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Taylor & Francis CRC Publisher Book "Cellulose-based Graft Copolymers : Structure and
Chemistry" edited by Dr Vijay Kumar Thakur
Chapter "Grafting of cellulose and cellulose derivatives by CuAAC click chemistry"
Faten HASSAN HASSAN ABDELLATIF, Jérôme BABIN,
Carole ARNAL-HERAULT, Anne JONQUIERES*
Laboratoire LCPM FRE 3564, ENSIC, Université de Lorraine, 1 rue Grandville, BP 20451, 54
001 Nancy Cedex France
___________________________________________________________________________
Abstract
Cellulose is one of the most abundant and inexpensive polysaccharides. Nevertheless, its
supramolecular structure, its insolubility in water and most organic solvents and its poor
processability have been limiting for several applications. Over the past few years, the copper(I)
catalysed Huisgen 1,3-dipolar cycloaddition of azides and alkynes (CuAAC) click chemistry has
offered new ways of modifying cellulose and its derivatives in particular mild conditions. This
review with 63 references focuses on this emerging field for the synthesis of advanced cellulose-
based materials and gels. The first part reports on cellulose pre-click modification for introducing
azide or alkyne groups necessary for the CuAAC click chemistry. Its application is then
reviewed for the preparation of different advanced cellulose-based macromolecular architectures:
i) crosslinked networks for structural materials and hydrogels, ii) block and graft copolymers,
and iii) dendronised celluloses. Recent works on cellulose-based polyelectrolytes are also
reviewed before addressing the particular issue of (nano)cellulose surface modification. Within
the frame of sustainable chemistry, CuAAC click chemistry certainly contributes to greatly
extend the scope and functionality of cellulose-based materials for a wide range of applications
in smart packaging, advanced food, health care, drug delivery, biomaterials, biocomposites,
biosensors, bactericidal materials, and biofunctional films.
___________________________________________________________________________
* Corresponding author. Email address: anne.jonquieres@univ-lorraine.fr, tel: +33 3 83 17 50 29,
fax: +33 3 83 37 99 77.
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Table of contents
1. Grafting of cellulose and cellulose derivatives by CuAAC click chemistry .............................. 3
1.1. Introduction .......................................................................................................................... 3
1.2. Pre-click modification of cellulose and cellulose derivatives for CuAAC click chemistry 4
1.3. Advanced crosslinked cellulose-based networks by CuAAC click chemistry .................... 6
1.3.1. Crosslinked cellulose-based structural materials .......................................................... 6
1.3.2. Crosslinked cellulose-based hydrogels ......................................................................... 8
1.4. Block and graft cellulosic copolymers by CuAAC click chemistry .................................... 9
1.5. Dendronised celluloses by CuAAC click chemistry .......................................................... 12
1.6. Cellulosic polyelectrolytes by CuAAC click chemistry .................................................... 15
1.7. Advanced cellulose (nano)materials by CuAAC surface modification ............................. 17
1.7.1. Advanced materials by cellulose surface modification .............................................. 17
1.7.2. Advanced materials by nanocellulose surface modification ....................................... 19
1.8. Conclusion ......................................................................................................................... 23
1.9. Acknowledgements ............................................................................................................ 24
1.10. References ........................................................................................................................ 24
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1. Grafting of cellulose and cellulose derivatives by CuAAC click chemistry
1.1. Introduction
Cellulose is one of the most abundant and inexpensive polysaccharides. Nevertheless, its
supramolecular structure, its insolubility in water and most organic solvents and its poor
processability have been limiting for several applications. Chemical modification of cellulose has
led to a variety of cellulosic derivatives with a broad property diversity and has opened new
scopes to cellulose-based materials. Chemical modifications have also offered new
functionalities to cellulose-based materials such as water or oil repellency, antibacterial
properties or stimuli-responsive behaviours.
Recently, important progress has been made for cellulose and its derivatives with new
modification strategies based on the copper(I) catalysed Huisgen 1,3-dipolar cycloaddition of
azides and alkynes (CuAAC) click chemistry (Fig. xx.1). Since the pioneering work of Sharpless
et al. on CuAAC click chemistry1; 2 and its first application to polymer chemistry in 2004,3
CuAAC click chemistry has become one of the best approaches to complex macromolecular
architectures and polymer modification.4-10
Insert Fig. xx.1 here
Over the past few years, CuAAC click chemistry has offered new ways of modifying cellulose
and its derivatives in particular mild conditions and with hydrolytically stable triazole linkers. Its
high reliability, efficiency and tolerance of different functional groups have contributed to
developing great structural and functional variety for cellulosic materials. According to the work
of Liebert et al. on the first modification of cellulose by CuAAC click chemistry11 and to related
recent reviews on polysaccharides,10; 12; 13 CuAAC click chemistry still offers opportunities for
4
polysaccharide modification with great potential for a wide range of applications in smart
packaging, advanced food, health care, drug delivery, biomaterials, biosensors, bactericidal and
antifouling materials, biofunctional films etc.
This chapter focuses on the emerging field of cellulose/cellulose derivatives grafting by CuAAC
click chemistry for the synthesis of advanced materials and gels. The first part reports on
cellulose pre-click modification for introducing azide or alkyne groups, which are necessary for
CuAAC click chemistry. The chapter then reviews the use of CuAAC click chemistry for the
preparation of different cellulose-based macromolecular architectures: i) crosslinked networks
for structural materials and hydrogels, ii) block and graft copolymers, and iii) dendronised
celluloses. Recent works on cellulose-based polyelectrolytes obtained by CuAAC click
chemistry are also reviewed before addressing the particular issue of advanced cellulose
(nano)materials by CuAAC surface modification.
1.2. Pre-click modification of cellulose and cellulose derivatives for CuAAC click chemistry
Pre-click modification of cellulose and cellulose derivatives is an essential step for CuAAC click
chemistry, which consists of introducing azido or alkyne groups onto the anhydroglucose rings
(Fig. xx.2).10; 14
Insert here Fig. xx.2
This functionalisation is mainly achieved after activation of the cellulose hydroxyl groups by
tosylation. The resulting p-toluene sulfonic ester of cellulose (tosyl cellulose) is one of the most
widely used intermediates in cellulose chemistry due to its high reactivity and its solubility in a
wide range of organic solvents.15 Tosylation of cellulose can be carried out by homogeneous
reaction of cellulose with p-toluene sulfonic acid in DMAc/LiCl. A wide range of substitution
degrees for the tosyl group (DStosyl) can be obtained by varying the molar ratio of p-toluene
5
sulfonic acid to the cellulose hydroxyl groups. Furthermore, cellulose tosylation takes place
preferably at position 6, as expected from the better reactivity of the corresponding primary
hydroxyl groups. Consequently, cellulose tosylation has been shown to be regioselective for
DStosyl < 1.14; 16 In addition, the tosyl group is an excellent leaving group to perform nucleophilic
substitutions with e.g. sodium azide or propargyl amine and introduce azido or propargyl groups,
respectively. Other miscellaneous techniques have been reported to a much less extent for
cellulose random pre-modification for the CuAAC click chemistry 14; 17-20.
