Controlled intramolecular collapse of linear polymer chains with crosslinkable groups is an efficient way to prepare single-chain nanoparticles in the size range of 5-20 nm. However, the nature of the crosslinking group is critical. In present study, poly(styrene-co-chloromethyl styrene) [P(St-co-CMS)] was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization and then was converted into polystyrene azide (PSAN(3)). Polystyrene containing benzoxazine side groups [P(St-co-BS)], which can be used as the precusor for the later intramolecular collapse, was obtained from PSAN3 and 3-(4-(prop-2-ynyloxy) phenyl)-3,4-dihydro-2H-benzo [e][1,3] oxazine (P-APPE) via the method of click chemistry. The sub-20 nm polymeric nanoparticles with well-defined structure via thermally intramolecular crosslinking of P(St-co-BS) were prepared. The structure change from the linear polymers to the single-chain nanoparticles was confirmed by nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), and gel permeation chromatography (GPC). The morphology and the dimension of the nanoparticles were characterized by using transmission electron microscope (TEM), atomic force microscopy (AFM), as well as dynamic light scattering (DLS). The results reveal that the size of the nanoparticles can be regulated by changing the molecular weight of the precursors and the amount of pendant benzoxazine groups by the use of controlled polymerization techniques. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5133-5141, 2011