Photochemical Polymerization of - ACS Publications - American

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Photochemical Polymerization of N‑Phenyl Mono-1,3-benzoxazines in Aqueous Media Jordi Salabert, Rosa María Sebastián,* and Jordi Marquet Department of Chemistry, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, 08193 Barcelona, Spain S Supporting Information *

ABSTRACT: Some N-phenyl monosubstituted mono-1,3benzoxazines can be polymerized under appropriate UV irradiation in aqueous solutions at room temperature. Photoinduced intramolecular electron transfer from the amine to the phenyl ring in aqueous media produces the mesolitic O−alkyl bond cleavage governed by “topologically controlled Coulombic interactions”. Irradiation of EWG-substituted benzoxazines produces opening of the oxazine ring as expected, but no propagation of the polymerization is observed probably due to the low nucleophilicity of the intermediate phenolic species. However, with more nucleophilic electron-donor substituted benzoxazines the photopolymerization proceeds smoothly, with excellent conversions and modest degrees of polymerization. The produced materials show a similar degree of polymerization to the observed in the non-cross-linked fraction when monobenzoxazines are polymerized under thermal conditions (>150 °C) but having a lower dispersity.

1. INTRODUCTION

Some years ago, some of us described a photochemical approach to the reductive cleavage of aminoalkyl nitrophenyl ethers.27,28 A significant achievement in this work was the cleavage of the O−alkyl bond instead of the alternative O−aryl bond cleavage or NO2 group reduction, a far easier processes with alkali metals in the ground state. Photoinduced intramolecular electron transfer from the amine to the phenyl ring produced predominant mesolytic O−alkyl bond cleavage. The limited motion freedom of the aminium radical cation with respect to the phenyl ring radical anion alters the normal electronic distribution of the radical anion, thus justifying the anomalous reactivity (“topologically controlled Coulombic interactions, TCCI”).29,30 These aminoalkyl phenyl ethers are structurally quite similar to benzoxazines, and the idea for this work was to test if the direct ring-opening polymerization of benzoxazines could be elicited photochemically at room temperature, taking advantage of the TCCI interactions that would be produced in the benzoxazine structure upon UV irradiation (Scheme 1).

1

Polybenzoxazines are a special kind of phenolic resins which monomers, 1,3-benzoxazines, are relatively easy and cheap to prepare.2 These materials have interesting properties such as near-zero volume changes during polymerization,3−7 low water absorption, 8 lower surface free energy than poly(tetrafluoroethylene),9 high char yield,10 very high chemical resistance,11 and adjustable glass transition temperature.12 Besides all these advantages, polymerization takes place at rather high temperatures (typically >180 °C) becoming one of their main shortcomings. Monomeric 1,3-benzoxazines undergo decomposition13 at high temperature and hydrolysis.14 Production of volatile compounds during polymerization can induce foaming and voids in the materials, together with contamination of the working place by toxic volatile organic compounds.13 Polymerization temperature can be modestly lowered by the addition of catalysts. Thus, catalysts have been employed to promote the ring-opening polymerization in different examples;15−24 however, as far as we know, no room temperature synthesis of polybenzoxazines has ever been described starting from monofunctional benzoxazines without the help of any catalyst or initiator. Moreover, and also to the best of our knowledge, no synthesis of polybenzoxazines in water has ever been attempted. In the closer cases reported in the literature, Ishida has described the room temperature polymerization of bifunctional benzoxazines with certain initiators (PCl5, PCl3, POCl3, TiCl4, AlCl3, and MeOTf) in chloroform by a complex mechanism,25 and Yagci has reported a photoinitiated cationic polymerization of a monofunctional benzoxazine by onium salts.26 © XXXX American Chemical Society

2. RESULTS AND DISCUSSION In the present work, seven benzoxazines have been irradiated in order to study the viability of the photochemical polymerization proposed. Synthesis of Benzoxazines 1−7. Benzoxazines 1−7 were prepared through Mannich reaction type, using solvent-free method A or solvent based method B (Scheme 2). Both Received: January 24, 2018 Revised: April 9, 2018

A

DOI: 10.1021/acs.macromol.8b00171 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Reductive Photocleavage of Aminoalkyl Nitrophenyl Ether (a) and Proposed Photochemical Ring-Opening of Benzoxazines To Proceed the Polymerization (b)

