Allyl type free radicals

Fig. 11. ESR spectra of allyl-type free radicals (dotted curve) and of mixed alkyl and allyl free radicals in irradiated polyethylene after heating to room temperature (solid curve). Decay of alkyl free radicals was calculated from height of peaks marked A [64],...

Table I. Ultraviolet Absorption Spectra of Allyl Type Free Radicals...

When polyethylene is irradiated at room temperature, allyl and polyenyl type free radicals are formed (2) and are much more stable than alkyl type free radicals (7, 13). Electron spin resonance (ESR) experiments have demonstrated that the allyl or polyenyl free radicals are quite stable, persisting for relatively long periods (hours or days)... 

That the mechanism of allylic bromination is of the free-radical type was demonstrated by Dauben and McCoy, who showed that the reaction is veiy sensitive to free-radical initiators and inhibitors and indeed does not proceed at all unless at least a trace of initiator is present. Subsequent work indicated that the species that actually abstracts hydrogen from the substrate is the bromine atom. The reaction is initiated by small amounts of Br. Once it is formed, the main propagation steps are... 

The types and reactions postulated for reactive intermediates in the radiation chemistry of polyethylene are reviewed. Ultraviolet spectroscopy is an important tool in complementing data obtained from electron spin resonance studies. Finally, the kinetics of growth and decay of the allyl and polyenyl free radicals as inferred from ultraviolet spectra are discussed. 

Allyl Free Radicals. Ayscough and Evans (3) have recently studied, by ESR measurements, the types of allylic free radicals produced by gamma-irradiation of several monomeric olefins. In irradiated polyethylene the allyl free radical is quite stable, persisting for several months at room temperature (31). The presence of these allyl free radicals is most noticeable in the case of high density polyethylene, and this type of free radical is undoubtedly the cause of the slow oxidation of polyethylene at room temperature, which lasts for 40 or more days after irradiation (10). Williams and Dole (40) could observe little if any oxidation of low density polyethylene when it was exposed to air after irradiation. By oxidation we mean formation of carbonyl groups as detected by infrared absorption studies at 1725 cm"1. Parenthetically, it should be noted that adding an oxygen. molecule to a free radical produces initially another type of free radical, a peroxy free radical, but in this paper we shall not discuss free radicals of this or any other types except those of hydrocarbons. 

This addition reaction is not restricted to a-hydroxyalkyl radicals, although this type of radical has been most widely investigated. Thus, allylic radical derived from 5MeCyt (Zhang and Wang 2003) and radicals derived from amino acids (Elad and Rosenthal 1969) are also reported to undergo this reaction. In DNA, they play a role in the formation of tandem lesions (Chap. 12.5), and it is likely that this kind of reaction contributes to free-radical-induced DNA/DNA and DNA/protein cross-linking. 

Methyl methacrylate (Fig. 1-4) and methacrylonitrile (6-5) are allylic-type monomers that do yield high molecular weight polymers in free radical reactions. This is probably because the propagating radicals are conjugated with and stabilized to some extent by the ester and nitrile substituents. The macroradicals are... 

The possible types of chain mechanisms for peroxodisulphate oxidation have been classified by Wilmarth and Haim according to the dominant initiation and termination steps, and the relative importance of sulphate radical-ions and hydroxyl radicals in the propagation steps. Some of the rate equations corresponding to the different types of mechanisms are the same, so the observation of a particular rate equation does not always permit a unique mechanism to be inferred. In certain cases the nature of the chain initiation step can be deduced from the effect of a free-radical scavenger on the reaction rate. Thus in the oxidation of 2-propanol, the addition of allyl acetate reduces the rate to that observed for the spontaneous decomposition of peroxodisulphate, indicating that the chain initiation step is the same as the rate-determining step of the spontaneous decomposition, viz. the fission of peroxodisulphate into sulphate radical-ions. 

The only low temperature concerted processes observed were of the jS(C-H) ester type eliminations to give olefin and acid (path a). Not unexpectedly, a Cope-type displacement of CO2 (path b) with allyl migration to form the alkyl allyl ether was not observed. At high temperatures the allyl carbonates without -hydrogens decomposed by simple bond fission free radical pathways. 

Dauben and McCoy demonstrated that the mechanism of allylic bromination is of the free-radical type, showing that the reaction is very sensitive to free-radical... 

The criterion of reasonablenesses not so vague as it might appear. The fact iiat a particular structure seems reasonable to us means that wc ha e previously encountered a compound whose properties are pretty well accounted for by a structure of that type the structure must, therefore, represent a fairly stable kind of arrangement of atoms and electrons. For example, each of the contributing structures for the allyl radical appears quite reasonable because we have encountered compounds, alkenes and free radicals, that possess the features of this structure. 

These types of mechanisms can be applied to most linear polymers. During the initiation reaction, the weaker bonds usually tend to dissociate first. It was noticed, for example, that the bonds (not including the bond to an sp carbon) of a carbon atom in a position to the double bond (the allyl carbon) are weaker than other C-C or C-H bonds. Therefore, the polymer containing an allyl carbon will be more likely to be involved in an initiation reaction. However, other reactions are not excluded in the free radical formation. 

Radicals in which the lone electron can be distributed around the molecule by resonance effects are particularly stable and occur in a number of oxidative reaction mechanisms. Examples of comparatively stable radicals of this type are the benzyl free radical and free radicals containing the allyl (or propenyl) group. These species can be stabilised as shown in Figure 8.5. 

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