Linear polystyrene of M(w) = 330 000 and M(w)/M(n) = 1.04 in which 50% and 100% of the phenyl residues were chloromethylated were heated in a 0.5 g/L ethylene dichloride solution in the presence of SnCl4 to convert them into an intramolecularly hyper-cross-linked soluble material. The spherical macromolecular species obtained exhibit low density and high surface area (up to 1000 m(2)/g) and swell strongly in all nonsolvents for linear polystyrene. Solutions in good polystyrene solvents exhibit low viscosity and reduced partial specific volume, but exceedingly high diffusion and sedimentation constants. According to size exclusion chromatography, dynamic light scattering, low angle X-ray scattering and electron microscopy, the diameter of the hyper-cross-linked species is estimated as about 12 nm in the dry state and about 17 nm in solution. In tetrahydrofuran, they associate reversibly, probably forming clusters consisting of 13 regularly packed particles. Because of the partial association, molecular weight measurements (using elastic light scattering and ultracentrifugation techniques) produce values slightly exceeding the 350 000-370 000 expected for the isolated particle.