Synthesis of high molecular weight poly(L -lactic acid) via melt/solid polycondensation: Intensification of dehydration and oligomerization during melt polycondensation

摘要

Melt/solid state polycondensation (MP/SSP) is a cost-effective route for synthesis of high molecular weight poly(L-lactic acid) (PLLA). However, the reaction rates in its four stages need to be enhanced greatly and the reaction times to be shortened largely before the MP/SSP technology can be industrialized. In this study, a new catalyst addition policy, i.e., adding TSA at the dehydration stage and SnCl_2 ·2H_2O at the MP stage, and more appropriate temperature and pressure programs were presented and applied in the MP process of LLA. The presence of TSA from dehydration appeared very effective for speeding up the dehydration and oligomerization stages as well as depressing racemization in the whole MP process. The polymerization degree (X_n) of oligomer was clearly increased, and the reaction time was shortened to a great extent. Direct using reduced pressure was also very helpful for intensifying the dehydration stage, only leading to LLA loss as little as 2%. A PLLA with M_w of 44,000 and optical purity of 96.8% suitable for subsequent SSP was produced after dehydration for 2 h, oligomerization for 2 h and MP for 4 h under appropriate conditions. And an interesting strong dependence of the M_w of final PLLA product on the X_n of the oligomer was observed.