Synthesis and characterization of polyphenol derived

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Synthesis and characterization of polyphenol derived from Schiff bases containing methyl and carboxyl groups in the structure a

a

a

İsmet Kaya , Emre Kartal & Dilek Şenol a

Polymer Synthesis and Analysis Lab, Faculty of Sciences and Arts, Department of Chemistry, Çanakkale Onsekiz Mart University, 17020 Çanakkale, Turkey Published online: 11 May 2015.

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Designed Monomers and Polymers, 2015 http://dx.doi.org/10.1080/15685551.2015.1041084

Synthesis and characterization of polyphenol derived from Schiff bases containing methyl and carboxyl groups in the structure İsmet Kaya*, Emre Kartal and Dilek Şenol Polymer Synthesis and Analysis Lab, Faculty of Sciences and Arts, Department of Chemistry, Çanakkale Onsekiz Mart University, 17020 Çanakkale, Turkey

Downloaded by [Canakkale Onsekiz Mart Universitesi] at 08:47 11 May 2015

(Received 5 November 2014; accepted 21 March 2015) In this study, a series of Schiff bases (CBAA4MP, CBAA5MP, and CBAA3MP) which differ from each other based on the position of methyl group were synthesized. The derived monomers were changed into their polymer kind (P-CBAA4MP, P-CBAA5MP, and P-CBAA3MP) by oxidative polycondensation in aqueous alkaline medium using NaOCl as the oxidant. The structures of the synthesized compounds were enlightened using FT-IR, UV–vis, 1H-NMR, and 13C-NMR analyses. The 1H-NMR and 13C-NMR results showed that polymerization preceded by C–C and C–O–C couplings of the monomer units (phenylene and oxyphenylene groups). The molecular weight dispersion of the polymers was designated by size exclusion chromatography analysis. Electrochemical (Eg0 ) and optical (Eg) band gaps of the synthesized substances are measured using CV and UV–vis techniques, in order of. Solid state electrical conductivities of both doped and undoped states of the synthesized polymers were evaluated. Also, the effects to the electrochemical band gaps values of ortho, meta, and para positions of methyl group in the structures of polymers were examined from CV measurements. Keywords: Schiff base polymers; polyphenols; oxidative polycondensation; optimum reaction conditions

1. Introduction Poly(imine)s, which are also named as poly(azomethine)s or Schiff base polymers, are known as conjugated polymers.[1] Polymerization of diverse kinds of phenol monomers has been reported up till know. Some of them are phenols bearing Schiff base in their structures. Oxidative polycondensation (OP) of monohydroxy or dihydroxy phenols having Schiff base units as side group has been quite studied in acidic and alkaline medium.[2–8] For a long time, Kaya and co-workers have studied on this class of PAMs with thermal, electrochemical, optical, and electrical properties.[9,10] Their conductivities have been raised by doping with iodine.[11] The effects of different substituents on the properties of oligo/polyphenols included azomethine (–CH=N) and active hydroxyl (–OH) groups have been searched in early days.[12,13] In these studies, the effects on the electrical conductivity, thermal stability, solubility, optical, and electrochemical band gaps have been debated. Coupling selectivity of oligo/polyphenols and their –CH=N– containing derivatives have also been worked and two coupling mechanisms for this class of PAMs have been noted including C–O–C and C–C couplings.[12,14] For these reasons, in this study we synthesized polymer derived from Schiff bases containing methyl and carboxylic acid in the structure. Then, the structures of compounds were confirmed by FT-IR, UV–vis spectra, 1H, and 13C-NMR analyses. *Corresponding author. Email: [email protected] © 2015 Taylor & Francis

