Triethylboron cocatalyst

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. 

Table 57 provides data for another example. A series of Cr/silica-titania catalysts, each activated in air at various temperatures, was tested with and without 10 ppm of triethylboron cocatalyst in the reactor. The average turnover per Cr atom was calculated from the polymer yield obtained in 1 h the values are listed as a function of activation temperature for both series of experiments. The average number of turnovers obtained from each catalyst was increased by the addition of cocatalyst, regardless of the activation temperature. 

The initial temperature of catalyst activation can also influence the amount of in situ branching obtained in the polymer. This is in agreement with the olefin-generating behavior of the organochromium catalysts (Figures 185 and 192, Table 55). Table 67 shows an experiment in which Cr/silica-titania was activated at 800 °C or at 650 °C, and then it was reduced and tested for polymerization activity with 5 ppm triethylboron cocatalyst. The 800 °C catalyst resulted in significantly lower polymer density than the 650 °C catalyst. This derives from two causes. The 800 °C... 

Four examples illustrating this point are shown in Figure 219, where data represent experiments in which various cocatalysts were used at a concentration of 8 ppm with a Cr(II) /silica-titania catalyst. Each cocatalyst produced a different a-olefin profile. Triethylboron produced a prominent 1-hexene spike with a much smaller Schulz-Flory background. The addition of a small amount of 02 with the triethylboron cocatalyst increased the relative amount of 1-hexene while decreasing the overall yield of a-olefins. At the other extreme, diethylsilane produced a sharp Schulz-Flory distribution with little additional 1-hexene. The behavior of diethylaluminum ethoxide was intermediate, as a large amount of 1-hexene was produced, superimposed onto a significant Schulz-Flory... 

This behavior is reversed if even a small amount of H2 is added to the reactor. Figure 243 shows an example of 0.2 MI polymers made with titanated Cr/silica in a pilot plant with triethylboron cocatalyst. The expected rise in MI during extrusion was almost completely curtailed... 

Cocatalysts, such as diethylzinc and triethylboron, can be used to alter the molecular-weight distribution of the polymer (89). The same effect can also be had by varying the transition metal in the catalyst chromium-based catalyst systems produce polyethylenes with intermediate or broad molecular-weight distributions, but titanium catalysts tend to give rather narrow molecular-weight distributions. 

Figure 201 shows the kinetics of polymerization with a Cr/silica-titania catalyst activated at low temperature (538 °C). Two curves are shown, representing polymerization first without a cocatalyst, and then with 10 ppm of triethylboron in the reactor [689]. The polymerization rate develops more rapidly in the latter case, which means that the average activity is much higher. 

In the next step, as represented by the data shown in Figure 209, triethylboron was added as the cocatalyst. When only 0.9 mol BEt3/mol Cr was added, it caused the response to shear stress of the polymer to increase still more than it had with ZnEt2 as the cocatalyst. This result indicates that BEt3 is a more effective cocatalyst than ZnEt2 with respect to the response of the polymer to shear stress. In the next series of experiments, the amount of BEt3 added to the reactor was raised still further. Each increase in the BEt3 concentration was found also to raise the response to shear stress of the resultant polymer, at all MI values. 

Thus, the cocatalyst tended to increase "shear thinning," and some cocatalysts were found to be more effective than others. Diethylzinc added a high-MW tail to the polymer MW distribution, but triethylboron enhanced both the low-MW and high-MW sides of the distribution. 

The first polymer grade had a specification HLMI of 10 and a specification density of 0.949 g mL 1. A Cr/silica catalyst, activated at 670 °C, was run with and without 4 ppm of triethylboron in the reactor. Upon addition of the cocatalyst, the activity of the catalyst increased nearly two fold. The polymer HLMI/MI ratio (response to shear stress) increased, which reflects the broader MW distribution of the polymer made in the presence of cocatalyst. The reaction temperature was maintained at 90.6 °C, but when BEt3 was added, H2 also had to be added to the reactor to raise the HLMI back to the target value of 10. This behavior indicates that the cocatalyst increased the polymer MW by the addition of a high-MW tail. 

Table 70 were obtained with a Cr/silica-titania catalyst activated at 650 °C and reduced in CO at 350 °C it was subsequently tested at 95 °C in the presence of 60 micromol/L of cocatalyst. In this example, diethylsilane was found to be least effective, and triethylboron with H2 the most effective for the selective production of 1-hexene. 

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