Dumbbell-shaped polyhedral oligomeric silsesquioxane (POSS) derivatives, in which two POSS units are linked through a bridge, have attracted attention in the last decade. Here, we prepared an unsymmetric dumbbell-shaped POSS derivative (3 Ph-iBu ) in which isobutyl-and phenyl-substituted POSS units are linked by a disiloxane unit and compared its thermal properties with those of the corresponding symmetric isobutyl-and phenyl-substituted dumbbell-shaped POSS derivatives (3 iBu-iBu and 3 Ph-Ph , respectively). The symmetric isobutyland phenyl-substituted dumbbell-shaped POSS derivatives, 3 iBu-iBu and 3 Ph-Ph , were almost completely phase-separated during a mixing process. This phase separation is due to the limited solubility of phenyl-substituted POSS compounds, which are only soluble in tetrahydrofuran (THF) and insoluble in hydrocarbons such as n-hexane and toluene, while the isobutyl-substituted POSS derivatives exhibit a wider spectrum of soluble solvents. The unsymmetric dumbbell-shaped POSS, 3 Ph-iBu , showed hybrid properties of solubility in solvents and thermal behaviors. Differential scanning calorimetric (DSC) analysis showed that enthalpy of the phase transition of 3 Ph-iBu was significantly lower than those of the mixture of 3 iBu-iBu and 3 Ph-Ph . No apparent melting behavior was observed above the phase transition. The thermal degradation of the weakest isobutyl substituents improves in the present single-component hybrid structure.

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