Polypeptoids bearing carboxylic acid groups on the N-substituent are useful building blocks for the construction of peptidomimetic supramolecular assemblies with stimuli-responsive properties. Towards this end, N-(3-tert-butoxy-3-oxopropyl) glycine derived N-carboxyanhydride (^tBuO₂Pr-NCA) has been successfully synthesized and polymerized using primary amine initiators to produce the corresponding poly(N-(3-tert-butoxy-3-oxopropyl) glycine) with molecular weights (M_n) of 5.6–59 kg mol⁻¹ and a narrow molecular weight distribution (PDI = 1.003–1.026). The polymerization was shown to proceed in a controlled manner, evidenced by the good agreement of the experimental molecular weight (M_n) with theoretical values and narrow molecular weight distribution in a wide range of monomer-to-initiator ratios ([M]₀: [I]₀ = 25 : 1–200 : 1), the linear increase of M_n with conversion and the second-order polymerization kinetics. The cloaked carboxyl groups on the poly(N-(3-tert-butoxy-3-oxopropyl) glycine) can be readily unveiled in mild acidic conditions to yield the poly(N-(2-carboxyethyl) glycine), a structural mimic of poly(glutamic acid). The poly(N-(2-carboxyethyl) glycine) polymer is a weak polyelectrolyte whose hydrodynamic size in water can be controlled by the solution pH.