Ring-Opening Metathesis Polymerization of Biomass-Derived Levoglucosenol

Article abstract of DOI:10.1002/anie.201814501

The readily available cellulose-derived bicyclic compound levoglucosenol was polymerized through ring-opening metathesis polymerization (ROMP) to yield polylevoglucosenol as a novel type of biomass-derived thermoplastic polyacetal, which, unlike polysaccharides, contains cyclic as well as linear segments in its main chain. High-molar-mass polyacetals with apparent weight-average molar masses of up to 100 kg mol^?1 and dispersities of approximately 2 were produced despite the non-living/controlled character of the polymerization due to irreversible deactivation or termination of the catalyst/active chain ends. The resulting highly functionalized polyacetals are glassy in bulk with a glass transition temperature of around 100 °C. In analogy to polysaccharides, polylevoglucosenol degrades slowly in an acidic environment.

Full text of DOI:10.1002/anie.201814501

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Accepted Article
Title: Ring-opening metathesis polymerization of biomass-derived
levoglucosenol
Authors: Tapas Debsharma, Felix N. Behrendt, André Laschewsky,
and Helmut Schlaad
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814501
Angew. Chem. 10.1002/ange.201814501
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Ring-opening metathesis polymerization of biomass-derived
levoglucosenol
Tapas Debsharma, Felix N. Behrendt, André Laschewsky, Helmut Schlaad*
Abstract: The readily available cellulose-derived bicyclic levoglu-
cosenol was polymerized via ring-opening metathesis polymerization
(ROMP) to yield polylevoglucosenol as a novel type of bio-based
thermoplastic polyacetal, which containsunlike polysaccha-
ridescyclic as well as linear segments in its main-chain. High molar
mass polyacetals with apparent weight-average molar masses of up
to 100 kg mol-1 and dispersities of ~2 were produced despite the non-
living/controlled character of the polymerization due to irreversible
deactivation or termination of the catalyst/active chain ends. The
resulting highly functionalized polyacetals are glassy in bulk with a
glass transition temperature of ~100 °C. In analogy to polysaccha-
rides, polylevoglucosenol degrades slowly in acidic environment.
successful metathesis polymerization of a purely sugar-derived
monomer. Sugar-related systems described so far include natural
macrocyclic glycolipids, which are polymerized via the
unsaturated fatty acid part,[6] or saccharides attached to a
norbornene or 7-oxanorbornene scaffold.[7] Also, a structurally
related carbohydrate lactone derived from gluconolactone was
polymerized by ROP using a tin-based catalyst to yield low molar
mass polyesters.[8]
Pure levoglucosenone
1 (which is also commercially
available, see SI) was readily obtained from the acid-catalyzed
pyrolysis of cellulose (Scheme 1).1 The attempts of free-radical or
anionic polymerization of 1 with azobisisobutyronitrile (AIBN) or
sec-butyl lithium (sBuLi) as initiators, respectively, were not
successful, producing low molar mass, oligomeric materials only
(data not shown). Also, despite its strained bicyclic ring structure,
1 did not undergo a ring-opening olefin metathesis polymerization,
possibly due to the interference of the keto moiety in proximity to
olefin.[9] We, therefore, decided to eliminate the keto group in
levoglucosenone by quantitative reduction, hoping that the
Fossil-based resources have been exploited commercially
since the industrial revolution. However, fossil resources are not
infinite on earth and exploitation will come gradually to a halt.
Moreover, fossil-based resources are recognized to cause all
kinds of pollution, whether it is global warming or plastic
contamination of aquatic wildlife. Hence, the move towards
renewable feedstocks and environmentally degradable systems,
preferentially by valorization of biomass waste, presents a major
challenge for chemistry, in particular for the production of
thermoplastic and degradable polymers.[1]
resulting bicyclic allylic alcohol levoglucosenol
2 could be
polymerized by ROMP to yield the polyacetal 3 (Scheme 1).[10]
Levoglucosenol 2 was readily obtained by reduction of 1 with
sodium borohydride in water,[4] followed by extraction with ethyl
acetate, and purification of the concentrate by sublimation. The
isolated product 2 was a mixture of two diastereomeric alcohols,
Cellulose is one of the most abundant products of biomass
and therefore an attractive renewable feedstock for the production
of value-added chemicals such as sugars, lactic acid, levulinic
acid, furans, etc.[2] Another special chemical that can be readily
the major isomer (95%) being the 1,6-anhydro-3,4-dideoxy--
D-
threo-hexopyranose (see SI).
obtained from cellulose is 1,6-anhydro-3,4-dideoxy-3--
D-pyra-
H
H
O
O
OH
O
O
a
b
c
nosen-2-one (IUPAC name: (1S,5R)-6,8-dioxabicyclo[3.2.1]oct-
2-en-4-one), also known as levoglucosenone.[3] Levoglucosenone
is a bridged heterocyclic vinyl ketone, which is used as a building
block in organic synthesis but is reluctant toward polymerization.
