Synthesis and Properties of Spiro-Centered Benzoxazines

Oct 13, 2015 - Molecular Engineering Institute, Kinki University Affiliation, 11-6 ... showed higher thermal stability with Tg in the range of 295–3...
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Synthesis and Properties of Spiro-Centered Benzoxazines Sini N. K, Motohisa Azechi, and Takeshi Endo* Molecular Engineering Institute, Kinki University Affiliation, 11-6 Kayanomori, Iizuka, Fukuoka, Japan S Supporting Information *

ABSTRACT: A series of thermally stable spiro-centered bis(benzoxazine) monomers (BPSPI/SPBC-Bz)s was synthesized using spirobiindane/spirobischroman phenol, different amines (aniline, p-toluidine, and 2-(4-aminophenyl)ethanol) and paraformaldehyde and was characterized by FT-IR, 1H and 13C NMR, and elemental analysis. The effect of incorporation of two different types of spiro-center on polymerization temperature of benzoxazine and thermal stability of resulting polymers was evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The curing occurred at high temperatures in both BPSPI-Bzs and SPBC-Bzs (275 °C), which lowered by incorporating hydroxyl group (250 °C) at the end. Spirobiindane based polybenzoxazines showed higher thermal stability with Tg in the range of 295−354 °C, initial degradation temperature at 10% weight loss at 397−405 °C and char yield of 37−42% at 600 °C compared to spirobischroman polybenzoxazines.

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INTRODUCTION Polybenzoxazines have gained great interest over commercial thermosetting resins due to their excellent thermal and mechanical properties. These polymers comprised of a phenol and tertiary amine Mannich bridge in the cross-linked network, which can be easily obtained by simply heating 1,3-benzoxazine monomers without addition of any catalysts and without generation of volatile byproducts. The most attractive feature is their design flexibility and versatility in molecular structure. Thus, polybenzoxazines can fulfill specific requirements for different applications such as aerospace, automotive, high temperature resistant adhesives and coatings, membranes etc. by selection of suitable functionalized phenols or amines.1−5 For example, thermal stability enhancement can be achieved by incorporation of reactive or bulky group such as allyl,6 nitrile,7 methacryloyl,8,9 addition imide,10−12 epoxide,13 furan,14 propargyl,15,16 acetylene,17 benzoxazole,18,19 fluorene,20−22 etc. The introduction of a spiro structure has been reported to increase the thermal properties of polymers. A spiro-center composes of two adjacent rings orthogonal to one another, with a tetrahedral bonding atom. It has a rigid noncoplanar structure with a twisted 90° angle which can restrict close packing and interchain interactions.23−28 Herein, we incorporated spiro skeletons such as spirobiindane and spirobichroman in to the benzoxazine backbone. The incorporation of such structures can reduce the main chain scission at high temperature due to presence of multibonds, thereby able to achieve thermally stable polybenzoxazines. To the best of our knowledge spirobichroman benzoxazine has not been reported so far. The thermal properties of spirobiindane benzoxazine (spirobisindane bisphenol/aniline) obtained in this study is © XXXX American Chemical Society

entirely different and significantly higher as compared to previously reported29 which possibly due to the high purity. The presence of oligomers or other impurities has been reported to affect the thermal properties of the polybenzoxazines.30 A systematic structure−property analysis is discussed in this paper by synthesizing a series of spiro-centered benzoxazine monomers using spirobiindane/spirobischroman diol, various mono amines such as aniline, p-toluidine, and 2-(4aminophenyl)ethanol and paraformaldehyde as shown in Scheme 1.

