Melt/solid polycondensation of l-lactic acid: an alternative route to poly(l-lactic acid) with high molecular weight

A high polymer of poly(l-lactic acid)(PLLA) is successfully obtained by the melt/solid polycondensation of l-lactic acid (LA) catalyzed by a tin chloride dihydrate/p-toluenesulfonic acid binary system. In this process, a polycondensate with a molecular weight of 20,000 Da is first prepared by ordinary melt-polycondensation, crystallized by heat-treatment around 105°C, and heated at 140 or 150°C for 10–30 h for further polycondensation. A high-quality polymer of PLLA can be obtained in high yield in a relatively short reaction time and its molecular weight exceeds 500,000 Da which is comparable with that of the PLLA obtained by the lactide method but has never been attained by the simple melt-polycondensation.