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

• Published in: New journal of chemistry Vol. 44; no. 4; pp. 1648 - 1655
• Main Authors: Lv, Chengdong, Zhou, Li, Yuan, Ruiting, Mahmood, Qaiser, Xu, Guangqiang, Wang, Qinggang
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 2020
• Subjects: Catalysts; Chemical synthesis; Molecular weight; Molecular weight distribution; Polylactic acid; Polymerization; Ring opening polymerization; Selectivity; Stereoselectivity; Thioureas; Ureas
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/C9NJ05074D

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 °C and −17% at −40 °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.