Copolymers of 2,6-dimethyl-phenol (DMP) and 2,6-diphenyl-phenol (DPP) were synthesized in the initial molar ratio of 100 : 0 (S(PPO)), 90 : 10 (Co-A), 75 : 25 (Co-B), 65 : 35 (Co-C), and 0 : 100 (PDPPO). Dense membranes of 30 mu m thickness were tested for single gas permeation and binary mixture separation of 55: 45 (in mol %) propylene-propane at 30 degrees C +/- 2 degrees C. Their performance was ultimately examined in the enrichment of propylene from a refinery off-gas mixture (ROG or also called as absorber tail gas, ATG) having the same composition as the ATG of a fluid catalytic cracking (FCC) unit of HPCL refinery, Visakhapatnam. The mixture contains C-1-C-5 hydrocarbons and nonhydrocarbons such as CO, CO2, H-2, and N-2. A detailed permeation study of the hydrocarbon part of ATG revealed that using S(PPO) and Co-A, propylene could be upgraded from similar to 29 mol % (on nonhydrocarbon free basis) to 62.2 and 74.4 mol % with propylene/propane selectivity ratio of 5.99 and 8.45, respectively. The structure of polymers was characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (Proton NMR), viscosity measurements. Scanning electron microscope (SEM), wide angle X-ray diffraction (WAXD), density and fractional free volume measurements were used for studying membrane morphology. Dynamic mechanical thermal analyzer (DMTA) and tensile testing were carried to find glass transition temperature (T-g) and mechanical properties. The relative differences observed in gas permeation of these polymers were correlated with the physical properties measured. S(PPO) and Co-A were identified as potential materials for the upgradation of propylene from refinery off-gas streams. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 2497-2507, 2013