Novel aromatic polyamides with main chain and pendant 1,2,4-triazole moieties and their application to the extraction/elimination of mercury cations from aqueous media

Abstract

This work describes five novel aromatic polyamides and copolyamides containing the triazole heterocycle attached by an amine linkage to the main and lateral chains. The polyamides were found to be amorphous and soluble in polar aprotic solvents. They demonstrated a film-forming capability, exhibited good thermal resistance and had high thermal transition temperatures (up to 375 °C). The triazole and amine groups imparted hydrophilicity to the polymers, giving rise to materials that improve their mechanical properties upon water uptake, and were efficient chelating/host units for heavy metal cations. These properties facilitated the preparation of solid polymer phases to be used for the extraction and elimination of environmentally deleterious cations from water environments. In regards to cation extraction from aqueous media with polyamide solid phases, the correct polymer structure designs led to the extraction of high percentages of Hg^II (neat 100%).

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Introduction

The outstanding thermal and mechanical resistance of wholly aromatic polyamides (aramids) classifies them into the category of high-performance organic materials. Thus, the commercial aromatic polyamides, poly(p-phenylene terphthalamide), poly(m-phenylene isophthalamide), and copoly(p-phenylene/3,4′-oxydiphenylene terephthalamide), are useful polymers applied in advanced technologies, such as flame-resistant and high tensile strength coatings and fibres to be used as fabrics and fillers in the aerospace and armaments industry, in asbestos substitutes, as electrical insulation, in bullet-proof body armour, as protective clothing, industrial filters, and sports fabrics. Moreover, they are increasingly being used as advantageous replacements for metals or ceramics in currently produced goods, or even as new materials in novel technological applications.^1–3

Their properties arise from their aromatic structure and amide linkages, which result in stiff, rod-like macromolecular chains that interact with each other via strong and highly directional hydrogen bonds. These bonds create effective crystalline microdomains, resulting in high-level intermolecular packing and cohesive energy. As a result, they exhibit extremely high transition temperatures that lie above their decomposition temperatures, are sparingly soluble in common organic solvents and, accordingly, can only be transformed upon solution.

Research efforts are therefore underway to take advantage of these properties, enhance their processability and solubility, and incorporate new chemical functionalities in the polyamide backbone or in the lateral structure, so that their applicability is expanded and remains on the forefront of scientific research.¹ Among others, the heterocycles are interesting chemical structures to modify the properties of aramids and other thermally stable polymers because they usually impart excellent thermal stability with improved solubility. As a consequence, research on aromatic polyamides with heterocycles in the main chain (oxadiazole,⁴ pyridine,⁵ diquinoline,⁶ phthalazinone,⁷ triazine,⁸ imide,⁹ benzofuran,¹⁰etc.) and in the pendant structure (carbazol,¹¹ imidazole,¹² benzoxazole,¹³ pyridine,¹⁴etc.) is widespread in the scientific literature.

Furthermore, the heterocycle properties and characteristics are expected to be retained by the polymers containing them. In this regard, the triazole derivatives are well-known binding sites for metals, especially for Hg^II, and applications of these kind of molecules for the removal of mercury cations from aqueous media, as well as sensing and recognition of mercury ions and the preparation of fluorescent on-off switchable chemosensors, have been described.^15–17 Thus, the inclusion of the triazole heterocycle into macromolecular chains may permit the preparation of polymers with good mechanical, thermal and chemical properties that can be transformed into materials capable of interacting selectively with cations, specifically mercury ions. These polyamides could then be useful in the development of sensing materials, permselective membranes and solid phases to extract environmentally polluting cations, among other applications.

We have been working recently on the development of new aromatic and aliphatic-aromatic polyamides with a la carte properties, such as enhanced solubility, superior thermal and mechanical properties, high water uptake, recognition and sensing capabilities, and environmentally harmful cation extraction capabilities.^18,19 The introduction of some kind of specific chemical functionality to the main or lateral chain can broaden the scope of the technological applications of the aromatic polyamides, giving rise to polymers with speciality properties, such as optical activity, electrochromicity and luminescence, special materials for membranes, and environmental or supramolecular-related technologies and applications.^1,2

Continuing with our line of research, we describe in this work the synthesis and characterisation of five novel aromatic polyamides and copolyamides with 1,2,4-triazole moieties in the main and lateral structures. The polymers have been prepared by the polymerisation of two kinds of new monomers: aromatic diacids (A-A monomers), and an amino acid (A-B monomer). The properties of the polymers, in terms of thermal and mechanical behaviour, water uptake and solubility, have been analysed and a comparison with commercial aromatic polyamides is presented.

Moreover, the behaviour of the pendant or main chain triazole heterocycle as a heavy-metal chelating group has given rise to the development of polyamide solid phases with affinity toward heavy-metal cations, especially Hg^II, in aqueous environments. As a result, we have studied the ability of these materials to eliminate environmentally toxic heavy metal cations as their nitrate salts from aqueous media. The choice of the polyamide backbone for extraction experiments comes from the outstanding thermal and mechanical stability of this type of polymers, their hydrophilicity (necessary to extract analytes from water media), as well as their poor solubility in water and in common organic solvents, which makes them especially useful as solid materials in molecular recognition because they can be regenerated and reused by a simple filtration and washing procedure.^1,19h–k

Experimental

All materials and solvents were commercially available and were used as received, unless otherwise indicated. N-Methyl-2-pyrrolidone (NMP, Aldrich) was vacuum-distilled twice over phosphorous pentoxide, and then stored over 4 Å molecular sieves. Lithium chloride (Aldrich) was dried at 400 °C for 12 h prior to use. Triphenylphosphite (TPP, Aldrich) was vacuum distilled twice over calcium hydride, and then stored over 4 Å molecular sieves. Pyridine (Merck) was dried at reflux temperature over sodium hydroxide for 24 h, and then distilled and stored over 4 Å molecular sieves. m-Phenylenediamine (MPD) and p-phenylenediamine (PPD) were commercially available (Aldrich) and were purified by double vacuum sublimation.

Intermediates and monomers

The two aromatic diacids were synthesised following the reaction conditions described in Scheme 1.

Scheme 1 Monomer synthesis and labels.

Diethyl 5-(3-benzoylthioureido)isophthalate (1). Approximately 54 mmol of ammonium thiocyanate, 45 g of silica gel and 125 mL of acetonitrile were placed in a 500 mL round bottom flask fitted with magnetic stirring. The suspension was stirred at room temperature for 30 min and then the solvent was removed under reduced pressure. 150 mL of 1,2-dichloromethane and 27 mmol of benzoyl chloride were added and stirred with silica gel supported ammonium thiocyanate for 2 h at room temperature. Afterwards, 54 mmol of ethyl 5-aminoisophthalate dissolved in 50 mL of 1,2-dichloromethane was added and the suspension was stirred at room temperature for 1 h, after which the solvent was removed under reduced pressure. The solid was extracted with 100 mL of N,N-dimethylacetamide and the solution was filtered to eliminate the insoluble silica gel. The clear solution was added dropwise to 500 mL of slightly acidified water to precipitate the diethyl 5-(3-benzoylthiouredio)isophthalate, which was filtered off and washed thoroughly with water. The crude product was then recrystallised from ethyl acetate.

