On the mechanism of activation of copper-catalyzed atom transfer radical polymerization

The mechanism of activation of atom transfer radical polymerization (ATRP) has been analyzed by inves-tigating the kinetics of dissociative electron transfer (ET) to alkyl halides (RX) in acetonitrile. Using a seriesof alkyl halides, including both bromides and chlorides, the rate constants of ET (kET) to RX by electrogen-erated aromatic radical anions (A•−) acting as outer-sphere donors have been measured and analyzedaccording to the current theories of dissociative ET. This has shown that the kinetic data fit very well the“sticky” dissociative ET model with the formation of a weak adduct held together by electrostatic inter-actions. The rate constants of activation, kact, of some alkyl halides, namely chloroacetonitrile, methyl2-bromopropionate and ethyl chloroacetate, by [CuIL]+(L = tris(2-dimethylaminoethyl)amine, tris(2-pyridylmethyl)amine, 1,1,4,7,7-pentamethyldiethylenetriamine) have also been measured in the sameexperimental conditions. Comparisons of the measured kactvalues with those predicted assuming anouter-sphere ET for the complexes have shown that activation by Cu(I) is 7–10 orders of magnitude fasterthan required by outer-sphere ET. Therefore, the mechanism of RX activation by Cu(I) complexes usedas catalysts in ATRP occurs by an inner-sphere ET or more appropriately by a halogen atom abstraction.