Various N-tert-butoxycarbonyl (N-Boc) amino acids, such as glycine, valine, and serine, were combined to 4-chloromethylstyrene. The resulted monomers were readily polymerized to give polystyrenes tethering an amino acid by a conventional free radical polymerization technique. After deprotection of the N-Boc group with trifluoroacetic acid, the obtained polystyrene derivatives were soluble in ethanol and spin-coated on a commercial poly(ether sulfone) porous membrane to fabricate a composite membrane for CO2 separation over H-2. The CO2 selective layer of synthesized PSt was ca. 200 nm thick, and the composite membranes showed preferential CO2 separation over H-2, while parent polystyrene passed through H-2 rather than CO2. Especially, the separation factor was raised after alkaline treatment to form free primary amine on the terminus of graft chain. Serine tethered polystyrene showed the highest (16) among them, which suggested the hydroxyl group would enhance the CO2 affinity of primary amine.