Organochromium alkyl compounds

Organoboration, of iminoboranes, 31 155-156 Organoboron alJcyls, 2 75 Organochromium alkyl compounds, 44 354-357 carbenes, 44 352-354 compounds, 44 344... 

The tetravalent chromium alkyl compounds were found to give catalysts that are somewhat more active than the catalyst made from the divalent chromium counterpart, under commercial reaction conditions (90-110 °C, 0.5-1.5 mol ethylene L ). Indeed, they were among the most active organochromium catalysts tested in our laboratory. Their overall 1-h yield was usually also superior to that observed with some of the best chromium oxide on silica-titania catalysts. Even when compared with chromium oxide systems used with a cocatalyst, the catalysts made with tetravalent chromium alkyls were equal or better in activity. Unfortunately, for commercial applications, these catalysts also tend to make some oligomers and wax as well. 

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

Many other organochromium compounds have since been synthesized and found to be active, including those with chromium exhibiting every valence up to Cr (IV). Chromocene is a well-studied example of an active divalent compound (52-55). Pentadiene-Cr(II) (56) is another, along with allyl-Cr(II) (52, 57). Allyl-Cr(III) is also active (52, 57-61). -Stabilized alkyls of Cr(II) and Cr(IV) such as trimethylsilylmethyl-Cr(IV), which also polymerizes ethylene when supported on an oxide carrier, have been synthesized and tested in this laboratory (57,62). All these organochromium catalysts are comparable in activity to the Cr(VI)/silica standard. 

Although organochromium catalysts are not well characterized, organochromium compounds are thought to bind to the support by reaction with surface hydroxyls as other types do. When Cr(allyl)3 or Cr(allyl)2 is used, propylene is released (59,60). Chromocene loses one ring (52-55), and / -stabilized alkyls of chromium lose the alkane (81). 

The formal valence of the starting organochromium compound has been varied from Cr(0) to Cr(TV), but it does not seem to make a major difference in the activity and other performance characteristics of the catalyst. In most cases, there is no induction time, because the chromium is already in a reduced state. Most of these catalysts can also be considered as already alkylated before contact with ethylene. Polymerization usually starts immediately on contact with ethylene, and the rate either remains constant or slowly declines during a 1-h polymerization rim. 

One is left to ponder initiation by other organochromium catalysts. Chromium allyls or 2,4-dimethylpentadienylchromium(II) could conceivably rearrange into p-l coordination upon addition of ethylene. However, chromocene must initiate the first chain in some other way, because the site must retain the ring. Thus, for chromocene catalysts, the initiation problem is similar to that described for chromium oxide. The diarene-chromium(O) and Cr(0)(CO)6 catalysts may also have this problem. Perhaps this is why these catalysts sometimes initiate polymerization more sluggishly than the chromium alkyls. However, there is also some evidence that the Cr(0) compounds can be oxidized by surface OH groups to leave a Cr-H group, which could also be considered an alkylated species. 

Indeed, one need not necessarily use a chromium alkyl for this purpose, as other organochromium compounds can also be used successfully. The open-ring chromocene, bis(2,4-dimethylpentadienyl) chromium(II), called Cr(DMPD)2, was tested and performed similarly in many respects. Figure 200 presents an example in which this organochromium compound, called Cr(DMPD)2, was added to the reactor along with Cr(VI)/silica (or at the right in the figure, just silica) calcined at 800 °C. 

Organochromium compounds are able to easily form stable carbene complexes as described in the above section [70-75]. The representative method is as follows hexacarbonylchromium reacts with a nucleophile to give an acylanion, then the chromium carbene is prepared by alkylation of the acyloxygen with a trialk-yloxonium salt or a diazomethane as shown in eq. (13.39). As this reaction was... 

Organochromium Compounds for Ethylene Polymerization Based on (Me)3CpCr(III) Alkyls... 

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