R-Silsesquioxane-Based Network Polymers by Fluoride Catalyzed Synthesis: An Investigation of Cross-Linker Structure and Its Influence on Porosity

Abstract

Silsesquioxane-based networks are an important class of materials that have many applications where high thermal/oxidative stability and porosity are needed simultaneously. However, there is a great desire to be able to design these materials for specialized applications in environmental remediation and medicine. To do so requires a simple synthesis method to make materials with expanded functionalities. In this article, we explore the synthesis of R-silsesquioxane-based porous networks by fluoride catalysis containing methyl, phenyl and vinyl corners (R-Si(OEt) 3 ) combined with four different bis-triethoxysilyl cross-linkers (ethyl, ethylene, acetylene and hexyl). Synthesized materials were then analyzed for their porosity, surface area, thermal stability and general structure. We found that when a specified cage corner (i.e., methyl) is compared across all cross-linkers in two different solvent systems (dichloromethane and acetonitrile), pore size distributions are consistent with cross-linker length, pore sizes tended to be larger and π-bond-containing cross-linkers reduced overall microporosity. Changing to larger cage corners for each of the cross-linkers tended to show decreases in overall surface area, except when both corners and cross-linkers contained π-bonds. These studies will enable further understanding of post-synthesis modifiable silsesquioxane networks.Materials 2020, 13, 1849 2 of 13 of reaction solvent and water content in modifying the overall pore size distributions of a single cross-linker system [29]. Though many reports speculate that the pore sizes obtained in sol-gel reactions are directly related to the bridging groups used, especially for rigid spacers [2,26,[30][31][32], we found that multiple pore size distributions (0.5 to 100 nm) could be obtained by simply changing the reaction solvent for the same bridge. For example, dichloromethane (DCM) favored micropores on the order of 1.2 nm and gel particles, while changing the reaction solvent to acetonitrile (ACN) gave pores centered around 3 nm and favored global gelation of the entire system. All the synthesized materials favored non-polar solvents as expected for organogels based on networked silsesquioxanes.Though high porosity materials with static or non-functionalizable pores are useful in many applications [33][34][35][36], it is often desirable to impart additional functionality to these materials for many specialized uses. such as in biology or environmental remediation. as capture and release agents [2,17,[37][38][39]. These functionalities may include hydrophilic groups, amino-acids and reversible chemistries such as "click" or complexation ligands [40][41][42][43][44]. Many researchers have been working on methods to synthesize active silsesquioxane-based networks, albeit primarily through preformed cage methods. In terms of preformed cage systems, Ervithayasuporn et al. have developed a recyclable methacrylate-POSS (polyhedral oligomeric silsesquioxanes) porous network which can efficiently bind Pd and acts as a catalyst f...

Formation of nanostructured silicas through the fluoride catalysed self-polymerization of Q-type functional silica cages

“…Our research group has developed highly porous POSS systems with cheap fluoride (F − ) catalysed sol–gel methods from R-alkoxysilanes. 15,16 In that work we found that solvent choice for the sol–gel chemistry could be used to vastly alter the porosity, surface areas, and textures of the synthesized networks. Inspired by that methodology and pure curiosity, we began exploring F − interactions with Q-cages, finding rapid reactivity to form polymeric materials.…”

“…From our recent studies, 15,16 catalyst to initiate the reaction. We observed that reactions in all solvent systems evolved large amounts of gas.…”

“…In our R-alkoxysilane work, 15,16 water content is an important reaction parameter. Therefore, water's influence in both DCM and ACN models was assessed-proving its miscibility influences the reaction in these solvents.…”

Formation of Nanostructured Silicas through the Fluoride Catalysed Self-Polymerization of Q-type Silsesquioxane Cages

“…21 Recently, our research group has focused on understanding fluoride catalysis for the synthesis of porous materials from R-alkoxysilanes. 22,23 Inspired by that methodology, we started examining the possibility of synthesizing porous materials directly from Q8M8 H without the need for additional crosslinkers. By this method, functionalization can be performed before polymerization, which can largely decrease the challenges in making functional porous materials and offer greater application opportunities.…”

“…From our past research, we found that solvent compatibility tends to have a large influence on fluoride catalyzed polymerization of R-alkoxysilanes, the amount of crosslinker can also drastically affect characteristics of resulting materials. 22,23 In those studies we established ideal solvent conditions for the polymerization processes, and therefore dichloromethane (DCM, Q8M8 H soluble) and acetonitrile (ACN, Q8M8 H insoluble) were determined to be ideal starting points for comparison here (Scheme 1, Fig. S1).…”