Benzoxazine Monomers and Polymers Based on 3,30-Dichloro-4,40-Diaminodiphenylmethane: Synthesis and Characterization

Article Benzoxazine Monomers and Polymers Based on 3,30-Dichloro-4,40-Diaminodiphenylmethane: Synthesis and Characterization

Viktoria V. Petrakova 1, Vyacheslav V. Kireev 1, Denis V. Onuchin 1, Igor A. Sarychev 1,2 , Vyacheslav V. Shutov 1, Anastasia A. Kuzmich 1, Natalia V. Bornosuz 1, Mikhail V. Gorlov 1, Nikolay V. Pavlov 1, Alexey V. Shapagin 3 , Ramil R. Khasbiullin 3 and Igor S. Sirotin 1,*

1 Faculty of Petroleum Chemistry and Polymeric Materials, Mendeleev University of Chemical Technology of Russia, Miusskaya Sq. 9, 125047 Moscow, Russia; [email protected] (V.V.P.); [email protected] (V.V.K.); [email protected] (D.V.O.); [email protected] (I.A.S.); [email protected] (V.V.S.); [email protected] (A.A.K.); [email protected] (N.V.B.); [email protected] (M.V.G.); [email protected] (N.V.P.) 2 All-Russian Scientiﬁc Research Institute of Aviation Materials, 105275 Moscow, Russia 3 Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences (IPCE RAS), 119071 Moscow, Russia; [email protected] (A.V.S.); [email protected] (R.R.K.) * Correspondence: [email protected]; Tel.: +7-(499)-978-91-98

Abstract: To reveal the effect of chlorine substituents in the ring of aromatic amine on the synthesis process of benzoxazine monomer and on its polymerization ability, as well as to develop a Citation: Petrakova, V.V.; Kireev, fire-resistant material, a previously unreported benzoxazine monomer based on 3,30-dichloro-4,40- V.V.; Onuchin, D.V.; Sarychev, I.A.; diaminodiphenylmethane was obtained in toluene and mixture toluene/isopropanol. The resulting Shutov, V.V.; Kuzmich, A.A.; benzoxazine monomers were thermally cured for 2 h at 180 ◦C, 4 h at 200 ◦C, 2 h at 220 ◦C. A Bornosuz, N.V.; Gorlov, M.V.; Pavlov, comparison between the rheological, thermal and fire-resistant properties of the benzoxazines based N.V.; Shapagin, A.V.; et al. 0 0 0 Benzoxazine Monomers and on 3,3 -dichloro-4,4 -diaminodiphenylmethane and, for reference, 4,4 -diaminodimethylmethane was Polymers Based on 3,30-Dichloro-4,40- made. The effect of the reaction medium on the structure of the oligomeric fraction and the overall Diaminodiphenylmethane: Synthesis yield of the main product were studied and the toluene/ethanol mixture was found to provide the and Characterization. Polymers 2021, best conditions; however, in contrast to most known diamine-based benzoxazines, synthesis in the 13, 1421. https://doi.org/10.3390/ pure toluene is also possible. The synthesized monomers can be used as thermo- and fire-resistant polym13091421 binders for polymer composite materials, as well as hardeners for epoxy resins. Chlorine-containing polybenzoxazines require more severe conditions for polymerization but have better fire resistance. Academic Editor: Jelena Vasiljević Keywords: benzoxazines; polybenzoxazines; diaminodiphenylmethane; 3,30-dichloro-4,40- Received: 20 December 2020 diaminodiphenylmethane; heterocycles; thermosetting binders Accepted: 20 April 2021 Published: 28 April 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Phenol-formaldehyde (phenolic) resins are widely used in construction [1], electron- iations. ics [2] and the aerospace sector [3] because of their low cost and relatively high performance, for example, in heat and fire resistance [4]. However, phenolic resins have a number of disadvantages that limit their use: the release of volatile substances during curing and the formation of micropores inside the material, which impair the properties of the final products or require autoclave processing to obtain high-performance material [5]. Ben- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. zoxazines are a novel analogue of phenolic resins with improved characteristics, mainly This article is an open access article due to polymerization without the release of volatile substances and wide possibilities of distributed under the terms and copolymerization with thermosets of other classes. Polybenzoxazines are characterized by a conditions of the Creative Commons number of advanced properties, including high mechanicals, dimensional stability and heat Attribution (CC BY) license (https:// resistance, near-zero curing shrinkage, and low moisture absorption [6,7]. The combination creativecommons.org/licenses/by/ of these properties have attracted a lot of attention from materials scientists, and many 4.0/). resin and prepreg manufacturers have developed and commercialized benzoxazine-based

Polymers 2021, 13, 1421. https://doi.org/10.3390/polym13091421 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1421 2 of 19

products for composites, coatings and the electronics industry [8,9]. Bifunctional monomers based on various architectures built up from diamines and monofunctional phenols seem to be more perspective compared to those based on diphenols and monoamines, since they are characterized by enhanced mechanical properties, thermal and fire resistance [5,10–15]. However, until recently, the synthesis of benzoxazine monomers based on diamines and monophenols was practically impossible. At the first stage of the condensation reaction hyperbranched crosslinked structures based on diamine and paraformaldehyde are formed, which can precipitate from the reaction mixture, thereby reducing the main product yield [16]. This circumstance excluded the possibility of carrying out a one-stage synthesis until it was shown that the proper selection of synthesis conditions allows researchers to obtain bifunctional monomers based on diamines and phenols [14]. In a number of recent works [14,17–21], it was shown that in certain conditions, namely, carrying out the reaction in a solvent mixture of toluene/ethanol (2:1), it is possible to obtain the target product in one stage. At temperatures above 180 ◦C, these monomers undergo thermal polymerization via the ring-opening of the oxazine cycle without the release of low-molecular byproducts. This process is accompanied by the formation of crosslinked polymers, and so the monomers can be used as binders for composite materials, both as a separate components of a thermosetting system, and as a co-monomers in systems with epoxy resins [22], bismaleimides, polyurethanes, cyan ethers, phthalonitriles, etc. to form highly crosslinked copolymers [23]. Benzoxazine monomers can be processed by resin transfer molding (RTM) and vacuum infusion due to the wide processing window [24–27]. The modern composite industry requires a variety of components for binders, for example, to obtain structural materials with reduced flammability and heat resistance. Thus, the search for new benzoxazine monomers with improved properties is an urgent task. We have chosen 3,30-dichloro-4,40-diaminodiphenylmethane (quamin, a chlorinated analog of the well-known industrial diamine 4,40-diaminodiphenylmethane) as the starting reagent for the synthesis of the P-d benzoxazine monomer. It is believed that the addition of chlorine atoms to the monomer structure will improve the fire and heat resistance properties of the materials [28–31]. An additional task was to reveal the effect of the chlorine substituents in the aromatic ring of the amine on the synthesis process of benzoxazine monomer and on its ability to undergo polymerization. The lower activity of quamin in condensation reactions with formaldehyde [32] allows us to make an assumption that special conditions (a mixture of toluene and ethanol solvents) are not necessary in this case. Electron-withdrawing chlorine atoms can also hinder the polymerization of benzoxazines [33]. These assumptions required experimental confirmation, which was the aim of this article. As a reference monomer, we used P-d based on 4,40-diaminodiphenylmethane. The second component for both diamines was phenol. The thermal and rheological characteristics of both diamine-based benzoxazines were investigated within this work.

2. Materials and Methods 2.1. Starting Materials 4,40-diaminodiphenylmethane (d) 97% (Alfa Aesar, Kandel, Germany), 4,40-diamino- 3,30-dichlorodiphenylmethane (quamin or q) 97% (Chimex Limited, Saint Petersburg, Russia), phenol puriﬁed by distillation, paraformaldehyde 91% (ERCROS, Barselona, Spain) used without cleaning; toluene, isopropanol (Komponent-Reaktiv, Moscow, Russia) were dried using molecular sieves and distilled before use.