Thermal stabilities of compounds were determined from TG-DTA measurements. The molecular weight values of polymers were determined from size exclusion chromatography (SEC) measurements. DSC analyses of the polyphenols pioneer to the determination of glass transition temperatures (Tg). Optical properties were identified using UV–vis spectra, and the optical band gaps were calculated from absorption edges. The HOMO–LUMO energy levels and electrochemical band gap values of the compounds were acquired using cyclic voltammetry (CV) measurements. Additionally, the electrical properties of doped and non-doped polymers and Schiff bases were designated. 2. Experimental 2.1. Materials 2-amino-4-methylphenol (2A-4MP, 98%), 2-amino-5methylphenol (2A-5MP, 98%), 4-amino-3-methylphenol (4A-3MP, 98%), 4-carboxybenzaldehyde (4-CBA, 98%), and all solvents were commercially obtained from Fluka and Merck Chemical Co. and used as supplied. 2.2. Synthesis of the Schiff bases Schiff bases monomers were synthesized by the condensation reaction of aromatic amine and aromatic

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aldehyde. Reactions were performed as follows: 1.23 g 2A-4MP, 2A-5MP, and 4A-3MP were separately dissolved in 50 mL of ethanol. 1.5 g of 4-CBA dissolved in 50 mL of ethanol, and it was added into each flask and reactions were maintained for 5 h with magnetic stirrer at room temperature. CBAA4MP and CBAA5MP were dark yellow precipitates, while the unprecipitated CBAA3MP was filtered, recrystallized from acetonitrile and dried in a vacuum desiccator. (Yields: 95, 91, 93%

Scheme 1.

Synthesis of the monomers.

Scheme 2.

Synthesis of the polymers.

for CBAA4MP, CBAA5MP, and CBAA3MP, respectively.) The reaction schemes of Schiff bases, their names and abbreviations of products are shown in Scheme 1. CBAA3MP: 1H-NMR (DMSO-d6): δ ppm,10.06 (s, 1H, –COOH), 9.07 (s, 1H, –OH), 8.55 (s, 1H, –CH=N), 8.02 (d, 2H, Ar-He), 7.99 (d, 2H, Ar-Hd), 7.06 (d, 1H, Ar-Hc), 6.67 (s, 1H, Ar-Hb), 6.61 (d,1H, Ar-Ha), 2.28 (s, 3H, –CH3). CBAA4MP: 1H-NMR (DMSO-d6): δ


Designed Monomers and Polymers

Scheme 3.

The combinations of the phenylene (C–C) and oxyphenylene (C–O–C) units.

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Scheme 4.

Possible radicalic types for CBAA5MP.

ppm,10.07(s, 1H, –COOH), 8.97 (s, 1H, –OH), 8.76 (s, 1H, –CH=N), 8.10 (d, 2H, Ar-He), 8.04 (d, 2H, Ar-Hd), 7.05 (s, 1H, Ar-Hc), 6.88 (d, 1H, Ar-Hb), 6.79 (d, 1H, Ar-Ha), 2.21 (s, 3H, –CH3). CBAA5MP: 1H-NMR (DMSO-d6): δ ppm,10.07 (s, 1H, –COOH), 9.03 (s, 1H, –OH), 8.78 (s, 1H, –CH=N), 8.11 (d, 2H, Ar-He), 8.03 (d, 2H, Ar-Hd), 7.18 (d, 1H, Ar-Hc), 6.72 (s, 1H, Ar-Ha), 6.62 (d,1H, Ar-Hb), 2.22 (s, 3H, –CH3). CBAA5MP: 13 C-NMR (DMSO-d6): δ ppm, 167.00 (C13-COOH), 156.73 (C8-C=NH), 151.61 (C1-ipso), 140.24 (C9-H), 137.88 (C3-ipso), 134.52 (C7-ipso), 132.50 (C12-H), 129.52 (C11-H), 128.70 (C10-H), 120.22 (C6-H), 118.77 (C5-H), 116.70 (C2-H), 20.85 (C4-CH3).

Figure 1.

