Mechanogeneration of Acid from Oxime Sulfonates

Article abstract of DOI:10.1021/jacs.6b00097

The generation of acid under mechanical force is potentially useful for initiating proton-catalyzed changes in polymeric materials. Here we demonstrate that oxime sulfonates - known photoacid generators - are also acid generators when activated mechanical

Full text of DOI:10.1021/jacs.6b00097

Communication
pubs.acs.org/JACS
Mechanogeneration of Acid from Oxime Sulfonates
Chikkannagari Nagamani,†,¶ Huiying Liu,† and Jeffrey S. Moore*,†,¶
¶
†Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at
Urbana−Champaign, Urbana, Illinois 61801, United States
S
* Supporting Information
Scheme 1. Oxime Sulfonate Mechanophore Generates a
Trifluoromethyl Ketone Moiety and an Acid Constituent,
Presumed To Be an Aryl Sulfonic Acid, upon Ultrasound-
Induced Mechanical Scission
ABSTRACT: The generation of acid under mechanical
force is potentially useful for initiating proton-catalyzed
changes in polymeric materials. Here we demonstrate that
oxime sulfonatesknown photoacid generatorsare also
acid generators when activated mechanically. NMR
analysis of products suggests that the ultrasound-induced
mechanochemical scission of the oxime sulfonate
mechanophore also generates a ketone functional moiety,
in addition to acid. Both acid and ketone moieties are
useful for developing stress-responsive polymeric materials
for autonomous self-healing applications.
cid-catalyzed reactions such as functional group depro-
A
tection, ester hydrolysis, and ring-opening polymerization
are ubiquitous.1 Coupling mechanical stress to acid catalysis
holds great promise for modifying macroscopic polymer
properties and may lead to unprecedented mechanoresponsive
materials and self-healing applications.2 Designing mechanoa-
cids, i.e., mechanophores that undergo force-promoted reaction
to produce acid from an initially nonacidic group, is the first
step toward realizing such mechanoresponsive materials.
Mechanophore design has progressed considerably during
the past decade.3 Mechanophores that elicit a wide array of
productive chemistry, such as mechanochromism,4 mechano-
luminescence,5 activation of latent metal catalysts,6 and
generation of reactive species (radicals, alkenes, ketenes,
etc.),3b,7 are well developed. Mechanoacids, however, are still
relatively scarce. There is only one known example of a
mechanoacid.6a The known mechanoacid, based on a gem-
dichlorocyclopropanated indene, is thermally unstable (de-
grades above 40 °C), which limits its practical utility for
materials applications due to poor processability.
•
(RSO2 ) radicals similar to photolytic activation (Scheme
2).8e These radicals in the presence of water lead to acid
Scheme 2. Proposed Pathways of Sulfonic Acid Generation
via Photolysis of Oxime Sulfonates
generation, presumably via an aryl sulfonic acid. For ease of
synthesis, the mechanophore (5, Chart 1) design included two
symmetrical oxime sulfonate groups, but since the mode of
proton formation involves a chain scission event, only one of
the groups is expected to respond to force activation. Given
that there is only a single mechanophore per polymer chain and
it requires analytical characterization of the products’ end-
While mechanoacids are scarce, photoacids8 are well known.
Several classes of photoacids exist,8b,c and a majority of them
contain weak bonds such as C−S, S−O,C−S, S−O, and C−I
that may be mechanically activated. In addition, many
11
groups using H and 13C NMR or other techniques, −CF3
was chosen as a substituent to take advantage of the highly
sensitive fluorine NMR for end-group analysis. CoGEF
(Constrained Geometries simulate External Force) calcula-
tions,12 which model the effect of external force on a
mechanophore placed near the center of a polymer chain,
predict preferential activation of the “S−O” bond in the oxime
sulfonate mechanophore under mechanical stress (Figure S1),
thus supporting our hypothesis.
1
photoacids display good thermal stability (100 °C < Tdec
<
225 °C).9 Activating photoacids under mechanical stress opens
up numerous possibilities to design thermally stable mecha-
noacids. Herein, we demonstrate, using an oxime sulfonate
photoacid as a mechanophore, that certain photoacids are
activatable by mechanical means to generate acid (Scheme 1).
