Synthesis and characterization of cellulose acetate grafted with ionic liquids containing different anions

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Introduction to Cu-catalyzed Azide-Alkyne Cycloaddition CuAAC

Among the cycloaddtion click reactions Sharpless et al used to consider the Cu(I) catalyzed Huisgen 1,3-dipolar cycloaddition of azides and alkynes (CuAAC) as the cream  and crop due to that the azides and terminal alkynes are fairly easy to introduce by chemical functionalization and they are extremely stable at standard conditions in addition to the absence of dimerization problems of the azide group [1]. They both can tolerate oxygen, water, common organic synthesis conditions, biological molecules, a large range of pH, and the reaction conditions of living systems (reducing environment, hydrolysis, etc.) [6].
Among the several advantages of CuAAC, this reaction is particularly attractive for bioscience. Firstly, as aforementioned, CuAAC proceeds well in aqueous medium and therefore may be efficiently performed under physiological conditions. Perhaps even more importantly, CuAAC is an extremely chemoselective reaction and can therefore be used for modifying highly functional biomolecules such as polypeptides, nucleic acids or polysaccharides [3].
In the absence of transition-metal catalyst, 1,3-dipolar Huisgen cycloadditions of azides and alkynes are, in most cases, not regioselective and usually rather slow at room temperature [12]. However, Meldal and coworkers reported that the use of catalytic amounts of copper(I), which can bind to alkynes, leads to fast, highly efficient and regioselective azide–alkyne cycloadditions at room temperature in organic medium [13]. Moreover, CuAAC can be successfully performed in polar media such as tert-butyl alcohol, ethanol or pure water led to a remarkable development of Huisgen cycloadditions in synthetic chemistry. Hence, CuAAC has been exponentially investigated within the last few years in organic synthesis, inorganic chemistry, polymer chemistry and biochemistry [3, 14]. Such rapid adoption of CuAAC in almost all areas of chemistry is rather unique and illustrates the versatility of this click reaction [3].

Mechanism of Cu-catalyzed Azide-Alkyne Cycloaddition (CuAAC)

The role of copper in the catalysis of the triazole formation has been subject to many debates since the discovery of this extremely useful cycloaddition, in which the catalyst accelerates the rate of reaction by 7 orders of magnitude [13].
[10] and updated by Bock et al.[15] indicating that using of the Cu(I) catalyst improve the regioselectivity of the reaction to get absolute 1,4-triazoles and the reaction is kinetically second order. This means that at least two cupper atoms are probably used as linkage center for two acetylene groups by µ bridge (Fig. 7).
From Figure 7, it was found that in the first step of the reaction, Cu(I) can insert itself into the terminal alkynes. It can coordinate with the π electron of terminal alkynes. This coordination is responsible for lowering the pKa of the acetylene proton, thus allowing deprotonation of the alkynes in aqueous solvent without the addition of a base. The structure of the resulting Cu(I) acetylide is very difficult to predict and approximately 35 structures have been reported for this intermediate . In the case of using a non basic solvent, a base, has to be added [6].

Catalytic systems for Azide-Alkyne Cycloadditions (AAC)

A variety of methods have been used to generate active catalysts that affect on 1, 3 dipolar cycloaddition reactions. One of the most common methods is the reduction of Cu(II) salts e.g. CuSO4.5H2O in the presence of a reducing agent such as sodium ascorbate to obtain the active Cu(I). There are large number of reducing agents that can be used with a reasonable success such as hydrazine and tris(2-carboxyethyl)phosphine. This catalytic system is preferred in many cases because it is very cheap, can be performed in water and does not need deoxygenated atmosphere. But the main drawback of this system is that the reducing agent may also reduce Cu(II) to Cu(0) , this limitation can be overcome by using a proper ratio of the reducing agent to the catalyst and addition of stabilizing agent such as tris(hydroxypropyl triazolyl methyl) [6].
Another source of Cu(I) is the direct addition of Cu(I) salts such as copper halides, CuI and CuBr. Copper halides are largely applicable in polymer and biological reactions. They are soluble in organic solvent and the reaction rate increases comparing to CuSO4.5H2O. But purity of the catalyst system is a more important criterion, so using of CuSO4.5H2O is more reliable than copper halides due to high purity of CuSO4.5H2O. In case of CuI, it is often selected when special anhydrous condition is required, and can be purified because of its partial solubility in intermediate polar solvent such as acetonitrile, THF, and acetone.
Another catalytic system consists of oxidizing copper metal with an amine salt, but this method has a lot of problems. First, it needs long reaction time and using of large amount of copper, which makes it more expensive comparing to the other methods. In addition, it requires acidic environment that may affect negatively on the acidic sensitive functional groups [6].
Using of Cu(I) modified zeolites as catalytic systems are very promising, since they are high porous, have high surface area, high thermal/hydrothermal stability and high site of selectivity [13].

