Dynamic polymers containing one acylhydrazone linkage and dynamic behavior thereof

Article abstract of DOI:10.1016/j.polymer.2013.03.023

Dynamic polystyrene (d-PS) and poly(n-butyl acrylate) (d-PBA) with an acylhydrazone linkage at each chain center were synthesized by atom transfer radical polymerization (ATRP) of corresponding monomers using a bis-bromoisobutyrate initiator bearing one a

Full text of DOI:10.1016/j.polymer.2013.03.023

Polymer 54 (2013) 2647e2651
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Dynamic polymers containing one acylhydrazone linkage and dynamic behavior
thereof
,
b
Zizhen Xu a, Peng Zhao a, Yongming Chen a , Guohua Deng
*
a State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, PR China
b School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Dynamic polystyrene (d-PS) and poly(n-butyl acrylate) (d-PBA) with an acylhydrazone linkage at each
chain center were synthesized by atom transfer radical polymerization (ATRP) of corresponding
monomers using a bis-bromoisobutyrate initiator bearing one acylhydrazone bond. Kinetic studies
demonstrated that the polymerization followed a first-order mechanism. When treated with trifluoro-
acetic acid or heated at 120 ꢀC in solution, the polymer chains of d-PS and d-PBA were found to be
cleaved partially into blocks of half original length, i.e. PS/2 and PBA/2, implying that a dynamic equi-
librium of forming and breaking acylhydrazone bond was established.
Received 5 December 2012
Received in revised form
9 March 2013
Accepted 10 March 2013
Available online 16 March 2013
Keywords:
Atom transfer radical polymerization
Block copolymers
Ó 2013 Elsevier Ltd. All rights reserved.
Dynamic covalent chemistry
1. Introduction
two segments have been recently reported for preparation of thin
membranes with well-defined nanopores [25]. After self-
Recently dynamic covalent chemistry (DCC) [1] has attracted
considerable attention as a powerful methodology for developing
new materials with dynamic exchanged components [2e4].
Reversible covalent bonds, also called dynamic covalent bonds, play
a key role in DCC to prepare such dynamic covalent polymers. In
addition to normal properties of conventional polymers such as
strength and stability, these dynamic polymers have shown the
ability to exchange components between different polymer chains
by stimuli-activating the exchange reaction of the dynamic cova-
lent bonds [2e4]. This feature presents amazing opportunities to
modify or control intrinsic properties of these dynamic polymers
after formation [5e29]. Generally, dynamic covalent polymers were
synthesized by polyaddition or polycondensation of small mole-
cules [5e13,19,20,23] or polymer building blocks [25,26], or by
chain growth polymerization of monomers with crosslinkers con-
taining reversible covalent bonds [14e18,21,22,24]. Dynamic poly-
mers containing acylhydrazone bonds [5,7,8,10,12,13,28], Dielse
Alder cycloadduct [6,9], imines [25], alkoxyamine units [14e16,20e
23], trithioesters [18,24] or disulfide bonds [19,27,29] were re-
ported to reveal self-healing [17,18,24,27e29], mesh size control
[14], tunable mechanical [10] or optical properties [11]. Moreover,
block copolymers with dynamic covalent linkage as the junction of
assembled into nanostructured cylindrical morphology in thin
films, the discontinuous cylindrical domains can be removed by
cleaving the oxy-imine bonds to give the pores with functionalized
walls. Dynamic polymers, polystyrene-b-poly(ethylene glycol) and
polystyrene-b-polyisoprene, with H-bonding linkage as the junc-
tion of the two segments were also reported and demonstrated
microphase separation like their covalent analogues [30].
Herein, we report synthesis and properties of well-defined ho-
mopolymers with a dynamic covalent linkage located at the chain
center given by controlled radical polymerization. The synthetic
route of the polymers is shown in Scheme 1. A covalent reversible
acylhydrazone bond is incorporated in the initiator (compound 2,
Scheme 1) for ATRP before polymerization. Kinetics of the ATRP of
styrene and n-butyl acrylate were investigated, proving that the
polymerizations were “living”/controlled ones. Such novel polymers
demonstrated interesting equilibrium cleavage at dynamic linkages
when treated with trifluoroacetic acid or heated at 120 ꢀC in solution.
