Polymeric gas separation membranes are a viable solution to mitigate greenhouse gas emissions directly linked to global warming due to their environment-friendly synthetic process and low cost. Poly(vinyl alcohol) (PVA) synthesized via non-petroleum routes is an eco-friendly material with several advantages for membrane applications such as good film-forming properties, good compatibility, and water solubility. Despite these benefits, the gas barrier property of PVA prevents its application in gas separation membranes. Therefore, we synthesized a graft copolymer consisting of PVA main chains and poly(oxyethylene methacrylate) (POEM) side chains via one-pot free radical polymerization and applied it to a highly CO2 permeable thin film composite membrane. Synthesis of the PVA-g-POEM graft copolymer was confirmed by Fourier transform infrared and proton nuclear magnetic resonance spectroscopy. X-ray diffraction and differential scanning calorimetry analyses revealed that the crystallinity of PVA-g-POEM decreased gradually with increasing POEM content, with the PVA-g-POEM graft copolymer becoming amorphous at a POEM content of 64 wt%. The composite membrane prepared with PVA-gPOEM exhibited a high CO2 permeance (347.3 GPU) and moderate selectivity (21.6 for CO2/N-2). This performance is superior to other PVA-based membranes reported to date and is close to the target area for commercialization. The improved separation performance is due to the intermingled CO2-philic POEM side chains and the decrease in crystallinity of PVA.