The average activation-deactivation equilibrium constant, < K >, was determined on a theoretical basis for controlled free-radical copolymerizations operating via a reversible termination mechanism (i.e., nitroxide-mediated polymerization or atom transfer radical polymerization), using the terminal model for the activation-deactivation equilibrium and the terminal model or the implicit penultimate unit effect model for the propagation reaction. From the equation, it was shown that the addition of a small fraction of an appropriate comonomer to a monomer with a very large activation-deactivation equilibrium constant, K, might lead to strong reduction of < K >, providing the added comonomer exhibits a low K. In nitroxide-mediated polymerization, the monomers with a very high K, such as the methacrylic esters, do not lead to controlled polymerization in the presence of nitroxides like SG1, despite the absence of disproportionation reaction between the nitroxide and the growing radical, because of the too fast irreversible self-termination of the propagating radicals present in high concentration. The polymerization stops at low conversion. Consequently, a reduction of K might lead to an enhanced quality of control. The method was indeed successfully applied to the SG1-mediated polymerization of methyl methacrylate at 90 degrees C. By adding only 4.4 or 8.8 mol % of styrene, the polymerization could be carried out to large conversions, while exhibiting all the features of a controlled system.