The synergistic effect of rigid and flexible substituents on insertion polymerization with α-diimine nickel and palladium catalysts

Abstract

α-Diimine catalysts with rigid steric hindrance groups demonstrated great potential in the field of olefin polymerization. We have recently focused on developing bulky yet flexible alkyl-substituted α-diimine catalysts and their application in the olefin insertion polymerization. In this contribution, we described the synthesis and characterization of a series of unsymmetrical α-diimine ligands bearing flexible cycloalkyl and rigid diphenylmethyl moieties and the corresponding Ni(II) and Pd(II) complexes. The unsymmetrical Ni(II) complexes exhibited very high catalytic activities (up to 1.4 × 10⁷ gmol⁻¹ h⁻¹) and yielded polyethylene with very high molecular weights (M_n up to 967 kg mol⁻¹) and branching densities (70–92/1000 C) in the ethylene polymerization. The obtained polyethylene products were excellent thermoplastic elastomers (SR up to 83%). On the other hand, the corresponding Pd(II) complexes showed moderate catalytic activities and generated polyethylene with high molecular weights (M_n up to 422 kg mol⁻¹) and high branching densities (64–82/1000 C). Moreover, in the ethylene/polar monomer copolymerization, the Pd(II) complexes demonstrated moderate catalytic activities and generated moderate-to-high molecular-weight polar functional copolymers (M_n up to 92 kg mol⁻¹) with tunable incorporation ratios (up to 11.57 mol%) and high branching densities (65–85/1000 C). Compared with the rigid and bulky diphenylmethyl-substituted Ni(II) or Pd(II) catalysts, the novel catalysts bearing flexible cycloalkyl and rigid diphenylmethyl substituents showed a remarkably higher catalytic activity (up to 10 times), a higher molecular weight, a higher branching density, and a better elastic recovery under the given experimental conditions for the Ni(II) species and exhibited much better incorporation ratios (up to 7 times) of the polar monomer for the Pd(II) species. Most interestingly, the introduction of flexible cycloalkyl groups greatly enhanced the chain growth of the Ni(II) catalytic system and facilitated the synthesis of the high-molecular-weight polymer compared with the rigid and bulky diphenylmethyl-substituted Ni(II) catalyst in a short time. In addition, the size of the ligand's cycloalkyl ring and its electronic properties significantly influenced the ethylene (co)polymerization.