Synthesis and characterization of poly[(ferulic acid)-alt-(glycine)]

Article abstract of DOI:10.1016/j.polymer.2017.02.007

We investigated the copolymerization of the biomass-derived monomers ferulic acid (FA) and glycine (Gly) to develop a highly thermostable aromatic-aliphatic poly(ester-amide). N-feruloylglycine methyl ester (FAMG), N-feruloylglycine (FAG), and N-(4-O-acet

Full text of DOI:10.1016/j.polymer.2017.02.007

Accepted Manuscript
Synthesis and characterization of poly[(ferulic acid)-alt-(glycine)]
Tatsuya Goto, Daisuke Ishii, Yukiko Enomoto-Rogers, Akio Takemura, Tadahisa
Iwata
PII:
S0032-3861(17)30120-9
10.1016/j.polymer.2017.02.007
JPOL 19408
DOI:
Reference:
To appear in: Polymer
Received Date: 27 September 2016
Revised Date: 31 January 2017
Accepted Date: 3 February 2017
Please cite this article as: Goto T, Ishii D, Enomoto-Rogers Y, Takemura A, Iwata T, Synthesis and
characterization of poly[(ferulic acid)-alt-(glycine)], Polymer (2017), doi: 10.1016/j.polymer.2017.02.007.
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Research paper
Synthesis and characterization of poly[(ferulic acid)-alt-(glycine)]
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Tatsuya Goto , Daisuke Ishii , Yukiko Enomoto-Rogers , Akio Takemura , and Tadahisa
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Iwata *
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Science of Polymeric Materials,
Department of Biomaterial Sciences,
Graduate School of Agricultural and Life Sciences,
The University of Tokyo
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1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
*Corresponding author
(e-mail: atiwata@mail.ecc.u-tokyo.ac.jp
Tel: +81-3-5841-5266
Fax: +81-3-5841-1304)
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Abstract
We investigated the copolymerization of the biomass-derived monomers ferulic acid (FA)
and glycine (Gly) to develop a highly thermostable aromatic-aliphatic poly(ester-amide).
N-feruloylglycine methyl ester (FAMG), N-feruloylglycine (FAG), and
N-(4-O-acetylferuloyl)glycine (AcFAG) were synthesized by amide coupling reactions
between FA and the methyl ester of Gly, followed by protection/deprotection of the end
groups. We attempted to polymerize the three types of monomers in bulk or solution state to a
poly[(ferulic acid)-alt-(glycine)] (P(FA-alt-Gly)) alternate copoly(ester-amide) of FA and Gly.
Only AcFAG yielded P(FA-alt-Gly) containing a high-molecular weight fraction of more than
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10 g/mol. Poly(AcFAG) (PAcFAG) is an amorphous polymer with a glass transition
temperature (T ) of 189 °C, a 5 % weight-loss temperature (Td5%) of 297 °C, and a 50 %
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weight-loss temperature (Td50%) at 495 °C.
Keywords: ferulic acid, glycine, alternating copolymer
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1. Introduction
Plastics that include rigid aromatic components such as poly(p-phenylene terephthalamide)
TM
TM
(Kevlar ), poly(p-phenylene-2,6-benzobisoxazole) (Zylon ), or
TM
poly(p-hydroxybenzoate-co-2,6-naphthoate) (Vectran ) are known as high-performance
materials, and have high thermal durability and tenacity. Owing to their thermotropic or
lyotropic liquid crystalline nature, these aromatic thermoplastics have been processed into
highly orientated fibers [1, 2]. These conventional aromatic thermoplastics have been
commercialized on the basis of petroleum chemistry.
Recently, increasing concerns about the environmental impacts of conventional
petroleum-based plastics have prompted the expanded development and utilization of
bio-based plastics [3]. The currently commercialized bio-based plastics include aliphatic
polyesters such as poly(lactic acid) or poly(hydroxyalkanoate)s, and polysaccharide
derivatives such as cellulose acetate [3-5]. However, the development of bio-based aromatic
plastics has also been expanding. In plants, p-hydroxycinnamic acids such as p-coumaric acid
(4-hydroxycinnamic acid), ferulic acid (FA; 4-hydroxy-3-methoxycinnamic acid), and caffeic
acid (3,4-dihydroxycinnamic acid) are metabolic intermediates in the biosynthesis of lignin or
various polyphenols. Agricultural wastes such as rice bran and extracted coffee beans are
promising sources of these p-hydroxycinnamic acids [5].
