A facile way for the synthesis of hyperbranched poly(4-chloromethylstyrene) [P(4-CMS)] with adjustable molar mass by classic atom transfer radical polymerization (ATRP) and mechanistically similar procedures is presented. Subsequently, the chlorine functional groups have been modified to obtain polymers with different polarities. On the one hand, the polymer was end-capped with unpolar groups (e.g., methyl, phenol ether) to obtain chemically inert substances. On the other hand, more complex functional groups have been introduced through azide groups by 1,3-dipolar cycloaddition reaction ("click chemistry"). Furthermore, a method for the introduction of ester groups under mild conditions using cesium carboxylates is presented, which also allowed the preparation of so-called hyperstars by attaching COOH functionalized polystyrene chains onto the P(4-CMS) as core molecule. All these reactions were carried out in high or very high yields. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2224-2235, 2010