Well-defined core–shell nanoparticles with a phthalimide core are synthesized using reversible addition–fragmentation chain transfer (RAFT)-mediated dispersion polymerization-induced self-assembly (PISA) of a novel vinyl benzyl phthalimide monomer, namely (2-(4-vinylbenzyl)iso-indoline-1,3-dione) (VBzPHT). Diblock copolymers consisting of poly(poly(ethylene glycol) methyl ether methacrylate)-block-(2-(4-vinylbenzyl)iso-indoline-1,3-dione) (PEGMA₁₅-b-PVBzPHT_n) self-assemble into nanoparticles during polymerization in methanol at 70 °C, when using poly(methyl ether methacrylate)-macro RAFT agent (macro-CTA, DP = 15, M_n = 7600 g mol⁻¹, Đ = 1.14) and 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator. While maintaining a constant chain length of the solvophilic macro-CTA agent, we show that varying the solvophobic PVBzPHT block length (DP = 15–200) under concentrated conditions (15 wt%) achieves self-assembled structures of increasing size. These nanoparticles, ranging from 95 to 389 nm in hydrodynamic diameter, are assessed using both dynamic light scattering and dry state transmission electron microscopy. Finally, we investigate hydrazine-responsive deprotection of phthalimide bearing amphiphilic (PEGMA₁₅-b-PVBzPHT_n) block copolymers, leading to the formation of poly(poly(ethylene glycol) methyl ether methacrylate)-block-vinyl benzyl amine (PEGMA₁₅-b-(PVBzNH₂)_n). This increased solvophilicity leads to complex aggregated assemblies in situ. The insights of this study offer guidelines to the preparation of well-defined nanoparticles through PISA, with unique post-synthetic responsiveness amenable to applications in drug delivery and biocatalysis.