Primary radicals, recombination

As is shown in [14,19] at intensive laser emission the intiation rate diminishes, which (according to [14,16] )is due to high rate of primary radicals recombination. But the extistence of nonlinear effects can t be excluded [20]. But it is not considered. 

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

The factor of/reflects the fact that some of the free radicals immediately recombine into stable molecules that do not initiate polymerization. A primary radical formed by the chemical initiator reacts with monomer to form a propagating radical that contains one monomer unit ... 

The primary step in the photolysis of methylcobalamin is homolytic fission to give the Co(II) cobalamin and methyl radicals. Recombination can occur, i.e., the reaction is reversed, unless the radicals and/or Co(II) are removed by further reactions ... 

In radical template polymerization, when only weak interaction exists between monomer and template and pick-up mechanism is commonly accepted, the reaction partially proceeds outside the template. If macroradical terminates by recombination with another macroradical or primary radical, some macromolecules are produced without any contact with the template. In fact, such process can be treated as a secondary reaction. Another very common process - chain transfer - proceeds simultaneously with many template polymerizations. As a result of chain transfer to polymer (both daughter and template) branched polymers appear in the product. The existence of such secondary reactions is indicated by the difficulty in separating the daughter polymer from the template as described in many papers. For instance, template polymerization of N-4-vi-nyl pyridine is followed, according to Kabanov et aZ., by the reaction of poly(4-vinylpyridine) with proper ions. The reaction leads to the branched structure of the product ... 

Although it is well established that polyisobutylene degrades under radiation, the main-chain scission radicals were never observed as primary radicals in any of these ESR studies. A possible explanation is that the two free radicals formed by chain scission are unable to migrate from the reaction site. The two end-group radicals are then likely to react with each other by either recombination or disproportionation. 

The primary donor triplet state 3P The triplet state of the primary donor is formed by recombination of the primary radical pair P,+4>X of prereduced bRCs in which the ET to the quinones is blocked ... 

The very slow rate of photolysis of hexachloroacetone in the liquid phase is attributed to cage effects and recombination of the primary radicals. This is supported by the fact that the rate of disappearance of hexachloroacetone is accelerated by the addition of oxygen which would combine with the radicals as they were formed. A second consequence of... 

Secondary rearrangements apparent isomerizations through radical recombination reactions. In the rearrangement reactions considered so far, the isomerization step is the primary photochemical process, except when a biradical is formed as an intermediate for in that case the primary photochemical process is really a dissociation, even though the fragments cannot separate. There are however cases of overall isomerizations which result from the recombinations of separated free radicals formed through a process of photodissociation. The photo-Fries reaction is an important example of this mechanism, and is illustrated in Figure 4.43. 

Photodissociation from the LMCT excited state produces the primary radical-ion pair within a solvent cage. Collisions among the primary products lead to recombination. The recombination competes with the diffusion out of the solvent cage. In this primary process, the fragments may never attain a separation of as much... 

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