Silsesquioxanes: Advanced Model Supports in Developing Silica-Immobilized Polymerization Catalysts

Abstract

To gain more insight, at a molecular level, into processes taking place at silica surfaces, suitable model systems for silica surface silanol sites are of great importance. The major advantage of such homogeneous model systems is that their structural properties and reactivity can be studied in detail using a wide range of powerful techniques such as single crystal X-ray diffraction and multinuclear solution NMR spectroscopy. Several model supports for silica have been studied, ranging from mono-, di- and polysilanols to silsesquioxanes. The latter are probably the most realistic homogeneous model systems for silica surface silanol sites known to date and provide an excellent opportunity to finetune synthetic procedures to modify silica surfaces or to anchor metal complexes to the support. In this chapter, the usefulness of silsesquioxanes to provide information about synthetic strategies, stability and reactivity of silicasupported (co)catalysts will be outlined. Physisorption, grafting and tethering are immobilization techniques that will be discussed. As will be shown, metallasilsesquioxanes not only contribute to a molecular level understanding of known heterogeneous catalyst systems; with activities sometimes even exceeding that of commercial silica-supported catalysts, they are interesting catalysts themselves.