Copolymerization of hepta-1,6-diene with ethylene using a cobalt-bis(imino)pyridine complex affords a polymer that contains trans-1,2-five-membered rings in the repeating unit from hepta-1,6-diene. The ratio of the repeating units of diene to those of ethylene is controlled from 3: 97 to 50:50, depending on the reaction conditions. Isotope-labeling reactions reveal that the major chain transfer pathway involves beta-hydrogen elimination of the polymer end formed after ethylene insertion.