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

A family of pyridine-oxazoline-ligated cobalt complexes L₂CoCl₂3a-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 AlEt₂Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 10⁶ 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.