Abstract: A carbosilane-based hybrid star polymer having a carbosilane core of large size and addition polymer chains emanating from the core, is described. The carbosilane core is formed from a central silane nucleus and built up by repeated alternating hydrosilylation of reactive vinylic sites followed by vinylation of silicon halide reactive sites. The core is reacted with living addition prepolymers to form the hybrid star polymer. The minimum core size to give the desired properties corresponds to that having at least about 48 reactive silicon halide sites on the periphery. These highly regular hybrid star polymers have unique solution and gelling agent properties and form highly structured and rigid gels, and also act as viscosity modifiers. The large carbosilane core having either vinyl or silicon halide reactive sites can be used as a reactive intermediate.