Bio-based vinylphenol family: Synthesis via decarboxylation of naturally occurring cinnamic acids and living radical polymerization for functionalized polystyrenes

Article abstract of DOI:10.1002/pola.29453

A series of bio-based vinylphenols or hydroxystyrenes is prepared by simple decarboxylation of various naturally occurring cinnamic acids such as o-, m-, and p-coumaric; caffeic; ferulic; and sinapinic acids, which possess hydroxy groups and other substituents at different positions on the aromatic ring. After protection of the phenolic moieties with trialkylsilyl groups, reversible addition–fragmentation chain-transfer polymerization is accomplished with cumyl dithiobenzoate to afford various bio-based hydroxyl-protected polystyrenes with controlled molecular weights and narrow molecular weight distributions. Subsequent deprotection of the silyl groups under mild conditions results in a series of well-defined functionalized polystyrenes possessing different numbers (mono-, di-, tri-) of hydroxy groups at different positions (o, m, p). The obtained functionalized polystyrenes show unique thermal properties depending on the substituents, and those with phenol and catechol groups serve as reducing agents for silver ions.

Full text of DOI:10.1002/pola.29453

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Bio-Based Vinylphenol Family: Synthesis via Decarboxylation of Naturally
Occurring Cinnamic Acids and Living Radical Polymerization for
Functionalized Polystyrenes
1
Hisaaki Takeshima,1 Kotaro Satoh
,
1,2 Masami Kamigaito
1Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8603, Japan
2Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Correspondence to: K. Satoh (E-mail: satoh@cap.mac.titech.ac.jp); M. Kamigaito (E-mail: kamigait@chembio.nagoya-u.ac.jp)
Received 2 July 2019; Revised 19 July 2019; accepted 21 July 2019
DOI: 10.1002/pola.29453
ABSTRACT: A series of bio-based vinylphenols or hydroxystyrenes
is prepared by simple decarboxylation of various naturally occur-
ring cinnamic acids such as o-, m-, and p-coumaric; caffeic; fer-
ulic; and sinapinic acids, which possess hydroxy groups and
other substituents at different positions on the aromatic ring.
After protection of the phenolic moieties with trialkylsilyl groups,
reversible addition–fragmentation chain-transfer polymerization
is accomplished with cumyl dithiobenzoate to afford various bio-
based hydroxyl-protected polystyrenes with controlled molecular
weights and narrow molecular weight distributions. Subsequent
deprotection of the silyl groups under mild conditions results in a
series of well-defined functionalized polystyrenes possessing dif-
ferent numbers (mono-, di-, tri-) of hydroxy groups at different
positions (o, m, p). The obtained functionalized polystyrenes
show unique thermal properties depending on the substituents,
and those with phenol and catechol groups serve as reducing
agents for silver ions. © 2019 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2019
KEYWORDS: bio-based polymer; cinnamic acid; hydroxystyrene;
catechol; functionalization of polymers; gallol; guaiacol; living
polymerization; phenol; RAFT; radical polymerization; syringol
compounds can be used as functional parts for adhesion,
reduction, curing, and so forth. Indeed, the catechol contained
in natural proteins is a key component for adhesion, as
observed in mussel adhesive proteins.11–16 However, in lignin,
many of the hydroxy groups are consumed when linking the
components.
INTRODUCTION Renewable resources are now receiving atten-
tion as raw materials for polymers, mainly from the viewpoint
of sustainability.1–6 In addition, their characteristic structures
often lead to unique bio-based polymers with unique proper-
ties, which are difficult to realize by simple petroleum-based
compounds and are attractive in view of new high-
performance or functional polymers. Furthermore, special
functions can be added by controlling the polymer structure
using precision polymerization.7
The cinnamic acid family has a common structure, namely, (E)-
3-phenyl-2-propenoic acid, which has phenyl and carboxylic
acid groups attached to the 1,2-position of the vinyl group and
thus can be regarded as a vinyl compound possessing both
styrenic and acrylic structures.17 However, due to steric hin-
drance around the 1,2-disubstituted vinyl group, cinnamic acids
rarely homopolymerize.18 Although they are copolymerized
with styrene and acrylate, their incorporation into the resulting
copolymers is relatively low.17,19 Alternatively, hydroxy-
functionalized cinnamic acids such as ferulic acid are polymer-
ized via condensation polymerization into bio-based polyesters
and polycarbonates using the hydroxy and carboxylic
groups,20–23 though they lose the hydroxy functional groups
upon polymerization.
A family of cinnamic acids widely and abundantly seen in
nature is the series of (E)-3-phenyl-2-propenoic acids differing
in their ring substituents, such as hydroxy and methoxy
groups.8–10 A series of these compounds, which belong to the
family of phenylpropanoids, are biologically synthesized via
step-by-step enzymatic reactions mediated by various
enzymes in plants, whereas most of them are finally converted
into lignin, that is, the second largest natural polymer product,
via natural polymerization using the phenolic and vinylic
groups for the linking reactions. The phenol, catechol, and
other related phenolic hydroxy groups in these natural
Additional supporting information may be found in the online version of this article.
© 2019 Wiley Periodicals, Inc.
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Recently, we reported
a facile synthesis of hydroxy-
functionalized styrenes, such as 4-vinylguaiacol (4VG) and
4-vinylcatechol (4VC), via decarboxylation of naturally occur-
ring ferulic and caffeic acids, which both belong to the
cinnamic acid family, upon heating to 100ꢀC just in the pres-
ence of triethylamine.24–26 Furthermore, after protection of
the phenolic functional groups with trialkylsilyl groups, the
protected 4VG and 4VC were polymerized via reversible
addition–fragmentation chain-transfer (RAFT) polymeriza-
tion27 to result in hydroxy-functionalized polystyrenes, that is,
poly(4VG) and poly(4VC), with controlled molecular weights
after deprotection of the silyl groups.
