The objective of the present study was to determine the best molecular balance between the two hydrogenated polybutadiene (HPB) and polymethylmethacrylate (PMMA) blocks that promotes an HPB-b-PMMA diblock copolymer with efficient compatibilization activity in a low-density polyethylene (LDPE)/PMMA immiscible blend. The model blend selected, LDPE/PMMA, is "more immiscible" than the LDPE/polystyrene pair largely reported in open literature. The blends having a composition of 80LDPE/20PMMA exhibit a droplet-in-matrix phase morphology whereas in 20LDPE/ 80PMMA a co-continuous phase morphology was developed. In the droplet-in-matrix phase morphology, the emulsifying efficiency of the copolymer was evaluated based on the maximum reduction of the PMMA droplet size it is able to promote. Whereas, in the co-continuous phase morphology, the copolymer was evaluated based on its ability to stabilize the maximum phase co-continuity. The sequences of the best emulsifying copolymer revealed are not symmetrical. An HPB-b-PMMA where the ratio of molar mass of the blocks, M-n (HPB)/M-n (PMMA), is within 1.8-1.95 exhibits a much better interfacial activity in LDPE/PMMA blends than a copolymer of much lower ratio (longer PMMA block). This is ascribed to the much higher interactions (cohesive energy density) encountered in PMMA (PMMA of the copolymer and PMMA phase of the blend) compared with the LDPE side (HPB of the copolymer and LDPE phase of the blend). (c) 2005 Wiley Periodicals, Inc.