Fully Aromatic High Performance Thermoset via Sydnone-Alkyne Cycloaddition

Article abstract of DOI:10.1021/jacs.6b03381

We have developed an efficient synthetic platform for the preparation of a new class of high performance thermosets based on the 1,3-dipolar cycloaddition of a bifunctional sydnone with a trifunctional alkyne. These processable materials possess outstandi

Full text of DOI:10.1021/jacs.6b03381

Communication
pubs.acs.org/JACS
Fully Aromatic High Performance Thermoset via Sydnone−Alkyne
Cycloaddition
Nisha V. Handa,†,‡ Shaoguang Li,†,‡ Jeffrey A. Gerbec,†,⊥ Naoko Sumitani,# Craig J. Hawker,*,†,‡,§
and Daniel Klinger*,‡,∥
§
†Mitsubishi Chemical Center for Advanced Materials, Department of Chemistry and Biochemistry, Materials Department, and
‡Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States
⊥Mitsubishi Chemical USA, Inc., Chesapeake, Virginia 23320, United States
#Mitsubishi Chemical Group Science and Technology Research Center, Inc., Yokohama 227-8502, Japan
∥Institute of Pharmacy, Freie Universitat Berlin, Konigin-Luise-Str. 2-4, 14195 Berlin, Germany
̈
̈
S
* Supporting Information
only a limited number of nonaromatic thermosets being
prepared using azide−alkyne click chemistry.6
ABSTRACT: We have developed an efficient synthetic
platform for the preparation of a new class of high
performance thermosets based on the 1,3-dipolar cyclo-
addition of a bifunctional sydnone with a trifunctional
alkyne. These processable materials possess outstanding
thermal stability, with Td5% of 520 °C and a weight loss of
<0.1% per day at 225 °C (both in air). Key to this
performance is the stability of the starting functional
groups that allows for reactive B-staging via simple thermal
activation to give fully aromatic and highly cross-linked
polypyrazole-based thermosets.
This study aims to translate the efficiency of 1,3-dipolar
cycloadditions to the formation of fully aromatic thermosets with
outstanding thermal properties.7 Upon approaching this task, we
were drawn to alternative 1,3-dipoles such as the mesoionic
heterocycles of munchnones8 and sydnones.9 Their cyclo-
̈
addition with alkynes produce aromatic pyrrole10 or pyrazole
units.9 Especially the stable (N-phenyl) sydnones have been
established as unusual but promising cycloaddition substrates for
pyrazole formation.11 Based on these considerations, we focused
on the reaction of multifunctional sydnones with multifunctional
alkynes to produce fully aromatic cross-linked materials12,13
containing highly thermally stable pyrazole cross-linking points
(Scheme 1a).12 This unique ability to access robust aromatic
linkages under catalyst-free conditions with the generation of
CO2 as the only side product illustrates the utility of sydnone
chemistry for the preparation of high performance materials.14 In
addition, this approach allows considerable flexibility in
molecular design due to the synthetic versatility inherent in
either the alkyne or the sydnone monomers.
rowth in the aerospace and microelectronic industries
Gincreasingly requires the development of lightweight
materials with excellent chemical, thermal, and mechanical
robustness.1 Thermoset polymers, as densely cross-linked
networks of organic repeat units, offer a modular and flexible
synthetic approach to such materials also allowing desired
properties to be tuned via the macromolecular structure.2
Specifically, rigid networks with high contents of aromatic units3
lead to increased heat resistance and improved thermo-
mechanical properties with fully aromatic high performance
thermosets (HPTs) offering significant promise.4 However, the
synthesis of fully aromatic structures is a major challenge since it
requires the efficient formation of aromatic connecting points
between aryl monomers without using metal catalysts.
To address the criteria of efficient and noncatalytic chemistry,
cycloaddition reactions were selected for thermoset formation.
In contrast to polycondensations that are prevalent in thermoset
synthesis,4c cycloaddition reactions avoid the formation of
significant side products with the most successful example
being the use of cyclopentadienone-based 4 + 2π cycloaddition
chemistry for the formation of Dow’s low-k semiconductor
dielectric SiLK materials.4d In this study our attention was
directed to 1,3-dipolar cycloadditions, which have generated
significant interest within the organic synthesis and bioconjuga-
tion communities due to their high efficiency, tolerance to
functionality, mild reaction conditions, and fast reaction rates.5
Despite the potential of 1,3-dipolar cycloadditions, their
application toward HPT development is underexplored with
The synthetic strategy is based on the reaction of a bifunctional
sydnone BS with a trifunctional alkyne TA to produce the fully
aromatic thermoset TS, consisting of aryl/pyrazole units
(Scheme 1b). TA was prepared using literature procedures and
BS was obtained in good yield from 1,4-diaminobenzene.
