Transfer Constants to Monomer

Chain transfer to monomer is the main reaction controlling molecular weight and molecular weight distribution. The chain-transfer constant to monomer, C, is the ratio of the rate coefficient for transfer to monomer to that of chain propagation. This constant has a value of 6.25 x lO " at 30°C and 2.38 x 10 at 70°C and a general expression of 5.78 30°C, chain transfer to monomer happens once in every 1600 monomer... 

Tabic 6.14 Selected Values for Transfer Constants to Monomer ... 

The transfer constants to monomer, hydrogen, and other components, km, kh, kk, are depending on reaction temperature. Because of the magnitude of kh, the MWD can be controlled via the hydrogen concentration, cu. ... 

One of the most striking features of CCT is the exceptionally fast rate at which it takes place. The molecular weight of a polymer can be reduced from tens of thousands to several hundred utilizing concentrations of cobalt catalyst as low as 100—300 ppm or 10 3 mol/L. The efficiency of catalysis can be measured as the ratio between the chain-transfer coefficients of the catalyzed reaction versus the noncatalyzed reaction. The chain-transfer constant to monomer, Cm, in MMA polymerization is believed to be approximately 2 x 10 5.29 The chain-transfer constant to catalyst, Cc, is as high as 103 for porphyrins and 104 for cobaloximes. Hence, improved efficiency of the catalyzed relative to the uncatalyzed reaction, CJCu, is 104/10 5 or 109. This value for the catalyst efficiency is comparable to many enzymatically catalyzed reactions whose efficiencies are in the range of 109—1011.18 The rate of hydrogen atom transfer for cobaloximes, the most active class of CCT catalysts to date, is so high that it is considered to be controlled by diffusion.5-30 32 Indeed, kc in this case is comparable to the termination rate constant.33... 

When compared to other monomer systems, there is a relatively high chain-transfer-constant-to-monomer in the polymerization of vinyl chloride. 

The same group [114] has also reported that M increases with increasing [M].q. From experimental results, the following dependence was obtained M oc [M]°4 . This relationship is ascribed to variation of the chain transfer to monomer (Cm). The chain transfer constant to monomer, Cm is constant at saturation pressures and decreases with decreasing pressure at subsaturation conditions. 

Table 8 Selected values for transfer constants to monomer...

Table 8.10. Values of transfer constants to monomer Ptnn) for some vinyl and related monomers at 60°C...

It is apparent why the zero-one limiting case is useful to interpret and predict data it contains only two rate coefficients, p and k. These can be obtained unambiguously by a combination of initiating using y-radiolysis and then following the rate after removal from the radiation source (these data are very sensitive to radical loss processes, especially the exit rate coefficient k), and steady-state rate data in a system with chemical initiator, which is sensitive to both p and k. Moreover, the transfer model for exit makes specific predictions as to the dependence of the value of k on, for example, particle size (Ugelstad Hansen, 1976). Specifically, this model predicts that k should vary inversely with particle area, and that the actual value of k can be predicted a priori from rate parameters measured by quite different means, such as the transfer constant to monomer. This size dependence, and the quantitative accord between model and experiment, have been verified for the exit rate coefficient (Morrison et al., 1994). 

Transfer Constants to Monomers, Polymers, Catalysts and Initiators, Solvents and Additives, and Sulfur Compounds in Free Radical Polymerization+... 

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