Low density and high density polythenes are obtained under different conditions. LDP is obtained by the polymerisation of ethene under high pressure of 1000 to $2000 \mathrm{~atm}$ at a temperature of $350 \mathrm{~K}$ to $570 \mathrm{~K}$ in presence of traces of dioxgen or a peroxide initiator.
While high density polythene is formed when addition polymerisation of ethene takes place in a hydrocarbon solvent in the presence of a catalyst at about $333 \mathrm{~K}$ to $343 \mathrm{~K}$ under a pressure of $6-7 \mathrm{~atm}$.
LDP has highly branched structure and is chemically inert and tough but flexible and a poor conductor of electricity. HDP consists of linear molecules and has a high denisty due to close packing. It is also chemically inert and more tough and hard.