Significantly, both synthetic approaches were scalable to 10+
gram scales from readily available starting materials (see SI).15
Initially the respective monomers were either mixed in solution
or melt processed; however, significant phase separation was
observed and low cross-reactivity obtained. To guarantee high
cross-linking densities, a prepolymer approach was developed.
This efficient B-staging strategy involves two steps: (1) the
preparation of partially cross-linked, soluble oligomers with
retention of reactive sydnone and alkyne units and (2)
processing and final curing of the oligomers at elevated
temperatures to produce the fully cross-linked thermoset
(Scheme 2).
Received: April 1, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b03381
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Journal of the American Chemical Society
Communication
thermoset formation that unreacted sydnone and alkyne
functional groups are retained in the soluble oligomers even
after isolation and prolonged storage in either solution or the
solid state. The presence of these unreacted groups was
confirmed by solution state 1H and solid state 13C NMR
spectroscopy (1H NMR, see SI; 13C NMR, see Figure 2). This
retention of reactivity coupled with a wide processing window
and reproducible molecular weight control for the oligomer
formation allows high solid content solutions and melt
processable systems to be used for the preparation of fully
cross-linked thermosets.
Scheme 1. (a) Exemplary Cycloaddition Reaction of
Sydnones and Symmetric Alkynes to Give Aromatic
Pyrazoles; (b) Synthetic B-Staging Pathway to the Formation
of New Pyrazole-Based Thermoset TS via Cycloaddition
a
Reaction of Bis-sydnone BS with Tris-alkyne TA
The final step in this process requires the optimization of
curing conditions for thermoset formation. For enhanced
thermal stability, the highest degree of cross-linking, i.e., the
maximum conversion of reactive groups, is required. Differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) studies were therefore performed on the oligomeric
precursor OL to fully understand the relationship between curing
conditions and functional group conversion. DSC analysis
revealed the existence of a broad exothermic peak between 170
and 390 °C, which can be attributed to the exothermic cross-
linking reaction (see SI). These observations were further
supported by TGA experiments on OL, which showed a series of
consecutive weight losses starting at lower temperatures prior to
the onset of final degradation at ∼500 °C (see SI). Assuming that
weight loss is due to CO2 generation upon cycloaddition to give
pyrazole-based cross-linking units, these results indicate that the
final curing of OL to a fully cross-linked thermoset TS occurs
over a broad temperature range starting at ca. 170 °C.
a
Note: only 3-substituted pyrazole units are shown. Potential 4-
substitued isomers are omitted for clarity.
In systematically optimizing this curing protocol, monitoring
the CO2 evolution from oligomers OL was studied in detail using
thermal gravimetric analysis−mass spectrometry (TGA−MS).
Precured oligomer films (60 °C for 24 h) were analyzed
following heating profiles at 150, 200, 250, and up to 300 °C for 5
h each. As shown in Figure 1, CO2 evolution and corresponding
weight loss were observed at the beginning of each temperature
increase with CO2 evolution decreasing at each stage.
Scheme 2. Schematic Representation of the B-Staging
Approach for the Preparation of a Pyrazole-Based High
Performance Thermoset Reaction of Bis-sydnones with Tris-
alkyne Gives Oligomers with Free Reactive Groups Available
for Final Thermal Curing
These findings suggest that for a constant curing temperature,
the formation of a cross-linked network decreases chain mobility,
therefore hindering further reactions. Increasing the temperature
increases chain mobility and enables further cross-linking.17 After
The development of suitable reaction conditions for both the
initial oligomerization and subsequent curing was critical in this
B-stage strategy. For oligomerization, maintaining solubility and
reactivity was achieved by optimizing the time and temperature
of the solution reaction between bis-sydnone BS and tris-alkyne
TA. It was found that reaction times between 15 and 24 h at T =
150 °C give essentially a quantitative yield of soluble oligomers
OL with molecular weights of ∼4 kDa (determined by GPC, see
SI). Increases in reaction time and temperature (T > 180 °C)
lead to higher molecular weight, partially cross-linked materials.
