Design of High-Performance Polybenzoxazines with Tunable Extended Networks Based on Resveratrol and Allyl Functional Benzoxazine

A novel resveratrol-based bio-benzoxazine monomer (RES-al) containing an allyl group has been synthesized using resveratrol, allylamine, and paraformaldehyde via Mannich condensation reaction, and its chemical structures have been characterized by FT-IR spectroscopy and NMR techniques. The polymerization behavior of this benzoxazine resin has been investigated using in situ FT-IR and differential scanning calorimeter (DSC) measurements, and the thermal-mechanical properties of its corresponding polybenzoxazines are evaluated by DMA and TGA. We show that by controlling the curing process of the oxazine ring, the C=C bond in resveratrol, and the allyl group in RES-al, the cross-linking network of the polybenzoxazine can be manipulated, giving rise to tunable performance of thermosets. As all curable functionalities in RES-al are polymerized, the resulted polybenzoxazine exhibits a good thermal stability with a Tg temperature of 313 °C, a Td5 value of 352 °C, and char yield of 53% at 800 °C under N2.

Resveratrol has been widely investigated, since it was isolated from white hellebore in 1940 [22][23][24][25]. The phenolic hydroxyl groups in resveratrol make it a good candidate for developing various thermosets [17,18]. For instance, resveratrol-based epoxy, cyanate ester, and benzoxazine thermosetting resins have been extensively reported, and all of those resveratrol-derived thermosets show high thermomechanical properties, indicating them as promising alternatives to petroleum-based thermosetting systems [22,26].
The present work reports a new resveratrol-derived tri-oxazine functional benzoxazine monomer containing an allyl group. The ring-opening polymerization behavior of the obtained benzoxazine monomer was investigated. In addition, the effects of resveratrol and allyl groups on thermal properties of the derived benzoxazine have been evaluated.

Characterization
1 H, 13 C, 1 H-1 H NOESY and 1 H-13 C HMQC NMR spectra were obtained on a Bruker AVANCE II 400 MHz (Bruker, Fällanden, Switzerland) spectrometer in DMSO-d 6 . Tetramethyl silane was used as an internal standard. FT-IR spectrophotometer (Nicolet Nexus 670, Nicolet, WI, USA) was carried out to further confirm the existence of each functionality in benzoxazine. Each spectrum was obtained in a resolution of 4 cm −1 , and the frequency range for the scans was set from 4000 to 400 cm −1 . Elementary analyzer (Elementar Vario EL-III, Hanau, Germany) was also adopted to confirm the chemical structure. A NETZSCH differential scanning calorimeter (DSC) (Model 204f1, NETZSCH, Selb, Germany) was performed to obtain DSC thermogram of each sample. The temperature ramp rate was set as 10 • C/min in N 2 (a N 2 flow rate of 60 mL/min). In addition, dynamic mechanical analysis (DMA) was performed by a NETZSCH DMA/242E analyzer (NETZSCH, Selb, Germany) at a heating rate of 3 • C/min. Thermal gravimetric analysis (TGA) was performed on a NETZSCH STA449-C thermogravimetric analyzer (NETZSCH, Selb, Germany) under both N 2 and air atmospheres at a flow rate of 40 mL/min. Moreover, the heating ramp rate for TGA was set as 10 • C/min.

Synthesis of Tri-Functional Benzoxazine (RES-al)
Into a 100 mL flask were mixed resveratrol (1.14 g, 5 mmol), allylamine (0.86 g, 15 mmol), paraformaldehyde (0.99 g, 33 mmol), and 35 mL of toluene. The mixture was stirred at 100 • C for 6 h. Then, the mixture was cooled down, and the organic layer in solution was washed by 2 N NaOH solution for 3 times. Afterwards the solution was further washed by distilled water. After removing the solvent, the crude product was purified through recrystallization in acetone/toluene mixtures (1:1). At last, white crystals were obtained (yield ca. 82%). 1

Polymerization Procedures of RES-al
Polybenzoxazines were obtained using different stepwise polymerization procedures via RES-al. RES-al was first cured in a mold by multiple curing steps of 1 h each at 140, 180, and 220 • C to produce a partially polymerized sample, namely, poly(RES-al)-1. Heating treatment of poly(RES-al)-1 at 260 • C for 1 h in the same mold generated poly(RES-al)-2, the further heating of which at 300 • C for 1 h gave rise to a fully cured thermoset, poly(RES-al)-3.

