Synthesis

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 Cage Octasilsesquioxanes via Modification Reactions of Existing POSS

One method for obtaining functionalized cage silsesquioxanes is to carry out typical reactions known from organic chemistry within the side arms of POSS. It is important to note that due to the sensitivity of the siloxane group to high pH, reactions in strongly basic environments lead to the degradation of the silsesquioxane core. Studies by V. Ervithayasuporn's team indicate that octakis(3-chloropropyl)octasilsesquioxane (1), when reacted with potassium carbonate in DMF at 60 °C, undergoes reorganization during which cages composed of 10 and 12 silicon atoms are formed (Scheme 4). During functionalization reactions, the Si–C bond may also be broken, which can occur under the influence of factors such as N-bromosuccinimide, m-chloroperbenzoic acid, hydrogen peroxide, or fluoride anions. Due to the presence of at least eight reactive sites in the cubic POSS molecules, reactions chosen for their functionalization are usually those with high efficiency, so that each side arm can be properly modified.

Synthesis of POSS via Corner Capping of Partially Condensed Silsesquioxane Cages

Cage silsesquioxanes are formed in reactions between open-cage silsesquioxanes containing fewer than eight silicon atoms and a reactive silane. The typical substrates for this reaction are compounds with the general formula R₇Si₇O₉(OH)₃ (e.g., compound 4 shown in Scheme 5), which contain seven non-reactive R groups and three silanol groups, or their sodium salts Na₃[R₇Si₇O₉(O)₃], along with trichloro- or trialkoxysilanes (methoxy or ethoxy). The reaction proceeds in the presence of a base, most commonly triethylamine, tetraethylammonium hydroxide, or tetramethylammonium hydroxide.

This method enables the synthesis of asymmetric silsesquioxanes containing seven non-reactive groups and one reactive group, such as: epoxy, vinyl, 3-chloropropyl, or 3-aminopropyl groups. The reaction for forming an asymmetric silsesquioxane with seven isobutyl side groups and one 3-aminopropyl group (compound 5) is shown in the Scheme below.