Thiophene-Rich Benzoxazines with an Amide Moiety: Integration of Structural and Hydrogen Bonding Influence on the Polymerization Mechanism by Experimental and Computational Studies

Abstract

The future development of benzoxazine thermosetting resins as advanced polymeric materials depends on designing more attractive chemical structures and reaching a deeper understanding of the structure−property relationship at the molecular level. Herein, we synthesized a new series of benzoxazines containing the thiophenecarboxamide or thiopheneacetamide functionality at the ortho-and para-positions with respect to oxygen in the oxazine ring. Successful syntheses of thiophenerich benzoxazine monomers and their corresponding molecular characterizations were confirmed by Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR), and high-resolution mass spectroscopy. Ring-opening polymerization processes of benzoxazines and thermal properties of their resulting polybenzoxazines were studied by differential scanning calorimetry, in situ FT-IR spectroscopy, and thermogravimetric analysis. In addition, a combined experimental and computational study using both 1D and 2D NMR as well as density functional theory calculations provided kinetic insights into the effects of structural and hydrogen bonding on the polymerization mechanism. With the designed varieties of hydrogen bonds in these novel benzoxazines, it has been found that oxygen in the oxazine ring involved in stronger hydrogen bonding can generate a relatively weaker O−CH 2 bond, leading to the reduced ring-opening polymerization temperature. The fundamental investigations presented in the current work provide us a deeper understanding of the hydrogen bonding interaction in benzoxazine as well as its role in polymerization behavior. Moreover, these newly obtained thiophene-rich benzoxazines also give us ample opportunities to develop high-performance polybenzoxazine thermosets with promising applications.

Oxazine Ring-Containing Polycyclic Monomers: A Class of Benzoxazine Thermosetting Resins with Intrinsically Low Curing Temperature

“…Specifically, SA-thiq exhibits the lowest polymerization temperature reported hitherto for pure benzoxazine resins without adding any catalysts and/or initiators. ,,,− Figure compares the maximum curing peak temperature of the monobenzoxazines synthesized in this study with those of recently reported low-temperature curing monobenzoxazine resins. The chemical structures of those compared resins are illustrated in Figure S14.…”

“…Comparison of T p of the monobenzoxazines synthesized in this study with those of recently reported low-temperature curing monobenzoxazine resins. ,,,− …”

“…The chemical structures of those compared resins are illustrated in Figure S14. Prior to this work, a strategy for lowering the polymerization temperature of benzoxazines has been well established by introducing intramolecular hydrogen bonding in benzoxazines with amide or hydroxyl groups. , However, this series of polycyclic benzoxazines can even show lower polymerization temperatures than those of amide- or hydroxyl-containing benzoxazine monomers in the absence of any type of acidic group. Herein the presence of highly basic tertiary amine within the polycyclic benzoxazine structure as well as the enhanced ring tension resulting from the direct linkage of benzene and aliphatic rings to the oxazine ring can collectively serve as intrinsic drivers to promote the ring-opening polymerization of the oxazine ring.…”

Hydrogen Bonding-Guided Strategies for Thermal Performance Modulation in Biobased Oxazine Ring-Substituted Benzoxazine Thermosets

“…In this context, leveraging the labile acidic proton, inherent in monomers, has received attention in benzoxazine chemistry. Thermal scission of intramolecular hydrogen bonding frees the acidic proton from −OH (phenolic, alcoholic, and carboxylic) − and amide linkage , to catalyze ROP at low temperatures in the primitive-generation benzoxazines. New synthetic advances in benzoxazine monomers bearing latent catalyst entities to facilitate polymerization have been explored in the past years. − Recently, synthetic accessibility to monomers containing functionalized oxazine rings motivated the preparation of fourth-generation, formaldehyde-free, low-temperature polymerizable PBZ networks.…”

Catalytic Role of Nanoconfinement inside MIL-125 (Ti) on the Ring-Opening Polymerization of Simple Benzoxazines

“…This unique blend of advantages positions PBzs as compelling contenders for high-performance applications. Accordingly, the recent research is predominantly focusing on synthesizing monomers from renewable resources, − designing monomers with special functionalities, − blending with other monomers or polymers, , reinforcing PBzs with a diverse range of fillers, , and synthesizing polymers with arranged hydrogen bonding. − In addition to these diverse fields in benzoxazine chemistry, the combination of benzoxazines with metal–organic frameworks (MOFs) has recently gained increased attention. MOFs originate from self-assembled metal or metal oxide nodes/clusters and organic linkers, and they are crystalline structures containing ordered nanosized pores .…”