The synthesis of gold nanoparticles (2-4 nm) carrying a single functional polystyrene chain and its characterization by gel permeation chromatography are reported. This has been achieved by a new type of macromolecular azo-initiator based on telechelic polystyrene with containing alpha,omega-methylcoumarin endgroups and an azo group in the middle of the polystyrene. The structure and the near-quantitative functionality of the initiator have been verified by performing NMR, GPC, and UV-vis measurements. This macroinitiator has been used to initiate a surface polymerization of 4-vinylthiophenol molecules immobilized on the surface of gold nanoparticles. As a product, gold nanoparticles carrying exactly one polystyrene chain have been synthesized with functionalization degrees of up to 90% (crude yield). Proof and quantification of the functionalization degree have been demonstrated by application of a GPC setup with a diode array detector for online UV-vis spectra.