Synthesis and Thermal Properties of Difunctional Benzoxazines with

Article pubs.acs.org/Macromolecules

Synthesis and Thermal Properties of Difunctional Benzoxazines with Attached Oxazine Ring at the Para-, Meta-, and Ortho-Position Sini Nalakathu Kolanadiyil,† Masaki Minami,‡ and Takeshi Endo*,† †

Molecular Engineering Institute, Kindai University, 11-6 Kayanomori, Iizuka, Fukuoka, Japan Specialty Chemical & Material Company, JX Nippon Oil & Energy Corporation, 8 Chidori, Yokohama, Japan

‡

S Supporting Information *

ABSTRACT: To investigate the role of position of oxazine ring in the benzoxazine backbone on their ring-opening polymerization (ROP) and thermal stability of resulting polybenzoxazine, we have synthesized difunctional monomers solely containing benzoxazine moieties (BZ2) with attached oxazine ring at the para-, meta-, and ortho-position. The ROP was examined by DSC analysis, which revealed a reduction in curing temperature in the order of meta (225 °C) < ortho (239 °C) < para (251 °C). The differences in structural and geometrical parameters were investigated by NMR (1H, 13C, 1H−1H NOESY) and X-ray crystallography analysis. The electronic effects and the intramolecular interaction between oxazine ring and aromatic hydrogen were higher when the attached oxazine ring was at the meta-position. The differences in their positioning also changed their ROP mechanism, an unusual intramolecular polymerization was observed in meta, while in ortho and para the polymerization proceeded in a regular manner. A curing mechanism responsible for lower curing temperature and faster ROP in meta has been proposed, which involves an intramolecular electrophilic substitution of iminium ion, resulting in aza-cyclic rings along with classical phenolic Mannich bridges in the networked structure. The cured resin showed a high Tg and in the order of para (291 °C) > meta (270 °C) > ortho (266 °C). Even though meta-PBZ2 displayed an earlier degradation with Td10 of 358 °C as compared to para and ortho (Td10: 373 °C) due to aza-cyclic rings, the main backbone degradation was observed to be coinciding in all PBZ2s at 417 °C with a char yield of 57% at 600 °C. Thus, changing position of oxazine ring to “meta” in the backbone is a beneficial strategy to have a low curing benzoxazine without sacrificing the thermal stability of resulting polybenzoxazine.

■

INTRODUCTION Polybenzoxazines, a relatively new phenolic-type thermosetting resin, in which the cross-linked network is constructed by linking phenolic moiety with Mannich bridges (−CH2−NR− CH2−). These resins has gained enormous interest in researchers from both academia and industry due to their outstanding properties including near zero volumetric shrinkage upon curing, minimal water absorption, low flammability, high glass transition temperature usually much higher than its curing temperature, fast physical and mechanical property buildup even at low conversion, etc. These unique properties are due to the existence of inter/intra molecular hydrogen bonding in the cross-linked network, specifically the very stable six-membered intramolecular OH- - -N hydrogen bonding. Moreover, the simplicity in benzoxazine monomer synthesis © XXXX American Chemical Society

make them more attractive, which involves Mannich condensation of phenol, primary amine, and formaldehyde.1−5 The tremendous molecular design flexibility offers myriad of modification in to the structure simply by changing suitably functionalized phenol/amines to develop polybenzoxazines with desired properties for particular applications.6−13 Unlike traditional phenolics, the starting monomer 1,3-cyclic benzoxazine can undergo thermally accelerated ring-opening polymerization without the use of harsh acid/base catalyst. However, their ring-opening polymerization (ROP) temperature usually appear at a very high range (200−270 °C), especially when it is Received: March 6, 2017 Revised: April 13, 2017

A

DOI: 10.1021/acs.macromol.7b00487 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Structure of Para-, Meta-, and Ortho-Positioned Bis-Benzoxazines

Scheme 2. Synthesis of Para- (p-BZ2), Meta- (m-BZ2), and Ortho-Positioned (o-BZ2) Bis-Benzoxazines

H2O in the case of methylol23) which can cause voids in the cured product and affect their mechanical properties. Recently, we have focused on developing benzoxazine monomers solely containing oxazine moieties instead of opting for other functional groups, in order to explore their potential in overcoming the drawbacks associated with these resins, especially their high curing temperature and weight loss before/ during ring-opening polymerization.14 In our initial efforts, we found that incorporation of more number of closely connected benzoxazine moieties in the backbone (mono to tetra) could prevent the earlier monomer evaporation and reduce the volatilization of imine during ROP. Moreover, introducing an intermediate in a tetrabenzoxazine was found to be the best solution for a complete relief from this issue. However, introducing more oxazine rings was not beneficial in accelerating the ring-opening polymerization at low temperature, regardless of the shift observed in their curing exotherm (Tp: 264 to 237 °C (mono to tetra)). In the present work, we have investigated how the position of oxazine moiety in the

a pure benzoxazine. Because of this, an excessive monomer volatilization was experienced before starting polymerization in the case of a typical monofunctional benzoxazine, 3-phenyl-3,4dihydro-2H-1,3-benzoxazine.14 Various efforts have been reported to reduce the polymerization temperature either by using an external catalytic/initiator system15−19 or by introducing special groups into the benzoxazine backbone. Inbuilt structure has always been of special interest since it could avoid the miscibility issues during blending and also nullify the losing of their inherent properties. Introduction of catalytic functional group such as phenolic hydroxyl,20−22 methylol,23−25 hydroxyethyl,26 amide,27,28 carboxylic,29 formyl,30 cyanate ester,31 allyl,32 etc., have been successful in reducing the curing temperature of a monobenzoxazine, while it had little/no effect when introduced into bis-benzoxazines.32−34 The disadvantage of introducing such catalytic groups is that they can reduce the monomer shelf life and also some of them releases undesired byproducts during ROP (e.g., CO2 in the case of formyl30 and B


Article

Macromolecules

Figure 1. 1H NMR spectra of para-, meta-, and ortho-bis-benzoxazines.

toward downfield from 8.35 to 9.19 ppm (para to ortho, Figure S1). Para-positioned bis-benzoxazine monomer (p-BZ2) was readily obtained by refluxing the reactants p-HPAMP, aniline, and paraformaldehyde for 2 days in chloroform. In case of meta-positioned benzoxazine, the synthesis was initially carried out by same procedure, refluxing the reactants m-HPAMP, aniline, and paraformaldehyde in chloroform. However, an offwhite sticky powdery formation was observed after 45 min of reaction, and the amount of these insoluble product was increased with reaction time. Most of the product obtained (after 24 h reaction) was found insoluble in the reaction solvent and their 1H NMR (Figure S2a) revealed an excessive oligomerization. This can be attributed to the delay in phenolic hydroxyl participation in Mannich condensation, which led to a simultaneous ring-opening addition reaction of already formed oxazine moiety (by aminomethyl phenol ring closure) in the presence of free m-phenolic hydroxyl (Figure S3).19 The soluble fraction indicated the signals corresponding to a mixture of products including bis-benzoxazine (m-BZ2), monobenzoxazine, unreacted triazine, and oligomers (Figures S2a and S3). The targeted compound m-BZ2 was extracted from this mixture by purification, but the yield was only 6.87% (column chromatography using hexane:ethyl acetate (20:1) and then by recrystallization using hexane:ethyl acetate (10:1)). A simultaneous ring-closure and Mannich condensation is necessary to avoid this oligomerization and was achieved by using triethylamine catalyst, which improvised the reactivity of phenolic hydroxyl.14 With triethylamine (0.25 mol), a homogeneous reaction was observed without any visible oligomerization, and the desired compound m-BZ2 was obtained after 24 h of reaction (Figures S2b and S4). The yield of m-BZ2 after purification was 36.3%, which was later

backbone can influence their ring-opening polymerization and thermal stability of resulting polybenzoxazines. To do so, we have designed difunctional benzoxazine monomers with attached oxazine ring at the para-, meta-, and ortho-position as shown in Scheme 1. A detailed structure−property analysis has done to understand the influencing parameters and discussed in detail.

