Synthesis and characterization of biobased polyesters derived from vanillin-based schiff base and cinnamic acid derivatives

Article abstract of DOI:10.1246/cl.160075

Renewable resources-based homo- and copolyesters were prepared from novel vanillin-based Schiff base and biobased cinnamic acid derivatives by transesterification. Chemical structures of the Schiff base and resulting polyesters were confirmed by FT-IR, ¹HNMR, and ¹³CNMR. Glass-transition temperatures of polymers were determined by DSC and showed over 100 °C. Photoluminescence properties were investigated for the Schiff base and polyesters. For polymers, broader emission spectra were observed compared with monomers, which probably originated from intramolecular charge transfer in each monomeric unit.

Full text of DOI:10.1246/cl.160075

Received: January 25, 2016 | Accepted: January 28, 2016 | Web Released: February 6, 2016
CL-160075
Synthesis and Characterization of Biobased Polyesters Derived
from Vanillin-based Schiff Base and Cinnamic Acid Derivatives
Hong Sun, Shinji Kanehashi,* Kousuke Tsuchiya, and Kenji Ogino*
Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
-24-16 Nakacho, Koganei, Tokyo 184-8588
2
(
E-mail: kanehasi@cc.tuat.ac.jp, kogino@cc.tuat.ac.jp)
Renewable resources-based homo- and copolyesters were
prepared from novel vanillin-based Schiff base and biobased
cinnamic acid derivatives by transesterification. Chemical
structures of the Schiff base and resulting polyesters were
confirmed by FT-IR, H NMR, and C NMR. Glass-transition
temperatures of polymers were determined by DSC and showed
over 100 °C. Photoluminescence properties were investigated
for the Schiff base and polyesters. For polymers, broader
emission spectra were observed compared with monomers,
which probably originated from intramolecular charge transfer
in each monomeric unit.
CHO
OH
COOH
COOH
COOH
a
b
c
OMe
OMe O N
OMe
-
+
2
Cl H
3
N
OMe
1
13
OH
1
OH
2
OH
3
OMe
MeO
d
e
O
Copolymer
N
OH
HO
4
Keywords: Biobased polyester | Schiff base
|
Fluorescence
OMe
Recently, replacement of oil-based raw materials with
renewable resources has received much attention due to growing
environmental demands. The use of natural renewable resources
derived from plant-based raw materials would be an effective
solution for this concern. The development of novel functional
polymer materials using renewable resources has been important
Scheme 1. Synthesis of 3-[(3,4-dimethoxybenzylidene)amino]-
-hydroxy-5-methoxybenzoic acid (DAHMBA 4) and homo- and
copolymers with p-coumaric acid and ferulic acid. (a) KOH,
b) HNO3, AcOH, (c) palladium on carbon (Pd/C, 10% loading),
ethanol, HCl, H2, (d) veratraldehyde, ethanol, (e) p-coumaric acid
or ferulic acid, acetic anhydride, disodium hydrogen phosphate.
4
(
1
and expected as a high-potential approach of green chemistry.
Renewable resources, vanillin, 4-hydroxy-3-methoxyben-
zaldehyde and cinnamic acid derivatives like p-coumaric acid
dehyde). The chemical structure of this Schiff-base monomer,
1
(
trans-4-hydroxycinnamic acid, CA) and ferulic acid (trans-4-
DAHMBA 4 was characterized by FT-IR, H NMR, and
1
3
