Regioselectively Functionalized Cellulose Ethers
-Preparation, Characterization and Properties-
A member of
Dominik Fenn, Nicolas Illy, Andreas Koschella, Thomas Heinze
Introduction
Esterification- and etherification reactions of cellulose under
heterogeneous conditions mostly lead to products with a distribution
of substituents within the repeating unit and along the polymer chains.
Beside glucose, 7 differently functionalized repeating units can be
detected, which may contribute to the polymer's properties, e.g.,
rheological behavior and thermoreversible gelation (Fig. 1).
As summarized in Fig. 2 the work presented in the poster is focused on:
Fig. 1: Functionalized as repeating units in cellulose derivatives.
2,3,6-tri-O
Substituent
Different raw materials for cellulose derivatization are applied regarding, e.g., source, DP, sugar composition, and
crystallinity, that may influence the reactivity. Especially the crystallinity affects the accessibility of the hydroxyl
functions for the reagent, while the content of alien polysaccharides, e.g. hemicelluloses, may influence the product
properties. The DP determines the viscosity of cellulose derivative solutions and the processability of cellulose under
homogeneous reaction conditions.
Cellulose derivatives with an uniform distribution of substituents are indispensable for the understanding of structureproperty-relationships. The synthesis of 2,3-di- and 3-mono-O-functionalized cellulose derivatives is carried out by
using protective group techniques.1 Triphenylchloromethane reacts with the primary OH group of the anhydroglucose
unit (AGU), while thexyldimethylchlorosilane (TDMS-Cl) forms 2,6-di-O-TDMS cellulose of high purity. In addition, a
multi step synthesis for 6-O-functionalized cellulose ethers has been developed.2 With regard to 3-O-functionalized
cellulose ethers it has been found that 3-mono-O-methyl cellulose is insoluble in organic solvents, while increasing the
alkyl chain renders the polymer soluble (Tab. 1).
•
Activation of pulps and assessment of structural changes and reactivity
•
Preparation of regioselectively functionalized cellulose ethers
•
Characterization of the structure and properties of cellulose ethers with different functionalization patterns.
Tab. 1: Solubility of 3-O-functionalized cellulose ethers
Soluble ina
DMA
Cellulose
Ethanol
DMSO
Methyl3
-
-
-
THF H2O
-
-
Ethyl4
-
+
+
-
+
Allyl3
-
+
+
-
-
n-Pentyl5
+
+
+
+
-
iso-Pentyl5
+
+
+
+
-
Dodecyl5
-
-
-
+
-
It has been demonstrated that the n-pentyl- and
isopentyl ethers possess aggregates in solution. In
contrast,
the
dissolved
3-O-dodecyl
ether
is
molecularly dispersed.
Pulp activation and assessment of reactivity
Tab. 2: Specification of the pulps used for the investigations.
Code
Pulp
The influence of pretreatments on the properties of different pulps
(Tab. 2) was investigated, i.e.,
• Mercerization of 3.0 g pulp in 60 ml NaOH (18%) for 1 h at room
temperature followed by washing with water until neutral reaction
(Treatment 1).
• Stirring of 3.0 g pulp in 150 ml H3PO4 (85%) for 20 min at room
temperature, addition of 150 ml tetrahydrofuran (THF), stirring for
40 min at room temperature and washing with THF and water until
neutral reaction (Treatment 2).
Crystallinity
The degree of crystallinity of the samples was determined by
means of 13C-CP/MAS NMR spectroscopy of the freeze-dried
samples.
Both NaOH and H3PO4, lead to a significant decrease of the
crystallinity (Tab. 3). However, NaOH is the more effective reagent
for decrystallizing of pulps.
B
45.4
31.1
36.2
C
46.1
25.7
38.0
D
45.0
37.0
43.6
1433
0.00
B
1157
14.80
Beech Sulfite
C
604
5.40
Spruce Sulfite
D
504
3.40
,.8
8%
11.,8 %
8%
%
Both NaOH and H3PO4 lead to a notable decrease of the hemicellulose
content (Fig. 3). NaOH is more effective than H3PO4.
native
unbehandelt
NaOH
Phosphorsäure
H
3PO 4
1144
1400
1200
1200
1000
800
800
600
400
400
200
00
A
A
B
B
C
C
D
D
O
HO
O
OH
1. R-I, NaH
(THF)
2. TBAF
(THF, DMSO)
O
O
HO
(N,N-Dimethylacetamide/LiCl)
O
8
4 6
1
O
11 10
CH3
2, 5
O
9
Fig. 4: Solutions of pulp A in
DMA/LiCl before (top) and
after
(bottom)
NaOHtreatment.
