This article describes an effective strategy for retarding the simultaneous polymer formation during the ethylene oligomerization with bis(imino) pyridine iron catalysts, by addition of siloxanes as modifiers into such systems. The concurrent effects of a suitable siloxane [e. g., tetraethyl orthosilicate (TEOS), cyclohexylmethyldimethoxysilane (CHMMS), or dicyclopentyldimethoxysilane (DCPMS)] are to increase the activity for the soluble oligomers and dramatically decrease the activity for the insoluble polymers, thus synergistically making a pronounced reduction of the polymer share in the total products. Based on the experimental facts when commercial methylaluminoxane (MAO), trimethylaluminum (TMA)-depleted MAO, and trialkyl aluminums (e. g., TMA) are applied as co-catalyst, respectively, the functional mechanism of silox-anes is preliminarily discussed. It is proposed that TMA containing in the commercial MAO makes little contribution to the final product but lowers the activity. And, there may be a close relationship between the anionic MAO cages and the insoluble polymer production. The influence of siloxanes exert on the catalyst systems could be a comprehensive result of the interactions between siloxanes and the catalytic components, through the modulation on both the electronic and steric effects of the active centers. (C) 2014 Wiley Periodicals, Inc.