Cocatalysts aluminophosphate

Even in the absence of polymerization, a significant loss in rate was possible. In a second run, fresh Cr/aluminophosphate catalyst was charged to the reactor at 90 °C with 5 ppm triethylboron cocatalyst. It was allowed to stir in the isobutane diluent for 96 min, then ethylene was added for the first time. Polymerization started immediately, but the reaction occurred at about 50% of the expected rate. This result indicates that at least some of the sites were unstable although polymerization had not yet started. In the absence of initial polymerization, however, the rate loss was not as great as that shown in Figure 170. Cr(VI) can also be reduced by isobutane, which is known to lower activity. 

Cr/aluminophosphate catalysts behave in a similar way. Figure 203 shows the activity improvement obtained with several catalysts with various P/Al ratios. BEt3 is a particularly effective cocatalyst for enhancement of the activity of such catalysts. The data of Figure 203 show that... 

FIGURE 203 Activity improvement of Cr/aluminophosphate catalysts upon addition of 5 ppm BEt3 cocatalyst, as a function of the activation temperature. 

But to correctly compare these polymers, one must also remember that MW and MW breadth can also have a minor effect on the polymer density. Therefore to make an accurate comparison, the change in density (or the amount of branching incorporated) must be considered (from the density before to that after cocatalyst was added). By this measure (Table 66), Cr/silica is still clearly the most effective catalyst, and there is little difference between the Cr/aluminophosphates—partly a result of little a-olefin generation and partly a result of poor incorporation efficiency. 

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