Aqueous Slurry Polymerization of Ethylene by Phosphinephenolato Nickel(II) Complexes: Catalyst Tailoring and Synthesis of Bimodal Polyethylene

A straightforward approach for achieving sustainable polymerizations involves conducting the polymerization process in green solvents. This approach aims to transform the general practice wherein the majority of olefin polymerizations are performed using hydrocarbon solvents, such as toluene and hexane. In this study, a series of phosphinephenolato nickel(II) catalysts bearing 2′,4′,6′-tri(tert-butyl)-1,1′-biphenyl and 2′,6′-di(iso-propoxy)-1,1′-biphenyl substituents were designed and employed in both toluene and aqueous slurry polymerization of ethylene. These catalysts demonstrated exceptionally high turnover frequency (TOF) (up to 2.0 × 106 h–1 in toluene and 3.3 × 105 h–1 in water), producing polyethylene waxes and high molecular weight polyethylene with tunable molecular weights (Mw = 4.6–1749.0 kg·mol–1). Impressively, the controlled polymerization performance facilitated by water-soluble H2N-PEG modified complexes 4-H2N-PEG and 6-H2N-PEG enabled the efficient synthesis of bimodal polyethylene with adjustable molecular weight distributions, comprising both polyethylene wax and high-density polyethylene. The bimodal polyethylene exhibits excellent mechanical properties, with an elongation at break of 1200% and a tensile strength of 36.7 MPa, and good flowability during handling and processing, supported by a dense spherical morphology. Moreover, the aqueous polymerization process allowed water recycling across four consecutive cycles, highlighting the environmental sustainability of aqueous slurry polymerization and its promising potential for industrial application.