Crosslinking of fluoroelastomers by "click" azide-nitrile cycloaddition

Article abstract of DOI:10.1002/pola.27549

A fluoroelastomer that bears pendant nitrile groups is crosslinked with a telechelic bis-azido fluorinated curing agent by "click" azide-nitrile cycloaddition. Thermogravimetric analyses of the resulting press cured films reveal an improvement by 20 C of the thermal degradation profile under air, compared to that of the corresponding uncured fluoroelastomer.

Full text of DOI:10.1002/pola.27549

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Crosslinking of Fluoroelastomers by “Click” Azide–Nitrile Cycloaddition
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Guillaume Tillet, Gerald Lopez, Ming-Hong Hung, Bruno Ameduri
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Institut Charles Gerhardt de Montpellier (ICGM)—UMR5253, Ingenierie et Architectures Macromoleculaires (IAM), Ecole Natio-
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nale Superieure de Chimie de Montpellier (ENSCM), 8 Rue de l’Ecole Normale, 34296 Montpellier, France
2DuPont Performance Polymers, P.O. Box 80293, Wilmington, Delaware 19880-0293
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Correspondence to: B. Ameduri (E-mail: bruno.ameduri@enscm.fr)
Received 29 September 2014; accepted 11 January 2015; published online 00 Month 2015
DOI: 10.1002/pola.27549
KEYWORDS: “click” chemistry; crosslinking; elastomers; films; fluoropolymers
Fluoroelastomers exhibit excellent resistance to heat,
weather, oil, solvents, and other chemicals.1 Commercially
available materials are either fluorosilicones or fluorophos-
phazenes, as well as fluorocarbon copolymers based on
vinylidene fluoride (VDF), hexafluoropropylene, tetrafluoro-
ethylene (TFE), and chlorotrifluoroethylene.2 Other common
fluoroelastomers include the copolymers of TFE with one or
more hydrocarbon olefins such as propylene (P), and also
the copolymers of TFE (and VDF) with a perfluoro(alkyl
vinyl ether) such as perfluoro(methyl vinyl ether) (PMVE).
minal acetylenes and azides) has vastly increased in broad-
ness and application in the field of polymer science.6
However, those type of 1,3-dipolar cycloaddition are still
scarcely employed in the fluoropolymers area.7
We describe here an appealing crosslinking method for fluo-
roelastomers via the formation of tetrazoles by using Huis-
gen cycloaddition between an azide and a nitrile.8 The
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chosen polymer was DuPontTM KalrezV perfluoroelastomer
(Fig. 1).9
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DuPontTM KalrezV perfluoroelastomer is a terpolymer based
on TFE, PMVE, and perfluoro(8-cyano-5-methyl-3,6-dioxa-
oct-1-ene) (8-CNVE), that contain weight proportions ranging
from 65 to 80%, 20 to 34%, and 1 to 3%, respectively.
Many fluoroelastomers require the incorporation of a cure
site monomer into their polymeric chains to enable an effi-
cient crosslinking. Fluoroelastomers are usually cured by
nucleophiles such as diamines, bisphenols, or with perox-
ides.3 Nevertheless, some curatives are toxic, whereas others
containing reactive bromine or iodine atoms can release
harmful byproducts during the curing reaction. Other cure
site monomers (e.g., those containing double bonds at both
ends of the molecule) may be so reactive that they disrupt
polymerization of the fluoroelastomer by altering the poly-
merization rate, by terminating polymerization, or by causing
undesirable chain branching. Lastly, the incorporation of a
cure site monomer into a fluoroelastomer chain may induce
a negative impact on the physical properties and the chemi-
cal resistance, once crosslinked.2 New curing systems for flu-
oroelastomers that avoid the aforementioned limitations are
thus in demand and this is the objective of this article.
This fluoroelastomer is used as high performance seal in
areas ranging from aerospace, pharmaceuticals, and electron-
ics.10 However, the conventional method used to carry out
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the crosslinking of DuPontTM KalrezV perfluoroelastomer
requires too much energy.11 In fact, the crosslinking reaction
via the cyclotrimerization of nitrile groups and the subse-
quent formation of triazines12 interchain bridges (Supporting
13
ꢀ
Information Fig. S1) is carried out at 300 C for two days.
To decrease the energetic cost, the Huisgen cycloaddition
between the pendant cyano groups of 8-CNVE and a bis-
azido fluorinated telechelic was considered, with the expec-
tation that it would decrease both the reaction temperature
and the reaction time.
