Biomimetic ABC triblock copolymers of poly-[2-(methacryloyloxy)ethyl phosphorylcholine]-b-poly[2-(dimethylamino)ethyl methacrylate]-b-poly(2-hydroxypropyl methacrylate) (PMPC-b-PDMA-b-PHPMA) were synthesized by RAFT aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) in the presence of a PMPC-b-PDMA macromolecular chain transfer agent (macro-CTA). This ABC triblock copolymer deploys well-known biocompatible PMPC and PDMA for the coordination of Ag+ ions to form silver nanoparticles in situ on reduction, and PHPMA for assembling (core) in water. The synthesis of PMPC-b-PDMA-b-PHPMA starts when both the reactive steric stabilizer of PMPC25-b-PDMA(4) macro-CTA and HPMA monomer are dissolved in water. The growing PHPMA is not soluble in water and begins to assemble based on three-layer onion micelles, in which the outer and inner shells are PMPC and PDMA, respectively. In the synthesis of PMPC25-b-PDMA(4)-b-PHPMA(z) at a constant 25% (w/w) solids concentration, the resultant assemblies change from spheres to worms to jellyfishes to vesicles when the targeted PHPMA chain length increases from 100mer to 400mer at full monomer conversion. Furthermore, in the synthesis of identical PMPC25-b-PDMA(4)-b-PHPMA(400) copolymers, the assembly morphology can be controlled from vesicles to spheres through worms by varying the solids concentration in the polymerization mixture, decreasing from 25% (w/w) to 15% (w/w) at full monomer conversion. Thus, the final morphology can be tuned by the degree of polymerization of HPMA and the solids concentration in the polymerization mixture. Using the resultant three PMPC25-b-PDMA(4)-b-PHPMA(400) assemblies as scaffolds, Ag(0) nanoparticles (Ag-NPs) are obtained through in situ reduction of AgNO3 facilitated by electrostatic interactions between the Ag+ ions and PDMA moieties. The resultant Ag-NPs loaded in the assemblies exhibit excellent stability, dispersibility, and activity of catalyst for the reduction of p-nitrophenol. The order of rate constants for the reduction using Ag-NPs loaded in the assemblies is worms > vesicles > spheres, which corresponds to the order of the surface areas of the assemblies of PMPC25-b-PDMA(4)-b-PHPMA(400). These results can be achieved thanks to the kinetically frozen PMPC25-b-PDMA(4)-b-PHPMA(400) assemblies with identical compositions.