A polymer of intrinsic microporosity (PIM-1) containing nitrile groups was reacted by [2 + 3] cycloaddition reaction to give a polymer substituted with tetrazole groups (TZ-PIM), and then was further methylated to give a new PIM substituted with methyl tetrazole groups (MTZ-PIM). In contrast to TZ-PIMs, the MTZ-PIMs had distinctly improved solubility characteristics, enabling a more detailed investigation of the degree of conversion for the cycloaddition reaction and the structures of TZ-PIMs, which showed the presence of two kinds of tetrazole rings. Compared with PIM-1, the MTZ-PIM showed higher gas permselectivity with a corresponding decrease in gas permeability for pure gas pairs such as O₂/N₂ and CO₂/N₂, and for mixed gases, such as CO₂/N₂. Data for selectivity coupled with high gas permeability is close to the Robeson 2008 upper-bound performance limit for the O₂/N₂ and CO2/N2 pure gas pairs, and exceeds the upper-bound for the CO₂/N₂ mixed gas pair. ; peer reviewed: yes ; NRC Pub: yes