Branched poly(styrene-co-dodecyl acrylate) (BPSDA) was prepared by the atom transfer radical copolymerization of styrene with dodecyl acrylate using p-cholomethyl styrene as initiator-monomer (inimer) and CuCl/Bpy (2,2'-bipyridine) complex as catalyst. The remarkable discrepancies between the molecular weight determined by gel permeation chmotagraphy and multiangel laser light scattering reveals the highly branched structure of the resulting copolymer. Furthermore, the composition was analyzed by hydrogen nuclear magnetic resonance (H-1 NMR), which is consistent with the feed ratio of monomers. Blending of the branched product with high density polyethylene (HDPE) was attempted in haake mixer. The rheological, mechanical, and thermal stability properties of the resulting blends were studied. Compared with pure HDPE, the complex viscosity of blend with addition of 4 wt% BPSDA decreased by 15.9%. While the elongation at break decreased by 5.5% and tensile strength decreased by 4.2%. SEM (scanning electron microscopy) revealed that the average particle size of disperse phase in HDPE/4% BPSDA blend is 0.45 mu m in diameter. Differential scanning calorimetry characterization showed that the addition of BPSDA accelerated the relative crystallization rate but decreased the final absolute degree of crystallinity. No obvious change of thermal stability of the blends was observed relative to pure HDPE. (C) 2008 Wiley Periodicals, Inc.