The aromatic hydroxyl acids mentioned above have been polymerized by direct
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polycondensation using thionyl chloride as both solvent and catalyst, or by ester exchange
after the acetylation of the phenolic hydroxyl group [6, 7]. The melting points of
poly(coumaric acid) and poly(caffeic acid) have been determined, but that of polyFA has not ;
moreover, polyFA is not soluble in popular organic solvents, and is not processable by heat or
dissolution. Pion et al. saturated the C=C bond of FA by the addition hydrogen, and the
saturated FA was converted into bio-based plastic polymers with excellent thermal and
mechanical properties like those of poly(ethylene telephthalate) (PET) [8]. Copolymerization
is an efficient method of altering and controlling the properties of polymers. Wang et al.
synthesized hyperbranched poly[(caffeic acid)-co-(ferulic acid)], which is a highly
heat-resistant, liquid crystalline polymer with improved processability [9]. Furthermore,
studies have been carried out on copolymers of FA, 4-hydroxy benzoic acid, and
1,6-hexanediol [10] or lactic acid [11]. FA-based precursors formed by the dimerization of FA
with diols, dicarboxylic acids, or diamines have been used as monomers for
poly(ester-urethane)s [12], poly(anhydride-ester)s [13, 14], epoxy resins [15, 16],
bisguaiacol-type oligomers [17], and polyene polymers [18]. The obtained polymers had
better solubility in organic solvents but exhibited no crystallinity owing to their random
monomer sequences and/or large repeating units.
Strict control of the backbone sequence and appropriate monomer design are necessary for
the further control of polymer properties including thermal and mechanical properties, and
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crystallinity. For example, Tabata and Abe prepared alternating copolymers of
3-hydroxybutyric acid (3HB) and 2-hydroxypropionic acid (2HP) [19]. In their study,
enantiomers of 3HB and 2HP were arbitrarily chosen and coupled after precisely protecting
the end-functional groups, i.e., the hydroxy and carboxyl groups, to ensure a completely
alternating monomer sequence. The copolymerization was carried out after deprotecting the
end-functional groups of the coupled 3HB-2HP precursor. As a result, the thermal properties
of the copolymers have been controlled over a wide range. However, it has been quite difficult
to obtain FA-based copolymers with well-controlled backbone structure and monomer
sequences such as alternating copolymers. Recently, Nguyen et al. reported the preparation of
poly(ethylene ferulate) as an alternate poly(ester-ether) of FA [20]. To the best of our
knowledge, no other alternate copolymers have been prepared from FA or other
hydroxycinnamic acid derivatives.
In the present study, we attempted to obtain a sequence-controlled copolymer of FA by using
glycine (Gly), an aliphatic amino acid with the simplest molecular structure, as the
counterpart. To form an alternating sequence, FA and the methyl ester of Gly were first
coupled through an amide bond. Thereafter, three alternate monomers with different
combinations of end-protecting states were prepared. Furthermore, we attempted
polymerization of the alternate monomers in bulk or solution states. Finally, we investigated
the molecular weight, backbone structure, and physical properties (solubility, thermal
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properties, and crystallinity) of the obtained poly(ferulic acid-alt-glycine) (P(FA-alt-Gly)).
2. Experimental
2.1. Materials
Ferulic acid (FA; trans-4-hydroxy-3-methoxycinnamic acid, 97%) and glycine methyl ester
hydrochloride (MG) were purchased from Wako Pure Chemicals, Tokyo, Japan. All other
reagents were commercially obtained and used without further purification.