■

RESULTS AND DISCUSSION Difunctional benzoxazine monomers (BZ2) solely containing benzoxazine moieties with attached oxazine ring at various positions para, meta, and ortho were synthesized via a tandem reaction as shown in Scheme 2. In order to vary the position of the attached oxazine ring in the backbone, the starting phenolic compound was changed to 2-(4-hydroxy(phenylaminomethyl)phenol (p-HPAMP), 2-(3-hydroxy(phenylaminomethyl)phenol (m-HPAMP), and 2-(2-hydroxy(phenylaminomethyl)phenol (o-HPAMP) for para-, meta-, and ortho-BZ2, respectively, which were obtained by reducing the Schiff base product based on 4-/3-/2-aminophenol and salicylaldehyde (Scheme S1). The mechanism of bis-benzoxazine formation involves a ring closure between the aminomethyl and neighboring phenolic hydroxyl group at first and followed by Mannich condensation between phenolic group and aniline, in the presence of stoichiometric amounts of paraformaldehyde.14,35 The position of the substituents in HPAMPs showed a strong influence on phenolic hydroxyl participation in Mannich condensation reaction. The phenolic hydroxyl reactivity was in the order of para > meta > ortho. This can be attributed to the steric hindrance due to the presence of bulky aminomethyl phenol on the meta-/orthoposition of phenolic hydroxyl. Moreover, the intramolecular hydrogen bonding between aminomethylphenol and phenolic hydroxyl is stronger in ortho and meta than para, evidenced by a shift in proton resonance signals of phenolic hydroxyl (a′) C


Article

Macromolecules

Figure 2. 13C NMR spectra of para-, meta-, and ortho-bis-benzoxazines.

Figure 3. ORTEP representations of crystal structures of (A) para-BZ2, (B) meta-BZ2, and (C) ortho-BZ2 (right side: with atom labeling, H atoms are omitted for clarity; left side: showing H- - -H intramolecular contacts). Displacement ellipsoids are shown at the 50% probability level. D


Article

Macromolecules

metrical parameters of the compounds are provided in the Supporting Information as Tables S1−S3, and the ORTEP views are shown in Figure 3. The characteristic irregular chair conformation was observed in all benzoxazine compounds. In para and meta, the orientations of two benzoxazine moieties in the backbone were observed in the same direction, while in ortho both are anti to one another due to steric hindrance. Generally, due to the distortion of bond length and bond angle, the oxazine ring possesses some ring strain, which is considered as the driving force for ring-opening polymerization under certain conditions and reported to occur through the break down of the −O−CH2− bond.5,37−39 It was of interest to understand the geometrical parameter differences in bis-benzoxazines induced by change of position of oxazine ring in the backbone, as any changes could potentially influence their ring-opening polymerization temperature. The −O−CH2−bond was found slightly elongated in m and o as compared to p and was 1.453(3)/1.460(3) Å (O1− C7/O2−C15) for meta, 1.454(1)/1.455(1) Å (O1−C8/O2− C15) for ortho, and 1.434(5)/1.435(4) Å (O1−C8/O2−C16) for para. Moreover, bond angles C−O−CH2 and O−CH2−N was found more deviated in meta and were almost same for both oxazine ring in the backbone, 114.5(2)° and 112.1(2)° for C6−O1−C7 and O1−C7−N1 and 114.4(2)° and 112.0(2)° for C13−O2−C15 and O2−C15−N2, i.e., a difference of 2.4°. While in ortho, the bond angles were 113.95(8)° and 114.77(9)° for C5−O1−C8 and O1−C8−N1 (0.8° difference) and 114.30(8)° and 113.31(8)° for C14−O2−C15 and O2− C15−N2 (0.9° difference). In the case of para, these were 115.4(3)° and 113.5(3)° for C5−O1−C8 and O1−C8−N1 (1.9° difference) and 112.7(3)° and 112.9(3)° for C12−O2− C16 and O2−C16−N2 (0.2° difference). The deviation observed in meta is unusual when compared it with most of the benzoxazine crystal data reported in the literature.40,41 Nonetheless, no significant difference was observed in the torsion angle of C−O−CH2−N, which was 41.04(4)/− 44.6(4)° for para, −44.6(3)/42.7(3)° for meta, and 42.8(1)/ 42.7(1)° for ortho. Additionally, the crystal structure of m-BZ2 revealed intramolecular H- - -H contacts between oxazine ring and aromatic hydrogen as indicated by PLATON CIF report (version 2016, PLAT410-ALERT 2C). The interactions observed between H7B- - -H14 (1.98 Å), H8A- - -H10 (1.96 Å), H15A- - -H22 (1.97 Å), and H16B- - -H18 (1.97 Å) are in accordance with those observed in the 1H−1H NOESY NMR spectrum (Figure S6). Effect of Position of Attached Oxazine Ring on RingOpening Polymerization. The effect of change in position of attached oxazine ring on thermal behavior of benzoxazine monomers was studied by DSC analysis, and the scans are shown in Figure 4. The results are also summarized in Table 1. A similar melting temperature was observed for para- and metabenzoxazine around 124 °C, while ortho showed relatively high Tm at 143 °C due to dense packing of molecules (Figure S8). A striking difference on curing exotherm was found due to regioisomerism and curing temperature was in the order of meta < ortho < para. In para-positioned benzoxazine, the curing exotherm was observed with onset (T0) at 236 °C and a maximum peak of curing temperature (Tp) at 251 °C. By changing the position of attached oxazine ring to meta, the curing exotherm was entirely shifted to a lower temperature region with T0 and Tp at 202 and 225 °C, respectively. The curing exotherm in ortho-positioned benzoxazine appeared in between with T0 and Tp at 221 and 239 °C. It is reasonable to