Synthesis and Structural Confirmation of RES-al
The resveratrol-derived tri-oxazine containing benzoxazine resin, RES-al, was successfully obtained by a modified Mannich condensation from resveratrol, allylamine, and paraformaldehyde (Scheme 1). The NMR proton and carbon assignments for the oxazine ring and the allyl group in RES-al are determined by both 2D 1 H-1 H NOESY and 1 H-13 C HMQC NMR spectra ( Figures S1 and S2). As can be seen in Figure 1, the characteristic proton resonances in Ar-CH 2 -N group from the oxazine ring of RES-al show overlapped doublets at 3.93 and 4.04 ppm. Besides, the triplet signals for the O-CH 2 -N group can be observed at 4.84, 4.85, and 4.91 ppm, respectively; the characteristic proton resonances of the allyl group in RES-al can be observed around 3.4, 5.2, and 5.9 ppm, respectively [27].

Synthesis and Structural Confirmation of RES-al
The resveratrol-derived tri-oxazine containing benzoxazine resin, RES-al, was successfully obtained by a modified Mannich condensation from resveratrol, allylamine, and paraformaldehyde (Scheme 1). The NMR proton and carbon assignments for the oxazine ring and the allyl group in RESal are determined by both 2D 1 H-1 H NOESY and 1 H-13 C HMQC NMR spectra ( Figures S1 and S2). As can be seen in Figure 1 Figure 2 shows the 13 C NMR spectrum, which was provided to confirm the chemical structure of this monomer. As can be observed in Figure 2, two sets of triplet signals at the range of 45-50 and 81-83 ppm are characteristic carbon resonances for Ar-CH2-N-and -O-CH2-N-, respectively [4]. In addition, other typical carbon resonances of allyl group in RES-al can also be found around 54, 118, and 135 ppm, respectively [17]. Therefore, the above NMR results fully support that we have successfully obtained the target benzoxazine compound.

Synthesis and Structural Confirmation of RES-al
The resveratrol-derived tri-oxazine containing benzoxazine resin, RES-al, was successfully obtained by a modified Mannich condensation from resveratrol, allylamine, and paraformaldehyde (Scheme 1). The NMR proton and carbon assignments for the oxazine ring and the allyl group in RESal are determined by both 2D 1 H-1 H NOESY and 1 H-13 C HMQC NMR spectra ( Figures S1 and S2). As can be seen in Figure 1 Figure 2 shows the 13 C NMR spectrum, which was provided to confirm the chemical structure of this monomer. As can be observed in Figure 2, two sets of triplet signals at the range of 45-50 and 81-83 ppm are characteristic carbon resonances for Ar-CH2-N-and -O-CH2-N-, respectively [4]. In addition, other typical carbon resonances of allyl group in RES-al can also be found around 54, 118, and 135 ppm, respectively [17]. Therefore, the above NMR results fully support that we have successfully obtained the target benzoxazine compound.  Figure 2 shows the 13 C NMR spectrum, which was provided to confirm the chemical structure of this monomer. As can be observed in Figure 2, two sets of triplet signals at the range of 45-50 and 81-83 ppm are characteristic carbon resonances for Ar-CH 2 -N-and -O-CH 2 -N-, respectively [4]. In addition, other typical carbon resonances of allyl group in RES-al can also be found around 54, 118, and 135 ppm, respectively [17]. Therefore, the above NMR results fully support that we have successfully obtained the target benzoxazine compound. FT-IR spectrum was obtained in this study to further confirm the existence of each functionality in this allyl-containing benzoxazine derived from resveratrol. As depicted in Figure 3, the band at 3077 cm −1 is attributed to the C-H stretching vibration in allyl group. In addition, the characteristic bands at around 1605 cm −1 are from the C=C stretching of resveratrol in benzoxazine structure. Moreover, the presence of the oxazine ring aromatic ether is evidenced by characteristic peak (C-O-C antisymmetric stretching) at 1236 cm −1 [28]. Furthermore, the typical band at 930 cm −1 is related to the oxazine ring [29].

