Core cross-linked star (CCS) clusters consisting of linear poly(methyl methacrylate) (PMMA) segments surrounding multiple, dispersed poly(divinylbenzene) (PDVB) cores have been synthesized by conventional free-radical polymerization via a "core-first" method. This unique macromolecular structure is akin to a cluster of several CCS polymers (a star polymer with a single, densely cross-linked core) that are covalently bonded together by linear core-to-core PMMA segments and retain excellent solubility even at very high molecular weights. This study provides evidence to the proposed "core-first" mechanism and subsequent CCS cluster formation by comparison to linear PMMA and model CCS polymers synthesized by atom transfer radical polymerization (ATRP). Characterization by UV-vis spectrophotometry, proton nuclear magnetic resonance (H-1 NMR), dynamic light scattering (DLS), and thermal analysis verified the formation of PMMA-PDVB CCS clusters and elucidated their macromolecular structure. Molecular weights of between 5000 and 20 000 kDa were achievable by selection of the appropriate formulation. Light scattering studies showed that CCS clusters have a hydrodynamic radius of between 20 and 50 nm, and this size is tunable on the basis of solvent choice.