Formaldehyde-Free Polybenzoxazines for High Performance Thermosets

Romain Tavernier, Lerys Granado, Gabriel Foyer, Ghislain David, and Sylvain Caillol*

Article

Formaldehyde-Free Polybenzoxazines for High Performance

Thermosets

Cite This: https://dx.doi.org/10.1021/acs.macromol.0c00192

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sı Supporting Information

ABSTRACT: We report the synthesis and ring-opening polymer-

ization of new formaldehyde-free benzoxazines. The polybenzox-

azines obtained displayed high thermal stability and high char

yields. Data from the literature combined with our analyses by

diﬀerential scanning calorimetry and thermogravimetric analyses

gave deeper understanding about the use of aromatic aldehydes

instead of formaldehyde for the generation of polybenzoxazines.

Using pyrolysis coupled with gas chromatography/mass spectrom-

etry (Py-GC/MS) at diﬀerent temperatures, we provided

qualitative data to propose some polymerization and degradation

mechanisms associated with these new structures. A dialdehyde

was also used for the ﬁrst time in order to obtain difunctional

monomers, instead of using diamines or bisphenols. Interestingly, we demonstrated that formaldehyde, which is a CMR

carcinogenic, mutagenic, and/or reprotoxic) substance, could be avoided for the synthesis of polybenzoxazines without any loss of

(

thermal performance. Finally, some interesting structure−properties relationships are herein discussed. In particular, the use of

benzyl amines, rather than aromatic amines, was found to signiﬁcantly increase the char yields.

INTRODUCTION

reactive amines. The three-step method is used with less

reactive amines or to prevent the formation of oligomers by

■

During the last 30 years, research on polybenzoxazine has been

growing fast, at the forefront of the innovations in phenolic

thermosets. Compared to the century old phenol form-

aldehyde, benzoxazine chemistry presents several advantages:

among them, we ﬁnd condensation-free cross-linking, a near

zero volumetric shrinkage, high glass transition temperature,

16,17

using salicylaldehyde to form an imine.

A secondary amine

is then obtained by imine reduction, forming the ﬁrst part of

the heterocycle. The last step consists in the formation of the

oxazine ring by reacting formaldehyde with the phenol moiety

and the secondary amine.

−3

high degradation temperatures, and high char yields.

These synthetic methods were used to obtain a wide range

of benzoxazine monomers, which allowed researchers to ﬁnd

the parameters having an inﬂuence on thermal properties of

polybenzoxazine networks. For instance, electron-withdrawing

groups on the phenol moiety have a strong eﬀect on the ring-

opening temperature. High aromatic content and high cross-

link densities are required to get high degradation temper-

atures and improved char yields. For instance, Endo and

Nalakathu Kolanadiyil investigated the inﬂuence of the oxazine

ring number in benzoxazine monomers on their thermal

stabilities, evidencing that a higher content of oxazine rings

contrast, their industrialization suﬀers from the inconvenient

crystallinity of the monomers/prepolymers and the require-

ment of elevated temperatures to achieve the cross-linking of

the thermoset during curing. It is believed that these issues

being solved, polybenzoxazines can ﬁnd many applications in

cutting-edge technologies, such as chelation agents for metal

recovery in aqueous media, high performance composites,

spatial radiation insulating shields, porous materials for CO

capture, anticorrosion coatings, or even self-healing and

shape-memory polymers.

Since the ﬁrst benzoxazine synthesis, performed by Holly

improves thermal stability of cured thermosets. However, an

and Cope, benzoxazine chemistry has shown high versatility.

Traditionally obtained from the condensation of formaldehyde

between a phenolic moiety and a primary amine, benzoxazine

synthesis is therefore compatible with a wide range of chemical

functionalities. Andreu et al. have identiﬁed three main

increase of the monomers functionality leads to a decrease of

Received: January 26, 2020

Revised: March 19, 2020

pathways to obtain 1,3-benzoxazine architectures, two of

them appearing to be predominant in the articles published at

2−15

this time.

The one-pot method is the most practical one,

used either in bulk or in solution, and developed for the most

XXXX American Chemical Society

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the reactivity, thus requiring higher curing temperatures. The

design of benzoxazine monomers is therefore crucial to reach

the desired thermal properties.

HRMS (m/z, positive mode, [M + H] ): C14

16NO; calculated

214.1226; found 214.1236

Synthesis of 2,2′-(((1,3-Phenylenebis(methylene))bis(azanediyl))-

bis(methylene))diphenol (Scheme 1, Compounds A and B, R = m-

One of the biggest drawback of benzoxazine is the use of the

harmful formaldehyde in the synthesis of monomers. Form-

aldehyde substitution is challenging since the use of aliphatic

aldehydes is detrimental to the ﬁnal thermal stability. So far,

only two examples of formaldehyde-free 1,3-benzoxazines have

been published, for thermoset applications. Ohashi et al. were

the ﬁrst team to report the synthesis of two 2-substituted

Xylylene). First 17.91 g of salicylaldehyde was dissolved in 50 mL of

methanol. Then 10.03 g of m-XDA was added to the mixture, which

was then reﬂuxed for 2 h. After the reaction was cooled to room

temperature, solvent was removed under reduced pressure. The

corresponding imine was isolated for characterization purpose.

