The activation rate parameter (k(act)) in atom transfer radical polymerization (ATRP) for a poly(ethylene-co-butylene) (PEB) macroinitiator bearing a bromoisobutyryl initiating group catalyzed by CuBr/2L and CuCl/2L (L = N-n-pentyl-2-pyridylmethanimine) has been determined using the nitroxide exchange reaction with hydroxy-TEMPO in combination with gradient polymer elution chromatography (GPEC). With GPEC and H-1 NMR analyses, the fraction of activated chains that is terminated via combination with hydroxy-TEMPO was estimated at 95%. Concentrations of the activating copper(l) complexes during the exchange experiments have been determined using UV-vis spectroscopic measurements. Compared to bromide, the use of chloride as the copper counterion results in a strong increase in k(act) by more than a factor of 2. The improved control in ATRP for mixed halide initiating systems as R-Br/CuCl compared to R-Br/CuBr systems is therefore not entirely attributed to an increased ratio of initiation to propagation rate through halogen exchange, but also finds its origin in an increased rate of activation.