Defect-free mixed-matrix membranes (MMMs) were prepared by incorporating hydrophilic metal-organic polyhedra (MOPs) into cross-linked polyethylene oxide (XLPEO) for efficient CO₂ separation. Hydrophilic MOPs with triethylene glycol pendant groups, which were assembled by 5-tri(ethylene glycol) monomethyl ether isophthalic acid and Cu(II) ions, were uniformly dispersed in XLPEO without particle agglomeration. Comparing with conventional neat XLPEO, the homogenous dispersion of EG₃ -MOPs in XLPEO enhanced CO₂ permeability of MMMs. Upon increasing the amount of EG₃ -MOPs, the membrane performance such as CO₂ /N₂ selectivity was steadily improved because of unsaturated Cu(II) sites at paddle-wheel units, which was confirmed by Cu K-edge XANES and TPD analysis. Therefore, such defect-free MMMs with unsaturated metal sites would contribute to enhance CO₂ separation performance.