Synthesis, polymerization kinetics and thermal properties of para

Reactive and Functional Polymers 129 (2018) 23–28

Contents lists available at ScienceDirect

Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react

Synthesis, polymerization kinetics and thermal properties of para-methylol functional benzoxazine

T

Kan Zhanga,⁎, Lu Hanb, Pablo Froimowiczc, Hatsuo Ishidab,⁎ a

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, United States c Design and Chemistry of Macromolecules Group, Institute of Technology in Polymers and Nanotechnology (ITPN), UBA-CONICET, School of Engineering, University of Buenos Aires, Av. Gral. Las Heras 2214 (P.C. C1127AAR), Buenos Aires, Argentina b

A R T I C L E I N F O

A B S T R A C T

Keywords: Benzoxazine para-functional Methylol Polybenzoxazine Hydrogen bond

Monofunctional benzoxazine with para-methylol functionality has been synthesized via Mannich condensation. The chemical structure of the synthesized monomer is confirmed by 1H nuclear magnetic resonance (NMR), 13C NMR and Fourier transform infrared spectroscopy (FT-IR). Polymerizations are monitored by in situ FT-IR, 1H NMR and differential scanning calorimetry (DSC). Activation energy of the polymerization is also studied by DSC. The apparent activation energy of the polymerization is calculated to be 79.8 kJ mol− 1 and 81.5 kJ mol− 1 according to the Kissinger and Ozawa methods, respectively. In addition, thermogravimetric analysis (TGA) result shows that the polybenzoxazine derived from para-methylol functional benzoxazine possesses excellent thermal stability with an initial decomposition temperature of 395 °C for a 5% weight loss and a char yield of 60% at 800 °C under N2.

1. Introduction Over the past few years, the applications for polymers with high thermal stability have increased drastically due to technological advancements. Polybenzoxazines [1–6], as a novel type of thermosets, have been extensively reviewed because of their various attractive properties including high thermal stability [7], high char yield [8], high glass transition temperature (Tg) [9], near-zero volumetric change during polymerization [10], good mechanical [11] and dielectric properties [12], low water absorption [13] and low flammability [14]. The most outstanding characteristic of this class of polymers is their extraordinarily rich molecular design flexibility that allows designing a variety of molecular structures to tailor the desired properties. Moreover, benzoxazine monomers can easily be synthesized via Mannich reaction involving condensation from various phenols, amines, and formaldehyde. By taking advantages of benzoxazine chemistry, various benzoxazines containing different functional groups such as amide [15–17], imide [7,18], allyl [19,20], nitrile [21], furyl [22], benzoxazole [23] have been developed to be studied and applied in high performance fields. Benzoxazines containing hydroxyl group have drawn attention due to their lower polymerization temperature in comparison with the traditional benzoxazine resins [24,25]. Polymerization of these

⁎

Corresponding authors. E-mail addresses: [email protected] (K. Zhang), [email protected] (H. Ishida).

http://dx.doi.org/10.1016/j.reactfunctpolym.2017.06.017 Received 9 December 2016; Received in revised form 26 June 2017; Accepted 27 June 2017 Available online 28 June 2017 1381-5148/ © 2017 Elsevier B.V. All rights reserved.

monomers usually proceeds smoothly at relatively low temperature probably due to the presence of hydrogen bonds established by the available OH groups [25]. Among these hydroxylated compounds, methylol functional benzoxazines have been reported as a smart precursor for chemical modification and polymeric designing [26–30]. Jin et al. developed a methacryloyl-benzoxazine-containing monomer from methylol functional benzoxazine and studied its photopolymerization behavior as well as thermal polymerization [26]. Furthermore, methylol functional benzoxazines were synthesized as precursors for poly (benzoxazine-co-urethane)s, which show high thermal stability and excellent mechanical integrity [27]. In addition, Baqar et al. synthesized a series of methylol functional benzoxazine monomers with different isomeric hydroxybenzyl alcohols and experimentally demonstrated the unique simultaneous addition and condensation polymerization behavior of the ortho isomer [28,29]. Hydrogenbonding interactions and their effect on the polymerization mechanism of ortho-methylol functional benzoxazine were further investigated using highly pure samples [30]. This para-methylol benzoxazine has been much less studied than its isomer, the ortho-methylol. It must be emphasized however that paramethylol benzoxazine also presents the mentioned advantages when compared with other traditional benzoxazines, although to a lesser extent than the ortho-methylol isomer. Thus, this work aims at studying


K. Zhang et al.

Scheme 1. Synthesis of an unsubstituted benzoxazine (PH-a) and the para-methylol functional benzoxazine (pHBA-a).

