Synthesis and Characterization of Bio‐based Polybenzoxazine Hybrids From Vanillin, Thymol, and Carvacrol

Abstract

Recently, the fast advancement of bio‐based polymers has boosted the interest in green and sustainable materials. In this context, polybenzoxazine‐derived renewable resources have been widely investigated due to their environmental benefits and high mechanical and thermal properties. This study focused on synthesizing hybrid benzoxazines from bio‐phenolic compounds— vanillin, thymol, and carvacrol— combined with Jeffamine D‐230 and paraformaldehyde. The chemical structures of hybrid benzoxazines (Van‐JD, Thy‐JD, and Car‐JD) were characterized by Fourier transform infrared spectroscopy (FT‐IR) and nuclear magnetic resonance spectroscopy (¹H NMR and ¹³C NMR). Furthermore, the curing behavior and thermal stability of synthesized bio‐phenolic‐based benzoxazines were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

In this study, we synthesized hybrid benzoxazines from the bio‐phenolic compounds—vanillin, thymol, and carvacrol— Jeffamine D‐230, and paraformaldehyde through a Mannich reaction. The thermal polymerizations of the hybrid benzoxazines were analyzed using differential scanning calorimetry (DSC).

1. Introduction

Benzoxazines are a class of heterocyclic compounds consisting of a benzene ring fused with an oxazine ring. These compounds have gained significant industrial importance due to their unique properties and variety of applications. Among the various isomers, the 1,3‐benzoxazine is prominent as the only known polymerizable form, making it especially valuable for advanced thermoset synthesis. 1,3‐Benzoxazines can be synthesized via the Mannich reaction, which involves a phenolic compound, primary amine, and an aldehyde, and can be carried out using either solution‐based or solvent‐free methods. The first synthesis of 1,3‐benzoxazine was reported by Holy and Cope in 1944.^{[ 1 ]} The polymerization of 1,3‐benzoxazine was discovered in 1973, and Higgenbottom pioneered the development of cross‐linked polybenzoxazines during the 1980s.^{[ 2 ]} Additionally, the reaction kinetics of polybenzoxazines was studied by Riese et al.^{[ 3 ]} Ning and Ishida synthesized polybenzoxazines and characterized their thermal and mechanical properties in 1994.^{[ 4 ]}

Generally, polybenzoxazines are obtained thermally by cationic ring‐opening polymerization of 1,3‐benzoxazines using non‐catalytic conditions.^{[ 5 ]} Depending on the functional groups in the 1,3‐benzoxazine structures, they are polymerized between 180 °C–250 °C. However, using catalysts can significantly reduce the polymerization temperature, making the process more energy‐efficient and environmentally friendly and minimizing thermal stress on processing equipment. Additionally, catalysts also help maintain curing temperatures within standard industrial limits. Several catalytic systems have been used for this purpose, such as acid sources,^{[ 6 ]} thiols,^{[ 7 ]} sulfur,^{[ 8 ]} salts,^{[ 9 ]} metal‐organic‐frameworks,^{[ 10 ]} metal ligands,^{[ 11 ]} and amines.^{[ 12 ]}

Polybenzoxazines have emerged as a promising alternative to traditional resins like epoxy, polyester, and phenolic resins. Polybenzoxazine resins are highly valued for their superior properties, including excellent thermal stability,^{[ 13a,b ]} non‐flammability,^{[ 13a ]} low water absorption,^{[ 14 ]} negligible volume change upon curing,^{[ 15 ]} chemical resistance, and low smoke production during combustion.^{[ 16 ]} These features make them highly desirable for composite materials, coatings, adhesives, and encapsulants, especially in the automotive, aerospace, and electronics industries.^{[ 17 ]}

