Alkoxy macroradical

The thermal or photochemical homolysis of tertiary hydroperoxides leads to the formation of alkoxy macroradicals 3-scission of alkoxy macroradicals may occur. This leads to a, 3-unsaturated ketones on the butadiene component and induces the scission of the butadiene-SAN grafts. The macroradical so formed on the SAN macrophase is the precursor, after isomerization, of the oxidation of the styrenic component according to... 

This chain can be end-capped with a halide perester (Equation 5.22), which provides a chain with a potential radical-forming site at one end. Thermal decomposition of this group in a second-stage reaction, in the presence of another monomer, generates an alkoxy macroradical from which to grow the second block, but also produces a second radical fragment likely to produce some homopolymer as a contantinant... 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

The next stage posited is scission of the hydroperoxide which, in theory, can take place in one of two ways 0-0 bond scission to form an alkoxy macroradical and HO, or C-O bond scission to form an alkyl macroradical and HOOa... 

The next step is thermal scission of the hydroperoxide to form an alkoxy macroradical (and HO° species) that may abstract hydrogen atom to form a hydroxyl group. This species can undergo chain scission to form an acid and aldehyde chains ends. 

Thus a short-Uving (ca. 1 ps) alkoxy type macroradical was only present in our experimental system. This macroradical was extremely unstable and degraded quickly to fragments with lower molar mass (compare Scheme 1). 

As has been established [45], one or both of the nitrogen-containing products and alkoxy radicals decompose at 90 °C at a significant rate to produce terminal alkyl macroradicals ... 

In the solid phase, the recombination of radicals in the cage is highly efficient, whereas the escape of NO and RO radicals from the cage is quite low. Alkoxy radicals RO generated in reactions (Equation 3.77) decompose or enter into substitution reactions with neighbouring macromolecules to form chain R and end R alkyl macroradicals as well as low-molecular alkyl radicals r. The appearance of the latter radicals in the reaction of PP hydroperoxide with NO was verified in [66] ... 

Alkoxy radicals may decompose or enter into substitution reactions with macromolecules to form chain Rc and end R alkyl macroradicals, and low molecular weight alkyl radicals r, which with NO give nitroso eompounds ... 

The mechanism of peroxide crosslinking is shotvn in Scheme 16.38. Upon heating, the peroxide decomposes into the primary alkoxy radicals, tvhich may further react to secondary radicals. These radicals abstract hydrogen atoms from the EPM chain. In the case of EPM, crosslinks are formed by combination of two macroradicals, whereas in the case of EPDM crosslinks are formed via combination, but also via addition of the macro-radical to the pendent unsaturation of a second EPDM chain. The formation of C-C bonds explains the higher thermal stability of peroxide-cured EPDM in comparison with sulfur-vulcanized EPDM with its labile S-S crosslinks. The reactivity for peroxide cure increases in the series ENB DCPD < VNB, because of the decreased steric hindrance at the unsaturation. The efficiency of peroxide curing is enhanced by the addition of co-agents, that is, chemicals with two or more unsaturated bonds, which are actually built... 

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. 

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]. 

The source of macroradicals RO- is macromolecules with hydroperoxide groups from which alkoxy radicals are formed in reactions of the type... 

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