Toward Elucidating the Role of Number of Oxazine Rings and

Nov 11, 2016 - We have developed novel di-, tri-, and tetrafunctional benzoxazine monomers (BZ2, BZ3, and BZ4), solely containing benzoxazine moieties...
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Toward Elucidating the Role of Number of Oxazine Rings and Intermediates in the Benzoxazine Backbone on Their Thermal Characteristics Sini N. K. and Takeshi Endo* Molecular Engineering Institute, Kindai University, Kayanomori, Iizuka, Fukuoka, Japan S Supporting Information *

ABSTRACT: We have developed novel di-, tri-, and tetrafunctional benzoxazine monomers (BZ2, BZ3, and BZ4), solely containing benzoxazine moieties arranged one after the other on the backbone, to discover the role of additional oxazine moiety on thermal properties. A tandem reaction was adapted, at first a ring-closure and then subsequent Mannich condensation by reacting 2-((4-hydroxyphenyl)aminomethyl)phenol (HPAMP) and aniline/4-amino-2-((phenylamino)methyl)phenol/4-amino-2-(((4-hydroxy-3-((phenylamino)methyl)phenyl)amino)methyl)phenol in the presence of paraformaldehyde to obtain BZ2/BZ3/BZ4. Tetrafunctional benzoxazine monomers containing an intermediate group (R-BZ4) were also synthesized by reacting HPAMP and p-phenylenediamine/ 4,4′-diaminodiphenylmethane (PDA/DDM) to understand the differences in thermal properties due to the distance created by those between two benzoxazine moieties. The observations (in DSC and TGA analysis) with increase in number of oxazine rings include (i) reduction in curing temperature from 264 °C (BZ1) to 237 °C (BZ4), however an intermediate distance between BZ4 increased curing temperature to 245 °C (DDM); (ii) slower curing rate, a complete curing was achieved only by heating above 200 °C; (iii) decreased weight loss during ring-opening polymerization, BZ1:90% to BZ4:2.4%; (iv) increase in Tg up to 315 °C; and (v) similar thermal stability for BZ2 to BZ4. Introduction of an intermediate between two oxazine rings in a tetrafunctional benzoxazine (R-BZ4) was found to be the best strategy to obviate weight loss during curing as well as to enhance the thermal degradation temperature of resulting polybenzoxazine (PDDM: Td10 of 395 °C).

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INTRODUCTION Polybenzoxazines are phenolic-type thermosetting resins and can be obtained by ring-opening polymerization (ROP) of cyclic 1,3-benzoxazine monomer by simply heating with no added catalyst, resulting in a cross-linked network containing phenolic Mannich bridges as a repeating unit. Because of the existence of these Mannich bridges (−CH2−NR−CH2−) along with additional intramolecular (OH---N and OH---O) and intermolecular (OH---O) hydrogen bonds in the network structure, these resins exhibit excellent thermal and mechanical properties, which makes them an exceptional candidate for high performance applications over traditional phenolics and epoxies.1−4 However, ring-opening polymerization of a typical monofunctional benzoxazine, 3-phenyl-3,4-dihydro-2H-1,3benzoxazine (BZ1), is limited to yield a low molecular weight linear polymer. In 1994, Ishida et al. developed difunctional benzoxazines based on bisphenols, primary amines, and paraformaldehyde, mainly 6,6′-(propane-2,2-diyl)bis(3-phenyl3,4-dihydro-2H-1,3-benzoxazine), the ROP of which can result in a high molecular weight cross-linked polybenzoxazine with better properties.5,6 Since then, there has been a growing devotion toward achieving further improvement in properties by utilizing the extraordinary molecular design flexibility in benzoxazine synthesis. Desired properties can be attained via a simple © XXXX American Chemical Society

structural modification by changing the starting phenols/ amines. Various cross-linkable or bulky groups such as imide, furan, nitrile, acetylene, benzoxazole, fluorene, spiro-center, etc., were introduced to enhance the thermal stability.7−16 Special side groups such as carboxylic, phenolic hydroxyl, methylol, hydroxyethyl, amide, formyl, cyanate ester, etc., have succeeded in reducing the curing temperature of monofunctional benzoxazine (BZ1) to a certain extent, while providing a better thermal stability.17−23 Moreover, flexible group containing bisbenzoxazines and main-chain polybenzoxazine precursors were synthesized to overcome the brittleness in the cured resins.24−28 Over recent years, the benzoxazine research has entered a new arena on synthesizing benzoxazine monomers containing three or four oxazine rings in the backbone by utilizing multifunctional phenol/amine as starting materials.29−34 It is believed that polyfunctionality in these monomers would result in an infinite polymeric network structure having better properties. For instance, thermally stable polybenzoxazines with excellent flame retardancy were obtained from Received: September 7, 2016 Revised: October 22, 2016

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triazine/phosphorus-based tribenzoxazines. A furan-containing fluorene-based tetrabenzoxazine monomer, having both bisphenol- and diamine-type oxazine rings in the backbone, was reported, and the cured product exhibited extremely high glass transition temperature (Tg) of 440 °C. Very recently, we have reported a thermally stable polybenzoxazine based on a trifunctional benzoxazine monomer 1,3,5-tris(3-phenyl-3,4dihydro-2H-benzo[1,3]oxazin-6-yl)benzene (Tg: 375 °C; Td10: 424 °C; char yield at 600 °C: 74%).35 Besides these, dendritic benzoxazine structures with more than five oxazine rings in the backbone were also developed by coreacting terminal group containing monobenzoxazine with various inorganic core components.36−39 The increasing interest toward multifunctional benzoxazines intrigued us to systematically investigate the real contribution of each additional oxazine rings in the backbone and what extent it is capable to improve the thermal properties. To gain a real insight, the benzoxazine monomers has to be designed in a particular way that there should not be any other contributing substituents in the backbone. In the present study, we have synthesized di-, tri-, and tetrafunctional benzoxazine monomers solely arranged one after the other on a monobenzoxazine as shown in Scheme 1. Such precisely designed main-chain type

