Hydrothermal polymerization towards fully biobased polyazomethines

Article abstract of DOI:10.1039/d0cc03026k

Microwave assisted polycondensation for the synthesis of (partially) biobased polyazomethines in water (hydrothermal polymerization) was investigated for the first time in this study. The polyazomethines prepared via this environmentally friendly and simple method show comparable characteristics as the polymers prepared via traditional methods in organic solvents.

Full text of DOI:10.1039/d0cc03026k

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Hydrothermal polymerization towards fully
biobased polyazomethines†
Cite this:DOI: 10.1039/d0cc03026k
a
a
a
b
Guotai Li, Kui Yu, Jurrie Noordijk, Monique H. M. Meeusen-Wierts,
b
b
b
Received 26th April 2020,
Accepted 7th July 2020
Bert Gebben, Petra A. M. oude Lohuis, Anton H. M. Schotman and
Katrien V. Bernaerts
a
*
DOI: 10.1039/d0cc03026k
rsc.li/chemcomm
2
2
23–26
Microwave assisted polycondensation for the synthesis of (partially) Morton and Unterlass.
Polyimides were obtained effi-
biobased polyazomethines in water (hydrothermal polymerization) ciently in two hours via polycondensation in hot water under
was investigated for the first time in this study. The polyazo- pressure using autoclaves or microwave reactors. Moreover,
methines prepared via this environmentally friendly and simple imine-based covalent organic frameworks (COFs) were also
2
7,28
method show comparable characteristics as the polymers prepared realized by this hydrothermal method recently,
especially
via traditional methods in organic solvents.
the newly published work by Zamora and coworkers providing a
simple and green synthetic protocol.
These results were inspiring to make PAz via hydrothermal
2
9
Polyazomethines (PAz) or poly(Schiff base)s have attracted
considerable attention of researchers during the last two dec- polymerization due to the similarity of the synthesis methods
ades, due to their interesting characteristic properties such as: to get PAz, polyimides and imine-linked COFs. They are con-
high thermal stability, liquid crystalline properties, excellent ventionally prepared via condensation reaction in organic
1
–3
mechanical properties, metal-chelating ability, etc. Especially solvents with removal of water. Although there are already
the conjugated aromatic PAz, which usually show special opto- some studies describing the polycondensation of PAz under
electric conductivity, have valuable potential applications in high pressure, i.e., from microwave reactors, they were still
4
–8
electrical fields.
The strategy to prepare polyazomethines is straightforward, organic solvents.
typically through polycondensation from dialdehyde and dia- reports on PAz, only very few of them took biobased monomers
performed via the conventional method with non-renewable
3
0–34
Furthermore, among these tremendous
34–36
mine monomers with the release of water. By derivatization of into consideration.
The increasing environmental concerns,
the monomers, various types of PAz can be expediently along with depleting fossil oil reserves, put higher requirements
9
obtained. Since the first report in 1923, polyazomethines both on resource renewability and synthetic sustainability of
1
0
11,12
13
14
37
containing moieties like phenyl, furan,
ester, ether,
polymer materials. As such, to develop a green and environmen-
1
5
16
17
18
19
silane, thiophene, ferrocene, pyridine, triazole etc. tally friendly method to prepare (partially) biobased polyzao-
were developed successfully. However, most of these azo- methines is a growing demand both in industrial factories and
methine polymerizations were proceeded in solvents like for scientific studies. Herein, we report our preliminary results of
dimethylsulfoxide (DMSO), N-methyl-pyrrolidone (NMP), hexa- the hydrothermal polycondensation towards PAz.
methylphosphoramide (HMPA) or toluene, characterized by
high boiling temperatures, non-renewability or even toxicity.
The study was started by comparing different polycondensa-
tion methods for the preparation of aromatic PAz with stoichio-
2
0
As far as we know there is only one report where water was used metric amounts of the common monomers p-phenylenediamine
as the solvent for PAz preparation, however it is only limited and terephthaldehyde (Table 1). Four known conditions were
2
1
35,38–40
to hydrophilic monomers. Recently, hydrothermal polymer- selected from literature
to be compared with catalyst free
ization for the synthesis of polyimides was established by microwave assisted polycondensation in water. Among the
selected conditions, the solvent N-methylpyrrolidone (NMP)/CaCl
was also used in analogy with the procedure to make another
class of liquid crystalline polymers, namely polyaramids. CaCl2
is known to improve solubility and acts as a dehydrating agent to
capture released water.
2
a
Biobased Materials, Faculty of Science and Engineering, Maastricht University,
P.O. Box 616, 6200 MD Maastricht, The Netherlands.
40
E-mail: katrien.bernaerts@maastrichtuniversity.nl
b
Teijin Aramid Research Institute QRI, Velperweg 76, P.O. Box 5153, 6802 ED
Arnhem, The Netherlands
All the PAz obtained from the different conditions are yellow
or orange solids in accordance with the reported literature. The
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc03026k
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Table 1 Polymerization conditions exploration for PAz synthesis from p-phenylenediamine and terephthaldehyde
a
À1
a
Entries
Conditions
Yield (%)
M
n
(g mol
)
Ð
Ref.
