Weldable, Reprocessable, and Water-resistant Polybenzoxazine Vitrimer Crosslinked by Dynamic Imine Bonds

Traditional polybenzoxazine thermosets cannot be reprocessed or recycled due to the permanent crosslinked networks. The dynamic exchangeable characteristics of imine bonds can impart the networks with reprocessabilities and recyclabilities. This study reported a weldable, reprocessable, and water-resistant polybenzoxazine vitrimer (C-ABZ) crosslinked by dynamic imine bonds. It was synthesized through a condensation reaction between an aldehyde-containing benzoxazine oligomer (O-ABZ) and 1,12-dodecanediamine. The resulting C-ABZ was able to be welded and reprocessed due to the dynamic exchange of imine bonds. The tensile strengths of the welded C-ABZ and the reprocessed C-ABZ after three cycles of hot-pressing were 76.7, 81.3, 70.8, and 58.1 Mpa, with corresponding tensile strength recovery ratios of 74.1 %, 78.6 %, 68.4 %, and 56.1 %, respectively. Furthermore, the polybenzoxazine backbone significantly improved the water resistance of the imine bonds. After immersing in water for 30 days at room temperature, the weight gain of C-ABZ was less than 1 % with corresponding tensile strength and tensile strength retention ratio of 59.5 Mpa and 57.5 %, respectively. Although the heat resistance of C-ABZ decreased slightly with increased hot-pressing cycles, a glass transition temperature (Tg, tanδ) of 150 °C was retained after the third hot-pressing. Overall, these findings demonstrate that the C-ABZ possesses excellent comprehensive performances.

Deciphering the Self-Catalytic Mechanisms of Polymerization and Transesterification in Polybenzoxazine Vitrimers

The use of internal catalysts has emerged as a pivotal design principle to facilitate dynamic exchanges within covalent adaptable networks (CANs). Polybenzoxazines, specifically, have shown considerable potential in generating vitrimers through thermally induced transesterification reactions catalyzed internally by tertiary amines. This study aims to investigate the chemical complexities of transesterification reactions within benzoxazine vitrimers. To achieve this, model molecules using various phenolic acids and amino-alcohol derivatives were synthesized as precursors. The structure of these model molecules was fully elucidated by using nuclear magnetic resonance (NMR). Differential scanning calorimetry (DSC) and rheology experiments evidenced the accelerated network formation of the precursors due to the presence of aliphatic -OH groups. Thermogravimetric analysis coupled with microcomputed gas chromatography (TGA-μGC) was used to provide evidence of transesterification reactions. The results showed that the spatial proximity between tertiary amine and hydroxyl groups significantly enhances the rate exchange, attributed to a neighboring group participation (NGP) effect. Interestingly, kinetic experiments using complementary NMR techniques revealed the thermal latency of the tertiary amine of benzoxazine toward transesterification reactions as its opening is needed to trigger the dynamic exchange. The study highlights the crucial role of steric hindrance and tertiary amine basicity in promoting the dynamic exchange in an internally catalyzed system.

Recent advances in the development of green furan ring-containing polymeric materials based on renewable plant biomass

Fossil resources are rapidly depleting, forcing researchers in various fields of chemistry and materials science to switch to the use of renewable sources and the development of corresponding technologies. In this regard, the field of sustainable materials science is experiencing an extraordinary surge of interest in recent times due to the significant advances made in the development of new polymers with desired and controllable properties. This review summarizes important scientific reports in recent times dedicated to the synthesis, construction and computational studies of novel sustainable polymeric materials containing unchanged (pseudo)aromatic furan cores in their structure. Linear polymers for thermoplastics, branched polymers for thermosets and other crosslinked materials are emerging materials to highlight. Various polymer blends and composites based on sustainable polyfurans are also considered as pathways to achieve high-value-added products.

Crystallizable Aliphatic Chains Enhanced Covalent Adaptable Networks: Fast Reprocessing and Improved Performance

Covalent adaptable networks (CANs) exhibit recyclability such as reprocessing, but it's a challenge to address the contradiction between reprocessing rate and performance. Here we innovatively introduce pendent aliphatic chain anhydride monoesters into epoxy CANs based on transesterification, which efficiently accelerates the reprocessing without sacrificing thermal and mechanical properties. The transesterification rate is raised on account of the flexible aliphatic chain-promoted segment movement and dynamic transfer auto-catalysis. When the carbon number reflecting the length of the pendent chain is 12, the epoxy CAN exhibits the fastest stress relaxation or reprocessing. Computation via molecular dynamics simulation demonstrates that the increased segmental mobility from the pendent aliphatic chains contributes to the enhanced reprocessability. Interestingly, the crystallization of the pendent aliphatic chains maintains or even improves the thermal and mechanical properties. Thus, introducing flexible and crystallizable aliphatic side chain is an innovative and efficient approach to accelerate dynamic reactions and network arrangement while improving performance. This article is protected by copyright. All rights reserved.

