The stress-strain diagrams and ultimate tensile properties of uncompatibilized and compatibilized hydrogenated polybutadiene-block-poly(methyl methacrylate) (HPB-b-PMMA) blends with 20 wt % poly(methyl methacrylate) (PMMA) droplets dispersed in a low-density polyethylene (LDPE) matrix were studied. The HPB-b-PMMA pure diblock copolymer was prepared via controlled living anionic polymerization. Four copolymers, in terms of the molecular weights of the hydrogenated polybutadiene (HPB) and PMMA sequences (22,000-12,000, 63,300-31,700, 49,500-53,500, and 27,700-67,800), were used. We demonstrated with the stress-strain diagrams, in combination with scanning electron microscopy observations of deformed specimens, that the interfacial adhesion had a predominant role in determining the mechanism and extent of blend deformation. The debonding of PMMA particles from the LDPE matrix was clearly observed in the compatibilized blends in which the copolymer was not efficiently located at the interface. The best HPB-b-PMMA copolymer, resulting in the maximum improvement of the tensile properties of the compatibilized blend, had a PMMA sequence that was approximately half that of the HPB block. Because of the much higher interactions encountered in the PMMA phase in comparison with those in HPB (LDPE), a shorter sequence of PMMA (with respect to HPB but longer than the critical molecular weight for entanglement) was sufficient to favor a quantitative location of the copolymer at the LDPE/PMMA interface. (C) 2004 Wiley Periodicals, Inc.