In this article, we applied this approach to a series of naturally
occurring cinnamic acids, including not only ferulic and caffeic
acids but also o-, m-, and p-coumaric and sinapinic acids, to syn-
thesize an array of bio-based hydroxy-functionalized polysty-
renes with controlled molecular weights (Scheme 1). We thus
investigated the synthesis of a series of hydroxystyrenes with
different numbers of hydroxy and methoxy substituents at vari-
ous positions of the aromatic ring via decarboxylation of the
cinnamic acids and then the preparation of well-defined
functionalized bio-based polystyrenes via RAFT polymerization
of the silyl-protected monomers, followed by deprotection. Fur-
thermore, the effects of the substituents on the thermal proper-
ties and reducing abilities of the polystyrenes were evaluated.
EXPERIMENTAL
Materials
p-Coumaric acid (TCI, Tokyo, Japan, >98.0%), m-coumaric acid
(TCI, >99.0%), o-coumaric acid (Sigma-Aldrich, St. Louis, MO,
USA, >97%), ferulic acid (Sigma-Aldrich, >99%), caffeic acid
(Kanto Chemical, Tokyo, Japan, >98.0%), sinapinic acid (TCI,
>98.0%), triethylamine (Et3N: TCI, >99.0%), N,N-dimethylf-
ormamide (DMF: Kanto Chemical, >99.5%; H2O < 0.001%),
copper(II) hydroxide (FUJIFILM Wako Chemical, Osaka, Japan,
>90.0%), 1,10-phenanthroline monohydrate (TCI, >99.0%), tert-
butyldimethylsilyl chloride (TBDMSCl: TCI, >98%), imidazole
(Kishida Chemical, Osaka, Japan, 99%), triethylsilane (Sigma-
Aldrich, 99%), and tris(pentafluorophenyl)borane [B(C6F5)3:
TCI, >97%] were used as received. 1,2,3,4-Tetrahydr-
onaphthalene (FUJIFILM Wako Chemical, 97%) was distilled
over calcium hydride under reduced pressure before use. Tolu-
ene (Kanto Chemical, >99.5%; H2O < 10 ppm) and tetrahydrofu-
ran (THF: Kanto Chemical, >99.5%; H2O < 0.001%) were dried
and deoxygenized by passage through columns of Glass Contour
Solvent Systems before use. α,α-4,40-Azobisisobutyronitrile
(AIBN) (Kishida Chemical, >99%) was purified by recrystalliza-
tion from methanol. Cumyl dithiobenzoate (CDB) was synthe-
sized according to the literature.28,29
SCHEME
1
Synthesis of bio-based hydroxyl-functionalized
polystyrenes via decarboxylation of naturally occurring cinnamic
acids followed by silylation, RAFT polymerization, and deprotection.
[Color figure can be viewed at wileyonlinelibrary.com]
Synthesis of TBDMS4VP (1)
p-Coumaric acid (25.0 g, 152.4 mmol), triethylamine (43.0 mL,
308.5 mmol), and DMF (86 mL) were placed in a three-necked
round-bottom flask equipped with a Dimroth condenser at
room temperature. The flask was put into an oil bath and
heated to 100ꢀC with stirring. The conversion of p-coumaric
acid was calculated from the concentration of residual p-
coumaric acid measured by proton nuclear magnetic reso-
nance (1H NMR). After 24 h, p-coumaric acid was completely
consumed (>99.9%) and converted into p-hydroxystyrene or
4-vinylphenol (4VP). The flask was cooled to 0ꢀC, and
2
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TBDMSCl (25.2 g, 167.5 mmol) was slowly added to the mix-
ture. The reaction mixture was stirred at 20ꢀC. After 24 h, ice
water was slowly added to stop the reaction. The reaction
mixture was diluted with n-hexane and washed with water.
The organic layer was concentrated by rotary evaporation.
The residue was purified by distillation under reduced pres-
sure (62ꢀC/16 Pa). TBDMS4VP (1) was obtained as a colorless
converted into o-hydroxystyrene or 2-vinylphenol (2VP). The
flask was cooled to 0ꢀC, and TBDMSCl (23.3 g, 154.7 mmol)
was slowly added to the mixture. The reaction mixture was
stirred at 20ꢀC. After 12 h, ice water was slowly added to stop
the reaction. The reaction mixture was diluted with n-hexane
and washed with aqueous HCl, aqueous NaHCO3, and water.
The organic layer was dried with Na2SO4 and concentrated by
rotary evaporation. The residue was purified by distillation
under reduced pressure (67ꢀC/31 Pa). TBDMS2VP (3) was
1
liquid (34.0 g, 95%). H NMR (400 MHz, CDCl3, 25ꢀC, δ)
[Fig. S1(A)]: 0.20 (s, 6H, SiCH3), 0.98 (s, 9H, SiC(CH3)3), 5.12
(d, 1H, trans, CH2═CH), 5.60 (d, 1H, cis, CH2═CH), 6.65 (dd,
1H, CH2═CH), 6.79 (d, 2H, ArH), 7.28 (d, 2H, ArH). 13C NMR
(125 MHz, CDCl3, 25ꢀC, δ) [Fig. S2(A)]: –4.3 (SiCH3), 18.4 (SiC
(CH3)3), 25.8 (SiC(CH3)3), 111.8 (CH2═CH), 136.5 (CH2═CH),
120.3, 127.5, 131.1, 155.6 (phenyl).
obtained as
a
colorless liquid (33.1 g, 91%). 1H NMR
(400 MHz, CDCl3, 25ꢀC, δ) [Fig. S1(C)]: 0.21 (s, 6H, SiCH3),
1.02 (s, 9H, SiC(CH3)3), 5.22 (d, 1H, trans, CH2═CH), 5.67 (d,
1H, cis, CH2═CH), 7.05 (dd, 1H, CH2═CH), 6.79 (d, 1H, ArH),
6.93 (dd, 1H, ArH), 7.13 (dd, 1H, ArH), 7.50 (d, 1H, ArH). 13C
NMR (125 MHz, CDCl3, 25ꢀC, δ) [Fig. S2(C)]: −4.0 (SiCH3),
18.5 (SiC(CH3)3), 26.0 (SiC(CH3)3), 113.7 (CH2═CH), 132.1
(CH2═CH), 119.8, 121.5, 126.2, 128.8, 129.2, 153.0 (phenyl).