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RESULTS AND DISCUSSION

The spirobiindane series of benzoxazine monomers (BPSPIBz)s were synthesized successfully by Mannich condensation reaction between spirobiindane diol (BPSPI−OH), aniline/ptoluidine/2-(4-aminophenyl)ethanol and paraformaldehyde using toluene at reflux temperature. In case of spirobichroman benzoxazine monomers (SPBC-Bz)s, the synthesis was not successful by using solvents such as toluene, 1,4-dioxane, 2methoxy ethanol etc., but utilizing p-xylene at 130 °C was found to be suitable for synthesis of SPBC-Ph-Bz and SPBCMe-Bz.31 The successful synthesis of spirobichroman benzoxazine based on 2-(4-aminophenyl)ethanol (i.e., SPBC-EthOHBz) was achieved only by using a base catalyst, 4-dimethyl aminopyridine in 1,4- dioxane at 90 °C for 8h. Without this Received: September 8, 2015 Revised: September 28, 2015

A

DOI: 10.1021/acs.macromol.5b01970 Macromolecules XXXX, XXX, XXX−XXX

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a small amount of phenolic impurity (either from starting monomer or oligomer) in benzoxazine is known to shift the curing exotherm to a lower temperature region, due to the catalytic effect of phenolic OH on the ring-opening polymerization of oxazine.30 The high curing temperature of spirobenzoxazines thus suggests the high purity. The proposed structure of ring-opening polymerization of spiro-benzoxazines are shown in Scheme 2. The benzoxazine monomers with spirobiindane unit showed a curing exotherm with onset (To) and maximum peak of temperature (Tp) at 249 and 273 °C for BPSPI-Ph-Bz, 245 and 272 °C for BPSPI-Me-Bz, and 253 and 265 °C for BPSPIEthOH-Bz, respectively. In case of BPSPI-Ph-Bz and BPSPIMe-Bz, a shoulder peak at 262 °C in the curing exotherm was observed, while BPSPI-EthOH-Bz showed a single exotherm right after melting. All spirobischroman based benzoxazines showed a single exotherm with To and Tp at 250 and 275 °C for SPBC-Ph-Bz, 246 and 274 °C for SPBC-Me-Bz, and 230 and 252 °C for SPBC-EthOH-Bz, respectively. No noticeable effect was observed by attaching a methyl group on the p-position of amine, however a decrease in curing temperature was observed by attaching CH2CH2OH group, which can be explained as the neighboring group participation of hydroxyl group on ringopening polymerization of oxazine.34 It was previously reported by our group that the addition of equimolar amount of 2-(N,Ndimethylamino)ethanol into 3,6-dimethyl-3,4-dihydro-2H[1,3]-benzoxazine has accelerated the rate of polymerization.34 The hydroxyl group in SPBC/BPSPI-EthOH-Bz is also capable to activate ring opening polymerization of benzoxazine in an intermolecular manner. This could be another possible reason for lowering the polymerization temperature. Comparatively spirobischroman based benzoxazine containing CH2CH2OH group (SPBC-EthOH-Bz) showed lowest curing temperature. The maximum curing temperature (Tp) in both spirobenzoxazines was found to be in the order EthOH < Me< Ph. The heat of polymerization was found to be increased (122−143 J g−1) by attaching CH3 and CH2CH2OH groups in case of BPSPI-Bzs while no specific order was observed in case of SPBC-Bzs. Thermal Properties of Cured Benzoxazine Resins. Glass transition temperature (Tg) of isothermally cured spirobenzoxazines was determined by DSC (Figure 3). The cured resins showed a very high Tg above 290 °C imparted by the presence of rigid spiro-center, which restricts the internal rotations and segmental motion of polymer chains. Poly(BPSPI-Bz)s exhibited a Tg in the range of 295−354 °C