2.3. Synthesis of (3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl) methanol (abbreviated as pHBA-a)

the polymerization behavior and kinetics of the para-methylol benzoxazine resin, complementing our previous publications [28–30]. It is known that phenolic impurities coming from the synthesis of benzoxazine monomers act as efficient initiators and/or catalysts. Therefore, only single crystal samples exhibiting sharp melting endothermic peaks in the thermograms obtained by DSC are used in this study. Unlike the complex polymerization mechanism proposed for ortho-methylol functional benzoxazine, para-methylol benzoxazine seems to polymerize following a much simpler mechanism. Furthermore, the thermal properties of the polybenzoxazine obtained upon polymerization are also investigated and presented in the present work.

Into a 50 mL round flask were added 20 mL of toluene, aniline (1.4 g, 15 mmol), pHBA (1.86 g, 15 mmol), and paraformaldehyde (0.96 g, 30 mmol). The mixture was refluxed for 6 h. Then the product was concentrated using a rotary evaporator and redissolved in chloroform followed by base-washed then once with water. The chloroform solution was dried over sodium sulfate anhydrous and recrystallized from chloroform to yield white crystals (yield: 75%). 1H NMR (600 MHz, DMSO-d6, 25 °C) δ, ppm: 4.34 (d, 2H, –CH2-OH,), 4.62 (s, 2H, Ar-CH2-N, oxazine), 5.00 (t, 1H, –OH), 5.40 (s, 2H, O-CH2-N, oxazine), 6.65–7.21 (m, 8H, Ar). FT-IR spectra (KBr), cm− 1: 3250 (stretching of OH of methylol), 1497 (stretching of trisubstituted benzene ring), 1236 (Ar-O-C asymmetric stretching), 944 (out-of-plane C-H of benzene ring to which oxazine ring is attached). Anal. Calcd. For C15H15NO2: C, 74.69%; H, 6.22%; N, 5.84%. Found: C, 74.13%; H, 6.37%; N, 5.69%.

2. Experimental 2.1. Materials 4-Hydroxybenzyl alcohol (pHBA) (98%), aniline, and paraformaldehyde (96%) were used as received from Sigma-Aldrich. Toluene, hexane, sodium hydroxide, and sodium sulfate were obtained from Fisher Scientific and used as received. 3-phenyl-3,4-dihydro-2Hbenzo[e] [1,3]oxazine (abbreviated as PH-a) was prepared following the methods developed in our laboratory, which were published elsewhere [29].

2.4. Polymerization of benzoxazine monomers Polymerizations were carried out by heating at 160, 180, 200 and 220 °C for 2 h each step, thus obtaining poly(pHBA-a) and poly(PH-a). 3. Results and discussion

2.2. Characterization

3.1. Preparation of para-methylol functional benzoxazine monomer

Varian Oxford AS300 nuclear magnetic resonance (NMR) spectrometer was used to obtain 1H and 13C NMR spectra using the number of transient of 64 and 1024, respectively at the proton frequency of 300 MHz and its corresponding carbon frequency of 150.864 MHz. For 1 H NMR quantitative analysis, a relaxation time of 10 s was adopted. Bomem Michelson MB100 Fourier transform infrared (FT-IR) spectrophotometer was used to acquire FT-IR spectra. The spectrophotometer was equipped with a deuterated triglycine sulfate (DTGS) detector and dry air purge unit and was operated at a resolution of 4 cm− 1 in the frequency range of 4000–400 cm− 1. Transmission spectra were obtained by averaging 64 scans for each thin film cast on a KBr plate of the partially polymerized samples. Elemental analysis (carbon, hydrogen, and nitrogen) was performed in a CE EA-1112 Analyzer. Samples were dried before the measurement. A TA Instruments differential scanning calorimetry (DSC) model 2920 was used with a temperature ramp rate of 10 °C/min and a nitrogen flow rate of 60 mL/min for all tests of DSC study. To determine the activation energy of benzoxazine polymerization, samples (2.0 ± 0.5 mg) were scanned at the different heating rates of 2, 5, 10, 15, and 20 °C/min. All samples were sealed in hermetic aluminum pans with lids. Thermogravimetric analyses (TGA) were performed on a TA Instruments High Resolution 2950 Thermogravimetric Analyzer that was purged with nitrogen at a flow rate of 40 mL/min. A heating rate of 10 °C/min was used.

