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

Abstract

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 1 H, 13 C, and 31 P 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 cationactivated 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.

Solvent-free Synthesis of Alkynyl-Based Biobased Benzoxazine Resins with Excellent Heat Resistance

“…benzoxazine monomers. 17,18 Phenol sources and amine sources are the two most important aspects. For the phenolic source, daidzein, 19 coumarin derivatives, 20,21 apigenin, 22 and resveratrol 23 have certain advantages in the preparation of high heatresistant benzoxazine resin, which is due to the rigid structure of the phenolic source and the unique cross-linking sites in the monomer.…”

“…Benzoxazine has a high advantage in structural design. Benzoxazine monomers are usually synthesized from phenol sources, amine sources, and aldehyde derivatives as raw materials. − So, different raw materials can be used to achieve a controllable synthesis of benzoxazine monomers. , Phenol sources and amine sources are the two most important aspects. For the phenolic source, daidzein, coumarin derivatives, , apigenin, and resveratrol have certain advantages in the preparation of high heat-resistant benzoxazine resin, which is due to the rigid structure of the phenolic source and the unique cross-linking sites in the monomer.…”

Design of a Low-Temperature Ring-Opening Benzoxazine System Using a Supramolecular Hydrogen-Bond Structure

“…For example, the five-membered H-bonding between the o-amide and oxazine ring as well as the six-membered H-bonding between the o-hydroxymethyl and oxazine ring were successfully incorporated, which could reduce the T r‑o by over 30 °C. Meanwhile, the strength of such intramolecular H-bonds is reported to be positively correlated with their effects on lowering the T r‑o of the oxazine ring. , Although H-bonds are easy to be introduced, benefitting from the molecular design flexibility of benzoxazine, the strength of H-bonds is usually limited and temperature dependent, leading to unsatisfactory T r‑o reduction effects. , In addition, the negative influence of the intermolecular H-bonds on the melting point generally results in unfavorable processing properties due to the formation of a hydrogen-bonding network . Therefore, a strategy with high efficiency and readily processibility to lower the T r‑o of benzoxazine is urgently needed.…”

“…Meanwhile, the strength of such intramolecular H-bonds is reported to be positively correlated with their effects on lowering the T r-o of the oxazine ring. 21,30 Although H-bonds are easy to be introduced, benefitting from the molecular design flexibility of benzoxazine, 31 the strength of H-bonds is usually limited and temperature dependent, leading to unsatisfactory T r-o reduction effects. 32,33 In addition, the negative influence of the intermolecular H-bonds on the melting point generally results in unfavorable processing properties due to the formation of a hydrogen-bonding network.…”

Novel Latent Catalyst for Ring-Opening Polymerization of 1,3-Benzoxazines Triggered by a Dual Ionic/Non-ionic Monomer Partnership

“…Polybenzoxazines have been recognized as a suitable high-performance thermosetting polymer alternative to phenolic resins. , They possess fulfilling, outstanding properties related to industrial expectations, such as heat resistance, , high char yield, − mechanical strength, , stable dielectric constants, − low humidity uptake, high glass transition temperatures, and negligible volumetric shrinkage. , The application of polybenzoxazines in various fields of the industry is incontestable and still requires further attention. Among other well-known applications, recently polybenzoxazines can find many unconventional applications in cutting-edge technologies, such as chelation agents for metal recovery, , high-performance composites, , space radiation insulating shields, porous materials for CO 2 capture, − anticorrosion coatings, − or even self-healing and shape-memory polymers. − …”