Living Polymerization Of Momoners Catalyzed By N-Heterocyclic Olefin-based Frustrated Lewis Pairs

The field of the“frustrated Lewis pair”?FLP?chemistry has continued to receive sustained intense interest ever since the seminal work reported by Stephan and Erker.The application of FLPs has now been well established in the small molecule chemistry,such as activation of small molecules,catalytic hydrogenation reactions,and new reactivity/reaction developments.In the area of macromolecular synthesis promoted by a Lewis pair?LP?,since the report on the polymerization by FLPs has also attracted increasing attention.However,there are still several undressed key issues in the polymerization of lactones and polar monomers by LPs.For the ring-opening polymerization of lactones,1)the degree of polymer control needs significant improvement,2)the molecular weight of the polyester is still limited,3)detailed mechanistic studies of this polymerization are lacking.For the polymerization of polar olefins,1)low control of polymerization,2)the initiation efficiency is very low,2)chain termination side reactions easily occur.In addition,neither of them has developed an efficient living polymerization system,so this paper attempts to solve the above difficulties.Conclusions are summarized as follows:In Chapter ?,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?-valerolactone??-VL?and?-caprolactone??-CL?in this study,achieving medium to high molecular weight linear?co?polyesters,with M_w up to 855 kg/mol and narrow molecular weight distributions??as low as 1.02?.Detailed investigations into the structures of key reaction intermediates,kinetics,and polymer structures have led to the polymerization mechanism,in that initiation involves nucleophilic attack of 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 homologs attack the incoming Al?C₆F₅?₃activated monomer to generate the tetrahedral intermediate,followed by the rate-determining ring-opening step to regenerate the zwitterionic species that reenters 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 distributin??=1.06–1.15?have been successfully synthesized using this method,regardless of the comonomer addition order.In Chapter ?,the living polymerization of conjugated polar alkenes such as methacrylates by a non-interacting,authentic frustrated Lewis pair?FLP?has remained elusive ever since the report on the FLP-promoted polymerization in 2010.Here we report that the polymerization of alkyl methacrylates by a FLP system based on strongly nucleophilic NHO Lewis base and sterically encumbered,but modestly strong Lewis acid MeAl?4-Me-2,6-^tBu₂-C₆H₂O?₂ is not only rapid but also living.This living polymerization was indicated by the formation of a linear,living chain,capped with NHO/H chain-ends,without backbiting-derived cyclic chain-ends.The true livingness of this FLP-promoted polymerization has been unequivocally verified by five lines of evidence,including predicted polymer number-average molecular weight(M_n,up to 351kg·mol^-1)coupled with low dispersity?D=1.05?values;obtained high to quantitative initiation efficiencies;observed linear increase of polymer M_n vs.monomer conversion and monomer-to-initiator ratio;found precision in multiple chain extensions;and formed well-defined diblock and ABA triblock copolymers with narrow molecular weight distributions??=1.09–1.13?,regardless of the comonomer addition order.