Free radical polymerization with catalytic chain transfer: Using NMR to probe the strength of the cobalt-carbon bond in small molecule model reactions

This work examines cobalt-carbon bond formation between the cobalt (II) macrocycle, (tetrakis(p-methoxyphenyl)porphyrinato)cobalt (II), (TAP)Co, and a variety of radicals derived from vinyl compounds to facilitate a better understanding of the various factors affecting the cobalt-carbon bond strength in catalytic chain transfer polymerization. The reaction of (TAP)Co with the following vinylic molecules was studied: methacrylonitrile, cyclohexene, methyl methacrylate, styrene, methyl acrylate, vinyl acetate, vinyl benzoate, methyl crotonate, cis-2-pentenenitrile, and ethyl α-hydroxymethacrylate. Different concentrations of each vinylic compound were added to (TAP)Co and 2,2′-azobis(isobutyronitrile) in CDCl ₃ at 60°C. The ratio of Co(III) to Co(II) and the nature of the radical bound to the cobalt macrocycle were determined via nuclear magnetic resonance measurements. Several factors are shown to affect the reaction of the radical and the cobalt (II) species (and hence the strength of the cobaltcarbon bond in the resulting compound). These factors are as follows: the number of pathways by which a radical may be derived from the vinyl compound; the variety of radicals that can be produced from the vinylic molecule; the stability of the radical(s) generated; and the relative propagation rate of the vinyl compound. A discussion on the relevance of this study to the behavior of different monomers in catalytic chain transfer reactions is included.