Scheme 2. Preparation of Benzoxazines 1−7 Using Method A or B

Figure 1. UV−vis spectra collection for benzoxazines 1−7 in CHCl3 at ≈2 × 10−5 M. Colored lines (compound): dark red (1), black (2), green (3), orange (4), yellow (5), blue (6), and clear blue (7).

methodologies were based on Ronda et al.’s previous work.31 Benzoxazines 1, 3, 5, 6, and 7 were isolated in slightly higher reported yields.31,32 However, when using triazine 833 for the preparation of 2, instead of reported aniline34 lower yields were obtained. Benzoxazine 4 is a new compound and has been completely characterized (see Supporting Information). Only benzoxazine 1, containing a NO2 group in the phenolic ring, was prepared using 1,4-dioxane as solvent; under bulk conditions only polymer was isolated. In general, the solventfree method is better for low melting point phenols (below 110−120 °C), affording higher yields at shorter reaction times in more sustainable conditions; however, depending on the final product, polymerization could occur. We were interested in promoting a ring-opening of the heterocyclic moiety of the benzoxazines by irradiation. First of all, UV−vis absorption spectra of monomers were recorded in CHCl3 at similar concentrations (≈2 × 10−5 M) showing

parallel trends (Figure 1, see also Supporting Information). In all cases two strong absorption bands in the ranges 238−248 and 277−310 nm were observed. p-NO2 benzoxazine (red line in Figure 1) showed stronger absorption than the others in the range 280−350 nm; for that reason benzoxazine 1 was the first monomer studied. Moreover, a third band appeared centered at 423 nm that it may be due to the extended conjugation of the phenolic ring. Taking into account this information, for the present work, a Hg medium pressure lamp was selected, which has a broad emission spectra in the interval from 250 up to 450 nm. Combined with a Pyrex filter, the real irradiation range was between 280 and 450 nm, where benzoxazines absorb. Attempts To Photoinitiate the Polymerization of Electron-Withdrawing Substituted Benzoxazines, pNO2, 1, and p-CN, 2. Marquet et al.27−30 set H2O/CH3CN 7:3 at pH 10 as the best conditions to carry out the photoreductive cleavage of the O−alkyl bond in 4-nitrophenyl B


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described.35 Compound 10 was not described in the literature and therefore was characterized. EWGs help the ring aperture making weaker the O−C bond but also reduce the nucleophilicity of the aromatic ring. Benzoxazines with p-NO2 and p-CN suffered a rapid ringopening but were not able to propagate a polymerization in competition with the water trapping of the iminium intermediate. The first attempt was done irradiating with a 400 W lamp; however, in order to slow down the initiation step a less powerful lamp (125 W) was used, but no significant changes were observed (Table 1, entries I and II). The rest of experiments in this work were performed with the less powerful lamp. In all cases the evolution of the reaction was followed by the disappearance of benzoxazine signals around 4.5 and 5.5 in 1 H NMR spectra. When the amount of water in the reaction medium was reduced (H2O/CH3CN 3:7), conversion of 1 decreased at longer reaction times (Table 1, entry III) being amine 9 still observed as the main product. Nonetheless, we decided to work in pure organic solvent, using anhydrous acetonitrile with benzoxazine 1. In this experiment not only the corresponding imine 1135 was isolated (32% yield) but also aldehyde 1236 was obtained (12% yield) after column chromatography (Table 1, entry IV) (Scheme 4); their 1H NMR spectra (see Supporting

ethers (Scheme 1) Polar solvents are usually required to facilitate the intramolecular electron transfer. For this reason, in this work the same solvent mixture was the first choice to start with (Table 1). Even though it has been reported that 1,3Table 1. Attempts To Photopolymerize Benzoxazines 1 and 2a entry Id II III IV V

e

VI VII VIII IX

f

Xg XI XII

−R

solvent

t (h)

convb (%)

NO2 (1) NO2 (1) NO2 (1) NO2 (1) NO2 (1) NO2 (1) NO2 (1) NO2 (1) NO2 (1) NO2 (1) CN (2) CN (2)

H2O/CH3CN 7:3

1.5

91

9 (38)

H2O/CH3CN 7:3

2

84

9 (40)

H2O/CH3CN 3:7

7

18

9

CH3CN (anh)

