Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry

Polyhedral Silsesquioxanes Research on compounds containing the Si–O bond has long been dominated by silicon dioxide, by minerals built from repeating SiO₂ units, and by silicones composed of repeating R₂SiO units (R = alkyl or phenyl). Over the past 20 to 30 years, however, interest in silsesquioxanes based on the RSiO₁.₅ unit has increased markedly.

Silsesquioxane nomenclature governs how chemists name and communicate the structures of polyhedral oligomeric silsesquioxane (POSS) compounds and related organosilicon frameworks. The term silsesquioxane derives from the Latin silicium (silicon), sesqui (one and a half), and oxygenium (oxygen), reflecting the characteristic oxygen-to-silicon atomic ratio of 1.5. Because systematic IUPAC designations for polyhedral silsesquioxanes are unwieldy — reaching 40 or more characters for even the simplest T 8 cage — the field relies on the concise M, D, T, and Q silicon atom classification and on the compact R n T n shorthand notation.

Polyhedral oligomeric silsesquioxane (POSS) chemistry, based on compounds of the general formula RSiO 1.5 , was established in 1946 when D. W. Scott reported the first synthesis of octamethylsilsesquioxane (Me 8 T 8 ) during the thermal rearrangement of branched-chain methylpolysiloxanes. A. Barry subsequently confirmed the crystallinity and oligomeric nature of these compounds in 1955 through powder X-ray diffraction and cryoscopic molar mass determination. J. Brown then extended this work by isolating discrete polyhedral cage compounds containing 8, 10, and 12 silicon atoms, designated T 8 , T 10 , and T 12 , demonstrating that the rigid Si 8 O 12 inorganic core combined with peripheral organic substituents confers both exceptional thermal resistance and broad compatibility with organic polymer matrices. Later developments, from cage functionalization to catalysis and functional materials, are summarized in POSS Silsesquioxane Chemistry: Synthesis to Applications .