Click chemistry step growth polymerization of novel α-azide-ω- alkyne monomers

Article abstract of DOI:10.1039/b805164j

A novel step growth polymerization A-B strategy based on the click chemistry polyaddition of tailor-made α-azide-ω-alkyne low molar mass monomers was developed, leading to polytriazole (co)polymers with tunable structures and properties. The Royal Society

Full text of DOI:10.1039/b805164j

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Click chemistry step growth polymerization of novel a-azide-x-alkyne
monomersw
Sandra Binauld,ab Denis Damiron,b Thierry Hamaide,b Jean-Pierre Pascault,a
Etienne Fleurya and Eric Drockenmuller*b
Received (in Cambridge, UK) 27th March 2008, Accepted 5th June 2008
First published as an Advance Article on the web 21st July 2008
DOI: 10.1039/b805164j
A novel step growth polymerization A–B strategy based on the
click chemistry polyaddition of tailor-made a-azide-x-alkyne
low molar mass monomers was developed, leading to polytria-
zole (co)polymers with tunable structures and properties.
Herein, we develop a novel A–B polyaddition strategy based
on the click chemistry step growth polymerization of tailor-
made low molar mass heterofunctional monomers (Scheme 1).
First, three stable a-azide-o-alkyne monomers were synthe-
sized using straightforward synthetic procedures. Then, the
step growth (co)polymerization of these monomers yielded
(co)polymers with tunable structures and properties.
Since its introduction in 2001 by Sharpless and co-workers,1 the
concept of click chemistry has been a wide source of inspiration
for the design of advanced multifunctional polymer materials.2
One of the most studied and reliable click reaction so far has
been the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition
of an azide (R–N3) and an alkyne (R–CRCH). This enhanced
regioselective chemical pathway is tolerant to humidity, oxygen,
a wide range of functionalities, and proceeds with almost
quantitative yields under mild conditions in different reaction
media. All these advantages have been extensively applied to
the different fields of materials science and polymer chemistry,
allowing the synthesis and functionalization of a wide range of
macromolecular architectures, e.g., 2D and 3D organic and
inorganic substrates, linear, block and star-like copolymers,
dendrimers and dendritic architectures, hyperbranched mate-
rials as well as hydrogels and polymer networks.3
In order to adjust the thermal properties, the distance
between two triazole linkages, as well as the controlled degra-
dation of the resulting polymers, different a-azide-o-alkyne
monomers were prepared and characterized by 1H and 13C
NMR. Monomer 2 was synthesized from methyl 4-hydroxy-
benzoate using sequential alkylation, reduction, bromination
and azidation procedures.8 Monomer 3 was prepared by DCC/
DPTS esterification of 4-pentynoic acid and 11-azidoundeca-
nol, 1.9 Monomer 4 was obtained from nucleophilic substitu-
tion of propargyl bromide by sodium 11-azidoundecanolate in
anhydrous THF. This reaction proceeded with high yield and
fidelity without being affected by the surrounding azide func-
tionality. It therefore represents a versatile and efficient single-
step pathway for the alkylation of azide derivatives. Monomers
2–4 are stable in the absence of copper(I) catalyst, as no traces
of addition products were observed even after prolonged
storage at ca. À4 1C. One advantage of the chosen structures
is that all synthesized monomers have a (nC + nO)/nN ratio
higher than 3, and could therefore be manipulated without any
significant risk of explosion.8
The elaboration of linear polytriazoles by catalyzed or
uncatalyzed click chemistry step growth polymerization of
dialkyne and diazide monomers, oligomers or polymers, has
been thoroughly investigated.4 However, the main drawback
of these A–A/B–B strategies resides on the difficulty to achieve
stoichiometry between antagonist click functionalities, broad
distributions of linear and cyclic chains with limited degree of
polymerization being generally obtained. To a smaller extent
A–B polyaddition strategies have also been investigated.
Matyjaszewski and co-workers reported the click coupling of
a-azide-o-alkyne polystyrenes, yielding mixtures of linear and
cyclic macromolecules.5 Similar architectures were also ob-
tained by Liskamp and co-workers using microwave-assisted
cycloaddition of peptide-based A–B monomers in dilute
conditions ([monomer] o 100 mg mLÀ1).6 This approach
was more recently used to design well-defined polystyrene
and poly(N-isopropyl acrylamide) macrocycles.7
a
´
Universite de Lyon, CNRS UMR5223, Ingenierie des Materiaux
Polymeres, INSA de Lyon, Laboratoire des Materiaux
´
´
`
´
Macromole´culaires, Villeurbanne, F-69621, France
b
´
Universite de Lyon, CNRS UMR5223, Ingenierie des Materiaux
Polymeres, Universite de Lyon 1, Laboratoire des Materiaux
´
´
`
´
´
`
Polymeres et Biomateriaux, Villeurbanne, F-69622, France.
