Chemoenzymatic polycondensation of para-benzylamino phenol

Article abstract of DOI:10.1515/chempap-2015-0242

Para-Benzylamine substituted oligophenol was synthesized via enzymatic oxidative polycondensation of 4-(benzylamino)phenol (BAP). Polymerization involved only the phenolic moiety without oxidizing the sec-amine (benzylamine) group. Chemoselective polycondensation of BAP monomer using HRP enzyme yielded oligophenol with sec-amine functionality on the side-chain. Effects of various factors including solvent system, reaction pH and temperature on the polycondensation were studied. Optimum polymerization process with the highest yield (63 %) and molecular weight (M_n = 5000, degree of polymerization ≈ 25) was achieved using the EtOH/buffer (pH 5.0; 1 : 1 vol. ratio) at 25 °C in 24 h under air. Characterization of the oligomer was accomplished by ¹H NMR and ¹³C NMR, Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), ultraviolet-visible spectroscopy (UV-Vis), cyclic voltammetry (CV) and thermogravimetric analysis (TGA). The polymerization process involved the elimination of hydrogen from BAP, and phenolic-OH end groups of the oligo(BAP), confirmed using ¹H NMR and FT-IR analyses. The oligomer backbone possessed phenylene and oxyphenylene repeat units, and the resulting oligomer was highly soluble in common organic solvents such as acetone, CHCl₃, 1,4-dioxane, N,N-dimethylformamide (DMF), tetrahydrofurane (THF) and dimethylsulfoxide (DMSO). Oligo(BAP) was thermally stable and exhibited 5 % and 50 % mass loss determined by thermogravimetric analysis at 247°C and 852°C, respectively.

Full text of DOI:10.1515/chempap-2015-0242

Chemical Papers
DOI: 10.1515/chempap-2015-0242
ORIGINAL PAPER
Chemoenzymatic polycondensation of
-benzylamino phenol
b
a
c
aPinar Yildirim, Ersen Gokturk, Ersen Turac, Haci O. Demir,
dErtugrul Sahmetlioglu*
aDepartment of Chemistry, Nigde University, 51240 Nigde, Turkey
bDepartment of Chemistry, Mustafa Kemal University, 31001 Hatay, Turkey
cDepartment of Chemistry, Kahramanmaras Sutcu Imam University, 46100 Kahramanmaras, Turkey
dDepartment of Chemistry, M. C¸ıkrıkc¸ıo˘glu Vocational College, Erciyes University, 38039 Kayseri, Turkey
Received 31 July 2015; Revised 6 October 2015; Accepted 7 October 2015
para-Benzylamine substituted oligophenol was synthesized via enzymatic oxidative polyconden-
sation of 4-(benzylamino)phenol (BAP). Polymerization involved only the phenolic moiety without
oxidizing the sec-amine (benzylamine) group. Chemoselective polycondensation of BAP monomer
using HRP enzyme yielded oligophenol with sec-amine functionality on the side-chain. Effects of
various factors including solvent system, reaction pH and temperature on the polycondensation were
studied. Optimum polymerization process with the highest yield (63 %) and molecular weight (Mn
= 5000, degree of polymerization ≈ 25) was achieved using the EtOH/ buffer (pH 5.0; 1 : 1 vol. ratio)
at 25◦C in 24 h under air. Characterization of the oligomer was accomplished by 1H NMR and 13C
NMR, Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC),
ultraviolet-visible spectroscopy (UV-Vis), cyclic voltammetry (CV) and thermogravimetric analysis
(TGA). The polymerization process involved the elimination of hydrogen from BAP, and phenolic
-OH end groups of the oligo(BAP), confirmed using 1H NMR and FT-IR analyses. The oligomer
backbone possessed phenylene and oxyphenylene repeat units, and the resulting oligomer was highly
soluble in common organic solvents such as acetone, CHCl3, 1,4-dioxane, N,N-dimethylformamide
(DMF), tetrahydrofurane (THF) and dimethylsulfoxide (DMSO). Oligo(BAP) was thermally sta-
ble and exhibited 5 % and 50 % mass loss determined by thermogravimetric analysis at 247◦C and
852◦C, respectively.