catalyst, the formation of SPBC-EthOH-Bz was observed after 4 days in 1,4 dioxane with yield of less than 2%. The structure of the spiro-centered benzoxazines was confirmed by FT-IR, 1H and 13C NMR spectroscopy. Figure 1 shows 1H NMR spectra of representative BPSPI-Ph-Bz and SPBC-Ph-Bz. The characteristic proton resonance signals due to oxazine ring, O−CH2−N and Ar−CH2−N in BPSPI-Ph-Bz were observed at 5.26−5.33 ppm (doublet) and 4.6 ppm (singlet) respectively.32 In case of SPBC-Ph-Bz, the oxazine protons appeared as a two sets of doublet resonances, O− CH2−N was at 4.96 and 5.05 ppm and Ar−CH2−N at 3.77 and 4.22 ppm. These unusual structural behavior can be attributed to the magnetic nonequivalence of adjacent protons in Ar− CH2−N and O−CH2−N due to the presence of spiro-center. 1 H−1H COSY NMR spectrum of SPBC-Ph-Bz further revealed the coupling interaction between neighboring protons in oxazine ring. The oxazine protons were assigned to one single carbon resonance in the 1H−13C HMQC NMR spectra of BPSPI-Ph-Bz and SPBC-Ph-Bz, confirms the absence of isomers in the structure. The carbon resonance signal due to O−CH2−N and Ar−CH2−N was observed at 79 and 51 ppm for BPSPI-Ph-Bz and 78 and 46 ppm for SPBC-Ph-Bz (Supporting Information, Figures S3−S8). The other findings confirm the successful formation of the structure was the presence of aliphatic protons due to spirobiindane at 2.17−2.29 (CH2) and 1.30−1.34 (CH3) ppm and spirobichroman at 2.03−2.15 (CH2) and 1.33−1.59 (CH3) ppm. The carbon resonance signal due to tetrahedral carbon was observed at 57 ppm in spirobiindane and 98 ppm in spirobichroman benzoxazines. The detailed 1H and 13C NMR spectra of all other spirobenzoxazine synthesized in this study is given in Supporting Information (Figures S9−S16). Curing Behavior. The melting and curing behavior of the spiro structured benzoxazine monomers was studied by DSC and the thermograms are shown in Figure 2. The spirobiindane bis(benzoxazine)s showed high melting temperatures (Tm) in the range of 224−249 °C while spirobischroman unit showed low in the range of 186−199 °C, which defines the ease of processability of spirobischroman over spirobiindane. A double melting endotherm was observed for both spiro-benzoxazines derived from p-toluidine (BPSPI-Me-Bz (214 and 227 °C) and SPBC-Me-Bz (166 and 186 °C)), which can be referred to the existence of two different populations of crystallite sites.33 The curing exotherm due to the ring opening polymerization of oxazine ring was observed at a high temperature region (240− 275 °C) in both spiro-centered benzoxazines. The existence of B

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Figure 1. 1H NMR spectra of BPSPI-Ph-Bz and SPBC-Ph-Bz in CDCl3.

whereas Poly(SPBC-Bz)s showed a Tg in the range of 294−332 °C. These values are much higher in comparison to conventional bisphenol A based benzoxazine (150 °C) (Figure S18) and also the fluorene based bis-benzoxazine (228 °C).6,20 Spirobichroman polybenzoxazines showed a decrease in Tg due to the high conformation mobility of six-membered hetrocyclic spirobichroman unit compared to more rigid spirobiindane. Incorporation of CH3 and CH2CH2OH group increased Tg of poly(SPBC-Bz)s while it showed a reverse effect in case poly(BPSPI-Bz)s. Figure 4 shows TGA thermograms of poly(BPSPI-Bz)s, poly(SPBC-Bz)s, and poly(B-a) under nitrogen atmosphere. Thermal stability was determined by initial decomposition temperatures, the temperature at 5% (Td5) and 10% weight loss (Td10) and char yield at 600 °C. The Td5 and Td10 values of

poly(BPSPI-Ph-Bz), poly(BPSPI-Me-Bz) and poly(BPSPIEthOH-Bz) were observed at 375 and 405 °C, 371 and 397 °C, and 373 and 399 °C respectively. In case of poly(SPBC-PhBz), poly(SPBC-Me-Bz), and poly(SPBC-EthOH-Bz), the Td5 and Td10 values were found at 364 and 377 °C, 358 and 372 °C, and 356 and 369 °C respectively. As anticipated, the incorporation of spiro skeleton into the backbone structure significantly improved thermal stability as compared to typical bisphenol A based benzoxazine (Td5 326 and Td10 344 °C). This can be attributed to the formation of highly cross-linked network consists of intra- and intermolecular hydrogen bonds along with a rigid and bulky spiro backbone. No significant changes were observed in the initial decomposition temperatures by the incorporation of aliphatic side groups but it increased char yield at 600 °C. Poly(BPSPI-Bz)s showed a C

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Figure 2. DSC scan of (a) BPSPI-Bzs and (b) SPBC-Bzs.

