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

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• 作者：Qianyi Wang ; Wuchao Zhao ; Jianghua He ; Yuetao ZhangEugene Y.-X. Chen
• 刊名：Macromolecules
• 出版年：2017
• 出版时间：January 10, 2017
• 年：2017
• 卷：50
• 期：1
• 页码：123-136
• 全文大小：737K
• ISSN：1520-5835

文摘

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 (M_w 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.