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RESULTS AND DISCUSSION

Novel benzoxazine monomers solely containing di- (BZ2), tri- (BZ3), and tetra- (BZ4) oxazine rings were successfully synthesized by a tandem reaction method. The synthesis route, structure of the reactants used, and final benzoxazine monomers are given in Scheme 2. The starting phenol 4-hydroxy(phenylaminomethyl)phenol (HPAMP) (Scheme 2) can undergo a combination of reactions in the presence of stoichiometric amounts of paraformaldehyde, at first by the ring-closure between the aminomethyl and neighboring phenolic hydroxyl group and subsequent classical Mannich-condensation reaction with primary amines. This new synthetic protocol utilizing HPAMP has been introduced by Yagci et al. in 2013 to synthesize an asymmetric bisbenzoxazine containing both aliphatic and aromatic units.40 Now, we have given a boost to this approach for designing multibenzoxazine monomers in a close proximity in the main backbone. As represented in Scheme 2, BZ2 was synthesized by simply reacting HPAMP with aniline in the presence of paraformaldehyde. To synthesize tri- and tetrabenzoxazine, we have newly designed primary amines similar to HPAMP, i.e., 4-amino-2((phenylamino)methyl)phenol (NH2-PAMP) and 4-amino-2(((4-hydroxy-3-((phenylamino)methyl)phenyl)amino)methyl)phenol (NH2-H(PAM)2P), by a stepwise procedure utilizing 5-nitrosalicylaldehyde as a starting monomer, as described in the Experimental Section. The obtained amines can also undergo a ring closure between neighboring amino methyl and phenolic hydroxyl and then Mannich condensation with other phenolic reactants. Since our aim was to design benzoxazine monomer containing three/four consecutive oxazine rings in the backbone, we have reacted HPAMP and NH2-PAMP/NH2-H(PAM)2P together in the presence of paraformaldehyde by refluxing in chloroform, which resulted in BZ3 and BZ4. Intermediate tetrafunctional benzoxazine monomers PDA-BZ4 and DDM-BZ4 (R-BZ4) were also synthesized in a similar way using p-phenylenediamine and 4,4′-diaminodiphenylmethane, respectively, as the amine reactants. The reaction was relatively difficult due to very well known triazine network gel formation in these aromatic diamines in the presence of paraformaldehyde even at room temperature. Solvents such as chloroform and toluene/ethanol (2/1) were reported to stave off this gelation and turn it into a soluble low molecular weight triazine network, which could easily undergo thermal dissociation and react with bisphenols.27,28 However, the reaction rate of HPAMP toward these triazine network was found to be very low in both solvents and was ameliorated by addition of triethylamine catalyst. All the benzoxazine monomers were well purified, first by column chromatography and then by recrystallization, since any traces of impurities (amines, phenols, and oligomers) could influence the ring-opening polymerization.35,41 The structure and purity of the monomers were confirmed by using NMR (1H and 13C), IR, and elemental analysis. Figure 1 shows the overlapped 1H NMR spectra of oxazine region in BZ2, BZ3, and BZ4. The characteristic proton resonance signals due to oxazine ring were observed as multiple signals in all benzoxazine monomers due to the presence of multiple oxazine rings and the asymmetry in the structure. For instance, in BZ2 the proton resonance due to −O−CH2−N− and −Ar−CH2−N− for the first oxazine ring, which is attached to the pendant phenyl ring (marked as 1 and 1′), was observed

Scheme 1. Structures of Mono−Tetra Benzoxazines Synthesized in This Study

consecutive multibenzoxazines are being reported for the first time, and this will help to get a closer look on the effect of number of oxazine rings and estimate their extent on properties. Moreover, intermediates such as phenylene and diphenylmethane have also been introduced in between two oxazine rings (PDA and DDM-BZ4, Scheme 2) in a tetrafunctional benzoxazine to elucidate the difference in thermal properties created by these intermediate distance. A comparative structure−property analysis has been discussed on the basis of number of oxazine rings and intermediates in the benzoxazine backbone on ring-opening polymerization and thermal stability of corresponding polybenzoxazines. B

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Figure 1. Expanded oxazine region in 1H NMR spectra of BZ2, BZ3, and BZ4 (full spectra can be viewed in the Supporting Information; Figures S1−S6). C

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Macromolecules at 5.26 and 4.55 ppm, respectively,42 while the second oxazine ring protons (marked as 2 and 2′) were observed at 5.24 and 4.51 ppm. In BZ3, the first (1, 1′) and third oxazine (3, 3′) protons were observed at almost a similar position to BZ2 at 5.27, 4.54 ppm and 5.25, 4.52 ppm, respectively, whereas the middle oxazine protons (2 and 2′) were observed toward upfield at 5.17 and 4.45 ppm. A similar trend was found in BZ4: the first and fourth oxazine protons were consistent at 5.26, 4.54 ppm and 5.24, 4.52 ppm, and the proton resonances due to middle oxazine rings 2, 2′ and 3, 3′ were observed toward upfield as overlapping peaks at 5.17, 4.45 ppm and 5.16, 4.44 ppm. To further ascertain the structure, we have analyzed 1 H−1H NOESY 2D NMR (Figures S1−S3); interestingly, we could see a number of oxazine intramolecular cross-interactions with aromatic hydrogen, especially for the attached additional oxazine rings, and these interactions were intensified by insertion of more number of oxazine rings. For instance, in BZ3, both 2 and 2′ have interaction with position b and c; 2′, 3, and 3′ have interaction with position b′; 3 and 3′ have interaction with c′. Furthermore, the NOE interaction between 2′ and position d in BZ2, 3′ and d in BZ3, and 4′ and d in BZ4 reconfirms the structure as well as the positions we assigned for each oxazine ring in Figure 1. In addition to these findings, the integral ratio between oxazine and aromatic protons (around 6.67−7.24 ppm, Figures S4−S6a) are also in a good agreement in these monomers, which therefore confirms the successful formation of di-, tri-, and tetrafunctional benzoxazine monomers with consecutive oxazine ring. It is noteworthy to mention that an evident shift in oxazine proton signals, in particular for −O−CH2−N− (a shift of ∼0.02 ppm toward downfield, Figure S7), was observed with increasing concentration (0.0028−0.15 mol L−1) in the 1H NMR spectra of BZ3 and BZ4, while no such observance was in BZ2 (Figure S8). This suggests the aggregation of molecules at high concentration with higher oxazine functionality which possibly renders the adjacent oxazine moieties to have an intermolecular interaction. The monomer structure was also confirmed with the help of 13C NMR (Figures S4−S6b), which showed multiple carbon