2.3. The synthesis of the polymers The synthesized monomers were transformed into their polymer derivatives in an aqueous alkaline medium using NaOCl as oxidant. Reactions were maintained in 250 mL two-necked round-bottom flask, and 50 mL of water and 1 mol/L KOH were added to monomers. When the monomers reacted with NaOCl in alkaline medium, radicals formed black color precipitates. Reactions were carried out at 80 °C for 24 h. The reaction mixtures were neutralized by HCl solution (1 M, 37%). The yields were washed by hot water in order to separate mineral salts. The synthesis reactions of polymers are shown in Scheme 2.

FT-IR spectra of CBAA5MP and P-CBAA5MP synthesized by OP.

Designed Monomers and Polymers Table 1.

5

FT-IR spectral data of the synthesized compounds. Compounds


−1

Functional groups (cm )

CBAA4MP

P-CBAA4MP

CBAA3MP

P-CBAA3MP

–OH C–H (aromatic) C–H (aliphatic) –OH (carboxylic acid) C=O –C=N C=C C–O

3364 3068 2865 2665–2544 1683 1624 1605 –

3203 3050 2980 2923–2861 1690 1553 1513 1253

3398 3055 2841 2632–2530 1686 1608 1573 –

3392 3075 2874 2664–2546 1675 1623 1589 1285

P-CBAA3MP: 1H-NMR (DMSO-d6): δ ppm,10.07 (s, 1H, –COOH), 9.03 (s, 1H, terminal –OH), 8.57 (s, 1H, –CH=N), 8.09 (d, 2H, Ar–He), 7.96 (d, 2H, Ar-Hd), 7.07 (d, 1H, Ar-Hc), 6.71 (s, 1H, terminal Ar-Hb, terminal ArHa), 2.28 (s, 3H, –CH3). P-CBAA4MP: 1H-NMR (DMSO-d6): δ ppm,10.08 (s, 1H, –COOH), 8.94 (s, 1H, terminal –OH), 8.74 (s, 1H, –CH=N), 8.10 (d, 2H, Ar–He), 7.97 (d, 2H, Ar-Hd), 7.22 (s, 1H, Ar-Hc), 6.88 (s, 1H, terminal Ar-Hb), 6.83 (s, 1H, terminal Ar-Ha),

Figure 2.

1

H-NMR spectra of CBAA5MP and P-CBAA5MP.

2.52 (s, 3H, –CH3). P-CBAA5MP: 1H-NMR (DMSO-d6): δ ppm,10.10 (s, 1H, –COOH), 9.04 (s, 1H, terminal –OH), 8.78 (s, 1H, –CH=N), 8.13 (d, 2H, Ar-He), 8.02 (d, 2H, Ar-Hd), 7.26 (s, 1H, Ar-Hc), 6.83 (s, 1H, terminal Ar-Ha),6.63 (s, 1H, terminal Ar-Hb), 2.33 (s, 3H, –CH3). P-CBAA5MP: 13C-NMR (DMSO-d6): δ ppm,180.68 (C13-COOH), 167.18 (C8-C=NH), 149.51 (C1-ipso), 141.60 (C9-H), 137.19 (C3-ipso), 134.78 (C7-ipso), 131.38 (C12-H), 130.32 (C11-H), 129.93 (C10-H),

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Figure 3.

13

C-NMR spectra of CBAA5MP and P-CBAA5MP.

122.60 (C6-H), 116.99 (C5-ipso), 115.99 (C2-ipso), 21.75 (C4-CH3), new peaks: 144.73, 143.01 (C–O–C coupling), 128.23, 125.70, 124.91 (C–C coupling). Reaction mechanism for the OP of P-CBAA5MP in aqueous alkaline medium and resonance structures of the radicals are exhibited in Scheme 3. Since the compound contains two hydroxyl groups, dispersion of the radicals in the cyclic structure is shown one-sided. Radicals are positioned in ortho and para, thus couplings during OPs maintain in the specified directions. For an instance of radicals produced in the synthesis of Schiff bases, CAA5MP is shown in Scheme 4.

