Influence of nitroxide structure on the 2,5- and 2,6-spirodicyclohexyl substituted cyclic nitroxide-mediated free-radical polymerization of styrene

The 2,6-spirodicyclohexyl substituted nitroxide, cyclohexane-1-spiro-2′-(3′-5′-dioxo-4′-benzylpiperazine- 1′-oxyl)-6′-spiro-1″-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 × 10^-5 s^-1, 5.19 × 10⁶ M^-1 s^-1, and 3.08 × 10^-12 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.