Ethylene is copolymerized with one or more mono .alpha.-olefins, especially butene and/or hexene, using as catayst a Group IVA metal hydrocarbyl compound supported on particulate alumina, the purity ratio (i.e. the molar ratio of Group IVA metal to Group I to III metal) of the said compound being at least 100/1 and, preferably, 500/1. By using these very pure compounds, especially in the case of zirconium tetrabenzyl, the incorporation of co-monomer in the copolymer product may be enhanced.