2.2. Nuclear magnetic resonance (NMR)
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NMR spectra were recorded on a JEOL JNM-ECA500 FT-NMR spectrometer, using
tetramethylsilane (TMS) as an internal standard. Methanol-d (99.8 % atom D, Cambridge
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Isotope Laboratories, Andover, MA, USA) was used as a solvent for FAMG, FAG, and
AcFAG. Chloroform-d (99.8 % atom D, Cambridge Isotope Laboratories), trifluoroacetic
acid-d (Sigma-Aldrich, Co., St. Louis, MO, USA), and dimethyl sulfoxide-d (99.9 % atom D,
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Isotec Co., Miamisburg, OH, USA) were used as solvents for the FAMG, FAG, and AcFAG
polymerization products, respectively. Furthermore, some of the polymerization products that
were not dissolved in the solvents mentioned above were dissolved in phenol-d (98 % atom
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D, Sigma-Aldrich)/chloroform-d = 4/6 (v/v). The number of accumulated scans was 8 or 16
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for the H spectra and 600 to 1024 for the C spectra of FAG and AcFAG. However, 16384
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scans were accumulated to obtain a C spectrum of the AcFAG polymerization product. The
relaxation delay after detection of FID was set to 2 s.
2.3. Fourier-Transformed Infrared Spectroscopy (FT-IR)
FT-IR spectra of the samples were obtained using the KBr method on a Nicolet Magna 6700
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spectrometer ranging from 600 to 4000 cm with a resolution of 4 cm over 64 scans. The
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absorbance of the spectra was normalized by the absorption at 1510 cm , which is assigned to
the stretching of the C=C bonds in the aromatic ring.
2.4. Gel Permeation Chromatography (GPC)
GPC measurements were performed on a Shimadzu LC-10 system in which Shodex K-806M
and K-802 columns (7.8 mm i.d.×300 mm for each columns) were linearly attached.
Chloroform / phenol = 9 / 1 (v / v) was eluted at a flow rate of 0.8 mL/min. The injection port,
column oven and RI detector were kept at 40 °C. The molecular weights of the polymer
samples, calibrated by using monodisperse polystyrene (PS) standards (peak-top molecular
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weight (M ) ranging from 1.32×10 to 3.15×10 ; SM-105, Shodex Co. Ltd., Tokyo, Japan),
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AcFAG and AcFAG oligomers (AcFAG: M = 293 at 25.0 min., AcFAG dimer : M = 526 at
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24.5 min., AcFAG trimer : M = 760 at 23.6 min., AcFAG tetramer : M = 993 at 23.1 min.,
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AcFAG pentamer : M = 1226 at 22.4 min.), or FAG and FAG oligomers (FAG: M = 251 at
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25.0 min, FAG dimer : M = 484 at 24.6, FAG trimer : M = 718 at 23.9 min.)
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p
2.5. Differential Scanning Calorimetry (DSC)
DSC measurements were performed in a DSC8500 calorimeter (Perkin Elmer Inc., Waltham,
MA, USA) under nitrogen flow. Indium was used as a calibration standard for temperature
and heat capacity. As sample weighing approximately 3 mg was sealed in an aluminum pan.
The empty pan was used as the reference. The heating scan to estimate melting points and
enthalpy of fusion was carried out 0 to 300 °C at a heating rate of 10 °C/min.
2.6. Thermogravimetry-Differential Thermal Analysis (TG-DTA)
The TG-DTA data for the precursors (FAMG, FAG, AcFAG) and products of polymerization
(PFAG and PAcFAG) were obtained using a STA 6000 Simultaneous Thermal Analyzer
(Perkin Elmer, Inc.) under a nitrogen flow. The weight-loss curves and DTA thermograms
were acquired at temperatures ranging from 30 to 900 °C at a heating rate of 10 °C/min.
