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[Display omitted] • Kinetic control of pore formation in polyimides were analyzed. • The pore size can be engineered by tailoring the DBP and NH 2 -POSS concentrations. • The DBP is easily removed with common organic solvents and leaves pores in the film. • The NH 2 -POSS acts as an additional nucleating agent during the PI chain crystallization. • The resultant PPI/NH 2 -POSS x provides desirable dielectric properties in a broad frequency and temperature ranges. Fabricating porous polymer films with precise morphological control via a conventional phase separation method is challenging. Herein, porous polyimide (PPI) film formation kinetics was thoroughly investigated to understand the underlying phenomena during the thermally induced phase separation. The porous films were prepared using a poly(amic acid) (PAA) solution with a small-molecular phase dispersion agent (i.e., porogen), dibutyl phthalate (DBP), which is miscible with the PAA chains. Using amine-functionalized polyhedral oligomeric silsesquioxane (NH 2 -POSS) as an additional nucleating agent, sponge-like NH 2 -POSS end-capped PPI (PPI/NH 2 -POSS x) films were fabricated with tunable pore size (16.3–76.9 μm) and porosity (42.6–69.4 %). This ensured control over the porous structure of the PPI/NH 2 -POSS x film, providing desirable low dielectric properties (a minimum of 2 at 1 MHz) in broad frequency and temperature range as well as high thermal stability (T g = 363–385 °C, T 5% = 519–559 °C), and good mechanical strength (17–23 MPa). It was found that the porous structure and physical properties of the PPI/NH 2 -POSS x films can be readily engineered by tailoring the initial content of a removable porogen and cage silsesquioxane. This work will provide a new perspective on fabricating temperature-resistant low-dielectric porous polymer films with tunable pore sizes via thermal-driven spontaneous phase separation with efficient processability. [ABSTRACT FROM AUTHOR]