Norbornene polymerization and copolymerization with 1-alkenes by neutral palladium complexes bearing aryloxide imidazolin-2-imine ligand

Abstract

A series of novel aryloxide imidazolin-2-imine bidentate neutral Pd(II) complexes with different substituents on the imidazolin-2-imine ligand were synthesized and characterized. In the single crystal X-ray diffraction analysis, the complexes Pd1 and Pd2 adopted almost square-planar geometries, while Pd3 bearing the bulkier 2,6-bis(diphenylmethyl)-4-methylphenyl substituent was obtained as the allyl adducted base-free η³ coordination mode. In the presence of very low EtAlCl₂ loadings (100 equiv.), these Pd(II) complexes exhibited high activity (∼10⁶ g mol⁻¹ h⁻¹) and good thermal stability (up to 100 °C) toward the addition polymerization of norbornene. Most importantly, these Pd(II) catalysts were able to promote the copolymerization of norbornene with 1-dodecene and 1-octadecene with high activity (>10⁵ g mol⁻¹ h⁻¹) to yield high-molecular-weight (M_n up to 5.59 × 10⁴ g mol⁻¹) copolymers. The 1-alkene incorporation of resulting copolymers was modulated in a wide range (5.1–15.3%) by changing the comonomer feed ratio, accompanied by the control of the glass transition temperature (T_g = 231.6–309.6 °C). These complexes are rare examples of neutral Pd(II) complexes toward the efficient synthesis of norbornene-based cyclic olefin copolymers (COCs).