Disproportionation radical

The main reason that the decreases as the polymerization temperature increases is the increase in the initiation and termination reactions, which leads to a decrease in the kinetic chain length (Fig. 17). At low temperature, the main termination mechanism is polystyryl radical coupling, but as the temperature increases, radical disproportionation becomes increasingly important. Termination by coupling results in higher PS than any of the other termination modes. 

The second method is to generate the quinone methide via radical coupling reactions, analogously to the way they are produced via radical coupling during lignification (Fig. 12.4c) orto exploit radical disproportionation reactions (Fig. 12.4d). 

In order to document the radical disproportionation reaction, we have used FT-IR spectroscopy to characterize the irradiation products. Upon irradiation of 1 in pentane, the formation of the characteristic peak near 2100 cm-1 due to Si-H stretching vibrations was readily apparent. The IR spectrum obtained in perdeuterated pentane was identical, suggesting that radical processes other than abstraction from the solvent are involved. Furthermore the ESR spectrum obtained in this solvent is identical to that already described. This raises the question whether the initially formed silyl radicals really abstract hydrogen from carbon with the formation of carbon-based radicals as suggested (13), particularly in light of the endothermicity of such a process. 

Scheme 5 The Russel scheme of secondary peroxyl radical disproportionation.

Chemiluminescence in the liquid-phase hydrocarbon oxidation was discovered. It was proved to be the result of secondary peroxyl radicals disproportionation R. F. Vasi ev, O. N. Karpukhin, and V. Ya Shlyapintokh [69]... 

It can be seen that primary and secondary R02 radicals disproportionate with the participation of the a-C—H bond. This explains why the substitution of D in the a-position for H retards the recombination of R02 [/tn//tD =1.9 for ethylbenzene, h/ d = 2.1 for styrene, and h/ d=1-37 for diphenylmethane [179]). Because of this, R02 radicals of unsaturated compounds with a double bond in the a-position to the peroxyl free valence disproportionate more rapidly than structurally analogous aliphatic peroxyl radicals (at 300 K, 2kt = 2x 107 and 3.8 x 106 L mol-1 s-1 for R02 radicals of cyclohexene and cyclohexane, respectively [180]). Among the products of secondary peroxyl radicals disproportionation, carbonyl compound and alcohol were found in a ratio of 1 1 at room temperature (in experiments with ethylbenzene [181], tetralin [103], and cyclohexane [182-184],... 

Hydrogen peroxide was identified as the product of secondary peroxyl radical disproportionation [187-192], It cannot be explained by the concerted mechanism of tetroxide decomposition. 

This mechanism of homolytic decomposition of formed tetroxide explains all known facts about secondary peroxyl radicals disproportionation ... 

It can be seen that primary and secondary R02 radicals disproportionate in the cage involving the a-C—H bond, which explains why the substitution of D in the a-position for H retards the disproportionation of R02 . Because of this, R02 radicals of unsaturated compounds with a double bond in the a-position to the peroxyl free valence disproportionate more rapidly than structurally analogous aliphatic peroxyl radicals [195],... 

The increase in the yield of alcohol among the products of peroxyl radicals disproportionation with increasing temperature is the result of acceleration of hydrotrioxide decomposition to the alcoxyl radical and H02 . The proposed scheme is valid for the disproportionation of tertiary peroxyl radicals as well (see earlier). The rate constants of disproportionation of primary and secondary peroxyl radicals are presented in Table 2.17. 

Rate Constants of Primary and Secondary Peroxyl Radicals Disproportionation in Hydrocarbon Solutions (Experimental Data)... 

CL arises in the very exothermic reaction of secondary and primary peroxyl radicals disproportionation [221,222] ... 

If quenching is a diffusion-controlled process (k 3xl09L mol 1 s ), the lifetime t 3x 10-7 s coincides with the lifetime of triplet acetophenone (product of peroxyl radical disproportionation in oxidized ethylbenzene). 

