Two late transition metal catalysts based on 2,6-bis(imino)pyridine iron(II) were synthesized by introducing methyl substitution (catalyst A) and t-Butyl substitution (catalyst B) at the ortho position of the aryl rings of the ligand. Comparative ethylene polymerizations using the catalysts showed quiet different behaviors. The activity of catalyst A was higher than that of catalyst B in all of the polymerization conditions used. The highest activities of these catalysts were obtained at almost 25 °C. Activities of the catalysts increased with increasing both monomer pressure and [Al]: [Fe] molar ratio. Multi modal peaks were appeared in the DSC analysis of oligomers obtained by catalyst A, while in the DSC analysis of the polymer obtained using catalyst B unimodal peaks were appeared. It is suggested that catalyst A containing less bulkier substitution produced polymer with different molecular weight fractions and different melting points. Besides, the t-butyl group in catalyst B resulted in producing polyethylene (PE) with single sharp DSC peaks; the latter is due to the formation of highly linear polyethylene. Polymerization temperatures affected the pattern of DSC thermograms in terms of number and shape of the peaks of the obtained polymers by catalyst A. Catalyst A produced linear oligomers contained liner olefinic part with a number average molecular weight in the range of 260 to 361.

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