Reduction was then performed by dissolving the imine in 50 mL of

methanol, and 5.61 g of sodium borohydride was added in small

portions, at 0 °C, in order to control the foaming. After complete

addition of the sodium borohydride, the reaction mixture was heated

to reﬂux for another 2 h. After cooling to room temperature, the

reaction mixture was quenched by precipitation in distilled water. The

resulting viscous amine was then recovered by liquid−liquid

extraction with ethyl acetate, organic phase was dried over magnesium

sulfate and solvent was removed under reduced pressure to aﬀord the

desired product. 24.81 g of a viscous pale-yellow oil were recovered

that further crystallized as a white solid. Yield = 97%.

benzoxazines using the salicylaldehyde route, namely, a

monobenzoxazine based on benzaldehyde and aniline and a

bisbenzoxazine (one monomer with two oxazine rings) with

the p-phenylenediamine. The authors reported good thermal

performances after curing, with rather high char yields (25%

and 48%, respectively). More recently, Pereira et al. reported

the polymerization of two hydrogenated cardanol-based

benzoxazines using benzaldehyde and valeraldehyde, but the

preparation of the starting phenolic compounds still required

the use of formaldehyde and their thermal performances were

HRMS (m/z, positive mode, [M + H] ): C H N O ; calculated

22 25

349.1911; found 349.1927.

Synthesis of Benzoxazine Monomers. Synthesis of 1,3-

Bis((2H-benzo[e][1,3]oxazin-3(4H)-yl)methyl)benzene (Ph-mxda)

(Scheme 1, Compound C, R = m-Xylylene and R = H). First,

rather limited. Overall, we found a lack in the literature of

alternatives to formaldehyde for synthesizing more sustainable

benzoxazines with high thermal performances.

1.03 g of 2,2′-(((1,3-phenylenebis(methylene))bis(azanediyl))bis-

In response, we aim in this work at providing new

formaldehyde-free benzoxazines structures using the afore-

mentioned salicylaldehyde reaction pathway. Monofunctional

benzaldehyde and benzylamine are reacted to produce

monobenzoxazines. Moreover, difunctional terephthalaldehyde

(methylene))diphenol was dissolved in 20 mL of toluene. Then,

.17 g of paraformaldehyde was added to the mixture, which was then

reﬂuxed for 2 h with a Dean−Stark apparatus. After the mixture

cooled to room temperature, solvent was removed under reduced

pressure. Product was recovered as a transparent viscous oil. Yield

99%.

HRMS (m/z, positive mode, [M + H] ): C H N O ; calculated

(TPA) and m-xylylenediamine (m-XDA) are employed to

synthesize bisbenzoxazines. We propose herein a systematic

study on the eﬀect of the aromatic aldehyde and amine, and

their functionality, on the syntheses and properties, in close

comparison with previous literature data. We provide a

qualitative analysis for the determination of polymerization

mechanisms, in particular by analyzing the released com-

pounds during polymerization with a gas chromatography

technique. Finally, we show that these new benzoxazines

display very high thermal performances, showcasing one

important beneﬁt of the formaldehyde substitution.

73.1911; found 373.1918. NMR H (CDCl , 7.26 ppm): δ = 7.38

(1H), 7.27−7.35 (m, 3H), 7.16 (t, 2H), 6.82−6.95 (m, 6H), 4.89 (s,

2H), 3.98 (s, 2H), 3.94 (s, 2H). NMR C (CDCl , 77.16): δ =

154.26, 138.56, 129.55, 128.67, 128.16, 127.87, 127.79, 120.78,

120.11, 116.58, 82.39, 55.60, 49.81.

Synthesis of 1,3-Bis((2-phenyl-2H-benzo[e][1,3]oxazin-3(4H)-yl)-

methyl)benzene (Ph-mxda[2]ba) (Scheme 1, Compound C, R =

m-Xylylene and R = Ph). First, 10.99 g of 2,2′-(((1,3-phenylenebis-

(

methylene))bis(azanediyl))bis(methylene))diphenol was dissolved

in 100 mL of toluene. Then, 3.36 g of benzaldehyde was added to the

mixture, which was then reﬂuxed for 19 h with a Dean−Stark

apparatus. After the mixture cooled to room temperature, solvent was

removed under reduced pressure. Product was precipitated in

cyclohexane and centrifugated to aﬀord 14.28 g of the product as a

yellow vitreous solid. Yield = 86%.

EXPERIMENTAL SECTION

■

Materials. Salicylaldehyde and terephthalaldehyde were purchased

from TCI. Benzylamine, paraformaldehyde, m-xylylenediamine (m-

XDA), sodium borohydride, and benzaldehyde were purchased from

Sigma-Aldrich. Anhydrous magnesium sulfate, toluene, ethyl acetate,

dichloromethane, cyclohexane, and methanol were purchased from

VWR. Deuterated chloroform was purchased from Eurisotop. All

solvents and reagents were used without further puriﬁcation.

HRMS (m/z, positive mode, [M + H] ): C H N O ; calculated

525.2537; found 525.2557. NMR H (CDCl , 7.26 ppm): δ = 7.65−

7.68 (m, 4H), 7.37−7.41 (m,5H), 7.30−7.34 (m, 5H), 7.21−7.24 (m,

2H), 7.05 (d, 2H), 6.90 (d, 4H), 6.01 (d, 2H), 3.86 (m, 8H). NMR

C (CDCl , 77.16): δ = 153.72, 153.70, 139.57, 139.24, 139.22,

139.13, 139.12, 139.09, 129.87, 129.12, 129.10, 18.97, 128.96, 128.91,

128.81, 128.77, 128.56, 128.53, 128.25, 128.13, 128.11, 128.08,

128.07, 127.97, 127.93, 127.83, 127.63, 127.60, 127.42, 126.75, 90.49,

Synthesis of Salicylamines. Synthesis of 2-((Benzylamino)-

methyl)phenol (Scheme 1, Compounds A and B, R = Bz). Here,

1.40 g of salicylaldehyde was dissolved in 50 mL of methanol. Then,

0.0 g of benzylamine was added to the mixture, which was then

90.46, 53.55, 53.45, 46.97, 46.91.

