Reversible addition-fragmentation chain transfer (RAFT) polymerization was used to produce poly(methyl acrylate) (pMA) loops grafted onto silica nanoparticles using doubly anchored bifunctional RAFT agents 1,4-bis(3'-trimethoxysilylpropyltrithiocarbonylmethyl)benzene (Z-group approach) and 1,6-bis(op-2-trimethoxysilylethylbenzyltrithiocarbonyl)hexane (R-group approach) as mediators. In both cases, molecular weights of the resulting surface-confined polymer loops increased with monomer conversion, whereas the grafting density was significantly higher in the case of the R-group supported RAFT polymerization due to mechanistic differences of the RAFT process at the surface. This result was evident from thermogravimetric analysis and supported by scanning electron microscopy. Polymer loops with molecular weights up to 53,000 g mol(-1) were accessible with polydispersities of about 2.0 without and 1.5 with the addition of free RAFT agent. LTV signals of the detached pMA loops measured via size exclusion chromatography were shifted to higher molecular weights compared with the corresponding RI signals, indicating branching reactions caused by the close proximity of growing radicals and polymer at the surface of the silica nanoparticles. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7656-7666,2008