Synthesis and properties of some aromatic polyamides with coumarin chromophores

Reactive & Functional Polymers 69 (2009) 27–35

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Synthesis and properties of some aromatic polyamides with coumarin chromophores Marioara Nechifor ‘‘Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, Iasi 700487, Romania

a r t i c l e

i n f o

Article history: Received 6 August 2008 Received in revised form 9 October 2008 Accepted 12 October 2008 Available online 18 October 2008 Keywords: Coumarinated aromatic polyamides Synthesis Characterization UV exposure

a b s t r a c t A novel monomer diacid, 6,60 -methylenebis{2-oxo-8-{2-[(2-oxo-2H-chromen-7-yl)oxy]acetoxy}-2Hchromene-3-carboxylic acid}, having two substituents (2-oxo-2H-chromen-7-yl)oxyacetate in the aromatic moiety, was synthesized and used in a direct polycondensation reaction with various aromatic diamines using triphenyl phosphite and pyridine as condensing agents to give a series of new aromatic polyamides with photosensitive coumarin pendent groups. Polyamide properties were investigated by DSC, TGA, GPC (gel permeation chromatographic analysis), and wide-angle X-ray scattering, viscosity and solubility measurements. The introduction of bulky side chains in the structure of aromatic polyamides led to moderate inherent viscosity values (0.40–0.87 dLg1) and increased solubility of these polymers in aprotic polar solvents such as NMP (N-methylpyrrolidone), DMAc, DMSO and DMF, and in less polar solvents like Py and THF. The good solubility of these polyamides was in agreement with their amorphous character as evidenced by X-ray diffraction diagrams. Gel permeation chromatography evidenced high molecular weights (49,400–63,900 gmol1) which allowed transparent, flexible and tough films to be cast from polymer solutions. These aromatic copolyamides showed good thermal properties associated with glass transition temperatures (Tg) in the range of 221–257 °C and the onset of decomposition in air above 390 °C. UV illumination (k > 300 nm) of the polymer films induced crosslinking between polyamide molecules through a [2p + 2p] photocycloaddition at the C@C bond of coumarin moieties. Information concerning the photoreactive property of coumarin-containing polymers was obtained by studying the changes in the UV absorption spectra and IR spectra of irradiated polymeric films. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Aromatic polyamides are widely used in technical applications as high-performance polymers because of their unique combination of outstanding thermal, mechanical, optical, and chemical properties. Due to their exceptional technological importance, they have attracted considerable effort in order to improve their solubility, because these polymers usually present limited solubility in most organic solvents, and high melting and glass transition temperatures, which decrease their processability and restrict further applications. To overcome these shortcomings, various approaches have been introduced and then applied to obtain thermally stable, high molecular weight and organic-soluble aromatic polyamides. The synthetic routes employed to avoid these important problems have intended to modify the polyamide structure and the main method used for this purpose was the attachment

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of different functional groups (or substituents) onto the polymer backbone capable of reducing the chain rigidity and further increasing their tractability [1–6]. The introduction of bulky pendent groups into the side chains of the polymer breaks its symmetry and regularity, and thereby may reduce the crystallinity and enhance the solubility. New monomer structures bearing bulky substituents bring about soluble aromatic polyamides and, at same the time, new polymer properties designed for advanced applications. Polymers having reactive functional groups have gained great attention from both academic and industrial fields because their synthesis provides an approach to a subsequent modification of the polymer structure to achieve tailored macromolecules for specific end applications. Photopolymers are studied for their macromolecular properties and for the properties of the photosensitive group. Among them, polymers having photocrosslinkable functional groups, such as cinnamoyl, chalcone, thymine or coumarin, represent an active field of research in polymer science because of their technological applications in the fields of photolithography

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M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

[7], non-linear optical materials [8], advanced microelectronics [9], liquid crystalline materials [10], holographic elements [11] and electrophotographic coatings [12]. Coumarin (2H-chromen-2-one), and its analogues as well, has attracted considerable attention from organic and medicinal chemists due to its photophysical and photochemical behavior. A number of natural and synthetic compounds containing the coumarin nucleus have been reported to exhibit a wide spectrum of biological activity and fluorescence in the visible light range. Crosslinking by photodimerization of coumarins and the reversibility of the photodimerization/photocleavage of coumarin dimers have found applications in various fields of science and engineering [13]. The present article reports on some soluble polyamides based on a novel condensation monomer, 6,60 -methylenebis{2-oxo8-{2-[(2-oxo-2H-chromen-7-yl)oxy]acetoxy}2H-chromene-3-carboxylic acid, bearing chromenyl pendent groups that create functionality to the polymer side chains and impart their properties to the macromolecules. It is expected that the presence of voluminous pendent groups to result in a less ordered polymer matrix, to enhance the solubility and processing characteristics of these polymers while maintaining a good thermal stability. Coumarin was employed as a photosensitive group because it was confirmed that upon irradiation above 300 nm of thin films of various polymers with coumarin side chains results in a simple [2p + 2p] photodimerization of the reactive C@C double bonds yielding in the formation of rigid cyclobutane rings and network structures which causes the insolubilization of the polymer films.

sion spectra of the polymers were obtained on a Perkin Elmer LS55 fluorescence spectrophotometer, at room temperature. Samples were degassed by bubbling nitrogen through the solution for 30 s. In order to avoid any intermolecular interactions, all experiments were carried out at a concentration of 105 M of the polymers in anhydrous 1,4-dioxane. Mass spectral data were obtained using an Agilent 6210 TOF LC/MS. The inherent viscosities of polymer solutions (0.5% w/v) in DMF were determined at 25 °C using an Ubbelohde suspended level viscometer. Solubilities were determined at a 1% (w/w) concentration. Elemental analyses were run in a Perkin Elmer 2400 CHNSO analyzer. Melting points of the monomers were measured on a Melt-Temp II (Laboratory Devices) apparatus without correction. Gel permeation chromatographic analysis (GPC) was carried out on a PL-EMD 950 evaporative mass detector instrument. Dimethylformamide (DMF) was used as mobile phase after calibration with polystyrene standards of known molecular weights. Thermogravimetric analysis was performed in air at a heating rate of 10 °C/min using a Q-1500D system derivatograph. The DSC analyses were done on a Mettler DSC 112E Instrument, with heating and cooling rate of 10 °C/min. The temperature at the midpoint of the corresponding heat-capacity jump in the second heating cycle was taken as Tg. Wide-angle X-ray scattering measurements were performed on a Bruker AXS-D8 Avance Xray diffractometer using CuKa radiation (k = 1.54 Å), at 36 KV and 30 mA in samples of the polymer films. The measurements were performed at 2h between 5° and 60°. 2.3. Monomer Synthesis

