Silsesquioxane - Chemical Structure and Synthesis

Chemical Structure and Synthesis

The structure of silsesquioxanes depends on the preparation method. To simplify, a silicon with a hydrolytically stable organic substituent and three easily hydrolyzed groups such as chlorine or alkoxy groups, which are reacted with water and an acid or base catalyst. The final structure depends on the function of the concentration of initial monomer, concentration of water, temperature, type of catalyst, and the nature of the non-hydrolyzing substituent. This can be seen in the following equations. Solvent hydrogen bonding can have a large effect on rates and types of molecular condensations.

The basic silsesquioxane synthesis methods introduced by those who pioneered its formation in the silicone industry typically involved producing trichlorosilane precursors. These precursors were often formed by the reaction of methylene chloride or hydrogen chloride with silicon metal in the presence of a metal catalyst. The subsequent reaction used to form the silsesquioxanes were typically metal-catalyzed hydrosilylation reactions with chloro- or alkylsilanes or organometallic coupling reactions with chlorosilanes. The choice of metal catalyst is dependent on the selected R substituents to be attached (for instance, with the addition of alkyl groups larger than methyl or organo-functional groups, platinum is used as a catalyst). Some work has been done for limited alterations of substituent addition, but in general there is to date no known method to direct or control the formation of particular substitutional isomers when introducing two or more different substituents in a silsesquioxane cage synthesis.

When characterizing silsesquioxanes the typical methods involved are x-ray diffraction, nuclear magnetic resonance spectroscopy (proton, carbon and silicon), and infrared radiation spectroscopy, though SEM and TEM have been used for visualization in crystal growth studies. Features of general importance when describing silsesquioxane compounds are the number of RSiO_3/2 units in the cage compounds and the degree to which the silsesquioxane is condensed. In a fully condensed silsesquioxane compound, the general formula is RaSiaO(1.5a–0.5b)(OH)b with b=0, indicating that all oxygen atoms in the compound are bridging silicon atoms. A less condensed silsesquioxane compound (b>0) is indicative of the compound having less coordinated Si-O-Si connections, thus in general how condensed the compound is gives the nature of the macroscopic molecule (i.e. polymeric forms are typically highly condensed- almost fully connected networks). It is worth noting that when the cage formations of silsesquioxanes are produced, though many different sizes are possible (i.e. T₈, T₁₀, T₁₂), the most preferred formation are the cubic T8 compounds due to the high stability of the Si₄O₄ rings in the cage.

Many polymeric forms of silsesquioxanes have been developed with varying molecular weights and synthesis methods. The first high molecular weight tractable polymeric silsesquioxane was a ladder type repeating unit, seen in Figure 3, polyphenylsilsequioxane, reported by Brown et al. in 1960. Brown’s findings were used as a basis for further research and synthesis variations by plethora of additional research groups investigating polyphenylsilsequioxanes. Though many alterations were made, the origin synthesis proposed by Brown involved a three step process outlined as follows: (1) the hydrolysis of phenyltrichlorosilane in a solvent with excess water to give a hydrolyzate, (2) equilibration of the hydrolyzate with potassium hydroxide at a low concentration and temperature to give the prepolymer, and (3) equilibration of the prepolymer at a high concentration and temperature to give the final polymeric form. It was found that the critical factors to increase the polymer weight were high concentration and temperature during the equilibration of the prepolymer. Another noteworthy milestone in the silsesquioxane polymeric materials is the development of soluble and stable polymethylsilsesquioxane with high molecular weights by Japan Synthetic Rubber. This polymer which, unlike its phenyl derivative, gels easily during the course of its synthesis, has found a wide range of alternative applications including cosmetics, resins, and chemical amplification resist for electron beam lithography.

Another scientific development in the field of silsequioxanes was the first synthesis of hydridosilsesquioxanes by Frye and Collins. Hydridosilsesquioxanes are a silsesquioxane type with only hydrogen substituents on the silicon, and are thus purely inorganic compounds. Initial synthesis methods involved the adding benzene solutions of trichlorosilane to a mixture of benzene, concentrated sulfuric acid, and fuming sulfuric acid to yield the T₁₀-T₁₆ oligomers. The T₈ oligomer was also synthesized, but by the reaction of trimethylsilane with a mixture of acetic acid, cyclohexane, and hydrochloric acid. It has been found that these compounds can be converted to silica coatings for application in environmental protection, and for application as an interlayer dielectric for integrated circuits.