Facile catalyst-free synthesis, exchanging, and hydrolysis of an acetal motif for dynamic covalent networks

Article abstract of DOI:10.1039/c9ta04073k

Dynamic covalent networks offer the favorable features of cross-linked polymers as well as the functions of reprocessing, recycling, self-healing, reshaping, and welding; however, it is a challenge to design readily monomer-recovery, highly malleable, catalyst-free, dynamic materials. Here we report the first design of acetal dynamic networks to address this challenge. Acetal dynamic networks were built via the catalyst-free "click" addition of polyol and a commercial divinyl ether without releasing small molecules. Small-molecule model compounds demonstrated thermally-induced acetal exchange without a catalyst. There are two mechanisms for the dynamic exchange of acetal: one is the metathesis of acetal, another is transacetalization. Acetal dynamic covalent networks exhibited excellent malleability and recyclability. They presented rapid stress relaxation at high temperatures. Hot press recovery can be achieved in 10 min at 150 °C. Meanwhile, the starting material was recovered with 92% recovery in 1 h under hot water treatment at 100 °C, and could be recross-linked with a commercial divinyl ether to obtain an acetal network. The acetal networks recovered by the two methods maintained the original structure and performance. An acetal dynamic linkage will open up a new way for the development of catalyst-free dynamic covalent networks and enrich acetal chemistry.

Full text of DOI:10.1039/c9ta04073k

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Journal of
Materials Chemistry A
Materials for energy and sustainability
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This article can be cited before page numbers have been issued, to do this please use: Q. Li, S. Ma, S.
Wang, W. Yuan, X. Xu, B. Wang, K. Huang and J. Zhu, J. Mater. Chem. A, 2019, DOI:
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0.1039/C9TA04073K.
Volume
Number 12
March 2018
Pages 4883-5230
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Journal of Materials Chemistry A
Facile Catalyst-free Synthesis, Exchanging, Hydrolysis of Acetal Motif
V
i
o
ew Article Online
r
DOI: 10.1039/C9TA04073K
Dynamic Covalent Networks
1
, 2
1
1, 2
1, 2
1
1
1
Qiong Li , Songqi Ma *, Sheng Wang , Wangchao Yuan , Xiwei Xu , Binbo Wang , Kaifeng Huang ,
Jin Zhu1*
1Key Laboratory of Bio-based Polymeric Materials Technology and Application of Zhejiang Province, Ningbo
Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R.
China;
2University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
*
Corresponding Authors: (Songqi Ma) E-mail: masongqi@nimte.ac.cn; Tel 86-0574-87619806; Fax 86-0574-
6685186. (Jin Zhu) E-mail: jzhu@nimte.ac.cn.
8
ABSTRACT: Dynamic covalent networks offer favorable features of cross-linked polymers as well as
functions of reprocessing, recycling, self-healing, reshaping, and welding; however, it is a challenge to design
readily monomer-recovery, highly malleable, catalyst-free dynamic materials. Here we report the first design
of acetal dynamic networks to address this challenge. Acetal dynamic networks were built via catalyst-free
“
click” addition of polyol and a commercial divinyl ether without releasing small molecules. Small-molecule
model compounds demonstrated thermally-induced acetal exchange without catalyst. There are two
mechanisms for the dynamic exchange of acetal: one is metathesis of acetal, another is transacetalization.
Acetal dynamic covalent networks exhibited excellent malleability and recyclability. They presented rapid
stress relaxation at high temperature. Hot press recovery can be achieved in 10 min at 150 °C. Meanwhile, the
starting material was recovered with 92% recovery in 1 h under hot water treatment at 100 °C, and could be
recross-linked with a commercial divinyl ether to obtain acetal network. The acetal networks recovered by the
two methods maintained the original structure and performance. Acetal dynamic linkage will open up a new
way for the development of catalyst-free dynamic covalent networks and enrich the acetal chemistry.
1
. INTRODUCTION
The irreversible structure of cross-linked polymers endows their remarkable material properties
simultaneously brings about embarrassment in reprocessing or recycling them.1, 2 Dynamic covalent networks
1

Journal of Materials Chemistry A
Page 2 of 24
involve altering bond arrangement and topology through reversible reactions responding to external stimuli
View Article Online
DOI: 10.1039/C9TA04073K
such as heat, light and pH, while maintain performance stability of conventional covalent networks in the
absence of stimuli.3, 4 In 2011, Leibler’s group innovatively introduced Lewis acid catalyst into epoxy
5
networks to achieve dynamic nature that they can be thermally reprocessed like viscoelastic liquids without
6
breaking network integrity, such new class of materials were coined as vitrimers. Since then, great efforts
have been dedicated to exploit novel dynamic materials based on reversible linkages including disulfides7,
8
9
10, 11
boroxine exchange,12 silyl ether
Diels-Alders reaction , olefin metathesis , dioxaborolane metathesis,
exchange,13 siloxane equilibration,14 imine exchange,15, 16 transcarbamoylation,17 vinylogous urethane/urea
transamination,18, 19 transalkylation of C-N bonds, diselenide, etc. which conquer the recycle puzzle of
permanent covalent cross-links as well as bring new functions such as self-healing, reshaping, welding, and so
on.22, 23 Despite these important advances, it is still a challenge to design readily monomer-recovery, highly
malleable catalyst-free dynamic materials without releasing small molecules during cross-linking and without
using catalyst during reprocessing and recycling. The vast majority of dynamic covalent networks or vitrimers
required high temperature and long time to be reprocessed, and most of them needed specific catalysts to
activate the exchange processes, which might arise the risk of catalyst instability and thermal degradation of
20
21
1
5
matrix. Moreover, the great majority of dynamic covalent networks had trouble being degraded to recycle
monomers (or starting materials). The currently reported catalyst-free, monomer-recovery dynamic covalent
networks are mainly based on Schiff base 15, 24, boroxine 12 and diketoenamine , as shown in Table 1. However,
Schiff base, boroxine and diketoenamine were often formed during the curing process and produce small
molecule water, which would easily remain in the cross-linked networks and affect their properties. Without
using catalyst, Schiff base networks also showed high malleability, while our previous work15, 16 demonstrated
that high-performance Schiff base networks also needed high temperature to be reprocessed, resulting in
oxidization and enbrittlement during the reprocess. Helm’s group 25 recently developed closed-loop recyclable
diketoenamine dynamic networks. Diketonamine bonds formed almost spontaneously in solvent-free raw
materials and could be used to make a wide variety of materials in principle. In addition, diketoamines enable
plastics to be recycled in the energy-saving process to regenerate the original monomers for the re-manufacture
of thermoset plastics that are nearly identical to the original materials. However, the diketoamine networks
25
2

