Bridged polysilsesquioxanes. Molecular-engineered hybrid organic-inorganicmaterials

The class of hybrid organic-inorganic materials called bridged polysilsesqu ioxanes are used for everything from surface modifiers and coatings to cata lysts and membrane materials. This paper examines how bridged polysilsesqui oxanes 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 syn thetic 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 un derstood. More recently, however, the focus has shifted to building functio nality into the bridging groups to make materials with controlled porosity that are capable of selective adsorption or catalysis or electronic and opt ic effects. This is the area where the full potential of bridged polysilses quioxanes as molecular-engineered materials is being explored.