2.4. Characterization techniques The solubility tests were done in different solvents using 1 mg sample and 1 mL solvent at 25 °C. The infrared and ultraviolet-visible spectra were measured by Perkin Elmer FT-IR Spectrum one and Analytikjena Specord 210 Plus, respectively. The FT-IR spectra were recorded using universal ATR sampling accessory (4000–550 cm−1). 1H and 13 C-NMR spectra (Bruker AC FT-NMR spectrometer operating at 400 and 100.6 MHz, respectively) were also recorded using deuterated DMSO-d6 as a solvent at 25 °C. The tetramethylsilane was used as internal standard. Thermal data were obtained using a Perkin Elmer

Table 2. The number average molecular weight (Mn), weight average molecular weight (Mw), PDI, and percentage values of the synthesized polymers. Total Compounds P-BAA4MP P-BAA5MP P-CBAA3MP

I. Fraction

II. Fraction

Mw

Mn

PDI

Mw

Mn

PDI

%

Mw

Mn

PDI

%

23,300 16,760 31,300

19,650 11,200 20,900

1.19 1.50 1.50

51,250 27,200 –

44,300 19,300 –

1.16 1.41 –

15 40 –

18,400 9800 –

15,300 5780 –

1.20 1.70 –

85 60 –



Figure 4.

Absorption spectra of the synthesized monomers and polymers.

Diamond Thermal Analysis system. TG-DTA measurements were made between10 and 1000 °C (in N2, rate 10 °C/min). DSC analyses were carried out using Perkin Elmer Pyris Sapphire DSC. DSC measurements were made between 25 and 420 °C (in N2, rate 10 °C/min). The number average molecular weight (Mn), weight average molecular weight (Mw), and polydispersity index (PDI) were determined by SEC techniques of Shimadzu Co. For SEC investigations a SGX (100 Å and 7 nm diameter loading material) 3.3 mm i.d. ×300 mm column, DMF (0.4 mL/min) as solvent, polystyrene standards were used. A refractive index detector and UV detector were used to analyze the products at 25 °C. The surface morphology of nanoparticles was monitored using a Jeol JSM-7100F Schottky field emission scanning electron microscope. Table 3. Electrochemical and optical parameters of the synthesized compounds. Compounds CBAA4MPP P-CBAA4MP CBAA5MP P-CBAA5MP CBAA3MP P-CBAA3MP a

a

HOMO −5.76 −5.69 −5.75 −5.48 −5.67 −5.51

b

LUMO (eV) −3.14 −3.16 −3.13 −2.78 −2.99 −3.15

Highest occupied molecular orbital. Lowest unoccupied molecular orbital. c Electrochemical band gap. d Optical band gap. b

7

c 0

Eg (eV) 2.62 2.53 2.62 2.70 2.69 2.36

2.5. Optical and electrochemical properties UV–vis absorption spectra of the synthesized substances were recorded by Analytikjena Specord 210 Plus at 25 °C. The absorption spectra were recorded using DMSO at 25 °C. The optical band gaps (Eg) were calculated from their absorption edges. CV measurements were carried out with a CHI 660 C Electrochemical Analyzer (CH Instruments, Texas, USA) at a potential scan rate of 20 mV/s. All the experiments were performed in a dry box filled with argon at room temperature. The electrochemical potential of Ag was calibrated with respect to the ferrocene/ferrocenium (Fc/Fc+) couple. The half-wave potential (E1/2) of (Fc/ Fc+) was measured in acetonitrile solution of 0.1 M tetrabutylammonium hexafluorophosphate, and was 0.39 V with respect to Ag wire. The voltammetric measurements were carried out in acetonitrile for the Schiff bases and acetonitrile/DMSO mixture (v/v:5/1) for the polymers.

d

Eg (eV) 2.88 2.63 2.92 1.90 2.91 2.26

2.6. Fluorescence measurements A Shimadzu RF-5301PC spectrofluorophotometer was used in fluorescence measurements. Emission and excitation spectra of the synthesized compounds were obtained in DMF for monomers and polymers. Measurements were made in a wide concentration range between 3.125 and 100 mg/L to determine the optimal fluorescence concentrations. Slit width of all the measurements was 5 nm.

