The development of single-chain polymeric nanoparticles (SCNP) has been of great scientific interest in recent years. Recently, we have developed a robust system to form SCNP at high polymer concentration (ca. 100 mg mL(-1)) via organocatalyzed ring-opening polymerization (ROP). In this approach, linear polymer precursors functionalized with pendent polymerizable caprolactone moieties undergo self-cross-linking in the presence of organocatalyst and alcohol initiator. Following on from our previous communication, we report in here a more in-depth fundamental investigation to better understand our system. For this, we have synthesized various linear random copolymer precursors (i.e., poly(oligo(ethylene glycol) acrylate) (P1), polystyrene (P2), and poly(methyl acrylate) (P3)) by reversible addition fragmentation chain transfer (RAFT) polymerization, and their abilities to form SCNP at high polymer concentration were evaluated. It was found that only P1, which contains oligo(ethylene glycol) side chains, was able to successfully form SCNP while the other linear precursors resulted in multichain aggregates, indicating the importance of side-chain brushes in aiding SCNP formation at high polymer concentration. Furthermore, we tested several multifunctional alcohol initiators (mono-, di-, and tetrahydroxy) and found that the initiator structure has no effect on the SCNP formation process. In addition, we investigated the effect of initiator concentration and observed that the particle size can be reduced (from 7.6 to 6.6 nm) when the initiator and linear precursor are in equimolar concentration. It is anticipated that the information derived from this study may lead to the development of new SCNP for targeted (bio)applications.