Formation and interconversion of organo-cobalt complexes in reactions of cobalt(II) porphyrins with cyanoalkyl radicals and vinyl olefins.
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)CH(3))CH(2)P) was estimated as 1.15 x 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 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 approximately 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 (*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.
Peng
Chi-How
CH
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
Li
Shan
S
Wayland
Bradford B
BB
D000475:Alkenes / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D003198:Computer Simulation; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D008956:Models, Chemical; D009570:Nitriles / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry*; D013997:Time Factors; D014753:Vinyl Compounds / Q000737:chemistry*
D000475:Alkenes; D005609:Free Radicals; D009570:Nitriles; D009942:Organometallic Compounds; D011166:Porphyrins; D014753:Vinyl Compounds; D003035:Cobalt