Kinetics of the free radical polymerization of styrene at 110 degrees C has been investigated in the presence of C-phenyl-N-tert-butylnitrone (PBN) and 2,2'-azobis(isobutyronitrile) (AIBN) after prereaction in toluene at 85 degrees C. The effect of the prereaction time and the PBN/AIBN molar ratio on the in situ formation of nitroxides and alkoxyamines (at 85 degrees C), and ultimately on the control of the styrene polymerization at 110 degrees C, has been investigated. As a rule, the styrene radical polymerization is controlled, and the mechanism is one of the classical nitroxide-mediated polymerization. Only one type of nitroxide (low-molecular-mass nitroxide) is formed whatever the prereaction conditions at 85 degrees C, and the equilibrium constant (K) between active and dormant species is 8.7 x 10(-10) mol L-1 at 110 degrees C. At this temperature, the dissociation rate constant (k(d)) is 3.7 x 10(-3) s(-1), the recombination rate constant (k(c)) is 4.3 x 10(6) L mol(-1) s(-1), whereas the activation energy (E-a,E-diss), for the dissociation of the alkoxyamine at the chain-end is similar to 125 kJ mol(-1). Importantly, the propagation rate at 110 degrees C, which does not change significantly with the prereaction time and the PBN/AIBN molar ratio at 85 degrees C, is higher than that for the thermal polymerization at 110 degrees C. This propagation rate directly depends on the equilibrium constant K and on the alkoxyamine and nitroxide concentrations, as well. (c) 2007 Wiley Periodicals, Inc.