Living Radical Polymerizations Mediated by Metallo-Radical and Organo-Transition Metal Complexes

Cobalt porphyrin derivatives are observed to participate in the control of living radical polymerization (LRP) by two distinctly different mechanisms. Cobalt (II) porphyrin (Co(II)) metallo-radicals mediate a LRP by reacting with polymeric radical (P·) to produce a quasi-equilibrium with an organometallic complex (Co-P) which gives living character through the persistent radical effect. Diamagnetic organo-cobalt complexes in combination with an additional radical source such as AIBN produces an alternate form of LRP. The rates of polymerization by the organo-cobalt mediated route are relatively fast and approach that of regular radical polymerization because the radical concentration is determined by the organic radical source. The cobalt (II) metallo-radical concentration is vanishingly small in the organo-cobalt mediated process which removes the need for ligands that use large steric requirements to suppress cobalt (II) catalyzed chain transfer. Radical polymerizations of alkyl acrylates, acrylic acid and vinyl acetate mediated by the organo-cobalt porphyrin route are used in illustrating the principal mechanistic features of this process.