(2024) 18:152
ÇANAKÇI ﻿BMC Chemistry
https://doi.org/10.1186/s13065-024-01264-6

BMC Chemistry

RESEARCH

Open Access

Investigation of thermal properties
and structural characterization of novel
boron‑containing Schiff base polymers
Dilek ÇANAKÇI1*

Abstract
Three novel Schiff base polymers were synthesized by using the Schiff base monomer obtained using aldehyde
containing boric acid and amines containing hydroxyl groups as the starting materials. The polymers were synthesized at the same temperature and using the same amount of oxidizing agent by the oxidative polycondensation
method. The characterization of all polymers and monomers was achieved by using Fourier transform infrared
spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–vis), nuclear magnetic resonance spectroscopy (1H-NMR),
liquid chromatography-mass spectrometry (LC–MS), gel permeation chromatography (GPC) and scanning electron
microscopy (SEM). The thermal stability of these compounds was studied by employing thermogravimetric analysis(TGA), and thermodynamics parameters such as activation energy ­(Ea), enthalpy (∆H), entropy (∆S), and Gibbs freeenergy(∆G) for the decomposition process were calculated using the Flynn–Wall–Ozawa method.
Keywords Schiff base polymer, Thermal stability, Thermodynamics parameters, Characterization
Introduction
Polymers are long-chain, high-molecular-weight compounds that are formed from multiple same or different atomic groups with chemical bonds with various
degrees of regular structure. Each polymer chain is connected to other neighboring chains with Van der Waals
interactions. This bond creates a much weaker attraction
in comparison to covalent bonds. With the help of thisinteraction, every chain is intertwined in the form of acoil in the melt and constitutes the macroscopic polymer
structure. The type of the monomer forming the polymer,
the presence of side groups on the polymer chain and theinteractions of these structures affect the chemical, physical and mechanical properties of the polymer. Polymers*Correspondence:
Dilek ÇANAKÇI
canakcidilek@adiyaman.edu.tr
1
Vocational School of Technical Sciences, Adıyaman University,
Adıyaman 02040, Turkey

are found in numerous structures on products that we
use today. The new monomer that was used for polymer
synthesis in this study was a Schiff base monomer. Schiffbases are known as having good nitrogen donor ligands(> C=N–). These Schiff base ligands provide the metal ion
with one or more electron pairs during the formation of
the coordination compound. Due to these properties thatthey have, Schiff bases are considered a very importantorganic class. Schiff bases and transition metal complexes
have a wide variety of applications in many fields including analytical, biological and inorganic chemistry. Since
Schiff base monomers have very good biological activity, they are used as anticancer [1, 2], antimicrobial [3, 4],antioxidant [5], analgesic [6–9], anti-inflammatory [10],anticonvulsant [11], antituberculosis [12] and anthelmintic [13] agents in the medical field. Schiff bases are also
used in many applications as a catalyst, dye, pigment,
polymer stabilizer and intermediates in synthesis [14].
In this study aldehydes containing boric acid in theirstructure were used in the synthesis of the Schiff bases.

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﻿ MC Chemistry(2024) 18:152

Nowadays, boric acid and many compounds containing boric acid as a functional group are used in various
industrial fields. For example, boric acid is used in glass
production to produce specialized glasses and glass fibers as it increases the resistance of the glass against heat,chemicals and mechanical effects. It may be added tosoaps and detergents due to its water softening and antimicrobial properties. It is also used as a binder in ceramic
production. In recent years, boronic acid functionalizedmaterials have been developed for selective separation[15]. For example, enrichment of glycoproteins with boric
acid has led to the synthesis of new compounds, such asaffinity chromatography materials [16, 17], magnetic
beads [18, 19], nanoparticles [20] microplates [21, 22]
and mesoporous materials [23, 24]. Other areas whereboric acid is used are the medicine, agriculture and petrochemical industries [15].
Today, there are many polymers containing different functional groups in their structure. Among thesepolymers, polymers containing the element B synthesized using the oxidative polycondensation method arevery few. In order to reduce this deficiency, an aldehyde
containing B in its structure was used in our monomer
synthesis stage. Three novel Schiff bases containing
boric acid in their structures and three novel polymers
by using these Schiff base monomers were synthesizedin this study. The synthesized Schiff base monomers
and polymers were extensively characterized to obtain
information on their physical, chemical and morphological properties. To investigate their thermal stability, inparticular, thermogravimetric analyses were conductedat three different heating rates. The obtained data were
examined by using the Flynn–Wall–Ozawa method, andthe thermodynamic parameters of the compounds during their decomposition reactions were determined.

Experimental
Materials and measurements

3-Formyl-4-methoxyphenylboronic acid, 4-aminophenol, 2-amino-5-methylphenol, 2-amino-4-chlorophenol,hydrochloric acid, ethanol and tetrahydrofuran (THF)
were used for the Schiff base monomers synthesis. Potassium hydroxide and sodium hypochlorite were used for
the Schiff base polymers synthesis. The solvents used in
the absorption analysis of the synthesized compounds
were dimethyl sulfoxide (DMSO), ethanol and 1, 4-dioxane (All chemical obtained were from Merck & Country).
The FT-IR data were obtained by using a Spectrum 100
FT-IR instrument Perkin Elmer. The absorption spectra
were recorded in three solvents at different polarities on a
Lambda 35 ES UV/Vis Spectrophotometer Perkin Elmer,
where the scan range was 200–700 nm. The 1H NMR
spectra were recorded with an Avance II 400 M NMR

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spectrometer Bruker at ambient temperature, and tetramethyl silane (TMS) was used as the internal standard(DMSO-d6 was used as solvent). Mass spectrometry was
performed using an LC/MS-TOF Spectrometer Agilent.
The molecular weights of the obtained polymers wereanalyzed using a VISCOTEK 270 max GPC/SEC System
Malvern at a 1 mL/min THF flow rate. The morphology
and grain size of all materials, as well as their elemental
compositions, were examined using a JSM-6510 Series
Scanning Electron Microscope. Thermogravimetric analyses (TGA) were carried out on a DTG 60H-DSC 60 Shimadzu device under ­N2 gas flow for 5, 15, 45 °C/min at
20–1000 °C.
Synthesis of compounds
Synthesis of the Schiff base monomer

The reaction for Schiff base monomer synthesis wasstandardized according to the following procedure:
2 mmol of 3-formyl-4-methoxyphenylboronic acid
(0.358 g) was dissolved in 20 mL of ethanol and 0.1 mL
of HCl in a flask. To this solution was added the amino
compound [4-aminophenol for BS-OH, 2-amino5-methylphenol for BS-CH3 or 2-amino-4-chlorophenol
for BS-Cl (2 mmol)] dissolved in 10 mL of DMSO, andthe reaction mixture was refluxed for 6 h at 80 °C until
the color of the solution turned completely orange. At
the end of the period the solution in the flask was kept atroom temperature for one day and orange-colored crystals were obtained. After the Schiff base crystals were
filtered, they were recrystallized using an ethanol/water
mixture and air-dried (Fig. 1). The synthesized Schiffbase monomers were BS-OH: 3-(((4-hydroxyphenyl)
imino)methyl)-4-methoxyphenylboronic acid, BS-CH3:
3-(((2-hydroxy-4-methylphenyl) imino)methyl)-4-methoxyphenylboronic acid and BS-Cl: 3-(((5-chloro-2-hydroxyphenyl)imino) methyl)-4-methoxyphenylboronicacid.
The BS-OH yield was 60.44% as an orange crystal; LC/
MS-TOF (m/z): 271; FT-IR (KBr, ν ­cm−1): 3323, 3188
(O–H), 3052 (C–H, aromatic), 1639 (C=N), 1603 (C=C,aromatic); UV–visible (DMSO, nm): 260, 355; 1H NMR(400 MHz, dmso) δ 10.35 (s, 1H), 10.26 (s, 1H), 9.18 (s,1H), 8.64 (s, 1H), 8.15 (s, 1H), 8.05 (s, 1H), 7.64 (s,1H),
7.27 (d, J = 8.5 Hz, 1H), 7.17 (d, J = 18.9 Hz, 1H), 6.95 (d,J = 8.3 Hz, 1H), 6.85 (d, J = 7.8 Hz, 1H), 4.00 (s, 3H).
The BS-Cl yield was 51.86% as an orange crystal; LC/
MS-TOF (m/z): 305; FT-IR (KBr, ν ­cm−1): 3330, 3211
(O–H), 3055, 2885 (C–H, aromatic), 1645 (C=N), 1602
(C=C, aromatic); UV–visible (DMSO, nm): 231, 296; 1H
NMR (600 MHz, dmso) δ 11.06 (s, 1H), 10.35 (s, 1H),
9.11 (s, 1H), 8.54 (s, 1H), 8.16 (s, 1H), 8.04 (s, 1H), 7.41(s, 1H), 7.22 (s, 1H), 7.18 (s, 1H), 7.06 (s, 1H), 3.96 (s, 3H).

