The activation rate parameters of polystyrene (kSact) and poly(butyl acrylate) (kBAact) macroinitiators in ATRP catalyzed by CuBr/4,4'-di-n-heptyl-2,2'-bipyridine have been determined at 110 C using a combination of nitroxide exchange reactions with HPLC analysis. The method is based on exchange of the bromide end group with a hydroxy-TEMPO end group. The depletion of the macroinitiator is monitored as a function of time. The method is applicable in different solvents, which implies the possibility to investigate the effect of solvent characteristics. Solvent polarity has a negative influence on kSact and a positive effect on kBAact. The ability of the solvent to act as ligand in the copper complex hampers the activation process for both macroinitiators, while the presence of hydroxy groups in the solvent increases both activation rate parameters.