Elastomeric polymers and compounded elastomers having reduced amounts of leachables and improved resistance to γ-irradiation sterilization suitable for pharmaceutical stopper and seal applications. These polymers and compounded elastomers maintain their physical performances upon being exposed to γ-irradiation over time and typical use conditions. The polymer is prepared by reacting a mixture of i) a C4 to C7 isoolefin monomer, ii) a styrene based monomer, and optionally iii) a C4 to C14 multiolefin monomer wherein the polymer contains 5 to 15 wt % of styrene derived units.