A series of [2-(1-aryliminobenzylidene)-9-arylimino-5,6,7,8-tetrahydrocyclohepta[b]pyridyl]cobalt chlorides (aryl = 2,6-Me₂Ph (Co1), 2,6-Et₂Ph (Co2), 2,6-i-Pr₂Ph (Co3), 2,4,6-Me₃Ph (Co4), and 4-Me-2,6-Et₂Ph (Co5)), incorporating a fused seven-membered ring and an imino C-phenyl group, was synthesized and characterized. Upon activation with methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), Co1–Co5 all showed high activities toward ethylene polymerization with the Co4/MAO system showing the highest activity (8.65 × 10⁶ g(PE) mol⁻¹(Co) h⁻¹). Significantly, these systems exhibited good thermal stability (up to 80 °C) as well as long catalytic lifetimes. The polyethylenes obtained all contained vinyl end-groups as well as narrow polydispersities. These phenyl-modified cobalt precatalysts provide a functional system for generating a unique class of vinyl-polyethylenes due to their ease of preparation, high catalytic activities and thermally stable polymerization.