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The morphology of self-assembled block copolymer aggregates is highly dependent on the relative volume fraction of the hydrophobic block. Thus, a dramatic change in the volume fraction of the hydrophobic block can elicit on-demand morphological transitions. Herein, a novel hydrophobic monomer containing a photolabile nitrobenzyl (Nb) protecting group was synthesized and incorporated into a block copolymer with poly(ethylene glycol) methacrylate. This motif allows for the hydrophobic volume fraction of the amphiphilic block copolymer to be dramatically reduced in situ to induce a morphological transition upon irradiation with UV light. Two amphiphilic block copolymers, Nb 94 and Nb 176, with hydrophobic weight fractions of 80% and 86%, respectively, were synthesized and their self-assembly in water studied. Nb 94 assembled into vesicles with R h = 235 nm and underwent a morphological transition after 21 minutes of UV irradiation to spherical micelles with R h = 27 nm, determined by dynamic light scattering and confirmed by transmission electron microscopy. At intermediate irradiation times (14–20 min), Nb 94 vesicles swelled to a larger size, but underwent a morphological transition over the course of hours or days, depending on the exact irradiation time. Nb 176 assembled into large compound vesicles with a hydrodynamic radius ( R h ) of 973 nm, as determined by dynamic light scattering (DLS), which decreased to ca. 700 nm after 300 minutes of UV irradiation with no apparent morphological transition. This study elucidates the mechanism and kinetics of the morphological transitions of block copolymer assemblies induced by a change in the hydrophobic volume fraction of the polymer.