2.2. Synthesis of Benzoxazine Monomers Based on Diamines of Various Structures The calculated amount of diamine, phenol and solvent was loaded (Table1) into a 500 mL round-bottom ﬂask equipped with a magnetic stirrer and a reﬂux condenser. The dissolution of solid reagents was carried out at 60 ◦C, then the calculated amount of paraformaldehyde was loaded. The temperature was raised to 80–90 ◦C and the synthesis was carried out for 8 h. Then, the solvents were removed and the product was dried at Polymers 2021, 13, 1421 3 of 19

90 ◦C in a vacuum oven for 6 h. The benzoxazine monomers were obtained as yellow glassy substances with a softening point temperature range of 80–100 ◦C in a 90–95% yield. Resulting monomers were used for further polybenzoxazines synthesis without additional puriﬁcation.

Table 1. The amounts of starting reagents for the synthesis of benzoxazine monomers based on diamines of various structures in various solvents.

Benzoxazine Monomer Reagent P-q P-d Toluene Toluene/Isopropanol = 2:1 Toluene/Isopropanol = 2:1 g 26.715 20 20 Diamine mol 0.100 0.075 0.100 g 18.822 14.091 18.99 Phenol mol 0.200 0.150 0.200 g 14.52 10.87 14.65 Paraformaldehyde, 91% (10% Excess) mol 0.440 0.330 0.440 Solvent mL 150 150 150

2.3. Curing of Benzoxazines Benzoxazine monomers were cured according to the following regime: 2 h at 180 ◦C, 4 h at 200 ◦C, 2 h at 220 ◦C; all samples were degassed at 130 ◦C for 1 h before curing. The completeness of the curing process was controlled by the absence of an exothermic effect on the DSC thermogram.

2.4. Measurements 1 13 H, C NMR spectra were measured in a CDCl3 solution using a Bruker AV-600 spectrometer (Bruker Corporation, Bremen, Germany) operating at frequencies of 600 and 162 MHz, respectively. Chemical shifts are reported in parts per million and referenced to the signals of deuterated solvents. 1H NMR chemical shifts are reported relative to the signals of tetramethylsilane. The spectra were processed using the MestReNova Lab package (version 12.0.4, MESTRELAB RESEARCH, S.L., Santiago de Compostela, Spain). Differential scanning calorimetry (DSC) was measured on a Netzsch DSC 204 F1 Phoenix instrument (Netzsch, Selb, Germany) in a nitrogen atmosphere (50 mL/min) at a heating rate of 10 ◦C/min. Thermogravimetric analysis (TGA) with quadrupole mass spectrometry (QMS) were carried out on a Netzsch TG 209 F1 Iris (Netzsch, Selb, Germany) and QMS 403 D Aeolos (Netzsch, Selb, Germany), respectively. TGA was carried out at a heating rate of 20 ◦C/min and an inert atmosphere ﬂow rate of 50 mL/min. The temperature of transport capillary was 230 ◦C. The curves were processed using the Netzsch Proteus Thermal Analysis version 6.1.0 (Netzsch, Selb, Germany). QMS was carried out in analog scan mode on the detector CH-TRON and in mass mode SCAN-F. The detector voltage was 1200 V. The holding time for the measurement was 0.5, 1 and 2 s. The curves were processed using the Inﬁcon AG QUADSTAR v7.02 (Netzsch, Selb, Germany). Flammability tests were carried out in accordance with Vertical Burning Test UL-94 (ASTM D3801–20a, Northbrook, IL, USA) [34] on 5 samples. The dimensions of the samples were 127 mm × 12.7 mm × 2 mm. Studies of rheological properties were carried out on the Anton Paar Modular Rheome- ter MCR 302 (Graz, Austria) in oscillation mode at frequency of 1 Hz, shear strain of 10% and a gap 0.5 mm using plate-plate geometry. FTIR spectra were recorded on a Nicolet 380 Fourier spectrophotometer with an ATR accessory. Polymers 2021, 13, x FOR PEER REVIEW 4 of 20

Studies of rheological properties were carried out on the Anton Paar Modular Rhe- ometer MCR 302 (Graz, Austria) in oscillation mode at frequency of 1 Hz, shear strain of Polymers 2021, 13, 1421 10% and a gap 0.5 mm using plate-plate geometry. 4 of 19 FTIR spectra were recorded on a Nicolet 380 Fourier spectrophotometer with an ATR accessory. The morphology and the char yield of the P-q-based polymer samples were investi- gatedThe by morphologyscanning electron and the microscopy char yield on of a the JSM-6510 P-q-based device polymer (JEOL, samples Tokyo, were Japan) investi- at an gatedaccelerating by scanning voltage electron of 15 kV. microscopy on a JSM-6510 device (JEOL, Tokyo, Japan) at an acceleratingSEM images voltage and of FTIR 15 kV. spectra were obtained with the use of char samples after ignition duringSEM images vertical and test FTIR according spectra wereto UL-94 obtained standard. withthe use of char samples after ignition duringElemental vertical testanalyzer according for sulfur, to UL-94 chlorine, standard. nitrogen and carbon Multi EA 5000 (Analytik Jena ElementalAG, Jena, Germany) analyzer for was sulfur, used. chlorine, nitrogen and carbon Multi EA 5000 (Analytik Jena AG,Investigation Jena, Germany) of elemental was used. composition of samples was carried out on scanning-elec- Investigation of elemental composition of samples was carried out on scanning- tron microscope (Jeol JSM-U3, Tokyo, Japan) with energy dispersive X-ray spectrometer electron microscope (Jeol JSM-U3, Tokyo, Japan) with energy dispersive X-ray spectrometer (Eumex, Heidenrod, Germany) in accelerating voltage of 15 kV. Chemical elements were (Eumex, Heidenrod, Germany) in accelerating voltage of 15 kV. Chemical elements were identified by the characteristic Kα-lines of the X-ray spectrum. identiﬁed by the characteristic Kα-lines of the X-ray spectrum.

3.3. Results and Discussion 3.1.3.1. Synthesis of Benzoxazine MonomersMonomers BasedBased onon DiaminesDiamines TheThe preparationpreparation ofof benzoxazinesbenzoxazines basedbased onon di-di- andand polyaminespolyamines isis complicatedcomplicated byby thethe peculiaritypeculiarity ofof thethe ﬁrstfirst stagestage ofof thethe reaction.reaction. TheThe formationformation ofof hyperbranchedhyperbranched triazinetriazine chainschains (Figure(Figure1 1),), whichwhich cancan lead lead to to the the reaction reaction mass mass gelation gelation [ [16],16], eithereither leadingleading toto aa decreasedecrease inin thethe yieldyield ofof thethe mainmain product,product, oror completecomplete impossibilityimpossibility ofof one-stepone-step synthesissynthesis ofof benzoxazinesbenzoxazines basedbased onon diamines.diamines.

FigureFigure 1. Possible side reaction during the production of benzoxazine monomers based on diamines.

0 DespiteDespite thethe aforementioned aforementioned fact, fact benzoxazines, benzoxazines based based on 4,4 on-diaminodiphenylmethane/ 4,4′-diaminodiphenylme- quamine,thane/quamine, phenol phenol and paraformaldehyde and paraformaldehyde were obtainedwere obtained successfully successfully (Figure (Figure2) by two2) by methods:two methods: in toluene in toluene and in and a mixture in a mixture of toluene of toluene and isopropanol and isopropanol according according to the scheme to the Polymers 2021, 13, x FOR PEER REVIEW 5 of 20 belowscheme (Figure below2 ).(Figure The structure 2). The structure of the obtained of the obtained benzoxazine benzoxazine monomers monomers was characterized was char- 1 13 usingacterizedH andusing C1H NMR and 13 spectroscopy.C NMR spectroscopy.

