Poly(ethene)

The manufacture of polythene using Ziegler-Natta catalysts takes place under very different conditions to LDPE, and results in a more rigid plastic.

The use of a titanium catalyst, together with an aluminium co-catalyst, enables polymer chains with little or no branching to be produced. These will pack together closer, giving a higher density material. Typically, HDPE may have at most 10 branches per 1000 carbon atoms.

diagram: catalyst used to make HDPE

Chain length and branching

Depending on production methods, the lengths of the chains produced can vary enormously, giving rise to polymer molecules with widely different molecular weights, and different properties.

Low molecular weight HDPE (less than 80,000) is very brittle. Commercial HDPE is usually in the range 80,000 to 1,200,000, representing carbon chains between 5,000 and 1 million atoms in length.

A higher degree of branching gives a softer and more elastic polymer.

Mechanism

Unlike LDPE, the reaction is not a free radical polymerisation. The catalysts used are very efficient, and 1g of catalyst can produce as much as 1000 kg of HDPE. After polymerisation the catalyst may be destroyed using water or alcohol. These very small amounts of catalyst residue remain in the polymer, and are not recovered.

The same mechanism occurs in making LLDPE from Ziegler-Natta catalysts, and so the same reactors may often be used for both.

diagram: mechanism of Ziegler-Natta catalyst with HDPE

For further information on this mechanism, see the Catalysis site.