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 the 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 oC demonstrated that the hafnium complex possessed very good activity for the production of polymers with ultra-high molecular weight (Mw 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 Mw and mol% 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.

Synthesis of imino-enamido hafnium and zirconium complexes: A new family of olefin polymerization catalysts with ultrahigh molecular weight capabilities

246th National Meeting (2013)

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