In the presence of tetraethylaluminoxane (TEAO), iron complexes were used to catalyze ethylene polymerizations with extremely high activities and generally produced polyethylene with a bimodal molecular weight distribution (MWD). This bimodal MWD of polyethylene was mainly derived from residual triethylaluminum in TEAO and was produced through a mechanism of chain transfer to aluminum. Ethyl-aluminoxane and tetraisobutylaluminoxane also were used to polymerize ethylene with high activities in the presence of iron complexes, and only polyethylene with a unimodal MWD was produced. The ratio of the rate constant of chain transfer to aluminum (k(trA)) to the rate constant of chain propagation (k(p)) was determined to be 0.12 for ([ArN==C(Me)](2)C5H3N)FeCl2 when Ar was 2,6-diisopropylphenyl (1) and 2.48 for {[ArN==C(Me)](2)C5H3N}FeCl2 when Ar was 2,6-dimethylphenyl (2); these values are far larger than those for metallocene-based catalysts. This explains why an iron complex usually produces polyethylene with a broader MWD than metallocene-based catalysts. Additionally, it can be concluded from the great difference between 1 and 2 with respect to k(trA)/k(p) that an iron complex with less congested aryl substituents is subjected to chain transfer to aluminum. (c) 2005 Wiley Periodicals, Inc.