Controlled radical polymerization of vinyl monomers mediated by cobalt(II) porphyrin and organo -cobalt complexes

Controlled/living radical polymerization (CRP/LRP) provides a versatile route for the synthesis of well-controlled homo and block copolymers with predetermined molecular weight (Mn), narrow molecular weight distribution (low PDI or Mw/M n), various architectures, and useful end-functionalities. Atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition-fragmentation chain transfer (RAFT) are discussed as the most successfully developed CRP methods. Cobalt-mediated radical polymerization (CMRP) and other organometallics (Te, Sb, Ti, etc.) mediated radical polymerization are currently more valuable on the synthesis of controlled polymers of specific monomers. Cobalt porphyrin complexes are effective to control the radical polymerization of acrylates and vinyl acetate (VAc) through reversible termination (RT) and/or degenerative transfer (DT) pathways. Sulfonated cobalt(II) and organo-cobalt porphyrins can also mediate a controlled radical polymerization of acrylic acid (AA) in water. Homo and block copolymers of methyl acrylate (MA), acrylic acid, and vinyl acetate with a wide range of molecular weight (10,000∼200,000) and low polydispersity (PDI = Mw/M n; 1.08∼1.40) have been synthesized under an undemanding condition (333K in benzene or water). The observed rate of radical polymerization in the presence of organo-cobalt complexes approaches that of regular radical initiated polymerization, but the absolute rate is slightly reduced. 1H NMR resonances of the organic group attached to cobalt metal center are shifted to high field region (0 ∼ −6 ppm) due to the porphyrin ring current and thus provide a route to observe the formation and transformation of the organo-cobalt species. Mechanistic model for studying the mechanism and evaluating the thermodynamic and kinetic factors of CMRP process has been established by the evolution of cobalt species shown in 1H NMR spectrum and kinetic simulation. MA and VAc polymerizations were used to demonstrate the two mechanisms, RT and DT, which occur in the cobalt porphyrins mediated CRP. The reaction schemes for cobalt porphyrin mediated methyl acrylate and vinyl acetate polymerizations are the same, but differences in thermodynamic and kinetic parameters result in a different propensities for alternate reaction pathways. The sequence in the formation of organo-cobalt complexes indicates that bond homolysis is important for the inter-conversion of organo-cobalt species formed in methyl acrylate CRP but associative radical exchange is the exclusive pathway for the transformation of organo-cobalt species in vinyl acetate CRP. The greater radical stabilization energy and lower energy of the singly occupied molecular orbital (SOMO) in the MA radical (·CH(CO 2CH3)CH3) contribute to the observed better control of methyl acrylate CRP than that for vinyl acetate CRP. The attempt on extending the application of cobalt porphyrin mediated controlled radical polymerization has never been ceased. The formation of organo-cobalt complexes with vinyl monomers such as styrene (Sty), vinylidene fluoride (VDF), N-vinyl pyrrolidone (VP), acrylonitrile (AN), and vinyl benzoate (VBz) has been observed and currently cobalt porphyrin complexes show a limited control on radiacal polymerization of styrene (Sty), N-vinyl pyrrolidone (VP), and vinyl benzoate (VBz). Homo and block copolymers with predetermined molecular weight ( Mn), low polydispersity (PDI or M w/Mn), and useful functionalities have various applications on self-assembling nanostructure, materials science, and biological research. Block copolymers of poly(methyl methacrylate)-b-poly(styrene) (PMMA-b-PSty) and homopolymers of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) were synthesized by the technique of reversible addition-fragmentation chain transfer (RAFT) and applied to the studies of biological science and nanoparticles formation