Chain transfer activation parameters

The rate constants for chain transfer and propagation may well have a different dependence on temperature (i.e. the two reactions may have different activation parameters) and, as a consequence, transfer constants are temperature dependent. The temperature dependence of Clr has not been determined for most transfer agents. Care must therefore he taken when using literature values of Clr if the reaction conditions are different from those employed for the measurement of Ctr. For cases where the transfer constant is close to 1.0, it is sometimes possible to choose a reaction temperature such that the transfer constant is 1.0 and thus obtain ideal behavior. 3... 

There has been nothing like the enthusiasm for the application to these systems of the theoretical equations, which we have noted in the previous sections and will encounter in the next. Nevertheless, a number of features are present which are qualitatively consistent with the discussions in Sec. 5.8.1 and which are in part illustrated in Table 5.11. There is a correlation of rate constant with the driving force of the internal electron transfer. -pjjg p. itro-phenyl derivative is a poorer reducing agent when protonated and k is much less than for the unprotonated derivative. Consequently disproportionation (2A 3) becomes important. Although there are not marked effects of structural variation on the values of A , the associated activation parameters may differ enormously and this is ascribed to the operation of different mechanisms."" The resonance-assisted through-chain operates with the p-... 

TABLE 3-14 Activation Parameters for Chain Transfer in Styrene Polymerization (60°C) I... 

The parameter E, gives the probability that the termination of active species takes place by combination (for = 0, termination takes place by chain transfer or disproportionation for = 1, termination takes place by combination exclusively). 

The influence of the l,3,5-trimethyl-hexahydro-l,3,5-triazine (TMT) on the radical polymerization of methyl methaciylate (MMA) was studied. The kinetic parameters were obtained (reaction orders and activation energy of polymerization). It is established that the triazine is the slight chain transfer during to polymerization initiated by azobisisobutyionitiile (AIBN) and the component of initiating system if the peroxide initiator is used. Polymers synthesized in the presence of TMT have the higher content of sindio and isotactic sequences in macromolecule. 

In a classic 1978 paper [5,6], L.L. Bohm reported on the experimental parameters needed to establish steady-state polymerization conditions in order to eliminate monomer transport phenomena from the experimental results. As pointed out by Bohm, suspension or slurry polymerization takes place if the polymerization temperature is lower than the polyethylene solubility temperature and, therefore, the semicrystalline polymer precipitates from the suspension medium as the polymerization proceeds. The important physical process is the mass transfer of ethylene, comonomer and hydrogen (chain transfer reagent used to control polymer molecular weight) from the gas phase through the suspension medium and into the growing polymer particle to the active site. In order to obtain correct kinetic results, concentration gradients and temperature gradients within the polymer particle need to be removed from the polymerization process to achieve the necessary steady-state polymerization conditions. 

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