We hypothesized that the “S−O” (ca. 63 kcal/mol)10 or “N−
O” (ca. 48 kcal/mol)10 bonds in oxime sulfonates, which are
weaker than the C−C (ca. 83 kcal/mol)10 or C−O (ca. 85
kcal/mol)10 bonds, are mechanically sensitive toward bond
Received: January 5, 2016
•
homolysis, generating sulfonyloxyl (RSO3 ) or sulfonyl
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b00097
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Journal of the American Chemical Society
Communication
Chart 1. Structures of Polymers and Bifunctional Initiator
Figure 1. Rate of polymer chain scission under pulsed ultrasound. A 1
mg/mL polymer solution was sonicated with pulsed ultrasound (8.7 W
cm−2) under argon atmosphere at 6−9 °C. Measurements were
performed in triplicate, and the error bars represent standard deviation
at each data point.
10−5 min−1 kDa−1), which provides substantial evidence for
preference to cleavage at the mechanophore versus random
cleavage along the polymer backbone.
Mechanochemical activation of the oxime sulfonate
mechanophore in polymer 1 was further confirmed by using
19F NMR spectroscopy. Upon ultrasonication, polymer 1 shows
two new peaks at −66.60 and −71.53 ppm in the 19F NMR
spectrum (Figure 2). The intensity of these two peaks increases
To experimentally verify mechanical activation of the oxime
sulfonate mechanophore, we synthesized a series of polymers
shown in Chart 1 using Cu (0) mediated single-electron-
transfer living-radical polymerization (SET-LRP) (see the
Supporting Information for synthetic details). The oxime may
exist as (E) and (Z) isomers. To exclude effects of the
stereochemical configuration on mechanochemical scission
rates,10 the oxime sulfonate mechanophore 5 was obtained
exclusively as (E,E)-isomer starting from the corresponding
(E)-oxime. We chose to synthesize the (E,E)-isomer of the
mechanophore since the (E)- and (Z)-isomers of the precursor
oxime were obtained in 9:1 ratio, respectively. Stereochemistry
of the oxime and the oxime sulfonate mechanophore is
confirmed by X-ray crystal structure, and 1H and 19F NMR data
(see the Supporting Information for details). Polymer 1 exhibits
excellent thermal stability in both solution and solid-state
(Figure S2).
Figure 2. 19F NMR spectra of polymers. CFCl3 (δ = 0 ppm) was used
as an internal standard.
Pulsed ultrasound was used to probe mechanochemical
activation of the polymers. Aliquots of sonicated polymer
solution removed at regular intervals were analyzed by gel
permeation chromatography (GPC). Polymer 1 with “chain-
centered” mechanophore showed an expected decrease in
molecular weight (MW) with sonication time, and the MW
reached to approximately one-half (Mn = 103.2 kDa) the
original polymer MW (Mn = 189.4 kDa) in about 30 min
(Figure S3). In contrast, the low MW polymer 1low MW (Mn =
38.2 kDa; PDI = 1.13) for which chain scission is less
susceptible to mechanochemical activation showed negligible
change in MW (Mn = 36.7 kDa) upon ultrasonication for 60
min, which confirms that the activation is indeed of mechanical
origin. Furthermore, the rate of chain scission in polymer 1 is
significantly faster compared to that of the PMA control (Mn =
188.7 kDa; PDI = 1.07) of comparable MW but without a
mechanophore (Figure 1). Under identical sonication con-
ditions, the rate constant for cleavage of polymer 1 (k′ = 13.6 ×
10−5 min−1 kDa−1), determined from the slope of the curve, is
nearly twice as larger as that for the PMA control (k′ = 7.1 ×
steadily with increasing sonication time, with a concomitant
decrease in the intensity of the peak at −66.53 ppm
corresponding to the uncleaved polymer 1 (Figure S4).