2. Experimental 2.1. Materials All chemicals were supplied by Sigma–Aldrich Co. 2,3-dihydroxybenzaldehyde, 1,3,5-trioxane, 2,2-dimethyl-1,3-dioxane-4,6dione (Meldrum’s acid), 7-hydroxy-2H-chromen-2-one (7hydroxycoumarin) and ethyl bromoacetate were used as received. Reagent grade solvents were dried and purified as follows. N-Methylpyrrolidone (NMP) and pyridine (Py) were refluxed in an inert atmosphere in the presence of phosphorous pentoxide (P4O10) and freshly powdered calcium hydride (CaH2), respectively, for 2 h, and distilled under vacuum and stored over 4 Å molecular sieves. Triethylamine was distilled over potassium hydroxide (KOH) pellets and then stored over 5 Å molecular sieves. Lithium chloride (LiCl) and calcium chloride (CaC12) were dried for 8 h at 170 °C under vacuum. Triphenyl phosphite (TPP) was purified by vacuum distillation. p-Phenylenediamine (13a) was purified by ?sublimation at reduced pressure, benzidine (13b) was recrystallized from water/ethanol (1:1), and m-phenylenediamine (13g) was vacuum distilled prior to use. All other aromatic diamines, including 4,40 -methylenedianiline (13c), 4,40 -oxydianiline (13d), 4,40 -sulfonyldianiline (13e) and 4,40 -(hexafluoroisopropylidene)dianiline (13f) were of high purity when received and used without further purification.

2.2. Physical measurements Spectral measurements of 1H and 13C NMR were performed using a Bruker Avance DRX spectrometer at 400 and 100 MHz, respectively, in DMSO-d6 as solvent. The chemical shifts were expressed in d values compared to Me4Si, which was used as an internal standard. Infrared spectra were recorded on a Bruker Vertex 70 Fourier transform infrared spectrometer. Electronic absorption spectra were measured using a SPECORD M42 spectrophotometer. UV irradiation was carried out with a 500 W mercury arc lamp without a filter, in air at room temperature. The fluorescence emis-

3,3’-Methylenebis(5,6-dihydroxybenzaldehyde) (3). A solution of 2,3-dihydroxybenzaldehyde (1) (34.5 g, 0.25 mol) and 1,3,5-trioxane (2) (2.7 g, 0.03 mol) in glacial acetic acid (50 mL) was heated to 90 °C under stirring and nitrogen atmosphere. To this, a solution of concentrated sulfuric acid (0.2 mL) in glacial acetic acid (5 mL) was added dropwise. The temperature of reaction mixture was maintained at 90 °C for 12 h and then it was cooled and poured onto a large amount of ice water. The resulting precipitate was filtered, washed with plenty of hot distilled water to remove acetic acid. The solid was extracted three times with diethyl ether to remove the excess of 2,3-dihydroxybenzaldehyde, and then dried in a vacuum oven at room temperature, and crystallized twice from acetone to give 14.75 g (overall yield 41%) of compound (3); m.p. = 188–189 °C. MS: m/z (%) = 288 (M+, 21), 252 (15), 230 (11), 107 (83), 94 (15), 77 (9). ANAL. Calcd for C15H12O6 (288.254): C, 62.50%; H, 4.19%. Found: C, 62.34%; H, 4.28%. IR (KBr, cm1), m: 3463, 3328 (OH), 3105–3000 (C@CH arom), 2909 (CH2), 2885 (H–CO), 2760, 1655 (HC@O), 844, 810 (CH2). 1H NMR: d = 10.97 (d, 2H, OH), 9.81 (s, 2H, CH@O), 6.91 (d, 2H, ortho to OH), 6.86 (d, 2H, ortho to CHO), 5.80 (d, 2H, OH), 3.71 (s, 2H, CH2). 13C NMR: d = 197.8 (C@O), 148.2 (C2), 141.3 (C3), 130.8 (C5), 121.9 (C6), 118.5 (C4), 117.6 (C1), 42.0 (CH2). 6,60 -Methylenebis(8-hydroxy-2-oxo-2H-chromene-3-carboxylic acid) (6). A mixture of 3,3’-methylenebis(5,6-dihydroxybenzaldehyde) (3) (14.4 g, 0.05 mol), Meldrum’s acid (4) (14.4 g, 0.1 mol), piperidine (0.20 mL, 0.002 mol), acetic acid (0.11 mL, 0.002 mol) and ethanol (150 mL) was stirred at room temperature for 30 min and then refluxed for 5 h. The reaction mixture was cooled to room temperature and chilled in a freezer for 1 h. The crystallized solid was filtered, washed with ethanol and dried in vacuo. The yield in (6) was 90% (19 g), m.p. = 321–324 °C. MS: m/z (%) = 424 (M+, 10), 336 (6), 280 (49), 222 (37), 170 (12), 91 (12), 77 (8). ANAL. Calcd for C21H12O10 (424.316): C, 59.44%; H, 2.85%. Found: C, 59.37%; H, 2.90%. IR (KBr, cm1), m: 3400 (OH free), 3330 (OH, phenol), 3150 (OH, carboxyl), 3120–3040 (C@CH arom), 2910 (CH2), 2400 (OH, carboxyl), 1725, 1670 (C@O, pyrone ring and carboxyl), 1598 (C@C vinylene), 1565, 1500 (C@C arom), 972 (out-