The properties of cellulose derivatives generally differ for randomly or regioselectively
substitution of the hydroxyl groups of the anhydroglucose rings. Regioselectivity is thus an
important issue in cellulose chemistry. The introduction of the azido or propargyl groups at
positions 2 and 3 requires regioselective cellulose modification as initially reported by Koschella
et al. for propargyl groups.21; 22 Regioselective modification involves protecting group chemistry
and additional synthetic steps, which are justified only when specific positions are targeted for
the azido or propargyl groups.23 As way of example, Fig. xx.3 shows the regioselective cellulose
modification for introducing propargyl groups at position 3 according to Fenn et al..24 In the later
work, thexyldimethylsilyl protecting groups were chosen due to their high selectivity for
positions 2 and 6 in homogeneous conditions. 2,6-di-O-thexyldimethylsilyl cellulose was then
treated with allyl halide (e.g. propargyl bromide) to produce 3-O-propargyl cellulose
regioselectively. Xu et al. reported the same strategy to prepare an azido amphiphilic
regioselective cellulose derivative (3-O-azidopropoxy poly(ethylene glycol)-2,6-di-O-
thexyldimethylsilylcellulose).25
Insert Fig. xx.3 here
Pre-click modification of the terminal reducing or non-reducing ends of cellulose derivatives has
6
been very rarely reported for CuAAC click chemistry and has offered new opportunities for
designing original graft or block cellulosic copolymers.26; 27 Nakagawa et al. prepared methyl
cellulose with a terminal alkyne group at the non-reducing end by reacting methyl 1,2,3- tri-O-
methyl celluloside with propargyl bromide after activation of the terminal hydroxyl group by
NaH.26 Cellulose triacetate derivatives with terminal azido groups at the reducing end were
obtained as CuAAC precursors by Nakagawa et al.26 and Enomoto-Rogers et al.27 on the basis of
a multi-step procedure initially reported by Kamitakahara et al. in a work not related to CuAAC
click chemistry.28
1.3. 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.14 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.
1.3.1. 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
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cellulose crosslinking (Fig. xx.4).14 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.
Insert Fig. xx.4 here
This promising approach was then extended to crosslinking of azido-cellulose with
propargylated starch.29 In the later 4-step strategy, 6-azido 6-deoxy cellulose with a DSazide of
0.4 obtained under microwave irradiation, was reacted with propargylated starch with a DSalkyne
of 2.2. The crosslinked cellulose/starch networks were obtained in high yield (83%). SEM
characterisation showed the different morphologies obtained after each step and the continuity
of the polysaccharide network after crosslinking as shown in Fig. xx.5.
Insert Fig. xx.5 here
In another recent related work by Peng et al., CuAAC click chemistry was reported for
crosslinking of cellulose and chitosan, which is another important polysaccharide (Fig. xx.6).17
Propargyl-celluloses with different DSalkyne from 0.25 to 1.24 were reacted with azido-chitosans
with DSazide from 0.02 to 0.46 in DMSO/H2O with CuSO4, 5H2O in presence of sodium
ascorbate at room temperature for 48 h. FTIR characterization showed the significant decrease of
the alkyne and azido bands after click chemistry. Complementary CP/MAS 13C NMR
experiments proved the formation of triazole rings. The crosslinked cellulose/chitosan networks
had improved thermal stability compared to cellulose, chitosan and even cellulose/chitosan
complex. SEM pictures obtained after fracture of the crosslinked networks revealed the striking
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presence of hollow tubes of millimiter size, which were not observed for the corresponding
cellulose/chitosan complex (Fig. xx.7) .
Insert Fig. xx.6 here
Insert Fig. xx.7 here
Agag et al. prepared original crosslinked networks of cellulose and polybenzoxazine from azido-
cellulose and an alkyne-functionalised benzoxazine monomer (Fig. xx.8).30In the later work,
benzoxazine monomer units were first grafted onto cellulose by CuAAC click chemistry as
confirmed by FTIR and 1H NMR. Thermal curing of this cellulosic derivative at high
temperature (200°C/1h) led to the ring opening polymerisation (ROP) of the benzoxazine
monomer units. TGA analysis showed that the resulting crosslinked cellulose/polybenzoxazine
networks were much more resistant to heat than virgin cellulose, with a residual weight increased
by one order of magnitude at 800°C.
Insert Fig. xx.8 here
1.3.2. 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..10; 13
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.31
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. xx.9). Rheologic measurements showed that the gelation time strongly decreased
9
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.
Insert Fig. xx.9 here
Following another interesting synthetic pathway combining classical radical polymerisation,
polymer modification and CuAAC click chemistry, Zhang et al. synthesised a series of original
temperature-responsive hydrogels from an azido-cellulose with a DSazide of 0.95 and an alkyne-
functionalised thermo-responsive copolymer (poly(N-isopropyl acrylamide-co-hydroxyl ethyl
methacrylate) P(NIPAM-co-HEMA)).32 The grafting of the alkyne-functionalised P(NIPAM-co-
HEMA) onto the azido-cellulose was carried out in DMSO in presence of CuBr and
N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) at room temperature with different
amounts of the thermo-responsive grafts (Fig. xx. 10). After freeze-drying and cryo-fracture of
the crosslinked hydrogels, SEM pictures revealed discontinuous porous inner structures. The
average pore sizes decreased from 40 and 10 µm when the amount of the thermo-responsive
grafts increased from 50 to 80 wt%, leading to higher crosslinking degrees.
Insert Fig. xx.10 here
1.4. Block and graft cellulosic copolymers by CuAAC click chemistry
To the best of our knowledge, the synthesis of block copolymers from cellulose or its derivatives
by CuAAC click chemistry has been reported only once so far. In 2012, Nakagawa et al.
prepared amphiphilic diblock copolymers from cellulose triacetate with a terminal azido group at
position 1 and methyl cellulose with a terminal alkyne group at position 4 (Fig. xx.11).26 CuAAC
10
click chemistry in presence of CuBr, sodium ascorbate and PMDETA initially led to diblock
copolymers with two hydrophobic blocks made of low molecular weight cellulose triacetate and
methyl cellulose. After cleavage of the acetate groups by sodium methanolate in organic
medium, the cellulose triacetate block was converted into a hydrophilic cellulose block. An
amphiphilic diblock copolymer was finally obtained with thermoreversible gelation properties.