ÇANAKÇI ﻿BMC Chemistry(2024) 18:152

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Fig. 1 Preparation of Schiff base monomers (BS-OH, BS-CH3 and BS-Cl, a; DMSO, ethanol, HCl reflux at 80 °C for 6 h

The BS-CH3 yield was 61.21% as an orange crystal;
LC/MS-TOF (m/z): 286; FT-IR (KBr, ν ­cm−1): 3322,
3188 (O–H), 3052 (C–H, aromatic), 1639 (C=N), 1600
(C=C, aromatic); UV–visible (DMSO, nm): 224, 263,
359; 1H NMR (600 MHz, dmso) δ 10.61 (s, 1H), 10.35(s, 1H), 9.76 (s, 1H), 8.57 (s, 1H), 8.16 (s, 1H), 8.05 (s,1H), 7.18 (d, J = 11.4 Hz, 1H), 6.82 (s, 1H), 6.67 (s, 1H),
3.91 (s, 1H).

Synthesis of the Schiff base polymer

All three novel Schiff base polymers were synthesized
by a method reported by us in the literature [25]. After
placing 1 mmol of the monomer in a flask, 10 mL of
KOH solution (2 mol/L) was added. The mixture wasstirred under reflux for 30 min at 70 °C until a homogeneous solution was obtained. After obtaining a clear
appearance, 1.2 mL of NaOCl (0.12 mol) was added toÇANAKÇI B
﻿ MC Chemistry(2024) 18:152

the solution, the temperature was raised to 95 °C andthe solution was stirred at this temperature for 8 h. At
the end of this period, HCl (37%, 0.5 ml, 0.04 mol) was
added to the solution to terminate the polymerization
process. The formed polymers were separated fromthe liquid phase by filtration. The polymers that wereobtained were washed with benzene to remove unpolymerized monomers remaining in the structure, andthen dried in an oven at 105 °C (Fig. 2). The obtained
polymer was purified twice from ethanol/water. The
synthesized Schiff base polymers were PBS-OH:
poly-(3-(((4-hydroxyphenyl)
imino)methyl)-4-methoxyphenylboronic acid), PBS-Cl: poly-(3-(((2-hydroxy4-methylphenyl)imino)methyl)-4-methoxyphenylboronic acid) and PBS-CH3: poly-(3-(((5-chloro-2-hydroxyphenyl)imino)methyl)-4-methoxyphenylboronicacid).

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The PBS-OH yield was 59.52% as a black powder;
FT-IR (KBr, ν ­cm−1): 3331, 3231 (O–H), 2887 (C–H, aromatic), 1645 (C=N), 1607 (C=C, aromatic); UV–visible(DMSO, nm): 247, 270, 518; 1H NMR (400 MHz, dmso)
δ 10.27 (s, 20H), 10.07 (s, 8H), 9.83 (s, 9H), 9.39 (s, 11H),
9.09–7.67 (m, 197H), 7.44 (s, 11H), 7.27 (s, 15H), 7.13 (s,15H), 7.07 (dd, J = 26.7, 11.6 Hz, 74H), 6.98 (s, 16H), 6.88
(d, J = 8.7 Hz, 31H), 6.72 (s, 21H), 5.21 (d, J = 1674.3 Hz,
41H), 3.75 (s, 10H), 3.64–3.08 (m, 73H).
The PBS-Cl yield was 54.37% as a black powder; FT-IR(KBr, ν c­m−1): 3238 (O–H and C–H, aromatic), 1649
(C=N), 1583 (C=C, aromatic); UV–visible (DMSO,
nm): 220, 255, 573; 1H NMR (400 MHz, dmso) δ 10.76(s, 16H), 10.29–8.02 (m, 21H), 8.44 (s, 13H), 7.24 (d,J = 18.1 Hz, 10H), 7.06 (t, J = 16.3 Hz, 3H), 6.94 (s, 10H),
6.90 (s, 10H), 6.44 (dd, J = 28.8, 13.3 Hz, 2H), 5.08–2.36(m, 101H).

Fig. 2 Preparation of the Schiff base polymer PBS-OH with a polycondensation reaction (b) KOH solution, NaOCl, 95 °C, 8 hÇANAKÇI ﻿BMC Chemistry(2024) 18:152

The PBS-CH3 yield was 36.48% as a black powder;
FT-IR (KBr, ν c­m−1): 3239 (O–H and C–H, aromatic),
1643 (C=N), 1615 (C=C, aromatic); UV–visible (DMSO,
nm): 241, 299; 1H NMR (400 MHz, dmso) δ 10.27 (s, 5H),
9.39 (s, 5H), 7.53 (d, J = 10.3 Hz, 5H), 7.05 (d, J = 14.6 Hz,
20H), 7.00 (s, 5H), 6.93 (dd, J = 29.8, 16.2 Hz, 22H), 6.79
(s, 7H), 6.74 (s, 8H), 6.68 (s, 7H), 6.61 (s, 7H), 6.35 (d,J = 13.2 Hz, 6H), 3.81 (s, 19H), 3.76 (d, J = 15.7 Hz, 17H),
3.70 (s, 11H), 3.36 (s, 464H).

Results and discussion
LC/MS analysis

A LC/MS analysis method was used to determine themolecular weights of the Schiff base monomers. The
obtained analysis data showed that a large number ofpeaks were formed. According to the LC/MS spectra, the molecular ion values (m/z) for BS-OH, BS-Cl
and BS-CH3 were determined to be 271, 305 and 286(Fig. 3), respectively. Other peaks in the spectrum wereformed as a result of the separation of elements fromthe monomers during ionization. The first group toleave the structure of monomers was HCl bound to theschiff base structure. Then the hydroxyl groups, methyl
and chlorine groups were separated from the structure,
respectively. In the last step, the structure of the benzene
rings was destroyed as a result of ionization. Degraded

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parts combined between each other and formed compounds having greater molecular weights. This situation
caused to be seen more peaks in the LC/MS spectra than
expected.
GPC analysis

Polymers chains usually consist of the several different
types of atoms that are covalently bonded to each other.
Each chain is connected to the other chains by Van der
Waals interactions, which create much lower forces of
attraction compared to the covalent bonds. Thanks tothis interaction, each chain is intertwined with eachother coil, forming a macroscopic polymer structure.
The molecular weight and distribution of the polymer
are important for its chemical and physical properties.
Gel permeation chromatography was used to determine
the molecular weights of the synthesized polymers. The
data obtained showed that the molecular weights (Mnand Mw, g/mol) for PBS-OH were 4.832 and 6.040, for
PBS-Cl were 4.911 and 7.220, for PBS-CH3 were 5.209
and 6.825, respectively. The polydispersity index (PDI)
values of the polymers indicated that about 20–25 monomers were involved in the polymerization of each chain(PDI: 1.25 for PBS-OH, 1.47 for PBS-Cl and 1.31 for PBSCH3). That the polydispersity index was greater than one
shows that, although the repeated monomer units ofFig. 3 The liquid chromatography-mass spectrometry (LC/MS) chromatograph of monomer BS-CH3

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each polymer were the same, they were heterogeneous interms of chain length.
FT‑IR analysis