At lower reaction temperatures (T < 120 °C), minimal coupling
was observed with high amounts of unreacted monomer still
present. The formation of pyrazoles was observed by comparing
1H NMR spectra of OL and model compound MO (see SI).
Additional studies on MO (see SI) suggest that the ratio between
the 3- and 4-substituted regioisomers is around 10:1 in the
oligomers, as suggested by the literature.16 It was important for
Figure 1. Analysis of thermoset curing by TGA−MS under nitrogen
atmosphere. The stepped heating protocol of the thermal curing of OL
can be correlated to the evolution of CO2 and the corresponding weight
loss.
B
DOI: 10.1021/jacs.6b03381
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Journal of the American Chemical Society
Communication
the final heating step (up to 300 °C for 5 h), TGA showed no
further weight loss prior to onset of degradation, indicating
complete cross-linking. As a result, the optimized cure conditions
for thermoset TS involve initial drop-casting of oligomer films
from N-methyl-2-pyrrolidone (NMP) and removal of residual
solvent. Films and bulk samples were then fully cured by
subsequent annealing at 150, 200, 250, and up to 300 °C for 5 h.
While TGA and TGA−MS serve as experimental tools to
indirectly determine final curing, solid-state 13C NMR was
employed to evaluate the reactive moieties in both the oligomeric
prepolymer, OL, and the final thermoset, TS. As can be seen in
Figure 2, distinct peaks can be observed for the starting bis-
Figure 3. (a) Thermogravimetric analysis (TGA) of TS in air. (b)
Isothermal TGA of TS at 225 °C over 47 days in air.
tan δ of TS (Figure 4). E′ at room temperature was observed to
be 3.3 GPa, highlighting the high stiffness of TS with only
Figure 2. Solid-state 13C NMR comparison of thermoset TS with
oligomer OL and monomers (BS and TA) reveals the presence of free
reactive groups in the oligomeric precursor and their disappearance
upon complete cross-linking via formation of pyrazoles.
sydnone, BS, and tris-alkyne, TA, at ∼95 ppm (a) and ∼85 ppm
(b,c), respectively. After initial B-staging, these peaks decrease in
intensity for OL and disappear for the fully cured thermoset TS.
Significantly, this disappearance is coincident with the
appearance of new resonances at 150 and 105 ppm
corresponding to the pyrazole cross-linking units (see SI for
spectral data of pyrazole model compound). Further support for
this sequence of reactions is obtained from Fourier transform
infrared (FT-IR) measurements, which show the reduction and
eventual disappearance of the carbonyl stretch for the sydnone
units at ∼1750 cm−1 on going from monomer to thermoset (see
SI). This combination of characterization methods demonstrates
the robustness of the proposed sydnone-alkyne cross-linking
chemistry for pyrazole-based thermosets.
Having developed a facile and efficient process for the
formation of fully aromatic thermosets, their thermal and
mechanical properties were investigated. As expected, the
thermal stability of TS was exceptionally high with Td5% observed
at 520 °C (Figure 3a) (60% char at 800 °C in N2; see SI)). Of
particular note is the thermal stability in air with Td5% greater than
500 °C and negligible long-term weight loss of 0.003 wt %/h
(0.072 wt %/day) after aging at 225 °C for over 45 days (Figure
3b). To put this performance in context, conventional
polybenzoxazines (Td5% ≈ 300 °C)18 and polyimide thermosets
(Td5% ≈ 450 °C) show significant long-term weight loss at
elevated temperatures over much shorter time periods,19 clearly
demonstrating the potential of sydnone−alkyne thermosets for
high performance applications.
Figure 4. Dynamic mechanical analysis of free-standing films of TS.
Temperature dependence of storage modulus (E′), loss modulus (E″),
and tan δ for the determination of Tg.