Insert Fig. xx.11 here
The preparation of graft copolymers from cellulose or cellulose derivatives has been widely
investigated and, over the past few years, a few bibliographic reviews have pointed out the recent
progress made for controlling this grafting by various techniques.10; 12; 33-35 Nevertheless, CuAAC
click chemistry has been rarely reported for the preparation of graft copolymers from cellulose
and its derivatives so far. This click chemistry is currently emerging as a promising method for
cellulose grafting and designing original materials with very different properties. Graft cellulosic
copolymers are obtained by grafting of cellulosic derivatives in homogeneous solution. The
grafting of (nano)cellulose surfaces is considered in a specific following subsection of this
chapter.
By combining controlled radical polymerisation (ATRP) with CuAAC click chemistry, Li et al.
prepared two closely related series of ethyl cellulose (EC) graft copolymers. For the first series
of graft copolymers, an EC macroinitiator with brominated side groups was used for the
"grafting from" of polystyrene (PS) grafts with controlled lengths (Fig. xx.12).19 The bromine
terminal atoms of the PS grafts were then substituted for azido groups. These azido groups were
further reacted with an alkyne-monomethoxy poly(ethylene glycol) by CuAAC click chemistry,
leading to ethyl cellulose derivatives with diblock grafts PS-b-PEG. For the second series of
copolymers, the bromine atoms of the EC macroinitiator were partially converted into azide
11
groups. The resulting "clickable" macroinitiator was then used for the one-pot "grafting from" of
styrene by ATRP and "grafting onto" of -alkyne monomethoxy-PEG by CuAAC click
chemistry. This second smart strategy took advantage of the orthogonality of ATRP and CuAAC
click chemistry for obtaining EC copolymers grafted by both PS and PEG grafts in a single
synthesis step.
Insert Fig. xx.12 here
Amphiphilic cellulosic graft copolymers were obtained by Xu et al. with PEG grafts bearing
azido terminal groups for click chemistry.25 The multi-step synthesis allowed the regioselective
grafting of PEG with allyl terminal groups at position 3 with thexyldimethylsilyl protecting
groups at positions 2 and 6. The allyl terminal groups of the PEG grafts were then converted into
azido groups in 3 steps. Honeycomb films were then prepared by the breathing figures method
and their pore sizes were controlled by the Xn of the functional PEG grafts. The honeycomb
films were grafted by biotin by CuAAC click chemistry, leading to advanced functional porous
films for bio-applications. Very recently, further studies showed the availability of the grafted
biotin for conjugation with fluorescent avidin inside the pores of the honeycomb films (Fig.
xx.13).20 CuAAC click chemistry also enabled to functionalize the honeycomb films with
Quantum Dots (QDs), which were again located inside the pores as shown by confocal
fluorescence microscopy. In another related work by the same team, the grafting of an alkynated
quaternary ammonium compound led to honeycomb films with antifouling and antibacterial
properties for Escherichia coli 36.
Insert Fig. xx.13 here
Negishi et al. reported the regioselective grafting of different alkyne-functionalised maltoside
(Mal) and lactoside (Lac) oligosaccharides onto azido-cellulose at position 6.37 The grafting was
12
carried out by CuAAC click chemistry in DMSO in presence of CuBr2, ascorbic acid, and
propylamine at room temperature for 12h with yields ranging from 65 to 79%. Water solubility
of the grafted copolymers was strongly influenced by the nature of the spacer between cellulose
and the grafted oligosaccharides. The water solubility was much lower in case of a hydrophobic
spacer although molecular dynamics simulations showed that hydrophilic and hydrophobic
spacers led to the same type of sheet-like structures for the grafted copolymers.
Compared with the former works on cellulosic copolymers grafted by CuAAC click chemistry,
another originality of the recent work of Enomoto-Rogers et al. was to consider grafted
copolymers with cellulosic grafts rather than cellulosic main chains (Fig xx.14).27 In this new
work, an amorphous polymethacrylate with alkyne side groups was initially prepared in three
steps using protecting group chemistry. The "grafting onto" of cellulose triacetate grafts with
azido groups at the reducing end was then carried out by CuAAC click chemistry. After cleavage
of the acetate groups by sodium methanolate, a polymethacrylate comb copolymer was obtained
with cellulosic grafts. An analysis by wide angle X-ray diffraction showed that a film of this
grafted copolymer was semi-crystalline with a diffraction pattern corresponding to that of
regenerated cellulose.
Insert Fig. xx.14 here
1.5. Dendronised celluloses by CuAAC click chemistry
The grafting of dendrons onto cellulose by CuAAC click chemistry has led to a variety of
biofunctional materials over the past few years. Dendrons are fractal structures with increasing
number of functional groups from one generation to the next one. Dendronization of cellulose
can thus increase its chemical functionality tremendously and offer interesting prospects for a
wide range of bio-applications (e.g. biosensors, drug delivery systems, biocatalysts etc).
13
In their first attempt to prepare dendronised cellulose, Pohl and Heinze regioselectively grafted
polyamido amine (PAMAM) dendrons of first and second generations, with an azido moiety at
their focal point, onto an alkyne-cellulose with a DSalkyne of 0.48 at position 6 (Fig. xx.15).16 The
grafting rate decreased from 70 to 50 % for the dendrons of first and second generations,
respectively, most likely due to strong steric hindrance. All the dendronised cellulose derivatives
were soluble in aprotic dipolar solvents (DMSO, DMF, DMAc).
Insert Fig. xx.15 here
Pohl et al. also investigated the reverse strategy consisting in grafting PAMAM dendrons with an
alkyne moiety at their focal point onto an azido-cellulose with a DSazido of 0.75 at position 6. In
homogeneous conditions in DMSO, grafting rates of 89, 75 and 41% were obtained for the
dendrons of first, second and third generations, respectively. Here again, steric hindrance limited
the grafting rate of the bulkiest dendrons but, overall, the new work greatly improved the number
of grafted dendrons and the functionality of the dendronised celluloses. Heterogeneous grafting
in MeOH was also remarkably well achieved with comparable grafting rates as those obtained in
homogeneous conditions.38
The regioselective grafting of PAMAM dendrons onto cellulose at position 3 required more
demanding protecting group chemistry.24 Thexyldimethylsilyl groups were chosen as protective
groups due to their high selectivity for positions 2 and 6, leaving position 3 available for
introducing alkyne side groups with a DSalkyne of 1. After deprotection, alkyne-cellulose was
successfully grafted with azido-functionalised PAMAM dendrons of first and second generations
but the grafting rates were fairly low (25%).
