Living Ring-Opening Polymerization of Lactones by N‑Heterocyclic Olefin/Al(C6F5)3 Lewis Pairs: Structures of Intermediates, Kinetics, and Mechanism

The strong Lewis acid Al­(C6F5)3, in combination with a strong Lewis base N-heterocyclic olefin (NHO), cooperatively promotes the living ring-opening (co)­polymerization of lactones, represented by δ-valerolactone (δ-VL) and ε-caprolactone (ε-CL) in this study. Medium to high molecular weight linear (co)­polyesters (Mw up to 855 kg/mol) are achieved, and most of them exhibit narrow molecular weight distributions (Đ as low as 1.02). Detailed investigations into the structures of key reaction intermediates, kinetics, and polymer structures have led to a polymerization mechanism, in that initiation involves nucleophilic attack of the Al­(C6F5)3-activated monomer by NHO to form a structurally characterized zwitterionic, tetrahedral intermediate, followed by its ring-opening to generate active zwitterionic species. In the propagation cycle, this ring-opened zwitterionic species and its homologues attack the incoming monomer activated by Al­(C6F5)3 to generate the tetrahedral intermediate, followed by the rate-determining ring-opening step to regenerate the zwitterionic species that re-enters into the next chain propagation cycles. Owning to the living features and lack of transesterification side reactions possessed uniquely by this Lewis pair polymerization system, well-defined di- and triblock copolymers with narrow molecular weight distributions (Đ = 1.06–1.15) have been successfully synthesized using this method, regardless of the comonomer addition order.本研究中，强路易斯酸（Lewis acid）三(五氟苯基)铝[Al(C₆F₅)₃]与强路易斯碱氮杂环烯烃（N-heterocyclic olefin, NHO）形成的协同路易斯对，可催化以δ-戊内酯（δ-valerolactone, δ-VL）和ε-己内酯（ε-caprolactone, ε-CL）为代表的内酯的活性开环（共）聚合。该体系可获得中等至高分子量的线性（共）聚酯，其重均分子量$M_w$最高可达855 kg/mol，且多数产物的分子量分布宽度极窄，多分散性指数$Đ$最低仅为1.02。通过对关键反应中间体、反应动力学及聚合物结构的详细表征分析，本研究阐明了该聚合反应的完整机理：引发阶段由NHO对被Al(C₆F₅)₃活化的单体进行亲核进攻，形成经结构确证的两性离子型四面体中间体，随后该中间体开环生成具有催化活性的两性离子物种。在链增长循环中，该开环得到的两性离子物种及其同系物，可进攻被Al(C₆F₅)₃活化的单体以生成新的四面体中间体，随后经过决速步开环反应，再生出可进入下一轮链增长循环的两性离子物种。得益于该路易斯对聚合体系独有的活性聚合特征与无酯交换副反应的优势，无论共聚单体的添加顺序如何，本方法均可成功合成结构明确、分子量分布窄的二嵌段及三嵌段共聚物，其多分散性指数$Đ$介于1.06至1.15之间。