2.7. Wide-angle X-ray diffraction (WAXD)
WAXD measurements were carried out on a Rotaflex RU-200 X-ray diffractometer
(Rigaku Corporation, Tokyo, Japan) operating at 50 kV and 100 mA. The samples were
exposed to nickel-filtered CuKα irradiation (λ = 0.15418 nm) for 20 min in a vacuum camera
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with a camera length of 44 mm. The scattered X-ray intensity from the sample was recorded
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on an imaging plate (127×127 mm ; 50 µm resolution; BAS) and the read-out was recorded
on an imaging plate reader (R-AXIA, Rigaku Corporation). The calibration of the camera
length was carried out (020)/(200) diffraction of sodium fluoride (d = 0.23166 nm). The
two-dimensional image of the scattering profile was circularly averaged and transformed to a
2θ profile.
2.8. N-feruloylglycine methyl ester (FAMG)
Glycine methyl ester hydrochloride (2.29 g, 18.2 mmol), 1-hydroxybenzotriazole (HOBt)
(3.66 g, 27.1 mmol), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride
(EDC-HCl) (4.24 g, 22.1 mmol) and triethylamine (11.4 mL, 81.0 mmol) were added to a
solution of FA (3.51 g, 18.0 mmol) in 200 mL of dichloromethane at 0 °C. The reaction
mixture was stirred overnight at room temperature. The crude product was obtained by
concentration of the reaction mixture after washing with 2M hydrochloric acid, dehydration
over sodium sulfate, and filtration. Finally, FAMG was obtained by recrystallization of the
crude product in dichloromethane, filtration, and vacuum drying (2.37 g, 48.7 % yield). The
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melting point = 125 °C; H-NMR (500 MHz, methanol-d ) δ (ppm) = 3.74 (s, 3H), 3.89 (s,
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3H), 4.05 (s, 2H), 6.34 (d, J = 16.0 Hz, 1H), 6.80 (d, J = 8.0 Hz, 1H), 7.04 (d, J = 8.5 Hz, 1H),
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7.14 (s, 1H), and 7.47 (d, J = 15.5 Hz, 1H); C-NMR (500 MHz, methanol-d ) δ (ppm) =
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42.1, 52.6, 56.4, 111.6, 116.5, 117.9, 123.4, 128.1, 142.9, 149.3, 150.1, 169.5,and 171.9.
2.9. N-feruloylglycine (FAG)
To a solution of FAMG (2.65 g, 10.0 mmol) in 70 mL of methanol, 20 mL of 1 M aqueous
sodium hydroxide was added. The mixture was stirred at room temperature for 3 h and
neutralized with 10 mL of 1 M sulfuric acid. The mixture was then diluted with anhydrous
methanol, dehydrated over sodium sulfate, and evaporated to obtain FAG (2.40 g, 95.4 %
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yield) as a solid. The melting point = 206 °C; H-NMR (500 MHz, methanol-d ) δ (ppm)=
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3.89 (s, 3H), 4.03 (s, 2H), 6.51 (d, J = 15.5 Hz, 1H), 6.80 (d, J = 8.0 Hz, 1H), 7.04 (d, J = 10.5
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Hz, 1H), 7.14 (s, 1H), and 7.48 (d, J = 15.5 Hz, 1H); C-NMR (500 MHz, methanol-d ) δ
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(ppm) = 42.0, 56.4, 111.6, 116.5, 118.2, 123.4, 128.3, 142.7, 149.2, 150.0, 169.4,and 173.1.
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2.10. N-(4-O-acetylferuloyl)glycine (AcFAG)
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To a solution of FAG (2.26 g, 9.0 mmol) in 9 mL of 2.5 M aqueous sodium hydroxide, 1.1
mL (11.6 mmol) of acetic anhydride was added at 0 °C. The solution was stirred for 10 min at
20 °C and for a further 20 min at 30 °C. The mixture was neutralized with 1 M sulfuric acid
and extracted with chloroform. The reaction product in the chloroform layer was dehydrated
over sodium sulfate, concentrated using an evaporator, and recrystallized in ethyl
acetate/hexane = 1/2 (v/v). Finally, AcFAG (1.04 g, 39.4 % yield) was obtained as a solid. The
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melting point = 150 °C; H-NMR (500 MHz, methanol-d ) δ (ppm) = 2.25 (s, 3H), 3.85 (s,
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3H), 4.04 (s, 2H), 6.66 (d, J = 15.5 Hz, 1H), 7.04 (d, J = 8.0 Hz, 1H), 7.16 (d, J = 8.0 Hz, 1H),
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7.26 (s, 1H), and 7.53 (d, J = 18.0 Hz, 1H); C-NMR (500 MHz, methanol-d ) δ (ppm) =
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20.4, 42.0, 56.4, 112.5, 121.7, 124.2, 135.3, 141.6, 142.5, 153.0, 168.8, 170.5, and 172.9.