Important characteristic peaks of the synthesized compounds were seen in the IR spectra of the Schiff basemonomers and polymers (Figs. 4 and 5, respectively)
Stretching vibration peaks, which prove the existence of-OH groups both attached to the benzene ring and in the
structure of boronic acid, were observed in the monomer
spectra at 3323–3188 ­cm−1 for BS-OH, 3330–3231 ­cm−1
for BS-Cl and 3323–3188 ­cm−1 for BS-CH3 [26–29]. The
medium broad absorption peak observed at 3052 ­cm−1in the spectrum of BS-OH was assigned to the aromatic
C–H stretching vibration. In the BS-Cl and BS-CH3
spectra, this peak was formed more broadly, respectively,
at 3035–2885 and 3052 ­cm−1. This was because the typesof groups attached to the benzene ring and the locations
to which they were attached were contradicts. This difference affected the position of the peaks caused by thevibrations of the other groups in the same way.
Even if the main structure of the monomers was
the same, the differentiation of the groups in the benzene ring increased or decreased the peak intensities
as expected. The strong absorption band observed at
1639 ­cm−1 in the spectrum of BS-OH was assigned to
the C=N of the azomethine groups stretching vibrations,Fig. 4 FT-IR spectrum of Schiff base monomers

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indicating the formation of the Schiff base monomer,
and the peak at 1379 ­cm−1 was attributed to the C–Nstretching vibration [30]. Moreover, the strong peak at
1603 ­cm−1 was assigned to the aromatic C=C stretching vibration of benzene rings. In the BS-Cl and BS-CH3
spectra, the stretching vibration peak of azomethinecorresponding to Schiff base formation was observed at
1645 and 1639 ­cm−1, respectively. The C–N stretchingvibration peak that supported the expected was a bondformation was 1385 ­cm−1 for BS-Cl and 1373 ­cm−1 for
BS-CH3. The aromatic C=C stretching vibration belonging to the benzene rings forming the main structure of
the novel compounds was found at 1602 and 1600 ­cm−1,
respectively, in the BS-Cl and BS-CH3 FT-IR spectra.
One of the best spectroscopic techniques used to study
the microstructure of polymers is FT-IR spectroscopy.
The difference between the spectra of the synthesizedpolymers and the spectra of their monomers may be seenclearly. In the spectra of the polymers, the peaks werebroader, and the number of the peaks formed may belower. This was because polymerization formed a macromolecular structure, and the total number of groups
therefore more peaks increased due to the repeating
units. At the same time, peaks were formed in similar
regions as a result of the vibration of the groups, and thepeak appearance widened as a result of the overlap of thepeaks.

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Fig. 5 FT-IR spectrum of Schiff base polymers

In the polymer spectra, it was seen that the –OH andaromatic C–H peaks overlapped and formed quite wide
peaks between 2500 ­cm−1 and 3600 ­cm−1. A similar situation was valid for the C=C and C=N stress vibration
peaks, too. These peaks were formed at 1645–1607 ­cm−1
for PBS-OH, 1582 ­cm−1 for PBS-CI and 1613 ­cm−1 for
PBS-CH3 [25].
1

H‑NMR analysis

The 1H NMR spectra of the novel Schiff base monomers
and polymers in DMSO were analyzed with TMS as thestandard. The structures of the monomers were characterized from the assignments of the observed chemical
shifts to the corresponding protons. The imine proton
signal (c in Figs. 6 and 7), proving Schiff base monomer formation, was formed at 8.64 ppm, 8.54 ppm and8.57 ppm for BS-OH, BS-Cl and BS-CH3, respectively[31]. The hydroxyl group protons in the structure of the
Schiff base monomers generated successive several signals between 9 and 11 ppm. Furthermore, the methoxy
proton peaks (d) in the structure were formed at 4 ppm
for BS-OH, 3.96 ppm for BS-Cl and 3.91 ppm for BS-CH3[32]. The peak observed next to the methoxy proton peakbelonged to the water molecule, and the signal observed
around 2.5 ppm belonged to the DMSO. Additionally, thepresence of aromatic rings was proven by the singlet anddoublet signals of aromatic protons between 6.60 and
8.15 ppm.
The 1H NMR spectra of the polymers showed that thepolymers were formed. As a result of polymer formation,
the number of benzene rings, hydroxyl groups and methoxy protons in the structure increased. So, the same typeof proton signals overlapped with each other. All proton
signals formed a hill view by generating multiple signals
in the regions where they should be. Since polymerization took place over the hydroxyl groups and the benzene
rings, the hydroxyl proton signals were lower and did notform a hill view. The hydroxyl proton signals occurredas mostly three singlets for PBS-Cl in the range of 9.68–
10.76 ppm. Imine and methoxy proton signals occurredin the spectra between 8.74 and 9.40 ppm and between
3.58 and 3.94 ppm, respectively. The most significant evidence supporting polymer formation in the spectra was
the wide hill appearance formed by the overlap of benzene proton peaks in the range of 5.48–8.44 ppm.
SEM and EDX analysis

A scanning electron microscope (SEM) with an Energy
Dispersive X-Ray Analyzer (EDX) were used to illuminate
the surface morphology of the synthesized Schiff basemonomers and polymers and analyze the elemental composition of compounds. The instrument was operated
with various magnifications for obtaining the best images

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Fig. 6 1H NMR spectra (400 MHz, dmso) of Schiff base monomers

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Fig. 7 1H NMR spectra (400 MHz, dmso) of Schiff base polymers

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Fig. 8 Scanning electron microscopy (SEM) image of: synthesizedcompounds: a BS-OH; b PBS-OH; c BS-Cl; d PBS-Cl

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attached to the benzene ring affected the morphologiesof the compounds (Figs. 8 and 9).
Especially when the morphological structure of
BS-CH3 was evaluated in terms of roughness, pore sizeand geometry parameters, it was seen that it was quite
different from BS-OH and BS-Cl. All three monomers
had a fibrous structure on their surface, and there werealso crystallized particles independent of the matrix
structure. However, the surface structure of BS-CH3 wasmore stratified than the other two monomers. BS-CH3
had the most multi- fibrous structure among the monomers, and so its pore size and number were higher thanthe others. The SEM images obtained at different magnifications proved that the morphology of the Schiff basemonomers changed as a result of the polymerization. The
biggest difference that can be seen in the SEM images ofmonomer and polymer is that the monomers lost theirfibrous crystal structure as a result of polymerization.
In addition, the layer structure and the number of poresmonomers decreased. When the SEM images of the polymers were examined in terms of the number of pores, it
was seen that the order was in the form of PBS-OH, PBSCl and PBS-CH3 from largest to smallest. All polymers
had small particles of various sizes on their surfaces. In
particular, SEM images of PBS-CH3 showed a large number of small particles independent of the main structure.
Qualitative and quantitative elemental analysis of themonomers was performed using the Energy Dispersive X-Ray Analysis (EDX) method. The data obtained
showed that the monomers contained the expected elements (C, O, N, B) and that HCl was attached to the main
structure during Schiff base synthesis. The percentage of
Cl in the compounds was about 20% for all three monomers (Fig. 10). This result showed that approximately 2
HCl groups were bound to each of the Schiff base monomers. As a result of polymerization, th HCl, which
depends on the structure of the monomer, was separated
from the main structure.
UV–vis analysis

Fig. 9 Scanning electron microscopy (500 KX) image of:the synthesized compounds: a BS-CH3; b PBS-CH3of the samples (20 kV: 2.00 and 5.00 KX). The SEM
images clearly showed that, even if the main chemical
structure of the monomers was similar, different groups

UV spectroscopic analysis of the synthesized Schiff basemonomers and polymers was performed by using solvents with three different polarities to investigate theelectromagnetic beam absorption properties in the UV–
vis region. The absorption spectra of the compounds
obtained using dimethyl sulfoxide (DMSO), dimethylformamide (DMF) and ethanol solvents are given in Fig. 11.
All spectra showed that the compounds underwent π → π*, n → π * and charge-transfer electronic transitions at
various wavelengths in the polar protic (DMSO andEthanol) and polar aprotic (DMF) solvents. The bands
seen in the spectra are given in Tables 1 and 2. The Schiffbase monomers and polymers formed π → π * ­(E2 andÇANAKÇI ﻿BMC Chemistry(2024) 18:152