resonance signals corresponding to each oxazine ring in the backbone. In BZ2, the carbon chemical shifts for 1 and 1′ were observed at 79.5 and 50.7 ppm and 2 and 2′ were at 80.5 and 51.1 ppm. Similarly in BZ3, 1 and 1′ were at 79.4 and 50.6 ppm, 2 and 2′ (middle oxazine ring) at 80.5 and 51.3 ppm, and 3 and 3′ at 80.4 and 51.1 ppm. In BZ4, the carbon signals due to O−CH2−N (1−4) and Ar−CH2−N (1′−4′) were in the ranges of 79.4−80.49 and 50.7−51.3 ppm, respectively. The structure of intermediate tetrafunctional benzoxazines (PDA and DDM-BZ4) was also verified by NMR analysis, and a representative 1H NMR spectrum confirming the structure of PDA-BZ4 is given in Figure 2. Unlike BZ4, both PDA- and DDM-BZ4 only showed two proton and carbon resonance signals corresponding to O−CH2−N (1 and 2) and Ar−CH2−N (1′and 2′) (see Experimental Section and Figures S9, S10a,b) that can be accounted for the symmetry introduced by the intermediates (phenylene and diphenylmethane) in these benzoxazines. Effect of Number of Oxazine Rings on Ring-Opening Polymerization. The thermal behavior of mono- to tetrafunctional benzoxazine monomers was studied by DSC analysis, and the thermograms are shown in Figure S14 and Figure 3. A trend of increase in melting temperature (58−158 °C) and a decrease in curing temperature (∼27 °C) was found with increase in number of oxazine rings in the backbone, i.e., from BZ1 to BZ4. For instance, BZ2 showed a curing exotherm with onset (T0) at 236 °C and a maximum peak of temperature (Tp) at 251 °C, showing the dominancy of the attached oxazine ring, which decreased the curing maxima by 13 °C (BZ1: Tp 264 °C, Figure S14). A further decrease in curing exotherm was observed in BZ3 with T0 and Tp at 224 and 242 °C, likewise in BZ4 at 215 and 237 °C. Nevertheless, the extent of reduction in maximum curing temperature was found to be low with each additional oxazine ring; therefore, any further incorporation of benzoxazine moiety (a fifth oxazine) will merely show an identical effect with a curing maxima around 230 °C. The reduction in curing temperature from mono- (264 °C) to tetrabenzoxazine (237 °C) is presumptively due to the

Figure 2. 1H NMR spectrum of PDA-BZ4 in CDCl3. D

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reduction happens even after introducing the intermediates between oxazine rings, we have investigated the curing behavior of R-BZ4. Comparative DSC scans of BZ4, PDA-BZ4, and DDM-BZ4 are shown in Figure 4. The introduction of phenylene intermediate showed a negligible change in curing temperature (T0: 219 °C; Tp: 238 °C; ΔH: 298.8 J/g); however, a noticeable difference was observed due to the intermediate distance created by DDM. Apparently, the curing temperature was shifted to a higher temperature with T0 and Tp at 228 and 245 °C (ΔH: 305.2 J/g) compared to BZ4 (215 and 237 °C). These results manifest that the lower curing temperature in BZ4 was mainly because of the sequential arrangement of benzoxazine moieties in the backbone; introducing a bulky intermediate can alter the curing exotherm to a higher temperature. Despite their close proximity and inter/intramolecular interactions, BZ4 still possess a high curing temperature in comparison to many functionalized monobenzoxazines (e.g., carboxylic, hydroxyl, o-amide, etc.) reported in the literature.17−23 Moreover, there has been a recent report claiming an extensive decrease in curing temperature (T0: 140 °C, Tp: 190 °C) and a faster curing rate with higher number of oxazine functionality (mono to tetra) in the backbone, as a result of interplay between neighboring oxazine rings.34,45 However, herein we have not observed such a decrease or faster curing rate; in fact, BZ2 to BZ4s need to be heated near their curing maxima (∼240 °C) to achieve a complete polymerization (Figure 5), which can be attributed to the purity of our compounds. The presence of impurities including unreacted starting amines/phenols, and in particular oligomers, is known to bring down the ring-opening polymerization to a lower temperature and also increase the curing rate due to their catalytic nature.35,41 In the current study, the benzoxazine monomers are solely arranged (BZ1 to BZ4), and there is no aforementioned influential impurities, thus revealing the true role of oxazine functionality on ROP. Next, we have determined the curing cycle in each benzoxazine monomers. Figure 5a shows the DSC scans of benzoxazine monomers cured at 180 °C for 5 h; a residual curing exotherm

Figure 3. DSC scans of di-, tri-, and tetra-benzoxazine monomers (heating rate: 10 °C min−1).

presence of intramolecular cross-interaction between oxazine ring and aromatic hydrogen. These interactions were higher with higher oxazine functionality as indicated by their 1H−1H NOESY NMR spectra (Figures S1−S3). Recently, Ishida et al. have proved the catalytic ability of intramolecular hydrogen bonding between oxazine ring and the o-methylol/amide group to lower the curing temperature of benzoxazine.43,44 The current context is not the same, but we assumed that these interactions may impel the oxazine ring attached at the end to open first (fourth oxazine moiety in BZ4) rather than the oxazine attached to phenyl pendant (first oxazine moiety in BZ4), which then assists in further ring-opening; thus, there is a shift in curing exotherm from BZ1 to BZ4. The slight broadness in curing exotherm and the reduction in heat of polymerization (ΔH) (359.4, 311.1, and 284.5 J/g in BZ2, BZ3, and BZ4, respectively) with higher number of oxazine rings further supports this assumption. To elucidate whether this

Figure 4. DSC scans of tetrafunctional benzoxazine monomer containing intermediates (heating rate: 10 °C min−1). E

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Figure 5. DSC scans of benzoxazine monomers cured at (A) 180 °C for 5 h and (B) 200 and 220 °C, each for 2 h, and (C) 250 °C for 2 h.