Synthesis of TBDMS3VP (2)
m-Coumaric acid (20.5 g, 124.7 mmol), 1,10-phenanthrioline
(2.3 g, 12.7 mmol), Cu(OH)2 (1.22 g, 12.5 mmol), and DMF
(275 mL) were placed in a three-necked round-bottom flask
equipped with a Dimroth condenser at room temperature.30
The flask was put into an oil bath and heated to 150ꢀC with
stirring. The conversion of m-coumaric acid was calculated
from the concentration of residual m-coumaric acid measured
by 1H NMR. After 66 h, the conversion of m-coumaric acid
reached 89%. The reaction mixture was diluted with diethyl
ether and washed with aqueous citric acid and water. After
drying with Na2SO4, the solvents were removed by rotary
evaporator to yield m-hydroxystyrene or 3-vinylphenol (3VP)
as a liquid (13.3 g, 88%).
Synthesis of TBDMS4VG (4), (TBDMS)2VC (5), (TES)2VC
(6), and (TIPS)2VC (7)
According to the literature, TBDMS4VG (4) was synthesized
from ferulic acid,24 and (TBDMS)2VC (5), (TES)2VC (6), and
(TIPS)2VC (7) were synthesized from caffeic acid25 [total
yield: 98% (4), 98% (5), 91% (6), and 92% (7)].
Synthesis of TBDMS4VS (8)
Sinapinic acid (24.9 g, 111.1 mmol), triethylamine (31.0 mL,
222.4 mmol), and DMF (56 mL) were placed in a three-necked
round-bottom flask equipped with a Dimroth condenser at
room temperature. The flask was put into an oil bath and
heated to 100ꢀC under stirring. The conversion of sinapinic
acid was calculated from the concentration of residual sin-
The obtained 3VP (7.9 g, 66.0 mmol) and imidazole (10.0 g,
146.7 mmol) were dissolved in DMF (37 mL) in a three-
necked round-bottom flask. The flask was cooled to 0ꢀC, and
TBDMSCl (11.0 g, 73.0 mmol) was slowly added to the mix-
ture. The reaction mixture was stirred at 20ꢀC. After 12 h, ice
water was slowly added to stop the reaction. The reaction
mixture was diluted with toluene and washed with water. The
organic layer was concentrated by rotary evaporation. The
residue was purified by silica gel column chromatography
(Silica Gel 60N, n-hexane). TBDMS3VP (2) was obtained as a
colorless liquid (14.6 g, 94%). 1H NMR (400 MHz, CDCl3,
25ꢀC, δ) [Fig. S1(B)]: 0.20 (s, 6H, SiCH3), 0.99 (s, 9H,
SiC(CH3)3), 5.23 (d, 1H, trans, CH2═CH), 5.71 (d, 1H, cis,
CH2═CH), 6.66 (dd, 1H, CH2═CH), 6.74 (dd, 1H, ArH), 6.88 (s,
1H, ArH), 7.00 (d, 1H, ArH), 7.17 (dd, 1H, ArH). 13C NMR
(125 MHz, CDCl3, 25ꢀC, δ) [Fig. S2(B)]: –4.2 (SiCH3), 18.4 (SiC
(CH3)3), 25.8 (SiC(CH3)3), 114.0 (CH2═CH), 136.9 (CH2═CH),
117.9, 119.6, 119.7, 129.5, 139.2, 156.0 (phenyl).
1
apinic acid measured by H NMR. After 2 h, sinapinic acid was
completely consumed (>99.9%) and converted into
4-vinylsyringol (4VS). The flask was cooled to 0ꢀC, and
TBDMSCl (16.8 g, 111.7 mmol) was slowly added to the mix-
ture. The reaction mixture was stirred at 20ꢀC. After 14 h, ice
water was slowly added to stop the reaction. The reaction
mixture was diluted with n-hexane and washed with aqueous
HCl, aqueous NaHCO3, and water. The organic layer was dried
with Na2SO4 and concentrated by rotary evaporation. The res-
idue was purified by silica gel column chromatography (Silica
Gel 60N, n-hexane
~ n-hexane/ethyl acetate = 97/3).
TBDMS4VS (8) was obtained as a white solid (30.4 g, 93%).
1H NMR (400 MHz, CDCl3, 25ꢀC, δ) [Fig. S1(D)]: 0.13 (s, 6H,
SiCH3), 1.00 (s, 9H, SiC(CH3)3), 3.81 (s, 6H, OCH3), 5.15 (d, 1H,
trans, CH2═CH), 5.61 (d, 1H, cis, CH2═CH), 6.62 (dd, 1H,
13
CH2═CH), 6.61 (s, 2H, ArH). C NMR (125 MHz, CDCl3, 25ꢀC,
δ) [Fig. S2(D)]: −4.5 (SiCH3), 18.9 (SiC(CH3)3), 25.9 (SiC
(CH3)3), 55.9 (OCH3), 112.0 (CH2═CH), 137.2 (CH2═CH), 103.6,
130.5, 134.7, 151.8 (phenyl).