The successful synthesis of a monofunctional benzoxazine monomer has been achieved using a primary amine (aniline), formaldehyde, and 4-hydroxybenzyl alcohol (pHBA). The reaction is depicted in Scheme 1. The chemical structures of pHBA-a were confirmed by 1H NMR, 13C NMR, FT-IR and elemental analysis. Fig. 1 shows the 1H NMR spectrum of pHBA-a. Typically, benzoxazine monomers have two singlets of equal integration values in 1H NMR spectra due to the two –CH2– groups forming the oxazine ring. The signals attributed to these two methylenes, Ar-CH2-N- and –O-CH2-N- are observed at 4.62 and 5.40 ppm, respectively. Also, the 1H NMR spectrum shows the presence of the methylol group (–CH2OH), by combining the signal of –CH2-O and hydroxyl attached to a methylol group at 4.34 and 5.00 ppm, respectively. 13 C NMR analysis was also performed to further confirm the structures of pHBA-a, the spectrum is shown in Fig. 2. The characteristic carbon resonances of oxazine ring appear at 49.70 and 79.85 ppm for Ar-CH2-N- and –O-CH2-N-, respectively. The resonance at 63.25 ppm is assigned to the carbon of methylol group (–CH2OH). FT-IR spectrum of pHBA-a using the KBr pellet method is shown in Fig. 3. The FT-IR spectrum of pHBA-a shows a broad band at 3250 cm− 1, corresponding to the stretching vibrational mode of the –OH group. In addition, a characteristic band of asymmetrically trisubstituted benzene ring is observed at 1497 cm− 1 confirming the presence of a methylol group as a constitutive part of the benzoxazine monomer. Also, the existence of a benzoxazine ring aromatic ether in 24


K. Zhang et al.

Fig. 1. 1H NMR spectrum of pHBA-a.

Fig. 4. DSC thermogram of pHBA-a.

between the calculated and observed data for the purified sample, showing that the targeted compound, pHBA-a, was obtained in high purity. This last result along with all the previous spectroscopic information are consistent with the successful synthesis of the target compound. 3.2. Polymerization behavior of benzoxazine monomer

Fig. 2.

13

DSC was used to study the polymerization behavior of pHBA-a. A thermogram of the pure monomer is shown in Fig. 4. The thermogram shows a sharp endothermic peak at 100 °C, thus supporting the notion of high purity of pHBA-a also observed by elemental analysis. It has been reported that upon heating, pHBA does exhibit a broad exothermic peak with onset at 154 °C and the maximum centered at 200 °C, which is due to the condensation reaction between two molecules of pHBA via the methylol group [33]. However, in the case of pHBA-a, the onset of the ring-opening polymerization was observed at 201 °C with the maximum centered at 216 °C. Thus, no peak corresponding to a methylol mediated condensation reaction was observed despite that pHBA-a contains para-methylol functionality as part of its structure. According to the hypothesis of the condensation reactions for phenolic resins, [34,35] the condensation reaction between molecules of pHBA proceeds by an initial dehydration reaction that forms a quinone methide, followed by the establishment of an equilibrium between its quinoid and benzenoid structure, which in turn reacts with another pHBA molecule. This process is illustrated in Scheme 2. Unlike in pHBA, where uncharged and zwitterionic species are in equilibrium, all intermediates in the ring opening structures of benzoxazines are zwitterionic and no dehydration reaction is observed suggesting that quinone methide cannot be formed from pHBA-a. Moreover, it is unfavorable for the zwitterionic intermediate to release water to form quinone methide structures due to the non-existence of free hydrogen atoms. Therefore, no condensation reactions can take place before the ring-opening polymerization of pHBA-a as also depicted in Scheme 2. In addition, comparing with the unsubstituted PH-a, pHBA-a presents significantly lower temperatures for both the onset and maxima of polymerization temperature [29]. In general, electron-withdrawing substituents lead to a more acidic phenol species, resulting in a stronger catalytic effect taking place [36]. However, the methylol group has neither a strong electron-withdrawing nor acid character. The role of this methylol group has been investigated and the existence of intermolecular hydrogen-bonding involving the methylol group and the oxazine ring seems to accelerate the ring-opening polymerization in an intermolecular manner [30]. In order to qualitatively study the structural evolution during

C NMR spectrum of pHBA-a.