Scheme 2. Thermal Activated Ring-Opening Polymerization of (a) BPSPI-Ph-Bz and (b) SPBC-Ph-Bz

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CONCLUSIONS A series of spirobiindane and spirobichroman based benzoxazines have been successfully prepared and effect of structure on the thermal properties was evaluated. With the introduction of these rigid spiro centers, the polybenzoxazines exhibited a very high glass transition temperature (290−360 °C) and excellent thermal stability compared to traditional polybenzoxazines. Spirobiindane polybenzoxazines showed superior thermal stability compared to spirobichroman because of higher rigidity. Incorporation of CH2CH2OH group found to be effective in lowering curing temperature in both spirobiindane- and spirobichroman-based benzoxazines.

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EXPERIMENTAL SECTION

Materials. Bisphenol-A, resorcinol, aniline, ethyl acetate, methanol, 1,4-dioxane, tetrahydrofuran (THF) and sodium carbonate were purchased from Wako Pure Chemical Industry (Osaka, Japan). 4methyl-3-penten-2-one (mesityl oxide), 2-(4-amino phenyl)ethanol, ptoluidine, p-xylene, trifluoromethanesulfonic acid, 4-dimethyl amino pyridine (DMAP) and paraformaldehyde were purchased from Tokyo Chemical Industry (Tokyo, Japan). Dichloromethane, chloroform, hexane, and toluene were purchased from Kanto Chemical co. Inc. (Japan). Iron(III) chloride was purchased from Sigma-Aldrich (Japan). All the reagents were used as received. Silica gel 60 (70−230 mesh ASTM) for column chromatography was purchased from Merck (Japan). The starting monomer, 3,3,3′,3′-tetramethyl-2,2′,3,3′-tetrahydro-1,1′-spirobi[indene]-6,6′-diol (BPSPI−OH) and 4, 4, 4′, 4′tetramethyl-2, 2′-spirobi [chroman]-7, 7′-diol (SPBC−OH) for

Figure 3. DSC scans of cured resins.

higher char yield of 37−42% compared to poly(B-a) (30%), while almost similar char yield was observed for poly(SPBCBz)s (28−36%). Spirobiindane polybenzoxazines exhibited approximately 25−30 °C higher degradation temperature and high char yield at 600 °C compared to spirobichroman. This can be attributed to high structural rigidity of spirobiindane over spirobichroman unit. D

