Effect of site isolation on the preparation and performance of silica-immobilized Ti CGC-inspired ethylene polymerization catalysts

Titanium constrained-geometry-inspired complexes (CGCs) are assembled using several different synthetic protocols on aminosilica scaffolds. In particular, a new synthetic method is reported that utilizes spatially patterned amines on the silica surface to create Ti–CGC sites that are substantially more active for ethylene polymerization than materials prepared using traditional methods. The materials are characterized using multiple techniques including thermogravimetric analysis (TGA), nitrogen physisorption, FT-Raman spectroscopy, 29Si and 13C cross-polarization magic-angle spinning (CPMAS) NMR spectroscopy, and diffuse–reflectance UV–visible (UV–vis) spectroscopy. While the new patterning protocol allows for quantitative addition of the cyclopentadienyl group to the surface amines and near quantitative metallation with the titanium source, these steps result in subquantitative additions on aminosilica surfaces prepared via traditional techniques (at high amine loadings) and excess titanium addition (at low amine loadings). Using methylaluminoxane (MAO) as a co-catalyst, the patterned catalyst is more productive than the control materials that were prepared using traditional techniques. However, use of MAO causes significant leaching of the metal complex from the solid support. Using a tris(pentafluorophenyl)borane/trialkylaluminum system as co-catalyst alleviates the leaching problem, allowing for the productivity of the immobilized species to be evaluated. In the polymerization of ethylene, the patterned catalyst is shown to be up to 10 times more productive than the solid control materials. The patterned catalyst is also more active than the homogeneous analogue.