0.5
0.0
A
B
3
O
5 O
2
O 1
O
34
Cellulosea/DP
DSb
Cloud point
(°C, 4% in H2O)
Statistic
Ethyl
Pulp
9
Solubility
Organic media
Ethyl
1.05 Yes
DMSO
Propyl
0.67 Yes
DMSO
Propyl
1.09 No
DMA, DMF, DMSO, NMP, ethanol, methanol
Butyl
0.80 No
DMA, DMF, DMSO, NMP
N,N-Dimethylacetamide (DMA), N,N-dimethylformamide (DMF),
dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP)
4 6
CH3
1
Fig. 6: Preparation of 3-O-functionalized cellulose ethers.
Tab. 4: Degree of substitution (DS) and solubility of 3-Ofunctionalized cellulose ethers prepared from spruce sulfite pulp with
DP 560.
6
2, 510
34
9
ppm
100
80
O
9
3
8
2O 1
10
O
• Treatment of pulps with NaOH and H3PO4 increases the reactivity due to decrystallization
and extraction of hemicelluloses. H3PO4 should be used only if the strong decrease of the DP
is intended.
• 3-O-ethyl- (DS up to 1) and 3-O-propyl celluloses (DS up to 0.7) are water soluble
derivatives with a uniform structure as confirmed by means of NMR spectroscopy.
• Investigation of the thermoreversible gelation showed a distinct relationship between
clouding temperature and functionalization pattern. The macromolecules of 3-mono-O-ethyl
cellulose behave as random coils in solution.
1.1-1.6
~30
Ethyl
Avicel/300
0.67
56
Regioselective
3-mono-O-Ethyl
SSP/560
1.05
63
3-mono-O-Ethyl
Avicel/300
1.00
63
3-mono-O-Propyl
SSP/560
1.08
34
6
40
sulfite pulp (SSP), degree of polymerization (DP)
of substitution
cTaken from 9
bDegree
CH3
20
With increasing temperature flocculation of the aqueous solutions of
cellulose ethers occurs (Tab. 5). Ethyl cellulose with statistic
functionalization pattern flocculates at about 33°C. Already small
deviations from the statistic functionalization pattern influence the
thermal behavior remarkably. Ethyl cellulose prepared in DMA/LiCl in
the presence of solid NaOH (non-statistic distribution of substituents)
becomes insoluble at 56°C, which is the same range as the cloud point
temperature of 3-mono-O-ethyl cellulose.
• Regioselective functionalization is still a challenging goal in synthesis and in order to
understand the structure-property-relationships of cellulose derivatives.
55-65
Ethylc
aSpruce
Fig. 7: Dept135-NMR spectra of different 3-mono-O-alkyl-2,6-di-O-acetyl
celluloses prepared from spruce sulfite pulp recorded in CDCl3.
Conclusion/Summary
33
1.3-2.5
Non-statistic
10
7
O
5 O
O
60
1.3
Methylc
CH3
8
CH3
O
DS 1.05
Fig. 5: Silylation of different pulps in DMA/LiCl
before („) and after („) NaOH-treatment.
11
O
OH
R= CH2CH3
DMSO: Dimethylsulfoxide
THF:
Tetrahydrofuran
CH2CH2CH3
TBAF:
Tetrabutylammonium fluoride trihydrate
CH2CH2CH2CH3
TDMS-Cl: Thexyldimethylchlorosilane
D
Cellulose ether
O
RO
C
Functionalization
pattern
8
CH3
7
O
Si
Water
1.0
Tab. 5: Thermoreversible gelation of cellulose ethers depending on the type of
substituent and the functionalization pattern.
NMR-spectroscopic measurements showed the structural uniformity of
the derivatives synthesized (Fig. 7). The substituents are exclusively
bound to position 3 of the AGU and the samples are free of impurities.
OH
O
3-O-Ether DS
1.5
Pulp
Fig. 3: Dependence of DP and hemicellulose
content of the pulps on the treatment.
DS 1.07
OH
2.0
Pulp
Regioselectively functionalized cellulose ethers were prepared by
protecting positions 2 and 6 of the AGU followed by etherification of
position 3 with alkyl iodides in the presence of NaH (Fig. 6).
Repeated conversion with tetrabutylammonium fluoride trihydrate
for deprotection yielded the corresponding cellulose ethers (Tab. 4).
The samples readily dissolve in organic solvents of different polarity.
In addition, 3-O-ethyl- and 3-O-propyl cellulose are soluble in water.
Si
NMR spectroscopy of the freeze-dried samples.7
Reactivity: The pulps were dissolved in N,N-Dimethylacetamide (DMA)/LiCl and treated
with TDMS-Cl (4 mol per mol AGU) in the presence of imidazole (4.7 mol per mol AGU)
for 24 h at 100°C.
