How do we polymerize ethylene into polyethylene

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Polyethene (also known as polyethylene, PE) is, along with polyvinyl chloride, the most common industrial plastic. Everyday objects such as plastic bags, freezer bags, buckets, laundry baskets, beverage crates, garbage cans, etc. are usually made of polyethene, but pipes and cable insulation are also often made of this material.

PE is a rather soft thermoplastic at room temperature. You can easily scratch the surface of an object made of PE with your fingernail. However, the substance is very stable towards chemicals: salt solutions, alkalis and most inorganic acids do not attack the material, even nail polish remover is sold in polyethene bottles. Without added color pigments, polyethene is translucent, but not completely transparent. Unless the material is processed into a very thin film. The density of polyethene is between 0.90 and 0.95 g / cm, depending on the production method3i.e. it floats in water.

PE arises formally through the polymerisation of ethene, a gaseous substance that is obtained through the petrochemical processing of petroleum.

The reaction equation is very simple to formulate, but actually making this material is extremely difficult. Only at 100 ° C and a pressure of approx. 100 MPa (that is a thousand times the normal air pressure) does ethene react in a radical polymerization to form what is known as high-pressure polyethene. However, chain branches also occur in this reaction. If a hydrogen atom is abstracted from the growing chain by a radical molecule, a new radical center is formed from which the chain can also continue to grow.

The resulting polyethene is tough and elastic. It has a density of around 0.9 g / cm3 and is referred to as PE-LD (from English: low density). It can be found in foil and plastic bags.

Another possibility for the synthesis of polyethene was later discovered by the scientist Karl Ziegler. In the presence of a catalyst, the ethene reacts with much lower external pressure to form a polyethene with unbranched chains and higher density (approx. 0.95 g / cm3) and crystallinity. The catalyst of this Ziegler-Natta polymerization is a trialkylaluminum compound. In other words, a substance in which an aluminum atom is linked to organic components. This polyethene is called PE-HD (from English: high density). It is also much more stable and can therefore be processed into pipes and other components.

Nowadays, both types of PE are produced and processed in large quantities.


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