Page 3 of 24
Journal of Materials Chemistry A
must be depolymerized by treatment with strong acids, which may be harmful to environment.
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DOI: 10.1039/C9TA04073K
Table 1. Comparison of dynamic covalent networks.
Polymer performance
Bond exchange
Activation
energy (kJ
Recycling
Networks based on different dynamic
Young’s
Tensile
T
g
Catalys
t
Monomers
recovery
bond
modulus strength(
Remolding
Ref.
(
°C)
-
1
(
GPa)
MPa)
27.2-
33.3
mol )
6
6-
This
0
.8-1.1
1.8
126-136
None
150 °C/10 min
yes
No
7
1
0
work
TBD,
Zn(Ac)
8
55
88
240 °C/3 min
5
2
,
P(Ph)
3
1
25
1.8
38
10-64
90
30
30-129
65
None
None
None
None
150 °C/15 min
180 °C/2-10 min
150 °C/30min
160 °C/60 min
Yes
Yes
No
25
26
27
20
5
5-
0
.13-1.0
2.4
1
35
8
7
-11
3-8
0.008
140
No
1
80-
9
8
8
1.8
0.7
28
40
None
None
No
10
28
2
00 °C/seconds
R
B
Ph
O
B
R
Ph
O
B
Ph
HO
HO
OH
B
O
B
O
B
O
B
O
6
32.9
79.5
80 °C/24 h
Yes
Ph
B
Ph
Ph
OH
R1
R3
S
R1
R2
S
S
S
R2
R4
S
S
R4
37.3-
40.6
1
.99
11.0
-
106
-
None
Ru
100 °C/1 h
-
No
No
No
29
9
S
S
R3
R1
R1
R2
R2
R3
R4
R4
R1
R1
R4
R4
R3
R3
R2
R2
0.94-1.1
MPa
-
R3
R1
R2-OH
R1-OH
R2
R1
R1
R1
Si
Si
125
1.8
20.3
81,174
None
190 °C/1 h
13
R1
R2
S
R3
R4
R3
S
S
R
S
R
R2
R4
R1
R2
66
0.04
6
113
None
None
160 °C/40 min
80 °C/1 h
No
No
30
31
X
X
X=BrSO
-
H
H
H
H
R2-NH2
R1-NH2
31-230
KPa
0.45-
0.80
N
N
N
N
R2
R2
R1
R1
76.6-
52-80
R
R
-
51.1
Recently, our laboratory has focused on developing recyclable cross-linked polymers based on acetal motif.
The traditional way of acetal formation is the acid-catalyzed acetalization of alcohol with aldehyde with
3

Journal of Materials Chemistry A
Page 4 of 24
elimination of water. Acetal can also be synthesized via acid-catalyzed “click” addition of vinyl ether with
View Article Online
DOI: 10.1039/C9TA04073K
hydroxyl group without releasing small molecules.32, 33 Acetal can be cleaved under acidic condition so it was
incorporated into cross-linked networks to achieve degradability and recyclability.34, 35 Acid-catalyzed acetal
3
6
37
exchange was also employed to produce acetal dendrimers, dynamic chiral crown ether, and cyclophane
formation.38 With the good degradability and dynamic exchangeability, acetal motif should be suitable to
produced facilely monomer-recovery dynamic covalent networks. However, acids as catalyst to synthesize
acetal or trigger the acetal exchange would catalyze the degradation of acetal and severely iMPact the stability
of acetal network.
Here, we report novel dynamic covalent networks based on acetal via the thermally-accelerated rather than
the acid-catalyzed alcohol-vinyl ether “click” reaction, the acetal dynamic networks are catalyst-free, robust,
highly malleable and readily recyclable to monomers by only hot water. The catalyst-free alcohol-vinyl ether
“
click” reaction was systematically investigated and utilized to synthesize acetal dynamic networks using a
typical synthesized polyol (poly (styrene-co-styrene-OH) 13, 39) and a commercial divinyl ether as the starting
material. The cross-linking process as well as thermal and mechanical properties were studied. The malleability,
reprocessability as well as the dynamic exchange mechanism of the acetal dynamic networks were
systematically investigated with the help of small-molecule models. The starting material polyol and other
monomers were recovered from the acetal dynamic networks by hot water treatment, and the degradation
mechanism was investigated as well.
2
2
. Experimental sections
.1 Materials. 4-Methylbenzyl chloride, ethylene glycol, styrene, azodiisobutyronitrile (AIBN), 1,1,3,3-
tetraethoxypropane and 1,1,3,3-tetramethoxypropane were purchased from Aladdin-reagent Co., China. 4-
Vinybenzyl chloride and 1,4-cyclohexanedimethanol divinyl ether (CVDE, mixture of cis/trans isomers, purity:
≥
98%) were obtained from Sigma-Aldrich Co. China. Trichloromethane (CHCl ), dichloromethane,
3
tetrahydrofuran and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Anhydrous
solvents were dried by activated molecular sieve before use. All other chemicals were used as received unless
noted.
4

