AbstractSuccessive chloromethylation‐anionic grafting sequences on polystyrene have led to well‐defined hyperbranched macromolecules. Polymers with branching functionalities f>5000 and molar masses over 107g/mol were thus prepared while maintaining a ratio of mass‐ to number‐average molar masses Mw/Mn= 1,1‐1,3. Three series of styrene polymers were prepared by varying the density of grafting sites along the chain, resulting in controllable stiffening of the molecular structure. Characterization of the branched molecules by size‐exclusion chromatography/low‐angle laser light scattering and light scattering confirmed a geometric increase in molar mass for successive generations, and a low ratio of mass‐ to number‐average molar masses. Static and dynamic light scattering experiments showed that the molecules behave like hard spheres in dilute solutions. Measurements in the semidilute range, however, showed a progressive structural stiffening effect as the branching