Synthesis of 3-Benzyl-2-phenyl-3,4-dihydro-2H-benzo[e][1,3]-

oxazine (Ph-ba[2]ba) (Scheme 1, Compound C, R = Bz and R

1 2

reﬂuxed for 2 h. After the reaction was cooled to room temperature,

solvent was removed under reduced pressure. The corresponding

imine was isolated for characterization purposes. Reduction was then

performed by dissolving 10.52 g of the imine in 50 mL of methanol,

and 3.68 g of sodium borohydride was added in small portions, at 0

= Ph). First, 1.02 g of 2-((benzylamino)methyl)phenol was dissolved

in 20 mL of toluene. Then 0.50 g of benzaldehyde was added to the

mixture, which was then reﬂuxed for 24 h with a Dean−Stark

apparatus. After the mixture cooled to room temperature, solvent was

removed under reduced pressure. Product was then recrystallized

from toluene, leading to 0.89 g of a colorless powder. Yield = 54%.

°C, in order to limit foaming. After complete addition of the sodium

borohydride, the reaction mixture was heated to reﬂux for another 2

h. After cooling to room temperature, the reaction mixture was

quenched by precipitation in distilled water. The resulting viscous

amine was then recovered by liquid−liquid extraction with ethyl

acetate, the organic phase was dried over magnesium sulfate, and

solvent was removed under reduced pressure to aﬀord the desired

product. A total of 15.30 g of yellow oil was recovered. Yield = 77%.

HRMS (m/z, positive mode, [M + H] ): C H NO; calculated

302.1539; found 302.1548. NMR H (CDCl , 7.26 ppm): δ = 7.65 (d,

2H), 7.27−7.40 (m, 8H), 7,20 (m, 1H), 7.01 (d, 1H), 6.88 (d, 2H),

6.00 (s, 1H), 3.85 (m, 4H). NMR C (CDCl , 77.16): δ = 153.72,

139.21, 138.96, 128.89, 128.77, 128.53, 128.50, 128.09, 127.92,

127.85, 127.28, 126.75, 120.73, 119.92, 116.64, 90.53, 53.48, 46.91.

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Synthesis of 1,4-Bis(3-benzyl-3,4-dihydro-2H-benzo[e][1,3]-

Results are displayed as a mean for six samples with 1σ standard-

deviation.

Insoluble fraction was determined by plunging cured samples in 10

mL of dichloromethane in sealed vials, for 48 h at room temperature.

Samples were then dried 24 h under vacuum and weighed in order to

determine the insoluble content. Results are displayed as a mean of at

least three samples with 1σ standard deviation.

oxazin-2-yl)benzene (Ph-ba[2,2′]tpa) (Scheme 1, Compound C,

R = Bz and R = p-phenylene). First, 1.01 g of 2-((benzylamino)-

methyl)phenol was dissolved in 20 mL of toluene. Then, 315 mg of

terephthalaldehyde was added to the mixture, which was then reﬂuxed

for 24 h with a Dean−Stark apparatus. After the mixture cooled to

room temperature, the product spontaneously recrystallized as a

colorless solid and was rinsed with toluene. It was then dried under

reduced pressure, leading to 576 mg of a colorless powder. Yield =

Nomenclature. According to the benzoxazine abbreviation

proposed by Ohashi et al., the ﬁrst letters represent the phenolic

moiety, “Ph” being the phenol, after the hyphen is represented the

amine moiety, “ba” symbolizing the benzylamine, and mxda the m-

xylylenediamine. The square brackets are used to precise the position

where the oxazine ring is substituted, and ﬁnally, after those brackets

are displayed, the abbreviation of the aldehyde, i.e., “ba” stands for

benzaldehyde and tpa for terephthalaldehyde. Thus, benzylamine and

benzaldehyde based monobenzoxazine is described as Ph-ba[2]ba, m-

xylylenediamine and benzaldehyde-based bisbenzoxazine is Ph-

mxda[2]ba, and benzylamine and terephthalaldehyde-based benzox-

azine is Ph-ba[2,2′]tpa. For comparison purpose, one formaldehyde-

based benzoxazine was synthesized with the same method from m-

xylylenediamine, abbreviated Ph-mxda, this structure having been

6%.

HRMS (m/z, positive mode, [M + H] ): C H N O ; calculated

25.2537; found 525.2546. NMR H (CDCl , 7.26 ppm): δ = 7,65 (s,

H), 7.24−7.37 (m, 10H), 7.20 (m, 2H), 7.01 (d, 2H), 6.88 (d, 2H),

.98 (d, 2H), 3.84 (m, 8H). NMR C (CDCl , 77.16): δ = 153.66,

53.63, 139.04, 139.01, 138.89, 138.89, 128.78, 128.76, 128.74,

28.61, 128.50, 127.94, 127.85, 127.54, 127.29, 127.28, 126.87,

26.86, 120.75, 119.87, 119.86, 116.62, 90.42, 90.35, 53.64, 53.57,

6.87, 46.83.

Curing of Benzoxazine Samples. Between 80 and 120 mg of

benzoxazine monomer was weighed in small aluminum pans, which

were then inserted in an oven preheated at 200 °C, for 6 h at

atmospheric pressure.

described ﬁrst by Setiabudi from Huntsmann in a patent.

Measurements. H and ¹³C nuclear magnetic resonance (NMR)

spectra were recorded on a Bruker AC 400 NMR spectrometer, in

RESULTS AND DISCUSSION

■

CDCl using nondeuterated residual solvent as reference.

Monomers Synthesis and Characterization. All form-

aldehyde-free benzoxazines have been synthesized according to

the three-step method developed by Ronda et al., as illustrated

in Scheme 1. First, salicylaldehyde was reacted with a

High resolution mass spectrometry measurements (HRMS) were

performed on a Waters Synapt G2-S high resolution mass

spectrometer equipped with an ESI (electrospray) ionization source.

