Synthesis, Characterization, and Catalytic Behaviors in Isoprene Polymerization of Pyridine-Oxazoline-Ligated Cobalt Complexes.

A family of pyridine-oxazoline-ligated cobalt complexes L 2 CoCl 2 3a - h were synthesized and characterized. Determined via single-crystal X-ray diffraction, complexes 3a and 3d, ligated by two ligands, displayed a distorted tetrahedral coordination of a cobalt center. The X-ray structure indicated the pyridine-oxazoline ligands acted as unusual mono-dentate ligands by coordinating only to N oxazoline . Upon activation with AlEt 2 Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 10 6 g·mol Co -1 ·h -1 ), affording cis -1,4- co -3,4-polyisoprene with molecular weights of 4.4-176 kg mol -1 and a narrow Ð of 1.79-3.42, suggesting a single-site nature of the active sites. The structure of cobalt catalysts and reaction parameters, especially co-catalysts and the reaction temperature, all have significant influence on the polymerization activity but not on the microstructure of polyisoprene.