Alkoxy radicals, oxidative degradation

A detailed study of the oxidation of propane also led Knox and Norrish [4] to the conclusion that the main oxidation route was via the successive degradation of aldehydes. Their scheme differed from that shown above, however, since it did not include the intermediate formation of an alkylperoxy radical and hydroxyl radicals were thought to be the principal chain carriers rather than the alkyl, alkylperoxy and alkoxy radicals. 

Organophosphorus compounds Phosphites are the main organophosphorus compounds used to control oxidative degradation of lubricants. They eliminate hydroperoxides, peroxy and alkoxy radicals, retard the darkening of lubricants over time and also limit photodegradation. These performance characteristics may be of importance for polyalphaolefins, hydrocracked or severely hydrotreated base stocks and white oils. 

Mechanism. Degradation of a-linolenic acid (a-lin) as proposed by (29,50,21) is demonstrated in Figure 6. The initial step is a hydrogen abstraction from an a-linolenic molecule by a radical species that was formed as a result of herbicidal action. In the following radical-chain reaction the w-3 alkyl peroxide is formed via the peroxy radical. Subsequently, this peroxide is decomposed in a Fenton-type reaction to the oj-3 alkoxy radical in the presence of transition metals that can undergo one-electron transfer reaction, e.g., Cu(I/II), Fe(Il/III), Ti(IIl/IV), or Ce(III/IV). The w-3 alkoxy radical can split by 8-sclssion to an unsaturated aldehyde and the ethyl radical. The latter is either oxidized to ethylene or reduced to ethane. 

The general mechanism for the atmospheric degradation of VOCs follows a pathway in which the initially produced RO2 radical is transformed into an alkoxy radical, RO. The fate of the alkoxy radicals determines the nature of the first stable products of oxidation, the degree of fragmentation of the original carbon chain and the potential for photo-oxidant formation. 

Both long and short molecular chains undergo degradation in an identical manner. The secondary radicals that form as a result of the decomposition of the alkoxy radical accelerate the oxidation process. It transforms into a chain process with an accompanying formation of various compounds. The products are alcohols, aldehydes, and ketones. The aldehydes are easily oxidized into peracids that degrade further into radicals. 

For a long time, the similarity between thermal and photolytic products was a mystery because Norrish photoprocesses are absent in the former situation. By using low MW model compounds, chemical derivatization, and C-NMR techniques, it has been assessed that a large fraction of the carbonylduring oxidative degradation of PP are a-methylated acids. Such a carboxylic structure can only originate from the oxidation of macroalkyl radicals [Eq. (51)1, which can be formed either by a Norrish I photoprocess or by j -sdssion of alkoxy radicals (Ref. 10, p. 583). 

The continuous decrease of unsaturation during photo-oxidative degradation can be explained by the attack on a double bond by alkoxy (RO", PO ) or peroxy (ROO , P02 ) radicals [729, 798] ... 

F. Gugumus [21] provides an alternative view of the thermal oxidation reactions in polymers. Various possibilities arising from inter- and intramolecular reactions between hydroperoxide groups, peroxy radicals, and alkoxy radicals are postulated. The author underlines the plausible over-estimation of degradation attributed to -scissions in polypropylene (PP) and offers alternative (non (3-scission) routes that result in formation of 1,2-dioxetane which can account for auto-oxidation, chain scissions and enhanced chemiluminescence of PP oxidation products. An illustration of this proposed scheme is provided in Scheme 6.4. 

Figure ll-B-28. Degradation scheme of alkoxy radicals generated dnring the oxidation of 2,4-dimethyl-2-pentanol (Atkinson and Aschmann, 1995). 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

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