Synthesis of High-Molecular-Weight Branched Polyethylene Using a Hybrid “Sandwich” Pyridine-Imine Ni(II) Catalyst

Abstract

Most pyridine-imine Ni(II) and Pd(II) catalysts tend to yield low-molecular-weight polyethylene and ethylene-based copolymers in olefin insertion polymerization, as the unilateral axial steric structure of such complexes often cannot provide effective shielding of the metal center. In this study, we synthesized a series of hybrid “semi-sandwich” and “sandwich” type pyridine-imine Ni(II) complexes by incorporating diarylmethyl or dibenzosuberyl groups onto 8-aryl-naphthyl motif. The as-prepared Ni(II) complexes afforded highly branched polyethylene with high molecular weights (level of 105 g/mol), and moderate activities (level of 105 g/(molh)) in ethylene polymerization. Most interestingly, compared to “semi-sandwich” Ni(II) complexes bearing (2-diarylmethyl-8-aryl)naphthyl units, the “full-sandwich” counterpart containing (2-dibenzosuberyl-8-aryl)naphthyl motif was able to produce higher-molecular-weight polyethylene with higher branching density. In addition, the effect of remote non-conjugated electronic substituents in diarylmethyl groups of the Ni(II) system was also observed in ethylene polymerization.

Flexible Axial Shielding Strategy for the Synthesis of High-Molecular-Weight Polyethylene and Polar Functionalized Polyethylene with Pyridine-Imine Ni(II) and Pd(II) Complexes

“…Our latest findings revealed that the Pd­(II)-based pyridine-imine catalysts were also efficient in the olefin and polar monomer copolymerization for producing high incorporation ratio polar functionalized polyolefins with tunable molecular weights. − , In this contribution, polar functionalized E-MA copolymers with very high incorporation ratios (up to 21.7 mol %) were yielded by complexes Pd1–Pd3 (Table ). These Pd­(II)-based complexes showed low copolymerization activities (level of 10 3 g mol –1 h –1 ), which were notably lower than their homo-polymerization activities, due to the formation of six-membered cyclic intermediate after the insertion of MA (Table vs 2 ). ,, As expected, doubling the initial MA concentration resulted in further reduced copolymerization activities, but improved the incorporation ratios. Complex Pd1 containing unsubstituted naphthyl in the 8-position yielded high branching density (67–70/1000 C) E-MA co-oligomers with very low molecular weights (0.3 kg/mol).…”

“…These Pd(II)-based complexes showed low copolymerization activities (level of 10 3 g mol −1 h −1 ), which were notably lower than their homo-polymerization activities, due to the formation of six-membered cyclic intermediate after the insertion of MA (Table 3 vs 2). 47,51,57 As expected, doubling the initial MA concentration resulted in further reduced copolymerization activities, but improved the incorporation ratios. Complex Pd1 containing unsubstituted naphthyl in the 8-position yielded high branching density (67−70/1000 C) E-MA co-oligomers with very low molecular weights (0.3 kg/mol).…”

“…This small methyl substituent will be beneficial for suppression of the chain transfer reactions during the polymerization process. Complexes Ni4 and Pd4 were synthesized as reference catalysts according to known literature. , …”

“…Complexes Ni4 and Pd4 were synthesized as reference catalysts according to known literature. 45,51 2.2. Ethylene Polymerization Catalyzed by Ni(II) Complexes.…”

Direct Synthesis of Partially Chain-Straightened Propylene Oligomers and P-MA Co-Oligomers Using Axially Flexible Shielded Iminopyridyl Palladium Complexes

“…The generated Ni(II)/Pd(II) complexes were apt to retard the chain transfer in polymerization. Therefore, the high-molecular-weight (co)polymers were produced [ 49 , 50 , 51 , 52 ]. We also found the Ni(II) and Pd(II) complexes bearing iminopyridyl ligands with dibenzhydryl and 8-alkyl/aryl-naphthyl substituents could induce ethylene (co)polymerization to form high-molecular-weight (co)polymers [ 52 , 53 , 54 ].…”