Reversible crosslinking of lignin via the furan-maleimide Diels-Alder reaction

Article abstract of DOI:10.1039/c5gc01319d

Two distinct functionalization schemes for Kraft lignin (KL) were developed to selectively incorporate furan and/or maleimide motifs as chain ends. The incorporation of furan functionalities was carried out by the selective and quantitative reaction of the lignin's phenolic OH groups with furfuryl glycidyl ether (FGE). Maleimide groups were introduced by esterifying the lignin's aliphatic and phenolic OH groups with 6 maleimidohexanoic acid (6-MHA), offering a high loading despite a somewhat incomplete conversion. Furan- and maleimide-functionalized lignins were subsequently combined to generate crosslinking via the Diels-Alder (DA) [4 + 2] cycloaddition reaction. The formation of the DA adduct was confirmed by ¹H NMR. Under appropriate conditions, the formation of a gel was apparent, which turned back into the liquid state after performing the corresponding retro-DA reaction upon heating to 120 °C. This study reveals the significant versatility and potential of the developed strategy for the utilization of lignin-based recyclable networks.

Full text of DOI:10.1039/c5gc01319d

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Reversible crosslinking of lignin via the furan-maleimide Diels-Alder reaction
a,b
b
a,*
b,*
Antoine Duval, Heiko Lange, Martin Lawoko, Claudia Crestini
a
Wallenberg Wood Science Center (WWSC), Department of Fiber and Polymer Technology, KTH
Royal Institute of Technology, SE-100 44 Stockholm, Sweden
b
Department of Chemical Sciences and Technologies, University of Rome ‘Tor Vergata’, Via della
Ricerca Scientifica 1, 00133 Rome, Italy
*
Corresponding authors:
lawoko@kth.se, Tel: +46 8 7908047; crestini@stc.uniroma2.it, Tel: +39 067259 4734
Abstract
Two distinct functionalization schemes for Kraft lignin (KL) were developed to selectively incorporate
furan and/or maleimide motifs as chain ends. The incorporation of furan functionalities was carried
out by the selective and quantitative reaction of the lignin’s phenolic OH groups with furfuryl glycidyl
ether (FGE). Maleimide groups were introduced by esterifying the lignin’s aliphatic and phenolic OH
groups with 6 maleimidohexanoic acid (6-MHA), offering a high loading despite a somewhat
incomplete conversion. Furan- and maleimide-functional lignins were subsequently combined to
generate crosslinking via the Diels-Alder (DA) [4 + 2] cycloaddition reaction. The formation of the DA
1
adduct was confirmed by H NMR. Under appropriate conditions, the formation of a gel was
apparent, which turned back into the liquid state after performing the corresponding retro-DA
reaction upon heating to 120 °C. This study displays the significant versatility and potential of the
developed strategy for the utilization of lignin-based recyclable networks.
1

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Introduction
Lignin is a widely abundant biopolymer, naturally synthesized in vascular plants by the
1
polymerization of phenylpropane units. In softwood, the lignin content can reach 30% by weight. It
is structurally a rather complex polyphenol, of large variety and diversity, originating from the fact
that different wood species and variable extraction processes impose different structural
characteristics in lignin.
2
Paper pulp processes, among which the Kraft process predominates, produce industrially large
-
1 3
amounts of lignin, estimated to about 70 Mt yr . Nevertheless, most of it is burnt in plants to
produce energy, thus limiting the amount of commercially available Kraft lignin (KL) to only
-
1 4
1
00 000 t yr . However, the interest for lignin in chemistry and polymer science has been fast
growing in the past few years, motivated by the need for alternatives to petroleum-based aromatic
compounds, and by the development of biorefinery processes expected to significantly increase the
5
,6
commercial availability. Progresses in controlled lignin depolymerization, involving the
7
-9
development of specific catalysts, could for example allow the production of phenol and
derivatives.
The direct use of lignin as a polymer is made difficult by its complex and variable macromolecular
structure. Despite huge efforts devoted towards its structural characterization during the last
decades, a clear idea of its molecular structure emerged only in the last years with the development
of advanced heteronuclear 2D NMR techniques. More specifically, the characterization and
quantitative evaluation of the different lignin interunit bondings, coupled with wet chemistry
methods, allowed recently to elucidate unprecedented structural details and to have a detailed map
1
0,11
of its different functionalities.
Furthermore, recent advances in the isolation of low polydispersity
1
2-14
15,16
lignin fractions, either by membrane filtration
enhance its potential use.
or solvent fractionation,
are contributing to
2

