Abstract
Observation of the formation and interconversion of organo-cobalt complexes ((TMP)Co-R) is used to reveal mechanistic features in the living radical polymerization (LRP) of methyl acrylate (MA) mediated by cobalt porphyrins. Both dissociative and associative exchange of radicals in solution with organo-cobalt complexes contribute to controlling the radical polymerization. The sequence of organo-cobalt species formed during the induction period for the (TMP)Co-R mediated LRP of MA indicates that homolytic dissociation is a prominent pathway for the interconversion of organo-cobalt complexes which contrasts with the corresponding vinyl acetate (VAc) system where associative radical exchange totally dominates these processes. The dissociation equilibrium constant (K d(333 K) ) for organo-cobalt complexes formed in methyl acrylate polymerization ((TMP)Co-CH(CO 2 CH3)CH 2 P) was estimated as 1.15 × 10 -10 from analysis of the polymerization kinetics and 1 H NMR. The ratio of the rate constants (333 K) for the cyanoisopropyl radical (·C(CH 3 ) 2 CN) adding with monomer (k 1 ) to the process of transferring a hydrogen atom to (TMP)Co II · (k 2 ) was evaluated for the methyl acrylate system as 2 × 10 -3 which is larger than that for vinyl acetate LRP (9 × 10 -5 ). Kinetic analysis places the rate constant for associative radical interchange (333 K) at ∼7 × 10 5 M -1 s -1 . The larger radical stabilization energy and lower energy of the singly occupied molecular orbital (SOMO) for methyl acrylate based radicals (·CH(CO 2 CH 3 )CH 2 P) compared to vinyl acetate contribute to the observed prominence of organo-cobalt homolytic dissociation and much smaller chain transfer which result in substantially better control for living radical polymerization of methyl acrylate than that observed for vinyl acetate. © 2009 American Chemical Society.