1
2
3
4
5
NMP/11 wt% CaCl
NMP/HMPA = 4/1, 120 1C, 24 h
DMSO, 180 1C, 24 h
2
, 120 1C, 24 h
91
72
90
89
98
900
900
1000
800
1.5
1.4
1.6
1.4
1.5
40
38
35
39
Toluene, ZnCl
2
(6 mol%), TsOH (2 mmol%), reflux, 12 h
H
2
O, 200 1C, microwave, 1 h
900
This work
a
À1
Measured by GPC in concentrated sulfuric acid (1 mg mL ) within 72 h.
n
number average molecular weight (M ) of the obtained poly- 200 1C (1 h) in the microwave (Table 1 entry 5; IR spectrum in
mers was measured by GPC in concentrated sulfuric acid. Fig. S7, GPC overlay in Fig. S9, ESI†). The molecular weight
Model experiments (p. S8 and S9, ESI†) and NMR studies over results under different conditions were also confirmed by the
time of the polymer (Fig. S8(c), ESI†) indicate that the polymers measurement of the reduced viscosity (Zred) in 96% sulfuric
4
4,45
are stable in H
2
SO
4
during the GPC handling time. The results acid (Table S2, ESI†).
Moreover, PAz prepared via this green
5
,41
in Table 1 are in line with what is expected from literature.
polymerization method have the highest yield due to the simple
Due to the limited solubility of the polymers in the reaction purification process: polymer is obtained by simply washing
solvents, they precipitate out during reaction what hampers the precipitated polymer powder with water and drying after
further polymerization and limits molecular weights. FT-IR the reaction, without the need to remove high boiling solvents.
(
Fig. S7, ESI†) confirms the presence of a residual carbonyl Hydrothermal polymerization also shows the highest efficiency
À1
peak at 1689 cm , typical for a terephthaldehyde end group, with the shortest reaction time among all these conditions. All
referring to traces of aldehyde monomer or low molecular in all, the preliminary experiments indicate the valuable
weight polymer with clear visibility of the end groups, similar potential of this green polymerization method towards poly-
as observed in literature for PAz.
shows that all the aldehyde peaks from the monomer disap-
4
2,43
NMR analysis further azomethines for further investigation.
In a next step, hydrothermal polymerization was used to
peared and shift to a higher ppm in the polymer, indicating make partially biobased polymers based on a dialdehyde
that the terephthaldehyde monomer was completely consumed monomer derived from 5-hydroxymethylfurfural (HMF), a ver-
4
6,47
and that low molecular weight polymer with aldehyde end satile biobased building block from carbohydrate sugars
4
8
groups was generated. The new peak around 9.82 ppm was which is used widely to construct polymer materials. Furan-
assigned as imine and further confirms the polymerized struc- 2,5-dicarbaldehyde (biobased monomer 1, Scheme S1, ESI†)
ture (Fig. S8, ESI†). The solvent based procedures resulted in derived polyazomethines were reported in literature with good
1
1,12
similar molecular weights as the polymerization in hot water at properties.
Moreover, this kind of PAz have a better solu-
bility, which makes it possible to measure their molecular
weight with GPC in hexafluoroisopropanol (HFIP with 0.19%
Table 2 Synthesis of partially biobased polyazomethines based on furan- NaTFA). The influence of the reaction time and temperature of
2,5-dicarbaldehyde in hot water
the hydrothermal polymerization were explored (Table 2). In all
cases, PAz were obtained successfully as orange solids. The
appearance of new characteristic peaks in the Fourier transform
À1
infrared (FT-IR) spectra at 1614 cm indicates the successful
Schiff-base formation between dialdehydes and diamines
a
n
3
À1
a
Entries Time (min) T (1C)
M
(Â 10 g mol
)
Ð
Yield (%)
(Fig. S10(d), ESI†). NMR spectra also confirm the success of
1
2
3
4
5
6
7
8
9
1
1
1
1
2
5
10
30
60
90
120
10
10
200
200
200
200
200
200
200
200
100
130
160
120
5.56
5.49
5.58
6.12
5.97
6.46
5.85
5.84
5.72
5.71
5.64
5.39
1.16 95
1.15 97
1.16 96
1.18 95
1.17 97
1.26 97
1.17 91
1.15 94
1.19 96
1.18 96
1.17 94
1.70 75
the polymerization from the disappearance of the peaks of the
aldehyde group (9.95 ppm) in biobased monomer 1 and the
phenyl protons in p-phenylenediamine (7.97 ppm), while new
aldehyde end groups (10.10 ppm) and distinguished imine
bonds (around 9.60 ppm) were generated in the polymers
(Fig. S10(e) and (f), ESI†). Moreover these polymers have a very
high thermal stability (T (5%) = 362 1C; T (max) = 390 1C)
d
d
0
1
2
indicated from TGA, but no Tm or Tg was observed in DSC
Fig. S10(b) and (c), ESI†). At 200 1C, the polymerization was first
10
1440
b
(
carried out from 1 min to 120 min (entries 1–8 in Table 2). To
our surprise, even after only 1 min reaction time, PAz could be
a
Determined by GPC with RI detection in HFIP/0.19% NaTFA, cali-
b
brated with PMMA standards. Polymerization was performed in NMP/
1 wt% CaCl
3
À1
1
2
.