c
ꢀ 2015 Institute of Chemistry, Slovak Academy of Sciences
Keywords: enzymatic oxidative polymerization, horseradish peroxidase enzyme, oligophenol, hy-
drogen peroxide
Introduction
formaldehyde resins) by peroxidase-catalyzed coupling
of phenols without employing a toxic formaldehyde
comonomer (Tonami et al., 1999; Goretzki & Ritter,
1998; Uyama et al., 1997). The possibility to produce
phenolic polymers in aqueous ambient conditions pro-
vides an important option considering green chem-
istry aspects. Peroxidase catalyzed polymerization in-
volves direct coupling of phenolic monomers through
primarily ortho coupling with a possibility of para link-
age, and carbon–oxygen and carbon–carbon coupling
Enzymatic oxidative polycondensation of phenols
has been extensively studied by many polymer re-
search groups in the last decades. Syntheses of a vari-
ety of polyaromatics using aniline and phenol deriva-
tives were accomplished enzymatically using oxidore-
ductase enzymes under mild reaction conditions pro-
viding good yields. This method provides an alterna-
tive way to the synthesis of phenolic resins (phenol–
*Corresponding author, e-mail: sahmetlioglu@gmail.com
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P. Yildirim et al./Chemical Papers
of phenols resulting in the formation of oxyphenylene
and phenylene repeat units in the product (polyphe-
nol) (Dordick et al., 1987).
the polycondensation leading to extra branching with
an amine group. The present study is focused on enzy-
matic oxidative polycondensation of para-benzylamine
functionalized phenol (BAP) with a secondary amine
functionality and the determination of its chemoselec-
tivity. Anilinic –NH2 group was protected by a ben-
zyl group to obtain the sec-amine functionality. Be-
cause the benzyl group is considered to be one of the
most important amine protecting groups cleaved by
catalytic hydrogenation (Wuts & Greene, 2006), its re-
moval from the oligomer by postpolymerization mod-
ification might provide free amine groups for further
functionalization of oligophenol.
BAP monomer was chemoselectively polymerized,
and the polymerization only involved the pheno-
lic moiety without the sec-amine group oxidation
as a side reaction. Enzymatic polycondensation was
achieved using the HRP enzyme and H2O2 as the oxi-
dizer in an organic solvent/buffer (pH 5–8; 1 : 1 vol. ra-
tio) at different temperatures under air. An oligophe-
nol containing the sec-amine group on the side-chain
with moderate molecular weight (Mn = 5000, de-
gree of polymerization (DP) ≈ 25) was produced in
a high yield (63 %). The optimum polymerization
process was achieved using the EtOH/buffer (pH 5;
1 : 1 vol. ratio) at 25◦C under air. Characterization of
the oligomer was accomplished using UV-Vis, FT-IR,
1H NMR and 13C NMR, Thermogravimetric analysis
(TGA), and cyclic voltametry (CV) and gel perme-
ation chromatography (GPC) analyses.
Catalytic activity of enzymes is reduced in organic
solvents due to their denaturation. Utilization of or-
ganic solvents is also not preferred in environmen-
tally benign polymer synthesis (Zhang et al., 2013).
Phenolic compounds are weakly soluble in aqueous
conditions, which limits the use of aqueous media in
the enzymatic oxidative polymerization. Dimers and
trimers of phenols formed during the polymerization
are even less soluble in water, and precipitation oc-
curs without further polymerization. Therefore, aque-
ous organic solvents have been used to overcome the
low solubility of phenols in water during the poly-
merization. The effect of the solvent composition on
the coupling selectivity (regioselectivity) was investi-
gated, yielding more soluble polyphenols (Uyama et
al., 1994). Controlling water content in the organic sol-
vent also provides a significant control over the poly-
dispersity and polymer size of the product (Zheng et
al., 2015; Eker et al., 2009; Tanaka et al., 2010).
Enzymes can be derived from renewable feedstocks
(for example, horseradish peroxidase (HRP) is found
in the roots of horseradish), and they are consid-
ered as environmentally benign and green catalysts
compared to classical initiators (Vietch, 2004). En-
zymatic polycondensation of phenols was first re-
ported by Dordick et al. (1987), who reported en-
zymatic oxidative polycondensation of phenols us-
ing the HRP enzyme in aqueous organic solvents.
