Recombinations processes disproportionation

Diffusion of particles in the polymer matrix occurs much more slowly than in liquids. Since the rate constant of a diffusionally controlled bimolecular reaction depends on the viscosity, the rate constants of such reactions depend on the molecular mobility of a polymer matrix (see monographs [1-4]). These rapid reactions occur in the polymer matrix much more slowly than in the liquid. For example, recombination and disproportionation reactions of free radicals occur rapidly, and their rate is limited by the rate of the reactant encounter. The reaction with sufficient activation energy is not limited by diffusion. Hence, one can expect that the rate constant of such a reaction will be the same in the liquid and solid polymer matrix. Indeed, the process of a bimolecular reaction in the liquid or solid phase occurs in accordance with the following general scheme [4,5] ... 

Radicals initially formed in solution by a bond homolysis will be held together briefly in a cage of solvent molecules. Because radical recombinations and disproportionations are so fast, they can compete with diffusion of the radicals through the layer of solvent molecules that surround them, with the consequence that some of the radicals formed never become available to initiate other processes in the bulk of the solution.98 These recombinations are termed geminate recombinations, and the phenomena that arise from this behavior are cage effects. 

Peroxyl radicals which do not decay by one of the unimolecular processes discussed above must disappear bimolecularly. In contrast to many other radicals, they cannot undergo disproportionation. Hence they are left to decay via the recombination process, the results of which is a tetroxide intermediate [reaction (46) an exception may be their reaction with 02- cf. reaction (7)].,... 

Let us summarize the above analysis. The initial comprehensive mechanism includes 17 elementary processes (no distinction being made between radical recombinations and disproportionations). Among these processes, 8 at least are negligible, 3 are non-determining and 3 quasi-non-determining, 3 are determining and, finally, 2 out of these three are determined. As has been noted, numerical sensitivity analyses carried out on more complex mechanisms [70, 74, 93, 94, 117] lead to the same type of conclusion. 

It is seen that free radical micromolecular or macromolecular initiators have been successfully employed for the synthesis of di-, tri- or multiblock copolymers. However, once again, the structure of these block copolymers depends upon the termination step of the polymerization, and especially on the recombination or disproportionation of macroradicals produced. Besides, such a method also generates homopolymers. Separation and purification of these different structures are usually very difficult or even impossible. Moreover, the copolymers obtained usually exhibit a broad polydispersity, a defect inherent in the classical radical process. 

The recombination and disproportionation of ethyl-radicals are considered to be the main termination processes, viz. 

Termination is the last step in a radicalic chain scission process generated by heat. The radical reactions can be terminated by disproportionation or recombination. The disproportionation reaction can be schematically written as follows ... 

In essentially all the reactions discussed so far, the radicals were generated by thermal or photochemical homolysis. There is another way to produce radicals and this is represented in Scheme 8.1. It consists in removing an electron from an electron-rich species represented by an anion or by adding one electron to an electron-deficient entity, now represented by a cation. It is possible, of course, to oxidise a radical to a cation or reduce it to an anion. This constitutes an alternative way of destroying radical character, in addition to recombination and disproportionation. Such transformations are referred to as redox processes they are exceedingly important in radical chemistry and their impact on organic synthesis can hardly be overstated. 

One of the big surprises of the recent past has been the observation that radical-radical termination processes via recombination or disproportionation have no activation energies and very high A-factors. A-factors for recombination reactions of alkyl recombinations vary from lO10-4 1/mole-sec for CH3 radicals (28) to a low of about 109 5 for tert-... 

The MALDI-TOF-MS analysis of the polyMMA, obtained in linear mode shows only one series of peaks, whose interval was regular, ca. 100, the molar mass of MMA unit. It indicates the absence of irreversible chain termination processes via recombination or disproportionation. According the proposed mechanism of polymerization the absolute masses of the peaks should be equal... 

Disproportionation Reactions. At this point it is interesting to discuss another type of recombination process, namely disproportionation. For example... 

Buschow (1994) analyzed these effects thermodynamically using the van t Hoff relation. The (Nd,Zr)2FeuB and/or Co- and Zr-containing regions have the higher equilibrium hydrogen partial pressure (Ph2)> which means that in the phases with additives the completion of the disproportionation process becomes difficult, and also the recombination process should be accelerated under the same conditions. The recombination reaction depends on thermally activated bulk diffusion which increases with temperature (McGuiness et al. 1990). 

When discussing crosslinking, recombination is the only useful process. Disproportionation leads to a formation of unsaturation (double... 

The radicals are removed by the process of termination this may be by either recombination or disproportionation. 

Radical polymerization requires an initiator such as AiBN and proceeds by a radicai chain process to give a largely regioregular, but generally atactic or somewhat syndiotactic polymer. Termination involves recombination or disproportionation. The process is tolerant of impurities, but molecular weight distributions are broad. 

The most efficient intramolecular secondary processes competing with the acyl-alkyl diradical recombination in five-membered and larger cyclic ketones are hydrogen shifts resulting in the disproportionation of the diradical to either ketenes or unsaturated aldehydes [cf. (5) (4) (6)]. 

Butenyl (C4H7) mainly disappears by reacting with ethyl radicals. The leftover ethyl radicals either recombine (faster process) or undergo disproportionation (slower process) ... 

Critical appraisal of the method, 23,26 using attempts to synthesize nonsymmetrically substituted lanthionines, resulted in rearrangement of the products, presumably due to phosphine-catalyzed disproportionation of the unsymmetrical disulfides. This reaction should proceed more rapidly than the desulfurization process and is thought to occur because sulfur extrusion takes place via a reversible reaction by recombination of ionic intermediates (Scheme 3). 21-22-24 Thus, the reaction of nonsymmetrical cystine derivatives results in the formation of a mixture of three different lanthionines. 

The most important processes that remove radicals are (1) combination of radicals with each other, either by direct bond formation (recombination) or by hydrogen atom transfer from one to the other (disproportionation), and (2) electron transfer between a radical and an oxidizing or reducing agent. 

It is known that for phenyl alkyl ketones both the Type I and Type II reactions occur only from triplet state. The radical pair formed from a-cleavage undergo disproportionation resulting in Type I products or recombine to regenerate the ground state ketone. The latter process reduces the efficiency of Type I product formation. 

In competition, the C(6)-yl and C(5)-yl radicals may disproportionate, possibly via an adduct [reactions (80) and (81)]. This yields the hydrate via an enol [reaction (83)]. The other product is the glycol [reaction (82)]. In the original paper (Al-Sheikhly and von Sonntag 1983), it has been proposed that it maybe formed in an ET reaction. Due the considerable rearrangement energies involved in ET reactions as compared to radical recombination reactions, it is now considered that this ET reaction might occur via an addition/elimination process [reactions (80) and (81)] such as has also been found for other systems. 

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