| Organic peroxides possess one or more oxygen–oxygen bonds, which decompose thermally to produce two radicals. The rates of decompositions can be enhanced by specific promoters or activators being mostly multivalent metals such as iron, cobalt or vanadium. They significantly decrease the energy necessary to break the oxygen–oxygen bond. Such accelerated decompositions occur below the normal application temperatures of the peroxides usually resulting in generation of only one useful radical, instead of two.
Azo initiators are symmetrical azonitrile compounds decomposing thermally by cleavage of the two carbon–nitrogen bonds to form two alkyl radicals and a nitrogen molecule. The resulting tert-alkyl radicals are generally more stable than most of the radicals generated from peroxide initiators. In addition, these radicals do not abstract hydrogens from the polymer backbones, which results in formation of linear polymers since the branched grafting is suppressed. Azonitrile decomposition rates show minor solvent effects and are not affected by transition metals, acids, bases, and many other components of the environment. These results in two advantages of azoinitiatos; their decomposition rates predictable and they cane be used for curing resins that contain a variety of extraneous materials. A drawback of azonitriles over peroxides, however, is the formation of toxic tetrasubstituted succinonitrile derivatives due to recombination of the alkyl radicals. |
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