Catalysts for Polyethylene

Because of its ordered structure, an isotactic polymer has higher melting point and tensile strength than the atactic polymer. The melting points of isotactic polymers are typically between 165 and 171°C, while those of atactic polymers are less than 0°C. 

As already mentioned, until about the beginning of 1990 only heterogeneous catalysts have been used for the polymerization of ethylene and propylene. For ethylene polymerization the catalysts used are essentially of three types. These are the Phillips catalyst, the Union Carbide catalyst, and the Ziegler catalyst. 

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Coordination catalysts contain a metal atom that is activated by a variety of electron-withdrawing species that are attached to it. Coordination catalysts for polyethylene fall into three main categories ... 

There is considerable information that points to the cationic nature of the Ziegler ethylene polymerization. Tabata, Shibano, Sobue and Hara (76) have found that the polymerization of ethylene at —78° with cobalt —60 irradiation shows the characteristics of cationic polymerization. Roha, Kreider, Frederick and Beears (77) found that an active Ziegler catalyst for polyethylene from a non-reduced trialkyl-aluminum-titanium tetrachloride system requires an electrophilic com-... 

Karapinka, Smith, Carrick (79) studied the use of methyltitanium trichloride as a catalyst for polyethylene. Alone it was inactive for the polymerization of polyethylene. It required the predecomposition to titanium trichloride at 120° or the addition of titanium trichloride to produce an active catalyst. Vanadium tetrachloride also produced an active catalyst. Aluminum bromide failed to activate the catalyst, whereas trialkylaluminum which reacts to produce alkylaluminum chlorides was effective. 

In Section 6.2, it was mentioned that one of the commercial catalysts for polyethylene (HDPE) is made by the reaction of chromocene with silica according... 

The Mg/Ti catalysts for polyethylene have been the most widely studied for historical reasons. The most comprehensive kinetic scheme for ethylene polymerization was provided recently by B6hm85) (see Fig. 27). 

A considerable improvement in Ziegler-Natta catalysis was achieved starting from the end of the 1960 s with the development of the so-called high yield catalysts for polyethylene, based on activated MgCl which gave rise to the well known advantages in the industrial production of the polymer. 

Table 1.3 Principal characteristics of transition metal-containing catalysts for polyethylene production.

Though Phillips catalysts are by far the most important supported chromium catalysts for polyethylene, there are other commercially important examples of such catalysts. These were developed primarily in the 1970s by the Union Carbide Corporation (6), now part of the Dow Chemical Company. UCC chromium catalysts for polyethylene will be discussed in section 5.4. 

Figure 5.2 Surface chromate structures resulting from treatment of silica with CrO. Calcination at high temperatures in air (or Op insures that Cr remains primarily in the +6 oxidation state and results in Phillips catalyst for polyethylene. Final activation occurs in reactor through reactions with ethylene (see text for details).

Two recent developments in non-metallocene single site catalysts for polyethylene are noteworthy ... 

Ligands with coordinating N-O atoms phenoxy-imine-catalysts for polyethylene 1096... 

Using authentic MgCl2 as the support led to even better catalysts for polyethylene, whereas the performance for polypropylene was ambivalent high productivity (>150 kg of polymer per gram of Ti) but poor stereoselectivity (less than 40% highly isotactic polymer) [11], However, the addition of proper Lewis bases to the catalyst formulation (Table 1), as components of the solid precatalyst ( internal donor ) or complexed with the Al-alkyl cocatalyst ( external donor ), improved both the productivity (up to 2-3 tons of polymer per gram of Ti) and the stereoselectivity (>95% highly isotactic polymer) [11, 38, 39]. 

A Japanese patent issued to Watanabe and Okamoto [301] describes preparation and illustrates an iron containing catalyst for polyethylene preparation. It is shown here as an illustration ... 

Some support materials can be rendered Lewis acidic enough to ionize dialkyl metallocenes. Marks and co-workers have reported (33) that alnmina dried at very high temperatures can react at least to some small degree with both thorium-and zirconium-based metallocene dimethyl species to yield active catalysts for polyethylene. The resulting cationic metal center is believed to remain coordinated to the surface through an Al-O-M Lewis acid/base linkage, at least prior to exposure to ethylene. Hybrid surface/cocatalyst systems based on aluminum alkyl-treated clays have been developed (34) in which the solid substrate appears to play some role in promoting polymerization activity far beyond that expected for non-methyl aluminoxane- or trialkylaluminum-activated catalysts. 

Other organic lithium compounds are catalysts for polyethylene, polyethylene-terephthalate films and fibers, and various other polymers. Some lithium-organics... 

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