Polyethylene resins having improved environmental stress crack resistance, stiffness and impact resistance is made by a process comprising feeding both a chromium oxide catalyst and a silyl chromium catalyst into a polymerization reactor. The chromium oxide catalyst and the silyl chromium catalyst are on separate supports. The chromium oxide catalyst is 25-50 weight percent and the silyl chromium catalyst is 50-75 weight percent of the total weight of catalyst. The catalysts may be added separately or as a single mixture.