2.11. Bulk polymerization of FAMG and AcFAG
FAMG (0.2 g) or AcFAG (0.5 g) was loaded into a stoppered glass tube (inner diameter of
10 mm) and heated at 200 °C under vacuum (< 3 kPa) for 1 to 16 h to allow polymerization.
The dark brownish products were removed from the glass tube using a micro spatula.
2.12. Solution polymerization of FAG
A representative procedure for the solution polymerization of FAG is as follows, FAG (50
mg, 0.2 mmol) was dissolved in thionyl chloride (1 mL, 12 mmol) at 0 °C in a 5 mL recovery
flask. The solution was stirred and refluxed at 65 °C for 1 to 16 h to allow polymerization.
After polymerization, the solution was concentrated to remove excess thionyl chloride and
dried in vacuo to yield a solid product.
2.13. FAMG bulk polymerization for 1 h
Available in the Supplement data
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2.14. Poly(AcFAG) (PAcFAG) from AcFAG bulk polymerization for 16 h
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The data for PAcFAG are as follows, yield: 83.4 %; conversion: 99 %, M = 1.8 × 10 , M =
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3.4 × 10 , DP = 7.7 (GPC: chloroform/phenol = 9/1 (v/v), calculated for AcFAG oligomers);
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glass transition temperature (T ) = 189 °C (DSC); 5 % weight-loss temperature (Td5%) =
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297 °C, 50 % weight-loss temperature (Td50%) = 495 °C (TGA); H-NMR (500 MHz,
pentafluorophenol/CDCl ) δ (ppm) = 2.3-2.5 (br, 3H), 3.6-4.1 (m, 5H), and 6.6-7.2 (br, 5H);
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C-NMR (500 MHz, phenol-d /CDCl ) δ (ppm) = 20.6, 44.5, 55.8, 110.5, 146.7, 152.6, 168.4,
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171.3, and 175.3 (other C signals overlapped with the solvent signals, and/or too small to
detect)
2.15. Poly(FAG) (PFAG) from FAG solution polymerization for 16 h
The data for PFAG are as follows, yield: 141 % (excess yield was caused by residual
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reagents (-S=O- detected by FT-IR at 1068 cm )); conversion: 98 %, M = 1.0 × 10 , M =
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1.5 × 10 , DP = 4.1 (GPC: Chloroform/phenol = 9/1 (v/v), calculated for FAG oligomers);
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H-NMR (500 MHz, pentafluorophenol/CDCl ) δ (ppm) = 2.3-2.5 (br, 3H), 6.5-8.0 (m, 5H),
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and 6.6-7.2 (br, 5H); C-NMR (500 MHz, phenol-d /CDCl ) δ (ppm) = 44.5, 55.8, 146.7,
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152.6, 167.8, and 170.5 (other C signals overlapped with the solvent signals, and/or too
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3. Results and discussion
3.1. Synthesis of monomers
Dimethylterephtalate and bisphenol-A have been used as monomers for bulk polymerization
into aromatic polyesters through transesterification [21]. FA can be polymerized in solution
via acid chloride as intermediate [6], and acetyl ferulic acid can be polymerized in bulk to
form polyesters through deacetylation and transesterification [7].
Therefore, in the present study, three types of alternate monomers (FAMG, FAG, and
AcFAG) were designed and synthesized as shown in Scheme 1. FAMG was synthesized by
coupling FA and methyl glycine using EDC-HCl as a coupling agent, and the carboxyl group
was protected by the methyl group forming ester. FAG was obtained by removing the methyl
group of FAMG using NaOH, and there were no elimination groups. AcFAG was synthesized
by acetylation of FAG using acetic anhydride, and the phenolic group was protected by an
acetoxy group.
