Synthesis of β-O-4-type artificial lignin polymers and their analysis by NMR spectroscopy

Article abstract of DOI:10.1039/b805460f

We describe the synthesis and NMR spectroscopic analysis of three artificial lignin polymers containing only the β-O-4 substructure: syringyl-type homopolymer, p-hydroxyphenyl-type homopolymer and guaiacyl/syringyl-type heteropolymer. Using gel permeation chromatography, the weight-average degree of polymerization (DP_w) of the three polymers was determined as 19.2, 38.6, and 13.9, respectively. The polymers were prepared based on the synthetic methodology of guaiacyl-type homopolymer, and were fully characterized using ¹H-, ¹³C-, and ¹H- ¹³C NMR spectroscopy of the acetylated and non-acetylated forms. The spectra of guaiacyl/syringyl-type heteropolymers were in good agreement with those of the β-O-4 substructure of milled wood lignin obtained from the hardwood of Japanese white birch. The Royal Society of Chemistry.

Full text of DOI:10.1039/b805460f

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis of b-O-4-type artificial lignin polymers and their analysis
by NMR spectroscopy†
Takao Kishimoto,*‡ Yasumitsu Uraki and Makoto Ubukata
Received 1st April 2008, Accepted 30th May 2008
First published as an Advance Article on the web 25th June 2008
DOI: 10.1039/b805460f
We describe the synthesis and NMR spectroscopic analysis of three artificial lignin polymers containing
only the b-O-4 substructure: syringyl-type homopolymer, p-hydroxyphenyl-type homopolymer and
guaiacyl/syringyl-type heteropolymer. Using gel permeation chromatography, the weight-average
degree of polymerization (DPw) of the three polymers was determined as 19.2, 38.6, and 13.9,
respectively. The polymers were prepared based on the synthetic methodology of guaiacyl-type
homopolymer, and were fully characterized using 1H-, 13C-, and 1H–13C NMR spectroscopy of the
acetylated and non-acetylated forms. The spectra of guaiacyl/syringyl-type heteropolymers were in
good agreement with those of the b-O-4 substructure of milled wood lignin obtained from the
hardwood of Japanese white birch.
pulse sequences are often used for qualitative, and recently,
Introduction
quantitative analysis of lignins.5–10 Typically, the chemical shifts
are assigned by comparison with those obtained from lignin model
dimers. Despite the numerous and readily available NMR spectral
data for lignin model compounds,11 1H- and 13C-NMR spectral
data for structures consisting of b-O-4 substructures12–14 remain
limited. In some cases, the chemical shifts of model compounds,
especially those of aromatic carbons, differ significantly from those
of isolated lignins. In some cases, the p-methoxyphenyl structure,
such as veratrylglycerol-b-guaiacyl ether, is inadequate for non-
phenolic b-O-4 structures. In contrast, b-O-4 type artificial lignin
polymers that actually consist of non-phenolic structures should
provide more accurate NMR spectral data.
Herein, we describe the synthesis of three artificial lignin poly-
mers composed exclusively of the b-O-4 substructure: syringyl-
type homopolymer, p-hydroxyphenyl-type homopolymer, and
guaiacyl/syringyl-type heteropolymer. The polymers, both as
the acetylated and non-acetylated forms, were fully analyzed
using modern NMR techniques. The guaiacyl-type homopolymer,
which was previously reported,2 was re-analyzed in order to
obtain additional NMR spectral data. The NMR spectra of
the guaiacyl/syringyl-type heteropolymer, which was synthesized
as an artificial hardwood lignin, were in good agreement with
those of the b-O-4 substructure of milled wood lignin (MWL)
from the hardwood of Japanese white birch (Betula platyphylla
var. japonica Hara). We believe that the NMR spectral data of
our artificial lignin polymers could prove to be useful for the
NMR spectroscopic analysis of isolated lignins from softwoods,
hardwoods, and herbaceous plants. Furthermore, the polymers
could serve as appropriate polymeric lignin model compounds
that possess both phenolic and non-phenolic structures.
