Redox initiators with alkyl hydroperoxides

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

Applying the steady-state approximation to the radicals R and OH- a rate law of the form above may be derived where [Cu J ofCu L j. The inverse dependence on [L l indicates that [Cu L] does not react with HgOj at a significant rate. A chain reaction has also been postulated in the oxidation of copper(n) dialkyldithiophos-phates with alkyl hydroperoxides. Initially, the redox process yields alkoxy and alkyl peroxy radicals. The kinetics of the initial reaction are second-order. For the dithiophosphate, the reaction is first-order with respect to each reagent, whereas for the corresponding dithiocarbamate complex there is a zero-order dependence on the metal species and a second-order dependence on the hydroperoxide concentration. 

There are other initiator systems of lesser commercial importance. Cumene hydroperoxide is reported to cure acrylic adhesives in the presence of alkyl or pyridyl thioureas [105]. These initiators have been combined with a phosphated acrylate to promote adhesion to metal [106]. Thiourea-based initiators can be applied as a one-part on galvanized metal, where the metal surface provides the second part of the redox initiator [107]. 

Other initiators such as alkyl peroxides and hydroperoxides and azo compounds allow the generation of initiator radicals over a wide range of temperatures (50-200 °C). Redox systems (e.g., Fe or N,N-dimethylaniline with a peroxide) extend this temperature range down to 0 °C and lower. Ultraviolet radiation, usually in the presence of an initiator or photsensitizer, is useful for coatings, imaging and printed cicuit board applications. Other techniques include ionizing radiation and electroinitiation. 

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