Thermolysis of Polyhedral Oligomeric Silsesquioxane (POSS) Macromers and POSS−Siloxane Copolymers

The pyrolysis of four polyhedral oligomeric silsesquioxane (POSS) macromers, Cy8Si8O11(OH)2, Cy8Si8O11(OSiMe3)2, Cy6Si6O9, and Cy8Si8O12 (where Cy = c-C6H11) and two POSS−siloxane copolymers [Cy8Si8O11−(OSiMe2) n O−] (n = 1, oligomer average 5.4) has been studied in argon, nitrogen, and under vacuum from 30 to 1000 °C. Product gases were analyzed by TGA-FTIR and mass spectroscopy. Analysis of the chars was conducted using cross-polarized (CP) and magic angle spinning (MAS) NMR spectroscopy, X-ray diffraction, density measurements, and gas adsorption analysis. All of the POSS macromers showed a propensity toward sublimation, while the POSS-siloxane copolymers underwent a complex depolymerization−decomposition process. For the copolymer [Cy8Si8O11−(OSiMe2)5.4O−] this process included the evolution of cyclic dimethylsiloxanes at 400 °C, cyclohexyl hydrocarbons from 450 to 550 °C, and H2 liberation from 700 to 1000 °C. Loss of the silsesquioxane “cage” structure occurred upon heating from 450−650 °C and after the evolution of most of the pyrolysis gases. Changes in both char porosity and density accompanied the structural rearrangements. The activation energies for pyrolysis under argon or nitrogen was 56 ± 9 kcal/mol and for oxidation in air was 20 ± 4 kcal/mol.