Styrene-EPDM-acrylonitrile tripolymer (EPDM-g-SAN) was synthesized by the graft copolymerization of styrene (St) and acrylonitrile (An) onto ethylene-propylene-diene terpolymer (EPDM) with "phase inversion" emulsification technique. The high impact strength engineering plastics AES was the blend of SAN resin and EPDM-g-SAN, which occupied good weathering and yellow discoloration resistivity. The effects of An percentage in comonomer and the weight proportion of EPDM to St-An on graft copolymerization behavior and AES notched impact strength were studied. The results showed that monomer conversion ratio (CR) exhibited a peak when the An percentage changed, and the maximum value was 97.5%. Grafting ratio (GR) and grafting efficiency (GE) enhance as well. The notched impact strength of AES presented a peak with the maximum value of 53.0 Kj/m(2), when An percentage was at the range of 35-40%. The spectra of FTIR showed that St and An were graft onto the EPDM. DSC analysis illuminated that T-g of EPDM phase in the blends was lower than that of the pure EPDM. TEM and SEM micrographs indicated that the polarity of g-SAN of EPDM-g-SAN was the main factor effect the particle morphology, in terms of size, distribution and isotropy. When weight ratio of St to An was 65/35, the polarity of g-SAN chains was appropriate, and the EPDM-g-SAN particles dispersed well in the SAN matrix. The supper impact toughness is interpreted in terms of EPDM phase cavitation and enhanced plastic shear yielding. The highest toughness occurs at an optimum EPDM-g-SAN phase particle size which is about 0.2 mu m in SAN resin matrix. (c) 2007 Wiley Periodicals, Inc.