Water soluble dendronised celluloses were also prepared by Pohl et al. from a
carboxymethylated azido-cellulose with a DSazide of 0.81 by CuAAC click chemistry in water at
14
ambient temperature (Fig. xx.16).39 The grafting of alkyne-functionalised PAMAM dendrons
proceeded mainly at position 6 with grafting rates varying from 63 to 48 % from the first to the
third generation of dendrons. A thorough physical chemical investigation showed that the
grafting of dendrons had no or a weak influence on the cellulosic chain stiffness or conformation
in solution.
Insert Fig. xx.16 here
Novel biofunctional cellulosic films were also developed by the same team from dendronised
cellulose containing a high number of amino groups, which are particularly appropriate for
further modification with biomolecules.40 In a first approach, azido-cellulose was first grafted
with alkyne-dendrons containing terminal amine groups in homogeneous solution and the
resulting dendronised cellulose was then simply blended with cellulose acetate. The second
approach considered simple surface grafting of an azido-cellulose film with the same dendrons in
mild conditions. After careful removal of the residual copper ions by complexation with
diethyldithiocarbamate trihydrate to avoid any detrimental interference with the targeted
biomolecules, the cellulosic films were successfully biofunctionalised with a glucose oxidase
enzyme in two steps (Fig. xx. 17).
Insert Fig. xx. 17 here
Highly versatile dendronised cellulosic surfaces were also developed by Montanez et al..41 After
its functionalization by azido groups, cellulose paper was grafted with functional dendrons of
first to fifth generations by CuAAC click chemistry in mild conditions (Fig. xx.18). The dendron
terminal groups were then chemically modified to obtain a very high number of orthogonal
chemical functions at the periphery. By this high precision multi-step strategy, each hydroxyl
group initially present at cellulose surface eventually led to 64 dendritic OH and 32 dendritic
15
azido groups. Biofunctionalisation was then successfully achieved with different alkyne-
functionalised molecules including amoxicillin and mannose. In the latter case, the biofunctional
surface allowed the detection of lectin protein at a concentration as low as 5 nM. This
dendronised cellulosic platform is particularly promising for bio- and chemical sensors, with
respect to the high number of the orthogonal functions present at the periphery and its versatility
to address a wide range of sensing applications.
Insert Fig. xx.18 here
1.6. Cellulosic polyelectrolytes by CuAAC click chemistry
Polysaccharide-based polyelectrolytes have become very important in formulation of aqueous
solutions for a wide range of applications. Today, food and health care industries largely rely on
their particular properties in solution and in particular on their strong thickening effect at very
low polymer concentration owing to the polyelectrolyte effect.
As firstly shown by Liebert et al., CuAAC click chemistry offered new pathways to original
cellulosic polyelectrolytes.11 Unlike cellulosic esters polyelectrolytes, these new polyelectrolyte
were stable towards hydrolysis owing to the high stability of the triazole linkers between
cellulose and the ionic side groups.
In a first work, 6-azido-6-deoxycelluloses with DSazide ranging from 0.88 to 0.99 were grafted
with an alkyne-functionalised carboxylic acid methyl ester and 2-ethynylaniline (Fig. xx.19).11
Both molecular reagents were considered as precursors for anionic and cationic groups,
respectively, after appropriate modification of the grafted cellulose derivatives. High degree of
substitution (0.86) was obtained for the carboxylic acid methyl ester in very mild conditions
(25°C/molar ratio of 1 with respect to the azido groups). The efficient grafting of 2-
ethynylaniline required much higher temperature (70°C) and a large excess in molecular reagent
16
(3 equiv.). Both cellulosic derivatives were soluble in DMF or DMSO. Their modification with
strong base or acid led to anionic and cationic cellulosic polyelectrolytes, respectively.
Insert Fig. xx.19 here
In another work by the same team, this approach was transposed to the grafting of
acetylenedicarboxylic acid dimethyl ester by means of click chemistry followed by
saponification of the ester side groups.42 The CuAAC click reaction of 6-azido-6-deoxycellulose
with the alkyne-functionalised diester was performed with a grafting rate of 62% and enabled to
increase greatly the number of anionic groups obtained after saponification compared to that of
the first work. Furthermore, the later anionic cellulosic polylectrolytes displayed tensio-active
properties and formed interesting ionotropic gels by precipitation in aqueous solutions containing
multivalent ions (Ca2+, Al3+) or a cationic polyelectrolyte (poly(diallyldimethyl ammonium)
chloride (polyDADMAC)).
Another interesting approach to novel cationic cellulosic polyelectrolytes based on azido-
cellulose and alkyne-functionalised ionic liquids was reported by Gonsior and Ritter.43 Cellulose
is known to be soluble in a few ionic liquids including 1-ethyl-3-methyl imidazolium acetate
(emim)(ac) but its viscosity is fairly high in these particular solvents. In the later work, 6-azido
6-deoxy cellulose with a DSazido of ca. 1 was grafted with three different alkyne-functionalised
ionic liquids (i.e. 1-methyl, 1-butyl, and 1-benzyl 3- propargyl imidazolium bromides) in
DMSO/H2O by CuAAC click chemistry at 25C for 48 h (Fig. xx.20). Almost quantitative
grafting was reached in these conditions with very high degrees of substitution for the grafted
ionic liquids. Rheology experiments were carried out for virgin cellulose and the corresponding
cationic polyelectrolytes in the ionic liquid 1-ethyl-3-methyl imidazolium acetate used as a
solvent. The results showed that cellulose grafting with ionic liquids decreased solution viscosity
17
by at least one order of magnitude compared to that of virgin cellulose. Furthermore, the solution
viscosity decreased with the size of the alkyl substituent on the imidazolium ionic liquids, the
methyl substituent thus providing the lowest solution viscosity.
Insert Fig. xx.20 here
1.7. Advanced cellulose (nano)materials by CuAAC surface modification
Surface modification of cellulose has been developed for a long time and a few recent
bibliographic reviews have pointed out the new progress made for its control by various
techniques.10; 12; 33-35. The modification of nanocelluloses is another important emerging issue for
advanced biomaterials and biocomposites.44-48 Compared to other modification techniques,
CuAAC click chemistry has been rarely used for (nano)cellulose surface modification so far.
Nevertheless, recent works in this field show that this click chemistry offers a tremendous
potential for designing original materials with very different properties and functionalities from
nano to macro-scales.