Polymerization Behaviors of the Benzoxazine Monomer
The polymerization behavior during the thermal treatment is shown by the DSC thermograms of RES-al in Figure 4a. An endothermic peak at 79 °C suggests the good purity of this allyl-containing resveratrol-based benzoxazine monomer. RES-al exhibits a very broad exothermic peak with a maximum at 212 °C, which is very different from other well-known mono-or bis-benzoxazine resins [30][31][32]. The exothermic peak consists of three overlapped peaks, including the ring-opening polymerization, and other two reactions of resveratrol and allyl groups in benzoxazine. The three oxazine rings in the same molecule are expected to alter the curing reaction due to the fully FT-IR spectrum was obtained in this study to further confirm the existence of each functionality in this allyl-containing benzoxazine derived from resveratrol. As depicted in Figure 3, the band at 3077 cm −1 is attributed to the C-H stretching vibration in allyl group. In addition, the characteristic bands at around 1605 cm −1 are from the C=C stretching of resveratrol in benzoxazine structure. Moreover, the presence of the oxazine ring aromatic ether is evidenced by characteristic peak (C-O-C antisymmetric stretching) at 1236 cm −1 [28]. Furthermore, the typical band at 930 cm −1 is related to the oxazine ring [29]. FT-IR spectrum was obtained in this study to further confirm the existence of each functionality in this allyl-containing benzoxazine derived from resveratrol. As depicted in Figure 3, the band at 3077 cm −1 is attributed to the C-H stretching vibration in allyl group. In addition, the characteristic bands at around 1605 cm −1 are from the C=C stretching of resveratrol in benzoxazine structure. Moreover, the presence of the oxazine ring aromatic ether is evidenced by characteristic peak (C-O-C antisymmetric stretching) at 1236 cm −1 [28]. Furthermore, the typical band at 930 cm −1 is related to the oxazine ring [29].

Polymerization Behaviors of the Benzoxazine Monomer
The polymerization behavior during the thermal treatment is shown by the DSC thermograms of RES-al in Figure 4a. An endothermic peak at 79 °C suggests the good purity of this allyl-containing resveratrol-based benzoxazine monomer. RES-al exhibits a very broad exothermic peak with a maximum at 212 °C, which is very different from other well-known mono-or bis-benzoxazine resins [30][31][32]. The exothermic peak consists of three overlapped peaks, including the ring-opening polymerization, and other two reactions of resveratrol and allyl groups in benzoxazine. The three oxazine rings in the same molecule are expected to alter the curing reaction due to the fully

