The kinetics and mechanism of reactions of aliphatic stable nitroxyl radicals with alkyl and peroxide radicals during styrene oxidation

Abstract

A kinetic study (EPR, microvolumometry), quantum chemical analysis (DFT B3LYP/6 31G*), and kinetic simulation of the antioxidant activity of aliphatic stable nitroxyl radicals during styrene oxidation was performed. The key reactions constituting the detailed mechanism of the process were analyzed. It was shown that the inhibiting action of nitroxyl radicals was caused by their reaction not only with alkyl radicals but also with substrate peroxy radicals, which resulted in the regeneration of nitroxyl radicals in chain termi nation steps.

Kinetics and Mechanism of Reactions of Aliphatic Stable Nitroxide Radicals in Chemical and Biological Chain Processes

“…Styrene's oxidation can be effectively inhibited by hydroxylamines. This assertion confirms with distinct induction period on oxygen consumption's kinetic curve (typical one represented at Figure 3 (Pliss et al, 2012)). Rate constants of such antioxidants' reactions with peroxide radicals can be determined from inhibited oxidation's rate dependence on time according to the following equation (Loshadkin et al, 2002):…”

“…As it has already been mentioned above, reactions of >NO • with peroxide radicals of 1-and 1,1-ethylene substituted monomers were discovered recently (Pliss et al, 2010a(Pliss et al, , 2010b(Pliss et al, , 2012. Authors (Pliss et al, 2010a(Pliss et al, , 2010b on the basis of kinetic data of styrene's and (meth)acrylates' oxidation in presence of piperidine, pyrroline, and imidazoline >NO • suggested that nitroxide radicals inhibit the oxidation process via >NO • reaction with substrate's both alkyl and peroxide radicals, and moreover >NO • regeneration occurs during chain termination process in accordance with reactions:…”

“…With special experiments it was shown that value of inhibiting factor e.g. for >NO • (III) and >NO • (IV) is more than 7 (Pliss et al, 2012), so it suggests that >NO • regeneration process occurs upon chain termination. We note that this effect doesn't depend on styrene's concentration.…”

“…In (Pliss et al, 2012) >NO • antioxidant activity was studied during styrene's oxidation using a complex of kinetic methods in combination with quantum-chemical calculations and kinetic modeling. The choice of styrene is caused by the following circumstances.…”

“…If substrate is M then chains are propagated by polyperoxide radicals (MO2 • ) through its addition to monomer's double bond in accordance with the reaction: MO2 • + M  M • (Mogilevich & Pliss, 1990 At the same time a probability of direct reaction of nitroxide with peroxide radicals is considered in a number of works (Barton, 1998;Goldstein & Samuni, 2007;Offer & Samuni, 2002;Pliss et al, 2010aPliss et al, , 2010bPliss et al, , 2012. Therefore it is necessary to review the most significant results in the field of kinetics and mechanism of elementary reactions of piperidine, pyrroline, and imidazoline >NO • w i t h a c t i v e p a r t i c l e s o f c h e m i c a l a n d b i o c h e m i c a l oxidation processes.…”