Advanced crosslinked cellulose-based networks by CuAAC click chemistry

CuAAC click chemistry has brought new simple ways of crosslinking cellulose and cellulose derivatives on the basis of azido or propargyl-cellulose. Compared to former crosslinking strategies using difunctional agents, the new approaches based on CuAAC click chemistry offered specific advantages for designing crosslinked cellulose-based newtorks.[26]. The chemical stability of the triazole ring formed by reaction of azido with alkyne groups was one of the important advantages compared to the weakness of former ester-containing crosslinking bridges towards hydrolysis. The new strategy also allowed a much better control of network formation by avoiding intramolecular reactions of the azido or alkyne groups. In this new strategy, the azido or alkyne side groups had to react with complementary groups on other polysaccharide chains to form the crosslinking bridges, leading to improved three-dimensional networks.

Crosslinked cellulose-based structural materials

Over the past few years, new strategies based on CuAAC click chemistry have led to original advanced crosslinked structural materials from cellulose alone or cellulose combined with other polysaccharides. A nice example was reported by Faugeras et al. with a simple approach for cellulose crosslinking (Fig.14).[26] In this work, azido-cellulose with a DSazide of 1.5 was reacted with propargyl-cellulose with a DSalkyne of 1.3. CuAAC click chemistry was performed in DMSO/H2O with CuSO4, 5H2O in presence of sodium ascorbate (reducing agent) during 7 days at room temperature or activated by microwave irradiation. Scanning electron microscopy revealed very significant differences between the azido- or propargyl-celluloses, and the resulting crosslinked porous networks.

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Crosslinked cellulose-based hydrogels

CuAAC click chemistry has also offered innovative pathways to crosslinked polysaccharide-based hydrogels, which have been recently reviewed by Elchinger et al. and Uliniuc et al..[22, 25] Amongst them, cellulose-based hydrogels have remained really scarce so far, despite their high potential as crosslinked bio-based hydrogels.
With this respect, Koschella et al. have reported the carboxymethylation of azido- and alkyne-celluloses leading to water soluble cellulose derivatives for CuAAC click chemistry.[43] Transparent hydrogels were then readily obtained by adding CuSO4, 5 H20 and ascorbic acid to aqueous solutions containing both cellulose derivatives with equimolar ratio of azido and alkyne groups (Fig. 19). Rheologic measurements showed that the gelation time strongly decreased with increasing DSazide, DSalkyne and copper catalyst concentration. Some of the freshly prepared hydrogels could further swell in water up to a water content of ca. 100%. However, in these challenging conditions, these hydrogels lost their mechanical withstanding and disintegrated. According to the authors, improving click chemistry for these systems could lead to advanced stimuli responsive cellulose-based hydrogels.

Table of contents :