Commercially known as Jeffamines, polyetheramines are versatile compounds available in various chain lengths and functional groups. The long alkyl chains in Jeffamine contribute to the flexibility and durability of the structures they form. These compounds are widely utilized as curing agents in epoxy resins, enhancing flexibility and strength. Jeffamines find extensive applications in the coatings industry for protective, decorative, and flooring purposes, as well as in the adhesive and composite industries.^{[ 18 ]} Jeffamines are frequently used in benzoxazine chemistry. Generally, they play a role in improving the mechanical properties of benzoxazine structures by imparting flexibility to them, which are typically rigid and brittle. Additionally, the polyether backbone of Jeffamines imparts an oily form to most of the Jeffamine‐based benzoxazines at room temperature, facilitating easier handling and processing in composite fabrication. The low melting point of the resins generally enhances their applicability in advanced material systems, where uniformity is critical. Moreover, Jeffamines influence the polymerization temperature of benzoxazines, allowing better control over curing conditions. Another important feature of Jeffamine‐based benzoxazines is the low evaporation of monomers upon curing, which is a serious problem for some benzoxazine monomers. Jeffamines such as Jeffamine D‐230 have relatively short chain lengths and can increase the crosslink density of polybenzoxazines. Longer chain Jeffamines, that is, D‐5000, have been reported to produce waxy, poorly crosslinked materials due to their tendency to form telechelic‐like structures. The use of Jeffamines as an amine group in the synthesis of benzoxazines was first studied by Yildirim et al.^{[ 19a ]} In addition, Agag et al. synthesized Jeffamine‐based benzoxazine, and the resulting polymer contains linear oligomers that are liquid at room temperature and can undergo crosslinking.^{[ 19b ]} Following this study, further research has been conducted on the gas permeability,^{[ 20 ]} corrosion resistance properties,^{[ 21 ]} shape memory behavior,^{[ 22 ]} and self‐healing abilities of Jeffamine‐based benzoxazines.^{[ 23 ]}

The increasing importance of Green Chemistry, the reduction of carbon footprint, and the integration of sustainable, renewable resource materials into daily life represent sustainable choices. Reducing or eliminating the use of petroleum‐based chemicals is important in this context. This research direction has also recently been seen in benzoxazine chemistry. In the last decade, green alternatives have been preferred to substitute petroleum‐based phenols, naphthols, and their derivatives in the benzoxazine synthesis.^{[ 24 ]} Numerous studies have utilized bio‐based phenolic compounds like cardanol,^{[ 25 ]} guaiacol,^{[ 26 ]} eugenol,^{[ 27 ]} catechol,^{[ 28 ]} magnolol,^{[ 29 ]} sesamol,^{[ 30 ]} umbelliferone,^{[ 31 ]} resveratrol,^{[ 32 ]} and vanillin,^{[ 33 ]} as well as primary amine compounds like furfurylamine,^{[ 34 ]} stearylamine,^{[ 26} , ²⁷ , ^{35 ]} and dopamine.^{[ 36 ]} The shift toward bio‐based phenols and amines is in accordance with Green Chemistry principles. While compounds like cardanol and vanillin have been extensively studied, thymol^{[ 37 ]} and carvacrol^{[ 38 ]} remain relatively less studied. Apart from this, the combination of Jeffamines with bio‐phenols has yet to be fully investigated. While the Jeffamines are not classified as bio‐based amines, they have several advantages, such as relatively lesser toxicity compared to many primary amines, ease of handling, and commercial availability. Their high solubility in alcohols and moderate solubility in water further enhance their suitability for bio‐hybrid systems. Given these properties, Jeffamines are considered good alternatives to bioamines in the synthesis where hybrid systems are preferred. In this context, “hybrid” refers to resins synthesized from a combination of bio‐sourced phenolic compounds and an industrially produced Jeffamine. Accordingly, in this study, we aimed to synthesize bio‐phenolic‐based benzoxazines using vanillin, thymol, and carvacrol in combination with polyetheramines, and then investigate their thermal properties and curing behavior to explore their potential in bio‐hybrid systems.

2. Results and Discussion

As highlighted in the introduction, the synthesis of resins from bio‐phenols supports the objectives of the principles of Green Chemistry. While fully bio‐based materials represent one aspect of this approach, another involves combining bio‐phenols with comparatively harmless, low, or non‐toxic chemicals to prepare biohybrid resins. We utilized vanillin, thymol, and carvacrol as naturally occurring phenolic compounds to achieve the latter approach. These compounds were then reacted to a Mannich reaction with polyether diamine (Jeffamine D‐230) and paraformaldehyde to synthesize bio‐phenolic‐based benzoxazines (Van‐JD, Thy‐JD, Car‐JD, respectively) (Scheme 1).

