Comparative Study of Melt and Azeotropic Polymerization on the Structural Evolution and Properties of Quasi-Alternating Polyester Amides

The ester–amide exchange reaction in polyester amides (PEAs) presents a long-standing challenge, complicating the understanding of the relationship between structural evolution and material properties during polymerization. In this study, two polymerization strategies─melt polymerization and azeotropic polymerization─were employed to synthesize PEAs from diamide diol monomers. The evolution of molecular weight, structural regularity, crystallization behavior, and thermal stability was systematically investigated. Melt polymerization enabled rapid molecular weight growth (up to 23.2 kg/mol within 12 h) but also promoted ester–amide exchange, which disrupted structural regularity and altered crystallinity. In contrast, azeotropic polymerization maintained a low ester–amide exchange rate (<10%), preserving the microstructural integrity while achieving moderate molecular weights. Spectroscopic and thermal analyses (NMR, FTIR, XRD, and DSC) revealed that ester–amide exchange during melt polymerization reduced crystallinity, induced crystal form transitions, and impacted thermal behavior. In both systems, thermal stability improved with increasing molecular weight, with melt-polymerized PEAs exhibiting slightly higher initial decomposition temperatures (T_d5). These findings highlight the pivotal role of polymerization strategy in tailoring the structure and properties of PEAs, providing valuable insights for the design of high-performance, biodegradable materials.