Unexpected o-aryl tBu group effect on suppression of chain transfer in pyridine–imine Ni(ii) and Pd(ii) catalyzed ethylene (co)polymerization

Abstract

Pyridine–imine systems often require steric hindrance at the right location to suppress the process of chain transfer in ethylene polymerization. Herein, a series of o-alkyl (Me, Et, iPr and tBu) pyridine–imine ligands with 2,4-bis(dibenzhydryl) substituents were synthesized and characterized. The corresponding Ni(II) and Pd(II) complexes were prepared and applied in polyethylene synthesis. The pyridine–imine Ni(II) complexes activated by excess alkyl aluminum chloride showed high activities (2.40–11.28 × 10⁶ g mol⁻¹ h⁻¹) in ethylene polymerization while the corresponding Pd(II) complexes activated by NaBArF exhibited moderate activities (1.3–23.4 × 10⁴ g mol⁻¹ h⁻¹). Moreover, an MA incorporation ratio of up to 23 mol% was obtained with relatively lower activity in ethylene–MA copolymerization catalyzed by the Pd(II) complexes. Most notably, the molecular weights of polyethylene and ethylene/MA copolymers yielded by o-tert-butyl Ni(II) and Pd(II) complexes were over an order of magnitude higher than those with other o-alkyl (Me, Et, iPr) substitutions. In addition, all the resulting polyethylene and ethylene/MA copolymers have high branching density, and hyperbranched structures (sec-butyl groups) were also observed in Pd(II)-catalyzed ethylene (co)polymerization.