The polymerization mechanism of ultrasonically initiated emulsifier-free emulsion copolymerization of n-butyl acrylate (BA) and acrylamide (AM) was investigated. A four-step polymerization mechanism of the ultrasonically initiated emulsifier-free emulsion was put forward based on the monomer conversion and the main reaction locus. Improving the power output would increase the monomer conversion and the rate of polymerization. However. when the reaction temperature was 30 degreesC and the concentration of Na2SO4 was 0.1%, the monomer conversion and the rate of polymerization achieved maximum. The FTIR spectra showed that the sample obtained by this way was the copolymer of BA and AM. but not the blend of poly(butyl acrylate) and polyacrylamide. (C) 2004 Elsevier Ltd. All rights reserved.