6

70

CH3CN (anh)

6

90

11 (32) and 12 (12) 12 (70)

H2Oh

4

0

MeOH (anh)

6

0

bulk

3

0

H2O/CH3CN 7:3

2

20

9

H2O/CH3CN 7:3

2

85

9

H2O/CH3CN 7:3 H2Oh

1 4

80 0

10 (55)

prodc (%)

Scheme 4. Photoinitiated Oxazine Ring-Opening of 1 Led to Formation of Imine 11 and Aldehyde 12 in Anhydrous CH3CN

a

Reaction conditions: 20−30 mg of benzoxazine in 50 mL of solvent irradiated under 125 W Hg medium pressure lamp at rt. bDetermined by 1H NMR (see Supporting Information for further information). c Isolated yield by column chromatography. Only major products were isolated. d400 W Hg medium pressure lamp. eAir flushed. fpH 4 (acetic/acetate buffer). gpH 10 (bicarbonate/carbonate buffer). hSDS addition.

Information) were compared with the ones previously described.35,36 When air was bubbled into the solution, aldehyde 12 was the main product with 70% of yield (Table 1, entry V). Nitroaromatics are good photooxidating agents, and this could justify the obtaining of the imine 11 and the aldehyde 12 when performing the photoreaction in the absence of water.37 Attemps to use pure water (Table 1, entries VI and XII) or methanol (anh) (Table 1, entry VII) as solvent or to perform the photoreaction in bulk (Table 1, entry VIII) were unsuccessful, the remaining corresponding benzoxazine unaltered. The low solubility of monomers in the polar solvents could be the reason for no reaction. The effect of pH was also evaluated carrying out the photoreaction replacing water by aqueous buffer solutions using 7:3 proportion. Acid medium (pH 4) decreased conversion from 84 to 20% (Table 1, entry IX). This result suggests that the amine protonation precludes the photoinduced ringopening by probably preventing the photoinduced intramolecular electron transfer. It is an important fact that supports our proposed mechanism. The best results obtained in our previously reported work concerning the cleavage of C−O bond of aminoalkyl phenyl ethers were performed at pH 10.27,28 We tried also our photochemical reaction at these basic pH’s; however, no significant effect over conversion was observed compared to the neutral conditions (Table 1, entry X). Further experiments were performed at neutral pHthe

benzoxazines undergo hydrolysis in aqueous solutions at room temperature, this is a slow process.13 In our hands, in the conditions reported in Table 1, and using the indicated reaction times, our starting 1,3-benzoxazines remained unaltered in the absence of irradiation (blank experiments). When irradiated, benzoxazines with strong electron-withdrawing groups (EWG) such as NO2 and CN underwent a photoinduced reaction, but they did not polymerize (Table 1, entries I, II, and XI). Instead of polymer, the corresponding phenol amines 935 and 10 were obtained very probably due to photochemical promoted oxazine ring-opening, trapping by water of the resulting iminium salt, and formaldehyde loss from the resulting aminal (Scheme 3). These small molecules were isolated by flash column chromatography. 1H NMR of compound 9 was compared with the one previously Scheme 3. Photoinduced Ring-Opening of Benzoxazines 1 and 2