Yield: 90%. M.p.: 148 °C. ¹H-NMR, δ_H (400 MHz, DMSO-d⁶, Me₄Si): 12.71 (1H, s, NH); 11.76 (1H, s, NH); 8.52 (2H, s, Ph); 8.34 (1H, s, Ph); 8.03 (2H, d, J 7.2, Ph); 7.72–7.68 (1H, m, Ph); 7.59–7.56 (2H, m, Ph); 4.39 (4H, q, J 7.2, CH₃CH₂); 1,38 (6H, t, J 7.2, CH₃CH₂). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 180.85, 169.05, 165.41, 140.09, 133.22, 133.02, 131.73, 130.29, 129.75, 129.45, 127.83, 62.44, 15.11. EI-LRMS m/z: 400 (M⁺, 14), 237 (24), 192 (16), 105 (100), 77 (66). FTIR [wavenumbers (cm⁻¹)]: ν_N–H: 3397; ν_{C–H(s, as)}: 2985, 2902; ν_C=O: 1722, 1670; δ_N–H: 1573; ν_Ar C=C: 1529; ν_C=S: 1309; ν_C–O: 1240.

5-(5-Phenyl-4H-1,2,4-triazol-3-ylamino)isophthalic acid (2). About 15 mmol of diethyl 5-(3-benzoylthiouredio)isophthalate were dissolved at room temperature in 150 mL of chloroform in a 250 mL round bottom flask fitted with magnetic stirring and a condenser. Afterwards, 75 mmol of hydrazine monohydrate was added dropwise and the reaction mixture was refluxed for 3 h. Then, chloroform was removed under reduced pressure and the solid obtained was washed thoroughly with distilled water. The product, diethyl 5-(5-phenyl-4H-1,2,4-triazol-3-ylamino)isophthalate, was filtered off and recrystallised from methanol.

Yield: 72%. M.p.: 228 °C. ¹H-NMR, δ_H (400 MHz, DMSO-d⁶, Me₄Si): 13.99 (1H, s, NH); 9.89 (1H, s, NH); 8.59 (2H, s, Ph); 8.04–8.00 (3H, m, Ph); 7.60–7.53 (3H, m, Ph); 4.40 (4H, J 7.2, CH₃CH₂); 1.39 (6H, t, J 7.2, CH₃CH₂). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 166.210, 161.10, 153.39, 143.81, 131.84, 131.05, 129.97, 128.07, 126.69, 121.18, 120.85, 61.96, 15.10. EI-LRMS m/z: 380 (M⁺, 100), 335 (14), 104 (45), 77 (16). FTIR [wavenumbers (cm⁻¹)]: ν_N–H: 3341; ν_{C–H(s, as)}: 2980, 2904; ν_C=O:1722; δ_N–H:1612; ν_Ar C=C:1590; ν_C=N:1549; ν_C–N:1468; ν_C–O: 1236.

In a 1 L Erlenmeyer flask fitted with a condenser and magnetic stirring, 10 mmol of 5-(5-phenyl-4H-1,2,4-triazol-3-ylamino)isophthalate was dissolved in 300 mL of methanol. About 75 mL of 20% aqueous sodium hydroxide was added and the mixture was then refluxed for 3 h, after which some methanol was removed under reduced pressure. 10% aqueous HCl solution was added drop wise to the mixture to adjust to pH 3 to precipitate 5-(5-phenyl-4H-1,2,4-triazol-3-ylamino)isophthalic acid (2). The product was filtered off, washed thoroughly with distilled water and MeOH, and dried in a vacuum oven overnight at 120 °C.

Yield: 92%. M.p.: 338 °C (d). ¹H-NMR δ_H (400 MHz, DMSO-d⁶, Me₄Si): 13.43 (3H, br s, NH, COOH); 9.87 (1H, NH); 8.54 (2H, s, Ph); 8.00–8.03 (3H, m, Ph); 7.59–7.50 (3H, m, Ph). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 168.01, 160.32, 154.34, 143.57, 132.74, 130.98, 130.07, 128.83, 126.81, 121.87, 121.41. EI-LRMS m/z: 324 (M⁺, 100), 181 (9), 119 (14), 104 (20), 77 (9). FTIR [Wavenumbers (cm⁻¹)]: ν_N–H: 3328; ν_acid O–H: broadband (3204, 2812); ν_C=O:1712; δ_N–H:1608; ν_Ar C=C:1590; ν_C=N:1549; ν_C–N:1484.

Ethyl 4-(3-(4-nitrobenzoyl)thioureido)benzoate (3). This compound was prepared and purified in a similar manner to that of diethyl 5-(3-benzoylthioureido)isophthalate (1).

Yield: 95%. M.p.: 179 °C. ¹H-NMR δ_H (400 MHz, DMSO-d⁶, Me₄Si): 12.61 (1H, s, NH); 9.25 (1H, s, NH); 8.38 (2H, d, J 8.8, Ph); 8.10–8.07 (4H, m, Ph); 7.86–7.84 (2H, J 8.8, Ph); 4.57 (2H, q, J 7.2, CH₂CH₃); 1.39 (3H, t, J 7.2, CH₂CH₃). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 177.63, 166.00, 165.38, 151.05, 141.43, 137.20, 130.74, 129.19, 128.97, 124.62, 123.28, 61.45, 14.60. EI-LRMS m/z: 373 (M⁺, 59), 179 (19), 165 (20), 150 (100), 120 (51), 104 (48). FTIR [Wavenumbers (cm⁻¹)]: ν_N–H: 3338; ν_{C–H(s, as)}: 2985, 2896; ν_C=O: 1723, 1693; δ_N–H: 1604; ν_Ar C=C: 1597; ν_C=S: 1309; ν_{NO2 (s, as)}: 1534, 1347; ν_C–O: 1242.

4-(5-(4-aminophenyl)-4H-1,2,4-triazol-3-ylamino)benzoic acid (4). 15 mmol of ethyl 4-(3-(4-nitrobenzoyl)thioureido)benzoate (3) was dissolved at room temperature in 150 mL of chloroform in a 250 mL round bottom flask fitted with magnetic stirring and a condenser. Afterwards, 75 mmol of hydrazine monohydrate was added drop wise and the reaction mixture was refluxed for 15 h. The solid obtained was filtered off and then it was stirred with slightly acidified water. The product, ethyl 4-(5-(4-nitrophenyl)-4H-1,2,4-triazol-3-ylamino)benzoate, was filtered and washed thoroughly with distilled water and EtOH.

