Microwave‐assisted solvent‐free synthesis of novel benzoxazines: A faster and environmentally friendly route to the development of bio‐based thermosetting resins

Abstract

Microwave-assisted organic synthesis (MAOS) is a well-established technique that has been used in the enhancement of chemical reactions. Here, the versatility of MAOS is explored describing an environmentally friendly one-pot route to novel bio-based benzoxazines under solvent-free conditions. The lignin derivative, guaiacol, along with paraformaldehyde and different conjugated and nonconjugated amines are successfully fused into guaiacol-derived 3,4-dihydro-2H-1,3-benzoxazines. The reactions conducted under microwave irradiation are completed much faster than those under traditional heating, reducing the reaction time from hours to only 6 min, with good yields. The chemical structures of novel benzoxazines are confirmed by 1 H and 13 C NMR spectroscopy, FTIR, and HR-MS. The thermal behavior of the resins are evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), showing that these polymers have good thermal stability and wide processing-window, with onset temperature of polymerization above 230 8C. These results indicate dramatic improvement over the traditional methodologies for the production of this class of resins, which are usually obtained by time-consuming procedures and in the presence of toxic solvents. Therefore, MAOS can be considered a green and efficient strategy for the synthesis of eco-friendly benzoxazines.

Synthesis and Polymerization of Naphthoxazines Containing Furan Groups: An Approach to Novel Biobased and Flame-Resistant Thermosets

“…As far as it can be ascertained, the synthesis of the Naft-Fu-H and Naft-Fu-Fu compounds is not reported in the literature and even the Naft-H-Fu compound is reported in only one single work [25], where the thermal properties or polymerization behavior of this compound was not explored. The synthesis of Naft-Fu-H was conducted under microwave heating, with satisfactory yield (60%), in a few minutes (7 min), especially if compared to the syntheses of oxazinic compounds conducted under traditional heating described in literature, which normally require several hours of reaction, thus demonstrating the efficiency of this method in the synthesis of this type of compound, as already reported in previous studies [3].…”

“…In addition to the advantages of traditional phenolic polymers, such as high thermal resistance, dimensional stability, good electrical properties, flame retardancy, and low smoke generation, polyoxazines exhibit their own attractive characteristics, such as low moisture absorption, good chemical resistance, high glass transition temperature (T g ), low shrinkage, and improved mechanical and thermal properties compared to traditional phenolic polymers. In addition, they offer great flexibility of molecular design and do not generate by-products during their polymerization [2,3].…”

Biobased Benzoxazine Derived from Daidzein and Furfurylamine: Microwave‐Assisted Synthesis and Thermal Properties Investigation

“…[41][42][43] Besides the phenolic hydroxyl groups,d aidzein also possesses the benzopyrones tructure, whichi sa ble to improve the thermal-mechanical properties of thermosets. [48,49] Inspired by this result, unlike the traditional oil-bath heating method, the microwaveh eating methodw as also tried to synthesize the benzoxazine monomer in this work. It is usually taken as the renewable amine sourcef or benzoxazine synthesis and the furan ring in furfurylamine can improvet he crosslink density and char yield of the resulting resins.…”

“…synthesized as eries of biobased benzoxazine monomers from guaiacol successfully,t hus opening up an ew methodf or biobased benzoxazine synthesis. [48,49] Inspired by this result, unlike the traditional oil-bath heating method, the microwaveh eating methodw as also tried to synthesize the benzoxazine monomer in this work. After the synthesis process was investigated,t he structural features and curing behaviors of synthesized Dz-f as well as the properties of cured resin p(Dz-f), in terms of thermodynamic properties and thermal stability,w ere investigated in detail.…”

Oligomeric phthalonitriles and tetrakis(phenylethynyl)benzene blends with improved processing and thermal properties

“…The most thoroughly studied high‐temperature thermosets that are structurally stable and exhibit high thermal and oxidative stability can be categorized into four main classes: benzoxazine (Bz), cyanate ester (CE), polyimide (PI), and PN resins. While Bz resins generally exhibit near‐zero shrinkage upon polymerization, high char yields, low water absorption, and low dielectric constants, this comes at the cost of high curing temperature (200 °C or higher), difficulty in processing, and brittleness of the thermoset in pure Bz polymers.…”