Fig. 3. FT-IR spectrum of benzoxazine monomer.

the monomers is indicated by the band centered at 1236 cm− 1, which is due to the C-O-C asymmetric stretching modes [31]. Furthermore, the oxazine-related mode is seen in the spectrum at 944 cm− 1 [32]. Elemental analysis was also performed and the excellent agreement 25


K. Zhang et al.

Scheme 2. Proposed condensation and polymerization mechanism of pHBA and pHBA-a.

Fig. 5. FT-IR spectra of pHBA-a after polymerization at different temperatures.

polymerization, FT-IR and 1H NMR analyses were carried out and the spectra for pHBA-a are displayed in Figs. 5 and 6, respectively. As shown in Fig. 5, the characteristic absorption bands of the oxazine moieties observed at 1236 cm− 1 (C-O-C asymmetric stretching modes) and 944 cm− 1 (oxazine-related band) gradually disappear upon increasing the temperature from room temperature to 180 °C, supporting the ring-opening polymerization of benzoxazine monomer. Fig. 6 shows the 1H NMR spectra of the thermally treated pHBA-a at different temperatures using deuterated DMSO (DMSO-d6). As can be seen in Fig. 6, 1 H NMR spectrum of pHBA-a after polymerization at 160 °C presents complicated signals due to different types of –CH2– units. Signals apart from the ones at 5.4 and 4.6 ppm attributed to the benzoxazine structure may be ascribed to the residual oxazine rings at the chain ends of some oligomers or polybenzoxazines of low molecular weight. A broad peak between 4.7 and 4.2 ppm is probably due to the presence of phenolic –CH2– groups, associated to the Mannich bridge after ringopening polymerization. Also, a new signal at 9.0 ppm showing the presence of phenolic –OH groups gradually increases from 120 to 160 °C, further suggesting the phenolic structure is formed upon ringopening polymerization of pHBA-a. Thus, the ring-opening polymerization of pHBA-a can straightforwardly be described as shown in

Fig. 6. 1H NMR spectra of pHBA-a after various thermal treatment.

Scheme 3. Ring-opening polymerization of pHBA-a.

Scheme 3, neglecting the possible crosslink reactions for simplicity. Figs. 7 and 8 show the DSC thermograms of pHBA-a and the plot obtained from conversion versus temperature, respectively, using different heating rates in both cases. The figures indicate that the initial polymerization temperature (Ti) and maxima peak temperature (Tp) 26


K. Zhang et al.

Fig. 9. Representations of Kissinger and Ozawa methods for calculating the activation energy of polymerization for pHBA-a.

Fig. 7. DSC curves of pHBA-a at different heating rates.

Table 1 The activation energy of pHBA-a obtained by Kissinger and Ozawa methods. Sample

pHBA-a

Ea (kJ/mol) Kissinger

Ozawa

79.8

81.5

for other monofunctional benzoxazine monomers [39,40]. This is because the polar and potentially nucleophilic hydroxyl group highly activates the ring-opening of the oxazine moiety.

3.3. Thermal properties of methylol functional polybenzoxazine Various polybenzoxazines with different functionalities exhibiting excellent thermal properties have been investigated in recent years [41–45]. TGA was used to study the thermal stability of poly(pHBA-a) and results are shown in Fig. 10. A summary of the main thermal properties, Td5 and Td10, defined as the temperatures at which a weight loss of 5 and 10% respectively occurred, and char yield, defined as the residual weight at 800 °C under N2 atmosphere, are presented in Table 2. For polybenzoxazines based on monofunctional benzoxazine monomers, the initial decomposition temperatures are lower than for polybenzoxazines of difunctional benzoxazine monomers. This is due to

Fig. 8. Conversion vs. temperature plot for pHBA-a polymerized at different heating rates.

shift to higher temperatures with the increase of heating rate as normally occurs with benzoxazine resins. The activation energy of polymerization was determined using the well-known Kissinger and Ozawa methods [37,38]. According to the Kissinger method, the activation energy can be calculated using Eq. (1) as follows:

β AR ⎞ E ln ⎛⎜ 2 ⎞⎟ = ln ⎛ − a E RT T a P p ⎝ ⎠ ⎝ ⎠ ⎜

⎟

(1)

where β is the heating rate, A is the frequency factor, Tp is the temperature of the exothermic peak, Ea is the activation energy, and R is the gas constant. If the plot of ln(β/Tp2) against 1/Tp is linear, Ea can be obtained from the slope of the corresponding straight line. Another theoretical treatment, namely, the Ozawa method, can also be applied to the thermal data using Eq. (2) as follows:

ln β = −1.052

Ea +C RTP

(2)

where C is a constant. Fig. 9 shows the plot of ln(β/Tp 2) or ln(β) against 1/Tp for pHBA-a according to the Kissinger and Ozawa methods. Interestingly, the Ea value was estimated to be as low as 79.8 (Kissinger) and 81.5 (Ozawa) kJ/mol as shown in Table 1. This Ea value is lower than those reported

Fig. 10. Thermogravimetric analysis of poly(pHBA-a) and poly(PH-a).

