regular linking of cellulose nanocrystals via click chemistry: synthesis and formation of cellulose...
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Regular Linking of Cellulose nanocrystals via Click Chemistry:
Synthesis and formation of Cellulose Nanoplatelet Gels
TEMPO-mediated Oxidation of Cellulose Nanocrystals (CNCs)
TEMPO –mediated oxidation selectively oxidizes the primary hydroxyl groups while leaving unaffected the secondary hydroxyl groups
OO
OO
O
OO
O H
O H
O H
O HOHOH
OH
OH
OHO H
H O O C
H O O C
H O O C
H O O C
OO
OO
O
OO
O H
O H
O H
O HOHOH
OH
OH
OHO H
N a O O C N aO O C
N aO O C N aO O C
(n-2)/2
(n-2)/2
2,2,6,6-tetramethyl-1-piperidinyloxy-radical (TEMPO)
NaBr
NaOCl
pH 10R.T.
Montanari et al. Macromolecules, 2005, 38, 1665-1671
Fourier Transform Infrared Spectroscopy (FTIR)
C=O band
CNCsTEMPO-oxidized CNCs
1730 cm-1
New signal at 1730 cm-1 indicates successful oxidation reaction
Degree of Oxidation (DO)
• DO was determined by using acid-base titration and FTIR
• Excellent correlation was found between these two methods
• Results are in good agreement with the recent literature (Montanari et al., Macromolecules, 2005, 38, 1665-1671, Saito et al., Biomacromolecules, 2006, 7, 1687-1691)
Strong Acid – Weak Acid Titration
50 mg of dry TEMPO-oxidized material was suspended in 15 ml of 0.01 M HCl.After 10 min of stirring the suspension was titrated with 0.01M NaOH.
162 x C x (V2-V1)w-36 x C x (V2-V1)
= 0.19
DO=
Where,
C= NaOH conc.V1, V2= amount of NaOH (L)W= weight of the O.D. sample (g)
Value 36 is the difference betweenthe molecular weight of an AGUand that of the sodium salt of aglucuronic acid
Weak acid region
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14 16 18 20 22 24 26
pH
NaOH (ml)
Da Silva Perez et al., Biomacromolecules, 2003, 4, 1417-1425
FT-IR
DO=I1730
I1050
≈ 0.20
COOH (1730)
Cellulose backbone (1050)
Habibi, Y., Chanzy, H., and Vignon, M. R. Cellulose, 2006, 13(6), 679-687.
• In general, 1,3-dipolar cycloaddition reactions have long been popular in the generation of carbohydrate mimetics in the homogeneous reaction environment.1
• The reaction has gained increasing attention after discovering that the 1,3-dipolar cycloaddition between azides and terminal alkynes can be catalysed by Cu(I) salts.2,3
• Several authors have described the use of this novel click-chemistry concept for the generation of carbohydrate mimetics and derivatives.4,5,6
Click Chemistry
1. Gallos, J. K. and Koumbis, A. E. Curr. Org. Chem., 2003, 7(5), 397–426.2. Tornøe, C. W.; Christensen, C.; and Meldal, M. J. Org. Chem., 2002, 67(9), 3057–3064.3. Rostovtsev, V. V.; Green, L. G.; Fokin V. V.; and Sharpless, K. B. Angew.Chem., Int. Ed., 2002, 41(14), 2596–2599.4. Huisgen, R. Pure Appl. Chem., 1989, 61(4), 613−628.5. Kolb, H. C; Finn, M. G; and Sharpless, K. B. Angew. Chem., Int. Ed., 2001, 40(11), 2004−2021.6. Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel, A.; Voit, B.; Pyun, J.; Fréchet, J. M. J.; Sharpless, K. B.; and Fokin, V. V. Angew. Chem., Int. Ed., 2004, 43(30), 3928 −3932.
Huisgen Cycloaddition
• Cellulose nanoplatelet gels (Click chemistry)• Grafting click precursors onto TEMPO-oxidized CNCs• Huisgen reaction (1,3-dipolar cycloaddition)
The Huisgen Cycloaddition is the reaction of a dipolarophile with a 1,3-dipolar compound that leads to 5-membered (hetero)cycles.