Scheme 1.

Synthesis of bio‐phenolic‐based benzoxazines.

To elucidate the molecular structures of the synthesized hybrid benzoxazines, detailed Fourier transform infrared (FT‐IR) and nuclear magnetic resonance (NMR) spectroscopic analyses were performed.

Accordingly, the IR characteristics of the compounds are as follows: The FT‐IR spectrum of Van‐JD, the benzoxazine derived from vanillin, Jeffamine D‐230, and paraformaldehyde, is presented in Figure 1 (an alternative overlaid figure is also presented as Figure S1). The typical aldehyde C─H and C═O stretching vibration bands are visible at 2760, 2724, and 1683 cm⁻¹, respectively. This indicates that the aldehyde group of vanillin remained throughout the Mannich reaction and subsequent cyclization. The characteristic mode of the substituted benzene C═C stretching band is seen at 1492 cm⁻¹. In addition, bands of the benzoxazine ring structure of Van‐JD appeared at 1290, 1237, and 1017 cm⁻¹ because of antisymmetric and symmetric stretching modes of C─O─C, respectively. Besides, aliphatic C─O─C stretching bands are visible at 1137 and 1090 cm⁻¹. Further confirmation of the oxazine ring is visible at 973 cm⁻¹ as out of the absorption mode of the benzene ring attached to the oxazine ring.

Figure 1.

FT‐IR spectra of Van‐JD, Thy‐JD, and Car‐JD.

Thymol and carvacrol are structural isomers that share the same molecular formula but differ in the arrangement of their atoms. Given their chemical similarity, the benzoxazine derivatives synthesized from these two isomers (Thy‐JD and Car‐JD) exhibit remarkably similar structural characteristics, which were determined by FT‐IR spectroscopic analysis. The FT‐IR spectra (Figure 1) of both benzoxazines (Thy‐JD and Car‐JD) are consistent with their respective structures. The C═C stretching bands of the substituted benzene appear at 1490 cm⁻¹ in Thy‐JD and 1489 cm⁻¹ in Car‐JD. The bands at 958 cm⁻¹ (in the Thy‐JD spectrum) and 964 cm⁻¹ (in the Car‐JD spectrum) are attributed to the out‐of‐the‐plane vibration of the benzene ring attached to the oxazine ring. In the context of 1,3‐benzoxazines, these out‐of‐plane vibrational modes are considered diagnostic peaks. They serve as fingerprints to confirm the successful formation of a 1,3‐oxazine ring.

In the ¹H NMR analysis of Van‐JD, the C─H signal (a) corresponding to the aldehyde group and the proton signals (b) of the OCH₃ group from vanillin were observed as singlets at 9.80 and 3.93 ppm, respectively. Additionally, the protons (c and d) associated with the oxazine ring appeared as multiplets in the 5.17–5.10 ppm and 4.21–4.13 ppm, respectively. Signals corresponding to the protons (e and f) from Jeffamine D‐230 were also observed in the spectrum (Figure 2). The signal for the aldehyde carbon appears at 190.9 ppm, while the carbon signals for the oxazine ring are observed at 82.5 and 82.4 ppm (OCH₂N) and 55.95 ppm (NCH₂). The carbon signal for the OCH₃ group is located at 56.0 ppm. Additionally, carbon signals from Jeffamine are observed in the range of 75.4–72.5 ppm for OCH₂ and at 55.92, 47.4, and 47.3 ppm for NCHCH₃ in the spectrum of ¹³C NMR of Van‐JD (Figure 2).

Figure 2.

¹H NMR and ¹³C NMR spectra of Van‐JD.

The protons associated with the oxazine ring are identifiable in the ¹H NMR spectra of Thy‐JD and Car‐JD (Figure 3). For the Thy‐JD, the multiplet at 4.96–4.89 ppm corresponds to the NCH ₂O group, while the multiplet signal in the range of 4.02–3.94 ppm is attributed to the NCH ₂ protons of the oxazine ring. Similarly, for the Car‐JD compound, the NCH ₂O protons appear as a multiplet in the 5.02–4.96 ppm range, and the NCH ₂ protons are observed as a multiplet between 4.22 and 4.14 ppm. Notably, additional minor peaks can be attributed to the Jeffamines D‐230 component, and are consistent with the repeating ─(C(CH₃)─O) _n ─ unit. Jeffamine D‐230 has an average ‘n’ value of 2.5 and this variability is inherent to the industrially produced Jeffamines. The ¹³C NMR spectra of both compounds (Figure S2) are consistent with their structures. The carbons of the oxazine ring are observed at 80.3 and 80.2 ppm for the NCH₂O carbons in both spectra and at 56.1 and 55.9 ppm for the NCH₂ carbons in the spectra of the Thy‐JD and Car‐JD compounds, respectively.

