Abstract: Ethylene is polymerized or copolymerized using a pressure of at least 300 kgm/cm.sup.2 and a multi-zone (i.e. two or more) reaction vessel wherein the feed to the first zone is fresh ethylene, which is preferably mixed with recycle gas from the low pressure hopper and into at least a subsequent zone there is introduced a feed containing return gas recycled from the separator. By this technique the composition of the gas feed to the various zones can be varied, for example the hydrogen content of the gas streams to the various zones can be varied and this can produce a polymer of broader molecular weight distribution.