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

Abstract

Titanium constrained-geometry-inspired complexes (CGCs) are assembled on aminosilica surfaces using two metallation protocols, a single-step method using a piano-stool complex, and a multi-step surface grafting approach that uses amine elimination from a M(N(Et 2)) 4 precursor. Two scaffolds are used in the study, one that has spatially patterned amines on the silica surface and another that has a distribution of different amine species prepared by a traditional amine-grafting approach. The materials are characterized by thermogravimetric analysis, elemental analysis, diffuse reflectance UV–vis spectroscopy, and subsequently, are evaluated in the polymerization of ethylene. Using both metallation protocols, the catalysts prepared on the patterned aminosilica are more productive than the materials prepared using traditional supports. The materials made via the single-step metallation method are more productive in the polymerization of ethylene per mole of metal atoms than the materials made using the amine elimination approach. However, on all supports, the single-step method results in significantly lower metallation efficiency than the amine elimination route, which allowed nearly quantitative surface functionalization. As a result, the materials made via amine elimination have a substantially higher metal loading that gives a higher catalytic productivity per gram of catalyst. It is also shown that the catalysts made via the amine elimination method are less susceptible to complex leaching caused by the co-catalyst methylalumoxane than the materials made via the single-step approach. Furthermore, when alkylaluminum/borane co-catalysts are used, leaching of active metal species from the support is not detected.