Chromocene catalyst

A chromocene catalyst supported on silica has been studied (94), with ethylene adding to a Cr—H bond. It is remarkable that the chromium atom, as well as the migrating hydrogen atom, appears to be essentially neutral. For a discussion, see the original article (94). 

Figure 5.5 Compounds used to produce supported chromium catalysts developed by Union Carbide for use in gas phase processes for LLDPE and HOPE. Catalysts must be supported, usually on silica, for optimal performance. Chromocene catalyst is used without a cocatalyst BTSC uses diethylaluminum ethoxide as cocatalyst.

Chromocene catalyst has excellent hydrogen response for molecular weight control. 

Chromocene catalysts have been reported to benefit when the silanol groups are replaced by silanes [582], Because this method involves... 

Chromocene catalysts respond well to H2, producing saturated end-groups and shifting the MW distribution almost intact to lower-MW values. They incorporate comonomer poorly (perhaps another indication of either crowding, or less electron deficiency on the chromium), and such catalysts almost completely exclude the larger comonomers such as 1-hexene and 1-octene. Probably because of the lack of vinyl end-groups (and perhaps also the lack of comonomer response), the polymers made by chromocene catalysts are almost completely devoid of LCB. 

Chromocene catalysts are not very sensitive to the choice of support used. They tend to produce polymers having the same narrow MW distribution. All these characteristics are different from those of the other organochromium catalysts and of chromium oxide catalysts. They are attributed to the influence of the remaining Cp ligand, which probably provides a more crowded and electron-rich environment than is formed on the other catalysts. 

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. 

Metal cyclopentadienyl complexes can also be used as cocatalysts, with the intent of creating chromocene-like structures on the surface of the catalyst, as shown in Scheme 46. Chromocene catalysts, which contain mono-attached chromium species incorporating one cyclopentadienyl ligand, are noted for their sensitivity to H2. It is believed that Cr/silica catalysts can be modified to make this species by the addition of metal cyclopentadienyls to the reactor, such as LiCp or MgCp2 [695],or by use of a combination of cyclopentadiene or indene with an aluminum alkyl cocatalyst [696]. When these modified catalysts are allowed to polymerize ethylene in the presence of a remarkable broadening of the polymer MW distribution is observed, mainly as a result of a shift of the low-MW part of the MW distribution. The chromocene surface species is known for its ability to incorporate H2 (thus lowering the polymer MW) and also to reject 1-hexene. Thus, these unusual cocatalysts have the potential to reverse the normal branch profile of polymers made with Cr/silica catalysts (i.e., to put more branches into the longer chains). 

H2 is often added to the reactor to decrease the polymer MW. The MW reduction is thought to occur by simple hydrogenolysis, as shown in Scheme 14. Chromium oxide on silica is not as sensitive to H2 as some other catalysts, such as Ziegler or chromocene catalysts. However, its H2 sensitivity is also not unusual, as many Ballard (zirconium or titanium) catalysts fall into the same category [297,376]. The sensitivity of chromium oxide catalysts can vary considerably, depending on the support, suggesting that various sites respond quiet differently. 

Ligand environment at active sites plays a significant role in polymerization behavior. Ligand effects in diene polymerization (8 7, 8 ) and work with supported chromocene catalysts (98) dramatically illustrate this point. 

Silica supported chromium catalysts that polymerize ethylene to polyethylene with as many as 12 methyl branches/1000 carbon atoms have been reported. The small amount of branching observed in the ethylene homopolymers prepared by these supported chromocene catalysts was attributed to a chain isomerization process (a) Karol, F. J. Karapinka, G. L. Wu,... 

Karapinka et al. to UCC. The chromocene catalyst on aluminum phosphate support was similar to that of Phillips Petroleum Co., invented by Hogan and Banks... 

Fig. 16. Chromocene catalyst. (Here indicates that the actual mechanisms are as yet unknown.)...

The activity of the chromocene catalyst prepared on 670 C silica is approximately 35,000 g PE/g Cr. A significant increase in catalyst activity is foimd in a commercial reactor where very high piuity raw materials are employed [28] and operate at a higher ethylene pressme. Under these conditions the chromocene catalyst can achieve an activity of 0.5-1.0 x 10 g PE/g Cr. 

McDaniel reported that deposition of chromocene onto an aluminophosphate support that was previously dehydrated at 600 C, in place of silica, provided a catalyst that produced polyethylene with a significantly narrower molecular weight distribution than the silica-supported chromocene catalyst [33]. For example, he found that a polyethylene sample with a Melt Index (I of 1.0 produced with the catalyst in which chromocene was supported on aluminophosphate exhibited a polydisper-sity (MyM ) value of 4.2, while a similar polyethylene sample prepared by Karol et at [28] with a catalyst in which the chromocene was deposited on silica exhibited a polydispersity value of 10.2, clearly showing a much more narrow molecular weight distribution for the polymer prepared with the aluminophosphate-supported catalyst. Moreover, a MyM value of 4.2 is comparable to polyethylene prepared with commercial Ziegler-type (titanium-based) catalysts that are used to provide polyethylene for applications that require a relatively narrow MWD. 

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