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M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

of-plane bending vibration of C–H in the vinylene linkage). 1 H NMR: d, ppm = 13.19 (br, s, 2H, COOH), 10.15 (br, s, 2H, OH), 8.64 (s, 2H, H4), 7.33 (s, 2H, J = 2.4 Hz, H5), 7.15 (s, 2H, J = 2.4 Hz, H7), 3.93 (s, 2H, CH2). 13C NMR: d, ppm = 164.0 (C@O), 156.4 (C2), 148.6 (C4), 146.2 (C8), 143.7 (C9), 138.8 (C6), 121.1 (C7), 119.2, 118.8 (C3, C10), 116.5 (C5), 44.1 (CH2). Ethyl [(2-oxo-2H-chromen-7-yl)oxy]acetate (9). To a well-stirred solution of 7-hydroxy-2H-chromen-2-one (7) (24.3 g, 0.15 mol) and freshly calcined potassium carbonate (62.1 g, 0.45 mol) in absolute acetone (300 mL), ethyl bromoacetate (8) (21.7 mL, 0.195 mol) was added in a dropwise fashion at 50–56 °C. The reaction mixture was stirred and refluxed for 5 h under a nitrogen atmosphere. The cooled reaction solution was filtered to remove the inorganic material, poured into icewater, and acidified to pH = 5–6 (H2SO4 solution 1 N). The resulting yellowish precipitate was filtered off, crystallized from ethanol and dried to give 31.62 g of compound (9) as white crystals (85% yield), m.p. 112–113 °C. MS: m/z (%) = 248 (M+, 31), 175 (22), 146 (6), 118 (100), 90 (43), 63 (30), 51 (14), 39 (5). ANAL. Calcd for C13H12O5 (288.233): C, 62.90%, H, 4.87%. Found: C, 62.83%; H, 4.79%. IR (KBr, cm1), d: 3080 (Ar–H stretching), 2982 and 2949 (CH3 and CH2, respectively, asymmetric stretching), 2897 and 2852 (CH3 and CH2, respectively, symmetric stretching), 1732 (C@O stretching vibration in the ester linkage), 1709 (C@O stretching in pyrone ring), 1613 (C@C stretching adjacent to C@O), 1574–1512 (m(C@C) in benzene ring), 1495– 1436 (Ar stretching and CH2 bridge), 1402 (cis C@C stretching), 1378 (CH3 deformation), 1280–1239 (C(C@O)-O and @C–O–C asymmetric stretching), 1149–1070 (C(C@O)–O and @C–O–C symmetric stretching), 988 (out-of-plane bending vibration of C–H in the vinylene linkage). 1H NMR: d, ppm = 7.96 (d, J = 9.6 Hz, 1H, H4), 7.61 (d, J = 8.9 Hz, 1H, H5), 6.96 (dd, J = 2 and J = 8.9 Hz, 1H, H6), 6.94 (d, J = 2 Hz, 1H, H8), 6.28 (d, J = 9.6 Hz, 1H, H3), 4.81 (s, 2 H, Ar–O–CH2), 4.16 (q, J = 7.4 Hz, 2H, CH2), 1.21 (t, J = 7.4 Hz, 3H, CH3). 13C NMR: d, ppm = 162.4, 162.1 (C2, COO), 160.7 (C7), 155.8 (C9), 143.2 (C4), 128.8 (C5), 112.7, 112.4 (C3, C10), 111.0 (C6), 100.1 (C8), 66.8 (Ar–O–CH2), 61.9 (CH2), 14.3 (CH3). [(2-Oxo-2H-chromen-7-yl)oxy]acetic acid (10). A solution of (9) (31 g, 0.125 mol) in propan-2-ol (150 mL) was treated with NaOH (48 g, 1.2 mol) in water (400 mL). The resulting mixture was heated (90–100 °C) and stirred for 12 h, cooled to room temperature, transferred into icewater and acidified with concentrated hydrochloric acid to pH  2. The resulting solid was filtered off, crystallized from ethanol and dried in vacuo to yield compound (10) (25 g, 91%) as a colorless solid, m.p. 215–217 °C. MS: m/z (%) = 220 (M+, 18), 175 (29), 146 (9), 118 (100), 90 (49), 63 (27), 51 (19), 39 (13). ANAL. Calcd for C11H8O5 (220.179): C, 60.00%; 3.66%. Found: C, 59.91%; H 3.58%. IR (KBr, cm1), m: 3540, 3412 (COOH), 3074 (Ar–H stretching), 1720 (C@O stretching in pyrone ring), 1698 (C@O, carboxyl), 1602 (C@C stretching adjacent to C@O), 1569–1508 (m(C@C) in benzene ring), 1490–1430 (Ar and CH2 stretches), 1398 (cis C@C stretching), 1129 (Ar–O–CH2), 984 (out-of-plane bending vibration of C–H in the vinylene linkage). 1 H NMR: d, ppm = 13.09 (br s, 1H), 7.97 (d, J = 9.6 Hz, 1H), 7.62 (d, J = 8.9 Hz, 1H), 6.98 (dd, J = 2.0 and 8.9 Hz, 1H), 6.96 (d, J = 2.0 Hz, 1H), 6.28 (d, J = 9.6 Hz, 1H), 4.89 (s, 2H, CH2). 13C NMR: d, ppm = 162.4 (C2), 160.6, 160.3 (COO, C7), 155.8 (C9), 143.2 (C4), 129.0 (C5), 112.7, 112.4, 111.9 (C3, C10, C6), 100.9 (C8), 66.2 (CH2). [(2-Oxo-2H-chromen-6-yl)oxy]acetyl chloride (11). Anhydrous dimethylformamide (2 mL) was added to a suspension of (10) (24.2 g, 0.11 mol) in freshly distilled thionyl chloride (80.25 mL, 1.1 mol), and the mixture was refluxed for 5 h while being stirred in a N2 atmosphere. The excess of thionyl chloride was stripped off by distillation in vacuo, and the residual product crystallized from n-hexane, and the white product was stored in a dark bottle; the yield was 24.13 g (92%), m.p. = 101–103 °C. MS: m/z (%) = 238