However, the simple reduction of the ketone functionality yields
the alcohol levoglucosenol,[4] which we found to be a suitable
monomer for the ring-opening metathesis polymerization
(ROMP)[5] using a derivative of the 2nd generation Grubbs catalyst.
To the best of our knowledge, this is the first example of a
Cellulose
1/n
O
H
O
H
O
OH
n
1
2
3
Scheme 1. Synthesis of levoglucosenone 1 by pyrolysis of cellulose and
subsequent reduction to levoglucosenol 2. Production of polyacetal 3 by ring-
opening metathesis polymerization of 2. Reagents and conditions: (a) 1%
H3PO4, 300 °C, Kugelrohr oven, (b) NaBH4, H2O, r.t., (c) C6, 1,4-dioxane, r.t.
First attempts to polymerize 2 involved the use of 1st, 2nd,
and 3rd generation Grubbs catalysts (Figure 1, C1-C3) as well as
1st and 2nd generation Hoveyda-Grubbs catalysts (C4-C5), all of
which failed (as did the attempted free-radical polymerization
initiated by AIBN). However, the polymerization turned out to be
successful with the mono-ortho-substituted N-heterocarbene
(NHC) derivative of the 2nd generation Grubbs catalyst C6, which
is known as a highly efficient catalyst for the ring closing
metathesis (RCM) of bulky tetrasubstituted olefins.[11] Metathesis
polymerizations were performed in 1,4-dioxane solution at room
temperature (Table 1). Monomer conversion (xp) reached 50-60%
(by 1H NMR spectroscopy) within 24 h, and, depending on the
catalyst loading ([2]0 = 4M, [2]0/[C6] = 66-1000), the polymers
exhibited apparent weight average molar masses (Mwapp) in the
range of 29 to 100 kg.mol-1 and dispersities (Ð) of 1.8-2.9 (by size
*
T. Debsharma, Dr. F. N. Behrendt, Prof. Dr. A. Laschewsky, Prof. Dr. H.
Schlaad
Institute of Chemistry, University of Potsdam
Karl-Liebknecht-Straße 24-25, Potsdam 14476, Germany
E-mail: schlaad@uni-potsdam.de
Prof. Dr. A. Laschewsky
Fraunhofer Institute of Applied Polymer Research IAP
Geiselbergstraße 69, 14476 Potsdam, Germany
Supporting information: Experimental details and synthetic procedures.
ESI-MS and NMR data of compounds 1 and 2 (1H and 13C NMR), 3 (ESI-
MS and 1H, 13C, HSQC, HMBC, NOESY, COSY NMR), and 4 (1H, 13C,
and HSQC NMR). DSC/TGA curves and SEC traces of polymer 3 (Table
1). 1H NMR spectra and SEC traces recorded during kinetic investigation
and SEC traces for equilibration and degradation experiments.
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exclusion chromatography (SEC) with polystyrene calibration).
The polymer 3 was found to be soluble in 1,4-dioxane, N,N-
dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP),
dimethylsulfoxide (DMSO), or sulfolane but insoluble in dichloro-
methane (DCM), tetrahydrofuran (THF), toluene, dimethyl-
carbonate, alcohols, or water. The glass transition temperature
The occurrence of different isomers can be well expected
because 2 is an asymmetric (chiral) monomer, which can add to
the growing chain in different orientations (sequences), in addition
to the cis-trans isomerism (cis/trans ratio not determined) at the
main-chain double bond. Since the NMR spectrum of the
hydrogenated polymer 3 (obtained by the reaction with H2/Pd-C
(Tg) of 3, as determined by differential scanning calorimetry (DSC), with 98% conversion of double bonds to alkane  polymer 4) still
is ~100 °C, and the polymer is thermally stable up to ~220 °C (by
thermogravimetric analysis (TGA), see SI).
showed two signals for OH (1H) and four signals for C1 (13C) (see
SI), the multiplicity of signals does not originate from cis/trans but
from sequence isomers, i.e., head-to-tail, head-to-head, and tail-
to-tail.
N
N
N
N
P
Ru
P
~
Cl
Cl
Cl
6
Ru
N
1
Ru
P
N
Cl
Cl
Ph
Cl
Ph
Br
Ph
4
Br
2
C3
C1
OH
C2
5
3
N
N
N
N
P
Cl
Cl
Cl
1,4-Dioxane H2O
DMSO
Ru
P
Ru
Ru
O
Cl
Cl
Ph
Cl
O
H4 H3
OH
H1 H5
H2,H6 H6’
C4
C5
C6
Figure 1. Screened Ru-based catalysts C1-C6 (Ph = phenyl) for ring-opening
metathesis polymerization of levoglucosenol 2
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
Table 1. Polymerization of 2 ([2]0 = 4M) with catalyst C6 in 1,4-dioxane solution
at room temperature for 24 h.