Page 5 of 24
Journal of Materials Chemistry A
2
.2 Synthesis of 2-(4-Vinyl-benzyloxy)-ethanol (1) 13, 39. 4-Vinybenzyl chloride (15.26g, 0.1mol) was added
View Article Online
DOI: 10.1039/C9TA04073K
to a mixture of ethylene glycol (165.5 g, 2.65 mol), NaOH (0.1 mol) and H O (0.1 mol) in a round-bottom
2
flask. The mixture was stirred at 70 ºC for 24 h. After cooing to room temperature, 300 mL of water was added
to the solution, followed by being extracted three times with dichloromethane and washed three times with
deionized water. The organic layer was dried by anhydrous magnesium sulfate, and dichloromethane was
evaporated to give a yellow liquid 1 (15.2 g, yield: 84%).
1
H NMR (400 MHz, DMSO-d ): δ = 7.45-7.25 (m, 4H), 6.75 (dd, 1H), 5.82 (d, 1H), 5.25 (d, 1H), 4.5 (s, 2H),
6
1
3
3
.60-3.49 (m, 4H); C NMR (100 MHz, DMSO-d ): δ = 138.8, 136.8, 136.6, 128.2, 126.4, 114.3, 72.4, 72.2,
6
6
0.8.
2
.3 Synthesis of Poly (styrene-co-styrene-OH) (PSS-OH) 13, 39. Styrene (5.21 g, 0.05 mol), 1 (8.91 g, 0.05
mol) were poured into a three-necked flask with 50 mL toluene. Then initiator AIBN (0.4926 g, 3 mol%) was
added into the mixture. The solution was stirred at room temperature for 1 h, bubbled N for 60 min, and heated
2
at 80 °C for 12 h. The reaction was quenched by liquid nitrogen and cooled to room temperature. 1000 mL of
hexane was added slowly into the reaction mixture to precipitate the copolymer. After filtration, the
precipitation was purified twice by being dissolved in minimum amount of CH Cl and precipitated by MeOH.
2
2
After drying in a vacuum oven at 60 °C for 24 h, a yellow solid PSS-OH was obtained (10.2 g, yield: 72%).
1
H NMR (400 MHz, DMSO-d ): δ = 7.6-6.9 (m), 4.53 (br, s), 3.76-3.60 (m), 2.28 (br, s), 2.0-0.85 (m).
6
2
.4 Synthesis of 2-(4-Methylbenzyloxy) ethanol (2). 4-Methylbenzyl chloride (14.06 g, 0.1 mol) was added
to a mixture of ethylene glycol (165.5 g, 2.65 mol), NaOH (0.1 mol) and H O (0.1 mol) in a round-bottom
2
flask. The mixture was stirred at 70 ºC for 24 h. After cooing to room temperature, 300 mL of water was added
to the solution, followed by being extracted three times with dichloromethane and washed three times with
deionized water. The organic layer was dried by anhydrous magnesium sulfate, and dichloromethane was
evaporated to give a yellow liquid 2 (14.3 g, yield: 86%).
1
H NMR (400 MHz, DMSO-d ): δ = 7.25-7.15 (m, 4H), 4.65 (t, 1H), 5.25 (d, 1H), 4.5 (s, 2H), 3.44-3.44 (m,
6
1
3
4
H), 2.29 (s, 3H); C NMR (100 MHz, DMSO-d ): δ = 136.9, 136.0, 129.2, 128.0, 72.4, 72.2, 60.8, 21.2.
6
2
.5 Preparation of the Acetal Dynamic Networks. PSS-OH (2.0 g) was dissolved in trichloromethane (5
5

