Effects of tetraalkylaluminoxane activators on ethylene polymerization catalyzed by iron-based complexes

Five different tetraalkylaluminoxanes, tetraisobutylaluminoxane (A1), ethyltriisobutylaluminoxane (A2), diethyldiisobutylaluminoxane (A3), triethylisobutylaluminoxane (A4) and tetraethylaluminoxane (A5), were prepared to polymerize ethylene in the presence of iron complexes [((2,6-RPh)NC(Me))2C5H3N]FeCl2 (RMe, M1; Riso-Pr, M2). Except M1/A1 and M2/A1, all other catalyst systems produced polyethylene with pronounced broad or bimodal molecular weight distribution (MWD). It was demonstrated that the low-MW fractions of these broad- or bimodal-MWD polyethylenes result from the chain transfer reaction to the alkylaluminium containing the Al-Et groups in the activator. As the Al/Fe ratio and the amount of the Al-Et groups in the activators increased in the order: A1 < A2 < A3 < A4 < A5, the polymerization activity and the low-MW fraction of broad- or bimodal-MWD polyethylene increased and the high-MW fraction correspondingly reduced.