Chain transfer constants definition

A plot of 1/DPn against [S]/[M] should yield a straight line with slope k /kp and intercept l/(DPn)o, where (DPn)o is the average chain length measured in the absence of transfer agent. The form of this equation leads to the definition of a chain-transfer constant C for each species, including monomer, in Eq. (19). 

The early work on chain transfer has been reviewed by HQl and Doak (/) and the chain transfer constants v4iich had been reported or could be estimated from data published up to that time are given in that reference. At that time, there were only scattered data on chain transfer constants and many of them had been determined at uncertain pressures, uncertain temperatures or under phase conditions which made the concentraticms of the reactants uncertain. There was furthermc e considerable variaticm in the pressures and temperatures at which the different chain-transfer agents had been studied, making it difficult to draw definite ccmclusions about thdr relative reactivities. 

It should be mentioned that expression (3.7) for the number average DP can be easily derived without solving the full set (3.6). By definition, Pn is equal to the total number of moles of monomer converted into polymer, MqX, divided by the total number of macromolecules. The latter increases in time due to chain transfer at a constant rate ktsSR and hence, at time t it is lo + ktsSIot. Replacing time with conversion yields formula (3.7). 

It turned out that in many cases one uses the term of living polymerization for processes that only partially fell within the definition given by Szwarc. Therefore, introduce the concept of the pseudo-living or quasiliving polymerization. These concepts apply when the termination and transfer of chain rate constants are equal to zero and the condition ki>kw is not satisfied, or the propagation reaction is reversible, or a reversible chain transfer to polymer takes place. 

In chain reactions involving three termination steps (two uncrossed and one crossed) the quantity = 1c /(1c 1c )1/2 is frequently used to interrelate the cross-termination constant with the two uncrossed termination constants. For many different types of radical < is found to be about 1 (or alternatively, if the statistical factor of 2 favoring the crossed termination process is ignored in the definition of the rate constants, < 2). In the present reaction system —3-6, in agreement with the value obtained by Russell at 90°C. (26). The crossed termination constant itself is somewhat less than half the value found for kt. This seems reasonable since only one hydrogen atom will be available for transfer in the crossed termination, compared with the two that are available in the self-reaction of two tetralylperoxy radicals. In addition, steric hindrance to reaction should be greater for the crossed termination than for Reaction 8. The products are presumably cumyl alcohol, a-tetralone [3,4-dihydro-l(2H)naphthalenone], and oxygen (28). 

A theoretical analysis of charge distribution within supercomplexes (or clusters in which the movement of diffusible carriers is restricted) has been developed by Lavergne et al [4]. This theory predicts the evolution of the redox state of the carriers under continuous illumination or flash excitation for any cluster stoichiometry. The predictive power of this treatment is illustrated by the analysis of the light-induced oxidation of primary and secondary donors in isolated centers of Rhodopseudomonas viridis (Fig. 3). In this case, it is definitely established that the secondary donors (cytochromes) are irreversibly bound to the reaction center. In the absence of mediators, no electron exchange is expected to occur between photocenters. In the presence of 200yM ascorbate, only two of the four cytochromes (cyt 556 and cyt 559) are in their reduced state prior to the illumination. As expected, the apparent equilibrium constant between P and the cytochromes measured during the course of illumination is much lower than that computed from the value of the redox potentials (K = 50 for cyt 559 and K 1500 for cyt 556). The fit between the experimental data and the theoretical simulation (dashed lines) is excellent and clearly demonstrates that the measurement of electron transfer reactions under weak illumination is a powerful tool to characterize the degree of structuration of a photosynthetic electron transfer chain. 

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