Controlled vinyl-addition-type polymerization of norbornene initiated by several cobalt complexes having substituted terpyridine ligands

A series of cobalt(II) complexes having terpyridine derivatives such as 2,2′:6′,2″-terpyridine (1), 4,4′,4″-tBu3-2,2′:6′,2″-terpyridine (2), 5,5″-Me2-2,2′:6′,2″-terpyridine (3), 6,6″-Me2-2,2′:6′,2″-terpyridine (4) and 6,6″-(3,5-Me2C6H3)2-2,2′:6′,2″-terpyridine (5) was synthesized. The structures of 1, 3, and 4 were confirmed by X-ray crystallography. The coordination sphere around the cobalt center in 1 can be described as pseudo square pyramidal. On the other hand, complex 4 has pseudo trigonal bipyramidal structure. Upon activation with d-MAO (dried-methylaluminoxane), these complexes showed high activities for the polymerization of norbornene (NBE). In particular, polymerization of NBE with 4/d-MAO system at room temperature resulted in quantitative yield within several hours to give the polymers with relatively narrow molecular weight distributions and controlled molecular weight. The polymerizations of NBE with these cobalt catalyst systems proceeded in vinyl addition polymerization, which was confirmed by 1H NMR spectra of the resulting polymers.