hydroxy-3-methoxycinnamic acid, FA) are obtained from lignin.
Recently, a variety of biobased functional polymers have been
C NMR, as shown in Figures S1 and S2. No obvious meting
point of DAHMBA was observed until 230 °C before the
pyrolysis.
2­6
synthesized.
Novel polyaldimines containing Schiff base
structure improved their tensile strength, transparency, and water
vapor transmission properties. Furthermore, polymers prepared
Prepared DAHMBA was highly soluble in common organic
solvents such as tetrahydrofuran (THF), N,N-dimethylacetamide
(DMAc), methanol, and acetone, while it showed low solubility
in chloroform, toluene, and hexane.
7
form cinnamic acid derivatives involving CA, FA, and caffeic
8­11
acid showed thermotropic and photoluminescence properties.
In the present study, we synthesized novel Schiff bases
derived from vanillin and homo- and copolyesters with CA or
FA by transesterification. We also focused on the influence
of methoxy groups in the Schiff base on the polymerization
reaction and the polymer characteristics. Since all the starting
materials were derived from renewable resources, polymers with
extremely high biobased contents can be produced.
All homo- and copolymers were synthesized according to
the literature. In a typical synthesis, poly(DAHMBA-co-
8
CA) (50/50) was prepared by transesterification as follows.
DAHMBA (2.1 mmol) and CA (2.1 mmol) were added into
5 mL of acetic anhydride containing disodium hydrogen phos-
phate and the mixture was stirred at 150 °C for 3 h. The resultant
mixture was heated at 200 °C for 30 min under stirring, and was
vacuumed for 15 min. The product was dissolved in chloroform
and purified by reprecipitation with methanol. The final product
was dried overnight at 80 °C under vacuum. The copolymer
(50/50) was soluble in common organic solvents like THF,
DMAc, and chloroform, while it was insoluble in methanol and
acetone, which are good solvents for the monomers.
As described in Scheme 1, novel Schiff base, 3-[(3,4-di-
methoxybenzylidene)amino]-4-hydroxy-5-methoxybenzoic acid
(DAHMBA 4) was prepared from over 4 steps. Firstly, 4-
hydroxy-3-methoxybenzoic acid (vanillic acid) (1) was prepared
by the Cannizzaro reaction of vanillin. Nitration of vanillic acid
1
2
was conducted using HNO3, according to the literature. The
nitro group of 4-hydroxy-3-methoxy-5-nitrobenzoic acid (5-
nitrovanillic acid) (2) was converted into amino group by Pd/C
Chemical composition and molecular weight of the homo-
and copolymers are summarized in Table 1. The composition
of each monomer (DAHMBA, CA, or FA) in the copolymers
(10%) and H2 gas and afforded 3-amino-4-hydroxy-5-methoxy-
1
benzoic acid (5-aminovanillic acid) hydrochloride (3). Finally,
DAHMBA 4 was synthesized by the reaction of 3-aminovanillic
acid hydrochloride 3 and 3,4-dimethoxybenzaldehyde (veratral-
(50/50) determined by H NMR was almost the same as that in
the feed, indicating successful copolymerization. FA-containing
homo- and copolymers showed relatively lower molecular
Chem. Lett. 2016, 45, 439–441 | doi:10.1246/cl.160075
© 2016 The Chemical Society of Japan | 439