Figure 3.

¹H NMR spectra of Thy‐JD and Car‐JD.

In conclusion, the spectroscopic data presented herein provides strong evidence for the successful synthesis of the desired bio‐phenolic‐based benzoxazines. The characteristic IR bands, proton, and carbon NMR signals observed for the synthesized compounds are in excellent agreement with the proposed molecular structures. Table 1 summarizes the key spectral data for easy reference.

Table 1.

The FT‐IR and NMR data of synthesized Van‐JD, Thy‐JD, and Car‐JD.

	FT‐IR (ν, cm−1) a)	1H NMR (δ, ppm) a)	13C NMR (δ, ppm) a)
Van‐JD	1492, 1290, 1237, 1017, 973	5.17–5.10 (NCH 2O) 4.21–4.13 (NCH 2)	82.5–82.4 (NCH2O) 56.0 (NCH2)
Thy‐JD	1490, 1280, 1247, 1222,1030, 958	4.96–4.89 (NCH 2O) 4.02–3.94 (NCH 2)	80.3–80.2 (NCH2O) 56.1 (NCH2)
Car‐JD	1489, 1250, 1233, 1040, 964	5.02–4.96 (NCH 2O) 4.22–4.14 (NCH 2)	80.3–80.2 (NCH2O) 55.9 (NCH 2)

^a) Belong to the oxazine ring in the structure.

At high temperatures, benzoxazine monomers can undergo thermal cationic ring‐opening polymerization (cROP) (see Supporting Information, Scheme S1 for a proposed cROP mechanism). The cROP of benzoxazine monomers is a complex thermal process critically influenced by multiple molecular and compositional factors. The polymerization temperature depends on the chemical nature of functional groups within the benzoxazine structure, the monomer's purity, and the concentration of oxazine units. The polymerization process of benzoxazines is an exothermic event, and differential scanning calorimetry (DSC) provides a powerful analytical technique for monitoring this exothermic polymerization process. Accordingly, DSC thermograms of Van‐JD, Thy‐JD, and Car‐JD show a single exothermic peak, which is the typical curing behavior of benzoxazines (Figure 4). The maximum curing temperatures were recorded as 207.1 °C for Van‐JD, 262.5 °C for Thy‐JD, and 256.1 °C for Car‐JD. Based on the DSC thermograms, the ΔH values were determined to be 56.3 J/g for Van‐JD, 43.3 J/g for Thy‐JD, and 24.1 J/g for Car‐JD based on the DSC thermograms.

Figure 4.

DSC thermograms of Van‐JD, Thy‐JD, and Car‐JD.

According to the DSC analysis, the ΔH varies between Thy‐JD and Car‐JD despite similar structures. In the curing process of benzoxazines, the methylene carbon forms a cation that reacts with the ortho and/or para position of the phenolic moiety through electrophilic aromatic substitution. For Thy‐JD, the presence of a bulky isopropyl group in close proximity to the para position generates a steric hindrance, retarding the substitution compared to Car‐JD, where less hindrance exists. This increased reaction difficulty in Thy‐JD likely results in a higher activation energy (E _a) and ΔH.

Here it should be noted that Van‐JD, Thy‐JD, and Car‐JD have broad exothermic curing bands. Ideally, a DSC thermogram of a chemical reaction shows a sharp exothermic peak. However, in practice, peak broadening or distortion often occurs due to overlapping reactions, like decomposition, depolymerization, and rearrangements, which often broaden or distort peaks. For example, in the case of benzoxazines, ether‐amine (─N─CH₂─O─Ar) linkage rearrangement to (─N─CH₂─Ar) is known for some polybenzoxazines with thermal effects. Accordingly, the band broadening observed in Figure 4 is likely due to the abovementioned overlapping effects.

