Novel triphenylamine-based polyamides: Efficient preparation via benzoxazine-isocyanide-chemistry at room temperature and electrochromic properties investigation

Article abstract of DOI:10.1016/j.dyepig.2020.108206

A type of novel triphenylamine (TPA)-based diisocyanide derivative, 4-isocyano-N-(4-isocyanophenyl)-N-(4-methoxyphenyl)aniline (M1), was successfully synthesized via Hofmann treatment of its primary amine precursor (2). A series of polyamides (P1–P3), whi

Full text of DOI:10.1016/j.dyepig.2020.108206

Journal Pre-proof
Novel triphenylamine-based polyamides: Efficient preparation via benzoxazine-
isocyanide-chemistry at room temperature and electrochromic properties investigation
Chang Yang, Wei Shi, Xin Chen, Kesong Zhang, Xinyu Zhou, Xu Sun, Sheng Ding,
Sai Liu, Zhengfeng Xie
PII:
S0143-7208(19)32490-8
DOI:
https://doi.org/10.1016/j.dyepig.2020.108206
DYPI 108206
Reference:
To appear in: Dyes and Pigments
Received Date: 23 October 2019
Revised Date: 10 January 2020
Accepted Date: 13 January 2020
Please cite this article as: Yang C, Shi W, Chen X, Zhang K, Zhou X, Sun X, Ding S, Liu S, Xie Z,
Novel triphenylamine-based polyamides: Efficient preparation via benzoxazine-isocyanide-chemistry at
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Novel triphenylamine-based polyamides: efficient preparation via
benzoxazine-isocyanide-chemistry at room temperature and electrochromic
properties investigation
Chang Yang, Wei Shi *, Xin Chen, Kesong Zhang, Xinyu Zhou, Xu Sun, Sheng Ding,
Sai Liu, Zhengfeng Xie
Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, College of
Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu,
610500, China
* corresponding author: Tel: +86 028 83037306; fax: +86 028 83037305
E-mail address: shiwei80@swpu.edu.cn, xjuwshi@gmail.com (Wei Shi)
1

Abstract
A type of novel triphenylamine (TPA)-based diisocyanide derivative,
4-isocyano-N-(4-isocyanophenyl)-N-(4-methoxyphenyl)aniline (M1), was
successfully synthesized via Hofmann treatment of its primary amine precursor (2). A
series of polyamides (P1-P3), which containing TPA units in backbones, were
successfully synthesized via efficient benzoxazine-isocyanide-chemistry (BIC) at
room temperature by using M1 and bifunctional benzoxazines as starting monomers.
1
Chemical structures of P1-P3 were verified by FT-IR, H NMR and gel permeation
chromatography (GPC) analyses. Polymers’ films were prepared by solution coating
(using DMSO as solvent), and electrochemical properties of P1-P3 were investigated
by cyclic voltammetry (CV). P1-P3 exhibited single-step highly reversible oxidation
process, reflecting their good electrochemical activity. Electrochromic properties of
polymers were further analyzed by spectroelectrochemical techniques. New
absorption bands at ~750 nm appeared in UV-vis spectra of P1-P3 when the applied
voltage reached ~ 0.65 V, accompanied by the obvious visual color change (from
colorless to blue-green). Response time and coloration efficiency of P1-P3 were also
investigated here. The work carried out in this study proved that BIC can act as
powerful tool to construct novel functional polyamides.
Keywords: benzoxazine-isocyanide-chemistry (BIC), triphenylamine, polyamide,
electrochromism
2

1 Introduction
Electrochromism (EC) refers to the reversible alteration of optical properties
such as absorption, transmittance and reflectivity of materials under an applied
electric field [1]. EC materials (ECMs) have been widely used in many fields,
including architectural and smart windows [2], anti-glare mirrors [3] and displays [4],
etc. Among various ECMs, organic polymers have been acknowledged as a type of
high-performance ECMs because of their good cycle stability [5], fast switching
speed [6], simple processing [7], and sometimes multiple color changing capability
(switching between different redox states) [8, 9]. To date, many polymer-based ECMs
have been reported, such as polythiophene [10], polyaniline [11], polycarbazole [12],
polypyrrole [13], polyfluorene [14] and polyarylamine [15], etc.
Thanks to their capability to form stable cationic radicals and low oxidation
potential [16-18], aromatic amine-based polymers are representative polymeric ECMs.
Among of them, the polymers containing triphenylamine (TPA) fragments are the
most popular samples, which have been intensively investigated and with satisfactory
comprehensive performance [19-21]. According to the main chain structures of
poly-TPAs, they can be divided into conjugated and non-conjugated ones. Jeong et al
synthesized a series of TPA-based conjugated polymers (CPs) by Suzuki coupling
reaction [22], the obtained polymers exhibited good thermal stability and
electrochromic properties. Xu et al prepared
a
class of TPA-based
donor-π-bridge-acceptor type CPs via Stille coupling polymerization [23], and the
3

