Treatment of 4-acetylstyrene (an AB(2) monomer) with dihydridocarbonyltris(triphenylphosphine)ruthenium leads to hyperbranched poly(4-acetylstyrene) whose structure has been determined by H-1 and C-13 NMR spectroscopy. The hyperbranched material has been characterized by GPC, LR, UV, TGA, DSC, and elemental analysis. Polymerization occurs by Ru-catalyzed addition of C-H bonds which are adjacent to the activating acetyl group across the C-C double bond of the vinyl group in both a Markovnikov and an anti-Markovnikov manner.