1.7.1. Advanced materials by cellulose surface modification
Adapting a procedure reported by Hafrén et al. for cellulose paper grafting with a fluorescent
probe,49 Krouit et al. grafted cellulose Avicel powder by an aliphatic polyester in heterogeneous
conditions.50 The cellulosic powder was first functionalised with alkyne side groups with
relatively long spacers in C11 for improving grafting efficiency. The "grafting onto" was then
perf-diazido-polycaprolactone (PCL) by CuAAC click chemistry in mild
conditions. In addition to surface grafting, the PCL difunctionality may have also led to limited
surface crosslinking. These grafted cellulosic powders were considered as interesting precursors
for the development of fully biodegradable cellulosic composites.51
A combination of controlled radical polymerisation with CuAAC click chemistry was another
18
interesting approach for the controlled surface grafting of cellulose. In a first work by Haddleton
et al.,52 an azide-functionalised fluorescent polymethacrylate (PMMA) oligomer with controlled
molecular weight was obtained by controlled radical copolymerisation (ATRP) of methyl
methacrylate (MMA) and a fluorescent methacrylate comonomer. This fluorescent oligomer with
an azido terminal group was then grafted onto an alkyne-functionalised cotton by CuAAC click
chemistry. An azido-monomethoxy poly(ethylene glycol) was also grafted onto the same cotton
derivative.
A thorough comparison of the methods of "grafting from" and "grafting onto" by CuAAC click
chemistry has been recently reported by Hansson et al. for the cellulose surface grafting with
PMMA grafts of different lengths.53 In this work, PMMA was first "grafted from" a cellulose
paper bearing initiator groups by controlled radical polymerisation (ARGET ATRP) (Fig. xx.21).
Following a procedure well known for the control of the graft molecular weight by the "grafting
from" method, the use of an alkyne-functional sacrificial initiator enabled to obtain free alkyne-
functionalised PMMA oligomers with the same molecular weights as those of the PMMA grafts.
These alkyne-functional PMMA oligomers were then "grafted onto" an azide-functionalised
cellulose paper. A comparison of both surface grafting methods with identical grafts showed that
the grafting density on the cellulosic surface was higher for the method of "grafting from", which
also enabled a better control of the graft content. The method of "grafting onto" was limited by
lower grafting density and efficiency for the longest grafts, mainly owing to the limited
accessibility of the alkyne terminal groups of the corresponding PMMA oligomers.
Insert Fig. xx.21 here
Recently, Filpponen et al. have proposed a very different strategy for cellulose surface
modification based on a combination of adsorption of a "clickable" carboxymethylcellulose on
19
cellulose surface with sequential CuAAC grafting in mild conditions (Fig. xx.22).18; 54 In a first
step, a carboxymethylcellulose (CMC) modified with azido or alkyne groups was adsorbed on
various cellulosic surfaces (i.e. regenerated cellulose, cellulose paper, cellulose nanofibrils
(CNFs)) by simple coating or dipping in a functionalised CMC aqueous solution. Low CMC
degree of substitution (DSazide or DSalkyne) and the presence of electrolytes in the aqueous
solution were both important for a good adsorption of the functionalised CMC onto cellulosic
surfaces.54 The grafting of alkyne-functionalised bovine serum albumin (BSA), azido-
functionalised fluorescent probe and monomethoxy PEG was then performed in mild conditions
by CuAAC click chemistry. The versatility of this approach appears particularly interesting for
modifying a variety of cellulosic surfaces.
Insert Fig. xx.22 here
1.7.2. Advanced materials by nanocellulose surface modification
Cellulose nanomaterials represent a new class of cellulosic materials (e.g. cellulose nanocrystals
(CNC) , nanofibrils (CNF) etc.) with one dimension in the nanometer scale, which offer
particularly high prospects for a wide range of applications.44-47 Taking advantage of their high
aspect ratio and outstanding mechanical properties, these renewable cellulosic materials have
been mainly used as fillers to reinforce polymer materials in composites. In particular, their
incorporation in biopolymers has led to original biocomposites for a sustainable composite
industry.51 Cellulose nanomaterials have also brought new functionalities to cellulose and offered
new pathways to advanced materials for the paper, food, health care, cosmetics and
pharmaceutical industries.
So far, chemical modification of cellulose nanomaterials has been investigated mainly for
improving their compatibility with polymer matrices in composite materials and has been the
20
subject for a few recent reviews.44; 45; 47; 48 Nevertheless, chemical modification of cellulose
nanomaterials by CuAAC click chemistry has been rarely reported so far and has not been
reviewed yet.
1.7.2.1. Advanced nanomaterials by cellulose nanocrystals modification
Cellulose nanocrystals (CNCs) are rod-like or whisker nanoparticles with high aspect ratio
(typically 3-5 nm in width, 50-500 nm in length) obtained by cellulose acid hydrolysis.
Filpponen and Argyropoulos described their surface functionalisation with azido and alkyne
groups in two steps.55 In the first step, hydroxyl groups on the CNC surface were converted into
carboxylic acid groups by TEMPO-mediated hypohalite oxidation. In the second step, these
carboxylic acid groups were reacted with azido- or alkyne-functionalised amines by
ethylcarbodiimide (EDC) coupling in presence of N-hydroxy succinimide (NHS). CuAAC click
chemistry of the azide and alkyne-surface functionalised CNCs led to the formation of
nanoplatelet gels. TEM analysis showed that the nanoplatelets formed after click chemistry
retained the rectangular shape of virgin CNCs and that they were the result of a highly regular
CNCs packing. The same team further investigated the CuAAC coupling of other azide and
alkyne-surface functionalised CNCs for new nanoplatelet gels.56 TEM analysis showed the
importance of solvent polarity for the CNCs packing during nanocellulose gelification induced
by CuAAC click chemistry. The CNCs were uniformly oriented in the nanoplatelets prepared in
water, while they were rather randomly oriented for those prepared in organic solvent (DMF).
Filpponen et al. also reported smart photoresponsive CNCs obtained by CuAAC grafting with
coumarin and anthracene fluorescent probes (Fig. xx.23).57 Fluorescence microscopy confirmed
the fluorescence of the grafted CNCs. Furthermore, UV-photoinduced reversible cycloadditions
of coumarin and anthracene resulted in the formation of photoresponsive cellulosic nano-arrays.
21
Insert Fig. xx.23 here
Taking advantage of another photoresponsive character, Feese et al. developed photobactericidal
cellulose nanoparticles by CuAAC grafting of azide-CNCs with an alkyne-functionalised
porphyrin.58 The porphyrin photosensitizer was then activated upon illumination and generated
cytotoxic species including singlet oxygen. The resulting photobactericidal effect was the most
intense for the bacteria Staphylococcus aureus, which represent one of the most dangerous
threats to human health. In another work by Eyley and Thielemans, an alkyne-functionalised
imidazolium ionic liquid was grafted onto azide-functionalised CNCs for anion exchange
applications.59 These cellulose nanoparticles grafted with imidazoliuim species could also have
interesting bactericidal properties.