Lignin, a major cell wall component in woody plants, is the
second most abundant biopolymer on earth after cellulose.
The complex structure of lignin arises from three precursors –
whereas the main constituent of softwood lignin is a guaiacyl
(G) unit, hardwood lignin consists mainly of guaiacyl and syringyl
(S) units. In addition to these units, herbaceous plants contain p-
hydroxyphenyl (H) units.
To investigate the structure and property of this complex
biopolymer, various lignin model compounds have been syn-
thesized to date. We have synthesized artificial lignin polymers
composed exclusively of a b-O-4 substructure,1,2 which are linear
with two- or three-carbon side chains and can be used as polymeric
lignin models with well-defined structures. Such polymers have
been found to be useful for the identification of characteristic ion
peaks induced by time-of-flight secondary ion mass spectroscopic
(ToF-SIMS) analysis of lignin.3 A guaiacyl-type b-O-4 artificial
lignin polymer with a three-carbon side chain was successfully
applied for the identification of degradation products of pyrolysis-
GC (gas chromatography) analysis of lignin.4 As lignin models, our
guaiacyl-type artificial lignin polymers have been found to be more
appropriate thanthecommon lignin modeldimer, guaiacylglycerol
guaiacyl ether.4
Modern 1H–13C-correlation 2-D NMR spectroscopic tech-
niques such as heteronuclear multiple quantum coherence
(HMQC) and heteronuclear single quantum coherence (HSQC)
Laboratory of Wood Chemistry, Research Group of Bioorganic Chemistry,
Division of Applied Bioscience, Research Faculty of Agriculture, Hokkaido
University, Sapporo, 060-8589, Japan. E-mail: takao@pu-toyama.ac.jp;
Fax: +81-766-56-2498; Tel: +81-766-56-7500 (ext. 567)
† Electronic supplementary information (ESI) available: NMR spectral
data for acetylated and non-acetylated forms of polymers 2a–2c; HMQC
spectra of polymers 2b–2d; a typical synthetic procedure for monomers.
See DOI: 10.1039/b805460f
‡ Present address: Bioorganic Chemistry Laboratory, Department of
Biotechnology, Faculty of Engineering, Toyama Prefectural University,
Imizu 939-0398, Japan. Fax +81-766-56-2498 Tel +81-766-56-7500 (ext.
567) E-mail: takao@pu-toyama.ac.jp
Results and discussion
Synthesis of artificial lignin polymers
The synthesis of b-O-4 type artificial lignin polymers was
based on the synthetic methodology of the guaiacyl-type (G)
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Scheme 1 Synthesis of b-O-4 type artificial lignin polymers.
homopolymer (polymer 2a),2 which involves the polyconden-
sation of a brominated monomer and the subsequent reduc-
tion of the carbonyl polymer, as shown in Scheme 1. The
three monomers, ethyl 2-bromo-3-(4-hydroxy-3-methoxyphenyl)-
3-oxopropanoate (monomer a), ethyl 2-bromo-3-(4-hydroxy-3,5-
dimethoxyphenyl)-3-oxopropanoate (monomer b), and ethyl 2-
bromo-3-(4-hydroxyphenyl)-3-oxopropanoate (monomer c), were
synthesized from the corresponding 4-hydroxyacetophenone
derivatives, instead of the benzaldehyde derivative which was
used in previous investigations.2 4-Hydroxyacetophenone deriva-
tives were benzylated with benzyl chloride, coupled with ethyl
carbonate15,16 in the presence of NaH, debenzylated with Pd/C,
and brominated with bromine. A typical synthetic procedure for
monomers is available as ESI†, and details of the discussion was
reported elsewhere.17
Table 1 GPC analysis of acetylated b-O-4 type artificial lignin polymers
2a–2d
Polymer
Yielda (%)
Mn
Mw
DPn
DPw
2ab (G)
2b (S)
33
24
64
17
3390
3690
3700
2550
5460
5950
9650
4120
12.1
11.9
14.8
8.6
19.5
19.2
38.6
13.9
2c (H)
2d (G/S)
a Overall yields of polymers 2a–2d from monomers a–d. b Data taken from
ref. 2.
