Autopolymerization of styrene-N-butylmaleimide mixtures at 125 or 140 degrees C in the presence of a stable nitroxyl radical [2,2,6, 6-tetramethylpiperidin- 1-yloxyl (TEMPO)] was found to proceed in a pseudoliving manner. Unimolecular initiators, which were originated by trapping self-generated radical species with TEMPO, took part in the process. Under the studied experimental conditions, the TEMPO-controlled autopolymerization with a varying comonomer ratio provided virtually alternating copolymers of narrow molecular weight distributions. The molecular weights of the copolymers increased with conversions. The obtained styrene-N-butylmaleimide copolymers containing TEMPO end groups were used to initiate the polymerization of styrene. The polymerization yielded poly(styrene-co-N-butylmaleimide)-polystyrene block copolymers with various polystyrene chain lengths and narrow molecular weight distributions. The compositions, molecular weights, and molecular weight distributions of the synthesized block copolymers and the initial poly(styrene-co-N-butylmaleimide) precursors were evaluated using nitrogen analysis, gel permeation chromatography, and H-1- and C-13-NMR spectroscopy.