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Figure 4. TGA profiles of (a) poly(BPSPI-Bz)s and (b) poly(SPBC-Bz)s. synthesis of spirobisindane/spirobichroman bis-benzoxazine was synthesized according to the literature with little modification (see Supporting Information).23,35 To compare thermal stability of spirobenzoxazines, bisphenol A benzoxazine (B-a) was synthesized according to the literature and purified by column chromatography (eluent: hexane/ethyl acetate (9/1).36 Synthesis of Spirobiindane Benzoxazine Monomers. Synthesis of 6,6,6′,6′-Tetramethyl-3,3′-diphenyl-3,3′,4,4′,6,6′,7,7′-octahydro-2H,2′H-8,8′-spirobi[indeno[5,6-e][1,3]oxazine] (BPSPI-Ph-Bz). In a 100 mL one neck round-bottom flask equipped with a condenser and guard tube were added BPSPI−OH (5 g, 16 mmol), aniline (3.02 g, 32 mmol), paraformaldehyde (1.94 g, 64 mmol) and toluene (50 mL), and the mixture was refluxed for 50 h. After removal of toluene under reduced pressure, the yellow crude product was recrystallized from ethyl acetate to obtain benzoxazine as pure white shiny solid crystals (yield: 4.2 g, 7.74 mmol, 47%): mp 208−226 °C. 1H NMR (400 MHz, CDCl3), δ: 7.26−7.31 (m, 4H), 7.11−7.13 (m, 4H), 6.93 (t, J = 7.2 Hz, 2H), 6.78 (s, 2H), 6.27 (s, 2H), 5.31 (d, J = 10.4 Hz, 2H, O−CH2−N), 5.27 (d, J = 10 Hz, 2H, O−CH2−N), 4.66 (s, 4H, Ar−CH2−N), 2.27 (d, J = 13.2 Hz, 2H, CH2), 2.19 (d, J = 13.2 Hz, 2H, CH2), 1.34 (s, 6H, CH3), 1.30 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3), δ: 153.8 (s), 150.5 (s), 148.7 (s), 144.9 (s), 129.3 (s), 121.2 (s), 119.7 and 119.6 (d), 118.1 (s), 112.5 (s), 79.06 (s, O− CH2−N), 59.6 (s, CH2), 57.4 (s), 51.0 (s, Ar−CH2−N), 43.0, 32.1, and 30.6 (s, C(CH3)2). ATR-FTIR (υ, cm−1): 1218 and 1068 (C−O− C), 1185 (C−N−C), 950 and 917 (out of plane C−H stretch of benzene attached to oxazine). Anal. Calcd for C37H38N2O2: C, 81.88; H, 7.06; N, 5.16. Found: C, 81.77; H, 7.14; N, 5.06. Synthesis of 6,6,6′,6′-Tetramethyl-3,3′-di-p-tolyl3,3′,4,4′,6,6′,7,7′-octahydro-2H,2′H-8,8′-spirobi[indeno[5,6-e][1,3]oxazine] (BPSPI-Me-Bz). A mixture of BPSPI−OH (5 g, 16 mmol), ptoluidine (3.47 g, 32 mmol), paraformaldehyde (1.94g, 64 mmol), and toluene (50 mL) was refluxed for 48 h. By cooling the reaction mixture pale yellow crystals were obtained in toluene. The product was further purified by recrystallization from ethyl acetate which yielded white crystals. (yield: 4.5 g, 7.89 mmol, 49%): mp 206−219 °C. 1H NMR (400 MHz, CDCl3), δ: 7.03−7.11 (m, 8H), 6.77 (s, 2H), 6.26 (s, 2H), 5.29 (d, J = 10.4 Hz, 2H, O−CH2−N), 5.25 (d, J = 10.4 Hz, 2H, O− CH2−N), 4.63 (s, 4H, Ar−CH2−N), 2.17−2.29 (m, 10H, CH3 overlapped with protons of CH2), 1.30 (s, 6H, CH3), 1.34 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3), δ: 153.7 (s), 150.5 (s), 146.4 (s), 144.8 (s), 130.8 (s), 129.8 (s), 119.6 (s), 118.4 (s), 112.4 (s), 79.54 (s, O−CH2−N), 59.65 (s, CH2), 57.4 (s), 51.18 (s, Ar−CH2− N), 43.0 (s), 30.6 and 31.9 (s, C(CH3)2), 20.67 (s, CH3). ATR-FTIR (υ, cm−1): 1212 and 1066 (C−O−C), 1188 (C−N−C), 948 and 912 (out of plane C−H stretch of benzene attached to oxazine), 2947 and 2855 (aliphatic C−H). Anal. Calcd for C39H42N2O2: C, 82.07; H, 7.42; N, 4.91. Found: C, 82.17; H, 7.45; N, 4.96. Synthesis of 2,2′-((6,6,6′,6′-Tetramethyl-6,6′,7,7′-tetrahydro2H,2′H-8,8′-spirobi[indeno[5,6-e][1,3]oxazin]-3,3′(4H,4′H)-diyl)bis(4,1-phenylene))diethanol (BPSPI-EthOH-Bz). A mixture of BPSPI−

