Synthesis, Modification and Characterization of New Phenolic Resins linked to Tetrabromophthalimide

The present work aimed to synthesize new phenol resins via incorporation of structural modification through introducing new phenolic compounds containing cyclic imide moiety in reaction with formaldehyde. The synthesis of these new resins involved three steps. First, one of the three N - (hydroxyphenyl)tetrabromophtalamic acids 1 - 3 were processed via a reaction between tetrabromophthalic anhydride and aminophenols. Amic acids 1 - 3 were dehydrated in the second step by smelting, producing the identical N - (hydroxyphenyl)tetrabromophthalimides 4 - 6 . The new imides represent the new phenolic component that was presented in condensation reaction with formaldehyde in the third step, creating the target phenolic resins 7 - 9 . The work also involved curing the new resins through esterification of phenolic OH groups by treatment with benzoyl chloride. The chemical structures of prepared compounds were confirmed according to FT-IR, 1 H NMR, and 13 C NMR spectral data. As a conclusion, the present work supply of new phenolic resins and the presence of the cyclic imide (tetrabromophthalimide) moiety in their structures exhibit high softening points and resistance to solubility, which fit with some applications, while subsequent curing through esterification exhibits better solubility and lower softening points, which fit with other applications.


Introduction
Phenolic resins are the most significant thermosetting resins produced via acid-or basecatalyzed polycondensation of formaldehyde and phenolic compounds [1][2][3]. Because of their excellent heat properties, strong mechanical strength, water resistance, heat resistance, and chemical stability [4,5], these resins have been extensively applied in many applications like automotive, aerospace, varnishes for wood, adhesives, phenolic foam, and surface coatings [6][7][8][9]. Because of all these important properties and the wide spectrum of various applications, it is not surprising that a great number of studies and research have been conducted to incorporate structural modifications [10] using other aldehydes or different phenols [11] or adding different additives [12] to acquire new resins having new properties. Three new phenolic resins bearing a tetrabromophthalimidyl moiety were synthesized in this work by polycondensation of N-(hydroxyphenyl)tetrabromophthalimide with formaldehyde under conditions similar to those used in resol preparation. The synthesized phenolic resins were adjusted by esterification with benzoyl chloride in the presence of pyridine. Both the newly synthesized and modified resins exhibit good properties that may serve different applications, like plasticization and the application of nonflammable materials or as flame-retardants.

Experimental
All the used synthetic compounds were purchased from Aldrich, Merck, and BDH companies. The FT-IR spectral data of the combined mixtures and resins were recorded using KBr discs on a Shimadzu FTIR-8400 Fourier Infrared Transform spectrophotometer. The 1 H NMR and 13 C NMR spectral data were registered on a Bruker 400 MHz apparatus utilizing tetramethylsilane as the internal standard and DMSO-d6 as a solvent. The melting points of the prepared compounds were not entirely settled by Gallenkamp capillary softening point equipment, while the melting points of synthesized and modified resins were determined by a thermal microscope Riecher thermover.

Synthesis of N-(hydroxyphenyl)tetrabromophthalamic acids 1-3
A solution of tetrabromophthalic anhydride (2.32 g, 5 mmol) in dry acetone (20 mL) was added dropwise to ortho-, meta-, and para-aminophenol (0.54 g, 5 mmol), respectively, in dry acetone (10 mL) with stirring and cooling [11,13]. After completion of the addition, the mixture was stirred at room temperature for two hours. The solid crude material was then filtrated, washed with dry ether, and recrystallized in a suitable solvent.

Synthesis of N-(hydroxyphenyl)tetrabromophthalimides 4-6
The titled compounds 4-6 were synthesized by dehydrating the phthalamic acids 1-3. The dehydration was performed through the fusion of amic acids 1-3 until complete melting [11], followed by heating at ten degrees above the amic acid melting point for one hour. The framed solid was cleansed by recrystallization from dioxane or chloroform.

Synthesis of N-(tetrabromophthalimidyl)phenol formaldehyde resins 7-9
A mixture of the prepared imides 4-6 (1.11 g, 2 mmol), formaldehyde (3 mL, 81.4 mmol), and ammonia solution (1.5 mL, 25%) was refluxed for 6 hours with blending [11]. The mingled materials contained two layers; the upper one was discarded while the lower layer was treated with glacial acetic acid until neutralization, and then dried in an oven at 80 °C. The output resin was purged by liquefying in DMF and then precipitating with water.

