Thermal Behavior of Functionalized Polybenzoxazines: Part 2, Directive Influence of Ethynyl Group

This paper describes the investigation of the influence of the substitution position of ethynyl groups in benzoxazines on their ring-opening polymerization (ROP) and the thermal stability of the resulting polybenzoxazines. A series of benzoxazines derived from bisphenol A with ethynyl groups at various positions are synthesized and structurally characterized using Fourier transform infrared (FT-IR) spectroscopy and ¹H nuclear magnetic resonance (¹H-NMR) spectroscopy. Monitoring of the curing behavior via in situ FT-IR and differential scanning calorimetry analyses reveals the occurrence of position-dependent effects during curing. Kinetic studies of the curing process, conducted via the Kissinger and Ozawa methods, indicate that the presence of ethynyl groups not only promotes the ROP but also reduces the activation energy required for the process. Compared to conventional ethynyl-free polybenzoxazine, ethynyl-functionalized polybenzoxazines exhibit superior thermal stability, including increased glass transition temperature. In particular, the introduction of the ethynyl groups at the meta position provides the greatest enhancement of the thermal properties, reaching an increase in the char yield of approximately 21%. This position also allows reducing the curing temperature, underscoring its critical role in the development of high-performance polybenzoxazines.