In this study, polysilsesquioxane (PSQ) containing phthalimido side-chain groups (PSQ-PI) with solubility and high thermal stability was prepared by the two-step reaction of an ammonium-group-containing soluble ladder-like PSQ (PSQ-NH3Cl) with phthalic anhydride. First, the reaction of PSQ-NH3Cl with phthalic anhydride was performed in the presence of triethylamine in a DMSO/water mixed solvent to obtain PSQ containing phthalamic acid side-chain groups (PSQ-PA). Next, PSQ-PI was prepared by refluxing a solution of PSQ-PA in DMF with stirring. PSQ-PI was soluble in organic solvents such as chloroform, DMF, and DMSO. The film of PSQ-PI could be prepared by drying its solution in DMF. The UV-Vis spectrum of PSQ-PI cast film showed high transmittance in the visible wavelength region. The pyrolysis temperatures of PSQ-PI under nitrogen flow were relatively high (T-d5 = 394 degrees C and T-d10 = 438 degrees C) because PSQ-PI has both a thermally stable Si-O-Si main-chain and phthalimido side-chains. (C) 2015 Elsevier Ltd. All rights reserved.