The polymerization of ethylene with late transition metal complexes typically provides highly branched amourphous poly(ethylene)s through a chain-walking mechanism, while the chain-straightening polymerization of alpha-olefins gives semicrystalline polymers. Herein, we report the polymerization of 1-hexene, 1-octene, and 1-dodecene catalyzed by an alpha-diimine Ni(II) complex in combination with diethylaluminum chloride. The effect of monomer chain length and monomer concentration on the productivity, polymerization livingness, selectivity of monomer insertion, and polymer structure/properties is investigated. The polymerization results indicate the possibility of precise microstructure control, depending on the monomer employed and monomer feedstock concentration, which in turn strongly affects the physical polymer properties. Additionally, di- and triblock copolymers were successfully fabricated in an easy, high-yielding route, at room temperature. The synthesis was accomplished through the sequential monomer addition from 1-dodecene and ethylene, without requiring any intermediate separation step. Each block exhibited distinct features from semicrystalline to amorphous, taking advantage of the different mechanism involved in the polymerization of 1-dodecene and ethylene. Investigation on mechanical behavior by uniaxial stretching until failure and step-cycle tensile tests showed that the block copolymers behave as thermoplastic elastomers with different performances depending on the composition, length of the blocks, and crystallinity. We demonstrate that this approach is a route to fabricate thermoplastic elastomers from readily accessible starting materials.