2. Experimental
2.1. Materials
CuBr was purified by washing crude chemical with acetic acid,
absolute ethanol and diethyl ether, and then dried under vacuum.
Chloroform, butyl acrylate (n-BA, Aldrich), N,N-dimethylformamide
(DMF, Beijing Chemical Reagent Co.) and styrene (St, >99%, Beijing
* Corresponding author. Tel.: þ86 10 62659906; fax: þ86 10 62559373.
E-mail address: ymchen@iccas.ac.cn (Y. Chen).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.03.023

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Z. Xu et al. / Polymer 54 (2013) 2647e2651
O
O
O
reflux
+
OH
HO
NH NH2
HO
NH
N
CH
OH
H
1
O
Br
O
O
O
O
Br
Br
O
O
NH
N
CH
CH
O
Br
TEA
2
O
O
Br
NH
N
O
Br
m
m
O
O
or
d-PS750 (2m=750)
or
CuBr/PMDETA
O
O
O
Br
O
NH
N
CH
O
Br
n
n
O
O
O
O
(d-PBA100: 2n=100)
(d-PBA234: 2n=234)
Scheme 1. Synthesis of functional initiator 2 and dynamic polymers d-PS and d-PBA.
Chemical Reagent Co.) were dried over calcium hydride for 24 h at
room temperature and distilled before use. Triethylamine was dried
by KOH. Tetrahydrofuran was dried by refluxing over Na prior to use.
2-Bromoisobutyryl bromide (98%, Aldrich), N,N,N0,N00,N00-pentam-
ethyl-diethylenetriamine (PMDETA, 99%, Aldrich), 2,2,2-trifluoro
acetic acid (TFA, Beijing Xingjin Chemical Works), 4-hydroxy
benzohydrazide (98%, Alfa Aesar) and other reagents were used as
received without further purification.
temperature for 30 min. After 2-bromoisobutyryl bromide (11.2 g,
48.7 mmol) was added into the mixture, the reaction was kept for
12 h. Then the solvent was evaporated, the crude product was
dissolved in 100 mL of alcohol. After cooling to 4 ꢀC for 4 h in
refrigerator, white power was precipitated. The collected product
was dried in vacuum for 48 h until constant weight. Yield 64%. 1H
NMR (d, ppm): 1.9 (s, 12H), 7.3(d, 4H), 7.9-8.1 (d, 4H), 8.6 (2H), 12(s,
1H); mp 243e244 ꢀC; Element analysis: Calcd for C22H22Br2N2O5:
C 47.68, H 4.00, N 5.05. Found: C 47.68, H 4.11, N 5.04.
2.2. Measurements
2.6. Preparation of polystyrene d-PS750
Size exclusion chromatography (SEC) was composed of Waters
515 HPLC pump and Waters 2414 refractive index detector equip-
ped with three Waters styragel columns HT2, HT4 and HT5. DMF
with LiBr (1 g/L) was used as eluent at a flow rate of 1.0 mL/min at
50 ꢀC. PS standards were used for the calibration. Nuclear magnetic
resonance spectroscopy (NMR) spectra were recorded on a Bruker
DMX400 spectrometer operated in the Fourier transform mode. d6-
DMSO was used as the solvent. Infrared spectra were recorded on a
Nicolet AVA-TAR 330 FT-IR spectrometer.
