Silicone-based impact modifiers were prepared in a previous study. The modifiers were composed of silicone/acrylic rubber cores and grafted acrylic shells. They improved the toughness of poly(vinyl chloride) (PVC) and poly(methyl methacrylate). The silicone emulsion that was used to produce the silicone-based impact modifiers was prepared via two routes: emulsion polymerization and bulk polymerization of octamethyltetracyclosiloxane. Many silicone-based impact modifiers were produced that had different silicone/acrylic rubber characteristics. Through a toughness examination of modified PVC, the best composition of the silicone-based impact modifiers was obtained, and the silicone content in the rubber composition was 25 wt %. The morphology of the silicone-based impact modifiers, determined by transmission electron microscopy, was as follows: core and second shell polymers were mainly poly(butyl acrylate), and the first shell polymer was silicone. The silicone-based impact modifiers were blended with engineering resins such as PVC, polycarbonate (PC), poly(butylene terephthalate) (PBT), and PC/PBT mixtures. The impact strength under standard conditions and after weathering test conditions for blends of the silicone-based impact modifiers were investigated with respect to two commercially available acrylic and methyl methacrylate/butadiene/styrene impact modifiers. The results showed good weatherability and good toughness under low-temperature conditions for the silicone-based impact modifiers. (C) 2004 Wiley Periodicals, Inc.