Construction and Catalytic Behavior of Nickel Catalysts Bearing Poly(Schiff Base) With Different Conjugated Structure for Ethylene Oligomerization

Series of conjugated poly(Schiff base) ligands (POPA, PMPA, and PTPA) with different construction patterns and complexing capacities were synthesized successfully using aromatic dialdehydes with different substituent positions. The corresponding poly(Schiff base) nickel complexes (POPA@Ni, PMPA@Ni, and PTPA@Ni) for ethylene oligomerization were obtained through coordinating the poly(Schiff base) ligands with (DME)NiCl 2 . The position of the substituent in aromatic dialdehyde significantly affected not only the molecular construction pattern of poly(Schiff base) but also the catalytic behavior of the corresponding nickel(II) complexes in ethylene oligomerization. Two linear poly(Schiff base) ligands (PMPA and PTPA) with layered‐sheet arrangement morphology and a certain degree of crystallinity were successfully synthesized using terephthalaldehyde and isophthalaldehyde as materials, and the corresponding nickel complexes (PMPA@Ni and PTPA@Ni) were obtained by intramolecular complexation. However, the macrocyclic Schiff base ligand (POPA) with a uniform smooth spherical shape and no crystallinity was synthesized using o ‐phthalaldehyde as a material, and the corresponding nickel complex (POPA@Ni) was synthesized via intramolecular complexation. Upon activation with methylaluminoxane (MAO), PMPA@Ni and PTPA@Ni bearing the linear poly(Schiff base) ligands exhibited higher catalytic activity and selectivity for C 4 olefin than the nickel complex (POPA@Ni) bearing the cyclic poly(Schiff base) ligand because of the confinement effect of the cyclic conjugated construction pattern.