Journal of Materials Chemistry A
Page 6 of 24
mL), CVDE (with the molar ratios of double bond to hydroxyl group of 1:1 and 0.95:1) was added to the
View Article Online
DOI: 10.1039/C9TA04073K
solution at 80 ºC. After about 30 min, the solution thickened and was casted into a Teflon mold. The mold was
placed in a 80 °C oven under reduced pressure (0.03 mmHg) for 12 h to get the precured acetal networks. After
being pressed at 150 °C for 10 min with a plate vulcanizer, followed by being postcured for 12 h at 80 °C,
acetal dynamic networks PC-0% OH (double bond : hydroxyl group = 1:1) and PC-5% OH (double bond :
hydroxyl group = 0.95:1) was obtained. Non-chlorinated solvents can be used as alternative options. As an
example, toluene was used as the non-chlorinated alternative to trichloromethane to prepare acetal network
PC-0% OH-Toluene with the same curing process.
2
.6 Reprocess of Acetal Dynamic Networks. The reprocess recycling test was carried out with a plate
vulcanizer. The films were cut into small pieces, placed between two steel sheets covered with two polyimide
films to avoid acetal dynamic networks adhering on the steel sheets and hot pressed at 150 °C under a pressure
of 15 MPa for 15 min (PC-0% OH), for 10 min (PC-5% OH). After cooling to room temperature, reprocessed
films were obtained.
2
.7 Gel content test. Two acetal networks (around 200 mg) were separately put into a Soxhlet extractor for
extracting with acetone for 24 h and dried in a blast oven at 60 °C for 24 h. m is initial mass, m is final mass
0
1
after drying; the gel content is calculated by m /m .
1
0
2
.8 Swelling measurement. PC-5% OH networks were immersed in THF, toluene, acetone at 40 °C for 12h,
and in H O for 2h. m is dried mass, m is swelling mass ; the swelling percentage is calculated by m /m .
2
0
1
1
0
2
.9 Small-Molecule Acetal Exchange Experiments. A mixture of two model acetals (1,1,3,3-
tetraethoxypropane (3) and 1,1,3,3-tetramethoxypropane (4)) with molar ratio of 1:1 was placed in a round-
bottom flask, and flushed with dry nitrogen before heating, then heated at 150 °C under enclosed condition.
1H NMR experiments after various times (15 min, 30min, 45min, 8h) were performed. A mixture of model
alcohol ((2-(4-methylbenzyloxy) ethanol) (2)) and 4 with molar ratio of 4:1 was placed in a round-bottom
1
flask, and flushed with dry nitrogen before heating, then heated at 150 °C under enclosed condition. H NMR
experiments after various times (15 min, 30min, 45min, 8h) were performed.
2
.10 Degradation Recycling and Monomer Recovery of the Acetal Dynamic Networks. Take PC-0% OH
6

Page 7 of 24
Journal of Materials Chemistry A
as an example, 200 mg of PC-0% OH was placed in 10 mL of deionized water in a 25 mL vial. The degradation
View Article Online
DOI: 10.1039/C9TA04073K
behavior was investigated by adjusting temperature. After keeping at 100 °C for 1 h, the sample was turned
into sticky solid and floated on water. The floated solid was washed with deionized water twice and dried
under vacuum at 60 °C for 24 h to obtain yellow solid powder PSS-OH with recovery of 92%. The water
1
solution was evaporated, and 1,4-cyclohexanedimethanol was obtained and determined by H NMR. Based on
the envisioned mechanism of degradation, acetaldehyde should also be produced during the degradation
process. In order to capture the low boiling acetaldehyde, D O was used as the alternative to H O to degrade
2
2
1
the PC-0% OH at 90 °C under an enclosed condition for 4 h and examined the H NMR technology directly.
The weight loss after degradation in water at different temperature (25 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C,
9
0 °C, 100 °C) for 1 h was measured. Take three averages for each experiment. The water resistance of acetal
dynamic network was also investigated by immersing the sample in 25 °C water for 48 h. The recovered PSS-
OH was recross-linked with CVDE with molar ratio of hydroxyl group to double bond of 1:1.
1
13
13
1
2
.11 Characterizations. H NMR and C NMR, C- H-COSY HMQC spectra were collected on a Bruker
AVANCE III spectrometer using DMSO-d as the solvent. Gel permeation chromatography (GPC) traces were
6
obtained on an Agilent 1100 SEC system using a PLGel Mixed-C column from Polymer Labs (Amherst, MA).
-1
THF was used as eluting solvent at a flow rate of 1.0 mL min . Number averaged and weight averaged
molecular weight distributions (M and M , respectively) of samples were measured with respect to
n
w
polystyrene (PS) standards purchased from Aldrich (Milwaukee, WI). Elemental analyses were performed on
an Elementar Vario EL analyzer. HPLC spectra were examined through an HPLC apparatus with single C18
bonded silica column and a UV/Vis absorption detector (LDC, Spectromonitor 3000) with CH OH/H O (v/v
3
2
=
70:30) as an isocratic elution system. DSC measurement was implemented under a nitrogen atmosphere
using Mettler-Toledo TGA/DSC I differential scanning calorimeter (METTLER TOLEDO, Switzerland) to
test the kinetics of catalyst-free alcohol-vinyl ether “click” reaction of small-molecule model alcohol 2 and
-1
CVDE (the mixture was approximately 4 mg) by heating from 25 °C to 160 °C at 5 °C min . The cured samples
-1
(
around 5 mg) were heated from -10 to 180 ºC at a heating rate of 50 ºC min , kept at 180 ºC for 3 min, and
-1
-1
cooled to 20 ºC at a cooling rate of 50 ºC min , and then heated to 180 ºC at a heating rate of 20 ºC min . The
non-isothermal DSC curves for glass transition temperatures were from the second heating processes.
7

