Mechanism and kinetics of the imidazolidinone nitroxide-mediated free-radical polymerization of styrene

Styrene radical polymerizations mediated by the imidazolidinone nitroxides 2,5-bis(spirocyclohexyl)-3-methylimidazolidin-4-one-1-oxyl (NO88Me) and 2,5bis(spirocyclohexyl)-3-benzylimidazolidin-4-one-1-oxyl (NO88Bn) were investigated. Polymeric alkoxyamine (PS-NO88Bn)-initiated systems exhibited controlled/living characteristics at 100-120 °C but not at 80 °C. All systems exhibited rates of polymerization similar to those of thermal polymerization, with the exception of the PSNO88Bn system at 80 °C, which polymerized twice as quickly. The dissociation rate constants (k_d) for the PS-NO88Me and PS-NO88Bn coupling products were determined by electron spin resonance at 50-100 °C. The equilibrium constants were estimated to be 9.01 × 10^-11 and 6.47 × 10^-11 mol L^-1 at 120 °C for NO88Me and NO88Bn, respectively, resulting in the combination rate constants (k_c) 2.77 × 10⁶ (NO88Me) and 2.07 × 10⁶ 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 k_d and k_c were 4-8 and 25-33 times lower, respectively, than the reported values for PSTEMPO at 120 °C, indicating that the 2,5-spirodicyclohexyl rings have a more profound effect on the combination reaction rather than the dissociation reaction.