3.2. Polymerization of FAMG, FAG, and AcFAG
The polymerization of the three alternate monomers (FAMG, FAG, AcFAG) was attempted
under various conditions to obtain completely alternate copolymer poly[(ferulic
acid)-alt-(glycine)], as shown in Scheme 1.
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Scheme 1. Synthesis of (a) FAMG, FAG, and AcFAG, and (b) poly[(FA)-alt-(glycine)].
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FAMG was subjected to bulk polymerization under vacuum at 200 °C. However, the
H-NMR spectrum of the product was almost identical to that of FAMG, and the signal from
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the methyl ester group protecting the carboxyl terminus of the glycine was still observed. In
addition, unexpected signals were observed at 1.6, 5.8, and 6.1 ppm. Similar peaks were also
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observed in the H-NMR spectra of polystyrene analogs polymerized from naturally occurring
β-methylstyrenes such as anethole and isoeugenol [22]. This suggests that some part of
FAMG was radically polymerized by the cleavage of the vinylene group adjacent to the
aromatic ring. However, considering our aim was to obtain linear poly(ester-amide)s solely by
the polycondensation reaction at both ends of the monomer, it was important that we avoided
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side reactions such as radical polymerization. Therefore, we concluded that FAMG is not a
suitable monomer for the production of poly[(ferulic acid)-alt-(glycine)].
FAG comprise a free carboxyl group at one end and a free hydroxyl group at the other, and
is therefore capable of polycondensation by chlorination of the carboxyl group. According to
the direct polymerization of FA by Elias et al. [6], the solution polymerization of FAG was
carried out by chlorination using thionyl chloride. The solution turned brown after 1 h
yielding the solid polymerization product. In the IR spectra of the polymerization products
obtained at different reaction times (Figure 1a), the intensity of the C=O stretching band at
-1
1720 cm , which can be attributed to the carboxyl group of FAG, decreased with increasing
-
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reaction time. Moreover, a new band appeared 1 h after the start of the reaction at 1740 cm ,
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and increased in intensity. This new band at 1740 cm can be attributed to the C=O of the
aromatic esters, namely, the esterified carboxyl group of the glycine unit. The C=O stretching
-1
band at 1720 cm due to monomers disappearing after 16 h is indicative of the complete
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conversion of FAG to PFAG. Figure 2a shows the H-NMR spectrum of the polymerization
product of FAG in TFA-d. The broad peak at 7 ppm can be attributed to the aromatic and
vinyl protons (5H) of the FA unit. Another broad peak at 4 ppm is derived from the
methylene protons (2H) of the glycine unit and the methoxy protons (3H) of the FA unit. The
broadened peak is indicative of the successful polymerization of FAG. Furthermore, the ratio
of the peak area of aromatic and vinyl protons to the sum of methylene and methoxy protons
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was 1:1. This result indicates that the obtained product consisted of FA and glycine in the
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same ratio, and that the aimed-for alternate copolymer PFAG had been obtained.
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Figure 1. FT-IR spectra of (a) PFAG and (b) PAcFAG polymerized for 1, 2, 4, 8, and 16 h
from the corresponding monomers.
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Figure 2. H-NMR spectra of (a) PFAG polymerized for 16 h, and (b) PAcFAG polymerized
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for 16 h.
Finally, we investigated the bulk polymerization of AcFAG. The reaction mixture turned
brown after 30 min, indicating the formation of a solid product. As shown in the IR spectra in
Figure 1b, the intensity of the C=O stretching bands attributable to the acetyl group (1760
-1
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cm ) and the carboxyl groups (1740 cm ), which are the terminal groups of AcFAG,
decreased with increasing reaction time. This supports the complete polymerization of AcFAG.
However, a new band, which can be attributed to esterified carbonyl carbons, appeared at
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1710 cm , and its intensity gradually increased with increasing reaction time. The H-NMR
spectrum of the obtained product is shown in Figure 2b. As discussed in the previous section,
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the ratio of the peak areas of the peak at 7 ppm (assigned to aromatic and vinyl protons) and
the peak at 4 ppm (assigned to methylene and methoxy protons) was 1:1, indicating the
successful polymerization of AcFAG and the production of alternate copolymer PAcFAG.