2d exhibited a low DP – a detailed explanation, however, would
require further investigations.
NMR spectroscopic analysis of artificial lignin polymers
Syringyl-type (S) homopolymer (polymer 2b) and p-
hydroxyphenyl-type (H) homopolymer (polymer 2c) were synthe-
sized from monomers b and c, respectively. Guaiacyl/syringyl-
type (G/S) heteropolymer (polymer 2d) was synthesized from a 1 :
1 (mol/mol) mixture of monomers a and b. Polymerization was
carried out using potassium carbonate in anhydrous DMF at rt
for 24 h to afford polymers 1b–1d with yields of 88%, 97%, and
90%, respectively. In contrast to the b-O-4 type polymers with two-
carbon side chains,1 precipitation of the resulting polymer was not
observed during polymerization in the cases of polymers 1b–1d
with three-carbon side chains. Because of incomplete reduction
of the carbonyl group of polymers 1b–1d at rt in some cases, the
reduction was carried out at 50 ◦C, in which polymers 1b–1d were
suspended in methanol, then treated with sodium borohydride.
The crude products were purified by precipitation in ether, in
order to remove the low molecular weight compounds, to afford
polymers 2b–2d with yields of 27%, 66%, and 18%, respectively.
The low yield of polymer 2d is attributable to its low molecular
weight, as described below.
Selected 1H- and 13C-NMR spectral data for the acetylated
and non-acetylated forms of polymers 2a–2c are summarized
in Tables 2–5 (full spectral data are available as ESI†). The
signals were assigned by HMQC and heteronuclear multiple
bond coherence (HMBC) NMR spectra, and compared against
those of dimer lignin model compounds, if available. In some
cases, the erythro- and threo-forms were distinguishable and were
read separately. Correlations between Hb and C4, as shown by
the HMBC spectra of polymers 2a–2d, indicated the successful
formation of b-O-4 linkages between the monomer units (data
not shown). However, several minor signals could not be assigned
for polymers 2a–2c in Fig. 1. Some of them might be assigned
to end units, but it is likely that some unexpected side-reactions
took place at the non-phenolic end units, when polymers 1a–1d
were treated with NaBH4. Further investigations are necessary to
clarify this point.
The 13C-NMR signals at 60.1, 71.5, and 86.7 ppm in syringyl-
type polymer 2b were assigned to the c-, a-, and b-carbons,
respectively in Fig. 1. The c- and a-carbons (60.1 and 71.5 ppm)
are comparable to those of the threo-form of syringylglycerol b-
syringyl ether (60.2 and 71.6 ppm); the b-carbon (86.7 ppm) is also
observed at a similar position (87.2 ppm).11 The signals at 59.6,
72.2, and 86.0 ppm in polymer 2b were assigned as the c-, a-, and
b-carbons, respectively, of the erythro-form. In the case of polymer
2b, the erythro- and threo-forms exist in nearly equal amounts.