improved to 45.8% by increasing the molar ratio of triethyl amine to 1 mol. In the case of o-HPAMP, no bis-benzoxazine formation was observed even after refluxing the reactants in chloroform for several days. The major product was monofunctional benzoxazine and triazine intermediate (Figure S5a); interestingly, the obtained mono-oxazine was found intact throughout the reaction as compared to m-HPAMP, in which most of the mono-oxazine formed was subjected to oligomerization. This suggests a monobenzoxazine with m-phenolic hydroxyl will be more prone to ring opening. A successful synthesis of orthopositioned bis-benzoxazine (o-BZ2) was achieved by utilizing triethylamine catalyst (1 mol); however, a longer reaction time was needed for a complete conversion, and the reactants were refluxed for 4 days in chloroform (Figure S5b). The yield of oBZ2 after purification was 42%. An excess amount of triethylamine (2 mol) reduced the reaction time to 3 days (yield: 38%); however, some shoulder peaks were observed along with oxazine signals. We also examined the catalytic effect of triethylamine on p-BZ2 synthesis, and no significant impact on reaction time was observed, but a better yield of 52% was attained, while the yield without catalyst was 30%. The structure of the benzoxazine monomers was confirmed by 1H NMR, and the spectra are given in Figure 1. The number of oxazine rings is the same in all benzoxazine monomers, and only position of attached oxazine ring has changed; accordingly, the proton resonance signals also varied. The oxazine signals (−O−CH2−N−, −Ar−CH2−N−) appeared at 5.26, 4.55 (1, 1′) and 5.24, 4.51 ppm (2, 2′) for p-BZ2,36,14 5.30, 4.53 (1, 1′) and 5.28, 4.55 ppm (2, 2′) for m-BZ2, and 5.46, 4.62 (1, 1′) and 5.28, 4.52 ppm (2, 2′) for o-BZ2; these assignments were supported by the intramolecular interactions observed in the 1 H−1H NOESY NMR analysis (Figures S6 and S7). Our previous study indicated that the intramolecular interaction between oxazine and aromatic hydrogen lowers the curing exotherm. These interactions were higher with increasing oxazine functionality, and a corresponding shift in curing exotherm was also observed.14 Herein we have noticed a similar kind of intramolecular interaction. In meta, the attached oxazine ring, 2 and 2′ showed interaction with both 3 and c, while in ortho, 2 and 2′ showed only one interaction, i.e., with c. The aromatic protons in BZ2s were observed in the range of 6.69− 7.24, 6.54−7.25, and 6.71−7.27 ppm depending on the para-, meta-, and ortho-position, respectively. The structure of the benzoxazine monomers was further verified by 13C NMR (Figure 2); the carbon resonance signals due to O−CH2−N and Ar−CH2−N were observed at 79.5, 50.7 (1, 1′) and 80.5 and 51.1 ppm (2, 2′) for p-BZ2, 79.46, 50.08 (1, 1′) and 79.60, 50.37 (2, 2′) ppm for m-BZ2, and 79.88, 50.36 (1, 1′) and 80.49, 50.45 (2, 2′) ppm for o-BZ2. The chemical shifts due to Ar−C−N− was at 142.5, 148.5 ppm for p-BZ2, 148.3, 148.4 ppm for m-BZ2, and 137.3, 147.2 ppm for o-BZ2. The carbon resonance signals due to Ar−C−O− was observed at 149.6, 154.4 ppm for p-BZ2, 154.39, 155.06 ppm for m-BZ2, and 148.3, 154.1 ppm for o-BZ2. The elemental analysis results further supports the structure and the purity of the BZ2s obtained. Single X-ray Analysis. Structures of the BZ2s were also confirmed with the single crystal X-ray diffraction analysis. Colorless crystals of p-, m-, and o-benzoxazine monomers suitable for the analysis were obtained by slow diffusion of hexane into solution of compounds in ethyl acetate at room temperature. The crystallographic details and selected geoE


Article

Macromolecules

two oxazine rings in the backbone (O1- - -O2 distance: m- 5.41 Å, o- 5.07 Å), the phenolic group generated by opening attached oxazine moiety can initiate the ROP of the oxazine ring attached to phenyl pendant in intramolecular manner, while it may be difficult in the case of para (O1- - -O2 distance: 6.61 Å). Moreover, the ring-opening polymerization is also highly influenced by the electronic effects and the steric hindrance caused by the surrounding groups,5 which explains why meta has lowest curing exotherm compared to ortho and para. In the 13 C NMR spectrum of meta Ar−C−N− (148.3 ppm) and Ar− C−O− (154.3 ppm) signals were observed toward downfield as compared to ortho and para, possibly due to deshielding (Figure 2), such cases are reportedly favorable for ring-opening polymerization.42 A resorcinol based benzoxazine (T0 and Tp: 179 and 229 °C) and m-phenylenediamine-based benzoxazine (T0 and Tp: 160 and 193 °C) have also been reported with a low polymerization temperature due to electronic effects played by their specific structure.43,44 In a more related study by Ishida et al., where they changed the position of methylol/amide functional group on a monobenzoxazine (p-, m-, o-), the ROP was found favorable in the case of ortho-positioned benzoxazine because of the intramolecular hydrogen bonding between methylol/amide with oxazine ring (methylol: o- (196 °C) < m- (214 °C) < p(231 °C); amide: o- (187 °C) < p- (241 °C).25,28 In our case the intramolecular interactions were at a shorter distance in the case of meta 1.98/1.97 Å (H7B- - -H14/H15A- - -H22) as compared to para and ortho, 2.18/2.16 Å (H8B- - -H10/ H16A- - -H18), and 2.36/2.16 Å (H8A- - -H10/H15A- - -H22), respectively (Figure 3). More importantly, in meta, the aromatic hydrogen which has intramolecular interaction with oxazine ring, i.e., H14 (Scheme 3, marked as position 2) can be highly active since it is located between the electronegative oxygen and nitrogen atom and therefore can act as a stimulant for ringopening polymerization. We hypothesized that this active H14 in meta is also capable to undergo intramolecular electrophilic substitution with iminium ion as shown in Scheme 3i since it is at the ortho position to the O of oxazine ring, while no such possibility exists in the case of ortho- and para-BZ2. This can facilitate a faster polymerization, which explains the relatively narrow curing exotherm and its shift to a lower temperature in

Figure 4. DSC thermograms of para-, meta-, and ortho-bisbenzoxazines (heating rate: 10 °C min−1).

Table 1. DSC Data of Bis-Benzoxazines bis-benzoxazine

Tm (°C)

T0 (°C)

Tp (°C)

ΔH (J g−1)

parametaortho-

123 124 143

236 202 221

251 225 239

359.4 411.5 378.2

assume that the elongated −O−CH2− bond in meta and ortho made them easy to ring open as compared to para, thus a reduction in curing temperature. However, many other key parameters including proximity between two oxazine moieties (m- or o-position), most importantly the intramolecular interaction between aromatic hydrogen and attached oxazine ring are also responsible for the changes in curing temperature. The accepted ring-opening mechanism in benzoxazine suggests the initiation by protonation of oxygen atom, which results in a zwitterionic intermediate/iminium ion, followed by electrophilic substitution of these iminium ion on another molecule.41 The reaction proceeds autocatalytically, where the phenolic group generated promotes the opening of neighboring oxazine moiety. In meta and ortho, due to the close proximity between

Scheme 3. Proposed Ring-Opening Polymerization Mechanism in meta-BZ2

F


Article

Macromolecules Scheme 4. Structures of Poly(BZ2)s (A) Para, (B) Meta, and (C) Ortho

Figure 5. DSC thermograms of (A) p-, (B) m-, and (C) o-BZ2s cured at 180 °C for different time intervals.

the case of meta (Figure 4 and Figure S9). The high heat of polymerization in meta (ΔH = 411.5 J g−1) as compared to ortho (ΔH = 378.2 J g−1) and para (ΔH = 359.4 J g−1), further pointing toward the unusual thermal event happening in the case of meta. Taking all these into consideration, herein we propose that the ring-opening polymerization in meta initiated by protonation of the oxygen atom in the attached oxazine ring in the

presence of active hydrogen (H14), which leaves an active site on position 2; then the iminium ion undergoes intramolecular substitution on that active site and forms a four-membered azacyclic ring as shown in Scheme 3i. The phenolic group thus generated will help in opening of the oxazine ring attached to phenyl pendant and the chain propagates by suffering electrophilic attack of iminium on another molecule and results in a cross-linked structure containing phenolic Mannich bridges G


