New hyperbranched poly(carbosilarylenes) are described that exhibit low glass transition temperatures and high thermal stabilities. The incorporation of Si-O-Si linkages into the branches leads to a polymer of increased flexibility but considerably lower thermal stability. The hyperbranched polymers are prepared via a one-pot hydrosilylation polymerization of aromatic AB(3) monomers. The monomers are prepared from commercially available reagents using lithium-halogen exchange reactions followed by treatment with chlorosilanes. The glass transition temperatures of the hyperbranched polymers range from -45 degrees C for the siloxane-containing polymer to 12 degrees C for the 1,4-substituted polymer. Number-average molecular weights (GPC) range from 2560 to 5600. The degree of branching in these polymers was determined by quantitative Si-29 NMR spectroscopy and found to be very close to the theoretical value of 0.44 for AB(3) systems.