The d-PS750 was prepared by atom transfer radical polymeri-
zation (ATRP) of styrene (St): Initiator 2 (10 mg, 18
mmol), PMDETA
(7.8 L, 36 mol), St (7.51 g, 72.1 mmol) and DMF (0.1 mL) were
m
m
charged into a Schlenk tube. The mixture was degassed by three
freezeseevacuateethaw cycles and backfilled with nitrogen. As
soon as CuBr (5.2 mg, 36
mmol) was introduced, the tube was
vacuumed and flame-sealed. Polymerization was carried out in an
oil bath thermostated at 65 ꢀC for 36 h, and then the reaction
mixture was exposed to the air and diluted with THF. After passing
through an alumina column and removing the solvents on a rotary
evaporator, the residues were precipitated into an excess of
methanol and purified by repeat precipitation. The final product d-
PS750 was dried under vacuum over night.
2.3. Polymer preparation
The synthetic routes of dynamic polymers are shown in
Scheme 1.
2.4. Preparation of compound 1
2.7. Preparation of poly(n-butyl acrylate)s d-PBA100 and d-PBA234
4-Hydroxybenzohydrazide (6.3 g, 41.4 mmol) and 4-
hydroxybenzoaldehyde (5.0 g, 40.9 mmol) were dissolved in
150 mL of alcohol, and then refluxed for 8 h. After cooling to 4 ꢀC for
4 h in refrigerator, white power was precipitated from alcohol and
The d-PBA100 was prepared by ATRP of butyl acrylate (n-BA):
Initiator 2 (100 mg, 181 mmmol), PMDETA (78 mL, 360 mmol), n-BA
(4.53 g, 36.2 mmol) and DMF (0.1 mL) were charged into a Schlenk
tube. The mixture was degassed by three freezeseevacuateethaw
cycles and backfilled with nitrogen. As soon as CuBr (52 mg,
obtained by filtration. Yield 87%. 1H NMR (
d, ppm): 6.8 (d, 4H), 7.4(d,
2H), 7.8 (d, 2H), 8.2 (s, 1H), 9.9-10.1 (2H), 11.4 (s, 1H); mp 288e
290 ꢀC.
360
mmol) was introduced, the tube was vacuumed and flame-
sealed. Polymerization was carried out in an oil bath thermo-
stated at 65 ꢀC for 36 h, and then the reaction mixture was exposed
to the air and diluted with THF. After passing through an alumina
column and removing the solvents on a rotary evaporator, the
residues were precipitated into an excess of mixture of methanol
2.5. Preparation of compound 2
Compound 1 (5.0 g, 19.5 mmol) and TEA (11.8 g, 117.0 mmol)
were dissolved in 150 mL of THF, and then stirred at room

Z. Xu et al. / Polymer 54 (2013) 2647e2651
2649
Fig. 3. Kinetic plot of the polymerization of St initiated from compound 2 by ATRP at
65 ꢀC in DMF, [2]:[St]:[CuBr]:[PMDETA] ¼ 1:200:2:2. VDMF:VSt ¼ 1:6.
Fig. 1. 1H NMR spectra of compound 1 and 2 in d6-DMSO.
and water (v/v ¼ 1:1) and purified by repeat precipitation. The final
product d-PBA100 was dried under vacuum over night.
For d-PBA234: Initiator 2 (50 mg, 90.5
180 mol), DMF (0.1 mL), n-BA (4.53 g, 36.2 mmol) and CuBr
(26 mg, 180 mol).
mmol), PMDETA (39 mL,
m
m
2.8. Acidolysis of d-PBA234
The d-PBA234 (20 mg) and DMF (1 mL) were charged into a
centrifugal tube. After TFA (50 L) was added and mixed homoge-
m
nously, the solution was checked by SEC at different time.
2.9. Pyrolysis of d-PBA234
The d-PBA234 (200 mg) and DMF (10 mL) were charged into a
25 mL flask. Pyrolysis was carried out in an oil bath thermostated at
120 ꢀC. The solution was checked by SEC at different time.
Fig. 4. Kinetic plot of the polymerization of n-BA initiated from compound 2 at 65 ꢀC
in DMF, [2]:[n-BA]:[CuBr]:[PMDETA] ¼ 1:200:2:2. VDMF:Vn-BA ¼ 1:6.
