Role of Amine Structure and Site Isolation on the Performance of Aminosilica-Immobilized Zirconium CGC-Inspired Ethylene Polymerization Catalysts

Abstract

The homogeneous zirconium complex (CpSiMe2Cl)ZrCl3 is immobilized onto three different aminosilica supports, resulting in supported constrained-geometry-inspired zirconium complexes that, when combined with methylalumoxane (MAO), are active for the homopolymerization of ethylene to produce linear polyethylenes. The structure of the immobilized metal species is shown to depend on both the nature of the amine groups on the surface (primary vs secondary) and the amine surface density (densely loaded vs site-isolated). Diffuse-reflectance ultraviolet−visible (UV−vis) spectra of the metallated solids indicates that a true, immobilized constrained-geometry catalyst (CGC) is likely only produced on the silica that contains isolated primary amine sites. The polymerization data support the hypothesis that a traditional constrained-geometry Zr catalyst exhibits higher catalytic activity compared to a more open metal center that is likely the primary species produced on a densely loaded aminosilica surface. MAO causes significant leaching of the immobilized catalysts, possibly due to the cleavage of the Si−O−Si surface linkage.