Crosslinking of cellulose by olefin metathesis

Article abstract of DOI:10.1081/CAR-120019013

New crosslinked microcrystalline cellulose plastic films were made by olefin metathesis. Cellulose was first acylated under microwave irradiation by 10-undecenoyl chloride in lithium chloride/N,N-dimethylacetamide homogeneous solution to yield unsaturated fatty acid cellulose esters with a degree of substitution (DS) varying from 1 to 2. After studying the variation of cellulose DS vs. microwave irradiation time, the unsaturated fatty acid cellulose esters were subjected to olefin metathesis using Grubbs's catalyst. Crosslinking ratio T (0% < T < 90%) and gel formation during cross metathesis of cellulose esters with different DS are discussed. Structure elucidation was mainly achieved from IR and ¹H NMR spectroscopic data.

Full text of DOI:10.1081/CAR-120019013

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Crosslinking of Cellulose by Olefin Metathesis
N. Joly a , R. Granet a & P. Krausz a
a Laboratoire de Chimie des Substances Naturelles , Faculté des Sciences et Techniques ,
Université de Limoges , 123, Avenue Albert Thomas, 87060, Limoges Cedex, France
Published online: 20 Aug 2006.
To cite this article: N. Joly , R. Granet & P. Krausz (2003) Crosslinking of Cellulose by Olefin Metathesis, Journal of
Carbohydrate Chemistry, 22:1, 47-55, DOI: 10.1081/CAR-120019013
To link to this article: http://dx.doi.org/10.1081/CAR-120019013
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JOURNAL OF CARBOHYDRATE CHEMISTRY
Vol. 22, No. 1, pp. 47–55, 2003
Crosslinking of Cellulose by Olefin Metathesis
*
N. Joly, R. Granet, and P. Krausz
Laboratoire de Chimie des Substances Naturelles, Faculte´ des Sciences et
Techniques, Universite´ de Limoges, Limoges Cedex, France
ABSTRACT
New crosslinked microcrystalline cellulose plastic films were made by olefin
metathesis. Cellulose was first acylated under microwave irradiation by 10-
undecenoyl chloride in lithium chloride/N,N-dimethylacetamide homogeneous solu-
tion to yield unsaturated fatty acid cellulose esters with a degree of substitution (DS)
varying from 1 to 2. After studying the variation of cellulose DS vs. microwave
irradiation time, the unsaturated fatty acid cellulose esters were subjected to olefin
metathesis using Grubbs’s catalyst. Crosslinking ratio T (0% < T < 90%) and gel
formation during cross metathesis of cellulose esters with different DS are discussed.
1
Structure elucidation was mainly achieved from IR and H NMR spectroscopic data.
Key Words: Cellulose; Crosslinking; Metathesis; Microwave; Plastic films.
INTRODUCTION
Because cellulose is considered to be the most abundant renewable polymer on
earth, numerous studies have been carried out targeting the use of cellulose as an
alternative to non-renewable petroleum based chemicals for the synthesis of po-
tentially biodegradable materials. Cellulose modifications, often achieved by fatty acid
*
Correspondence: P. Krausz, Laboratoire de Chimie des Substances Naturelles, Faculte´ des
Sciences et Techniques, Universite´ de Limoges, 123, Avenue Albert Thomas–87060 Limoges
Cedex, France; Fax: (33)05-55-45-72-02; E-mail: Krausz@unilim.fr.
47
DOI: 10.1081/CAR-120019013
0732-8303 (Print); 1532-2327 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