(M+, 27), 175 (18), 146 (11), 118 (97), 90 (44), 63 (28), 51 (18), 39 (8). ANAL. Calcd for C11C7ClO4 (238.625): C, 55.36%; H, 2.95%; Cl, 14.85%. Found: C, 55.23%; H, 3.06%; Cl, 14.77%. IR (KBr, cm1), m: 3070 (Ar–H stretching), 1765 (COCl), 1725 (C@O stretching in pyrone ring), 1595 (C@C stretching adjacent to C@O), 1564– 1502 (m(C@C) in benzene ring), 1487–1425 (Ar and CH2 stretches), 1395 (cis C@C stretching), 1125 (Ar–O–CH2), 982 (out-of-plane bending vibration of C–H in the vinylene linkage). 1H NMR: d, ppm = 7.97 (d, J = 9.6 Hz, 1H), 7.62 (d, J = 8.9 Hz, 1H), 6.98 (dd, J = 2.0 and 8.9 Hz, 1H), 6.96 (d, J = 2.0 Hz, 1H), 6.28 (d, J = 9.6 Hz, 1H), 4.91 (s, 2H, CH2). 13C NMR: d, ppm = 174.9 (CO), 162.7, 162.4 (C7, C2), 155.3 (C9), 143.2 (C4), 129.7 (C5), 112.7, 112.4, 112.0 (C3, C6, C10), 101.6 (C8), 72.1 (CH2). 6,60 -Methylenebis{2-oxo-8-{2-[(2-oxo-2H-chromen-7-yl)oxy]acetoxy}-2H-chromene-3-carboxylic acid} (12). A mixture of 6 (18.65 g, 0.044 mol) and triethylamine (16.7 mL, 0.12 mol) in 1,4-dioxane (150 mL) was cooled to 0 °C. To this was added dropwise a solution of (11) (23.85 g, 0.1 mol) in 1,4-dioxane (75 mL), with vigorous stirring. The reaction mixture was stirred at room temperature for 24 h under a nitrogen blanket. The mixture was chilled to 5 °C, and the by-product, triethylamine hydrochloride, was separated by filtration. The crude product was extracted with ethyl acetate and the extract was dried over MgSO4. After removal of solvent, the resultant solid was recrystallized twice from ethanol, resulting in 29.48 g of (12) (yield 81%), m.p. = 269–271 °C. MS: m/z (%) = 826 (M+, 18), 736 (6), 678 (8), 663 (23), 501 (39), 422 (34), 336 (11), 289 (17), 222 (21), 162 (48). ANAL. Calcd for C43H24O18 (826.645): C, 62.47%; H, 2.92%. Found: C, 62.37%; H, 2.79%. IR (KBr, cm1), m: 3412 (OH free), 3150 (OH, carboxyl), 3110–3020 (Ar–H stretching), 2902 (Ar–CH2–Ar), 2400 (OH, carboxyl), 1730 (C@O, carbonyl pyrone and ester), 1650 (C@O, carboxyl), 1615 (C@C vinylene), 1565, 1500 (C@C arom), 1477–1434 (Ar and CH2 stretches), 1400 (cis C@C stretching), 1263–1230 (C(C@O)–O and @C–O–C asymmetric stretching), 1127–1090 (C(C@O)–O and @C–O–C symmetric stretching), 982 (out-of-plane bending vibration of C–H in the vinylene linkage). 1H NMR: d, ppm = 13.27 (br, s, 2H, COOH), 8.64 (s, 2H, H4), 7.63 (d, J = 9.5 Hz, 2H, H40 ), 7.31–7.37 (d, J = 2.4 Hz and J = 8.5 Hz, 4H, H5 and H50 , respectively), 6.90 (d, J = 2.4 Hz, 2H, H7), 6.84–6.82 (dd, J = 2.1 Hz and J = 8.5 Hz, 4H, H60 and H80 ), 6.26 (d, J = 9.5 Hz, 2H, H30 ), 4.81 (d, 4H, CH2), 4.40 (s, 2H, CH2). 13C NMR: d, ppm = 168.2 (C@O),

Fig. 1. 1H NMR and

13

C NMR spectra of the diacid monomer (12).

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M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

164.0 (COOH), 162.4 (C20 ), 156.2 (C2), 159.5 (C70 ), 155.8 (C90 ), 148.6 (C4), 145.0 (C8), 143.7, 143.2 (C9, C40 ), 139.2 (C6), 128.8 (C50 ), 121.8 (C7),119.0, 118.6, 118.0 (C3, C10, C5), 112.7, 112.4 (C30 , C60 , C100 ), 100.3 (C80 ), 66.8 (OCH2), 44.0 (CH2). Fig. 1 provides the 1H NMR and 13C NMR spectra of dicarboxylic acid (12).

3. Results and discussion

(molar ratio 2,3-dihydroxybenzaldehyde: formaldehyde = 2.78:1) is necessary to avoid polymeric by-products formation. Reaction of dialdehyde (3) with Meldrum’s acid (4) to give biscoumarin acid (6) involves a Knoevenagel condensation in the presence of piperidinium acetate as a catalyst followed by the cyclization of the resulting 5,5’-{methylenebis[(5,6-dihydroxy-3,1-phenylene)methylylidene]}bis(2,2-dimethyl-1,3-dioxane-4,6-dione) (intermediate (5)). Piperidinium acetate was prepared in situ, by mixing equivalent quantities of piperidine and acetic acid. o-Hydroxybenzylidene Meldrum’s acid derivative (5) can be isolated by simple filtration or, without interruption, cyclised under reflux to give (6) (Scheme 1). 7-Chlorocarbonylmethoxycoumarin (11) was prepared using a three step procedure from 7-hydroxycoumarin (7) as a starting material, as shown in Scheme 2. Alkylation of (7) with ethyl bromoacetate in dry acetone in the presence of anhydrous potassium carbonate, as base, produced ethyl ester (9). Its saponification with 3N NaOH in aqueous isopropanol and subsequent acidolysis afforded coumaryloxyacetic acid (10). The latter was converted to the corresponding coumaryloxyacetic acid chloride by reacting with excess of thionyl chloride and using DMF as catalyst. The acid chloride (11) was then reacted with HO-substituted biscoumarin acid (6) in 1,4-dioxane at 0 to 5 °C in the presence of triethylamine as HCl scavenger added at a 20 mol % excess compared to acid chloride to obtain the diacid monomer (12).

