The Spatial and Electronic Effects of Substituent Groups on the Thermal Curing of Bio-Based Benzoxazines

To explore the influence of substituent groups on thermally induced curing,eight new bio-based benzoxazines containing different substituent groups with different electron negativity and volumes were synthesized.The thermal curing of these bio-based benzoxazines was studied in detail.Combined with the curing reaction kinetics,simulation and calculation of Highest Occupied Molecular and Lowest Unoccupied Molecular values,the spatial and electronic effects of different substituent groups on the curing of benzoxazine was explored.It was found that when the substituent was located at the position directly connected to the N atom,the steric hindrance effect of the group was dominant.When the substituent group was located on the benzene ring connected to the O atom,both the electronic effect and the spatial effect influenced the curing of benzoxazine.When an electron-withdrawing group was connected ortho position to the O atom,the curing reaction was promoted due to the decreased electron cloud density of O-on the oxazine ring,making the C-O bond easier to break.When an electron-donating group was connected to the meta position of the O atom it also promoted the curing reaction,possibly because it increased the electron cloud density of the+CH2 reaction site and thereby facilitated electrophilic substitution via attack of+CH2 on the cross linking reaction centre.This work provides a deeper understanding of how spatial and electronic effects of substituents affect the curing of benzoxazine.

Effects of molecular structure parameters on ring-opening reaction of benzoxazines

A colorless monoclinic crystal of dichloro-benzoxazine was obtained and examined by single crystal X-ray diffraction analysis. At the same time, molecular modeling analysis of nine benzoxazine compounds with different substituting groups was performed. Both single crystal X-ray diffraction analysis and molecular modeling analysis provide a detailed picture of molecular structure on molecular level and show good consistence with each other. On the basis of structural analysis, the effects of molecular structure parameters on ring-opening polymerization of ben-zoxazines have been explored.

One-pot multicomponent synthesis of novel tricyclic pyrrolo[2,1-c][1,4]benzoxazines

A simple and efficient synthetic protocol has been developed using a one-pot multicomponent reaction （MCR） involving 2-aminophenols, dialkyl acetylenedicarboxylates, and ethyl bromopyruvate in poly（ethyleneglycol）. This protocol was utilized to prepare novel pyrrolo[2,1-c][1,4]benzoxazines in excellent yields.

SYNTHESIS OF AROMATIC DIAMINE-BASED BENZOXAZINES AND EFFECT OF THEIR BACKBONE STRUCTURE ON THERMAL AND FLAMMABILITY PROPERTIES OF POLYMERS

Three kinds of novel aromatic diamine-based benzoxazines containing naphthalene, propane-2,2-diyldibenzene and neopentyl groups in the backbone, respectively （designated as BAPNCP, BAPBACP and BAPNPGCP, respectively）, were synthesized and characterized. In addition, the effects of backbone structures on curing behaviors of the monomers and thermal and flammability properties of the resulting polymers were systematically studied. The results indicated that BAPNPGCP displayed the highest enthalpy of the curing reaction associated with the ring-opening of benzoxazine, which was due to the effect of benzoxazine ring content per unit mass. Interestingly, the 5 wt% weight loss temperature and char residue after thermogravimetric test for poly（BAPNPGCP） were 8 ℃ and 7% higher than those of poly（BAPBACP）. Meanwhile, the total heat release of poly（BAPNPGCP） was less than half of that for poly（BAPBACP）, indicating the substantial effect of benzoxazine ring content on flammability and char formation. Furthermore, it was found that poly（BAPNCP） gave the best thermal stability and flame retardancy, which was due to the synergistic effect between naphthalene group and benzoxazine ring content. This study provides new insight into the curing behavior of benzoxazine and further understanding on the high performance of polybenzoxazine.