Reprocessable Polybenzoxazine Thermosets with High Tgs and Mechanical Strength Retentions Using Boronic Ester Bonds as Crosslinkages

In order to obtain reprocessable polybenzoxazine thermosets with high heat resistance and mechanical strength retentions, network structures without irreversible parts were constructed via crosslinking benzoxazine oligomers using boronic ester cross-linkers. Firstly, the benzoxazine monomer containing carbon–carbon double bonds was synthesized via the Mannich reaction. After thermal ring-opening polymerization, the benzoxazine oligomer containing carbon–carbon double bonds (OBZ) was yielded. Through the thiol-ene click reaction of the OBZ and dithiol cross-linker bearing boronic ester bonds, the polybenzoxazine thermosets using boronic ester bonds as crosslinkages (OBZ-BDB) were successfully synthesized. The structures of OBZ and OBZ-BDB were characterized by SEC, ¹H NMR, and FT-IR measurements. Reprocessing experiments showed that OBZ-BDB has remarkable reprocessability. The retention rates of the tensile strengths through three generations of reprocessing were 98%, 95%, and 84%, respectively. Meanwhile, OBZ-BDB cross-linked by boronic ester bonds had brilliant thermal properties. The T_g of the original OBZ-BDB was 224 °C. With the increase of the reprocessing generations, the T_gs basically remained unchanged.

Catalyst-free transesterification vitrimers: activation via α -difluoroesters

Transesterification vitrimers often require high catalyst loadings to achieve 3D networks reprocessable at moderately high temperature. The addition of an activating group close to the ester bonds allows to synthesize...

Solvent-free synthesis of a formaldehyde-free benzoxazine monomer: study of its curing acceleration effect for commercial benzoxazine

A new 2-substituted benzoxazine bearing a phenol was blended with commercial benzoxazine for improving curing and thermomechanical properties.

Exploiting the Reversible Covalent Bonding of Boronic Acids for Self-Healing/Recycling of Main-Chain Polybenzoxazines

In this work, a new strategy for the synthesis of self-healable/recyable polybenzoxazine networks under mild conditions, by exploiting dynamic B–O bond exchanges is presented. The process is based on mixing...

Bio-Based Bisbenzoxazines with Flame Retardant Linker

This work explores the strategy of incorporating a highly substituted reactive flame retardant into a benzoxazine moiety. For this purpose, a DOPO-based flame retardant received a chain extension via reaction with ethylene carbonate. It was then reacted with phloretic acid to obtain a diphenol end-capped molecule, and further reacted with furfurylamine and paraformaldehyde to obtain a benzoxazine monomer via a Mannich-like ring closure reaction. This four-step synthesis yielded a partly bio-based halogen-free flame retardant benzoxazine monomer (DOPO-PA-fa). The successful synthesis was proven via NMR, IR and MS analysis. The polymerization behavior was monitored by DSC and rheological analysis both showing the polymerization starts at 200 °C to yield pDOPO-PA-fa. pDOPO-PA-fa has a significant thermal stability with a residual mass of 30% at 800 °C under ambient atmosphere. Furthermore, it reached a V-0 rating against small flames and an OI of 35%. Blended with other benzoxazines, it significantly improves their thermal stability and fire resistance. It emphasizes its potential as flame retardant agent.

Self-Polymerization Promoting Monomers: In Situ Transformation of Disulfide-Linked Benzoxazines into the Thiazolidine Structure

Polybenzoxazines obtained especially from green synthons are facing challenges of the requirement of high ring-opening polymerization (ROP) temperature of the monomer, thus affecting their exploration at the industrial front. This demands effective structural changes in the monomer itself, to mediate catalyst-free polymerization at a low energy via one-step synthesis protocol. In this regard, monomers based on disulfide-linked bisbenzoxazine were successfully synthesized using cystamine (biobased) and cardanol (agro-waste)/phenol. Reduction of the disulfide bridge in the monomer using dithiothreitol under mild conditions in situ transformed the oxazine ring in the monomer, via neighboring group participation of the -SH group in a transient intermediate monomer, into a thiazolidine structure, which is otherwise difficult to synthesize. Structural transformation of ring-opening followed by the ring-closing intramolecular reaction led to an interconversion of O-CH₂-N containing a six-membered oxazine ring to S-CH₂-N containing a five-membered thiazolidine ring and a phenolic-OH. The structure of the monomer with the oxazine ring and its congener with the thiazolidine ring was characterized by spectroscopic methods and X-ray analysis. Kinetics of structural transformation at a molecular level is studied in detail, and it was found that the reaction proceeded via a transient 2-aminoethanethiol-linked benzoxazine intermediate, as supported by nuclear magnetic resonance spectroscopy and density functional theory studies. The thiazolidine-ring-containing monomer promotes ROP at a substantially low temperature than the reported mono-/bisoxazine monomers due to the dual mode of facilitation of the ROP reaction, both by phenolic-OH and by ring strain. Surprisingly, both the monomer structures led to the formation of a similar polymer structure, as supported by thermogravimetric analysis and Fourier transform infrared study. The current work highlights the benefits of inherent functionalities in naturally sourced feedstocks as biosynthons for the new latest generation of benzoxazine monomers.

Propargylamine: an attractive amine source for designing high-performance benzoxazine resins with low polymerization temperatures

A series of benzoxazine resins using propargylamine as the amine source were synthesized to achieve highly thermally stable thermosets with low polymerization temperatures.

Light induced crosslinking of main chain polybenzoxazines

Photo-induced cationic ring opening reactions were performed on main chain polybenzoxazine precursors using iodonium salts.

Dissociate transfer exchange of tandem dynamic bonds endows covalent adaptable networks with fast reprocessability and high performance

A covalent adaptable network combining continuous reprocessability and high performance was achieved via dissociate transfer exchange (DTE) of tandem dynamic bonds.