Chapter 1 Bibliography
Grafting of cellulose and cellulose derivatives by CuAAC click chemistry
1. Introduction to click chemistry
1.1. Definition of click chemistry
1.2. General classification of click reactions
1.3. Click chemistry based on Cu-catalyzed Azide-Alkyne Cycloaddition (CuAAC)
1.3.1. Introduction to Cu-catalyzed Azide-Alkyne Cycloaddition CuAAC
1.3.2. Mechanism of Cu-catalyzed Azide-Alkyne Cycloaddition (CuAAC)
1.3.3. Catalytic systems for Azide-Alkyne Cycloadditions (AAC)
1.3.4. Copper free Azide-Alkyne Cycloadditions (AAC)
2. Grafting of cellulose and cellulose derivatives by CuAAC click chemistry
2.1. Introduction
2.2. Pre-click modification of cellulose and cellulose derivatives for CuAAC click chemistry 20
2.3. Advanced crosslinked cellulose-based networks by CuAAC click chemistry
2.3.1. Crosslinked cellulose-based structural materials
2.3.2. Crosslinked cellulose-based hydrogels
2.4. Block and graft cellulosic copolymers by CuAAC click chemistry
2.5. Dendronised celluloses by CuAAC click chemistry
2.6. Cellulosic polyelectrolytes by CuAAC click chemistry
2.7. Advanced cellulose (nano)materials by CuAAC surface modification
2.7.1. Advanced materials by cellulose surface modification
2.7.2. Advanced materials by nanocellulose surface modification
3. Conclusion
4. Acknowledgements
5. References
Chapter 2 PLA grafting onto cellulose acetate by « click » chemistry. Application to new bio-based membranes for ethyl tert-butyl ether (ETBE) bio-fuel purification by pervaporation.
1. Introduction
2. Material and methods
2.1. Materials
2.2. CA functionalization and grafting
2.2.1. Synthesis of 6-azidohexanoic acid
2.2.2. Synthesis of α-alkyne PLA1640
2.2.3. Modification of CA with azide side groups
2.2.4 Grafting of the azido CA by « click » chemistry with α-alkyne PLA1640
2.2.5. Physicochemical characterization
2.3. Membrane preparation for pervaporation and sorption experiments
2.4. Sorption experiments
2.5. Pervaporation experiments
3. Results and discussion
3.1. Synthesis and characterization of cellulose acetate grafted with PLA
3.1.1. Synthesis of azido cellulose acetate
3.1.2. Synthesis of α-alkyne PLA
3.1.3. Synthesis and characterization of cellulose acetate grafted with PLA
3.2. Sorption properties of cellulose acetate grafted with PLA for ETBE purification
3.3. Pervaporation properties of cellulose acetate grafted with PLA for ETBE purification
4. Conclusion
5. References
Supporting information
Synthesis of 6-azido hexanoic acid
Chapter 3 Grafting of cellulose acetate with ionic liquids for biofuel purification by a membrane process . 
Chapter 3 Part 1 Grafting of cellulose acetate with ionic liquids using click chemistry
1. Introduction
2. Experimental
2.1. Materials
2.2. Methods
2.2.1. Synthesis of 1- methyl-3-propargyl imidazolium bromide
2.2.2. Grafting of imidazolium ionic liquid onto cellulose acetate by « click » chemistry
3. Results and discussion
4. Conclusion
5. References
Chapter 3 Part 2 Grafting of cellulose acetate with ionic liquids for biofuel purification by a membrane process: Influence of the cation.
Abstract
Table of contents
1. Introduction
2. Experimental
2.1. Materials
2.2. Synthesis and characterization of cellulose acetate grafted with different ionic liquids
2.2.1. Synthesis of a bromo-cellulose acetate derivative
2.2.2. Reaction of bromo-cellulose acetate with different nucleophiles for cellulose acetate grafting by different ionic liquids
2.2.3. Polymer characterization
2.3. Membrane preparation for pervaporation and sorption experiments
2.4. Sorption experiments
2.5. Pervaporation experiments
3. Results and discussion
3.1. Synthesis and characterization of cellulose acetate grafted with different ionic liquids
3.1.1. Synthesis of cellulose acetate grafted with different ionic liquids
3.1.2. Characterization of cellulose acetate grafted with different ionic liquids
3.2. Membrane properties for ETBE biofuel purification by pervaporation
3.2.1. Sorption properties of cellulose acetate grafted with different ionic liquids
3.2.2. Pervaporation properties of cellulose acetate grafted with different ionic liquids
3.2.3. Chemical physical analysis of the membrane properties based on ionic liquid polarity parameters
4. Conclusion
5. Acknowledgements
6. References
Appendices
Appendix A. Permeability calculation and data for ETBE purification by pervaporation
Appendix B. 1H NMR characterization of the bromo-cellulose derivative.
Chapter 3 Part 3 Grafting of cellulose acetate with ionic liquids for biofuel purification by a membrane process : Influence of the anion.
1. Introduction
2. Experimental
2.1. Materials
2.2. Synthesis and characterization of cellulose acetate grafted with ionic liquids containing different anions
2.2.1. Synthesis of cellulose acetate grafted with ionic liquids containing acetate anion
2.2.2. Synthesis of cellulose acetate grafted with different ionic liquids containing Tf2N or BF4
 anions
2.2.3. Polymer characterization
2.3. Membrane preparation for pervaporation and sorption experiments
2.4. Sorption experiments
2.5. Pervaporation experiments
3. Results and discussion
3.1. Synthesis and characterization of cellulose acetate grafted with different ionic liquids by anion exchange
3.1.1. Cellulose acetate grafted with imidazolium or ammonium ionic liquids with acetate counter anions
3.1.2. Cellulose acetate grafted with imidazolium and ammonium ionic liquids with fluorinated counter anions
3.1.3. Morphology characterization of the different cellulosic materials based on DSC and synchrotron SAXS
3.2. Sorption properties of cellulose acetate grafted with ionic liquids containing different anions for ETBE purification
3.2.1 Influence of the ionic liquid anion on sorption properties of cellulose acetate grafted with different ionic liquids for ETBE purification
3.3. Pervaporation properties of cellulose acetate grafted with different ionic liquids for ETBE purification
3.4. Kamlet-Taft analysis of the membrane properties based on ionic liquid polarity parameters
3.4.1. Choice of the Kamlet-Taft parameters used for the physico-chemical analysis
3.4.2. Kamlet-Taft analysis of the sorption and pervaporation properties
4. Conclusion
5. Acknowledgements
6. References

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