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Membrane technologies can offer dramatically higher energy efficiency than thermally driven separations such as distillation. The fabrication of robust, solvent-stable active layers on inexpensive supports is essential for the widespread utilization of this technology by industry. Here we show that polymer membranes incorporating a perfluoroalkyl side chain onto a hydrocarbon backbone provide remarkable enhancements in performance and stability in the dehydration of ethanol by pervaporation, even surpassing commercial perfluoropolymers. To rapidly generate these robust thin film composite membranes, we use a method termed spin coating ring-opening metathesis polymerization (scROMP) that combines the polymerization and deposition of the membrane selective layer into a 2-min process with under 1 mL of solvent per 36 cm2 of polymer. Here, the scROMP of 5- (perfluoro-n-alkyl)norbornenes (NBFn) with perfluoroalkyl side chain lengths (n) of 4, 6, 8, and 10 is used to generate semifluorinated films on polyacrylonitrile (PAN) supports. pNBFn membranes exhibit greater solvent stability than their nonfluorinated polynorbornene (pNB; n =0) counterpart while retaining excellent thermal stability, as evidenced by reduced swelling in polar and nonpolar solvents and <1 % mass loss in thermogravimetric analysis up to 130 ◦C. Molecular simulations show that the fluorocarbon side chains orient parallel to the surface in the bulk but more normal to the surface at the interface, consistent with experimental IR spectroscopy and wetting measurements. Of the polymers studied, pNBF8 shows the greatest performance in ethanol dehydration, obtaining a selectivity of 180 and a water permeance of 1000 GPU, while sustaining high performance for >40 h of continuous operation.