Attenuated total reﬂectance Fourier transform infrared absorption

spectroscopy (ATR-FTIR) measurements were carried out with a

Nicolet 6700 spectrometer from Thermo-Scientiﬁc, equipped with a

mercury−cadmium−tellurium detector, in the middle infrared range

with a resolution of 4 cm⁻and 32 scans were coadded to each

spectrum.

Scheme 1. General Reaction Pathway for the Synthesis of

,3-Benzoxazines

Diﬀerential scanning calorimetry (DSC) was measured on a DSC-3

F200 Maia (Netzsch GmbH) equipped with an intracooler module.

−1

The atmosphere was dry nitrogen at a ﬂow rate of 70 mL·min . The

temperature sensor was calibrated with biphenyl, indium, bismuth,

−

and CsCl standards at 10 °C·min . High-pressure stainless-steel pans

and lids (100 MPa, sealed at 3 N·cm) were used to prevent signal

from volatile evaporation. Pans were weighed before and after the

analysis to verify that there were no leaks during the analysis. Between

and 12 mg of samples was weighed into the pans.

Thermogravimetric analysis (TGA) was performed with a TGA-3

libra (Netzsch GmbH). Between 10 and 12 mg of monolithic sample

−

was weighed in platinum pans. The atmosphere was 40 mL·min

nitrogen. The heating rate was 10 °C·min⁻to monitor the thermal

performances of cured resins from room temperature (RT) to 900 °C.

The Py-GC/MS analytical setup consisted of an oven pyrolyzer

connected to a GC/MS system. A Pyroprobe 5000 pyrolyzer (CDS

Analytical) was used to pyrolyze the samples in a helium environment.

This pyrolyzer is supplied with an electrically heating platinum

ﬁlament. One coil probe enables the pyrolysis of samples (less than 1

mg) placed in quartz tube between two pieces of quartz wool. The

sample was successively heated at 200, 300, 400, 500, 600, and 900

C. Each temperature was held for 15 s before gases were drawn to

the gas chromatograph for 5 min. The pyrolysis interface was coupled

to a 450-GC gas chromatograph (Varian) by means of a transfer line

heated at 270 °C. In this oven the initial temperature of 70 °C was

−

held for 0.2 min, and then raised to 310 °C at 10 °C·min . The

column is a Varian Vf-5 ms capillary column (30 m × 0.25 mm) and

−

helium (1 mL·min ) was used as the carrier gas; a split ratio was set

to 1:50. The gases were introduced from the GC transfer line to the

ion trap analyzer of the 240-MS mass spectrometer (Varian) through

the direct-coupled capillary column. Identiﬁcation of the products was

achieved comparing the observed mass spectra to the U.S. National

Institute of Standards and Technology mass spectral library.

stoichiometric amount of selected amine (1:1), in order to

form the corresponding imine A (which was isolated for

characterization purpose). Then, subsequent imine was

reduced by sodium borohydride in order to get the

aminomethylphenol B. Aza-acetalization, i.e., aromatic alde-

hyde condensation with aminomethylphenol, resulted in the

desired 2-substituted 1,3-benzoxazine C. Three formaldehyde-

Solid content was evaluated for all monomers after curing at 200

C. For each monomer, six samples were weighed before and after

curing program, in order to determine the residual solid content.

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Figure 1. (a) H NMR spectra of Ph-mxda (1), Ph-mxda[2]ba (2), Ph-ba[2]ba (3) and Ph-ba[2,2′]tpa (4) (full spectra available in Supporting

Information) in chloroform-d with according structures and proton assignments. (b) FTIR spectra of benzoxazine monomers in the ﬁngerprint

region (shifted vertically).

free benzoxazines have been successfully synthesized with good

yields a monobenzoxazine based on benzylamine and

benzaldehyde (R = −CH Ph and R = Ph), a bisbenzoxazine

singlet at 6.00 ppm for Ph-ba[2]ba, whereas methylene groups

display more complex multiplicities. Since they are constrained

in the heterocycle, methylene protons from the ring are not

synthesized from m-xylylenediamine and benzaldehyde (R =

equivalent, and should appear as a doublet. Moreover, the

formation of the oxazine ring generates a stereocenter on the 2-

position, thus the signal from the methylene is observed as an

overlapped quadruplet with the signal of the other methylene

of the benzylic amine residue, between 3.80 and 3.91 ppm. For

2-substituted bisbenzoxazine Ph-mxda[2]ba, we observe the

O−CH−N signal as two singlets at 6.01 and 6.02 ppm,

illustrating the presence of diastereomers (each benzoxazine

ring generates asymmetry). The methylene protons signals

show a complex multiplicity ranging from 3.80 to 3.90 ppm.

The same multiplicity is observed in Ph-ba[2,2′]tpa as protons

from O−CH−N on the ring with two singlets at 5.97 and 5.98

ppm, and methylenes with a broad multiplet from 3.79 to 3.89

−

CH PhCH −, meta position and R = Ph) and another

bisbenzoxazine, based on benzylamine and a dialdehyde,

terephthalaldehyde (R = CH Ph and R = −Ph−, para

position). Previous authors used Lewis or Brønsted acids to

catalyze the condensation reaction between aldehyde and

imine. For instance, Tang et al. studied the aza-acetalization of

aromatic aldehydes with catalysts such as boron triﬂuoride,

23,24

25,26

stain tetrachloride,

trimethylsilyl chloride,

and other

Lewis or Brønsted acids. They showed in overall that the

nucleophilicity of the amine moiety is a key-parameter of this

reaction. Ohashi et al. used a Brønsted acid catalyst for the

synthesis of bisbenzoxazine from p-phenylenediamine. It

shall be noted however that steric parameters have a strong

inﬂuence on the amine reactivity, especially for secondary

ppm. In C NMR spectra (Figures S5 −S8) methylene carbon

O−CH −N for Ph-mxda is observed at 82.39 ppm. We can

amines. In our case, we did not use any catalyst since we

obtained the desired bisbenzoxazine structures in reasonable

time scales (≤26 h) with good yields.

observe the signal of O−ArCH−N carbon on the closed ring at

90.51 ppm for Ph-ba[2]ba. For Ph-mxda[2]ba, O−ArCH−N

carbons appear at 90.46 and 90.49 ppm, and for Ph-

ba[2,2′]tpa, respectively at 90.42 and 90.36 ppm. The

presence of two peaks conﬁrms the existence of diastereomers

in formaldehyde-free bisbenzoxazines.

