The syntheses and selected structure property relationships of two series of polycarbosiloxanes (HB-PCSOX) and polycarbosilanes (HB-PCS) prepared by the platinum-catalyzed bimolecular non-linear hydrosilylation polymerizations of commercially available 1,3-diallyl- or 1,3-divinyl- disiloxanes or disilanes and tri- or tetra-functional siloxysilanes or silanes (i.e., A(x) + B-y polymerization systems where x = 2 and y = 3 or 4) are described. The polymerizations were controlled to avoid gelation and ensure preparation of non-crosslinked soluble polymer products by adjusting the molar ratios of the reacting A (allylsilyl, Si-CH2 CH=CH2, or vinylsilyl,. Si-CH=CH2) and B (silyl, Si-H) reactive functional groups such that r <= 1/[(x - 1)(y - 1)] or r >= (x - 1)(y - 1) where r = [A]/[B]. The polymers were characterized by IR, H-1, C-13 and Si-29 NMR, SEC, DSC and TGA. Their molecular weights were found to increase in the following order of the side-group substitution in the 1,3-divinyldisiloxane monomer used: EtO > Me > MePh > Ph-2, consistent with the increased electron-donation to the vinyl groups causing their increased reactivity in hydrosilylation. These polymers represent unique yet easily and economically available multifunctional nanoscopic dendritic building blocks for more complex 3D nanostructured materials for a variety of applications in electronics, photonics, lithography, specialty coatings, etc. (C) 2012 Elsevier Ltd. All rights reserved.