Catalyst-Free Low-Temperature Melt Processable Polyimide Resin with Exceptional Thermal Stability

Simultaneously achieving low-temperature processability and ultrahigh thermal stability in polyimides (PIs) remains a critical challenge. Herein, a catalyst-free molecular design strategy is proposed by combining different dianhydrides (EBPA, α-BPDA, and 6FDA) with diamines (BAFL, TFDB, and mPDA). Among them, the combination of internal alkyne dianhydride (EBPA) and terminal ethynyl end-capper (4-PEPA) enables intrachain-terminal cooperative cross-linking, and the resulting PI exhibits the lowest curing temperature and the highest glass transition temperature (Tg). The optimized PI resin achieves a Tg above 490 °C and Td5% exceeding 550 °C at a curing temperature of only 350 °C. Further incorporation of disulfide units generates sulfur radicals upon heating, establishing a self-accelerating co-cross-linking network. The optimized PI exhibits a record-low curing temperature of 310 °C while maintaining a Tg of ∼420 °C and Td5% above 550 °C under a nitrogen atmosphere. DFT calculations confirm the role of alkyne-disulfide synergy in promoting early radical activation and network formation. The resin shows a compressive strength of 194 MPa at room temperature and retains a value of 92 MPa after 15 min of exposure at 300 °C, providing a promising strategy to low-temperature processable yet thermally robust PI resins for aerospace and advanced thermal protection sectors.