H NMR spectra are displayed in Figure 1a and in Figures

S1 −S4 , of the Supporting Information. In formaldehyde-based

benzoxazine, characteristic peak of the O−CH −N benzox-

azine ring is observed as a singlet at 4.89 ppm whereas

methylene groups of the oxazine ring or from the benzylic

amine residue appear at 3.98 and 3.94 ppm. For formaldehyde-

free benzoxazine, the characteristic peak resulting from the

formation of the heterocycle, i.e., O−CH−N proton, gives a

FTIR spectroscopy was performed for all the monomers and

acquired spectra are displayed in Figure 1b. Symmetric and

asymmetric C −O−C stretching bands can be respectively

−1

found at 1022 and 1218 cm . For the formaldehyde-based

benzoxazine, Ph-mxda, we observe the characteristic band of

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Figure 2. (a) DSC thermograms of Ph-mxda, Ph-mxda[2]ba, Ph-ba[2], and Ph-ba[2,2′]tpa at 10 °C/min. (b) IR of cured polybenzoxazine

−

between 800 and 2000 cm . (c) Py-GC/MS chromatograms of all benzoxazines, at 200 °C, with the corresponding m/z (Da).

−1

31,32

oxazine ring skeletal vibration at 921 cm .

The actual

reported for m-phenylenediamine-based benzoxazine. We

vibration assignment of this complex band is challenging and

has been recently reported elsewhere. All these characteristic

bands are also observed for formaldehyde-free benzoxazines.

noticed that the use of m-substituted monomers tends to be

favorable to produce desirable liquid monomers at room

temperature. We also noticed that monomers having more

aromatic moiety lead to higher melting temperature, e.g., 56

°C for Ph-a, 70−75 °C for Ph-ba, (2 rings), and 200 °C Ph-

ba[2,2′]tpa and Ph-pda[2]ba (5 rings).

−

C −O−C bands are located at 1031 and 1219 cm for Ph-

−

mxda[2]ba, 1028 and 1231 cm for Ph-ba[2]ba, and 1033

−

and 1224 cm

for Ph-ba[2,2′]tpa. Also, the speciﬁc

benzoxazine ring band related to the O−C−N bonds is

Following the melting transition, the exothermic peak is

characteristic of polybenzoxazine ROP. One notes that m-XDA

benzoxazines exhibit two overlapped exotherms suggesting two

reaction pathways (one single peak is observed for others).

Most of our benzoxazines polymerize in the range 200−300

°C. However, Ph-mxda[2]ba polymerizes at a lower temper-

ature (peak maximum at 194 °C). Ren et al. showed that meta-

oriented bisbenzoxazines have lower polymerization temper-

located respectively at 929 cm⁻for Ph-mxda[2]ba, 932 cm

−1

for Ph-ba[2]ba, and 930 cm⁻¹for Ph-ba[2,2′]tpa. It is

interesting to note that this characteristic band is split into two

sharp signals, compared to the formaldehyde-based benzox-

−

azine, which displays a broader unique band at 921 cm . This

is consistent with the work of Han et al. showing that the

characteristic benzoxazine related mode is aﬀected by

substitution in position 2 of the oxazine ring.₁Finally, the

very similar signals between 1610 and 1450 cm⁻are attributed

to the aromatic skeleton vibrations for all monomers. In

addition, mass spectrometry was also used to complete the full

characterization of the synthesized structures (Figures S9 −

S14 ). Overall, the structural characterizations conﬁrmed

obtaining of the desired structures.

atures. It could be attributed to the electronic conﬁguration

of meta-oriented amine which could favor the ring opening.

This phenomenon is increased by the electronic eﬀect of the

aromatic cycle attached to the oxazine ring, which enhances

the ring-opening kinetics.

Generally, bisbenzoxazines exhibit higher molar enthalpy

than their monobenzoxazines counterpart, having more

reactive moieties per monomer (higher functionality). In the

case of monofunctional monomers, polymerization enthalpies

values are higher for formaldehyde-free benzoxazine bearing a

phenyl ring on the 2-position. However, the trend is reversed

for bisbenzoxazines and 2-substituted bisbenzoxazines, the

latter have lower polymerization molar enthalpies (ca. 50%).

For instance, Ph-mxda has a polymerization enthalpy of 82 kJ·

Polymerization Behavior and Mechanisms. Ring-open-

ing polymerization (ROP) has been investigated by DSC

(Figure 2a). Table 1 compares our data with the literature.

First, most of the thermograms show an endothermic

transition due to monomer melting. Interestingly, Ph-mxda

and Ph-mxda[2]ba were conﬁrmed to remain amorphous

liquids, whereas the other solid benzoxazine monomers

displayed a neat melting transition. Similar observation were

−

−1

mol and Ph-mxda[2]ba 47 kJ·mol . This is conﬁrmed when

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Table 1. DSC Data for All Benzoxazines Studied

−1

All data were generated from DSC at 10 °C·min . Polymerization temperature (T_p_o_l_y_m) is the temperature at peak maximum). References 11, 19,

4, and 35 are mentioned in the body of this table but linked here.