introduction of different electrophilic groups brought different electrochromic
behaviors to polymers. Toppare’s group synthesized a series of CPs based on TPA,
benzotriazole, and thiophene via Stille coupling [24], and the obtained polymers
exhibited fast switching response. However, highly expensive palladium-catalysts
were inevitably used in the preparation of these TPA-based conjugated materials
[24-26]. Conjugated TPA polymers can also be prepared by electrochemical
polymerization protocol [8, 27]. However, the low utilization of monomer [28],
nonuniform film formation and the difficulty in large-scale preparation [29] greatly
restrict the application of electrochemical polymerization [30]. TPA-based polyamides
(TPA-PAs) are one of the most widely studied non-conjugated polymeric ECMs.
There is no requirement for metal catalysts in the syntheses of TPA-PAs [19], and the
solubility, film-formability and electrochemical properties of polymers can be
effectively improved by combining TPA units with polyamide (PA) backbones [31,
32]. The most common method for the preparation of TPA-PAs is the condensation
between TPA-based diamines and dicarboxylic acid monomers [33]. Sun et al
reported a series of TPA-based PAs by the condensation between TPA-derivatived
diamines and dicarboxylic acid monomers, and the obtained polymers exhibited high
electrochromic absorption/fluorescence contrast, outstanding switching stability, and
fast response speed [34, 35]. Such amine-carboxylic acid condensation usually
requires strict purity of monomers, and must be carried out under high reaction
o
temperature (>100 C), which tend to cause unwilling side reactions [36]. PAs can
also be prepared by direct condensation between diamines and di-acylchloride at
4

ambient temperature [36]. Nevertheless, there is much difficulty in the preparation,
purification and preservation of acylchlorides, and the corresponding structural
designing flexibility is poor [37], which greatly limits its utilization in TPA-PAs.
Therefore, there is an urgent demand for developing efficient synthesis method for
TPA-PAs under mild conditions.
In 2018, our group reported a new method for synthesizing PAs by using
aromatic isocyanides and benzoxazines as raw materials and octylphosphoric acid
(OPA) as catalyst [38]. The specific reaction between benzoxazines and isocyanides
(Benzoxazine-Isocyandie Chemistey, BIC) displayed many advantages, such as
smooth occurrence at room temperature, metal-catalyst free, short polymerization
time (6 h) and insensitive to water and oxygen. Based on BIC reaction, very recently
our group designed a class of tetraphenylethylene-contaning functional PAs and
explored their applications in the fields of optical probing and heterogeneous catalysis
[39]. These works have provided successful experience for the construction of
functional PAs via BIC reaction. With TPA-derivatived isocyanide compound as
starting monomer, it is expected that TPA groups can be introduced into PAs skeleton
under mild conditions via BIC reaction to construct corresponding ECMs. To the best
of our knowledge, such work has not been reported by far.
In this study, we designed and synthesized a type of novel TPA-derivatived
diisocyanide
monomer,
4-isocyano-N-(4-isocyanophenyl)-N-(4-methoxyphenyl)aniline (M1). A series of PA
derivatives containing TPA functional units (P1-P3) were successfully prepared by
5

BIC reaction under mild condition by using different bifunctional benzoxazines and
M1 as monomers. All these PAs showed good solubility and thermal stability, and can
form homogeneous and transparent films by coating of their solution. Cyclic
voltammetry (CV) of polymers’ films displayed obvious electrochemical activity.
Upon oxidation, the color of polymer films changed from colorless to blue-green.
Based on this, the electrochromic behaviors of P1-P3 were subsequently investigated
here.
2 Experimental section
2.1 Materials
p-Anisidine, cesium fluoride, 4-fluoronitrobenzene, palladium/carbon (10%),
octylphosphonic acid (OPA), tetrabutylammonium hydroxide, paraformaldehyde,
tetrabutylammonium perchlorate (TBAP) and other reagents and chemicals were
purchased from Adamas or Acros and used as received. Indium tin oxide (ITO)-coated
glass (50 mm × 7 mm × 1 mm, 5 Ω per square, Sigma-Aldrich) were used for
substrates to load polymer films. 4-Methoxy-N, N-bis(4-nitrophenyl)aniline (1) and
N-(4-aminophenyl)-N-(4-methoxyphenyl)benzene-1, 4-diamine (2) [40, 41], 1,
4-bis(2-(4-(6-nonyl-2H-benzo[e][1, 3]oxazin-3(4H)-yl)phenyl)propan-2-yl)benzene
(B1) [38], bis(4-(6-nonyl-2H-benzo[e][1, 3]oxazin-3(4H)-yl)phenyl)methane (B2) [38]
and 6, 6'-(propane-2, 2-diyl)bis(3-hexyl-3, 4-dihydro-2H-benzo[e][1, 3]oxazine) (B3)
[38] were synthesized according to previously reported methods.
2.2 Instruments
FT-IR spectra were recorded on a WQF-520 FT-IR spectrometer in the range of
6

4000-500 cm-1. 1H and 13C NMR spectra were measured on a Bruker AVANCE III HD
NMR spectrometer at 400 MHz and 101 MHz using tetramethylsilane (TMS) as an
internal reference in CDCl3. High resolution mass spectra (TOF-MS) were performed
on Waters Q-TOF Premier. Melting point was measured by X-4 digital melting-point
microscope (Beijing Tech Instrument Co., China). Thermogravimetric (TG) analyses
were recorded on METTLER TOLEDO TGA-DSC1 (under nitrogen, with a heating
o
rate of 10 C/min). Gel permeation chromatographic (GPC) data were collected from
a Waters 1515 instrument using tetrahydrofuran (THF) as an eluent with a flow rate of
1.0 mL/min. UV-vis absorption spectra were recorded by a SHIMADZU UV-1800
spectrophotometer. Cyclic voltammetry (CV) were conducted using a CHI 760e
electrochemical workstation at a scan rate of 50 mV/s-1 with a nitrogen-saturated
solution of 0.1 M TBAP in acetonitrile (CH3CN). Electrochemical impedance
spectrum (EIS) tests were obtained with a CHI 760e electrochemical workstation at
the frequency range of 0.1 to 100 kHz in 0.1 M TBAP/CH3CN. Theoretical
calculations were performed within the spin-unrestricted density functional theory
(DFT) framework as implemented in DMol3 code [42]. The generalized gradient
approximation with the Becke (exchange) and the Lee−Yang−Parr (correlation)
functional (BLYP) [43], together with double-numeric quality basis set (DNP), was
used throughout all calculations. The convergence tolerance for energy change, max
force, and max displacement were 2×10−5 Ha, 0.004 Ha Å−1, and 0.005 Å,
respectively.
2.3 Synthesis of 4-isocyano-N-(4-isocyanophenyl)-N-(4-methoxyphenyl)aniline
7