Polymerization Behaviors of the Benzoxazine Monomer
The polymerization behavior during the thermal treatment is shown by the DSC thermograms of RES-al in Figure 4a. An endothermic peak at 79 • C suggests the good purity of this allyl-containing resveratrol-based benzoxazine monomer. RES-al exhibits a very broad exothermic peak with a maximum at 212 • C, which is very different from other well-known mono-or bis-benzoxazine resins [30][31][32]. The exothermic peak consists of three overlapped peaks, including the ring-opening polymerization, and other two reactions of resveratrol and allyl groups in benzoxazine. The three oxazine rings in the same molecule are expected to alter the curing reaction due to the fully asymmetric structure of RES-al. Besides, the curing process of the C=C bond in allyl group could also extend the polymerization behavior of RES-al. Figure 4b shows the TGA thermogram of RES-al. A 5.5% of weight loss can be found before 300 • C for RES-al, which is much lower than the resveratrol and aniline based tri-benzoxazine resin. This suggests that the additional cross-linkage from the allyl group in RES-al helps to reduce the degradation during the curing processes. However, it is difficult to calculate the enthalpy of the polymerization of each reaction due to the overlapped polymerization behaviors involved during the polymerization of RES-al. Moreover, the additional degradation behavior also affects the enthalpy of the polymerization.
Polymers 2020, 12, x 5 of 10 asymmetric structure of RES-al. Besides, the curing process of the C=C bond in allyl group could also extend the polymerization behavior of RES-al. Figure 4b shows the TGA thermogram of RES-al. A 5.5% of weight loss can be found before 300 °C for RES-al, which is much lower than the resveratrol and aniline based tri-benzoxazine resin. This suggests that the additional cross-linkage from the allyl group in RES-al helps to reduce the degradation during the curing processes. However, it is difficult to calculate the enthalpy of the polymerization of each reaction due to the overlapped polymerization behaviors involved during the polymerization of RES-al. Moreover, the additional degradation behavior also affects the enthalpy of the polymerization. To systematically investigate the polymerization behaviors of this newly obtained benzoxazine resin, in situ FT-IR analyses were then employed, which utilized the typical band at 1236 cm −1 (C-O-C asymmetric stretching) and 930 cm −1 (oxazine ring related mode) to analyze the ring-opening polymerization behavior of RES-al (see Figure 5). The phenolic hydroxyl band in the region of 3700-3200 cm −1 obviously increases at higher temperature. Meantime, both the characteristic C-H stretching vibration of the substituted allyl group at 3077 cm −1 and the C=C stretching of resveratrol at 1605 cm −1 decrease after heating, and completely disappear at 300 °C. These observations suggest that the exothermic peak from the above DSC curve of RES-al involves three types of curing routes, including the oxazine ring polymerization, the olefin polymerization through the double bonds from resveratrol, and from the allyl functionalities. To systematically investigate the polymerization behaviors of this newly obtained benzoxazine resin, in situ FT-IR analyses were then employed, which utilized the typical band at 1236 cm −1 (C-O-C asymmetric stretching) and 930 cm −1 (oxazine ring related mode) to analyze the ring-opening polymerization behavior of RES-al (see Figure 5). The phenolic hydroxyl band in the region of 3700-3200 cm −1 obviously increases at higher temperature. Meantime, both the characteristic C-H stretching vibration of the substituted allyl group at 3077 cm −1 and the C=C stretching of resveratrol at 1605 cm −1 decrease after heating, and completely disappear at 300 • C. These observations suggest that the exothermic peak from the above DSC curve of RES-al involves three types of curing routes, including the oxazine ring polymerization, the olefin polymerization through the double bonds from resveratrol, and from the allyl functionalities. DSC was also applied to investigate the thermal behaviors of RES-al based thermosets. Figure 6 displays DSC thermograms of RES-al and its related thermosets. Clearly, the exothermic peak gradually decreased in intensity after the further heating treatment and fully disappeared for poly(RES-al)-3, suggesting the completion of the thermally activated polymerization behaviors in RES-al. In addition, the exothermic peak at 280 and 308 °C of poly(RES)-al-1 and poly(RES-al)-2 may represent the reactions of resveratrol and allyl groups after the polymerization of oxazine ring. In general, it is considered that there are two reaction approaches involved in the polymerization of benzoxazine. The first step is the ring-opening polymerization and followed with the electrophilic substitution. The final polybenzoxazine may have phenolic structure, phenoxyl structure, or both [4]. As the reaction site on the aromatic ring are highly substituted in the left part of the molecule in RESal, the oxazine ring in the left part could be polymerized to form phenoxyl structure. Therefore, we finally give a proposed polymerization behavior of RES-al as shown in Scheme 2.  DSC was also applied to investigate the thermal behaviors of RES-al based thermosets. Figure 6 displays DSC thermograms of RES-al and its related thermosets. Clearly, the exothermic peak gradually decreased in intensity after the further heating treatment and fully disappeared for poly(RES-al)-3, suggesting the completion of the thermally activated polymerization behaviors in RES-al. In addition, the exothermic peak at 280 and 308 • C of poly(RES)-al-1 and poly(RES-al)-2 may represent the reactions of resveratrol and allyl groups after the polymerization of oxazine ring. In general, it is considered that there are two reaction approaches involved in the polymerization of benzoxazine. The first step is the ring-opening polymerization and followed with the electrophilic substitution. The final polybenzoxazine may have phenolic structure, phenoxyl structure, or both [4]. As the reaction site on the aromatic ring are highly substituted in the left part of the molecule in RES-al, the oxazine ring in the left part could be polymerized to form phenoxyl structure. Therefore, we finally give a proposed polymerization behavior of RES-al as shown in Scheme 2. DSC was also applied to investigate the thermal behaviors of RES-al based thermosets. Figure 6 displays DSC thermograms of RES-al and its related thermosets. Clearly, the exothermic peak gradually decreased in intensity after the further heating treatment and fully disappeared for poly(RES-al)-3, suggesting the completion of the thermally activated polymerization behaviors in RES-al. In addition, the exothermic peak at 280 and 308 °C of poly(RES)-al-1 and poly(RES-al)-2 may represent the reactions of resveratrol and allyl groups after the polymerization of oxazine ring. In general, it is considered that there are two reaction approaches involved in the polymerization of benzoxazine. The first step is the ring-opening polymerization and followed with the electrophilic substitution. The final polybenzoxazine may have phenolic structure, phenoxyl structure, or both [4]. As the reaction site on the aromatic ring are highly substituted in the left part of the molecule in RESal, the oxazine ring in the left part could be polymerized to form phenoxyl structure. Therefore, we finally give a proposed polymerization behavior of RES-al as shown in Scheme 2.