Curing of resins 7-9 to the corresponding poly N-(benzoyloxyphenyl) tetrabromophthalimidyl formaldehyde 10-12
Benzoyl chloride (4 mL, 30 mmol) was added slowly to the mixture of synthesized phenolic resins (10 mmol), DMF (25 mL), and pyridine (10 mL) with stirring at 5 °C [11]. The resultant mixture was stirred for 6 hours at room temperature before filtration. The filtrate was evaporated, and the solid crude material was refined by dissolving in DMF and precipitating with water.

Results and discussion
The synthesis and modification of phenolic resins is still an important topic that researchers are interested in, and many studies have been published on it. Thus, the objective of the current work is the synthesis and change of modern phenolic resins considering the presentation of a new phenolic moiety. In the polymer condensation with formaldehyde, the compound N-(hydroxylphenyl)tetrabromophthalimide is formed. The incorporation of a tetrabromophthalimide component in the new phenolic resins improves their chemical and thermal stability since polyimides are thermally and chemically stable polymers [14][15][16][17] with high mechanical strength. Furthermore, it has been reported that the incorporation of cyclic imides into many polymeric chains improves their thermal and substance stabilities [18]. As a result, we chose the tetrabromophthalimide moiety for incorporate in the new resins with the aim of producing phenolic resins with improved properties. The synthesis of the target phenolic resins was performed in three steps, as shown in Scheme 1. In the initial step, three N-(hydroxyphenyl) tetrabromophthalamic acids 1-3 were prepared via the reaction of para-, meta-, and ortho-amino phenols with tetrabromophthalic anhydride. In the second stage, amic acids 1-3 were dried out by combination, yielding N-(hydroxyphenyl)tetrabromophthalimides 4-6, which were then polycondensed with excess formaldehyde under heat and basic conditions, yielding the target phenolic resins 7-9.  The first stage in the base-catalyzed condensation reaction producing a mixture of monomethylol, dimethylol, and trimethylol phenols. The second stage involved a condensation reaction between methylol groups leading to link formation between phenolic rings through methylene or ether bridge producing resol phenolic resins as shown in Scheme 2. Scheme 2: synthetic mechanism of resol phenolic resins As shown in Scheme 3, the general mechanism steps of this condensation reaction involved producing a mixture of monomethylol, dimethylol, and trimethylol phenols, then involving the condensation reaction between methylol groups, leading to link formation between phenolic rings via a methylene or ether bridge. The physical properties of the prepared amic acids 1-3, imides 4-6, and resins 7-9 are listed in Tables 1, 2, and 3, respectively. The physical properties of the prepared resins 7-9 clearly indicated that the presence of tetrabromophthalimide components in their structures gave them great warm strength, as demonstrated by their high softening points (more than 360 °C). Their opposition against dissolvability in numerous solvents consequently the tars were insoluble in many solvents, including chloroform, acetone, tetrahydrofuran, diethyl ether, dioxane, and benzene, while they showed acceptable solubility in DMF, DMSO, and hot ethanol. The prepared compounds were characterized using FT-IR, 1 H NMR, and 13 CNMR spectroscopy. The FT-IR spectral data of the prepared amic acids 1-3 show distinct retention groups at 3421-3433 cm -1 , 3193-3375 cm -1 , which correspond to v(O-H) carboxyl and v(N-H) amide, respectively [19]. The absorption bands for v(C=O) carboxyl and v(C=O) amide appeared at 1710-1718 cm -1 and 1649-1670 cm -1 , respectively. The FT-IR spectral data of N-(hydroxylphenyl)tetrabromophthalimides 4-6 revealed two absorption bands at 1770-1776 cm -1 , 1720-1724 cm -1 which were attributed to the asymmetric v(C=O) imide and the symmetric v(C=O) imide, respectively. The spectra also reveal distinct new absorption bands at 1377-1390 cm -1 due to v(C-N) imide. The FT-IR spectral data of the target phenolic resins 7-9 showed two new strong absorption peaks at 1236-1238 cm -1 and 1006 cm -1 , which belong to the v(C-O-C) ether. Other distinct peaks for asymmetric and symmetric v(C-H) aliphatic were found at 2921-2985 cm -1 , 2871-2875 cm -1 [19]. The appearance of these absorption bands is an excellent proof of the formation of methylene and ether bridges, and