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As a multifunctional polymer displaying many aliphatic and phenolic OH groups, lignin actually offers
1
7
plenty of opportunities for functionalization. The control of the lignin multifunctional character
constitutes a key step to achieve upgrade and valorization, and remains today a challenge
contributing to the difficulty of developing lignin-based materials. The multifunctional character of
lignin has naturally been exploited to form polymer networks, in an attempt to produce bio-based
resins. The crosslinking of lignins with aldehydes, diacids or diisocyanates has for example been
1
7
18
reported, as well as radical polymerization. Lignin-based fibers have also been generated, e.g. by
1
9
electrospinning of lignins. Nevertheless, all these materials contain either chemical bonds that are
permanent, which implies for instance that the materials can’t be recycled or used in more
sophisticated applications that require, for example, reversible matrix-generation, or no real
chemical bonds at all, a fact that comes with other disadvantages. An important field of research in
2
0,21
polymer materials is thus concerned with the establishment of reversible crosslinking capabilities,
2
2
23-25
which opens a path toward recyclable networks and self-healing materials.
Material science
applications come into play, in which a temporarily switching from a liquid to a solid state is
desirable, i.e., solidifyable liquids, e.g., for use in technical devices that are in principal flexible, but
carrying reversible stiffness, and that are thus able to reach locations difficult to access with stiff
devices; applications range from stabilizing cavities in crashed houses in disaster settings to
stabilizing smaller cavities emerging during natural aging of materials, e.g., aging of wood. The
combination of a temporary crosslinking realized on a lignin-backbone allows the generation of novel
polymer-based adhesives and coatings exhibiting a double, i.e., a temporary and a permanent curing
system: such functional polymers be temporarily fixed for trial purposes or accuracy checks, before a
2
6,27
‘classical’, permanent curing system is activated.
The Diels-Alder (DA) [4+2] cycloaddition between furan and maleimide moieties, which respectively
act as diene and dienophile, represents, among others, an interesting tool to generate bonds in a
2
8
reversible fashion. It presents several advantages, such as the low temperatures required to
perform the forward and retro reactions, at 60-70 and 110-120°C, respectively, which are compatible
3

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with the thermal stability of most polymers. In addition, a wide range of furan monomers can be
prepared from furfural and hydroxymethylfurfural, respectively obtained from bio-based polymers
2
9
(C5 or C6 sugars in polysaccharides).
In this study, we aimed at creating a novel temperature responsive material combining the
multifunctional character of lignin with the attractive features of the furan-maleimide DA reaction.
To do so, KL was selectively modified at its phenolic chain ends with furfuryl glycidyl ether (FGE) or 6-
maleimidohexanoic acid (6-MHA), to expose pendant furan and maleimide groups, respectively. The
3
1
1
functionalizations were monitored by a set of spectroscopic ( P and H NMR, UV and FTIR) and
chromatographic (SEC) techniques. Finally, furan- and maleimide-grafted lignins were combined in
order to perform the DA forward reaction and generate lignin-based gels, which reversibly turned
back to the liquid state on performing the retro-DA reaction.
Experimental part
Materials
3
0
Softwood Kraft lignin (KL) was obtained from the Lignoboost process. Furfuryl glycidyl ether (FGE,
9
6%), 6-maleimidohexanoic acid (6-MHA, 90%) and oxalyl chloride were purchased from Sigma-
Aldrich and used as received. N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich) was pre-dried
over 4 Å molecular sieves, distilled under vacuum and stored over 4 Å molecular sieves.
Dichloromethane (DCM, puriss. p.a., Sigma-Aldrich) was refluxed over CaCl for 12h, distilled off and
2
stored over 4 Å molecular sieves.
Lignin functionalization with furan moieties
4

