The mechanism of activation of atom transfer radical polymerization (ATRP) has been analyzed by investigating the kinetics of dissociative electron transfer (ET) to alkyl halides (RX) in acetonitrile. Using a series of alkyl halides, including both bromides and chlorides, the rate constants of ET (k(ET)) to RX by electrogenerated aromatic radical anions (A(center dot-)) acting as outer-sphere donors have been measured and analyzed according 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 interactions. The rate constants of activation, k(act), of some alkyl halides, namely chloroacetonitrile, methyl 2-bromopropionate and ethyl chloroacetate, by [(CUL)-L-I](+) (L = tris(2-dimethylaminoethyl)amine, tris(2-pyridylmethyl)amine, 1,1,4,7,7-pentamethyldiethylenetriamine) have also been measured in the same experimental conditions. Comparisons of the measured k(act) values with those predicted assuming an outer-sphere ET for the complexes have shown that activation by Cu(I) is 7-10 orders of magnitude faster than required by outer-sphere ET. Therefore, the mechanism of RX activation by Cu(I) complexes used as catalysts in ATRP occurs by an inner-sphere ET or more appropriately by a halogen atom abstraction. (C) 2013 Elsevier Ltd. All rights reserved.