Branched a-olefin, 4-methyl-1-pentene (4 MP), was polymerized with classical alpha-diimine nickel complexes in the presence of MAO. Influences of structure of a-diimine nickel catalysts and polymerization parameters including temperature and [Al]/[Ni] mole ratio were evaluated. At 0 degrees C, 4-methyl-1-pentene can be polymerized in a living/controlled manner. The obtained poly(4-methyl-1-pentene)s are amorphous elastomers with low glass transition temperature (T-g). Nuclear magnetic resonance (NMR) and distortionless enhancement by polarization transfer (DEPT) analyses show that various types of branches and microstructural units are present in the polymers. On the basis of assignment of microstructures, mechanistic models that involves the 1,2- and 2,1-insertion, and chain walking were constructed. The influences of temperature and [Al]/[Ni] mole ratio on branching degree, branch type, and insertion pathways were also discussed in detail. (C) 2010 Elsevier Ltd. All rights reserved.