(M1)
Compound 2 (305 mg, 1 mmol), dichloromethane (10 mL) and aqueous sodium
hydroxide (10 mL, 33%) were transferred into a two-necked bottle, and
tetrabutylammonium hydroxide (26 mg, 0.1 mmol) was added as a surfactant.
Chloroform (165 µL, 2 mmol) was added to the reaction system by pipette, and the
mixture was refluxed for 3 h under nitrogen atmosphere. The solution was cooled to
room temperature and washed three times by deionized water. The organic phase was
separated and dried over by MgSO4. The crude product was purified by
chromatography (silica gel, ethyl acetate/petroleum ether/triethylamine = 3:30:1, v/v/v)
and dried in vacuum for 24 h at 50 oC to get final product as tawny powder (108 mg,
33% yield). m. p. 105-106 oC. FT-IR (KBr, cm-1): 2924, 2852, 2120, 1594, 1500, 1289,
1
1243. H NMR (400 MHz, CDCl3) δ (TMS, ppm) 7.25-7.20 (d, J = 4.7 Hz, 4H),
7.06-7.01 (d, J = 4.4 Hz, 2H), 7.01-6.96 (d, J = 4.7 Hz, 4H), 6.92-6.87 (d, J = 4.4 Hz,
2H), 3.82 (s, 3H). 13C NMR (101 MHz, CDCl3) (ppm): 163.39, 157.76, 147.83,
138.49, 128.16, 127.52, 122.42, 120.50, 115.46, 55.36. TOF-MS calcd. for
C21H15N3O: 325.1221. found [M1+H]+: 326.1290, [M1+Na]+: 348.1128, [M1+K]+:
364.0863.
2.4 Syntheses of TPA-PAs (P1-P3)
P1-P3 were synthesized according to the procedures reported by our group [38].
A typical procedure for the preparation of P1 was given as an example here: OPA
(15.5 mg, 0.08 mmol), water (7.2 µL, 0.4 mmol) and B1 (186.6 mg, 0.2 mmol) were
dissolved in 2 mL of chloroform in a two-necked flask, and stirred for 5 min in ice
8

bath. M1’s chloroform solution (2 mL, 0.1 M) was added dropwise to the reaction
system and kept stirring for 0.5 h in ice bath, and stirred for 6 h at room temperature.
The resultant solution was concentrated and added dropwise to 100 mL of cold ethyl
ether. The precipitate was collected by filtration and dried under vacuum for 24 h to
give polymerization product.
P1: brown powder, 170 mg (70% yield). FT-IR (KBr, cm−1): 3287, 2956, 1676,
1
1604, 1500, 1278, 1244. H NMR (400 MHz, CDCl3) δ (TMS, ppm) 9.11 (hydroxyl
protons), 7.57-6.46 (aromatic protons), 4.50 (-CH2 protons), 3.80 (-CH2 protons), 3.68
(-OCH3 protons), 1.85-0.22 (-CH2 and –CH3 protons). GPC: Mn = 11832, PDI = 1.85.
P2: yellow powder, 140 mg (65% yield). FT-IR (KBr, cm−1): 3287, 2956, 1676,
1
1604, 1500, 1278, 1244. H NMR (400 MHz, CDCl3) δ (TMS, ppm) 9.11 (hydroxyl
protons), 7.50-6.49 (aromatic protons), 4.43 (-CH2 protons), 3.80 (-CH2 protons), 3.72
(-OCH3 protons), 1.76-0.28 (-CH2 and –CH3 protons). GPC: Mn = 7398, PDI = 1.88.
P3: tawny powder, 96 mg (55% yield). FT-IR (KBr, cm−1): 3287, 2956, 1676,
1
1604, 1500, 1278, 1244. H NMR (400 MHz, CDCl3) δ (TMS, ppm) 9.11 (hydroxyl
protons), 7.48-6.54 (aromatic protons), 3.84-3.45 (-CH2 and –OCH3 protons), 3.17
(-CH2 protons), 2.56 (-CH2 protons), 1.73-0.70 (-CH2 and –CH3 protons). GPC: Mn =
4125, PDI = 1.88.
2.5 Electrochemical performance investigation
ITO glasses were washed subsequently with pure water, acetone, and ethanol by
ultrasonication. Each PA sample (6 mg) was dissolved in 2 mL of DMSO, and the
o
resultant solution was dropped onto ITO and vacuum dried at 70 C for 12 h to
9