Journal of Materials Chemistry A
Page 8 of 24
Thermogravimetric analysis (TGA) was performed on a Mettler-Toledo TGA/DSC1 thermogravimetric
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DOI: 10.1039/C9TA04073K
analyzer (METTLER TOLEDO, Switzerland) under nitrogen atmosphere from 50 to 800 ºC at a heating rate
-1
of 20 ºC min with approximately 6 mg sample. The FTIR spectra were examined using a Micro-FTIR
Cary660 (Agilent, America). The dynamic mechanical analysis (DMA) was performed on a Q800 DMA (TA
instruments, American) under a tensile mode at a frequency of 1 Hz. Samples with dimensions of 20 mm
(length) × 5 mm (width) × 0.3 mm (thickness) were used to test the dynamic mechanical properties from
−
1
−
100°C to 180 °C at a heating rate of 3 °C min . Stress relaxation: Samples with dimensions of 20 mm × 0.5
-3
mm × 0.3mm were initially preloaded by 1 × 10 N force to maintain straightness. After reaching the testing
temperature, it was allowed 10 min to reach thermal equilibrium. The specimen was stretched by 2% on the
DMA machine and the deformation was maintained throughout the test. The decrease of the stress was
recorded. Creep experiments: A constant stress of 0.2 MPa was applied to the specimen after temperature
equilibration for 10 min. Tensile test was conducted on an Instron 5567 Electric Universal Testing Machine
-1
(
Instron, America) with gauge length of 20 mm at a cross-head speed of 5 mm min at room temperature. The
X-ray photoelectron spectroscopy (XPS) spectra was performed on an AXIS ULTRA apparatus (Kratos,
England).
3
. Results and discussion
3
.1 Synthesis of the Acetal Dynamic Networks
In this work, acetal dynamic networks were synthesized through catalyst-free “click” addition of polyol and
divinyl ether. First, a typical polyol (poly(styrene-co-styrene-OH (PSS-OH)) was synthesized (Scheme 1).13, 39
-1
Its average molecular weight (Mn) determined by GPC is around 7030 g mol (Fig. S4), and the molar
1
proportion of 2-(4-vinylbenzyloxy) ethanol (1) in PSS-OH examined by H NMR is around 45% (Fig. S5).
Then, to understand the catalyst-free alcohol-vinyl ether “click” reaction clearly and to guide the cross-linking
process, we studied the reaction of model alcohol 2 and 1,4-cyclohexanedimethanol divinyl ether (CVDE).
The non-isothermal DSC curve of the mixture of 2 and CVDE exhibited an strong exothermic peak between
around 55 ºC and 100 ºC (Fig. 1b), which means that the reaction proceeded quickly at this temperature range
1
and slowly below 55 ºC. The real-time H NMR technology was also used to monitor the reaction during the
8

Page 9 of 24
Journal of Materials Chemistry A
heating process (Fig.1a). The peaks at 6.5, 4.2 and 4.0 ppm belonging to the double bonds of CVDE gradually
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DOI: 10.1039/C9TA04073K
decreased, and the peaks at around 4.6 ppm representing acetal linkages appeared, which indicates the
formation of acetal from vinyloxy group and hydroxyl group. After reaching 55 °C, the change of the
characteristic peaks became more apparent. When being heated to 80 °C, double bonds were almost consumed.
1
3
1
C- H-COSY HMQC of alcohol 2 and CVDE at 80 °C (Fig. S8-S9) represented multi-peaks at 4.6 ppm belong
to only one carbon at 100 ppm. These results indicate that alcohol can react with vinyl ether to afford acetal
without catalyst, and the reaction proceeded very slowly at room temperature but it could be thermally
accelerated. Based on these results, acetal dynamic networks PC-0% OH and PC-5% OH were prepared
through the cross-linking of PSS-OH and CVDE with molar ratios of hydroxyl group to double bond of 1:1
and 1:0.95, respectively (Scheme 2). FTIR and DSC technologies were used to determine the cross-linking
degree. As shown in the FTIR spectra of PC-0% OH and PC-5% OH (Fig. 2), there are no peaks belonging to
-1
the double bonds from vinyl ether, and the broad band in the 3200–3600 cm region ascribed to the O–H
stretching vibrations almost disappeared for PC-0% OH and became very weak for PC-5% OH coMPared with
the FTIR spectra of their starting materials PSS-OH and CVDE. The non-isothermal DSC curves of PC-0%
OH and PC-5% OH shows no curing peaks (Fig. S10). These results demonstrate that the cross-linking
reactions between PSS-OH and CVDE were completely proceeded. Solvent resistance measurements were
also carried out to manifest the implementation of the covalently cross-linked structure in the acetal dynamic
networks. The swelling experiment were performed in various solvents at 40 ºC. The acetal dynamic networks
were insoluble but only swollen in common organic solvents such as tetrahydrofuran, toluene, and acetone
with swelling degree of 176%, 160% and 147%, respectively (Fig. S11 and Table S1), and possessed high gel
content of above 96% (Table S2), indicating robust cross-linking interaction.
Scheme 1. Synthetic route of PSS-OH.
9

Journal of Materials Chemistry A
Page 10 of 24
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DOI: 10.1039/C9TA04073K
Fig.1 Catalyst-free alcohol-vinyl ether “click” reaction of small-molecule models: a) Scheme of “click”
1
reaction of 2 and CVDE and Real-time H NMR monitoring of “click” reaction of model alcohol 2 and CVDE
during the heating process; b) Non-isothermal DSC curve of “click” reaction of 2 and CVDE.
Scheme 2 Preparation of the acetal dynamic networks.
1
0