39−41

Scheme 2. Illustration of Ring-Opening of the Oxazine Ring in (a) Formaldehyde-Based Benzoxazines

Phenyl-Substituted Benzoxazine

and in (b) 2-

comparing Ph-pda and Ph-pda[2]ba, with respectively 51 and

After full curing at 200 °C, all polybenzoxazine were veri ﬁ ed

to be fully cross-linked, (no residual enthalpy in DSC, Figure

S15 ). Furthermore, they were con ﬁrmed to be perfectly

insoluble in a good solvent (Table S1).

−

1 kJ·mol of total enthalpies. Those lower polymerization

enthalpies may be explained by the steric hindrance that may

reduce the reactive species diﬀusion in 2-substituted

benzoxazines compared to formaldehyde-based ones. Those

results are in accordance with the work of Pereira et al., which

also reported lower polymerization molar enthalpies for

formaldehyde-free benzoxazines (with lower activation ener-

FTIR spectroscopy has been used to compare the monomer

and polymer structures. For all the synthesized monomers, we

can observe the disappearance of the speciﬁc benzoxazine band

−1

around 920−950 cm after curing, conﬁrming the ring

opening of the structures (Figure 2b). For the formaldehyde-

gies).

based benzoxazine poly(Ph-mxda), we can observe a residual

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Article

Figure 3. (a) TGA and DTG thermograms of all synthesized benzoxazines, under nitrogen, at 10 °C/min. (b) Comparison of temperature at 5%

degradation and char yield of the synthesized polybenzoxazines with literature. (c) Py-GC/MS chromatograms of Ph-mxda[2]ba under the

isothermal treatments (T reported next to the curves) with associated m/z (Da).

band around 1220 cm⁻¹while the 1022 cm⁻¹band have

disappeared, and a free OH band is observed as a shoulder

around 3200 cm⁻(Figure S16). This observation is in line

with a Mannich-type conformation of the cured network

one, poly(Ph-mxda[2]ba), 81.7 ± 0.3 wt % remains in the

network, showing that replacing formaldehyde by benzalde-

hyde does not seem to impact degassing during polymer-

ization. More volatiles are produced during the curing of Ph-

ba[2]tpa, since only 68.0 ± 0.5 wt % remains in the cured

samples. For the monobenzoxazine, however, dry content is

only 14.7 ± 0.8 wt %, which is consistent with the fact that this

is the lightest monomer and that monobenzoxazines are known

to only produce small oligomers, which can be volatilized or

degraded during the polymerization.

(Scheme 2), as demonstrated by Wang et al. For 2-

substituted benzoxazines, a Mannich-type structure is also

present for poly(Ph-mxda[2]ba), as conﬁrmed by the

−

presence of the free OH band at 3200 cm and the 1200

−

−1

cm band, while the 1028 cm band decreased but did not

completely disappear, meaning that there is a mixed structure

of N,O-acetal and Mannich-type. Poly(Ph-ba[2,2′]tpa) also

displayed a mixed structure, since there is on one hand a broad

band of free OH, showing the Mannich-type structure, and two

bands at 1178 and 1017 cm⁻that are characteristics of the

N,O-acetal structures, on the other hand. However, in that

case, those bands were slightly shifted compared to the

The structure of those volatiles can be of precious help in

order to understand the curing mechanism. Direct pyrolysis-

MS has been used in the literature in order to characterize the

volatile during polymerization. This technique has the

advantage to prevent chemical recombination that could

occur by trapping the volatiles in a solvent or by

−

monomer (1224 and 1033 cm ). Bands attributed to the

aromatic skeletons are still observed for all materials, in the

condensation. We thus performed a pyrolysis coupled to

GC/MS at 200 °C, where separation performed by gas

chromatography allowed better MS resolutions. We observed

that formaldehyde-free benzoxazines and Ph-mxda had a

slightly diﬀerent behavior in terms of volatile releasing. Figure

2c represents the chromatograms of benzoxazine monomers

after pyrolysis at 200 °C for 15 s, with associated m/z for main

peaks. Retention times, m/z and proposed chemical structures

of detected compounds are reported in Table S2 with some of

the associated MS spectra (Figures S20 −S22 ). For Ph-mxda,

toluene was observed (91 Da) that could result from solvent

trapped into the viscous monomer or fragmentation.

Methylphenol was also observed (108 Da) and the two main

peaks can be attributed to the reactive imines (134 and 148

Da) formed from the scission of the monomer on the oxazine

−

range 1610−1450 cm . The enlargement of those signals

indicates additional substitution that occurred during cross-

linking. Due to the low residual mass after thermal treatment,

poly(Ph-ba[2]ba) could not be analyzed by FT-IR.

Sometimes described for polyaddition resin and volatile-free

thermosets, it is known that ring-opening polymerization

produces volatile organic compounds, such as amines or

imines, which identiﬁcation is a clue for comprehension in the

18,37,38

polymerization mechanism.

In our case, dry content was

calculated by comparing the weight of a sample before and

after curing at 200 °C, in order to determine the volatiles

formation during polymerization. For poly(Ph-mxda), dry

content is 81.0 ± 0.7 wt %, and for the benzaldehyde-based

Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

deﬁnitive conclusions. Three input categories of structures are

Article

ring, as reported by several publications. ⁸^,³⁷⁻⁴⁰Another imine

can also be found with a lower quantity, at 159 Da,

corresponding to the scission on the two nitrogen atoms of

the m-XDA residue. All MS spectra of imines or amines

proposed are shown in the Supporting Information .

