Cage recombination product

A second approach has been to use an unsymmetrical initiator which allows the two radicals of interest to be generated simultaneously in equimolar amounts.175 In this case, analysis of the cage recombination products provides information on cross termination uncomplicated by homotermination. Analysis of products of the encounter reaction can also give information on the relative importance of cross and homotermination. However, copolymerization of unsaturated products can cause severe analytical problems. 

The polarization of the olefinic protons of the in-cage recombined product (1) was derived from the originally polarized cyclopropyl protons via the intramolecular dipolar cross relaxation. 

Thermal decomposition of r-butyl rrans,frans-2,3-diphenylcyclopropanepercarboxylate (92) in ethylbenzene yielded (10 %) exclusively rra/is,rrans-2,3-diphenylcyclopropyl r-butyl ether (93) as a cage recombination product, whereas the cis,cis isomer (94) gave a low yield of recombination product consisting of 1 % trans,trans 93 and 1.5 % cis,cis 95. The low yield and the loss of stereoselectivity in the latter case are thought to be due to a steric effect ... 

Approximately 5% of radicals undergo cage recombination when dicumyl hyponitrite (35) is decomposed in bulk MMA or S at 60 CC.72 Dicumyl peroxide, the product of cage recombination is likely to be stable under the conditions where hyponitrites are usually employed. Nonetheless, its formation is a concern since... 

A slow rate of p-scission also means that the main cage recombination process will be cage return to reform the peroxydicarbonate. Dialkyl peroxides are typically not found amongst the products of peroxydicarbonate decomposition. In these circumstances, cage recombination is unlikely to be a factor in reducing initiator efficiency. 

The low conversion initiator efficiency of di-r-butyl pcroxyoxalatc (0.93-0.97)1-1 is substantially higher than for other peroxyeslers [/-butyl peroxypivalale, 0.63 /-butyl peroxyacetate, 0.53 (60 °C, isopropylbenzene)195]. The dependence of cage recombination on the nature of the reaction medium has been the subject of a number of studies. 12I,1<>0 20CI The yield of DTBP (the main cage product) depends not only on viscosity but also on the precise nature of the solvent. The effect of solvent is to reduce the yield in the order aliphatic>aromatie>protic. It has been proposed199 that this is a consequence of the solvent dependence of p-scission of the f-butoxy radical which increases in the same series (Section 3.4.2.1.1). 

Three types of model study have been performed. The first approach has been to decompose a mixture of two initiators (/.< . one to generate radical A, the other to generate radical B). With this method experimental difficulties arise because the two types of radical may not be generated at the same rate and because homotermination products from cage recombination complicate analysis. 

The thermal decomposition of azoalkanes bearing geminal a-cyano and a-trimethylsiloxy groups has been the subject of a report. The symmetrical compound (107) decomposes near room temperature to afford entirely C—C dimers, whereas the unsymmetrical azoalkane (108) requires heating to 75 °C. A NMR product study of photolysed (107) in the presence of TEMPO showed that the fate of caged t-butyl-l-trimethylsiloxy-l-cyanoethyl radical pairs is disproportionation (17%), cage recombination (20%), and cage escape (63%). 

Bartlett has shown that photodecomposition of azocumene in benzene in the presence of a variety of radical scavengers yields 35% bicumyl as the product of cage recombination in both direct and sensitized photolyses.462 Decomposition of azo-l-cyanocyclohex-ane, 42, yields 1-cyanocyclohexyl radicals which couple in two ways,... 

A kinetic scheme is most easily worked out for the pure polymerization.106 It is useful first to make certain simplifying approximations and definitions. We replace Scheme 6 by Scheme 8, where In is an initiator producing radicals R , M is the monomer, Mn is the growing polymer chain, and M —Mm is combination product and Mn( H) are disproportionation products. If all radicals R produced by the initiator were available to start chains, we could write, from the first two reactions, kinetic Equation 9.40 for rate of change of R-concentration. Because of cage recombination, only some fraction f of the... 

Both a and b are nonchain processes because the yields of the final products (54 and 35%, respectively) do not change upon addition of p-dinitrobenzene, tert-butylnitrone as well as in the presence of oxygen. Substitution of MeOH for DMSO as the solvent suppresses route a entirely. As suggested (Al-Khalil Bowman 1984), the presence of a proton in the methanol solvent cage stabilizes the nitro thiocyanate anion radical. Such stabilization prevents inner-cage recombination and contributes to diffusion of the electron-transfer products into the solution volume. 

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