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Abstract: The preparation of polymer nanoparticles with a uniform size and shape, beyond spheres, is an unresolved problem. Here we report a living aqueous catalytic polymerization, resulting in particles grown by a single active site and composed of a single ultra high molecular weight polyethylene (UHMWPE) chain. The control on a molecular level (M_w/M_n = 1.1–1.2) and at the same time on a particle level (PDI < 0.05) together with the immediate deposition of the growing chain on the growing nanocrystal results in a distinct evolution of the particle morphology over time. These uniform nanocrystals are obtained as concentrated aqueous dispersions of > 10 wt-% (N ≈ 10¹⁹ particles L⁻¹) polymer content. Key to this robust procedure to single chain nanoparticles are long-lived water-stable Ni(II) catalysts that do not undergo any chain transfer. These findings are a relevant step towards polymer materials based on nanoparticle assembly.