Thermal oxidation scheme 8-scission

It is well known that thermal oxidation in polyethylene (PE) in the presence of oxygen leads to sudden and deep embrittlement, in which oxidative chain scission plays a key, but perhaps indirect, role. We have developed a kinetic model, derived from a branched radical chain mechanistic scheme, able to predict accurately molecular structural changes (7). 

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. 

PEG can be severely degraded in air. Its melting point and heat of fusion are reduced by as much as 13 °C and 32 kJ kg"1, respectively [81]. The thermal degradation of PEG in air follows a random chain scission oxidation mechanism, and could be suppressed by addition of an antioxidant, 2, 2,-methylene-bis (4-methyl-6-tert-butylphenol) (MBMTBP), due to the reaction of MBMTBP with ROO radicals formed in the propagation step [79]. Low-molecular-weight esters including formic esters are produced as the main products of the thermal degradation of PEG (Scheme 3.17) [80]. 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

Photolysis of 3-buten-l-ol nitrite affords no cyclized products (Cy5/Cy4) neither does 5-hexen-l-ol nitrite (Cy6/Cy7). The same result is obtained on peroxydisulphate oxidation of 5-hexen-l-ol. In the Cy6/Cy7 case an important competitive pathway is probably 1,5-intramolecular ally lie hydrogen abstraction and, indeed, esr spin trapping by nitrosodurene " provides evidence of this. Cyclization in the Cy6/Cy7 case was considered to explain the reaction products of tetrahalogeno-o-benzoquinones with 2,3-dimethylbut-2-ene but was discarded in favor of a direct cycloaddition process on the basis of spin trapping and deuteration experiments. As discussed before, cyclization in the Cy3/Cy4 case must be difficult to observe because of the high j5-scission rate of oxyranylalkyl radicals. Nevertheless, this pathway has been used recently to explain the formation of diepoxides in the thermal-, photochemical-, or ferrous-salt-induced decomposition of unsaturated cyclic peroxides. In view of the multistep scheme involved this conclusion must await further confirmation. 

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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