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KL (500 mg) was dissolved in water containing NaOH (100 mg, 2.5 mmol, corresponding to 1 eq. to
total acidic groups in KL, i.e. phenolic OH and COOH). After 1h of stirring, furfuryl glycidyl ether (FGE)
was added (from 0.25 to 1.5 eq. to KL phenolic OH) and the reaction mixture was stirred at 50 °C
overnight. After cooling to room temperature and acidifying to pH2 using 10% (v/v) aqueous HCl, the
suspension was centrifuged to recover the precipitated lignin. The functionalized lignin was then
washed 3 times with 50 mL acidified water (pH2) and subsequently freeze-dried.
Lignin functionalization with maleimide moieties
6
-maleimidohexanoic acid (6-MHA) was first converted into the corresponding acyl chloride via
reaction with oxalyl chloride: 6-MHA (0.5 to 2.5 eq. to KL phenolic OH) was dissolved in DCM
2.5 mL). Oxalyl chloride (1 eq. to 6-MHA) was added and the reaction was conducted at room
temperature under an inert atmosphere for 2 h. Gaseous by-products (HCl, CO and CO ) were
(
2
allowed to escape the reaction mixture, and the resulting acyl chloride was used without further
purification.
2 3
KL (500 mg) was dissolved in dry DMF containing pulverized K CO (347 mg, 2.5 mmol, corresponding
to 1 eq. to total acidic groups in KL, i.e. phenolic OH and COOH). After 1 h of stirring, the solution of
acyl chloride in DCM was added dropwise and the reaction mixture was stirred at 50 °C for 1 h, thus
allowing the DCM to evaporate.
After cooling to room temperature, 10 mL of water were added and the solution was acidified to pH
6
using 10% (v/v) aqueous HCl, thus allowing the functionalized lignin to precipitate. The suspension
was centrifuged to recover the precipitated lignin, and the functionalized lignin was washed 3 times
with 50 mL water (adjusted to pH6) before being freeze-dried.
3
1
P NMR
3
An accurately weighed amount of lignin (about 30 mg) was dissolved in 400 µL of anhydrous CDCl /
pyridine solution (1:1.6 v/v). 100 µL of a standard solution of cholesterol (0.1
5
3
M in anhydrous CDCl /

Green Chemistry
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pyridine solution) containing Cr(III) acetylacetonate as relaxation agent was then added. Finally,
00 µL of 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (Cl-TMDP, 95%, Sigma-Aldrich) were
1
added and the mixture was stirred at room temperature for 2 h. The mixture was then transferred
into 5 mm NMR tubes and the spectra were measured on a Bruker 300 MHz spectrophotometer (256
scans at 20 °C). All chemical shifts reported are relative to the reaction product of water with Cl-
TMDP, which gives a sharp signal in pyridine/CDCl at 132.2 ppm. Quantitative analysis was
3
2
9
performed based on previous literature reports.
1
H NMR
An accurately weighed amount of lignin (about 20 mg) was dissolved in 500 µL of DMSO-d
6
. 100 µL of
a standard solution of 2,3,4,5,6-pentafluorobenzaldehyde in DMSO-d were then added, and the
6
mixture was transferred into 5 mm NMR tubes. The spectra were acquired on a Bruker 300 MHz
spectrometer (64 scans at 20 °C).
FTIR
FTIR spectra were measured on a Perkin Elmer Spectrum 100 FTIR spectrometer. The spectra were
-1
acquired on KBr pellets as the average of 32 scans between 450 and 4000 cm with a resolution of
-
1
4
cm .
Phenolic content by UV spectroscopy
The phenolic content of the lignins was measured according to the previously reported UV method,
which is based on the difference of absorption of the phenolic moieties in neutral solution and the
3
2,33
corresponding phenolate anions in alkaline environments.
and 400 nm on a Shimadzu UV-1800 spectrophotometer.
Spectra were recorded between 200
Size-exclusion chromatography (SEC)
6

