Methyl methacrylate and butyl acrylate monomers are copolymerized by atom-transfer radical polymerization, affording polymers with well-controlled molecular weight and low polydispersity. A kinetic analysis of this system is compared with the corresponding free-radical polymerization system. The copolymerization rate follows an opposite trend to that observed in conventional copolymerization. This fact is attributed to a smaller population of radicals generated in the reaction, since the relative fraction of propagating radicals is the same as that in classical copolymerization.