Actually, Huisgen was the first to understand the breadth of
the 1,3-dipolar cycloaddition between an azide and a termi-
nal or an internal alkyne to give a 1,2,3-triazole.4 Such a
reaction was described as “the cream of the crop of click
chemistry” by Sharpless and coworkers.5 Nowadays, the
metal catalyzed azide/alkyne “click” reaction (a variation of
the Huisgen 1,3-dipolar cycloaddition reaction between ter-
A model study has been first carried out to determine the
feasibility of such a “click” reaction (Scheme 1).
In a first step, 1-iodoperfluorohexane (C6F13I) was ethylen-
ated in tert-butanol using di-(4-tert-butylcyclohexyl) peroxy-
dicarbonate as the radical initiator.14 The structure of
compound
1
was confirmed by 1H and 19F NMR
Additional Supporting Information may be found in the online version of this article.
C
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FIGURE
1 Chemical
structure
of
DuPontTM
KalrezV
perfluoroelastomer.
spectroscopies. The 19F NMR spectrum of 1 (Supporting
Information Fig. S2) displayed the characteristic peak at
2122.6 ppm attributed to the CF2CH2 pattern while it did
not exhibit any peak at 259 ppm (characteristic of a CF2I
end-group in C6F13I). In agreement with the literature,15 this
step afforded 1 in about 80% yield. The second step is a
nucleophilic substitution of the terminal iodine group by an
azido group, yielding 2 in 94% yield (Scheme 1).16 Azido
compound 2 was then characterized by Fourier transform
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SCHEME 2 Crosslinking of DuPontTM KalrezV perfluoroelastomer
by “click” azide–nitrile cycloaddition.
The above method was then applied to crosslink DuPontTM
1
infrared spectroscopy (FTIR) and H NMR spectroscopy. The
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KalrezV perfluoroelastomer. Bis-azido fluorinated curing
FTIR spectrum of 2 (Supporting Information Fig. S3) dis-
played the characteristic azide band at 2110 cm21. The 1H
NMR spectrum of 2 (Supporting Information Fig. S4) showed
an expected downfield shift of the protons adjacent to the
terminal azido group. The various tests (Supporting Informa-
tion Table S1) of “click” reactions between 2 and 8-CNVE
were carried out using a 1.1-fold excess of 2 with respect to
8-CNVE. The reactions were monitored by 19F NMR spectros-
copy. The most appropriate reaction conditions afforded 3 in
agent 5 was synthesized according to a similar procedure as
that of perfluoroazido compound
2
(Scheme 2).7(a),17
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DuPontTM KalrezV perfluoroelastomer and 5 were solubilized
in perfluoro(butyltetrahydrofuran) and the resulting solution
was casted to obtain a film after evaporation. The evapora-
tion step has to be slow to prevent any formation of bubbles
withiꢀn the film. Therefore, a pre-evaporation was carried out
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at 5 C for 24 h in a cold room then at 35 C for 24 h in an
oven. Finally, the film was press cured at 150 ꢀC for 24 h
ꢀ
90% yield in the absence of any catalyst at 150 C for 24 h.
The 19F NMR spectrum of 3 superimposed with that of 8-
CNVE is presented in Supporting Information Figure S5. The
fluorine atoms adjacent to the nitrile group (i.e. “g” signal at
2109.4 ppm, Supporting Information Fig. S5 top spectrum)
in 8-CNVE underwent an upfield shift after the reaction (i.e.,
“g” signal at 2112.6 ppm, Supporting Information Fig. S5
bottom spectrum). The characteristic signals corresponding
to the C6F13 group were also identified.
(Scheme 2).
The crosslinked film obtained from a 5:perfluoroelastomer
0.5:5 weight ratio was analyzed by FTIR spectroscopy
FIGURE 2 TGA thermograms under air of uncured DuPontTM
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SCHEME 1 “Click” azide–nitrile cycloaddition reaction between
8-CNVE and perfluoroazido compound 2.
KalrezV perfluoroelastomer and cured films, where Kalrez x:y
represents the 5:perfluoroelastomer weight ratio.
2
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3 (a) A. L. Logothetis, Prog. Polym. Sci. 1989, 14, 251–272; (b)
A. Taguet, B. Ameduri, B. Boutevin, Adv. Polym. Sci. 2005, 184,
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(Supporting Information Fig. S6). The corresponding FTIR
spectrum displayed a small band at 1675 cm21 due to the
presence of C@N stretching vibrations in tetrazole.18 As
an evidence of efficient crosslinking, the film was totally
insoluble in perfluoro(butyltetrahydrofuran).