remove the solvent. Surfaces of obtained polymer films were coated with Nafion
o
solution (2.5 wt%) as the binding agent, and vacuum dried at 40 C for 5 h to obtain
final films (with areas of ~ 21 mm × 7 mm). The spectroelectrochemical cell was
composed of a quartz cuvette with a three electrode system (ITO-supported polymer
film as the working electrode, a platinum wire as the counter electrode, and an
Ag/AgCl electrode as the reference electrode) for subsequent electrochemical and
electrochromic performance tests.
3 Results and discussion
3.1 Synthesis of M1 and P1-P3
Due to the occurrence of para-coupling dimerization between TPA segments
during oxidation process, the electrochemical reversibility of TPA-PAs is usually
unsatisfactory when there are unsubstituted benzene rings in TPA structure [44-46].
Such para-coupling dimerization can be prevented by the introduction of occupying
substituents into the para-position of TPA, improving the electrochromic stability thus
[47]. Liou’s group developed a series of PAs with the occupation of different
substituents at the para-position of TPA, and their electrochromic properties were
studied in depth [48-53]. Experimental results showed that the stabilization of TPA
cationic radicals can be realized by introducing electron-donating substituents, and
PAs with methoxyl group (-OCH3) at the para-position of TPA group displayed better
electrochromic stability and chromogenic efficiency than corresponding values
regarding other substituents [46]. Along with this line, -OCH3 was also selected here
to act as the occupation site in TPA, and the synthetic route of M1 is displayed in
10

Scheme 1. Using a triphenylamine-based primary amine derivative (2) as precursor,
the corresponding diisocyanide monomer (M1), which with a methoxyl substituent at
the para position was synthesized via simple and convenient Hofmann reaction.
O2N
NO2
H2N
NH2
CN
NC
F
NH2
CsF, DMSO
Reflux, 130 o
N
N
Pd/C, EtOH
Hydrazine monohydrate
CHCl3, NaOH
CH2Cl2
N
+
C
NO2
OCH3
OCH3
1
OCH3
2
OCH3
M1
CH3
O
H2N
N
+
NH2
HO
+
(CH2O)n
Reflux
N
O
B1
CH3
H2N
NH2
+
HO
+
(CH2O)n
N
N
Reflux
O
O
B2
CH3
N
O
NH2
O
N
+
HO
OH
+
(CH2O)n
Reflux
B3
H
OH
O
C
CHCl3, OPA
rt
N
N
C
O
N
H
N
B1
+
M1
+
H2O
n
N
C9H19
OH
OCH3
P1
C9H19
O
H
N
C
C
N
H
N
CHCl3, OPA
rt
n
O
C9H19
B2
+
M1
+ H2O
N
OH
N
C9H19
OCH3
HO
P2
N
CHCl3, OPA
rt
B3
+
M1
+ H2O
HO
OH
N
H
H
N
C
O
N
C
O
N
n
OCH3
P3
Scheme 1. Synthetic route of P1, P2 and P3.
FT-IR spectrum of M1 (Figure 1(a)) shows a distinct absorption signal at ~ 2120
cm-1, which belongs to the stretching vibration of isocyanide (-NC) group [54]. The
signals at ~ 2924 and ~ 2852 cm-1 correspond to the stretching vibration of methoxyl
group (-OCH3). The characteristic absorption of benzene skeleton (C=C) appears at
11

~1594 and ~ 1500 cm-1. The distinctive vibration at ~ 1289 and ~ 1243 cm-1 can be
1
assigned to the stretching of C-N (in TPA) and C-O (in-OCH3) bonds. In H NMR
spectrum of M1 (Figure (1b)), four groups of aromatic proton signals appear at δ
7.25-7.20, 7.06-7.01, 7.01-6.96 and 6.92-6.87 ppm, respectively, and their
assignments are shown in the inset of Figure 1(b). The characteristic peak at δ 3.82
ppm corresponds to the –OCH3 group (e). The protons integral is consistent with the
chemical structure of M1. As shown in Figure 1(c), the characteristic peak at δ
163.39 ppm corresponds to -NC carbon (a). The signal at δ 157.76 ppm is assigned to
the aromatic carbon (i), which directly links to -OCH3. Peaks at δ 147.38–115.38 ppm
are attributed to other carbons on the benzene ring (corresponding assignment can be
traced in Figure 1(c)), and the signal at δ 55.36 ppm corresponds to the –OCH3
carbon (j). Three distinctive mass signals appear in the TOF-MS spectrum of M1
(Figure S1) at 326.1290 [M1+H]+, 348.1128 [M1+Na]+ and 364.0863 [M1+K]+,
respectively, which is in good agreement with the theoretical value (C21H15N3O, calcd.
325.1221). The above analyses solidly prove that M1 has been successfully prepared.
(b)
(a)
e
2852
b
2924
a
b
a
1594
CDCl3
1
c
d
c
d
2120
1243
1289
1500
4000 3500 3000 2500 2000 1500 1000 500
7.3 7.2 7.1 7.0 6.9 6.8
Chemical Shift (ppm)
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5
Wavenumber ( )
cm-1
Chemical Shift (ppm)
12

