Synthesis and thermal properties of new dumbbell-shaped isobutyl-substituted POSSs linked by aliphatic bridges

Five new dumbbell-shaped polyhedral oligomeric silsesquioxanes (POSSs), in which two identical silicon cages are linked to various length aliphatic bridges, were prepared by corner capping reaction between hepta isobutyltricycloheptasiloxane trisilanol (HIBT) and suitable bis(triethoxysilyl) derivatives. The products obtained were characterized by elemental analysis and H-1 NMR spectroscopy, and the results were in very good agreement with the expected ones. Degradations were carried out in flowing nitrogen and in static air atmosphere, and temperatures at 5 % mass loss (T (5 %)) and residues at 700 A degrees C were determined to investigate the resistance to the thermal degradation. The T (5 %) values were lower in oxidative atmosphere than in inert environment, and increased linearly as a function of organic bridge length in either used atmosphere. The residues at 700 A degrees C were higher in static air than in flowing nitrogen. The results obtained for various dumbbell-shaped POSSs were discussed and compared with each other. A comparison with the results previously obtained with the corresponding un-bridged phenyl, hepta isobutyl-POSSs showed a higher resistance to the thermal degradation of bridged POSSs.