27


K. Zhang et al.

References

Table 2 Summary of TGA thermogram results. Sample

Td5 (°C)

Td10 (°C)

Char Yield (%)

Poly(pHBA-a) Poly(PH-a)

395 258

436 323

60 40

[1] F. Holly, A.C. Cope, J. Am. Chem. Soc. 66 (1944) 1875–1879. [2] X. Ning, H. Ishida, J. Polym. Sci. A Polym. Chem. 32 (1994) 1121–1129. [3] H. Ishida, T. Agag (Eds.), Handbook of Benzoxazine Resins, Elsevier, Amsterdam, 2011. [4] C.P.R. Nair, Prog. Polym. Sci. 29 (2004) 401–498. [5] N.N. Ghosh, B. Kiskan, Y. Yagci, Prog. Polym. Sci. 32 (2007) 1344–1391. [6] T. Takeichi, T. Kawauchi, T. Agag, Polym. J. 40 (2008) 1121–1131. [7] K. Zhang, J. Liu, S. Ohashi, X. Liu, Z. Han, H. Ishida, J. Polym. Sci. A Polym. Chem. 53 (2015) 1330–1338. [8] H.J. Kim, Z. Brunovska, H. Ishida, Polymer 40 (1999) 6565–6573. [9] K. Zhang, J. Liu, H. Ishida, Macromolecules 47 (2014) 8674–8681. [10] H. Ishida, H.Y. Low, Macromolecules 30 (1997) 1099–1106. [11] S.B. Shen, H. Ishida, Polym. Compos. 17 (1996) 710–719. [12] J. Wu, Y. Xi, G.T. McCandless, Y. Xie, R. Menon, Y. Patel, D.J. Yang, S.T. Iacono, R.M. Novak, Macromolecules 48 (2015) 6087–6095. [13] H. Ishida, D. Allan, J. Polym. Sci. B Polym. Phys. 34 (1996) 1019–1030. [14] M.W. Wang, C.H. Lin, T.Y. Juang, Macromolecules 46 (2013) 8853–8863. [15] T. Agag, C.R. Arza, F.H.J. Manrer, H. Ishida, J. Polym. Sci. A Polym. Chem. 49 (2011) 4335–4342. [16] T. Agag, J. Liu, R. Graf, H.W. Spiess, H. Ishida, Macromolecules 45 (2012) 8897–8991. [17] K. Zhang, H. Ishida, Polym. Chem. 5 (2015) 2541–2550. [18] S.N. Kolanadiyil, J. Bigwe, I.K. Varma, React. Funct. Polym. 73 (2013) 1544–1552. [19] M. Ergin, B. Kiskan, B. Gacal, Y. Yagci, Macromolecules 40 (2007) 4724–4727. [20] T. Agag, T. Takeichi, Macromolecules 34 (2001) 7257–7263. [21] H. Qi, H. Ren, G. Pan, Y. Zhuang, F. Huang, L. Du, Polym. Adv. Technol. 20 (2009) 268–272. [22] Y.L. Liu, C.I. Chou, J. Polym. Sci. A Polym. Chem. 43 (2005) 5267–5282. [23] K. Zhang, Q.X. Zhuang, Y.C. Zhou, X.Y. Liu, G. Yang, Z.W. Han, J. Polym. Sci. A Polym. Chem. 50 (2012) 5115–5123. [24] B. Kiskan, B. Koz, Y. Yagci, J. Polym. Sci. A Polym. Chem. 47 (2009) 6955–6961. [25] R. Kudoh, A. Sudo, T. Endo, Macromolecules 43 (2010) 1185–1187. [26] L. Jin, T. Agag, Y. Yagci, H. Ishida, Macromolecules 44 (2011) 767–772. [27] M. Baqar, T. Agag, H. Ishida, S. Qutubuddin, Polymer 52 (2011) 307–317. [28] M. Baqar, T. Agag, H. Ishida, S. Qutubuddin, J. Polym. Sci. A Polym. Chem. 50 (2012) 2275–2286. [29] M. Baqar, T. Agag, R. Huang, J. Maia, S. Qutubuddin, H. Ishida, Macromolecules 45 (2012) 8119–8125. [30] K. Zhang, P. Froimowicz, L. Han, H. Ishida, J. Polym. Sci. A Polym. Chem. 54 (2016) 3635–3642. [31] T. Agag, T. Takeichi, Macromolecules 36 (2003) 6010–6017. [32] J. Dunkers, H. Ishida, Spectrochim. Acta 51 (A) (1995) 1061–1074. [33] M. Baqar, T. Agag, H. Ishida, S. Qutubuddin, React. Funct. Polym. 73 (2013) 360–368. [34] A. Knop, L.A. Pilato (Eds.), Phenolic Resins, Springer, Berlin, 1985. [35] M. Higuchi, T. Urakawa, M. Morita, Polymer 42 (2001) 4563–4567. [36] R. Andreu, J.A. Reina, J.C. Ronda, J. Polym. Sci. A Polym. Chem. 46 (2008) 3353–3366. [37] H.E. Kissinger, Anal. Chem. 29 (1957) 1702–1706. [38] T. Ozawa, J. Therm. Anal. 2 (1970) 301–324. [39] S. Ohashi, J. Kilbane, T. Heyl, H. Ishida, Macromolecules 48 (2015) 8412–8417. [40] K. Zhang, H. Ishida, Polymer 66 (2015) 240–248. [41] J. Wang, X. Fang, M.Q. Wu, X.Y. He, W.B. Liu, Eur. Polym. J. 47 (2011) 2158–2168. [42] X. He, J. Wang, N. Ramdani, W.B. Liu, L.J. Liu, Y. Yang, Thermochim. Acta 564 (2013) 51–58. [43] J. Wang, H. Wang, J.T. Liu, W.B. Liu, J. Therm. Anal. Calorim. 114 (2013) 1255–1264. [44] C. Jubsilp, K. Punson, T. Takeichi, S. Rimdusit, Polym. Degrad. Stab. 95 (2010) 918–924. [45] H. Wang, J. Wang, X. He, T. Feng, N. Ramdani, M. Luan, W. Liu, X. Xu, RSC Adv. 4 (2014) 64798–64801. [46] I. Hamerton, S. Thompson, B.J. Howlin, C.A. Stone, Macromolecules 46 (2013) 7605–7615.