OH (5 g, 16 mmol), 2-(4-amino phenyl)ethanol (4.45 g, 32 mmol), paraformaldehyde (1.94g, 64 mmol), and toluene (50 mL) was refluxed. After 30 h, a white precipitate started to form, and the reaction mixture was further refluxed for 10 h for the complete precipitation of the compound. Then, it was filtered by suction, washed with ethanol, dried (60 °C for 12 h), and recrystallized from tetrahydrofuran. Further purification was done by recrystallization using THF:hexane (70:30) mixture, followed by washing with hot methanol to obtain compound as white powder. (yield: 3.2 g, 5.07 mmol, 31%): mp 229−250 °C. 1H NMR (400 MHz, DMSO-d6), δ: 7.03 (d, J = 8.8 Hz, 4H), 6.97 (d, J = 8.8 Hz, 4H), 6.88 (s, 2H), 5.95 (s, 2H), 5.30 (d, J = 10.4 Hz, 2H, O−CH2−N), 5.24 (d, J = 10.4 Hz, 2H, O−CH2−N), 4.58 (s, 4H, Ar−CH2−N), 4.53 (t, J = 5.2 Hz, 2H, OH, disappeared after D2O exchange), 3.46−3.51 (m, 4H, CH2), 2.57 (t, J = 7 Hz, 4H, CH2), 2.15 (d, J = 13.2 Hz, 2H, CH2), 2.01 (d, J = 12.8 Hz, 2H, CH2), 1.25 (s, 6H, CH3), 1.19 (s, 6H, CH3). 13C NMR (100 MHz, DMSO-d6), δ: 153.8 (s), 150.1 (s), 146.6 (s), 144.4 (s), 132.0 (s), 130.0 (s), 120.69 and 120.65 (d), 117.8 (s), 111.4 (s), 78.92 (s, O−CH2−N), 62.89 (s, CH2), 59.50 (s, CH2), 57.3 (s), 50.23 (s, Ar−CH2−N), 43.0, 30.8, and 32.0 (s, C(CH3)2), 38.7 (s, CH2). ATRFTIR (υ, cm−1): 1211 and 1068 (C−O−C), 1190 (C−N−C), 949 (out of plane C−H stretch of benzene attached to oxazine), 2954, 2929, and 2854 (aliphatic C−H), 3558 (sharp, OH) and 3468 (broad, OH). Anal. Calcd for C41H46N2O4: C, 78.06; H, 7.35; N, 4.44; Found: C, 77.75; H, 7.38; N, 4.39. Synthesis of Spirobichroman Benzoxazine Monomers. Synthesis of 6,6,6′,6′-Tetramethyl-3,3′-diphenyl-3,3′,4,4′,6,6′,7,7′octahydro-2H,2′H-8,8′-spirobi[chromeno[6,7-e][1,3]oxazine] (SPBC-Ph-Bz). Into a 100 mL one neck round bottomed flask were added SPBC−OH (2 g, 5.87 mmol), aniline (1.09 g, 11.75 mmol), paraformaldehyde (0.70 g, 23.5 mmol), and p-xylene (30 mL). The reaction mixture was heated at 130 °C for 15 h. After removal of solvent under reduced pressure, the reddish yellow crude product was purified by column chromatography using dichloromethane:hexane (70:30) as eluent. However, some yellow impurity remains with crystals. In order to further purify SPBC-Ph-Bz, it was again passed through silica column eluting with a chloroform:hexane (70:30) mixture and then recrystallized from hexane to yield white crystals (yield: 1.26 g, 2.19 mmol, 37%): mp 185−192.4 °C. 1H NMR (400 MHz, CDCl3), δ: 7.07−7.12 (m, 6H), 6.77 (t, J = 7.4 Hz, 2H), 6.65 (d, J = 8.4 Hz, 4H), 6.50 (d, J = 8.4 Hz, 2H), 5.05 (d, J = 10 Hz, 2H, O− CH2−N), 4.96 (d, J = 10.8 Hz, 2H, O−CH2−N), 4.22 (d, J = 17.2 Hz, 2H, Ar−CH2−N), 3.77 (d, J = 16.8 Hz, 2H, Ar−CH2−N), 2.14 (d, J = 14 Hz, 2H, CH2), 2.05 (d, J = 14 Hz, 2H, CH2), 1.33 (s, 6H, CH3), 1.59 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3), δ: 153.1 (s), 148.0 (s), 146.8 (s), 129.2 (s), 125.0 (s), 124.0 (s), 120.7 (s), 117.4 (s), 110.8 (s), 110.03 (s), 98.36 (s, O−C−O), 78.67 (s, O−CH2−N), 46.72 (s, CH2), 46.25 (s, Ar−CH2−N), 32.39, 33.52 and 30.50(s, C(CH3)2). ATR-FTIR (υ, cm−1): 1252 and 1037 (C−O−C), 1165 (C−N−C), 915, 943, and 968 (out of plane C−H stretch of benzene attached to oxazine), 2954, 2928, and 2859 (aliphatic C−H). Anal. E

