Dissociative electron transfer to activated alkyl halide initiators relevant to living radical polymerization

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX•- through an outer sphere electron transfer (OSET) process. These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C–X bond dissociation. Here it is clearly shown that injection of one electron into RX produces only a weakly associated charge induced donor acceptor type radical anion complex without any significant covalent sigma-type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited. The comparison of the experimental activation free energies with theoretically computed values shows a close agreement between theory and experiment. The agreement with experimental data, which fit very well the sticky model, is a relevant support for the appropriateness of this simplified model, which considers only the ion-dipole interactions.