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Polynorbornenes have drawn much attention due to their unique physical properties such as high glass transition temperature, optical transparency, low dielectric properties and birefringence. Due to higher tolerance toward oxygen functionalities in the monomer and the polymer than early transition metals, recent research activities have focused on the development of late transition metal catalysts with enhanced performance. Reported catalytic systems are mainly based on Ni and Pd complexes containing various bidentate P^O, N^O, P^P and N^N ligands with bulky aryl substituents. The complexes with tri or higher dentate ligands are generally known to be inactive or less active for olefin polymerizations than ones with bidentate ligands. Bipyridines, especially 2,2'-bipyridines utilized as a metal chelating ligand due to its robust redox stability and ease of functionalization are extensively reviewed. For the potentially tetradentate bipyridine derivatives, the following advantages are expected; higher stability of starting metal complexes and polymerization intermediates due to increased chelate effect, conservation of symmetry around the active center to produce stereospecific α-olefin polymer, and blocking or retardation of β-hydrogen elimination to produce high molecular weight polymers. Therefore, we could expect the new type of complexes with high activity and stability. Herein we report the novel and well-characterized nickel complexes containing potentially tetradentate bipyridine ligands, which are highly active towards norbornene polymerization. 6,6'-bis(alkoxymethyl)-2,2'-bipydines were prepared with moderate yields by the substitution reaction of mesyl compounds with alkoxides (Scheme 1). Ni(DME)Br2 (DME = dimethoxyethane) has been generally used as a starting material. However, due to higher solubility of NiCl2 than the products, new nickel complexes had been conveniently prepared by the simple reaction between NiCl2 and functionalized bipyridine ligands refluxed in EtOH with moderate yields (Scheme 2). Unfortunately, due to paramagnetic character, these can be characterized only by elemental analysis and X-ray crystallography. Ni(bpy)Cl2 was prepared for the comparison. 8

Highly Active Nickel Complexes Containing Functionalized Bipyridine Ligands for Norbornene Polymerization