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The present investigation is undertaken to provide a deeper understanding of the 5‐ethylidene‐2‐norbornene (ENB) homopolymerization. To this end, binuclear and mononuclear nickel α‐diimine catalysts, as well as modified methylaluminoxane (MMAO) as a cocatalyst, are selected and utilized. Al/Ni ratio of 1000 coupled with a temperature 25 °C and a polymerization time 12 h are chosen as the optimal statistics. It is shown that the highest obtained activity for binuclear and mononuclear catalysts are 511 and 478 kg polymer/mol, respectively. Thereafter, the characterization of the obtained poly 5‐ethylidene‐2‐norbornene (PENB) is performed by fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Concerning the glass transition of cyclic polyolefin, DSC analysis reveals that catalyst structure is a determining factor (Tg binuclear: 176 °C and Tg mononuclear: 140 °C). As validated by dynamic mechanical thermal analysis (DMTA), it is realized that the alteration in the catalyst structure exerts a far‐reaching influence on the polymer architecture. Surprisingly unexpectedly, excluding addition polymerization, an in‐depth examination of polymer structure uncovers the potentially tenable participation of both cationic and ring‐opening polymerization, albeit, with a diversified degree of contribution. Lastly, PENB is functionalized with three functional groups, and then the successful grafting of polar functional groups into modified cyclic polyolefin backbone is confirmed by NMR and FTIR spectroscopy.

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