3. Results and discussion
study, and its structure and synthetic method are shown in Scheme
1. 1H NMR spectra of compound 1 and 2 (Fig. 1) show that the two
peaks at 10.1 ppm at 9.9 ppm from the aromatic eOH groups of
compound 1 in curve A are invisible in curve B, while one peak at
2.1 ppm from the eCH3 groups of compound 2 is observed in curve
B. In FT-IR spectra (Fig. 2) of compound 1 and 2, the absorption
peaks of carbonyl group from compound 1 and 2 appear at
1639 cmꢁ1 and 1653 cmꢁ1 respectively. The peak at 1755 cmꢁ1 (in
curve b) is ascribed to the carbonyl group of the ester in compound
2. All the results prove that compound 2 was prepared successfully.
Homopolymers, d-PS and d-PBA, were synthesized by ATRP with
the initiator 2 (Scheme 1). Fig. 3 shows ATRP of St in DMF at 65 ꢀC
3.1. Preparation of d-PS and d-PBA
Compound 2 containing an acylhydrazone bond and two 2-
bromoisobutyrate groups was used as an initiator for ATRP in this
b
a
1653
1755
1639
Table 1
Synthesis of d-PS and d-PBA initiated by 2.
Entry
Feed ratioa Monomer Time (h) Mn (kDa)b PDIb Yield (%)
d-PS750
d-PBA100 1:200:2:2
d-PBA234 1:400:2:2
1:4000:2:2 St
36
7
8
78
12.5
29.9
1.23 15c
1.1
50d
1.14 58.5d
n-BA
n-BA
a
[2]:[M]:[CuBr]:[PMDETA], 65 ꢀC.
2000
1800
1600
1400
1200
1000
b
c
Given by SEC (DMF system) calibrated by polystyrene standards.
Given by weighting method.
Wavenumber (cm-1)
d
Given by 1H NMR by comparing the protons of monomers and polymers in the
Fig. 2. FT-IR spectra of (a) compound 1 and (b) compound 2.
reaction mixture.

2650
Z. Xu et al. / Polymer 54 (2013) 2647e2651
Fig. 5. (A) SEC traces of d-PBA234 in DMF solution (20 mg/mL) mixed with TFA (1/60, v/v) for different time at room temperature (a: 0 h; b: 6 h; c: 12 h; d: 47.5 h; e: 72 h; f: 84 h; g:
95 h); (B) Decomposed d-PBA234 for different time.
and the plot of ln ([M]0/[M]) against polymerization time is linear
and the conversion of St increases with increasing polymerization
time. Similarly, ATRP of n-BA was carried in DMF at 65 ꢀC and its
kinetic is shown in Fig. 4. The plot of ln ([M]0/[M]) against poly-
merization time in Fig. 4 is also linear. As shown in Fig. 3, Mw/Mn
(PDI) of d-PS formed during the polymerization process increased
gradually from 1.14 to 1.42, indicating the radical coupling termi-
nation occurred. For d-PBA system in Fig. 4, the PDI is 1.07e1.10,
demonstrating no obvious termination. All the results indicated
that the polymerizations initiated by 2 are controlled. Thus, we
prepared several samples, d-PS750, d-PBA100 and d-PBA234, for
further investigation, and the results were shown in Table 1.
(Fig. 5A), the peak of d-PBA234 (curve a) was at elution time
26.5 min, and its intensity decreased along with increasing acid-
olysis time (curve b to g in Fig. 5A). Meanwhile, another peak
appeared at about 27.7 min with hydrolysis and then increased
rapidly. Since the latter peak had a half molecular weight, it is
ascribed to the hydrolyzed product PBA234/2. The relative amount
of remained d-PBA234 and produced PBA234/2 was determined by
comparing their individual peak areas after deconvolution in the
SEC curves and then the percentage of decomposed d-PBA234 with
different acidolysis time (Fig. 5B) was calculated based on the ratio:
[the peak area of PBA234/2]/[the area of whole trace] ꢂ 100%. The
results show that the amount of cleaved d-PBA234 increased with
increasing acidolysis time and reached to 77% after 95 h. The
decomposing rate of the acylhydrazone linkage was fast in 6 h and
then it slowed down, indicating that the decomposing and the
reforming reactions of the acylhydrazone bond between d-PBA234
and PBA234/2 were built gradually.
