Nickel-Catalyzed Reductive Homodimerization of Brominated Polystyrene Chains

A nickel-based catalytic system leading to high yields (approximate to 80%) of polymer chain coupling has been developed. The extent of coupling (X-c) is found to be relatively high (>75%) and consistent across a wide range of catalyst loadings (0.1 - 1.0 equivalent with respect to the chain end), comparing favorably with other chain-end joining methods. A mechanistic cycle is proposed to occur with benzylic chain-end radical intermediates, consistent with the addition of a nitroso radical trap substantially lowering coupling yields. The effect of the polymer chain end (Cl vs Br) and the halide associated with the nickel catalyst is also probed.