Monodisperse aromatic N-methyl benzamide-based molecules (repeat number: x = 5–10, molar mass distribution M_w/M_n = 1.02–1.04) connected with terephthalic acid at both sides (MAB_x–x) were rapidly prepared by a step-wise condensation reaction using the reported one-pot dendrimer synthetic method. These were copolymerized with NH₂-terminated poly(propylene oxide) (PPO_y) using a condensation reagent at ambient temperature, and the relationship between the structure and the properties was evaluated. The copolymers, poly(MAB_x–x-b-PPO_y), had M_n values up to 12 500 g mol⁻¹ (M_w/M_n = 1.6), with only a single T_g at −70 °C by DSC, assignable to the pure PPO domain. Only for the copolymer films consisting of a monodisperse MAB_x–x segment, did the dynamic mechanical analysis (DMA) show two T_g values at −70 and above 50 °C, indicating the clear phase segregation of the copolymers. The mechanical properties of the copolymer films were mostly dependent on the fraction of the monodisperse hard segment, and the tensile modulus and elongation at break varied from 3.3 to 32.5 MPa and 150 to 540%, respectively.