Modulating Thermostability and Productivity of Benzhydryl-Substituted Bis(imino)pyridine-Iron C2H4 Polymerization Catalysts through ortho -CnH2n−1 (n=5, 6, 8, 12) Ring Size Adjustment

Four types of benzhydryl-containing bis(arylimino)pyridine-iron(II) chloride complex, [2-{CMeN(2,4,6-Me3C6H2)}-6-{CMeN(Ar)}C5H3N]FeCl2 (Ar=2,4-(CHPh2)2-6-(C5H9)C6H2 Fe1, 2,4-(CHPh2)2-6-(C6H11)C6H2 Fe2, 2,4-(CHPh2)2-6-(C8H15)C6H2 Fe3 and 2,4-(CHPh2)2-6-(C12H23)C6H2 Fe4), discriminable by the ring size of the ortho-CnH2n−1 (n=5, 6, 8, 12) group, have been prepared in good yield (>85 %) via the reaction of the corresponding N,N,N′-ligand, L1–L4, with ferrous chloride tetrahydrate. All new complexes were characterized by FT-IR spectroscopy and elemental analysis. The molecular structure of Fe3 emphasizes the distorted pseudo-square pyramidal geometry bestowed to the metal center and the steric unevenness provided by the inequivalent N-mesityl and N-2,4-dibenzyhydryl-6-cyclooctylphenyl groups. Extremely high activities for ethylene polymerization were achieved at temperatures between 60 °C and 70 °C following pre-treatment of the iron(II) precatalyst with either MMAO or MAO, with the relative performance being: Fe2>Fe1>Fe3>Fe4. Notably, ortho-cyclohexyl Fe2, under activation with MMAO, afforded the highest level of performance of this study reaching 2.82×107 g PE per mol (Fe) per h at 70 °C forming strictly linear and narrowly dispersed polyethylene (Mw/Mn=2.0) with moderate molecular weight (11.3 kg per mol). Furthermore, runs performed using Fe1/MMAO at 90 °C saw the catalytic activity drop by around only 40 % highlighting the remarkable thermostability of these catalysts. By comparison, polymerizations performed using MAO as co-catalyst were less controlled with broader dispersities (Mw/Mn range: 4.8–10.1), while the molecular weights were generally higher (55.7–150.9 kg per mol).