A series of 6FDA/PMDA-TAB polypyrrolone copolymers has been shown to exhibit performance lying on or above the so-called O-2/N-2 upper bound membrane separation tradeoff limit. Further analysis of diffusion in these high performance materials can potentially guide future structural enhancements, which may lead to improved separation properties. This work compares and analyzes diffusion in this attractive family of hyperrigid polymers using entropic and energetic selectivities. When the polymer structure and penetrant sizes are examined, possible polymer matrix configurations can be discussed with regard to the entropy of the molecules in the normal and activated states. This analysis extends to O-2/CH4 and N-2/CH4 entropic selectivities, which corroborate the trend observed with the O-2/N-2 gas pair, but the changes are more pronounced due to the larger difference in penetrant size.