Synthesis of Imino-Enamido Hafnium and Zirconium Complexes: A New Family of Olefin Polymerization Catalysts with Ultrahigh-Molecular-Weight Capabilities

Bidentate, nitrogen-containing molecules are of interest as ligands in transition-metal-catalyzed olefin polymerization. A computational evaluation of a series of 1,2-bis-imines, 1,2-imine-enamines, and 1,2-bis-enamines of cyclic and acyclic derivatives was conducted to understand the relative thermodynamic stabilities of these compounds. The five- and six-membered-ring 1,2-imine-enamines were found to be more stable than the other tautomers, while the 1,2-bis-imines were calculated to have the lowest energy among four- and seven-membered-ring and acyclic derivatives. On the basis of the computational results, literature examples, and prior experience with imino-amido catalysts, 1,2-imine-enamines containing a cyclohex-2-enylidene backbone were targeted as ligands for a new family of polyolefin catalysts. The desired ligands were prepared in three steps, starting from the commercially available 1,2-cyclohexanedione, and subsequently converted into hafnium and zirconium complexes. An ethylene/1-octene copolymerization study conducted at 120 °C demonstrated that the hafnium complex possessed very good activity for the production of polymers with ultrahigh molecular weight ( M w of 1037 kDa) and moderate 1-octene incorporation. This molecular weight is 20 times higher than that produced by the titanium constrained geometry catalyst (CGC) under the same polymerization conditions. The imino-enamido zirconium complex exhibited slightly lower activity than that observed for the hafnium catalyst, yielding an ethylene/1-octene copolymer with M w and mol % 1-octene incorporation of 509 kDa and 6.4, respectively. The polymerization reactions with these catalysts conducted in the presence of diethylzinc led to a sharp decrease in the observed polymer molecular weights, indicative of effective chain transfer. These imino-enamido complexes exhibit higher activity, produce higher molecular weight polymers, incorporate higher levels of 1-octene, and demonstrate significantly improved thermal stability relative to analogous imino-amido complexes reported previously.