Determining the extent of ROP is crucial for understanding the reaction dynamics of benzoxazine monomers. DSC enables precise measurement of monomer conversion percentage through a systematic analytical approach. According to Equation (1), the ROP reaction conversion for any given temperature (T _α) can be calculated as the ratio of the areas under the ROP exotherm (T _α) to the total heat of reaction.

$$\alpha =\frac{\mathrm{\Delta }{H}_{T\alpha }}{\mathrm{\Delta }{H}_{\mathrm{total}}}$$ (1)

In Figure 5, the conversion plots of all benzoxazines against temperature showed a characteristic S‐shaped curve. This S‐shape indicates a gradual increase in conversion as the temperature rises. During the initial stage, the process begins slowly, corresponding to the initiation of the ring‐opening reaction. This is followed by a second stage where conversion rises rapidly indicating an acceleration likely driven by an autocatalytic process. In this process, reaction products act as catalysts to speed up the reaction and produce the observed sharp increase in conversion. Finally, the reaction slows and eventually approaches a plateau as it completes.

Figure 5.

Conversion % versus temperature of bio‐phenolic‐based benzoxazines.

Understanding the kinetic parameters of benzoxazine polymerization is fundamental for both theoretical molecular design and practical industrial applications. Such parameters provide critical insights into the thermochemical behavior of polymeric systems, revealing the complex molecular mechanisms that govern chemical reactions. Hence, the activation energy (E _a) is an important kinetic parameter, which acts as a quantitative measure of the energy barrier that must be overcome for the polymerization to proceed. In the context of benzoxazine curing, E _a directly determines the molecular resistance to ring‐opening polymerization.

The E _a of the monomer can be determined using the Kissinger and Ozawa equations,^{[ 39 ]} which are derived from the Arrhenius equation (k = A exp(−E _a/RT)).

These calculations are based on the peak temperatures obtained from scans performed at different heating rates. Accordingly, the Arrhenius derivative is used for the Kissinger approach and is expressed as Equation (2).

$$\ln \frac{\beta }{{T_p}^2}=\ln \frac{AR}{E_a}-\frac{E_a}{R{T}_p}$$ (2)

where β, A, R, and T _p, are the heating rate, the frequency factor, the gas constant, and the maximum temperature of the exotherm, respectively. The plot of ln (β/T _p ²) slope versus 1/T _p provides the E _a value for the curing process. The modified Ozawa method can also be applied, which complements the Kissinger approach and validates and cross‐references activation energy calculations. The E _a from the Ozawa method is determined using Equation (3).

$$\ln \beta =-1.052\frac{E_{\mathrm{a}}}{R{T}_{\mathrm{p}}}+C$$ (3)

where C is the constant, the E _a can be derived from the slope of the plot ln β against 1/T _p.

All the corresponding plots and thermograms generated from different heating rates are presented in Figure 6. Accordingly, the E _a values were calculated using Kissinger and Ozawa methods and are summarized in Table 2 and Figure 6. For Van‐JD, E _a was found to be 90.98 kJ/mol using the Kissinger method and 94.00 kJ/mol using the Ozawa method. For Thy‐JD, the E _a value was calculated to be 140.46 kJ/mol using the Kissinger method and 141.98 kJ/mol using the Ozawa method. For the Car‐JD, the E _a is 82.96 kJ/mol using the Kissinger method and 87.11 kJ/mol using the Ozawa method.

Figure 6.

DSC thermograms and Kissinger and Ozawa graphs for (a) Van‐JD, (b) Thy‐JD, and (c) Car‐JD.

Table 2.

The calculated E _a values from DSC plots.

	The E a Value (Kissinger) in kJ/mol	The E a Value (Ozawa) in kJ/mol
Van‐JD	90.98	94.00
Thy‐JD	140.46	141.98
Car‐JD	82.96	87.11

In addition to kinetic studies, the thermal stabilities of the synthesized benzoxazine monomers after curing were evaluated using thermogravimetric analysis (TGA). In the case of vanillin‐based polybenzoxazine (from the Van‐JD), the T _5% value was 254 °C, and the T _10% value was 275 °C. For the thymol‐based polybenzoxazine (from the Thy‐JD) exhibited T _5% and T_10% values of 290 °C and 312 °C, while the carvacrol‐based polybenzoxazine from Car‐JD showed T _5% and T _10% values of 231 °C and 248 °C, respectively (Figures S3–S5).