CDCl3
(c)
d
c
h
g
j
e
i
f
a
b
170 160 150 140 130 120 110 100 90 80 70 60 50
Chemical Shift (ppm)
Figure 1. FT-IR (a), 1H NMR (b) and 13C NMR (c) spectra of M1.
With M1 in hand, the preparation of corresponding PAs (P1-P3) was carried out
subsequently with M1 and bifunctional benzoxazines (B1-B3) as starting monomers
(Scheme 1). The molecular design strategy and utilization of B1-B3 as starting
monomers mainly based on the following considerations: I) flexible alkyl chains
present in the structures of B1-B3 can endow targeted polymers with good solubility
in chloroform (as polymerization solvent), preventing the precipitation of
polymerization product from reaction system to terminate propagation prematurely;
II) investigation regarding the influence brought by the aromatic (for B1 and B2) and
alkyl (for B3) N-substituents (on benzoxazine) to BIC-polymerization and subsequent
electrochromic properties could be helpful to get more insight about BIC reaction and
further performance modification. On the other hand, benefiting from its high
reactivity, mild reaction condition and great molecular designing flexibility, BIC
reaction holds the prospective potential in the construction of novel functional PAs.
Based on this, with OPA as catalyst, corresponding PAs (P1-P3) were facilely
obtained by stirring M1 and bifunctional benzoxazines (B1-B3) in chloroform at
13

room temperature for 6 h.
From FT-IR spectra of resultant polymers (Figure 2), it can be seen that the three
polymers exhibit similar spectral characteristics. The stretching vibration signal of
phenolic hydroxyl (-OH) and amide (-C=O-NH) groups appear at ~ 3287 cm-1, and
the absorption of carbonyl group (C=O) (in amide segment) locates at ~ 1676 cm-1.
The vibration absorption of C-N-C bonds in TPA and tertiary amine moieties can be
observed at ~ 1278 cm-1, and the signal at ~ 1244 cm-1 corresponds to the C-O-C
stretching of methoxyl group.
P1
P2
P3
3287
1676
1244
1278
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber ( )
cm-1
Figure 2. FT-IR spectra of P1-P3.
1H NMR spectra of P1-P3 are shown in Figure 3. Slight proton signals at δ ~
9.11 ppm appear in the spectra of P1-P3 (marked as “a”). This can be assigned to the
phenolic hydroxyl (-OH) groups [38], which were formed during BIC polymerization.
As revealed by Figure 3, 1H NMR spectra of P1 and P2 highly resemble to each other,
which stems from their similar backbone structures. Aromatic protons of these two
polymers appear at δ 7.40-6.14 ppm, and the peaks at δ 4.58-3.55 ppm belong to
methylene (-CH2-, adjacent to the tertiary amine, b&c) and methoxyl (-OCH3, d)
14

groups in polymers’ structures. Hydrogen signals of alkyl side chains appear at δ
1.85-0.22 ppm. The aromatic protons in the spectrum of P3 appear at δ 7.48-6.54 ppm.
Distinct form that of P1 and P2, there are three groups of N-adjacent methylene
protons (b’, c’ and e’) appear in the spectrum of P3, and their chemical environment
are significantly affected by the presence of non-aromatic hexyl substituent on the
tertiary amine segment. The signals of a portion of methylene protons (b’) mix with
that of –OCH3 (d’), appearing at δ 3.84-3.45 ppm. Other two groups of -CH2- (c'&e')
appear at δ 3.17 (c') and 2.56 (e') ppm, respectively. Moreover, in the spectrum of P3,
two sets of tiny -CH2- signals, which correspond to the benzoxazine ring, appear at δ
4.79 (square) and 3.90 (circle) ppm. This indicates that there are some residual
benzoxazines on the chain ends of P3, suggesting that the degree of polymerization of
P3 is lower than that of P1 and P2. From the mechanism of BIC [38], the first step of
the reaction relies on the H+-promoted ring opening of benzoxazine to form iminium
(I) intermediate (Scheme 2). When the substituent (R) on the N atom is an aromatic
group, the conjugation effect is helpful to stabilize I, facilitating its formation thus.
During the polymerization of P3, the non-aromatic alkyl chain brings ambiguous
influence to the stability of intermediate I, and unfavorable to subsequent chain
propagation. Such hypothesis was also supported by molecular weights comparison
between P1-P3 (will be discussed in following section). The above structural analyses
indicate that the target polymers have been successfully prepared.
15

CDCl3
*
d
c
b
a
P1
*: ethyl ether
**: unkown impurity
*
d
**
c
b
**
a
P2
P3
b' d'
e'
c'
a'
10
9
8
7
6
5
4
3
2
1
Chemical Shift (ppm)
Figure 3. 1H NMR spectra of prepared PAs.
Scheme 2. Mechanism about the acid-promoted ring-opening of benzoxazine to
form iminium I.
3.2 Solubility, molecular weights, thermal and UV-vis absorption properties of
P1-P3
The solubility of P1-P3 was evaluated here, and the results are summarized in
Table 1. P1-P3 not only display good solubility in polar solvents (NMP, DMSO, DMF,
etc.), but also completely soluble in weak polar solvents (CHCl3 and THF). Good
solubility and solvent adaptability of P1-P3 can be attributed to the simultaneous
introduction of flexible alkyl chains and polar groups (phenolic hydroxyl and amides)
into polymer structures. In addition, it is reported that the introduction of helical TPA
group can effectively improves the solubility of polymers [55]. The excellent
solubility ensures that these polymers can be fabricated by simple and convenient
16

