Synthesis, Structures, and Norbornene Polymerization Behavior of Neutral Nickel(II) and Palladium(II) Complexes Bearing Aryloxide Imidazolidin-2-imine Ligands

A series of novel aryloxide imidazolidin-2-imine bidentate neutral Ni(II) and Pd(II) complexes bearing five- and six-membered chelate ring structures were synthesized and characterized. X-ray diffraction analysis results revealed that all of the Ni(II) complexes (Ni1-Ni3) and Pd(II) complexes (Pd1 and Pd3) adopted an almost square-planar geometry. In the presence of various cocatalysts such as MAO, MMAO, Et₂AlCl, and EtAlCl₂, all of the Ni(II) and Pd(II) complexes exhibited remarkably high activities (up to 2.6 × 10⁷ g of PNB (mol of M)^-1 h^-1) toward the addition polymerization of norbornene. These catalyst systems produced high-molecular-weight polynorbornene (PNB) with narrow molecular weight distribution, except for the insoluble PNB obtained with Pd1-Pd3/MAO systems. The Pd(II) complexes showed particularly good thermostability with a high activity of 1.56 × 10⁷ g of PNB (mol of Pd)^-1 h^-1 even at 80 °C. These complexes are rare examples of neutral Ni(II) and Pd(II) complexes bearing aryloxide-functionalized imidazolidin-2-imine ligands in the field of olefin polymerization.