These results clearly confirm that the scission occurred with
high mechanochemical selectivity at or in close proximity to the
mechanophore since fluorine signal in 1 comes from only the
oxime sulfonate mechanophore and not the PMA polymer
backbone. Furthermore, no new peaks were observed in 19F
NMR spectra upon ultrasonication of four different controls,
which include (i) polymer 1low MW; (ii) polymer 2 with “chain-
end” mechanophore; (iii) small molecule mechanophore 5; and
(iv) a physical mixture of 5 and PMA polymer (Figure S5).
To test acid generation upon ultrasonication of polymer 1,
pH measurements were carried out in CH3CN/H2O (99:1 v/v)
mixture using a pH meter for organic solvents equipped with a
H+ ion sensitive glass electrode. Solutions of polymer 1 showed
a steady decrease in pH with increasing sonication time and the
pH changed from 6.0 to 3.6 in 60 min (Figure 3). In sharp
contrast, PMA control showed a minimal change in pH (6.0 to
5.7) upon ultrasonication for 60 min. These results provide our
B
DOI: 10.1021/jacs.6b00097
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 4. 15N NMR spectra of polymers 1* (before and after
sonication) and 4*. The chemical shifts are reported relative to
CH3NO2 standard.
Figure 3. Change in pH with sonication time. The ultrasonicated
polymer solution (1 mg/mL in CH3CN) was concentrated, dried
under vacuum, and CH3CN/H2O (99:1 v/v) was added to obtain a 3
mg/mL polymer solution for pH measurements. Measurements were
performed in triplicate, and the error bars represent standard deviation
at each data point.
(ca. −360 ppm) in the 15N NMR spectrum of the
ultrasonicated polymer 1* disfavors the “N−O” bond scission
path A (Scheme 2), which is consistent with the prediction of
CoGEF simulations. Low concentration of byproducts, coupled
with low sensitivity of 15N NMR, also limit the detection ability.
We are currently investigating alternate detection methods to
identify other possible byproducts.
best evidence for the generation of acid upon mechanical
activation of the oxime sulfonate mechanophore.
In conclusion, we have demonstrated that an oxime sulfonate
photoacid is also mechanically activatable to generate acid. This
finding suggests that a thorough exploration of the photoacids
is a reasonable approach to discover new mechanoacids that are
thermally stable. Given the alignment of our experimental
results with CoGEF calculations, a reasonable first step in this
pursuit is to computationally survey the known photoacids for
their tendency to undergo mechanochemically induced bond
homolysis.
We now turn to analyze the products formed from
mechanochemical scission of the oxime sulfonate mechano-
phore in polymer 1. A detailed study of the scission products is
underway. Our preliminary findings are presented here. 19F and
15N NMR techniques are valuable in identifying some of the
scission products. Mechanochemical scission of the oxime
sulfonate mechanophore in polymer 1 exhibits two new peaks
at −66.60 and −71.53 ppm in the 19F NMR spectrum (Figure
2). The peak at −71.53 ppm matches well with that of
authentically prepared trifluoromethyl ketone-terminated poly-
mer 3 (Chart 1), thus confirming the formation of a ketone
moiety. Ketones are also the most commonly observed
byproducts of the photolysis of oxime sulfonates.8 The peak
at −66.60 ppm is tentatively assigned to the sulfonic acid-
terminated PMA polymer. Since the chemical shifts of the
authentically prepared sulfonate ester-terminated polymer 2
(−66.62) and the authentically prepared (E)-oxime-terminated
polymer 4 (−66.58) are very close, it was difficult to
unequivocally assign the peak at −66.60 ppm to either a
sulfonic acid- or an (E)-oxime-terminated polymer based on
the 19F NMR data. Hence, 15N NMR was employed.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b00097.