Scheme 3. Ring-Opening Polymerization of Benzoxazine Monomers (a) BZ1, (b) BZ2, (c) BZ3, (d) BZ4, and (e) PDA−BZ4

was observed in all benzoxazine monomers, except for BZ1. The ΔH of residual oxazine was in the order of DDM > PDA > BZ4 > BZ3 > BZ2. The extent of polymerization was highly dependent on the number of oxazine rings in the backbone,

and the amount of ring opened structure was 87% in BZ2, 72% in BZ3, and 64% in BZ4. Approximately 50% of unreacted benzoxazine monomers were found in the case of intermediate tetrafunctional benzoxazines. These results thus suggest that F

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Figure 6. TGA vs DSC plots of uncured benzoxazine monomers (heating rate: 10 °C min−1, under a N2 atmosphere): (A) BZ1, (B) BZ2, (C) BZ3, (D) BZ4, (E) PDA−BZ4, and (F) DDM−BZ4.

thus, the unopened oxazine moiety will be more in BZ4 compared to BZ1. Therefore, it requires longer time for ringopening polymerization at low temperature. It was found difficult to obtain a completely cured product in R-BZ4 even by heating for 15 h at 180 °C (Figure S15). The curing rate was further assessed by heating at 200 and 220 °C for 2 h each (Figure 5b), and a complete ROP was observed in BZ2; however, a very small amount of residual oxazine exotherm was observed in BZ3 (8 J/g; 97% conversion), BZ4 (14.8 J/g; 94% conversion), and R-BZ4s (PDA: 25.8 J/g (91% conversion) and DDM: 32.6 J/g (89% conversion)). A complete disappearance of exotherm in benzoxazine with higher oxazine functionality, in particular R-BZ4, was observed only by heating at 250 °C for 2 h (Figure 5c). The cured resins

the benzoxazine monomer with higher number of oxazine rings requires longer duration or a higher temperature for curing. The ring-opening polymerization of benzoxazine monomer is considered as an autocatalytic process, at first initiation by protonation of oxygen atom in the oxazine ring, which results in an iminium ion (zwitterionic intermediate).46 Then, electrophilic attack of this iminium ion on another molecule occurs as shown in Scheme 3a; the phenolic groups thus generated act as an initiator for further ROP. Hence, the higher the oxazine functionality, the more the number of zwitterionic intermediates and reactive sites (marked with red circle). For instance, in BZ4 all four consecutive oxazine rings will create cationic intermediates and undergo electrophilic substitution on the reactive sites in another BZ4 molecule (Scheme 3d); G

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Macromolecules showed a high Tg and found to be increasing (PBZ1:111 °C18 to PBZ4:315 °C) with increase in number of oxazine rings in the backbone. This can be attributed to the formation of additional cross-links endowed by higher oxazine functionality along with the presence of inter- and intramolecular hydrogen bonding in the phenolic−Mannich bridge network. A further enhancement in Tg (∼5−7 °C) was observed in PPDA and PDDM−BZ4, which can be ascribed to the restriction of segmental motion in the polymer network by the intermediates. Weight Loss during Ring-Opening Polymerization. Endo et al. previously enlightened the outgassing issue in benzoxazine monomers during curing due to the dissociation of the zwitterionic intermediate accompanied by the volatilization of imine product (Ph−NCH2), thereby forming voids containing cured material.47 We have investigated the weight loss during the ring-opening polymerization in each benzoxazine monomers to explore the differences created by introduction of additional oxazine rings and intermediates. Figure 6 shows a comparison of TGA and DSC scans of uncured benzoxazine monomers. In BZ1 (Figure 6a), the initial weight loss started around 140 °C, and an excessive amount of monomer evaporation was observed (almost 75%) before even starting polymerization. A further ∼15% of weight loss was observed where the curing exotherm appeared in the DSC scan (241−272 °C), which is presumably due to the cleavage of zwitterionic intermediate during ROP. The zwitterionic intermediate in BZ1 is also very unstable and can easily disintegrate into two products: (a) phenolic species and (b) N-methyleneaniline as shown in Scheme 3a. Some of these phenolic species could participate in polymerization by suffering electrophilic attack with another zwitterionic intermediate; otherwise, they will volatilize along with the imine, leading to high weight loss. Such decomposed fragments, especially amines, have been detected in the pyrolysis mass spectroscopy analysis of aniline-based benzoxazines, even at the end stages of curing.48 In BZ2 (Figure 6b), the weight loss started only above 200 °C, and the monomer evaporation was extensively reduced (∼4%) due to the presence of additional oxazine ring. The weight loss due to the zwitterionic intermediate degradation during ROP (232−274 °C) was also found relatively low (∼11%). This can be possibly due to the tethering of the cross-links on the para/ortho position of the oxazine attached to the pendant phenyl ring, before it opens as shown in Scheme 3b (steps 1 and 2). Therefore, elimination of N-methyleneaniline was only plausible cause for weight loss in BZ2, and the stable phenolic fragment will undergo further electrophilic substitution and form PBZ2. A very remarkable decrease in weight loss was observed (Figure 6c,d) with the incorporation of more number of consecutive oxazine rings in the backbone (BZ3:3.3% and BZ4:2.4%), further supporting the hypothesis mentioned above. These results highlights the advantage of closely connected benzoxazine moieties, which helped to prevent the undesired monomer evaporation before curing as well as the stabilization of zwitterionic intermediate by providing additional cross-links (Scheme 3c,d), which eventually reduced the volatilization of imine during ROP. As anticipated, there was no weight loss in the case of intermediate benzoxazines (Figure 6e,f), mainly because of the tethering of the zwitterionic intermediates by cross-links from both sides (Scheme 3e), making them difficult to disintegrate. To provide further insight into the weight loss in these monomers during their actual curing cycle, we have cured the benzoxazines inside a TGA instrument under a N2 atmosphere.

A complete volatilization was observed over a period of 30 min in the TGA scan of BZ1 during curing at 180 °C (Figure S16), suggesting the difficulty of obtaining a completely cured product of BZ1 without an excessive monomer loss. Figure 7 shows

Figure 7. TGA profiles of di-, tri- and tetrabenzoxazine monomers during ring-opening polymerization at 250 °C for 2 h.

the TGA profiles of di- to tetrabenzoxazine monomers during curing at 250 °C for 2 h. The weight loss in BZ2 to BZ4 was observed only at the initial stages of curing within the time span of ∼10 min before reaching 250 °C, thereafter no significant weight loss. The values observed (15.5% for BZ2, 4.8% for BZ3, and 4.2% for BZ4) were consistent with those witnessed in their monomer TGA scans. It has to be noted that BZ2 cured at a relatively low temperature, 230 °C for 2 h, also showed a similar weight loss (16.8%), indicating such weight loss at the initial stages cannot be avoided by altering the curing temperature (Figure S17). There was no weight loss observed in the TGA scans of intermediate benzoxazines during curing (at 250 °C for 2 h), similar to their monomer TGA scans. Thus, designing monomers like R-BZ4 would be an efficient strategy to avoid the volatilization during curing and obtain a void-free product. Moreover, the wastage of material during curing due to earlier monomer evaporation can be avoided, which is a very important aspect in industrial production. Thermal Stability of Polybenzoxazines. Thermal stability of polybenzoxazines (PBZ)s was evaluated by TGA under a N2 atmosphere, and the scans are shown in Figure 8. The initial decomposition temperatures, the temperature at 5% (Td5) and 10% weight loss (Td10), in PBZ1 were observed at 271 and 326 °C, respectively. PBZ2 showed a high Td5 of 350 °C and Td10 of 373 °C, a significant increase of ∼80 and 50 °C, respectively, due to their highly cross-linked network structure with additional inter- and intramolecular hydrogen bonding. Moreover, Mannich bridges in PBZ1 (Scheme 3a) are very weak and reported to decompose into amine fragments easily before 300 °C,49 while in PBZ2 the Mannich bridges formed by ROP of attached oxazine ring are more stable, since those are tied within the cross-links (Scheme 3b). No significant increase (only ∼3 °C) in thermal stability was observed by incorporation of more than two oxazine rings in the backbone. PBZ3 showed a similar Td5 and Td10 as PBZ2 at 353 and 377 °C, respectively. In contrast, PBZ4 showed a low Td5 and Td10 values at 343 and 366 °C, which can be attributed to both Tg and initial thermal decomposition falling at the same region. The char yield at 600 °C was in the order of H