Synthesis of TBDMS2VP (3)
o-Coumaric acid (25.4 g, 154.7 mmol), triethylamine (43.0 mL,
308.5 mmol), and DMF (85 mL) were placed in a three-necked
round-bottom flask equipped with a Dimroth condenser at
room temperature. The flask was put into an oil bath and
heated to 120ꢀC with stirring. The conversion of o-coumaric
acid was calculated from the concentration of residual o-
coumaric acid measured by 1H NMR. After 37 h, o-coumaric
acid was almost completely consumed (>99.9%) and
Synthesis of TBDMS(TES)24VGa (9)
n-Hexane (32 mL), Et3SiH (6.0 mL, 37.7 mmol), and B(C6F5)3
solution in toluene (0.80 mL, 50 mM), which was prepared in
another flask by solubilizing B(C6F5)3 (39.8 mg, 0.08 mmol)
with toluene (1.5 mL), were placed in a three-necked round-
bottom flask equipped with a dropping funnel at room
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temperature. TBDMS4VS (8) (10.0 mL of 1.89 M solution in n-
hexane, 18.9 mmol) was put in the dropping funnel. The flask
was cooled to 10ꢀC, and then TBDMS4VS (8) was slowly
dropped into the mixture at 10ꢀC. After the addition of
TBDMS4VS (8), the reaction mixture was stirred at 20ꢀC. The
conversion of TBDMS4VS (8) was calculated from the concen-
tration of the methoxy group measured by 1H NMR. After
20 h, the methoxy group was quantitatively consumed to yield
TBDMS(TES)24VGa (9), although it contained a small amount
of the hydrosilylated product (olefin/hydrosilylated
form = 1/0.05, reaction yield = 95%). The reaction mixture
was diluted with n-hexane and washed with water. After con-
centration by rotary evaporation, the residue was purified by
silica gel column chromatography (Silica Gel 60N, n-hexane)
to yield TBDMS(TES)24VGa (9) as a colorless viscous liquid
poly(3) (170 mg in 5.6 mL of THF, Mn = 12,600, Mw/
Mn = 1.08), the TBDMS group was deprotected using acetic
acid (0.09 mL) and tetrabutylammonium fluoride (TBAF)
(1.45 mL of 1.0 M solution in THF) at 40ꢀC. After stirring for
36 h, the reaction mixture was diluted with ethyl acetate and
washed with aqueous HCl and water. After removing the sol-
vent, precipitation was conducted for purification to give poly
(2VP) (65.0 mg, 75%).
Reduction of Silver Ions
A representative example of poly(4VC) is given below. Poly
(4VC) was dissolved in dried and deoxygenated THF by the
syringe technique under dry argon to prepare the polymer solu-
tion ([catechol group]0 = 200 mM). AgNO3 was dissolved in
degassed and distilled water by the syringe technique under dry
argon to prepare the solution ([AgNO3]0 = 2.0 mM). The poly-
mer solution (2.6 mL) was placed in a quartz cell under dry
argon, and then AgNO3 solution (0.13 mL) was added to the
polymer solution under vigorous stirring at 25ꢀC. The cell was
immediately capped, and ultraviolet–visible (UV–vis) measure-
ments were started. The color of the solution changed to yellow,
and a peak attributed to Ag nanoparticles (~400 nm) gradually
appeared.
1
(6.31 g, 68%). H NMR (400 MHz, CDCl3, 25ꢀC, δ) [Fig. S1(E)]:
0.12 (s, 6H, SiCH3), 0.76 (q, 12H, SiCH2CH3), 0.97 (t, 18H,
SiCH2CH3), 1.00 (s, 9H, SiC(CH3)3), 5.09 (d, 1H, trans,
CH2═CH), 5.50 (d, 1H, cis, CH2═CH), 6.52 (dd, 1H, CH2═CH),
13
6.54 (s, 2H, ArH). C NMR (125 MHz, CDCl3, 25ꢀC, δ) [Fig. S2
(E)]: −4.0 (SiCH3), 5.2 (SiCH2CH3), 6.8 (SiCH2CH3), 18.6 (SiC
(CH3)3), 25.9 (SiC(CH3)3), 111.7 (CH2═CH), 136.8 (CH2═CH),
111.3, 130.0, 138.4, 148.5 (phenyl).
RAFT Polymerization
Measurements
1H and 13C NMR spectra were recorded on a JEOL ECS-400 spec-
trometer operating at 400 and 125 MHz, respectively. The
number-average molecular weight (Mn) and molecular weight
distribution (MWD; Mw/Mn) of the product polymers were
determined by SEC in THF at 40ꢀC on two polystyrene gel col-
umns (Tosoh Multipore HXL-M [7.8 mm i.d. × 30 cm] × 2; flow
rate 1.0 mL min−1) or in DMF containing 100 mM LiCl at 40ꢀC
on two hydrophilic polymer gel columns (Tosoh α-M + α-3000
[7.8 mm i.d. × 30 cm]; flow rate 1.0 mL min−1) (for polymers
after deprotection) connected to a JASCO PU-2080 precision
pump and a JASCO RI-2031 detector. The columns were cali-
brated against standard polystyrene samples (Varian;
Mp = 580–3053000, Mw/Mn = 1.02–1.23). The glass transition
temperature (Tg) of the polymers was recorded on a Q200 dif-
ferential scanning calorimeter (TA Instruments Inc.). The sample
was first heated to 210ꢀC at 10ꢀC min−1, then equilibrated at
this temperature for 10 min, and finally cooled to −10ꢀC at
5ꢀC min−1. After being held at this temperature for 5 min, the
sample was then heated to 260ꢀC at 10ꢀC min−1. All Tg values
were obtained from the second scan after removing the thermal
history. UV–vis absorption spectra were recorded on a JASCO V-
550 spectrometer.