3.1. Monomer synthesis

3.2. Polymer synthesis

Target compounds (3), (6), (12) and (14a–g) were synthesized according to the known procedures [14–17] and general synthetic routes for their preparation are depicted in Schemes 1–3. The dialdehyde (3) was obtained in 41% yield by treating 2,3-dihydroxybenzaldehyde (1) with 1,3,5-trioxane (2) in acetic acid solution containing a catalytic amount of sulfuric acid. Traces of acetyl sulfate, which probably is formed, act as a catalyst for the decomposition reaction of trioxane to formaldehyde. The excess of (1)

Direct polycondensation of aromatic diamines with dicarboxylic acids in the presence of triphenyl phosphite is a well known convenient method to prepare aromatic polyamides due to phosphorylation of the diacid. Pyridine is employed invariably to activate TPP through a complex formation. Addition of the inorganic salts increases both the solubility of the polymers in reaction media and viscosity of resulting polymers, and, consequently, the yield and molecular weight of the polyamides. In addition, the di-

2.4. Polyamide syntheses The polyamides were prepared by the triphenyl phosphite-activated method, using the following general procedure. 1,4-Phenylenediamine (13a) (0.54 g, 0.005 mol) and diacid (12) (4.14 g, 0.005 mol) were dissolved in a mixture of NMP (50 mL) and Py (10 mL) containing CaCl2 (3 g) and LiCl (1 g) by heating with stirring at 80 °C, under an inert atmosphere in order to avoid humidity from air. The temperature was then raised to 110 ± 5 °C and a solution of triphenyl phosphite (3.15 mL, 0.012 mol) in 5 mL anhydrous pyridine was added dropwise. The solution was heated for further 5 h. After cooling, the obtained polymer solution was trickled into a large amount of stirring methanol. The precipitated polymer was washed successively with methanol and hot water, collected by filtration, and dried under vacuum at 80 °C for 12 h. The yield was almost quantitative. The inherent viscosity of the polymer (14a) in DMF was 0.69 dL/g, measured at a concentration of 0.5 g/dL at 25 °C. Polyamides (14b–g) were synthesized under essentially the same conditions as mentioned above.

O

O

O O

H

AcOH H

+ O HO

H HO

O OH

OH OH

OH

1

2

3 O

O O 3 + O

EtOH piperidinium acetate

O

O

O O

O HO

O

O

OH O OH

OH 4

5 reflux

COOH

HOOC O

O

O

O

OH

OH 6

Scheme 1. Synthesis of 6,60 -methylenebis(8-hydroxy-3-carboxycoumarin) (6).

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M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

O

O

OH

O

+

Br

K2CO3

O

O

O

O

8

7

O

acetone

O

9 2-propanol NaOH

O O

reflux

O

O

O

SOCl2

O

O

Cl

O OH

11

6

+ 11

1,4-dioxane

10 COOH

HOOC

TEA

O

O

O

O O

O

O

O O

O

O

O 12

O

O

Scheme 2. Synthesis of the diacid monomer (12).

OC 12

+

H2N R1

N H O

13

O

O

O 14

O

R: O

p-C6H4

NH

OR

OR

R1

R1

CO

TPP NH2 pyridine

O

polyamide 14

a b

O

R1 SO2

polyamide 14

e

CF3 f

c O

d

CF3 m-C6H4

g

Scheme 3. Synthesis of polyamides (14a–g).

rect polycondensation route avoids moisture-sensitive acid chlorides or isocyanates used in conventional methods, thus providing significant advantages to manufacturing operations. All polyamidations proceeded in homogeneous, transparent and viscous solutions throughout the reaction, and the polymers were isolated as fibers or powders in almost quantitative yields (Scheme 3). The chemical structures of the synthesized polymers were confirmed by elemental analysis, FT-IR, 1H and 13C NMR spectroscopy. The elemental analysis of these polyamides evidenced, in all cases, found carbon values lower than calculated ones for the proposed

structures. The explanation resides in the hygroscopic character of the amide group. The moisture uptake was calculated by the following expression (Eq. (1)):

moisture uptake ¼

W  W0  100 W0

ð1Þ

where W is the weight of the polymer sample after standing at room temperature and W0 is the weight of the polymer sample after drying in vacuum at 100 °C for 8 h.

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M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

The amount of water absorbed in these polyamides ranged from 2.14 (polyamide 14f) to 3.94% (polyamide 14d). The correct values of the carbon, hydrogen and nitrogen were calculated using the following equations (Eqs. (2) and (3)):

for C and N :

corrected value ¼ ð100 þ moisture uptakeÞ  found value

for H :

