Abstract: A catalyst system useful for polymerizing olefins is disclosed. The catalyst system comprises an activator and a Group 4 metal complex. The complex incorporates a dianionic, tridentate heterocyclic-8-anilinoquinoline ligand. In one aspect, a supported catalyst system is prepared by first combining a boron compound having Lewis acidity with excess alumoxane to produce an activator mixture, followed by combining the activator mixture with a support and the dianionic, tridentate Group 4 metal complex. The Group 4 metal complexes are easy to synthesize, support, and activate, and they enable facile production of high-molecular-weight polyolefins.