N-15-labeled nitroxides are employed to trap propagating radicals in the copolymerization of styrene and methyl acrylate. The resulting polymeric alkoxyamines are analyzed by N-15 NMR. The assignment of the observed bands to the two possible end groups of the propagating copolymer chain is achieved by comparison of the spectra with the homopolymer alkoxyamines. Copolymers in the suitable molar mass range for the current investigation are obtained by the formation of initiating radicals and nitroxyl radical traps in situ, from a low molar mass N-15-labeled alkoxyamine.