Formation and Interconversion of Organo-Cobalt Complexes in Reactions of Cobalt(II) Porphyrins with Cyanoalkyl Radicals and Vinyl Olefins

摘要

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 (Kd(333 K)) for organo-cobalt complexes formed in methyl acrylate polymerization ((TMP)Co-CH(CO_2CH_3)CH_2P) was estimated as 1.15 × 10~(-10) from analysis of the polymerization kinetics and ~1H NMR. The ratio of the rate constants (333 K) for the cyanoisopropyl radical (_¤C(CH_3)_2CN) 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 x 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_2CH_3)CH_2P) 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.