Bridged Polysilsesquioxanes. Molecular-Engineered Hybrid Organic−Inorganic Materials

The class of hybrid organic−inorganic materials called bridged polysilsesquioxanes are used for everything from surface modifiers and coatings to catalysts and membrane materials. This paper examines how bridged polysilsesquioxanes are prepared, processed, characterized, and used. In particular, it describes how attaching several “inorganic” cross-linking trialkoxysilanes on organic bridging groups permits facile formation of network polymers and gels with high levels of chemical functionality. There are a number of synthetic entries into bridged polysilsesquioxane monomers that have allowed a multitude of different bridging groups to be integrated into xerogels (dry gels) or aerogels (supercritically dried “air gels”). Much of the research to date has been successfully focused on engineering of the size of pores through the choice of the bridging groups. For example, materials with some of the highest known surface areas in porous materials have been prepared, and parameters allowing control of the pore size distributions are well understood. More recently, however, the focus has shifted to building functionality into the bridging groups to make materials with controlled porosity that are capable of selective adsorption or catalysis or electronic and optic effects. This is the area where the full potential of bridged polysilsesquioxanes as molecular-engineered materials is being explored.