C


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was also tested for benzoxazines 1 and 7 (Table 1, entry VIII, and Table 2 entry VIII). After long irradiation times (4−8 h) no polymer formation was observed in any case, recovering the benzoxazine unaltered. An explanation for this behavior should be the same as the one given for attempted photoreactions in highly concentrated solutions, confirming the deleterious effect of bimolecular processes from the benzoxazine excited state. In any case, results described in Table 2 demonstrate that the initial hypothesis holds true and that the O−alkyl photocleavage originally described in aminoalkyl nitrophenyl ethers27−30 can be an interesting synthetic tool in ring-opening polymerization of benzoxazines. In Scheme 5 a summary of proposed mechanisms for different experimental conditions and monomers is shown. In polar solvents, where benzoxazines must be soluble, a photoinduced intramolecular electron transfer from the amine to the alkoxyphenyl ring should occur producing the O−alkyl mesolytic bond cleavage. If the alkoxyphenyl ring of another benzoxazine is nucleophilic enough to attack the iminium intermediate, the polymerization proceeds (Scheme 5, path A, R = H, Cl, F, CH3, and OMe). However, when electronwithdrawing substituents are present (Scheme 5, path B, R = NO2 and CN), the corresponding benzoxazines are not nucleophilic enough, and a retro-Mannich-type reaction is observed in water media, obtaining the corresponding benzylamine and not the polymer. When the amine of the monomer is protonated, working a pH 4, the electron transfer is not possible, and the ring-opening process does not occur. Emulsion Photopolymerization of Nonsubstituted or Resonance Electron-Donor-Substituted Benzoxazines in Water. In order to achieve more sustainable conditions, the next photopolymerizations were attempted in pure water, in the absence of organic solvent. Because of benzoxazines are not soluble in water, sodium dodecyl sulfate (SDS) as emulsifier agent was used. As described in Table 3, benzoxazines 3, 4, and 7 were successfully photopolymerized, affording short oligomers (n = 3−5) (Table 3, entries I, II, and V). Benzoxazines 5 and 6 were not soluble enough and remained unreacted (Table 3, entries III and IV). Especially remarkable is experiment V since relatively high molecular weight oligomers are obtained. Reaction of 7 at higher temperatures was tested (Table 3, entry VI), resulting in a decrease in conversion from 100 to 77% being the dispersion increased. Experiments irradiating benzoxazine 7 at different times (Table 3, entries V−VIII) are a hint that perhaps we are at least partially in the presence of step-growth polymerization.38 Monomer disappears very fast, but the final growing of the polymer (oligomer) is slower. So we could also conclude that the light initiating steps are quite fast. In light of GPC data for these water only experiments, oligomer sizes were similar to those obtained in CH3CN:H2O mixtures. Comparison of Average Molecular Weights for the Thermal and the Photochemical Polymerization of Benzoxazines 3−7. In order to compare photochemically prepared polybenzoxazines with thermically prepared ones, bulk 1,3-benzoxazines 3−7 were heated at 200 °C for 4 h at open air, and the resulting polymers were analyzed by GPC (Table 4). When we prepared the samples to be analyzed by GPC, the first important observed difference was that the polymers of photochemical origin were totally soluble in THF, whereas in

one obtained directly by dissolving reactants in the desired solvents. The results reported in Table 1 demonstrate the photochemical ring-opening of benzoxazines. The reaction seems to correspond to the previously reported “topologically controlled Coulombic interactions in mesolitic bond cleavages”.27−30 However, opened products, but not polymerization, were observed. The resulting phenols containing EWGs, after opening of the oxazine ring, most probably were not nucleophilic enough to compete with water in the trapping of the resulting iminium ions to sustain the propagation. Photopolymerization of Nonsubstituted or ElectronDonor-Substituted Benzoxazines in Water/ACN or THF. Following the hypothesis that electron-poor benzoxazines 1 and 2 were not nucleophilic enough to sustain propagation, more nucleophilic nonsubstituted and electron-donor-substituted benzoxazines 3−7 were tested under the best conditions, in H2O/CH3CN 7:3 as solvent mixture (Table 2). Table 2. Photopolymerization of Benzoxazines 3−7a entry

-R

solvent

time (h)

convb (%)

I II III IV V VI VIId VIII

Cl (3) F (4) H (5) Me (6) MeO (7) MeO (7) MeO (7) MeO (7)

H2O/CH3CN 7:3 H2O/CH3CN 7:3 H2O/CH3CN 7:3 H2O/CH3CN 7:3 H2O/CH3CN 7:3 THF:H2O 9:1 THF:H2O 9:1 bulk

2.5 4 4 2.5 2 4 4 8

94 78 93 78 87 76 0 0

Mnc (D) 711 583 855 831 617 736

(1.07) (1.14) (1.22) (1.26) (1.14) (1.17)

a

Reaction conditions: 20−30 mg of benzoxazine in 50 mL of solvent (∼2.5 × 10−3 M) irradiated under 125 W Hg medium pressure lamp at rt. bDetermined by 1H NMR (see Supporting Information for further information). cNumber-average molecular weight and polydispersity, determined by GPC. dBenzoxazine concentration was increased up to 0.2 M.

