Styrene radical polymerizations mediated by the imidazolidinone nitroxides 2,5-bis(spirocyclohexyl)-3-methylimidazolidin-4-one-1-oxyl (NO88Me) and 2,5-bis(spirocyclohexyl)-3-benzylimidazolidin-4-one-1-oxyl (NO88Bn) were investigated. Polymeric alkoxyamine (PS-NO88Bn)-initiated systems exhibited controlled/living characteristics at 100-120 degreesC but not at 80 degreesC. All systems exhibited rates of polymerization similar to those of thermal polymerization, with the exception of the PSNO88Bn system at 80 degreesC, which polymerized twice as quickly. The dissociation rate constants (k(d)) for the PSNO88Me and PS-NO88Bn coupling products were determined by electron spin resonance at 50-100 degreesC. The equilibrium constants were estimated to be 9.01 X 10(-11) and 6.47 X 10(-11) mol L-1 at 120 degreesC for NO88Me and NO88Bn, respectively, resulting in the combination rate constants (k(c)) 2.77 X 10(6) (NO88Me) and 2.07 X 10(6) L mol(-1) s(-1) (NO88Bn). The similar polymerization results and kinetic parameters for NO88Me and NO88Bn indicated the absence of any 3-N-transannular effect by the benzyl substituent relative to the methyl substituent. The values of kd and k(c) were 4-8 and 25-33 times lower, respectively, than the reported values for PS-TEMPO at 120 degreesC, indicating that the 2,5-spirodicyclohexyl rings have a more profound effect on the combination reaction rather than the dissociation reaction.