A supramolecular four-armed star copolymer was prepared from the pseudorotaxane complex of a crown-centered polystyrene and a bis(2,2,6,6-tetramethylpiperidinyl-1-oxy) viologen compound and n-butyl methacrylate (BMA) by nitroxide-mediated polymerization of the latter. Solvent fractionation with dioxane/THF/MeOH removed a small amount of a high molecular weight poly(BMA) homopolymer. The molecular weight increase from its monomodal size exclusion chromatographic trace gives direct evidence of the four-armed star copolymer formation; there is no residual starting polystyrene. NMR spectroscopy shows the two components of the copolymer and confirms the presence of the central [2]rotaxane unit. Differential scanning calorimetry displays two glass-transition temperatures, which correspond closely to those of polystyrene and poly(BMA).