Original hybrid CNC-protein nanoparticles have also been reported by Karaaslan et al.
recently.60 In this work, CNCs alkyne-functionalised at their reducing ends were coupled with
azido-functionalised -casein micelles by CuAAC click chemistry in water. AFM and TEM
imaging showed a variety of shapes for the CNC/ -casein conjugates obtained in these
conditions. Such hybrid polysaccharide-protein nanoparticles could be interesting building
blocks for new biomaterials.
1.7.2.2. Advanced nanomaterials by cellulose nanofibrils modification
Cellulose nanofibrils (CNFs) can be prepared from a variety of cellulose sources.44; 45 These
nano-objects have outstanding mechanical properties with a Young's modulus similar to that of
Kevlar. Owing to their original properties, CNFs can also, in particular conditions, lead to
optically transparent cellulosic paper very different from the white paper obtained from common
cellulose (Fig. xx.24) .
Insert Fig. xx.24 here
22
Smart pH-responsive or fluorescent cellulose nanofibrils (CNFs) have been obtained by CNF
surface modification by CuAAC click chemistry in mild conditions in aqueous media.61 The
CNF surface was first functionalised with azido groups by reaction with 1-azido-2,3-
epoxypropane in presence of sodium hydroxyde. The azido-CNFs were then used as a nano-
platform for introducing amino or fluorescent side groups. Fig. xx.25 showed an image for a
fluorescent pale yellow-green CNF film obtained after grafting of a fluorescent probe. AFM
images confirmed that the nanofibrillated structure was well maintained after surface
functionalisation. Furthermore, the CNFs with amine side groups displayed pH-responsive
rheological properties. The addition of acetic acid to their aqueous suspension led to lower
viscosity and a very strong decrease in the storage and loss moduli. In that case, amine
protonation was responsible for strong ionic repulsions and a collapse of the CNF network.
Insert Fig. xx.25 here
CNFs functionalised by amine groups by CuAAC click chemistry were also reported by Luong
et al. for the development of graphene/cellulose nanocomposite papers with high electrical and
mechanical performance.62 These nanocomposites were obtained by filtering stable dispersions
of reduced graphite oxide and amine-functionalised CNFs. The very good dispersion of graphene
obtained in this work was partially due to the nucleophilic addition of the CNFs amine groups
with epoxy groups present on graphene. CNFs functionalisation by CuAAC click chemistry
provided an efficient compatibilisation system for graphene contents up to 10 wt%. A strong
enhancement of the mechanical properties was already observed after addition of graphene in
very low amount (0.3 wt%). Furthermore, the electrical conductivity of the graphene/CNF
nanocomposite papers strongly increased from 4.79 104 to 71.8 S m1 with their graphene
content from 0.3 to 10 wt%. The high flexibility, stability, mechanical and electrical
23
conductivity performance of the new graphene/CNF nanocomposites offer interesting prospects
for portable electronics and electromagnetic shielding devices.
1.8. Conclusion
Over the past few years, CuAAC click chemistry has offered new ways of modifying cellulose
and its derivatives in particular mild conditions and with hydrolytically stable triazole linkers. Its
high reliability, efficiency and tolerance of different functional groups have contributed to
developing great structural and functional variety for cellulosic materials.
A wide range of copolymer architectures have been easily obtained from azido or alkyne-
functionalised cellulosic derivatives: structural or hydrogel crosslinked networks, block and graft
copolymers and dendronised celluloses. New cationic and anionic polyelectrolytes have also
been easily prepared with greatly improved hydrolytic stability compared to former
polyelectrolyte cellulosic esters. The recent progress made on cellulose nanomaterials
modification have shown another great potential for the developement of new cellulosic
materials for advanced applications from nano to macro-scale.
CuAAC click chemistry has also brought a new range of functionalities to cellulose and its
derivatives. Thermo, pH, and photo-responsive cellulosic (nano)materials have been obtained by
simple grafting of responsive moieties in homogeneous or heterogeneous conditions. Fluorescent
and (photo) bactericidal properties were also easily provided to cellulosic (nano)materials by
CuAAC click chemistry.
Within the frame of sustainable chemistry, CuAAC click chemistry will certainly contribute to
greatly extend the scope and functionality of cellulose-based materials for a wide range of
applications in smart packaging, advanced food, health care, drug delivery, biomaterials,
biocomposites, biosensors, bactericidal materials, and biofunctional films.
24
1.9. Acknowledgements
The authors would like to thank the ELEMENT Erasmus Mundus Programme for the PhD
scholarship awarded to Mrs Faten Hassan Hassan Abdellatif.
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List of Figures
Figure xx.1. Copper(I) catalysed Huisgen 1,3-dipolar cycloaddition of alkynes and azides
(CuAAC). Source: Reprinted from Reference6, Copyright (2008), Figure 1 p 954, with
permission from Wiley-VCH.
Figure xx.2. Pre-click modification of cellulose for CuAAC click chemistry. An example
according to Faugeras et al.10; 14 (Original Figure)
Figure xx.3. Regioselective pre-click modification of cellulose for CuAAC click chemistry.
Example of the synthesis of 3-O-propargyl cellulose using thexyldimethylsilyl protecting
groups.24. Source: Reprinted from Reference24, Copyright (2009), Figure 1 p 349, with
permission from Elsevier.
Figure xx.4. Preparation of crosslinked cellulose by CuAAC click chemistry.14 Source: Reprinted
from Reference14, Copyright (2012), Figure 1 p 248, with permission from Elsevier.
Figure xx.5. SEM pictures (500×) of: (A) unmodified starch; (B) propargyl starch; (C)
unmodified cellulose; (D) azido cellulose; (E) crosslinked cellulose/starch network obtained by
CuAAC click chemistry.29 Source: Reprinted from Reference29, Copyright (2012), Figure 2 p
1889, with permission from Elsevier.
Figure xx.6. Preparation of crosslinked networks of cellulose and chitosan by CuAAC click
chemistry.17 Source: Reprinted from Reference17, Copyright (2012), Scheme 1 p 5202, with
permission from Wiley Periodicals, Inc.
Figure xx.7. Examples of SEM pictures showing hollow tubes for the crosslinked networks of
cellulose and chitosan obtained by CuAAC click chemistry.17 Source: Adapted from Reference17,
Copyright (2012), Figure S1 in Supporting information, with permission from Wiley Periodicals,
Inc.
30
Figure xx.8. Synthesis of benzoxazine-functional cellulose by CuAAC click chemistry,
considered as an interesting intermediate for preparing crosslinked cellulose/polybenzoxazine
networks.30 Source: Reprinted from Reference30, Copyright (2012), Figure 2 p 1348, with
permission from Wiley Periodicals, Inc.
Figure xx.9. Freshly prepared hydrogels by CuAAC click chemistry of azido- and alkyne-
carboxymethyl celluloses in various conditions.31 Source: Reprinted from Reference31, Copyright
(2011), Figure 3 p 160, with permission from Elsevier.