Without acid catalyst, the exchange reaction between acylhy-
drazone and acylhydrazine/aldehyde also occurred but with much
less efficient at 120 ꢀC [12]. Dynamic behaviors of the polymer d-
PBA234 at high temperature without acid catalyst were further
investigated here. As shown in Fig. 6, d-PBA234 in DMF solution
(20 mg/mL) gradually decomposed to 72.6% after pyrolysis at
120 ꢀC for 7 d. The decomposing rate was fast in the first one day
and then slowed down gradually. Apparently, d-PBA234 decom-
posed slower under pyrolysis conditions than under acidic condi-
tions at mild temperature, which is in agreement with the
exchange efficiency of polyacylhydrazones in literature [12].
Because pyrolysis needs no outside additive chemicals, it may be
useful to trigger segment exchange by heating.
3.2. Dynamic behavior of d-PBA under acid environment or at high
temperature
It is known that acylhydrazone bond is reversible under a mild
acidic condition [31]. Polyacylhydrazones formed by condensation
of small monomers were found to reversibly exchange their
monomeric units under acidic and heating conditions after for-
mation [13]. In this study the polymer d-PBA234 was chosen for
acidolysis investigation in DMF solution (polymer concentration
20 mg/mL) at room temperature. When TFA (1/60, v/v) was added
to the solution, the acylhydrazone linkage at the central point of d-
PBA234 chain was activated and gradually decomposed into acyl-
hydrazine and aldehyde groups, thus generating half polymer
chains PBA234/2. Since the acid is also a catalyst to form acylhy-
drazone linkage, the reverse process of reforming d-PBA234 by
condensation of the acylhydrazone and aldehyde groups at the
ends of PBA234/2 was also active. Therefore, an equilibrium be-
tween the d-PBA234 and its cleaved products PBA234/2 should be
gradually established. As monitored by SEC at different time
Estimated by extrapolation in Figs. 5B and 6B, about 20% of d-
PBA234 may be presented when the equilibrium is built. Namely it
Fig. 6. (A) SEC traces of d-PBA234 in DMF solution (20 mg/mL) heated for 120 ꢀC at different time (a: 0 d; b: 1 d; c: 2.5 d; d: 3 d; e: 4 d; f: 5 d; g: 6 d; h: 7 d); (B) Decomposed
d-PBA234 for different time.

Z. Xu et al. / Polymer 54 (2013) 2647e2651
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2651
took a very long time to build the equilibrium for cleavage and
reformation. This result contrasted to the case of small hydrazones
formed by hydrazine derivatives bearing electron withdrawing
groups and aromatic aldehydes. Under close to neutral conditions
(6 < pH < 8) in water, formation and hydrolysis of these small
hydrazones is very rapid, taking several minutes to hours to reach
an equilibrium [32]. From the decomposing ratio curve of d-PBA234
in Fig. 6B, it can be seen that an equilibrium was not reached even
after 7 days at 120 ꢀC. This slow rate to reach an equilibrium should
be attributed to the macromolecular tails tethered to the dynamic
linkage.
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Dynamic polymers d-PS and d-PBA containing one dynamic
acylhydrazone linkage located in the chain center were prepared by
ATRP. The dynamic acylhydrazone linkage was introduced in the
initiator before polymerization. Kinetic studies indicated that the
polymerizations initiating by the initiator are “living”/controlled
ones. These dynamic polymers were partially cleaved into halves of
their original chain length when treated with trifluoroacetic acid or
heated at 120 ꢀC in solution, indicating an equilibrium between
polymers through the formation and scission of the acylhydrazone
bond was established although it took more time than that of small
molecules.
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Financial support from NSF China (21090350 and 21090353) is
gratefully acknowledged.
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