Palladium nanoparticles of improved stability against aggregation were prepared using 5-arm star-shaped block copolymers with a poly(ethylene oxide) (PEO) core and a poly(-caprolactone) (PCL) corona as templates. The PEO core of these star-shaped block copolymers could be swollen with palladium acetate (Pd(OAc)2) in N,N-dimethylformamide (DMF), and Pd nanoparticles were obtained by reduction with NaBH4. This procedure resulted in well defined Pd nanoparticles that were stabilized against aggregation due to a barrier of PCL chains. Transmission electron microscopy (TEM) experiments strongly suggested that one Pd nanoparticle was formed inside each star-shaped block copolymer. The stability of the Pd nanoparticles with respect to aggregation was strongly dependent on the length of the PCL chains. Subsequently, these nanoparticles were evaluated for their capability to act as catalysts in Heck coupling reactions. These reactions were performed in an automated synthesizer and monitored by GC–MS revealing that these particles are highly active for the catalytic reactions mentioned. Up to 99% conversion was observed for the coupling of styrene with 4-bromoacetophenone within 24 h at a catalyst loading of 0.1 mol%.