RAFT polymerization was conducted by syringe under dry nitro-
gen in sealed glass tubes. A representative example of the poly-
merization of TBDMS4VP (1) with CDB as the RAFT agent in the
presence of AIBN is given below. 1 (2.58 mL, 10.0 mmol), CDB
(0.45 mL of 222 mM solution in toluene, 0.10 mmol), AIBN
(0.25 mL of 100 mM solution in toluene, 0.025 mmol),
1,2,3,4-tetrahydronaphthalene (0.22 mL) as an internal stan-
dard, and toluene (1.5 mL) were placed in a 25 mL glass tube
equipped with a three-way stopcock at room temperature. The
total volume of the reaction mixture was 5.0 mL. After mixing,
the mixture was aliquoted into six baked glass tubes, and the
tubes were sealed by flame under a nitrogen atmosphere. The
tubes were immersed in a thermostatic oil bath at 60ꢀC. At cer-
tain intervals, the reaction tubes were cooled to −78ꢀC to termi-
nate the polymerization. Monomer conversion was determined
from the concentration of residual monomer measured by 1H
NMR, with 1,2,3,4-tetrahydronaphthalene as an internal stan-
dard (e.g., 48 h, 53%). The quenched reaction solutions were
evaporated to dryness to give poly(1) (Mn = 12,000, Mw/
Mn = 1.10). The obtained polymer was purified by preparative
size exclusion chromatography (SEC) for 1H NMR analysis.
Deprotection of Silyl Groups
Deprotection of the TBDMS groups was performed according
to previous papers.24,25 Representative examples are given
below. Poly(1) (140 mg, Mn = 9900, Mw/Mn = 1.10) was dis-
solved in THF (7.7 mL), followed by the addition of
hydrochloric acid (0.70 mL, 12 M) at room temperature. After
stirring for 48 h, the solution was diluted with ethyl acetate
and washed with aqueous NaHCO3 and water. The organic
layer was evaporated to remove the solvents. The residue was
dissolved in acetone and purified by precipitation into n-
hexane three times to give poly(4VP) (75.2 mg, 100%). For
RESULTS AND DISCUSSION
Synthesis of Various Bio-Based Functionalized Styrenes
from Cinnamic Acids: Decarboxylation and Silylation
A series of hydroxystyrenes with different numbers of substitu-
ents at different positions on the aromatic ring were first syn-
thesized by decarboxylation of naturally occurring cinnamic
acids, such as o-, m-, and p-coumaric; ferulic; caffeic; and sin-
apinic acids (Scheme 2). According to our previous reports on
the synthesis starting from ferulic and caffeic acids, the
4
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FIGURE 1 Time-conversion curves for RAFT polymerization of
series of silyl-protected hydroxystyrenes: [M]0/[CDB]0/
a
[AIBN]0 = 2000/20/5.0 mM (1–6, 8) or 1000/10/2.5 mM (7, 9)
in toluene at 60ꢀC. [Color figure can be viewed at
wileyonlinelibrary.com]
triethylsilyloxy groups via reaction with Et3SiH in the presence
SCHEME
2
Synthesis and total yield of silyl-protected
15,24
of B(C6F5)3
to result in the silyl-protected 4-vinylgallol
hydroxystyrenes via decarboxylation of cinnamic acids followed by
silylation. [Color figure can be viewed at wileyonlinelibrary.com]
(4VGa) (9), which has three trialkylsilyl-protected hydroxyl
groups at the m, p, and m-positions, in good yield (68%). A
series of mono-, di-, and tri-hydroxystyrenes with trialkylsilyl-
protected groups were thus prepared from naturally occurring
cinnamic acids via successive simple reactions in relatively
high yields.
decarboxylation of o-, m-, and p-coumaric acids and sinapinic
acid was similarly investigated upon heating in the presence of
triethylamine in DMF at 100–120ꢀC. o- and p-Coumaric acids
and sinapinic acid were quantitatively converted into o-
hydroxystyrene (2VP), p-hydroxystyrene (4VP), and 4-hydroxy-
3,5-dimethoxystyrene (4VS), respectively, without any sponta-
neous thermal polymerizations. However, almost no decarboxyl-
ation occurred for m-coumaric acid with triethylamine upon
heating. These results suggest that the o- and p-hydroxy groups
work efficiently for the simple base-induced decarboxylation of
cinnamic acids due to the resonance effects. However, decarbox-
ylation of m-coumaric acid into m-hydroxystyrene (3VP) was
successfully achieved by the other common method using a cop-
per catalyst.30
RAFT Polymerization of Bio-Based Silyl-Protected
Hydroxystyrenes
RAFT polymerization of a series of bio-based silyl-protected
hydroxystyrenes was investigated using CDB as an RAFT agent,
which is generally effective for the controlled radical polymeri-
zation of styrenes,27 in the presence of AIBN in toluene at 60ꢀC
(Fig. 1). All monomers were consumed up to high conversions,
although the polymerization rates depended on the positions,
numbers, and bulkiness of the substituents on the aromatic ring.
In particular, highly bulky trisubstitutions with all trialkylsilyl
groups at the 3,4,5-position (9) and one substitution with bulky
TBDMS at the 2-position near the vinyl group (3) significantly
retarded the polymerizations. The first-order plots for the
monomers with slow rates gradually deviated from the linearity
due to the depletion of AIBN for the long reactions (Fig. S3).
After decarboxylation, the hydroxy groups were protected with
various trialkylsilyl groups via reactions with trialkylsilyl chlo-
ride in the presence of appropriate bases. Notably, in situ direct
silylation of the obtained hydroxystyrenes was achieved not
only for 4VG and 4VC but also for 2VP, 4VP, and 4VS upon sim-
ple addition of the silyl chloride in the presence of pre-existing
triethylamine, which was used for the decarboxylation, in high
yields. The total yields were thus very high (>90%). The iso-
lated 3VP, which was prepared by decarboxylation using the
copper catalyst, was similarly converted into the silyl-
protected form in the presence of imidazole as another base.
