<p>A series of nonsymmetric tetranuclear spiro α-diol-β-diimine nickel complexes with a non-coordinating aryl group (Site 1) and a coordinating side-arm tertiary amine group (Site 2) were designed, synthesized, and characterized for studying the roles of Ni(II)s in the cluster in olefin polymerizations. Upon activation with MMAO, these complexes show moderate to high activity in ethylene homo- and co-polymerization with 3-buten-1-ol and 9-decen-1-ol. These nonsymmetric tetranuclear complexes exhibit bimodal catalytic behavior besides great activity, up to 2569 kg/(mol Cat h atm). The <i>M</i>_w distribution of PE from bimodal to monomodal and <i>M</i>_w of the low molecular weight (LMW) fraction from 11 to 66 kg/mol can be efficiently tuned by the introduced coordinating side arm tertiary amine or aryl group at Site 2. Polymer ¹H NMR shows vinyl end group exists in PEs with LMW fraction and its absence in PEs without LMW fraction, suggesting the Site 2 Ni center with side-arm tertiary alkyl amine leads to LMW PE and β-H elimination is the chain termination pathway. DFT studies were carried out supporting the different roles of the Ni sites bearing different steric and electronic environments in the tetranuclear complex on activity, polymer dispersity and <i>M</i>_w of the LMW fraction in ethylene polymerization.</p>