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Journal of Materials Chemistry A
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Fig. 2 FTIR spectra of CVDE, PSS-OH, PC-0% OH and PC-5% OH.
.2 Thermal Properties of the Acetal dynamic networks
3
DMA curves (Fig. 3) and DSC curves (Fig. S12) of acetal dynamic networks and Table 2 exhibited the glass
transition temperature (T ) of PC-0% OH is higher than that of PC-5% OH. In general, the T of the cross-
g
g
linked networks is interrelated with their cross-link density and the rigidity of their chain segment.40, 41 The
cross-link density (ν ) of the acetal dynamic networks was calculated from the DMA data by the following
c
equation (1):42
퐸푅
휈푐 =
(1)
3
푅푇
where E was the plateau modulus in the rubbery state at T + 60 ºC (Pa), R was the gas constant, and T was
R
g
the Kelvin temperature of T + 60 ºC. As shown in Table 2, the ν value of PC-0% OH is higher than that of
g
c
PC-5% OH. Since the chemical structures of PC-0% OH and PC-5% OH are very similar, corresponding to
their similar rigidity, the higher T of PC-0% OH than that of PC-5% OH was mainly from the higher cross-
g
link density of PC-0% OH than that of PC-5% OH.
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Fig. 3 DMA curves of the acetal dynamic networks a) PC-0% OH b) PC-5% OH.
Table 2. Thermal properties of the acetal dynamic networks.
Tg (ºC)
νc
Td5% (ºC)
Td30% (ºC)
Ts (ºC)
Sample
-3
(
mol m )
DMA DSC
air N2
air N2
air N2
PC-0% OH
PC-5% OH
71
66
67
54
990
241 263
236 262
344 363
340 362
148 158
839
146 158
The thermal stability of the acetal dynamic networks was evaluated by TGA in air and nitrogen atmospheres,
and their curves were showed in Fig. 4. The initial decomposition temperature of 5% weight loss (Td5%) as well
as the temperature of 30% weight loss (Td30%) were summarized in Table 2. The statistic heat-resistant index
(
Ts) was calculated according to the equation (2):43
푇푠 = 0.49 [푇푑5% +0.6(푇푑30% ― 푇푑5%
)
(2)
The acetal dynamic networks showed Td5% of above 236 °C under air atmosphere and above 262 °C under
nitrogen atmosphere, which demonstrates that they were stable below these temperatures. The two acetal
dynamic networks showed similar thermal stability as can be seen from their similar Td5% Td30% and T values.
s
The slightly higher thermal stability of PC-0% OH might be from its higher cross-link density. In order to
determine the stability during the reprocess or malleability measurement, isothermal TGA curves of PC-5%
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Journal of Materials Chemistry A
OH at 180 °C under nitrogen were also measured (Fig. S13). There was no obvious weight loss after heating
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for 60 min, which indicates that the acetal dynamic networks would be sable during the reprocess or
malleability measurement.
Fig. 4 TGA curves of the acetal dynamic networks a) under air; b) under nitrogen.
3
.3 Malleability of the Acetal Dynamic Networks
To evaluate the potentiality of acetal network as a new type of dynamic covalent network, PC-0% OH and
PC-5% OH were subjected to both stress relaxation inspection and creep analysis through DMA. Fig. 5 depicts
time-dependent relaxation stress of PC-0% OH (Fig. 5a) and PC-5% OH (Fig. 5b) at different temperature.
The stress of both samples was found to decrease exponentially with time, which can be traced by a Maxwell
2
7
model with a single characteristic relaxation time (τ*) . τ* was determined from the stress relaxation curves
as the time required to reach 1/e of the initial stress, which enabled the coMParison of relative stress-relaxation
rates at different temperatures. As expected, both samples achieved absolute stress relaxation under high
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temperature, validating that the covalent networks present viscoelastic liquid characteristic from the
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exchangeable acetal linkages. Moreover, the relaxation speed of acetal dynamic networks quickened with the
increase of the temperature from 150 °C to 180 °C. For instance, for PC-5% OH, the τ* reduced from1043 s
to 89 s when the temperature raised from 150 °C to 180 °C. The samples with different hydroxyl content
showed discrepant relaxation properties, excess hydroxyl group-containing PC-5% OH was more susceptible
to stress relaxation than PC-0% OH, indicating that the residual hydroxyl group can accelerate the dynamic
exchange reaction of acetal linkages. For example, the τ* of PC-0% OH at 180 °C is 123 s, while PC-5% OH
spent only 89 s to achieve the same goal. The τ* obeyed the Arrhenius equation with the temperature for both
samples (Fig. 5c), which indicates that the acetal dynamic networks have vitrimer characteristics.18, 44
Topology-freezing temperature (T ) and activation energy (E ) were determined using the methodology
v
a
reported in literature6, 13. The measured values of τ*s were plotted versus 1000/T. The plots were fit to the
Arrhenius law.
Fig. 5 a-b) Stress relaxation curves of acetal dynamic networks a) PC-0% OH and b) PC-5% OH; c) Arrhenius
analysis of the characteristic relaxation time τ* versus 1000/T for acetal dynamic networks; d) Creep curves
of PC-5% OH.
From Fig. 5c and Fig. S14, the E s of acetal exchange for PC-0% OH and PC-5% OH were calculated to be
a
-1
-1
1
36 kJ mol and 126 kJ mol , respectively. The E’ of PC-0% OH from 150 °C to 180 °C is 8.5 MPa, and that
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Journal of Materials Chemistry A
1
2
of PC-5% OH in the same temperature range is 7.1 MPa (Fig. 3). Because η is 10 Pa at T , τ*s at T of PC-0%
v
v
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DOI: 10.1039/C9TA04073K
5
5
OH and PC-5% OH were calculated to be 3.1 × 10 s and 3.8× 10 s, respectively. Using these values, T was
v
computed to be 99 °C for PC-0% OH and 89 °C for PC-5% OH. In agreement with the τ* trend, the PC-0%
OH showed higher E and T than PC-5% OH. To further verify the malleability of acetal dynamic networks,
a
v
PC-5% OH was subjected to creep analysis at different temperature (Fig. 5d and Fig. S15). The sample
deformed under an external force and reverted to the original shape after withdrawing loading at 80 °C.
However, the PC-5% OH sample could generate an irreversible deformation at 100 °C (above its T ). Upon
v
external loading at 150 °C, a greater creep occurred. The significant viscoelastic response is attributed to the
triggered dynamic exchange reactions of acetal motif at elevated temperature, which permits the topology
rearrangements of cross-linked networks to accommodate external loading. The exchange reactions are inferior
and even frozen at low temperature, hence the stiff networks behave like the classic thermosets. These results
indicate the excellent thermal-induced malleability of acetal networks.
3
.4 Reprocess Recycling of the Acetal Dynamic Networks
Thermosets with a permanent covalent cross-linked architecture generally cannot be recycled, reprocessed or
even reshaped, while the processing of thermoplastics demands to precisely control temperature because they
are featured with a narrow glass transition temperature and thus a very abrupt drop in viscosity. Since acetal
dynamic networks exhibit a gradual decay in viscosity that follows Arrhenius law, it could be recycled without
strict temperature control or specific molds. In order to actually examine the reprocessability of the acetal
dynamic networks, the samples were chopped and then employed to hot-pressing. The chopped acetal dynamic
networks could be re-compressed into complete films using a plate vulcanizer at 150 °C (15 min for PC-0%
OH and 10 min for PC-5% OH). The procedure was repeated twice and the reprocessed films were
characterized with tensile test, FTIR measurement. Fig. 6a and Fig. 6b show the representative stress-strain
curves of the original, first reprocessed and second reprocessed acetal dynamic networks, and the data are
summarized in Table S3. Original PC-0% OH showed Young modulus of 1.1 GPa and tensile strength of 33.3
MPa, and original PC-5% OH exhibited Young modulus of 1.0 GPa and tensile strength of 28.8 MPa. After
being cut into small pieces and hot pressed through two generations, there was no apparent decrease of
mechanical properties for both samples. For example, the Young modulus and tensile strength of the second
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reprocessed PC-0% OH reached 968 MPa and 29.7 MPa. The tensile properties reveal that the reprocessed
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DOI: 10.1039/C9TA04073K
samples could revert a great large extent of original mechanical properties. It is essential to recognize that
repeated chopping and hot-pressing molding will lead to unrecoverable chain scission within the network,
which was dominantly devoted to subtle divergence in mechanical properties between the recycled sample and
original sample. The FTIR spectra (Fig. 7) of these reprocessed PC-5% OH sample perfectly matched with
that of the original sample, demonstrating the chemical structure of the cross-linked network was barely revised
after recycling by hot-pressing. To further identify the chemical components of the acetal dynamic networks
before and after reprocessing, the surface chemical structures of the samples were examined by XPS (Fig.
S17). Obviously, there is only little change in chemical composition during reprocess. The slight increase of
oxygen content might be due to the oxidation of the samples during the hot press process. Meanwhile, TGA
analysis proved that the acetal dynamic network was stable enough to be recycled at elevated temperature as
mentioned above (Fig. S13). These results indicate that the thermal and mechanical properties as well as the
chemical structures of the acetal dynamic networks were preserved well during the reprocess.
Fig. 6 a-b) Representative tensile stress-strain curves of a) PC-0% OH and b) PC-5% OH; c) Diagram of
reprocessing recycle mechanism (top) and images of cut and reprocessed PC-5% OH (bottom).
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Fig. 7 FTIR spectra of the original and reprocessed PC-5% OH.
3
.5 Dynamic Exchange Mechanism of the Acetal Dynamic Networks
Small-molecule model compounds were used to investigate the dynamic exchange mechanism of the acetal
dynamic networks. Under the acid catalysis, there are two mechanisms for the dynamic exchange of acetal:
4
5
one is metathesis of acetal that is the acetal exchange between two different acetal structures ; another is
transacetalization, that is, a hydroxyl group reacts with an acetal and replace part of the acetal structure to form
a new acetal.36 To investigate the dynamic exchange of acetal without acid catalyst, the acetal exchange
kinetics of small-molecule model compounds were studied. Two model acetals 3 and 4 were chose to certify
metathesis of acetal (Fig. 8a and Fig. S18-S19) and model acetal 4 and model alcohol 2 were mixed to ensure
transacetalization (Fig. 8b and Fig. S20-S21). Both reactions were carried out at 150 ºC and monitored by
1
NMR technology. Fig 8 exhibited the H NMR. New acetal peaks at around 4.72 ppm. 4.85 ppm and 4.99 ppm
and 4.51 ppm, 4.63 ppm appeared for both reactions, which is indicative of the thermally-induced dynamic
1
3
1
exchange of acetal following the mechanism of metathesis of acetal and transacetalization. C- H-COSY
HMQC of original acetal 3 and acetal 4 mixture at 150 ºC for 0mins and 8h (Fig. S22a) exhibited new acetal
peak C appeared and new carbon had appeared around 100 ppm, which is corresponding to the new acetal
1
3
1
peak. This phenomenon can also be seen in the C- H-COSY HMQC of original alcohol 2 and acetal 4 (Fig.
S22b).
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1
Fig. 8 Catalyst-free acetal exchange mechanisms: a) metathesis of acetal and the H NMR spectra of the
1
reaction system at 150 ºC for different time; b) transacetalization and the H NMR spectra of the reaction
system at 150 ºC for different time.
3
.6 Degradation Recycling and Monomer Recovery of the Acetal Dynamic Networks
The acetal dynamic networks could be readily recycled by only hot water. Take PC-0% OH as an example,
after immersing in boiling water for 1 h, PC-0% OH turned into a sticky solid and floated on the water, and
1
after cooling down and drying, further turned into a yellow solid (Fig. 9a) which showed almost identical H
NMR and SEC spectra to the original PSS-OH (Fig. 10a and Fig. S23). The water layer was evaporated and
1
1
,4-cyclohexanedimethanol was obtained and determined by H NMR spectroscopy (Fig. 10b). Based on the
envisioned mechanism of degradation, acetaldehyde should also be produced during the degradation process
Fig. 9b). In order to capture the low boiling acetaldehyde, D O was used as the alternative to H O to degrade
(
2
2
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8