this proves the success of phenolic resin formation. The FT-IR spectral data of amic acids 1-3, imides 4-6, and phenolic resins 7-9 are listed in Tables 4, 5, and 6, respectively. The 1 H NMR and 13 C NMR spectral data were also used to prove the chemical structures of the prepared compounds. The 1 H NMR spectrum of amic acid 1 revealed mutilate signals at 6.84-8.12 ppm due to aromatic protons, signals between 11.42 and 11.44 ppm back to NH proton and signals from 12.02 to 12.44 ppm belong to OH protons [19]. The 13 C NMR spectrum of compound 1 exhibited signals at 129.1-144.2, 172.9, and 172.9 ppm, which are attributed to the aromatic carbons, C=O amide and C=O carboxyl carbons, respectively. The 1 H NMR spectra of the prepared imides 4 and 5 showed signals at 6.87-7.94, and 9.86-9.87 ppm, which belong to aromatic and OH phenolic protons, respectively. The 13 C NMR spectra of imides 4 and 5 showed signals at 111.8-154.4 and 158.1-166.4 ppm for aromatic carbons and C=O imide carbons, respectively. On the other hand, the 1 H NMR spectra of the prepared phenolic resins (7, 8, and 9) showed signals at 4.09-4.81 and 5.04-5.17 ppm that belong to OCH2 and CH2 protons, respectively. More signals appeared at 6.87-8.65 and 8.25-10.0 ppm, which belong to aromatic protons and OH protons, respectively. The 13 C NMR spectra of phenolic resins (7, 8, and 9) showed signals at 70.2-79.8 ppm due to OCH2 and CH2 carbons, signals at 100.3-159.6 ppm attributed to the aromatic carbons, and signals at 156.7-169.1 ppm belonging to C=O imide carbons [19]. All the details of the 1 H NMR and 13 C NMR spectra of the preparation are shown in Tables 7 and 8. This work also included the modification of the pre-arranged resins 7-9 via the presentation of a phenolic hydroxyl group in an esterification reaction with benzoyl chloride in the presence of pyridine. During the esterification response, phenolic hydroxyl bunches are completely changed over to benzoyl bunches, as displayed in Scheme 4.   Table 10. The 1 H NMR spectral data of compounds 10-12 show signals at 1.91 and 2.09 ppm that belong to the protons of the methylene group (CH2), a singlet signal at 3.02 ppm that belongs to the OCH2 ether group protons, and multiple signals at 7.30-7.48 ppm due to aromatic protons. The 13 C NMR spectra of modified phenolic resins 10-12 showed a signal at 21.3 ppm due to the carbon of the methylene group (CH2), 53.9 ppm for the carbon of the OCH2 ether group, 121.3-150.8 ppm due to the aromatic carbons, 163.4 ppm of the C=O group, and 169.7 ppm attributed to the C=O ester carbon. It is recognizable that the actual properties of the altered tars demonstrate that the esterification strategy was appropriate for working on the actual properties of pitches 7-9. Since the existence of C-O-C, ester securities in the integrated ester moiety have displayed adjusted saps delicate quality and adaptability, prompting the decline of their conditioning focuses and protection from dissolvability. In this way, the relieved gums showed low mellowing focuses adjacent to great solvency in numerous solvents, including chloroform, acetone, THF, DMF, and DMSO. These new properties make the relieved gums suitable for applications such as glues and coatings [20,21]         Compound number 13 C NMR spectral data (ppm)

Conclusion
The present work supplies us with new phenolic resins containing the important tetrabromophthalimide component. The presence of this component in the new resins exhibits good thermal stability along with resistance to solubility in many organic solvents, and these properties give these resins the possibility of being introduced in certain applications. Besides, it is noticeable that the modification of the new resins via esterification of phenolic (OH) groups. This is because treatment with benzoyl chloride has a clear effect on the resin's physical properties; thus, the presence of ester groups in the modified resins decreases their melting points and enhances their solubility in other organic solvents, and this fits with other applications, as plasticization. Furthermore, several studies have shown that the presence of multiple halogen substituents in the same molecule increases its flame resistance. Therefore, we expected the new resins to possess good flame resistance due to the presence of four bromo groups in their molecules, and this will give these resins the opportunity to be used in applications of non-flammable materials or as flame-retardants.