Highly active and cis-1,4 selective polymerization of 1,3-butadiene catalyzed by cobalt(II) complexes bearing α-diimine ligands

A series of cobalt(II) complexes bearing α-diimine ligands were synthesized and characterized by elemental and spectroscopic analysis. These complexes had the general formulas [ArN=C(Me)–(Me)C=NAr]CoCl2 (Ar = C6H5, 3a; 4-MeC6H4, 3b; 4-MeOC6H4, 3c; 4-FC6H4, 3d; 4-ClC6H4, 3e; 2-MeC6H4, 3f; 2-EtC6H4, 3g; 2-iPrC6H4, 3h; 2,4,6-Me3C6H2, 3i; 2,6-Et2C6H3, 3j; 2,6-iPrC6H3, 3k). 2,6-Bis[(2,6-diisopropylphenylimino)ethyl]pyridine CoCl2 (4a) was also synthesized for comparison. The structures of complexes 3i, 3k, and 4a were further analyzed by X-ray crystallography. When the Co(II) complexes were activated with ethylaluminum sesquichloride, they exhibited high catalytic activity for 1,3-butadiene polymerization. The polymers produced have high cis-1,4 stereoregularity (up to 98.0%) and high molecular weights (Mn = 1 × 104–1 × 105). The substituent ligand affected both catalytic activity and stereoselectivity through an electronic effect while steric hindrance by the substituent was not important. The effects of the polymerization conditions, such as polymerization time, temperature, different alkylaluminum compounds used as cocatalyst, and [Al]/[Co] molar ratio, on polymerization behavior were investigated.

Graphical AbstractA series of cobalt(II) complexes ligated by α-diimine have been synthesized and characterized. The catalyst system [α-diimine]CoCl2/ EASC shows high catalytic activity and cis-1,4 selectivity (up to 98%) for 1,3-butadiene polymerization.Figure optionsDownload full-size imageDownload as PowerPoint slide