Even for the two-step reactions, the total yield was 83%. Fur-
thermore, two methoxy groups in the TBDMS-protected 4VS
(8) derived from sinapinic acid were converted into two
Figure 2 shows the Mn, Mw/Mn, and SEC curves of the poly-
mers obtained from the monosubstituted monomers (1–3)
derived from o-, m-, and p-coumaric acids. Mn values of the
obtained polymers all increased in direct proportion to mono-
mer conversion, indicating that all these polymerizations
proceeded in living fashion, although the slopes were slightly
different, most likely due to the difference in the hydrody-
namic volumes of the resulting polymers depending on the
substituted positions. The MWDs were narrow for the p- and
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FIGURE 2 Mn, Mw/Mn, and SEC curves of the polymers obtained by RAFT polymerization of monosubstituted silyl-protected
hydroxystyrenes derived from o-, m-, and p-coumaric acids: [M]0/[CDB]0/[AIBN]0 = 2000/20/5.0 mM in toluene at 60ꢀC. [Color figure
can be viewed at wileyonlinelibrary.com]
FIGURE 3 Mn, Mw/Mn, and SEC curves of the polymers obtained by RAFT polymerization of di-substituted silyl-protected
hydroxystyrenes derived from ferulic and caffeic acids: [M]0/[CDB]0/[AIBN]0 = 2000/20/5.0 mM (4–6) or 1000/10/2.5 (7) mM in toluene
at 60ꢀC. [Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 4 Mn, Mw/Mn, and SEC curves of the polymers obtained by RAFT polymerization of tir-substituted silyl-protected
hydroxystyrenes derived from sinapinic acid: [M]0/[CDB]0/[AIBN]0 = 2000/20/5.0 mM (8) or 1000/10/2.5 mM (9) in toluene at 60ꢀC.
[Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 6 H NMR spectra [(CD3)2CO, 50ꢀC] of the polymers
obtained after deprotection of poly(1) (a), poly(2) (b), poly(3) (c),
poly(4) (d), poly(5) (e), and poly(8) (f). [Color figure can be
viewed at wileyonlinelibrary.com]
FIGURE
5
1H NMR spectra (CDCl3, 55ꢀC) of the polymers
obtained by RAFT polymerization of 1 (a), 2 (b), 3 (c), 4 (d), 5 (e),
and 8 (f). [Color figure can be viewed at wileyonlinelibrary.com]
these results for the disubstituted monomers are summarized in
Figure 3 for comparison, although some of the results were pre-
viously reported elsewhere.24,25 The MWDs were fairly narrow
(Mw/Mn ~ 1.1) for 4–6. However, for 7, which possess two
highly bulky TIPS groups at the m- and p-positions, the MWD
was relatively broad. However, these distributions became
narrower (Mw/Mn = 1.8 ! 1.5) with monomer conversion,
again indicating slow interconversion due to steric hindrance.
m-substituted monomers (1 and 2) (Mw/Mn ~ 1.1). The o-
substituted monomer (3) resulted in relatively broad MWDs,
which became narrower (Mw/Mn = 1.8 ! 1.3) as the polymer-
ization proceeded. These results suggest that the steric hin-
drance around the propagating radical species slowed the
interchanging reaction between the propagating radical and
the RAFT terminal, as similarly observed in the RAFT poly-
merization of highly bulky methacryaltes.31
m-, p-, and m-Trisubstituted monomers, that is, 4VS protected
with TBDMS (8) and 4VGa protected with two TES and one
TBDMS (9), which were both derived from sinapinic acid as
described above, also afforded the polymers with controlled
molecular weights (Fig. 4). In particular, the TBDMS-silyl
For m- and p-disubstituted monomers (4–7), which were
derived from ferulic and caffeic acids, all Mn values were con-
trolled and nearly the same, independent of the substituents. All
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FIGURE
8
DSC curves of the polymers obtained after
deprotection of silyl groups. Polymer [Mn(Calcd)]: poly(4VP)
(10,400), poly(3VP) (5700), poly(2VP) (8800), poly(4VG) (8200),
poly(4VC) (12,900), poly(4VS) (8800). [Color figure can be
viewed at wileyonlinelibrary.com]
the repeating monomer units and the RAFT ω-terminal (x and
z) were very close to the calculated values (Mn[calcd]), assum-
ing that one RAFT agent produces one polymer chain. In con-
trast, Mn(SEC) values based on polystyrene calibration were
lower than both Mn(calcd) and Mn(NMR) due to the difference
in the hydrodynamic volumes of the polymers.24,25 Thus, CDB
works as an appropriate RAFT agent for controlling the radical
polymerizations of a series of silyl-protected hydroxystyrenes
regardless of the number of substituents on the aromatic ring,
except for excessively bulky ones.
FIGURE 7 DSC curves of the polymers obtained by RAFT
polymerization of silyl-protected hydroxyl styrenes. Polymer
[Mn(SEC), Mw/Mn]: poly(1) (9900; 1.10), poly(2) (8900; 1.07), poly
(3) (12,600; 1.58), poly(4) (19,500; 1.13), poly(5) (11,300; 1.06),
poly(6) (21,700; 1.08), poly(7) (11,600; 1.86), poly(8) (10,300;
1.06), poly(9) (6400; 1.88). [Color figure can be viewed at
wileyonlinelibrary.com]
protected 4VS (8), which possesses two less bulky methoxy
groups at the two m-positions, resulted in polymers with very
narrow MWDs (Mw/Mn ~ 1.1) and controlled molecular
weights that increased in direct proportion to monomer con-
version. In contrast, the three-trialkylsilyl-protected 4VGa (9),
which has two bulkier TES groups at the two m-positions,
resulted in polymers with broader MWDs, indicating that two
bulkier substituents at both meta-positions prevent efficient
reversible chain transfer during the RAFT process.
