Synthesis of PMMA-PBD-PMMA (MBM) triblock copolymers with a 1,3-diisopropenyl-benzene- (DIB-) based initiator is studied. Diethyl ether, tert-butyl methyl ether, N,N,N’,N’-tetramethylethylenediamine (TMEDA) and THF were found to be efficient polar additives for obtaining PMMA-PBD-PMMA triblock copolymers, but these triblock copolymers displayed a high 1,2-microstructure of the PBD block. A combination of the initiator seeding technique and weakly polar additives has allowed MBM triblock copolymers viith high 1,4-microstructure of PBD to be obtained. These copolymers exhibited a definitely higher upper service temperature than conventional thermoplastic elastomers, i.e. SBS or SIS Kratons. The influence of chain microstructure, copolymer composition and block length on the mechanical properties has been studied. Short polystyrene blocks of ca. M-n, 2000 have been incorporated in between the PBD central block and the PMMA outer sequences. A higher tensile strength was observed for these MBM-like triblock coplymers. Contamination by a diblock structure expectedly had a very depressive effect on the bulk properties, and the stress-strain behavior mainly depended on the PMMA content and secondarily on molecular weight.