Question

Low density polythene and high density polythene, both are polymers of ethene but there is marked difference in their properties. Explain

Answer 1

Low-density polythene

• It is obtained by the polymerisation of ethene under high pressure of $1000\text{to}2000$ atmospheres at a temperature of $350K\text{to}570K$, in the presence of dioxygen or a peroxide initiator (catalyst).
• It is obtained through free-radical addition and $H$-atom abstraction.
• It has a highly branched structure, and thus chains stay away from each other, giving less density to the polymer.
• These polymers have a straight-chain structure with some branches as shown below:

• It is chemically inert and tough but flexible and a poor conductor of electricity.
• It is used in making squeeze bottles, toys, insulation of electricity cables and flexible pipes.
• It is less hard and tougher than HDPE.

High-density polythene

• It is formed when the addition polymerisation of ethene takes place in the presence of catalysts such as the Ziegler-Natta catalyst at a temperature of $333K\text{to}343K$ and pressure of $6-7K$ atmospheres.
• It consists of linear molecules and has a high density due to the close packing of chains.

• These polymers are also called linear polymers.
• It is harder and tougher than LDPE.
• It is used for manufacturing buckets, dustbins, bottles, pipes etc.