Abstract
The N,N,N'-ferrous chloride complexes,[2-{CMeN(2,4-(CHPh)2-6-FC6H2))-6-(CMeNAr)C5H3N]FeCl2(Ar=2,6-Me2C6H3 Fe1,2,6-Et2C6H3 Fe2,2,6-iPr2C6H3 Fe3,2,4,6-Me3C6H2 Fe4 and 2,6-Et2-4-MeC6H2 Fe5),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 ℃,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 ℃.Generally,the resulting polyethylenes were highly linear(Tm range:122-132 ℃),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 by1H-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.
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