Steric and electronic modulation of iron catalysts as a route to remarkably high molecular weight linear polyethylenes

Five structurally related bis(arylimino)pyridine-iron( ii ) chloride complexes, [2-[CMeN{2,6-{(4-FC 6 H 4 ) 2 CH} 2 -4-NO 2 }]-6-(CMeNAr)C 5 H 3 N]FeCl 2 (Ar = 2,6-Me 2 C 6 H 3 Fe1 , 2,6-Et 2 C 6 H 3 Fe2 , 2,6-i-Pr 2 C 6 H 3 Fe3 , 2,4,6-Me 3 C 6 H 2 Fe4 , and 2,6-Et 2 -4-MeC 6 H 2 Fe5 ), incorporating one N -2,6-bis{di(4-fluorophenyl)methyl}-4-nitrophenyl group and one distinct N -aryl group, have been prepared in good yield through the interaction of the corresponding free ligands ( L1-L5 ) with FeCl 2 ·4H 2 O. All ferrous complexes were paramagnetic which was manifested by broad and highly shifted peaks in their 1 H NMR spectra. The marked steric imbalance imposed by the two inequivalent N -aryl groups was a key feature highlighted in the molecular structures of representative complexes Fe1 and Fe2 . Upon activation with either MAO or MMAO, Fe1-Fe5 all exhibited high activities for ethylene polymerization with good thermal stability [activities as high as 1.58 × 10 7 g (PE) mol −1 (Fe) h −1 at 60 °C], affording especially high molecular weight linear polyethylenes (3.92 × 10 5 g mol −1 at 70 °C; T m > 130 °C). To the best of our knowledge, the molecular weights of the polyethylenes produced by the current class of iron catalysts exceed the highest values reported for related bis(imino)pyridine-iron catalysts to date; changes in the ortho -R 1 substitution pattern offered some additional fine control of the molecular weight. Moreover, the nature of the aluminoxane co-catalyst employed had a noticeable effect on the polymer end group composition. When using MAO, unsaturated polymers containing both vinyl and n -propyl end groups were evident, whereas with MMAO, fully saturated polymers were generated containing both isobutyl and n -propyl end groups. The depicted N , N , N -iron( ii ) chloride precatalysts, upon activation with either MAO or MMAO, not only display excellent thermal stability but are also capable of generating exceptionally high molecular weight linear polyethylenes.