Activation rate constants (k(act)) for a variety of initiators for Cu-mediated ATRP were measured with Cu(I)Br(PMDETA) at various temperatures (i.e., -40 to + 60 degrees C. Reactions of less active alkyl halides were more accelerated by increased temperatures than reactions of more active initiators. Straight Eyring and Arrhenius plots were obtained, from which the activation parameters (i.e., Delta H-+/-, Delta S-+/-, E-a, and In A) were determined. The activation entropies Delta H-+/- are in between 26.0 and 38.7 kJ mol(-1) with highly negative activation entropies (Delta S-+/- = -156 to - 131 J mol(-1) K-1), which indicate greatly ordered structures of the transition states for these reactions.