Thermal Properties of Thermosets Derived from RES-al
The thermomechanical performances of RES-al derived thermosets were evaluated by DMA, and the glass transition temperature (Tg) of each thermoset was determined by the maximal of the tan δ peak. As depicted in Figure 7  The thermal stabilities of RES-al based thermosets were evaluated by TGA under both N2 and air conditions (see Figures 8 and 9). Notably, the temperatures of Td5 and Td10 of these thermosets are much higher than some other reported thermosets generated from traditional benzoxazine resins [33,34]. Herein, some initial weight loss in both nitrogen and air can be observed for poly(RES-al)-1 and poly(RES-al)-2, which is because of the decomposition of defect structures in their cross-lined networks [35], but poly(RES-al)-3 showed even better thermal stability, with the highest Td5 of 352 °C (in both N2 and air), and Td10 of 378 °C in N2 and 380 °C in air. The improved thermal stability of thermoset is mainly attributed to the multiple cross-linked polymer structure generated from the oxazine ring-opening polymerization and the additional polymerization of the C=C groups in resveratrol and allyl groups, which also significantly reduces the defects in terminal chain-ends. Poly(RES-al)-3 also exhibits a high char yield value of 53% at 800 °C in N2. All data related to thermal properties of thermosets are summarized in Table 1. We should also point out that the enhanced thermal stability of poly(RES-al) is also partially attributed to the removing of some defects in the polybenzoxazine network during the high temperature polymerization. As a result, the excellent thermal properties suggest that resveratrol-based thermosets reported here are promising materials for high-temperature applications.

Thermal Properties of Thermosets Derived from RES-al
The thermomechanical performances of RES-al derived thermosets were evaluated by DMA, and the glass transition temperature (T g ) of each thermoset was determined by the maximal of the tan δ peak. As depicted in Figure 7, poly(RES-al)-1, poly(RES-al)-2, and poly(RES-al)-3 show T g temperatures at 230, 274, and 313 • C, respectively. In all of the cases, the T g values increase with the polymerization temperature and are all higher than their corresponding polymerization temperatures. Poly(RES-al)-3 exhibits higher storage modulus value than both poly(RES-al)-1 and poly(RES-al)-2 from room temperature to 300 • C. The excess crosslink points attributed from the C=C in allyl and resveratrol significantly increase the overall cross-linking density, which increases the T g of RES-al based thermosets.

