The file associated with this record is under embargo until 12 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above. ; 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. ; This work was supported by National Natural Science Foundation of China (Nos. U1362204, 51473170, 21473160 and 21374123). GAS thanks the Chinese Academy of Sciences for a Visiting Scientist Fellowship. ; Peer-reviewed ; Post-print