Abstract
Detailed mechanistic studies reveal that halogen exchange (HE) in ATRP can occur not only by a radical pathway (atom transfer) but also by an ionic pathway (S N 2 reaction) because Cu (I) (L)X and Cu (II) (L)X 2 complexes contain weakly associated halide anion that can participate in the S N 2 reaction with alkyl halide (ATRP initiator). Both pathways were kinetically studied, and their contributions to the HE process were quantitatively evaluated for seven alkyl halides and three Cu (I) (L)Cl complexes. Radical pathway dominates the HE process for 3° and 2° alkyl bromides with more active complexes such as Cu (I) (TPMA)Cl. Interestingly, ionic pathway dominates for 1° alkyl bromides and less active ATRP catalysts. These studies also revealed that degree of association of alkyl halide anion depends on the structure of copper complexes. In addition, radical pathway is accompanied by the reverse reactions such as deactivation of radicals to alkyl bromides and also activation of alkyl chlorides, reducing the efficiency of halogen exchange. © 2011 American Chemical Society.