Employing enediynes as crosslinking precursors, a novel yet efficient strategy, namely photo-triggered Bergman cyclization, was integrated with intramolecular chain collapse to yield polymeric nanoparticles with the size regime below 20 nm. Enediyne motif was designed delicately to possess a high photo-reactivity, with the double bond locked in a methyl benzoate ring while triple bonds substituted with phenyls. Single electron transfer-living radical polymerization was conducted to provide linear acrylate copolymers with controlled molecular weights and narrow polydispersities. Poly(butylarylate-co-5) went through UV-irradiation with a concurrent Bergman cyclization, resulting in well-defined ultrafine polymeric nanoparticles. Results from NMR, Raman scattering, photoluminescence and UV-vis spectra corroborated the presence of conjugative structures in the polymeric nanoparticles, indicating the occurrence of photo-induced Bergman cyclization. A series of other acrylate-based nanoparticles were investigated to confirm the applicability of such a unique strategy in thermal sensitive but UV-stable polymeric structures, making photo-Bergman cyclization a promising tool towards polymeric nanoparticles. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5330-5338, 2011