Abstract Copolymerizations of ethylene (E) with 1,5-hexadiene (1,5-HD) has been carried out with rac-1,2-ethylenebis(1-indenyl)Zr(NMe 2) 2 [ rac-(EBI)Zr(NMe 2) 2] and isopropylidene(cyclopentadienyl)(9-fluorenyl)ZrMe 2 ( iPr(Cp)(Flu)ZrMe 2) compounds combined with Al( i-Bu) 3/[Ph 3C][B(C 6F 5) 4] as a cocatalyst system. Microstructures of copolymers were determined by 1H, 13C NMR, Raman spectroscopy and X-ray diffraction (XRD). The isospecific rac-(EBI)Zr(NMe 2) 2/Al( i-Bu) 3/[Ph 3C][B(C 6F 6) 4] catalyst showed much higher polymerization activity than syndiospecific iPr(Cp)(Flu)ZrMe 2/Al( i-Bu) 3/[Ph 3C][B(C 6F 6) 4] catalyst; however, the latter catalyst showed much higher comonomer reactivity, resulting in r E=17.44 and r 1,5-HD=0.02( r E× r 1,5-HD=0.35), than the latter catalyst showing r E=4.48 and r 1,5-HD=0.12( r E× r 1,5-HD=0.54). The 1,2-inserted 1,5-HD led to cyclic backbones by the intramolecular reaction. The intramolecular cyclization was independent of the polymerization temperature. Detailed investigation on the structure of copolymers by 13C NMR and distortionless enhancement by polarization transfer (DEPT) spectroscopies showed that alternating structural units become predominant as the amount of 1,5-HD incorporated into the copolymer increased. The qualitative and quantitative analyses of the structure of copolymers were also made by XRD and Raman spectroscopy.