Copolymerizations of ethylene (E) with 1,5-hexadiene (1,5-HD) has been carried out with rac-1,2-ethylenebis(l-indenyl)Zr(NMe2)(2) [rac-(EBI)Zr(NMe2)(2)] and isopropylidene(cyclopentadienyl)(9-fluorenyl)ZrMe2 (iPr(Cp)(Flu)ZrMe2) compounds combined with AI(i-Bu)(3)/ [Ph3C][B(C6F5)(4)] as a cocatalyst system. Microstructures of copolymers were determined by H-1, C-13 NMR, Raman spectroscopy and X-ray diffraction (XRD). The isospecific rac-(EBI)Zr(NMe2)(2)/Al(i-Bu)(3)/[Ph3C][B(C6F6)(4)] catalyst showed much higher polymerization activity than syndiospecific iPr(Cp)(Flu)ZrMe2/Al(i-Bu)(3)/[Ph3C][B(C6F6)(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) X r(1.5-HD) = 0.35), than the latter catalyst showing r(E) = 4.48 and r(1.5-HD) = 0.12(r(E) X 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 C-13 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.