Polyhedral Oligomeric Silsesquioxane

Quantum-mechanical calculations of octhydrooctasilsesquioxane showed that the highest occupied molecular orbital (HOMO) of this compound consists of atomic orbitals of free electron pairs of oxygen atoms, while the lowest energetically unfilled molecular orbital (LUMO) The lowest unoccupied molecular orbital) is spherical and is located in the center of the silsesquioxane core as shown in figure belowe. The calculations also showed that the energy gap between the HOMO and LUMO orbitals is approximately 6-7 eV. This value is higher than the limit for conductivity (3 eV), which proves that the silsesquioxane core is an insulator. Figure of molecular orbitals: (a) HOMO and (b) LUMO octhydrocarbonoctasilsesquioxane Taking into account the low electronegativity of silicon atoms (1.90 compared with 2.55 for the carbon atom according to the Pauling scale), it can be assumed that the POSS core will behave like an electron donor group. However, experimental studies have shown that the silses...

There are many synthetic methods for the preparation of POSS, other than the methods used in the first attempts to synthesize this type of compounds. These methods can be divided into three categories. The first is the hydrolysis and condensation of silanes, the second is the preparation of a polyhedron compound from silanols containing less than eight silicon atoms, the third is the modification of the side arms of the already existing polyhedric silsesquioxane or the substitution of a hydrogen atom in octahydrooctasilsesquioxane. Synthesis of POSS Compounds   Synthesis of POSS Compounds

Polyhedral Silsesquioxanes Research on compounds containing the Si–O bond has for years been dominated by silicon dioxide, minerals containing repeating SiO₂ fragments, and silicones composed of repeating R₂SiO units (R = alkyl or phenyl). Over the past 20–30 years, there has been a noticeable increase in studies on silsesquioxanes containing the RSiO₁.₅ unit. These compounds, due to the presence of both an inorganic fragment and an organic group, possess hybrid properties. The inorganic Si–O–Si fragment imparts chemical and thermal resistance to these compounds, while the organic R group increases their solubility and provides appropriate reactivity. A wide range of polymeric structures with the general formula (RSiO₁.₅)ₙ can be synthesized, but the most interesting are those with polyhedral architectures.⁴ Polyhedral oligomeric silsesquioxanes (POSS) are three-dimensional organosilicon compounds with the general empirical formula (RSiO₁.₅)ₙ (where R = H, alkyl, alkenyl, or aryl; n =...

The English name silsesquioxane comes from Latin and can be translated as: sil icium (silicon), sesqui (one and a half), ox ygenium (oxygen), which means that the ratio of atoms to silicon atoms is one and a half. The use of system names for polyhedra silsesquioxanes can be cumbersome, therefore the literature uses the nomenclature for siloxanes. This classification distinguishes five silicon atoms (see figure below). Type "M" symbol one connected with three organic groups and an atom connected. A silicon atom of type "D" is linked to two atoms, type "T" is linked to three atoms, and type "Q" is linked to four atoms manually. Si–O connections can form siloxane (Si–O–Si) or silanol (Si–OH) groups. To distinguish them, a superscript is used, which defines the number of Si–O–Si bonds formed by the silicon atom, eg T 3 represents a silicon atom containing one organic group and three siloxane bonds. Polyhedral silsesquioxanes have T 3 t...

Research on compounds containing Si–O bond for years has been dominated by silicon oxide, minerals containing the repeating fragment of SiO 2 and silicones made of repeating units of R 2 SiO (R = alkane or phenyl). Over the past 20–30 years, research has been developing on silsesquioxanes containing the RSiO 1.5 group. Due to the presence of an inorganic fragment and an organic group, these compounds have hybrid properties. The inorganic Si–O–Si fragment gives these compounds chemical and thermal resistance, while the organic R group increases their solubility and gives them appropriate reactivity. It is possible to obtain many polymeric structures with the general formula (RSiO 1.5 ) n , but the most interesting are those with polyhedron structure. 1 The first reports of compounds composed of repeating CH 3 SiO 1.5 groups appeared in 1946 in the work of D. Scott. 2 Scott described the thermal depolymerization of polymethylsiloxane obtained during the condensation of trichlorometh...