Chemical shift (ppm)
a
app
b
d
Figure 2. 1H NMR (500 MHz) spectrum of polyacetal 3 (entry 2) in DMSO-d6;
 5.90–5.79 (m, 1H), 5.79–5.65 (m, 1H), 5.37–5.10 (m, 1H), 4.78–4.64 (m, 1H),
4.58–4.42 (m, 1H), 4.05–3.87 (m, 2H), 3.54–3.42 (m, 1H).
xp
Mw
Tg
Entry
[2]0/[C6]
Ð c
(%)
(kg.mol-1)
(°C)
1
2
3
4
66
100
200
1000
60
59
59
50
29
37
1.8
100
100
92
~
6
1
2.0
2.2
2.9
2
47
5
3
4
100
98
DMSO
C4 C3
C1
C5 C2 C6
C5 C2C6
aMonomer conversion, determined by 1H NMR spectroscopy. bApparent weight-
average molar mass, determined by SEC. cDispersity index, determined by
SEC. dGlass transition temperature, determined by DSC.
The molecular structure of polymer 3 was resolved via ESI-
MS and NMR techniques (see SI). Mass spectrometric analysis
revealed a molar mass of the repeating unit of 128 Da, which is in
agreement with the expected structure of 3 shown in Scheme 1.
All proton and carbon signals in 1H and 13C NMR spectra (Figure
2 and 3, respectively) were assigned to the polyacetal 3 with the
help of various 2D NMR experiments (see SI). It is important to
note that the three stereocenters of monomer 2 (at carbons C1,
C2, and C5; cf. Scheme 1 and Figure 2) were preserved during
metathesis polymerization (3 (entry 2): []D = -35°; 1,4-dioxane).
Still, the 1H NMR spectrum (Figure 2) showed in total 8
different types of protons and, interestingly, two multiplet peaks of
the same intensity ratio (~0.5 H) at 5.13-5.16 ppm and 5.27-5.31
ppm. These disappeared upon the addition of D2O (see SI),
suggesting that these two signals arise from the hydroxyl protons.
This peak assignment was confirmed with HMBC and HSQC
measurements (see SI). The appearance of the hydroxyl protons
in two regions as two sets of multiplets indicates the existence of
different isomeric structures in the polymer chain. Also, the 13C
NMR spectrum (Figure 3) revealed four distinct peaks of the
characteristic acetal carbon (C1) at ~106 ppm, which corresponds
to four isomeric configurations.
C4 C3
C1
200
180
160
140
120
100
80
60
40
20
0
Chemical shift (ppm)
Figure 3. 13C NMR (150 MHz) spectrum of polyacetal 3 (entry 2) in DMSO-d6;
 132.7, 131.1, 129.9, 128.5, 105.7, 76.4, 71.5, 69.2.
We moved on with a kinetic investigation of the poly-
merization and optimization of reaction conditions. As mentioned
above, the monomer conversion just reached 50-60% (Table 1)
(note: unreacted monomer
quantitatively in pure form by sublimation, see SI), which is
indicative of termination reactions and hence non-living
2 could be recovered almost
a
character of the polymerization. Indeed, the pseudo-first-order
time-conversion plot (Figure 4a) for the polymerization of 2 with
C6 at room temperature is not linear but could be fitted with a
model of unimolecular termination (see equation in Figure 4a).
Furthermore, the evolution of the polymer molar mass (Mw
app
,
1
SEC) with monomer conversion (xp, H NMR spectroscopy, see
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Table 2. Polymerization of 2 with catalyst C6 ([2]0/[C6] = 100) in 1,4-dioxane
solution at different monomer concentrations ([2]0) and reaction temperatures
(T) for 24 h.a
SI) deviates strongly from linearity (Figure 4b), and molar masses
are much higher than calculated from the monomer-to-catalyst
ratio. High molar mass polymers were formed in the early stage
of the reaction, i.e., xp < 20%, and the chains grew shorter with
increasing depletion of monomer, as it is typically observed for
non-living processes.[12] Compliantly, the resulting polymer 3
exhibits a Schulz-Flory type molar mass distribution with a
dispersity of Ð ~ 2 (Table 1 and Figure 4c).
b
app c
Entry
[2]0
T
xp
Appearance
Mw
Ð d
(°C)
90
(%)
12
(kg.mol-1)
2.3
5
6
4 M
4 M
4 M
4 M
4 M
2 M
2 M
2 M
2 M
1 M
Liquid
Liquid
1.6
1.7
1.9
2.0
2.2
60
40
28
0
33
49
59
62
2
18
33
37
87
7
Liquid
2
LiquidGele
LiquidGele
Liquid
Such kinetic behavior suggests that the rate of initiation is
much slower as compared to the monomer addition (propagation)
step, and active chains are continuously produced and grow very
quickly until they undergo irreversible termination or deactivation.
