The synthesis, characterization, and structure regulation of hyperbranched polycarbosiloxane with dendritic boron cores were realized in this paper. First, dendritic boron core was synthesized via hydroboration with borane dimethylsulfide and bis (allyloxy)dimethylsilane. Then, the hyperbranched polycarbosiloxanes with dendritic boron cores were synthesized via hydrosilylation with AB(2) type monomer of bis(allyloxy) methylsilane and dendritic boron cores. The molecular structures of the dendritic boron core and resulting hyperbranched polymers were characterized by using Fourier transform infrared spectroscopy, H-1 nuclear magnetic resonance, and C-13 nuclear magnetic resonance spectroscopies. Size exclusion chromatography/multiangle laser light scattering analysis reveals that the structures of hyperbranched polycarbosiloxane can be regulated effectively by incorporation of functional dendritic boron cores. Compared with hyperbranched polycarbosiloxane of the same molecular weight level, the hyperbranched polycarbosiloxane with dendritic boron cores presents narrower molecular weight distribution as well as much smaller hydrodynamic radius and intrinsic viscosity. Thermalgravimetric analyzer analysis indicates that both the decomposition temperature and ceramic yields are increased as the results of the incorporation of dendritic boron cores into hyperbranched polycarbosiloxane. (c) 2006 Wiley Periodicals, Inc.