Copper-Catalyzed vs thermal step growth polymerization of starch-derived α-w-alkyne dianhydrohexitol stereoisomers: to click or not to click?

Article abstract of DOI:10.1021/ma9024784

The synthesis of α-azide-ω-alkyne 1,4:3,6-dianhydrohexitols with controlled stereochemistry from starch-derived isosorbide, isomannide, and isoidide as well as a detailed study of their polyaddition by CuAAC in solution or by catalyst and solvent-free 1,3

Full text of DOI:10.1021/ma9024784

Macromolecules 2010, 43, 17–19 17
DOI: 10.1021/ma9024784
Copper-Catalyzed vs Thermal Step Growth
Polymerization of Starch-Derived R-Azide-ω-Alkyne
Dianhydrohexitol Stereoisomers: To Click or Not To
Click?
Celine Besset,†,‡ Sandra Binauld,† Mathias Ibert,‡
ꢀ
Patrick Fuertes,‡ Jean-Pierre Pascault,† Etienne Fleury,†
Julien Bernard,*,† and Eric Drockenmuller*,†
†INSA de Lyon, Ingeꢀnierie des Mateꢀriaux Polymeꢁres
ꢀ
(IMP, UMR CNRS 5223), Universite Claude Bernard Lyon 1,
‡
69622 Villeurbanne Cedex, France and Division Recherche,
Roquette Freꢁres, 62080 Lestrem Cedex, France
Received November 10, 2009
Revised Manuscript Received November 19, 2009
Within a few decades materials science has considerably
widened its scope of applications with a panel of efficient and
versatile synthetic methods affording an astonishing diversity
of complex multifunctional macromolecular architectures. Re-
cently, the highly efficient, orthogonal and regiospecific copper(I)-
catalyzed azide-alkyne cycloaddition (CuAAC),1 one of the
most reliable example of the “click chemistry” concept,2 has
opened new perspectives for the design of polymer materials with
targeted properties and the modification of 2D and 3D sub-
strates.3 A tremendous palette of functionalized (co)polymers,
macrocycles, block copolymers, star and hyperbranched poly-
mers, dendrimers, polymer bioconjugates, and networks has thus
been described. The benefits of CuAAC have also been widely
applied to the field of step growth polymerization, allowing the
design of a new class of macromolecules with 1,4-disubstituted
1,2,3-triazole within the repeating unit. Outpassing the limita-
tions of thermal 1,3-dipolar Huisgen polyaddition in solution
(low reactivity, high temperatures, limited yields),4 CuAAC
polyaddition of diazide and dialkyne monomers has been exten-
sively investigated.5 Emerging strategies have focused on the
CuAAC polyaddition of R-azide-ω-alkyne monomers yielding
polytriazole (co)polymers with tunable structure and properties.6
However, these approaches are generally limited by the poor
solubility and the low polymerization degree (DPn) of the result-
ing materials. Also, while copper(I) catalysis ensures a regio-
selective polyaddition, the binding properties of the multiple
triazole rings along the polymer backbone complicate catalyst
extraction. Such residuals may alter the properties of the resulting
materials and jeopardize their potential applications.7
Herein, we present the synthesis of R-azide-ω-alkyne 1,4:3,6-
dianhydrohexitols with controlled stereochemistry from starch-
derived isosorbide, isomannide, and isoidide as well as a detailed
study of their polyaddition by CuAAC in solution or by catalyst-
and solvent-free 1,3-dipolar Huisgen cycloaddition. The resulting
library of isomeric dianhydrohexitol/triazole-based polymers
allows a fine investigation of the structure/properties relation-
ship. Particularly, a fundamental insight into the influence of
monomer stereochemistry (RR, RS, SR, SS) as well as polyaddi-
tion regiospecificity (1,4- or 1,4/1,5-triazoles) on the physico-
chemical properties of the resulting polymers is thus accessible.
Finally, the questionable benefits of copper catalysis over thermal
Figure 1. Synthesis of R-azide-ω-alkyne 1,4:3,6-dianhydrohexitol ste-
reoisomers from isosorbide, isoidide, and isomannide.
Huisgen cycloaddition in the field of step growth polymerization
are discussed.
1,4:3,6-Dianhydrohexitols, especially isosorbide and isoman-
nide, can be readily obtained through sequential hydrogenation
and dehydration of
D-glucose and D
-mannose, respectively.8
In the context of fossil fuels rising costs and rarification, these
diols issued from abundant and renewable agro-resources have
attracted considerable attention in the field of materials science.
Indeed, 1,4:3,6-dianhydrohexitols have been thoroughly investi-
gated as bio-sourced monomers for the step growth polymeriza-
tion industry.9 However, the relatively low reactivity of the
secondary alcohols has encouraged the development of deriva-
tives with enhanced reactivity.10 In this context, we have deve-
loped the synthesis of R-azide-ω-alkyne 1,4:3,6-dianhydrohexitol
stereoisomers in order to generate polytriazoles with controlled
stereochemistry by azide-alkyne cycloaddition. Such AB þ AB
step growth polymerization strategy has many advantages such
as circumventing stoichiometry issues inherent to AA þ BB
systems, designing polymers with a high weight fraction of bio-
sourced monomer, and handling monomers with a reasonable
content of nitrogen atoms.11 R-Azide-ω-alkyne dianhydrohexi-
tol stereoisomers 9-12 (RR, RS, SR, SS) were thus obtained
from isosorbide, isoidide, and isomannide using a three-step
synthetic pathway (Figure 1). Nucleophilic substitution of tailor-
made monotosylates 1-4 by sodium azide yielded azide deriva-
tives 5-8 with configuration inversion of the adjacent carbon
atom. Alkylation of the remaining hydroxyl group by propargyl
bromide was then performed at room temperature to avoid any
undesirable step growth polymerization. Monomers 9-12 were
stored at -20 ꢀC, a temperature below which no traces of
coupling products could be observed by 1H NMR after several
months of storage.
Monomers 9-12 were then polymerized by CuAAC step
growth polymerization in solution to investigate the influence
of monomer stereochemistry on the physicochemical properties
of the resulting polytriazoles (Figure 2). Relatively high mono-
mer concentrations (1 M in DMSO) were employed in order to
limit the formation of cyclic species.6c Under these condi-
tions, polymerization of each monomer resulted in the forma-
tion of a physical gel that could be rapidly dissolved by
*Corresponding authors. E-mail: julien.bernard@insa-lyon.fr (J.B.);
eric.drockenmuller@univ-lyon1.fr (E.D.).
r 2009 American Chemical Society
Published on Web 12/03/2009
pubs.acs.org/Macromolecules

