Boroxyl-based radical initiators and polymerization

This paper discusses a new family of living free radical initiators, alkylperoxy-dialkylborane (C-O-O-BR₂), which show living polymerization of acrylate and methacrylate monomers at ambient temperature. The C-O-O-BR₂ species can be prepared by in situ selective mono-oxidation of an asymmetrical trialkylborane (-C-BR₂) with a control amount of oxygen. Apparently, in the presence of polar monomers the C-O-O-BR₂ engages spontaneously hemolytic cleavage to form active alkoxyl radical (C-O*) and "stable" boroxyl radical (*O-BR₂), due to the delocalization of the free radical with the empty p-orbital of boron. The alkoxyl radical is active in initiating the polymerization of vinyl monomers. On the other hand, the stable borinate radical may form 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.