Phenoxycycloalkylimine ligated zirconium complexes for ethylene polymerization: Formation of vinyl-terminated low molecular weight polyethylenes with high efficiency

Bis(phenoxyimine)Zr complexes 1-8 containing a series of cycloalkyl groups on the imine-N's were synthesized (1: cyclopropyl; 2, 3: cyclobutyl; 4: cyclopentyl; 5, 7: cyclohexyl; 6, 8: 2-methylcyclohexyl). X-ray crystallographic analyses suggested that complexes 3, 5, and 8 assume an octahedral coordination geometry with a trans-phenoxy-O, cis-imine-N, and cis-Cl disposition and that the cycloalkyl groups on the imine-N's influence steric environments around the chlorine bound sites (i.e., potential polymerization sites). Upon activation with MAO at 25 degrees C, these complexes produced low-to-high molecular weight polyethylenes (PEs) (M-w 1900-960000, M-w/M-n 1.6-4.9) with very high efficiency (22-290 kg of PE/(mmol of cat. h)), which is comparable to or exceeds that seen with Cp2ZrCl2/MAO (28 kg of PE/(mmol of cat. h)). The cycloalkyl group has a profound effect on both catalytic activity and product molecular weight, indicating the critical importance of the substituent on the imine-N for polymerization catalysis. The catalytic activity increased with an increase in the steric bulk of the cycloalkyl substituent, albeit too much steric bulk reduced the activity. The product molecular weight was also related to the steric bulk of the cycloalkyl group, in that increased steric bulk normally resulted in higher molecular weight PEs. The PEs produced with complexes 1-5/MAO (Al/Zr molar ratio = 1250) possess a high degree of vinyl unsaturation at one of the two polymer chain ends (M-w 2000-14000, vinyl selectivity, 90-96 mol %). Polymerizations performed at a much higher Al/Zr molar ratio of 12500 confirmed the marked preference of these complexes for beta-H transfer as the chain termination mechanism (M-w 1900-14000, vinyl selectivity, 90-95 mol %). The vinyl-terminated PEs were readily transformed to the corresponding epoxy- and diol-terminated PEs, which are valuable materials for PE- and polar polymer-based block and graft copolymers. Ethylene pressure studies on complexes 1, 2, 4, and 5 revealed a first-order dependence on ethylene for both the rate of chain propagation and the rate of chain transfer. On the basis of this polymerization behavior together with X-ray analyses and DFT calculation studies, we concluded that beta-H transfer to an incoming monomer is responsible for the formation of vinyl-terminated PEs. The calculations revealed that the complexes disfavor beta-H transfer to the Zr metal due to the extreme instability of a metal hydride species that is produced in such a chain transfer process. Therefore, the unique polymerization catalysis and distinctive polymer formation with phenoxycycloalkylimine ligated Zr complexes were demonstrated.