Network completeness enables angstrom-scale transport pathways in polymer membranes.

Achieving high molecular selectivity without compromising mechanical robustness remains a central challenge in separation science. Here we report polymer networks containing covalently embedded diffusion channels that integrate porous frameworks within the polymer matrix. By systematically tuning crosslinking motifs, we introduced network completeness as a quantitative design descriptor that connects bridge connectivity to separation performance. Our density-probe approach provided experimental evidence of sub-3 Å pores, a feature previously only speculated or simulated. The hydrogen-selective ms-oDMB-DB50 membrane surpasses state-of-art polymer performance thresholds while maintaining stable operation over extended periods. This design rule combines engineering practicality with effective diffusion channels arising from a reticular-style network architecture and density-probe readouts. Our results establish a framework for reticular-style design in polymers and identify structural attributes that enable dual-domain functionality for advanced molecular separations.