Isotope effects and the mechanism of atom transfer radical polymerization

The mechanism of free-radical polymerization and atom transfer radical polymerization (ATRP) of methyl methacrylate was studied by a combination of C-13 kinetic isotope effects and theoretical calculations. Free-radical polymerization initiated by AIBN and ATRP initiated by 1:2:1 CuBr:2,2'-bipyridine: ethyl 2-bromoisobutyrate exhibit statistically indistinguishable C-13 isotope effects. This supports identical chain-propagation steps in ATRP and free-radical polymerization. The free-radical additions leading to these isotope effects were modeled in DFT calculations of transition structures for the addition of 2-methoxycarbonyl-2-propyl radical to methyl methacrylate. Comparison of predicted isotope effects with the experimental values supports the approximate accuracy of the DFT transition structures vs the physical transition state but suggests that the predicted transition structures are slightly too early. This suggestion is supported by comparison of DFT predictions with high-level ab initio predictions in the simpler addition of methyl radical to ethylene. In light of the evidence here and in the literature supporting a free-radical addition at the core of the ATRP mechanism, the often-proposed intervention of metal-radical complexes in either ATRP or the mechanistically parallel atom transfer radical addition reactions is critically evaluated.