Atactic polystyrene of M = 330,000 Da and M-w/M-n = 1.04 was subjected to chloromethylation. By the heating of chloromethylpolystyrene with SnCl4 in a very dilute solution in ethylene dichloride, the polymeric coils were converted into intramolecularly hypercrosslinked macromolecules, nanosponges, of molecular weight of about 370,000 Da and a diameter of about 17 nm. When in solution, the nanosponges tend to reversibly self-assemble into regular clusters. Size exclusion chromatography and sedimentation analysis corroborate the suggestion that the clusters consist of 13 spherical subunits and thus acquire a molecular weight of about 5.0 x 10(6) Da and a diameter of about 45 nm.