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 delta-valerolactone (delta-VL) and epsilon-caprolactone (epsilon-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 ((D) over bar 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 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 weight distributions ((D) over bar = 1.06-1.15) have been successfully synthesized using this method, regardless of the comonomer addition order.