Figure 2. Synthesis of benzoxazine mo monomersnomers based on diamines.

A low-intensity signal with a chemical shift at δH = 5.1 ppm in the 1H NMR spectra of the products obtained via this method (Figure 3, Table 2) indicates a small number of triazine structures.

A G

C H G J L F B F H K J I L K I C d-clorophorm B D D 7.30 7.20 7.10 7.00 6.90 6.80 D A

E D Toluene

Toluene/ Isopropanol 2:1

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 δH, ppm Figure 3. 1H NMR spectrum of benzoxazine based on quamine in various solvents.

Table 2. 1Н and 13C NMR spectroscopy data for benzoxazine monomers.

Proton Chemical Shifts Δh (ppm) Carbon Chemical Shifts Δc (ppm) Sample Oxazine Ring Diamine Oxazine Ring Diamine CH2N CH2O CH2 CH (Ar) CH2N CH2O CH2 CH (Ar) P-d 4.70 5.44 3.96 6.96–7.49 50.15 79.39 40.01 116.69–154.17 P-q 4.58 5.30 3.78 6.81–7.35 50.71 80.31 39.73 116.75–153.88

In the second method, a toluene/isopropanol mixture with 2:1 volume ratio was used. Due to the hydroxyl groups affinity to isopropanol and increased general solvation, triazine structures are not formed (Figures 3 and 4, Table 2) [35].

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Polymers 2021, 13, 1421 5 of 19 Figure 2. Synthesis of benzoxazine monomers based on diamines.

1 1 A low-intensitylow-intensity signal with with a a chemical chemical shift shift at at δδHH == 5.1 5.1 ppm ppm in in the the H HNMR NMR spectra spectra of ofthe the products products obtained obtained via via this this method method (Figure (Figure 3, Table3, Table 2)2 indicates) indicates a small a small number number of oftri- triazineazine structures. structures.

A G

C H G J L F B F H K J I L K

I clorophorm

- C d B D D 7.30 7.20 7.10 7.00 6.90 6.80 D A

E D Toluene

Toluene/ Isopropanol 2:1

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 δH, ppm FigureFigure 3.3. 11H NMR spectrum of benzoxazine based on quamine in various solvents.

1 13 Table 2.2. 1HН and 13CC NMR NMR spectroscopy spectroscopy data data for for benzo benzoxazinexazine monomers. monomers.

Proton Chemical Shifts Δh (ppm) Carbon Chemical Shifts Δc (ppm) Proton Chemical Shifts ∆h (ppm) Carbon Chemical Shifts ∆c (ppm) Sample Oxazine Ring Diamine Oxazine Ring Diamine Sample Oxazine Ring Diamine Oxazine Ring Diamine CH2N CH2O CH2 CH (Ar) CH2N CH2O CH2 CH (Ar) CH N CH O CH CH (Ar) CH N CH O CH CH (Ar) P-d 4.702 5.442 3.962 6.96–7.49 50.152 79.392 40.01 2 116.69–154.17 PP-d-q 4.704.58 5.445.30 3.963.78 6.96–7.496.81–7.35 50.1550.71 80.31 79.39 39.73 40.01 116.75 116.69–154.17–153.88 P-q 4.58 5.30 3.78 6.81–7.35 50.71 80.31 39.73 116.75–153.88 In the second method, a toluene/isopropanol mixture with 2:1 volume ratio was used. Due Into the secondhydroxyl method, groups a affinity toluene/isopropanol to isopropanol mixture and increased with 2:1 volumegeneral ratiosolvation, was used. tria- Duezine tostructures the hydroxyl are not groups formed afﬁnity (Figure to isopropanols 3 and 4, Table and increased2) [35]. general solvation, triazine structures are not formed (Figures3 and4, Table2)[35]. The yields of benzoxazines in the two preparation methods were up to 90–95%. To ex- clude the formation of compounds with the Mannich bridge, a 10% excess of paraformaldehyde was used, its loading was carried out gradually and only after complete dissolution of quamine and phenol, the optimal reaction temperature was used (80–90 ◦C). Benzoxazine monomer based on 4,40-diaminodiphenylmethane (P-d) was obtained by the analogous method in toluene/isopropanol solvents mixture.

Based on the data obtained from 1H NMR spectroscopy, it can be concluded that some branched triazine structures insoluble in toluene are formed. Thus, carrying out the reaction in a toluene/isopropanol solvent mixture gives a much cleaner product than in pure toluene medium. However, unlike a number of halogen-free diamines [14,17–21], the direct synthesis of P-q from quamine, phenol and paraformaldehyde in toluene medium is still possible despite the presence of a slightly larger number of byproducts in comparison with the toluene/ethanol solvent mixture. PolymersPolymers2021 2021, 13, 13, 1421, x FOR PEER REVIEW 66 of of 19 20

A I J G C H K D E L B D

H G

7 5 8 0 6 0 7 4 5

8 8 1 1 8 4 2 5 5 7 7 3

......

3 7 7

. . .

7 0 3 4 7 0 6 2 6

2 2 0 0 5 4 3 3 2 2 1 9

1 1 8 5 1 1 1 1 1 1 1 3

B C

H F I G D E A

J L K

160 150 140 130 120 110 100 90 80 70 60 50 40 δC, ppm FigureFigure 4. 4.13 13CC NMR NMR spectrum spectrum of of benzoxazine benzoxazine based based on on quamine quamine in in toluene/isopropanol toluene/isopropanol 2:1.2:1.

TheThe structures yields of benzoxazines of the obtained in monomersthe two preparation and polymers methods were were conﬁrmed up to 90% using–95%. FTIR To spectroscopyexclude the formation and elemental of compounds analysis with (Table the3 Mannich). In the bridge, FTIR a spectra10% excess of monomersof paraform- (Figurealdehyde5), thewas absorptionused, its loading bands was correspond carried out to variousgradually vibrations and only inafter the complete oxazine ring:disso- −1 atlution 944 and of quamine 1215–1205 and cm phenol,–tothe thestretching optimal reaction vibrations temperature of the C–N–C was used and C–O–C (80–90 bonds,°С). −1 respectively;Benzoxazine in the regionmonomer of 1489–1486 based on cm 4,4′-diaminodiphenylmethane–to the stretching vibrations (P of-d) the was C–H obtained bonds −1 inby the the –CH analogous2– groups; method at 751–745 in toluene/isopropanol cm –to the bending solvents vibrations mixture. of –CH2– groups. In the FTIR spectraBased on of polybenzoxazines,the data obtained thefrom intensities 1H NMR of spectroscopy, these bands decrease it can be signiﬁcantly, concluded andthat −1 asome broadened branched band triazine at 3600–3380 structur cmes insolubleappears, in corresponding toluene are formed. to the Thus, stretching carrying vibrations out the ofreaction phenolic in hydroxyla toluene/isopropanol groups linked solvent by hydrogen mixture bonds. gives The a much absorption cleaner bands product in the than FTIR in Polymers 2021, 13, x FOR PEER REVIEW 7 of 20 spectrapure toluene of benzoxazine medium. However, and related unlike polybenzoxazines a number of halogen correspond-free diamines well to the[14,17 literature–21], the datadirect [36 synthesis–39]. of P-q from quamine, phenol and paraformaldehyde in toluene medium is still possible despite the presence of a slightly larger number of byproducts in compar- 944 1489–1486 944 ison with the1489–1486 toluene/ethanol solvent mixture. 1215–1205 (b) 1215-–1205 (a) The structures of the751–745 obtained monomers and polymers were confirmed751–745 using FTIR spectroscopy and elemental analysis (Table 3). In the FTIR spectra of monomers (Figure 3380 5), the absorption bands correspondP-q to various vibrations in the oxazine ring: at 944 and 1215–1205 cm−1 –to the stretching vibrations of the C–N–C and C–O–C bonds, respectively; in the region of 1489–1486 cm−1 –to the stretching vibrations of the C–H bonds in the – CH2– groups; at 751–745 cm−1 –to the bending vibrations of –CH2– groups. In the FTIR

Absorbance spectra of polybenzoxazines, the intensities of these bands decrease significantly, and a −1 broadened band at 3600–3380 cmP-d appears, corresponding to the stretching vibrations of phenolic hydroxyl groups linked by hydrogen bonds. The absorption bands in the FTIR spectra of benzoxazine and related polybenzoxaz ines correspond well to the literature 4000 3000 2000 1000 4000 3000 2000 1000 data [36–39]. –1 Wavenumder (cm ) FigureFigure 5. 5.FTIR FTIR spectra spectra of of benzoxazine benzoxazine monomers monomers ( a()a) and and polymers polymers ( b(b).).

