Living Polymerization of Ethylene Catalyzed by Titanium Complexes Having Fluorine-Containing Phenoxy−Imine Chelate Ligands

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Abstract
Seven titanium complexes bearing fluorine-containing phenoxy−imine chelate ligands, TiCl2{η2-1-[C(H)NR]-2-O-3-tBu-C6H3}2 [R = 2,3,4,5,6-pentafluorophenyl (1), R = 2,4,6-trifluorophenyl (2), R = 2,6-difluorophenyl (3), R = 2-fluorophenyl (4), R = 3,4,5-trifluorophenyl (5), R = 3,5-difluorophenyl (6), R = 4-fluorophenyl (7)], were synthesized from the lithium salt of the requisite ligand and TiCl4 in good yields (22%−76%). X-ray analysis revealed that the complexes 1 and 3 adopt a distorted octahedral structure in which the two phenoxy oxygens are situated in the trans-position while the two imine nitrogens and the two chlorine atoms are located cis to one another, the same spatial disposition as that for the corresponding nonfluorinated complex. Although the Ti−O, Ti−N, and Ti−Cl bond distances for complexes 1 and 3 are very similar to those for the nonfluorinated complex, the bond angles between the ligands (e.g., O−Ti−O, N−Ti−N, and Cl−Ti−Cl) and the Ti−N−C−C torsion angles involving the phenyl on the imine nitrogen are different from those for the nonfluorinated complex, as a result of the introduction of fluorine atoms. Complex 1/methylalumoxane (MAO) catalyst system promoted living ethylene polymerization to produce high molecular weight polyethylenes (Mn > 400 000) with extremely narrow polydispersities (Mw/Mn < 1.20). Very high activities (TOF > 20 000 min-1 atm-1) were observed that are comparable to those of Cp2ZrCl2/MAO at high polymerization temperatures (25, 50 °C). Complexes 2 − 4, which have a fluorine atom adjacent to the imine nitrogen, behaved as living ethylene polymerization catalysts at 50 °C, whereas complexes 5 − 7, possessing no fluorine adjacent to the imine nitrogen, produced polyethylenes having Mw/Mn values of ca. 2 with β-hydrogen transfer as the main termination pathway. These results together with DFT calculations suggested that the presence of a fluorine atom adjacent to the imine nitrogen is a requirement for the high-temperature living polymerization, and the fluorine of the active species for ethylene polymerization interacts with a β-hydrogen of a polymer chain, resulting in the prevention of β-hydrogen transfer. This catalyst system was used for the synthesis of a number of unique block copolymers such as polyethylene-b-poly(ethylene-co-propylene) diblock copolymer and polyethylene-b-poly(ethylene-co-propylene)-b-syndiotactic polypropylene triblock copolymer from ethylene and propylene.

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