Various organic and inorganic electron donor com-
pounds including phenols, amines, indoles, phenolic
acids and sulfonates can be oxidized using a perox-
idase enzyme catalyst and H2O2. There is a huge
amount of structurally diverse phenols which have
been subjected to enzymatic oxidative polycondensa-
tion (Uyama & Kobayashi, 2002). Additionally, phe-
nol derivatives having different functional groups can
also be chemoselectively polymerized using the enzy-
matic polycondensation method. For example; when
a phenolic compound with a methacryloyl group was
subjected to enzymatic polycondensation, chemoselec-
tivity of the enzyme enabled to polymerize only the
phenolic moiety without involving the methacryloyl
group in the polycondensation process, and a polyphe-
nol with a methacryloyl group on the side-chain was
efficiently produced (Uyama et al., 1998). Moreover,
enzymatic polycondensation has also been efficiently
performed to synthesize conducting polymers (Nabid
& Entezami, 2003a, 2003b) and to remove phenol
from wastewater (Moulay, 2009; Pradeep et al., 2012;
Narayan & Pushpa, 2012).
Experimental
4-Aminophenol (Merck, Germany), benzaldehyde
(Merck), sodium borohydride (Merck), phosphate
buffers (pH = 5.0, 6.0, 7.0, 8.0 and 9.0), hydrogen per-
oxide (Sigma–Aldrich, USA), N,N-dimethylformamide
(DMF, Merck), dimethylsulfoxide (DMSO, Merck),
tetrahydrofuran (THF, Merck), chloroform (CHCl3,
Merck), dichloromethane (DCM, Merck), acetone
(Merck), methanol (Merck), ethanol (Sigma–Aldrich)
and 1,4-dioxane (Merck) were used without further
purification. HRP enzyme was purchased from Ap-
pliChem (Germany) and used as received.
1H NMR and 13C NMR spectra (Bruker-Instru-
ment-NMR Spectrometer DPX-400; Germany) were
recorded at 25◦C using DMSO-d6, and TMS was
used as an internal standart. Chemical shift is given
in δ relative to TMS. FT-IR spectra were recorded
on a Perkin–Elmer spectrum 400 (USA) spectrom-
eter equipped with an ATR probe in the region
of 4000–400 cm−1 at 64 scans per sample. UV-Vis
spectroscopy experiments were carried out on a Shi-
madzu 160A (Japan) instrument using quartz cu-
vettes (path length: 1 cm, volume: 3 mL, solvent:
DMF). Cyclic voltammograms were obtained at RT in
tetra-n-butylammonium tetrafluoroborate (TBAFB;
0.1 M)/THF electrolyte-solvent coupled with a sys-
In literature; aminophenols with primary amine
functionalities (for example; 4-aminophenol which
contains both phenolic –OH and anilinic –NH2 groups)
were polymerized employing the HRP enzyme (Shan
et al., 2003). However, amine oxidation was involved in
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iii
Fig. 1. Synthesis of (E)-4-(benzylideneamino)phenol.
Fig. 2. Synthesis of the BAP monomer.
tem consisting of an electrochemical analyzer (CH In-
struments 660 B, USA) and a CV cell containing Pt
plates as the working and counter electrodes, and an
Ag wire as the pseudo reference electrode. TGAs were
performed under N2 using a Perkin–Elmer Pyrisdia-
mond 6.0 instrument. About 5–10 mg of each sample
were heated at 10◦C min−1 from RT to 1000◦C. Gel
permeation chromatography (GPC) was performed at
40◦C using a Perkin–Elmer Series 200 instrument with
an internal differential refractive index detector and
two TSK gel AM SEC Gel columns using a solution
of 0.01 M LiCl in HPLC grade DMF as the mobile
phase at the flow rate of 1.0 mL min−1. Calibration
was performed with narrow polydispersity polystyrene
(PS) standards.
Fig. 3. Chemoselective polymerization of the BAP monomer
using the HRP enzyme.
found to be fully soluble in 1,4-dioxane, THF, DMF,
DMSO, acetone, chloroform and EtOH and insoluble
in water. FT-IR of BAP: 3350 cm−1 (-NH stretch),
3000 cm−1 (broad, -OH−),1 1484 cm−1 (aromatic –
Synthesis of (E)-4-(benzylideneamino)phenol
(E)-4-(benzylideneamino)phenol was synthesized
according to the procedure reported before (Fig. 1)
(Kaya & Gu¨l, 2004). 4-Aminophenol (2.725 g, 25.0
mmol) and benzaldehyde (2.65 g, 25.0 mmol) were
stirred in MeOH (50 mL) at RT. The mixture was
then refluxed at 70◦C for 2 h. After the reaction
was completed, the solvent was evaporated. (E)-4-
(benzylideneamino)phenol was crystallized in MeOH
for further purification.