For p-hydroxyphenyl type polymer 2c, the signals at 59.8, 71.2,
and 83.1 ppm were assigned to the c-, a-, and b-carbons of
the erythro-form, respectively. The small resonance at 70.9 ppm
was assigned to the a-carbon of the threo-form. In the case of
polymer 2c, the erythro-form is predominant. The difference in
the chemical shifts of the side chain carbons between the erythro-
and threo-forms of polymer 2c is smaller than that of polymer
GPC analysis of artificial lignin polymers
The number-average and weight-average molecular weights of the
acetylated form of the artificial lignin polymers were determined
using gel permeation chromatography (GPC). Acetylation was
performed using acetic anhydride and pyridine at rt. The degree
of polymerization (DP) was calculated from the average molecular
weight. The results, along with those of polymer 2a, are summa-
rized in Table 1. The weight-average degree of polymerization
(DPw) of polymer 2b was 19.2, which is quite similar to that
of polymer 2a. Polymer 2c exhibited the highest DPw, which
may be attributed to the accessibility of monomer c due to the
absence of methoxy groups. The guaiacyl/syringyl heteropolymer
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Table 2 Selected 1H-NMR spectral data for b-O-4 type artificial lignin polymers in DMSO-d6 (ppm)
Proton assignment
Polymer
a
b
c
1
2
3
4
5
6
2aa (G, erythro)
2a (G, threo)
2b (S, erythro)
2b (S, threo)
2c (H, erythro)
2c (H, threo)
4.75
4.28
3.47–3.63
3.20, 3.47–3.63
3.39, 3.73
3.23, 3.65
3.50–3.66
—
—
—
—
—
—
7.02
—
—
—
—
6.85
6.91
—
—
—
—
—
—
6.94
6.85
(6.85)b
6.70
(4.75)b
4.87
(4.28)b
4.12
(7.02)b
6.70
(6.94)b
—
4.90
4.02
6.78
—
6.85
6.91
6.78
4.73
4.29
7.25
7.25
(4.73)b
(4.29)b
3.21, 3.50–3.66
(7.25)b
(7.25)b
a Data taken from ref. 2. b Chemical shifts in the threo-forms could not be read separately because of low intensities. The chemical shifts may or may not
be the same as the erythro-forms.
Table 3 Selected 1H-NMR spectral data for acetylated b-O-4 type artificial lignin polymers in CDCl3 (ppm)
Proton assignment
Polymer
a
b
c
1
2
3
4
5
6
2aa (G, erythro)
2a (G, threo)
2b (S, erythro)
2b (S, threo)
2c (H, erythro)
2c (H, threo)
6.00
6.05
6.00
6.08
5.98
6.01
4.67
4.61
4.59
4.14, 4.37
3.95, 4.24
4.17, 4.42
3.85, 4.31
4.14, 4.31
3.94, 4.20
—
—
—
—
—
—
7.03
—
—
—
—
6.91
6.98
—
—
—
—
—
—
6.85
6.90
(7.03)b
6.61
(6.85)b
—
(6.90)b
6.61
4.52
6.61
7.31
7.34
—
6.91
6.98
6.61
7.31
7.34
4.73
(4.73)b
a Data taken from ref. 2. b Chemical shifts in the threo-forms could not be read separately because of low intensities. The chemical shifts may or may not
be the same as the erythro-forms.
Table 4 Selected 13C-NMR spectral data for b-O-4 type artificial lignin polymers in DMSO-d6 (ppm)
Carbon assignment
Polymer
a
b
c
1
2
3
4
5
6
2aa (G, erythro)
2a (G, threo)
2b (S, erythro)
2b (S, threo)
2c (H, erythro)
2c (H, threo)
71.4
70.8
72.2
71.5
71.2
70.9
83.6
84.4
86.0
86.7
83.1
83.1
59.8
59.9
59.6
60.1
59.8
59.9
135.0
134.7
138.0, 138.1
137.8
134.3
111.7
111.3
104.3
104.3
127.9
127.7
148.9
148.9
152.1
152.0
115.5
115.4
146.7
147.0
115.0
114.5
152.1
152.0
115.5
115.4
119.1
118.7
104.3
104.3
127.9
127.7
134.2, 134.3b
134.7, 134.8b
157.7
134.1
158.0
a Data taken from ref. 2. b The erythro- and threo-forms were tentatively assigned.
Table 5 Selected 13C-NMR spectral data for acetylated b-O-4 type artificial lignin polymers in CDCl3 (ppm)
Carbon assignment
Polymer
a
b
c
1
2
3
4
5
6
2aa (G, erythro)
2a (G, threo)
2b (S, erythro)
2b (S, threo)
2c (H, erythro)
2c (H, threo)
73.9
74.7
74.6, 74.7
76.2
73.8
79.7
80.1
80.7
80.7
78.1
78.1
62.4
63.1
62.8
63.8
62.3
62.8
131.3
131.0
133.2
132.8
129.5
129.4
111.9
111.5
104.5
104.5
128.9
128.9
150.6
147.0
118.3
119.7
119.4
104.5
104.5
128.9
128.9
150.3
(147.0)b
(118.3)b
152.8c
152.8c
135.0, 135.3c
136.4, 136.6, 136.8c
158.2
153.1, 153.2c
116.6
153.1, 153.2c
116.6
116.4
74.1
116.4
158.6
a Data taken from ref. 2. b Chemical shifts in the threo-forms could not be read separately because of low intensities. The chemical shifts may or may not
be the same as the erythro-forms. c The erythro- and threo-forms were tentatively assigned.
