This study reported the control of molecular weights of functional polymer chains attached to the surfaces of the narrowly distributed environmentally responsive particles obtained from a single step process. As a proof-of-concept, a pH-responsive poly(l-glutamic acid) (PLGA) macromonomer with a degree of polymerization (n) of 50 or 260 was prepared by the ring-opening polymerization of γ-benzyl-l-glutamate N-carboxy anhydride; this was performed using 4-vinylbenzylamine and was followed by hydrolysis. The pH-responsive PLGA macromonomer was further used as a stabilizer for the dispersion polymerization of styrene. Each macromonomer stabilized the dispersion polymerization, affording narrowly distributed polystyrene particles exhibiting pH-responsive aggregation or dispersion behavior, thereby indicating the graft of PLGA chains on the particle surface. Large particles with a low surface density were obtained using PLGA260. The particle sizes and surface glutamic acid densities were tunable over 0.8–2.0 μm and 0.4–1.8 unit/nm2, respectively, while maintaining narrow size distributions.