Poly(1,3,4-oxadiazole-ether) with reactive carboxylic acid pendants was synthesized from solution polymerization via nucleophilic displacement polycondensation among 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole (BFPOx) and 4,4′-bis(4-hydroxyphenyl) valeric acid (BHPA). Without altering the polymeric segments, benzimidazole modified poly(1,3,4-oxadiazole-ether)s were prepared by varying stoichiometric ratios of 1,2-phenylenediamine. The molecular structural characterization of these polymers was achieved by, FT-IR, NMR, TGA, elemental analysis, and analytical techniques. The weight-average molecular weight of virgin polymer with carboxylic acid functionality was determined by gel permeation chromatography (GPC) and was found to be 22400
High performance polymers are of outstanding significance as light emitting diodes, nonlinear optical devices, and photovoltaic cells [
Benzimidazoles can be easily constructed via classical synthetic route from the condensations of 1,2-phenylenediamine with carboxylic acid functional group. Indeed, polymers bearing the carboxylic acid functionality can be converted practically to benzimidazoles; polymers with various heterocyclic units are widely used and studied for their good dielectric and pyroelectric performances [
The present paper describes the synthesis of a poly(1,3,4-oxadiazole-ether) that comprises carboxylic acid moiety (Figure
Synthetic route to prepare benzimidazole modified poly(1,3,4-oxadiazole-ether) via VALPOx polymer.
4,4′-Bis(4-hydroxyphenyl) valeric acid and 4-fluoro benzoic acid were purchased from Sigma-Aldrich and used as received.
Elemental analyses were performed with a PerkinElmer PE 2400 CHN elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded by a Thermo Nicolet −5700, USA, spectrometer (diamond window method). The 1H & 13C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer using tetramethylsilane (TMS) as an internal standard reference. The solid-state 13C-NMR spectra were recorded on a Bruker DSX-500 solid-state NMR spectrometer with a magnetic field of 7.04 T and carbon frequency of 125.77 MHz (internal standard was glycine). Thermogravimetric analysis (TGA) was performed on a PerkinElmer Diamond TGA/DTA thermogravimetric analyzer at a heating rate of 10°C/min in a nitrogen atmosphere. The number- and weight-average molecular weights (
Method was adapted from the literature [
MS: 259.0 (MH+); calcd for C14H8F2N2O, 258.36. Anal. Calcd: C, 65.12; H, 3.12; F, 14.7; N, 10.85; O, 6.20. Found: C, 65.04; H, 3.01; N, 10.68. FT-IR (diamond window): 1652 (C=N of Ox ring), 1560, 1504, 1422, 1364, 1320, 1214, 1104, 1038, 992, 845, 821, 779, 749, 709 cm−1.
The synthesis of VALPOx with active free carboxylic acid groups was achieved from the nucleophilic displacement reaction of oxadiazole-activated bis(fluoride) monomer with 4,4′-bis(4-hydroxyphenyl) valeric acid.
A typical synthesis of this polymer was conducted in a three-neck flask equipped with a nitrogen inlet, stirrer, Dean-Stark trap, and condenser.
The flask was charged with 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole, 0.2582 g (1 mmol), 0.2684 g (1 mmol), K2CO3 (0.2903 g, 2.1 mmol), N-Methyl pyrollidine (10 mL), and toluene (10 mL); the flask was purged with moisture free nitrogen three times. The reaction mixture was then heated to 120°C for 2 h until the toluene was all condensed in the Dean-Stark trap. Upon dehydration, the polymerization was heated to 180°C for 22 h. The cooled viscous reaction mixture was diluted with 5 mL of NMP and then dropped into 300 mL of demineralised water containing 10% hydrochloric acid. The precipitated polymer was repeatedly washed with excess of water and dried in vacuum at 60°C.
Yield: 85%; FT-IR (diamond window, cm−1) 1708 (C=O, sym.), 1585 (C=O, asym.), 1504 (–C=N, sym.), 1488, 1243 (–C–O–C), 1070 (C–O, oxadiazole ring). 1H NMR (400 MHz, DMSO-
A direct synthesis of a modified VALPOx polyether with 30 mass % 1,2-phenylenediamine (VALPOx-B-30) is discussed. In a 100 mL, three-neck flask equipped with a mechanical stirrer and a nitrogen inlet/outlet, finely powdered 1.0 g (9.0 mmol) of VALPOx and 0.3 g of 1,2-phenylenediamine were added and stirred by adding 20 g of polyphosphoric acid. The reaction system was then evacuated and filled with nitrogen for three times to remove air and moisture. The reaction mixture was stirred at 120°C for 1 h until complete dissolution of the polymer and 1,2-phenylenediamine; then temperature was slowly raised to 150°C for a period of 12 h. The reaction mixture was cooled to about 60–70°C and poured in to 500 mL ice cold distilled water with proper stirring, The dark pinkish brown fibrous material separated was collected by filtration and washed several times with water and finally washed with hot water until the filtrate was neutral. The resulting dark brown polymer VALPOx-B-30 was dried under vacuum at 60°C for 24 h.
