Alkyl hydroperoxides induced decomposition

This is basically the same type of induced decomposition that occurs with other peroxide classes, eg, the dialkyl peioxydicaibonates and diacyl peroxides. Table 8. Commercial rerf-Alkyl Hydroperoxides ... 

Therefore, first-order, decomposition rates for alkyl hydroperoxides, ie, from oxygen—oxygen bond homolysis, are vaUd only if induced decomposition reactions... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

As electrophilic substitutes for peracids, the use of borate ester induced decomposition of alkyl hydroperoxides and molybdenum VI peroxy-complexes have been reported in the recent literature. Although these reagents have led to the epoxidation of olefins in greater than 90% yield there are no reports yet of their application to steroid olefins. 

Cobalt compounds are generally the more effective catalysts and consequently have received the most attention.34,112 131c Much information has been gained from studies of cobalt-catalyzed decompositions of alkyl hydroperoxides under nonautoxidizing conditions. One important point to be borne in mind in these studies is that radical-induced chain decomposition of the hydroperoxide, via reactions (55) and (56), is always in competition with decomposition via the foregoing cycle. 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

P. F. Wolf, R. K. Barnes, Borate ester-induced decomposition of alkyl hydroperoxides. The epoxidation of olefins by electrophilic oxygen, J. Org. Chem. 34 (1969) 3441. 

Polymer is degraded by heat, energy, UV or residues of catalyst and generates alkyl radicals. This alkyl radical reacts with oxygen and form peroxy radicals. These peroxy radicals abstract hydrogen from other polymer and forms alkyl radicals and hydroperoxide. The decomposition of hydroperoxide to alkoxy and hydroxyl radicals induces additional decomposition of the polymer chain. In order to stop the radical chain reaction of degradation, stabilisers such as phenolic antioxidant, phosphites, thioether and hindered amine light stabilisers (HALS) are added. 

Radical reaction pathways govern the crosslinking and degradative reactions of UHMWPE. For these reactions to occur, macroradicals must be induced in the polymer, for example by thermal decomposition of hydroperoxides or by high-energy radiation, which leads to homolytic bond scissions with the production of alkyl macroradicals. In previous chapters, we have often referred to crosslinking... 

---