Synthesis and characterization of regioselectively functionalized cellulose ethers
TDMS-Cl
Imidazole
24 h, 100°C
13C-CP/MAS
Degree of polymerization: Viscometry in cupriethylenediamine solution according
to ISO 5351.6
Hemicellulose content: Hydrolysis with HClO4 and HPLC-analysis of the sugar mixture.8
55.,2
2%
%
55.,4
4%
11.,7 %
7%
22.,9
%
9%
%
33.2
A
Beech Sulfite
Crystallinity:
The reactivity of the NaOH-treated samples was assessed by conversion of the pulps
dissolved in DMA/LiCl with TDMS-Cl in the presence of imidazole. NaOH-treated pulps
give clear solutions (Fig. 4). The silylation experiments showed clearly that NaOH
increases the reactivity of the cellulose (Fig. 5). The initial degree of substitution (DS)
of about 1.5 can be increased to 2 after NaOH-treatment yielding high-DP 2,6-di-OTDMS celluloses.
Degree of polymerization
H3PO4
29.6
Cotton Linters
Analysis
Reactivity
Treated with
NaOH
DPCU6 Hemicellulose (%)
From Fig. 3 it becomes obvious that NaOH does not affect the DP of the
pulp. In contrast, H3PO4 leads to a remarkable polymer degradation up to
60%. Starting from DP 1433 DP (pulp A) values as low as 300 result.
Crystallinity (%)
61.5
Controlled
synthesis
Degree of polymerization and hemicellulose content
Tab. 3: Change of crystallinity of
different pulps depending on the
treatment with NaOH or H3PO4.
A
Properties
Solubility
Viscosity
Thermoreversible gelation
Fig. 2: Relationships between the cellulose raw material and the properties of the derivatives prepared thereof.
(DMF), dimethylsulfoxide (DMSO), tetrahydrofuran (THF)
Native
Product
DP
DS
Functionalization pattern
Reactivity
Treatment
aN,N-Dimethylformamide
Pulp
Chemical
functionalization
33.,4
4
00.,0 %%
022
11.,9
%
%
9%
%
R=
Properties
DP
Crystallinity
Sugar composition
By
capillary
viscometry
(Ubbelohde
viscosimeter) the diluted solution of
3-mono-O-ethyl cellulose in the range of
0.001 g cm-3 to 0.01 g cm-3 was studied
(Fig. 8).
The data are analyzed with
Huggins-equation for diluted polymers:
ηred = k’[η]2c+[η]
ηinh = k’’[η]2c+[η]
leading to k‘= 0.532 and k‘‘= -0.097 for
3-mono-O-ethyl cellulose and k‘= 0.323 and
k‘‘= -0.123 for the ethyl cellulose with
statistic functionalization pattern.
References
1Philipp B, Klemm D, Papier 49 (1995) 3
2Kondo T, Carbohydr. Res. 238 (1993) 231.
3Koschella A, Heinze Th, Klemm D, Macromol. Biosci. 1 (2001) 49
4Koschella A, Fenn D, Heinze Th, Polym. Bull. 57 (2006) 33
5Petzold K, Klemm D, Heublein B, Burchard W, Savin G, Cellulose 11 (2004) 177
6ISO 5351 (2004) Pulps- Determination of limiting viscosity number in
cupriethylenediamine (CED) solution.
7Kono H, Yunoki S, Shikano T, Fujiwara M, Erata T, Takai M, J. Am. Chem. Soc. 124
(2002) 7506
8Heinze Th, Pfeiffer K, Angew. Makromol. Chem. 266 (1999) 37
9Dönges R, Brit. Polym. J. 23 (1990) 315
600
y = 26745x + 224,28
2
R = 0,9963
500
ηred ηinh
2,6-di-O
2,3-di-O
3,6-di-O
The regioselectively functionalized cellulose ethers published up to now were often prepared from cellulose with
average degree of polymerization (DP) <300 although samples with DP >500 may allow a better comparison with
technically important cellulose ethers.
Degree of substitution
Glucose
6-mono-O
2-mono-O
3-mono-O
400
300
200
y = -4867,2x + 227,39
2
R = 0,9873
100
0
0
0.002 0.004 0.006 0.008 0.01 0.012
Concentration (g cm-3)
Fig. 8: Plot of reduced and inherent
viscosities of 3-mono-O-ethyl cellulose vs.
concentration.
Contact:
Thomas Heinze
Center of Excellence for Polysaccharide Research
Friedrich Schiller University of Jena
Humboldtstrasse 10
D-07743 Jena
Germany
++49 3641 9 48270
++49 3641 9 48272
| Thomas.Heinze@uni-jena.de
www.uni-jena.de/chemie/institute/oc/heinze