Polymerization of 1,3-butadiene by bis(salicylaldiminate)cobalt(II) catalysts combined with organoaluminium cocatalysts

The one-pot reactions of salicylaldehyde or its derivatives with 2,6-dialkyl substituted anilines in the presence of metallating agent, Co(OAc)2.4H2O, yield a series of cobalt(II) complex precatalysts (Ar1-NC-Ar2-O)2Co (1, Ar1 = 2,6-dimethylaryl, Ar2 = aryl; 2, Ar1 = 2,6-diethylaryl, Ar2 = aryl; 3, Ar1 = 2,6-diisopropylaryl, Ar2 = aryl; 4, Ar1 = 2,6-dimethylaryl, Ar2 = 3-t-butylaryl; 5, Ar1 = 2,6-diethylaryl, Ar2 = 3-t-butylaryl; 6, Ar1 = 2,6-diisopropylaryl, Ar2 = 3-t-butylaryl) in high yields. The treatment of these cobalt(II) salicylaldimine complexes with ethylaluminum sesquichloride (EAS) forms highly active 1,3-butadiene (BD) polymerization catalysts, yielding polybutadienes of moderate molecular weights (MW = 33,900–44,500) with narrow molecular weight distributions (MWD = 1.29–2.36). All Co(II) complexes yield highly cis-1,4-polybutadienes (>94%) with negligible amounts of 1,4-trans (<2.32%) and 1,2-added (<3.37%) products in polymerizations at 30 °C combined with EAS. As the steric bulk at the cobalt center increases by changing the alkyl substituents on the ligand environment, 1,4-cis content decreases. The cis content and MW decrease as polymerization temperature increases. The polymerization yield and the cis content increase as BD concentration increases. The nonlinearity of [BD] to yield and [BD] to MW relationships demonstrates that the narrow MWD comes from uniform active species present in the polymerization system. The activation procedures of the Co(II) complexes in conjunction with EAS are studied by using UV-visible spectroscopy. Suitable correlations are found between polymerization activities and the changes in UV-visible absorption spectra of the complex.