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Journal of Materials Chemistry A
1
the PC-0% OH at 90 °C under an enclosed condition for 4 h and examined by H NMR spectroscopy directly
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(Fig. 10c). Thus, during the hot water treatment, the acetal cross-links were gradually hydrolyzed into hydroxyl
group and formyl group, the released PSS-OH floated on the water, and the CVDE segment turned into 1,4-
cyclohexanedimethanol and acetaldehyde dissolving in water (Fig. 9b). Although CVDE was not recovered
during the degradation, its degraded 1,4-cyclohexanedimethanol and acetaldehyde are important chemicals
which can be reused again. The recovered PSS-OH was recross-linked with CVDE with molar ratio of
hydroxyl group to double bond of 1:1, the recross-linked sample exhibited almost the same mechanical
properties as the original PC-0% OH (Fig. 11). Degradation experiments of PC-0% OH in water at different
temperatures (25 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C) for 1h were also carried out to determine
the water sensitivity of the acetal dynamic network. As shown in Fig. 12, after degradation for 1h, the acetal
dynamic network kept stable below 40 °C. This could be explained by the low swelling degree of the samples
in water at 40 °C (Table S1). At room temperature, the sample could exist stably even after being immersed in
water for 48 h (Fig. S24). Increasing temperature could quicken the degradation rate which was increased
sharply when reaching 90 °C. However, at 80 °C the rate of degradation was 19%, which indicates that the
acetal dynamic network had a certain resistance to water below 80 °C. After immersing in 90 °C and 100 °C
water for 1 h, the degradation degree was up to 65 % and 100 %, respectively. Although the degradation of
this material is environmental-friendly, the durability and stability of the material also need to be considered.
In order to improve the stability of the material, the hydroxyl content of the polyol compound can be decreased
or more hydrophobic groups can be introduced into the main chain to reduce the water absorption property.
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Fig. 9 Degradation recycling of the acetal dynamic network in hot water. a) Catalyst-free degradation process
of PC-0% OH by only hot water; b) Degradation mechanism and recross-linking process.
1
1
Fig. 10 a) H NMR spectrum of the original and recycled PSS-OH; b) H NMR spectrum of the recycled 1,4-
1
cyclohexanedimethanol; c) H NMR spectrum of PC-0% OH after degradation in 90 °C D O for 4 h.
2
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0