Yield: 80%; FT-IR (diamond window, cm−1) 3394 (N–H), 1600 (–C=N, sym), 1488, 1243 (–C–O–C), 1069 (C–O, oxadiazole ring). 13C NMR (125 MHz, solid state):
A similar procedure was followed for the synthesis of polymers VALPOx-B-10 and VALPOx-B-20. For example,VALPOx-B-10 describes the VALPOx polymer that incorporates 10 mass % of 1,2-phenylenediamine.
The synthesis of new polymer VALPOx with active carboxylic acid moiety was achieved via conventional aromatic nucleophilic substitution polymerization technique from 4,4′-bis(4-hydroxyphenyl) valeric acid and 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole; the polycondensation was carried out at elevated temperature in NMP/toluene azeotrope in presence of anhydrous pulverised potassium carbonate as catalyst. The carboxylic acid in the virgin polymer is successfully converted to benzimidazole pendants via polyphosphoric acid condensation route. The amount of 1,2-phenylenediamine with respect to VALPOx varied as 10, 20, and 30 mass %, and the polymers thus obtained were designated as VALPOx-B-10, VALPOx-B-20, and VALPOx-B-30, respectively. The incorporation of benzimidazole moiety in polymer was confirmed via FTIR spectroscopy (diamond window method) in the range of 400–4000 cm−1; in each scan, the amount of well-grounded sample was kept constant (1 mg) in order to estimate the changes in the intensities of the characteristic peaks with respect to the amount of 1,2-phenylenediamine; the peak at 1070 cm−1 was assigned to C–O stretching of oxadiazole and benzimidazole modified poly(1,3,4-oxadiazole-ether) shows 3394 cm−1 peak for N–H stretching; NMR spectroscopic studies of synthesized polymers are in agreement with the proposed structure. Signal broadening of the 1H NMR spectrum of the polymer VALPOxwas due to polymerization and is presented in Supplementary Material (See Supplementary Material available online at
Thermogravimetric analyses (TGAs) were performed for all the polymers and the results were presented in Figure
TGA plot of poly(1,3,4-oxadiazole-ether)s at a heating rate of 10°C/min under a nitrogen atmosphere.
Kinetic and thermodynamic parameters were determined using Broido’s method [
Kinetic and thermodynamic parameters of polyethers.
Samples* | Decomposition range (°C) |
|
|
|
|
|
---|---|---|---|---|---|---|
P | 350–640 | 1.582 | −9.93 | −6.39285 | −161.624 | 124.3015 |
P-B-10 | 410–610 | 1.256 | −10.17 | −6.52982 | −161.222 | 126.6415 |
P-B-20 | 420–590 | 2.919 | −9.18 | −6.46377 | −161.298 | 125.4526 |
P-B-30 | 422–610 | 1.953 | −9.68 | −6.56409 | −161.563 | 125.6582 |
Plots of −Ln(ln(−1/
Finely powdered polymer samples were compressed into pellets of thickness in the range of 0.5-0.6 mm and subjected to the dielectric measurements; Sandwiched polymer samples between two silver-plated stainless steel electrodes were analysed by impedance analyzer model HIOKI 3352-50 HiTESTER Version 2.3. Silver paint (ELTECKS preparation number 1228-C) was coated on both of the flat exteriors of the pressed tablet and the electrical contacts were made using the same silver paint to the silver electrodes. The electrical contacts were checked to verify the ohmic connection. The measurements were carried out at room temperature in between the 50 Hz–5 MHz. The capacitance value (
The variation of dielectric constant versus frequency at room temperature was plotted in Figure
Room temperature variation of dielectric constant (
Plot of loss factor (tan
Room temperature variation of loss tangent on log (frequency) for VALPOx and benzimidazole modified VALPOx polymers.
The frequency dependence of the ac conductivity for VALPOx and benzimidazole modified VALPOx polymers are shown in Figure
Room temperature variation of AC conductivity with log (frequency) for VALPOx and benzimidazole modified VALPOx polymers.
Poly(1,3,4-oxadiazole-ether)s with pendant benzimidazole units were synthesized and characterized; it was demonstrated that good thermally stable polymeric stuff can be synthesized from the pendant carboxylic acid functional poly(1,3,4-oxadiazole-ether) (VALPOx); reactive pendant carboxylic acid group is exploited to hold the benzimidazole moieties, by polyphosphoric acid condensation. These polymers with benzimidazole and oxadiazole heterocycles exhibit remarkable thermal stability with decomposition temperature above 410°C. Frequency dependence ac conductivity has been found to support the nearly constant loss (NCL) model in isothermal condition at room temperature. The dielectric constant
It is to state that neither authors nor their institution has financial or other relationships with any organization or people that may influence the author’s work. Authors are not being paid by any organization or agency related with the product. There is no conflict of interests to be declared.
The authors gratefully acknowledge Indian Institute of Science, STIC, and SICART for providing NMR, TGA, and GPC measurements.