Article

Macromolecules

in both an inter- and intramolecular manner due to their close proximity, but the exotherm was found to be overlapping with glass transition temperature (Tg). There was no obvious curing exotherm observed after heating for 3 h; however, a further heating, i.e., 180 °C for 5 h, increased the Tg, which hints that intermediates/unreacted monomers were available for additional cross-link formation (Figure 5C). A residual curing exotherm was apparent in para even after heating for 8 h, and a slow conversion of monomer was observed with time. After 3 h, para-BZ2 reaches vitrification (84% conversion), which limited the conversion with further heating. These observations reflect the low reactivity of oxazine ring at the para-position and their inability to initiate intramolecular ROP of oxazine attached to pendant phenyl (Figure 5A). To have further insight into the ring-opening polymerization, FTIR analysis of BZ2s isothermally heated at 180 °C for different time intervals was done. For a comparison, BZ2s cured near their curing maxima (i.e., meta: 220 °C for 2 h; para and ortho: 235 °C for 2 h) were also investigated (Figure 6). The absorption bands of oxazine ring at 928, 940 cm−1 (out-ofplane C−H stretch of benzene attached to oxazine), 1217 and 1032 cm−1 (C−O−C), and 1362 cm−1 (CH2 wagging) disappeared in meta after heating at 180 °C for 90 min, which confirms the complete ring-opening polymerization (Figure 6A). Moreover, multiple bands at 1618−1568 cm−1 were changed to two broad peaks at 1617 and 1579 cm−1 and the bands at 1493−1485 cm−1 shifted to 1454 cm−1, which suggests the cross-link formation by substitution of iminium ion on the aromatic ring as shown in Scheme 4B. However, in the case of ortho and para (Figure 6B,C) the oxazine absorption bands were present even after heating at 180 °C for 5 h, which shows the presence of residual oxazine rings and their low reactivity compared to meta. On the other hand, ortho and para cured at 235 °C showed disappearance of the oxazine absorption bands and a shift in aromatic absorption bands due to complete ring-opening polymerization. Additionally, in all cases a broad absorption band around 3367 cm−1 due to inter/intramolecular hydrogen bonding can be seen from the initial stages of curing, manifesting the formation of phenolic Mannich bridges and absence of phenoxy intermediates (Figures S10−S12).39,48 The aza-cyclic ring in the cured meta-BZ2 is similar to benzocyclobutene (BCB) structure; hence, the characteristic absorption band of BCB at 2832 cm−1 (−CH2− vibration) was used as a reference.49 In the IR spectra of meta-BZ2 cured at 180 and 220 °C (Figure S10), the absorption bands at 2922 and 2853 cm−1 can be seen. The C−H stretch of Mannich bridges also lies at this range. The relatively strong bands in meta compared to ortho and para (cured at 235 °C) can be attributed to the presence of aza-cyclic ring. A further structural investigation was needed to find definite confirmation about aza-cyclic ring formation. Since meta-BZ2 was found highly insoluble within 10 min of heating at 180 °C, solid state 13C NMR analysis was performed, and the spectra of uncured and isothermally cured meta-BZ2 are shown in Figure 7. The signals corresponding to oxazine ring, O−CH2−N and Ar−CH2−N, were observed in monomer at 81 and ∼47 ppm.50 After heating at 180 °C for 10−15 min the carbon signal at 81 ppm was reduced remarkably, and a new signal at 32 ppm appeared, which can be attributed to the formation of Mannich bridges (CH2−NR−CH2, designated as a). It was also observed that the signal due to Ar−CH2−N became broader and its intensity increased, possibly due to the presence of intermediate iminium

with a aza-cyclic ring as depicted as structure 1 (Scheme 3i). It is also quite possible that ROP proceeds in the general accepted mechanism, i.e., substitution of iminium ion (⟩N+CH2) on another benzoxazine molecule (at position 2 or 1) in intermolecular fashion and form phenolic Mannich bridges without any cyclic ring as depicted as structure 2 (Scheme 3ii). Thus, a combination of structures 1 and 2 would be expected in the cured product of meta. In case of para and ortho, a normal ROP is expected. The prominent active position for iminium ion (⟩N+CH2) substitution is at the ortho position to the O of oxazine ring (as shown in Scheme 4; para: position 1 and 2; ortho: position 1). However, it has been reported that if the ortho-position is not available it could undergo substitution on the free para/ meta-position to the O of oxazine ring. Conversely, if the ortho/ para-position is blocked, it could undergo substitution at the ortho/para-position to the N of the oxazine ring.5,20,35,45−47 Here, in the case of o-BZ2, only the ortho-position is blocked with another benzoxazine moiety; therefore, polymerization can proceeded by substitution on the para-position to the O of both oxazine ring and ortho-position to the O of attached oxazine ring as well. All possible extra sites available for substitution (marked with red circle) and corresponding polymeric structures are shown in Scheme 4. To understand the ring-opening mechanism, DSC analysis of isothermally cured benzoxazine monomers at 180 °C for different time intervals was done, and the scans are shown in Figure 5. The benzoxazine conversion with time was also calculated from the residual oxazine observed in the DSC scan, and the data are listed in Table 2. The conversion rate was Table 2. Conversion of Benzoxazine Monomers at 180 °Ca % of polymerization: (ΔHmonomer − ΔHresidual)/ ΔHmonomer × 100

a b

at 180 °C

para

meta

ortho

30 min 60 min 90 min 120 min 180 min 5h 8h

2.7 16.7 65.6 70.7 84.4 87.4 90.5

93.7 98.6 99.1 −b −b

21.4 80.3 92.1c 92.9c −c −c

ΔHmonomer: para: 359.4 J g−1; meta: 411.5 J g−1; ortho: 378.2 J g−1. No measurable ΔH. cCuring exotherm overlapped with Tg.

found in the order of meta > ortho > para. In meta, most of the oxazine ring underwent ring-opening within 30 min with a residual oxazine enthalpy of 25.8 J g−1 (93% conversion); this faster reaction is possibly because of the mechanism proposed in Scheme 3i. While in ortho and para the oxazine ring was found intact with only 21 and 2% of ring-opened structures, respectively. A complete ring-opening was achieved in the case of meta after heating for 90 min (99% conversion); however, a deviation in the DSC scan can be seen albeit of prolonged heating (before Tg), which may be due to the presence of azacyclic rings. There was no measurable heat of enthalpy; hence, the thermal breakdown of these cyclic rings at this temperature is uncertain (Figure 5B). In ortho, the ring-opening was found faster after heating for 60 min with a residual oxazine enthalpy of 74.5 J g−1 (80% conversion) showing the capability of phenolic hydroxyl generated to initiate the ROP of neighboring oxazine moieties H


Article

Macromolecules

Figure 7. Solid state 13C NMR spectra of m-BZ2 cured at 180 and 220 °C (*side bands: 16 and 229 ppm).

is preferred, hence more aza-cyclic ring in the cross-linked network than 220 °C. We have also analyzed solid state 13C NMR of uncured and isothermally cured para- and ortho-BZ2s. The observations were almost consistent with IR analysis; residual oxazine signals (80 and 48 ppm in ortho; 81 and 49 ppm in para) were observed at 180 °C and disappeared at 235 °C (Figures S13 and S14). Weight Loss during ROP. Weight loss during ROP was investigated by TGA under a N2 atmosphere (Figures S15− S17). In the TGA scans of uncured benzoxazine monomers, all BZ2s showed an earlier decomposition due to monomer evaporation before the onset of ROP (in meta and ortho around 184 °C and para 207 °C), and the total weight loss during ROP was found much lesser in the case of meta (5−7%) than para (15−25%) and ortho (17−21%). This can be attributed to the lower ring-opening polymerization temperature and faster curing rate in meta, thereby reducing the extent of monomer evaporation and volatilization of zwitterionic intermediate. The vapors of the volatiles which were observed on the top side wall of the test tube of the isothermally cured BZ2s were isolated by dissolving in DMSO-d6, and their 1H NMR spectra indicated signals corresponding to oxazine and imine (Figure S18).14,51 Thermal Properties of Polybenzoxazines. Thermal stability of polybenzoxazines was evaluated by TGA under a N2 atmosphere, and the scans are shown in Figure 8. The thermal decomposition of phenolic Mannich bridges was reported to happen by cleavage of either C−C or C−N bonds, which primarily results in volatilization of benzene derivatives, amines, phenols, and Mannich base compounds, and these products can undergo further degradation, recombination, or successive dehydrogenation to form secondary byproducts and finally char formation.52 The initial thermal decomposition temperature in PBZ2s was observed in the order of para = ortho > meta (Figure 8D). The temperature at 5% (Td5) and 10% weight loss (Td10) in para

