Divinylsiloxane-bisbenzocyclobutene (DVS-bisBCB) polymer has very low dielectric constant and dissipation factor, good thermal stability, and high chemical resistance. The fracture toughness of the thermoset polymer is moderate due to its high crosslink density. A thermoplastic elastomer, polystyrene-polybutadiene-polystyrene triblock copolymer, was incorporated into the matrix to enhance its toughness. The cured thermoset matrix showed different morphology when the elastomer was added to the B-staged prepolymer or when the elastomer was B-staged with the DVS-bisBCB monomer. Small and uniformly distributed elastomer domains were detected by transmission electron micrographs (TEM) in the former case, but TEM did not detect a separate domain in the latter case. A high percentage of the polystyrene-polybutadiene-polystyrene triblock copolymer could be incorporated into the DVS-bisBCB thermoset matrix by B-staging the triblock copolymer with the BCB monomer. The elastomer increased the fracture toughness of DVS-bisBCB polymer as indicated by enhanced elongation at break and increased K1c values obtained by the modified edge-lift-off test. Elastomer modified DVS-bisBCB maintained excellent electrical properties, high T-g and good thermal stability, but showed higher coefficient of linear thermal expansion values. (c) 2006 Wiley Periodicals, Inc.