Page 11 of 26

Fig. 10 EDX spectra and SEM image of a PBS-Cl and b BS-Cl

B band) and n → π * bands due to the benzene rings intheir structures. Additionally, BS-Cl and BS-OH formed
the charge-transfer transition band in the DMF andDMSO solvents, respectively. When the spectra of thepolymers and monomers were compared, it was seen that
there was 25 expected shift in the positions of the bands
belonging to the electronic transitions. This was because,
as a result of polymerization, many benzene rings formed
bonds with each other, and this led to the formation of
conjugate structures that would allow electronic transitions to occur more easily. The monomers formed bands
in the DMF solution in the 226–293 nm range due toπ → π * ­(E2 band) electronic transitions and in the rangeof 320–400 due to π → π * (B Band) electronic transitions. Additionally, BS-Cl, unlike the other monomers
in DMF solvent, formed a broad band at 572 nm due tothe charge-transfer-complex (CTC) transition. This result
indicated that the interaction of the BS-Cl monomer with
the DMF solvent was different from the other monomers.
This band was formed by the excitation of an electron in
the monomer structure with a photon and transferring of
this electron to the DMF. In this case, only partial chargetransfer occurred on the electronic basic level, and thiscreated the CTC transition band. The CTC transfer band

was formed between 516 and 576 nm in the spectra of allsynthesized polymers.
The CTC transfer band was formed at 554 nm for
PBS-OH, 570 nm for PBS-Cl and 576 nm for PBS-CH3[33]. The reason why PBS-OH and PBS-CH3 formed
bands in the visible region, different from their monomers, was that the electron density increased due to thepresence of numerous benzene rings (electron donors)
in the structure of the polymers, and therefore, easierelectron transfer to the molecule with less electron
density (electron acceptor) was provided. When the
spectra of the monomers and polymers in DMSO andethanol were compared to the spectra in DMF it was
clearly seen that there was a shift in the absorption
bands. BS-Cl, which formed the CTC band (572 nm) in
the DMF, did not form this band in DMSO and ethanol.
Additionally, PBS-Cl in DMSO and PBS-CH3 in bothDMSO and ethanol did not form the CTC band. PBSOH formed bands what close to each other in DMSO
and ethanol (DMSO λ: 518 nm, ethanol λ: 516 nm). It
may be clearly seen from the data that the energy transition of both the monomers and polymers depended
on the polarity of the solvent in which they weredissolved.

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Fig. 11 UV–vis spectra of the Schiff base monomers and polymers in the studied solvents: DMF, DMSO and Ethanol

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Thermogravimetric analysis

Thermogravimetric analysis is an analysis method thatexamines changes in the weight of compounds withtemperature. With the method of thermal analysis,changes that occur in the weights of a substance or its

derivatives under a temperature program are examined, and the heat absorbed or released in a reaction
is measured. To examine the thermal stability levels of
the synthesized Schiff base monomers and polymers,
TGA/DTA/DTG analyses were carried out at three

Table 1 UV–vis spectrum data of Schiff base monomers in DMF, DMSO and Ethanol
Compound

BS-OH

BS-Cl

BS-CH3

Solvent

A

λmaxa

C
εmaxb

λmaxa

D

λmaxa

εmaxb

DMF

249

1.49

261

1.38

349

0.84

–

–

DMSO

–

–

260

0.60

355

0.49

–

–

εmaxb

λmaxa

εmaxb

Ethanol

232

0.36

–

–

319

1.50

–

–

DMF

226

0.61

299

1.50

365

0.31

572

0.10

DMSO

231

1.50

296

0.76

–

–

–

–

Ethanol

229

1.49

286

1.44

320

0.27

–

–

DMF

–

–

274

0.93

360

0.42

–

–

DMSO

224

1.27

263

1.49

359

0.42

–

–

231

1.50

–

–

321

1.41

–

–

Ethanol
a

B

−1

b

The wavelength of the band (nm); molar absorptivity coefficient ­(mol−1l ­cm )

Table 2 UV–vis spectrum data of the Schiff base polymers in DMF, DMSO and Ethanol
CompoundSolvent

A

B

λmaxa
PBS-OHPBS-ClPBS-CH3

λmaxa

εmaxb

λmaxa

D
εmaxb

λmaxa

εmaxb

DMF

246

1.01

302

1.50

348

1.42

571

0.03

DMSO

247

1.32

270

1.49

–

–

518

0.17

Ethanol

247

1.50

253

1.48

345

0.38

516

0.15

DMF

236

1.00

302

1.50

349

1.42

570

0.03

DMSO

220

1.16

255

1.49

–

–

573

0.21

Ethanol

222

1.50

292

0.92

–

–

567

0.24

DMF

218

0.90

256

0.64

302

0.31

576

0.15

DMSO

241

0.21

299

0.12

–

–

–

–

224

0.18

–

–

301

0.09

–

–

T75

Tf

Ethanol
a

εmaxb

C

−1

b

The wavelength of the band (nm); molar absorptivity coefficient ­(mol−1l ­cm )

Table 3 Thermal data obtained of monomers by TGA (heating rate β: 5, 15, 45 °C/min.)Compound
BS-OH

BS-Cl

BS-CH3

i

β

Ti

T25

T50

Residue%

5

128

245

616

762

1000

11.74

15

140

277

712

–

1000

37.86

45

154

319

761

–

1000

38.74

5

97

214

359

719

1000

6.69

15

123

226

381

539

1000

0.98

45

137

250

409

599

1000

9.18

5

129

163

363

656

1000

15.84

15

149

218

426

921

1000

23.62

45

128

265

454

929

1000

21.08

İnitial decomposition temperature, ­T25, 50, 75: temperature for 25, 50, 75% weight loss, fend of decompositionÇANAKÇI B
﻿ MC Chemistry(2024) 18:152

Page 14 of 26

Table 4 Thermal data obtained of polymers by TGA (heating rate β: 5, 15, 45 °C/min.)Compound
PBS-OHPBS-ClPBS-CH3

β

Ti

T25

T50

T75

Tf

Residue%

5

80

338

544

689

1000

2.59

15

83

374

678

–

1000

29.73

45

95

391

677

–

1000

42.83

5

81

379

714

–

1000

48.44

15

73

504

847

–

1000

49.40

45

86

523

932

–

1000

51.92

5

86

347

642

935

1000

26.08

15

79

397

851

–

1000

46.20

45

97

428

998

–

1000

53.77

Ti: İnitial decomposition temperature, ­T25, 50, 75: Temperature for 25, 50, 75% weight loss, ­Tf: end of decomposition experiment

Table 5 Interpretation of the TGA curves recorded at 5, 15 and 45 °C/min.(Schiff base monomers and polymers)
Compound

Step

β5
Ta

BS-OH
BS-Cl

BS-CH3

β15
Tb

%c

β45

Ta

Tb

%c

Ta

Tb

%c

1st

128–243

212

24.56

14–278227

24.70

154–318

247

24.98

2nd

245–1000

664

63.70

279–1000

522

37.44

319–1000

603

36.28

1st

97–190167

17.59

123–236

186

22.56

137–277

202

23.75

2nd

195–405

322

39.46

237–477

386

40.35

276–514

409

44.89

3rd

409–1000

708

36.26

478–1000

799

36.11

515–1000

809

22.18

1st

129–170

148

18.66

149–219

168

19.98

128–265

184

24.86

2nd

178–427

342

41.52

230–470

409

33.94

267–460

428

27.89

3rd

430–1000

798

23.98

472–1000

481

22.46

462–1000

488

26.17

PBS-OH

1st

80–388266

26.54

83–553143

35.11

96–531

282

32.60

2nd

390–1000

638

70.87

554–1000

682

35.16

532–1000

702

24.57

PBS-Cl1st

81–625381

38.90

73–624304

27.94

86–666

627

26.73

2nd

626–1000

774

12.66

626–1000

783

22.66

668–1000

793

21.35

1st

86–552275

37.30

79–598297

34.48

97–626

326

33.84

2nd

554–1000

762

36.62

600–1000

712

19.32

628–1000

686

12.39

PBS-CH3

Ta: Mass loss temperature ranges, ­Tb: Maximum temperature on the DTA curve, %c: Percentage of lost mass of the sample in the temperature range

different heating rates and between 20 and 1000 °C. The
obtained results are presented in Tables 3, 4, 5.
In all monomers and polymers, mass loss took placewith the increase in temperatures. The reason for thismass loss up to 100 °C was the water absorbed by thecompounds. Figure 12 shows the TGA mass loss of the
BS-OH monomer at 5, 15 and 45 °C/min heating rate.
The thermal decomposition of the monomer took placein two steps at all three heating rates. In the thermal
analysis of BS-OH conducted at a rate of 5 °C/min,the decomposition steps occurred as a result of endothermic decompositions in the temperature intervalsof 128–243 °C and 245–1000 °C. As a result of theincrease in the heating rate, there was a differences inthe temperature intervals of the decomposition steps.
The temperature differences occurring in the decomposition steps are summarized in Table 5.