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starting material. These monomers underwent ring closure and subsequent classical Mannich condensation reaction in the presence of paraformaldehyde and yielded BZ2 to BZ4. Tetrafunctional benzoxazine monomers containing intermediates were also successfully synthesized to differentiate the change in thermal properties, by a similar reaction pathway utilizing primary amine such as p-phenylenediamine and 4,4′-diaminodiphenylmethane. The curing temperature was found to be decreasing from 264 °C (BZ1) to 237 °C (BZ4) with increase in number of oxazine rings in the backbone; however, an intermediate between BZ4 increased the curing temperature to 245 °C (DDM-BZ4). Moreover, DSC scan of benzoxazine monomers cured at different temperatures revealed that benzoxazines with higher oxazine functionality require longer time to cure at low temperature, i.e., higher the oxazine functionality, slower the curing rate. Based on our findings, the incorporation of more number of oxazine rings in the backbone is not an effective strategy to reduce the curing temperature or accelerate the ROP. Weight loss during ring-opening polymerization was also investigated, and a decrease in weight loss was observed (BZ1: 90% to BZ4: 2.4%) with increase in number of consecutive oxazine ring in the backbone. Furthermore, introducing an intermediate between two oxazine rings was found to be an efficient way to obviate weight loss during curing. The introduction of higher oxazine functionality resulted in polybenzoxazines with higher Tg of 315 °C; a further enhancement was achieved by incorporating intermediates PDA and DDM. Our TGA results suggest that presence of two/three oxazine rings in the benzoxazine backbone is sufficient to provide a thermally stable polybenzoxazine, with a maximum Td10 of 377 °C and char yield of 62% at 600 °C. No significant increase in thermal stability was observed by incorporating a fourth oxazine moiety in the backbone, however introducing an intermediate raised the initial degradation temperature to 395 °C.

Figure 8. TGA scans of polybenzoxazines, PBZ1−PBZ4.

PBZ4 = PBZ3 (62%) > PBZ2 (58%) > PBZ1 (44%). The extent of increase in char yield was 18% with incorporation of three oxazine moieties in the backbone, and no further increase was observed with a fourth oxazine ring. Comparative TGA scans of PBZ4 and intermediate polybenzoxazines P(R-BZ4) are shown in Figure 9. Thermal

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EXPERIMENTAL SECTION

Materials. 5-Nitrosalicylaldehyde, aniline, p-phenylenediamine, 4,4′-diaminodiphenylmethane, paraformaldehyde, sodium borohydride (NaBH4), sodium carbonate (Na2CO3), and triethylamine were purchased from Tokyo Chemical Industry, Japan. Sodium sulfate (anhydrous), ethanol (99.5), ethyl acetate, and tin(II) chloride dihydrate were purchased from Wako Pure Chemical Industry, Japan. Chloroform, dichloromethane (CH2Cl2), and hexane were purchased from Kanto Chemical Co. Inc., Japan. All the reagents were used as received. Silica gel 60, 0.063−0.200 mm (normal) and 35− 70 μm (flash) for column chromatography, was purchased from Merck, Japan. Monofunctional benzoxazine (BZ1) was synthesized according to Andreu et al.18 and purified by column chromatography (hexane:ethyl acetate (20:1)) and recrystallized from hexane to obtain white crystals (Figure S13). Anal. Calcd for C14H13NO: C, 79.59; H, 6.20; N, 6.63. Found: C, 79.74; H, 6.14; N, 6.62. Synthesis of Benzoxazine Monomers: Di-, Tri-, and Tetrabenzoxazines (BZ2, BZ3, BZ4, and R-BZ4). The common starting phenol, 2-((4-hydroxyphenyl)aminomethyl)phenol (HPAMP), was synthesized according to reported in the cited works.18,40 The amino compounds NH2-PAMP and NH2-H(PAM)2P were synthesized by a three-step procedure using 5-nitrosalicylaldehyde, comprised of an imine formation (step a), followed by reduction to corresponding benzyl amine (step b), and finally the reduction of nitro group to amine (step c). Synthesis of 4-Amino-2-((phenylamino)methyl)phenol (NH2-PAMP). NH2-PAMP was synthesized as follows: (a) aniline (13.93 g, 149.5 mmol) was added dropwise into a round-bottom flask containing 5-nitrosalicylaldehyde (25 g, 149.5 mmol) in ethanol

Figure 9. TGA scans of tetrafunctionalized polybenzoxazines.

decomposition temperature was increased by introduction of intermediates; PPDA and PDDM showed a high Td5 and Td10 of 360 and 384 °C and 371 and 395 °C, respectively (PBZ4: 343 and 366 °C). Compared to PBZ4, the only structural difference is the absence of phenyl pendant Mannich bridges in PPDA and PDDM-BZ4. The Mannich bridges in these are interlaced within the cross-links, making them more stable. The char yield at 600 °C was 62 and 65% in PDDM and PPDA, respectively.

