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(CO2CH3)CH2P) was estimated as 1.15 x 10(-10) from analysis of the polymerization kinetics and H-1 NMR. The ratio of the rate constants (333 K) for the cyanoisopropyl radical (center dot C(CH3)(2)CN) adding with monomer (k(1)) to the process of transferring a hydrogen atom to (TMP)Co-II center dot(k(2)) was evaluated for the methyl acrylate system as 2 x 10(-3) which is larger than that for vinyl acetate LRP (9 x 10(-5)). Kinetic analysis places the rate constant for associative radical interchange (333 K) at similar to 7 x 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 (center dot CH(CO2CH3)CH2P) 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.