The radiation yield depends on the temperature of oxidation and the initiation rate, i.e., the intensity of radiation IT [233], Radoxidation occurs as an initiated chain reaction at an elevated temperature when peroxyl radicals react more rapidly with hydrocarbon RH than disproportionate, kp(2kt) [RH]2 > (see Chapter 2)]. Radoxidation proceeds as a nonchain reaction at low temperatures when peroxyl radicals disproportionate more rapidly than react with hydrocarbon. The temperature boundary Tv between these two regimes of oxidation depends on the value of radiation intensity 7r. The values of Tv for irradiated heptane oxidation is as follows [233] ... 

The extent to which chain oxidation is inhibited depends on the activity and concentration of the antioxidant. A specific activity of an antioxidant as a retarding agent should be expressed per unit concentration of the inhibitor. If the antioxidant terminates chains, chain self-termination by the reaction of peroxyl radical disproportionation should be taken into account. As a result, one obtains the following expression for estimation of the activity F of the introduced amount of the antioxidant [18] ... 

Trisubstituted phenoxyl radicals disproportionate if the para-substituent bears the a-C—H bond. Two ways were proposed [3] the direct disproportionation and the formation of labile dimer followed by the decay. 

The values of rate constants of phenoxyl radical disproportionation are given in Table 15.10. 

Semiquinone radicals disproportionate with the formation of hydroquinone and quinone [2],... 

The reaction of AmO with H02 occurs with AH < A//c min and, subsequently, with a low activation energy (E=0.5RT) and a high rate constant. The latter is higher than 2kt for peroxyl radicals (see Chapter 6), which is important for cyclic chain termination. The inverse situation takes place in reactions of nitroxyl radical disproportionation with alkylperoxyl radicals. For these reactions we observe inequality AH > A//c min and, subsequently, relatively a high activation energy (E> 0.5RT) and a low rate constant. The latter are lower than 2kt for... 

One more mechanism responsible for the synergistic action of two phenols has recently been discovered during the study of disproportionation of phenoxyl radicals [42,43], This reaction is possible only for phenoxyls possessing C—H groups in the ortho- or para-position [43], For instance, 2,4,6-tris(l,l-dimethylethyl)phenoxyl is unable to disproportionate, whereas ionol radicals disproportionate. It was found that the cross-disproportionation of ionol and a-tocopherol radicals occurs much more rapidly than homodisproportionation (323 K [42]) ... 

Because the polymerization with the thermal iniferters previously described was performed at a high temperature, some side reactions might be unavoidable, e.g., ordinary bimolecular termination between polymer radicals, disproportionation between a polymer radical and a small radical leading to deactivation of the iniferter site, initiation by the radical generated from the iniferter sites, rearrangements of the structure of the iniferter sites, and spontaneous initiation of polymerization. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

The analogy between electron-transfer via addition/elimination (Eq. 2b,c) or abstraction/elimination (Eq. 2a, c) and classical solvolysis involving closed-shell molecules (nonradicals) is seen by comparing Scheme 1 with Scheme 3, in which XY, the precursor of the ions X and Y , is formally derived from the two radicals X and Y". Analogous to Scheme 1, on the way to the ionic products that result from the interaction between X and Y there are two possibilities if XY denotes a transition state, the reaction (Eq. 3a, a ) is a case of outer-sphere electron transfer. If, however, a covalent bond is formed between X and Y, the path (Eq. 3b, b ) is an example of inner- sphere electron transfer. Obviously, part b of the scheme describes the classical area of S l solvolysis reactions (assuming either X or Y to be equal to C) [9, 10]. If a second reaction partner for C (other than the solvent) is allowed for (the (partial) ions then represent transition states), then Eq. 3b also covers Sn2 reactions. If looked upon from the point of view of radical-radical reactivity, Eqs. 3a and b show well-known reactions radical disproportionation in Eq. 3a,a and combination in Eq. 3b. 

With radical sources other than acyl peroxides, the rearomatization of the a-complex can take place by various, not always well characterized, reactions, such as oxidation by metal salts, hydrogen abstraction by intermediate radicals, disproportionation, and induced decomposition. 

The reverse of disproportionation is referred to as com-proportionation. A special case of disproportionation (or dismutation) is radical disproportionation which serves as a termination process in the fate of free radicals. For example, 2 CH3CH2- CH3CH3 + CH2=CH2. 

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