Table 3. Elemental analysis data for benzoxazine monomers.

Elements Calculated, % Found, % P-d C 80.16 80.03 H 6.03 6.07 O 7.36 7.61 * N 6.45 6.29 P-q C 69.19 68.86 H 4.81 4.62 O 6.36 6.52 * N 5.56 5.66 Cl 14.08 14.34 * These values are obtained by difference.

The content of elements in benzoxazine monomers (Table 3), taking into account the measurement error, converges with the theoretical calculation, therefore, pure monomers were obtained. The obtained benzoxazine monomers are polymerized by the thermal method with the formation of an insoluble thermosetting polymer according to the scheme shown in Figure 8 (Figure S3 in Supplementary Materials).

3.2. Properties of Diamine-Based Benzoxazine Monomers The curing kinetics of various diamine-based benzoxazine monomers was investigated by using differential scanning calorimetry (DSC) (Figures 8 and S3, Table 4). The curves show that P-d polymerization proceeds under milder conditions than P-q. This may be due to the fact that the aromatic rings of the diamine in P-q are deactivated by chlorine atoms; therefore, the addition to the ortho position relative to the nitrogen atom will be somewhat difficult both sterically and energetically.

Table 4. Differential scanning calorimetry results.

Parameter P-d P-q Onset 224 242 Temperature Characteristics of Curing Exotherm (°C) Peak 234 247 End 246 253 Polymerization Heat (J/g) 310 295

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Table 3. Elemental analysis data for benzoxazine monomers.

The content of elements in benzoxazine monomers (Table3), taking into account the measurement error, converges with the theoretical calculation, therefore, pure monomers were obtained. The obtained benzoxazine monomers are polymerized by the thermal method with the formation of an insoluble thermosetting polymer according to the scheme shown in Figure 8 (Figure S3 in Supplementary Materials).

3.2. Properties of Diamine-Based Benzoxazine Monomers The curing kinetics of various diamine-based benzoxazine monomers was investigated by using differential scanning calorimetry (DSC) (Figure 8 and Figure S3, Table4). The curves show that P-d polymerization proceeds under milder conditions than P-q. This may be due to the fact that the aromatic rings of the diamine in P-q are deactivated by chlorine atoms; therefore, the addition to the ortho position relative to the nitrogen atom will be somewhat difﬁcult both sterically and energetically.

Table 4. Differential scanning calorimetry results.

Parameter P-d P-q Onset 224 242

Temperature Characteristics of Curing Exotherm (◦C) Peak 234 247 End 246 253 Polymerization Heat (J/g) 310 295

This statement is in a good agreement with early studies by Ishida and colleagues [40,41], who showed that benzoxazine monomers based on bisphenol A and aromatic amines polymerize not only according to the standard scheme with the formation of phenolic polymers with a Mannich bridge, but also with the participation of the p-position of the aromatic amine residue. The structure of the aromatic amine is important here. Amines with an activated aromatic ring (that is, with electron-donor substituents that increase the electron density in the o- and p-positions relative to the amino group) significantly increase the network density, while deactivated amines with a methyl group in the o- and p- positions, on the contrary, noticeably reduce it (Figure6, Figure 8 and Figure S3) [ 42]. In other words, if benzoxazine monomer has an electron-donor substituent in the m-position relative to the nitrogen atom or has no substituents in the aromatic ring of the diamine, then polymerization proceeds under milder conditions (lower temperature of the onset of polymerization and lower enthalpy of the curing reaction) and higher cross-link density and glass transition temperature are achieved in the resulting polymer (Figure 14). Polymers 2021, 13, x FOR PEER REVIEW 8 of 20

This statement is in a good agreement with early studies by Ishida and colleagues [40,41], who showed that benzoxazine monomers based on bisphenol A and aromatic amines polymerize not only according to the standard scheme with the formation of phenolic polymers with a Mannich bridge, but also with the participation of the p-position of the aromatic amine residue. The structure of the aromatic amine is important here. Amines with an activated aromatic ring (that is, with electron-donor substituents that increase the electron density in the o- and p-positions relative to the amino group) significantly increase the network density, while deactivated amines with a methyl group in the o- and p- positions, on the contrary, noticeably reduce it (Figures 6, 8 and S3) [42]. In other words, if benzoxazine monomer has an electron-donor substituent in the m-position relative to the nitrogen atom or has no substituents in the aromatic ring of the diamine, then polymerization proceeds under milder conditions (lower temperature of the onset of Polymers 2021, 13, 1421 polymerization and lower enthalpy of the curing reaction) and higher cross-link density8 of 19 and glass transition temperature are achieved in the resulting polymer (Figure 14).

4 potential reactive sites 2 deactivated potential reactive sites P-d P-q

Figure 6. Possible reactive sites in the aromatic ring of the diamines and inﬂuenceinfluence of substituents.

In this case case the the P-q P-q monomer monomer has has an an electr electron-withdrawingon-withdrawing chlorine in o-position and ◦ ◦ as a result polyP-q has reduced TTgg of 182 °CC compared compared to polyP-d’s 190 °C.C. Studies [[43,44]43,44] report that the presence of electro-acceptor groups in amine or phenol aromatic ring can significantlysignificantly affect the conditions for obtaining the benzoxazine monomer, and also can increase the polymerization temperature, which is observed during during the the curing curing of of P-q, P-q, too. too. To confirmconfirm the stability of thethe benzoxazinebenzoxazine monomers during curing in the selected mode, a thermogravimetric analysis both in air and argon atmosphere was carried out (Figure7 7).). Figures8 and9 show that the temperature of 5% mass loss for monomers P-d and P-q in air 407 ◦C and 333 ◦C, respectively, and in argon, 366 ◦C and 327 ◦C, respectively. These temperatures indicate the stability of the materials under the selected curing mode. The char yield at 800 ◦C for P-d and P-q in argon was 45% and 50%, respectively; in air, both monomers burned out without residue at 800 ◦C. To determine the optimal curing mode of the obtained P-q, as well as the possibility of using it in the production of polymer composite materials and the processing method, we measured temperature profile of viscosity at a heating rate of 2 ◦C/min (Figure8). It can be seen that the obtained benzoxazine monomer has a rather wide technological window (115–225 ◦C) at a low viscosity (<1 Pa·s), which makes it possible to process this monomer by the vacuum infusion method. Figure8 represents the comparison of the viscosity and DSC curves at the same heating rate of 2 ◦C/min. The dramatic increase in viscosity that corresponds to the gelation is observed right after the peak of DSC curve. To evaluate the melting stability of P-q, viscosity curves were obtained in rotational mode at 130 ◦C for 2 h at a shear rate 200 rpm. Figure9 shows that the P-q viscosity increase 2 times within 2 h; however, its values still remain below 1 Pa·s. While viscosity of P-d monomer was not altered in a distinguishable way. This aspect contributes to the possibility of the vacuum infusion processing of monomers having P-d base more preferred than P-q.