Polybenzoxazines, depending on their structure, generally exhibit char yields (Yc) ranging between 20% and 70%.^{[ 40 ]} However, the char formation in polybenzoxazine systems is a complex phenomenon sensitive to multiple molecular architectural factors. Polyether amines generally demonstrate a detrimental effect on char yield compared to aromatic amines. Their aliphatic ether structure tends to diminish thermal stability and lower the potential for robust char formation at elevated temperatures. Moreover, it is well known that during polymerization, carbocation intermediates attack phenolic groups' ortho and para positions, resulting in cross‐linked polymers. Polybenzoxazines with para positions occupied by substituents tend to have char yields below 30% due to lower cross‐linking.^{[ 18b ]} In addition, the unique structural characteristics of thymol and carvacrol introduce additional limitations to char yield because of aliphatic substituents positioned on aromatic rings, reduced cross‐linking potential due to steric hindrances of the substituent on aromatic rings and therefore interruption of optimal polymer network formation. For the synthesized polymers, the only char yield was 23% for the polymer from Thy‐JD at 800 °C. The derivative of the TGA data revealed maximum decomposition temperatures (T _max) for mass loss as 541 °C for Van‐JD, 375 °C for Thy‐JD, and 306.4 °C for Car‐JD (Table 3). Although the char formation is limited, the T _max and initial degradation temperatures of these polymers are relatively similar to many other polybenzoxazines.

Table 3.

Thermal properties of hybrid benzoxazines before and after thermal curing.

Benzoxazine	T 5% (°C)	T 10% (°C)	T max (°C)	Yc (%) a)
Van‐JD	254	275	541	≈0
Thy‐JD	290	312	375	23
Car‐JD	231	248	306.4	≈0

^a) At 800 °C

The structural modeling of polymer networks is well‐established with various methodologies developed in the literature. The Flory–Rehner model is widely recognized among these methods. This model can accurately estimate crosslinking densities to comprehend some mechanical behavior of polymer materials. This approach predicts crosslinking densities that correspond closely with experimental rheological data.^{[ 41} , ^{42 ]}

Therefore, in this study, we utilized the Flory–Rehner equation (Equation 4) to calculate the crosslinking density (ʋ) of polyThy‐JD and polyCar‐JD, based on their swelling properties. The equation is expressed as:

(4)

The variables in this equation are as follows: d _p; the polymer density, V ₁; the molar volume of the solvent (cm³/mol), ʋ _2m; the volume fraction of polymer, f; the cross‐link functionality, χ ₁; the Flory–Huggins solvent‐polymer interaction parameter, and M _c, the molecular weight between cross‐links (g/mol). The volume fraction in the swollen network (ʋ _2m) is calculated using Equation (5).

$${\upsilon }_{2m}=\frac{V_{\mathrm{p}}}{V_{\mathrm{g}}}=\frac{V_{\mathrm{g}}-V_{solvent}}{V_{\mathrm{g}}}=1-\frac{m_{\mathrm{solvent}}{d}_{\mathrm{g}}}{d_{\mathrm{solvent}}{m}_{\mathrm{g}}}$$ (5)

In this equation, d _s is the density of the solvent, d _g is the density of the swollen polymer, m _solvent is the mass of the solvent within the swollen polymer, m _g is the mass of the swollen polymer, V _p is the volume of the polymer, and V _g is the volume of the swollen polymer. Using these parameters, the crosslinking densities for polyThy‐JD and polyCar‐JD were estimated to be 0.0706 and 0.1324 mol/cm³, respectively. (The detailed calculation procedure is provided in the Supporting Information).

Moreover, to gain insights into the viscoelastic behavior of polyVan‐JD, rheological measurements were conducted at 45 °C. The amplitude sweep experiment revealed that this polymer exhibits Newtonian behavior, consistent with expectations for a thermosetting polymer. At this temperature, the storage modulus of polyVan‐JD was determined to be approximately 6.44 × 10⁷ Pa, while the loss modulus was around 3400 Pa (see Supporting Information Figure S6).