solution coating processes [30].
Table 1 Solubility and molecular weights of P1-P3.
Solventsa
GPCb
Mw
Sample
NMP DMF DMSO THF Dioxane CHCl3 CH3CN
Mn
PDI
++
++
++
++
++
++
++
++
++
++
++
++
++
++
+-
++
++
++
--
--
--
11832 21949 1.85
7398 13981 1.88
4125 7768 1.88
P1
P2
P3
a Qualitative solubility were tested by using 2 mg of polymers in 1 mL of solvent (++:
soluble at room temperature; +-: partially soluble; --: insoluble even on heating).
b Relative to polystyrene standard, using THF as the eluent.
Molecular weights of P1-P3 were evaluated by GPC, and the corresponding
retention curves and molecular weights values can be traced in Figure S2 and Table 1.
From Figure S2, a single signal peak appears in curves of P1-P3, and the retention
time is in the order of P1<P2<P3. It can be seen from Table 1 that the Mn of P3 (4125)
is significantly lower than the corresponding values of P1 (11832) and P2 (7398),
1
which consistent with the information reflected by their H NMR analyses. From
Table 1 one can note that P1-P3 have narrow PDIs (with PDI < 1.88), indicating that
the molecular chains length is relatively uniform in these polymers.
The thermal stability of P1-P3 was examined by TG analysis. TG curves are
shown in Figure 4 and the relevant data are summarized in Table 2. The 10% weight
loss temperatures of P1-P3 (in nitrogen) are 337, 323 and 292 oC, respectively. The 50%
o
weight loss temperature of these polymers are all higher than 400 C, reflecting the
satisfactory thermal stability of them. The char yields of P1-P3 at 800 oC are 22%, 28%
and 24%, respectively. Their relatively lower char yields can be attributed to the large
17

content of long alkyl chains in polymers structures, which were gasified at high
temperature.
100
P1
P2
P3
90
80
70
60
50
40
30
20
100 200 300 400 500 600 700 800
Temperature (oC)
Figure 4. TG curves of P1, P2 and P3.
Table 2 Thermal properties of the PAs.
Sample
P1
Td5% (oC)a
308
Td10% (oC)a
337
Char yield (%)b
22
28
24
300
323
P2
P3
274
292
a Decomposition temperature, recorded at a heating rate of 10 oC/min, under nitrogen.
b Residual weight percentage at 800 oC.
The UV-vis absorption of P1-P3 in solution was also investigated here. As
shown in Figure S3, these polymers exhibit absorption maxima at ~314 nm in DMSO,
which can be attributed to the π-π* transition in triphenylamine segments [56].
3.3 Electrochemical properties of P1-P3
The electrochemical properties of P1-P3 were characterized by CV. Due to the
similar main chain structure of these polymers, their films exhibit similar
electrochemical behaviors (Figure 5). A pair of highly reversible oxidation-reduction
18

processes appear in CV curves of P1-P3, indicating that the introduction of TPA
moiety endow these polymers with good electrochemical activity. The half-wave
oxidation potentials (E1/2) of P1-P3 locate separately at 0.84, 0.83, and 0.81 V, with
the oxidation onset potentials (Eonset) at 0.66, 0.64, and 0.64 V, respectively, which
corresponds to the electron-loss process of TPA moiety in polymer backbones [45].
One can also find from Figure 5 that the oxidation peak currents of P1-P3 are higher
than the corresponding reduction values, indicating that the electron donor in
polymers acts more prominently than the acceptor [57]. The highest occupied
molecular orbital (HOMO) energy levels can be estimated by Eonset [58] of P1-P3, to
be -5.46, -5.44 and -5.44 eV, respectively. The optical band gaps (Eg) of P1-P3 can be
evaluated by their respective UV-vis absorption onsets (Figure S4), and the relevant
data are listed in Table 3. The lowest unoccupied molecular orbital (LUMO) energy
levels of P1-P3 can be estimated by their respective HOMO and Eg values, to be -2.21
-2.22 and -2.17 eV, respectively.
0.0008
0.0006
0.0004
0.0002
0.0000
-0.0002
-0.0004
P1
P2
P3
Ox
Re
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential (V)
Figure 5. Cyclic voltammetric diagrams of P1, P2 and P3 films on an ITO-coated
glass substrate in CH3CN solutions containing 0.1 M TBAP at a scan rate of 50 mV/s.
19

In order to visually describe the orbital distribution of P1-P3, the repeating unit
of polymers were theoretically calculated by DFT. The corresponding distribution
images are shown in Figure 6 (since P1 and P2 have identical TPA-amide
substructural motif, we just simulated the orbital distribution of P1 and P3 here). As
can be seen from Figure 6, the HOMO orbitals of P1 (Figure 6(a)) and P3 (Figure
6(c)) focus on the electron-rich TPA region (as donor), while the LUMO orbitals
(Figure 6 (b)&(d)) spread on the junction part between TPA and amide moieties (as
acceptor), indicating that there is a certain degree of charge transfer in the molecule.
One can also find that aromatic (for P1) or alkyl (for P3) substituents in the tertiary
amine groups have no significant effect on the orbital distribution.
Figure 6. Calculated HOMO and LUMO orbital diagrams of P1 (a&b) and P3 (c&d).
20

