A para-substituted bis(3-aminopropyl)hexaisobutyl-polyhedral oligomeric silsesquioxane (POSS) was successfully synthesized via a selective corner-opening reaction of 3-aminopropylheptaisobutyl-POSS and a subsequent corner-capping reaction. The selectivity of the corner-opening reaction was studied by density functional theory calculations. Polymerization of the POSS monomer with pyromellitic dianhydride resulted in a yellow, self-standing film.