The aim of this work is the preparation of porous polymeric membranes for liquid separations stable in organic solvents, high temperature and/or extreme acid or basic conditions. Polymeric membranes with these properties could replace more traditional and energy-expensive separation processes like distillation, competing with ceramic membranes due to their easy processability and scalability. A limited library of polymers have been successfully used for decades in water-based applications. They are however unstable in organic solvents without an additional treatment, which is usually a crosslinking reaction. In this dissertation different highperformance polymeric membranes and crosslinking strategies are presented and discussed, allowing their use in harsh environments. We present for the first time the preparation of porous membranes using poly(oxindole) derivatives. These polymers were prepared by superacid catalyzed polyhydroxyalkylation, which is a novel one-pot, room-temperature, metal-free polymerization method. The obtained polymers were fully characterized and then manufactured into membranes by the non-solvent induced phase separation method. The crosslinking of these membranes was achieved by different protocols. First, we reacted the oxindole group in the polymer backbone with a variety of dibromides of different chemical structure. Secondly, we incorporated a propargyl side group, followed by a crosslinking in hot glycerol. Moreover, the strategy of crosslinking using propargyl as pendant group was successfully demonstrated in membranes made of poly(benzimidazole) and poly(triazolebisphenol-AF). And thirdly, we prepared membranes from hydroxyl-functionalized poly(oxindole), and conducted a controlled thermal oxidation, which resulted in the crosslinking by phenoxy radicals. In each case, the resulting membranes achieved insolubility in polar aprotic organic solvents, high resistance in acid medium and had high decomposition temperatures. In each case, the resulting membranes achieved not only insolubility in polar aprotic organic solvents and resistance to acid media but also showed high decomposition temperatures. Finally, we demonstrated for the first time the preparation of porous membranes based on recycled poly(ethylene terephthalate) plastic bottles and their potential application for separations in an organic solvent medium.