Chain initiation activation energy

In the case of catalysis by iron complexes with HMPA, which does not transformed in the course of oxidation, it is possible to estimate the apparent activation energies for micro steps of ethylbenzene oxidation - chain initiation (activation by O2) and propagation (Cat + R02 ) at two temperatures, 80 and 120°C. These are EJ w =24.53 and 13.03 kcal/mol and (Wp ) =21.46 and 17.63 kcal/mol in the absence and presence of HMPA, respectively. The gain in activation energy of the initiation reaction 11.5 kcal/mol via the coordination of HMPA is approximately equal to the energy 10 kcal/mol of ligand addition to metal acetylacetonates [139]. The difference in between the initiation and propagation reactions in the presence of HMPA is presumably responsible for tendency of oxidation selectivity to increase with decrease in temperature. 

The elimination process that occurs during the thermal degradation of poly(vinyl acetate) (PVAc) has been studied and it has been found that elimination of acetate groups initially begins slowly, but increases as degradation proceeds due to an additional process. The increase in rate was found to depend on the concentration of unsaturated groups in the polymer chain. The activation energy of the initial step was found to be 190 kj/mol, while that for the additional process was 130 kj/mol. The additional process of elimination... 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

Figure 7 shows the limiting maximum molecular weight of products from a reactor of fixed size varies directly with the frequency factor of the initiator at a fixed activation energy, while the limiting conversion varies inversely with the frequency factor. In addition, the length of the chain-transfer controlled zone is increased inversely with the frequency factor. 

It hag been shown that transition of a backbone carbon from the sp to sp state is promoted by tensile stresses and inhibited by compressive strains (10,44). The acceleration of the process of ozone oxidation of the polymers under load is not associated with the changes in supramolecular structure or segmental mobility of the chain. The probably reason of this effect is a decreasing of the activation energy for hydrogen abstraction (44). The mechanism of initial stages of the reaction of ozone with PP can be represented as ... 

Bond energy considerations indicate that the initiation reaction (4.2.2) should be quite slow because its activation energy must be quite high (at least equal to the bond dissociation energy). If one were dealing with an open sequence reaction mechanism, such a step would imply that the overall reaction rate would also be low because in these cases the overall reaction becomes approximately equal to that of the rate limiting step. In the case of a chain reaction, on the other hand, the overall reaction rate is usually much faster because the propagation steps occur many times for each time that an initiation step occurs. 

It would be desirable to be able to use data such as that given in Table 12 to predict Dt values for other methyl metallic alkyls and to set a pattern for ethyl and possibly higher alkyls. These dissociation energies should be approximately equal to the kinetic activation energy for the first stage of dissociation in a nonchain decomposition or to the activation energy of the initiation step in a chain decomposition. 

The BPO decomposes with a specific rate constant of about 10 /sec, an Arrhenius constant (A) of about 10 , and an activation energy of about 28 kcal/mol (about 115 kJ/mol). As noted earlier, not all radicals initiate new chains. Some terminate prior to initiation forming inactive products, mainly phenyl benzoate (Equation 6.5). Thus, as noted, an efficiency constant (/) is used that reflects the ratio of BPO that actually form chains. 

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