An electron spin resonance study of the second-order decay of 4-alkyl-2,6-di-t-butylphenoxyl radicals in solution

Abstract

The decay of 4-methyl-and 4-ethyl-2,6-di-t-butylphenoxyl radicals in benzene solution is first-order a t high radical concentrations and second-order a t low radical concentrations. The concentration range in which the changeover from first-to second-order reaction occurs is determined by the equilibrium constant for quinol ether dimer formation from the phenoxyl radicals. Observed second-order rate constants a t 22 "C for the loss of 4methyl-, 4-ethyl-, and 4-isopropyl-2,6-di-t-butylphenoxyl radicals are, respectively, 2200, 500, and 2 I mol-l s-l. The activation energy for the decay of 4-methyl-2.6-di-t-butyfphenoxyl in benzene solution is 3.2 =t 0.3 kcal mol-l. Ontario, CanadaA PREVIOUS e.s.r. investigation of the decay of 4met hyl-2,6-di-t-but ylphenoxyl (4-met hyl-DTBP) in benzene solution indicated that the reaction is first-order with a rate constant of 1.3 x A later flash photolysis study showed that the first-order decay is preceded by a rapid second-order reaction to give a high yield of phenoxyl dimer.2 At concentrations of phenoxyl in the range to 2 x 1 0 p 6 ~ the reaction again appears to be essentially second-order, the rate constant at 24 "C being estimated t o be 4-59 x lo2The decay of 4-ethyl-2,6-di-t-butylphenoxyl (4-ethyl-DTBP) was found under given reaction conditions to be first-order at 21-26 "C but second-order at 47 "C.* Weiner and Mahoney5 have recently proposed a mechanisni which accounts for the observed e.s.r. and flash photolysis results for the decay of 4-methyl-DTBP at high concentrations. The rapid initial decay observed by Land and Porter yields a quinol ether dimer and a low ' equilibrium ' concentration of phenoxyl radical. The observed first-order decay of phenoxyl involves either the direct conversion of 4-methyl-DTBP into phenol and quinone methide or the breakdown of the quinol ether dimer to the same products. In terms of either mechanism, the reaction order should change from one to two at sufficiently low concentrations of phenoxyl radical. E.s.r. measurements of the disappearance of 4-methyl-, 4-ethyl-, and 4-isopropyl-DTBP a t relatively low concentrations in benzene and vinyl acetate solutions confirm the general predictions of the proposed mechanisms and allow direct estimates of kinetic parameters t o be made. It has also been shown that 4-methyl-DTBP produced by flash photolysis of the corresponding phenol undergoes second-order decay at concentrations less than 1 0 p 6 ~. s-l at 25 "C.l EXPERIMENTAL Benzene (Fisher Scientific Company spectroanalysed grade) was used as supplied. Vinyl acetate was successively fractionally distilled, partially polymerised, and redistilled according to a procedure generally used in the preparation of monomer for free radical polymerisation.6 4-Ethyl-and 4-isopropyl-2,6-di-t-butylphenol were sub-

Degradation of γ-irradiated polyethylene-ethylene vinyl alcohol-polyethylene multilayer films: An ESR study

“…The Land e factors g for 1 e 3, and 5 in toluene are around 2.0048, as expected for phenoxyl radicals (g ¼ 2.0045) [23,24]. Assuming a weak anisotropy of signals for the PbO 2 oxidized powder as well as for the g-irradiated powder, the g-factors were straightforwardly determined and reported as 2.0048, 2.0043, 2.0044 and 2.0042, 2.0044, and 2.0046, respectively, confirming that the radical is a phenoxyl type radical.…”

Characterization of Reactive Intermediates Generated During Photolysis of 4-Acetoxy-4-aryl-2,5-cyclohexadienones: Oxenium Ions and Aryloxy Radicals

“…It is known that aryloxy radicals are relatively longlived species in solution that decay primarily by dimerization in the absence of radical traps. [21][22][23][24][25][26][27] In many cases in which there is a 4-alkyl or 4-aryl substituent, including 5a, the dimerization is reversible and biphasic decay kinetics can be observed. 21,24,26 A mechanism similar to that shown in Scheme 4 has been used to analyze these cases.…”

“…The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals. [21][22][23][24][25][26][27][28]…”

Mechanisms of action and reactivities of the free radicals of inhibitors

“…Quinonemethides were observed to accumulate symbatically with decay of ArO* 1-3.124,125,127 Quinone methides are the final products of oxidation of the are £13,10%; E13, ±1 kJ/mol; A13, 40%. bAt 283 K.starting ArOH with chemical agents 26,125,127,[134][135][136][137][138][139]. The kinetic and activation parameters for reaction 6-13 are given in Table…”