A series of novel three-arm star blocks consisting of three polyisobutylene-b-poly(methyl methacrylate) (PIB-b-PMMA) diblocks radiating from a tricumyl core were synthesized, characterized, and tested. The synthetic strategy involved three steps: the synthesis of Cl-t-tritelechelic PIE by living cationic isobutylene (IB) polymerization, the conversion of the Cl-t termini to isobutyryl bromide groups, and the initiation of Living radical methyl methacrylate (MMA) polymerization by the latter groups. The PIE and PMMA segment lengths (M-n's) could be controlled by controlling the conditions of the living cationic and radical polymerizations of IB and MMA, respectively. Core destruction analysis directly proved the postulated three-arm microarchitecture. The structures of the products were analyzed by H-1 NMR and Fourier transform infrared spectroscopies, and their thermal properties were analyzed by differential scanning calorimetry and thermogravimetric analysis. The presence of a low- and a high-temperature glass transition (T-g,T-PIB similar to -63 degrees C, T-g,T-PMMA similar to 120 degrees C) indicated a phase-separated micromorphology. Stress/strain analysis showed a tensile strength of up to similar to 22.9 MPa and an elongation of similar to 200%.