We have developed a highly efficient, one-step approach for the synthesis of core cross-linked star (CCS) polymers using commercial macromonomers (polyethylene glycol methyl ether methacrylates) as the arms via RAFT-mediated emulsion polymerization in aqueous media. This approach employs a small molecular chain transfer agent (CTA), commercial macromonomer, hydrophobic cross-linker, and optional hydrophobic spacing monomer as the polymerization recipe to synthesize CCS polymers via direct one-step polymerization in aqueous buffer solution. Various polymerization parameters, including buffer concentration and molar ratio of macromonomer/cross-linker/spacing monomer relative to CTA, were investigated. CCS polymers of high yield and low dispersity were obtained within 4 h under a wide range of conditions. Analysis of polymerization kinetics and macromolecular parameters of the generated polymeric species during the polymerization process led to insights into the mechanistic aspects of the CCS formation process, which was proposed to involve three stages, i.e., polymer chain growth, cross-linking to form CCS, and CCS growth. Finally, synthesis of CCS polymers using macromonomers of different molecular weights pointed to the necessity for optimization of the polymerization conditions for each macromonomer, possibly due to different polymerization rates and steric hindrance.