Block polymer of alternating copolymers, poly[(ST-a-MA)-b(IP-a-MMA)-b-(ST-a-MA)](I), has been synthesized in the presence of AlEt;Cl;,by using Mn;(CO);/CBr;. initiation system.The GPC results indicate that there is no unreacted prepolymer in I purified by precipitation andfractionation. The study of NMR shows that every block of polymer I has alternating structure. The dynamicmechanical properties of I measured by torsional braid method indicate that the polymer has two T;s,being 38 and 75℃, respectively. The transmission electron micrographs of I show they have two phasestructure and the domain size increases with increasing molecular weight of hard block.