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3.1. Solubility of the compounds The solubility tests of the polymers synthesized by OP in aqueous alkaline medium and Schiff bases are examined in various organic solvents. Solubility tests of Schiff base monomers and dark brown-black colored polymers were carried out in test tubes (1 mg/1 mL) at room temperature. Insoluble substances were heated. Generally, the polymers dissolved in polar solvents such as DMF and DMSO; however, they did not dissolve in apolar solvents such as n-heptane and n-hexane.


3.2. Spectral analyses of the synthesized compounds The infrared spectra of Schiff base CBAA5MP and its polymer were shown in Figure 1. The peak values of functional groups of other polymers were given in Table 1. FT-IR spectrum in Figure 1 shows that –OH stretch for the hydroxyl group, C=O stretching vibration peak and characteristic peak of imine bond (HC=N) of CBAA5MP are observed at 3394, 1675, and 1627 cm−1, in turn. –OH peak of carboxylic acid was observed at the wide range of 3000–2500 cm−1. Because of the polyconjugated structure, wider and broader peaks were remarked. The vibration frequencies of C=O stretching

Figure 5.

of carboxylic acid, and imine bond (–CH=N) were observed at 1690 and 1656 cm−1, respectively. 1 H-NMR spectra of the synthesized CBAA5MP Schiff base and its polymer are shown in Figure 2. The 1 H and 13C-NMR peak values of other materials were given in the synthesis part. As seen in the 1H-NMR spectrum of CBAA5MP, the peak signals for –OH, -Ha, and Hb are observed at 9.03, 6.72, and 6.62 ppm, respectively. The spectrum of the polymer indicates the elimination of H by C–C coupling and –OH proton by C–O–C coupling. Kaya et al. have prior to study the radical polymerization mechanism of polymers.[12] New peaks appeared as a result of binding are observed at 7.55, 7.05, and 7.00 ppm. As seen in Figure 3, 13 C-NMR results of P-CBAA5MP indicate that the polymerization mainly proceeds by coupling of C2 and C5 carbon as well as phenoxy (PhO ) radicals. Some other peaks for the coupling points are also observed at the 13 C-NMR spectra of the polymers. On the other hand, in the 13C-NMR spectrum of the polymer new peaks which do not appear in the spectrum of its monomer, are observed. SEC results of P-CBAA4MP, P-CBAA5MP, and PCBAA3MP are given in Table 2. As seen in Table 2, two fractions for P-CBAA4MP and P-CBAA5MP, and

Cyclic voltammograms of the synthesized monomers and polymers.



9

Figure 6. Emission spectra of solutions in THF, DMF, and DMSO of compounds (Slit width: λEx 5 nm, λEm 5 nm; concentration of the compounds: 0.01 mg/mL).

only one fraction for P-CBAA3MP are obtained from SEC measurements. According to SEC analyses results, P-CBAA4MP, P-CBAA5MP, and P-CBAA3MP were contained 96, 69, and 128 repeated units, respectively. 3.3. Optical and electrochemical properties UV–vis absorption spectra of the synthesized substances were recorded by Analytikjena Specord 210 Plus at 25 °C. As solvent, methanol and DMSO were used for monomers and polymers, respectively. Measurements were maintained at room temperature. In Figure 4, the absorption spectra of Schiff base polymers and monomers are overlapped. When spectra are examined, the synthesized polymers in aqueous alkaline medium have a lower band gap than their monomers, in other words, a redshift was observed. A decrease in band gap is a sign of increase in conjugation. P-CBAA5MP this new broadband was observed centered at 570 nm. This band is characteristic for the conductive polymers. Optical band gaps of monomers are calculated

Figure 7. The color appearences of solutions in DMSO of polymers.