The Creation of the Azido Functionality on the Nano Crystals
O
OO
O
OH
O
OH
OHOH
O
OO
O
Na
Na
NH2
OO
ON3
O
O
O
N3
NH
NH
O
O
O
N3
O
OO
O
OH
O
OH
OHOH
O
O
+
+-
-
+
MES pH 4
EDC/NHSrt, 24h
CNC 11-azido-3,6,9-trioxaundecan-1-amine (AZ)
CNC-AZ
N3 (2120 cm-1)C=O (1730 cm-1)
The Creation of the Alkyne Precursor on the Crystals
O
OO
O
OH
O
OH
OHOH
O
OO
O
Na
Na
NH2
NH
NH
O
OO
O
OH
O
OH
OHOH
O
O
+
+-
-
+
MES pH 4
EDC/NHSrt, 24h
CNC
CNC-PR
Propargylamine (PR)
The Formation of CNC-Based Click-Gels
NH
NH
O
OO
O
OHO
OH
OHOH
O
O
NH
NH
O
OO
O
OHO
OH
OHOH
O
O
O
O
O
N
NH
NH
O
O
O
N
O
OO
O
OHO
OH
OHOH
O
O
NN
N
N
NH
NH
O
OO
O
OHO
OH
OHOH
O
O
O
O
O
N3
NH
NH
O
O
O
N3
O
OO
O
OHO
OH
OHOH
O
O
NH
NH
O
OO
O
OHO
OH
OHOH
O
O
CNC-Click
CuSO4 x 5H2O
Ascorbic acid
CNC-PR
CNC-AZ
CNC-PR
N3 disappeared
Click Reaction
Precursors in H2O
Gel formation: Elemental analysis:
Sample % C % H % N % Oa
Tempo-ox. CNC 41.75 5.76 0.08 52.41
CNC 43.55 6.11 0.04 50.30
CNC-AZ 42.87 5.20 1.90 50.03
CNC-PR 43.20 5.29 0.79 50.72
CNC-Click 43.28 6.03 1.51 49.18
aO = 100 % - C (%) – H (%) – N (%)
CuSO4
Ascorbic acid
vigorous stirring (1 min)
The grafting densities of CNC-PR and CNC-AZ were calculated by using the DO value of 0.20 for Tempo-oxidized CNCsand the elemental composition of CNC-PR and CNC-AZ ; graftingdensities of CNC-PR and CNC-AZ were found to be ~50% and ~35%, respectively
Transmission Electron Microscopy
IndividualCNC
TEM images of TEMPO-oxidized cellulose nanocrystals (a) and cellulose nano-platelet gel (b). Scale bar 500 nm
a b
The average dimensions of the crystals were found to be approximately 10-20 nm width and 100-200 nm long.
Thermogravimetric Analysis
CNC-Click
CNC-Click has different thermal properties than its precursors
Benzoylation of Cellulose; Allowing the Visualization of Mol. Weight Distribution
Cell-OH + Benzoyl-Cl Cell-Bz
• Reaction is carried out in Ionic Liquid
• UV – active benzoyl group
• Benzoylated cellulose is completely soluble in THF (compatible with GPC)
NN
Cl+
Gel Permeation Chromatography
CNC-Click
1.00E+031.00E+041.00E+051.00E+061.00E+070
0.2
0.4
0.6
0.8
1
1.2
Mw
UV
Ab
sorb
ance
Solid line (CNC-Click); Dotted line (CNC-PR);Dashed line (CNC-AZ)
CNC-click showed significantly higher molecular weight (cross-linking)Than its precursors (CNC-AZ and CNC-PR)
Sample Mn (1 x103 gmol-1)
MW (1 x103 gmol-1)
Mp (1 x103 gmol-1)
PD
CNC 28 69 51 4.1
CNC-PR 21 72 53 3.5
CNC-AZ 15 53 35 3.4
CNC-Click 27 335 63 12.6
Summary
The primary hydroxyl groups in cellulose nanocrystals were first selectively oxidized to carboxylic acids which were further used as reactive sites for the amidation reactions to provide the precursors necessary for the Click chemistry reaction.
Click-chemistry has been utilized for the preparation of new gel-like cellulose nanomaterials.
Benzoylation of CNCs allows complete dissolution of material into THF and makes GPC analysis possible