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100 P-d P-q 90 80 a) 70

50 Residual weight (%) 40 100 200 300 400 500 600 700 800 Temperature (oC)

100

90 b)

40 Residual weight (%) 30

100 200 300 400 500 600 700 800 o Temperature ( C) Figure 7. TGA curves of benzoxazine monomers P-d and P-q: (a) in the argon; ((b)) inin thethe airair atmosphere.atmosphere.

Figures 8 and 9 show that the temperature of 5% mass loss for monomers P-d and P- q in air 407 °C and 333 °C, respectively, and in argon, 366 °C and 327 °C, respectively. These temperatures indicate the stability of the materials under the selected curing mode. The char yield at 800 °C for P-d and P-q in argon was 45 and 50%, respectively; in air, both monomers burned out without residue at 800 °C.

PolymersPolymersPolymers 2021 20212021, ,13, 1313, ,x, x 1421FOR FOR PEER PEER REVIEW REVIEW 101010 of of of 20 20 19

((aa)) ((bb))

FigureFigure 8. 8. The TheThe comparison comparison comparison of of viscosity viscosity (black) (black) and and DSC DSC (red) (red) curves curves of of P-q P-q ( (aa)) and and P-d P-d ( (bb)) at at a a heating heating rate rate 2 2 °C/min. ◦°C/min.C/min.

1.51.5 P-qP-q 1.01.0 P-dP-d

0.50.5 Dynamic Viscosity (Pa · s) · (Pa Viscosity Dynamic Dynamic Viscosity (Pa · s) · (Pa Viscosity Dynamic 1010 30 30 50 50 70 70 90 90 110 110 TimeTime (min)(min) Polymers 2021, 13, x FOR PEER REVIEW 11 of 20 Figure 9. Change in the viscosity of benzoxazine monomers at 130 ◦C. FigureFigure 9. 9. Change Change in in the the viscosity viscosity of of benzoxazine benzoxazine monomers monomers at at 130 130 °C. °C. 3.3. Thermal Properties and Fire Resistance of Diamine-Based Polybenzoxazines ToTo determine determine the the optimal optimal curing curing mode mode of of the the obtained obtained P-q, P-q, as as well well as as the the possibility possibility BenzoxazineBenzoxazine monomersmonomers werewere curedcured accordingaccording to to the the following following regime: regime: 22 hh atat 180180◦ °C,C, ofof using using it it in in the the production production of of polymer polymer co compositemposite materials materials and and the the processing processing method, method, 44 hh atat 200200◦ °C,C, 22 hh atat 220220◦ °C;C; allall samplessamples werewere degasseddegassed at at 130 130◦ °CC forfor 11 hh beforebefore curing. curing. The The wewe measuredmeasured temperaturetemperature profileprofile ofof viscosityviscosity atat aa heatingheating raterate ofof 22 °C/min°C/min (Figure(Figure 8).8). ItIt polymerizationpolymerization schemescheme isis shownshown inin FigureFigure 1010.. TheThe oxazineoxazine ringsrings areare openedopened toto formform aa cancan be be seen seen that that the the obtained obtained benzoxazine benzoxazine monomer monomer has has a a rather rather wide wide technological technological win- win- crosslinkedcrosslinked polymerpolymer underunder thethe high high temperature. temperature. dowdow (115–225 (115–225 °C) °C) at at a a low low viscosity viscosity (<1 (<1 Pa·s), Pa·s), wh whichich makes makes it it possible possible to to process process this this mon- mon- omeromer by by the the vacuum vacuum infusion infusion method. method. FigureFigure 8 8 represents represents the the comparison comparison of of the the viscosity viscosity and and DSC DSC curves curves at at the the same same heat- heat- inging rate rate of of 2 2 °C/min. °C/min. The The dramatic dramatic increase increase in in viscosity viscosity that that corresponds corresponds to to the the gelation gelation is is observedobserved right right after after the the peak peak of of DSC DSC curve. curve. ToTo evaluate evaluate the the melting melting stability stability of of P-q, P-q, viscosity viscosity curves curves were were obtained obtained in in rotational rotational modemode atat 130130 °C°C forfor 22 hh atat aa shearshear raterate 200200 rpm.rpm. FigureFigure 99 showsshows thatthat thethe P-qP-q viscosityviscosity in-in- creasecrease 2 2 times times within within 2 2 h; h; however, however, its its values values still still remain remain below below 1 1 Pa·s. Pa·s. While While viscosity viscosity of of P-dP-d monomer monomer was was not not altered altered in in a a distinguishabl distinguishablee way. way. This This aspect aspect contributes contributes to to the the pos- pos- sibilitysibility of of the the vacuum vacuum infusion infusion processing processing of of monomers monomers having having P-d P-d base base more more preferred preferred thanthan P-q. P-q.

3.3.3.3. Thermal Thermal Properties Properties and and Fire Fire Resistan Resistancece of of Diamine-Based Diamine-Based Polybenzoxazines Polybenzoxazines FigureFigure 10.10.Scheme Scheme ofof polymerizationpolymerization ofof benzoxazinebenzoxazine monomers.monomers.

Elemental analysis of the obtained polymers (Table 5) showed that the only target polymerization reaction takes place during polymerization; that is to say, destruction does

not occur with the release of byproducts.

Table 5. Elemental analysis data for polybenzoxazines and char residue.

Found, % Calculated, % Elements By Elemental Analysis Surface X-Ray Spectrometry 1 Polymer Char Polymer Char Polymer Char P-d C 80.16 - 79.97 87.78 78.68 91.94 H 6.03 - 5.99 2.05 - - O 7.36 - 7.79 2 4.41 2 21.32 8.06 N 6.45 - 6.25 5.76 - - P-q C 69.19 - 68.52 89.30 73.53 91.81 H 4.81 - 4.80 1.59 - - O 6.36 - 6.64 2 2.77 2 23.83 7.64 N 5.56 - 5.63 4.87 - - Cl 14.08 - 14.41 1.47 2.64 0.56 1 Measurement was carried out using scanning-electron microscope (Jeol JSM-U3, Tokyo, Japan) with energy dispersive X-ray spectrometer (Eumex, Heidenrod, Germany) in accelerating voltage of 15 kV. 2 These values are obtained by difference.

The glass transition temperatures (Tg) of the obtained cured samples were measured by DSC at a heating rate 10 °C/min. It turned out that Tg for P-d is higher than for P-q, being 190 °C and 182 °C, respectively (Figure 11). By the character of DSC curves we could define that the beginnings of the thermal destruction of the P-q and P-d polymers are very close to each other in the region of 250 °C. However, the dramatic rise of DSC heat flow of P-q indicates that thermal stability of P-q at temperatures 250–300 °C is lower than that of P-d.

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Elemental analysis of the obtained polymers (Table5) showed that the only target polymerization reaction takes place during polymerization; that is to say, destruction does not occur with the release of byproducts.

Table 5. Elemental analysis data for polybenzoxazines and char residue.

Found, % Calculated, % Elements By Elemental Analysis Surface X-ray Spectrometry 1 Polymer Char Polymer Char Polymer Char P-d C 80.16 - 79.97 87.78 78.68 91.94 H 6.03 - 5.99 2.05 - - O 7.36 - 7.79 2 4.41 2 21.32 8.06 N 6.45 - 6.25 5.76 - - P-q C 69.19 - 68.52 89.30 73.53 91.81 H 4.81 - 4.80 1.59 - - O 6.36 - 6.64 2 2.77 2 23.83 7.64 N 5.56 - 5.63 4.87 - - Cl 14.08 - 14.41 1.47 2.64 0.56 1 Measurement was carried out using scanning-electron microscope (Jeol JSM-U3, Tokyo, Japan) with energy dispersive X-ray spectrometer (Eumex, Heidenrod, Germany) in accelerating voltage of 15 kV. 2 These values are obtained by difference.

