An amphiphilic block copolymer, poly(methylacrylate)82-block-poly(N-(acryloyloxy)succinimide0.29-co-(N-acryloylmorpholine)0.71)155 (PMA82-b-P(NAS0.29-co-NAM0.71)155), was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization and then was supramolecularly assembled into micelles in aqueous solution, followed by chemical crosslinking throughout the shell region upon the introduction of 2,2′-(ethylenedioxy)-bis(ethylamine) as a crosslinker to afford well-defined shell crosslinked nanoparticles (SCKs). The number-averaged hydrodynamic diameters of the micelles and SCKs were (17 ± 4) nm and (16 ± 3) nm, respectively, by dynamic light scattering (DLS), and (15 ± 2) nm and (13 ± 2) nm, respectively, by transmission electron microscopy (TEM). In an attempt to narrow the particle size distributions, the dodecyl trithiocarbonate chain end of the block copolymer was replaced by a 2-cyanoisopropyl moiety. Each nanoparticle system was characterized by DLS, electrophoretic light scattering (ELS), TEM, and small-angle X-ray scattering (SAXS). SAXS was of particular importance, as it provided definitive observation and quantification of shell contraction and densification upon shell crosslinking. The direct incorporation of NAS into the block copolymers during their preparation allowed for unique crosslinking chemistry to proceed with added diamino crosslinkers. The primary advantages of this system include the ability to conduct in situ synthesis of SCKs that are crosslinked directly and derivatized easily by adding nucleophilic ligands before, during, or after the crosslinking.