A study of the elementary reactions involved in the catalytic activation process of alpha-diimine NiBr2 by methylaluminoxane (MAO), for alpha-olefins polymerization, was undertaken in toluene and chlorobenzene as solvents. The investigation is based on the comparative study of polymerization kinetics of hex-l-ene, UV-visible spectroscopic analysis of the catalytic system and structural analysis of the poly(hex-1-ene)s formed. Correlation between polymerization activities and changes in the W-visible absorption spectrum of the transition metal derivative were found. Spectroscopic data have underlined the important role of the monomer in the stabilization process of the nickel-based active species. Partial polymerization orders with respect to MAO, nickel derivative and monomer were determined. Surprisingly, an apparently negative monomer order of the polymerization corresponding to a decrease of the polymerization rate with an increase of monomer concentration in the reacting medium was observed. A tentative interpretation based on the presence of two types of propagating active species of different reactivity, resulting from the walking migration mechanism, is discussed.