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Prior to SEC, the lignin samples were acetobrominated as reported elsewhere to ensure complete
3
4
solubilization in THF. Briefly, 5 mg lignin was suspended in 1 mL glacial acetic acid/acetyl bromide
(9:1 v/v) for 2 h. The solvent was then removed under reduced pressure, and the residue was
dissolved in HPLC-gradeTHF and filtered over 0.45 µm syringe filter. SEC analyses were performed
using a Shimadzu instrument consisting of a controller unit (CBM-20A), a pumping unit (LC 20AT), a
degasser (DGU-20A3), a column oven (CTO-20AC), a diode array detector (SPD-M20A), and a
refractive index detector (RID-10A. Three analytical GPC columns (each 7.5 x 30 mm) were connected
in series for analyses: Agilent PLgel 5 µm 10000 Å, followed by Agilent PLgel 5 µm 1000 Å and Agilent
-
1
PLgel 5 µm 500 Å. HPLC-grade THF (Chromasolv®, Sigma-Aldrich) was used as eluent (0.75 mL min ,
at 40 °C). Standard calibration was performed with polystyrene standards (Sigma Aldrich, MW range
-
1
1
62 – 5 x 106 g mol ). Final analyses of each sample was performed using the intensities of the UV
signal at λ = 280 nm, employing a tailor-made MS-Excel-based table calculation.
Diels-Alder (DA) reactions
The functionalized lignins were dissolved in DMSO at a concentration of 15% (w/w), and heated in an
oven at 70 °C without stirring. When gelation occurred, the samples were further heated to 120 °C in
an oil bath to perform the retro-DA reaction.
Results and Discussion
Lignin functionalization with furan moieties
KL was functionalized with furfuryl glycidyl ether (FGE) to expose furan rings at the phenolic chain
end, as depicted in Scheme 1. The reaction was carried out in water containing a stoichiometric
amount of NaOH with respect to the total acidic groups in lignin (i.e. both COOH and phenolic OH
groups). This way, only carboxyl and phenolic OH groups are deprotonated, whereas aliphatic OH
7

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groups remain unaffected because of their much higher pK . Hence, phenolate ions are the only
a
reactive specie, whereas acids are inactivated as carboxylates. Increasing amounts of FGE have been
tested, from 0 to 1.5 molar equivalents to lignin phenolic OH groups. The content in functional
3
1
1
groups of the derivatized lignins has been monitored using P NMR, H NMR and UV spectroscopy.
All results are listed in Table 1.
Scheme 1. Reaction of lignin with furfuryl glycidyl ether (FGE). (1) and (2) are the possible products.
Only the furfurylated lignin (1) is actually formed under the reaction conditions.
3
1
The content in phenolic OH groups measured by P NMR and UV spectroscopy are similar (Table 1
3
1
and Figure 1a). All the P NMR spectra are available as supplementary information (Figure S1). The
depletion of phenolic OH groups follows the stoichiometry of FGE used for the functionalization,
indicating a good selectivity. The reaction is indeed quantitative for 1.5 eq. of FGE, since no more
phenolic OH groups can be detected. As can be seen in Figure 1b, the conversion of C5-condensed
phenolic groups seems slightly harder to achieve, most likely because of steric hindrance. A close
look at the spectra reveals that the 5-5 condensed structures, giving rise to a signal in the region
3
5
1
40.2 – 141.7 ppm, are the most recalcitrant to the functionalization (Figure S1).
8

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3
1
1
Table 1. Quantification of the functional groups in the lignins derivatized with FGE by UV, P and H
-
1
NMR (mmol g )
3
1
[a]
[b]
by UV
1
[c]
by P NMR
by H NMR
Sample
name
FGE
eq.
-
Cond. Uncond.
Total
Ph-OH
4.61
3.02
2.01
1.06
0.67
0.40
0.32
0.19
Total Ph-
OH
Al-OH
COOH
Furan
Ph-OH
2.14
1.50
1.02
0.62
0.43
0.29
0.23
0.14
Ph-OH
2.47
1.52
0.99
0.44
0.25
0.11
0.09
0.04
Initial
KL_FGE0
2.00
1.47
2.35
2.71
3.05
3.27
3.48
3.29
0.51
0.40
0.44
0.41
0.35
0.30
0.34
0.29
3.20 ± 0.10
2.86 ± 0.04
1.66 ± 0.02
1.05 ± 0.06
0.61 ± 0.12
0.34 ± 0.02
0.18 ± 0.12
0.09 ± 0.01
-
-
0
KL_FGE0.25 0.25
KL_FGE0.5 0.5
KL_FGE0.75 0.75
KL_FGE1
KL_FGE1.25 1.25
0.53 ± 0.05
1.07 ± 0.04
1.89 ± 0.09
2.03 ± 0.08
2.62 ± 0.11
2.36 ± 0.08
1
KL_FGE1.5
1.5
[
[
[
a]
Spectra available as supplementary information (Figure S1)
Averages and standard deviations of 3 measurements
Averages and standard deviations of the integrations of the regions 6.2 – 6.5 ppm and 7.4 – 7.7
b]
c]
ppm. Spectra available as supplementary information (Figure S2)
As shown in Scheme 1, the opening of the epoxide produces a new aliphatic OH, thus explaining the
3
1
increase in aliphatic OH content (Figure 1a). It gives rise to a new peak in the P NMR spectra at
1
46.3 ppm, which is distinct from the lignin aliphatic OH signal, as previously observed for a similar
3
6
reaction with propylene oxide. The regiochemistry of the reaction could not be evaluated by the
previously used method, because primary and secondary aliphatic OH groups signals overlap.
Selected samples were then phosphitylated with 2-chloro-1,3,2-dioxaphospholane and submitted to
3
1
37
P NMR, since this reagent allows a better resolution of aliphatic OH groups. The corresponding
spectra are given as supplementary information (Figure S2). They show the emergence of a new peak
in the characteristic region for secondary aliphatic OH groups, at 134.5 ppm, whereas the content in
primary aliphatic OH groups remains constant. The increase in secondary aliphatic OH thus measured
coincides with the global increase presented in Figure 1a, within a 5% error margin. This indicates
that this reaction proceeds with a high regioselectivity, yielding only compound (1), which is
expected to be the favored product resulting from the nucleophilic attack of the C-O bond by the
phenolate anion at the less hindered position.
9