ð2Þ

corrected value ¼ ð100  moisture uptakeÞ  found value

ð3Þ

By correcting the experimental values, the new found values correlated well with the calculated ones. Key structural features of polyamides (14a–g) were verified by FT-IR based on their characteristic absorption bands. A broad and strong band in the range of 3200–3450 cm1 was observed due to mN–H (amide A) which was overlapped by mOH from adsorbed water molecules. Coupled vibrations (dNH and mCO, amide I) were distinctly apparent as two bands in the 1660–1680 cm1 and 1710–1730 cm1 range, respectively. In addition, mixed vibrations involving OCN and NH groups appeared as a strong band at 1350 cm1 (amide III). The amide II band arose from NH deformation mode and was situated at 1550 cm1. The presence of coumarin unit in polyamide structures was evidenced by the relevant bands centered at 1730 cm1 and ascribed to the carbonyl stretching of both lactone rings and ester units of 7-coumaryloxyacetate side chains, and around 1615 cm1 due to vinylene C@C stretching vibration, and 1575 and 1400 cm1 characteristic to cis C@C stretching from pyrone ring, and around 980 cm1 (out-of-plane bending vibration of C–H in the vinylene linkage). Also, the IR spectra displayed absorption peaks between 12601230 cm1, and 1125–1080 cm1, associated with asymmetric and symmetric ester stretching, respectively. Bands attributed to Ar–H stretching, C@C ring stretch and di-substituted benzene appeared around 3110–3020, 1500 and 845 cm1, respectively. Bands located at 2900, 1480–1430 and 1125 cm1 are associated with Ar–CH2–Ar, Ar and CH2 stretches, and Ar–O–CH2, respectively. IR spectrum of polymer (14c) is represented in Fig. 2. 1 H NMR spectra were also used to identify the structure of polymers. Fig. 3 illustrates the 1H NMR spectrum of polyamide (14d) in DMSO-d6. Each proton of (14d) was assigned, and the proton assignments (ppm) are as follows: (a) 10.89 amide; (b) 8.97, 7.63 and 6.26 coumarin–vinylic; (c) 7.35, 7.14, 6.90 and 6.82–6.84 coumarin–aromatic; (d) 7.85 and 7.01 aromatic diamine; (e) 4.84 side chain meth-

Fig. 3. 1H NMR spectrum of the polyamide (14d).

ylene; (f) 4.40 backbone methylene. The integration values were in accordance with the expected structure. The 13C NMR spectrum of polymer (14d) consists of coumarin peaks at 162.4 (C20 ), 156.0 (C2), 159.5 (C70 ), 155.8 (C90 ), 148.8 (C4), 145.1 (C9, C8), 143.2 (C40 ), 140.2 (C6), 128.8 (C50 ), 122.0 (C7), 118.6, 118.0, 117.9 (C10, C5, C3), 112.7, 112.4 (C60 , C30 , C100 ), 100.3 (C80 ) ppm, aromatic carbons from diamine moiety at 156.7 (C100 ), 136.0 (C400 ), 121.8 (C300 ), 118.2 (C200 ) ppm, two signals at 66.8 and 44.0 ppm attributed to CH2O and CH2, respectively, and two peaks at 168.2 and 160.7 ppm ascribed to COO and CO–NH groups (Fig. 4). 3.3. Polymer characterization The polymers under study had moderate inherent viscosities of 0.40–0.87 dLg1 in DMF, and sufficient high molecular weights for casting thin and homogeneous films, as listed in Table 1. Coumari-

Fig. 2. FT-IR spectrum of polymer (14c).

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M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

Fig. 4.

13

C NMR spectrum of the polyamide (14d).

Table 1 Inherent viscosity (ginh) and GPC values of polyamides. Polymer code

ginha, dLg1

Mwb, gmol1

Mnc, gmol1

PD

14a 14b 14c 14d 14e 14f 14g

0.69 0.56 0.81 0.87 0.45 0.40 0.78

58,300 53,300 59,700 63,900 49,900 49,400 61,000

22,500 20,100 25,300 29,700 18,400 17,200 27,600

2.59 2.65 2.36 2.15 2.71 2.87 2.21

a Inherent viscosity, measured at a concentration of 0.5 g/dL in DMF, at 25 ± 0.1 °C. b Measured by GPC in DMF; polystyrene was used as standard. c Polydispersity by GPC (Mw/Mn).

nated polyamides exhibited weight-average molecular weights (Mw) and number-average molecular weights (Mn) values in the ranges 49,400–63,900 and 17,200–27,600 g/mol, respectively. The polydispersity indices (Mw/Mn) were between 2.15 and 2.87, i.e., in the standard range for polycondensation reactions, and indicate that the polymers have quite a narrow molecular weight distribution and a low quantity of oligomers. Aromatic diamines (13e) and (13f), having a reduced basicity due to the electron-withdrawing groups, resulted in polyamides with lower molecular weights. The synthesized polyamides showed good solubilities in common polar aprotic solvents such as NMP, DMAc, DMSO, DMF, 1,4dioxane and sulpholane. The polyamides are also soluble in less polar solvents like o-chlorophenol, m-cresol, pyridine and tetrahydrofuran (THF) except polymers (14a) and (14b), whose solubility is limited due to the para-substituted structure and high rigidity, and to the presence of rigid rod-like benzidine moiety, respectively. The polyamides (14d) and (14f), containing flexible linkages (ether and hexafluoroisopropylidene, respectively), and (14e), having polar sulfonyl group, and (14g), derived from asymmetrical mphenylenediamine revealed higher solubility in the solvents tested. All of the polymers are insoluble in common organic solvents such as chloroform, acetone, nitrobenzene, cyclohexanone, etc but they are soluble in concentrated sulfuric acid. All the coumarin-containing polyamides possess film-forming ability. Their solutions in NMP and DMF having a concentration of about 10% were cast onto glass substrates and followed by slow elimination of the solvent to yield thin, tough, flexible, transparent and colorless films. The good solubility of these polymers can be explained by the presence of

