Boroxyl-mediated living radical polymerization and applications

This article discusses a new family of boroxyl-based living radical initiators and its applications in the preparation of sophisticated block and graft copolymers. The chemistry is centered on the living radical initiator (i.e., alkylperoxy-dialkylborane (C-O-O-BR₂) species), which is formed by mono-oxidation of an asymmetrical trialkylborane (-C-BR₂) with oxygen. In the presence of polar monomers (including acrylates and methacrylates), the C-O-O-BR₂ engages spontaneous hemolytic cleavage at ambient temperature to form an active alkoxyl radical (C-O*) and a "stable" boroxyl radical (*O-BR₂). The alkoxyl radical is active in initiating the polymerization of vinyl monomers, and the stable boroxyl radical forms a reversible bond with the propagating radical site to prevent undesirable termination reactions. The living polymerization was characterized by predictable polymer molecular weight, narrow molecular weight distribution, and the formation of telechelic polymers and block copolymers by sequential monomer addition. Furthermore, this living radical initiator system benefits from several unique features of trialkylborane, including (a) easy incorporation into polymer chains (chain ends or side chains) and (b) in situ auto-transformation to living radical initiators (C-O-O-BR₂). The combination provides a one-pot reaction process to prepare some sophisticated block and graft copolymers with very different polymer blocks and useful properties.