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CONCLUSIONS The aim of this study was to provide an insight into the role of additional oxazine moiety in the benzoxazine backbone on their thermal properties. In order to pursue that, we have developed novel di, tri-, and tetrafunctional benzoxazine monomers (BZ2, BZ3, and BZ4) solely containing benzoxazine moieties arranged one after the other by utilizing phenol/amine containing (phenylaminomethyl)phenol (PAMP) backbone as I

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6.24 (dd, 1H, J = 8.4, 2.4 Hz), 6.44 (t, 1H, J = 7 Hz), 6.49 (d, 2H, J = 7.6 Hz), 6.54−6.57 (m, 2H), 7.95 (dd, 1H, J = 8.8, 2.8 Hz, ArNO2), 8.03 (d, 1H, J = 2.4 Hz, ArNO2), 8.56 (broad, 2H, OH). 13C NMR (100 MHz, DMSO-d6): δ 42.4 (CH2), 111.5, 112.6, 113.8, 115.4, 116.0, 116.1, 124.2, 124.5, 126.9, 128.5, 129.2, 140.1, 141.6, 146.8, 149.4, 162.0. (c) NH2-H(PAM)2P was synthesized by reducing NO2-H(PAM)2P (10 g, 27.3 mmol) in the presence of tin(II) chloride dihydrate (30.9 g, 136.9 mmol) at 85 °C for 2 h in ethanol (150 mL). After cooling the reaction mixture, the pH of the solution was changed to 8.5 by adding saturated Na2CO3 solution. The obtained white precipitate was suction filtered, and the amino compound was extracted using ethyl acetate and washed (first with distilled water, followed by brine and distilled water again). The solvent was dried over anhydrous sodium sulfate and evaporated off. The brown oily crude compound was purified by column chromatography by eluting with hexane:ethyl acetate (50:50), which resulted in a brown powder (yield: 4.4 g, 13.18 mmol, 48%). 1H NMR (400 MHz, DMSO-d6): δ 3.91 (d, 2H, J = 6 Hz, CH2), 4.04 (d, 2H, J = 5.6 Hz, CH2), 4.26 (s, 2H, NH2), 5.11 (t, 1H, J = 5.6 Hz, NH), 5.79 (t, 1H, J = 6 Hz, NH), 6.23 (dd, 2H, J = 8.74, 2.8 Hz), 6.42−6.47 (m, 3H), 6.51−6.54 (m, 4H), 6.99 (t, 2H, J = 7.8 Hz), 8.38 and 8.45 (s, overlapped, 2H, OH). 13C NMR (100 MHz, DMSO-d6): δ 42.4 (CH2), 43.8 (CH2), 111.8, 112.7, 113.8, 114.2, 115.2, 116.0, 116.1, 126.7, 127.0, 129.3, 141.1, 142.5, 146.5, 146.6, 149.5. Synthesis of 3′-Phenyl-2,3′,4,4′-tetrahydro-2′H-3,6′bibenzo[e][1,3]oxazine (Dibenzoxazine, BZ2). HPAMP (10 g, 46.49 mmol), aniline (4.32 g, 46.49 mmol), paraformaldehyde (4.6 g, 153.42 mmol), and chloroform (100 mL) were taken in a roundbottomed flask fitted with a condenser and guard tube (CaCl2), and the contents were refluxed. The completion of reaction was monitored by 1H NMR and confirmed after 48 h by almost disappearance of triazine signal at 4.82 ppm. The reaction mixture was cooled, and the solvent was evaporated off. The crude compound was purified by flash column chromatography using hexane:ethyl acetate (20:1) mixture as eluent and then recrystallized from hexane:ethyl acetate (20:1). White needlelike crystals obtained (yield: 4.7 g, 13.65 mmol, 30%); mp 122.7 °C. IR (solid) υ/cm−1: 1219 and 1032 (C−O−C), 1160 (C−N−C), 923, 934, and 971 (out-of-plane C−H stretch of benzene attached to oxazine). 1H NMR (400 MHz, CDCl3): δ 4.51 and 4.55 (s, 4H, Ar−CH2−N), 5.24 and 5.26 (s, 4H, O−CH2−N), 6.69 (d, 1H, J = 8.4 Hz), 6.78 (d, 2H, J = 7.6 Hz), 6.84−6.91 (m, 3H), 6.96 (d, 1H, J = 7.6 Hz), 7.05−7.11 (m, 3H), 7.21−7.24 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 50.7 and 51.1 (Ar−CH2−N), 79.4 and 80.5 (O−CH2−N), 117.05, 117.3, 117.6, 118.3, 119.6, 120.93, 120.97, 121.3, 121.5, 126.8, 127.9, 129.3, 142.5, 148.5, 149.6, 154.4. Anal. Calcd for C22H20N2O2: C, 76.72; H, 5.85; N, 8.13. Found: C, 76.84; H, 5.81; N, 8.04. Synthesis of 3″-Phenyl-2,2′,3″,4,4′,4″-hexahydro-2″H3,6′:3′,6″-terbenzo[e][1,3]oxazine (Tribenzoxazine, BZ3). A mixture of HPAMP (5 g, 23.24 mmol), NH2-PAMP (4.97 g, 23.24 mmol), paraformaldehyde (3.06 g, 102.2 mmol), and chloroform (50 mL) was refluxed for 48 h and cooled, and the solvent was evaporated off. The crude product was purified by column chromatography using eluent hexane:ethyl acetate (70:30) mixture and then recrystallized from hexane:ethyl acetate (1:1), which resulted in white shiny solid crystals (yield: 3.9 g, 8.17 mmol, 35%); mp 140−143.5 °C. IR (solid) υ/cm−1: 1211 and 1024 (C−O−C), 1152 (C−N−C), 904, 934, and 960 (out-of-plane C−H stretch of benzene attached to oxazine). 1H NMR (400 MHz, CDCl3): δ 4.45, 4.52, and 4.54 (s, 6H, Ar−CH2−N), 5.17, 5.25, and 5.27 (s, 6H, O−CH2−N), 6.69 (dd, 2H, J = 8.8, 2.8 Hz), 6.75 (t, 2H, J = 3.2 Hz), 6.78 (d, 1H, J = 8.4 Hz), 6.85−6.92 (m, 4H), 6.97 (d, 1H, J = 7.2 Hz), 7.05−7.12 (m, 3H), 7.21−7.24 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 50.6, 51.1, and 51.3 (Ar−CH2−N), 79.4, 80.4, and 80.5 (O−CH2−N), 117.0, 117.30, 117.37, 117.6, 118.3, 119.6, 119.7, 120.92, 120.98, 121.31, 121.35, 121.4, 126.8, 127.9, 129.3, 142.5, 142.6, 148.5, 149.60, 149.67, 154.4. Anal. Calcd for C30H27N3O3: C, 75.45; H, 5.70; N, 8.80. Found: C, 75.50; H, 5.62; N, 8.67. Synthesis of 3‴-Phenyl-2,2′,2″,3‴,4,4′,4″,4‴-octahydro2‴H-3,6′:3′,6″:3″,6‴-quaterbenzo[e][1,3]oxazine (Tetrabenzoxazine, BZ4). A mixture of HPAMP (2 g, 9.29 mmol),

