Melt polycondensation of L-lactic acid catalyzed by metal complexes bearing benzotriazole derivatives.

Direct melt polycondensation (MP) of L-lactic acid provides a low-cost approach for the production of poly(L-lactic acid) (PLLA). However, traditional MP process faces challenges of low catalyst efficiency and potential biotoxicity of the remaining catalysts. Herein, benzotriazole-based metal complexes of nontoxic metals (titanium, aluminum, magnesium, and iron) were synthesized under mild condition, and their catalytic performance in the MP of L-lactic acid was evaluated. Ti-based complexes supported by methyl-benzotriazole and tert-amyl-benzotriazole have excellent catalytic efficiency with PLLA of weight-average molecular weight of 29,000 g/mol obtained within 9 h, superior to traditional Ti-based catalysts and most other single-component catalysts. Simulation shows that the Ti-based complex can effectively activate the reaction by its Lewis acidity and decrease the reaction energy barrier via forming metastable intermediate products. The as-synthesized PLLA exhibits good thermal stability, low cell toxicity and good blood compatibility, demonstrating its potential for biomedical applications. This work presents a high efficiency, low-toxicity Ti-based catalyst for environmentally benign and efficient production of high-molecular weight PLLA.