Deprotection of Silyl Groups into Hydroxy-Functionalized
Bio-Based Polystyrenes
Deprotection of the silyl groups was then conducted using
hydrochloric acid or TBAF under mild conditions. The tri-
alkylsilyl groups in poly(1), poly(2), poly(4), poly(5), and poly
(8) were quantitatively deprotected by hydrochloric acid in
THF at room temperature to afford poly(4VP), poly(3VP), poly
(4VG), poly(4VC), and poly(4VS), respectively (Fig. 6), which
all possess intact RAFT ω-chain ends even after deprotection
due to the mild conditions. In contrast, for poly(3), the o-
silyloxy group was hardly deprotected by hydrochloric acid
under the same conditions due to steric hindrance close to the
main chain. However, the deprotection of poly(3) was suc-
cessfully achieved with TBAF in THF at 40ꢀC to result in poly
(2VP), although the RAFT ω-chain ends were not observed
after deprotection. SEC curves of the deprotected polymers
were similarly narrow (Fig. S4). In addition, the deprotection
of poly(9) into poly(4VGa), which has high antioxidant and
adsorption properties,32 was investigated using hydrochloric
acid or TBAF, but complete deprotection was difficult. The
appropriate deprotection method is still under investigation.
These results show that a series of mono-, di-, and tri-
substituted hydroxystyrenes protected with trialkylsilyl groups
were all polymerized in a controlled fashion by RAFT polymeri-
zation using CDB as the RAFT agent. However, bulkiness, partic-
ularly near the propagating radical species, not only retarded
the polymerization but also induced broadening of MWDs.
Figure 5 shows a series of 1H NMR spectra of poly(1), poly(2),
poly(3), poly(4), poly(5), and poly(8) obtained by RAFT poly-
merization using CDB. In all the spectra, small peaks attributed
to phenyl protons (β, x, y, and z) of the RAFT groups at both
the α- and ω-chain ends were observed in addition to large
peaks of the repeating monomer units. Mn values (Mn[NMR])
measured by the peak intensity ratio of aromatic protons (c) of
8
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FIGURE
9 UV–vis spectra of mixtures of AgNO3 and hydroxy-functionalized polystyrenes: [monomer unit]0 = 95 mM,
[AgNO3]0 = 0.095 mM, in THF at 25ꢀC. poly(4VP) (Mn = 10,400) (a), poly(4VG) (Mn = 14,200) (b), poly(4VC) (Mn = 13,500) (c), and poly
(4VS) (Mn = 17,600) (d). [Color figure can be viewed at wileyonlinelibrary.com]
similar result was reported for a series of poly(acetoxy-
These results indicate that a series of well-defined bio-based
substituted styrene)s with different substituted positions.33
hydroxy-functionalized polystyrenes with different numbers
of substituents at different positions can be obtained by RAFT
polymerization of silyl-protected hydroxystyrenes followed by
the deprotection of the silyl groups under mild conditions.
gen bonding between the phenolic hydroxy groups except for
After deprotection of the silyl groups, the Tg values of the
resulting poly(hydroxystyrene)s increased because of hydro-
poly(2VP) with an o-hydroxy substituent (Fig. 8). Among
them, poly(4VC) showed the highest Tg value (190ꢀC). The Tg
values of poly(4VG) and poly(4VS) were 116 and 129ꢀC,
Thermal Properties
The thermal properties of the obtained polymers were evalu-
ated using differential scanning calorimetry (DSC).
respectively. These values were lower than that of poly(4VP)
(174ꢀC) without additional substituents, most likely because
the m-methoxy groups adjacent to the p-hydroxy group pre-
vent hydrogen bonding interactions between the hydroxy
groups.34,35 A similar effect of methoxy substituents on Tgs
has been reported for bio-based epoxy resins.36
Almost all the silyl-protected polymers except for poly(7) and
poly(9) showed distinct glass transition temperatures (Tg),
varying widely from 20 to 145ꢀC and depending on the sub-
stituents (Fig. 7). Poly(1), poly(2), and poly(3), which have
the same TBDMS group but at different positions, showed sig-
nificantly different Tgs depending on the substituted positions.
The Tg of poly(1) with the p-substitution was 107ꢀC, while
poly(2) with the m-substitution showed the lowest Tg (54ꢀC),
and poly(3) with the o-substitution had the highest Tg value
(145ꢀC). These results suggest that the molecular motion of
the main chain is significantly affected by the positions of
TBDMS groups and that substitution nearest the main chain
heavily prevents the motion to result in the highest Tg. A
Reducing Properties
The reducing abilities of the obtained phenolic polymers
were evaluated by mixing the polymers with silver ions
(Ag+) because catechol groups can reduce Ag ions to form
Ag nanoparticles.37 When the solution of AgNO3 was
dropped into poly(4VC) solution in THF, the color of the
solution quickly changed to yellow. In the UV–vis spectrum,
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9 R. Vanholme, K. Morreel, J. Ralph, W. Boerjan, Curr. Opin.
Plant Biol. 2008, 11, 278.
a peak at 400 nm, which can be attributed to the plasmon
resonance of Ag nanoparticles, appeared and increased in
intensity (Fig. 9). This result indicates that Ag ions were
reduced by the catechol groups to produce Ag nanoparticles.
For other phenolic polymers, such as poly(4VG) and poly
(4VS), a similar peak at approximately 400 nm was observed,
but the intensity was lower than that for poly(4VC). Thus,
poly(4VC) has stronger reducing properties than the other
phenolic polymers.
10 M. N. Clifford, J. Sci. Food Agric. 2000, 80, 1033.
11 E. Kim, Y. Liu, W. T. Leverage, J.-J. Yin, I. M. White,
G. F. Payne, Biomacromolecules 2014, 15, 1653.
12 H. Watanabe, A. Fujimoto, A. Takahara, J. Polym. Sci. Part A:
Polym. Chem. 2013, 51, 3688.
13 J. H. Waite, M. L. Tanzer, Science 1981, 212, 1038.
14 H. Lee, S. M. Dellatore, W. M. Miller, P. B. Messersimith, Sci-
ence 2017, 318, 426.