(150 mL), stirring at 60 °C. The yellow homogeneous solution was suddenly turned to orange-red powdery suspension, confirming the imine formation. The heating was continued further for 4 h to ensure the completion of reaction and then cooled. (b) To this NaBH4 (2.82 g, 74.7 mmol) was added portionwise over a period of 15 min at RT; when the reduction was completed (∼15 min), the red suspension became homogeneous. Then distilled water was added (100 mL), and the compound was extracted with dichloromethane (3 × 100 mL). The organic layers were combined, washed again with distilled water (3 × 100 mL), and dried over anhydrous sodium sulfate. The solvent was concentrated to yield yellow crystals of NO2-PAMP (yield: 27.5 g, 112.6 mmol, 75%). 1H NMR (400 MHz, DMSO-d6): δ 4.21 (s, 2H, CH2), 6.2 (broad, 1H, NH), 6.47−6.53 (m, 3H), 6.95−7.03 (m, 3H), 7.98 (dd, 1H, J = 8.8, 2.8 Hz, ArNO2), 8.05 (d, 1H, J = 2.8 Hz, ArNO2), 11.23 (broad, 1H, OH). 13C NMR (100 MHz, DMSO-d6): δ 41.3 (CH2), 112.6, 115.6, 116.6, 124.1, 124.7, 128.1, 129.5, 140.1, 148.8, 162.1. (c) In a 500 mL round-bottomed flask equipped with a reflux condenser and a guard tube, NO2-PAMP (25 g, 102.4 mmol), tin(II) chloride dihydrate (115.5 g, 512.1 mmol), and ethanol (250 mL) were combined and heated at 85 °C for 2 h. The reaction mixture was then cooled, and the pH of the solution was changed to 8.5 by adding saturated Na2CO3 solution. The white precipitate thus obtained was suction filtered, which resulted in a white cake and the amino compound was extracted using ethyl acetate (400 mL). The organic layer was then washed with distilled water (2 × 250 mL), followed by brine, and then once again with distilled water. The solvent was dried over sodium sulfate and evaporated under reduced pressure to obtain light brown crystals (yield: 18 g, 84 mmol, 82%). 1H NMR (400 MHz, DMSO-d6): δ 4.04 (d, 2H, J = 6 Hz, CH2), 4.31 (s, 2H, NH2), 5.86 (t, 1H, J = 5.8 Hz, NH), 6.26 (dd, 1H, J = 8.2, 2.4 Hz), 6.44−6.52 (m, 5H), 6.99 (t, 2H, J = 7.8 Hz), 8.43 (s, 1H, OH). 13C NMR (100 MHz, DMSO-d6): δ 42.1(CH2), 112.6, 113.7, 114.8, 115.9, 116.0, 126.6, 129.2, 141.2, 146.2, 149.5. Synthesis of 4-Amino-2-(((4-hydroxy-3-((phenylamino)methyl)phenyl)amino)methyl)phenol (NH2-H(PAM)2P). NO2-H(PAM)2P was synthesized (a) by reacting 5-nitrosalicylaldehyde (7.8 g,

46.7 mmol) and NH2-PAMP (10 g, 46.7 mmol) in ethanol (100 mL) at 60 °C for 4 h. (b) Then, after cooling the reaction mixture, an excess amount of NaBH4 (1.76 g, 46.7 mmol) was added and stirred for 15 min. Distilled water was added; the yellow precipitate thus obtained was extracted with CH2Cl2 and washed with distilled water and dried over sodium sulfate. The solvent was concentrated to afford yellow crystals (yield: 12.4 g, 33.9 mmol, 72%). 1H NMR (400 MHz, DMSO-d6): δ 4.08 (d, 4H, J = 15.2 Hz, CH2), 5.83 (broad, 2H, NH), J

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7.49 kHz (1H) and at mixing times of 500 ms. 1H NMR spectra were also taken by changing the concentration of benzoxazine monomers (BZ2, BZ3, and BZ4) from 0.002 to 0.15 mol L−1. FTIIR spectra were recorded on a Thermo Scientific Nicolet iS10 spectrometer equipped with a Smart iTR diamond ATR sampling accessory in the range of 4000−500 cm−1. Elemental analysis was carried out with Yanaco CHN Corder MT-5. Melting point was determined using Yanaco Micro Melting Point Apparatus MP-500. Differential scanning calorimetry (DSC) analysis was carried out on a Seiko Instrument DSC-6200R (temperature range: 30−400 °C) at a heating rate of 10 °C min−1, under a nitrogen flow of 50 mL min−1 using a hermetic aluminum pan with 4 ± 0.5 mg of sample enclosed in it. Curing of benzoxazine monomers was done by heating the sample in a DSC pan from 30 °C to specified curing temperature at a heating rate of 20 °C min−1, held at that temperature for a specified time, and then cooled. The completion of curing was analyzed by a second heating from 30 to 400 °C at 10 °C min−1 heating rate. Thermogravimetric analysis (TGA) was performed on a Seiko Instrument TG-DTA 6200 (temperature range: 30−600 °C) using an aluminum pan under a nitrogen atmosphere (flow rate 200 mL min−1) at a heating rate of 10 °C min−1. Weight loss during ring-opening polymerization of benzoxazine monomers was evaluated by heating 8 ± 0.5 mg of sample in a TGA pan from 30 °C to curing temperature at 20 °C min−1 rate and held BZ1 at 180 °C for 2 h; BZ2, BZ3, BZ4, and R-BZ4 at 250 °C for 2 h and cooled. To evaluate the thermal stability of the cured resins, a second heating was performed from 30 to 600 °C at heating rate of 10 °C min−1, and the scans were recorded. A second heating was not possible in the case of BZ1 due to excess weight loss; therefore, we have cured it in a test tube by heating at 150 °C for 2 h, 170 °C for 2 h, and 190 °C for 4 h, and then TGA measurement was performed at a heating rate of 10 °C min−1.