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Fig. 11 Tensile stress-strain curves of the original PC-0% OH and PC-0% OH recross-linked from the recycled
PSS-OH.
Fig. 12 Weight loss of PC-0% OH after immersing in water at different temperatures for 1 h.
4
. Conclusions
A new class of dynamic covalent networks was successfully developed based on acetal motif. We found the
dynamic exchange of acetal without acid catalyst following two mechanisms of thermally-induced
transacetalization and metathesis of acetal and successfully utilized it to construct catalyst-free dynamic
covalent networks. Thermally-accelerated alcohol-vinyl ether “click” addition was developed to synthesize the
acetal dynamic networks which exhibited vitrimer characteristics with excellent thermal malleability and
reprocessability. The one with excess hydroxyl group showed higher malleability due to the higher rate of
transacetalization than that of metathesis of acetal. In addition, our strategy allowed for facile recovery of the
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original polyol by only hot water and reusing it to build acetal dynamic networks, but the acetal dynamic
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networks were stable in cold water (below 40 °C) and insoluble in common organic solvents. Although another
starting material divinyl ether could not be recovered from the networks, its degradation product diol and
acetaldehyde could be readily recycled. Our strategy has the potential to foster the development of dynamic
materials based on acetal, green and practical dynamic covalent networks as well as green acetal chemistries.
Studies on acetal dynamic networks with different structures as well as the structure-property relationships are
currently underway in our laboratory.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (No. 51773216), Youth
Innovation Promotion Association, CAS (No. 2018335).
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Table of Contents Entry
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DOI: 10.1039/C9TA04073K
Facile Catalyst-free Synthesis, Exchanging, Hydrolysis of Acetal Motif for
Dynamic Covalent Networks
Qiong Li, Songqi Ma*, Sheng Wang, Shenghua Zhou, Wangchao Yuan, Xiwei Xu, Binbo Wang, Kaifeng
Huang, Jin Zhu*
This work highlights the exploitation of acetal motif to build dynamic covalent networks.
1