Ethene was copolymerized (1) with 1,5-hexadiene with rac-ethylenebis(indenyl)zirconium dichloride/methylaluminoxane (MAO) used as a catalyst and (2) with 1,7-octadiene with bis(n-butylcyclopentadienyl)zirconium dichloride/MA0 and rac-ethylenebis(indenyl)hafnium dichloride (Et[Ind](2)HfCl2)/MAO used as catalysts at 80 degreesC in toluene. The copolymer microstructure and the influence of diene incorporation on the rheological properties were examined. Ethene and 1,5-hexadiene formed a copolymer in which a major fraction of the 1,5-hexadiene was incorporated into rings and a small fraction formed 1-butenyl branches. The copolymerization of ethene with 1,7-octadiene resulted in a higher selectivity toward branch formation. Some of the branches formed long-chain-branching (LCB) structures. The ring formation selectivity increased with decreasing ethene concentration in the polymerization reactor. Melt rheological properties of the diene copolymers resembled those of metallocene-catalyzed LCB homopolyethenes and depended on the vinyl content, the catalyst, and the polymerization conditions. At high diene contents, all three catalysts produced crosslinked polyethene. This was especially pronounced with Et[Ind](2)HfCl2, where only 0.2 mol % 1,7-octadiene in the copolymer was required to achieve significantly modified rheological properties,