Thermally Stable Aromatic Monodentate Phosphine Cobalt Complexes: Synthesis, Characterization, and Behavior in Diene Polymerization

ABSTRACT A series of monodentate phosphine ligands were studied to investigate their effects on the CoCl 2 (Ligand) 2 ‐MAO system catalyzing the polymerization of 1,3‐butadiene. Aromatic phosphine ligands with varying electronic effects predominantly yield 1,2‐structured polybutadiene (PB), while cis ‐structures are obtained with bulky aromatic phosphines and heteroatom‐containing phosphines. For polyisoprene, the polymerization under any cobalt complex condition results in a 1,4/3,4 mixed structure. Single‐crystal X‐ray diffraction analysis reveals that the tris(4‐methoxyphenyl) phosphine complex ( Co2 ) has a distorted tetrahedral structure. Even at 120°C Co2 maintains over 85% monomer conversion, making it the most thermally stable monodentate aromatic phosphine cobalt catalyst reported to date. The polymerization results indicate that this series of catalytic systems shows higher activity and molecular weight for 1,3‐butadiene polymerization than for isoprene polymerization. The variation in polymerization temperature (0°C–120°C) has little effect on the polymer's regioselectivity, but the stereoselectivity is significantly influenced by temperature. Some monodentate phosphine catalyst systems show moderate to high yields at high temperatures (120°C), demonstrating excellent thermal stability.