Selected Kinetic Data for Decomposition of Azo-Compounds

Water-soluble azo compounds include 4,4 -azobis(4-cyanovaleric acid) (29) and the amidinium hydrochlorides (22 and 23). 

Unsymmetrical azo-compounds find application as initiators of polymerization in special circumstances, for example, as initiators of living radical polymerization [e.g. triphenylmethylazobenzene (30) (see 9.3.4)], as hydroxy radical sources [e.g. a-hydroperoxydiazene (31) (see 3.3.3,1)1, for enhanced solubility in organic solvents [e.g. f-butylazocyclohexanecarbonitrile (32)J, or as high temperature initiators [e.g. t-butylazoformamide (33)]. They have also been used as radical precursors in model studies of cross-termination in copolymerization (Section 

While some details of the kinetics of radical production from dialkyldiazenes remain to be unraveled, their decomposition mechanism and behavior as polymerization initiators are largely understood. Kinetic parameters for some common azo-initiators are presented in Table 3.2. 

Thermolysis rates ( j) of dialkyldiazenes (15) show a marked dependence on the nature of R (and R ). The values of k increase in the series where R (=R ) is ary primary secondary tertiary allyl. In general, kA is dramatically accelerated by a-substituents capable of delocalizing the free spin of the incipient radical.49 For example, Timberlake7 has found that for the case of dialkyldiazenes, 

X-C(CH3)2-N=N-C(CHj)2-X that kd increases in the series where X is CH3 -OCH3 -SCH3 -CO R -CX -P h -CH=CH2 (see also Table 3.2). These results can be rationalized in terms of the relative stability of the radicals generated (R, R1 ). 

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Table 3.2 Selected Kinetic Data for Decomposition of Azo-Compounds ...

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