With Brookhart type alpha-diimine Ni(II) based catalysts, it is highly challenging to tune polymers branching level and branch-type distribution, which in turn strongly affects thermal and mechanical properties, through the aryl ortho-positions modification, while maintaining high turnover frequencies (TOFs). Herein, we are interested in performing a systematic investigation on the polymerization of 1-octene, 1-decene, and 1-octadecene catalyzed by a series of alpha-diimine nickel(II) complexes with methyl ligand backbone and different substituents in aryl positions (Ni1-Ni6). In addition to bulky isopropyl and tert-butyl substituents described in the original Brookhart's work, complexes with different aryl ortho- and para-substituted alpha-diimine ligands, including the less sterically demanding methyl and ethyl substituents, are investigated. The C-13 NMR spectra of the polymers have been assigned in detail, and some unique features have been identified and related to the chain-walking coordination/insertion mechanism. Changes in the ligand structure and monomer size have important effects on the numerous combinations of insertion and chain-walking paths from which different branches are installed. We have also carried out a comprehensive investigation of the mechanical behavior of the polymers by means of uniaxial stretching until failure, step-cycle, and creep tensile tests. Overall, the resulting polymers exhibited a broad spectrum of tensile properties, depending on their microstructure and crystallinity which in turn are strongly affected by, monomer length and type of alpha-diimine ligand. 1-Octene and 1-decene polymers behave as elastomers with excellent mechanical properties, i.e., high elongation at break (up to 2000%) and good strain recovery, while 1-octadecene polymers behave as plastomers.