2b. As shown in Tables 3 and 5, because the chemical shifts of
the b-protons and carbons of polymer 2c significantly differ from
those of polymers 2a and 2b in the acetate forms, the presence
of the p-hydroxyphenylglycerol-b-phenyl ether type substructure
in herbaceous plants can be readily detected using 2-D NMR
spectroscopy.
The 13C-NMR spectra of polymer 2d and milled wood lignin
from Japanese white birch in DMSO-d6 are shown in Fig. 2.
The 13C-NMR signals of polymer 2d are basically the sum of
polymers 2a and 2b with some exceptions. The side chain signals
at 83.3, and 87.1 ppm do not exist in polymers 2a nor 2b. The
signals at 83.3 and 87.1 ppm were assigned to the b-carbons of the
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1
As shown in the H-NMR spectra of the acetylated polymers
2a–2d in CDCl3 (Fig. 3), distinct NMR signals were observed for
the side chain of each polymer. Syringyl aromatic protons were
readily distinguishable from other aromatic protons in the b-O-4
structures. Although we expected that, for polymer 2d, the bulky
syringyl monomer b would be more difficult to polymerize than
the guaiacyl monomer a, the syringyl/guaiacyl ratio of 49 : 51,
which was calculated using the peak areas of syringyl (C2-H and
C6-H, 6.57–6.70 ppm) and guaiacyl aromatic protons (C2-H, C5-
H and C6-H, 6.78–7.08 ppm), indicated that monomers a and b
were incorporated in nearly equal amounts.
Conclusions
Three artificial lignin polymers containing only the b-O-4 sub-
structure were successfully synthesized and fully analyzed using
NMR spectroscopy. The resulting NMR spectral data could ben-
efit NMR spectroscopic analyses of isolated lignins; specifically,
the spectra of guaiacyl/syringyl-type heteropolymer 2d were in
good agreement with those of the b-O-4 substructure in milled
wood lignin from Japanese white birch. Our artificial lignin
polymers would serve as appropriate polymeric lignin models
in the investigations of reaction mechanisms during the various
processes, such as pulping, bleaching, and biodegradation.
Experimental
Measurements
1H- and 13C-NMR spectra were recorded using a JEOL JNM
EX-270 FT-NMR (270 MHz), a Bruker AMX500 FT-NMR
(500 MHz), or a JEOL FT-NMR ECA600 (600 MHz) spec-
trometer. HMQC and HMBC NMR spectra were recorded
using a Bruker AMX500 FT-NMR (500 MHz) or a JEOL FT-
NMR ECA600 (600 MHz) spectrometer. Chemical shifts (d) are
reported in ppm. The 13C-NMR spectra were processed using
exponential multiplication with 3 Hz line broadening. The average
molecular weights of the acetyl derivatives of polymers 2a–2d
were determined using gel permeation chromatography (GPC)
in tetrahydrofuran using a HITACHI Liquid Chromatograph L-
6200, equipped with UV (L-4000 Hitachi, 280 nm) and RI (JASCO
RI-2031 Plus) detectors, and Shodex GPC columns (KF-803L
and 802, 30 cm × 8.0 mm, connected in series) (tetrahydrofuran,
0.5 mL min−1, 40 ◦C). The molecular weights were calibrated using
polystyrene standards.
