Dialkyldiazenes

Polymeric azo-compounds and multifunctional initiators with azo-linkages are discussed elsewhere (see 3.3.3 and 7.6.1) as are azo compounds, which find use as iniferters (see 9.3.4). 

The kinetics and mechanism of the thennal and photochemical decomposition of dialkyldiazenes (15) have been comprehensively reviewed by Engel.The use of these compounds as initiators of radical polymerization has been covered by Moad and Solomon and Sheppard.The general chemistry of azo-compounds has also been reviewed by Koga et Koenig, and Smith. 

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

Unsymmetrical azo-compotmds 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. /-butylazocyclohexanecarbonitrile (32)J, or as high temperature initiators -biitylazoformamide (33)]. They have also been used as radical precursors in model studies of cross-termination in copolymerization (Section 

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Dialkyldiazenes (15, R—alkyl) are sources of alkyl radicals. While there is dear evidence for the transient existence of diazcnyl radicals (17 Scheme 3.18) during the decomposition of certain unsymmetrieal diazenes49 51 and of cis-diazenes,54 all isolable products formed in thermolysis or photolysis of dialkyldiazenes (15) are attributable to the reactions of alkyl radicals. 

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 <primaryfree spin of the incipient radical.49 For example, Timberlake7 has found that for the case of dialkyldiazenes,... 

There have been numerous studies on the kinetics of decomposition of A IRK. AIBMe and other dialkyldiazenes.46 Solvent effects on are small by conventional standards but, nonetheless, significant. Data for AIBMe is presented in Table 3.3. The data come from a variety of sources and can be seen to increase in the series where the solvent is aliphatic < ester (including MMA) < aromatic (including styrene) < alcohol. There is a factor of two difference between kA in methanol and k< in ethyl acetate. The value of kA for AIBN is also reported to be higher in aromatic than in hydrocarbon solvents and to increase with the dielectric constant of the medium.31 79 80 Tlic kA of AIBMe and AIBN show no direct correlation with solvent viscosity (see also 3.3.1.1.3), which is consistent with the reaction being irreversible (Le. no cage return). 

Alicyclic ew-dialkyldiazenes are very thermolabile when compared to the corresponding tram-isomers, often having only transient existence under typical reaction conditions. It has been proposed49 that the main light-induced reaction of the dialkyldiazenes is tram-cis isomerization. Dissociation to radicals and nitrogen is then a thermal reaction of the cis-isomer (Scheme 3.19),... 

An important ramification of the photolability of azo-compounds is that, when using dialkyldiazenes as thermal initiators, care must be taken to ensure that the polymerization mixture is not exposed to excessive light during its preparation. 

The proportion of useful radicals generated from common dialkyldiazenes is never quantitative typically it is the range 50-70% in media of low viscosity i.e. in low conversion polymerizations).3 88 89 The main cause of this inefficiency is loss of radicals through self-reaction within the solvent cage. 

For dialkyldiazenes where the a-positions are not fully substituted, tautomcrization to the corresponding hydrazonc may also reduce the initiator efficiency90 (Scheme 3.20). This rearrangement is catalyzed by light and by acid. 

Dialkyldiazenes are unstable and lose nitrogen with the formation of two alkyl radicals. This is the only process observed when the 1,1-dialkyIhydrazine is oxidised in alkaline solution where none of the diazenium ion survives [144]. Electrochemical oxidation of the cyclic dialkylhydrazine 32 in an unbuffered solution... 

Hydrazyl radicals derived from 1,2 diaryl- and 1,2-dialkyldiazenes have been postulated [186] and identified in homogeneous solution by ESR [187]. The observation that the product 15a, obtained from l-/er/-butyl-2-phenyldiazene, consists of only one isomer is in agreement with the expectation that the hydrazyl radical should be more stable when the unpaired electron is localized rather on the PhN than on the /BuN group. The oxidative IFET is assumed to proceed as described for the pho-todehydrodimerization. Heterocoupling of the hydrazyl and allyl radicals affords the allylhydrazine. Thus, formation of the addition product is a le lh+ process, whereas the by-products are formed via a 2e 2h+ process, irrespective whether the hydrazobenzene derivative is formed by subsequent reduction or disproportionation of the hydrazyl radical (Eqs. 34 and 35) ... 

N,N -Dialkylsulfamides (137), react with alkaline sodium hypochlorite to yield dialkyldiazenes (151) in non-aqueous media, the intermediate thiadiaziridine, 1,1-dioxides (152) were isolated (Scheme 62). 

The classical methods of generating authentic free radical intermediates include thermal and photochemical decomposition of diacyl peroxides (or peresters) and dialkyldiazenes. Both these types of precursor have been successfully used to produce cyclopropylmethyl-type radicals but, because neither compound can readily be incorporated into a ehain reaction, most applications have been in mechanistic or spectroscopic work. Bis(cyclopropylacetyl) peroxide (1) and /err-butyl cyclopropaneperoxyacetate (2) have frequently been used in EPR spectroscopy. ... 

The alkali hypochlorite reaction of A/ iV -dialkylsulphamides leads to the formation of dialkyldiazenes 430. Ohme s group393,394 has postulated thiadiaziridine-1,1 -dioxides (431) as intermediates in the reaction (equation 141). 

I he hyponitrites generally appear somewhat more efficient with respect to radical generation than the dialkyldiazenes (see 3.3.1.1). However, a proportion of radicals is lost through cage reaction with formation of the corresponding dialkyl peroxides or ketone plus alcohol (Scheme The disproportionation... 

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