C
(-OH bend), 1206 cm−1
—
C- stretch), 1400 cm
—
(aromatic C—N), 1168 cm−1 (C—O), 826 cm−1 (1,4-
para-disubstitution), 746 cm−1 (aromatic C—H out-
of-plane bend) and 699 cm−1 (H-bonded O—H out-
1
of-plane bending). H NMR (400 MHz, DMSO-d6), δ:
8.39 (s, Ha), 7.30 (m, Hf, Hg and Hh), 6.50 (d, Hb),
6.42 (d, Hc), 5.58 (s, Hd), 4.16 (s, He). 13C NMR
(200 MHz, DMSO-d6), δ: 48 (C-5), 114 (C-3), 116 (C-
2), 127 (C-9), 128 (C-7), 129 (C-8), 141 (C-6), 142
(C-4), 149 (C-1). UV-Vis/nm of BAP in DMSO: 269,
329.
Synthesis of 4-(benzylamino)phenol monomer
BAP (8.0 g, 0.04 mol) was dissolved in MeOH
(50 mL), and cooled down to 0◦C using an ice-bath.
Then, NaBH4 (0.38 g, 0.01 mol) was added to the
mixture in portions. The reaction mixture was stirred
for 2 h at RT. MeOH was evaporated, and HCl
(10 vol. %, 10 mL) was added dropwise to the re-
action mixture. The reaction mixture was then ex-
tracted with DCM, and washed with brine. The or-
ganic phase was dried over magnesium sulfate, and
solvent was evaporated and the resulting product
was further purified by flash column chromatogra-
phy (SiO2 column, elution with hexane/diethylether;
ϕr = 8 : 2; Fig. 2) (Xu & Wang, 2010). BAP was
Enzymatic polycondensation of BAP
BAP (0.05 g, 0.25 mmol) and HRP (1 mg) were
dissolved in a mixture of EtOH/phosphate buffer (at
the desired pH; 1 : 1 vol. ratio; 20 mL) under air.
Then, H2O2 (70 L, 34.5–36.5 %) was added to the
mixture seven times every 10 min at the desired re-
action temperature. After 24 h, oligomer precipitates
were collected by vacuum filtration and the product
was washed with MeOH and water to remove unre-
acted BAP and HRP. The obtained oligomer was dried
in an oven at 60◦C (Fig. 3) (Kumbul et al., 2015).
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Table 1. Enzymatic polycondensation of BAP by the HRP enzyme and H2O2 at 30◦C under air
Entrya
Solvent
Buffer pH
Yield/%
Mn/(g mol−1
)
PDI
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
EtOH
EtOH
EtOH
EtOH
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
5.0
6.0
7.0
8.0
5.0
6.0
7.0
8.0
5.0
6.0
7.0
8.0
58
53
44
48
60
54
54
58
38
21
29
38
4500
3700
2900
4300
4700
4300
4200
4600
1100
900
1.62
1.46
1.68
1.96
1.25
1.11
1.12
1.35
1.74
1.44
1.58
1.98
10
11
12
800
1000
a) All polymerization processes were carried out in a mixture of an organic solvent/phosphate buffer (1 : 1 vol. ratio) at 30 ◦C for
24 h under air; PDI – polydispersity index.
Oligo(BAP) was found to be completely soluble in
DMF and DMSO, partially soluble in THF, and in-
soluble in acetone, MeOH, EtOH, 1,4-dioxane, chlo-
roform and water. FT-IR of oligo(BAP): 3034 cm−1
The aim of this work was to study the enzy-
matic oxidative polycondensation of a phenol deriva-
tive containing the sec-amine functionality and the
detailed characterization of the resulting oligomer. A
phenol derivative with the para-benzylamine function-
ality (BAP) was chemoselectively polymerized under
enzymatic oxidative polymerization conditions, and
only the phenolic moiety was subjected to the oxida-
tive coupling without involving a side reaction with
the sec-amine group.