Figure 6. FTIR spectra of isothermally cured and uncured BZ2s (A) m, (B) o, and (C) p.

ions, which are not stable and can easily undergo substitution with further heating. A complete disappearance of O−CH2−N signal was observed after heating at 180 °C for 60 min along with an enhancement in the intensity of signal at 32 ppm, suggesting a complete opening of oxazine ring. However, the signal at 46 ppm (Ar−CH2−N) did not disappear even after prolonged heating for 3 h (marked as b, Figure 7), affirming the presence of aza-cyclic rings in the backbone (Scheme 3i). This signal was also present in the case of meta-BZ2 cured at 220 °C for 2 h, but with a very low intensity which may be due to differences in the ratio of structures 1 and 2 in the cured network. At 220 °C, both intra- and intermolecular electrophilic substitution as proposed in Schemes 3i and 3ii can proceed equally fast; however, at 180 °C the intramolecular substitution I


Article

Macromolecules

Figure 8. TGA scans of PBZ2s: (A) p-, (B) m-, and (C) o-, and (D) combined scans of PBZ2s obtained by curing at their Tp.

and ortho were observed at 350 and 373 °C. However, in meta Td5 and Td10 were observed at a relatively lower temperature, 333 and 358 °C. This can be due to the presence of aza-cyclic ring, which may disintegrate at an earlier temperature than classical phenolic Mannich bridges. After ∼20% of degradation, the thermal breakdown was found coinciding at 417 °C in all PBZ2s and then occurred in a similar manner with a char yield of ∼57% at 600 °C. The curing temperature had little impact on the thermal stability of m-PBZ2 and p-PBZ2 (∼10 °C difference; Figure 8A−C and Table S4), while a huge difference of ∼40 °C was observed in the case of ortho. o-PBZ2 obtained by curing at 180 °C showed a low Td5 and Td10 at 302 and 331 °C as compared to those obtained by curing at their maximum peak of temperature (Td5 and Td10: 348 and 372 °C). This can be attributed to the inability of the iminum ion to undergo electrophilic substitution on the extra active sites mentioned in Scheme 4C; therefore, the propagation of phenolic Mannich bridge may have proceeded through the preferred active site, i.e., ortho-position to the O of attached oxazine ring (position 1, Scheme 4C), resulting a relatively lower molecular weight polymer as compared to those obtained at 235 °C. In the 13C solid state NMR spectra of o-BZ2 cured at 180 °C, the signals at 119.8, 147.8 (Ar−C−N), and 161.3 ppm (Ar−C−O) confirm the presence of unreacted sites, and these signal intensities were decreased significantly at 235 °C (Figure S12), indicating the formation of additional cross-links through these active sites and hence higher thermal stability for o-PBZ2 obtained at 235 °C. Moreover, the six-membered intramolecular hydrogen bonds formed during polymerization is reportedly hinders the propagation of polymer chain by deactivating the reactive site.53,54 The intramolecular hydrogen bonding would be stronger in the case of ortho since the two

benzoxazine moieties are closely attached to each other; an additional five-membered hydrogen bonding between N and OH can form in the ring-opened structure (Scheme S2). This may have obstructed the propagation of polymer chain at 180 °C than at 235 °C. The glass transition temperature of PBZ2s was analyzed by DSC (Figure 9) and was found in the order of para (291 °C) >

Figure 9. DSC scans of PBZ2s (heating rate: 10 °C min−1).

meta (270 °C) > ortho (266 °C). These results do not follow the anomalous trend observed by Liu and Ishida for isomeric structure of BF-a based benzoxazine, i.e., o,o′ > o,p′ > p,p′.47 The ROP proceeded differently in all BZ2s due to regioisomerism, which changed their cross-linked network architecture as shown in Scheme 4. The existence of additional aza-cyclic rings in meta can lead to irregularities in the crosslinked structure and increase free volume, hence a lower Tg than para. However, in ortho and para only phenolic Mannich J


Article

Macromolecules

600 °C. The Tg followed the common regioselectivity order para (291 °C) > meta (270 °C) > ortho (266 °C). In conclusion, this study contributes to overcoming the major drawback of benzoxazine reported in the literature, which is their high curing temperature by simply changing the positioning of oxazine to “meta”, and this strategy can be applied to the existing benzoxazine monomers (bisphenol/ diamine based) to have a low curing benzoxazines with a balanced thermal properties for resulting polybenzoxazines.

bridges exist, but they showed a ΔTg of 25 °C, which may be due to the symmetrical and packed structure in para as compared to ortho where the regular active site (i.e., ortho position to the O of oxazine ring) for electrophilic substitution is being blocked by another benzoxazine moiety in the monomer; therefore, the cross-links formation will not be as symmetrical as in the case of para.

■

CONCLUSIONS To understand the influence of positioning of oxazine ring in the benzoxazine backbone on their thermal properties, we have synthesized benzoxazine monomers solely containing two benzoxazine moieties (BZ2), where the position of attached oxazine ring has changed at para-, meta-, and ortho-position. A tandem reaction was followed in which a ring closure and classical Mannich condensation between p-/m-/o-hydroxyphenyl aminomethylphenol, aniline, and paraformaldehyde to yield para-, meta-, and ortho-BZ2, respectively. A successful BZ2 synthesis was only possible by utilizing triethylamine catalyst in the case of meta and ortho, and the reaction rate was found to be meta > para > ortho. In the DSC analysis, the curing exotherm due to ROP of oxazine was found effectively reduced ∼25 °C by changing the position of oxazine ring in the backbone and was in the order of meta (225 °C) < ortho (239 °C) < para (251 °C). As a result, the weight loss during ROP was also less in meta (7%) than ortho and para (∼15−25%). By comparing structural and geometrical parameter differences using NMR (1H−1H NOESY, 13C) and X-ray crystallographic analysis, we found that the electronic effects were higher and intramolecular interactions (between aromatic hydrogen and oxazine ring) were at a shorter distance (1.96 Å) when the attached oxazine ring was at the meta position, which is responsible for the reduction in curing temperature. Moreover, the ROP was also found faster in meta and was able to achieve a completely cured product at 180 °C in 90 min, as compared to para and ortho where residual oxazine remained even after 5 h. We proposed a curing mechanism responsible for this behavior which follows an intramolecular electrophilic substitution of iminium ion with the formation of a four membered aza-cyclic rings along with phenolic Mannich bridges, which is well supported by solidstate 13C NMR. To the best of our knowledge such a cyclic ring formation during ROP is being reported for the first time in benzoxazine chemistry. We are currently in search for finding further evidence for the aza-cyclic ring formation and any possible rearrangement reaction of these cyclic ring to Mannich bridges or other linkages. It has been highlighted in the literature that ortho-substituted benzoxazines (e.g., methylol/amide) have favorable properties relative to their para and meta counterparts.23−25,27,28,47,55 However, our findings suggest that when the benzoxazine monomer substituted with another benzoxazine moiety instead of other functional group, meta has favorable properties than the ortho and para counterparts. Indeed, ortho-positioning showed a faster curing than para and achieved 92% polymerization at 180 °C in 90 min, but the resulting PBZ2 had a low thermal stability (∼40 °C lower) as compared to those cured at high temperature (235 °C). Overall, the initial thermal decomposition temperature (Td10) of a completely cured product was found to be para = ortho (373 °C) > meta (358 °C) due to earlier breakdown of cyclic rings in meta. Nevertheless, after 20% of degradation the thermal breakdown was found to be para = ortho = meta with a char yield of 57% at