Figure xx.10. Preparation of thermo-responsive crosslinked hydrogels from an azido-cellulose
and an alkyne-functionalised copolymer P(NIPAM-co-HEMA) by CuAAC click chemistry.32
Source: Reprinted from Reference10, Copyright (2011), Figure 12 p 1617, with permission from
MDPI AG. Journal Contact : MDPI AG
Polymers Editorial Office, Klybeckstrasse 64, 4057 Basel, Switzerland. E-Mail: polymers@mdpi.com Tel. +41 61
683 77 34; Fax: +41 61 302 89 18; Open Access Journal - Corresponding author: romain.lucas@unilim.fr
Figure xx.11. Preparation of amphiphilic cellulose-based diblock copolymers by CuAAC click
chemistry.26 Source: Reprinted from Reference26 , Copyright (2012), Figure 3 p 1318, with
permission from Springer.
Figure xx.12. Two strategies for the synthesis of ethyl cellulose graft copolymers with diblock
PS-b-PEG grafts or with both PS and PEG grafts by CuAAC click chemistry in homogeneous
solution.19 Source: Reprinted from Reference19, Copyright (2012), Figure 1 p 2170, with
permission from Wiley-VCH.
Figure xx.13. Bio-functional honeycomb films made of an amphiphilic cellulosic graft
copolymer with PEG-biotin grafts obtained by CuAAC click chemistry. Comparison of the
combined confocal fluorescent (pink) and optical (blue) images (a) before and (b) after
conjugation with avidin.20 Source: Reprinted from Reference20, Copyright (2013), Figure 6 p
31
731, with permission from the American Chemical Society.
Figure xx.14. Preparation of polymethacrylate copolymers grafted with cellulosic side chains by
CuAAC click chemistry.27 Source: Reprinted from Reference27, Copyright (2012), Figure 1 p
2239, with permission from Elsevier.
Figure xx.15. Preparation of dendronised cellulose with PAMAM dendrons of first generation
from an alkyne-functionalised cellulose at position 6 by CuAAC click chemistry.16 Source:
Reprinted from Reference16, Copyright (2008), Figure 2 p 1743, with permission from Wiley-
VCH.
Figure xx.16. Preparation of dendronised carboxymethyl cellulose with PAMAM dendrons of
first generation from an azido-functionalised cellulose at position 6 by CuAAC click chemistry.39
Source: Reprinted from Reference39, Copyright (2009), Figure 2 p 1103, with permission from
Elsevier.
Figure xx.17. Two-step biofunctionalization of an azido-cellulose film grafted by alkyne-
dendrons containing terminal amino groups, with a glucose oxydase enzyme.40 Source:
Reprinted from Reference40, Copyright (2009), Figure 6 p 387, with permission from the
American Chemical Society.
Figure xx.18. Schematic drawing of a biofunctional cellulose surface grafted with dendrons of
fifth generation with 32 azido groups at the periphery.41 Source: Adapted from Reference41,
Copyright (2011), Figure 1 p 2216, with permission from the American Chemical Society.
Figure xx.19. Preparation of anionic and cationic cellulosic polyelectrolytes by CuAAC click
chemistry and further modification by strong base or acid.11 Source: Adapted from Reference11,
Copyright (2006), Figure 1 p 210, with permission from Wiley-VCH.
Figure xx.20. Preparation of cationic polyelectrolytes by grafting of azido-cellulose with alkyne-
32
functionalised imidazolium ionic liquids.43 R = Methyl, butyl or benzyl. Source: Adapted from
Reference43, Copyright (2011), Figure 3 p 2637, with permission from Wiley-VCH.
Figure xx.21. Cellulose surface modification by the methods of "grafting from" and "grafting
onto" by CuAAC click chemistry with free alkyne-functional PMMA oligomers obtained during
"grafting from" in presence of an alkyne-functional sacrificial initiator.53 Source: Reprinted from
Reference53, Copyright (2013), Figure 1 p 65, with permission from the American Chemical
Society.
Figure xx.22. Cellulose surface modification based on a combination of adsorption of a
"clickable" carboxymethylcellulose with sequential CuAAC grafting in mild conditions.18
Source: Reprinted from Reference18, Copyright (2012), Figure 1 p 737, with permission from the
American Chemical Society.
Figure xx.23. Photoresponsive CNCs obtained by CNCs grafting with coumarin and anthracene
fluorescent moieties by CuAAC click chemistry.57 Source: Reprinted from Reference57,
Copyright (2011), Figure 1 p 36, published under Open Access CC BY 3.0 license. Contact:
info@intechopen.com. Corresponding author: Pr Dimitris Argyropoulos, North Carolina
University, USA, dsargyro@ncsu.edu
Figure xx.24. Optically transparent nanofiber paper (left) composed of 15 nm
cellulose nanofibers (upper left, scale bar in inset: 100 nm) and conventional
cellulose paper (right) composed of 30 µm pulp fibers (upper right,
scale bar in inset: 200 µm).63 Source: Reprinted from Reference63, Copyright (2009), Figure 1 p
1595, with permission from Wiley Interscience.
Figure xx.25 (a) Image of a fluorescent pale yellow-green CNF film obtained after surface
grafting of a fluorescent dye by CuAAC click chemistry. AFM phase-images of (b) unmodified
33
and (c) azide-functionalised NFC with scan size of 2 µm.61 Source: Reprinted from Reference61,
Copyright (2011), Figure 2 p 1206, with permission from Springer.
34
Figure xx.1. Copper(I) catalysed Huisgen 1,3-dipolar cycloaddition of alkynes and azides
(CuAAC). Source: Reprinted from Reference6, Copyright (2008), Figure 1 p 954, with
permission from Wiley-VCH.
35
O
H
O
H
HO
H
H
OH
HO
OH
O
H
O
H
HO
H
H
OH
HO
OSO2
CH3
TsCl/TEA, 8°C, 24h
Cellulose Tosyl Cellulose
O
H
O
H
HO
H
H
OH
HO
N3
NaN3/DMF
100°C, 24H
Azido Cellulose
H2N
80°C, 24-48H/DMSO
O
H
O
H
HO
H
H
OH
HO
HN
Propargyl Cellulose
Figure xx.2. Pre-click modification of cellulose for CuAAC click chemistry. An example
according to Faugeras et al.10; 14
36
Figure xx.3. Regioselective pre-click modification of cellulose for CuAAC click chemistry.
Example of the synthesis of 3-O-propargyl cellulose using thexyldimethylsilyl protecting
groups.24 Source: Reprinted from Reference24, Copyright (2009), Figure 1 p 349, with
permission from Elsevier.