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Figure 1. Content in phenolic and aliphatic OH groups of the lignins functionalized with increasing
amount of FGE (a) and evolution of the content in condensed and uncondensed phenolic OH groups
(b).
The decrease in phenolic OH is linearly correlated to the increase in aliphatic OH (Figure 2a). Indeed,
the formation of the phenyl ether bond goes along with the generation of a new aliphatic OH group
-
1
as a result of the epoxy ring opening (Scheme 1). Nevertheless, since data are expressed in mmol g ,
the correlation does not follow a 1 to 1 ratio, because of the increase in molecular weight associated
to the FGE introduction. The apparent decrease in phenolic OH groups is thus more important than
the concomitant increase in aliphatic OH.
1
0

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1
The degree of lignin functionalization, expressed as the loading in furan groups, was evaluated by H
1
NMR. The H NMR spectra of the functionalized lignins, given as supplementary information (Figure
S3), show two new peaks at 6.42 and 7.62 ppm, which are assigned to the H atoms from the furan
ring. Those peaks have been integrated in order to calculate the amount of furan groups bound to
1
the lignin. The values obtained from the H NMR are compared in Figure 2b to the corresponding
3
1
data obtained by P NMR, for which the increase in aliphatic OH groups has been taken as
representative of the furan loading. A good linear correlation is observed, which however deviates
1
from the y = x curve. The discrepancy can arise from the overlap of the furan signals in H NMR with
the aromatic protons from lignin.
Figure 2. Correlation between the decrease in phenolic OH (Δ(Ph-OH)) and the increase in aliphatic
3
1
OH (Δ(Al-OH)), as measured by P NMR (a), and correlation between the amount of linked furan
3
1
1
groups as measured by P and H NMR (b).
The molecular weight distributions (MWD) of the functionalized lignins are presented in Figure 3. The
n
inset shows the progressive increase of the number-average MW, M , with the amount of FGE used.
The increase is, as expected, quite low, and the differences between the MWD rather small.
Additional difficulties in discussing the magnitude of the MW increase arise from the fact that the
samples are acetobrominated prior to SEC analysis, to allow the dissolution in THF. In case the
1
1

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functionalization with FGE is incomplete, the remaining phenolic OH groups are thus converted into
acetyls, thus reducing the difference in MW with those functionalized with FGE and minimizing the
MW increase caused by FGE.
Figure 3. Molecular weight distributions of the lignins functionalized with 0 to 1.5 eq. FGE. The inset
n
shows the evolution of M with the amount of FGE used for functionalization.
Further characterization of the functionalized lignins was carried out by FTIR. The spectra, measured
for all the samples, are shown in Figure 4a. The main lignin peaks, such as the aromatic skeleton
-1 38
vibration at 1510 and 1600 cm , are visible on all the functionalized samples. In addition, new
-
1
peaks characteristic of the furan ring appear in the fingerprint region, at 600 and 750 cm . Their
intensity correlates well with the amount of FGE used for the functionalization (Figure 4b). The FTIR
data hint at a levelling off when more than 1.25 eq. FGE are used, in good agreement with the data
3
1
1
-1
from P and H NMR. A peak at 1150 cm , assigned to the ether bonds in FGE, is also clearly
discernible, though it overlaps with other lignin peaks and can thus not be used for quantifying
purposes.
1
2