Synthesis of a benzoxazine monomer containing maleimide and allyloxy groups

A novel benzoxazine monomer 3-（4-allyloxy）phenyl-3,4-dihydro-2H-6-（N-maleimido）-l,3-benzoxazine （AMB） was synthesized and structure was confirmed by FT-IR, 1H NMR. Thermal analysis （DSC） of AMB showed the introduction of allyloxy group melting point and exhibited a narrow and symmetric curing exothermic window.

Theoretic Study of 3-(4-N-Maleimido)-phenyl-2,4-dihydro-2H-1,3-benzoxazine Molecular Structure,Spectrum and Thermodynamic Properties

The geometric structures,electronic absorption spectrum,and thermodynamic pro-perties of 3-（4-N-maleimido）-phenyl-2,4-dihydro-2H-1,3-benzoxazine molecule were studied at the B3LYP/6-311＋G＊ level by density functional theory.The results show that three rings of this molecule are in different planes.In gas,absorption wavelength of the lowest energy excitation was obtained at 503 nm,and solvents made it blue-shifted by 3-7 nm,both corresponding to the electron transition of HOMO → LUMO.At 298.15 K,the standard molar formed enthalpy and free energy of the title compound molecule were-549.43 and-273.37 kJ·mol-1,respectively.

Synthesis of benzoxazine-based phenolic resin containing furan groups

A novel benzoxazine-based phenolic resin containing furan groups（PFB） was synthesized via simple two-step reactions and the structure of PFB was confirmed by FTIR and ~1H NMR spectra.Differential scanning calorimetry（DSC） showed that the polybenzoxazine cured from PFB had good heat resistance and lower polymerization temperature compared with that of benzoxazine-based phenolic resins.

Morphological, mechanical and thermal properties of cyanate ester/benzoxazine resin composites reinforced by silane treated natural hemp fibers

This present study deals with the reinforcement of thermosetting resin blends composed of cyanate ester(CE) and benzoxazine(BOZ) resins with natural hemp fibers(NHFs). These NHFs were initially treated by using a silane coupling agent(SCA) in order to chiefly enhance their distributions as well as adhesions within the CE/BOZ resin matrix,then incorporated with various weight amounts ranging from 5 wt% to 20 wt% with a regular interval of 5 wt%. The obtained results showed that at the maximum treated fiber loading(20 wt%), distinctive enhancements in the mechanical properties in terms of flexural strength and microhardness were obtained. Besides, the thermal stability and glass transition temperature(Tg) were appreciably enhanced and were higher than those of the pure CE/BOZ resin properties. With respect to the astonishing properties of the NHFs, these enhancements could be possibly due to the good dispersion and adhesion of the treated NHFs inside the CE/BOZ resin achieved upon using the SCA. Therefore,we believe herein that these renewable and cheap NHFs have considerable potential to be used as reinfocer materials for CE/BOZ resin composites to be used in various industrial sectors.

Synthesis of a novel benzoxazine containing benzoxazole structure

A high-purity benzoxazine（Boz-BOA） containing benzoxazole structure was successfully synthesized by three-step synthetic method using 2-（4-aminophenyl）-1H-benzoxazole-5-amine（BOA） and ortho-hydroxybenzaldehyde.The structure of Boz-BOA was confirmed by FTIR and;H NMR spectra.The DSC was utilized to probe the curing behavior of Boz-BOA and exhibited a narrow melting peak and curing exothermic peak.

Synthesis of 3-Phenyl-6-formyl-3, 4-dihydro-2H-1, 3-benzoxazine

A novel benzoxazine monomer containing aldehyde group, 3-phenyl-6-formyl-3, 4-dihydro-2H-1, 3-benzoxazine （Ald-B）, was synthesized via the Mannich condensation of formaldehyde, p-hydroxybenzaldehyde and aniline. Its structure was characterized by Fourier transform infrared spectroscopy （FTIR）, proton nuclear magnetic resonance （^1HNMR）. The side reaction in the synthesis is discussed.

