During the production of high impact polystyrene the rubber particle formation process is very important to control the final physical property balance. Besides the rubber viscosity, the presence of a copolymer to reduce the interfacial tension between the rubber and polystyrene phase is central. Such a copolymer can be added or can be made during the polymerization. In this study, it was attempted to create a block rubber in situ using ultrasound. Polybutadiene dissolved in styrene has been sonicated to create macroradicals. It was anticipated that these macroradicals would initiate the polymerization of styrene thus generating a poly(butadiene-block-styrene) acting as emulsifier during the production of high impact polystyrene. No evidence was found for the formation of a block copolymer but the higher reactivity and the resulting rubber particles indicate that besides rubber molecular weight reduction extra functionality was introduced on the rubber. No attempts were made to define the nature of the functionality. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1546-1551, 2009