While the first step in the thermal analysis conducted
at a rate of 15 °C/min occurred in the range of 145–
278 °C, the first step range of temperature in the thermal
analysis conducted at a rate of 45 °C/min became 154–
318 °C. A similar situation was applicable to the second
decomposition steps. The second decomposition steps
occurred for 15 and 45 °C/min in the respective intervals of 279–1000 °C and 319–1000 °C. It may be clearlyseen in the TGA curves of the compounds that the heating rate in the thermal analysis affected the obtained
data directly. There was also an increase in the remaining
residue amount of the substance in direct proportion to
the increase in the heating rate. After the analyses of the
BS-OH monomer at the heating rates of 5, 15 and 45 °C/min, the amounts of the residue at 1000 °C were respectively 11.74, 37.86 and 38.74%. The thermal decomposition of BS-Cl occurred in three steps at all applied heating
rates, in difference to the case in BS-OH. The startingÇANAKÇI ﻿BMC Chemistry(2024) 18:152

Page 15 of 26

Fig. 12 TGA curves for BS-OH at heating rates of 5, 15, 45 °C/min using an atmosphere of nitrogen at flow rate of 100 mL/min

Fig. 13 TGA curves for BS-Cl at heating rates of 5, 15, 45 °C/min using an atmosphere of nitrogen at flow rate of 100 mL/min

temperature of the first decomposition step of BS-Cl was
lower than that of BS-OH (Fig. 13). In the analyses of the
BS-Cl monomer at the heating rates of 5, 15 and 45 °C/min, the starting temperatures of decomposition were,respectively, 97, 123 and 137 °C. Another difference inthe thermal analysis data of BS-OH and BS-Cl was theamount of the remaining residue substance. As a resultof the thermal analyzes of BS-Cl, the remaining residueÇANAKÇI B
﻿ MC Chemistry(2024) 18:152

substance amounts were 6.69%, 0.98% and 9.18% at heating rates of 5, 15 and 45 °C/min, respectively.
The thermal decomposition of the BS-CH3 monomer
also took place in three steps, as in the case of the BS-Cl
monomer, (Fig. 14). The decomposition starting temperature of BS-CH3 was higher than those of both BS-OH
and BS-Cl. The decomposition starting temperaturesof the monomer at the heating rates of 5, 15 and 45 °C/
min were, respectively, 129, 149 and 158 °C. The amountof the remaining substance as a result of the thermal
decomposition of the BS-CH3 monomer was higher
than that of BS-Cl and lower than that of BS-OH. Afterthe analyses of the BS-CH3 monomer conducted at theheating rates of 5, 15 and 45 °C/min, the amounts of theremaining substance were found, respectively, as 15.84,
23.62 and 21.08%.
The data obtained as a result of the thermal analyses ofthe monomers showed that, although the main structuresof the monomers were similar, the fact that the positions
and types of the groups connected to the main structure
were different affected the monomers’ thermal stability
directly. Figure 15 shows the TGA curves of the PBS-OH
polymer at the 5, 15, 45 °C/min heating rates. As in thecase of its monomer, the thermal decomposition of the
PBS-OH polymer also took place in two steps.
The TGA data where listed on the all polymers showedthat the decomposition starting temperatures of the polymers were lower than those of the monomers. The reason

Page 16 of 26

for this was that the ratio of water absorbed by the polymers was higher than that absorbed by the monomers.
Moreover, in difference to their monomers, the PBSCl and PBS-CH3 polymers had decompositions in two
steps. The first decomposition steps of PBS-OH at the5, 15, 45 °C/min heating rates took place in the respective temperature intervals of 80–388 °C, 80–553 °C
and 96–531 °C. As a result of the thermal analyses, theamounts of remaining substance of PBS-OH at 1000 °C
were respectively 2.59, 29.73 and 42.83%. In a similar way
to its monomer, the amount of the remaining substancesof the polymer increased with increasing heating rate.
The amounts of the remaining substance of the PBS-Cland PBS-CH3 polymers also were higher than those ofboth their monomers and the PBS-OH polymer (Figs. 16
and 17). Approximately half of the mass remained undecomposed at all heating rates in the PBS-Cl polymer andat the heating rates of 15 and 45 °C in the PBS-CH3 polymer. The first reason for this result was that the resistanceshown by the polymers against temperature increases
were highly different. Although the repeating units of thepolymers were the same, the fact that the groups on theunits were different changed the characteristic propertiesof the polymers [33]. –OH and –Cl are electron-donatinggroups, while –CH3 is an electron withdrawing group.
This difference affected the aromatic structures on thepolymer chains and led them to have bonds with different polymer chains, therefore forming a more stable crossFig. 14 TGA curves for BS-CH3 at heating rates of 5, 15, 45 °C/min using an atmosphere of nitrogen at flow rate of 100 mL/minÇANAKÇI ﻿BMC Chemistry(2024) 18:152

Fig. 15 TGA curves for PBS-OH at heating rates of 5, 15, 45 °C/min using an atmosphere of nitrogen at flow rate of 100 mL/min

Fig. 16 TGA curves for PBS-Cl at heating rates of 5, 15, 45 °C/min using an atmosphere of nitrogen at flow rate of 100 mL/min

Page 17 of 26

ÇANAKÇI B
﻿ MC Chemistry(2024) 18:152

Page 18 of 26

Fig. 17 TGA curves for PBS-CH3 at heating rates of 5, 15, 45 °C/min using an atmosphere of nitrogen at flow rate of 100 mL/min

linkled structure. As the heat effect influenced the outerlayer of the spiral, the inner layers could remain undecomposed. The second reason was the boron compounds
forming and remaining in the environment what was asa result of the decomposition of the aromatic structures.
Boron is an element with a high thermal resistance and,
therefore, the thermal resistance of all compounds it
forms is also high.
Kinetic analysis

Based on thermogravimetric findings, several analysis
methods have been proposed to obtain information that
could be considered significant such as the activationenergy and the exponential factor. All kinetic information
can be obtained from experiments using different kinetic
models. All kinetic studies assume that the isothermaltransformation rate, dα/dt, is linearly related to the temperature-related rate constant (k) and the temperatureindependent conversion degree (α). That is,dα= k(T )f (α)dt(1)

This expression describes the conversion degree as afunction of the reduction in the reactant weight and therate constant at a constant temperature. According to the
Arrhenius equation:Eak(T ) = Aexp −RT(2)

The temperature dependence of the rate consent, k, isexpressed in Eq. (2). Therefore, when Eq. (1) is substituted
into Eq. (2), Eq. (3) is obtained (A is the exponential factor
assumed to be independent of temperature, E is the activation energy, T is the absolute temperature, and R is the gas
constant)Eadα= Aexp
f (α)
(3)
dt
RT
If the sample temperature changes by a controlled andconstant heating rate, β = dT/dt, as just β the change in theconversion degree, may be analyzed as a function of thetemperature. This temperature is dependent on the heating time. Thus, the transformation rate may be expressed
as follows:

dα
dα
=β
dt
dT

(4)

and by combination with the equation above, the following equation is obtained:


Ea
dα
A
= exp
f (α)
(5)
dT
β
RT

ÇANAKÇI ﻿BMC Chemistry(2024) 18:152

Page 19 of 26

Fig. 18 Ozawa’s plots of logarithm of heating rate (Logβ)versus reciprocal temperature (1000/T) at various conversion degrees(%) for BS-OH

Fig. 19 Ozawa’s plots of logarithm of heating rate (Logβ)versus reciprocal temperature (1000/T) at various conversion degrees(%) for BS-Cl

By determining the respective integral limit ranges of
0 and α for the mass transformation fraction values forthe thermal reaction start, ­Ti, and the temperature, T,
at any time, the following equation can be written:


α dα
AT
Ea
= ∫ expdTg(α) = ∫
(6)
β Ti
RT
0 f (α)Here, g(α) is the integral function of the transformation. By applying them in known kinetic methods,
these functions are used in the prediction of reaction
mechanisms from dynamic TGA curves. New anothermethod, known as the FWO method, which was developed independently by Flynn-Wall and Ozawa to calculate non-isothermal data, is the integral method [34].
Using the Doyle approach and the logarithmic formof Eq. (6), the linear equation of the FWO method is
obtained, by transforming the logarithmic expression
into the natural logarithm, the following equation is
obtained [35].

