Based on the correlation matrix between descriptor features in Fig. of Supporting Information, there are two main pairs of correlated features, which suggests that some of the top features do not have independent physical significance. Namely, descriptors 107 and 109 (Aliphatic Cycle Counts, Correlation Group A) are highly correlated with one another and relatively anti-correlated with features 15 and 16 (FpDensityMorgan, Correlation Group B). The steric space occupied by rings generally results in a lower molecular density, which explains the opposition between Group A and Group B. The features in Group A have a positive impact on gas permeability, while the features in Group B have a negative impact on gas permeability. This suggests that repeating units with more non-aromatic rings allow for larger free volume elements and lower densities, thereby higher gas permeabilities. This supports the emerging direction of polymer research on non-planar structures, such as, kink, spiro, cardo and pendant groups (-CF3), bulky and flexible groups (-O-), or different spatial linkage configurations in polyimides for enlarging their microporosities .