Nonsubstituted benzoxazine 5 and electron-donor-substituted benzoxazines 3, 4, 6, and 7 were successfully polymerized, affording oligomers (conversions: 78−94%; quantitative yields of polymers were obtained with respect to consumed benzoxazines) by irradiation from 2 to 4 h (Table 2, entries I−V). In these cases the nucleophilicity of the electron-rich phenol rings is enough to propagate the polymerization at room temperature. Products were analyzed by gel permeation chromatography (GPC) (Table 2, entries I−V), resulting in short oligomers with n = 3, 4, or 5. In an attempt to increase oligomer size, a higher benzoxazine concentration in the photoreaction was tested. To overcome the poor solubility of benzoxazines in the standard 7:3 H2O/ CH3CN solvent mixture, THF/H2O (9:1) was used. In this mixture of solvents photopolymerization of 7 was carried out, first at the standard concentration (∼2.5 × 10−3 M) (Table 2, entry VI) affording similar results than when using H2O/ CH3CN (7:3). When concentration was increased up to 0.2 M, curiously enough, conversion was near 0 (Table 2, entry VII), recovering the starting benzoxazine unaltered. This behavior suggests that this higher concentration results probably in predominant unproductive bimolecular energy transfer and/or electron transfer processes that successfully compete with the intramolecular electron transfer and mesolytic cleavage. In order to remove solvent dependence, bulk photopolymerization D


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Scheme 5. Proposed Mechanism for Photochemical Ring-Opening Polymerization of Benzoxazines Containing Different Substituents

the thermal ones only a fraction was soluble. The obvious interpretation for this behavior is that the thermal polymers are partially cross-linked while the photochemical ones are not, being the chemical structure of the last ones better defined. In Table 4, sizes (Mn) and dispersities of the photochemical polymers and the THF soluble fraction of the thermal ones are compared. Sizes are not significantly different (small oligomers in all the cases). It should be mentioned that it has been previously reported that monofunctional 1,3-benzoxazines generate mainly short oligomers when polymerized.26 Dispersities are significantly smaller in the photochemically produced oligomers, where the polymerization reaction is more controlled at room temperature. This fact is particularly evident in entries I and III. In the thermal conditions, at 200 °C, polybenzoxazines can also be degraded, being this effect a probable cause of the highest polydispersity. However, the values shown in Table 4 should be taken with care and as approximate ones, particularly the “dispersity” values of the photochemical experiments, since the low molecular weights of the polymers and the presence of residual monomer in the sample in these cases preclude a very accurate integration of the traces (see the Supporting Information where the range for integration in each case is indicated and the GPC traces of the monomeric benzoxazines are also described).

Table 3. Photopolymerization of Benzoxazines 3−7 in Watera entry

-R

time

convb (%)

Mn (D)c

I II III IV V VId VII VIII IX

Cl (3) F (4) H (5) Me (6) MeO (7) MeO (7) MeO (7) MeO (7) MeO (7)

4h 2h 2.5 h 4h 4h 4h 15 min 30 min 2h

55 86 0 0 100 77 80 87 88

635 (1.09) 616 (1.11)

840 530 472 449 511

(1.05) (1.18) (1.06) (1.10) (1.15)

a

Reaction conditions: 20−30 mg of benzoxazine, 350 mg of sodium dodecyl sulfate in 50 mL of solvent (∼2.5 × 10−3 M) irradiated under 125 W Hg medium pressure lamp at rt. bDetermined by 1H NMR (see Supporting Information for further information). cNumber-average molecular weight and polydispersity, determined by GPC. dReaction performed at 55 °C.

Table 4. Comparison of Molecular Weights and Dispersities between Polybenzoxazines Obtained Photochemicallya and Thermicallyb from 3−7 entry

benzoxazine

I II III IV V

3 4 5 6 7

Mn (D) photoc 711 583 855 831 617

(1.07) (1.14) (1.22) (1.26) (1.14)

Mn (D) thermalc,d 1261 1508 605 421 952

(4.00) (1.75) (2.48) (1.68) (1.41)

3. CONCLUSIONS The aim of this study was to demonstrate that by taking advantage of the topology of the structure of N-phenyl mono1,3-benzoxazines (probable operation of the TCCI effect),27,28 and the fact that they absorb in the UV range, it was possible to photochemically induce the O−alkyl bond cleavage and thus the ring-opening through an intramolecular electron transfer process. Deeming that the ring-opening step has been postulated as the key step in the benzoxazines polymerization,1

