High-density polyethylene with aluminum alkyls

Several commercial processes are used to produce high-density polyethylene. All employ more moderate pressures and most also use lower temperatures than the low-density polyethylene processes. The Ziegler-developed process uses the mildest conditions, 200-400 kPa (2 atm) and 50-75°C, to polymerize a solution of ethylene in a hydrocarbon solvent using a titanium tetrachloride/aluminum alkyl-based coordination catalyst. After quenching the polymerized mixture with a simple alcohol, the catalyst residues may be removed by extraction with dilute hydrochloric acid or may be rendered inert by a proprietary additive. The product is almost insoluble in the hydrocarbon solvent, so is recovered by centrifuging and drying. The final product is extruded into uniform pellets and cooled for shipping to fabricators. 

Ziegler-Natta catalysts consist of a complex of a base metal alkyl or halide with a transition metal salt. Base metals from groups I-III of the periodic table may be used in combination with transition metals from groups IV-VIII. Naturally, some of these combinations are preferred for one type of polymerization or another. A classic example of a Ziegler-Natta catalyst suitable for the polymerization of ethylene to high density polyethylene is the complex of triethyl aluminum (AlEts) with titanium tetrachloride (TiCU). The proposed reaction mechanism is shown in Figure 3. 

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