Catalytic Behavior of Cobalt Complexes Bearing Pyridine-Oxime Ligands in Isoprene Polymerization

Several cobalt(II) complexes Co1-Co3 bearing pyridine-oxime ligands (L1 = pyridine-2-aldoxime for Co1; L2 = 6-methylpyridine-2-aldoxime for Co2; L3 = phenyl-2-pyridylketoxime for Co3) and picolinaldehyde O-methyl oxime (L4)-supported Co4 were synthesized and well characterized by FT-IR, mass spectrum and elemental analysis. The single-crystal X-ray diffraction of complex Co2 reveals that the cobalt center of CoCl₂ is coordinated with two 6-methylpyridine-2-aldoxime ligands binding with N_pyridine and N_oxime atoms, which feature a distorted octahedral structure. These Co complexes Co1-Co4 displayed extremely high activity toward isoprene polymerization upon activation with small amount of AlClEt₂ in toluene, giving polyisoprene with high activity up to 16.3 × 10⁵ (mol of Co)^-1(h)^-1. And, the generated polyisoprene displayed high molecular weights and narrow molecular distribution with a cis-1,4-enriched selectivity. The type of cobalt complexes, cocatalyst and reaction temperature all have effects on the polymerization activity but not on the microstructure of polymer.