Dialkylenecarbonate-Bridged Polysilsesquioxanes: Hybrid Organic-Inorganic Sol-Gels with a Thermally Labile Bridging Grou
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ABSTRACT In this paper, we introduce a new approach for altering the properties of bridged
polysilsesquioxane xerogels using post-processing modification of the polymeric network. The bridging organic group contains latent functionalities that can be liberated thermally, photochemically, or by chemical means after the gel has been processed to a xerogel. These modifications can produce changes in density, solubility, porosity, and or chemical properties of the material. Since every monomer possesses two latent functional groups, the technique allows for the introduction of high levels of functionality in hybrid organic-inorganic materials. Dialkylenecarbonate-bridged polysilsesquioxane gels were prepared by the sol-gel polymerization of bis(triethoxysilylpropyl)carbonate (1) and bis(triethoxysilylisobutyl)carbonate (2). Thermal treatment of the resulting non-porous xerogels and aerogels at 300-350 'C resulted in quantitative decarboxylation of the dialkylenecarbonate bridging groups to give new hydroxyalkyl and olefinic substituted polysilsesquioxane monolithic xerogels and aerogels that can not be directly prepared through direct sol-gel polymerization of organotrialkoxysilanes. INTRODUCTION Bridged polysilsesquioxanes are a family of hybrid organic-inorganic materials prepared by sol-gel polymerization of molecular building blocks that contain a variable organic bridging group and at least two trialkoxysilyl groups [1-5]. The high level of functionality of these monomers results in their rapid gelation, even in dilute solution. Upon drying, the gels become highly condensed, solid materials (xerogels and aerogels). In contrast to silica gels or silsesquioxanes with pendant organic groups, (RSiOl 5)fl, the bridging organic group remains as an integral part of the network structure. A considerable effort has been made to establish links between the bridging organic group and selected aspects of xerogel morphology [1]. These studies have focused on surface area and pore size distribution, two important properties of bridged polysilsesquioxanes and sol-gels in general. The studies have produced insight as to how small perturbations in the organic fragment of the molecular building block affect the polymerization chemistry and final xerogel structure. In this paper, we introduce a new approach for altering the properties of bridged polysilsesquioxane xerogels. The method involves post-processingmodification of the polymeric network. The strategy is illustrated in Scheme 1. The bridging organic group contains latent functionality. Following polymerization and processing to a xerogel, the latent functionality can
99 Mat. Res. Soc. Symp. Proc. Vol. 576 ©1999 Materials Research Society
be liberated thermally, photochemically, or by chemical means. These modifications can produce changes in density, solubility, porosity, and or chemical properties of the material. Since every monomer is a potential functional group, the technique allows for the introduction of high levels of functionality in hybrid organic-inorganic mat
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