Controlling polyethylene branching via surface confinement of Ni complexes

Abstract

The heterogeneous surface support can play a key role in determining polymer microstructure, as we show for a novel variant of Ni-catalyst from the family of late-transition metal complexes; this extends the toolbox for novel catalytic solutions in industrial processes. Novel variants of single-atom catalysts (Ni-FO-Al@SiO2, Ni-FO-Si@SiO2, Ni-O-Al@SiO2, and Ni-O-Si@SiO2) were prepared in the form of unsymmetrical a-diimine Ni complexes (Ni-OH and Ni-FOH) and then characterized by inductively coupled plasma - optical emission spectrometry (ICP-OES) and X-ray photoelectron spectroscopy (XPS) analysis. Ethylene slurry-phase polymerization was performed both via self-supporting and covalent-tethering strategies to systematically study the surface confinement effects. High catalytic activity was maintained under the slurry-phase polymerization (as high as 3.9 × 106 g of PE (mol of Ni)−1 h−1). The crucial features of high molecular weight (>106 g mol−1) and high branching density (as high as 180.1BD/1000C) were found among the PE samples produced via heterogeneous polymerization. A detailed investigation suggested that surface functional groups, such as [sbnd]OH and [sbnd]Cl, coordinate with the active Ni species via their lone pairs and terminate the ethylene polymerization. Microstructure analysis of the PE confirm that the supporting substrate provides the chance to modulate the chain-walking behavior of these Ni catalysts. Systematic high-temperature 1H and 13C NMR analysis indicated that the PE branching density could significantly decrease by surface confinement from the solid substrate. Until now, such microstructure control has been mainly realized via the laborious synthesis of bulky a-diimine ligands.