Chromium Ziegler-Natta catalysts

Metallocenes give polyethylene producers a long list of opportunities to work on. They have already created polyethylene copolymers that compete well in applications that have been formerly the exclusive domain of the more costly, so-called high value plastics. Further, they are augmenting the chromium oxide and Ziegler-Natta catalysts systems that have been used for HDPE and LLDPE with metallocene catalysts. That creates even further... 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

Polymerization with Complex Catalysts. High density polyethylene reached a domestic production of 1.25 billion pounds in 1968. It is made either with a stereospecific Ziegler-Natta catalyst or on a supported chromium oxide catalyst. The latter forms a complex with the silica-alumina and is activated by treatment with air and steam at elevated temperature. The mechanism is such that electrons are donated to the catalyst in order to be returned under polymerizational-promoting conditions, consequently lowering the energy of the system ... 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

Description Catalyst or metallocene is injected directly into the reactor as received in the case of Ziegler-Natta or after activation for chromium catalyst. Chromium catalysts give broad molecular-weight products and supported Ziegler-Natta catalysts produce narrow molecular-weight distribution products. The BP metallocene catalysts offer exceptional strength and clarity normally associated with metallocenes but combined with easy processing. 

Another example for a defined pincer carbene chromium(III) catalyst for the oligomerisation of ethylene also comes from McGuinness et al. [482]. It is an example of a nonmetallocene polymerisation catalyst [483] that traditionally falls into the categories of chromium based Phillips and Union Carbide systems [484], nickel based SHOP catalysts [485] and titanium/aluminium based Ziegler-Natta catalysts [29], The chromium(III) pincer carbene catalysts are highly active for the oligomerisation of ethylene and produce mainly a-oleflns. 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

For polyethylene produced with transition metal catalysts, molecular weight distribution is dictated largely by the catalyst employed. Polydispersities typically range from 2-3 for polyethylene made with single site catalysts, 4-6 for polymer produced with Ziegler-Natta catalysts and 8-20 for polyethylene made with supported chromium catalysts. These differences are illustrated... 

If a copolymer such as VLDPE or LLDPE is the target resin, satisfactory comonomer incorporation must be achieved. This is manifested by the amount of comonomer incorporated (evidenced by density) and the distribution of comonomer in the polymer (evidenced by composition distribution). In general, supported chromium oxide catalysts incorporate comonomer more easily than Ziegler-Natta catalysts. 

Diluents must be inert toward the catalyst system and are usually saturated hydrocarbons such as propane, isobutane and hexane. Slurry processes typically operate at temperatures from about 80 to 110 °C and pressures of 200-500 psig. Polyethylene precipitates as formed resulting in a suspension of polymer in diluent. The catalysts most commonly used in slurry processes are chromium-on-silica or supported Ziegler-Natta catalysts. 

Most of the polyethylene made in gas phase processes employs Ziegler-Natta catalysts. There are, however, a few instances where supported chromium and single site catalysts are used. A simplified process flow diagram for the Unipol gas phase reactor process is shown in Figure 7.4. Principal operating features of the gas phase process are summarized in Table 7.5. 

Before the 1970s, Ziegler-Natta catalysts for a-olefin production were normally prepared from certain compounds of transition metals of Groups IV-VI of the periodic table (Ti, V, Cr, etc.) in combination with an organoraetallic alkyl or aryl (Table I). Practically all subhalides of transition metals have been claimed as catalysts in stereoregular polymerization. Only those elements with a first work function <4 eV and a first ionization potential <7 V yield sufficiently active halides, that is, titanium, vanadium, chromium, and zirconium (7, Only titanium chlorides have gained widespread acceptance in crystalline polyolefin production. 

Other kinds of coordination catalytic systems developed few years before the Ziegler-Natta catalysts were based on chromium and molybdenum oxides supported on SiOj AI2O3 and other supports. The catalysts were patented by Phillips Petroleum and Standard Oil companies of Indiana for the synthesis of polyolefins. Although Phillips catalysts were the first to produce a fraction of crystalline polypropylene, these systems were more useful for the production of polyethylene. In fact, the Phillips and the Ziegler-Natta catalysts are currently the two commercial systems that dominate the production of HDPE [2]. 

Sukhadia, A. M., Trade-offs in blown film LLDPE type resins from chromium, metallocene and ziegler-natta catalysts. Journal of Plastic Film Sheeting 2000,16, (1), 54-70. 

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