Gold nanoparticles of improved stability against aggregation were prepared using poly(ethylene oxide)-block-poly(epsilon-caprolactone) (PEO-b-PCL) star-block copolymers. A five-arm star-shaped macroinitiator (PEO) was utilized for the automated parallel controlled ring-opening polymerization of epsilon-caprolactone to prepare a series of PEO-b-PCL star-block copolymers with a constant PEO core linked to PCL blocks of variable length. The PEO core was swelled with KAuCl4 in N,N-dimethylformamide (DMF), and gold nanoparticles were subsequently obtained by reduction with NaBH4. Since the process was always templated by the same PEO core for all investigated polymers, the average dimension of the formed gold nanoparticles was in the same range for all star-block copolymers. In sharp contrast, the size distribution and long-term stability against aggregation of the gold nanoparticles dispersed in DMF were strongly dependent on the PCL block length, confirming the role of PCL blocks as stabilizing blocks for these nanoparticles.

We report on a simple procedure to tune the hydrophilicity of hybrid gold nanoparticles. The nanoparticles have been prepared in the core of a poly(ethylene glycol)-block-poly(epsilon-caprolactone) (PEG-b-PCL) five-arm star block copolymer. A hydrophilic corona was then added to these hybrid gold nanoparticles by direct chemisorption of trithiocarbonate-containing poly(acrylic acid) chains. These polymers were synthesized by RAFT polymerization with a trithiocarbonate as the chain-transfer agent. The efficiency of the grafting was evidenced by TEM, AFM, and DLS and by the successful transfer of these nanoparticles from organic solvent to water.

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(e-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 NaBH 4 . 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%. Experimental Chemicals and reagentsAll reagents were used without further purification unless stated otherwise. Solvents were purchased from Biosolve Ltd.