High mechanical strength, thermal stability, and flame retardancy are three crucial criteria for highperformance polymers to be suitable for aerospace applications. Most polymers, however, cannot meet the three criteria simultaneously. Herein, phosphonium bromide-terminated poly(ether imide)s (PEI-PhPPh 3 Br) simultaneously possessing high mechanical strength, thermal stability, and flame retardancy were synthesized by functionalizing dianhydrideterminated poly(ether imide)s (PEI-DA) with triphenyl-4aminophenylphosphonium bromide. With the judiciously designed end group, PEI-PhPPh 3 Br exhibited excellent tensile properties, thermal stability, and flame retardancy. Importantly, PEI-PhPPh 3 Br with a molecular weight of 12 kDa [PEI-PhPPh 3 Br (12k)] showed a tensile strength of 109 ± 4 MPa and a Young's modulus of 2.75 ± 0.12 GPa, much higher than those of the noncharged PEI analogue. Additionally, PEI-PhPPh 3 Br (12k) showed outstanding flame retardancy, better than the state-of-the-art commercial PEIs, as evidenced by the high limiting oxygen index of 51% and high char yield of 60% at 980 °C. The study herein provides a highly effective strategy to simultaneously improve mechanical strength, thermal stability, and flame retardancy, which are three important properties rarely possessed by most polymers.

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The glass transition temperature, thermal degradation temperature, and complex viscosity of metal sulfonated polyetherimides decrease with an increase in metal cation size.