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Figure 4. FTIR spectra of the functionalized lignins, depending on the amount of FGE used for
functionalization (a) and evolution of the furan peaks heights with the amount of FGE used for
functionalization (b).
Lignin functionalization with maleimide moieties
The second step of the work consisted in the grafting of maleimide groups onto lignin. This was
accomplished by esterification with 6-maleimidohexanoic acid (6-MHA), as shown in Scheme 2.
In order to increase the reaction yield, 6-MHA was first converted into the corresponding acyl
chloride by reacting with oxalyl chloride (Scheme 2a). The solution of acyl chloride was then directly
transferred into the lignin solution to perform the esterification (Scheme 2b). The reaction has been
1
3

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3
1
1
performed with increasing amounts of 6-MHA and followed by P NMR, H NMR and UV
spectroscopy. All data are given in Table 2.
Scheme 2. 2-steps maleimidation of lignin: conversion of 6-maleimidohexanoic acid (6-MHA) into the
corresponding acyl chloride (a) and esterification of lignin with 6-maleimidohexanoic acyl chloride
(b).
A preliminary study on the kinetic of the reaction showed that 1 h was sufficient to achieve the
maximum functionalization yield (supplementary information, Figure S4). The content in OH groups
3
1
was measured by P NMR, whereas UV spectroscopy was used to monitor the content in phenolic
3
1
OH groups. Some samples couldn’t be analyzed by P NMR, because they were insoluble in the
solvent mixture after phosphitylation. In this case only the UV data are reported in Figure 5a.
3
1
Nevertheless, when both P NMR and UV data are available, a good correlation is observed (Table
2
). In order to improve the reaction efficiency, larger reagent excess has to be used to reach high
degree of functionalization, which however does not lead to a complete depletion of phenolic OH
groups, since a plateau is observed above 2 eq. 6-MHA.
1
4

