Methyl radicals, chain termination

Traylor and Russell (30) have shown recently that similar reactions for the cumyloxy radical are important in cumene oxidation at 60 °C., and Hendry (12) has provided some quantitative data. At low concentrations of hydrocarbon, Reaction 9 is favored over Reaction 7 (propagation by tert-BuO ), and significant numbers of methyl radicals are formed and converted to Me02 radicals. Chain termination thus shifts from the slow termination by 2 tert-Bu02 (Reaction 6) to Reaction 10, which has a rate constant several hundredfold larger (21). The apparent order of the oxidation in isobutane is then 3/2 a similar relation applies to gas-phase oxidations and is discussed there. 

The above explanation of autoacceleration phenomena is supported by the manifold increase in the initial polymerization rate for methyl methacrylate which may be brought about by the addition of poly-(methyl methacrylate) or other polymers to the monomer.It finds further support in the suppression, or virtual elimination, of autoacceleration which has been observed when the molecular weight of the polymer is reduced by incorporating a chain transfer agent (see Sec. 2f), such as butyl mercaptan, with the monomer.Not only are the much shorter radical chains intrinsically more mobile, but the lower molecular weight of the polymer formed results in a viscosity at a given conversion which is lower by as much as several orders of magnitude. Both factors facilitate diffusion of the active centers and, hence, tend to eliminate the autoacceleration. Final and conclusive proof of the correctness of this explanation comes from measurements of the absolute values of individual rate constants (see p. 160), which show that the termination constant does indeed decrease a hundredfold or more in the autoacceleration phase of the polymerization, whereas kp remains constant within experimental error. 

Activation energies for chain termination are smaller than for chain propagation, but they are significantly greater than zero. This might not have been anticipated inasmuch as methyl radicals seem to combine in the gas phase without measurable activation energy. ... 

One of the mechanisms proposed recently for the photooxidation of PVC involves the formation of methyl radicals from the ends of branches and chains (94). It is extremely difficult to understand how such radicals could be produced, since all of the saturated chain ends and branch ends identified thus far contain a terminal chloromethyl arrangement (see above). 

The formed methyl radical adds dioxygen, and the methylperoxyl radical participates in chain termination ... 

Recently an analogous mechanism for cyclic chain termination has been established for quinones [47], Quinones, which can act as acceptors of alkyl radicals, do not practically retard the oxidation of hydrocarbons at concentrations of up to 5 x 10 3 mol L 1, because the alkyl radicals react very rapidly with dioxygen. However, the ternary system, /V-phenylquinonc imine (Q) + H202 + acid (HA), efficiently retards the initiated oxidation of methyl oleate and ethylbenzene [47]. This is indicated by the following results obtained for the oxidation of ethylbenzene (343 K, p02 = 98 kPa, Vi = 5.21 x 10-7 mol L 1 s 1). 

Table III shows that chains are carried by both alkylperoxy and alkoxy radicals. However, some of the tert-BuOH came from the initiation and termination steps, and the methyl radical balance is unsatisfactory.

CH4 + Cl- —> HC1 + CH3-CH3- + Cl2 —> CH3C1 + Clin practice, chain reactions are limited by so-called termination processes. In our example, chlorine atoms or methyl radicals are destroyed by reacting with one another, as shown in the following equations ... 

The overall rates of chain reactions usually are slowed very much by substances that can combine with atoms or radicals and convert them into species incapable of participating in the chain-propagation steps. Such substances are called radical traps, or inhibitors. Oxygen acts as an inhibitor in the chlorination of methane by rapidly combining with a methyl radical to form the comparatively stable (less reactive) peroxymethyl radical, CH300-. This effectively terminates the chain ... 

In presence of triphenyl phosphite or tri(4-methyl 6-f-butylphenyl) phosphite, (6) also acts as an oxidation inhibitor for polyenes. The complexes formed have not been studied, but the effect is attributed to termination of radical chains and the complexes are more active than the phosphites alone.29 Vanadyl acetylacetonate is also employed as a hardener for unsaturated polyesters.30... 

An equimolecular amount of tertbutyl oxide and tert-butyl peroxide radicals are formed, which can initiate the polymerization of styrene consequently, considering that the chain termination reaction of polystyrene proceeds by coupling, it results that most of the polystyrene molecules will contain one or two terminal peroxide groups. These have been used for a second step polymerization, e.g. of methyl methacrylate. In this case also, it seems that the macro end radical should be much more efficient for initiating polymerization than the small tert-butyl oxide radical (47). The styrene content of the block copolymer was about 35%. 

Read s group (86, 87) are studying some interesting co-oxidation reactions catalyzed particularly by RhCl(PPh3)3 in benzene at 20°C. Terminal olefins and triphenylphosphine are converted by 02 to the corresponding methyl ketones and the phosphine oxide. Radical chain processes were not detected, and such reactions normally do not yield methyl ketone products. Net oxygen-atom transfers were considered to result via the mechanism shown in Scheme 3. 

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