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Structural Tuning of Polymers of Intrinsic Microporosity via the Copolymerization with Macrocyclic 4‐tert‐butylcalix[4]arene for Enhanced Gas Separation Performance

4‐tert‐butylcalix[4]arene (CA) is chemically incorporated into polymers of intrinsic microporosity (PIM‐1) via a one‐step nucleophilic copolymerization reaction to study the unprecedented synergistic effects of polymer structural tuning and the unique 3D cavity of CA on gas separation performance. From the wide‐angle X‐ray diffraction (XRD) and positron annihilation spectroscopy (PALS), two opposing structural effects of CA incorporation on polymer chains are found, with one being expansive for fractional free volume (FFV) and the other contractive. The bulky cup‐shape CA cavity reduces the chain packing efficiency, which increases FFV and the gas permeability, but the smaller dihedral angle of CA shrinks FFV, leading to the enhanced gas selectivity. More interestingly, CA's unique 3D open cavities also favor selective passage of gas molecules. These combinative effects impart PIM‐CA copolymer membranes with attractive and tunable gas transport properties. The gas permeability improves dominantly at small CA loadings while the selectivity is enhanced significantly at higher loadings, allowing PIM‐CA membranes with ≥1% CA loadings to perform beyond 2008 Robeson upper bounds for H2/N2, H2/CH4, CO2/N2, and CO2/CH4 separations. As a cheap commercial product with high chemical versatility, CA can expand the spectrum of PIM‐based membrane designs for more efficient hydrogen and natural gas purification processes.