Fig. 1 13C-NMR spectra of b-O-4 type artificial lignin polymers 2a–2c in
ꢀꢀ
ꢀꢀ
DMSO-d6. 1ꢀ, 3ꢀ, 4ꢀ, 5ꢀ: phenolic end units; 1ꢀꢀ, 2ꢀꢀ, 6ꢀꢀ, a , bꢀꢀ, c : non-phenolic
end units; x: solvents.
syringylglycerol-b-guaiacyl (SG) and guaiacylglycerol-b-syringyl
(GS) ether-type substructures, respectively.13 While MWL con-
tains, in addition to the b-O-4 substructure, various other sub-
structures, the 13C-NMR spectra of polymer 2d and birch MWL
in DMSO-d6 are strikingly similar. Although the signals that
correspond to the guaiacyl unit, especially those of the b-carbons,
are weaker for birch MWL than for polymer 2d, major carbon
signals in polymer 2d correspond to those of b-O-4 substructures
of the hardwood lignin.
Polymerization
In contrast to the significant differences between the chemical
shifts of some aromatic carbons of the dimer models and those
of native lignins, the chemical shifts of artificial lignin polymer 2d
are quite similar to those of the isolated birch MWL, as described
above. For example, a signal that corresponds to the guaiacyl
aromatic carbon at the 5-position of the erythro-form of polymer
2d appears comparably to that of the MWL (115.2 ppm). In
contrast, the signal of the erythro-form of the veratrylglycerol-
b-guaiacyl ether is observed at 111.6 ppm.14 These results indicate
that, in respect to NMR spectroscopic analyses, our artificial lignin
polymers can serve as lignin models that are more accurate than
the common lignin model dimers.
To a 25 mL flask containing monomer b (0.97 g, 2.79 mmol),
which was dried at rt overnight under vacuum, was added finely
powdered dry K2CO3 (0.58 g, 4.19 mmol) in anhydrous DMF
(5 mL). After 24 h under a nitrogen atmosphere at rt, the reaction
mixture was poured into ice-water (120 mL), then adjusted to
pH 2–3 using 2 M HCl to precipitate the polymer. The polymer
was isolated by filtration, washed with water, and dried over P2O5
under vacuum to give polymer 1b (650 mg, 88%).
Similarly, polymers 1c (693 mg, 97%) and 1d (724 mg, 90%) were
synthesized from monomer c (1.0 g, 3.48 mmol) and a mixture
of monomers a (510 mg, 1.6 mmol) and b (560 mg 1.6 mmol),
respectively.
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Fig. 2 13C-NMR spectra of b-O-4 type artificial lignin polymer 2d and milled wood lignin (MWL) from Japanese white birch in DMSO-d6.
Reduction of polymers 1b–1d
2e), 114.8 (G, 5t), 115.2 (G, 5e), 118.8 (G, 6t), 119.1 (G, 6e), 134.3
(S, 4e), 134.8 (S, 4t; G, 1), 135.0, 135.2, 135.3 (G, 1), 137.8 (S,
1t), 138.1 (S, 1e), 145.4 (G, 4ꢀ), 146.9 (G, 4e), 147.0 (G, 4t), 149.2
(G, 3), 152.1, 152.4 (S, 3, 5). 4ꢀ: phenolic end units; e: erythro; t:
threo.
To a stirred suspension of polymer 1b (602 mg) in CH3OH
(10 mL) was carefully added NaBH4 (683 mg) portionwise, while
maintaining the reaction temperature at 50 ◦C. The suspension
gradually turned into a clear solution. After 24 h, the reaction
mixture was treated with acetic acid to decompose excess NaBH4,
then poured into 0.5 M HCl (200 mL). The resulting precipitate
was isolated by centrifuge, washed with water, lyophilized, then
dissolved in 1,2-dichloroethane–ethanol (2 : 1, v/v, 5 mL) or 1,4-
dioxane (5 mL). The resulting solution was poured into diethyl
ether (100 mL) to remove low molecular weight compounds. The
resulting precipitate was isolated by filtration, then dried in vacuo
to give polymer 2b (158 mg, 31%).
