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

A family of pyridine–oxazoline-ligated cobalt complexes L2CoCl2 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 Noxazoline. Upon activation with AlEt2Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 106 g·molCo−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.