logβ = log

AE0.457.E− 2.315 −Rg(α)
RTFig. 20 Ozawa’s plots of logarithm of heating rate (Logβ)versus reciprocal temperature (1000/T) at various conversion degrees(%) for BS-CH3(7)

Here β is the heating rate (°C/min). According to thisequation, the activation energy can be calculated fromthe slope in the plot of log β and (1000/T). The slope
is equal to the value of ‘− 0.457 E/R’. In this method,as there is a linearity between log β and ‘1000/T’, thecurves are parallel to each other for all percentagetransformations (Figs. 18, 19, 20, 21, 22, 23).
The activation energies and exponential factors
obtained from the plots drawn by using the Flynn–
Wall–Ozawa method for all monomers are given in
Tables 6, 7, 8 and for all polymers are given in Tables 9,10, 11.The obtained data showed that the activationenergies of the monomers and polymers changed

Fig. 21 Ozawa’s plots of logarithm of heating rate (Logβ)versus reciprocal temperature (1000/T) at various conversion degrees(%) for PBS-OH

during their thermal degradation process in relation
to the transformation percentages (Figs. 24 and 25).
As the transformation percentages of the monomersÇANAKÇI B
﻿ MC Chemistry(2024) 18:152

Page 20 of 26

Fig. 22 Ozawa’s plots of logarithm of heating rate (Logβ)versus reciprocal temperature (1000/T) at various conversion degrees(%) for PBS-Cl

Fig. 23 Ozawa’s plots of logarithm of heating rate (Logβ)versus reciprocal temperature (1000/T) at various conversion degrees(%) for PBS-CH3

and polymers increased, their activation energies also
increased. The BS-OH monomer showed the lowest
activation energy at the heating rate of 5 °C/min andthe transformation percentage of α = 10% (33.28 kJ/
mol). When the heating rates were compared by keeping the transformation percentages constant, it wasseen that there was mostly an increase in the activation energy. However, the opposite was observed when
the transformation percentage was 90%. There was adecrease in the activation energy with the increase in
the heating rate. The highest activation energy was
obtained at the heating rate of 5 °C/min and the transformation percentage of α = 10% (92.37 kJ/mol). It wasobserved that the activation energies of BS-Cl and
BS-CH3 were higher in comparison to BS-OH. Especially, BS-CH3 had a higher activation energy thanthe other two monomers at all three heating rates.
For BS-Cl, the lowest activation energy was obtainedat the heating rate of 5 °C/min (α: 10%, E­ a: 30 kJ/mol),while the highest one was obtained at the heating rateof 45 °C/min (α: 90%, ­Ea: 93.20 kJ/mol). For the activation values of BS-CH3, the highest one was obtained
likewise at the heating rate of 45 °C/min (α: 90%, E­ a:94.95 kJ/mol), and the lowest one was at 5 °C/min (α:10%, ­Ea: 39.59 kJ/mol).
Considering the activation energies of the polymers,
it was observed that these values increased as the transformation percentages increased from 10 to 90%, as in
the case of the monomers of these polymers. The highest activation energies were found at the heating rateof 15 °C/min for PBS-OH (α: 90%, ­Ea: 95.20 kJ/mol), at
45 °C/min for PBS-Cl (α: 90%, ­Ea: 97.60 kJ/mol) and at
15 °C/min for PBS-CH3 (α: 90%, ­Ea: 97.44 kJ/mol).
When the activation energies of the polymers based on
their heating rates were examined by keeping their transformation percentages constant, as opposed to the casefor their monomers, it was seen that there was a reduction in their activation energies at some transformation
percentages. This situation was observed especially in

Table 6 Activation energies and pre-exponential factors of BS-OH calculated from isoconversional method (Flynn–Wall–Ozawa) at 5,
15 and 45 °C/min
α

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9

β5

β15

Tα

Ea (kJ/mol)

A(s−1)

401

33.28

4.43 × ­105

35.19

6

2.78 × ­10

37.35

5

3.31 × ­10

43.32

3

3.03 × ­10

50.54

5

4.33 × ­10

54.36

8

2.76 × ­10

56.19

8

8.31 × ­10

74.03

6

424
450
522
609
655
677
892
1113

92.37

3.24 × ­10

17

3.67 × ­10

β45TαEa (kJ/mol)

A(s−1)

4.15 × ­105

445

36.93

3.99 × ­105

37.10

6

2.63 × ­10

465

38.59

2.53 × ­106

40.83

5

3.03 × ­10

492

40.83

3.03 × ­105

53.61

3

2.45 × ­10

652

54.11

2.42 × ­103

56.52

5

3.87 × ­10

701

58.18

3.76 × ­105

57.76

8

2.60 × ­10

721

59.84

2.51 × ­108

60.00

8

7.78 × ­10

738

61.25

7.62 × ­108

74.94

6

799

66.31

3.62 × ­106

17

1059

87.89

3.86 × ­1017

TαEa (kJ/mol)

A(s−1)

428

35.52

447
492
646
681
696
723
903
1100

91.30

3.20 × ­10
3.72 × ­10ÇANAKÇI ﻿BMC Chemistry(2024) 18:152

Page 21 of 26

Table 7 Activation energies and pre-exponential factors of BS-Cl calculated from isoconversional method (Flynn–Wall–Ozawa) at 5, 15
and 45 °C/min
α

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9

β5

β15
−1

Tα

Ea (kJ/mol)

A(s )

429

35.60

6.70 × ­108

37.93

8

1.02 × ­10

44.48

5

1.32 × ­10

49.46

8

7.28 × ­10

52.70

8

457
536
596
635
667
922
1024
1099

4.63 × ­10

11

55.36

8.45 × ­10

76.52

9

3.85 × ­10

84.99

7

1.04 × ­10

91.21

9

4.04 × ­10

β45TαEa (kJ/mol)

A(s−1)

7.79 × ­108

447

37.10

6.43 × ­108

38.18

8

1.01 × ­10

486

40.33

9.63 × ­107

40.50

5

1.45 × ­10

554

45.98

1.27 × ­105

49.05

8

7.34 × ­10

628

52.12

6.91 × ­108

53.95

8

676

56.10

4.35 × ­108

56.60

11

8.27 × ­10

701

58.18

8.04 × ­1011

59.17

9

4.98 × ­10

723

60.00

4.91 × ­109

73.62

7

1.20 × ­10

912

75.69

1.17 × ­106

88.47

9

1123

93.20

3.95 × ­109Tα

Ea (kJ/mol)

A(s )

369

30.62

460
488
591
650
682
713
887
1066

−1

4.53 × ­10

4.17 × ­10

Table 8 Activation energies and pre-exponential factors of BS-CH3 calculated from isoconversional method (Flynn–Wall–Ozawa) at 5,
15 and 45 °C/min
α

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9

β5

β15
−1

Tα

Ea (kJ/mol)

A(s )

477

39.59

9.76 × ­109

40.83

10

2.30 × ­10

45.23

9

2.08 × ­10

62.41

5

5.01 × ­10

69.63

9

7.23 × ­10

74.94

6

1.15 × ­10

79.34

6

492
545
752
839
903
956
1011
1077

83.91
89.39

6.10 × ­10

10

3.76 × ­10

14

2.69 × ­10

β45TαEa (kJ/mol)

A(s−1)

9.56 × ­109

505

41.91

9.22 × ­109

41.58

10

2.26 × ­10

520

43.16

2.17 × ­1010

42.41

9

2.22 × ­10

536

44.48

2.12 × ­109

45.56

5

6.86 × ­10

567

47.06

6.64 × ­105

60.25

9

8.36 × ­10

759

62.99

7.99 × ­109

67.23

6

1.28 × ­10

844

70.05

1.23 × ­106

73.62

6

924

76.69

6.31 × ­106

10

1020

84.66

3.73 × ­1010

14

1144

94.95

2.53 × ­1014

Tα

Ea (kJ/mol)

A(s )

487

40.42

501
511
549
726
810
887
990
1129

82.17
93.70

−1

6.57 × ­10
3.84 × ­10
2.56 × ­10

Table 9 Activation energies and pre-exponential factors of PBS-OH calculated from isoconversional method (Flynn–Wall–Ozawa) at 5,
15 and 45 °C/min
α

