Molecular-Level Control of Thermal and Morphological Transitions in Semi-Aromatic Polyamides by Cu(I)-Catalyzed Azide–Alkyne Click Polymerization

Developing solution or melt-processable aromatic polyamides while preserving their step-out thermal and physical properties, afforded by aromatic groups and intermolecular interactions, embodies a key step toward their broader deployment. However, the relationship between chemical structure and processability reflects a delicate balance between intermolecular interactions (H-bonding and π–π interactions), steric constraints, and chain conformations. Herein, we employ Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click polymerization of a series of aromatic dialkyne monomers with diazide-functionalized aromatic diamides. This approach afforded high-molecular-weight, solution or melt-processable, semi-aromatic polyamides. The aforementioned balance is tuned using the following sequence of backbone functionalities between aromatic groups in the dialkyne: none (biphenyl), methylene (−CH₂−), carbonyl (−C(O)−), isopropyl (−C(CH₃)₂−), and sulfonyl (−SO₂−). Temperature-dependent X-ray scattering and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements were used to elucidate the connection between H-bonding and phase behavior in these semi-aromatic polymers. It was found that varying the linking groups between the aromatic rings led to a progression from semicrystalline (biphenyl) to mesomorphic (−CH₂–, −C(O)–, −C(CH₃)₂−) and finally to amorphous (−SO₂−) morphologies. For first-generation polymers, denoted by the letter “a” (P1a–P5a), dialkynes bearing a −C(CH₃)₂– spacer segment led to a polymer with accessible melting transition (T_m < 300 °C), while a −CH₂– spacer segment led to a more ordered morphology and an inaccessible melting transition (T_m > 300 °C). Introducing two geminal dimethyl groups into the diazide monomer disrupted the chain packing, leading to amorphous polymers in all but the biphenyl derivative, and improved the thermal stability while the mechanical properties are commensurate with those of engineering thermoplastics. Consequently, the resulting second generation polymers, denoted by the letter “b” (P1b–P5b), were melt processable. We anticipate that this work signifies an advancement to predict the connection between molecular design to the desired thermal properties and morphology of semi-aromatic polyamides.