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48
Joly, Granet, and Krausz
Scheme 1. Reaction sequences for cellulose crosslinking. i—DMAP in LiCl/DMAc, microwave
irradiation; ii—Grubbs’s catalyst [RuCl2(PCy3)2 = CH–Ph]/CH2Cl2 under argon, 2h.
acylation,[1–3] have attracted strong interest since the discovery of solvents allowing
dissolution and homogeneous derivatization of cellulose.[4,5] More recently, our lab-
oratory has elaborated plastic films by acylation of cellulose in a lithium chloride/N,N-
dimethylacetamide (LiCl/DMAc) solvent system under microwave irradiation condi-
tions.[6] Another transformation, crosslinking of cellulose, has great importance in
textile[7] and paper research for studies on paper wet performance[8] and swelling be-
havior[9] of crosslinked cellulose derivatives.
The use of the olefin metathesis reaction has grown in organic synthesis since the
discovery by Grubbs and co-workers of a new type of catalyst based on ruthenium;[10]
this catalyst presents a low sensitivity to polar functions such as hydroxyl or carbonyl
groups. For example, sugar derivatives[11–14] or nucleoside analogues[15] have been
successfully synthesized with the help of this method.
The aim of this work is to obtain of new hydrophobic plastic films using olefin
cross metathesis of unsaturated fatty acid esters of microcrystalline cellulose dissolved
in LiCl/DMAc. These reactions are depicted in Scheme 1.
RESULTS AND DISCUSSION
The first step consists of cellulose acylation with 10-undecenoyl chloride by
microwave irradiation under homogeneous conditions. Then in a second step, we per-
formed olefin metathesis of the cellulose ester groups using Grubbs’s catalyst.

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Crosslinking of Cellulose by Olefin Metathesis
49
Table 1. Irradiation times, DS and molar yields for reaction with 6 equiv of 10-undecenoyl
chloride and 1 equiv of DMAP.
Microwave irradiation
time (min)
Entry
DSa
Molar yieldb (%)
1
2
3
4
5
6
1.5
2
1.0
1.4
1.7
1.9
2.0
2.0
90%
93%
95%
96%
92%
94%
2.5
3
3.5
4
aDS is the degree of substitution per glucose unit (DSmax = 3, standard error = 0.2).
bQuotient (ꢀ100) of amount of polymer isolated to the theoretical weight of cellulose ester
according to DS (standard error = 5%). Every data entry represents the average of three
independent experiments.
Acylation
In order to study the variation of DS as a function of microwave irradiation time,
the reaction was carried out with 6 equiv of 10-undecenoyl chloride per
anhydroglucose unit (73.8 mmol, 16 mL). Reaction times, DS and molar yields are
presented in Table 1. Molar yields, close to 100% for each experiment, strongly
support that on average acylation takes place on almost every anhydroglucose unit of
cellulose. It can be observed, in Figure 1, that DS values rise linearly from 0 to 1.9 for
Figure 1. DS variation versus microwave irradiation (1 equiv of DMAP, 6 equiv of 10-
undecenoyl chloride).

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50
Joly, Granet, and Krausz
irradiation times varying from 0 to 3 minutes and then reach a plateau (DS ꢁ 2.0) after
3.5 minutes or longer. According to the literature,[16] OH groups in position C-6 and C-
2 are favored for acylation to an equal degree over those in C-3, although we never
observed trisubstitution.
Cellulose esters were converted into plastic films by casting procedures as described
in the experimental section. These films have the same aspect and the similar mecha-
nical behavior as commercial plastics such as polypropylene or polyethylene: they are
transparent and non brittle either by torsion or folding. Concerning their thermal
behavior, it can be noted that they have a tendency to soften from 110°C to 250°C
without turning black.
Olefin Cross Metathesis of Unsaturated Fatty Acid Cellulose Esters
In all experiments, the ester solution concentration was 1 g/100 mL in CH2Cl2,
whatever the DS value might be. Too high concentration of C C double bonds (above
1 g of cellulose ester per 75 mL CH2Cl2) resulted in gel formation, even if the catalyst
ratio was very low (<0.5%). In every case, the increase in crosslinking ratio goes with
an increase of solution viscosity until gel formation. This rise in viscosity of the
solution proves that cellulose underwent substantial crosslinking during the metathesis
reaction. For the same experimental conditions, we chose 2 hours reaction time, as it
was observed that the crosslinking ratio did not change even for reaction times from 2
to 24 hours.
For each cellulose crosslinking, casting was directly done after reaction by slow
evaporation of CH2Cl2 at room temperature under atmospheric pressure. Plastic films
were obtained in every case presented below. It should be noted that these films cannot
Table 2. Metathesis results and solutions appearance for different esters.
Entry
Ester DS
1.4
Catalyst ratio (% mol.)
Ta (%)
Aspectb
1
0.1%
0.2%
0.4%
0.4%
0.25%
0.25%
0.4%
0.4%
0.2%
0.2%
0.3%
0.1%
0.15%
0.2%
0.4%
0.6%
19%
38%
67%
63%
47%
51%
83%
78%
44%
42%
63%
21%
35%
55%
91%
89%
LIQUID
LIQUID
GEL
2
3
4
GEL
5
1.7
LIQUID
LIQUID
GEL
6
7
8
GEL
9
1.9
2.0
LIQUID
LIQUID
GEL
10
11
12
13
14
15
16
LIQUID
LIQUID
VISCOUS
GEL
GEL
aCrosslinking ratio (standard error = 5%).
bBefore casting.