The glass transition temperatures (Tg) of the obtained cured samples were measured by ◦ DSC at a heating rate 10 C/min. It turned out that Tg for P-d is higher than for P-q, being 190 ◦C and 182 ◦C, respectively (Figure 11). By the character of DSC curves we could define Polymers 2021, 13, x FOR PEER REVIEW 12 of 20 that the beginnings of the thermal destruction of the P-q and P-d polymers are very close to each other in the region of 250 ◦C. However, the dramatic rise of DSC heat flow of P-q indicates that thermal stability of P-q at temperatures 250–300 ◦C is lower than that of P-d.

FigureFigure 11. 11. DSCDSC curves curves of of the the cured cured P-q P-q and and P-d. P-d.

TheThe determination determination of of the the fire-resistance ﬁre-resistance acco accordingrding to the UL-94 UL-94 standard standard showed that that polyP-dpolyP-d has aa categorycategory V-1,V-1, while while polyP-q polyP-q has has an an increased increased resistance resistance to burningto burning and and so can so canbe classiﬁed be classified in the in V-0the categoryV-0 category (Table (Table6). In 6). comparison In comparison with polyP-d,with polyP-d, the quamine-based the quamine- based polymer is characterized by near-zero burning times, even after several flame applications. The examples of the tested samples are presented in Figure 12. The multicellular foam on the surface of the polyP-q samples seems to an the insulating effect resulting in the limitation of the diffusion of oxygen to the surface of the underlying material. The obtained flammability experimental data are in good agreement with the calculated data on the limiting oxygen index (LOI) according to the Van Crevelen–Hovtyzer rule [45] (1): LOI = 17.5 + 0.4···CY (1) where CY is the coke yield according to the TGA data.

Table 6. Thermal properties of benzoxazines based on diamines.

Parameter PolyP-d PolyP-q Tg (°C) a 190 182 τ1, s 18 5 τ2, s 10 0 τ3, s 7 2 τ4, s 20 1 UL-94 τ5, s 5 0 τtot, s 60 8 Afterglow, s No No Particles or Drops No No Flammability Rating V-1 V-0 LOI b 36 40 TGA in Argon T5% (°C) 395 357 T10% (°C) 416 380 W800 (%) 46 57 W900 (%) 45 57 TGA in Air T5% (°C) 423 375 T10% (°C) 445 395

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polymer is characterized by near-zero burning times, even after several ﬂame applications. The examples of the tested samples are presented in Figure 12. The multicellular foam on the surface of the polyP-q samples seems to an the insulating effect resulting in the limitation of the diffusion of oxygen to the surface of the underlying material. The obtained ﬂammability experimental data are in good agreement with the calculated data on the limiting oxygen index (LOI) according to the Van Crevelen–Hovtyzer rule [45] (1):

LOI = 17.5 + 0.4··· CY (1)

where CY is the coke yield according to the TGA data.

Table 6. Thermal properties of benzoxazines based on diamines.

Parameter PolyP-d PolyP-q ◦ a Tg ( C) 190 182

τ1, s 18 5

τ2, s 10 0

τ3, s 7 2

τ4, s 20 1

UL-94 τ5, s 5 0

τtot, s 60 8 Afterglow, s No No Particles or Drops No No Flammability Rating V-1 V-0 LOI b 36 40 TGA in Argon ◦ T5% ( C) 395 357 ◦ T10% ( C) 416 380

W800 (%) 46 57

W900 (%) 45 57 TGA in Air ◦ T5% ( C) 423 375 ◦ T10% ( C) 445 395

W800 (%) 14 5

W900 (%) 12 1 a Measured by DSC (Supplementary Materials). b Calculated by Van Krevelen–Hovtyzer equation (char yield at 800 ◦C values were used for calculation) [45].

One of the limiting factors of plastics’ high-temperature usage is their tendency not only to soften, but also to undergo the thermally induced degradation. Thermal degradation is the upper limit of the polymer’s operating temperature; above this temperature polymers can degrade with the formation of low-molecular-weight products that can change their properties. The study of polybenzoxazines’ decomposition allows us to understand nature of the thermal stability of the material and prompt us to create new structures with greater thermal resistance. It was proposed that the thermal decomposition of polybenzoxazines occurs step- wise [46–48]. At the ﬁrst stage of the destruction, aromatic compounds are formed (benzene, derivatives of phenol, aniline). In the second step—low-molecular compounds (hydrocar- bons, carbon dioxide, aliphatic amines, etc.), followed by carbonization. Polymers 2021, 13, x FOR PEER REVIEW 13 of 20

W800 (%) 14 5 Polymers 2021, 13, 1421 W900 (%) 12 1 13 of 19 a Measured by DSC (Supplementary Materials). b Calculated by Van Krevelen–Hovtyzer equation (char yield at 800 °С values were used for calculation) [45].

(a) (b) Polymers 2021, 13, x FOR PEER REVIEW 14 of 20 Figure 12. Photos of the tested samples of polyP-q (a) and polyP-d (b).

One of the limiting factors of plastics’ high-temperature usage is their tendency not 100 only to soften, but also to undergo the thermally induced degradation. Thermal degradation is the upper limit of the polymer’s operating temperature; above this temperature 90 polymers can degrade with the formation of low-molecular-weightP-q products that can 80 change their properties. P-d a) The study of polybenzoxazines’ decomposition allows us to understand nature of the 70 thermal stability of the material and prompt us to create new structures with greater ther- 60 mal resistance. It was proposed that the thermal decomposition of polybenzoxazines occurs step- 50 wise [46–48]. At the first stage of the destruction, aromatic compounds are formed (ben- Residual weight (%) 40 zene, derivatives of phenol, aniline). In the second step—low-molecular compounds (hy- drocarbons, carbon dioxide, aliphatic amines, etc.), followed by carbonization. 30 In the present work, thermogravimetric analysis with a mass-detector was carried 100out. According200 to300 the TGA400 data (Figure500 13600 and Table 700 6), the800 5% and900 10% mass-loss of polyP-q occurs at lowerTemperature temperatures (oC) compared to polyP-d. However, probably due to the presence of chlorine atoms, polyP-q has a higher char yield. 100 b) 90

30 Residual weight (%) 20

100 200 300 400 500 600 700 800 900 Temperature (oC)

FigureFigure 13. TGA 13. TGA curves curves of polyP-d of polyP-d and and polyP-q: polyP-q: (a) ( ina) thein the argon; argon; (b )(b in) in the the air air atmosphere. atmosphere.

The structures formed during thermal destruction of polyP-q are described below (Table 7).

Table 7. The results of TG-MS analysis of volatile products of polyP-q in air and inert atmosphere.

№ Probable Structure m/z Calculated m/z Observed Intensity × 1012 Fraction a, % T, °C Air 1 +CH2–N–CH2+ 43.04 2 +CH2N = CH2 42.03 42.3 24 10.52 3 CH3N = CH2 43.04 343 4 CH3Cl 49.99 50.4 0.25 0.11 5 HClO 51.97 52.3 0.1 0.04 6 CO2 43.99 44.2 200 87.66 7 CH3CH2NH2 45.06 45.3 2.8 1.23 700 8 NO2 45.99 46.2 1 0.44 Argon 9 CO2 43.99 44.2 5.8 18.87 373

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In the present work, thermogravimetric analysis with a mass-detector was carried out. According to the TGA data (Figure 13 and Table6), the 5% and 10% mass-loss of polyP-q occurs at lower temperatures compared to polyP-d. However, probably due to the presence of chlorine atoms, polyP-q has a higher char yield. The structures formed during thermal destruction of polyP-q are described below (Table7).

Table 7. The results of TG-MS analysis of volatile products of polyP-q in air and inert atmosphere.