■

EXPERIMENTAL SECTION

Materials. 4-Aminophenol, 3-aminophenol, 2-aminophenol, salicylaldehyde, sodium borohydride (NaBH4), aniline, paraformaldehyde, and triethylamine were purchased from Tokyo Chemical Industry, Japan. Ethyl acetate, anhydrous sodium sulfate, and ethanol (99.5) were purchased from Wako Pure Chemical Industry, Japan. Dichloromethane, chloroform, and hexane were purchased from Kanto Chemical Co. Inc., Japan. All the reagents were used as received. The starting phenolic monomers, 2-((4-hydroxyphenyl)amino)methyl)phenol (p-HPAMP), 2-(((3-hydroxyphenyl)amino)methyl)phenol (m-HPAMP) and 2-((2-hydroxyphenyl)amino)methyl)phenol (o-HPAMP) were synthesized by reacting 4-aminophenol/3-aminophenol/2-aminophenol with salicylaldehyde as described in the Supporting Information.20 Synthesis of Bis-Benzoxazine Monomers 3′-Phenyl-2,3′,4,4′tetrahydro-2′H-3,6′-bibenzo[e][1,3]oxazine (p-BZ2), 3′-Phenyl2,3′,4,4′-tetrahydro-2′H-3,7′-bibenzo[e][1,3]oxazine (m-BZ2), and 3′-Phenyl-2,3′,4,4′-tetrahydro-2′H-3,8′-bibenzo[e][1,3]oxazine (o-BZ2). Into a round-bottomed flask equipped with a condenser and guard tube, the contents p-HPAMP or m-HPAMP or oHPAMP (5 g, 23.24 mmol), aniline (2.16 g, 23.24 mmol), paraformaldehyde (2.3 g, 76.71 mmol), triethylamine (2.4 g, 23.24 mmol), and chloroform (100 mL) were added and refluxed. The reaction was monitored by 1H NMR, and the completion was confirmed after 24 h in the case of m-BZ2, 48 h in the case of p-BZ2, and 96 h for o-BZ2. The crude compound was purified by flash column chromatography (Silica gel, 35−70 μm) using eluent hexane:ethyl acetate (20:1) followed by recrystallization using a hexane:ethyl acetate (10:1) mixture. p-BZ2. White needle like crystals (yield: 4.17 g, 12.11 mmol, 52%). The structural details of this compound are the same as reported in our recent publication.14 m-BZ2. White solid crystals (yield: 3.6 g, 10.4 mmol, 45%); mp 126.6 °C. 1H NMR (400 MHz, CDCl3): δ 4.53 and 4.55 (s, 4H, Ar− CH2−N), 5.28 and 5.30 (s, 4H, O−CH2−N), 6.54 (d, 1H, J = 2.4 Hz), 6.65 (dd, 1H, J = 8.2, 2.3 Hz), 6.78 (d, 1H, J = 8 Hz), 6.84−6.92 (m, 3H), 6.97 (d, 1H, J = 7.2 Hz), 7.06−7.11 (m, 3H), 7.21−7.25 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 50.08 and 50.37 (Ar−CH2−N), 79.46 and 79.60 (O−CH2−N), 106.5, 111.3, 113.9, 117.08, 118.3, 120.91, 120.94, 121.5, 126.8, 127.4, 127.9, 129.3, 148.36, 148.49, 154.39, 155.06. IR (solid) υ/cm−1: 928, 940, and 951 (out-of-plane C−H stretch of benzene attached to oxazine), 1217 and 1034 (C−O− C), 1114 (C−N−C). Anal. Calcd for C22H20N2O2: C, 76.72; H, 5.85; N, 8.13. Found: C, 76.65; H, 5.89; N, 7.99. o-BZ2. White solid crystals (yield: 3.38 g, 9.82 mmol, 42%); mp 145.6 °C. 1H NMR (400 MHz, CDCl3): δ 4.52 and 4.62 (s, 4H, Ar− CH2−N), 5.28 and 5.46 (s, 4H, O−CH2−N), 6.71−6.76 (m, 2H), 6.79 (d, 1H, J = 8 Hz), 6.86 (t, 1H, J = 7.37 Hz), 6.91−6.97 (m, 2H), 7.00−7.03 (m, 1H), 7.08−7.12 (m, 3H), 7.27 (t, 2H, J = 8 Hz). 13C NMR (100 MHz, CDCl3): δ 50.36 and 50.45 (Ar−CH2−N), 79.88 and 80.49 (O−CH2−N), 116.8, 118.2, 119.3, 120.7, 120.9, 121.1, 121.60, 121.69, 121.9, 126.8, 127.8, 129.4, 137.3, 147.2, 148.3, 154.1. IR (solid) υ/cm−1: 918, 931, and 938 (out-of-plane C−H stretch of benzene attached to oxazine), 1217 and 1012 (C−O−C), 1158 (C− N−C). Anal. Calcd for C22H20N2O2: C, 76.72; H, 5.85; N, 8.13. Found: C, 76.80; H, 5.88; N, 8.08. Measurements. Solution state NMR, 1H (400 MHz)/13C (100 MHz) spectra were recorded using a JEOL JNM-ECS 400 spectrometer in CDCl3/DMSO-d6 using tetramethylsilane as internal K


Macromolecules

■

reference. 2D NMR, 1H−1H nuclear Overhauser effect spectroscopy (NOESY) spectra were also collected in CDCl3 with the following specificationst2 dimension: 1024 data points; t1 dimension: 256 data points; X sweep: 7.49 kHz (1H); mixing time: 500 ms. Solid state NMR, 13C cross-polarization (CP)/magic angle spinning (MAS) (99.5 MHz) measurements were performed on a JEOL JNM-ECX 400 spectrometer, at a spinning speed of 10 kHz. Single-crystal X-ray diffraction data were collected using a Bruker APEXII ULTRA/CCD diffractometer. The structures were solved and refined using the Bruker SHELXTL software package. Elemental analysis was carried out using a Yanaco CHN Corder MT-5. A Yanaco micro melting point apparatus MP-500 was used to determine the melting point of benzoxazines. FTIR 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. For structural determination by IR and solid state NMR, ∼100 mg of benzoxazine monomer was taken in a test tube, which was sealed with a septum and fitted with a nitrogen balloon and heated at 180 °C or near their maximum peak of curing temperature for a specified time interval. Differential scanning calorimetry (DSC) analysis was carried out on a Seiko Instrument DSC-6200R using a hermetic aluminum pan with 4 ± 0.5 mg of sample enclosed in it. The temperature range was 30−400 °C, and the thermograms were recorded at a different heating rates of 2, 5, 10, 15, and 20 °C min−1 under a nitrogen flow of 50 mL min−1. Isothermal curing studies was also done by heating benzoxazine monomers in a DSC pan from 30 to 180 °C at a heating rate of 20 °C min−1, held at 180 °C for specified time (30 min, 60 min, 90 min, 2 h, 3 h, 5 h, or 8 h), and cooled. A second heating was performed from 30 to 400 °C at 10 °C min−1 rate to evaluate the conversion of monomer. The glass transition temperature (Tg) was determined by using DSC by curing the samples at their maximum peak of temperature (meta: 220 °C for 2 h; ortho: 235 °C 2 h; para: 250 °C for 2 h). Thermogravimetric analysis (TGA) was performed on a Seiko Instrument TG-DTA 6200 using an aluminum pan in the temperature range of 30−600 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere (flow rate 200 mL min−1). Weight loss during ringopening polymerization of benzoxazine monomers was evaluated by heating 8 ± 0.5 mg of sample in a TGA pan from 30 °C to the curing temperature at 20 °C min−1 rate. The weight loss during each curing cycle was investigated. meta: (i) 180 °C for 90 min; (ii) 180 °C for 3 h; (iii) 220 °C for 2 h, ortho: (i) 180 °C for 3 h; (ii) 180 °C for 5 h; (iii) 235 °C for 2 h, para: (i) 180 °C for 8 h; (ii) 250 °C for 2 h. Then, a second heating was performed to evaluate thermal stability with a temperature range of 30−600 °C at heating rate of 10 °C min−1.

