Radicals induced decomposition

Decomposition. Acetaldehyde decomposes at temperatures above 400°C, forming principally methane and carbon monoxide [630-08-0]. The activation energy of the pyrolysis reaction is 97.7 kj/mol (408.8 kcal/mol) (27). There have been many investigations of the photolytic and radical-induced decomposition of acetaldehyde and deuterated acetaldehyde (28—30). 

Two secondary propagating reactions often accompany the initial peroxide decomposition radical-induced decompositions and -scission reactions. Both reactions affect the reactivity and efficiency of the initiation process. Peroxydicarbonates and hydroperoxides are particularly susceptible to radical-induced decompositions. In radical-induced decomposition, a radical in the system reacts with undecomposed peroxide, eg ... 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

Primary and secondary dialkyl peroxides undergo thermal decompositions more rapidly than expected owing to radical-induced decompositions (73). Such radical-induced peroxide decompositions result in inefficient generation of free radicals. 

Because di-/ fZ-alkyl peroxides are less susceptible to radical-induced decompositions, they are safer and more efficient radical generators than primary or secondary dialkyl peroxides. They are the preferred dialkyl peroxides for generating free radicals for commercial appHcations. Without reactive substrates present, di-/ fZ-alkyl peroxides decompose to generate alcohols, ketones, hydrocarbons, and minor amounts of ethers, epoxides, and carbon monoxide. Photolysis of di-/ fZ-butyl peroxide generates / fZ-butoxy radicals at low temperatures (75), whereas thermolysis at high temperatures generates methyl radicals by P-scission (44). 

Diacyl peroxides (20, = alkyl oi aiyl) also undergo three types of radical induced decomposition (187) all of which produce the radical... 

As a consequence of their susceptibility to radical-induced decomposition, neat and concentrated solutions of diacyl peroxides undergo self-accelerating... 

Alkyl peroxyesters are much less sensitive to radical-induced decompositions than diacyl peroxides. Induced decomposition is only significant in peroxyesters containingnonhindered a-hydrogens or a, P-unsaturation (213,242). 

Transfer to initiator is generally of lesser importance than with the corresponding diacyl peroxides. They arc, nonetheless, susceptible to the same range of reactions (see 3.3.2.1.4). Radical-induced decomposition usually occurs specifically to give an alkoxy radical and an ester (Scheme 3.34). 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

It is also known that alkyl cobaloximes are subject to radical-induced decomposition.2 7 This suggests an alternative to the mechanism shown in Scheme 9.28 involving reversible chain transfer (Section 9.5). 

The kinetic form of the decomposition in various solvents indicates competing unimolecular homolysis of the peroxide link (a) and radical induced decomposition (b). Other diacyl peroxides behave similarly, except that, in the case of acetyl peroxide, induced dceomposition is much less important. More highly branched aliphatic or a-phenyl-substituted diacyl peroxides decompose more readily, partly because induced decomposition is more important again and partly because of the occurrence of decomposition involving cleavage of more than one bond (for a mechanistic discussion of these cases, see Walling et al., 1970). 

Hollaender and Neumann, 1971) as supporting a free-radical induced decomposition of intermediate phenyldi-imide (phenyldiazene) by way of the diazenyl radical [equation (48) cf. Hoffmann and Guhn, 1967]. 

The last-mentioned compound has been chosen in preference to benzoyl peroxide in many of the more recent kinetic investigations on account primarily of the freedom of its decomposition from side reactions such as radical-induced decomposition (see p. 113). It may also serve as a photoinitiator under the influence of near ultraviolet radiation, which... 

Radical grafting, 10 206 Radical-induced decompositions, 14 280 of dialkyl peroxydicarbonates, 14 289 Radical ozone reactions, 17 774 Radical polymerization, 22 40. See also Free-radical polymerization controlling, 14 297 of methacrylic ester polymers, 16 279-290... 

It is clear from a study of thermal and radical-induced decompositions of N-alkoxycarbonyldihydropyridines that radical processes are of minor importance, and that pyridine formation is probably a consequence of 1,2-elimination of formate (Scheme 6). It has also been concluded that the rate of 1,4-elimination of formate from iV-alkoxycarbonyl-l,4-dihydropyridines at higher temperatures is too rapid to be explained by a homolytic process. 

Thermal decomposition in three different ways, i.e. homolytic, polar and radical induced decomposition, as well as intermolecular reaction of sulfonyl peroxides are the main reactions displayed by sulfonyl peroxides. When bis(arylsulfonyl) peroxides are allowed to decompose at 25-40 °C in chloroform, homolytic 0—0 bond fission followed by hydrogen abstraction from the solvent results in the formation of the corresponding arylsnlfonic acids. Mixed acyl sulfonyl peroxides undergo complicated thermal decomposition in solution, and have been used commercially as polymerization initiators, since they provide a source of free radicals at a relatively low temperature . 

By studying the reaction in the presence and absence of a radical acceptor, 2,6-di-terf-butyl-4-methylphenol, radical-induced decomposition was selectively eliminated and the separate terms in Equation K were evaluated. Using barium dibenzyl dithiophosphate the following ratio was obtained ... 

Two secondary propagating reactions often accompany the initial peroxide decomposition radical-induced decompositions and /3-scission reactions. Both reactions affect the reactivity and efficiency of the initiation process. 

Because di-tert-alkyl peroxides are less susceptible to radical-induced decompositions, they tire safer and more efficient radical generators Ilian primary or secondary dialkyl peroxides. They are the preferred clialkyl peroxides for generating free radicals for commercial applications. 

Thermal decompositions of peroxycarboxylic acids and dieir salts can proceed by free-radical and nonradical padrs. Often the decomposition products and the rate are affected by the nature of the solvent. Peroxycarboxylic acids undergo photodecomposition and radical-induced decomposition. They also are decomposed by a variety of metals, metal ions, and complexes. 

Diacyl peroxides (16. R1 = R2 = alkyl or aryl) also undergo radical induced decomposition either by direct radical displacement on the oxygen-oxygen bond, or by radical addition to, or abstraction from, the hydrocarbyl group adjacent to die peroxide. 

These low temperature peroxides are susceptible to radical-induced decompositions. This susceptibility largely accounts for the hazards associated with their production and storage. In contrast to diacyl peroxides... 

As a peroxide class, dialkyl peroxydicarbonates are very susceptible to radical-induced decompositions ... 

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. 

Cooper, W. The effect of structure of diacyl peroxides on their radical induced decomposition in vinyl monomers. J. Chem. Soc. 1952, 2408. 

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