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Cobalt-catalyzed chain transfer in methyl methacrylate (MMA) or methacrylonitrile (MAN) polymerizations employing very high concentrations of catalyst allows the detailed investigation of the reaction of primary radicals with monomers. The enabling reaction is the transfer of a hydrogen atom from a Co(III)−H species to monomer to generate the monomer radicals (H-MMA• and H-MAN•, respectively). These radicals react with additional monomer yielding the dimer radicals, H-MMA2 • and H-MAN2 •, respectively, or with the cobalt catalyst to yield the starting monomer. These reactions generate the respective dimers relatively free of higher oligomers. This approach has allowed the determination of the rates of reaction of the monomer radical (k p1) and dimer radical (k p2) with monomer. H-MAN• reacts with methacrylonitrile at a rate (k p1 = 340 ± 40 M-1 s-1 at 60 °C) which is considerably faster than does a high polymer radical (literature:  k p ∞ ≡ k p = 55 M-1 s-1 for MAN at 60 °C). For H-MMA• at 60 °C, k p1 = 14 000 ± 1500 M-1 s-1 and for H-MMA2 •, k p2 = 3600 M-1 s-1, relative to the literature value for H-MMA∞ •, k p = 843 M-1 s-1. It was also possible to determine the rate constants of catalytic chain transfer by the cobalt catalysts; in MMA free-radical polymerization at 60 °C, k s2 = 2.3 × 106 M-1 s-1 for the tetramethyl ether of [hematoporphyrinato IX]cobalt(II) and 2.8 × 106 M-1 s-1 for [tetra(p-methoxyphenyl)porphyrinato]cobalt(II).