Yield: 30%. M.p.: 310 °C. ¹H-NMR δ_H (400 MHz, DMSO-d⁶, Me₄Si): 14.00 (1H, s, NH); 10.05 (1H, s, NH); 8.38 (2H, d, J 8.4, Ph); 8.25 (2H, d, J 8.4, Ph); 7.92 (2H, d, J 8.4, Ph); 7.73 (2H, d, J 8.4, Ph); 4.29 (2H, q, J 7.2, CH₂CH₃), 1.33 (3H, t, J 7.2, CH₂CH₃). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 166.58, 157.11, 155.37, 148.45, 146.53, 136.75, 131.51, 127.65, 125.19, 121.78, 116.41, 61.08, 15.28. EI-LRMS m/z: 353 (M⁺, 100), 308 (49), 279 (4), 262 (15), 234 (5). FTIR [Wavenumbers (cm⁻¹)]: ν_N–H: 3341; ν_{C–H(s, as)}: 2985, 2903; ν_C=O:1672; δ_N–H:1610; ν_Ar C=C:1591; ν_C=N:1549; ν_C–N:1484; ν_{NO2 (s, as)}: 1531, 1338; ν_C–O: 1283.

In a 250 mL Erlenmeyer flask fitted with magnetic stirring and a condenser, 10 mmol of ethyl 4-(5-(4-nitrophenyl)-4H-1,2,4-triazol-3-ylamino)benzoate, 50 mmol of SnCl₂, 70 mL of ethyl acetate and 50 mL of ethanol were stirred together and heated under reflux. When the reaction mixture was completely dissolved, the reaction was kept at reflux for 2 h. Afterwards, solvents were removed under reduced pressure and the remaining solid was stirred with 150 mL of water. The pH was then made slightly basic (pH 7–8) by addition of 5% aqueous sodium bicarbonate. The resulting solid was filtered off and washed with water. The product, ethyl 4-(5-(4-aminophenyl)-4H-1,2,4-triazol-3-ylamino)benzoate, was extracted with methanol for 24 h in a Soxhlet.

Yield: 83%. M.p.: 241 °C. ¹H-NMR δ_H (400 MHz, DMSO-d⁶, Me₄Si): 13.40 (1H, br s, NH); 9.78 (1H, s, NH); 7.0 (2H, d, J 8.8, Ph); 7.73–7.70 (4H, m, Ph); 6.70 (2H, d, J 8.4, Ph); 5.63 (2H, s, NH); 4.29 (2H, q, J 7.2, CH₂CH₃); 1.33 (3H, t, J 7.2, CH₂CH₃). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 170.69, 164.45, 15.67, 155.60, 151.61, 135.44, 132.18, 124.61, 119.83, 119.49, 118.51, 64.90, 19.28. EI-LRMS m/z: 323 (M⁺, 100), 295 (13), 278 (35), 119 (33), 118 (24), 92 (8). FTIR [Wavenumbers (cm⁻¹)]: ν_N–H: 3367; ν_{C–H(s, as)}: 2978, 2903; ν_C=O:1681; δ_N–H:1607; ν_Ar C=C:1587; ν_C=N:1546; ν_C–O: 1279.

4-(5-(4-Aminophenyl)-4H-1,2,4-triazol-3-ylamino)benzoic acid (4) from 4-(5-(4-aminophenyl)-4H-1,2,4-triazol-3-ylamino)benzoate was prepared and purified in a similar manner to that of 5-(5-phenyl-4H-1,2,4-triazol-3-ylamino)isophthalic acid (2).

Yield: 89%. M.p.: 345 °C (d). ¹H-NMR δ_H (400 MHz, DMSO-d⁶, Me₄Si): 12.83 (1H, br s, COOH); 9.75 (1H, s, NH); 7.91 (2H, d, J 8.0, Ph); 7.75 − 7.71 (4H, m, Ph); 6.75 (2H, d, J 8.4, Ph), 5.96 (2H, br s, NH). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 168.42, 167.24, 160.16, 15.87, 150.98, 147.67, 147.37, 131.75, 131.59, 128.27, 121.59, 120.45, 116.20, 115.94, 115.85, 114.98. EI-LRMS m/z: 295 (M⁺, 100), 278 (2), 250 (4), 133 (8), 119 (38), 118 (27), 104 (5), 92 (7), 77 (6). FTIR [Wavenumbers (cm⁻¹)]: ν_N–H: 3373; ν_acid O–H: broadband (3070, 2781); ν_C=O:1679; δ_N–H:1613; ν_Ar C=C: 1586; ν_C=N: 1554.

1,3-bis(4-{[(4-{3-[(4-carboxyphenyl)amino]-4H-1,2,4-triazol-5-yl}phenyl)amino]carbonyl})benzene (5). In a 100 mL flask equipped with a nitrogen inlet and magnetic stirring, 20 mmol of 4-(5-(4-aminophenyl)-4H-1,2,4-triazol-3-ylamino)benzoic acid (4) was dissolved in 20 mL of NMP under a nitrogen blanket. Then, the solution was cooled to 0 °C by an ice-water bath and 10 mmol of trimethylsilyl chloride was added. Afterwards, 10 mmol of isophthaloyl chloride was added portionwise. The mixture was kept at 0 °C for 30 min, warmed to room temperature for 2 h, and then heated at 50 °C for 2 h. The final solution was added dropwise to 400 mL of water yielding a precipitate of the product, which was filtered off, washed thoroughly with water, extracted with acetone overnight and dried in a vacuum oven at 130 °C overnight.

Yield: 98%. ¹H-NMR δ_H (400 MHz, DMSO-d⁶, Me₄Si): 13.85 (2H, br s, NH); 10.73 (2H, br s, NH); 9.88 (2H, br s, NH); 8.62 (1H, s, Ph); 8.23 (2H, d, J 7.6, Ph); 8.03 (8H, m, Ph); 7.89 (4H, d, J 9.2, Ph); 7.78 (1H, t, J 7.6, Ph); 7.70 (4H, d, J 8.8, Ph). ¹³C-NMR, δ_C (100.6 MHz, DMSO-d⁶, Me₄Si): 171.09, 169.18, 167.15, 166.24, 166.11, 159.92, 154.35, 147.37, 146.56, 144.43, 135.99, 135.86, 131.96, 131.66, 129.76, 128.14, 127.42, 124.43, 123.17 121.38, 116.03, 115.84. FTIR [Wavenumbers (cm⁻¹)]: ν_N–H: 3301; ν_acid O–H: broadband (3045, 2844); ν_C=O:1681, 1650; δ_N–H:1607; ν_Ar C=C:1587; ν_C=N:1537.

Polymer synthesis

The polymer synthesis and acronyms are depicted in Scheme 2.

Scheme 2 Polymer synthesis and associated codes.