Non-isothermal curing kinetics and thermal properties of benzoxazine-phenolic copolymers

Using novolac phenolic resin, aniline and formaldehyde as raw materials, benzoxazine-phenolic copolymers with different percentages of benzoxazine rings were prepared. FT-IR was adopted to characterize the molecular structure of the novolac-type phenolic resin and the benzoxazine-phenolic copolymer BP31. In order to understand the curing process of the copolymers, the curing behavior and curing kinetic characteristics were studied by differential scanning calorimetry （DSC）, and the catalytical effect of phenolic hydroxyl on the curing behavior of copolymers was investigated. To investigate the thermal properties of this resin, the thermal degradation behaviors of the cured samples were studied by thermal gravimetric （TG） method, and glass-transition temperatures （Tg） of the cured copolymers were also evaluated by DSC. The dynamic Ozawa method was adopted to determine the kinetic parameters of the curing process as well. The activation energy is 78.8 kJ/mol and the reaction rate constant is in the range from 40.0 to 5.2 （K/min）＂ according to reaction temperatures. The Ozawa exponent decreases from 2.4 to 0.7 with the increase of reaction temperature, and curing mechanism is expounded briefly according to the results. TG result shows that the highest char yield of copolymers is 50.3%. The highest Tg of copolymers is 489 K, which is much higher than that of pure benzoxazine resin.

Terresoxazine,A Novel Compound with Benzoxazine Skeleton from Tribulus terrestris

Terresoxazine, a novel benzoxazine derivative was isolated from the fruits of Tribulus terrestris. Its structure was determined as 7-hydroxy-3, 3a-dihydro-5H-pyrrolo-[1,2-a] [3,1] -benzoxazin-1(2H)-one, on the basis of the spectral techniques and X-ray crystallographic analysis.

Recent Advances in Flame Retardant Bio-Based Benzoxazine Resins

Benzoxazines have attracted wide attention from academics all over the world because of their unique properties.However,most of the production and preparation of benzoxazine resins depends on petroleum resources now,especially bisphenol A-based benzoxazine.Therefore,owing to the environmental impacts,the development of bio-based benzoxazines is gaining more and more interest to substitute petroleum-based benzoxazines.Similar to petroleum-based benzoxazines,most of bio-based benzoxazines suffer from flammability.Thus,it is necessary to endow bio-based benzoxazines with outstanding flame retardancy.The purpose of this review is to summarize the latest advance in flame retardant bio-based benzoxazines.First,three methods of the synthesis of bio-based benzoxazines are introduced briefly.Furthermore,the curing mechanism of benzoxazine and the effect of branched chains on the curing behavior are also discussed and summarized.Subsequently,this review focuses on fully bio-based benzoxazines,partly bio-based benzoxazines,and bio-based benzoxazine composite materials in terms of flame retardancy as well as thermal stability and some other special properties.Finally,we give a brief comment on the challenges and prospects of the future development of flame retardant bio-based benzoxazines.

Formulating Novel Halogen-Free Synergistic Flame Retardant Epoxy Resins for Vacuum Assisted Resin Infusion Composites

The most common process to manufacture advanced composites is the costly autoclave.One of the out-of-autoclave alternatives is the low-cost vacuum assisted resin infusion(VARI)which produces quality parts with less pollution.Epoxy resin is a widely used composite matrix resin,but its high flammability limits its use as interior composite parts for vehicles.The usual flame retardant for epoxy involves halogen,which is effective but has high smoke toxicity.As a result,halogen-free flame retardant epoxy resin systems become dominant.In this paper,phosphorus flame retardant was combined with benzoxazine(BOZ)to produce synergistic effect and achieve satisfactory flame retardance,as well as mechanical improvement for the epoxy resin.Differential scanning calorimetry(DSC),dynamic mechanical analysis(DMA),thermal gravitational analysis(TGA),the cone calorimeter(CC),and limiting oxygen index(LOI)were used to characterize the resins.The results showed significant improvement on the flame retardance of the synergistically modified resins.Specifically,the carbon residue increased by 113.6%,and the char thickness increased by 6 to 7 times,compared to those of the flammable benchmark resin.The LOI reached 33 and passed the UL94 V-0 vertical burn rating.The modified resins also exhibited adequate stability and viscosity suitable for VARI processes.