Moreover, in the case of PAcFAG, a peak attributable to the methyl protons of the acetyl
group in AcFAG was observed at 2.2 ppm. The acetyl group of AcFAG remained at the
terminal of PAcFAG after the reaction. Therefore, the number-averaged degree of
polymerization of PAcFAG was 5.3, calculated from the ratio of protons of the acetyl group
(3H) to the protons of the methoxy (3H) and the methylene (2H) groups, (DP = Iacetyl
/
1
3
Imethoxy+methylene × 5/3 (Figure 2)). In the C-NMR spectrum of polymerization product (Figure
3), three carbonyl peaks were observed at 168.4 (FA-glycine amide bond), 171.3 (glycine
aromatic ester), and 175.3 ppm (acetyl group). Therefore, the amide bond between FA and
glycine was retained.
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05
06
1
3
3
Figure 3. C-NMR spectra of PAcFAG polymerized for 16 h
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3.3 Solubility and molecular weight of poly[(ferulic acid)-alt-(glycine)]s
The poly[(ferulic acid)-alt-(glycine)]s were completely soluble in phenol and
pentafluorophenol when heated to 130 °C. PAcFAG had low solubility in common organic
solvents, probably owing to the strong cohesion between the FA moieties of the polymer chain.
Poly(ferulic acid) is only dissolved in pentafluorophenol [7]. FA-based poly(ether-ester)s
(ethylene ferulate, polypropylene ferulate, and polyhexalene ferulate) are soluble in
hexafluoroisopropanol (HFIP) for GPC analysis [20]. In our study, we found that PAcFAG
was soluble in phenol and pentafluorophenol, but not in HFIP. The solubility of PAcFAG was
similar to that of poly(ferulic acid), but not to that of the FA-based poly(ether-ester)s. This
results from stronger intermolecular interaction and rigidness attributable to the
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hydrogen-bonding of the amide and ester bonds. Therefore, we determined the molecular
weights of PFAG and PAcFAG by GPC with chloroform/phenol = 9/1 (v/v) as the mobile
5
phase. The elution curve revealed PS standards peaks at 17.0 min (M = 1.57 × 10 ), 17.7 min
p
4
4
4
(M = 6.6 × 10 ), 18.4 min (M = 2.9 × 10 ), 19.2 min (M = 1.0 × 10 ), 20.6 min (M = 3.2 ×
p
p
p
p
3
3
10 ), and 21.8 min (M = 1.3 × 10 ). As shown in the elution curves from GPC analysis in
p
Figure 4, PFAG polymerized for 16 h contained a higher molecular weight fraction than that
polymerized for 1 h. However, the major portion of injected PFAG eluted after the PS
standards. This shows that the major polymerization products of PFAG comprised a mixture
of FAG oligomers. Therefore, we did not estimate the average molecular weight of the
polymerized product of FAG. However, PAcFAG polymerized for 16 h eluted within PS
standard ranges. The elution peak between 17 and 19.5 min corresponds to a fraction with a
4
M of 3.9 × 10 and a M / M ratio of 1.4. This suggests that the PAcFAG polymerized for 16
w
w
n
h contained a considerable amount of polymerized fraction. Behind the polymerized fraction,
five distinctive peaks assignable to the pentamer, tetramer, trimer, dimer, and monomer were
observed at 22.4, 23.1, 23.6, 24.5, and 25.0 min, respectively. The elution behavior
1
corroborates the H NMR spectrum (Figure 2). However, the degree of polymerization (DP =
1
5.3) estimated from the H NMR spectrum is lower than the DP calculated from the GPC data
1
(DP = 7.7). This arose because the peak of the residual acetic acid in the H NMR spectrum
overlapped that of the terminal acetyl ester and the integral value of the terminal group peak
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increased. Considering these results, we regard AcFAG to be the most suitable precursor for
obtaining poly[(ferulic acid)-alt-(glycine)] with sufficient molecular weight as a polymeric
material.
