Room temperature nickel-catalyzed reductive coupling of end-brominated polystyrene chains

Facile, room-temperature coupling of brominated polymer chains can be accomplished in high yields (similar to 80%) using a nickel-based catalytic cycle that also includes magnesium and zinc. The extent of coupling (X-c) was found to be similarly high when performed under vacuum or with a nitrogen purge, although its effectiveness varied significantly as a function of the solvent, reagents, and the structure of the polymer chain end. Kinetic studies show that even at room temperature, nearly all coupling occurs within the first similar to 2 h of the reaction when performed under nitrogen or vacuum, with accompanying color changes occurring in the catalytic system coinciding with catalytic activity. One step of the mechanistic cycle is proposed to occur via chain-end radical intermediates inserted into the catalytic cycle, consistent with the addition of a radical trap essentially thwarting the dimerization by capturing the polymer radical and preventing its inclusion in the reaction. The presence of aryl bromide functional groups, however, does not impede the coupling reaction, demonstrating the catalyst's preference for the chain-end alkyl bromides under our conditions. The impact and necessity of the other components were also explored, supporting the importance of the MgCl2 serving as a Lewis acid to promote the reactivity of the C-Br end groups.