A Facile Strategy for Intrinsic Low- D k and Low- D f Polyimides Enabled by Spirobifluorene Groups

Modified polyimides (MPIs) show great potential towards 5G communication applications, due to its excellent thermal stability, mechanical property and chemical stability as compared to most of polymers. Introducing fluoride groups or porous structure is favorable to ultra-low dielectric constant ( D k ) and dielectric loss ( D f ). However, the cost of the fluorinated MPIs is high and their synthetic processes are complicated, and porous MPIs suffer poor mechanical properties. Also, increasing the fraction of free volume is a very effective way to lower D k through introducing more ultra-low- D k air component. However, most of this kind of MPIs lag far behind the fluorinated MPIs and the porous MPIs in terms of ultra-low D f , hindering the application of MPIs in high-speed communication devices. Thus, it is highly desirable to develop intrinsic ultra-low- D k / D f MPIs at high frequency with less fluoric groups and nonporous structure. Herein, we introduce a facile and effective strategy to lower D k and D f through introducing rigid and large sterically hindered aromatic groups into MPIs. On the one hand, their large steric hindrance effect leads to low D k by increasing intrinsic free volume. On the other hand, the resulting highly stiff polymer chain and strong intermolecular interaction are favorable to reduce D f by inhibiting dipole orientations. Based on this strategy, the spirobifluorene groups are preferred. The as-prepared MPIs show excellent dielectric performance with low D k of 2.74–2.76 and low D f of 0.00599 at 10 GHz, to some extent, exceeding the multiple fluorinated MPI with D k / D f of 2.67/0.00663 at 10 GHz.