Termination rate activation temperature

With this information it is now possible to derive values of h and ki from the appropriate slope and intercept in Fig. 13 with the aid of Eq. (43). All these constants are listed in Table XXIII, together with the other constants derived above. In Table XXIII data are also given for the polymerization process at 40°C but is probably less accurate since fewer measurements of Pn and Q were obtained at this temperature. The values for the energy of activation obtained for the termination constants therefore can only be considered approximate. It is evident from these, however, that because kt increases more rapidly with temperature than kiKi or kiK2 that both D ] and Pn will decrease with increasing temperature. This is not clear from the experimental data because the temperature difference is too small although the trend is discernible. It also follows from Eq. (40) that since R is the product of two terms, fc2 which increases with temperature and 5° [E ] which decreases with increasing temperature, the rate versus temperature curve will have a maximum. 

Fig. 9. Increasing the calcining temperature, which dehydroxylates the surface, enhances the polymerization activity and especially the termination rate up to 925°C, where sintering destroys the silica base.

Fig. 14. The melt index of the polymer, which reflects the catalyst termination rate, is also promoted by titania, at least at the lower activation temperatures.

Hydroxyl groups can also be replaced by halide. Fluorided catalysts show improved activity at low calcining temperatures. The electronic environment is probably altered considerably because termination rates are always depressed. The MWD is sometimes narrowed, suggesting a more uniform... 

The gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

In general, during CVD growth, the rate of polymer chain propagation exceeds those of initiation and termination and is a strong function of the substrate temperature. This rate varies inversely with substrate temperature, i.e., decreases with increasing substrate temperature and vice versa. As mentioned earlier, chemical reactions on the surface, lateral diffusion of the incoming species, vaporization, are all characterized by activation processes and obey an Arrhenius temperature dependence of the form ... 

In the low-temperature oxidation chemistry, formation of HO2 is effectively a terminating step. Reaction (1) regenerates an active radical, OH, and reduces the termination rate. 

The rate of 7-ray polymerization was proportional to the square root of the CO2 concentration the molecular weight was unaffected. It was also believed that CO2 did not decrease the termination rate or increase the propagation rate but caused an increase in the initiation rate in the 7-ray initiated formaldehyde polymerization. The rate of polymerization was proportional to the square root of monomer concentration, but not of higher order as earlier suggested. Nakashio et al. [41] had apparently observed substantial spontaneous polymerization in their work. The temperature dependence of the rate found by Fukui et al. allowed an activation energy of 10.3 kcal mole to be calculated for this polymerization. 

In the electron transfer case, the termination of the Dh>A (or Ah>D) electron transfer process is caused by the solvent reorganization about the newly formed charge distribution. This reorganization dissipates electronic energy into a continuum of nuclear modes and establishes both sides of the redox reaction as distinct stable species, so that the direct and reverse processes have no memory of each other and proceed independently. This stabilization also implies that the transition must be thermally activated. In the rate expression (17.50) this process expresses itself via the factor F that depends on the reorganization energy and on the temperature through a distinct activation term. 

Figure 2 also plots the melt index (RMIP = relative melt index potential) of these same polymers. Since a high RMIP indicates a high termination rate, it also (like the activity) increased with increasing activation temperature up to the point of sintering. Other measures of the termination rate, such as the vinyl content of the polymer, also displayed this same pattern. 

Hydroxyl Population. All of these facts indicate a connection between the hydroxyl population on the silica surface and the catalyst s activity and relative termination rate. Figure 3 plots this decrease in the hydroxyl population. Silica, containing no chromium, was calcined at various temperatures and then reacted with CH3MgI solution. The amount of methane released was taken as an indication of the surface hydroxyl content. As the activation temperature was increased, the hydroxyl population decreased from over 4 OH/nm at 200 C to less than 1 OH/nm at 900 C. However, it never actually reached zero even at the highest temperatures studied, but was always significant compared to the coverage by chromium. 

R/R Activation. Figure 5 shows that the enhanced dehydroxyl-ation by carbon monoxide also had a pronounced effect on the termination rate during polymerization. In these experiments, two series of Cr/silica catalyst samples were activated and allowed to polymerize ethylene to a yield of about 5000g PE/g. In one series the catalyst samples were simply calcined five hours in air as usual at the temperatures shown. The relative melt index potential (RMIP) has been plotted against activation temperature and the expected increase up to the point of sintering was observed. 

H2 addition is different from other methods of chain termination. For example, when the reaction temperature is raised, the termination rate is multiplied by some factor, which is determined by the activation energy. If the responses of all the sites are approximately the same, the MW distribution is shifted intact to lower MW, without distortion. (To understand why, note that the GPC curve is plotted on a logarithmic MW scale, and the multiplication of the various termination rates by a constant factor amounts to an additive shift across the MW distribution.) However, when H2 is added to the reactor, a new termination pathway is opened, adding to the overall termination rate. If each site is affected equally, then a constant increment is added to the termination rate at each site, which means that the MW distribution should be distorted. High-MW sites should respond proportionately more than the others. 

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