marginal changes being observed up to ∼500 °C.20 The broad
peak in tan δ and E″ at ∼90 and 70 °C can be assigned to β
transitions due to the presence of loose chain ends and network
irregularities in these cross-linked structures.21 Similarly, the
maxima of tan δ and E″ reveal a very high Tg approaching 480 °C
for TS. All of these properties are improved when compared to
benchmark thermosetting polyimides such as PETI-5 (249 °C)19
and TriA-PI (343 °C).22
Thermomechanical analysis (TMA) of free-standing films of
TS (see SI) reveals a macroscopic thermal expansion coefficient
(CTE) of 47.5 μm/(m·°C) over a temperature range of 25−350
°C. This low thermal expansion is required for dimensional
stability in high performance materials and is comparable with
commercial poly(benzoxazine) and polyimide materials, which
have CTEs of ∼24 to 43 μm/(m·°C).23 Additionally, it was
interesting to observe the unexpectedly high adhesion of the
thermoset to a variety of substrates. For example, a lap shear
adhesion strength of ∼2.6 MPa was obtained for aluminum
To illustrate the mechanical performance of these materials,
dynamic mechanical analysis (DMA) showed enhanced temper-
ature-dependent storage modulus (E′), loss modulus (E″), and
C
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(2) (a) Amendt, M. A.; Chen, L.; Hillmyer, M. A. Macromolecules 2010,
43, 3924−3934. (b) García, J. M.; Jones, G. O.; Virwani, K.; McCloskey,
B. D.; Boday, D. J.; ter Huurne, G. M.; Horn, H. W.; Coady, D. J.;
Bintaleb, A. M.; Alabdulrahman, A. M. S.; Alsewailem, F.; Almegren, H.
A. A.; Hedrick, J. L. Science 2014, 344, 732−735. (c) Gardziella, A.;
Pilato, L.; Knop, A. Thermosets: Overview, Definitions, and
Comparisons. In Phenolic Resins; Springer: Berlin Heidelberg, 2000;
pp 109−121.
substrates. Given the fully aromatic structure, this is assigned to
the nitrogen-rich pyrazole cross-linking units.24 Finally, the
synthetic versatility of this new approach enables a range of
thermoset architectures to be obtained by simply changing the
molecular structure of the monomers. As shown in Figure 5,
(3) (a) Iqbal, M.; Norder, B.; Mendes, E.; Dingemans, T. J. J. Polym.
Sci., Part A: Polym. Chem. 2009, 47, 1368−1380. (b) Agag, T.; Takeichi,
T. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1878−1888.
(4) (a) Sroog, C. E. Prog. Polym. Sci. 1991, 16, 561−694. (b) Ishida, H.
Chapter 1 - Overview and Historical Background of Polybenzoxazine
Research. In Handbook of Benzoxazine Resins; Agag, H. I., Ed.; Elsevier:
Amsterdam, 2011; pp 3−81. (c) Hergenrother, P. M. High Perform.
Polym. 2003, 15, 3−45. (d) Martin, S. J.; Godschalx, J. P.; Mills, M. E.;
Shaffer, E. O.; Townsend, P. H. Adv. Mater. 2000, 12, 1769−1778.
(5) (a) Padwa, A. 1,3-Dipolar Cycloaddition Chemistry; Wiley: New
York, 1984. (b) Lutz, J.-F. Angew. Chem., Int. Ed. 2007, 46, 1018−1025.
(6) (a) Song, H. B.; Baranek, A.; Bowman, C. N. Polym. Chem. 2016, 7,
603−612. (b) Díaz, D. D.; Punna, S.; Holzer, P.; McPherson, A. K.;
Sharpless, K. B.; Fokin, V. V.; Finn, M. G. J. Polym. Sci., Part A: Polym.
Chem. 2004, 42, 4392−4403. (c) Wan, L.; Luo, Y.; Xue, L.; Tian, J.; Hu,
Y.; Qi, H.; Shen, X.; Huang, F.; Du, L.; Chen, X. J. Appl. Polym. Sci. 2007,
104, 1038−1042. (d) Ergin, M.; Kiskan, B.; Gacal, B.; Yagci, Y.
Macromolecules 2007, 40, 4724−4727.
(7) (a) Rode, V. V.; Bondarenko, Y. M.; D’Yachenko, A. V.; Krongauz,
Y. S.; Korshak, V. V. Polym. Sci. U.S.S.R. 1969, 11, 935−944.
(8) Reissig, H.-U.; Zimmer, R. Angew. Chem., Int. Ed. 2014, 53, 9708−
9710.
(9) Browne, D. L.; Harrity, J. P. A. Tetrahedron 2010, 66, 553−568.
(10) Lu, Y.; Arndtsen, B. A. Angew. Chem., Int. Ed. 2008, 47, 5430−
5433.
(11) Browne, D. L.; Vivat, J. F.; Plant, A.; Gomez-Bengoa, E.; Harrity, J.
