3,3-dimethyl-1,1-diphenyl azabutane-N-oxides with electron-releasing and electron-withdrawing substituents in one of the aromatic rings were synthesized and their effect on styrene polymerization was assessed. Styrene polymerization kinetic experiments were carried out in bulk at 100 and 120 degreesC with BPO or AIBN as initiators and diphenyl-azabutane type nitroxides as mediators. Results were compared with the kinetics using the preformed alkoxyamine. Different [nitroxide]/[initiator] ratios were evaluated at 100 degreesC and the optimum ratio = 1.75 was found. Polydispersities between 1.2 and 1.4 were obtained for polystyrene of M-n up to 25,000. Experiments to obtain molecular weights around 120,000 were also carried out using oligomeric alkoxyamines as mediators. The calculated equilibrium constant (K) of the styrene polymerization at 120 degreesC with diphenyl azabutane as mediator is larger than the previously reported for TEMPO 1, and similar to the reported for TIPNO 2 and DEPN 3, The polymerization with these nitroxides is faster than the polymerization using other nitroxides and substituents on the aromatic ring have no important effect on the control of polymerization; these experimental results agree with some electronic parameters obtained from molecular modeling of the nitroxides through semiempirical methods. (C) 2003 Published by Elsevier Ltd.