Regular Linking of Cellulose nanocrystals via Click Chemistry: Synthesis and formation of Cellulose Nanoplatelet Gels

H O O C H O H O

TEMPO-mediated Oxidation of Cellulose Nanocrystals (CNCs)

O O H O H O H O O C O H O H O O C O H O O O H O H O H O O C (n-2)/2 O H O O H pH 10 R.T.

2,2,6,6-tetramethyl-1-piperidinyloxy-radical ( TEMPO) NaBr NaOCl N a O O C H O H O O O H O H O N a O O C O H O N a O O C O H O O O H O H O N a O O C (n-2)/2 O H O O H TEMPO –mediated oxidation selectively oxidizes the primary hydroxyl groups while leaving unaffected the secondary hydroxyl groups

Montanari et al. Macromolecules, 2005, 38, 1665-1671

Fourier Transform Infrared Spectroscopy (FTIR) CNCs TEMPO-oxidized CNCs C=O band 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

12 10 8 6 4 2 0 0 2 4 6

Weak acid region

DO= 162 x C x (V2-V1) w-36 x C x (V2-V1) = 0.19

Where, C= NaOH conc.

V1, V2= amount of NaOH (L) W= weight of the O.D. sample (g)

8 10 12 14

NaOH (ml)

16 18 20 22 24 26

Value 36 is the difference between the molecular weight of an AGU and that of the sodium salt of a glucuronic acid

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.

Da Silva Perez et al.,

Biomacromolecules

,

2003

,

4

, 1417-1425

FT-IR

Cellulose backbone (1050) DO= I 1730 I 1050 ≈

0.20

COOH (1730) Habibi, Y., Chanzy, H., and Vignon, M. R. Cellulose, 2006, 13(6), 679-687.

Click Chemistry

•

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

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 Na + O O H O O OH

CNC

O O H O O OH O O Na + O + H 2 N O N 3 O O

11-azido-3,6,9-trioxaundecan-1-amine (AZ)

 C=O (1730 cm -1 ) O H O N 3 O O O H N O

MES pH 4 EDC/NHS rt, 24h

O OH

CNC-AZ

OH O H O O N H O O O O N 3 O  N 3 (2120 cm -1 )

The Creation of the Alkyne Precursor on the Crystals Na + O O H O O O OH

CNC

O H O O OH O O O Na + + H 2 N

Propargylamine (PR)

MES pH 4 EDC/NHS rt, 24h O H O H N O O OH

CNC-PR

O H O O N H OH O O

The Formation of CNC-Based Click-Gels

O H O H N O O OH O H O O N H CNC-PR OH O O O H O N 3 O O O H N O O OH CNC-AZ O H O O OH O N H O O O O N 3

 N 3 disappeared

O H O O NH O OH O H O O CNC-PR O NH O OH

H N O O H O

CuSO 4 x 5H 2 O Ascorbic acid

O OH O OH O H O O N N H N N O O O H N O O O O O H O OH O H O O

CNC-Click

OH O N H O O H O O NH O OH O O H O O O N N N NH O OH O

Gel formation:

Click Reaction

Elemental analysis: Sample Tempo-ox.

CNC CNC % C 41.75

43.55

% H 5.76

6.11

% N 0.08

0.04

% O a 52.41

50.30

CNC-AZ 42.87

5.20

1.90

50.03

Precursors in H 2 O CuSO 4 Ascorbic acid CNC-PR 43.20

5.29

0.79

50.72

vigorous stirring

(1 min)

CNC-Click 43.28

6.03

1.51

a O = 100 % - C (%) – H (%) – N (%)

49.18

The grafting densities of CNC-PR and CNC-AZ were calculated by using the DO value of 0.20 for Tempo-oxidized CNCs and the elemental composition of CNC-PR and CNC-AZ ; grafting densities of CNC-PR and CNC-AZ were found to be ~50% and ~35%, respectively

a

Transmission Electron Microscopy

b

Individual CNC TEM images of TEMPO-oxidized cellulose nanocrystals (a) and cellulose nano-platelet gel (b).

Scale bar 500 nm 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 N + N Cl Cell-Bz • Reaction is carried out in Ionic Liquid • UV – active benzoyl group • Benzoylated cellulose is completely soluble in THF (compatible with GPC)

Gel Permeation Chromatography

CNC-Click

0,4 0,2 1,00E+07 1,00E+06 1,00E+05

M w

1,00E+04

Solid line (CNC-Click); Dotted line (CNC-PR);

Dashed line (CNC-AZ) 0 1,00E+03 1,2 1 0,8 0,6

Sample CNC CNC-PR CNC-AZ CNC-Click M n (1 x10 3 gmol -1 ) M W (1 x10 3 gmol -1 ) M p (1 x10 3 gmol -1 ) PD 28 21 15 27 CNC-click showed significantly higher molecular weight (cross-linking) Than its precursors (CNC-AZ and CNC-PR) 69 72 53 335 51 53 35 63 4.1

3.5

3.4

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