Activation energy alkyl radical -scission

This reaction has been put forward to explain the observed fact that the number of chain scissions corresponds to the number of carboxyl groups formed in the oxidation of polyethylene. Activation energy of both processes is 140 kj/mol. The mechanism of such an elementary fragmentation reaction remains however uncertain. The reactions of a chain scission are likely to precede the isomerization of original secondary alkyl peroxy radicals. 

The alkyl radical initially formed reacts readily with oxygen to give the corresponding alkylperoxy radical, which may abstract hydrogen from a fuel molecule to form the alkylhydroperoxide or alternatively decompose to yield an aldehyde and an alkoxy radical. Some workers thought that this decomposition was preceded by an isomerization of the alkylperoxy radical, the activation energy of which had been estimated by Semenov [3] to be ca. 20 kcal. mole. Shtern was of the opinion that the major, if not the only, fate of the alkylperoxy radical was decomposition, but in contrast to other workers he believed that it must involve scission of a C—C bond and could not lead to the formation of a carbonyl compound and hydroxyl radical. 

It is assumed that this is also a rate determining step for the overall reaction. The activation energy of reaction (4) and the site density of oxygen active centers were the only adjustable parameters of the model. In general, a C-H bond scission for reactants and products of the methane dimerization process occurs by an Eley-Rideal (E-R) type mechanism to form a gas-phase alkyl radical and a hydroxyl surface site (HO ) ... 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

The alkoxy radical is usually described as a typical product of the thermal decomposition of hydroperoxides. Nevertheless, in the post-irradiation oxidation process at room temperature, it cannot originate from this reaction because all the formed products follow a kinetic similar to that of ketone formation [21]. The reaction between the alkyl macroradical and the peroxy macroradical forms peroxides (Scheme 9, Reaction 20), but we can also hypothesize Reaction 21, Scheme 9. Literature studies demonstrate that the alkoxy radical can give beta-scission (Reaction 28) forming a primary alkyl radical and CO, a product that is found during the irradiation of PE (Scheme 10, Reaction 29) [24]. The activation energy of this reaction is around 50kJ/mole. 

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