18 Macromolecules, Vol. 43, No. 1, 2010
Communication
postpolymerization addition of solvent. Despite the physical
gelation of the polymerization media, the polyadditions pro-
ceeded up to high conversions furnishing, after purification,
polytriazoles 13-16 with excellent yields (Table 1).
To determine appropriate storage conditions and thermal
processing of these materials, the concomitant evolutions of
monomer conversion and Tg with temperature during polyaddi-
tion of 9 and 12 were precisely monitored by DSC (Figure 5,
Figures S1 and S2 in the Supporting Information).
The regioselectivity of the CuAAC step growth polymeriza-
1
tion was clearly demonstrated by H NMR (example for 13 in
No significant influence of monomer stereochemistry on the
rate of polymerization could be observed for monomers 9 and 12.
It must be pointed out that polymerizations can be completed
within minutes by applying appropriate curing conditions. The
particularly high reactivity of monomers 9-12 at low tempera-
tures and the possibilities to perform precure steps and to process
solid reactive oligomers are key features of this versatile ap-
proach. Polyaddition of monomers 9 and 12 at different heat-
ing rates were monitored by DSC in order to determine the
polyaddition activation energy using the Kissinger method
(Figures S3 and S4 in the Supporting Information).14 Similar
values (74 and 79 kJ/mol for 9 and 12, respectively) were obtained
confirming that thermal azide-alkyne cycloaddition is not af-
fected by monomer stereochemistry.
Figure 3) with the appearance of a single signal at 8.16 ppm
characteristic of 1,4-disubstituted 1,2,3-triazoles. Also, the for-
mation of polytriazole chains was confirmed by the significant
downfield shift of most protons from the 1,4:3,6-dianhydrohexi-
tol moiety as well as the disappearance of the signals of alkyne
and azide groups.
Polytriazoles 13-16 were then characterizedby SEC in DMSO
at 120 ꢀC using universal calibration (dual detection RI/
viscosimetry) to corroborate 1H NMR analysis. Unfortunately,
while 13 and 15 readily dissolve in DMSO at 120 ꢀC, 14 and 16 are
hardly soluble at this temperature, most probably reflecting an
effect of monomer stereochemistry. In the case of 13 and 15, SEC
analysis confirmed the preparation of polymers with molar
masses (Mn) of ca. 7000 g/mol (Table 1). These relatively low
values of Mn and high values of PDI most likely stem from the
physical gelation of the polymerization medium that hampers the
significant chain growth generally occurring at the end of the
polyaddition process. DSC analysis of polytriazoles 13-16 high-
lights their amorphous behavior and the high Tg values resulting
from the rigidity of the 1,4:3,6-dianhydrohexitol and triazole
segments. Interestingly, the values of Tg are significantly im-
pacted by monomer stereochemistry, with a minimum of 140 ꢀC
for 13 (SYSZ) and a maximum of 173 ꢀC for 16 (RYRZ).
However, the relatively high thermal resistance of polytriazoles
13-16 was only slightly influenced by monomer stereochemistry
(Td10 = 314-340 ꢀC).
The nonregioselective character of this copper-free polyaddi-
tion process was highlighted by 1H NMR. Indeed, ca. 2:1 signal
Whereas rising efforts are dedicated to the development of
metal-free click reactions,12 the influence of azide-alkyne cyclo-
addition regioselectivity on polytriazole properties has been add-
ressed lately.13 In order to overcome problems inherent to the use
of copper catalyst and investigate the influence of polyaddition
regioselectivity on physicochemical properties of the resulting
polytriazoles, catalyst-free polyaddition of 9-12 was performed
in bulk (Figure 4). Interestingly, the monomer conversion and thus
the DPn of the resulting polytriazoles could be precisely adjusted
with the polyaddition temperature. Indeed, once the Tg of the
growing polytriazoles reaches the reaction temperature, the system
Figure 3. 1H NMR spectra (DMSO-d6) of monomer 9 and polytria-
zoles 13 and 17 obtained respectively by CuAAC polyaddition in
DMSO and, catalyst- and solvent-free polyaddition.
1
vitrifies impeding further chain growth. As shown by H NMR,
reactive solid materials with DPn of 3 and 5 are obtained from 9
after several weeks at respectively 4 and 25 ꢀC, while temperatures
above Tg¥ ensure quantitative conversion of the monomer and
obtaining high DPn polytriazoles.
Figure 4. Synthesis of polytriazoles 17-20 by catalyst- and solvent-free
azide-alkyne cycloaddition.
Figure 2. Synthesis of polytriazoles 13-16 by CuAAC polyaddition
in DMSO.
Table 1. Properties of Polytriazoles 13-20 Synthesized by CuAAC in DMSO or by Catalyst and Solvent Free Azide-Alkyne Cycloaddition
1,4/1,5a
Mn (g/mol)b
DPn
PDIb
T
g¥ (ꢀC)
Td10 (ꢀC)c
b
no.
condition
monomer
stereo.
yield (%)
13
14
15
16
17
18
19
20
DMSO
DMSO
DMSO
DMSO
bulk
bulk
bulk
bulk
9
SYSZ
SYRZ
RYSZ
RYRZ
SYSZ
SYRZ
RYSZ
RYRZ
95
95
94
88
100
100
100
100
100/0
100/0
100/0
100/0
2.0/1.0
1.9/1.0
1.8/1.0
1.8/1.0
6900
d
7500
d
11500
8900
31900
9900
33
d
36
d
55
42
152
47
28.7
d
21.4
d
5.5
1.3
1.9
3.3
140
146
141
173
132
136
124
159
323
325
314
340
359
354
360
356
10
11
12
9
10
11
12
a From 1H NMR. b From SEC (dual detection RI/viscosimetry) in DMSO at 120 ꢀC. c Temperature at 10 wt % loss. d Partially soluble in DMSO at 120 ꢀC.

