Balancing Processability and Performance: Benzoxazole Thermosets with Ultra-Low Dielectric Constants and High Thermal Stability

Polybenzoxazoles are promising high-performance materials for thermally stable dielectric components, microelectronic insulating layers, and aerospace-related applications owing to their exceptional thermal stability and mechanical properties; however, their poor solubility, high processing temperatures, and limited processability still restrict practical fabrication. This study presents the design and synthesis of two series of thermosetting benzoxazole monomers to address these limitations. These monomers incorporate cross-linkable arylethynyl and arylonitrile terminal groups, combined with either symmetric hexafluoroisopropylidene-bridged or asymmetric mono-benzoxazole architectures. The structure–property relationships governing solubility, curing behaviour, thermal stability, and dielectric properties are systematically investigated. The results show that incorporating hexafluoroisopropylidene units significantly enhances solubility and reduces dielectric constants, whereas nitrile-terminated systems exhibit superior thermal stability compared with their alkyne-terminated counterparts. Notably, the optimized asymmetric polybenzoxazole achieved a temperature at 5% mass loss of 602.2 °C, while the optimized symmetric polybenzoxazole exhibited an ultra-low dielectric constant of 1.83 at a frequency of 1 MHz. This work demonstrates a viable molecular design strategy for balancing solution processability, thermal stability, and dielectric performance in advanced polybenzoxazole thermosets.