Disproportionation rate constant

M] = monomer concentration [l] = initiator concentration [R ] = radical concentration f = initiator efficiency k-j g = chain transfer rate "constant" ktr = termination (combination or disproportionation) rate constant... 

All the above-mentioned processes are similar in a CH3CN-pyridine (v/v = 9/1) mixture32. However, the disproportionation rate of Co1 which is electrogenerated at — 1.30 V/SCE and its oxidative addition rate to various arylhalides are divided by a factor of two compared to the rate constants determined in DMF-pyridine. It has also been shown that the use of vinyl acetate stabilizes Co1 species. Under these conditions, the disproportionation rate constant is divided by a factor of seven whereas the oxidative addition rate constants are not much affected. 

In these cases k is a lower limit for molecular disproportionation rate constants since during the reaction a substantial amount of the donor is converted to tetralin (35b). 

Disproportionation equilibria have been studied for various systems. Cauquis and co-workers investigated by electrochemical means the matrix of equilibria corresponding to Scheme 2 for 3,7-dimethoxypheno-thiazine and its derivatives, and applied the measurement of the response of the equilibria to different conditions of basicity to the definition of a scale of basicity in acetonitrile. The disproportionation kinetics of the iron-thionine system were measured several years ago solvent effects on the disproportionation rate constant have been examined, and, lately, an indirect measurement of the synproportionation rate constant of thionine and leucothionine has been made. ... 

Most likely is smaller than the propagation constant, i,e, k < 250 M sec l, However, since the concentration of the monomer is at least 10 times larger than that of the monomeric radical-anions, the addition competes efficiently with the dimerization. Nevertheless, the resulting dimeric radical-anions, M.M , play no role in the pol3nnerization because their diffusion controlled disproportionation (rate constant 10 M lsec" ) destroys them as soon as formed. Hence, radical propagation is imperceptible in such systems. 

In general terms, this is represented as Eq. (6.7b), where ktd is the rate constant for termination by disproportionation or simply the disproportionation rate constant. 

The dependence of the disproportionation rate constant on ionic strength confirmed that the reacting species is doubly charged [30]. 

The disproportionation rate constant, which is pressure-independent, was estimated to be kdis<0.15 krec from the eightfold increase found for the overall rate constant k is+krec [13]. KJKec O was obtained by extrapolating the mole ratio 4n(R4)/n(P2H4) measured for PH3 photolysis to zero reaction time, where it becomes K JKec [11 ] critical remarks concerning... 

It seems that the rate constants of these processes have not been measured directly. The commonly measured value is the ratio A of the disproportionation rate constant to that of radical recombination. 

Accounting for high pressures (of several tens of torr and higher) and taking the second-order recombination rate constant kj.ec 10 cm mol s (see [219]), the disproportionation rate constant can be measured from the known absolute values and the temperature dependence of A. The results of such calculations for several alkyl radical pairs are given in Table 10. The activation energy for disproportionation reactions is seen to be usually low. 

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