Mechanistic Study Boosts Knowledge Of Radical Polymerization

Reductive cleavage of alkyl halides to initiate atom-transfer radical polymerization, also called living radical polymerization, proceeds via a concerted process rather than a stepwise mechanism, according to a study by researchers at Italy’s University of Padova, Australian National University, and U.S.-based General Electric Power & Water (J. Am. Chem. Soc., DOI: 10.1021/ja110538b). Atom-transfer radical polymerization is a metal-mediated technique prized for producing polymers with uniform chains. The mechanism for polymerization initiation, however, is poorly understood, with debate in the field about whether alkyl halide initiators are cleaved into R∂ and X^– in a single step through a concerted mechanism or go stepwise through a radical anion intermediate. The researchers used theoretical and experimental techniques to study dissociative electron transfer to three alkyl bromide initiators. They found that the initiators cleave without forming an intermediate and concluded that activation involves an inner-sphere electron transfer between the alkyl bromide and a metal catalyst, with the bromine atom acting as a bridge to the metal. A better understanding of this polymerization mechanism should help promote better initiator and catalyst design, the researchers say.