The stable free radical polymerizations of styrene were investigated with five 4-sulphonate-2,2,6,6-tetramethylpiperidine-N-oxyl stable radicals as mediators and benzoyl peroxide (BPO) as initiators at 125 A degrees C. The results indicated that the polymerizations proceeded in a "living"/controlled manner, i.e., the polymerization rates were first-order with respect to the monomer concentrations, molecular weights increased linearly with conversions and the molecular weight distributions were relatively low (M (w)/M (n) = 1.2-1.4), H-1 NMR analysis of the polymer chain-ends and successful chain extensions. The polymerization rates were faster than that of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl (OH-TEMPO) mediated ones. The effects of steric interference of different substitute groups at four-position of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), the molar ratios of stable radical to BPO and the temperature on the polymerizations were investigated.