3
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3
Figure 4. Elution curves for (a) PFAG and (b) PAcFAG (phenol/chloroform = 1/9 (v/v))
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3.4 Thermal properties and crystallinity
We carried out TGA analysis to investigate the thermal stability of the polymers at elevated
temperatures. The TGA thermograms for the monomers and polymers of AcFAG are shown in
Figure 5. Whereas the weight of AcFAG started to decrease at 160 °C, PAcFAG retained its
original weight up to approximately 300 °C. Above 300 °C, both the monomer and polymer
of AcFAG exhibited a similar weight reduction. Therefore, we considered their decomposition
behavior to be the same, and it comprised two steps. We attributed the first decomposition
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50
51
step at 300 °C to elimination by carbonization and the second step at 550 °C is the thermal
3
cracking of the aromatic compounds.
3
52
53
3
Figure 5. TGA curves of AcFAG and PAcFAG polymerized for 16 h (programming rate: 10
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°C / min, temperature range: 30-900 °C)
Figure 6 shows the DSC thermogram of PAcFAG polymerized for 16 h. Although the
melting endotherm was not observed in the 1st run, the glass transition was observed at 189
o
°C in the 2nd run. FA-based poly(ether-ester)s have T values in the range 51-113 C [20].
g
PAcFAG has a higher T than FA-based poly(ether-ester)s. This implies that PAcFAG is more
g
rigid than FA-based poly(ether-ester)s. In the POM investigation, we also observed the
thermoplastic deformation of PAcFAG at 300 °C (Figure 7).
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63
3
Figure 6. DSC curves of PAcFAG after polymerization for 16 h (programming rate: 10
3
64
65
°C/min, temperature range: 0–300 °C).
3
3
66
67
3
Figure 7. Images of (a) AcFAG, (b) PAcFAG polymerized for 16 h, and (c) PAcFAG
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polymerized for 16 h heated at 300 °C. Scale bar: 200 µm.
The amorphous nature of PAcFAG was also supported by the X-ray diffractogram of PAcFAG
(Figure 8), in which only an amorphous halo was observed. The improvements to the thermal
properties and crystallinity by varying the chain length of the amino-acid counter monomer
are under consideration.
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3
Figure 8. X-ray diffraction pattern and diffractgram of PAcFAG polymerized for 16 h
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4. Conclusion
We synthesized three novel alternate monomers (FAMG, FAG, and AcFAG) from FA and
glycine, and investigated their polymerization in bulk and solution. The NMR spectra, FT-IR
spectra and GPC curves indicated that AcFAG was the most suitable alternate monomer for
producing the aimed-for poly[(ferulic acid)-alt-(glycine)] copolymer. Poly[(ferulic
acid)-alt-(glycine)] derived from AcFAG (PAcFAG) was completely soluble in phenol and
pentafluorophenol when heated, but had low solubility in common organic solvents. PAcFAG
exhibited high thermal stability with a Td50% of 520 °C. The T of PAcFAG derived from its
g
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DSC was 189 °C and the results from the POM investigation, the DSC curves, and the X-ray
diffractogram indicated that it was amorphous.
In the future, to produce FA-based polymer materials with sufficient strength, the molecular
weight of the polymers must be increased by appropriate monomer design and
copolymerization methods. Furthermore, to ensure better processing, the solubility, thermal
plasticity, and thermal processability of the polymers must be improved by controlling the
structure of the polymers and by reducing their melting and softening points.
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgement
The authors would like to thank Prof. Dr. Hideki Abe and Dr. Koichiro Tachibana at the
RIKEN Center for Sustainable Resource Science for their assistance with the GPC
measurement.
Funding
This work was partially supported by a Grant-in-aid for Scientific Research (A) (No.
26248044(2014) (T.I.)) and by a Grant-in-Aid for Young Scientists (B) (No.25870921 (2013)
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(D.I.)) from Japanese Society for Promotion of Science.
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Appendix A. Supplementary data
Supplementary data related to this article are available.
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