Thermal Polymerization of Cyanate Ester-Benzoxazine:Study of a Functional Model Compound

A novel benzoxazine(BOZ)monomer is synthesized by a pot method with solvent-free to blend with cyanate ester(CE).A soluble intermediate is obtained after being cured for 20 h at 80℃.The two model compound and the blends are analyzed with the infrared radiation(IR),nuclear magnetic resonance(NMR)spectroscopy,and differential scanning calorimetry(DSC).The results show that an intermediate of the iminocarbonate and BOZ structures is formed by the ring-open BOZ reacting with the cyanate groups and ring-unopened BOZ.Moreover,rearrangement and ring-opening occur in the postcure of the intermediate to form the alkyl isocyanurate structure with polybenzoxazine.

Sustainable Polymers Derived From Naturally Occurring Materials

Nearly 95% of monomers or chemical intermediates used today are based on fossilized carbon such as coal and petroleum. This has resulted in a high rate of depletion of fossilized reserves, continuous escalation in petroleum prices, environmental impact with the increase in emission of greenhouse gases, and accumulation of non-biodegradable waste on earth. Current global main challenges are moving towards green sources - need for vast new and sustainable material resources;supplement, reuse and replace petroleum based polymeric materials;biodegradability of materials to prevent build up of waste;toxicity associated with the preparation, usage and environmental safety. Recent investigations are therefore, focused on procuring materials from the plant resources, agricultural waste and their utility in synthesis of polymeric materials. Amongst the polymers derived from natural resources poly(lactic acid) is a leading candidate. Commercial quantities of natural oil-based polyols such as castor, soya bean oil have been available over the past several years and currently used for synthesis of? polyesters, polyurethanes etc, but today many other? natural materials are also being investigated. It should be possible to produce sustainable polymers commercially and economically.

Designing a Phthalonitrile/Benzoxazine Blend for the Advanced GFRP Composite Materials

High performance resin must be used in the high performance glass fiber-reinforced plastic（GFRP） composites, but it is sometimes difficult to balance the processabilities and the final properties in the design of advanced thermoset GFRP composites. In this study, a phthalonitrile/benzoxazine（PPN/BZ） blend with excellent processability has been designed and applied in the GFRP composite materials. PPN/BZ blend with good solubility, low melt viscosity, appropriate gel condition and low-temperature curing behavior could enable their GFRP composite preparation with the prepreg-laminate method under a relatively mild condition. The resulted PPN/BZ GFRP composites exhibit excellent mechanical properties with flexural strength over 700 MPa and flexural modulus more than 19 GPa. Fracture surface morphologies of the PPN/BZ GFRP composites show that the interfacial adhesion between resin and GF is improved. The temperatures at weight loss 5%（T_（5%）） and char residue at 800 °C of all PPN/BZ GFRP composites are over 435 °C and 65% respectively. PPN/BZ GFRP composites with high performance characteristics may find applications under some critical circumstances with requirements of high mechanical properties and high service temperatures.

Smart synthesis of silver nanoparticles supported in porous polybenzoxazine nanocomposites via a main-chain type benzoxazine resin

In this article, novel silver nanoparticles immobilized on macroporous polybenzoxazine nanocomposites （Ag-poly（BA-ddm）^x main） were prepared as catalysts for catalytic reduction reaction. For this purpose, a main-chain type benzoxazine was cured in dimethyl sulfoxide by thermally activated ring-opening polymerization at 180℃ for 2days followed by the reduction of silver nitrate in homogeneous polybenzoxazine solution. The porous structure of the resin was confirmed by scanning electron microscope （SEM） analysis and N2 adsorption]desorption studies. Besides, transmission electron microscopy images showed that spherical particles （around 3-10 nm in sized） are dispersed in Ag-poly （BA-ddm）^x main network. The catalytic activity of the as-prepared nanocomposite has been investigated by photometrically monitoring the reduction of methylene blue by an excess of NaBH4. The kinetic data of the reduction reaction was explained by the assumption of a pseudo-first-order reaction with regard to methylene blue （MB）. As evidence by Ultraviolet （UV） spectral analysis, the Ag-poly（BA-ddm）^x main catalyst possesses excellent catalytic reduction of MB, and no deactivation or poisoning of the catalyst was observed. The results demonstrate that porous polybenzoxazine supported silver nanoparticles can be applied as reusable catalysts with satisfied catalytic activity.

