Iminoimidazole-based Co(II) and Fe(II) complexes: Syntheses, characterization, and catalytic behaviors for isoprene polymerization

A stereoselectivity switchable polymerization of isoprene has been developed, which is catalyzed by iminoimidazole-Co(II) and -Fe(II) complexes. The influence of substituents ranging from electron donating to the electron withdrawing on the iminoimidazole-Co(II) and -Fe(II) catalysts is investigated for isoprene polymerization. Two sets of iminoimidazole-Co(II) and -Fe(II) complexes have been prepared and fully characterized. X-ray crystallography analysis reveals that the complexes Co1 and Fe1 adopt distorted tetrahedral geometries. In the presence of AlEt 2 Cl as co-catalyst, all the Co(II) complexes are active and the catalytic activity is highly dependent on the molar ratio of Al/Co. All the Co(II) complexes exhibit higher activities at low Al/Co ratio. Compared with the Co(II) complexes, the Fe(II) complexes are essentially inactive under the identical condition. However, on activation with combination of AlEtCl 2 and [Ph 3 C][B(C 6 F 5 ) 4 ], both Co(II) and Fe(II) complexes display high activities with good conversions of isoprene (up to >99%). Additionally, low molecular weight and high trans -1,4-unit (>96%) selectivity are characteristics of the resultant polyisoprene. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57 , 767–775