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-

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The effect of 2,4,6-tri-I-butylphenol on the rate of polymerization of vinyl acetate and on the degree of polymerization of the resultant polymer has been determined. It is shown that the phenoxy radicals produced by hydrogen transfer do not reinitiate polymerization and that they disappear from solution by reaction with polyvinyl acetate radicals. The chain transfer constant is estimated to be 0.017 + 0.004 at 45" from molecular weight studies and 0.021 f 0.004 from rate studies. Stationary state concentrations of phenoxyl of the order of

4‐Methyl‐2,6‐di‐tert‐butylphenol strongly retards the free radical polymerization of vinyl acetate initiated by azobisisobutyronitrile. The chain transfer constant, estimated from rate data, is 0.020 ± 0.004 at 35°C and does not vary significantly with temperature. Molecular weight data lead to transfer constants of 0.023, 0.020, and 0.024 at 35, 45, and 55°C, respectively. A mean kinetic isotope effect of 9.8 ± 1.0 is observed for the phenol deuterated at the OH group, showing that the main attack of poly(vinyl acetate) radicals on the phenol involves hydrogen abstraction from this group. The activation energy for hydrogen abstraction is estimated to be 7.8 kcal/mole, and the rate constant at 50°C is 160 ± 40 1./mole‐sec. The stationary concentration of 4‐methyl‐2,6‐di‐tert‐butylphenoxyl in the polymerization mixture is proportional to the phenol concentration and is independent of the initiator concentration, as shown by electron spin resonance studies. Cross termination of poly(vinyl acetate) and phenoxy radicals occurs to a greater extent than mutual termination of these radicals. The rate constant for cross termination is close to 1 × 108 1./mole‐sec at 50°C; the activation energy for cross termination is 2.9 ± 1.3 kcal/mole.