A new controllable approach to synthesize hyperbranched poly(siloxysilanes) via hydrosilylation of A(2)- and B'B-x-type monomers was developed in this work. A(2) monomers (dimethylbis(dimethylsiloxy)siloxane and tetramethyldisiloxane), B'B-x monomers (methylvinyldiallylsilane and vinyltriallylsilane), and the resultant hyperbranched poly(siloxysilanes) were well characterized using FTIR, H-1 NMR, C-13 NMR, Si-29 NMR, and SEC/MALLS. The In situ FTIR results indicate that the controllable polymerization can be carried out quickly and the reaction process was obviously performed in two stages. At the first stage, silicon hydride selectively reacts with vinyl silane groups, which produces intermediate structures with one Si-H and two (or three) allyl groups. Consequently, at the second stage, these intermediates act as new AB(2) (or AB(3)) type monomers and continue to be self-polymerized to generate hyperbranched polymers. By this novel controllable approach, molecular weights and their polydispersity of the resulted hyperbranched poly(siloxysilanes) can be conveniently regulated via adjusting the process parameters, such as feeding ratio of two monomers. (c) 2008 Wiley Periodicals, Inc.