37
Figure xx.4. Preparation of crosslinked cellulose by CuAAC click chemistry.14 Source: Reprinted
from Reference14, Copyright (2012), Figure 1 p 248, with permission from Elsevier.
38
Figure xx.5. SEM pictures (500×) of: (A) unmodified starch; (B) propargyl starch; (C)
unmodified cellulose; (D) azido cellulose; (E) crosslinked cellulose/starch network obtained by
CuAAC click chemistry.29Source: Reprinted from Reference29, Copyright (2012), Figure 2 p
1889, with permission from Elsevier.
39
Figure xx.6. Preparation of crosslinked networks of cellulose and chitosan by CuAAC click
chemistry.17 Source: Reprinted from Reference17, Copyright (2012), Scheme 1 p 5202, with
permission from Wiley Periodicals, Inc.
40
Figure xx.7. Examples of SEM pictures showing hollow tubes for the crosslinked networks of
cellulose and chitosan obtained by CuAAC click chemistry.17 Source: Adapted from Reference17,
Copyright (2012), Figure S1 in Supporting information, with permission from Wiley Periodicals,
Inc.
41
Figure xx.8 : Synthesis of benzoxazine-functional cellulose by CuAAC click chemistry,
considered as an interesting intermediate for preparing crosslinked cellulose/polybenzoxazine
networks.30 Source: Reprinted from Reference30, Copyright (2012), Figure 2 p 1348, with
permission from Wiley Periodicals, Inc.
42
Figure xx.9. Freshly prepared hydrogels by CuAAC click chemistry of azido- and alkyne-
carboxymethyl celluloses in various conditions.31 Source: Reprinted from Reference31, Copyright
(2011), Figure 3 p 160, with permission from Elsevier.
43
Figure xx.10. Preparation of thermo-responsive crosslinked hydrogels from an azido-cellulose
and an alkyne-functionalised copolymer P(NIPAM-co-HEMA) by CuAAC click chemistry.32
Source: Reprinted from Reference10, Copyright (2011), Figure 12 p 1617, with permission from
MDPI AG.
44
Figure xx.11. Preparation of amphiphilic cellulose-based diblock copolymers by CuAAC click
chemistry.26 Source: Reprinted from Reference26, Copyright (2012), Figure 3 p 1318, with
permission from Springer.
45
Figure xx.12. Two strategies for the synthesis of ethyl cellulose graft copolymers with diblock
PS-b-PEG grafts or with both PS and PEG grafts by CuAAC click chemistry in homgeneous
solution.19 Source: Reprinted from Reference19, Copyright (2012), Figure 1 p 2170, with
permission from Wiley-VCH.
46
Figure xx.13. Bio-functional honeycomb films made of an amphiphilic cellulosic graft
copolymer with PEG-biotin grafts obtained by CuAAC click chemistry. Comparison of the
combined confocal fluorescent (pink) and optical (blue) images (a) before and (b) after
conjugation with avidin.20 Source: Reprinted from Reference20, Copyright (2013), Figure 6 p
731, with permission from the American Chemical Society.
47
Figure xx.14. Preparation of polymethacrylate copolymers grafted with cellulosic side chains by
CuAAC click chemistry.27 Source: Reprinted from Reference27, Copyright (2012), Figure 1 p
2239, with permission from Elsevier.
48
Figure xx.15. Preparation of dendronised cellulose with PAMAM dendrons of first generation
from an alkyne-functionalised cellulose at position 6 by CuAAC click chemistry.16 Source:
Reprinted from Reference16, Copyright (2008), Figure 2 p 1743, with permission from Wiley-
VCH.
49
Figure xx.16. Preparation of dendronised carboxymethyl cellulose with PAMAM dendrons of
first generation from an azido-functionalised cellulose at position 6 by CuAAC click chemistry.39
Source: Reprinted from Reference39, Copyright (2009), Figure 2 p 1103, with permission from
Elsevier.
50
Figure xx.17. Two-step biofunctionalisation of an azido-cellulose film grafted by alkyne-
dendrons containing terminal amino groups with a glucose oxydase enzyme.40 Source: Reprinted
from Reference40, Copyright (2009), Figure 6 p 387, with permission from the American
Chemical Society.
51
Figure xx.18. Schematic drawing of a biofunctional cellulose surface grafted with dendrons of
fifth generation with 32 azido groups at the periphery.41 Source: Adapted from Reference41,
Copyright (2011), Figure 1 p 2216, with permission from the American Chemical Society.
52
Figure xx.19. Preparation of anionic and cationic cellulosic polyelectrolytes by CuAAC click
chemistry and further modification by strong base or acid.11 Source: Adapted from Reference11,
Copyright (2006), Figure 1 p 210, with permission from Wiley-VCH.
53
Figure xx.20. Preparation of cationic polyelectrolytes by grafting of azido-cellulose with alkyne-
functionalised imidazolium ionic liquids.43 R = Methyl, butyl or benzyl. Source: Adapted from
Reference43, Copyright (2011), Figure 3 p 2637, with permission from Wiley-VCH.
54
Figure xx.21. Cellulose surface modification by the methods of "grafting from" and "grafting
onto" by CuAAC click chemistry with free alkyne-functional PMMA oligomers obtained during
"grafting from" in presence of an alkyne-functional sacrificial initiator.53 Source: Reprinted from
Reference53, Copyright (2013), Figure 1 p 65, with permission from the American Chemical
Society.
55
Figure xx.22. Cellulose surface modification based on a combination of adsorption of a
"clickable" carboxymethylcellulose with sequential CuAAC grafting in mild conditions.18
Source: Reprinted from Reference18, Copyright (2012), Figure 1 p 737, with permission from the
American Chemical Society.
56
Figure xx.23. Photoresponsive cellulose nanocrystals (CNCs) obtained by CNC grafting with
coumarin and anthracene fluorescent moieties by CuAAC click chemistry.57 Source: Reprinted
from Reference57, Copyright (2011), Figure 1 p 36, with permission from the INTECH Open
Acess Publisher.
57
Figure xx.24. Optically transparent nanofiber paper (left) composed of 15 nm
cellulose nanofibers (upper left, scale bar in inset: 100 nm) and conventional
cellulose paper (right) composed of 30 µm pulp fibers (upper right,
scale bar in inset: 200 µm).63 Source: Reprinted from Reference63, Copyright (2009), Figure 1 p
1595, with permission from Wiley Interscience.
58
Figure xx.25. (a) Image of a fluorescent pale yellow-green CNF film obtained after surface
grafting of a fluorescent dye by CuAAC click chemistry. AFM phase-images of (b) unmodified
and (c) azide-functionalised NFC with scan size of 2 µm.61 Source: Reprinted from Reference61,
Copyright (2011), Figure 2 p 1206, with permission from Springer.