Facile Mechanochemical Preparation of Polyamide-derivatives via Solid-state Benzoxazine-isocyanide Chemistry

With the exploration of novel sustainable protocol for functional polyamides'(PAs)construction as the starting point,herein,the small molecular model compound(M 1-ssBIC)was prepared firstly by manual grinding of monofunctional benzoxazine(1a)and isocyanide(1 b)via solid-state benzoxazine-isocyanide chemistry(ssBIC)to evaluate the feasibility of ssBIC.Linear PAs(P1-series polymers)were subsequently synthesized from biunctional benzoxazine(2a)and isocyanide(2b),and the influence of the loading of catalyst(octylphosphonic acid)(OPA)on the polymerization was investigated.Afterwards,two kinds of cross-linked PAs were successfully constructed via ssBIC by using trifunctional benzoxazine(3a)and cross-linked polybenzoxazine(4a)as reaction substrates,respectively,thus verifying the adaptability of ssBIC.Structural characterization indicates that amide,phenolic hydroxyl and tertiary amine substructures,with metal-complexing capability,have been successfully integrated into the obtained PAs.A type of representative PA/silver composite(P3-AgNPs)was prepared subsequently via in situ reduction treatment,and its application as recyclable reduction catalyst for organic pollutant p-nitrophenol(4-NP)was preliminarily investigated here to provide the example for possible downstream application of ssBIC.We think that this current work could provide a new pathway for the construction of functional PAs through facile and sustainable ssBIC protocol.

Microwave-assistant Syntheses, Crystal Structures and Safener Activities of Two Substituted Phenyl Isoxazole Derivatives

Two novel substituted phenyl isoxazole benzoxazine formamide derivatives were designed and synthesized with substituted o-aminophenol,1,2-dibromoethane and different phenyl substituted isoxazole formyl chloride as the raw materials via microwave assistant synthesis.The target compounds were characterized by IR,^(1)H NMR,^(13)C NMR and HRMS.Both single-crystal structures were further determined by X-ray diffraction.3-(2’-Chloro-6’-fluoro-phenyl)-4-(2’,3’-dihydro-1’,4’-benzoxazine)-5-methyl-isoxazole formamide (4a) crystallizes in orthorhombic system,P2_(1) space group with a=8.9414(18),b=10.834(2),c=17.706(4)A,V=1715.1(6)A^(3),D_(c)=1.444 Mg/m^(3),Z=4,F(000)=768,μ(Mo Kα)=0.255 mm^(-1),R=0.0406 and wR=0.1171.3-Phenyl-4-(6-methyl-2’,3’-dihydro-1’,4’-benzoxazine)-5-methyl-isoxazole formamide (4b) is of triclinic system,space group PI with a=7.7659(16),b=8.3626(17),c=13.484(3)A,α=76.04(3)°,β=100.63(3)°,γ=82.01(3)°,V=841.6(3)A^(3),D_(c)=1.319 Mg/m^(3),Z=2,F(000)=352,μ(Mo Kα)=0.090 mm^(-1),R=0.0672 and wR=0.2671.Both crystals are packed through C–H···O hydrogen bonding interaction.There is C–H···F hydrogen bond between 4a molecules,and C–H···N between 4b molecules.Bioassay results showed that compounds 4a and 4b exhibited detoxification on maize and restored maize growth index.