4 R. Huisgen, In 1,3-Dipolar Cycloaddition Chemistry; A. Padwa
Ed.; Wiley: New York, 1984. pp. 1–176.
5 H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. Int.
Ed. 2001, 40, 2004–2021.
Crosslinked films with various 5:perfluoroelastomer weight
ratios were analyzed by thermogravimetric analysis (TGA)
under air to compare their degradation profile to that of a
perfluoroelastomer film obtained from the same procedure
but without any addition of crosslinking agent 5 (Fig. 2).
Overall, the thermograms revealed an improvement by about
20 ꢀC of the thermal stability under air for the press cured
films with telechelic crosslinking agent 5. In contrast to poly-
meric tetrazoles that decompose with evolution of significant
6 (a) R. A. Evans, Aust. J. Chem. 2007, 60, 384–395; (b) D.
Fournier, R. Hoogenboom, U. S. Schubert, Chem. Soc. Rev.
2007, 36, 1369–1380; (c) W. H. Binder, R. Sachsenhofer, Macro-
mol. Rapid Commun. 2008, 29, 952–981; (d) E. Hwang, K. L.
Lusker, J. C. Garno, Y. Losovyj, E. E. Nesterov, Chem. Com-
mun. 2011, 47, 11990–11992; (e) A. Z. Samuel, S.
Ramakrishnan, Langmuir 2013, 29, 1245–1257; (f) J. Han, Y.
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amounts of gas at 120–250 ꢀC,19 DuPontTM KalrezV cross-
7 (a) A. Soules, B. Ameduri, B. Boutevin, G. Calleja, Macromo-
lecules 2010, 43, 4489–4499; (b) T. Cai, E. T. Kang, K. G. Neoh,
Macromolecules 2011, 44, 4258–4268; (c) R. Vukic´evic´, U.
linked perfluoroelastomers exhibit an improved thermal
behavior, thanks to the perfluorinated backbone that acts as
a protective shield for tetrazoles.
€
€
Schwadtke, S. Schmucker, P. Schafer, D. Kuckling, S.
Beuermann, Polym. Chem. 2012, 3, 409–414; (d) T. Cai, K. G.
Neoh, E. T. Kang, In Progress in Controlled Radical Polymeriza-
tion: Materials and Applications; K. Matyjaszewski, B. Sumerlin,
N. Tsarevsky, Eds.; ACS Symposium Series; American Chemi-
cal Society: Washington, DC, 2012; (e) Y.-W. Yang, J.
Hentschel, Y.-C. Chen, M. Lazari, H. Zeng, R. M. van Dam, Z.
Guan, J. Mater. Chem. 2012, 22, 1100–1106.
To summarize, on the basis of a model study, DuPontTM
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KalrezV perfluoroelastomer that bears pendant nitrile groups
was successfully crosslinked with a bis-azido fluorinated cur-
ing agent by “click” azide–nitrile cycloaddition. Thermogravi-
metric analyses of the resulting press cured films revealed
an improvement by about 20 ꢀC of the thermal degradation
profile under air, compared to that of the corresponding
uncured fluoroelastomer. This method is simple to imple-
ment and requires much less energy than that currently
used in the fluoroelastomers industry.
8 (a) Z. P. Demko, K. B. Sharpless, Org. Lett. 2001, 3, 4091–
4095; (b) L. Bosch, J. Vilarrasa, Angew. Chem. Int. Ed. 2007, 46,
3926–3930.
9 http://www.dupont.com/products-and-services/plastics-poly-
mers-resins/parts-shapes/brands/kalrez-perfluoroelastomer.html
(accessed July 15, 2014).
10 J. Francois, Chemie-Anlagen 1 Verfahren 2008, 41, 28–29.
11 A. L. Logothetis (E. I. Du Pont de Nemours & Co), U.S. Pat-
ent 5,677,389, October 14, 1997.
ACKNOWLEDGMENTS
The authors thank the DuPont Performance Polymers company
12 R. J. Dams, S. G. Corveleyn, W. M. A Grootaert, G. D.
Dahlke, M. A. Guerra (3M Innovative Properties Company), WO
2012005972, January 12, 2012.
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for sponsoring this work and for kindly providing KalrezV. 3M
and AkzoNobel are also acknowledged for supplying perfluor-
o(butyltetrahydrofuran) and di-(4-tert-butylcyclohexyl) perox-
ydicarbonate, respectively.
13 J. Scheirs, Modern Fluoropolymers: High Performance Poly-
mers for Diverse Applications; Wiley: Chichester, UK, 1997.
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15 J. Balague, B. Ameduri, B. Boutevin, G. Caporiccio, J. Fluo-
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