Gas transport and separation properties of a series of glassy polymers from different families are studied, and energetic and entropic contributions to diffusivity selectivity are estimated for the oxygen/nitrogen gas pair. Some polymers are found to entropically favor the diffusion of nitrogen over oxygen, despite an overall diffusivity selectivity favoring oxygen. General structural features of glassy polymeric matrices are identified which may be responsible for the magnitude of energetic and entropic contributions to diffusivity selectivity;. Overly well-packed structures only result in entropic and energetic factors working to oppose each other leading to average overall diffusivity selectivities and low absolute diffusivities and permeabilities. On the other hand, rigidity of the polymer backbone introduced by substituent groups, hindering chain motion and disrupting packing, often leads to lower energetic contributions and similar entropic effects for both gas molecules. Increasing the entropic contributions to diffusivity selectivity while maintaining high energetic Selectivities, observed for some polymers, would be the key to taking the next step in polymeric gas separation membrane materials. Polymers need to mimic the structure of molecular sieving media to successfully increase entropic selectivity values. This is a highly complex problem with subtle effects involving many structural features of the polymer matrix and the polymer-penetrant interaction.