High Temperature Iron Ethylene Polymerization Catalysts Bearing N, N, N′-2-(1-(2,4-Dibenzhydryl-6-fluorophenylimino)ethyl)-6-(1-(arylphenylimino)ethyl)pyridines

The N, N, N′-ferrous chloride complexes, [2-CMeN(2,4-(CHPh)2-6-FC6H2)-6-(CMeNAr)C5H3N]FeCl2 (Ar = 2,6-Me2C6H3Fe1, 2,6-Et2C6H3Fe2, 2,6-iPr2C6H3Fe3, 2,4,6-Me3C6H2Fe4 and 2,6-Et2-4-MeC6H2Fe5), each possessing one N-2,4-dibenzhydryl-6-fluorophenyl group, were readily synthesized from their respective unsymmetrical bis(imino)pyridines, L1–L5. Structural identification of Fe2 highlighted the variation in the steric properties provided by the dissimilar N-aryl groups. Following pre-treatment with either MAO or MMAO, complexes Fe1–Fe5 all displayed, at an operating temperature of 80 °C, high activities for ethylene polymerization with levels falling in the order: Fe4 > Fe1 > Fe5 > Fe2 > Fe3. Notably, Fe4/MAO displayed the highest activity of 1.94×107 gPE·molFe−1·h−1 of the study with only a modest loss in performance at 90 °C. Generally, the resulting polyethylenes were highly linear (Tm range: 122–132 °C), narrowly disperse and of low molecular weight (Mw range: 6.73–46.04 kg·mol−1), with the most sterically hindered Fe3 forming the highest molecular weight polymer of the series. End-group analysis by 1H- and 13C-NMR spectroscopy revealed saturated alkyl (n-propyl and i-propyl) and unsaturated vinyl chain ends indicative of the role of both β-H elimination and chain transfer to aluminum as termination pathways. By comparison with previously reported iron precatalysts with similar tridentate ligand skeletons, it is evident that the introduction of a large benzhydryl group in combination with a fluorine as the ortho-substituents of one N-aryl group has the effect of enhancing thermal stability of the iron polymerization catalyst whilst maintaining appreciable polymer molecular weight.