The degradation behavior has also been investigated using

Py-GC/MS, as shown in Figure 3c for Ph-mxda[2]ba and in

Figures S17 −S19 for all monomers. It shall be noted that the

coherence between the TGA and Py-GC/MS results was

aﬀected by the fact that TGA uses continuous heating ramp,

whereas a stepwise program was used for the pyrolysis. After

the initial pyrolysis at 200 °C, several pyrolysis steps have been

performed, by heating each sample to 300, 400, 500, 600, and

900 °C, with a cooling to room temperature between each

step. We observed that for the same sample, the volatile

releasing was very low compared to the curing step at 200 °C,

for all benzoxazines. However, as the temperature was raised,

the quantity of volatiles increased, which is correlated to high

degradation temperature of the cured networks. In early stages

of decomposition, we can observe the same volatiles than

during the polymerization. Those compounds could have been

For all formaldehyde-free monomers, volatiles were diﬀer-

ent, except for methylphenol that was released by all samples.

Retention times, m/z, proposed structures and corresponding

monomers are reported in Figure 2 c and in Table S3 . MS

spectra are reported in Figures S23 −S33 . Some ions were

detected in the three monomers, such as imine with one or two

phenyl groups (105 and 194 Da), or an amine with two phenyl

groups that could be part of the network with cross-linking on

the phenyl ring of the amine residue (211 Da). Other heavier

ions were detected corresponding either to the monomer for

Ph-ba[2]ba (300 Da) or opened fragments (286 or 311 Da)

for Ph-ba[2,2′]tpa. In the recent work of Pereira et al., TGA of

trapped in the network and are released over the T

of the

monomers showed early weight loss, around 200 °C. They

associated those losses to the degradation of pending alkyl

chain. However, when comparing both their TGA with DSC

data, we can notice that those ﬁrst weight losses correspond to

the ring-opening polymerization temperatures. Since the

second weight loss of monomers corresponds to the one

occurring for cross-linked monomers, the ﬁrst weight loss

corresponds to volatilization of reactive species during

polymerization and not early degradation.

material. They can also originate from the early degradation

of the network, as it has been shown before. All the

monomers displayed nearly the same degradation pathway,

releasing at high temperature aromatic fragments such as

benzene (78 Da), toluene (91 Da), xylene (106 Da), phenol

(

94 Da), benzonitrile (103 Da), methylphenol (108 Da),

methylbenzonitrile (117 Da), phenantrenol (194 Da, Figure

S34 ), pyrene (208 Da) which is characteristic of charring

materials, especially polybenzoxazines.

45,46

The higher is the

pyrolysis temperature, the lower is the molecular weight of the

released compounds.

As a conclusion, even if the formaldehyde-based poly(Ph-

mxda) still demonstrates maximum performances, these results

prove that benzaldehyde and terephthalaldehyde are eﬀective

and promising alternatives to formaldehyde.

Some interesting trends can also be envisioned about the

structure-thermal property relationships of salicylaldehyde

benzoxazines. Figure 4 proposes a short meta-analysis

comparing our results and literature data (detailed in Figure

The polymerization mechanism that is currently the most

accepted by the community implies the formation of a very

reactive imine, by the ring-opening of the heterocycle on the

39−41

oxygen atom (Scheme 2a).

The observation of imine

compounds that are volatilized during the polymerization are

in accordance with this mechanism. As they are observed both

in formaldehyde-based and formaldehyde-free benzoxazine, we

can conclude that the polymerization of 2-substituted

benzoxazines follow the same mechanism, i.e. the ring-opening

occurs via the formation of a reactive imine, which is reactive

toward nucleophilic ortho positions of the phenolic moiety

Scheme 2b). The cured networked structures as analyzed by

IR and the volatile structures are in accordance with the

accepted polybenzoxazines mechanisms.

c). Owing to the fact that we compare a relatively small

amount of data, we hereafter outline tendencies rather than

(

Thermal Stability and Degradation. The thermal

stability has been evaluated by TGA up to 900 °C. The

thermograms are shown in Figure 3b and the results compared

19,35,42

with literature data in Figure 3c.

Char yield at 800 °C

and temperature at 5% weight loss (T_d₅_%) are the usual

parameters to compare the thermal stability of thermosets. All

the synthesized polybenzoxazines displayed superior thermal

resistance, with char yields in the range of 48−65% and T

d5%

10−350 °C. Derivative TGA (DTG) showed the main steps

of materials degradation. Degradation patterns displayed up to

three steps for poly(Ph-ba[2]ba) and two main steps for all of

the other samples. Early degradations around 300 °C occurred

in poly(Ph-mxda) and poly(Ph-ba[2]ba), that account

respectively for 3% and 4% of weight. This early degradation

38,43

step is attributed to the cleavage of amine fragments

this is conﬁrmed by the Py-GC/MS discussed below. Around

00 °C, we observed DTG peaks for poly(Ph-ba[2]ba),

poly(Ph-mxda[2]ba) (Figure 3a) and poly(Ph-ba[2,2′]tpa)

Figure S19 ), that correspond to the starting of the network

degradation. Finally, for all the samples, the major weight loss

happened over 500 °C according to the DTG, and

corresponding to the char formation.

and

Figure 4. Comparison of (a) char yield at 800 °C and (b)

temperature at 5% degradation of amines (blue), monomer

functionality (gray), and aldehydes (green) categories. The column

bar values represent the average (of the data reported in Figure 3b)

along with one standard deviation as error bars. The star marks two

signiﬁcantly diﬀerent average values (analysis of variance: p-value =

0.078).

Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

The Supporting Information is available free of charge at

Article

compared (amine and aldehyde types and functionality of

monomers) with a focus on char yield and T_d₅_%as outputs.

The amine type seems to be the key factor to increase char

yields with a relatively good signiﬁcance (p-value <0.1). We

observe that benzylic amines (benzylamine and m-XDA) gave

rise to higher char yields than the aromatic aniline and p-PDA

thermal polymerization, which were in accordance with

previously reported mechanisms for formaldehyde-based

benzoxazines, leading to the hypothesis that ring-opening

polymerization occurs via the same mechanisms. IR evidenced

that the chemical structure of the network is similar for

formaldehyde and aromatic aldehyde-based benzoxazines.