Communication
Macromolecules, Vol. 43, No. 1, 2010 19
Supporting Information Available: Syntheses and charac-
terization methods, kinetic monitoring, and determination of
the polyaddition activation energy by DSC. This material is
available free of charge via the Internet at http://pubs.acs.org.
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input on the influence of polyaddition regioselectivity on the
physicochemical properties of polytriazoles. First, a significant
solubility enhancement was observed as, conversely to their
analogues 13-16, polytriazoles 17-20 are readily soluble in
DMSO at room temperature. SEC experiments underlined the
generation of polytriazoles with higher Mn and sharper molar
mass distributions than those obtained for 13-16, consistent with
a homogeneous polymerization medium. Whereas the absence of
catalytic system clearly facilitates the process, the consequent loss
of regioselectivity only slightly affects the thermal properties
of the resulting polytriazoles as a ca. 10-15 ꢀC decrease in the
Tg values was measured. As for 13-16, the influence of monomer
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159 ꢀC for 20 (RYRZ). Compared to 13-16, polytriazoles 17-
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=
354-360 ꢀC) independently of monomer stereochemistry. This
most probably stems from the higher Mn of 17-20 which reduce
the contribution of chain ends in the degradation mechanism.
As a conclusion, CuAAC polyaddition in DMSO and thermal
polyaddition of tailor-made R-azide-ω-alkyne dianhydrohexitol
stereoisomers afford bio-sourced polytriazoles having Tg values
highly dependent on monomer stereochemistry. Compared to
previous studies on thermal Huisgen polyaddition, the particu-
larly high reactivity of these monomers afforded the completion
of the reaction after short reaction times at moderate tempera-
tures. Also, the possibility to adjust DPn with reaction tempera-
ture and to process reactive solid oligomers obtained by precure
steps reflects the robustness of this polyaddition process. Finally,
whereas the benefits of CuAAC in polymer science are numerous,
its application to step growth polymerization of R-azide-
ω-alkyne 1,4:3,6-dianhydrohexitols is detrimental. Indeed, while
CuAAC polyaddition in solution generates poorly soluble low
Mn polytriazoles and requires several purification steps, thermal
polyaddition in bulk provides polytriazoles with enhanced solu-
bility, high Mn, and comparable thermal properties using tunable
and versatile processing conditions.
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