Table 3 Electrochemical and energy levels of P1-P3.
λonset (nm)a Eonset (V) E1/2 (V)b Eg (eV)c HOMO (eV)d LUMO (eV)e
Polymer
382
385
379
0.66
0.64
0.64
0.84
0.83
0.81
3.25
3.22
3.27
-5.46
-5.44
-5.44
-2.21
-2.22
-2.17
P1
P2
P3
a λonset: absorption onset wavelength in film state.
b E1/2: average potential of the redox couple peaks.
c Eg: Calculated by 1240/λonset
.
d
HOMO energy levels were calculated from Eonset using Ag/AgCl (4.8 eV) as
reference, with the formula of HOMO = -(Eonset + 4.8) eV.
e LUMO = HOMO + Eg.
The electrochemical stability of polymers was further studied by successive CV
tests. As shown in Figure 7, after 30 cycles of CV tests all curves illustrate certain
degree of decay in peak current, but still retain good oxidation-reduction reversibility.
The CV profiles display ambiguous alteration during successive redox cycles for all
polymers (Figure 7), suggesting that the introduction of methoxyl to the para-position
of TPA are favorable to stabilize the resultant cationic radicals and efficiently prevent
the coupling between TPA moieties.
(b)
(a)
1st
0.0008
0.0010
1st
2-29cycle
30th
2-29cycle
30th
0.0005
0.0000
-0.0005
0.0004
0.0000
-0.0004
0.0
0.2
0.4
0.6
Potential (V)
0.8
1.0
1.2
0.0
0.2
0.4
0.6
Potential (V)
0.8
1.0
1.2
21

(c)
0.0004
1st
2-29cycle
30th
0.0002
0.0000
-0.0002
0.0
0.2
0.4
0.6
Potential (V)
0.8
1.0
1.2
Figure 7. Cyclic voltammetric diagrams of P1 (a), P2 (b) and P3 (c) films for
continuous 30 cycles.
3.4 Electrochromic properties of P1-P3
During the CV testing of polymers, it can be clearly observed that P1-P3 films
illustrated obvious color change in the electrochemical redox process, revealing that
these polymers might possess electrochromic properties. The electrochromic
performance of polymers was further studied by spectroelectrochemical technique. As
shown in Figure 8(a), when the applied voltage increases to 0.65 V, P1 exhibits a new
absorption band at ~ 755 nm, which keeps pace with its oxidation initiation (Table 3).
This spectral change can be attributed to the formation of TPA+ radical [18] during the
oxidation process. With the applied voltage increasing from 0.64 to 0.84 V, the
absorbance at ~ 755 nm gradually raise, accompanied by the apparent color change of
the film (from colorless to blue-green, inset in Figure 8(a)), reflecting the formation
of more TPA cations [59]. With the further enhancement of driving potential (> 0.84
V), the absorbance begins to fall, and the whole change trend is similar to the
alteration of current in CV curve (Figure 7). As reflected by Figure 8(b) and Figure
8(c), P2 and P3 exhibit similar electrochromic behaviors as that of P1, which is
reasonable since they have similar main-chain structures.
22

0.20 V
0.40 V
0.60 V
0.65 V
0.70 V
0.72 V
0.74 V
0.76 V
0.78 V
0.80 V
0.82 V
0.84 V
0.86 V
0.88 V
0.90 V
0.95 V
0.20 V
0.40 V
0.60 V
0.65 V
0.70 V
0.72 V
0.74 V
0.76 V
0.78 V
0.80 V
0.82 V
0.84 V
0.86 V
0.88 V
0.90 V
0.95 V
(b)
(a)
Ox
Re
755 nm
Ox
Re
751 nm
400
500
600
700
800
900
1000
400
500
600
700
800
900 1000
Wavelength (nm)
Wavelength (nm)
0.20 V
0.40 V
0.60 V
0.65 V
0.70 V
0.72 V
0.74 V
0.76 V
0.78 V
0.80 V
0.82 V
0.84 V
0.86 V
0.88 V
0.90 V
0.95 V
(c)
Ox
755 nm
Re
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 8. Absorbance spectra of P1 (a), P2 (b) and P3 (c) thin-films on the
ITO-coated glass substrate in 0.1 M TBAP/CH3CN at different applied potentials
(insets is the visual color change images).
The transmittance changing behaviors of P1-P3 between neutral and oxidation
states were investigated further. Corresponding relationships between
transmittance/current density and time are shown in Figure 9 ((a), (c) and (e)), and
corresponding electrochromic performance data are summarized in Table 4 and Table
5. The maximum transmittance changes (∆Tmax) of P1-P3 are 22, 23 and 37%,
respectively, which are comparable to reported values of other TPA-base polymers [24,
60]. Electrochromic response time refers to the time required for a material to switch
from one color state to another (defined by converting 90% transmittance), including
23

the coloring time (tc) and bleaching time (tb), which is one of the important parameters
for evaluating electrochromic speed [61]. According to Figure 9 ((b), (d) and (f)), tc
values of P1-P3 are 6.2, 5.1 and 5.7 s, and tb values are 5.8, 4.7 and 5.1 s, respectively.
These response time values are relatively larger than those of other typical TPA-based
PAs (Table 4). Hydrogen bonding might be induced between polymer chains due to
the presence of polar amide and phenolic hydroxyl moieties in the structures of P1-P3,
reducing the free volume between polymer chains. As was reported by Sun et al, such
close packing between polymer chains is unfavourable for the transport of electrolyte
ions during the electrochromic process [34, 35], prolonging the switching time thus.
0
200
400
600
800
1000
(b)
(a)
90
75
70
65
60
55
50
0.000
80
70
60
50
-0.002
-0.004
-0.006
tc = 6.2 s
tb = 5.8 s
15 20
0
5
10
Time (s)
25
0
200
200
400
Time (s)
600
600
800
800
1000
0
400
1000
0.000
75
70
65
60
55
50
(c)
(d)
90
80
70
60
50
-0.004
-0.008
-0.012
-0.016
tc = 5.1 s
tb = 4.7 s
0
5
10
Time (s)
15
20
25
0
200
400
Time (s)
600
800
1000
24