■

ACKNOWLEDGMENTS This work was funded by JX Nippon Oil & Energy Corporation, Japan. S.N.K and T.E are thankful to Associate Prof. Michito Yoshizawa and Dr. Yoshihisa Sei (Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, Japan) for single X-ray crystallographic measurements.

■

REFERENCES

(1) Ning, X.; Ishida, H. Phenolic Materials via Ring-Opening Polymerization: Synthesis and Characterization of Bisphenol-A Based Benzoxazines and Their Polymers. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1121−1129. (2) Nair, C. P. R. Advances in Addition-Cure Phenolic resins. Prog. Polym. Sci. 2004, 29, 401−498. (3) Ghosh, N. N.; Kiskan, B.; Yagci, Y. Polybenzoxazines- New High Performance Thermosetting Resins: Synthesis and Properties. Prog. Polym. Sci. 2007, 32, 1344−1391. (4) Goward, G. R.; Sebastiani, D.; Schnell, I.; Spiess, W. H.; Kim, H. D.; Ishida, H. Benzoxazine Oligomers: Evidence for a Helical Structure from Solid-State NMR Spectroscopy and DFT-Based Dynamics and Chemical Shift Calculations. J. Am. Chem. Soc. 2003, 125, 5792−5800. (5) Ishida, H. Overview and Historical Background of Polybenzoxazine Research. In Handbook of Benzoxazine Resins; Agag, T., Ed.; Elsevier: Amsterdam, 2011; Chapter 1, pp 3−81. (6) Shih, H. K.; Chu, Y. L.; Chang, F. C.; Zhu, C. Y.; Kuo, S. W. A Cross-Linkable Triphenylamine Derivative as a Hole Injection/ Transporting Material in Organic Light-Emitting Diodes. Polym. Chem. 2015, 6, 6227−6237. (7) Katanyoota, P.; Chaisuwan, T.; Wongchaisuwat, A.; Wongkasemjit, S. Novel Polybenzoxazine-Based Carbon Aerogel Electrode for Supercapacitors. Mater. Sci. Eng., B 2010, 167, 36−42. (8) Li, H. Y.; Liu, Y. L. Polyelectrolyte Composite Membranes of Polybenzimidazole and Crosslinked Polybenzimidazole-Polybenzoxazine Electrospun Nanofibers for Proton Exchange Membrane Fuel Cells. J. Mater. Chem. A 2013, 1, 1171−1178. (9) Pakkethati, K.; Boonmalert, A.; Chaisuwan, T.; Wongkasemjit, S. Development of Polybenzoxazine Membranes for Ethanol-Water Separation via Pervaporation. Desalination 2011, 267, 73−81. (10) Taskin, O. S.; Kiskan, B.; Yagci, Y. Polybenzoxazine Precursors As Self-Healing Agents for Polysulfones. Macromolecules 2013, 46, 8773−8778. (11) Arslan, M.; Kiskan, B.; Yagci, Y. Benzoxazine-Based Thermosets with Autonomous Self-Healing Ability. Macromolecules 2015, 48, 1329−1334. (12) Nalakathu Kolanadiyil, S.; Bijwe, J.; Varma, I. K. Synthesis of Itaconimide/Nadimide-Functionalized Benzoxazine Monomers: Structural and Thermal Characterization. React. Funct. Polym. 2013, 73, 1544−1552. (13) Liu, Y.; Huang, J.; Su, X.; Han, M.; Li, H.; Run, M.; Song, H.; Wu, Y. Shape Memory Polybenzoxazines Based on Polyetheramine. React. Funct. Polym. 2016, 102, 62−69. (14) Sini, N. K.; Endo, T. Toward Elucidating the Role of Number of Oxazine Rings and Intermediates in the Benzoxazine Backbone on Their Thermal Characteristics. Macromolecules 2016, 49, 8466−8478. (15) Ishida, H.; Rodriguez, Y. Catalyzing the Curing Reaction of a New Benzoxazine-Based Phenolic Resin. J. Appl. Polym. Sci. 1995, 58, 1751−1760. (16) Sudo, A.; Hirayama, S.; Endo, T. Highly Efficient CatalystsAcetylacetonato Complexes of Transition Metals in the 4th Period for Ring-Opening Polymerization of 1,3-Benzoxazine. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 479−484. (17) Liu, C.; Shen, D.; Sebastian, R. M.; Marquet, J.; Schonfeld, R. Catalyst Effects on the Ring-Opening Polymerization of 1,3Benzoxazine and on the Polymer structure. Polymer 2013, 54, 2873−2878. (18) Sun, J.; Wei, W.; Xu, Y.; Qu, J.; Liu, X. D.; Endo, T. A Curing System of Benzoxazine with Amine: Reactivity, Reaction Mechanism and Material Properties. RSC Adv. 2015, 5, 19048−19057.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00487. Experimental details and NMR spectra of HPAMPs, NMR and IR spectra, DSC and TGA scans and X-ray crystallographic data of benzoxazines (PDF) Cif file for para-BZ2 (mercury); Cif file for meta-BZ2 (mercury); Cif file for ortho-BZ2 (mercury); Cif Platon report for para-, meta-, and ortho-BZ2 (ZIP)