NH2-H(PAM)2P (3.11 g, 9.29 mmol), and paraformaldehyde (1.56 g, 52.07 mmol) was refluxed in chloroform (60 mL); as the reaction proceeds, the powdery suspension slowly turned to homogeneous. After 48 h, the reaction mixture was cooled and then evaporated off the solvent. The crude product was purified by column chromatography, initially eluted with hexane:ethyl acetate (4:1) mixture, then ethyl acetate concentration was increased gradually to remove the side products, and finally BZ4 compound was collected by eluting with hexane:ethyl acetate (1:1) mixture. The obtained pale yellow powder was further purified by recrystallization using hexane:ethyl acetate (1:4) to result in a white fine powdery compound (yield: 0.6 g, 0.983 mmol, 10%); mp 158 °C. IR (solid) υ/cm−1: 1212 and 1024 (C−O−C), 1152 (C−N−C), 904, 937, and 961 (out-of-plane C−H stretch of benzene attached to oxazine). 1H NMR (400 MHz, CDCl3): δ 4.44, 4.45, 4.52, and 4.54 (s, 8H, Ar−CH2−N), 5.16, 5.17, 5.24, and 5.26 (s, 8H, O−CH2−N), 6.67−6.79 (m, 7H), 6.84−6.92 (m, 5H), 6.97 (d, 1H, J = 6 Hz), 7.05−7.12 (m, 3H), 7.21−7.25 (m, 2H). 13 C NMR (100 MHz, CDCl3): δ 50.7, 51.14, and 51.37 (Ar−CH2−N), 79.44, 80.40, and 80.49 (O−CH2−N), 117.0, 117.2, 117.33, 117.35, 117.6, 118.3, 119.5, 119.6, 119.7, 120.9, 121.2, 121.3, 121.5, 126.8, 127.9, 129.3, 142.51, 142.57, 142.6, 148.5, 149.5, 149.6. Calcd for C38H34N4O4: C, 74.73; H, 5.61; N, 9.17. Found: C, 74.75; H, 5.54; N, 9.09. Synthesis of Tetrabenzoxazines Containing Intermediates, R-BZ4 (R: -PDA, DDM). 1,4-Bis(2,4-dihydro-2′H-[3,6′-bibenzo[e][1,3]oxazin]-3′(4′H)-yl)benzene (PDA-BZ4). A mixture of HPAMP (3 g, 13.94 mmol), p-phenylenediamine (0.75 g, 6.97 mmol), paraformaldehyde (1.38 g, 4.60 mmol), triethylamine (0.70 g, 6.97 mmol), and chloroform (30 mL) was taken in a round-bottomed flask, then placed in a preheated oil bath (65 °C), and refluxed. After 2 h, anhydrous sodium sulfate (∼1 g) was added to sequester the byproduct water from the reactants. The reaction mixture was then continued refluxing for another 22 h, then cooled, and filtered the sodium sulfate. The solvent was evaporated off, and the transparent viscous crude product was purified by flash column chromatography using eluent hexane:ethyl acetate (2:1) mixture. The resultant white powder (38%) was recrystallized from hexane:ethyl acetate (1:4) (white fluffy crystals; yield: 0.85 g, 1.39 mmol, 20%); mp 180.5 °C. IR (solid) υ/cm−1: 1217 and 1034 (C−O−C), 1163 (C−N−C), 927, 952, and 976 (out-of-plane C−H stretch of benzene attached to oxazine). 1H NMR (400 MHz, CDCl3): δ 4.45 and 4.51 (s, 8H, Ar− CH2−N), 5.18 and 5.24 (s, 8H, O−CH2−N), 6.68 (d, 2H, J = 8.8 Hz), 6.74 (d, 2H, J = 2.4 Hz), 6.80 (d, 2H, J = 8 Hz), 6.86−6.89 (m, 4H), 6.96 (s, 2H), 6.98 (s, 4H), 7.11 (t, 2H, J = 7.6 Hz). 13C NMR (100 MHz, CDCl3): δ 51.09 and 51.15 (Ar−CH2−N), 80.24 and 80.53 (O−CH2−N), 117.0, 117.3, 117.5, 119.6, 120.1, 120.8, 120.9, 121.3, 126.8, 127.9, 142.4, 143.4, 149.5, 154.4. Anal. Calcd for C38H34N4O4: C, 74.73; H, 5.61; N, 9.17. Found: C, 74.89; H, 5.62; N, 9.01. Bis(4-(2,4-dihydro-2′H-[3,6′-bibenzo[e][1,3]oxazin]-3′(4′H)-yl)phenyl)methane (DDM-BZ4). 4,4′-Diaminodiphenylmethane (1.38 g, 6.97 mmol) was used as the primary amine to obtain DDM-BZ4; the rest of the synthesis procedure and purification method were as same as described above (white shiny fluffy crystals, yield: 1.1 g, 1.56 mmol, 22%); mp 172.2 °C. IR (solid) υ/cm−1: 1221 and 1032 (C−O−C), 1168 (C−N−C), 926, 959, and 976 (out-of-plane C−H stretch of benzene attached to oxazine). 1H NMR (400 MHz, CDCl3): δ 3.78 (s, 2H, CH2), 4.51 and 4.52 (s, 8H, Ar−CH2−N), 5.23 and 5.25 (s, 8H, O−CH2−N), 6.69 (d, 2H, J = 8.8 Hz), 6.76 (d, 2H, J = 2.8 Hz), 6.80 (d, 2H, J = 8.4 Hz), 6.88 (dd, 4H, J = 7.6, 3.6 Hz), 6.98 (d, 5H, J = 8.4 Hz), 7.029 (d, 5H, J = 8.8 Hz), 7.11 (t, 2H, J = 7.6 Hz). 13 C NMR (100 MHz, CDCl3): δ 40.33 (CH2), 50.84 and 51.16 (Ar−CH2−N), 79.71 and 80.56 (O−CH2−N), 117.0, 117.3, 117.5, 118.6, 119.6, 120.91, 120.96, 121.3, 126.8, 127.9, 129.6, 134.6, 142.4, 146.7, 149.6, 154.4. Anal. Calcd for C45H40N4O4: C, 77.12; H, 5.75; N, 7.99. Found: C, 77.19; H, 5.60; N, 8.01. Measurements. 1H (400 MHz)/13C (100 MHz) spectra were recorded on JEOL JME-ECS 400 NMR spectrometer, in CDCl3/ DMSO-d6 using internal reference as tetramethylsilane (TMS; δ= 0 ppm). 2D NMR, nuclear overhauser effect spectroscopy (NOESY) spectra were collected in CDCl3 over 1024 data points along the t2 dimension and 256 data points in the t1 dimension with X sweep of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01965. NMR spectra as well as DSC and TGA scans of benzoxazines (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(T.E.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank JX Nippon Oil & Energy Corporation for financial support, with special thanks to Mr. Masaki Minami for fruitful discussions.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01965 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01965 Macromolecules XXXX, XXX, XXX−XXX