We present an elegant approach for fabricating polymeric nanoparticles via the reversible noncovalent intramolecular cross-linking of single polymer chains. By protecting the well-studied 2-ureido-pyrimidinone moiety with an o-nitrobenzyl group, we have introduced a latent quadruple hydrogen bonding motif. Incorporating this group into linear poly(norbornenes) creates a class of supramolecular materials that can be readily dissolved and subsequently cross-linked upon irradiation with UV light, eliminating the need for selective solvent techniques. This is a facile method for producing well-defined particles with narrow polydispersity as confirmed by atomic force microscopy. By virtue of the nature of the intramolecular cross-linking (supramolecular interactions of pendant groups), these metastable nanoparticles are akin to folded biomacromolecules, representing the first step in linking synthetic polymers and self-folded biopolymers.