Radical self-reaction

The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

The detailed analysis of the phenoxyl radical self-reactions are described in reviews [2-6]. 

Here, v is the scan rate, k is the radical self-reaction rate constant, and Ep is the CV wave peak potential. The standard potentials obtained ranged from 1.28 V (4-02NPh0H) to 0.17 V (4-HOPhOH). Good agreement with the literature values was obtained in those cases where the data were available. 

Boyd, A. A., and R. Lesclaux, The Temperature Dependence of the Rate Coefficients for /3-Hydroxyperoxy Radical Self Reactions, Int, J. Chem. Kinet., 29, 323-331 (1997). 

Typical reactions that obey the rate law in Equation 8.15 are radical self-reactions, such as that in Equation 8.17, for which the rate constant is k = 2.4 x 109 M 1 s 1.20... 

Experiments performed at room temperature and at SF6 pressures of 250-1000 mbar show that the kinetics of decay of the C2F5 radicals does not depend on the total pressure. The reaction of the oxygen atoms produced in the hydroxyl radical self-reaction with the ethyl and hydroxyl radicals is not important in this pressure range and has been omitted in the reaction scheme. The value of k,+2 = (1.1 0.2) x 10"10 cm3molecule"1s"1 estimated by Fagerstrom et al.203 can be considered as an upper limit of the high-pressure limiting rate constant kl oo for reaction (1). 

The overall reaction sequence leading to CO2 formation, through the HCHO and CO intermediate "stable products, is shown in Figure 5.2. When NO, levels are sufficiently high that reaction of the peroxy radicals HO2 and CH3O2 with NO predominates over peroxy radical self-reactions, the methane oxidation chain depicted in Figure 5.2 can be written as... 

Table 6 collects the available HCFC-based peroxy radical self-reaction rate constants. There are two important points to keep in mind regarding this table. The first is that the peroxy radical self-reaction proceeds by two major channels ... 

TABLE 6 HFC- and HCFC-Based Peroxy Radical Self-Reactions... 

The data in Table 6 suggest a few general trends. The room-temperature HCFC-based peroxy radical self-reaction rate constants all lie in the rather... 

The decarbonylation of pivaloyl radicals in aqueous solution, k - 2.5 X 10 s 147) is somewhat slower than in most non-aqueous solvents 148), but sufficiently rapid to compete with the reaction with O2 in Eq. (21) under most conditions. In the absence of O2, most of the acyl radicals produced in Eq. (19) will dissociate CO, Eq. (20), and react with CraqOO Eq. (21). A small fraction may disappear in bimolecular radical self-reactions. 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

No new data have been reported for tertiary alkylperoxy radicals. It has been confirmed that the rate constant of the t-butylperoxy radical self-reaction is very... 

It is possible to rationalise the first trend in terms of steric effects associated with the increasing alkyl chain length and branching. The same trend is observed in the peroxy radical self-reaction kinetics. The second trend is more difficult to explain but may reflect a decrease in the RO-O bond strength caused by the electron withdrawing halogen atom. Ab initio theoretical studies are needed to shed further light on the reactivity trends. 

Percent product distribution cyclo-hexanone 67.0 1.2, cyclo-hexanol 29.2 0.5, unidentified bifiinctional compounds 3.8 1.2. The cyclo-hexoxyl radical undergoes remarkably little ring cleavage. Values for the branching ratio of the cyclo-hexylperoxy radical self-reaction and for the decomposition rate coefficient of the cyclo-hexoxyl radical are shown in Tables 1 and 2. ... 

Photoionization Kinetic study of allyl radical self-reaction. Selby et al. 

The self-reactions of thiyl radicals and the reactions of thiyl radicals with other organosulfur radicals and organic radicals proceed predominantly via combination, with disproportionation being of minor importance (see Table 7). Little information is available regarding the kinetics and mechanisms of thiyl radical reactions with organic radicals. The ensuing discussion focuses primarily on thiyl radical self-reactions. The reader is referred to Tables 7 and 8 and the relevant references for information regarding reactions other than thiyl radical selfreactions. 

Even though the rate of radical-radical reaction is determined by diffusion, this does not mean there is no selectivity in the termination step. As with small radicals, self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 43). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an rmsaturated end and the other has a saturated end (Scheme 43). 

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