Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide-mediated autopolymerization of styrene are discussed. At 120-125 degrees C and moderate 2,2,4,4-tetramethyl-1-piperidinyloxy (TEMPO) concentrations (0.02-0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (k(h))]. At higher TEMPO concentrations ([N center dot] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition k(ad), also becomes relevant. From previous experimental data and simulations, initial estimates of k(h) approximate to 1 and kad approximate to 6 x 10(-7) L mol(-1) s(-1) are obtained at 125 degrees C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction-period conditions, the simulations confirm the validity of the quasi-steady-state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well-known expression k(th)[M](3) (where [M] is the monomer concentration and k(th) is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. (c) 2006 Wiley Periodicals, Inc.