Isoselective ring-opening polymerization and asymmetric kinetic resolution polymerization of rac-lactide catalyzed by bifunctional iminophosphorane-thiourea/urea catalysts

摘要

A series of iminophosphorane-thiourea/urea bifunctional catalysts was 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 groups without undesired side reactions. The experiment data revealed that ROPs of rac-LA are a "controlled/living" process. The highest stereoselectivity (P-m) 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 a chain-end control mechanism when using rac/(R,R)-IPTU-1/IPU-1 as catalysts. Additionally, the ee values of -11% at 0 degrees C and -17% at -40 degrees C were achieved at about 50% conversion using (S)-IPU-2 as a chiral catalyst. The selectivity factor (s = k(L)/k(D)) of 1.6 indicated that a clear kinetic resolution occurred and the enantiomorphic site control mechanism was involved.