The catalyst used in the manufacture of polyethylene is a mixture of

Polythenes are linear or slightly branched long chain molecules. These are capable of repeatedly softening on heating and hardening on cooling and are thus thermoplastic polymers.

There are two types of polythene, low density polythene and high density polythene.

Low density polythene: It is obtained by the polymerisation of ethene under high pressure of 1000 to 2000 atmospheres at a temperature of 350 K to 570 K in the presence of traces of dioxygen or a peroxide initiator (catalyst). The low density polythene (LDP) is obtained through the free radical addition and H-atom abstraction. It has highly branched structure

High density polythene is formed when addition polymerisation of ethene takes place in a hydrocarbon solvent in the presence of a catalyst such as trialkyllaluminium and titanium tetrachloride (Ziegler-Natta catalyst) at a temperature of 333 K to 343 K and under a pressure of 6-7 atmospheres. High density polythene (HDP) thus produced, consists of linear molecules as shown below and has a high density due to close packing.

Hence, the catalsyt used in the manufacture of high density polythene is ${\mathrm{TiCl}}_4 \& \mathrm{Al}{\left({\mathrm{CH}}_3\right)}_3$