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In addition, a decrease in aliphatic OH groups is also measured, indicating that the esterification does
not show selectivity towards phenolic OH groups. The comparison of the reactivity of C5-condensed
and non-condensed phenolic units reveals practically no difference at high loading, indicating that
the limitation of the reactivity is not caused by steric hindrance.
3
1
Table 2. Quantification of the functional groups in the lignins derivatized with 6-MHA by UV and P
-
1
NMR (mmol g )
3
1
[a]
[b]
by UV
1
[c]
by P NMR
by H NMR
6
-MHA
6
-MHA
eq.
Cond.
Ph-OH
Uncond.
Ph-OH
Total
Ph-OH
Total Ph-
OH
Δ
Sample name
Al-OH
COOH
aliphatic
chain
(COOH)
Initial
-
0
2.00
1.40
2.14
1.45
na
2.47
1.50
na
4.61
2.95
na
0.51
0.28
na
3.20 ± 0.10
-
-
-
-
KL_6-MHA0
KL_6-MHA0.5
KL_6-MHA1
KL_6-MHA1.2
KL_6-MHA1.5
KL_6-MHA2
KL_6-MHA2.5
[
d]
0.5
1
na
2.24 ± 0.03
2.29 ± 0.01
2.11 ± 0.05
1.65 ± 0.04
1.27 ± 0.02
1.35 ± 0.05
1.33 ± 0.43
1.50 ± 0.24
na
0
0.96
0.86
na
1.28
0.93
na
1.50
1.11
na
2.78
2.04
na
0.59
0.65
na
0.06
na
0
1.2
1.5
2
2.39 ± 0.96
2.91 ± 0.51
3.07 ± 0.38
0.50
0.51
0.50
0.55
0.55
0.60
1.04
1.15
1.89
1.93
0.59
1.18
2.5
[
a]
b]
c
Spectra available as supplementary information (Figure S5)
Averages and standard deviations of 3 measurements
Spectra available as supplementary information (Figure S6)
[
[
[
d]
31
na = not analyzed, because of insolubility in the P NMR solvent mixture after the phosphorylation
reaction
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Figure 5. Content in phenolic and aliphatic OH groups of the lignins functionalized with increasing
amount of 6-MHA (a) and evolution of the content in condensed and uncondensed phenolic OH
groups (b).
3
1
P NMR spectra of the functionalized lignins show a strong new peak in the carboxylic acid region at
34.7 ppm (supplementary information, Figure S5). Even if it could in principle arise from unreacted
-MHA that survived during the workup, the MWD presented in Figure 6 do not show any additional
1
6
-
1
peak in the low molecular weight region (M = 211 g mol for 6-MHA), despite the absorption of the
1
maleimide group at the detection wavelength (280 nm). On the other hand, H NMR spectra,
presented as supplementary data (Figure S6), only show a slight increase in the carboxylic acid peak
1
located around 12 ppm. The new COOH groups, calculated from the integration of the H NMR
spectra, are presented in Figure 7. The increase is significant only for the samples functionalized with
1
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3
1
more than 2 eq. 6-MHA, but is always much lower than the one measured by P NMR (Table 2). It is
thus hypothesized that new carboxyl groups are formed during the phosphitylation reaction
3
1
necessary for acquiring the P NMR spectra. Especially, acidic conditions during the reaction could
lead to the opening of the maleimide ring, which in the presence of residual water could form a new
carboxyl group as chain end (Scheme 3).
Scheme 3. Possible opening of the maleimide ring under the acidic conditions of the phosphitylation
3
1
reaction, causing the observed increase in carboxyl groups by P NMR.
1
The observation of the H NMR spectra reveals the appearance of a strong new peak at 7.00 ppm,
assigned to the 2 H atoms of the maleimide ring (Figure S6). However, since it overlaps with the
aromatic protons of lignin, a precise quantification is impossible. The H atoms from the aliphatic
chain of 6-MHA are on the other hand well separated from the lignin protons, and have thus been
used to quantify the amount of 6-MHA grafted to the lignin. The results, given in Figure 7, correlate
3
1
fairly well with the decrease in OH groups obtained from P NMR.
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Figure 6. Molecular weight distributions of the lignins functionalized with 0 to 2.5 eq. 6-MHA. The
inset shows the evolution of M
n
with the amount of 6-MHA used for functionalization.
1
Figure 7. Functional groups calculated from H NMR. Spectra are available as supplementary
information (Figure S6). The aliphatic chain has been calculated as the average of the integration
regions 2.10 – 2.30 ppm (2H) and 1.00 – 1.80 ppm (6H).
n
The MWD of the maleimide-functionalized lignins (Figure 6) show an increase in M for the reaction
with up to 2 eq. of 6-MHA. The decrease for 2.5 eq. could be related to an incomplete attachment of
1
the 6-MHA, as shown by the increase in COOH groups measured by H NMR.
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The functionalization of lignin with maleimide groups can also be revealed by FTIR, as shown in
Figure 8. Specific peaks from the maleimide ring appear in the fingerprint region, at 696 and
-
1
8
30 cm . The carbonyl region is also strongly enhanced because of numerous contributions. The two
-
1
C=O groups from the maleimide ring give rise to the signal at 1705 cm . The formation of the ester
-
1
bond between lignin and 6-MHA should be responsible for the shoulder observed around 1770 cm .
Figure 8. FTIR spectra of the functionalized lignins, depending on the amount of 6-MHA used for
functionalization
Reversibly crosslinking lignin via the furan-maleimide Diels-Alder (DA) reaction
Once the lignins modified with furan and maleimide groups were prepared, the emphasis was placed
on the Diels-Alder (DA) reaction, depicted in Scheme 4. The forward reaction, i.e. the crosslinking
reaction, was performed at 70 °C, while the retro reaction was conducted at 120 °C.
Samples with similar loading factors in furan (KL_FGE1.25) and maleimide (KL_6-MHA2) were thus
mixed in DMSO. The contents in furan and maleimide were respectively 2.31 ± 0.30 and 2.48 ±
-
1
31
0
.16 mmol.g , as evaluated by the averages and standard deviations of the loadings measured by P
1
and H NMR. An array of experiments was carried out including different ratios of functionalized
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lignins and control experiments on the initial lignin or on the furan- and maleimide- lignins alone, as
reported in Table 3.
Scheme 4. Crosslinking of the functionalized lignins via the Diels-Alder reaction
Table 3. Compositions of the mixtures of furan- and maleimide- lignins used to study the Diels-Alder
reaction (15% w/w solutions in DMSO)
Initial KL (%
Furan- lignin
(% w/w)
Maleimide-
lignin (% w/w)
Sample name
Gelation
w/w)
100
50
50
0
Control_KL
Control_FGE
Control_MHA
FGE_MHA_1:2
FGE_MHA_1:1
0
50
0
33
50
0
0
50
66
50
no
no
no
yes
yes
0
In order to study the influence of the concentration, the FGE_MHA_1:1 sample was prepared at
concentrations from 1 to 15 % (w/w). After 4 days at 70 °C, the 15% solution had turned into a gel as
a result of the crosslinking, whereas the 10% one became very viscous and stuck to the walls, as
shown in Figure 9a. No visible change was observed at lower concentration. Upon heating at 120 °C,
the gel quickly turned back to the liquid state, as visible on Figure 9b, since the crosslinks are
removed via the retro Diels-Alder reaction.
2
0