by Eg = 1242/λonset formula and results are given in Table 3. Cyclic voltammograms data, oxidation (Eox), and the reduction (Ered) peak potentials of the synthesized compounds are given in Figure 5. The HOMO-LUMO energy levels and electrochemical band gaps (Eg0 ) were figured out from oxidation and reduction peak

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reaction one of the phenol hydroxyl groups (–OH) vanishes as a result of C–O–C coupling.[18] As seen in Table 3, electrochemical band gaps and optical band gaps values were found to be compatible. According to ortho, meta, and para positions of methyl group in phenyl structure, the electrochemical band gap values of P-CBAA3MP, P-CBAA4MP, and P-CBAA5MP were changed as 2.36, 2.53, and 2.70 eV, respectively.

Table 4. Fluorescence spectral data of the synthesized compounds with optimum concentrations in some employed solvents. Solvents

a

CBAA4MP

DMF DMSO THF DMF DMSO THF DMF DMSO THF DMF DMSO THF DMF DMSO THF DMF DMSO THF

490 510 320 490 510 320 490 510 320 490 510 320 490 510 320 490 510 320

P-CBAA4MP CBAA5MP P-CBAA5MP


λEx

Compounds

CBAA3MP P-CBAA3MP

λmax

b

c (Em)

522 537 420 526 543 417 537 530 437 521 531 414 585 577 427 581 568 440

IEm

240 67 81 343 80 303 186 82 76 154 106 260 207 52 85 56 26 76

3.4. Fluorescence measurements Fluorescence measurements of the synthesized Schiff bases and their polymers are received using DMF, THF, and DMSO. Measurements were done at diverse concentrations to identify the optimal concentrations. In Figure 6, the color of P-CBAA4MP, P-CBAA5MP, and P-CBAA3MP are shown as the same in three solvents. As seen in Figure 7, P-CBAA4MP and CBAA3MP had exhibited higher fluorescence intensity in DMF. However, P-CBAA5MP showed higher fluorescence in THF. The obtained results from Figure 7 are given in Table 4. According to solution polarities, λmax emission wavelength values of P-CBAA3MP, P-CBAA4MP, and PCBAA5MP were changed between 440–581, 417–543, and 414–531 nm, respectively.

a

Excitation wavelength for emission. Maximum emission wavelength. Maximum emission intensity.

b c

potentials.[15] The calculations were made using the following equations [16,17]: E HOMO ¼ ð4:39 þ Eox Þ

(1)

E LUMO ¼ ð4:39 þ Ered Þ

(2)

Eg0 ¼ E LUMO E HOMO

(3)

3.5. Thermal analysis Thermal decomposition data (TGA, DTG, DTA, and DSC) are summarized in Table 5. According to the Table 5, the initial degradation temperatures (Ton) of the monomers are higher than those of their polymers. But char (%) values of the polymers are higher than those of the monomers apart from P-CBAA3MP. This is because of the formation of C–O etheric bond during the OP reaction (C–O–C coupling). This weak bond is easily broken at moderate temperatures and makes the polymer thermally unstable.[18] According to ortho, meta, and para positions of methyl group in phenyl structure, the char values (at 1000 °C) of P-CBAA3MP, P-CBAA4MP,

The calculated electrochemical band gaps agree with the optical band gap values; as a result of the polyconjugated structure the polymers have lower band gaps. However, it is observed in all CVs that HOMO energy levels of the polymers are lower than those of the Schiff bases. As stated above, during the polycondensation

Table 5.

Thermal degradation values of the synthesized compounds.

Compounds

a

CBAA4MP P-CBAA4MP CBAA5MP P-CBAA5MP CBAA3MP P-CBAA3MP

222 164 245 176 226 178

a

Ton

Wmax.T

20% weight loss

50% weight loss

% Char at 1000 °C

DTA exo/endo

DSC cTg (°C)/dΔCp (J/g K)

248, 350, 449 206, 442 267, 346 191, 430, 538 240, 465 203, 464

246 193 260 278 372 201

310 427 294 734 567 406

4 28 4 46 46 24

–/238 –/– –/257 526/– –/240 525/211

– 116/0.186 – 132/0.037 – 149/0.209

b

The onset temperature. Maximum weight temperature. c Glass transition temperature. d Change of specific heat during glass transition. b



Figure 8.

