Isoselective Ring-Opening Polymerization and Asymmetric Kinetic Resolution Polymerization of rac-Lactide Catalyzed by Bifunctional Iminophosphorane-Thiourea/Urea Catalysts

A series of iminophosphorane-thiourea/urea bifunctional catalysts were synthesized and utilized for the isoselective ring-opening polymerization of rac-lactide. The ROPs were promoted efficiently, affording the polylactides with controlled molecular weight, narrow molecular weight distribution and well-defined end group without undesired side reactions. The experiment dates revealed that ROPs of rac-LA are “controlled/living” progress. The highest stereoselectivity (Pm) was up to 0.80 using rac-IPU-1 under mild reaction conditions. The mechanistic study indicated that stereoselective ROPs of rac-LA were mainly controlled by chain-end mechanism when use rac/(R, R)-IPTU-1/IPU-1 as catalysts. Additionally, The ee values of -11% at 0 °C and -17% at -40 °C were achieved at about 50% conversion using the (S)-IPU-2 as chiral catalyst. The selectivity factor (s = kL/kD) of 1.6 indicated that a clear kinetic resolution occurred and the enantiomorphic site control mechanism was involved.