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9

β5

β15
−1

Tα

Ea (kJ/mol)

A(s )

450

37.35

1.96 × ­105

43.74

11

2.45 × ­10

50.71

7

3.65 × ­10

59.17

5

6.69 × ­10

66.73

5

8.16 × ­10

74.03

6

4.66 × ­10

81.17

7

5.26 × ­10

87.89

8

527
611
713
804
892
978
1059
1159

96.19

5.32 × ­10

11

3.37 × ­10

β45TαEa (kJ/mol)

A(s−1)

2.04 × ­105

402

33.36

2.20 × ­105

42.49

11

2.48 × ­10

509

42.24

2.49 × ­1011

47.22

7

3.92 × ­10

575

47.72

3.88 × ­107

52.78

5

629

52.20

7.58 × ­105Tα

Ea (kJ/mol)

A(s )

432

35.85

512
569
636
715
812
916
1020
1147

−1

7.50 × ­10

691

57.35

9.49 × ­105

67.39

6

5.12 × ­10

765

63.49

5.43 × ­106

76.02

7

5.62 × ­10

849

70.46

6.06 × ­107

84.66

8

964

80.01

5.84 × ­108

11

1080

89.64

3.62 × ­1011

59.34

95.20

5

9.17105.52 × ­10
3.41 × ­10ÇANAKÇI B
﻿ MC Chemistry(2024) 18:152

Page 22 of 26

Table 10 Activation energies and pre-exponential factors of PBS-Cl calculated from isoconversional method (Flynn–Wall–Ozawa) at 5,
15 and 45 °C/min
α

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9

β5

β15
−1

Tα

Ea (kJ/mol)

A(s )

340

28.22

3.64 × ­106

37.18

7

1.15 × ­10

44.15

5

1.19 × ­10

52.70

5

6.97 × ­10

58.93

5

4.20 × ­10

64.24

5

1.50 × ­10

69.38

5

1.61 × ­10

75.86

5

9.86 × ­10

87.56

8

448
532
635
710
774
836
914
1055

2.02 × ­10

β45TαEa (kJ/mol)

A(s−1)

3.50 × ­106

366

30.37

3.38 × ­106

39.09

7

1.09 × ­10

488

40.50

1.05 × ­107

49.55

5

1.06 × ­10

584

48.47

1.09 × ­105

59.17

5

6.21 × ­10

709

58.84

6.24 × ­105

67.14

5

3.69 × ­10

835

69.30

3.57 × ­105

75.11

5

1.28 × ­10

950

78.85

1.22 × ­105

82.00

5

1.36 × ­10

1041

86.40

1.29 × ­105

88.14

5

8.49 × ­10

1111

92.21

8.11 × ­105

93.29

8

1176

97.60

1.81 × ­108Tα

Ea (kJ/mol)

A(s )

353

29.29

471
597
713
809
905
988
1062
1124

−1

1.89 × ­10

Table 11 Activation energies and pre-exponential factors of PBS-CH3 calculated from isoconversional method (Flynn–Wall–Ozawa) at
5, 15 and 45 °C/min
α

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9

β5

β15
−1

Tα

Ea (kJ/mol)

A(s )

448

37.18

2.64 × ­1016

45.65

9

4.01 × ­10

54.28

8

1.60 × ­10

62.91

6

2.13 × ­10

67.89

6

2.58 × ­10

72.62

7

1.13 × ­10

77.68

6

3.16 × ­10

82.66

3

2.35 × ­10

88.64

6

550
654
758
818
875
936
996
1068

2.07 × ­10

β45TαEa (kJ/mol)

A(s−1)

2.62 × ­1016

459

38.09

2.58 × ­1016

44.15

9

4.14 × ­10

526

43.65

4.19 × ­109

51.70

8

1.68 × ­10

596

49.46

1.76 × ­108

59.76

6

2.24 × ­10

659

54.69

2.45 × ­106

67.47

6

2.59 × ­10

725

60.17

2.91 × ­106

74.36

7

1.10 × ­10

806

66.89

1.22 × ­107

81.34

6

3.02 × ­10

902

74.86

3.28 × ­106

88.47

3

2.20 × ­10

1003

83.24

2.34 × ­103

97.44

6

1111

92.21

1.99 × ­106Tα

Ea (kJ/mol)

A(s )

452

37.51

532
623
720
813
896
980
1066
1174

all transformation percentages in the PBS-OH polymer.
As the heating rate increased from 5 to 45 °C/min, theactivation energy decreased (α: 90%, at 5 °C/min ­Ea: 66.73
kJ/mol; at 15 °C/min ­Ea: 59.34 kJ/mol; at 45 °C/min ­Ea:
57.35 kJ/mol).
In PBS-Cl, a similar result was observed when thetransformation percentage became 0.3 and 0.4. The activation energy found at the heating rate of 15 °C/min
was higher than those at both 5 °C/min and 45 °C/min(α: 30%, at 5 °C/min ­Ea: 44.15 kJ/mol; at 15 °C/min ­Ea:
49.55 kJ/mol; at 45 °C/min ­Ea: 48.47 kJ/mol). In difference to the other two polymers, in the PBS-CH3 polymer,
activation energies that were not in direct proportion
to the transformation percentage and heating rate wereobtained. The activation energy increased along with theincreasing heating rate when the transformation percentage was 10%, but it decreased when the transformation
percentage was 20%, 30%, 40% and 50%. In the interval
where the transformation percentage between was 60%
and 90%, the activation energy obtained at the heating−1

1.88 × ­10

rate of 15 °C/min was higher than those obtained at 5 and
45 °C/min (α:0.8, at 5 °C/min E­ a:82.66 kJ/mol; at 15 °C/
min ­Ea:88.47 kJ/mol; at 45 °C/min ­Ea:83.24 kJ/mol).
The most important reason for the changes in the activation energies of the polymers was the changes createdin the chain structures of the polymers as the ambient
temperature increased. Atoms found in the chains in theamorphous or crystalline regions of polymers below theglass transition temperature continue their atom movements that are also valid for small molecules such astranslation, vibration and shear. However, the polymer
chains cannot perform movements like bending andcurving below the glass transition temperature. Therefore, they cannot change their shape or position inside
the structure under external forces, and their covalentbonds break when forced excessively.
When the glass transition temperature is reached, theenergy required for the chains in amorphous regions toturn around chemical bonds is provided, and the polymer
chains can perform bending and curving movements.

ÇANAKÇI ﻿BMC Chemistry(2024) 18:152

Page 23 of 26

That is, throughout the thermal analysis above TGA thatwas conducted, the polymers used a part of the energy
taken from the outside to undergo their movements,
while they used another part of this energy for decomposition. Additionally, as a result of the increased energy,
the decomposed polymer units created new structures byrecombination.
Calculation of thermodynamic parameters

During all types of movements and chemical reactions,energy absorption, energy release or conversion of one
type of energy into another is possible. Examinationof the relationships between the different energy types
within a system constitutes the subject of thermodynamics. Thermodynamics determines the internal energy,
enthalpy, entropy and free energy values of the system
during a physical or chemical transformation and examines the dependability of these on the reaction conditions. The following equations were used to examine thechanges that occurred in the thermodynamic parameters
during the thermal decomposition analyses of the synthesized Schiff base monomers and polymers at the different heating rates [36].