the lower crosslinking density of monofunctional benzoxazines despite some expected reactivity from the extra functionality bore by the benzoxazine, as for example the aromatic ring of aniline moiety. As a result, the initial degradation could take place from the terminal Schiff base and secondary amides as structural defects of polybenzoxazines [46]. However, the decomposition temperatures of poly(pHBA-a) reflected on the Td5 and Td10 are 395 and 436 °C, respectively, which are significantly higher than polybenzoxazines without methylol functionality, poly(PH-a). Moreover, poly(pHBA-a) presents a char yield of 60% at 800 °C, which is considerably higher than that of poly(PH-a) (40%). TGA results suggest an enhanced thermal stability of the methylol functional polybenzoxazine in comparison with that of polybenzoxazine obtained from the unsubstituted resin (PH-a). The remarkable improvement for the thermal stability of poly(pHBA-a) might be in part due to the strong intermolecular hydrogen-bonding networks between the methylol and phenolic groups in polybenzoxazine. Further detailed studies regarding the additional intermolecular hydrogenbonding on the thermal properties of polybenzoxazine are currently under investigation. 4. Conclusions para-methylol functional benzoxazine monomer was successfully synthesized exploiting a solution method, and its chemical structure was confirmed by 1H NMR, 13C NMR and FT-IR. The polymerization behavior was systematically studied by FT-IR and 1H NMR at different polymerization stages. Phenolic moieties within the chemical structure of the product upon heating are formed after the ring-opening polymerization starts. DSC was also used to further study the polymerization of pHBA-a. The activation energy of polymerization was estimated to be 79.8 (Kissinger) and 81.5 (Ozawa) kJ/mol. It was found that the activation energy of polymerization of pHBA-a is lower than those of other reported monofunctional benzoxazine monomers. It is worth noticing the excellent thermal stability of the polybenzoxazine derived from para-methylol functional benzoxazine monomer, as demonstrated by TGA results. Acknowledgments K. Zhang is indebted to the partial support of National Natural Science Foundation of China (No. 51603093), the Science and Technology Agency of Jiangsu Province (No. BK 20160515), China Postdoctoral Science Foundation (No. 2016 M600369) and Jiangsu Postdoctoral Science Foundation (No. 1601015A).

28