Linear polystyrene chains were grown from the convex surface of two commercially available multiwall carbon nanotubes (MWCNTs) with similar diameter but different lengths. The MWCNTs were supplied from Bayer Material Science (R) (purity >95%, external diameter = 13-16 nm, length = 1-10 mu m, denoted MWCNTBMS95) and Future Carbon GmbH (purity >99%, external diameter = 15 nm, length = 5-50 mu m, denoted MWCNTFC99). The MWCNTs were oxidized with nitric acid, consecutively reacted with thionyl chloride, glycol or poly(ethylene glycol), 2-bromo-2-methylpropionyl bromide and finally with styrene under atom transfer radical polymerization (ATRP) conditions. The content of polystyrene grafted from the surface of the MWCNTs can be controlled by adjusting the molecular weight of the poly(ethylene glycol), the initiator concentration and the monomer to carbon nanotube weight ratio. Under comparable experimental conditions, a higher amount of polystyrene is grafted from the MWCNTBMS95 than from MWCNTFC99. The difference in dimensions and the state of aggregation of the carbon nanotubes influence the grafting from polymerization reactions, where relative shorter and tightly aggregated carbon nanotubes promote higher polymerizations yields than longer and less aggregated carbon nanotubes. The increase of the viscosity of the carbon nanotube dispersion decreases the polymer grafting content. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1035-1046, 2010