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Crosslinking of Cellulose by Olefin Metathesis
51
be dissolved in any organic solvent nor in water. Their thermal behavior is similar to
that of the starting cellulose ester except that they soften from 180°C to 250°C.
Results reported in Table 2 show that the metathesis reaction can be reproduced
with the same percentage of catalyst and leads to the same crosslinking ratio (compare
entries 3 and 4, 5 and 6, 7 and 8, 9 and 10). Whatever the DS of starting cellulose
ester, gels appear as soon as the crosslinking ratio T reaches 60%.
We studied the influence of catalyst percentage on the crosslinking ratio for a
sample with DS = 2.0. Reaction conditions were the same as those of the previous
experiments, and the catalyst molar ratio ranged from 0.1% to 0.6%. Results are
reported in Table 2 (entries 12 to 16). Very high values of crosslinking ratios were
obtained even in presence of a very low amount of catalyst. The corresponding graph
(Figure 2) representing T vs. metathesis catalyst ratio shows a linear increase in
crosslinking ratio (from 0 to 70%) when the Grubbs’s catalyst ratio increased from 0 to
0.3%. This rise is attenuated and T becomes constant (90%) for catalyst ratios up to
0.4%. It should be noticed that a viscous solution was obtained when T = 55%, and
that a gel appeared when metathesis was conducted in presence of 0.4% catalyst.
Spectrometric Analysis
Cellulose esters and crosslinked products were characterized by IR spectral analy-
sis. The IR spectra (Figure 3) of cellulose 10-undecenoate with DS = 2 (Figure 3a)
and the corresponding crosslinked product with T = 50% (Figure 3b) display the
features of both products and more especially characteristic of ester function bands at
1748 cm ꢂ 1, remaining hydroxyl groups of cellulose (3486 cm ꢂ 1), alkene functions
from the aglycone moieties (3076 cm ꢂ 1, 1640 cmꢂ 1, 909 cm ꢂ 1 and 723 cm ꢂ 1), and
alkyl chains (2926 cm ꢂ 1, 2855 cm ꢂ 1, 1463 cm ꢂ 1, 1417 cm ꢂ 1). Both alkyl and
carbonyl stretching bands of the crosslinked product displayed an increased width
compared to the corresponding bands of the starting cellulose ester; in addition,
Figure 2. Variation of crosslinking ratio (T) versus to Grubbs’s catalyst ratio (%mol) for a
DS = 2 ester. Experimental conditions: see Table 2.

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Joly, Granet, and Krausz
Figure 3. IR spectra of plastic films. a. cellulose ester with DS = 2; b. the corresponding
crosslinked cellulose ester with T = 50%.
crosslinking resulted in an increase of intensity of the 723 cm ꢂ 1 band, characteristic of
the cis-alkene isomer.
1
Concerning H NMR data, only starting cellulose esters can be analyzed due to the
1
insolubility of casted crosslinked products in organic solvents. The H NMR spectrum
of cellulose ester displays peaks at 5.80 ppm and 4.97 ppm corresponding to ethyl-
enic protons, peaks from 3.2 ppm to 5.5 ppm of anhydroglucose units and peaks
representing protons a to the carbonyl group (2.32 ppm), a to the C C double bond
(2.02 ppm), b to the carbonyl group (1.60 ppm), and the remaining aliphatic methylene
protons at 1.29 ppm. For the crosslinked products, we analyzed, after saponificationn
and TLC, the diacid by 1H NMR spectroscopy. We observed the same peaks re-
presenting aliphatic protons but the ethylenic protons are seen as two close peaks at
5.38 ppm and 5.34 ppm corresponding to protons from E and Z isomers of the diacid,
respectively. It can be noted that olefin metathesis of cellulose esters is not selective for
any of the two isomers because the E/Z molar ratio is very close to 1 in every case, as
was reported in other studies.[15]
EXPERIMENTAL
Materials
Microcrystalline cellulose (20 m), 4-dimethylaminopyridine (DMAP, Acros 99%),
bis(tricyclohexylphosphine)benzylidene ruthenium(IV) dichloride (Grubbs’s catalyst,
Strem), 10-undecenoyl chloride (Fluka ꢃ97%), and solvents were distillated under
conventional methods. Grubbs’s catalyst was stored under inert atmosphere at 4°C.
LiCl (Merck Eurolab 99%) was stored under vacuum.

