| Summary: | A family of pyridine–oxazoline-ligated cobalt complexes L<sub>2</sub>CoCl<sub>2</sub> <b>3a</b>–<b>h</b> were synthesized and characterized. Determined via single-crystal X-ray diffraction, complexes <b>3a</b> and <b>3d,</b> 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<sub>oxazoline</sub>. Upon activation with AlEt<sub>2</sub>Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 10<sup>6</sup> g·mol<sub>Co</sub><sup>−1</sup>·h<sup>−1</sup>), affording <i>cis</i>-1,4-<i>co</i>-3,4-polyisoprene with molecular weights of 4.4–176 kg mol<sup>−1</sup> 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.
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