11

SEM photographs of P-CBAA4MP, P-CBAA5MP, and P-CBAA3MP.

and P-CBAA5MP were changed as 24, 28, and 46%, respectively. The highest char was observed in P-CBAA5MP containing methyl group at para positions in phenyl structure. According to DSC curves of the synthesized polyphenols, the glassy transition temperatures (Tg) is ranked as P-CBAA3MP > P-CBAA5MP > P-CBAA4MP. 3.6. Morphological characterization Morphological properties of the synthesized polyphenols are acquired by scanning electron microscopy (SEM) technique. SEM image of dust forms of the polymers are

given in Figure 8. According to the SEM photographs, the synthesized polymer P-CBAA4MP consists of unhomogenous-condensed particles. P-CBAA5MP has platelike structures with edges of approximately 7 μm wide and 13 μm long. P-CBAA3MP has porous structures. This spongy material is preferred for some reasons such as shear strength, reducing sound events, and water retention. 4. Conclusions Novel polyphenol were synthesized from Schiff bases containing methyl and carboxylic acid in the structure.


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According to SEC results, the synthesized compounds have molecular weights about 16,000–30,000 g mol−1. Solubility test results of the synthesized polyphenols indicated the good solubility of CBAA4MP, CBAA5MP, and their polymers. As well as P-CBAA3MP and PCBAA4MP are soluble in polar organic solvents such as DMSO and DMF. According to UV–vis measurements, P-CBAA5MP has a quite low band gap value of 1.90, and the electrical conductivity measurement by doping with iodine gave better results. Consequently, the conductivity of P-CBAA5MP could be highly increased and used as the semi-conductive polymer in electronic, optoelectronic, and photovoltaic applications. Abilities of the polymers to be used in gas sensing materials were discussed. Fluorescence intensity of the synthesized compounds showed different fluorescence effects in different solvents. Thermal degradation characteristics of the synthesized polymers have lower stabilities than those of their Schiff base monomers due to C–O–C coupling with lower C–C coupling. DSC analyses of the polymers showed that the Tg values vary between 110 and 140 °C. Disclosure statement

[7]

[8]

[9]

[10] [11]

[12]

[13]

No potential conflict of interest was reported by the authors.

References [1] Yang CJ, Jenekhe SA. Effects of structure on refractive ındex of conjugated polyimines. Chem. Mater. 1994; 6:196–203. [2] Kaya İ, Çetiner A, Saçak M. Synthesis, characterization and thermal degradation oligomer and monomer/oligomer metal complex compounds of 2-Methylquinolin-8-ol. J. Macromol. Sci. Part A: Pure Appl. Chem. 2007;44:463–468. [3] Kaya İ, Erçağ A, Çulhaoğlu S. Synthesis and characterization of oligo-2-[(2-hydroxymethylphenylimino) methyl] phenol and oligo-2-[(2-hydroxymethylphenyl imino) methyl]-4-bromo-phenol. Turk. J. Chem. 2007;31:55–63. [4] Kaya İ, Yıldırım M. Synthesis, characterization, thermal stability, conductivity and band gap of a new aromatic polyether containing an azomethine as a side. J. Appl. Polym. Sci. 2007;106:2282–2289. [5] Kaya İ, Bilici A. Syntheses, structures, electric conduction, electrochemical properties and antimicrobial activity of azomethine monomer and oligomer based on 4-hydroxybenzaldehyde and 2-aminopyridine. Polimery. 2007;52:827–835. [6] Kaya İ, Bilici A, Saçak M. Synthesis, characterization, and antimicrobial properties of oligo-4-[(pyridine-3-yl-methy-

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