Primary alkoxy radicals atom abstraction

However, this process is thwarted because of a high activation energy hence. Reaction (4.7) plays a significant role at higher temperatures or under catalysed conditions, considered in Sections 4.2.2 and 4.2.3. Once formed, hydroxy and especially primary alkoxy radicals are so active that they abstract hydrogen atoms in non-selective reactions. Reactions (4.8. 10) ... 

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... 

However, the situation is not as clear-cut as it might at first seem since a variety of other factors may also contribute to the above-mentioned trend. Abuin et a/.141 pointed out that the transition state for addition is sterically more demanding than that for hydrogen-atom abstraction. Within a given series (alkyl or alkoxy), the more nucleophilic radicals are generally the more bulky (i.e. steric factors favor the same trends). It can also be seen from Tabic 1.6 that, for alkyl radicals, the values of D decrease in the series primary>secondary>tertiary (i.e. relative bond strengths favor the same trend). 

Alkylperoxy radicals are selective, and abstraction of a secondary H-atom is favored over abstraction of a primary H-atom. Alkoxy radicals, on the other hand, are less selective and can abstract primary hydrogens ... 

Many types of peroxides (R-O-O-R ) are also utilized, including diacyl peroxides, peroxydicarbonates, peroxyesters, dialkyl peroxides, and inorganic peroxides such as persulfate, the latter being used mainly in water-based systems. The rate of peroxide decomposition as well as the subsequent reaction pathway is greatly affected by the nature of the peroxide chemical structure, as illustrated for fert-butyl peroxyesters in Scheme 4.2. Pathway (a), the formation of an acyloxy and an alkoxy radical via single bond scission, is favored for structures in which the carbon atom in the a-position to the carbonyl group is primary (for example, terf-butyl peroxyace-tate, R = CHg). Pathway (b), concerted two-bond scission, occurs for secondary and tertiary peroxyesters (for example, terf-butyl peroxypivalate, R = C(CH3)3) [1, 2]. The tert-butoxy radical formed in both pathways may decompose to acetone and a methyl radical, or abstract a hydrogen atom to form tert-butanol. 

Primary and secondary peroxy radicals are more reactive in hydrogen abstraction than analogous tertiary radicals [64, 65]. The secondary peroxy radicals can abstract a labile hydrogen atom from another polyethylene molecule to form hydroperoxides species (Reaction 3 from Scheme 2.2), which are the compoxmds with higher contribution to the oxidation cycle, as well as another radical, through which the process can continue. In most polymers, the rate of this step in the chain reaction determines the overall rate of oxidation [66]. Finally, due to the thermal instability of the 0-0 bonds of these species (bond energy = 40 kcal/mol), they readily decompose into hydroxyl (OH ) and alkoxy (RO ) radicals, which ultimately result in the appearance of final... 

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