case, the solid-state reaction was performed by raising the temperature from 130 to 155 degreesC, or at a constant temperature of 140 or 150 degreesC, In the former case the molecular weight of PLLA increased linearly with the temperature increase, while in the latter case the molecular weight increased above 500 000 Da in a relatively short reaction time. The crystallization of the polycondensate was a key step of this polycondensation and the crystallinity of the PLLA product was well correlated with the increase in the molecular weight. This method can be a direct industrial method for the manufacture of PLLA from LA, which may potentially be an alternative to the currently adopted lactide method.