The following describes a typical polymerization reaction. In a 50 mL three-necked flask fitted with mechanical stirring, 10 mmol of diamine and 10 mmol of diacid (polymers I, II and IV), or 10 mmol of the amino acid (A-B monomer, polymer III), or 5 mmol of the amino acid, 5 mmol of MPD, and 5 mmol of isophthalic acid (copolymer V) and lithium chloride (1.4 g polymers I, II, and IV; 2.6 for polymer III and 2.0 g for copolymer V) were dissolved in a mixture of 6 mL of pyridine, 22 mmol of TPP and 20 mL of NMP (28 mL of NMP for copolymer III). The solution was stirred and heated at 110 °C under a dry nitrogen blanket for 4 h. The system was then cooled to room temperature and the solution was precipitated in 300 mL of methanol to render a swollen, fibrous precipitate. The polymer obtained was filtered and washed with methanol and distilled water, after which it was extracted with acetone for 24 h in a Soxhlet, and dried in a vacuum oven at 120 °C overnight. The yields were quantitative for all of the synthesised polymers.

Measurements. ¹H and ¹³C NMR spectra were recorded with a Varian Inova 400 spectrometer operating at 399.92 and 100.57 MHz, respectively, with deuterated chloroform (CDCl₃), or deuterated dimethylsulfoxide (DMSO-d₆) as solvent. Infrared spectra (FT-IR) were recorded with a Nicolet Impact spectrometer and inherent viscosities were measured with an Ubbelohde viscometer at 25 ± 0.1 °C with NMP as a solvent at a concentration of 0.5 g/dL.

Differential scanning calorimetry (DSC) data were recorded on a Perkin–Elmer Pyris I analyser with 10 mg of sample under a nitrogen atmosphere at a scan rate of 20 °C min⁻¹. Thermogravimetric analysis (TGA) data was recorded on a 5 mg sample under a nitrogen or oxygen atmosphere on a TA Instrumenta Q50 TGA analyser at a scan rate of 10 °C min⁻¹.

Polymer solubility was determined by mixing 10 mg of polymer with 1 mL of solvent, followed by stirring for 24 h at room temperature.

Polymer films were prepared by evaporation of cast solutions in NMP or DMA. Solutions of 8% by polymer weigh for polyamide I in NMP/DMA (50%, v/v) and 7% for polymer II and V in DMA were used. The solvent was eliminated by heating at 100 °C for 4 h in an air-circulating oven, and then at 120 °C for 4 h under vacuum (1 mm Hg). To determine the tensile properties of the polymers, strips (5 mm in width and 30 mm in length) were cut from polymer films of 30–45 μm thickness on a Hounsfield H10KM Universal Testing Dynamometer at 20 °C. Mechanical clamps were used and an extension rate of 5 mm min⁻¹ was applied using a gauge length of 10 mm. At least six samples were tested for each polymer and the data was then averaged.

Water absorption measurements were determined gravimetrically at room temperature. Powdered polymeric samples of about 300 mg, previously dried at 120 °C for 24 over phosphorus pentoxide, were placed in a closed box containing a saturated aqueous solution of NaNO₂ at 20 °C, which provided a relative humidity of 65%. The samples were periodically weighed over a period of 24 h and then allowed to remain in contact with this atmosphere for a further 8 days until they had equilibrated and presented no further changes in weight.

The solid-liquid extraction of Cr^III, Fe^III, Co^II, Ni^II, Cu^II, Zn^II Cd^II, Hg^II and Pb^II as their nitrate or chloride salts was carried out as follows. Approximately 10 mg of the appropriate polymer was shaken with 1 mL of an aqueous solution of the metal salt for a week at 25 °C (from previous kinetic experiments on extractions, equilibrium could be assumed in less than three days). The molar ratio of each cation to the triazole heterocycle subunits was 1 : 1. After filtration, the concentration of each cation in the liquid phase was determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500). Consecutive dilutions of sample aliquots with ultra-pure water/nitric acid (5% v/v) were performed to reach concentrations in the range of the calibration curve (from 0 to 40 ppb) and direct information regarding the extraction percentages of metal ions by polyamides was obtained.

Results and discussion

This work describes the properties and synthesis of aromatic polyamides and copolyamides bearing 1,2,4-triazole moieties linked to amine groups in the main and pendent chains. The triazole is a five-member cycle containing three nitrogens, which open the possibility to interchain interactions of this cycle with the polar amide and amine groups, thus deeply influencing the polymer properties and characteristics.

Monomer and polymer synthesis and characterisation

The reaction steps carried out to obtain the diacid monomers (2, 5) and the amino acid A-B monomer (4) were easy and well documented from an organic chemistry viewpoint, and produced the monomers from widely available and cheap chemicals in good yield and purity. The preparation procedure of intermediates 1 and 3, and the reaction mechanism, were previously described by Kodomari et al.²⁰ Purification by crystallization or solution-reprecipitation methods yielded monomers of high purity, as demonstrated by NMR spectra. The intermediates and monomers were characterised by IR, ¹H and ¹³C NMR spectroscopy, thereby fully confirming the chemical structure of all the products.

The polyamides have been synthesised by combining m-phenylenediamine or p-phenylenediamine with the diacid monomers 2 and 5, by homopolymerising the A-B monomer 4, or by copolymerizing the A-B monomer 4 with isophthalic acid and m-phenylenediamine. The polymers were synthesised according to the method described by Yamazaki et al. and others.²¹ Their structures are shown in Scheme 2, and the inherent viscosities are given in Table 1. As an illustrative example, Fig. 1 shows the chemical characterisation of polymer I. The inherent viscosity data, which was higher than 0.50 dL/g, and the ¹H and ¹³C spectra were consistent with a high degree of polycondensation, and the peaks were assigned on the basis of model compounds, the ab initio theoretical analysis (discussed below) and the monomer's spectra.

Table 1 Inherent viscosities and solubilities of the polymers

Polymer	ηinh (dLg^−1)	Solubility^a
		DMSO, DMA	DMF	NMP	m-cresol	THF, CHCl3, CH, AcOH	Oleum
a CH = cyclohexanone; ++ = Soluble at room temperature; + = soluble on heating; +− = partially soluble; − = insoluble. b Inherent viscosity in oleum. c Inherent viscosity in a solution of LiCl in NMP (0.1g mL^−1).
I	0.58	++	++	++	+−	—	++
II	1.04	++	++	+	—	—	++
III	0.38 ^b	−	−	−	−	−	++
	0.68 ^c
IV	0.50	++	+	++	—	—	++
V	0.56	++	+−	++	—	—	++

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Fig. 1 Characterisation of polyamide I: (a) IR-FT, (b) ¹H NMR, and (c) ¹³C NMR.

The 1,2,4-triazole heterocycle showed the tautomerism equilibrium depicted in Scheme 3, and exhibited acid/base behaviour consistent with their pK_as.²² The presence of the tautomers in the polymer solutions was detected by ¹H NMR experiments, although the spectrum of polyamide I depicted in Fig. 1 was obtained with selected experimental conditions in order to get a spectral pattern without the splitting of the triazole protons arising from the tautomerism of the heterocycle.