№ Probable Structure m/z Calculated m/z Observed Intensity × 1012 Fraction a, % T, ◦C Air + + 1 CH2–N–CH2 43.04 + 2 CH2N = CH2 42.03 42.3 24 10.52

3 CH3N = CH2 43.04 343

4 CH3Cl 49.99 50.4 0.25 0.11 5 HClO 51.97 52.3 0.1 0.04

6 CO2 43.99 44.2 200 87.66 700 7 CH3CH2NH2 45.06 45.3 2.8 1.23

8 NO2 45.99 46.2 1 0.44 Argon 5.8 373 9Polymers 2021, 13, x COFOR2 PEER REVIEW 43.99 44.2 18.87 15 of 20 8.6 445 3.0 373 50.2 3.2 438 3.98.6 445 445 10 CH3Cl 49.99 3.0 26.86 373 3.0 373 50.2 3.2 438 51.3 3.4 438 3.9 445 10 CH3Cl 49.99 26.86 4.03.0 445 373 51.3 2.9 3.4 373 438 + 11 CH = CH2CN 52.02 52.3 3.34.0 12.19 438 445 3.12.9 445 373 11 +CH = CH2CN 52.02 52.3 3.3 12.19 373 438

+ 3.1 445 12 CH2CH2Cl 63.0 63.3 3.0 3.93 438 373 445 12 +CH2CH2Cl 63.0 63.3 3.0 3.93 438 13 CH3CH2Cl 64.01 3.5 4.59 438 65.3 445 14 CH3OCl 65.99 4.2 5.50 445 13 CH3CH2Cl 64.01 3.5 4.59 438 + 65.3 1514 CHCH2NH3OCl3Cl 66.02 65.99 66.8 3.5 4.2 4.59 5.50 445 445 1615 HOCH+CH2NHCH2NH3Cl2OH 77.0566.02 66.8 3.5 4.59 438 445 77.4 3.5 4.59 1716 HOCH C2NHCHH + 2OH 77.0477.05 445 438 6 5 77.4 3.5 4.59 6 5+ 1817 C C6HH6 78.0577.04 78.4 3.6 4.72 438 445 18 C6H6 78.05 78.4 3.6 4.72 438 19 CH3CH2N = NCH2CH3 86.08 85.4 2.6 3.41 373 19 CH3CH2N = NCH2CH3 86.08 85.4 2.6 3.41 373 20 C6H5NH2 93.06 438 20 C6H5NH2 93.06 93.3 2.8 3.67 438 21 C6H5CH3 92.06 93.3 2.8 3.67 445 21 C6H5CH3 92.06 445 22 C H OH 94.04 95.5 2.5 3.28 373 22 C6H5OH 94.04 95.5 2.5 3.28 373

2323 98.1198.11 98.198.1 2.92.9 3.803.80 445 445

a a Volatile Volatile product product fraction isfraction equal to is the equal ratio ofto thethe intensity ratio of of the the intensity given substance of the togiven the sum substa of allnce intensities to the sum at whole of all temperature intensities range. at whole temperature range.

The result obtained is generally in satisfactory agreement with previously published studies. It can be noted that, both in air and in an inert atmosphere in the range of 300– 500 °C, the destruction of benzoxazine cycles occurs predominantly and a small amount of potentially toxic chlorine-containing substances is released. As can be seen from Table 5, at low temperatures from 343 °C and 700 °C in air, compounds are formed containing chlorine: HClO and CH3Cl are formed with fractions 0.04% and 0.11%, respectively. More compounds containing chlorine are formed in an argon atmosphere at different temperatures (373–445 °C). For example, CH3Cl (26.86%), +CH2CH2Cl (3.93%), CH3CH2Cl (4.59%), CH3OCl (5.5%), +CH2CH2NH3Cl (4.59%). Above 500 °C, there is almost no mass loss in the inert atmosphere. In air, thermo-oxidative destruction is most intense at 700 °C. The surface of the fractured polymer sample and char yield were examined using SEM. Figure 14a shows that the polymer has glassy bulky surface with no specific morphological features. The SEM image of the char residue (Figure 14b) showed that during the burning of polybenzoxazine, a dense foam protective layer with a pore diameter from 2.3 to 60.6 µm is formed, which prevents the polymer from further burning.

Polymers 2021, 13, 1421 15 of 19

The result obtained is generally in satisfactory agreement with previously published studies. It can be noted that, both in air and in an inert atmosphere in the range of 300–500 ◦C, the destruction of benzoxazine cycles occurs predominantly and a small amount of potentially toxic chlorine-containing substances is released. As can be seen from Table5, at low temperatures from 343 ◦C and 700 ◦C in air, compounds are formed containing chlorine: HClO and CH3Cl are formed with fractions 0.04% and 0.11%, respectively. More compounds containing chlorine are formed in an argon atmosphere at different ◦ + temperatures (373–445 C). For example, CH3Cl (26.86%), CH2CH2Cl (3.93%), CH3CH2Cl + ◦ (4.59%), CH3OCl (5.5%), CH2CH2NH3Cl (4.59%). Above 500 C, there is almost no mass loss in the inert atmosphere. In air, thermo-oxidative destruction is most intense at 700 ◦C. The surface of the fractured polymer sample and char yield were examined using SEM. Figure 14a shows that the polymer has glassy bulky surface with no speciﬁc morphological features. The SEM image of the char residue (Figure 14b) showed that during the burning Polymers 2021, 13, x FOR PEER REVIEWof polybenzoxazine, a dense foam protective layer with a pore diameter from 2.3 to 60.6 µ16m of 20

is formed, which prevents the polymer from further burning.

60.6 µm

2.3 µm

11.5 µm

11.5 µm 11.5 µm 24.6 µm

15.4 µm

(a) (b)

FigureFigure 14. 14.SEM SEM analysis analysis of polybenzoxazineof polybenzoxazine polyP-q polyP-q (a) and(a) and char char yield yield (b) ( surfaces.b) surfaces.

It isIt alsois also known known that that halogen halogen radicals radicals compete compete forfor thethe radical species species HO· HO ·andand H· H that· thatare are critical critical for for flame flame propagation propagation [49,50]. [49,50 Thus,]. Thus, the thehigher higher fire fireresistance resistance of polyP-q of polyP- com- q comparedpared to polyP-d to polyP-d can canbe explained be explained by two by twofactors. factors. The Thefirst firstfactor factor is the is release the release of low ofmolecular low molecular weight weight products, products, such as such CO2 as, NO CO2, 2CH, NO3Cl2 ,and CH HClO,3Cl and which HClO, cause which system cause cool- systeming and cooling removing and removing of high-energy of high-energy HO· and HO H·· radicals.and H· radicals. The second The factor second is factorthe formation is the formationof the char of thelayer char on layerthe polymer on the polymersurface, whic surface,h prevents which oxygen prevents pene oxygentration penetration into the bulk intoof thethe bulksample of the and sample thereby and stops thereby combustion. stops combustion. TheThe notable notable feature feature of of polyP-q polyP-q is is its its higher higher thermalthermal resistanceresistance in air, air, compared compared to to the theresults results in in argon argon (Figure (Figure 13). 13 ).This This behavior behavior usually usually occurs occurs when when thermo-oxidative thermo-oxidative reactions with the participation of atmospheric oxygen occur additionally in air, leading to the formation of densely crosslinked products. This is also confirmed by the results of TGA- MS analysis (Table 7). In an oxygen atmosphere at a temperature of about 340 °C, active particles (cations) are formed, which can additionally cross-link the polymer network. In an argon atmosphere, these particles are also formed, but already at higher temperatures. The formation of a char layer and small pores can be observed in the SEM images of the samples tested according to the UL-94 standard and the surface of the char residue examined using SEM (Figures 12 and 14). Quamine-based polybenzoxazine char residue was investigated by FTIR (Figure 15). It showed stretching vibrations of the C–Cl (800–700 cm–1) bond after burning of the polymer sample. This observation is consistent with relatively low content of chlorine-containing ions during TGA and with the formation of a dense char residue in the UL-94 vertical test and presumably indicates the cessation of combustion on the sample surface without the flame penetrating deep into the sample.