Irradiated at 125 W lamp in H2O/CH3CN 7:3 (Table 2). b200 °C, 4 h, THF soluble fraction for GPC. cNumber-average molecular weight and polydispersity, determined by GPC. dOnly THF soluble fraction was analyzed by GPC. a

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were performed in a Bruker BIFLEX with Reflectus Modus. ESI MS were obtained from a Bruker MicroTOF-Q using a direct inlet system. Gel permeation chromatography analysis were performed using Agilent Technologies 1260 Infinity hardware, using THF as solvent at 1 mL/min. GPC hardware was equipped with three gel columns: PLgel 5 μm Guard/50 × 7.5 mm), PLgel 5 μm 10000 Å MW 4K− 400K, and PL Mixed gel C 5 μm MW 200−3M. Synthesis of benzoxazines 1−7 using standard previously reported methods and their 1H NMR spectra are gathered in the Supporting Information.31−33 Triazine 8 was also previously described.33 General methods are indicated below: Method A. 1 equiv of the corresponding phenol, 1 equiv of paraformaldehyde, and 1 equiv of triazine 8 were poured into a roundbottom flask. Reactants were heated at 100 °C for different times (see Supporting Information); the corresponding product was purified from the reaction crude. Method B. 1 equiv of corresponding phenol, 1 equiv of paraformaldehyde, and 1 equiv of triazine 8 were poured into a round-bottom flask and dissolved in 1,4-dioxane. The mixture was heated at reflux. After the time indicated in the Supporting Information the corresponding product was purified from reaction crude. Benzoxazine 4 and compound 10 were new, and the characterization is indicated below: Benzoxazine 4. See complete synthesis and purification in the Supporting Information. 1H NMR (CDCl3, 400 MHz) δ (ppm): 4.61 (s, 2H), 5.34 (s, 2H), 6.75 (m, 2H), 6.83 (m, 1H), 6.96 (t, 1H, J = 7.3 Hz), 7.11 (d, 2H, J = 7.9 Hz), 7.28 (m, 2H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 50.4 (d, J = 1.5 Hz), 79.5, 112.7 (d, J = 23.0 Hz), 114.6 (d, J = 23.1 Hz), 117.8 (d, J = 7.9 Hz), 118.3, 121.6, 121.7 (d, J = 6.6 Hz), 129.3, 148.2, 150.2 (d, J = 2.1 Hz), 156.9 (d, J = 240 Hz). HR ESI-MS (m/z): [calcd]: [M + H]+, 230.0976, [expt]: [M + H]+, 230.0971. Compound 10. 1H NMR (CDCl3, 250 MHz) δ (ppm): 4.49 (s, 2H), 6.87 (m, 2H), 6.95 (d, J = 8.4 Hz, 1H), 7.00 (tt, J = 7.4 Hz, J = 1.1 Hz, 1H), 7.30 (m, 2H), 7.50 (d, J = 1.8 Hz, 1H), 7.53 (dd, J = 8.4 Hz, J = 1.8 Hz, 1H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 48.6, 103.0, 116.3, 117.7, 119.3, 121.7, 124.0, 129.5, 132.5, 133.5, 146.3, 161.2. HR ESI-MS (m/z): [calcd]: [M + Na]+, 247.0842, [expt]: [M + Na]+, 247.0830 Photopolymerization standard procedures: In solution: In a 50 mL Pyrex immersion reactor, benzoxazine (20−30 mg) was dissolved in 50 mL of solvent or solvent mixtures (H2O:CH3CN 7:3, THF:H2O 9:1, CH3CN, MeOH, or H2O). In the case of pure water, 350 mg of SDS was added. An immersion reactor was provided with magnetic stirring and was closed. A water-cooled 125 W Hg medium pressure lamp was used to irradiate reaction mixture. The product was extracted from water with 100 mL of CH2Cl2 (if SDS was added, 2 mL of saturated CaCl2 solution was added). Solvent was dried using Na2SO4 (anh) and removed under vacuum; product was analyzed by NMR, GPC, and IR and MS when required. In bulk: 50 mg of corresponding benzoxazine was dissolved in 1 mL of acetone. Solution was poured into a Petri dish, and solvent was allowed to evaporate; then the sample was irradiated. Product was analyzed by NMR. Thermal polymerization standard procedure: 50 mg of corresponding benzoxazine was added to vial and heated at 200 °C for 4 h. The product was analyzed by NMR and GPC.