3. Conclusion

This study demonstrates the successful synthesis of hybrid benzoxazines (Van‐JD, Thy‐JD, and Car‐JD) using vanillin, thymol, and carvacrol in combination with Jeffamine D‐230 through a Mannich reaction. The thermal polymerization behavior of the synthesized compounds was investigated through DSC analysis. Van‐JD exhibited the lowest curing temperature (207.1 °C), while Thy‐JD and Car‐JD demonstrated higher temperatures (262.5 °C and 256.1 °C, respectively). These variations are attributed to the structural differences of the bio‐phenolic monomers, influencing the energy barrier for ring‐opening polymerization. The conversion analysis revealed an autocatalytic process, characterized by an initial slow reaction phase, followed by rapid acceleration and eventual plateau as the reaction approached completion.

Overall, the combination of bio‐phenolic compounds with Jeffamines offers a promising pathway for developing biohybrid benzoxazine resins, which is in line with Green Chemistry principles. The widespread availability of Jeffamines further enhances the practicality of this approach. Moreover, the biohybrid nature of these resins makes them suitable candidates for sustainable applications in adhesives or coatings, which can contribute to a reduced environmental footprint.

4. Experimental Section

4.1. Materials

Vanillin (Merck), thymol (98%, Ambeed), carvacrol (98%, Ambeed), Jeffamine D‐230 (Sigma–Aldrich), paraformaldehyde (96%, Acros), chloroform (anhydrous, ≥99.5%, Sigma–Aldrich), sodium hydroxide (pellets, Sigma–Aldrich), and sodium sulfate (anhydrous, ≥99%, Sigma–Aldrich).

4.2. Nuclear Magnetic Resonance Spectroscopy

¹H NMR and ¹³C NMR spectra were recorded at room temperature on Agilent VNMRS 500/125 MHz, respectively. Chemical shifts (δ) are reported in ppm. Splitting patterns were described as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of doublet), and br (broad signal).

4.3. Fourier‐Transform Infrared Spectroscopy

Fourier‐transform infrared spectroscopy (FT‐IR) spectra were obtained using a Nicolet 6700 FT‐IR spectrometer from Thermo Fisher Scientific equipped with attenuated total reflection (ATR).

HR‐MS analyses were performed on Agilent 6230‐B TOF LC/MS in m/z.

4.4. Thermal Analysis

Thermal properties were investigated by differential scanning calorimeter (DSC) using a PerkinElmer Diamond DSC under nitrogen flow. Samples (3–5 mg) were placed in 30 µL aluminum pans. The instrument was programmed from 30 °C to 300 °C at a heating rate of 10 °C/min. Differential thermal analysis (DTA) and thermal gravity analysis (TGA) were performed on Perkin‐Elmer Diamond TA/TGA at a heating rate of 10 °C/min. from 50 °C to 900 °C under nitrogen.

4.5. The General Procedure of Synthesis of Hybrid Benzoxazine

A bio‐based phenolic compound (6.6 mmol) and Jeffamine D‐230 (3.3 mmol) were added to 40 mL of chloroform and stirred until a clear solution was formed. Then paraformaldehyde (13.2 mmol) was added to the solution and refluxed for 48 h. The mixture cooled to room temperature and washed with 1N sodium hydroxide solution (3 × 30 mL) and water (2 × 20 mL). The organic phase was dried with sodium sulfate, and a solvent was evaporated in a vacuum. The product was used as obtained.

