The radical copolymerization of styrene and n-butyl methacrylate mediated by 1-phenyl-1-(2',2',6',6'-tetramethyl-1'-piperidinyl-oxy)ethane in bulk at 125 C has been analyzed over a wide range of conversions and monomer feed compositions. Monomer reactivity ratios have been determined, and the Mayo-Lewis terminal model provides excellent predictions for the variations of the intermolecular structure over the entire conversion range. The kinetic analysis of this copolymerization system indicates an apparent propagation rate coefficient independent of the monomer feed composition as well as a limiting conversion that decreases as the styrene monomer feed decreases. This fact is attributed to side reactions leading to unsaturated end groups and the accumulation of nonactive adducts of n-butyl methacrylate. The number-average molecular weights linearly increase with conversion, and the copolymers present narrow molecular weight distributions.