Single-walled carbon nanotubes (SWNT) were functionalized with polystyrene (PSt) by grafting to and grafting from methods. PSt-N-3 With designed molecular weight and narrow molecular weight distribution was synthesized by atom transfer radical polymerization (ATRP) of styrene (St) followed by end group transformation and then added to SWNT. The grafting from functionalization was achieved by ATRP of St using 2-bromopropionate groups immobilized SWNT as initiator. Methyl 2-bromopropionate (MBP) was added as free initiator to control the chain propagation on SWNT during the polymerization. Raman and near-IR spectra show that PSt was covalently attached to the sidewalls of SWNT by the grafting to approach, and the degree of functionalization was about 1 in 48 SWNT carbon atoms as determined by thermogravimetric analyses (TGA). In the grafting from approach, size exclusion chromatography (SEC) results show that the molecular weight of free St increased linearly with St conversion, and the PSt cleaved from the SWNT after high conversion had the same molecular weight as the PSt produced in solution. TGA show that the amount of PSt grafted from the SWNT increased linearly with the molecular weight of the free PSt. By both methods, the final functionalized SWNT dissolved well in organic solvents, and the original SWNT bundles were broken into very small ropes or even individual tubes as revealed by AFM.