Scheme 3 Tautomerism and acidity of 1,2,4-triazoles.

The tautomerism equilibriums in the polyamides were studied by ¹H NMR, as depicted in Fig. 2. The results showed that at 20 °C and at relatively concentrated samples, two different sharp signals were observed for both the amino and triazole heterocycle protons in the ¹H NMR spectrum of polymer I. The increase in the experiment temperature from 20 °C to 100 °C caused a significant broadening of the proton signals of the triazole cycle and a collapse of the three signals of the amine proton. We believe that these results confirmed the tautomerism equilibrium, as at higher temperatures, the faster chemical proton interchange gave rise to a broadening in the signals of the cycle protons. Thus, the NMR pattern showed the behaviour of a unique overage cycle, due to the NMR time scale, thus giving rise to a relatively sharp unique signal of the amine proton. The broadening of these proton signals by decreasing the concentration of the ¹H NMR samples has also been detected, as depicted in Fig. 2.

Fig. 2 (a) Temperature dependence of the ¹H NMR spectra of polyamide I. (b) Concentration dependence of the ¹H NMR spectra of polyamide I: (upper) diluted conditions, 5.6 × 10⁻³ mol of structural unit/L; (lower) concentrated conditions, 8.4 × 10⁻² mol of structural unit/L.

The tautomerism was also studied theoretically (ab initio) by gas-phase density functional theory (DFT) calculations.²³ The estimated energies and ¹H NMR chemical shifts of the tautomers of the 1,2,4-triazole moiety of a polyisophthalamide model compound are shown in Table 2. The relative energy of the tautomers and the predicted chemical shifts were used to assign the NMR signals of the synthesised polymers.

Table 2 Gas phase DFT calculations of the energies and estimated ¹H NMR chemical shifts of three o the tautomers of the 1,2,4-triazole moiety of a polyisophthalamide model

Model compound	Energy (relative energy)^a/kJ mol^−1	Estimated ^1H NMR chemical shifts^a
		Ha	Hb
a : The geometry optimisation and energy evaluations were performed with Spartan software using density functional theory (DFT) calculations at the B3LYP/6-31G* level of theory.
M1	−4093735.74 (40.0)	5.66 (a1)	8.05 (b1)
M2	−4093766.31 (9.44)	5.73 (a2)	8.79 (b2)
M3	−4093775.75 (0.0)	5.54 (a3)	9.86 (b3)

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Properties of the polymers

Thermal properties. The thermal behaviour of the polyamides was evaluated by means of DSC and TGA. The polyamides showed from high to extremely high glass transition temperatures, ranging from 293 to 375 °C (Table 3).

Table 3 Thermal DSC and TGA data of the polymers

Polymer	DSC	TGA
	T g/°C	N2 atmosphere			O2 atmosphere
		T 5 /°C	T 10 /°C	Char yield (%)	T 5 /°C	T 10 /°C	Char yield^c (%)	LOI
a 5% weight loss. b 10% weight loss. c Limiting oxygen index.
I	293	380	400	50	380	400	—	37.5
II	306	390	405	46	385	405	—	35.9
III	375	400	420	50	390	415	3	37.5
IV	345	385	415	48	365	400	—	36.7
V	295	380	415	53	380	410	3	38.7

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Polyamides I and II showed strong similarity with a related polyamide in terms of chain stiffness, lateral volume and polarity of the bulky side group, as poly(m-phenylene 5-benzoylaminoisophthalamide) and poly(p-phenylene 5-benzoylaminoisophthalamide), which have a T_gs of 298 and 318 °C, respectively.¹⁰

Conversely, the T_g of polymer III was as high as 375 °C, showing the influence of the heterocycle in the structural unit, the rigidity of the backbone imposed by the rigid A-B monomer and the extra hydrogen bond interactions, as compared with conventional aramids with amine/amide, amide/heterocycle, heterocycle/heterocycle or amine/heterocycle subgroups.

The T_gs of polymer IV and copolymer V were closely related to the transition values of polymer III (375 °C) and poly(m-phenylene isophthalamide) (275 °C).

The thermogravimetric data of polymers, summarised in Table 3, confirmed their good thermal stability. The temperature of 10% weight loss of these polyamides under nitrogen and oxygen atmospheres was higher than 400 °C in all cases. TGA thermograms for polyamide II, depicted in Fig. 3, show a similar behaviour of the polyamides in both oxygen and nitrogen atmospheres, and had very similar decomposition temperatures. This was an important observation because most polymer applications occur under normal atmospheric environments.

Fig. 3 TGA of polyamide II under nitrogen and oxygen atmospheres (w.l.: weight loss percentage). The auto stepwise isothermal technique was carried out under nitrogen atmosphere.

Up to three degradation steps were detected, but most of the polymers showed only two. As an illustrative example, Fig. 3 depicts the TGA degradation pattern and the data interpretation of polymer II. When the second decomposition step was completed, the loss of the complete pendant group was observed. In order to improve weight change resolution, an auto stepwise isothermal experiment was performed for polyamide II (the use of the auto stepwise mode gives the highest possible degree of resolution between successive decomposition events, such as those associated with complex degradation processes). In this type of experiment the sample was heated at a constant rate until a weight loss was observed, continuing with the isothermal conditions until the decomposition event took place, and then resuming heating until the next weight loss. Using this approach, overlapping decomposition processes could sometimes be separated.

The char yield at a given temperature was intimately related with the thermal behaviour of the polymers. Thus, the char yield of these polymers in a nitrogen atmosphere was significant, being higher than 48% at 800 °C. The char yield could be used to estimate the limiting oxygen index (LOI) of the polymers using the experimental Van Krevelen equation (LOI = 17.5 + 0.4 CR, where CR is the char yield in % weight).²⁴ The polymers had LOI values higher than 36, meaning that our polymers could be classified as self-extinguishing polymers (LOI is defined as the minimum fraction of oxygen in a mixture of oxygen and nitrogen that will support combustion after ignition). As air contains approximately 21% oxygen, any material with a LOI lower than 21 will probably not burn in an open air situation.

Mechanical properties. Tough, transparent and almost colourless films were obtained by casting of polyamide I, II and V, which makes them suitable for testing as gas separation membranes, fixed-site carrier membranes for anion separation, ion selective membranes for ion selective electrodes or for anion purification, selective solid-liquid extraction of anions, or other technological applications related to the chelating capabilities of the triazole heterocycle.

