A novel heterocyclic diamine monomer based on pyrimidine ring, 2,4-bis(4-aminophenoxy)pyrimidine, was successfully synthesized. The obtained heterocyclic diamine was fully characterized and employed to synthesize a series of polyimides (PIs) by polycondensation with commercially available aromatic dianhydrides via the conventional two-step method. The effects of the pyrimidine rings into the main chain of the PIs were evaluated through the study of their solubility, thermal, mechanical, and optical properties. The inherent viscosities of the resulting poly(amic acid)s were in the range of 0.58–1.45 dL g−1. Meanwhile, the PIs had good thermal stability with the 5% weight loss temperatures of 445–475°C under nitrogen atmosphere as well as outstanding mechanical properties with tensile strength of 93–105 MPa and elongation at break of 3.6–4.2%. The cutoff wavelength of the PI films were in the range of 348–413 nm.
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