E-mail: eric.drockenmuller@univ-lyon1.fr
´
w Electronic supplementary information (ESI) available: Synthetic
procedures. See DOI: 10.1039/b805164j
Scheme 1 Synthesis and click chemistry step growth (co)polymeriza-
tion of a-azide-o-alkyne monomers 2–4.
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Fig. 2 SEC analysis of monomer 4 and resulting polytriazole 7 in
Fig. 1 1H NMR spectra of (a) monomer 4 and (b) polytriazole 7.
CHCl3 ((a) monomer, (b) precipitated polymer, (c) crude polymer).
standards. The same dilution-mediated distribution of cyclic
and linear species was observed for polytriazole 6 and 10.
Then, in order to adjust the solubility and the thermal
properties of the resulting materials, different copolymers were
prepared using every stoichiometric combination of mono-
mers 2–4, yielding copolymers 8–11. In addition to NMR and
SEC (when soluble in CHCl3), all polytriazoles were charac-
terized by DSC and ATG in order to investigate their thermal
properties in relation with the monomer(s) structure(s). The
solubility and thermal properties of (co)polymers 5–11 are
summarized in Table 1. Concerning polytriazole 5, the combi-
nation of the backbone rigidity, the hydrogen bonding capa-
city of triazole rings and the p-stacking of the phenyl groups
results in a material only partially soluble in DMSO. DSC
analysis of this polymer shows an amorphous behaviour for
temperatures ranging from 0 to 200 1C. However, its melting
temperature might be higher than its degradation temperature,
as generally observed for highly crystalline aromatic polymers
such as aramides.
First, monomers 2–4 were polymerized in chloroform
(0.2 M of monomers) using Cu(PPh3)3Br and DIPEA as
catalytic system (respectively 0.01 and 3 equivalents according
to the monomer). Resulting homopolymers were characterized
1
by H and 13C NMR as well as SEC analyses.
1
As an example, H NMR spectra of monomer 4 and corres-
ponding polytriazole 7 are presented in Fig. 1. The click chemistry
polyaddition of the monomer is confirmed by the appearance of
the characteristic signal of the triazole rings at 7.51 ppm, as well
as the disappearance of the alkyne signal at 2.41 ppm. Also,
during the polyaddition process the signals of methylene groups
adjacent to alkyne and azide functionalities at 3.25 and 4.12 ppm
were progressively shifted to 4.32 and 4.61 ppm, respectively. The
splitting of the signals at 7.51, 4.61 and 4.32 ppm (towards 7.56,
4.66 and 4.42 ppm, respectively) must be pointed out as they
highlight the presence of 13 mol% of a cyclic derivative arising
from monomer 4 intramolecular click reaction. This low molar
mass cycle could be easily isolated after precipitation of the
1
polymer in diethyl ether, as confirmed by H NMR analysis of
Polytriazoles 6 and 7 are semi-crystalline powders (Tm
=
the filtrate and the precipitate. The ratio of cyclic vs. linear species
could be easily tuned by varying the monomer weight fraction in
the reaction media (0.1–50 wt% of monomer). Further work
regarding the elaboration of cycles through cyclization of well-
defined heterofunctional oligomers is currently under progress.
SEC experiments performed in CHCl3 on monomer 4, crude
polymerization mixture and precipitated polymer 7 are plotted
on Fig. 2. It first appears that no monomer 4 remains in the
polymerization mixture, pointing out the complete conversion
characteristic of the click chemistry process. Corroborating 1H
NMR data, a significant amount of a lower molar mass
compound corresponding to the intramolecular click reaction
of monomer 4 appears at an elution volume of 8.4 mL. Also, as
discussed above, this compound could be isolated and was
absent from the SEC traces of precipitated polytriazole 7.
Other cyclic structures obtained from the intramolecular reac-
tion of heterofunctional dimers and oligomers are overlaid with
the smallest linear species at elution volumes of 7.5–8.2 mL.