4.6. Synthesis of Van‐JD

The product was obtained according to the general procedure of benzoxazine synthesis as a light yellowish oil. (Yield 71%). FT‐IR (ATR, cm⁻¹): ν _max 3081, 2970, 2869, 2760, 2724, 1683, 1592, 1492, 1394, 1290, 1232, 1137, 1090, 1017, 973, 954, 881, 860, 752, 743 cm⁻¹; ¹H NMR (500 MHz, CDCl₃): δ 9.80 (s, 1H, CHO), 7.26 (bs, 1H, Ar─H), 7.13 (bs, 1H, Ar─H), 5.17–5.10 (m, 2H, O─CH ₂─N), 4.21–4.13 (m, 2H, N─CH ₂), 3.93 (s, 3H, OCH ₃), 3.58–3.13 (m, OCH _2, and NCHCH₃), 1.18–1.03 (m, CH ₃); ¹³C NMR (125 MHz, CDCl₃): δ 190.9, 150.3, 148.7, 128.8, 124.0, 122.2, 121.1, 107.8, 82.5, 82.4, 82.3, 75.4, 75.3, 75.2, 75.1, 75.0, 74.8, 72.6, 72.5, 56.0, 55.9, 47.4, 47.3, 17.2, 17.1, 17.0, 16.4, 16.3. HRMS: Calcd for C₂₉H₃₈N₂O₈ [M + H]⁺ 543.2706, found 544.7143.

4.7. Synthesis of Thy‐JD

Blondish oily products yielded 78% according to the general procedure of benzoxazine synthesis. FT‐IR (ATR, cm⁻¹): ν _max 3074, 2963, 2930, 2869, 1606, 1582, 1489, 1446, 1373, 1310, 1250, 1223, 1151, 1106, 1046, 1004, 964, 946, 905, 806, 753 cm⁻¹; ¹H NMR (500 MHz, CDCl₃): δ 6.96 (d, J = 7.80 Hz, 2H, Ar─H), 6.69 (d, J = 7.75 Hz, 2H, Ar─H), 4.96–4.89 (m, 4H, O─CH ₂─N), 4.02–3.94 (m, 4H, N─CH ₂), 3.63–3.15 (m, $\sim$16H, OCH ₂, NCHCH₃, and CH(CH₃)₂), 2.14 (s, 6H, Ph─CH ₃), 1.19 (d, J = 6.85 Hz, 12H, CH(CH ₃)₂, 1.14–1.07 (m, 12H, CH ₃); ¹³C NMR (125 MHz, CDCl₃): δ 152.1, 133.8, 133.7, 132.7, 132.6, 123.3, 121.3, 120.0, 119.9, 80.3, 80.2, 75.4, 75.3, 75.2, 75.1, 75.0, 74.9, 74.8, 74.7, 72.5, 72.4, 72.3, 56.1, 56.0, 55.9, 46.4, 46.3, 26.2, 22.7, 18.0, 17.3, 17.2, 17.1, 16.2, 16.1. HRMS: Calcd for C₂₅H₅₄N₂O₅ [M + H]⁺ 583.4111, found 584.0853.

4.8. Synthesis of Car‐JD

The dark yellow oily product yielded 56% according to the general procedure of benzoxazine synthesis. FT‐IR (ATR, cm⁻¹): ν _max 3075, 2963, 2927, 2869, 1608, 1586, 1490, 1458, 1375, 1280, 1247, 1222, 1103, 1061, 1030, 982, 958, 942, 914, 809, 754 cm⁻¹; ¹H NMR (500 MHz, CDCl₃): δ 7.01 (d, J = 7.80 Hz, 2H, Ar─H), 6.79 (d, J = 7.70 Hz, 2H, Ar─H), 5.02–4.96 (m, 4H, O─CH ₂─N), 4.22–4.14 (m, 4H, N─CH ₂), 3.71–3.32 (m, $\sim$13H), 3.25–3.21 (m, 2H), 2.95–2.90 (m, 2H), 2.20 (s, 6H, Ph─CH ₃), 1.26 (d, J = 7.05 Hz, 12H, CH(CH ₃)₂), 1.20–1.14 (m, $\sim$15H, CH ₃); ¹³C NMR (125 MHz, CDCl₃): δ 152.9, 143.7, 128.5, 123.0, 118.3, 116.0, 80.3, 80.2, 75.5, 75.4, 75.3, 75.2, 75.1, 75.0, 74.8, 72.6, 72.5, 72.4, 55.9, 55.8, 55.7, 55.6, 45.8, 28.1, 23.5, 23.4, 17.3, 17.2, 16.3, 16.2, 15.7. HRMS: Calcd for C₃₃H₅₀N₂O₄ [M + H]⁺ 539.3849, found 539.5297.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

Supporting Information

Erdeger Merve, Gungor Fusun Seyma, Chemistry - An Asian Journal. 2025, 11111. 10.1002/asia.202401777

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.