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Macromolecules

a rate of 10 °C min−1. Thermo gravimetric analysis (TGA) was performed on a Seiko Instrument TG-DTA 6200 using an aluminum pan under nitrogen atmosphere (flow rate 200 mL min−1) at a heating rate of 10 °C min−1. Ring Opening Polymerization. The curing of spiro benzoxazines were performed at different temperature cycle under nitrogen atmosphere. The complete disappearance of exotherm was observed in DSC with the following curing cycle: BPSPI-Ph-Bz, 260 °C, 1 h 35 min; BPSPI-Me-Bz, 260 °C, 1 h 15 min; BPSPI-EthOH-Bz, 265 °C, 1 h; SPBC-Ph-Bz/SPBC-Me-Bz, 270 °C, 1 h; SPBC-EthOH-Bz, 250 °C, 1 h. B-a benzoxazine was cured at 180 and 200 °C each for 2 h.6,37

Calcd for C37H38N2O4: C, 77.33; H, 6.66; N, 4.87. Found: C, 77.35; H, 6.59; N, 4.88. Synthesis of 6,6,6′,6′-Tetramethyl-3,3′-di-p-tolyl3,3′,4,4′,6,6′,7,7′-octahydro-2H,2′H-8,8′-spirobi[chromeno[6,7-e][1,3]oxazine] (SPBC-Me-Bz). A mixture of SPBC−OH (2 g, 5.87 mmol), p-toluidine (1.26 g, 11.75 mmol), paraformaldehyde (0.70 g, 23.5 mmol), and p-xylene (30 mL) was heated at 130 °C for 15 h. The solvent was evaporated off and the crude product was purified by column chromatography twice (first using dichloromethane: hexane (40:60) as eluent and then from chloroform: hexane (70:30) mixture) and recrystallized it from hexane to yield white crystals (yield: 1.46 g, 2.42 mmol, 41%): mp 167.6−188.6 °C. 1H NMR (400 MHz, CDCl3), δ: 7.10 (d, J = 8.8 Hz, 2H), 6.89 (d, J = 8 Hz, 4H), 6.58 (d, J = 8.4 Hz, 4H), 6.50 (d, J = 8.4 Hz, 2H), 5.04 (d, J = 10.4 Hz, 2H, O−CH2−N), 4.93 (d, J = 10 Hz, 2H, O−CH2−N), 4.19 (d, J = 17.2 Hz, 2H, Ar− CH2−N), 3.75 (d, J = 17.2 Hz, 2H, Ar−CH2−N), 2.18−2.02 (m, 10H, CH3 overlapped with protons of CH2), 1.59 (s, 6H, CH3), 1.33 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3), δ: 153.1 (s), 146.8 (s), 145.7 (s), 130.1 (s), 129.8 (s), 124.9 (s), 123.8 (s), 117.6 (s), 110.7 (s), 109.9 (s), 98.27 (s, O−C−O), 78.93 (s, O−CH2−N), 46.72 (s, CH2), 46.53 (s, Ar−CH2−N), 32.38, 33.51, and 30.45 (s, C(CH3)2), 20.54 (s, CH3). ATR-FTIR (υ, cm−1): 1241 and 1035 (C−O−C), 1165 (C− N−C), 912, 923, and 934 (out of plane C−H stretch of benzene attached to oxazine), 2955, 2921, 2891, and 2867 (aliphatic C−H). Anal. Calcd for C39H42N2O4: C, 77.71; H, 7.02; N, 4.65. Found: C, 77.91; H, 7.14; N, 4.68. Synthesis of 2,2′-((6,6,6′,6′-Tetramethyl-6,6′,7,7′-tetrahydro2H,2′H-8,8′-spirobi[chromeno[6,7-e][1,3]oxazin]-3,3′(4H,4′H)-diyl)bis(4,1-phenylene))diethanol (SPBC-EthOH-Bz). To a 100 mL one necked round-bottom flask were added SPBC−OH (2 g, 5.87 mmol), 2-(4-amino phenyl)ethanol (1.61 g, 11.75 mmol), paraformaldehyde (0.70 g, 23.5 mmol), 4-DMAP (0.179 g, 1.46 mmol), and 1,4-dioxane (20 mL), the mixture was heated at 90 °C for 8 h and cooled, and the solvent was evaporated off. The crude product was purified