15 J. Heo, T. Kang, S. G. Jang, D. S. Hwang, J. M. Spruell,
K. L. Killops, J. H. Waite, C. J. Hawker, J. Am. Chem. Soc. 2012,
134, 20139.
CONCLUSIONS
In conclusion, a series of bio-based hydroxystyrenes were pre-
pared in high yield via simple decarboxylation of various natu-
rally occurring cinnamic acids, such as o-, m-, and p-coumaric;
caffeic; ferulic; and sinapinic acids. All the phenolic groups
were easily protected with trialkylsilyl groups in high yield.
RAFT polymerization of these silyl-protected styrenes
afforded the polymers with controlled molecular weights. The
silyl groups were easily deprotected to result in an array of
bio-based functionalized polystyrenes with different numbers
of hydroxy groups at different positions, which affect the ther-
mal and reducing properties of the polymers. Since the syn-
thesis of various hydroxy-functionalized polystyrenes from
petroleum-derived compounds is difficult, the utilization of
naturally occurring cinnamic acids is attractive and straight-
forward for functional polymers with various phenolic groups.
16 N. Patil, C. Jérôme, C. Detrembleur, Prog. Polym. Sci. 2018,
82, 34.
17 Y. Terao, K. Satoh, M. Kamigaito, Biomacromolecules 2019,
20, 192.
18 C. S. Marvel, G. H. McCain, J. Am. Chem. Soc. 1953, 75, 3272.
19 F. R. Mayo, C. Walling, Chem. Rev. 1950, 46, 191.
20 A. C. Fonseca, M. S. Lima, A. F. Sousa, A. J. Silvestre,
J. F. J. Coelho, A. C. Serra, Polym. Chem. 2019, 10, 1696.
21 T. Kaneko, T. H. Hi, D. J. Shi, M. Akashi, Nat. Mater. 2006,
5, 966.
22 T. Kaneko, D. Kaneko, S. Wang, Plant Biotechnol. 2010,
27, 243.
23 S. Wang, S. Tateyama, D. Kaneko, S. Ohki, T. Kaneko, Polym.
Degrad. Stab. 2011, 96, 2048.
24 H. Takeshima, K. Satoh, M. Kamigaito, Macromolecules 2017,
50, 4206.
ACKNOWLEDGMENTS
25 H. Takeshima, K. Satoh, M. Kamigaito, ACS Sustain. Chem.
Eng. 2018, 6, 13681.
This work was supported by JSPS Research Fellowships for
Young Scientists for H. Takeshima (no. 18J152S1), the JSPS
Core-to-Core Program B, the Asia-Africa Science Programs, the
JST-Mirai Program “Development for Innovative Adhesion
Technologies for Realizing Society 5.0,” and the Program for
Leading Graduate Schools “Integrative Graduate School and
Research Program in Green Natural Sciences.”
26 H. Takeshima, K. Satoh, M. Kamigaito, Polym. Chem. 2019,
10, 1192.
27 G. Moad, D. H. Solomon, The Chemistry of Radical Polymeri-
zation, 2nd ed.; Elsevier Science: Oxford, UK, 2012.
28 G. Moad, J. Chiefari, Y. K. Chong, J. Krstina,
R. T. A. Mayadunne, A. Postma, E. Rizzardo, S. H. Thang, Polym.
Int. 2000, 49, 993.
REFERENCES AND NOTES
29 S. H. Thang, Y. K. Chong, R. T. A. Mayadunne, G. Moad,
E. Rizzardo, Tetrahedron Lett. 1999, 40, 2435.
1 C. Tang, C. Y. Ryu, Eds., Sustainable Polymers Form Biomass;
Weinheim, Germany: Wiley-VCH, 2017.
30 S. Cadot, N. Rameau, S. Mangematin, C. Pinel, L. Djakovitch,
Green Chem. 2014, 16, 3089.
2 Y. Zhu, C. Romain, C. K. Williams, Nature 2016, 540, 354.
31 K. Ishitake, K. Satoh, M. Kamigaito, Y. Okamoto, Angew.
Chem. Int. Ed. 2009, 48, 1991.
3 M. R. Thomsett, T. E. Storr, O. R. Monaghan, R. A. Stockman,
S. M. Howdle, Green Mater. 2016, 4, 115.
32 K. Zhan, H. Ejima, N. Yoshie, ACS Sustain. Chem. Eng. 2016,
4, 3857.
4 V. Froidevaux, C. Negrell, S. Caillol, J.-P. Pascault, B. Boutevin,
Chem. Rev. 2016, 116, 14181.
33 K. Nakamura, T. Hatakeyama, H. Hatakeyama, Polym. J.
1983, 15, 361.
5 D. K. Schneiderman, M. A. Hillmyer, Macromolecules 2017, 50,
3733.
34 T. Hatakeyama, K. Nakamura, H. Hatakeyama, Polymer 1978,
19, 593.
6 H. T. H. Nguyen, P. Qi, M. Rostagno, A. Feteha, S. A. Miller,
J. Mater. Chem. A 2018, 6, 9298.
35 K. Nakamura, T. Hatakeyama, H. Hatakeyama, Polym. J.
1986, 18, 219.
7 A. H. E. Müller, K. Matyjaszewski, Eds., Controlled and Living
Polymerization: From Mechanisms to Materials; Weinheim, Ger-
many: Wiley-VCH, 2009.
36 E. D. Hernandez, A. W. Bassett, J. M. Sadler, J. J. La
Scala, J. F. Stanzione III., ACS Sustain. Chem. Eng. 2016, 4,
4328.
8 J. Ralph, K. Lundquist, G. Brunow, F. L. H. Kim, P. F. Schatz,
J. M. Marita, R. D. Hatfield, S. A. Ralph, J. H. Christensen,
W. Boerjan, Phytochem. Rev. 2004, 3, 29.
37 Y. Saito, H. Yabu, Chem. Commun. 2015, 51, 3743.
10
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