Thermal stability for difunctional monomers are quite similar,

leading to high char yields (58% for m-XDA and

benzaldehyde-based benzoxazine and 59% for benzylamine

and terephthalaldehyde-based benzoxazine) and high degrada-

tion temperatures. In addition m-XDA and benzaldehyde-

based structures showed lower polymerization temperature

and enthalpy, with no melting point and high thermal stability.

Structural characterization of cured materials revealed the same

structural arrangement of the networks, leading to the

conclusion that the addition of an aromatic ring on the 2-

position does not aﬀect polymerization mechanisms. Overall,

we showed that we can avoid the use of formaldehyde without

aﬀecting neither the cross-linking mechanisms, the network

structures nor the thermal stabilities of polybenzoxazines. With

more in-depth studies on the cross-linking mechanisms of

these benzoxazines and the selection of appropriate starting

materials, it is expected to obtain seriously competitive and

more sustainable polybenzoxazines for high performance

applications.

(

+12% char yield in average). The same trends are suggested

for the degradation temperatures but with a lower level of

signiﬁcance (+70 °C). These higher thermal performances,

which suggest higher cross-linking density, could be readily

explained with the cross-linking mechanisms. As shown in

Scheme 2, the imine intermediates would be more reactive in

the case of aromatic amine, but it would also be more

hindered, i.e., less available for nucleophilic attacks. On the

other hand, because the benzylic amines are less hindered, they

are supposedly more prone to react with nucleophilic moieties,

i.e., the phenolates, which would ﬁnally lead to higher cross-

link density and upgraded thermal performances.

The functionality of the benzoxazine monomers does not

seem to be an important factor. Both mono- and

bisbenzoxazine lead to similar thermal performances. In fact,

increasing the functionality of the amine could lead to lower

gelation time and lower volatile contents (observed in this

work between benzylamine and m-XDA). Yet, comparing

mono- and bisbenzoxazines, the reaction mechanisms are

expected to remain the same. Both cured materials would have

similar cross-linking density explaining the observed equivalent

thermal performances.

ASSOCIATED CONTENT

sı Supporting Information

■

From a perspective of sustainable development, it is

interesting to compare formaldehyde-based to benzaldehyde-

based benzoxazines. The herein compared benzaldehyde

benzoxazines display lower degradation temperatures than

formaldehyde ones (−80 °C). However, this trend is not

signiﬁcant, considering the dispersity of the data. In contrast,

very similar average char yields are recorded for the two

groups. Therefore, we consider that benzaldehyde (or

terephthalaldehyde) can eﬀectively replace the toxic form-

aldehyde without a signiﬁcant loss of thermal resistances.

Finally, among all synthesized and discussed benzoxazines,

the network Ph-mxda(2)ba is particularly attractive. With

benzaldehyde rather than formaldehyde, the synthesis does not

involve any CMR (carcinogenic, mutagenic, and/or repro-

toxic) precursor (yet, to be nuanced because of the known

sensitizing eﬀects of m-XDA). One very interesting feature is

that the monomer is liquid with no melting transition at

ambient conditions, which considerably facilitates the process-

ing conditions (along with other benzylic amine-based

benzoxazine). Furthermore, Ph-mxda(2)ba possesses the

lowest polymerization temperature of all systems herein

studied and discussed. We envision that such monomer

could be used with existing industrial processes. The advantage

of using bisbenzoxazine (i.e., m-XDA rather than benzylamine)

is its relatively high solid content after full curing (low VOC).

Ph-mxda(2)ba displayed very good thermal performances,

among the highest of the systems discussed.

https://pubs.acs.org/doi/10.1021/acs.macromol.0c00192.

Full H and C NMR spectra, ESI-HRMS of monomers,

dry contents and insoluble fractions of cured materials,

DSC of cured samples, and Py-GC/MS chromatograms

and mass spectra (PDF)

AUTHOR INFORMATION

■

Corresponding Author

Sylvain Caillol − ICGM, Universite Montpellier, CNRS,

ENSCM, 34296 Montpellier, France; orcid.org/0000-

003-3106-5547 ; Email: sylvain.caillol@enscm.fr

Authors

Romain Tavernier − ICGM, Universite

Montpellier, CNRS,

ENSCM, 34296 Montpellier, France; orcid.org/0000-

001-8186-9557

ys Granado − ICGM, Universite Montpellier, CNRS,

Ler

ENSCM, 34296 Montpellier, France; orcid.org/0000-

001-5812-8777

Gabriel Foyer − ArianeGroup, 33185 Le Haillan, France

Ghislain David − ICGM, Universite Montpellier, CNRS,

ENSCM, 34296 Montpellier, France; orcid.org/0000-

001-5839-6130

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.macromol.0c00192

SUMMARY

■

Notes

We demonstrated that the synthesis of formaldehyde-free 1,3-

benzoxazine is easily accessible in three steps with reasonable

yields. We showed that, overall, polymerization enthalpies of

such monomers are lower than formaldehyde-based ones,

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

−

−1

(

42−90 J·g versus 35−415 J·g , respectively). Pyrolysis-

The authors wish to thank the French “Direction Gen

l’Armement” for the funding of the Ph.D. work and the

ale de

GC/MS allowed identifying the volatile structures during

Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Mazzetto, S. E.; Lomonaco, D. Influence of Natural Substituents in

Article

Laboratory of “Mesures Physiques” of University of Mont-

pellier for the ESI-MS measurements.

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the Polymerization Behavior of Novel Bio-Based Benzoxazines. Mater.

Today Commun. 2019, 21, 100629.

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