A wide range of buffer pH, different solvent sys-
tems and reaction temperatures were studied to deter-
mine the optimum conditions of enzymatic polycon-
densation of BAP using HRP. All polymerization reac-
tions were performed in an organic solvent/phosphate
buffer (1 : 1 vol. ratio) under air using H2O2 as
the oxidizer. An appropriate solvent system and pH
enabling enzymatic oxidative polymerization of the
monomer were first determined (Table 1). To address
the effect of organic solvent, polymerization was per-
formed using three different aqueous organic solvents
(MeOH, EtOH and 1,4-dioxane); the results showed
that the enzymatic polymerization of BAP is effi-
ciently achieved in the EtOH/buffer (pH 5.0; 1 : 1
vol. ratio) at 30◦C in 24 h (Table 1, entry 5). The
other solvent systems provided lower molecular weight
and yield of the oligomer. Therefore, EtOH was cho-
sen as the polymerization cosolvent, and reaction pH
and temperature were changed to determine their role
in the polycondensation.
(broad, -OH), 1508 cm−1 (aromatic -C C- stretch),
—
—
1233 cm−1 (aromatic C—N), 1168 cm−1 (C—O),
824 cm−1 (1,4-para-disubstitution), 697 cm−1 (H-
bonded O—H out-of-plane bending) (Coates, 2000).
1H NMR (400 MHz, DMSO-d6), δ: 10.10 (s, Ha), 7.40
(m, Hf, Hg and Hh), 6.90 (s, Hc), 6.50 (s, Hb), 5.80
(s, Hd), 4.83 (s, He). UV-Vis/nm of oligo(BAP) in
DMSO: 273, 386.
Results and discussion
Phenolic compounds are weakly soluble in aque-
ous solutions, which limits the use of aqueous me-
dia for the enzymatic polycondensations of phenols.
Dimers and trimers of phenols formed during the poly-
merization are even less soluble in water. Therefore,
precipitation of oligophenols occurs without further
polymerization. On the other hand, polymerization of
phenolic compounds by peroxidase catalysts does not
produce polymers in a buffer without using organic
solvents. Hence, enzymatic polycondensation of phe-
nols is achieved in an aqueous organic solvent such
as acetone/water, 1,4-dioxane/water, etc. (Liu et al.,
2000). Addition of organic solvents to the reaction me-
dia causes polyphenols solution in the buffer enabling
thus polymerization progress.
Phenoxy radicals can be generated through one-
electron oxidation of a phenol derivative in the pres-
ence of the peroxidase enzyme and H2O2 as the ox-
idizer, and they can undergo radical coupling and
transfer reactions generating polymers (Kobayashi &
Higashimura, 2003). The obtained polyphenol struc-
tures are mainly composed of oxyphenylene and
phenylene repeat units formed by carbon–oxygen
(oxy–ortho), carbon–carbon (ortho–ortho, ortho–para
or para–para) coupling of phenols, respectively (Mita
et al., 2002).
In order to study the influence of reaction pH, poly-
merization was performed at four different pH condi-
tions (pH = 5.0, 6.0, 7.0 and 8.0). The HRP enzyme is
known to be catalytically inactive at pH ≈ 3.0; there-
fore, pH < 5.0 were not studied (Ikeda et al., 1998). A
similar behavior was also reported for the alkaline re-
gion and the HRP enzyme exhibits lower catalytic ac-
tivities at pH > 9.0; thus, neither pH values > 8.0 were
not considered for the polycondensation (Zheng et al.,
2015). Table 1 also summarizes the influence of pH on
the polymerization at 30◦C. Optimum polymerization
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v
Table 2. Influence of reaction temperature on the polyconden-
the polycondensation process; reaction temperature of
25◦C was found to be the best in terms of the high-
est yield (63 %) and number average molecular weight
(Mn = 5000 g mol−1) for the polymerization of BAP
using the EtOH/buffer (pH 5.0; 1 : 1 vol. ratio) and
H2O2 as the oxidizer (Table 2, entry 14). Higher or
lower reaction temperatures decreased both yield and
molecular weight of the product. The HRP enzyme is
thermally deactivated at ≥ 60◦C (Ghoul & Chebil,
2012); therefore, reaction temperatures higher than
55◦C were not emploied for the polymerization.
sation of BAP at pH 5.0
Entrya
T/◦C
Yield/%
Mn/(g mol−1
)
PDI
13
14
15
16
17
18
19
20
25
30
35
40
45
55
57
63
60
56
42
30
16
4500
5000
4700
4300
3400
1900
700
1.18
1.14
1.25
1.56
1.88
1.93
2.13
a) All polymerization processes were carried out in EtOH/buffer
(pH 5.0; 1 : 1 vol. ratio) for 24 h under air; PDI – polydispersity
index.