Thermal Properties of Thermosets Derived from RES-al
The thermomechanical performances of RES-al derived thermosets were evaluated by DMA, and the glass transition temperature (Tg) of each thermoset was determined by the maximal of the tan δ peak. As depicted in Figure 7  The thermal stabilities of RES-al based thermosets were evaluated by TGA under both N2 and air conditions (see Figures 8 and 9). Notably, the temperatures of Td5 and Td10 of these thermosets are much higher than some other reported thermosets generated from traditional benzoxazine resins [33,34]. Herein, some initial weight loss in both nitrogen and air can be observed for poly(RES-al)-1 and poly(RES-al)-2, which is because of the decomposition of defect structures in their cross-lined networks [35], but poly(RES-al)-3 showed even better thermal stability, with the highest Td5 of 352 °C (in both N2 and air), and Td10 of 378 °C in N2 and 380 °C in air. The improved thermal stability of thermoset is mainly attributed to the multiple cross-linked polymer structure generated from the oxazine ring-opening polymerization and the additional polymerization of the C=C groups in resveratrol and allyl groups, which also significantly reduces the defects in terminal chain-ends. Poly(RES-al)-3 also exhibits a high char yield value of 53% at 800 °C in N2. All data related to thermal properties of thermosets are summarized in Table 1. We should also point out that the enhanced thermal stability of poly(RES-al) is also partially attributed to the removing of some defects in the polybenzoxazine network during the high temperature polymerization. As a result, the excellent thermal properties suggest that resveratrol-based thermosets reported here are promising materials for high-temperature applications. The thermal stabilities of RES-al based thermosets were evaluated by TGA under both N 2 and air conditions (see Figures 8 and 9). Notably, the temperatures of T d5 and T d10 of these thermosets are much higher than some other reported thermosets generated from traditional benzoxazine resins [33,34]. Herein, some initial weight loss in both nitrogen and air can be observed for poly(RES-al)-1 and poly(RES-al)-2, which is because of the decomposition of defect structures in their cross-lined networks [35], but poly(RES-al)-3 showed even better thermal stability, with the highest T d5 of 352 • C (in both N 2 and air), and T d10 of 378 • C in N 2 and 380 • C in air. The improved thermal stability of thermoset is mainly attributed to the multiple cross-linked polymer structure generated from the oxazine ring-opening polymerization and the additional polymerization of the C=C groups in resveratrol and allyl groups, which also significantly reduces the defects in terminal chain-ends. Poly(RES-al)-3 also exhibits a high char yield value of 53% at 800 • C in N 2 . All data related to thermal properties of thermosets are summarized in Table 1. We should also point out that the enhanced thermal stability of poly(RES-al) is also partially attributed to the removing of some defects in the polybenzoxazine network during the high temperature polymerization. As a result, the excellent thermal properties suggest that resveratrol-based thermosets reported here are promising materials for high-temperature applications.

Conclusions
A new tri-benzoxazine resin based on resveratrol and allyl groups was synthesized and characterized in the current work. The thermally activated polymerization of RES-al proceeded in multiple curing approaches, including the ring-opening polymerization of oxazine ring and the polymerization of C=C bonds in both resveratrol and allyl groups in benzoxazine. Notably, the resulting fully polymerized thermoset showed prominent thermal stability with high Td5 (352 °C) and Td10 (378~380 °C) values in both N2 and air. This newly obtained thermoset also exhibited high Tg of 313 °C and a high char yield value of 53% at 800 °C under nitrogen atmosphere. The excellent thermal properties suggest the great potential of the thermosets derived from this newly obtained resveratrol and allyl-based tri-benzoxazine resin in high-performance areas.

Conclusions
A new tri-benzoxazine resin based on resveratrol and allyl groups was synthesized and characterized in the current work. The thermally activated polymerization of RES-al proceeded in multiple curing approaches, including the ring-opening polymerization of oxazine ring and the polymerization of C=C bonds in both resveratrol and allyl groups in benzoxazine. Notably, the resulting fully polymerized thermoset showed prominent thermal stability with high Td5 (352 °C) and Td10 (378~380 °C) values in both N2 and air. This newly obtained thermoset also exhibited high Tg of 313 °C and a high char yield value of 53% at 800 °C under nitrogen atmosphere. The excellent thermal properties suggest the great potential of the thermosets derived from this newly obtained resveratrol and allyl-based tri-benzoxazine resin in high-performance areas.

Conclusions
A new tri-benzoxazine resin based on resveratrol and allyl groups was synthesized and characterized in the current work. The thermally activated polymerization of RES-al proceeded in multiple curing approaches, including the ring-opening polymerization of oxazine ring and the polymerization of C=C bonds in both resveratrol and allyl groups in benzoxazine. Notably, the resulting fully polymerized thermoset showed prominent thermal stability with high T d5 (352 • C) and T d10 (378~380 • C) values in both N 2 and air. This newly obtained thermoset also exhibited high T g of 313 • C and a high char yield value of 53% at 800 • C under nitrogen atmosphere. The excellent thermal properties suggest the great potential of the thermosets derived from this newly obtained resveratrol and allyl-based tri-benzoxazine resin in high-performance areas.
Author Contributions: K.Z. and X.H. conceived and designed the experiments; Y.X. and R.Y. performed the experiments; K.Z., Y.X., and S.Y. wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding:
The authors thank the National Natural Science Foundation of China (52073125, 51603093).