Synthesis, characterization, and properties of a novel aromatic ester-based polybenzoxazine

Polybenzoxazines with molecular design flexibility have excellent properties by using suitable raw materials. A new benzoxazine monomer terephthalic acid bis-[2-(6-methyl-4H-benzo[e][1,3]oxazin-3-yl)]ethyl ester (TMBE) with bis-ester groups has been synthesized from the simple esterification reaction of terephthaloyl chloride and 2-(6-methyl-4H-benzo[e][1,3]oxazin-3-yl)-ethanol (MB-OH). The chemical structure of TMBE was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (1H-NMR, 13C-NMR). Polymerization behavior of TMBE was studied by differential scanning calorimetry (DSC) and FT-IR after each cure stage. The cross-linked polybenzoxazine (PTMBE) gave a transparent film through the thermal casting method. The dynamic mechanical analysis of PTMBE showed that the Tg was 110 °C. Thermogravimetric analysis reveals better thermal stability as evidenced by the 5% and 10% weight-loss temperatures (Td5 and Td10) of PTMBE, which were 263 and 289 °C, respectively, with a char yield of 27% at 800 °C. The tensile test of the film revealed that the elongation at break was up to 14.2%.


Introduction
As a new phenolic-type resin, polybenzoxazines possess attractive properties, such as high glass transition temperature, 1,2 ame retardancy, 3 low water absorption, 4 and low cost. 5 They are applied to the eld of halogen-free ame retardant laminates, vacuum pump rotors, printed circuit boards, friction materials, composites, and other elds. 6,7 Nevertheless, inherent brittleness of polybenzoxazines, especially in aromatic polybenzoxazines, limits their further development in the chemical industry.
There are generally three reported approaches to improve the toughness of polybenzoxazines. The rst approach is introducing rubber, 8,9 the exible heat-resistant linear polymer [10][11][12][13][14] into resins as the toughening agent to improve toughness. The second approach is designing inherently tough benzoxazine monomers containing so segments to afford tough polybenzoxazines. 15,16 The third approach is the toughening of polybenzoxazines by inorganic nanoparticles. [17][18][19] Among them, the designing of inherently tough benzoxazine monomers and polybenzoxazines prepolymers was a signicant approach to improve the exibility of polybenzoxazines from the molecular level, and to increase the degree of chemical crosslinking between the two phases.
The tough benzoxazine monomers were synthesized, including monofunctional and difunctional benzoxazines from available phenols containing exible groups, primary amines and formaldehyde. 4,[20][21][22][23] These monomers improve the exibility of polybenzoxazines with dramatically scarifying their heat resistance. Therefore, some main-chain type polybenzoxazines 24,25 were synthesized by Mannich reaction with polyether diamines of different molecular weights or PDMS with different molecular weights for the diamine terminal, bisphenol A, and formaldehyde. Such high-molecular-weight prepolymers exhibited signicantly improved toughness due to the so segments and generated a high cross-linking degree due to the polybenzoxazine component.
The other approach to improve toughness is indirectly preparing of polybenzoxazine prepolymers using poly-esterication, 14 coupling reactions, alternating copolymerization of donor-acceptor monomers, 26 Diels-Alder reaction, 27 polyetherication, 28 hydrosilylation 29 These synthetic approaches also necessarily yield high-performance polybenzoxazines with improved toughness. The novel polyesters were synthesized from the polycondensation reaction of bisbenzoxazine-diol, pyromellitic dianhydride, and 4,4 0 -(hexa-uoroisopropylidene)diphthalic anhydride. 14 The molecular weights of polyesters were in the range of 5800-7000 Da with benzoxazine moieties in the main chain exhibited high exibility induced by the so pentyl and esters groups and comparable thermal stability concerning low molar mass analogous. Aydogan 29 prepared polysiloxanes containing benzoxazine units in the main chain by hydrosilylation of 1,1,3,3tetramethyldisiloxane and diallyl functional benzoxazines. To further improve its exibility, polysiloxanes chain was extended by the reaction of poly (bisbenzoxazinedimethylsiloxane)s and octamethyl-cyclotetrasiloxane in the presence of tetrabutylammonium hydroxide as a catalyst.

Synthesis of MB-OH
MB-OH was synthesized according to the literature. 12 A suspension of paraformaldehyde (15.3 mL, 210 mmol), 30 mL 1,4dioxane were added to a 250 mL three-neck ask in an ice bath. Then 2-aminoethanol (6.0 mL, 100 mmol) was added drop-wise to the system. The mixture stirred at room temperature for about 30 min. Aer adding p-cresol (10.6 mL, 100 mmol), the temperature was gradually increased to 70 C, and stirring was continued for another 3.5 h. A yellowish viscous uid was obtained aer removing the solvent through a rotary evaporator. Then the viscous liquid was dissolved in dichloromethane, washed with 1 L of 0.1 N aqueous KOH and nally three times with 1 L of distilled water. The methylene chloride solution was dried with MgSO 4 , ltered, and concentrated under vacuum to afford crude product. The crude product was recrystallized with n-hexane to give pure white solid of MB-OH (9.53 g, 51%). 1