Fig. 4 shows the UV-Vis spectrum of the monomer:
n→π* transition of the amine (-NH-) group and π→π*
—
transitions of the phenyl rings (-C C-) of BAP were
—
detected at 269 nm and 329 nm, respectively (Moulay,
2009). UV-Vis spectrum of the oligomer (Table 2,
entry 14) illustrated in Fig. 5 displays similar ab-
sorption bands at 273 nm and 386 nm. The n→π*
transition band of the amine (-NH-) group and the
—
π→π* transition bands of the phenyl rings (-C C-) of
—
oligo(BAP) were detected as broad absorption bands
at 273 nm and 386 nm, respectively. The similar ab-
sorption bands obtained for the monomer and the
oligomer can be explained by the same resonance-
inductive effects. UV-Vis spectrum of oligo(BAP) also
displayed an absorption band up to 750 nm which is
probably due to the polyaromatic conjugated system
in its backbone (Wagner et al., 2002).
Figs. 6 and 7 provide the FT-IR spectra of the
monomer and oligo(BAP) obtained via enzymatic
polycondensation (Table 2, entry 14), respectively.
The broad phenolic -OH stretch at around 3000 cm−1
and the secondary amine (-NH-) stretching band at
3350 cm−1 confirm the presence of the main func-
tional groups of the monomer (Fig. 6). Similar bond
stretching bands were observed also for the oligomer.
Overlap of the stretching bands of amine (-NH-) and
phenolic -OH groups on the oligomer was observed as a
broad band at 3034 cm−1. Disappearance of the band
at 746 cm−1 from the oligomer spectrum indicates
the 1,2-substitution pattern where the repeat units
are coupled through ortho-positions of the phenolic
moiety, verifying thus the polymerization of trisubsti-
tuted benzene (Fig. 7) (Kupriyanovich et al., 2008).
FT-IR spectra of the monomer and oligomer show the
secondary amine N—H wag bands at 699 cm−1 and
697 cm−1, respectively (Stuart, 2004). If the secondary
amine group had oxidized during the polycondensa-
tion, the product would have contained extra tertiary
amine C—N stretches in the FT-IR spectrum of the
oligomer and the secondary amine N—H peak would
have disappeared or be very weak in the 1H NMR
spectrum of the product. Moreover, ortho positions of
the amine group can also be coupled to produce extra
substitution patterns in the structure of the oligomer;
however, such coupling was not observed in the FT-IR
spectra
Fig. 4. UV-Vis spectrum of BAP.
Fig. 5. UV-Vis spectrum of oligo(BAP) (Table 2, entry 14).
pH at 30◦C was found to be pH 5.0 in aqueous EtOH
(Table 1, entry 5). Polymerization yield and oligomer
molecular weight decreased at pH > 5.0. Lower yields
were due to leaving the low molecular weight mate-
rial in the solution during the work-up procedure to
eliminate unreacted BAP and HRP.
After optimization of the reaction pH and solvent
system parameters, polymerization temperature was
optimized for the EtOH/buffer and pH 5.0. Table 2
summarizes the influence of reaction temperature on
Fig. 8 shows the 1H NMR spectrum of the
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P. Yildirim et al./Chemical Papers
Fig. 6. FT-IR spectrum of BAP.
Fig. 7. FT-IR spectrum of oligo(BAP) (Table 2, entry 14).
Fig. 8. 1H NMR spectrum of BAP.
monomer. The singlet peak at δ 8.39 was attributed
zylamine (-NH-, Hd) and benzylic -CH2 (He) pro-
to the phenolic -OH (Ha) proton, the expected ben-
tons were observed as singlets at δ 5.58 and 4.16,
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vii
Fig. 9. 1H NMR spectrum of oligo(BAP) (Table 2, entry 14).
Fig. 10. 13C NMR spectrum of BAP.
respectively, verifying the structure of the monomer.
The peaks at δ 7.30 (Hf, Hg and Hh), 6.5 (Hb) and
6.42 (Hc) were assigned to aromatic protons of the
attributed to the polymerization reaction. Addition-
ally, the decrease in the intensity of the Hb protons
in the oligomer spectrum compared to the monomer
indicates that some coupling reactions occur at the
ortho-positions, which proves that these positions of
the phenolic ring are the most favored for coupling.