Polyamide tensile strength and the Young's modulus (Table 4) were found to be around 40–44 MPa and 1.3–1.7 GPa, respectively. The mechanical properties of these polyamides could be considered reasonably good for non-oriented films made by casting on a laboratory scale with no post-thermal treatment.^14,25

Table 4 Mechanical properties and moisture absorption of the polyamides

Polymer	Mechanical properties			Moisture absorption
	Tensile strength/MPa	Young's Modulus/GPa	Elongation (%)	Water uptake (%)	Molecules of H2O/repeating unit	Molecules H2O/equiv of amino-triazole ^a
a Considering an uptake of 0.6 water molecules per amide group. b Dry film. c Film stored in a 65% relative humidity atmosphere. d film immersed in pure water for 24 h.
I	—^b	—	—	13.3	2.9	1.7
	37 ^c	1.5	2.4
	43 ^d	1.2	3.8
II	44 ^b	1.7	4.4	13.3	2.9	1.7
	50 ^c	1.5	10
	55 ^d	1.4	18
III	—	—	—	12.3	1.9	1.3
IV	—	—	—	12.0	5.5	1.5
V	40 ^b^c^d	1.4	2.2	10.3	3.0	1.2

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The influence of water uptake on mechanical properties is shown in Table 4. Increasing levels of water absorption were found to reduce the Young's modulus and enhance the tensile strength and elongation in polyamide I and II, thus enhancing their tenacity. These results agreed with our previous experiences in film handling. Dry strips of polyamide I were fragile and could be transformed into a creasable material after storing in a 65% relative humidity (r.h.) atmosphere or after immersing in pure water. The storage at 65% r.h. or immersion in water of polyamide II enhanced its tensile strength by 13% and 23%, respectively, while the Young's modulus fell by 18% and the elongation increased by 310% when the film was immersed in water. Conversely, the mechanical behaviour of polyamide V remained nearly constant with increasing quantities of water uptake.

Strips of polyamide II, previously immersed in pure water until the water uptake equilibrium was achieved, were also repeated subjected to tensile tests before fracture. The chain orientation induced by mechanical stretching enhanced the tensile strength and Young's modulus by 66% and 50%, respectively.

Wide angle X-ray scattering (WAXS) and fourier-transform infrared spectroscopy. Polyamide crystallinity, as evaluated by WAXS, displayed an amorphous pattern in all cases. Fig. 4 shows the wide-angle X-ray scattering pattern for polyamides I and III. The average distance of the polymer chains in the amorphous region was observed to be 4.9 Å. Another amorphous halo that overlapped with the former could be detected for polyamide III, which corresponded to an average interchain distance of 5.5 Å. This result was consistent with the Fourier-transform infrared spectrum recorded for this polymer, which had two peaks centred at 1660 and 1700 cm⁻¹, and related to the resonation frequencies of amide carbonyl groups in two different amorphous regions (Fig. 5). The peak centred at higher frequencies probably related to the establishment of lower energy interchain hydrogen bonds derived from larger interchain distances.

Fig. 4 X-Ray diffraction patterns of polyamide I (black line) and III (grey line).

Fig. 5 FT-IR spectra of polymers II and III.

Solubility. The polymer solubilities are shown in Table 1. The isophthalamides (I and II) were soluble in aprotic polar solvents due to the bulkiness of the pendant structure, and showed that the polyamide with the all-meta phenylene backbone (I) had enhanced solubility compared with polyamide II, which contained a para-phenylene residue in the structural unit.

Conversely, the rigid and ordered polyamide structure derived from the polymerization of the amino acid monomer (polyamide III, monomer 4) exhibited an extremely low solubility, and could only be dissolved in concentrated sulfuric acid and in aprotic solvents with salts (LiCl) as solubility promoters. The polymer was not degraded by the sulfuric acid, as shown by the variation of the inherent viscosities of solutions of III in oleum (Table 5). The insolubility of this polymer compared well with the solubility of poly(p-phenylene terephthalamide).¹ On the other hand, the copolymerisation of the amino acid monomer (4) with equimolecular quantities of isophthalic acid and m-phenylene diamine (Scheme 2) gave rise to a random copolymer (V) with a structure resembling both polyamide III and poly(m-phenylene isophthalamide), and which had an enhanced solubility compared with III.

Table 5 Evolution of the inherent viscosities of solutions of polymer III in concentrated sulfuric acid (100%) with time

Solution time	ηinh/dL g^−1^a
a Polymer concentration = 0.5 g/dL.
Freshly prepared solution	0.38
24 h	0.35
72 h	0.35

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Water absorption. The hydrophilic or hydrophobic character of the organic molecules arose from the presence or absence of hydrophilic groups. Regarding aromatic polyamides, their polar amide groups interacted effectively with water, leading to partially hydrophilic materials that absorbed water from the environment. Moreover, the polyamides described in our work had extra polar and hydrophilic groups, with the triazole cycle and amine moieties increasing moisture uptake. The water molecules interacted with the amide groups, diminishing the amide-amide interactions that endowed the polyamides with their high cohesive energy and, consequentially, their high thermal transitions and resistance and superior mechanical properties. As a result, it is the water uptake that determines the final application of these high performance materials, in particular because the absorbed water diminished the T_g and influenced mechanical, electrical and dielectrical properties. Conversely, in other technological fields, such as membrane technology, greater water uptake in water treatment membranes, or even in dense gas separation membranes, implied better performance.

The polyamides described herein had from two (I, II, III) to four (IV) amide groups, and from one (I, II, III) to two (IV) triazole and amine moieties per structural unit, with the resulting copolymers having complex structures. As it has been previously outlined, the amide, the amine and the triazole cycle moieties are highly polar groups, which influenced the water uptake of the polymers through hydrogen bonding interactions with water.

The measurement of the isothermal absorption of water at 65% r.h. gave values that correlated to the polyamide chemical structure. Table 4 shows the water absorption percentages for polyamides, the molecules of water per structural unit, and estimated molecules of water per amine-triazole moiety. The latter was calculated on the basis of an uptake of 0.6 water molecules per amide group, predicted from the water absorbed by poly(m-phenylene 5-benzoylaminoisophthalamide).¹⁶

The polyamide water absorption rates were high, ranging between 10.3–13.3%, with the higher values corresponding to the polymers with pendant structures, probably due to the amorphous structure of the polyamides and also because of the higher interchain distances imposed by the bulkiness of the lateral groups.

Regarding the number of water molecules per repeating unit, a range between 1.9 and 5.5 was observed, with the higher values corresponding to the polymer with the higher content of polar moieties in the polymeric structural unit (IV). The remarkably low value corresponding to polymer III could be attributed to the order derived from the linear and regular polymer structure that gave rise to crystallinity and a higher chain packing.

The estimated water uptake per triazole-amine group varied between 1.2 and 1.7 molecules, which gave an idea of the hydrophilic character of these two groups and of the influence they have on the polyamides.

