Remote dibenzocycloheptyl substitution on a bis(arylimino)pyridyl-iron ethylene polymerization catalyst; enhanced thermal stability and unexpected effects on polymer properties

Four examples of para -dibenzocycloheptyl-substituted 2,6-bis(arylimino)pyridyl-iron( ii ) chloride complexes, [2,6-{(2-R 1 ,4-(C 15 H 13 ),6-R 2 C 6 H 2 )N&z.dbd;CMe} 2 C 5 H 3 N]FeCl 2 (R 1 = R 2 = Me Fe1 , Et Fe2 , iPr Fe3 , R 1 = Me, R 2 = Et Fe4 ), have been synthesized and characterized by a combination of IR spectroscopy, elemental analysis and single crystal X-ray diffraction. The molecular structures of Fe2 and Fe4 highlight not only their pseudo -square pyramidal geometries but also the variation in configuration of the two remote dibenzo-fused cycloheptyl rings. When activated with MAO or MMAO, Fe1-Fe4 displayed very high activities for ethylene polymerization by operating effectively at 70 °C/10 atm [up to 7.14 × 10 7 g(PE) mol −1 (Fe) h −1 ] producing high molecular weight linear polyethylene (as high as 98.7 kg mol −1 ) with a single melting temperature ( T m : >129.7 °C) as well as various levels of vinyl unsaturation. Even at temperatures up to 100 °C, the polymerization activity remained high [up to 1.46 × 10 7 g(PE) mol −1 (Fe) h −1 ] underlining the good thermal stability of this catalyst class. By contrast at P C2H4 = 1 atm, all polyethylenes produced using Fe1-Fe4 at run temperatures between 20-60 °C exhibited two melting point values with either MAO ( T m1 93.5-116.2 °C, T m2 124.9-130.0 °C) or MMAO ( T m1 65.1-85.9 °C, T m2 116.8-122.8 °C) in line with the bimodality of the lower molecular weight polymers. In particular, when MMAO was employed as a co-catalyst, the polyethylene displayed a wax-like composition and displayed a very narrow dispersity ( M w / M n range: 1.1-1.6). The title iron catalyst displays exceptionally high catalytic activity at 70 °C and high activity at 100 °C; wax-like properties are a feature of the polyethylenes as are the narrow dispersities.