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 PS-NO88Bn 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 ϫ 10 Ϫ11 and 6.47 ϫ 10 Ϫ11 mol L Ϫ1 at 120°C for NO88Me and NO88Bn, respectively, resulting in the combination rate constants (k c ) 2.77 ϫ 10 6 (NO88Me) and 2.07 ϫ 10 6 L mol Ϫ1 s Ϫ1 (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 PS-TEMPO at 120°C, indicating that the 2,5-spirodicyclohexyl rings have a more profound effect on the combination reaction rather than the dissociation reaction.
The 2,6-spirodicyclohexyl substituted nitroxide, cyclohexane-1-spiro-2Ј-(3Ј,5Ј-dioxo-4Ј-benzylpiperazine-1Ј-oxyl)-6Ј-spiro-1Љ-cyclohexane (BODAZ), was investigated as a mediator for controlled/living free-radical polymerization of styrene. The values of the number-average molecular weight increased linearly with conversion, but the polydispersities were higher than for the corresponding 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and 2,5-bis(spirocyclohexyl)-3-benzylimidazolidin-4-one-1-oxyl (NO88Bn) mediated systems at approximately 2.2 and 1.6 at 100 and 120°C, respectively. These results were reflected in the rate coefficients obtained by electron spin resonance spectroscopy; at 120°C, the values of the rate coefficients for polystyrene-BODAZ alkoxyamine dissociation (k d ), combination of BODAZ and propagating radicals (k c ), and the equilibrium constant (K) were 1.60 ϫ 10 Ϫ5 s Ϫ1 , 5.19 ϫ 10 6 M Ϫ1 s Ϫ1 , and 3.08 ϫ 10 Ϫ12 M, respectively. The value of k d was approximately one and two orders of magnitude lower, and that of K was approximately 20 and 7 times lower than for the NO88Bn and TEMPO adducts. These results are explained in terms of X-ray crystal structures of BODAZ and NO88Bn; the six-membered ring of BODAZ deviates significantly from planarity as compared to the planar five-membered ring of NO88Bn and possesses a benzyl substituent oriented away from the nitroxyl group leading to a seemingly more exposed oxyl group, which resulted in a higher k c and a lower k d than NO88Bn.
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