Vitamin B12 Derivative Enables Cobalt-Catalyzed Atom Transfer Radical Polymerization

Advances in controlled radical polymerizations by cobaltcomplexeshave primarily taken advantage of the reactivity of cobalt as a persistentradical to reversibly deactivate propagating chains by forming a carbon-cobaltbond. However, cobalt-mediated radical polymerizations require stoichiometricratios of a cobalt complex, deterring its utility in synthesizingwell-defined polymers. Here, we developed a strategy to use cobaltas a catalyst to control radical polymerizations via halogen atomtransfer with alkyl halide initiators. Using a modified, hydrophobicanalogue of vitamin B-12 (heptamethyl ester cobyrinate)as a cobalt precatalyst, we controlled the polymerization of acrylatemonomers. The polymerization efficiency of the cobalt catalyst wassignificantly improved by additional bromide anions, which enhancedthe deactivation of propagating radicals yielding polymers with dispersityvalues <1.2 using catalyst concentrations as low as 5 mol %. Weanticipate that the development of cobalt catalysis in atom transferradical polymerization will enable new opportunities in designingcatalytic systems for the controlled synthesis of polymers.