Free radical, mechanism recombination

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

One of the important possible mechanisms of MF action on biological systems is the influence of free radical production. Chemical studies predict that MFs may affect free radical reactions through the radical pair mechanism [201]. A reaction between two free radicals can generate a free radical pair in the triplet state with parallel electron spins. In this state free radicals cannot recombine. However, if one of the electrons overturns its spin, then free radicals can react with one another to form a diamagnetic product. Such electron spin transition may be induced by an alternative MF. 

Direct Liquefaction Kinetics Hydrogenation of coal in a slurry is a complex process, the mechanism of which is not fully understood. It is generaly believed that coal first decomposes in the solvent to form free raclicals which are then stabilized by extraction of hydrogen from hydroaromatic solvent molecules, such as tetralin. If the solvent does not possess sufficient hydrogen transfer capability, the free radicals can recombine (undergo retrograde reactions) to form heavy, nonliquid molecules. A greatly simplified model of the liquefaction process is shown below. 

The EE and phE mechanisms for neat polymers proposed by ourselves and others all involve the consequences of breaking bonds during fracture. Zakresvskii et al. (24) have attributed EE from the deformation of polymers to free radical formation, arising from bond scission. We (1) as well as Bondareva et al. (251 hypothesized that the EE produced by the electron bombardment of polymers is due to the formation of reactive species (e.g., free radicals) which recombine and eject a nearby trapped electron, via a non-radiative process. In addition, during the most intense part of the emissions (during fracture), there are likely shorter-lived excitations (e.g., excitons) which decay in a first order fashion with submicrosecond lifetimes. The detailed mechanisms of how bond scissions create these various states during fracture and the physics of subsequent reaction-induced electron ejection need additional insight. 

Scheme 2.24 Free radical dissociation-recombination mechanism...

The migrating group migrates with nearly complete retention of configuration. Thus, the reaction is intramolecular and concerted. However, there is evidence which supports the free radical dissociation-recombination mechanism (Scheme 3.27). 

The hydroxytelechelic polymers synthesis involving a free-radical mechanism employs polymerization initiators which are cleaved into free radicals bearing hydroxyl substituents, by heat, light or redox systems. These radicals initiate polymerization of monomers and can give hydroxyl-terminated polymers by recombination. 

The second mechanism is recombination of the recoil atom with ions or radicals produced by radiation. Because of the weaker intermolecular forces in molecular crystals, individual excited molecules are relatively more isolated and will decompose into fragments (usually free radicals) more readily than in ionic crystals. Assuming that 10 ev are necessary to produce a free radical pair in a molecular crystal, about ten such pairs would be produced per 100-ev hot spot. Since we have seen that for doses of 5 X 10 r every radioactive atom is included once, on the average, in a hot spot, it follows that every radioactive atom will have a good probability of being found in the vicinity of a free radical at least once. Although most of the free radicals would recombine with one another their concentration would probably still suffice to make this second mechanism an attractive possibility. 

In both the polymerizations, free radicals are the species that are responsible for the formation of bonds in the depositing materials. The growth mechanism, however, is not by the conventional chain-growth free-radical polymerization. In a conventional free-radical chain-growth polymerization, two free radicals and 10,000 monomer molecules yield a polymer with degree of polymerization 10,000, which does not contain free radicals. In contrast to this situation, in plasma polymerization and Parylene polymerization, 10,000 species with free radical(s) recombine to yield a polymer matrix that has an equivalent degree of polymerization, and contains numbers of unreacted free radicals (dangling bonds). 

Until 1950s, radiation-induced polymerization was considered to proceed only by the free-radical mechanism. In fact, the rate of ion generation by ionizing radiation is one to two orders of magnitude lower than that of free-radical formation. In contrast, the recombination constants for ions (ion and counter-ion) are approximately two orders of magnitude higher than those for free radicals. Hence, the stationary concentration of ions is approximately 100 times lower than that of free radicals. Consequently, radiation polymerization proceeds mainly by a free-radical mechanism. 

The rate of formation of ions is a factor of approximately 10-100 times smaller than that for free radicals. Conversely, the recombination constants are about 100 times larger for ions than for free radicals. Thus, it follows that the steady state concentration of ions is about 100 times smaller than that for free radicals. Consequently, the majority of radiation-initiated polymerizations proceed by a free radical mechanism. 

Photochemical reactions proceed via a free-radical mechanism. The radicals, which are formed near the light source if they do not diffuse quickly to react further with other species, will recombine, generating excess heat instead of a productive reaction. Large-scale photochemical reactions are usually performed with macro-scale lamps immersed in the reaction vessel. Issues involved in such design are scalability of light sources, heat and mass transfer in the processes, and safety concerns (e.g., explosions caused by excess heat). Radical recombination reduces the quantum efficiency of the overall process. By PI miniaturization the diffusion length is reduced, leading to an increase in frequency of collision with other molecules to produce the desired product. 

Termination is by the recombination of H and Cl. There is complete corroboration of this mechanism from photochemical studies. This is undoubtedly a free-radical chain propagation mechanism. 

A term describing certain combinations of mechanical action and chemical reactions exemplified by, but not confined to, the mastication of elastomers. In this process it is considered that the deforming forces break the molecular chains into two pieces, with formation of free radicals at the chain ends. Such radicals may recombine, or combine with oxygen or other... 

In cold mastication of rubber, a substance which prevents the recombination of the free radicals produced by the mechanical shearing forces. See Cold Mastication, Free Radical and Mechano-Chemical. 

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