Organochromium catalysts branching

In contrast, the other organochromium catalysts, which terminate mainly by -elimination, produce extremely broad MWD polyethylene. In fact, the range of products is so broad that in addition to high polymers, a good portion of the product is also oligomeric. The activity is not diminished by hydrogen. Side reactions must occur easily on such catalysts because the polymers frequently are considerably branched (all types) and have some internal unsaturation as well. 

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... 

In addition to incorporated oligomers, which produce even-numbered branches, methyl branching is also detected in small amounts in the polymers made with many organochromium (but not chromium oxide) catalysts. Chromocene is especially known for this behavior [303,654,679,680]. It is usually thought to result from (3-hydride elimination to the chromium, followed by reinsertion of the same chain or (perhaps a comonomer) in the backwards 2,1 position. The number of methyl branches formed is usually not large enough to have a significant effect on the resin density. 

In this way the subtleties of the normal commercial polyethylene grades are reproduced faithfully, but without the expense of purchasing, purifying, and storing a-olefin comonomer. Continuous addition of the organochromium compound directly into the reactor, or with the chromium oxide catalyst to a pre-contacting vessel which flows to the reactor, provides precise and instantaneous control of the resin density (i.e. level of branching). 

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