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Crosslinking of Cellulose by Olefin Metathesis
Cellulose Dissolution
53
A stock solution of microcrystalline cellulose in LiCl/DMAc was prepared by
adaptation of a method used elsewhere.[17] To remove trapped water, cellulose was
subjected to a solvent exchange: after storage in vacuum, the polymer was first dipped
into methanol (30 min) and then into DMAc (30 min). A solution of 6.7% (w/v) LiCl
in DMAc was prepared by stirring for 1 h at 60°C. Then cellulose was added and
stirred at 70°C until complete dissolution (approximately 3 h). The stock solution
concentration is 33 g/L LiCl/DMAc.
Acylation of Cellulose
DMAP was firstly dissolved at room temperature in cellulose solution before
adding 10-undecenoyl chloride, and the mixture was subjected to activation in a
domestic microwave oven for 1.5 to 4 min (power output = 180 W).
In a typical experiment, 2 g of cellulose (60 mL of the stock solution, 12.3 mmol
of anhydroglucose unit) was stirred with 1.5 g of DMAP (12.3 mmol, 1 equiv) until
complete dissolution. Then, 5 or 6 equiv of 10-undecenoyl chloride were added. After
microwave irradiation, the esterified product was precipitated by addition of water and
twice purified by dissolution in chloroform followed by precipitation with methanol.
Casting, Characterization and Degree of Substitution
Every product was cast and gave plastic films. Cellulose ester (white powder) was
dissolved in 30 mL chloroform and then the solvent was evaporated at room tem-
perature under atmospheric pressure.
All products were characterized by FT-IR spectroscopy using an FT-IR Perkin–
1
Elmer Spectrum 1000 apparatus and by H NMR in CDCl3 using a Bruker DRX-400
spectrometer (operating at 400 MHz). Chemical shift was measured in ppm with TMS
as the internal standard.
The degree of substitution (DS) was determined by pHmetric titration after
saponification of esters in 0.25 M NaOH at 50°C during 17 h or by 1H NMR
spectrometry, depending on the solubility of cellulose esters in chloroform as described
in one of our preceding papers.[18]
Crosslinking Reaction
The reaction took place under inert atmosphere. The unsaturated cellulose ester
was dissolved in dichloromethane with bubbling argon. Grubbs’s catalyst was removed
and stored under an inert atmosphere before its introduction into the ester solution.
Cellulose ester (1 g/100 mL CH2Cl2) was mixed with 0.1% to 0.6% of catalyst per
mole of alkene unit and stirred for 2 h. After reaction, the crosslinked products were
converted into plastic films by direct casting.
Experiments were carried out to study the reproducibility of olefin metathesis
applied to cellulose esters. For esters of identical DS, the same ratio of catalyst was
introduced in ester solution and the reaction mixture was stirred during 2 h under
inert atmosphere.

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Joly, Granet, and Krausz
Determination of the Crosslinking Ratio
The crosslinking ratio (T) was determined by saponification of the metathesis
product using a 0.5 M NaOH solution at 80°C during 24 h. After acidification with HCl
(pH 2) and extraction of the mixture of 10-undecenoic and 10-eicosendioic acids with
chloroform, these two compounds were separated by TLC (eluent CHCl3/EtOH 93/7)
and T was calculated according to the following formula:
nd
T ¼
ꢀ 100
nt
T is the quotient ( ꢀ 100) of mole number of diacid isolated after saponification (nd)
to the theoretical one (nt) for a complete reaction. T values can be related to un-
decylenic acid metathesis yields.
ACKNOWLEDGMENTS
We wish to thank the ‘‘Conseil Re´gional du Limousin’’ for financial support. We
also are grateful to Dr. Guilloton for help in manuscript preparation and to Dr. Zerrouki
for helpful discussion.
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Received February 7, 2002
Accepted November 26, 2002