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 CO 2 separation over H 2 . The CO 2 selective layer of synthesized PSt was ca. 200 nm thick, and the composite membranes showed preferential CO 2 separation over H 2 , while parent polystyrene passed through H 2 rather than CO 2 . Especially, the separation factor a CO 2 =H 2 was raised after alkaline treatment to form free primary amine on the terminus of graft chain. Serine tethered polystyrene showed the highest a CO 2 =H 2 (16) among them, which suggested the hydroxyl group would enhance the CO 2 affinity of primary amine.