The controlled graft modification of silica gel has been achieved by RAFT polymerization under ultrasonic irradiation. Silica gel was first dispersed in dry toluene under ultrasonic irradiation, and then reacted with 3-aminopropyldimethylethoxylsilane (KH550), and subsequently with S-1-dodecyl-S'-(alpha,alpha'-dimethyl-alpha ''-acetic acid)trithiocarbonate (DDACT) to serve as RAFT agent. The graft polymerization of methyl acrylate was carried out under ultrasonic irradiation (59 kHz, 90 W). The poly(methyl acrylate) (PMA) grafted silica gel was characterized by Fourier transform infrared spectrometry (FT-IR), field-emitting scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The results show that the graft polymerization was well controlled by the RAFT process, and the PMA modified silica gel particles were synthesized successfully. The graft content could reach 32% within 6 h though there was an inhibition of about 50 min in the graft polymerization. Each silica gel particle was coated with an outer layer of PMA chains, and no aggregation was detected in the SiO2-g-PMA dispersion. This work indicated that ultrasonic initiated RAFT polymerization can be a method to modify silica gel with "well-defined" graft polymers. (c) 2008 Elsevier B.V. All rights reserved.