The weight average molar mass of the linear polytriazoles were
estimated by deconvolution of the peak at ca. 6.8 mL yielding
Mn values of ca. 13 000 g molÀ1 according to polystyrene
115 and 107 1C, respectively) soluble in halogenated and polar
aprotic solvents. Compared to 5, this solubility difference
results from the longer size, the lower inter-chain cohesion
and the lower rigidity of the C11 segments separating two
consecutive triazole linkages. Both polymers have a glass
transition temperature (Tg) of ca. À10 1C, higher than poly-
ester and polyether analogues (À44 1C and À80 1C for PTMO
and C11 polyester, respectively).9 Thus, triazole groups parti-
cipate to the increase of Tg, most probably toward interchain
Table 1 Characteristics of polytriazoles 5–11
Sample
2 : 3 : 4a Tg/1C
Tm/1C Solubilityb
Mnc/g molÀ1
5
6
7
8
1 : 0 : 0
0 : 1 : 0
0 : 0 : 1
1 : 1 : 0
1 : 0 : 1
0 : 1 : 1
1 : 1 : 1
125
À12
À9
5
—
—
C, D, S
C, D, S
S
—
115
107
—
—
81
13 300
13 300
—
—
12 100
—
9
20
À17
S
10
11
a
T, C, D, S
S
0
—
b
Mol%. T = THF, C = CHCl3, D = CH2Cl2, S = DMSO.
Obtained by SEC in CHCl3 according to polystyrene standards.
c
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Fig. 3 DSC Thermograms of polytriazole homopolymers 5, 7 and
corresponding copolymer 9 obtained from monomers 2 and 4.
Fig. 4 TGA analyses of polymers 5–11.
hydrogen bonding interactions. However, Tg remains lower
than for corresponding polyamide derivatives (Tg = 50 1C for
C12 polyamide).
respectively) compared to polytriazoles based on aliphatic
monomers.
In conclusion, we developed a versatile strategy for the
elaboration of polytriazole (co)polymers by click chemistry
step growth polymerization of low molar mass a-azide-o-
alkyne monomers. This robust synthetic approach can be
extended to a wide range of linear polytriazoles and well-
defined cyclic architectures with targeted properties.
Moreover, the use of click chemistry enables further post-
functionalization of the resulting macromolecular architec-
tures since it allows to carry functionalities through their
synthesis.
The replacement of the ester linkage by an ether only
slightly changes the melting temperature (Tm) of 6 and 7,
indicating that thermal properties are mainly ruled by the
triazole ring units. Such high Tm values cannot be attributed to
crystallization of the C11 methylene segments only. Therefore
the whole repeating unit involving hydrogen bonding triazole
rings participates to the crystal structure.
Copolymers 8, 9 and 11 based on monomer 2 are amor-
phous with Tg values of ca. 5, 20 and 0 1C, respectively.
Similarly to the corresponding homopolymers 6 and 7, co-
polymer 10 is semi-crystalline (Tm = 81 1C). However, crystal-
lisable segments are shorter in this random copolymer, leading
to a reduction of the crystal size, that lowers the melting
temperature of 10 compared to the corresponding homopoly-
mers 6 and 7.
Notes and references
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The click chemistry step growth polymerization of A–B
monomers allows to easily combine different heterofunctional
monomers, leading to a range of polytriazoles with tunable
properties. Besides the solubility properties, Fig. 3 shows an
example of the influence of the monomer structures on the
thermal properties of the resulting materials. Indeed, DSC
thermograms of homopolymers 5, 7 and the corresponding
copolymer 9 highlight the change in the thermal and therefore
mechanical properties (shift of Tg and loss of crystallinity)
resulting from the combination of monomers with distinct
structures.
Finally, ATG analyses (Fig. 4) show that all synthesized
polytriazoles are relatively stable, resulting in degradation
temperatures (at 10 wt% loss under nitrogen) from 330 to
380 1C. Here again, the influence of the monomer combina-
tions on the final material properties is illustrated, since the
loss of weight varies significantly with the initial monomer
structure. For instance, polytriazoles containing benzyl mono-
mer 2 in a ratio of at least 50 mol% (e.g. polytriazoles 5, 8 and
9) present a high amount of ash (47, 30 and 20 wt%,
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This journal is The Royal Society of Chemistry 2008
4140 | Chem. Commun., 2008, 4138–4140