Poly(ε-caprolactone) (PCL) star polymer with a cross-linked microgel core was synthesized by atom-transfer radical polymerization (ATRP) of divinylbenzene (DVB) using mono-2-bromoisobutyryl PCL ester as a macroinitiator. Then poly(ε-caprolactone)-polystyrene (PCL-PS) heteroarm star polymer was produced subsequently by grafting PS from the core of PCL star polymer in which the initiating groups were inherited from PCL star formation using ATRP. Characterization by size exclusion chromatography (SEC) and 1H NMR demonstrated the successful preparation of the target star polymers. Since the PCL was linked with the core through an ester group, the bare microgels bearing carboxylic acid or ester groups were produced by hydrolysis or alcoholysis to degrade the PCL chains of the PCL star polymers, respectively. PS star polymer with a core functionalized with carboxylic acid groups was also prepared by the hydrolysis of PCL-PS heteroarm star polymer. To illustrate the unique characteristics of this novel core-functionalized PS star polymer, it was used to prepare monodispersed lead sulfide (PbS) nanoparticles.