Azonitrile initiators, decomposition rates

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

The rates of initiation depend on the type of activation ch.osen. In photochemical initiation, Vs = 2 0/, where I is the absorbed light intensity and 0 = the efficiency coefficient. With an average intensity, rates of initiation of approximately 10"7 mole l 1 s 1 are attained. In thermal activation, autoinitiation by interaction between oxygen and aldehyde gives low values of approximately 10 9 mole l-1 s"1 under standard laboratory conditions. When azonitrile is used, since the thermal decomposition rate of this product is approximately first order [62], Vi is given by... 

Table 2.2. Decomposition Rates of Some Azonitrile Initiators ...

Table 3.2 Decomposition rates of some azonitrile initiators...

The effects of the medium upon the dissociations and efficiencies of typical initiators have been reviewed with some discussion of poly functional initiators. Solvation effects have been considered for azoisobutyronitrile. The rate of decomposition of the azonitrile in dimethylformamide/glycerol mixtures depends upon the composition, but because of inhomogeneities resulting from limited solubility of the initiator and not because of effects such as differences between the viscosities of the various systems. The solubilities of typical peroxides over a range of pressures and. temperatures have been measured to assess problems that might arise from crystallization of initiators at high pressures. ... 

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