Facile access to ultra-highly branched polyethylenes using hybrid “sandwich” Ni(II) and Pd(II) catalysts

Abstract

The branching type and amount in polyolefins have a significant impact on the material properties. In this study, a family of “sandwich” or “semi-sandwich” α-diimine ligands with unilateral steric hindrance and the corresponding Ni(II) and Pd(II) complexes have been designed and synthesized. In the nickel-catalyzed ethylene polymerization, the developed Ni(II) catalysts exhibited moderate activities and yielded highly branched polyethylenes with high molecular weights (up to 473.2 kg/mol). Unexpectedly, the hybrid “sandwich” Ni(II) catalyst bearing 8-aryl- and 2-dibenzosuberyl-naphthyl fragments generated ultra-highly branched (146–168/1000C) polyethylene with a unique branching distribution pattern under conventional polymerization conditions. More uniquely, the branching density of the obtained polyethylene decreased gradually with increasing temperature, contrary to the previous literature reports. In the palladium-catalyzed ethylene polymerization, the Pd(II) catalysts displayed moderate activities and yielded hyperbranched polyethylenes with high molecular weights (up to 191 kg/mol). Similar to the nickel-catalyzed ethylene polymerization, the hybrid “sandwich” Pd(II) catalyst also yielded polyethylenes with higher branching densities (112–118 vs 88–95/1000C) and molecular weights (101–191 vs 59–130 kg/mol) than the corresponding “semi-sandwich” Pd(II) catalysts. Moreover, the hybrid “sandwich” Pd(II) catalyst generated E-MA copolymers with the highest branching densities and incorporation ratios among the catalysts. Overall, the hybrid sandwich structure facilitates the synthesis of higher branched polyethylene in the Ni(II) and Pd(II) catalyzed ethylene (co)polymerization reactions.