Synthesis and characterization of new polyphenols derived from o-dianisidine: The effect of substituent on solubility, thermal stability, and electrical conductivity, optical and electrochemical properties
Introduction
Polyazomethines (PAMs), with a wide range of applications, have had continual increasing interest due to having a lot of useful properties such as high thermal stability and excellent mechanical strength as well as their semi-conductivity and optoelectronic properties [1], [2], [3]. On the other hand, PAMs, especially aromatic derivatives suffer from low solubility. To solve this problem, many kinds of PAMs like poly(azomethine ether)s [4], poly(acrylate-azomethine)s [5], poly(azomethine carbonate)s [6], poly(amide-azomethine-ester)s [7] and poly(azomethine sulfone)s [8] have been synthesized so far. Additionally, PAMs containing methoxy substituents had been presented with fine solubility and high thermal stability [9]. PAMs derived from alcoxy-substituted benzidine had been previously presented as good soluble polymers [10]. The attempts to increase of solubilities of PAMs by side-group functionalization were clearly reviewed by Iwan and Sek [11]. According to this review chemical modification in structures of PAMs by side-group functionalization can weaken interchain interactions and consequently improve the solubilities. In the present study, we synthesized some soluble PAMs derived from o-dianisidine which has dimethoxy substituents at ortho positions of –NH2 groups of benzidine.
As a result, we obtained good soluble polyphenols containing azomethine groups. Many kinds of oligophenols had been previously investigated with their useful properties [11], [12], [13]. Also, substituent effect on radical-controlled oxidative polymerization rate of phenol monomers had been presented and the probable coupling mechanism of this reaction had been suggested [14]. According to these studies, radicalic oxidative polycondensation reaction of phenol derivatives proceeds by CC and CO coupling mechanism.
We previously reported several oligophenol derivatives of PAMs with a wide range of useful properties like paramagnetism, semi-conductivity, electrochemical cell, and resistance to high energy [15], [16], [17], [18], [19]. Moreover, some kinds of this class polymer are indicated to have antimicrobial activity as well as fine electrical properties [20], [21]. Also, it is known that the synthesis procedure of this class of PAMs is cheap and environmentally harmless [22]. This is because of using cheap oxidants (NaOCl, H2O2, air, etc.) in an aqueous medium. So, this method is important for green chemistry and it can be preferable to obtain polyconjugated azomethine polymers.
At the first part of this study, we synthesized the Schiff base monomers by condensation reaction of o-dianisidine with aromatic hydroxyl aldehydes containing different substituents. Then, we converted these compounds to polyphenol derivatives via oxidative polycondensation (OP) reaction using NaOCl as oxidant. We also followed the OP reaction of MHBADIAN monomer using a time-controlled spectrum mode of a UV–vis spectrophotometer. New growing peaks during polymerization were properly obtained. Solubility measurements of the synthesized compounds were carried out showing that the solubilities of the polymers are quite higher than those of the previously synthesized kinds due to containing methoxy groups. However, it is indicated that higher molecular weight of a polymer decreases its solubility even if it contains more solubility-increasing substituents. We characterized the synthesized compounds using FT-IR, UV–vis spectra, 1H and 13C NMR and SEC analyses. TG-DTA technique was used to determine the stabilities of thermal degradation. DSC analyses of the polyphenols were also carried out thus the glass transition temperatures (Tg) were determined. Optical properties were determined by 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 obtained by using cyclic voltammetry measurements. Additionally, electrical properties of doped and undoped polymers and Schiff bases were determined.
Section snippets
Materials
Salicylaldehyde, o-dianisidine, 4-hydroxybenzaldehyde, vanillin, 3-ethoxy-4-hydroxybenzaldehyde, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofurane (THF), dioxane, methanol, ethanol, acetonitrile, acetone, toluene, ethyl acetate, heptane, hexane, CCl4, CHCl3, H2SO4, KOH and HCl were supplied from Merck Chemical Co. (Germany) and they were used as received. Thirty percent aqueous solution of sodium hypo chloride, NaOCl was supplied by Paksoy Chemical Co. (Turkey).
Syntheses of the monomers
The Schiff
Solubilities and structures of the compounds
The synthesized monomers have light color-powder forms whereas their polyphenol derivatives are dark colored. The solubility test results are shown in Table 2. According to Table 2, MHBADIAN and EHBADIAN have higher solubilities in polar solvents such as methanol, ethanol, THF, acetonitrile and ethyl acetate compared to 2-HBADIAN and 4-HBADIAN. Yet, they are partly soluble in toluene, chloroform, CCl4, hexane and heptane. What is more, 2-HBADIAN and 4-HBADIAN are generally partly soluble in
Conclusions
Novel soluble polyphenol derivatives of azomethine compounds having polyconjugated structures were synthesized by oxidative polycondensation reaction. It was indicated that methoxy substituent made these polymers highly soluble. In contrast to their monomers, P-MHBADIAN and P-EHBADIAN were less soluble than the other polyphenols, while they have more methoxy/ethoxy substituents. This is because of their highly molecular weights. Only MHBADIAN is able to polymerize at low temperatures like room
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