Indeed, the deactivation of catalyst or active chain ends could be
observed with the naked eye by the occurrence of blackening of
the reaction mixture. Further, with ongoing reaction time, the
catalyst gives rise to chain length equilibration, e.g., via back-
biting, thus contributing to the reduction of polymer molar mass
(see equilibration experiment in SI).
8
f
f
9
60
40
28
5
-
-
10
11
12
13
16
28
74
13
Liquid
8.8
13
1.9
2.3
2.0
1.7
Liquid
LiquidGele
104
10
Liquid
13
aAll reactions were repeated at least once and results were consistent within
experimental errors. bMonomer conversion, determined by 1H NMR
spectroscopy (see SI). cApparent weight-average molar mass, determined by
SEC. dDispersity index, determined by SEC. eReaction solutions were initially
liquid but slowly turned into a gel (gel liquefied upon dilution with 1,4-dioxane).
fNot determined.
An interesting property of the polyacetal 3 is its amorphous
character and high glass transition temperature (Tg ~ 100 °C, see
Table 1), despite of the double bonds and stereocenters in the
main-chain. Hence it might serve as a thermoplastic polymer[10]
and for the fabrication of transparent polymer films (see Figure
4d). Also, it contains functionalizable hydroxyl and olefin groups
and degradable acetal units in the main-chain. A high molar mass
polymer 3 was found to degrade completely in 1,4-dioxane
solution at room temperature in the presence of p-toluenesulfonic
acid/water within about 40 days (see SI).
In summary, we introduced levoglucosenol, which is an
olefinic derivative of
a bicyclic carbohydrate, as a novel
sustainable monomer for producing thermoplastic polyacetals by
ROMP. The use of a derivative of 2nd generation Grubbs catalyst
(C6) yielded high molar mass polymers, the structure of which
was characterized in detail by MS and NMR techniques, as well
as by SEC and DSC. The polylevoglucosenol exhibited a
surprisingly high glass transition temperature of ~100 °C, which
might be explained by the combined stiffening effects of the
dioxolane ring in the main chain and hydrogen-bonding
interactions of the alcohol moieties. The polymerization does not
show living characteristics with respect to control over polymer
molar masses and their distributions, which is likely due to slow
initiation, chain equilibration, and limited stability of the used
catalyst. Ongoing research is devoted to the further optimization
of catalyst design and polymerization conditions, for instance by
variable temperature protocols,[13] aiming at implementing
living/controlled ROMP of levoglucosenol and derivatives thereof,
and exploring the post-polymerization modification potential of the
new polymer class.
Figure 4. Polymerization of 2 with catalyst C6 ([2]0 = 4M, [2]0/[C6] = 100) in 1,4-
dioxane at room temperature: (a) pseudo-first-order time-conversion plot, (b)
theor
evolution of weight-average molar mass (Mwapp) with conversion (xp) (Mw
=
2(Ð).[2]0/[C6].xp), (c) SEC-RI trace (eluent: NMP) of polymer 3 obtained at 24 h
after precipitation into DCM, and (d) photograph of polymer 3 isolated as a thin
film.
We further tried to optimize the reaction conditions in order
to maximize the monomer conversion and polymer molar mass;
results are summarized in Table 2. Best results were obtained at
[2]0 = 2-4 M at a temperature of 0-5 °C (temperature window is
limited by the freezing of the solution): xp = 62-74%, Mwapp = 87-
104 kg.mol-1 (entries 8 and 12 in Table 2). As before, the apparent
polymer molar masses were much higher than expected for a
living polymerzation mechanism (Mwtheor = 25.6 kg.mol-1 at 100%
conversion). However, both monomer conversions and molar
masses decreased drastically when the polymerizations were
conducted at higher temperature (> 40 °C) or dilution (1 M).
Acknowledgements
We would like to thank Sascha Prentzel (SEC), Dirk Schanzen-
bach (TGA/DSC), Angela Krtitschka, Matthias Heydenreich, and
Heiko Möller (NMR), Bernd Schmidt, and Katharina Zesch for
their support and contributions to this project.
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Keywords: metathesis – ring-opening polymerization –
sustainable – thermoplastic – degradable polymer
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Entry for the Table of Contents
COMMUNICATION
Ring-opening metathesis
polymerization (ROMP) of
Tapas Debsharma, Felix N.
Behrendt, André Laschewsky,
Helmut Schlaad*
levoglucosenol,
derives from a pyrolysis
product of cellulose,
yields a fully sustainable,
high molar mass
thermoplastic polyacetal.
which
Page No. – Page No.
Ring-opening metathesis
polymerization of biomass-
derived levoglucosenol
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