obtained with an M
n
of 5.56 Â 10 g mol . With increasing
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presence of typical amine and aldehyde carbonyl vibrations in
FT-IR (Fig. S12(d), ESI†) due to residual monomer or end
groups in low molecular weight polymer. The low yield was
caused by the stickiness of the obtained polymer, which
complicates work up by precipitation. These two polymers have
a high thermal stability with a T
do not show a melting point in DSC before degradation. At the
same time, Polymer II shows a glass transition temperature (T
at about 67 1C but no T was observed in Polymer I (Fig. S11(f)
d
(5%) of more than 300 1C but
g
)
g
and S12(c), ESI†). These green polymers and their corres-
ponding properties were assumed to provide beneficial infor-
mation to design more green conductive polymers, imine-based
COFs, liquid crystalline materials etc.
All in all, the successful synthesis of fully biobased polymers
further demonstrates that hydrothermal polymerization is an
environmentally benign and simple processing method that is
Scheme 1 Fully biobased polyazomethine synthesis via hydrothermal
polymerization.
n
reaction time, the M of PAz slightly increases in some cases. straightforward for the preparation of polyazomethines. Mean-
Additionally, the obtained polymers became darker upon while several factors such as the formation of a stable con-
increase of the reaction time. Considering efficiency and energy jugated aromatic product, polymer phase separation due to bad
5
1
saving, 10 min reaction time was selected to explore the influence solubility and higher pressure for solid-state polymerization
of temperature. When decreasing the temperature from 200 1C are hypothesized to explain the elongation of the polymer
entry 4 in Table 2) to 160, 130 and 100 1C (entries 9–11 in chains rather than the hydrolysis of the imine bond in the
Table 2), the M decreased slightly. Meanwhile, the yields are aqueous medium during hydrothermal polymerization. Further
very high (from 91%–97%) in all cases due to the earlier investigations are required to unravel the exact mechanism.
mentioned simplicity in purification. To further confirm the In summary, hydrothermal polymerization was successfully
(
n
advantages of hydrothermal polymerization in PAz synthesis, developed for the synthesis of polyazomethines. Within only
furan-2,5-dicarbaldehyde derived PAz was also prepared using 10 min microwave assisted polymerization in water, polyazo-
NMP/CaCl2 at 120 1C (Table 2, entry 12). This results in methines were obtained with excellent yields and comparable
3
À1
comparable M
and lower yield, what further proves the superiority of hydro- solvents. Furthermore, this green method was also applied for
thermal polymerization to prepare PAz. the preparation of biobased polyazomethines, derived from
After successful synthesis of partially biobased PAz using this vanillin, 5-hydroxymethylfurfural and cardanol. These biobased
green polymerization method, fully biobased PAz were targeted. polymers have a high thermal stability and M values ranging
n
(5.39 Â 10 g mol ) but longer reaction times properties to the polymers prepared conventionally in organic
n
À1
À1
Polymers based on biobased building blocks generally have a from 4000 g mol to 7000 g mol , and could provide useful
lower CO footprint compared to materials from fossil fuel-based information to design more sustainable imine-based materials.
2
sources. Combined with the hydrothermal polymerization only in This green polycondensation method avoids the use of organic
water, we aim to make the whole process more sustainable, not solvents and catalysts, ensures high efficiency and affords
only from the preparation point of view but also when considering excellent yield after a simple work up procedure, and as such
the raw material sources. We first synthesized the biobased holds valuable potential for polyazomethine synthesis in both
monomers: dialdehyde coming from the lignin derived building industrial production and scientific studies.
49
block vanillin (biobased monomer 2, Scheme S2, ESI†) and
Guotai Li (CSC No. 201709110151) and Kui Yu (CSC No.
diamine derived from the cashew nut shell liquid building block 201706630001) acknowledge the support provided by China
50
cardanol (biobased monomer 3, Scheme S2, ESI†). The side Scholarship Council (CSC) to study at Maastricht University.
groups in vanillin (OH, methoxy) and cardanol (long aliphatic We acknowledge the support of the revision from Dr Rienk
chain) decrease the stiffness of PAz and thus enhance the Eelkema at TuDelft.
solubility. Both of these monomers were obtained with moderate
to high yields (see ESI† for detailed procedure).
Conflicts of interest
With these biobased monomers in hand, hydrothermal
polymerization at 200 1C was applied to prepare fully biobased
There are no conflicts to declare.
1
PAz (Scheme 1). FT-IR and H NMR spectra of the monomers
and the obtained polymers were performed to confirm the
Notes and references
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