Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number-average molecular weight, Mu, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N-tert-butyl-alpha-phenylnitrone (PBN), N-methyl-alpha-phenylnitrone (PMN), and N-methyl-alpha-(4-bromo-phenyl) nitrone (pB-PMN), addition rate coefficients, k(ad,macro), in a similar magnitude to the styrene propagation rate coefficient, k(p), are found with spin capturing constants C-SC (with C-SC k(ad),(macro)/k(p)) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol(-1) were deduced for k(ad,macro,) congruent with a decreasing CSC with increasing temperature. Almost constant M. over up to high monomer to polymer conversions is found when C-SC is close to unity, while increasing molecular weights can be observed when the CSC is large. From temperatures of 100 degrees C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1098-1107, 2009