Atactic polystyrene of M = 330,000 Da and M-w/M-n = 1.04 was subjected to a complete chloromethylation. By heating the chloromethyl polystyrene with SnCl4, in a very dilute solution in ethylene dichloride, the polymeric coils were converted into intramolecularly hypercrosslinked macromolecules, called "nanosponges." These species have a molecular weight of about 370,000 Da and a diameter of about 17 nm. When in solution, the nanosponges display a tendency to reversibly self-assemble into regular clusters. Preparative size-exclusion chromatography isolates a fraction consisting predominantly of spherical clusters that are composed of 13 subunits and acquire a molecular weight of approximately 5.0 X 10(6) Da and a diameter of 45 nm. Scanning atomic force microscopy (AFM) provides images of individual nanosponges, N = 13 clusters, as well as higher spherical clusters. The regular spherical species most probably belong to the cluster series N = 1 + Sigma(10n(2) + 2), where n is the number of shells around the central nanosponge.