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 twoidentical silicon cages are linked to various length aliphatic bridges, were prepared bycorner capping reaction between hepta isobutyltricycloheptasiloxane trisilanol (HIBT) andsuitable bis(triethoxysilyl) derivatives. The products obtained were characterized byelemental analysis and 1H NMR spectroscopy, and the results were in very good agreementwith the expected ones. Degradations were carried out in flowing nitrogen and in static airatmosphere, and temperatures at 5% mass loss (T5%) and residues at 700°C weredetermined to investigate the resistance to the thermal degradation. The T5% values werelower in oxidative atmosphere than in inert environment, and increased linearly as afunction of organic bridge length in either used atmosphere. The residues at 700°C werehigher in static air than in flowing nitrogen. The results obtained for various dumbbellshapedPOSSs were discussed and compared with each other. A comparison with theresults previously obtained with the corresponding un-bridged phenyl, hepta isobutyl-POSSs showed a higher resistance to the thermal degradation of bridged POSSs.