0
200
400
600
800
1000
(f)
(e)
105
80
72
64
56
48
40
0.000
90
75
60
45
-0.002
-0.004
-0.006
-0.008
tc = 5.7 s
tb = 5.1 s
15 20
0
5
10
Time (s)
25
0
200
400
600
800
1000
Time (s)
Figure 9 Optical switching of P1 (between 0 and 0.84V) (a), P2 (between 0 and 0.82
V) (c) and P3 (between 0 and 0.82 V) (e) (with cycle time of 20 s) and the
corresponding optical switch times (b, d and f) (P1-P3 were monitored at 755, 751
and 755 nm, respectively).
Electrochromic coloration efficiency (CE) is another important parameter of
ECMs, which can be calculated by the formula of δOD = log(Tb/Tc) and CE = δOD/Q
(where Tb and Tc represent the transmittance before and after coloring; δOD is the
alteration of optical density, which is proportional to the amount of coloring center; Q
(mC/cm2) refers to the amount of charge injected or extracted per unit area) [62].
According to the transmittance-time curves obtained under the multipotential step
condition (Figure 9), δOD values of P1-P3 can be evaluated to be 0.1459, 0.1579 and
0.2659, respectively (Table 5). CE values of P1-P3 are 225, 212 and 247 cm2/C,
respectively (with corresponding Q values of 0.648, 0.744 and 1.075 mC/cm2).
25

Table 4 The summary of molecular weights and electrochromic performances of our
polymers and other typical TPA-based polyamides.
Response
λmonitored
∆Tmax
(%)
Decayc
(%)
timeb
Polymer
Mn
Reference
[34]
(nm)a
tc (s) tb (s)
45900
813
/
1.6
1.2
9.1
79400
69600
59400
20500
41200
970
545
900
800
421
60
/
5.65 0.98
/
[63]
[64]
[21]
[65]
[35]
4.1
4.6
3.2
1.3
1.0
2.0
1.0
0.5
/
/
2.88
83
/
/
/
This
work
This
work
This
work
11832
7398
4125
755
751
755
22
23
37
6.2
5.2
5.7
5.8
4.7
5.1
9.7
P1
P2
P3
64.6
33.2
a Given wavelength where the data were determined.
b Time taken for 90% of the full-transmittance change.
C Decay of coloration efficiency after 300 cyclic scans.
26

Table 5 Optical and electrochemical data collected from CE measurements of P1-P3.
Qb Qb CEc CEc
(1)
(50)
(1)
(50)
Polymer δaOD(1) δaOD(50)
(mC/cm2) (mC/cm2) (cm2/C) (cm2/C)
0.1459 0.1594
0.1579 0.1439
0.2659 0.1907
0.648
0.744
1.075
0.724
0.709
0.836
225
212
247
220
203
228
P1
P2
P3
a Optical density change at the given wavelength after cyclic scans.
b Injected charge is determined from the in situ experiments.
c Coloration efficiency is calculated from the equation of CE = δOD/Q.
Stability of electrochromic switching of P1-P3 has also been investigated here.
As shown in Figure S5, after 300 switching cycles all curves reveal certain degree of
decay, and the loss rates of CE for P1-P3 are evaluated as 9.7%, 64.6% and 33.2%,
respectively. To get insight about such difference in CE stability for our polymers,
electrochemical impedance spectroscopy (EIS) were studied to find if there is
relationship between impedance and CE. As can be seen from the corresponding
Nyquist curve (Figure S6), the semicircular diameters of three polymers in high
frequency region are close to each other, suggesting that there is similar charge
transfer resistances (Rct) for them. This suggests that the relatively poor
electrochemical stability of P2 and P3 is not due to their larger impedance. Given the
similar backbone structures of P1-P3, the lower molecular weight and presence of
residual active end groups might be responsible for the relatively poorer stability of
P2 and P3.
Finally, molecular weights and electrochromic performances of our polymers
and other reported TPA-based PAs are summarized here (Table 4) to make a direct
27

comparison between them. As reflected by Table 4, the comprehensive
electrochromic performances of our polymers are inferior to other typical PAs. We
speculate that this might be owing to the synergism of relatively smaller molecular
weights (Table 4), lower electrochromic site density (triarylamine substructure per
each repeating unit) and closer packing of polymer chains of our polymers than
others.
4 Conclusions
In summary, a serious of novel TPA-containing PAs (P1-P3) were successfully
synthesized at room temperature via efficient BIC reaction. Solubility tests indicated
that the introduction of TPA segment endowed polymers with good solubility in
common organic solvents, and the corresponding films can be prepared by
practical-simple solution coating process. P1-P3 showed a pair of highly reversible
oxidation-reduction peaks in CV tests. DFT simulation shown that P1-P3’s HOMO
orbitals focus on the electron-rich TPA region, while LUMO orbitals spread on the
conjugation part between TPA and amide moieties. When the applied potential
reaches ~ 0.65 V, P1-P3 exhibited new absorption bands in their UV-vis spectra at
755, 751 and 755 nm, respectively, and the appearance color of polymers changed
from colorless to blue-green accordingly. Response time, CEs and switching stability
of P1-P3 were studied in further electrochromic analyses. This current work supports
that the BIC reaction, which with the advantages of good substrates adaptability, high
structural designing flexibility, simple operation and mild reaction condition, could
provide an efficient pathway for the construction of novel functional PAs. It is
28