by column chromatography using ethyl acetate: hexane (70:30) as eluent and recrystallized from methanol to yield white crystals (yield: 1.28 g, 1.93 mmol, 32%): mp 201−205.6 °C. 1H NMR (400 MHz, CDCl3), δ: 7.12 (d, J = 8.8 Hz, 2H), 6.84 (d, J = 8.4 Hz, 4H), 6.51 (d, J = 8 Hz, 2H), 6.37 (d, J = 8.4 Hz, 4H), 5.17 (d, J = 12.4 Hz, 2H, O−CH2−N), 4.97 (d, J = 10.4 Hz, 2H, O−CH2−N), 4.22 (d, J = 17.2 Hz, 2H, Ar−CH2− N), 3.79 (d, J = 17.2 Hz, 2H, Ar−CH2−N), 3.66 (t, J = 5.4 Hz, 4H, CH2), 2.57−2.68 (m, 4H, CH2), 2.16 (d, J = 14 Hz, 2H, CH2), 2.07 (d, J = 14 Hz, 2H, CH2), 1.70 (s, 2H, broad OH, disappeared after D2O exchange), 1.61 (s, 6H, CH3), 1.33 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3), δ: 153.2 (s), 146.8 (s), 146.2 (s), 130.6 (s), 129.9 (s), 125.0 (s), 124.3 (s), 117.7 (s), 111.0 (s), 110.2 (s), 98.40 (s, O−C− O), 79.23 (s, O−CH2−N), 63.47 (s, CH2), 46.56 (s, CH2), 45.54 (s, Ar−CH2−N), 38.24 (s, CH2), 32.35, 33.90, and 30.61 (s, C(CH3)2). ATR-FTIR (υ, cm−1): 1248 and 1032 (C−O−C), 1166 (C−N−C), 912, 924, and 944 (out of plane C−H stretch of benzene attached to oxazine), 2989, 2964, 2935, and 2864 (aliphatic C−H), 3562 (OH, sharp). Anal. Calcd for C41H46N2O6: C, 74.30; H, 7.00; N, 4.23. Found: C, 74.14; H, 7.01; N, 4.22. Measurements. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on JEOL JME-ECS 400 NMR spectrometer, in CDCl3/ DMSO-d6 using tetramethyl silane as internal standard. 1H−1H correlated spectroscopy (COSY) and 1H−13C heteronuclear multiple quantum coherence (HMQC) were also examined. ATR-FTIR spectra was recorded by using a Thermo Scientific Nicolet iS10 spectrometer equipped with a Smart iTR diamond ATR sampling accessory. Elemental analysis was done by using Yanaco CHN Corder MT-5. Melting point of benzoxazine monomers were determined by using a Yanaco micro melting point apparatus, MP-500. Differential scanning calorimetry (DSC) analysis was done to determine curing behavior of benzoxazine monomers using Seiko Instrument DSC-6200R in an aluminum pan at a heating rate of 10 °C min−1 and under a nitrogen flow of 50 mL min−1. Glass transition temperature (Tg) was recorded by heating benzoxazine monomer in a DSC pan from 30 °C to curing temperature at a heating rate of 20 °C min−1, held at that temperature for a specified time, then cooled and again heated from 30 to 400 °C at

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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.5b01970. Experimental details for spirobiindane and spirobischroman bisphenols, 1H, 13C, COSY, and HMQC NMR spectra of spiro-centered benzoxazines, and a DSC scan of cured bisphenol-A benzoxazine (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 The authors thank Masaki Minami, JX Nippon Oil & Energy Corporation, for financial support. REFERENCES

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