P. A. J. Am. Chem. Soc. 2009, 131, 7762−7769.
(12) (a) Huisgen, R.; Grashey, R.; Gotthardt, H.; Schmidt, R. Angew.
Chem., Int. Ed. Engl. 1962, 1, 48−49. (b) Stille, J. K.; Bedford, M. A. J.
Polym. Sci., Part A-1: Polym. Chem. 1968, 6, 2331−2342. (c) Browne, D.
L.; Taylor, J. B.; Plant, A.; Harrity, J. P. A. J. Org. Chem. 2010, 75, 984−
987.
Figure 5. Expanding the sydnone−alkyne cross-linking strategy to
different monomers: Investigated monomer combinations to give
pyrazole-based thermosets and their respective Td5% values in air.
varying the combination of four readily accessible monomers
(see SI) allows the preparation of four different thermosets (TS
to TS-c), which all show exceptional heat resistance with Td5%
values exceeding 430 °C.
In summary, the highly efficient cycloaddition of bifunctional
sydnones with trifunctional alkynes represents a facile synthetic
approach to a new family of fully aromatic thermosets based on
pyrazole cross-linking units. Exceptionally high thermal and
mechanical performance clearly illustrates the successful trans-
lation of efficient cross-linking chemistry and functional group
stability to final material properties under standard processing
conditions. The availability of alternative monomers demon-
strates the potential of this tunable thermoset platform as a high
performance matrix for advanced technologies.
(13) Stille, J. K. Makromol. Chem. 1972, 154, 49−61.
(14) Intemann, J. J.; Huang, W.; Jin, Z.; Shi, Z.; Yang, X.; Yang, J.; Luo,
J.; Jen, A. K. Y. ACS Macro Lett. 2013, 2, 256−259.
(15) (a) Stille, J. K.; Bedford, M. A. J. Polym. Sci., Part B: Polym. Lett.
1966, 4, 329−331. (b) Stewart, F. H. C. Chem. Rev. 1964, 64, 129−147.
(c) Yashunskii, V. G.; Kholodov, L. E. Zh. Obshch. Khim. 1962, 32,
3661−5. (d) Asundaria, S. T.; Patel, K. C. J. Heterocyclic Chem. 2013, 50,
E136−E141.
(16) Foster, R. S.; Jakobi, H.; Harrity, J. P. A. Tetrahedron Lett. 2011,
52, 1506−1508.
(17) Spruell, J. M.; Wolffs, M.; Leibfarth, F. A.; Stahl, B. C.; Heo, J.;
Connal, L. A.; Hu, J.; Hawker, C. J. J. Am. Chem. Soc. 2011, 133, 16698−
16706.
(18) Agag, T.; Takeichi, T. Macromolecules 2003, 36, 6010−6017.
(19) Smith, J. G., Jr; Connell, J. W.; Hergenrother, P. M. Polymer 1997,
38, 4657−4665.
(20) Chang, H. C.; Lin, C. H.; Tian, Y. W.; Feng, Y. R.; Chan, L. H. J.
Polym. Sci., Part A: Polym. Chem. 2012, 50, 2201−2210.
(21) Feng, T.; Wang, J.; Wang, H.; Ramdani, N.; Zu, L.; Liu, W.; Xu, X.
Polym. Adv. Technol. 2015, 26, 581−588.
(22) Yokota, R.; Yamamoto, S.; Yano, S.; Sawaguchi, T.; Hasegawa, M.;
Yamaguchi, H.; Ozawa, H.; Sato, R. High Perform. Polym. 2001, 13, S61−
S72.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b03381.
■
S
Detailed synthetic procedures and characterization data
for the new compounds and additional experimental
results (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*hawker@mrl.ucsb.edu
*daniel.klinger@fu-berlin.de
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (MRSEC program,
DMR 1121053) and the Mitsubishi Chemical Company
(Mitsubishi Chemical Center for Advanced Materials) for
financial support. We also thank Philip M. Carubia (Cornell
Center for Materials Research) for performing DMA and TMA
analysis.
(23) Wang, M. W.; Lin, C. H.; Juang, T. Y. Macromolecules 2013, 46,
8853−8863.
(24) Matos-Per
2012, 134, 9498−9505.
́
ez, C. R.; White, J. D.; Wilker, J. J. J. Am. Chem. Soc.
REFERENCES
(1) Meador, M. A. Annu. Rev. Mater. Sci. 1998, 28, 599−630.
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DOI: 10.1021/jacs.6b03381
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX