Previous studies have examined polypyrrolone and polyimide membranes for gas separations. For the first time this study examines poly(pyrrolone-imide) copolymers for the O-2/N-2, CO2/CH4, and C3H6/C3H8 separation. Combining these two classes of polymers is designed to provide rigidity and desirable mechanical properties in addition to high-quality gas transport properties. Specifically, the copolymer 6FDA-TAB/DAM was studied while systematically varying the TAB/DAM ratio in order to quantitatively alter the structure of the polymer matrix. Most copolymers studied exhibited results near or above the "upper bound" for O-2/N-2, CO2/CH4, and C3H6/C3H8 separations. Surprisingly, a maximum in C3H6/C3H8 selectivity was found as a function of the TAB/DAM ratio, and this does not follow the trend expected based on previous literature data. A similar trend has also been observed in carbon molecular sieve materials with varying material structure. This demonstrates that certain materials may show excellent size selective properties for one application (O-2/N-2 for example) and exhibit undesirable separation properties for a different application (C3H6/C3H8 in this case).