Abstract
By dealing CrCl3∙3THF with the corresponding ligands (L1–L5), an array of fluoro-substituted chromium (III) chlorides (Cr1–Cr5) bearing 2-[1-(2,4-dibenzhydryl-6-fluoro- phenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine (aryl = 2,6-Me2Ph Cr1, 2,6-Et2Ph Cr2, 2,6-iPr2Ph Cr3, 2,4,6-Me3Ph Cr4, 2,6-Et2-4-MePh Cr5) was synthesized in good yield and validated via Fourier Transform Infrared (FT-IR) spectroscopy and elemental analysis. Besides the routine characterizations, the single-crystal X-ray diffraction study revealed the solid-state structures of complexes Cr2 and Cr4 as the distorted-octahedral geometry around the chromium center. Activated by either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the chromium catalysts exhibited high activities toward ethylene polymerization with the MMAO-promoted polymerizations far more productive than with MAO (20.14 × 106 g (PE) mol−1 (Cr) h−1 vs. 10.03 × 106 g (PE) mol−1 (Cr) h−1). In both cases, the resultant polyethylenes were found as highly linear polyethylene waxes with low molecular weights around 1–2 kg mol−1 and narrow molecular weight distribution (MWD range: 1.68–2.25). In general, both the catalytic performance of the ortho-fluorinated chromium complexes and polymer properties have been the subject of a detailed investigation and proved to be highly dependent on the polymerization reaction parameters (including cocatalyst type and amount, reaction temperature, ethylene pressure and run time).
Highlights
With a view to further explore the effect of ortho-fluoro group with stronger electron-withdrawing ability on catalytic performance of chromium catalysts, we report the synthesis route and characterization data of the ortho-fluoro substituted 2,4-bis(imino)pyridylchromium (III) chloride complexes (G, Chart 1) along with their ethylene polymerization behavior
The stoichiometric reactions of the diiminopyridine compounds (L1–L5) with CrCl3 ·3THF in dichloromethane being stirred for 10 h under room temperature gave corresponding 2-[1-(2,4dibenzhydryl-6-fluorophenyl-imino)ethyl]-6-[1-(arylimino)ethyl]pyridylchromium (III) chlorides [aryl
The FTIR spectra of these chromium complexes show that the ν(C=N)imine stretching frequencies fell in the range 1611–1618 cm−1 which compared to 1638–1643 cm−1 for the free ligands indicating an effective coordination between the imine-nitrogen and the chromium metal
Summary
Since silica-supported Phillips [1,2,3,4,5] and Union Carbide [6,7,8] catalyst systems became extensively used in commercial production of polyolefins, the development of Cr-based catalysts has been a critical research issue for researchers [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]. As homogeneous catalysts allow the production of polymers with narrow molecular weight distributions compared to heterogeneous ones [32,33,34,35,36,37], the exploration of promising homogeneous chromium precatalysts for ethylene oligomerization and/or polymerization by exploiting new ancillary ligands framework has gradually become one of the hot issues of research in this field [3,14,19,21,23,38]. The chromium complexes B and C (Chart 1) and their derivatives showed moderate or high activities towards ethylene oligo-/polymerization generating oligomers or a mixture of oligomers and polyethylene waxes [42,43,44,49,50,51], while all the cycloalkyl-fused bis(arylimino)pyridylchromium (III) complexes (D, Chart 1) were able to produce strictly linear polyethylenes or vinyl-terminated polyethylene waxes [52].
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