Efficient synthesis of highly optically pure l-lactide via cyclodepolymerization of poly(l-lactic acid)

l -Lactide ( L -LA) is an important intermediate for the industrial production of poly( l -lactic acid) (PLLA). However, it remains a challenge to afford L -LA in high yield, high efficiency, and high optical purity. Here, we report an efficient catalytic cyclodepolymerization (CDP) of PLLA under relatively mild temperatures (≤180 °C) and high vacuum to afford L -LA with high optical purity. Sixteen commercial metal and organic base catalysts are employed for CDP. For most metal catalysts, efficient PLLA CDP for L -LA with a high yield exceeding 90% can be achieved within 4 h. The crude L -LA is composed of >95% L -LA, which displays a high optical purity with a high ee % value over 99% after recrystallization. Various influencing factors on CDP are carefully investigated. Our results reveal that increasing the depolymerization temperature, time and catalyst loading can significantly boost the yield of crude L -LA, but excessive temperature and high catalyst loading tend to deteriorate the purity of L -LA. Notably, the recycled catalyst can be reused repeatedly for CDP with no evident loss of catalytic efficiency and stereoselectivity. The CDP of PLLA for L -LA follows an intramolecular backbiting mechanism, which accompanies with the occurrence of intermolecular transesterification. Moreover, the obtained high-purity L -LA can be further used to fabricate PLLA with high molecular weight and good crystallinity.