Fig. 10 displays the 13C NMR spectrum of the
monomer (BAP). The peaks at δ 48 and 149 can be
recognized as benzylic carbon (-CH2-, C5) and C1 car-
bon of phenol forming the monomer. Assignments of
other carbons are due to the aromatic carbons on the
structure. The 13C NMR data confirmed perfect agree-
ment of the monomer structure with the expected
1
monomer (Turac et al., 2008). H NMR spectrum of
the product (Table 2, entry 14) also shows similar pro-
ton peaks as its monomer. The peak at δ 10.1 origi-
nates from the phenolic -OH (Ha) terminal unit pro-
tons (Fig. 9) and the benzylamine (-NH-, Hd) and ben-
zylic -CH2 protons (He) were observed at δ 5.80 and
4.83, respectively. Aromatic protons of the oligomer
were slightly shifted to downfield and observed be-
tween δ 7.40, 6.90 and 6.50. Broadening of proton
NMR was observed in previous studies and it was
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viii
P. Yildirim et al./Chemical Papers
Fig. 11. Cyclic voltammograms of oligo(BAP) (Table 2, entry 14) in monomer-free electrolyte at different scan rates between
100 mV s−1 and 900 mV s−1
.
Fig. 13. TGA thermogram of oligo(BAP) (Table 2, entry 14).
Fig. 12. Relationship between anodic and cathodic peak cur-
rents vs. scan rate.
14) were evaluated using TGA under N2 in the range
of 25–1200◦C (Fig. 13). TGA demonstrated a vari-
ety of thermal decomposition responses to increased
temperatures. The mass loss of the sample starts at
around 100◦C, which corresponds to the removal of
water and indicates that the product was not dried
thoroughly before the experiment. Degradation of low
molecular weight compounds was not expected at this
temperature since the product has a narrow molec-
ular weight distribution. The product mass loss of
5 % at 247◦C and of 50 % at 852◦C indicates that
the oligomer is highly thermostable due to the long
conjugated oligomer backbone and that it possesses
high thermal stability against thermal decomposition;
about 10 % of the initial mass of the sample (carbona-
ceous residue) remained after its heating up to 1200◦C
(Kumbul et al., 2015).
chemical shifts (Fig. 10). 13C NMR spectrum of the
oligomer could not be taken successfully due to diffi-
cult shimming.
Electrochemical characterization of the oligomer
(Table 2, entry 14) is shown in Figs. 11 and 12; one
oxidation and one reduction peak were observed in the
anodic and cathodic regions, respectively, at –0.30 V
and 0.42 V vs. Ag/AgCl between –2.0 V and 2.0 V
(Fig. 11). The peaks are very well defined and re-
versible over several cycles (Cheraghi et al., 2009). An-
odic and cathodic peak currents (Ipa and Ipc) showed
a significant change at different scan rates between
100 mV s−1 and 900 mV s−1 (Fig. 12). Peak currents
(Ipa and Ipc) demonstrated a linear dependence on the
scan rate indicating that the redox process is surface-
controlled and the electrochemical process is reversible
even at high scan rates.
Molecular weight of the oligomers was measured
using GPC (DMF as the eluent phase with narrow
PS standards); number-average molecular weight of
the oligomers was in the range of several thousands.
Thermal properties of the product (Table 2, entry
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ix
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In conclusion, para-benzylamine functionalized
oligophenol was successfully synthesized via enzy-
matic oxidative polymerization of BAP in EtOH/buf-
fer (pH 5.0; 1 : 1 vol. ratio) and H2O2 as the oxidizer at
25◦C under air. The phenolic moiety was chemoselec-
tively polymerized under this reaction conditions and
a new class of oligophenols possessing the sec-amine
side-chain was produced in a high yield (63 %) and
with a moderate molecular weight (Mn = 5000, DP ≈
25). The resulting oligomer contained oxyphenylene
and phenylene repeat units. Thermal analysis results
obtained for the oligomer showed a 5 % mass loss at
247◦C and a 50 % mass loss at 852◦C, indicating that
the oligomer was highly thermostable due to the long
conjugated oligomer backbone. About 10 % of the ini-
tial mass of the oligomer (carbonaceous residue) re-
mained after its heating to 1200◦C, proving higher
resistance against high temperature. Cyclic voltam-
metry also confirmed the electroactive nature of the
oligomer. Further investigations can explore the ap-
plications of the synthesized sec-amine-functionalized
oligophenols.
Kumbul, A., Gokturk, E., Turac, E.,
& Sahmetlioglu, E.
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Acknowledgements. Ersen Gokturk would like to acknowl-
edge the Turkish Ministry of National Education for his Ph.D.
scholarship.
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