Substituent-induced torsional rigidity and isomeric ester geometry dictating the ultra-low dielectric constant and dissipation factor in polyimides

ABSTRACT Breaking the intrinsic trade-off between dielectric constant ( D k ) and dielectric loss ( D f ) in polymer dielectrics remains a central challenge in high-frequency material design. In this study, a molecular design framework is established to decouple D k and D f in poly(ester imide) (PEsI) systems by regulating torsional dynamics and dipole architecture simultaneously. Ortho-methyl substitution is introduced to impose torsional constraints along the imide backbone, while ester-isomer engineering enables precise modulation of dipole distribution and intermolecular packing through connectivity control. Combined experimental and computational analyses reveal that the dielectric response is governed not solely by fractional free volume but by the coupled interplay of packing efficiency, dipole orientation, and constrained chain dynamics. T–2MP exhibits a low D k of 2.52 at 10 GHz, indicating an optimal balance between steric expansion and chain alignment. Further, copolymerization enables the independent tuning of dielectric parameters, yielding T–2MP/B (1:1) with an ultra-low D f of 0.0022 and a D k of 2.84. This behavior originates from the favorable ester geometry of BPTP, which promotes dense packing, reduces the dipole moment, and suppresses electronic polarization. All PEsI copolymers maintain high thermal stability ( T d5% > 474 °C) and low thermal expansion (CTE < 14.4 ppm K –1 ). This work provides a generalizable molecular design strategy for overcoming the D k – D f trade-off in polymer dielectrics, offering a pathway toward high-performance materials for next-generation high-frequency electronics and advanced packaging.