Hyperbranched polysiloxane polymers with multivinyl functionality were designed and synthesized through a one-step and one-pot deactivation enhanced atom transfer polymerization (DE-ATRP) approach from the copolymerization of polydimethylsiloxane (PDMS) macromonomers and divinylbenzene (DVB). Various feed ratios of siloxane-based monomer and divinyl monomers were investigated. We showed that even at DVB concentrations as high as 80 mol % in the feed, 65% yield of hyperbranched polymer could be obtained without gelation because the DE-ATRP suppressed the rapid formation of macronetwork structures. The molecular weight, polydispersity, macromolecular structure of hyperbranched poly(DVB-co-PDMS) as well as its viscosity in silicone oil were characterized by GPC-MALLS, (1)H NMR and rheometer. By tracking the relationship between the radius of gyration, elution volume and molecular weight from MALLS analysis, solid evidences of the highly branched and condensed structure of the polymers were obtained. Furthermore, the oil thickening experiments demonstrate that this hyperbranched polymer can act as a well-controlled viscosity-modifier for Silicone oils, which potentially will have important application in coating, cosmetic and pharmaceutical products. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 629-637, 2012