Diffusion-controlled rate constant general discussion

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

The majority of radical reactions of interest to synthetic chemists are chain processes [3,4]. For those readers who are not familiar with this chemistry, some general aspects of radical chain reactions are discussed here. Scheme 1 represents the simple addition of a thiol to a carbon-carbon double bond as an example of a chain process. Thus, RS radicals, generated by some initiation processes, undergo a series of propagation steps generating fresh radicals. The chain reactions are terminated by radical combination or disproportionation. In order to have an efficient chain process, the rate of chain transfer steps must be greater than that of chain termination steps. Since the termination rate constants in the liquid phase are controlled by diffusion (i.e. 10 M s ) and radical... 

The emission from a controlled-release formulation is generally limited by a diffusion process which is controlled by the concentration gradient across a barrier to free emission and the parameters of the barrier itself (3). The rate of release follows approximate zero order kinetics if the concentration gradient remains constant i.e., the rate is independent of the amount of material remaining in the formulation except near exhaustion. A large reservoir of pheromone is generally used to attain a zero order release. Most formulations, however, tend to follow first order kinetics, in which the rate of emission depends on the amount of pheromone remaining. With first order kinetics, In [CQ/C] = kt where CQ is the initial concentration of pheromone, C is the residual pheromone content at time t, and k is the rate of release. When C 1/2 CQ, the half-life, of the formulation is 0.693/k. Discussions of the theoretical basis for release rates appear elsewhere (4- 7)... 

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