Solid-liquid extraction. If a solution containing different target or guest molecules was in contact with a solid phase that could interact selectively with these guest molecules, and therefore act as a solid host, a solid-liquid extraction takes place. In our experiments, the triazole heterocycle moieties of the solid polyamides acted as hosts, or even as chelators, for cations dissolved in water. The selectivity and effectiveness of the extraction could be studied in terms of extraction percentage (% E), distribution coefficient (Kd′), and selectivity. The distribution coefficient is commonly defined as the concentration ratio of the cation in both phases and is a measure of the capacity of the material to extract cations under competitive conditions (see eqn (1), where V is the solution volume in litres and n the molar amount of polymer structural units).^{18, 26}

The solid-liquid extraction selectivity, α_S,L, is the ratio of two distribution coefficients (eqn (2)). The Hg^II cation was taken as the reference, as it has the highest distribution coefficient (Kd′_M2).

Table 6 depicts the solid-liquid extraction results for the elimination of several cations from aqueous solution using polyamide I as the extractant. We chose this polyamide because of its high water uptake as well as its all-meta phenylene polymer backbone conducting to lower linearity and less rigid macromolecular structure. Extraction levels between 25 and 36% were found for tested cations, except for Hg^II, which had an extraction level from aqueous solution close to 100%.

Table 6 Solid-liquid extraction of metal cations from aqueous solution by solid-phase polyamide I: extraction percentage (% E), distribution coefficient (Kd′) and selectivity (α) in competitive and non-competitive conditions

Salt	% E	Kd′/L mol^−1	α × 10^3 (Kdi′/KdHg′)
Cr(NO3)3	36	21	8
Fe(NO3)3	30	16	6
Co(NO3)2	35	21	8
Ni(NO3)2	35	21	8
Cu(NO3)2	36	21	8
ZnCl2	25	14	5
Cd(NO3)2	34	20	8
Hg(NO3)2	99	27 × 10^2	1 × 10^3
Pb(NO3)2	31	16	6

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The extraction level of mercury cations from aqueous media by solid polyamide phases points to future applications in the fields of water decontamination, membrane preparation for selective cation transport and in sensing materials, among other applications.

Conclusion

This work describes the synthesis and characterization of five new aromatic polyamides bearing the triazole heterocycle in the pendant or main chain structures. These polyamides showed a good thermal behaviour and good solubility that makes them suitable for solution transformation. These properties, in turn, enable tough colourless transparent films to be cast, which have properties which unusually enhance with increasing water content in the material. The polar triazole and the amide groups greatly increased the hydrophilicity of the polyamides, making these polymers promising materials for the production of water treatment or gas separation membranes. Moreover, the triazole group was found to be a chelating, or host, unit for toxic heavy metal cations, thus making these polyamides effective solid extractant phases in the extraction and elimination of mercury cations from water media (elimination of nearly 100% of the Hg^II cations was observed in the experimental conditions).

Acknowledgements

The authors gratefully acknowledge the financial support provided by the Spanish Ministerio de Educación y Ciencia – Feder (MAT2008-00946/MAT) and by the Caja de Burgos.

