POSS Silsesquioxane History: Synthesis of T8, T10, and T12 Cages

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 (Me₈T₈) 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 T₈, T₁₀, and T₁₂, demonstrating that the rigid Si₈O₁₂ inorganic core combined with peripheral organic substituents confers both exceptional thermal resistance and broad compatibility with organic polymer matrices.

Scott's 1946 Discovery of RSiO_1.5 Compounds and the Origins of Silsesquioxane Chemistry

The first reports of compounds composed of repeating CH₃SiO_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 trichloromethylsilane with dichlorodimethylsilane. This reaction yielded a white solid tentatively identified as octamethylsilsesquioxane containing traces of other polycyclic methylsilsesquioxanes, though this identification was not confirmed by structural analysis at the time of publication.

Barry's Structural Characterization of Crystalline Organosilsesquioxanes (1955)

Subsequent investigations focused on the identification of the compounds formed in these reactions. In 1954, A. Barry,³ through the hydrolytic condensation of organically substituted silanes, synthesized compounds analogous to those reported by D. Scott. These compounds contained either alkyl groups or a benzene ring as organic substituents. Using powder X-ray diffraction, Barry established the crystallinity of the resulting products, a finding that precluded the formation of a polymeric material. The presence of oligomeric compounds was established through thorough analysis of the experimental data: the silicon-to-organic-group ratio was determined by elemental analysis, the molar mass was determined cryoscopically, and the formation of closed cyclic structures devoid of silanol groups was confirmed by infrared spectroscopy.³

Brown's Synthesis of T₈, T₁₀, and T₁₂ Polyhedral POSS Cage Compounds

Later work by J. Brown focused on the synthesis of polyhedral silsesquioxanes of various cage sizes.^4,5 Brown employed the hydrolytic condensation of trimethylethoxysilane, using a solution of potassium hydroxide in aqueous methanol as the catalyst. The product obtained after crystallization was found to be a mixture of cages with different core sizes, i.e., T₈, T₁₀, and T₁₂. A structural description of the obtained particles was furthermore proposed, indicating the formation of polyhedra. Applying analogous reaction conditions to the hydrolysis of trichlorophenylsilane, Brown obtained a mixture of phenylsilsesquioxanes containing 8, 10, and 12 silicon atoms.⁵

Thermal Stability of Polyhedral Silsesquioxanes Relative to Sol-Gel Polymers

In 1990, the thermal properties of polyhedral silsesquioxanes were investigated for the first time. These compounds were found to exhibit high thermal resistance of silsesquioxanes, substantially exceeding that of the polymeric materials produced by the sol-gel technique.⁶ This finding prompted the broader use of silsesquioxanes as polymer additives to enhance thermal resistance. In the 1990s, research on silsesquioxanes focused primarily on developing synthetic methods for silsesquioxanes bearing reactive pendant groups and on their subsequent functionalization. Current research is directed toward the functionalization of POSS molecules to achieve previously unknown properties.

Structure of Octamethyloctasilsesquioxane (Me₈T₈) and the Si₈O₁₂ Core

Figure 1. Structure of octamethyloctasilsesquioxane (Me₈T₈).

References

(1) Cordes, D. B.; Lickiss, P. D.; Rataboul, F. Recent Developments in the Chemistry of Cubic Polyhedral Oligosilsesquioxanes. Chem. Rev. 2010, 110, 2081–2173.

(2) Scott, D. W. Thermal Rearrangement of Branched-Chain Methylpolysiloxanes. J. Am. Chem. Soc. 1946, 68, 356–358.

(3) Barry, A. J.; Daudt, W. H.; Domicone, J. J.; Gilkey, J. W. Crystalline Organosilsesquioxanes. J. Am. Chem. Soc. 1955, 77, 4248–4252.

(4) Vogt, L. H.; Brown, J. F. Crystalline Methylsilsesquioxanes. Inorg. Chem. 1963, 2, 189–192.

(5) Brown, J. F.; Vogt, L. H.; Prescott, P. I. Preparation and Characterization of the Lower Equilibrated Phenylsilsesquioxanes. J. Am. Chem. Soc. 1964, 86, 1120–1125.

(6) Laine, R. M.; Rahn, J. A.; Youngdahl, K. A.; Babonneau, F.; Hoppe, M. L.; Zhang, Z. F.; Harrod, J. F. Synthesis and High Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium-Catalyzed Redistribution of Methylhydridooligo- and -Polysiloxanes. Chem. Mater. 1990, 2, 464–472.

Key Review Article Cordes, D. B.; Lickiss, P. D.; Rataboul, F. "Recent Developments in the Chemistry of Cubic Polyhedral Oligosilsesquioxanes." Chemical Reviews, 2010, 110, 2081–2173.
DOI: 10.1021/cr900201r
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