A thermoplastic polymer and an article of manufacture consisting essentially of this polymer are described herein which possess excellent mechanical properties at both room and elevated temperatures. The polymer is obtained by modifying a block copolymer composed of a conjugated diene compound and an alkenyl arene compound. The block copolymer is first selectively hydrogenated and thereafter modified by grafting carboxyl containing functional groups primarily in the alkenyl arene block via metalation. The mechanical properties may be varied and controlled by varying the degree of functionalization (amount of carboxyl groups), and the degree of neutralization of the carboxyl groups from an acid to a metal carboxylate salt.