References

1. J. M. García, F. C. García, F. Serna and J. L. de la Peña, Prog. Polym. Sci., 2010, 35, 623–686.
2. M. Trigo-López, P. Estévez, N. San-José, A. Gómez-Valdemoro, F. C. García, F. Serna, J. L. de la Peña and J. M. García, Recent Pat. Mater. Sci., 2010, 2, 190–208.
3. (a) P. E. Cassidy, Thermally Stable Polymers, Dekker, New York, 1980; H.H. Yang, Aromatic High-Strength Fibers. Wiley, New York, 1989; (b) J. K. Fink, High Performance Polymers, William Andrew Inc, New York, 2008; L. Vollbracht, Aromatic Polyamides, in Comprehensive Polymer Science, ed. G. Allen, B. Bevington, G. V. Eastmond, A. Ledwith, S. Russo and P. Sigwald, Pergamon Press, Oxford, 1989, vol. 5, ch. 22, pp. 373–383; (c) J. Gallini, Polyamides, aromatic, in Encyclopedia of Polymer Science and Technology, John Wiley & Sons, New York, 2005, vol. 3, pp. 558-84; (d) S. Ozawa and K. Matsuda, Aramid copolymer fibers, in Handbook of Fiber Science and Technology. Vol III: High Technology Fibers: Part B, ed. M. Lewin and J. Preston, Marcel Dekker Inc., New York, 1989, vol. 3, ch. 1, pp. 1–34; (e) D. Tanner, J. A. Fitzgerald, P. G. Riewald and W. F. Knoff, Aramid structure/property relationships and their role in applications development, in Handbook of Fiber Science and Technology. Vol III: High Technology Fibers: Part B, ed. M. Lewin and J. Preston, Marcel Dekker Inc., New York, 1989, vol. 3, ch. 2, pp. 35–80; (f) H. H. Yang, Nomex aramid fiber, in Handbook of Fiber Science and Technology. Vol III: High Technology Fibers: Part C, ed. M. Lewin and J. Preston, Marcel Dekker Inc., New York, 1993, vol. 3, ch. 2, pp. 77–178; (g) J. Peterson, Aromatic Polyamides, in Encyclopedia of Polymer Science and Engineering, ed. H. F. Mark, N. M. Bikales, C. G. Overberger and G. Menges, John Wiley & Sons, Inc., New York, 1988, vol. 11, pp. 381–409.
4. (a) A. Abdolmaleki, Polym. Degrad. Stab., 2007, 92, 292–928; (b) I. Sava, M. D. Iosip, M. Bruma, C. Hamciuc, J. Robison, L. Okrasa and T. Pakula, Eur. Polym. J., 2003, 39, 725–738.
5. (a) K. Faghihi and Z. Mozaffari, J. Appl. Polym. Sci., 2008, 108, 1152–1157; I. In and S. Y. Kim, Polymer, 2006, 47, 547–552; (b) B. Tamami and H. Yeganeh, Polymer, 2001, 42, 415–420; (c) S. Mehdipour-Ataei and H. Heidari, Macromol. Symp., 2003, 193, 159–167.
6. F. A. Bottino, G. Di Pasquale, L. Scalia and A. Pollicino, Polymer, 2001, 42, 3323–3332.
7. (a) J. Tan, C. Wang, W. Peng, G. Li and J. M. Jiang, Polym. Bull., 2009, 62, 195–207; (b) L. Cheng and G. Jian, J. Appl. Polym. Sci., 2004, 92, 1516–1520.
8. (a) R. R. Pal, P. S. Patil, M. M. Salunkhe, N. N. Maldar and P. P. Wadgaonkar, Polym. Int., 2005, 54, 569–575; (b) A. D. Sagar, R. D. Shingte, P. P. Wadgaonkar and M. M. Salunkhe, Eur. Polym. J., 2001, 37, 1493–1498; (c) H. S. Patel, R. R. Shah and K. C. Patel, Int. J. Polym. Mater., 2007, 56, 499–510.
9. (a) S. H. Hsiao, C. P. Yang, C. W. Chen and G. S. Liou, Eur. Polym. J., 2005, 41, 511–517; (b) S. Bhuvana, M. Madhumathi and M. Sarojadevi, Polym. Bull., 2006, 57, 61–72; (c) K. I. Aly, M. H. Wafadan and M. A. Hussein, J. Appl. Polym. Sci., 2009, 112, 513–523.
10. A. Banihashemi and H. Firoozifar, Eur. Polym. J., 2003, 39, 281–289.
11. S. H. Hsiao, C. W. Chen and G. S. Liou, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 3302–3313.
12. (a) M. Ghaemy, H. Behmadi and R. Alizadeh, Chin. Chem. Lett., 2009, 20, 961–964; (b) V. Ayala, E. M. Maya, J. M. García, J. G. de la Campa, A. E. Lozano and J. de Abajo, J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 112–121.
13. J. H. Kim, Y. H. Kim, Y. J. Kim, J. C. Won and K. Y. Choi, J. Appl. Polym. Sci., 2004, 92, 178–185.
14. J. J. Ferreiro, J. G. de La Campa, A. E. Lozano and J. de Abajo, J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 5300–5311.
15. (a) S. Elshani, P. Apgar, S. Wang and C. M. Wai, J. Heterocycl. Chem., 1994, 31, 1271–1274; (b) L. Uzun, A. kara, B. Osman, E. Yilmaz, N. Besirli and A. Denizli, J. Appl. Polym. Sci., 2009, 114, 2246–2253; (c) A. Kumar and P. S. Pandey, Tetrahedron Lett., 2009, 50, 5842–5845; (d) J. Zhan, D. Tian and H. Li, New J. Chem., 2009, 33, 725–728.
16. H. C. Hung, C. W. Cheng, I. T. Ho and W. S. Chung, Tetrahedron Lett., 2009, 50, 302–305.
17. (a) B. Liu, G. C. Guo and J. S. Huang, J. Solid State Chem., 2006, 179, 3136–3144; (b) G. Mahmoudi, A. Morsali and L. G. Zhu, Z. Anorg. Allg. Chem., 2007, 633, 539–541; (c) K. C. Chang, I. H. Su, A. Senthilvelan and W. S. Chung, Org. Lett., 2007, 9, 3363–3366.
18. V. Calderón, F. C. García, J. L. de la Peña, E. M. Maya and J. M. García, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 2270.
19. (a) V. Calderón, F. García, J. L. de la Peña, E. V. Maya, A. E. Lozano, J. I. de la Campa, J. de Abajo and J. M. García, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 4063–4075; (b) V. Calderón, G. Schwarz, F. García, M. J. Tapia, A. J. M. Valente, H. D. Burrows and J. M. García, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 6252–6269; (c) J. M. García, F. C. García and F. Serna, J. Polym. Res., 2007, 14, 341–350; (d) F. Serna, F. García, J. L. de la Peña and J. M. García, High Perform. Polym., 2008, 20, 19–37; (e) N. San-José, A. Gómez-Valdemoro, F. C. García, F. Serna and J. M. García, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 4026–4036; (f) N. San-José, A. Gómez-Valdemoro, F. C. García, J. L. de la Peña, F. Serna and J. M. García, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 5398–5407; (g) M. J. Tapia, A. J. M. Valente, H. D. Burrows, V. Calderón, F. García and J. M. García, Eur. Polym. J., 2007, 43, 3838–3848; (h) V. Calderón, F. Sena, F. García, J. L. de la Peña and J. M. García, J. Appl. Polym. Sci., 2007, 106, 2875–2884; (i) N. San-Jose, A. Gómez-Valdemoro, F. Clemente García, V. Calderón and J. M. García, React. Funct. Polym., 2008, 68, 1337–1345; (j) N. San-José, A. Gómez-Valdemoro, P. Estevez, F. C. García, F. Serna and J. M. García, Eur. Polym. J., 2008, 44, 3578–3587; (k) A. Gómez-Valdemoro, V. Calderón, N. San-José, F. C. García, J. L. de la Peña and J. M. García, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 670–681; (l) N. San-José, A. Gómez-Valdemoro, V. Calderón, J. L. de la Peña, F. Serna, F. C. García and J. M. García, Supramol. Chem., 2009, 21, 337–343.
20. M. Kodomari, M. Suzuki, K. Tanigawa and T. Aoyama, Tetrahedon Lett., 46, pp. 5841–5843.
21. (a) N. Yamazaki, F. Higashi and J. Kawataba, J. Polym. Sci., Polym. Chem. Ed., 1974, 12, 2149–2154; (b) D. J. Liaw, B. Y. Liaw and M. Q. Jeng, Polymer, 1998, 39, 1597–1607; (c) D. J. Liaw, B. Y. Liaw and C. Y. Chung, Acta Polym., 1999, 50, 135–140; (d) D. J. Liaw and B. Y. Liaw, Macromol. Chem. Phys., 1999, 200, 1326–1332; (e) D. J. Liaw, B. Y. Liaw, J. R. Chen and C. M. Yang, Macromolecules, 1999, 32, 6860–6863; (f) D. J. Liaw, B. Y. Liaw and C. M. Yang, Macromolecules, 1999, 32, 7248–7250; (g) D. J. Liaw, P. N. Hsu, W. H. Chen and S. L. Lin, Macromolecules, 2002, 35, 4669–4676; (h) D. J. Liaw, F. C. Chang, M. K. Leung, M. Y. Chou and K. Muellen, Macromolecules, 2005, 38, 4024–4029; (i) D. J. Liaw, C. C. Huang and W. H. Chen, Polymer, 2006, 47, 2337–2348.
22. J. A. Joule and K. Mills, Heterocycle Chemistry, Blackwell Science, Oxford, 2000.
23. Spartan'08. Wavefunctin, Inc. Irving, CA; Spartan'08 for Windows, Macintosh and Linux. Tutorial and User's Guide. Wavefunctin, Inc. Irving, 2008.
24. D. W. Van Krevelen and K. te Nijenhuis, Properties of Polymers. Their Correlation with Chemical Structure; Their Numerical Estimation and Prediction from Additive Group Contributions, Elsevier, Amsterdam, 4th edn, 2009, pp. 855–57.
25. (a) A. Marcos-Fernández, A. E. Lozano, J. de Abajo and J. G. de la Campa, Polymer, 2001, 42, 7933–7941; (b) E. Ferrero, J. F. Espeso, J. G. de la Campa, J. de Abajo and A. E. Lozano, J. Polym. Sci., Part A: Polym. Chem., 2002, 40, 3711.
26. (a) B. Dunjic, A. Favre-Réguillon, O. Duclaux and M. Lemaire, Adv. Mater., 1994, 6, 484–486; (b) A. Duhart, J. F. Dozol, H. Rouquette and A. Deratani, J. Membr. Sci., 2001, 185, 145–155.

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