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Figure 9. Solutions of functionalized lignins (furan/maleimide 1:1 w/w) in DMSO (1, 10 or 15% w/w)
after heating at 70 °C (a) and 15% w/w solution before and after heating at 70 °C, and after heating
at 120 °C (b)
Gelation was shown to occur after 36h when furan- and maleimide- lignins were combined (Figure
S7), in 1:1 or 1:2 (w/w) ratios. After performing the retro reaction at 120 °C, the solutions were
cooled to room temperature and the gels were formed again at 70 °C, thus confirming the full
reversibility of the crosslinking.
To assess that the gelation is related to the formation of the DA adduct between furan and
1
maleimide, similar experiments were performed in DMSO-d
6
, followed by acquisition of H NMR
spectra, reported in Figure 10. In this case, the concentration was chosen in order not to get a gel,
which allowed measuring accurate NMR spectra without solubility issues. The signal of the furan
protons at 7.62 ppm decreases as the corresponding double bonds are lost during the formation of
the DA adducts (Scheme 4). The signal of the maleimide double bond at 7.00 ppm is also reduced,
but it overlaps with the lignin aromatic signals, making the quantification difficult. The newly formed
double bond of the DA adduct overlaps with the signal of the furan protons at 6.40 ppm. The
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formation of the DA adduct also generates a new peak at 5.10 ppm, which is clearly discernible in the
inset in Figure 10. After 63h, the conversion reached 30% as partially expected by the low
concentration chosen. It has to be pointed out here that due to the high degree of functionalization
of the single lignins the gelation process is expected to occur even with a relatively low conversion of
the furan and maleimide groups.
1
Figure 10. H NMR spectra of F50_M50, measured after different time at 70 °C.
Conclusion
A synthetic strategy for the specific functionalization of Kraft lignin was developed to get furan and
maleimide groups respectively linked at the chain ends. The reaction of KL with furfuryl glycidyl ether
(FGE) showed a high degree of selectivity towards the phenolic OH groups, and was performed in
quantitative yields in the presence of a slight excess of FGE. Maleimide groups were attached by the
esterification of KL with 6-maleimidohexanoic acid (6-MHA). With a 2 fold excess of 6-MHA, high
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loading could be achieved. In this case the reaction was not highly selective and besides phenolic OH,
also some of the aliphatic OH groups of KL were functionalized.
The furan- and maleimide-grafted lignins were then combined to generate a mixture suitable for
crosslinking via the Diels-Alder (DA) [4 + 2] cycloaddition. The lignin solutions in DMSO turned into a
gel at 70°C as a result of the crosslinking, and quickly turned back to the liquid state after generating
1
the retro-DA reaction at 120 °C. H NMR confirmed the formation of the DA adduct, though the
conversions were relatively low.
This study thus shows the successful exploitation of the DA reaction to generate reversible
crosslinking in lignin. Based on these results, further efforts should now be devoted to the
preparation of solid materials, as well as to the study of their properties, and potential recyclability
3
9
or self-healing ability. The developed lignin crosslinking chemistry will be further coupled with other
potential applications of lignins, such as the preparation of adhesives, where the thermal reversibility
2
6
acts as a temporary curing element, in combination with a ‘classical’ permanent curing system
attached to yet unused functional elements present in the lignin, e.g., remaining hydroxyl groups,
double bonds, etc.
Acknowledgments
The Knut and Alice Wallenberg Foundation is gratefully acknowledged for the funding of this work.
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Graphical Abstract for TOC
Lignoboost lignin was independently functionalized with furan and maleimide groups. Mixed,
these functionalized lignins show all features of reversible networks.
1