Core-shell poly(butadiene-graft-styrene) (PB-g-PS) rubber particles were synthesized with different initiation systems by emulsion grafting polymerization. These initiation systems included the redox initiators and an oil-soluble initiator, 1,2-azobisisobutyronitrile (AIBN). Then the PB-g-PS impact modifiers were blended with polystyrene (PS) to prepare the PS/PB-g-PS blends. In the condition of the same tensile yield strength on both samples, the Izod test showed that the notched impact strength of PS/PB-g-PS(AIBN) was 237.8 J/m, almost 7 times than that of the PS/PB-g-PS(redox) blend, 37.2 J/m. From transmission electron microscope (TEM) photographs, using the redox initiators, some microphase PS zones existed in the core of PB rubber particles, which is called "internal-grafting." This grafting way was inefficient on toughening. However, using AIBN as initiator, a great scale of PS subinclusion was seen within the PB particle core, and this microstructure increased the effective volume fraction of the rubber phase with a result of improving the toughness of modified polystyrene. The dynamic mechanical analysis (DMA) on both samples showed that the glass transition temperature (T,,) of rubber phase of PS/PB-g-PS(AIBN) was lower than that of PS/PB-g-PS(redox). As a result, the PB-g-PS(AIBN) had better toughening efficiency on modified polystyrene than the PB-g-PS(redox), which accorded with the Kerner approximate equation. (c) 2006 Wiley Periodicals, Inc.