Synthesis of TMBE
Into a 100 mL dry round-bottom ask equipped with a calcium chloride drying tube, 20 mL dry methylene chloride, MB-OH (1.9 g, 10 mmol), triethylamine (1.7 mL, 1.4 mmol) were added and mixed at room temperature for about 30 min. Then terephthaloyl chloride (1.0 mL, 5.0 mmol) was added at 5 C. The mixtures were stirred at room temperature for 2 d and kept reuxing for 4 h. A yellow transparent solution was obtained aer cooling to room temperature. The solution was washed with 1 L of 0.5 N aqueous NaOH and nally three times with 1 L of distilled water. The methylene chloride solution was dried with MgSO 4 , ltered, and concentrated under vacuum to afford crude product (1.6 g, yield: 61%). The product was puried by chromatography (v methylene chloride : v ethyl acetate ¼ 2 : 1) to afford white power solid. 1

Polymerization of TMBE
The amount of TMBE was melted, stirred and transferred to a rectangular aluminum foil mold and then cured at 120 C/2 h, 140 C/2 h, 160 C/2 h, 180 C/2.5 h in an air-circulating oven. Aer that, samples were allowed to cool slowly to room temperature to be tested.

Characterizations
FTIR spectra were obtained with Bruker Tensor 27 FTIR spectrometer (Bruker, GM) in which samples were preparing as KBr pellets. 1 H-NMR and 13 C-NMR spectra were recorded on a Bruker Avance 300 instrument (Bruker) using CDCl 3 as the solvent and TMS as the internal standard. Differential scanning calorimetry (DSC) was conducted using a DSC SP instrument (Rheometric Scientic) at a heating rate of 10 C min À1 under nitrogen. Thermogravimetric analysis (TGA) was performed on a TGA/DSC STARe System instrument (Mettler-Toledo) at a heating rate of 10 C min À1 under N 2 atmosphere. The gas ow rate was 100 mL min À1 . The dynamic mechanical analysis (DMA) of samples was carried out using a Mettler-Toledo DMA/ SDTA861e instrument (Mettler-Toledo). The specimen with dimensions of approximately 3.00 Â 2.20 Â 0.8 mm 3 was tested by a shear mode at 1 Hz in the temperature range of 30-200 C at a heating rate of 3 C min À1 . Tensile measurement were also recorded with Material Testing Machine Model UTM5105 at a crosshead speed of 1 mm min À1 . Each sample with a dimension of approximately 20.0 Â 5.0 Â 1.0 mm 3 was tested from an average of at least 5 tests.

Preparation of TMBE and its polymer
Scheme 1 illustrates our strategy for the preparation of TMBE and PTMBE. First, according to reference, we synthesized containing hydroxyl benzoxazine (MB-OH). Second, using triethylamine as deacid reagent, dry dichloromethane as solvent, TMBE was obtained via the esterication reaction of MB-OH and terephthaloyl chloride at a molar ratio of 2 : 1. TMBE was easily soluble in common low-boiling organic reagents, such as dichloromethane, chloroform, and tetrahydrofuran. Finally, PTMBE was formed by the cross-linking of TMBE under thermal curing reaction with polymerization temperature of 180 C.

Characterization of TMBE
The chemical structure of TMBE was conrmed by NMR and FT-IR spectral analysis. As can be seen from Fig. 1 where the 1 H NMR spectrum of TMBE is presented, the appearance of the protons resonating at 4.05 ppm (-O-*CH 2 -N) and 4.88 ppm (Ar-*CH 2 -N) was clear evidence for the benzoxazine ring formation on TMBE. Moreover, the peaks at 3.18 ppm and 4.52 ppm assigned to protons of -N-*CH 2 -Cand C-*CH 2 -Owere observed, respectively. The integral ration of these peaks was close to the theoretical ratio of 2 : 2 : 2 : 2. The signal peak appeared at 2.27 ppm corresponding to the Ph-*CH 3 . The peak appeared at 8.10 ppm corresponding to the protons of the benzene ring linked to the ester group. The peaks appeared at 6.68 ppm, 6.75 ppm, and 6.95 ppm assigned to protons of benzene ring linked to the oxazine ring.
The 13 C NMR spectrum presented from Fig. 2  FT-IR spectra also give evidence for the formation of benzoxazine containing bis-ester groups (Fig. 3). The characteristic peak at 932 cm À1 due to the C-H out-of-plane vibration in the benzene ring where an oxazine ring attached was clearly observed. Additionally, the strong peak at 1720 cm À1 was characteristic absorption of the carbonyl in terephthalate. Scheme 1 Synthetic route to TMBE and PTMBE. Fig. 1 1 H- This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 6953-6959 | 6955

