Combination, disproportionation of radicals

The mutual reaction of two radicals can lead to their simple connection (combination) 

Combination and disproportionation are, of course, preceded by the occurrence of a different transition complex. It seems that the transition complex leading to the disproportionation of small radicals (ethyl) is more compact than that from which the combined complex is generated. Nevertheless, they both have a loosened structure. The mechanism of disproportionation is not known in detail. The high A factor indicates that a simple abstraction of the / hydrogen by one radical from the other is not probable. The transition complex may be polar [2] a direct proof is not accessible. The rate is determined by diffusion [3] the effect of solvent polarity on the activation parameters cannot be measured. The value of fct (comprising combination and disproportionation) of two small radicals is of the order of 109 mol-1 dm3 s l at temperatures around 273 K. 

Fortunately the observed deviations are not usually very large. When differences between the results of calculations and experimental measurements appear, it can not at present always be safely stated to what extent they are caused by variations in kt or by termination of, or by, primary radicals (non-ideality of polymerization). 

North confirmed the controlling effect of diffusion in the bimolecular termination of two macroradicals [4], It is useful to remember that diffusion is manifested by several overall effects, each of which can be considered independently the main of these are 

Some authors consider diffusion (a), (b) as consecutive processes, and assume the existence of colliding pairs [7-9]. Other models stress the importance of segmental diffusion of the active ends in a common volume of the two colliding macro molecules [10-12]. A common drawback of the mathematical models is the lack of a generally formulated expression for the effective diffusion coefficient of the active end in a coiling chain. Most models try to solve this difficulty by introducing suitable parameters with some physical meaning. 

Higher by two or more orders than is usual for a bimolecular reaction of the two simple molecules (species). 

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