Acetylation of polymers 2b–2d
Polymer 2b (65 mg) was acetylated using acetic anhydride and
pyridine (1 : 1, v/v, 4 mL) at rt. After allowing the reaction
to proceed for 24 h, it was diluted with ethyl acetate, washed
with brine, dried over anhydrous Na2SO4, and concentrated to
dryness in vacuo. The resulting polymers were used without further
purification. Polymers 2c and 2d were acetylated following similar
procedures.
Polymer 2d (acetate): 1H-NMR (CDCl3): d 1.91, 1.94, 2.01, 2.06,
2.07, 2.08, 2.13 (s, a,c-OCOCH3), 2.29, 2.32 (s, Ph-OCOCH3), 3.75,
3.79 (s, OCH3), 3.86 (S, Cc-Ht), 4.17 (G, S, Cc-He), 4.21 (G, Cc-
Ht), 4.29 (S, Cc-Ht), 4.37 (G, Cc-He), 4.42 (S, Cc-He), 4.54 (S,
Cb-Ht), 4.59 (G, Cb-Ht, S, Cb-He), 4.67 (G, Cb-He), 6.00 (G,
S, Ca-He), 6.08 (G, S, Ca-Ht), 6.60, 6.67 (S, 2, 6), 6.80–7.10 (G,
aromatics); 13C-NMR (CDCl3): 20.8, 21.0 (a,c-OCOCH3), 55.9,
56.1 (OCH3), 62.5 (G, ce), 62.8 (S, ce), 63.2 (G, ct), 63.9 (S, ct),
74.0 (G, ae), 74.6, 74.7 (G, at, S, ae), 74.4, 76.0 (a), 76.3 (S, at), 79.8
(G, be), 80.2 (G, bt), 80.7 (S, b), 104.4, 104.8 (S, 2, 6), 111.7 (G,
2t), 112.1 (G, 2e), 118.4 (G, 5), 119.9 (G, 6), 131.2 (G, 1t), 131.5
(G, 1e), 132.0, 132.2, 132.4 (GS or SG, 1), 132.7 (S, 1t), 133.2 (S,
Similarly, polymers 2c (336 mg, 66%) and 2d (114.8 mg, 18%)
were synthesized from polymers 1c (630 mg) and 1d (625 mg),
respectively.
Polymer 2d. 1H-NMR (DMSO-d6): 3.21 (Cc-Ht), 3.36, 3.48–
3.84 (Cc-H), 3.71, 3.73, 3.75 (OCH3), 4.01 (S, Cb-Ht), 4.10 (S,
Cb-He), 4.27 (G, Cb-H), 4.77 (G, Ca-H), 4.84 (GS or SG, Ca-H),
4.89 (S, Ca-H), 6.70 (S, C2-He, C6-He), 6.75 (S, C2-H, C6-H), 6.79
(S, C2-Ht, C6-Ht), 6.87 (G, C6-H), 6.94 (G, C5-H), 7.03 (G, C2-
H);13C-NMR (DMSO-d6): d 55.6, 55.9 (OCH3), 59.6, 59.9, 60.0
(c), 70.8 (G, at), 71.5 (S, at; G, ae), 71.8, 71.9 (SG or GS, a), 72.2
(S, ae), 83.3 (SG, b), 83.7 (G, be), 84.4 (G, bt), 86.1 (S, be), 86.8
(S, bt), 87.1 (GS, b), 104.3, 104.7 (S, 2, 6), 111.4 (G, 2t), 111.7 (G,
2986 | Org. Biomol. Chem., 2008, 6, 2982–2987
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Preparation of milled wood lignin (MWL)
Milled wood lignin (MWL) was prepared from wood meal
(<40 mesh) from Japanese white birch according to the standard
procedure with dry-milling under nitrogen for 2 d, and purification
by the method of Lundquist.18 The Klason lignin content of the
resulting MWL was 82.8%. Sugar content, which was measured
by the standard alditol acetate method using gas chromatography,
was 4.6%.
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1e), 135.1, 135.3, 136.4, 136.7, 136.8 (S, 4), 147.0, 147.3 (G, 4),
150.6, 150.8 (G, 3), 152.8, 153.1 (S, 3, 5), 168.8 (Ph-OCOCH3),
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