the pendent rigid coumarin groups. Because of these voluminous groups, the packing of the polymer chains in tight structures through hydrogen bonding between amide groups is prevented and, consequently, the solvent molecules can easily diffuse into the polymer chains. The increased solubility of these polyamides is mainly due to their amorphous character. All the X-ray diffraction diagrams of polyamides gave predominantly amorphous patterns. Thermal properties of all polymers were assessed by dynamic thermal analysis including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Quenching from temperature of 350 °C to room temperature yielded amorphous samples so that in most cases the glass transition temperatures (Tg’s) easily could be observed in the second heating traces of DSC. The DSC curves exhibited only clean second-order transitions and no endotherms until 350 °C that could be assigned to melting transitions. These results indicate that these polyamides possessed an amorphous nature. The resulting polymers showed Tg’s in the 221–257 °C range, depending on the diamine monomer used in the synthesis. The initial decomposition temperatures (Td’s), the temperatures of 10% weight loss (T10’s) and the residual weight values in static air were determined from original thermograms and tabulated in Table 2. The temperature for 10% weight loss, which is an important criterion to evaluate the thermal stability of polymers, was above 390 °C, indicating their high thermal stabilities, which may be due to the presence of relative rigid aromatic heterocyclic backbone. There is a small weight loss at the beginning of the TG curves of the polymers, in the temperature range from 100 to 200 °C, which is due to the hygroscopic moisture released (amide groups are involved in extensive hydrogen bonding with adsorbed water molecules) and to the residual solvent evaporation (NMP). A sharp weight loss is recorded on heating up to 420 °C and corresponds to the degradation of methylene groups, which are vulnerable to thermo-oxidative processes. The exothermic effect (500–550 °C) observed in the DTA curves, which constitutes the second decomposition step of the pyrolysis process and which dominates in thermograms, is due to heat decomposition of the coumarin rings. The nature of the diamine moiety did not greatly influence the thermal resistance of the polymers. 3.4. Optical properties Information concerning the photoreactive property of coumarin-containing polymers was obtained by studying the changes in the UV absorption spectra and FT-IR spectra of irradiated polymeric films. Solutions of polymers (14a-g) (approximately 1% w/v in 1,4dioxane) were cast onto quartz plates. Following evaporation in

Table 2 Thermal characterization of polymers (14a–g). Polymer code

Tga, °C

Tdb, %

T10c, °C

Char yieldd, (%)

14a 14b 14c 14d 14e 14f 14g

252 248 221 226 233 257 240

379 382 370 374 372 383 377

403 406 393 398 395 408 401

25 26 21 22 23 26 24

a Glass transition temperature, from the second heating traces of DSC measurements. b Temperature of the first onset on the TGA curve. c Temperature at which a 10% weight loss was recorded by TGA. d Aerobic residual weight at 600 °C.

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M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

air and vacuum drying, the bulk films were exposed to UV light irradiation from an ultraviolet lamp in the absence of a filter for varying lengths of time. Before UV light irradiation the spectra exhibited an absorption maximum near 320 nm (kmax) attributed to p–p* transition of the olefinic double bond from coumarin chromophores in the polymer structure. A decrease in intensity of the absorbance was observed during UV illumination of the polymer film. This decrease is attributed to the reduction of conjugation length and generation of cyclobutane ring by the electron redistribution. Although the photodimerization can take place both in intra- and intermolecular manner, the insolubilization in organic solvents of irradiated polymeric films indicates the development of an intermolecular process. This was an additional evidence for the occurrence of a photocrosslinking reaction. With respect to the photochemistry of coumarin derivatives, it is known that the reversibility of the photodimerization/photocleavage process allows coumarin dimers to revert to their original structure. Although it has been reported [18,19] that photocleavage of dimerized photoproducts can be achieved by exposure to UV light above 300 nm wavelength, there is no direct evidence for the cleavage of dimers from the UV data discussed here. The intensity decrease at 319 nm was proportional to the consumption of the 3,4-olefinic bonds in the coumarin structure, while an increase in the absorbance at 248 nm is an indicative of cleavage of the cyclobutane rings and reversal to the original lactone structure. The absorption spectrum of (14g), as shown in Fig. 5, is broad in the 275–350 nm spectral regions with a maximum at 319 nm and a shoulder around 300 nm. At the same time, Fig. 5 displays UV spectral changes of polyamide (14g) in film state under continuous irradiation. Based on the above discussion, the change of A319 values reflects the degree of crosslinking of polymer (14g), therefore the crosslinking density of (14g) can be estimated using the following expression (Eq. (4)):

DC ¼

ðA319 Þ0  ðA319 Þt  100 ðA319 Þ0

ð4Þ

where DC (%) is the degree of crosslinking, (A319)0 and (A319)t are the absorbance values of polymer (14g) at 319 nm before irradiation and after irradiation for t min, respectively. Fig. 6 depicts the dependence of the A319 and DC values of polymer (14g) on UV irradiation time t.

Fig. 5. Change of UV absorption spectrum of polymer (14g) in film state during UV irradiation.

Fig. 6. Time-dependent changes in absorbance at 319 nm and crosslinking degree (CD) of polymer (14g) under ultraviolet irradiation.

The capability of photocrosslinking of the new polymers was also demonstrated using FT-IR spectroscopic analysis. Film specimens of the synthesized polymers were deposited from 1,4-dioxane solution (2%) onto KBr windows. After the solvent was evaporated, the films were stored under vacuum at 80 °C for 8 h to remove the last traces of solvent. The samples were subjected to UV light irradiation and the IR spectra were taken at different time intervals. The differences between IR transitions of unirradiated and irradiated samples can be attributed to the [2p+2p] cycloaddition process. As the dimerization reaction proceeded, several distinct changes induced by photoirradiation were evident. Stretching vibrations of C@C (1615 cm1) and cis C@C (1575 cm1) in the vinylene linkage from a-pyrone ring decreased in intensity, due to the destruction of the conjugation within coumarin ring during photoirradiation. Simultaneously, the emergence of a new nonconjugated C@O stretching vibration (1765 cm1), next to its original vibration (1730 cm1) was observed, suggesting the disappearance of enone conjugation, which means the disruption of the double bond in pyrone ring and the generation of cyclobutane photoproduct, consistent with the results obtained by UV–vis spectral analysis. According to the literature, the shift to higher wavenumbers is attributed to the predominant formation of the syn head-to-head dimer [20]. With photocleavage, the two C@O peaks merge and the double bond peaks increase in size and shift back to their original positions The recovery of the transition at 1730 cm1 was not observed in the case of the polymers under discussion. The above observations in UV and IR studies ascertain the [2p + 2p] photocycloaddition reaction of the reactive olefin double bond of the polymer on UV exposure. Additional evidences for the occurrence of the photocrosslinking reaction are as follows: (a) the UV exposed polymer films became insoluble in organic solvents, including amide-type solvents in which polymers were initial soluble; (b) DSC analysis of the irradiated films evidenced no glass transition region and no exotherms in their thermograms measured in the range 20–350 °C (the photocrosslinking of coumarin side chains brings about a reduced free volume and a confined movement of polymer chains and these restrictions lead to an increased Tg); (c) thermal behaviour of the crosslinked polymers showed no significant weight loss up to 420 °C, and their Td’s and char yields in air were around 30–35° and about 4–6%, respectively, higher than those of parent polymers, indicating an improved thermal stability. Coumarin derivatives are widely used as blue fluorophores with commercial exploitation in a broad range of applications due to their strong fluorescence in the UV and visible region. For example, some authors recommended that coumarin derivatives substituted

M. Nechifor / Reactive & Functional Polymers 69 (2009) 27–35

35

4. Conclusion

Fig. 7. Fluorescence spectrum of (14g) in dilute solution (kex = 315 nm).

