Polyethylenes catalysts

The introduction of metallocene and single-site polyethylene catalyst systems may eventually change the demand for higher olefins used as comonomers. Some sources indicate that their use will increase the demand for comonomers, but others feel that they will reduce comonomer use. At any rate, it is not expected that their introduction will have a significant effect on comonomer growth rates for the period 1992—1996 (see Olefin polymers). 

Single-screw extruders, 23 398, 16 723 Single-site catalyst, 16 82, 26 545 in HDPE production, 20 154-155 Single-site polyethylene catalyst systems, 17 725... 

However, in contrast to fuels, petrochemicals intermediates must be produced at extremely high purities. For example, CO at ppm levels will poison polyethylene catalysts, and acetylene in ethylene at this level will produce a crosslinked polymer that will have unsatisfactory properties. Therefore, the chemical engineer must produce these intermediates with extremely high purities, and this requires both careful attention to minor reactor products and to efficient separation of them from the desired product. These factors are also important in the economics of petrochemicals. 

ESR Studies of Surface Species. ESR has been applied widely in heterogeneous catalysis as a technique for examining the nature and concentration of possible catalytic sites when a material is activated either chemically or thermally (64, 65, 66). ESR studies on the Phillips polyethylene catalyst, Cr03/Si02, are a classical example of this application (67). The interpretation of such ESR studies is questionable since the chemical changes during activation are not well understood, and the nature of the surface species may have to be assumed. 

The unique nature of the alkyl attached to titanium in a polyethylene catalyst has been indicated by Gray (80). Methyltitanium trichloride has an infrared spectrum which is unique and different from the bridged or unbridged methyl of methylaluminum chlorides. Although methyltitanium trichloride is not an effective catalyst to polymerize ethylene, this unique character is an indication of a difference which is developed further in the effective polyethylene catalysts. 

The low-pressure processes, such as slurry, solution, or gas phase, can produce types I, II, III, and IV polyethylenes. Catalysts used in... 

Gibson and workers [27] at BP have reported a high-throughput approach directed towards the optimization of chromium polyethylene catalysts. The study aimed to improve the performance of a previously discovered chromium catalyst. In this study, the ca. 200-member hemi-salen ligand library was complexed in situ to a soluble chromium precursor [p-tolylCrCl2(thf)3], followed by addition of 180 equiv. of MAO, and exposure to 1 atm of ethylene for 15 min. Relative activities were determined from the polyethylene yields, and a new highly active chromium catalyst was uncovered. The Coates group has reported a clever approach to the... 

An important characteristic of heterogeneous polyethylene catalysts is the phenomenon of particle replication. Particle size distribution (psd) and morphology of the catalyst are reproduced in the polymer. If the catalyst is finely divided, the polymer will also be fine and may cause handling problems. If the catalyst contains agglomerates of oversized particles, so too will the polymer. Morphology replication is illustrated in Figure 3.2. Figure 3.3 shows how psd of the catalyst is mirrored in the polymer. 

Titanium tetrachloride was the logical choice as the raw material for early Ziegler-Natta catalysts. TiCl is a clear, colorless, hygroscopic liquid that fumes upon exposure to ambient air. TiCl (aka "tickle 4") was (and still is) manufactured in enormous volumes as a precursor to titanium dioxide used as a pigment for paint. Consequently, TiCl was readily available and relatively inexpensive. Also, TiCl had been shown by Ziegler and coworkers to produce some of the most active polyethylene catalysts. Though sometimes called a catalyst, it is more accurate to call TiCl a "precatalyst," since it must be reduced and combined with a cocatalyst to become active. 

Metal Alkyls in Polyethylene Catalyst Systems 49 4.2.1 Reducing Agent for the Transition Metal... 

Reaction of dialkylmagnesium compounds with selected chlorinated compounds produces finely divided MgCl that can be used as a support for polyethylene catalysts. Other reagents may be used to produce different inorganic magnesium compounds, also suitable as supports. Examples are shown in Figure 4.1. Treatment of these products with transition metal compounds results in a supported "precatalyst." Typically, the transition metal is subsequently reduced by reaction with an aluminum alkyl and the solid catalyst isolated. The solid catalyst and cocatalyst (usually TEAL) may then be introduced to the polymerization reactor. 

As discussed in section on p. 37, fine particles are usually not desirable for Ziegler-Natta catalysts. However, most polyethylene catalysts produced by the method described in this section are used in solution processes where psd and morphology are less important than in gas phase or slurry processes (see Chapter 7). 

Organoboron compounds constitute a broad and rich area of organometallic chemistry and a detailed discussion is inappropriate for an introductory text on polyethylene. However, several organoboron compounds are crucial for selected polyethylene catalyst technologies. For example, arylboranes are used as cocatalysts for single site catalyst systems and will be discussed in Chapter 6 (see section 6.3.2). The purpose of this section is to introduce the trialkylborane that is a component of 3 generation Phillips catalyst systems (Chapter 5) employed in industrial processes in for linear polyethylene. 

Table 1.3 Polyethylene catalysts containing transition metals 22...

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. 

In general, aliphatic substituents enhance the activity of polyethylene catalysts. In this series of... 

The strategies for catalyst innovaticHis are also shown to be greatly dependent on the progress in surface science of PhUlips-type catalysts. More and more novel Cr-based polyethylene catalysts with better performance and products of improved structure and properties can be expected through successive surface modifications of either the chromate species or the silica support on the traditional Phillips catalyst... 

Standard of Indiana Catalyst. The first low pressure polyethylene catalyst invented (46), the Standard of Indiana catalyst system, saw relatively little commercial practice. Their 1951 patent discloses reduced molybdenum oxide or cobalt molybdate on alumina for ethylene polymerization, preferably in aromatic solvents. Later, work concerning the use of promoters was also disclosed. 

Ziegler Catalysts. For his work in the discovery of a new class of highly active catalysts for polymerization of ethylene, propylene, and dienes, Karl Ziegler shared the 1963 Nobel Prize in Chemistry with Guilio Natta whose contributions were predominantly related to polypropylene. Today, these catalysts together with the Phillips catalyst are responsible for the majority of the world s polyethylene production. Loosely defined, Ziegler catalysts are polyethylene catalysts derived from transition-metal halides and main group metal alkyls (46,50-53). In modem... 

Complexes with Open Pentadienyl Ligands. Open-chain analogues of the Cp ligand have only received limited attention. Compoimds 21 (87) and 22 (88) have been featured in patent examples as moderately active polyethylene catalysts after activation hy [Cp2Fe][B(C6F5)4] and MAO, respectively. 

Cyclopentadienylchromium Compounds. Chromocene, a paramagnetic and somewhat labile metallocene, forms a polyethylene catalyst after chemisorption on silica (108). The reaction generates an equivalent of CsHe, and the propagating site has been proposed (109) to be as shown in equation 13. The characteristics of this ill-characterized catalyst are facile chain transfer to H2, poor comonomer incorporation, and fairly broad molecular weight distribution (MWD). This catalyst is still in commercial use. 

Polyethylene Catalysts without Propylene Polymerization Data. 108... 

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