The preparation of star block copolymers with a permanently hydrophilic block and an “environmentally“ sensitive block, which can change its nature from hydrophilic to hydrophobic, leads to nanometer-sized responsive materials with unique properties. These polymers are able to undergo a conformational change or phase transition as a reply to an external stimulus resulting in the formation of core-shell nanoparticles which further tend to aggregate. Star-shaped copolymers with different cores were synthesized via atom transfer radical polymerisation (ATRP). The core-first method chosen as synthetic strategy allows good control over the polymer architecture. First of all the multifunctional initiators were prepared by esterification reaction of the hydroxyl groups with 2-chloropropionyl chloride. Using pentaerythritol, -cyclodextrine as core molecules, which possess a well-defined number of functional groups up to 21, allows to define the number of arms per star polymer. Polyglycerols as hyperbranched polyethers were employed to create star polymers with up to 48 arms. In order to prepare stimuli-responsive multi-arm copolymers, containing a stimuli-responsive (poly(N-isopropylacrylamide) (PNIPAAm)) and a non-responsive block (poly(N,N-dimethylacrylamide) (PDMAAm)), consecutive ATRP was carried out. The polymers were characterized intensively using NMR spectroscopy, size exclusion chromatography (SEC) and differential scanning calorimetry (DSC). Whereas the temperature-depending aggregation behavior in aqueous solution as a function of the number of arms and various block length ratios was investigated by light scattering methods.