A highly gas permeable polymer with exceptional size selectivity is prepared by fusing triptycene units together via a poly­merization reaction involving Tröger's base formation. The extreme rigidity of this polymer of intrinsic microporosity (PIM-Trip-TB) facilitates gas permeability data that lie well above the benchmark 2008 Robeson upper bounds for the important O₂/N₂ and H₂/N₂ gas pairs.

Organic–organic composite membranes are prepared by in situ crystallization of cage molecules in a polymer of intrinsic microporosity. This allows a direct one‐step route to mixed‐matrix membranes, starting with a homogeneous molecular solution. Extremely high gas permeabilities are achieved, even after ageing for more than a year, coupled with good selectivity for applications such as CO₂ recovery.

Nitrile groups in the polymer of intrinsic microporosity PIM-1 were reduced to primary amines using borane complexes. In adsorption experiments, the novel amine–PIM-1 showed higher CO₂ uptake and higher CO₂/N₂ sorption selectivity than the parent polymer, with very evident dual-mode sorption behavior. In gas permeation with six light gases, the individual contributions of solubility and diffusion to the overall permeability was determined via time-lag analysis. The high CO₂ affinity drastically restricts diffusion at low pressures and lowers CO₂ permeability compared to the parent PIM-1. Furthermore, the size-sieving properties of the polymer are increased, which can be attributed to a higher stiffness of the system arising from hydrogen bonding of the amine groups. Thus, for the H₂/CO₂ gas pair, whereas PIM-1 favors CO₂, amine–PIM-1 shows permselectivity toward H₂, breaking the Robeson 2008 upper bound.