A new biscoumarin diacid was successfully prepared and several aromatic polyamides having bulky and rigid coumarin groups in their side chains were produced by a direct polycondensation reaction with various aromatic diamines. These polymers exhibited the ability to redissolve in polar aprotic solvents and showed high thermal stability. The polymers had high molecular weights as evidenced by their easily discernable Tg values and film-forming properties. There was no evidence of crystallinity in any of the thermograms. Irradiation of the polymer films by UV light rendered dimerization of the coumarin groups and resulted in gradual crosslinking, which induced insolubilization of the polymers. An additional conclusion of this report is that the polyamides presented intense fluorescence. These polyamides are showing considerable promise for processable and thermal resistant materials, and can be regarded as potential negative-type photoresist materials. References

at positions 3, 4, 6, 7 and 8 are suitable for use as dye laser media [21,22]. Among coumarin derivatives, dimeric coumarins (also called biscoumarins) present particular optical properties, such as a molar absorption coefficient much higher than that of monocoumarins, absorption and emission spectra shifted to longwavelengths, and a very high fluorescence quantum yield. These physical characteristics are also influenced by the nature and position of substituents [23,24]. The polymers discussed here present in their repeating units a dicoumarin-3-carboxylic acid moiety in which the two fluorophores are linked through a methylene bridge at position 6. In addition, another two coumarin residues belong to the monomer as substituents at position 8 and tethered by an oxyacetoxy spacer. Therefore, aromatic coumarinated polyamides under study are highly fluorescent. The emission spectra of the polymer solutions showed only one band with a maximum located at 387 nm, irrespective of the structure of aromatic diamine moiety, which means that the PL maximum corresponding to coumarin chromophore in the backbone could not be detected by exciting with absorbance maxima (Fig. 7). The fluorescence spectra of polymer films showed red shifts of 18 nm in the emission maxima and are characteristically broader as compared with those in dioxane solution.

[1] D.J. Liaw, B.Y. Liaw, J.R. Chen, C.M. Yang, Macromolecules 32 (1999) 6860. [2] M. Inoki, F. Saitou, A. Kozuka, K. Izumoto, Y. Kasashima, F. Akutsu, J. Macromol. Sci. Pure Appl. Chem. A40 (2003) 45. [3] S.H. Hsiao, K.H. Lin, Polymer 45 (2004) 7877. [4] I. Sava, C. Hamciuc, Des. Monomers Polym. 8 (2005) 249. [5] L. Liu, S.H. Li, H.C. Lee, K.Y. Hsu, React. Funct. Polym. 66 (2006) 924. [6] S. Mallakpour, M. Kolahdoozan, React. Funct. Polym. 68 (2008) 91. [7] J.W. Jeong, Y. Kwon, Y.S. Han, L.S. Park, Mol. Cryst. Liq. Cryst. 443 (2005) 59. [8] L.Z. Zhang, Y. Li, Z.X. Liang, Q.S. Yu, Z.G. Cai, React. Funct. Polym. 40 (1999) 255. [9] K. Mizoguchi, E. Hasegawa, Polym. Adv. Technol. 7 (1996) 471. [10] D.M. Song, K.H. Jung, J.H. Moon, D.M. Shin, Opt. Mater. 21 (2002) 667. [11] Y. Ohe, H. Ito, N. Watanabe, K. Ichimura, J. Appl. Polym. Sci. 77 (2000) 2189. [12] N.S. Allen, L. Khan, M. Edge, M. Billings, J. Vercs, J. Photochem. Photobiol. A: Chem. 116 (1998) 235. [13] S.R. Trenor, A.R. Shultz, B.J. Love, T.E. Long, Chem. Rev. 104 (2004) 3059. [14] C.S. Marvel, N. Tarkoy, J. Am. Chem. Soc. 79 (1957) 6000. [15] A. Song, X. Wang, K.S. Lam, Tetrahedron Lett. 44 (2003) 1755. [16] Y. Chujo, K. Sada, T. Saegusa, Macromolecules 23 (1990) 2693. [17] F. Higashi, Y. Taguchi, N. Kokubo, H. Ohta, J. Polym. Sci.: Part A: Polym. Chem. Ed. 19 (1981) 2745. [18] S.R. Trenor, T.E. Long, B.J. Love, Macromol. Chem. Phys. 205 (2004) 715. [19] Y. Chen, J.L. Geh, Polymer 37 (1996) 4481. [20] P.O. Jackson, M. O’Neill, W.L. Duffy, P. Hindmarsh, S.M. Kelly, G.J. Owen, Chem. Mater. 13 (2001) 694. [21] B.B. Raju, T.S. Varadarajan, J. Photochem. Photobiol. A: Chem. 85 (1995) 263. [22] T.L. Arbeloa, F. L Arbeloa, J.L. Arbeloa, J. Lumin. 68 (1996) 149. [23] H. Zhang, T. Yu, Y. Zhaob, D. Fan, L. Qian, C. Yang, K. Zhang, Spectrochim. Acta Part A 68 (2007) 725. [24] H. Ammar, S. Fery-Forgues, R. El Gharbi, Dyes Pigments 57 (2003) 259.