Main observation and conclusionrnRing-opening polymerization (ROP) of cyclic esters in the presence of stannous octoate (Sn(Oct)2) is the main way to obtain biode-gradable aliphatic polyesters,an important family of biodegradable polymers which have been widely used and still rapidly developed in the fields of biomedical polymers and environment-friendly materials.The underlying mechanism is thought via a coordina-tion-insertion way,but the pathway is still open owing to the absence of direct experimental evidence.Herein,we inquire this issue through density functional theory (DFT) calculations.According to our DFT calculations and the following Curtin-Hammett evaluation,the carbonyl oxygen has a significant advantage over the ester oxygen,and thus the ring is opened mainly through pathway A instead of pathway B.The stannous octoate is identified as a catalyst rather than an initiator.We eventually summarize the main stages dur-ing the whole polymerization of lactide.