Free radical induced decomposition

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]. 

Toluene flow system. A minor amount of free radical induced decomposition was evident. 

This conclusion is further supported by calculations of the reaction rate of the free radical induced decomposition of /-butyl alcohol. The most reasonable mechanism is the self inhibiting chain process... 

As well as purely thermal scission, then, there must also be a free-radical-induced decomposition. This decomposition can be described kinetically by a separate rate constant k ... 

Free Radical-Induced Decomposition of Allylic Peroxides and Hydroperoxides... 

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). 

Dilly O, loemJ B, Vos A, Munch JC (2004) Bacterial diversity in agricultural soils during litter decomposition. Appl Environ Microbiol 70 468-474 Dizdaroglu M (1991) Chemical determination of free radical-induced damage to DNA. Free Radical Biol Med 10 225-242 Eaton RW, Ribbons DW (1982) Metabolism of dimethylphthalate by Micrococcus sp. strain 12B. JBacteriol 151 465-467... 

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... 

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... 

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 . 

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. 

The free radical inducing peroxide decomposition may be obtained from another substance—free radical generator. In this case, the primary reaction is the one that generates active sites, and the secondary decomposition one may proceed via formation of previously unobserved products. 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

The decomposition of hydroperoxides can also be induced by raising the temperature and is promoted by metal catalysis. Free radicals formed during the reaction (4) and (5) can also take part in radical induced decomposition of hydroperoxides... 

The latter observations with methyl oleate, together with thermodynamic considerations and EPR evidence for free radical intermediates, suggest an alternative explanation for the dramatic increase in oxidation rates once hydroperoxides accumulate, namely that bimolecular decomposition may be specific to allylic hydroperoxides and proceed via LOO radical-induced decomposition rather than by dissociation of hydrogen-bonded dimers (280). Reaction sequence 63 is analogous to Reactions 49 and 50a, where one slowly reacting radical reacts with a... 

Kinetics of thermal decomposition of dialkyl peroxides in solution as well as the gas phase have been reviewed by Molyneux and Frost and Pearson . The decomposition of dialkyl peroxides is moderately free from induced decomposition, compared to other types of peroxides. As seen from Table 65, the first-order rate coefficient increases by about 16 % when the initial peroxide concentration is increased about 5 fold at reasonably high peroxide concentrations. The increase in the rate coefficient is attributed to an induced decomposition where hydrogen atom abstraction generates the radical (I). Further reaction of (I) produces isobutylene oxide and the f-butoxy radical, viz. 

As would be predicted from Equation 1, the rate of dissociation of free radical initiators is decreased by the application of pressure. Thus azobisisobuty-ronitrile dissociates with a rate constant equal to 4.47 X 1(H sec." at 1500 atm. but at 1 atm. the dissociation rate constant is 5.5 X 10 sec. (8). Studies concerning the effect of pressure on the decomposition of benzoyl peroxide reveal that the rate of this reaction also decreases with increasing pressure (II, 18). The extent to which the radical-induced decomposition of this peroxide at high pressures affects the rate is not clear, but it appears that some complications arise from this cause. 

In the polymer industry phenolic compounds are important stabilizers they are used to prevent the free-radical induced polymerization of monomers (e.g. methyl methacrylate, styrene) during transit and as stabilizers for polymer systems where radical induced decomposition and decay mechanisms operate. Examples of the latter include polyolefins, such as polyethylenes or polypropylenes, and polyvinyl chlorides. However, these uses of phenols and phenolic polymers are not considered in this chapter (refer to Chapter 12). 

Homolytic processes Evidence also has been presented for the radical-induced decomposition of thiolsulfinates (6,7). Homolytic cleavage is facilitated by the weak S-S bond ( 40kcal). The availability of sulfidic sulfur for radical attack is indicated by the observation that thiolsulfinates strongly retard the free radical polymerization of vinyl monomers (8). 

The lability of allylic iodides and their sensitivity to free radical-induced isomerization, polymerization, and decomposition reactions has discouraged kinetic studies of their substitution and isomerization reactions. 

CD3CC1=CD.CD3) were also formed, their origin being revealed by the phase of the polarization. Polarization in radicals escaping the primary cage was in some cases retained in the products of the decomposition of further diazoalkane which the free radicals induced (Roth, 1972a). 

Among the several possible mechanisms which have been expressed, a free-radical reaction could be a dominant process for plasma depositions [36, 37]. Hence, two types of reaction, i.e., plasma-induced polymerisation and plasma-state polymerisation are presumed. The plasma-induced polymerisation is the conventional free-radical induced polymerisation of molecules containing unsaturated carbon-carbon bonds. The plasma-state polymerisation depends on the presence of ions, electrons and other species which are energetic enough to break any bond. The resulting decomposition products of the plasma recombine by a free-... 

Wellington (1980) discussed the interaction of hydroperoxides with iron as a transition metal catalyst which produces more free radicals which may initiate further radical-induced decomposition of the polymer. In order to remove the oxygen from solution, reducing agents such as sulphite and bisulphite may be present however, these two may react with iron to produce radicals. For example, the reaction between bisulphite (HSO") and Fe as follows ... 

In a broad sense radical-induced decomposition may be considered as the reaction of any substrate promoted by a free radical, but the more common usage is to restrict consideration to those cases in which the molecule undergoing reaction is a free radical initiator (1, ). Thus radical-induced decomposition may be de-fTned as reaction of a free radical with the primary source of the radical. This phenomenon is particularly noticeable when a free radical initiator does not react with first-order kinetics, but is consumed by the radicals it generates in chain processes. This case was originally elucidated for the kinetics of benzoyl peroxide decomposition in solvents such as ethyl ether ( - ), and was found to occur with radical displacement (the SH2 reaction) (S) by solvent derived radicals on the peroxidic oxygen of benzoyl peroxide as shown in Fig. 1. 

Triphenylsulfonium salts do not undergo free radical induced chain decomposition. However, Ledwith has reported that diphenyl-4-thiophenoxyphenylsulfonium salts give high quantum yields of diphenyl sulfide when irradiated at 304 nm in THF ( = 1,5) and cyclohexene oxide (4> = 3) indicating that these particular compounds also undergo free radical induced chain decomposition. 

Azobisnittiles are efficient sources of free radicals for vinyl polymerizations and chain reactions, eg, chlorinations (see Initiators). These compounds decompose in a variety of solvents at nearly first-order rates to give free radicals with no evidence of induced chain decomposition. They can be used in bulk, solution, and suspension polymerizations, and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

---