Phosphaphenanthrene-Functionalized Benzoxazines Bearing Intramolecularly Hydrogen-Bonded Phenolic Hydroxyl: Synthesis, Structural Characterization, Polymerization Mechanism, and Property Investigation

Hydrogen bonding in thermosetting resins can have a significant influence on the polymerization processes and the properties of corresponding thermosets, but its role in the polymerization of benzoxazine resins remains unclear. Here, we synthesized two novel phosphaphenanthrene-functionalized benzoxazine monomers from 2-6-oxido-6H-dibenz-[c,e][1,2]oxaphosphorin-1,4-dihydroxy phenylene, aniline/furfurylamine, and paraformaldehyde and investigated the structures by 1H, 13C, and 31P NMR, Fourier transform infrared (FT-IR), elemental analysis, and high-resolution mass spectrometry. Ring-opening polymerizations of both monomers were then studied by differential scanning calorimetry (DSC) and in situ FT-IR spectroscopy, and the resulting thermosets exhibited high thermal stability and low flammability. Density functional theory (DFT) calculations suggested that intramolecular hydrogen bonds are preferably formed between the phenolic −OH and the P═O in the phosphaphenanthrene functionality for both monomers, which is in line with the experimental results. We proposed a cation-activated ring-opening polymerization mechanism where intramolecular hydrogen bonding plays a pivotal role. The combination of experimental and computational effort provides molecular-level insights into intramolecular hydrogen bonding and its role in polymerization mechanisms in benzoxazine chemistry as well as a new angle for the design of high-performance thermoset polymers.