Polymers 2021, 13, 1421 16 of 19

reactions with the participation of atmospheric oxygen occur additionally in air, leading to the formation of densely crosslinked products. This is also conﬁrmed by the results of TGA- MS analysis (Table7). In an oxygen atmosphere at a temperature of about 340 ◦C, active particles (cations) are formed, which can additionally cross-link the polymer network. In an argon atmosphere, these particles are also formed, but already at higher temperatures. The formation of a char layer and small pores can be observed in the SEM images of the samples tested according to the UL-94 standard and the surface of the char residue examined using SEM (Figures 12 and 14). Quamine-based polybenzoxazine char residue was investigated by FTIR (Figure 15). It showed stretching vibrations of the C–Cl (800–700 cm−1) bond after burning of the polymer sample. This observation is consistent with relatively low content of chlorine-containing ions during TGA and with the formation of a dense char residue in the UL-94 vertical test Polymers 2021, 13, x FOR PEER REVIEWand presumably indicates the cessation of combustion on the sample surface without17 of the 20

ﬂame penetrating deep into the sample.

800–700 Absorbance

4000 3000 2000 1000 –1 Wavenumder (cm ) FigureFigure 15.15. FTIR spectrum ofof thethe charchar residueresidue ofof P-q-basedP-q-based polybenzoxazine.polybenzoxazine.

BasedBased onon thethe data data of of elemental elemental analysis analysis and and IR IR spectroscopy spectroscopy of theof the coke coke residue residue (Table (Ta-5, Figureble 5, Figure 15), it can 15), be it seencan be that seen the that residue the containsresidue acontains small amount a small of amount chlorine of atoms. chlorine According atoms. toAccording the TGA-MS to the data TGA-MS in air (Table data7 in), itair can (Table be seen 7), thatit can only be 0.11%seen that and only 0.04% 0.11% of the and chlorine- 0.04% containingof the chlorine-containing products CH3Cl andproducts HClO CH are3Cl formed, and HClO respectively. are formed, The data respectively. obtained indicate The data a lowobtained lability indicate of chlorine a low atoms lability in the of compositionchlorine atoms of polybenzoxazine, in the composition which of polybenzoxazine, is a positive factor inwhich terms is of a safetypositive and factor environmental in terms of friendliness. safety and environmental friendliness. TheThe lowerlower temperaturetemperature ofof thethe onsetonset ofof polyP-qpolyP-q decompositiondecomposition inin anan inertinert atmosphereatmosphere isis mostmost likelylikely duedue toto thethe easiereasier cleavagecleavage ofof bondsbonds inin thethe polymerpolymer duedue toto thethe presencepresence ofof electron-withdrawingelectron-withdrawing chlorinechlorine atomsatoms inin thethe diaminediamine structure.structure. OnOn the contrary, thethe higherhigher thermalthermal resistance of of polyP-q polyP-q in in air air indicates indicates a agreater greater proportion proportion of ofcrosslinking crosslinking and and re- resultedsulted char char formation. formation. This This fact fact is is indirectly indirectly confirmed confirmed by by the the presence of aa significantsignificant amountamount ofof residualresidual chlorinechlorine inin thethe charchar residue,residue, asas wellwell asas a relatively lowlow chlorinechlorine contentcontent inin thethe combustioncombustion products.products. Thus, the formation of char probably plays a major role in the mechanism of combus- Thus, the formation of char probably plays a major role in the mechanism of combustion and thermooxidative destruction of polyP-q. tion and thermooxidative destruction of polyP-q. 4. Conclusions 4. Conclusions A new, previously not reported in the literature benzoxazine monomer based on 3,30-dichloro-4,4A new, previously0-diaminodiphenylmethane not reported in the literature (quamine) benzoxazine and phenol monomer (P-q), was based synthesized on 3,3′- indichloro-4,4 a high yield′-diaminodiphenylmethane (90–95%). P-q has a fairly (quamine) wide technological and phenol (P-q), window was withsynthesized a viscosity in a ofhigh less yield than (90%–95%). 1 Pa·s at a temperatureP-q has a fairly range wide of technological 120–200 ◦C, sowindow it can bewith used a viscosity as a binder of less in thethan production 1 Pa·s at a oftemperature polymer composite range of 120–200 materials °C, by so vacuum it can be infusion used as ora binder RTM. Compared in the pro- toduction the well-known of polymer commercialcomposite materials benzoxazine by va monomercuum infusion P-d (obtained or RTM. fromCompared phenol to and the 4,4well-known0-diaminodiphenylmethane), commercial benzoxazine this new monome polymerr P-d has (obtained a higher charfrom yield phenol (57% and for 4,4 polyP-′-dia- qminodiphenylmethane), and 46% for polyP-d atthis 800 new◦C) polymer and UL-94 has V-0a higher fire resistancechar yield rating.(57% for The polyP-q obtained and 46% for polyP-d at 800 °C) and UL-94 V-0 fire resistance rating. The obtained results indicate the possibility to applicate P-q as a co-monomer in the production of fire-resistant binders for polymer composite materials, except for areas where the use of halogen-containing flame-retardants is not allowed.

Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: 1H NMR spectrum of benzoxazine based on 4,4′-diaminodiphenylmethane in toluene/isopropanol 2:1, Figure S2: 13C NMR spectrum of benzoxazine based on 4,4′-diaminodiphenylmethane in toluene/isopropanol 2:1, Figure S3: The DSC curves describing the curing process of diamines-based benzoxazines (heating rate 10 deg/min), Figure S4: DSC curves of polybenzoxazines based on diamines (heating rate 10 deg/min), Figure S5: Mass spectrum of degradation of polybenzoxazine P-q in air at 343 °C, Figure S6: Mass spectrum of degradation of polybenzoxazine P-q in argon at 373 °C, Figure S7: Mass spectrum of degradation of polybenzoxazine P-q in argon at 438 °C, Figure S8: Mass spectrum of degradation of polybenzoxazine P-q in argon at 445 °C. Author Contributions: I.S.S. and V.V.P.–planning of the experiments, writing of the manuscript; I.A.S., V.V.P., N.V.P. and A.A.K.–synthesis of monomers and polymers, interpretation of the results;

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results indicate the possibility to applicate P-q as a co-monomer in the production of ﬁre-resistant binders for polymer composite materials, except for areas where the use of halogen-containing ﬂame-retardants is not allowed.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/polym13091421/s1, Figure S1: 1H NMR spectrum of benzoxazine based on 4,40- diaminodiphenylmethane in toluene/isopropanol 2:1, Figure S2: 13C NMR spectrum of benzoxazine based on 4,40-diaminodiphenylmethane in toluene/isopropanol 2:1, Figure S3: The DSC curves describing the curing process of diamines-based benzoxazines (heating rate 10 deg/min), Figure S4: DSC curves of polybenzoxazines based on diamines (heating rate 10 deg/min), Figure S5: Mass spectrum of degradation of polybenzoxazine P-q in air at 343 ◦C, Figure S6: Mass spectrum of degradation of polybenzoxazine P-q in argon at 373 ◦C, Figure S7: Mass spectrum of degradation of polybenzoxazine P-q in argon at 438 ◦C, Figure S8: Mass spectrum of degradation of polybenzoxazine P-q in argon at 445 ◦C. Author Contributions: I.S.S. and V.V.P.—planning of the experiments, writing of the manuscript; I.A.S., V.V.P., N.V.P. and A.A.K.—synthesis of monomers and polymers, interpretation of the results; N.V.B., V.V.S. and D.V.O.—DSC analysis and rheology; A.V.S. and R.R.K.—Elemental analysis and TGA-MS; M.V.G.—editing of the manuscript, I.S.S. and V.V.K.—conceptualization, general manage- ment and editing of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the Russian Science Foundation under grant 19-73-10204. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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