Table 1. Characteristics of synthesized polymers
Composition/mol %
¹
1
¹1
Polymer
M /g mol
n
M /g mol
w
PDI
Tg/°C
Tm/°C
in feed
in copolymera
Poly(DAHMBA)
Poly(DAHMBA-co-CA)
Poly(CA)
Poly(DAHMBA-co-FA)
Poly(FA)
100/0
50/50
0/100
50/50
0/100
100/0
42/58
0/100
47/53
0/100
b
4900
3000
2400
2700
1100
10300
5400
2700
5400
2000
2.1
1.8
1.1
2.0
1.8
108
119
120
116
112
®b
®
188
®
®
a
1
Determined by H NMR measurement in DMSO-d6. Not observed.
Table 2. Optical properties of monomers and polymers
Monomer Polymer
CA FA
Wavelength
DAHMBA
FA
DAHMBA
CA
CA (50/50)
FA (50/50)
a
­
­
abs/nm
em/nm
277
383
311
382
321
387
300
424
311
468
326
471
309
438
298
471
a
aIn chloroform solution (0.1 g L¹1).
weights compared with corresponding CA-containing polymers.
This is probably due to the steric hindrance caused by methoxy
group adjacent to the hydroxy group.
(
a)
(b)
(c)
DAHMBA
CA
FA
Monomer Polymer
Monomer Polymer
Monomer Polymer
According to Table 1, the molecular weights of
poly(DAHMBA) and the copolymers were higher than those
of homopolymers, poly(CA) and poly(FA), indicating that
DAHMBA is more reactive in transesterification despite the
presence of methoxy group adjacent to the hydroxy group. Kan
et al. prepared a similar Schiff-base-type monomer (BAHBA)
without the three methoxy groups, and investigated its polymer-
Figure 1. Photographs of chloroform solutions (0.1 g L¹1
irradiated by 365 nm UV light. Monomers and polymers of (a)
DAHMBA, (b) CA, and (c) FA.
)
8
ization. Judging from molecular weights of the homopolymers,
the reactivity of DAHMBA is considered similar to or higher
than that of BAHBA. This fact also indicates that steric
hindrance of the methoxy group is not severe compared with that
of FA.
(a)
(b)
(c)
DAHMBA
CA
Poly (CA)
FA
Poly (FA)
Thermal properties of these polymers are also summarized
in Table 1. The glass-transition temperatures (Tg’s) of the
homopolymers are 108, 120, and 112 °C for poly(DAHMBA),
poly(CA), and poly(FA), respectively. The Tg of poly(FA) is in
Poly (DAHMBA)
350
400
450
500
550
600
350
400
450
500
550
600
350
400
450
500
550
600
9
good agreement with the reported value of 113 °C. In addition,
Wavelength/nm
Wavelength/nm
Wavelength/nm
the melting of poly(CA) was observed at 188 °C. This value was
8
Figure 2. Photoluminescence spectra of monomers and poly-
¹
32 °C lower than the reported value. This deviation could be
1
mers in chloroform (0.1 g L ). (a) DAHMBA, (b) CA, and (c) FA.
caused by the different molecular weight. Since the solubility of
poly(CA) presented here was slightly different from the reported
poly(CA), poly(CA) in this study is suggested to have lower
molecular weight than the reported one. Glass-transition temper-
spectra of copolymers exhibited superimposed absorptions of
two homopolymers, indicating that no specific interaction exists
between two moieties in the ground state.
atures of both copolymers lie in the range of T ’s for the
g
corresponding homopolymers.
Figure 2 presents the photoluminescence spectra for chloro-
form solutions of monomers and homopolymers. As indicated
in Table 2, polymerization induced considerable red shift of
Figure 1 presents photographs of the chloroform solution
of monomers and their homopolymers, irradiated by 365 nm
UV light. DAHMBA, as well as CA and FA, showed blue
photoluminescence, while BAHBA without methoxy group
shows weak visible photoluminescence.8
The optical (UV­vis absorption and photoluminescence)
properties are summarized in Table 2. The maximum absorbance
wavelengths (­abs) of poly(DAHMBA) and poly(FA) were red-
shifted compared with those of respective monomers, while that
of poly(CA) was the same as that of its monomer. Absorption
maximum emission wavelength (­ ), accompanied by broad-
em
ening of emission. The solutions emit purple to pale-blue
fluorescence with large Stokes shifts. The largest Stokes shift
was observed in the poly(FA) solution. Since all monomers
involve donor­π-conjugation­acceptor systems, the intramolec-
ular charge-transfer state contributes to fluorescent properties. In
polymers, excitons can interact with chromophores or dipoles in
the vicinity, leading to large Stokes shift and peak broadening.
440 | Chem. Lett. 2016, 45, 439–441 | doi:10.1246/cl.160075
© 2016 The Chemical Society of Japan

CA and FA. All monomers and polymers showed photolumi-
nescence in chloroform solution on irradiation by 365 nm UV
light. Large Stokes shift was observed in the poly(FA) solution.
The photoluminescence property of poly(DAHMBA-co-FA)
(
a)
(b)
Poly
DAHMBA)
Poly
(DAHMBA)
50/50
Poly(CA)
50/50
Poly(FA)
(
(
50/50) was more strongly influenced by FA than DAHMBA.
All polymers showed high glass-transition temperatures (over
00 °C). Thermal gravimetric analysis of polymers also revealed
1
good thermal stability until 250 °C. Hence, these biobased
polyesters can be used as novel engineering thermoplastics.
Figure 3. Photographs of chloroform solutions (0.1 g L¹1
irradiated by 365 nm UV light. (a) Poly(DAHMBA-co-CA) and
b) poly(DAHMBA-co-FA).
)
(
Supporting Information is available on http://dx.doi.org/
1
0.1246/cl.160075.
(
a)
(b)
Poly(CA)
Poly(FA)
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Chem. Lett. 2016, 45, 439–441 | doi:10.1246/cl.160075
© 2016 The Chemical Society of Japan | 441