Summary: We report for the first time the synthesis of unimolecular polystyrene-based nanoparticles (PS-NPs) under normal air atmosphere by direct carbon-carbon “click” chemistry (i.e., “self-click” chemistry). First, statistical poly(styrene-co-4-vinylbenzyl chloride) copolymers synthesized by RAFT polymerization were decorated in a facile manner with alkyne groups by reaction with potassium propiolate, providing well-defined nanoparticle precursors (PS-Prec). Next, individual PS-Prec chains were collapsed irreversibly to unimolecular nanoparticles of ca. 12 nm in size by intrachain Cu(II)-catalyzed alkyne dimerization under high-dilution conditions, as revealed by SEC, H1-NMR, DLS and TEM measurements. Based on predictions from a recent compressible thermodynamic model, the resulting nanoparticles could be promising materials for the development of homogeneous, miscible PS-NPs / polycarbonate all-polymer nanocomposites.