photochemical activation of the O−alkyl mesolitic cleavage could lead to the polymerization of benzoxazines at room temperature and in aqueous solutions. We have found that nitro- and cyano-substituted monobenzoxazines (the more similar structures to the ones where the TCCI effect had been reported) undergo ring-opening when irradiated with UV light in aqueous media. However, no polymerization was observed being the amine, the imine, and the aldehyde, the observed products, all coming from the retroManich reaction on the iminium cation intermediate. The absence of propagation has been attributed to the low nucleoplicity of the nitro- or cyano-substituted aromatic ring. When more electron-donor substituents (Cl, F, and OMe) were studied, ring-opening and polymerization were observed. The photoreaction could even be carried out in pure water, only with the addition of an emulsifier such as SDS. As far as we know, those are the first examples of room temperature polymerization of benzoxazines (temperatures over 150 °C are normally needed) with the additional advantage of the polymerization reaction being successful in an environmental friendly solvent such as water. Comparison of the properties of the materials obtained by thermal and photochemical polymerization offers some interesting observations. Thus, thermal polymerization of substituted benzoxazines 3−7 produces materials that are only partially soluble in THF whereas photochemical polymerization produces materials totally soluble in THF. This indicates a higher content of cross-linked material in the thermal polymer. GPC studies of the THF-soluble fractions indicate similar average number molecular weights. However, a second and significant difference becomes apparent; the photochemical polymers show a much lower dispersity that their thermal counterparts, being their final structures are better defined. The final conclusion would be that model substituted monobenzoxazines can be photochemically polymerized in aqueous solutions at room temperature. This can open a new research avenue in the efforts to reduce the polymerization temperature and the associated energy waste in the industrial production of commercial polybenzoxazines from bisbenzoxazines. Further studies will be carried out in that direction.

4. EXPERIMENTAL SECTION Materials and Methods. All commercially acquired reagents were used as received. When required, solvents were dried used standard procedures. Reactions requiring an inert atmosphere were conducted under nitrogen or argon using standard Schlenk techniques. All other reactions were performed employing standard organic synthesis protocols. Thin-layer chromatography (TLC) was performed using Merck aluminum backed plates of TLC silica gel 60 F254; the plates were revealed using UV light. Standard flash column chromatography was accomplished using silica gel (60 Å pore size, 230−400 μm mesh size). Lamps and immersion reactors used for irradiation: 125 W medium pressure lamp 3010/PX0686 and 400 W medium pressure lamp 3040/PX0686 both from Photochemical Reactors Ltd. Immersion reactors and filters are made of Pyrex. Spectra were recorded using Bruker spectrometers DPX-250, DXP360, and AVANCE-III 400 (250 MHz (1H); 62.5 MHz (13C), 360 MHz (1H); 90 MHz (13C) and 400 MHz (1H); 100 MHz (13C), respectively). 1H and 13C chemical shifts are reported in ppm relative to tetramethylsilane, using residual proton and 13C resonances from solvent as internal standards. Infrared spectra were recorded using a Bruker Tensor 27 instrument equipped with an ATR Golden Gate cell and a diamond window. UV−vis spectra were recorded using a Hewlett-Packard 8453 and quartz cuvette (1 cm). MALDI-TOF MS

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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.8b00171. Experimental procedures for the synthesis of benzoxazines 1−7; UV−vis spectra of 1−7 and a table summarizing the most important data; 1H NMR of compounds 1−7 and 9−12; 13 C NMR and HRMS F


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chromatogram of new compounds 4 and 9; 1H NMR of polymeric mixtures and GPC chromatograms (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(R.M.S.) E-mail [email protected]; Tel 0034935814288; Fax 0034935812477. ORCID

Rosa María Sebastián: 0000-0001-5519-9131 Jordi Marquet: 0000-0003-2210-8154 Author Contributions

R.M.S. and J.M. contributed equally. Funding

This work was financial supported from Spanish Government (CTQ2015-65439-R, CTQ2014-51912-REDC). We also acknowledge support from the Generalitat de Catalunya (2014SGR1105 and 2017SGR465). Notes

The authors declare no competing financial interest.

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