The emulsion polymerization of styrene in the presence of hydrophilic poly(methacrylic acid-co-poly(ethylene oxide) methyl ether methacrylate), P(MAA-co-PEOMA), macromolecular RAFT (reversible addition-fragmentation chain transfer) agents possessing a trithiocarbonate reactive group and 19 ethylene oxide subunits in the grafts was performed to create in situ P(MAA-co-PEOMA)-b-polystyrene amphiphilic block copolymer self-assemblies. The system was studied using the following conditions: a pH of 5, two different compositions of the MAA/PEOMA units (50/50 and 67/33, mol/mol), different molar masses of the macroRAFT agents, and various concentrations of the latter targeting different molar masses for the polystyrene block. This work completes a previous one performed at pH 3.5, under otherwise similar experimental conditions, for which only spherical particles were obtained [Zhang et al. Macromolecules 2011, 44, 7584]. For both MAA/PEOMA compositions, the system led to different nano-object morphologies such as spherical micelles, nanofibers, and vesicles, depending directly on the molar masses of the hydrophilic and hydrophobic blocks. A pH of 5 was shown to be the best compromise to achieve nonspherical particles while keeping a good control over the chain growth.