The present invention includes compositions suitable for air barriers such as innerliners where adhesion to tire carcass materials (e.g., SBR) and flexibility are desirable, as well as low air permeability. The invention includes a tire innerliner made by combining a filler; a sulfur cure system; optionally at least one secondary rubber; and at least one halogenated terpolymer of C.sub.4 to C.sub.8 isoolefin derived units, C.sub.4 to C.sub.14 multiolefin derived units, and p-alkylstyrene derived units. Examples of suitable fillers include modified carbon black, carbon black, silica, exfoliated clays, and combinations thereof. The present invention also includes a method of producing an elastomeric terpolymer composition comprising combining in a diluent C.sub.4 to C.sub.8 isoolefin monomers, C.sub.4 to C.sub.14 multiolefin monomers, and p-alkylstyrene monomers in the presence of a Lewis acid and at least one initiator to produce the terpolymer. Examples of suitable initiators include cumyl compounds and or halogenated organic compounds, especially secondary or tertiary halogenated compounds such as, for example, t-butylchloride, 2-acetyl-2-phenylpropane (cumyl acetate), 2-methoxy-2-phenyl propane (cumylmethyl-ether), 1,4-di(2-methoxy-2-propyl)benzene (di(cumylmethyl ether)); the cumyl halides, particularly the chlorides, such as, for example 2-chloro-2-phenylpropane, cumyl chloride (1-chloro-1-methylethyl)benzene), 1,4-di(2-chloro-2-propyl)benzene (di(cumylchloride)), and 1,3,5-tri(2-chloro-2-propyl)benzene (tri(cumylchloride)); the aliphatic halides, particularly the chlorides, such as, for example, 2-chloro-2,4,4-trimethylpentane (TMPCl), and 2-bromo-2,4,4-trimethylpentane (TMPBr).