H = E − RTm(8)G = H − STm

(9)



Ah
S = 2.303R Log
kTm

Fig. 24 The activation energy ­(Ea) of compounds changes with α (aBS-OH, b BS-Cl, c BS-CH3)



(10)

where ­Tm is the DTA peak temperature, h is the Planck’s
constant, k is the Boltzmann constant, and R is the universal gas constant. The data obtained as a result of thecalculations are given in Table 12.
Enthalpy refers to the energy change in a constantpressure environment. The heat given to the constantpressure system is expended not only for the increasein the internal energy but also for the work done against
the environment. As enthalpy is a state function, it isFig. 25 Changes in the activation energy ­(Ea) of the polymer compounds with α (a PBS-OH, b PBS-Cl, c PBS-CH3) at the three heating rateÇANAKÇI B
﻿ MC Chemistry(2024) 18:152

Page 24 of 26

Table 12 Degradation kinetic parameters for Schiff base compounds at 5, 15 and 45 °C/min, determine by Ozawa method
Compound

BS-OH
BS-Cl

BS-CH3

PBS-OHPBS-ClPBS-CH3
a,c

Step

β5

β15

∆Ha

− ∆Sb

149.15143 × ­10–2

2nd

327.51–2

129 × ­10

1st

35.60

79.2 × ­10–2

2nd

202.02–2

87.5 × ­10

3rd

252.73

1st

β45
∆Gc

∆Ha

− ∆Sb

144 × ­10–2

833

116.36

144 × ­10–2

865

394.16–2

82.8 × ­10

1046

387.61

128 × ­10–2

1508

109.31

95.2 × ­10–2

545

123.4295.9 × ­10–2

574

719

218.78–2

85.8 × ­10778

226.4286.4 × ­10–2

812

120 × ­10–2

1429

162.0971.4 × ­10–2

923

168.9071.9 × ­10–2

937

39.59

56.5 × ­10–2

275

82.00

57.1 × ­10–2

333

129.5657.7 × ­10–2

390

2nd

148.4872.5 × ­10–2

591

148.2361.8 × ­10–2

564

110.0562.4 × ­10–2

544

3rd

397.2394.1 × ­10–2

1403

316.7262.7 × ­10–2

784

326.3563.1 × ­10–2

805

1st

131,8031.8 × ­10–2

298

237.71

146 × ­10–2

845

296.39104 × ­10–2

873

2nd

465.21

69.1 × ­10–2

1093

323.28106 × ­10–2

1335

240.1186.9 × ­10–2

1078

1st

354.82

139 × ­10–2

1263

244.26

154 × ­10–2

1132

247.50156 × ­10–2

1651

2nd

163.42–2

96.3 × ­101168

338.55–2

141 × ­10

1827

355.07157 × ­10–2

2028

1st

267.92

66.1 × ­10–2

629

260.62

66.2 × ­10–2

636

312.9992.8 × ­10–2

864

2nd

321.62134 × ­10–2

1708

341.62130 × ­10–2

1622

250.32129 × ­10–2

1487

1st

∆Gc

∆Ha

− ∆Sb

842

113.46

1536
383

∆Gc

kJmol−1; bJmol−1 ­K−1

path-independent and an exact differential function.
The enthalpy function is not dependent on the changing
order of the variables (T, P), but it is only dependent on
their initial and final states. The enthalpy change between
two states is shown as ΔH in Table 12. Considering theenthalpy changes of the monomers and polymers during
thermal decomposition, it was seen that their enthalpies decreased in direct proportion to the temperature
increase. The highest enthalpy value among the monomers was in the third step of the thermal analysis of the
BS-CH3 monomer conducted at the heating rate of 5 °C/
min (ΔH: 397.23 kJ/mol).
The lowest enthalpy value was in the first step of thethermal analysis of BS-Cl conducted at 5 °C/min (ΔH:
35.60 kJ/mol). As in the case of the monomers, there
were changes in the enthalpy values of the polymers
based on the heating rate. The highest (at 5 °C/minheating rate, ΔH: 465.21 kJ/mol) and lowest (at 5 °C/
min heating rate, ΔH: 131.80 kJ/mol) enthalpy values
belonged to PBS-OH. Entropy is expressed as the disorderliness of a system, and it is known that a certain
energy is spent for a change in entropy. What changesystem at a certain pressure, a certain temperature andwith a certain composition has a single entropy value.
The entropy change between two states is shown as ΔS,
and entropy change also has a single value. Entropychange is dependent on the heat (q) and temperature(T) values. Thermal degradation reactions enhance thedisorder of a system as they are facilitated by providing heat to the system from the outside [27]. The negative ∆S values that were calculated showed (Table 12)that the resistance of the bonds in all monomers andpolymers, except for the BS-OH monomer, against
thermal decomposition was higher in the first step thanthe other steps at all heating rates. The highest andlowest entropy values were found among the monomers in, respectively, BS-CH3 (at 5 °C/min heating
rate, ΔS: 56.5 × ­10–2 kJ/mol) and BS-OH (at 45 °C/minheating rate, ΔS: 144 × ­10–2 kJ/mol). Among the polymers, these values belonged, respectively, to PBS-OH(at 5 °C/min heating rate, ΔS: 31.8 × ­10–2 kJ/mol) andPBS-CH3 (at 45 °C/min heating rate, ΔS: 156 × ­10–2 kJ/
mol). The opposite of events where entropy decreases
while enthalpy increases is spontaneous. In events
where both enthalpy and entropy change, the tendencyof spontaneity is determined by the enthalpy, entropy
and absolute temperature. Depending on this, theentropy may be positive, negative or zero. As a result,
events where the enthalpy goes towards the minimum,
and the entropy goes to the maximum determine ΔG
as a measure of the increase in the tendency of spontaneity (irreversibility). The free energy changes (ΔG)
of the thermal decomposition steps of the monomers
and polymers were positive. This result showed that thedecomposition reactions were irreversible. The highest ΔG value occurred in BS-OH among the monomers(at 5 °C/min heating rate, ΔG: 1536 kJ/mol) and PBSCH3 among the polymers (at 5 °C/min heating rate, ΔG:
1708 kJ/mol) in the second step. The lowest ∆G value
occurred in BS-CH3 among the monomers (at 5 °C/minheating rate, ΔG: 275 kJ/mol) and PBS-OH among thepolymers (at 5 °C/min heating rate, ΔG: 298 kJ/mol) in
the first step. The differences in the free energy changesÇANAKÇI ﻿BMC Chemistry(2024) 18:152

show that the steps in the decomposition reactions
carry out specific decompositions.

Conclusions
Three novel Schiff base monomers containing boron andthree novel polymers by using these monomers as thebasic materials were synthesized. There was boron in thearomatic structures of the synthesized compounds. The
reason for using boron compounds in the basic materials
was that the resistance of boron against heat is high, andit was expected that the boron would transfer this property to the synthesized compounds. For the characterization of the synthesized monomers and polymers, eight
different analysis methods were used. The obtained data
proved the synthesizability of the designed compounds.
TGA/DTA/DTG analyses of the synthesized compounds
were carried out at the heating rates of 5, 15 and 45 °C/
min. This way, the resistance of the monomers and polymers against heat at different heating rates was investigated. As a result of the TGA analyses conducted in theinterval of 20 to 1000 °C, a substantial amount of substances remained undecomposed in both the monomers
and polymers. Organic compounds mostly decompose
completely at temperatures over 600 °C. The reason forthere being substantial amounts of substances remaining
in the environment as a result of the thermal analysis was
the element boron in the structures of the compounds.
The Flynn–Wall–Ozawa method was used to find thethermodynamic parameters of the compounds. Whenthe ∆H, ∆S and ∆G data of the compounds were examined, the highest ∆H values were in BS-CH3 among themonomers and in PBS-OH among the polymers. The
total ∆H values in all three polymers were higher thanthose of their monomers. The negative ∆S and positive∆G values of the compounds showed that the compounds
had high resistance against heat, and their decomposition reactions were irreversible. The data showed that theentropy values of the polymers were higher than those of
the monomers. This was an indication that the thermal
decomposition of the polymers occurred more slowly
than that of the monomers. Consequently, although themain structures of the monomers were similar, the factthat the groups connected to the structures were different led the compounds to have different thermal stability values. As a result of polymerization, the thermal
stability property of the monomers was transferred tothe polymers, and as a result of the increase of molecules
decrease when polymerized, their resistance of against
heat increased. When the ∆H, ∆S and ∆G values of thepolymers were compared, the best thermal stability ranking was PBS-OH ˃ PBS-CH3 ˃ PBS-Cl. Thermal degradation analyzes performed at different heating rates provedthat the thermal stability of the Schiff base polymers were

Page 25 of 26

higher than that of the Schiff base monomers, especiallyat high temperatures. This result shows that our newly
synthesized Schiff base polymers could be used for thermal insulation, especially in new generation electronic
devices.
Acknowledgements
Not applicable.
Author contributions
Dilek Çanakçı carried out all synthesis stages, Dilek Çanakçı carried out writing
and review of the manuscript.
Funding
Not applicable.
Availability of data and materials
No datasets were generated or analysed during the current study.

Declarations
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Received: 14 May 2024 Accepted: 7 August 2024

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