In this article, we compare two routes for carrying out in situ nitroxide-mediated polymerization of styrene using the C-phenyl-N-tert-butylnitrone (PBN)/2,2'-azobis(isobutyronitrile) (AIBN) pair to identify the best one for an optimal control. One route consists in adding PBN to the radical polymerization of styrene, while the other approach deals with a prereaction between the nitrone and the free radical initiator prior to the addition of the monomer and the polymerization. The combination of ESR and kinetics studies allowed demonstrating that when the polymerization of styrene is initiated by AIBN in the presence of enough PBN at 110 degrees C, fast decomposition of AIBN is responsible for the accumulation of dead polymer chains at the early stages of the polymerization, in combination with controlled polystyrene chains. On the other hand, PBN acts as a terminating agent at 70 degrees C with the formation of a polystyrene end-capped by an alkoxyamine, which is not labile at this temperature but that can be reactivated and chain-extended by increasing the temperature. Finally, the radical polymerization of styrene is better controlled when the nitrone/initiator pair is prereacted at 85 degrees C for 4 h in toluene before styrene is added and polymerized at 110 degrees C. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47:1085-1097, 2009