CO was not a comonomer but an inhibitor in ethylene polymerization catalyzed over [1,2-bis(2,6-diisopropylphenylimino)]acenaphthene nickel(II) dibromide (1)/MAO. The average number of active sites, [(C) over bar*], and the average rate constant for chain propagation, (k) over bar (p) of the (1)/MAO system was determined using CO inhibition method based on the assumption that two molecules of CO coordinate to each active center. At 0 degrees C, the average number of active centers, (C) over bar% was increased with the Al/Ni ratio, and (k) over bar (p) was not influenced by the Al/Ni ratio. Up to Al/Ni ratio of 3000, the average number of active sites was saturated. Single active site was present at the high ratio of Al/Ni and its reactivity with CO is uniform at 0 degrees C. The maximum average activity was 5262.07 (kg-PE/mol-Ni/atm/hr) and 65.7% of (1) was converted to form active cation complexes at 0 degrees C and Al/Ni molar ratio of 5000, while 36.5% of (1) was activated at the Al/Ni molar ratio of 250. Above 30 degrees C, the complicated CO poisoning behavior was observed because the reactivity and stoichiometry of active centers with CO were not uniform and their thermal stability was very poor. (c) 2005 Elsevier Ltd. All rights reserved.