Recombination of radicals

As a stable radical, however, NO can also catalyze the recombination of radicals (X,Y) at higher concentrations, eventually inhibiting overall oxidation [45] ... 

Shislov14 16 observed radiothermoluminescence associated with the recombination of radical-anion pairs in y-irradiated DMSO-d6 (peak at 105 K) and DESO (peak at 153 K). The equation of the reaction giving the indicated luminescence can be written in general as follows ... 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

Pyrolysis of more complex molecules proceeds via production of free radicals. Then formula (4.5) fails, because reactions of creation and recombination of radicals in these systems are irreversible. Therefore, the steady-state concentration of active particles in these systems depends on conditions of pyrolysis, determining the first or the second order of recombination of active particles, and is governed by the following equations [8]... 

The problem of retention of asymmetry of the formed free radical in the fast geminate recombination of radicals was studied by photolysis of the optically active azo-compound PhMeCH—N=NCH2Ph [88,89]. The radical pair of two alkyl radicals was initiated by the photolysis of the azo-compound in benzene in the presence of 2-nitroso-2-methylpropane as a free radical acceptor. The yield of the radical pair combination product was found to be 28%. This product PhMeEtCCH2Ph was found to be composed of 31% 5,5 -(-)(double retention), 48% meso (one inversion), and 21% R.R(+) double inversion. These results were interpreted in terms of the competition between recombination (kc), diffusion (kD), and rotation (kml) of one of the optically active radicals with respect to another. The analysis of these data gave kxo[Pg.126]

The higher the viscosity of the solvent, the higher the amount of the parent molecules formed due to the geminate recombination of radicals. The observed rate constant of decomposition of the initiator decreases with an increase in viscosity [3,90], This was observed in the decomposition of peresters and diacetyl peroxide in various solutions. Subsequently, the fraction fT of the radical pairs recombining to the parent molecule increases with an increase in the viscosity ... 

In linear EP of bifunctional monomers, such as S, with water soluble initiators, the monomer droplets do not compete with micelles in capturing radicals from the aqueous phase because the total surface area of the droplets is much smaller than that of micelles and growing particles. Nevertheless, if some radicals enter monomer droplets, rapid termination takes place. Therefore, polymerization in monomer droplets is negligible [88]. However, if in the crosslinking EP of 1,4-DVB a few radicals are captured by monomer droplets, they can polymerize completely because the recombination of radicals is suppressed by the gel effect. Moreover, in thermal initiation or in initiation by hydrophobic initiators, such as AIBN, radicals are formed predominantly in the hydrophobic phase, i.e. in monomer droplets and in micelles, and crosslinking EP is initiated in the organic phase. 

TEMPO combines with the radical chain and keeps the concentration of the growing radical chain low, such that the recombination of radicals is suppressed. This type of radical polymerisation is called Atom Transfer Radical Polymerisation (ATRP). It has the properties of a living polymerisation, as the molecular weight increases steadily with time and one can make block polymers this way by adding different monomers sequentially. 

The rate constants for recombination of radicals, k, were determined using the chemiluminescence method, which consists in recording the intensity of luminescence induced by recombination of R02 radicals, when the system passes from one steady state to another (6). A change in steady state was induced by introducing an initiator (Figure 1). The k6 value was obtained from... 

Termination can be realized both by macroradicals on the template (template-template termination) or by recombination of radicals on the template with macroradicals or oligoradicals not connected with the template (cross-termination). For some systems, it is difficult to decide whether they are type I or type II. The intermediate systems can also exist. 

Still open also is the problem of para-products formation. Practically all comparable data are given in Table II. They exhibit a surprising independence of yields of para-product (63) from the polarity of the solvent. The not-too-convincing explanation that both Path A and Path B are equally participating in para-product formation is at hand. It is clear that more quantitative data are necessary to elucidate this problem. Experimentally found spin densities of photoexcited phenoxy radicals (Table III) would allow a preferential recombination of radicals in the para-position (cf. Eqs. 67 + 68 -> 73). 

Figure 3.9b portrays homolytic bond formation by the recombination of radicals and is accompanied by charge transfer from A to B. The radicals must be singlet coupled. The interaction of triplet-coupled electron pairs is repulsive and does not lead to bond formation. The reverse process describes homolytic bond cleavage and results in singlet-coupled free radicals. 

With so many uncertainties, it is hardly surprising that the difficulties inherent in a successful application of the diffusion equation (or molecular pair analysis) to recombination probability experiments are very considerable. Chemically induced dynamic polarisation (Sect. 4) is a fairly new technique which may assist in the study of recombination of radicals following their diffusive separation from the solvent cage. 

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