Polymerization of TMBE
The DSC heating-scan thermogram of TMBE was depicted in Fig. 4. The sharp endothermic peak appearing at 139 C corresponds to the melting temperature (T m ) of TMBE. TMBE showed unimodal curing behaviour as observed from the single exothermic peak, which is from the ring-opening polymerization of the benzoxazine ring. The onset and peak top temperatures of the peak are 176 and 231 C, respectively. The polymerization temperature of TMBE is similar to the values reported to other traditional benzoxazine compounds. 30 DSC result of curing TMBE show the presence of aliphatic chains (C4) in the benzoxazine structure has little effect on the polymerization behavior of TMBE and only a lower temperature is required for polymerization.
The polymerization behavior of TMBE was also monitored by FTIR aer curing at each temperature (Fig. 5). With the increase of temperature, the intensity of the characteristic absorption peak at 932 cm À1 due to benzoxazine structure gradually decreased due to the opening of the oxazine ring and disappeared aer the 180 C for 2.5 h. The intensity of absorption bands at 1502 cm À1 due to the trisubstituted benzene ring (stretching) also decreased. Meanwhile, some new absorption bands appeared at 3442 cm À1 due to the phenolic hydroxy and at 1481 cm À1 due to the tetrasubstituted benzene ring, suggesting that the ring-opening polymerization of TMBE occurred. Both DSC and IR spectroscopy indicates the formation of polybenzoxazine by polymerization at 180 C.

Thermal property of PTMBE
Dynamic mechanical analysis (DMA) was performed to study the viscoelastic properties of PTMBE (Fig. 6). At 30 and 80 C, the storage modulus (G 0 ) values are 168 and 100 MPa, respectively, whereas the loss modulus (G 00 ) values are 11 and 27 MPa, respectively. For poly(BA-a) (bisphenol-A and aniline-based polybenzoxazine), the storage modulus G 0 of 1.9 Â 10 3 MPa is higher than that of PTMBE due to their high aromatic content. It has been clearly demonstrated from DMA that the exibility of PTMBE is better than poly(BA-a). The T g was observed at 110 and 89 C from the maximum loss factor and maximum loss modulus, respectively. The decrease of T g can be attributed to the exible alkyl chain in PTMBE because of a lower energy barrier for motion. TGA was performed to determine the thermal stability of PTMBE. Fig. 7 shows the TGA thermograms of PTMBE. In nitrogen, 5 and 10% weight loss temperatures (T d5 and T d10 ) of PTMBE were 263 and 289 C, respectively. The char yield of PTMBE was up to 27.0% at 800 C. The T d5 and T d10 of PTMBE were decreased by about 50 C compared with poly(BA-a). The char yield of PTMBE at 800 C has a slight drop compared with poly(BAa). PTMBE shows the lower thermal stability than poly(BA-a) attributed as mentioned previously to the exible aliphatic chain.

Mechanical properties of PTMBE
The stress-strain curve was showed to study the mechanical property for the PTMBE lm (Fig. 8). The PTMBE lm showed elongation at a break of 14.2% and the tensile strength of 11.3 MPa. It is reported that the PB-a lm is very brittle with elongation at break of 1.3%. 30 The elongation at break of PTMBE was larger than that of PB-a. Namely, PTMBE had remarkably improved toughness, and the lm is easy to bend. Interestingly, in spite of the introduction of the aromatic ester group, PTMBE still showed the high elongation at break comparing with polybenzoxazines 20 only containing linear long aliphatic chains (C12). The toughness enhancement of PTMBE is attributed to the long exible alkyl chains, which enhances the mobility of segments under load and thereby increases the ultimate elongation. The tensile modulus of PTMBE is 248.0 MPa, lower than that of PBA-a (4.3 GPa) with rigid backbone. 30 Therefore, the incorporation of the long aliphatic chain and aromatic ester group into PTMBE network structure gave exible polymer lm without excessively scarifying its heat resistance.
A yellowish PTMBE lm (7 Â 6 cm 2 ) with 150-200 mm in thickness was obtained through the thermal casting method in Fig. 9(a). A larger-sized lm (20 Â 20 cm 2 ) was also prepared in our lab according to the same procedure. Besides, the PTMBE lm had remarkably improved toughness, and is easy to bend in Fig. 9(b).   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 6953-6959 | 6957

Conclusions
By utilizing a clean and facile route, we have prepared a highpurity aromatic ester-based benzoxazine monomer and its polymer. Thermally activated polymerization of benzoxazine monomer provided exible, uniform polymer lm. The novel polybenzoxazine lm exhibited signicantly improved toughness due to the long aliphatic chain. The storage modulus and glass transition temperature of PTMBE were 168 MPa and 110 C, respectively. Regarding thermal stability, PTMBE showed lower thermal stability than poly(BA-a) due to the aliphatic hydrocarbon chain. It is anticipated to nd applications of PTMBE as a self-healing material.

Conflicts of interest
There are no conicts to declare.