■

Article

AUTHOR INFORMATION

Corresponding Author

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

Takeshi Endo: 0000-0002-0659-217X Notes

The authors declare no competing financial interest. L


Article

Macromolecules

(40) Wattanathana, W.; Nonthaglin, S.; Veranitisagul, C.; Koonsaeng, N.; Laobuthee, A. Crystal Structure and Novel Solid-State Fluorescence Behavior of the Model Benzoxazine Monomer: 3,4Dihydro-3,6-dimethyl-1,3,2H-benzoxazine. J. Mol. Struct. 2014, 1074, 118−125. (41) Ranjith, S.; Thenmozhi, S.; Manikannan, R.; Muthusubramanian, S.; Subbiahpandi, A. 3,3′-(p-Phenylene)Bis(3,4Dihydro-2H-1,3-Benzoxazine). Acta Crystallogr., Sect. E: Struct. Rep. Online 2009, 65, o581. (42) Kawaguchi, A. W.; Sudo, A.; Endo, T. Thiol-Functionalized 1,3Benzoxazine: Preparation and Its Use as a Precursor for Highly Polymerizable Benzoxazine Monomers Bearing Sulfide Moiety. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1448−1457. (43) Arnebold, A.; Schorsch, O.; Stelten, J.; Hartwig, A. ResorcinolBased Benzoxazine with Low Polymerization Temperature. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1693−1699. (44) Ren, S.; Yang, X.; Zhao, X.; Zhang, Y.; Huang, W. Wei Huang, An m-Phenylenediamine-Based Benzoxazine with Favorable Processability and its High-Performance Thermoset. J. Appl. Polym. Sci. 2016, 133, 43368. (45) Wang, M. W.; Jeng, R. J.; Lin, C. H. Study on the Ring-Opening Polymerization of Benzoxazine through Multisubstituted Polybenzoxazine Precursors. Macromolecules 2015, 48, 530−535. (46) Van, A.; Chiou, K.; Ishida, H. Use of Renewable Resource Vanillin for the Preparation of Benzoxazine Resin and Reactive Monomeric Surfactant Containing Oxazine Ring. Polymer 2014, 55, 1443−1451. (47) Liu, J.; Ishida, H. Anomalous Isomeric Effect on the Properties of Bisphenol F-based Benzoxazines: Toward the Molecular Design for Higher Performance. Macromolecules 2014, 47, 5682−5690. (48) Sudo, A.; Kudoh, R.; Nakayama, H.; Arima, K.; Endo, T. Selective Formation of Poly(N,O-acetal) by Polymerization of 1,3Benzoxazine and Its Main Chain Rearrangement. Macromolecules 2008, 41, 9030−9034. (49) Cheng, Y.; Yang, J.; Jin, Y.; Deng, D.; Xiao, F. Synthesis and Properties of Highly Cross-Linked Thermosetting Resins of Benzocyclobutene-Functionalized Benzoxazine. Macromolecules 2012, 45, 4085−4091. (50) Russell, V. M.; Koenig, J. L.; Low, H. Y.; Ishida, H. Study of the Characterization and Curing of Benzoxazines Using 13C Solid-State Nuclear Magnetic Resonance. J. Appl. Polym. Sci. 1998, 70, 1413− 1425. (51) Sudo, A.; Du, L. C.; Hirayama, S.; Endo, T. Substituent Effects of N-Alkyl Groups on Thermally Induced Polymerization Behavior of 1,3-Benzoxazines. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2777− 2782. (52) Hemvichian, K.; Ishida, H. Thermal Decomposition Processes in Aromatic Amine-Based Polybenzoxazines Investigated by TGA and GC-MS. Polymer 2002, 43, 4391−4402. (53) Laobuthee, A.; Chirachanchai, S.; Ishida, H.; Tashiro, K. Asymmetric Mono-oxazine: An Inevitable Product from Mannich Reaction of Benzoxazine Dimers. J. Am. Chem. Soc. 2001, 123, 9947− 9955. (54) Bai, Y.; Yang, P.; Song, Y.; Zhu, R.; Gu, Y. Effect of Hydrogen Bonds on the Polymerization of Benzoxazines: Influence and Control. RSC Adv. 2016, 6, 45630−45635. (55) Han, L.; Zhang, K.; Ishida, H.; Froimowicz, P. Study of the Effects of Intramolecular and Intermolecular Hydrogen-Bonding Systems on the Polymerization of Amide-Containing Benzoxazines. Macromol. Chem. Phys. 2017, 1600562.

(19) Nalakathu Kolanadiyil, S.; Azechi, M.; Endo, T. Synthesis of Novel Tri-Benzoxazine and Effect of Phenolic Nucleophiles on Its Ring-Opening Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2811−2819. (20) Andreu, R.; Reina, J. A.; Ronda, J. C. Studies on the Thermal Polymerization of Substituted Benzoxazine monomers: Electronic effects. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3353−3366. (21) Lin, C. H.; Feng, Y. R.; Dai, K. H.; Chang, H. C.; Juang, T. Y. Synthesis of a Benzoxazine with Precisely Two Phenolic OH linkages and the Properties of its High-Performance Copolymers. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2686−2694. (22) Zhang, W.; Froimowicz, P.; Arza, C. R.; Ohashi, S.; Xin, Z.; Ishida, H. Latent Catalyst-Containing Naphthoxazine: Synthesis and Effects on Ring-Opening Polymerization. Macromolecules 2016, 49, 7129−7140. (23) Baqar, M.; Agag, T.; Huang, R.; Maia, J.; Qutubuddin, S.; Ishida, H. Mechanistic Pathways for the Polymerization of MethylolFunctional Benzoxazine Monomers. Macromolecules 2012, 45, 8119− 8125. (24) Baqar, M.; Agag, T.; Ishida, H.; Qutubuddin, S. Polymerization Behavior of Methylol-Functional Benzoxazine Monomer. React. Funct. Polym. 2013, 73, 360−368. (25) Zhang, K.; Froimowicz, P.; Han, L.; Ishida, H. HydrogenBonding Characteristics and Unique Ring-Opening Polymerization Behaviour of Ortho-Methylol Functional Benzoxazine. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3635−3642. (26) Kudoh, R.; Sudo, A.; Endo, T. A Highly Reactive Benzoxazine Monomer, 1-(2-Hydroxyethyl)-1,3-benzoxazine: Activation of Benzoxazine by Neighboring Group Participation of Hydroxyl Group. Macromolecules 2010, 43, 1185−1187. (27) Agag, T.; Liu, J.; Graf, R.; Spiess, H. W.; Ishida, H. Benzoxazole resin: A novel class of thermoset polymer via smart benzoxazine resin. Macromolecules 2012, 45, 8991−8997. (28) Froimowicz, P.; Zhang, K.; Ishida, H. Intramolecular Hydrogen Bonding in Benzoxazines: When Structural Design Becomes Functional. Chem. - Eur. J. 2016, 22, 2691−2707. (29) Andreu, R.; Reina, J. A.; Ronda, J. C. Carboxylic AcidContaining Benzoxazines as Efficient Catalysts in the Thermal Polymerization of Benzoxazines. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6091−6101. (30) Sini, N. K.; Bijwe, J.; Varma, I. K. Renewable Benzoxazine Monomer from Vanillin: Synthesis, Characterization, and Studies on Curing Behavior. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 7−11. (31) Ohashi, S.; Kilbane, J.; Heyl, T.; Ishida, H. Synthesis and Characterization of Cyanate Ester Functional Benzoxazine and its Polymer. Macromolecules 2015, 48, 8412−8417. (32) Oie, H.; Sudo, A.; Endo, T. Acceleration Effect of N-Allyl Group on Thermally Induced Ring-Opening Polymerization of 1,3-Benzoxazine. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 5357−5363. (33) Sini, N. K.; Azechi, M.; Endo, T. Synthesis and Properties of Spiro-Centered Benzoxazines. Macromolecules 2015, 48, 7466−7472. (34) Sini, N. K.; Bijwe, J.; Varma, I. K. Thermal Behaviour of BisBenzoxazines Derived From Renewable Feed Stock ’Vanillin’. Polym. Degrad. Stab. 2014, 109, 270−277. (35) Imran, M.; Kiskan, B.; Yagci, Y. Concise Synthesis and Characterization of Unsymmetric 1,3-Benzoxazines by Tandem Reactions. Tetrahedron Lett. 2013, 54, 4966−4969. (36) Dunkers, J.; Ishida, H. Vibrational Assignments of 3-alkyl-3,4dihydro-6-methyl-2H-1,3-Benzoxazines in the Fingerprint Region. Spectrochim. Acta, Part A 1995, 51, 1061−1074. (37) Liu, X.; Gu, Y. Effects of Molecular Structure Parameters on Ring-Opening Reaction of Benzoxazines. Sci. China, Ser. B: Chem. 2001, 44, 552−560. (38) Wang, Y. X.; Ishida, H. Synthesis and Properties of New Thermoplastic Polymers from Substituted 3,4-Dihydro-2H-1,3-benzoxazines. Macromolecules 2000, 33, 2839−2847. (39) Chutayothin, P.; Ishida, H. Cationic Ring-Opening Polymerization of 1,3-Benzoxazines: Mechanistic Study Using Model Compounds. Macromolecules 2010, 43, 4562−4572. M


Synthesis and Thermal Properties of Difunctional Benzoxazines with

Recommend Documents