Bulk radical polymerization of styrene in the presence of nitronyl nitroxides (2-(4-substituted phenyl)-4,4,5,5-tetramethyl-4,5-dihydroimidazolyl-1-oxyl 3-oxide) was studied. All nitronyl nitroxides, like other nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine 1-oxyl radical (TEMPO), act as reversible radical scavengers. The efficiency of controlling the polymerization is affected by the substituent at the 4'-position. The efficiency increases with electron donating strength of 4'-substituents, at least at the beginning of the reaction. However, the thermal stability of nitronyl nitroxides decreases in the same order. Thus, TEMPO is more suitable than nitronyl nitroxides for controlled/''living'' radical polymerization of styrene.