Ziegler-Natta catalysts polyethylene production

Ziegler-Natta catalysts are used commercially for the production of stereoregular polymers, especially isotactic polypropylene and high-density linear polyethylene. The resultant polymers have number and weight average molecular weights (Mn and Mw, respectively) that are defined as... 

Polyethylene polymers history of, 17 701-702 simulations of, 16 747 Polyethylene products, Ziegler-Natta catalysts for, 26 540-543 Polyethylene resins, 17 700-703 applications for, 17 703 Polyethylenes, classification of, 17 701t. 

Chemicals. Polymerization Catalysts are used in the production of polymers, such as linear and low-density polyethylene (LLDPE). An example of these catalysts are Ziegler Natta catalysts, which are combinations of titanium halides with aluminium and magnesium alkyls. 

Production of polyethylene and polypropylene by the Ziegler-Natta catalysts, prepared from Ti chlorides and organoalanes. 

Production of Polyethylene and Polypropylene by Ziegler-Natta Catalysts... 

Polyethylene and polypropylene (metallocenes). Metallocenes of different types are being used in a variety of commercial processes to make polymers with different properties than traditional Ziegler-Natta catalysts. The metallocene catalysts can be optimized for chain length and sterochemical control of the product polymer. 

The Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

In only a few polymerization processes are metallocene catalysts used in a soluble form. Supported metallocene catalysts are preferred for the production of polyethylene or isotactic polypropylene on an industrial scale, especially in the slurry and gas-phase processes. To use them in existing technological processes (drop-in technology) as replacements for the conventional Ziegler-Natta catalysts, the metallocenes have to be anchored to an insoluble powder support, including silica, alumina, and magnesium dichloride (208-217). Various methods of anchoring catalysts to supports are possible (Fig. 25) ... 

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 ... 

Present-day Ziegler-Natta catalysts are supremely suitable for the production of linear polyethylene and of highly isotactic polypropylene. They are also used to produce the softer ethylene-propylene copolymers, used for packaging and related purposes. Due to the presence of distinct catalyst sites in typical Ziegler-Natta catalysts, these copolymers suffer from non-uniformity however, and copolymers which contain increased amounts of higher ot-olefins, desirable for certain applications, cannot easily be made with these catalysts. 

Copolymers of ethylene with a-olefins, such as the short-chain branched LLDPE (linear low-density polyethylene) impact materials or the EPD (ethylene-propylene-diene copolymer) rubbers represent major percentages of the total polyolefin production, due to their desirable mechanical properties. Solid-state MgCl2-supported Ziegler-Natta catalysts however, have unfavourable reactivity... 

The Ziegler-Natta catalysts that are used most industrially are solids that are suspended in the reaction medium. The polymerization reaction that is favored depends on the catalyst components and on their state of aggregation and other details of their preparation. Heterogeneous catalytic systems appear to be necessary for the production of isotactic polyolefins, but soluble catalysts have been used for the synthesis of polyethylene and syndiotactic polypropylene. 

Ziegler-Natta catalysts are the most important transition metal catalysts for production of polyethylene as well as other poly-a-olefins. Indeed, at this writing, it would not be practicable to manufacture the quantities of stereoregular... 

Diethylzinc is employed in several ways in polyethylene production. Its earliest application was as a chain transfer agent for molecular weight control (31, 32). Today, chain transfer in Ziegler-Natta catalyst systems is achieved chiefly by hydrogenolysis (previously discussed in connection with the mechanism of chain termination in section 3.7). Use of diethylzinc for molecular weight control for polyethylene is no longer significant. 

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