This thesis focuses on understanding the factors that affect the properties of polyetherimide (PEI) and improving the properties. As a high-performance thermoplastic resin, the first challenge in PEI application is its high processing temperature and viscosity. Therefore, two supramolecular strategies were applied to not only solve the problem of high processing temperature or viscosity but also enhance the mechanical and flame retardancy. In addition, the yellow to amber color of PEIs limits its applications in high-tech fields such as microelectronics and optoelectronics. Thus, a fundamental study of how end group and molecular weight affect the optical properties of PEIs provides a better knowledge of the mechanism and an effective strategy for designing PEIs. To lower the processing viscosity while maintaining or even improving the mechanical properties of PEI, the first strategy was to synthesize PEI oligomers, and incorporate self-complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) units at the chain ends. Surprisingly, the UPy imparted PEI with a Mn as low as 8 kDa (8k-PEI) with great film formability. Excitingly, 8k-PEI-UPy exhibited an outstanding Young's modulus higher than those of state-of-the-art high-molecular-weight (high-MW) commercial PEIs. Therefore, the incorporation of UPy was proved to be an effective method to synthesize low-molecular-weight, high-mechanical-strength PEIs. Although low-molecular-weight PEI-UPy had high mechanical properties, its limited thermal stability and potentially low flame retardancy, however, restricted its applications in areas such as aerospace and aircrafts. Hence in another strategy, which utilize the phosphonium ionic groups were incorporated into PEI oligomers targeting at achieving high thermal stability, flame retardancy, and mechanical properties simultaneously. Functionalization of dianhydride-terminated PEI by tetraphenylphosphonium bromide afforded the synthesis of phosphonium bromide terminated PEI (PEI-PhPPh3Br), which simultaneously exhibited excellent thermal stability up to ~400°C, outstanding flame retardancy evidenced by high char yield and extremely high limiting oxygen index, and a very high mechanical strength. The study thus provides an efficient strategy to simultaneously enhance the thermal and mechanical properties as well as flame retardancy. Furthermore, the low-molecular-weight PEI-PhPPh3Br had good processability due to its strong shear thinning. In addition to the thermal and mechanical properties and flame retardancy, the end groups affect the optical properties, especially the yellowness, of PEIs. Understanding how end group and molecular weight affect the yellowness, of PEIs is critical for their applications in fields including optoelectronics and microelectronics. Thereby, PEIs with different Mn and various end groups including electron-withdrawing and electron-donating were prepared and characterized. Electron-withdrawing end groups reduced the yellowness and increased the transparency of PEI, regardless of the Mn. Electron-donating end groups increased the yellowness of PEIs with dependence on the Mn. The Mn affected the yellowness of PEIs by changing end group density and the probability of charge-transfer complex formation. The systematic study reveals the correlations among yellowness, end group, and molecular weight of PEIs. / MS / One small step for end groups, one giant leap for properties. Simply tuning the repeating units at the polymer chain ends drastically changes the properties of the polymers. This thesis focuses on the modification of the end groups in low-molecular-weight polyetherimides, a class of high-temperature high-performance engineering thermoplastics, to achieve improved and tunable properties, such as mechanical strength, flame retardancy, and optical properties. On one hand, low-molecular-weight polyetherimides enabled low processing temperatures to decrease the processing cost. On the other hand, the incorporation of noncovalent hydrogen bonding interactions improved the mechanical strength of low-molecular-weight polyetherimides and maintained their thermal stability. This study for the first time showed the incorporation of multiple hydrogen bonds was effective to generate low-molecular weight but high-mechanical-strength polyetherimides. Although multiple hydrogen bonds improved the mechanical properties of polyetherimides, the thermal stability was inadequate for industrial melt processing at elevated temperatures. Alternatively, by incorporating noncovalent electrostatic interaction groups, the polyetherimides showed not only improved mechanical properties but also high thermal stability. Excitingly, their flame retardancy and melt processability were also significantly improved. This polyetherimide has great potential for applications such as aircrafts and aerospace. The end groups affected not only the thermal, mechanical, and rheological properties, but also the optical properties of polyetherimide. Polyetherimide has an intrinsic yellow color originated from the charge transfer complexes that are formed between electron-rich and electron-deficient moieties in the polymer chains. By tuning the concentrations of the different end groups, we controlled the strength of the charge transfer complexes and thus the yellowness of the films. Through a systematic study, a 3D contour was constructed and revealed the relations among the yellowness, the end group, and the molecular weight of polyetherimides. The 3D contour provides guidelines for designing polyetherimides with suitable molecular weights and adequately low yellowness.

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