We prepared flexible free-standing films of trifluoropropyl-substituted open-cage silsesquioxane-pendant polysiloxane by optimizing the sol-gel reaction conditions of tris(dimethoxysilyl-ethyl-dimethylsiloxy)-heptatrifluoropropyl-substituted open-cage silsesquioxane (1). The polycondensation of 1 was fully achieved even at 50 °C for 6 h under vacuo. ²⁹Si CP-MAS NMR analysis indicated that the flexible free-standing films, polycondensed at 50 °C and 180 °C, included cyclotrisiloxane (D₃) and linear siloxane (D_linear) structures. The elastic modulus and decomposition temperature at 5% mass weight loss (T_d5) of the product by polycondensation at 180 °C under N₂ were significantly greater than those for the 50 °C product. Significant changes in the UV‒vis spectra of the resulting transparent films were not observed even after 13 days of UV irradiation in air. In contrast, UV irradiation of the isobutyl-substituted counterpart under air clearly caused a decrease in its transmittance due to autoxidative degradation.