A sol-gel polymerization method was developed to make polyimide (PI) silsesquioxane (SSQ) nanoparticles as functional, soft dielectric materials. The surface functionalization of the polymer chain backbone and chain ends of poly(trimellitic anhydride chloride-co-4,4 '-methylenedianiline), PMR-15 resin, with para-(chloromethyl)-phenylethyltrimethoxy silane yielded a novel sol-gel reactive sites functionalized PMR-silane precursor. Upon base-catalyzed hydrolysis and condensation of the PMR-silane precursor, spherical, raspberry-like PMR-SSQ nanoparticles were obtained in considerably good yield. Controlling the particle size distribution was attempted upon adjusting the molar ratio between the silane precursor and the base, as well as in the presence of a catalytic amount of silica sols. Particle composition, thermal stability, and morphology were confirmed from Fourier transform infrared, thermogravimetric analysis, and scanning electron microscopy analyses. Nanoparticles, visualized under transmission electron microscopy exhibit a nanoporous structure. The Brunauer-Emmett-Teller analysis confirmed that the average pore dimension is ranged from 2 to 5 nm. The dielectric constant of PMR-SSQ nanoparticles was as low as 1.95, compared to dielectric constants of 3.05 and 3.13 for PMR-15 and PMR-silane, respectively. Thus, the base-catalyzed sol-gel polymerization of alkoxysilylated PI offers a novel synthetic path to make functional nanoporous PI nanostructures that possess ultralow dielectric constants. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 57, 562-571.