Two UV-curable hyperbranched poly(siloxysilane)s (I and III) containing vinyl and allyl end groups were synthesized via polyhydrosilylation with methylbis(methylethylvinylsiloxy)silane and methylbis(dimethylallylsiloxy)silane monomers. A cationic LTV-curable hyperbranched polymer (II-Ep) with epoxy end groups was prepared via the hydrosilylation of hyperbranched polymer II with Si-H terminated groups and glycidyl methacrylate, and II was also obtained via the polyhydrosilylation of AB(2)-type monomer methylvinylbis(methylethylsiloxy)silane. All hydrosilylation reactions were catalyzed by Pt/C or chloroplatinic acid. Three AB(2)-type monomers were synthesized via the hydrolysis of functional chlorosilane, which was prepared with Grignard reagents and dichlorosilane. The molecular structures of the polymers were characterized with H-1 NMR, Fourier transform infrared, and gel permeation chromatography, and the UV-curing behaviors of the polymers under different atmospheres and with different photoaccelerators were also investigated. The thermostability of uncured and cured polymers was examined with thermogravimetric analysis, and the data indicated that the orders of the onset decomposition temperatures for the cured polymers and the residue weights were as follows: III (380 degrees C) > I (320 degrees C) > II-Ep (280 degrees C) and I (70.4%) > III (64.1%) > II-Ep (60.9%), respectively. (c) 2005 Wiley Periodicals, Inc.