Formation, dissociation, and radical exchange of organo-cobalt complexes in mediating living radical polymerization

Reactions of organic radicals (•R) with cobalt(II) metallo-radicals (Co•) and organo-cobalt complexes (Co-R′) have a central role in several pathways that produce control for radical polymerizations. Interactions of organic radicals with cobalt(II) metallo-radical produce a cobalt hydride (Co-H) via the β-hydrogen abstraction. Subsequent addition of the cobalt hydride with the olefin monomers to produce organometallic complexes (Co-R′) can give catalytic chain transfer. Reversible homolysis of the organo-metal bond in the absence of β-H transfer gives the living radical polymerization (LRP) by a reversible termination (RT) mechanism. When the exchange of freely diffusing radicals in solution with the dormant organic units in organometallic complexes is fast, a LRP can occur by a degenerative transfer (DT) mechanism. Methyl acrylate (MA) and vinyl acetate (VAc) polymerization mediated by cobalt porphyrin complexes are used to illustrate the features of these two LRP pathways. Kinetic and thermodynamic properties for several of the central reactions are evaluated by following the time evolution of the 1H NMR for organo-cobalt porphyrin complexes.