Ultra‐Selective Molecular‐Sieving Gas Separation Membranes Enabled by Light–Heat Dual Crosslinking of Polyimide for Helium Extraction From Natural Gas

ABSTRACT Helium extraction from natural gas demands ultra‐high He/CH 4 selectivity (>1000), yet this benchmark remains largely unmet by polymeric membranes due to the permeability‐selectivity trade‐off and the challenge of constructing sub‐4 Å sieving channels. Here, we report an orthogonal light–heat dual‐crosslinking strategy to program the free‐volume topology of polyimide membranes. A photoreactive precursor (6FCC‐X) was synthesized via a click reaction by incorporating photosensitive units (C═C─S) and varying amounts of thermally labile─COOH groups into the backbone. UV irradiation first triggers [2+2] cycloaddition to construct a dense and uniform super‐crosslinked network, which defines the primary molecular‐sieving framework. Subsequent thermal decarboxylation generated rigid biphenyl linkages that are proposed to function as molecular‐strut‐like structural motifs, enhancing chain rigidity, improving micropore connectivity, and refining sub‐4 Å sieving constrictions. This sequential topology programming decouples helium transport from methane exclusion, which enables the optimized 6FCC‐3‐CR membrane to achieve a He permeability of 33.9 Barrer and an exceptional He/CH 4 selectivity of 3767, outperforming most reported polymer‐based membranes and rivaling advanced carbon molecular sieve membranes. This work establishes a molecular‐strut‐mediated topology‐programming principle for polymer molecular sieves, offering a general route to reconcile fast transport with precise molecular exclusion.