Formation and interconversion of organo-cobalt complexes in reactions of cobalt(ll) porphyrins with cyanoalkyl radicals and vinyl olefins
Journal
Inorganic Chemistry
Journal Volume
48
Journal Issue
11
Pages
5039-5046
Date Issued
2009
Author(s)
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 2CH3)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 ? 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. ? 2009 American Chemical Society.
Other Subjects
alkene;cobalt;free radical;nitrile;organometallic compound;porphyrin;vinyl derivative;article;chemical model;chemistry;computer simulation;kinetics;synthesis;time;Alkenes;Cobalt;Computer Simulation;Free Radicals;Kinetics;Models, Chemical;Nitriles;Organometallic Compounds;Porphyrins;Time Factors;Vinyl Compounds
Type
journal article