The paper describes changes in the structure, morphology, mechanical and thermal properties of porous film samples of poly(4,4'-oxidiphenylene)pyromellitimide prepared as a result of selective destruction of urethane blocks in copolymers composed of pyromellitimide blocks and polyurethane blocks. The initial samples of the new composition of statistical copoly(urethane-imide)s (CoPUIs) were prepared via polycondensation methods using pyromellitic dianhydride (PMDA), 4,4'-oxidyaniline (ODA), 2,4-toluylenediisocyanate (TDI), as well as polycaprolactone (PCL) and poly(1,6-hexanediol/neopentylglycol-alt-adipic acid) (ALT) as monomers. The molar ratio of imide and polyurethane blocks in CoPUI was 10:1. The initial films were heated up to 170 °C to complete the polycondensation processes, after which they were subjected to thermolysis and hydrolysis. The thermolysis (thermal degradation) of copolymers was carried out by heating the initial samples to temperatures of 300 °C or 350 °C. Then, the thermolized films were subjected to chemical degradation in hydrolytic baths containing an aqueous solution of potassium hydroxide. As a result, urethane blocks were destroyed and removed from the polymer. The resulting products practically did not contain polyurethane links and, in chemical composition, were practically identical to poly(4,4'-oxidiphenylene)pyromellitimide. NMR and IR spectroscopy, atomic force microscopy, X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and dynamic mechanical analysis and mechanical properties testing were used to determine the differences in the structure and properties of the initial copolymers and targeted products. The effect of the conditions of destructive processes on the structure, morphology and mechanical properties of the obtained porous polyimide films was determined. From a practical point of view, the final porous films are promising as membranes for filtering aggressive amide solvents at high temperatures.