<p>Modified polyimides (MPIs) possess excellent thermal stability, chemical stability, and mechanical properties, and are considered to be a kind of dielectric material for high-frequency communication. Enhancing the rigidity of the polymer chains and intermolecular interactions can ensure low <i>D</i>_k/<i>D</i>_f at high frequency, which is attributed to the effective restriction of dipole orientations. However, it is difficult to achieve tight chain packing in an overly rigid polymer chain, whereas an overly flexible polymer chain might be insufficient to restrain small-scale molecular motions below <i>T</i>_g. To balance the trade-off between the rigidity of the polymer chains and tight chain packing, MPI was developed with a rigid-soft structure based on a naphthalene-alkyl-based diamine. On the one hand, incorporating the soft unit can enhance the movability of polymer chains to achieve dense chain packing for polyimides (PIs). On the other hand, the presence of rigid aromatic units can enhance intermolecular interactions and further restrict the motion of polar imide groups below <i>T</i>_g. As a result, the resultant MPI can prevent small-scale molecular motion below <i>T</i>_g. In contrast to the reference PI-TFMB-6FDA, <i>D</i>_k/<i>D</i>_f is significantly reduced from 2.72/0.0075 to 2.73/0.005 at a high frequency of 10 GHz. Furthermore, the rigid-soft structure endows PIs with good thermoplasticity owing to the good chain flexibility above <i>T</i>_g. In addition, PIs based on rigid-soft structures can preserve favorable thermal stability.</p>