Chromocene-Based Catalyst

A catalyst based on chromocene, Bis(cyclopentadienyl)chromium, was developed in the 1960s at Union Carbide by G. L. Karapinka and cowork-ers and was the first commercial chromium-based catalyst that was prepared with an organochromium compound containing Cr-carbon bonds in the starting material. In addition, the starting material based on Cr(II), did not need to be oxidized to a Cr(VI) species to obtain a high activity ethylene polymerization catalyst. 

This Cr-based catalyst has several characteristics that make this particular catalyst unique in comparison to the Phillips catalyst and the Bis(triphenylsilyl)chromate catalyst. These unique characteristics relative to these two other Cr-based catalysts include  

The elimination of approximately one cyclopentadiene per chromocene is reported [22] at a deposition temperature of 25°C after one hour using 0.010 g chromocene in 30 ml decane and 0.5 g of Davison Grade 952 silica previously dehydrated at 800°C. 

The deposition of chromocene into Grade 56 silica that was previously dehydrated at 100°C-800°C was investigated [25] using either decane or 

Data Point Solvent (slurry) Temperature (°C) Chromocene (mmol/g silica) Excess Chromocene (mmol/g silica)  

---

A typical catalyst containing 2.5% of chromium in n-hexane (20 mg 1" ) activated by aluminium diethyl ethoxide (0.3 mg 1" ) gave a yield of 140 g polymer per h at 88—91°C and a monomer pressure of 300 p.s.i.g. The chromocene based catalysts are also of high activity, this being dependent on the conditions of catalyst preparation. Dehydration at 670°C gave the highest yield of polymer. Detailed kinetics were not carried out, but yields of 130—1670 g polymer per mmole Cr per 100 p.s.i. ethylene per h at 60°C were reported. 

Examination of Figure 3.22 shows that the chromocene-based catalyst has extremely high hydrogen response resulting in a highly-saturated polymer chain with almost exclusively methyl end groups, while the catalyst... 

Figure 3.21 Chain transfer reactions that are used to control polyethylene molecular weight. The chromocene-based catalyst exhibits very high hydrogen response. Cp ligand on Cr not shown.

Effect of Silica Dehydration Temperature on the Chromocene-Based Catalyst... 

Thermal aging of the chromocene-based catalyst in an inert atmosphere at 100-600°C resulted in the complete loss of the cyclopentadienyl ligands that provides a modified catalyst with only 10-30% of the original activity and introduces a high level of terminal unsaturation into the polyethylene [29]. 

Table 3.7 The effect of silica dehydration temperature on chromocene-based catalyst activity.

Karol and coworkers [34] reported silica-supported chromium-based catalysts based on Bis(indenyl) chromium (II), Bis(fluorenyl) chromium (II) and Bis(9-methyIfluorenyI) chromium (II) in place of chromocene. These catalysts were prepared at room temperature by reacting the dehydrated silica with a hexane solution containing one of the Cr(II) compounds. The Bis(indenyl)-based catalyst exhibited good activity under some polymerization conditions, while the Bis(fluorenyl)-based catalyst was significantly less active than the chromocene-based catalyst. Table 3.8 summarizes some polymerization data for these catalyst systems. 

The indenyl-based and fluorenyl-based experimental catalysts each required approximately three times as much hydrogen to produce a similar polymer molecular weight as the chromocene-based catalyst, while... 

Chromocene (CrCp2) supported on silica is used to generate certain chromium-based catalysts for the polymerization of ethylene (e.g., Phillips and Union Carbide catalysts). The nature of the organometallic species responsible for the catalysis is not known with certainty, though it is noteworthy that some Crm alkyls such as [Cp Cr(CH2Ph)(THF)2]+BPh catalyze the polymerization of ethylene.19... 

A. Noshay, F.J. Karol, Chemical activation of silica supports for chromocene-based polyethylene catalysts, in R.P. Quirk (Ed.), Transition Metal Catalyzed Polymerization, Proc. Int. Symp., 2nd, Akron, OH, 1986, Cambridge University Press, Cambridge, 1988, pp. 396-416. 

Bis(triphenylsilyl)chromate, [(CgHj)3SiO]2CrO and an aluminum alkyl and the other catalyst is based on chromocene, CrlCjHj). These two Cr-based catalysts, in addition to a Phillips-type catalyst, were used in the 1960s to commercialize the gas-phase process for the manufacture of various grades of HDPE. This process was licensed by Union Carbide around the world under the trade name UNIPOL Process. 

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. 

Supported chromium catalysts were developed by Union Carbide Corporation in the 1970s using different chromium precursors than are used in standard Phillips catalysts (6,11). The most important of these are based on chromocene and bis(triphenylsilyl)chromate, depicted in Figure 5.5. These catalysts are used in the Unipol gas phase process for LLDPE and HOPE and are different from standard Phillips catalysts in several respects ... 

Furthermore, the polymers produced by the two catalyst types are also different. Indeed, in terms of polymer properties, the organochromium catalysts can be subdivided into two major classes based on their behavior—(a) chromocene and some of its derivatives, and (b) all others. These properties are summarized in Table 54. 

A competitor of Phillips catalyst, based on chromium oxide supported on silica, is the Union Carbide catalyst, which is prepared by the reaction of chromocene with silica. When chromocene, [Cp2Cr ], reacts with SiO2-(800)> it gives [(=SiO)Cr(Cp)] according to mass balance analysis (Scheme 42 and Table 12), and this surface complex is highly active in ethylene polymerization. ... 

---