Macroalkyl radicals formation

Radiation chemistry in polymer research, 168-169 Reactive macroalkyl radicals, formation, 409 Reactive modifiers addition of reactive antioxidants on rubbers, 417 adhesion, 420,422 demanding applications, 414,416 improving additive performance during melt processing, 412 polymer bound antioxidant, 418-419/ Reduced poly(vinyl chloride),... 

The central feature of this mechanism is, therefore, that the phenoxyl radical is reversibly reduced and re-oxidised this leads to the continuous consumption of macroalkyl radicals. The phenoxyl radical can, therefore, react with polypropylene radicals and compete with PP-MA adduct formation in the stabilised polymer (Figure 3, curve MA-S). 

PVC behaves quite differently during processing. The initial step is the same, the formation of macroalkyl radicals, but the latter can undergo a rapid unzipping of hydrogen chloride from the initial... 

Under certain conditions, notably during polymer processing where the oxygen concentration is low, and in light, where the rate of formation of macroalkyl radicals is relatively high, reactions (3.8) and (3.9) may operate together to produce a catalytic antioxidant effect. 

Primary process of formation of primary and secondary macroalkyl radicals. 

Alko l radicals and chlorine atoms are highly reactive in hydrogen abstraction leading to the formation of macroalkyl radicals (P), which in turn react with almost zero activation energy with ground state oxygen which is itself a diradical (reaction 2) ... 

Post-irradiation oxidation of UHMWPE at room temperature results in the formation of ketones as main products of the oxidation cycle, together with hydroperoxides and variable amounts of acids, alcohols, esters, and lactones [21]. It has been shown [21] that hydroperoxides in UHMWPE are stable at room temperature and start to decompose at temperature higher than 70°C, therefore ketones and the other oxidized species observed during post-irradiation oxidation cannot result only from the decomposition of hydroperoxides. Considering these results, it can be supposed that ketones are also formed during the first step of oxidation of UHMWPE as a consequence of a direct reaction between macroalkyl radicals and oxygen. A modified Bolland s cycle is proposed in Scheme 9 [21]. 

Acids are produced by scission of the polymeric chain, with a mechanism that has not yet been elucidated [21, 28-29]. The alkoxy radical formed through Reaction 21 can decompose via (3-scission, according to Reactions 28 and 29 (Scheme 10), with the formation of a methyl chain end, whose increase has been observed in the post-irradiation oxidation process, and of a carbonyl radical, which in turn decomposes giving a primary macroalkyl radical and carbon monoxide (CO), commonly found among the products of irradiation or thermo-oxidation of PE [2, 30]. Primary alkyl macroradicals react with oxygen to form primary hydroperoxides, then the hydroperoxides decomposition results in the formation of acids, as already stated in the literature [28]. 

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