Fluorinated 2,6-bis(arylimino)pyridyl iron complexes targeting bimodal dispersive polyethylenes: probing chain termination pathways via a combined experimental and DFT study

In this work, fluorinated 2,6-bis(arylimino)pyridyl iron(II) complexes, [2-[CMeN(2,4-{(4-FC6H4)(2)CH}(2)-6-F}]-6-(CMeNAr)C5H3N]FeCl2 (Ar = 2,6-Me2C6H3 Fe1, 2,6-Et(2)C(6)H(3 )Fe2, 2,6-(Pr2C6H3)-Pr-i Fe3, 2,4,6-Me3C6H2 Fe4, and 2,6-Et-2-4-MeC(6)H(2 )Fe5) and [2-[CMeN{2-{(4-FC6H4)(2)CH}-4-{(C6H5)CHAr'}-6-F}]-6-(CMeN (2,6-(Pr2C6H3)-Pr-i))C5H3N]FeCl2 (Ar' = 3-{(4-FC6H4)(2)CH)(2)-4-NH2-5-FC6H2 Fe6), verified with different steric substituents, were synthesized and characterized. The molecular structures of Fe2 and Fe3 were determined by X-ray diffraction, revealing a pseudo-square-pyramidal geometry. High activities were achieved toward ethylene polymerization in each iron complex case. The sterically least demanding ligand enhanced the activity of its complex Fel with the highest activity up to 16.8 x 10(6) g of PE (mol of Fe)(-1) h(-1) at 70 degrees C, while the bulkiest ligand led to the formation of the highest molecular weight of the resulting polyethylene using Fe6. Generally, the resulting polyethylenes are highly linear and most of them have a tendency to display bimodal distributions by virtue of the presence of multiple sites or competing chain transfer reactions. Endgroup analysis of polyethylenes confirms that the end groups include both unsaturated vinyl-end groups and saturated n-propyl or i-butyl, revealing the co-existence of two chain termination pathways including primary chain transfer to aluminium and secondary beta-H transfer. The chain termination processes were interpreted with the 1D sequence inverse-gated decoupled C-13 NMR measurement of the resulting polyethylenes and DFT calculations along with the relevant polymerization mechanism.

Automated summary

In this study, fluorinated 2,6-bis(arylimino)pyridyl iron(II) complexes were synthesized and characterized. The complexes showed high activity in ethylene polymerization, with the least sterically demanding ligand exhibiting the highest activity. The endgroup analysis of the resulting polyethylenes revealed the co-existence of primary chain transfer to aluminum and secondary β-H transfer as chain termination pathways.