Membrane technology has been considered a promising strategy for carbon capture to mitigate the effects of increasing atmospheric CO₂ levels because CO₂-philic membranes have demonstrated significant application potential, especially, for CO₂/light gas separation. In this regard, poly(ethylene oxide) (PEO), which is a representative CO₂-philic material, has attracted extensive research attention owing to its specific dipole–quadrupole interaction with CO₂. Herein, we report a facile one-step synthesis protocol via the in situ polymerization of highly flexible polyethylene glycol to overcome the limitations of PEO, including high crystallinity and poor mechanical strength. The robust structure derived from intricate entanglements between short PEO chains and the polymer matrix enables an extremely high loading of linear polyethylene glycol (up to 90 wt%). Consequently, the separation performance easily surpasses the upper-bound limit. Moreover, the high structural stability allows for the concurrent increase of CO₂ permeability and CO₂/light gas selectivity at high feed pressure (up to 20 bar). This study provides a promising strategy to simultaneously improve the toughness and gas separation properties of all-polymeric membranes, demonstrating significant potential for industrial carbon capture and gas purification.