Facile Strategy Enabling Fluorine-Free Polyimides with Ultralow Dielectric Loss and Thermal Expansion Close to Copper

Polymer dielectrics for emerging high-speed interconnects must unite a low dielectric constant (D_k) with an ultralow dissipation factor (D_f) while retaining excellent thermal and mechanical robustness. We report a fluorine-free, linear-backbone strategy that pairs an ester-type dianhydride, 1,4-phenylene bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylate) (TAHQ), with biphenyl diamines to access ester-ether copolyimides via solution polycondensation and thermal imidization. The homopolyimides from m-tolidine/TAHQ (PI-0) and 3,4-ODA/TAHQ (PI-1) establish that 3,4-ODA is intrinsically favorable for reducing high-frequency loss; building on this, we incorporate 3,4-ODA into the m-tolidine/TAHQ system to obtain copolyimides PI-X (X = mole fraction of 3,4-ODA in the diamine feed). At 10 GHz, the PI-X films achieve D_f down to 0.0013 with D_k around 3.1-3.4, giving D_f × √D_k values of 0.0024-0.0030 that are lower than those of both parent homopolyimides. The PI-X series also exhibits reduced in-plane coefficients of thermal expansion (CTE). The minimum CTE reaches 11.8 ± 2.8 ppm/°C (n = 4), while PI-0.625 shows a CTE of 17.2 ± 1.6 ppm/°C (n = 4), which closely matches that of copper (∼17 ppm/°C). This close CTE match is beneficial for mitigating interfacial stress, warpage, and reliability issues in copper-clad laminates and related devices. Without resorting to complex monomer design or multistep diamine syntheses, simple copolymerization with 3,4-ODA simultaneously suppresses high-frequency dielectric loss and lowers CTE, while preserving outstanding thermomechanical performance (T_d5% = 485-500 °C; tensile strength = 68-157 MPa). These results position TAHQ-based, fluorine-free ester-ether copolyimides incorporating 3,4-ODA and m-tolidine as practical ultralow-loss dielectrics with copper-matched CTEs for high-frequency electronic hardware.