A process for making an ethylene homopolymer in the presence of an oxide-supported chromium catalyst is disclosed. A small amount of an α-olefin contacted with the catalyst before polymerizing ethylene or introduced into an ethylene homopolymerization unexpectedly boosts process productivity. When used at part per million levels, the α-olefin improves productivity while maintaining desirable polymer properties. The invention is particularly valuable for making HDPE resins useful for blow molding applications.