Syndiotactic poly(propylene-α-olefin) copolymers with controlled microstructure and desired properties for oxygen enrichment have been synthesized via metallocene catalysts. The controlled incorporation of comonomers as pendant branch mainly improved the oxygen permeability, but the oxygen/nitrogen selectivity dropped around 20%. By controlling the amount and length of the branches in the syndiotactic polypropylene, the permeability of the membrane was enhanced up to 12-fold and it is comparable to typical commercial polymers used for this application. In contrast with conventional commercial polypropylene, copolymers with a large amount of comonomer were soluble in cyclohexane, allowing the manufacture of composite membranes with high permeance.