■
S
Experimental details, synthetic procedures, and character-
ization data (1H, 13C, and 19F NMR, and MS) (PDF)
X-ray crystallographic data for compounds 9 (CCDC
1443858), 15 (CCDC 1443857), and 16 (CCDC
1443856) (CIF)
AUTHOR INFORMATION
Corresponding Author
*jsmoore@uiuc.edu
■
Following similar synthetic procedures, analogous 15N-
labeled polymers 1* (Mn = 162.6 kDa; PDI = 1.11) and 4*
(Mn = 68.2 kDa; PDI = 1.15) were synthesized using 15N-
labeled hydroxylamine (see the Supporting Information for
details). Due to the low sensitivity of 15N NMR as well as the
low concentration of 15N atoms per polymer chain, gel-phase
NMR was used to record 15N NMR.13 The authentically
prepared (E)-oxime-terminated polymer 4* shows a peak at
−2.68 ppm in the 15N NMR spectrum (Figure 4). However,
upon ultrasonication, polymer 1* exhibits no peak at −2.68
ppm, confirming that the oxime-terminated moiety is not one
of the products that we isolated.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(NSF CHE 13-00313). The authors gratefully acknowledge
Lingyang Zhu for assistance with 15N NMR and Jeffery Alan
Bertke for assistance with X-ray crystallographic data collection.
REFERENCES
■
Based on the proposed path B in Scheme 2, NO is another
possible byproduct. A peak corresponding to NO (ca. 400 to
600 ppm) was not observed in the 15N NMR spectrum of the
ultrasonicated polymer 1*. NO is a gaseous, short-lived, open
shell structure known to convert into other nitrogenous
compounds upon reaction with oxygen or water, which make
its detection difficult. Failure to observe peaks corresponding to
(1) (a) Delcroix, D.; Martin-Vaca, B.; Bourissou, D.; Navarro, C.
Macromolecules 2010, 43, 8828−8835. (b) Teng, C. J.; Weber, W. P.;
Cai, G. P. Polymer 2003, 44, 4149−4155. (c) Bouzide, A.; Sauve, G.
Synlett 1997, 1997, 1153−1154. (d) Snider, B. B. Acc. Chem. Res. 1980,
13, 426−432.
(2) (a) Dearborn, E. C.; Fuoss, R. M.; White, A. F. J. Polym. Sci. 1955,
16, 201−208. (b) Liu, H. H.; Chen, W. T.; Wu, F. T. J. Polym. Res.
2002, 9, 251−256. (c) Kean, Z. S.; Craig, S. L. Polymer 2012, 53,
+
imine (ca. −60 to −80 ppm), NH3 (ca. −380 ppm) or NH4
C
DOI: 10.1021/jacs.6b00097
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
1035−1048. (d) Black, A. L.; Orlicki, J. A.; Craig, S. L. J. Mater. Chem.
2011, 21, 8460−8465. (e) Diesendruck, C. E.; Moore, J. S. Self-Healing
Polymers: From Principles to Applications 2013, 193−214.
(3) (a) Li, J.; Nagamani, C.; Moore, J. S. Acc. Chem. Res. 2015, 48,
2181−2190. (b) Berkowski, K. L.; Potisek, S. L.; Hickenboth, C. R.;
Moore, J. S. Macromolecules 2005, 38, 8975−8978. (c) Larsen, M. B.;
Boydston, A. J. J. Am. Chem. Soc. 2013, 135, 8189−8192. (d) Peterson,
G. I.; Larsen, M. B.; Boydston, A. J. Macromolecules 2012, 45, 7317−
7328.
(4) (a) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.; Nishihara, M.;
Takahara, A.; Otsuka, H. Angew. Chem., Int. Ed. 2015, 54, 6168−6172.
(b) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough,
D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martinez, T. J.; White, S.
R.; Moore, J. S.; Sottos, N. R. Nature 2009, 459, 68−72. (c) Wang, Z.
J.; Ma, Z. Y.; Wang, Y.; Xu, Z. J.; Luo, Y. Y.; Wei, Y.; Jia, X. R. Adv.
Mater. 2015, 27, 6469−6674.
(5) (a) Chen, Y.; Spiering, A. J. H.; Karthikeyan, S.; Peters, G. W. M.;
Meijer, E. W.; Sijbesma, R. P. Nat. Chem. 2012, 4, 559−562. (b) Ma,
Z. Y.; Wang, Z. J.; Teng, M. J.; Xu, Z. J.; Jia, X. R. ChemPhysChem
2015, 16, 1811−1828.
(6) (a) Diesendruck, C. E.; Steinberg, B. D.; Sugai, N.; Silberstein, M.
N.; Sottos, N. R.; White, S. R.; Braun, P. V.; Moore, J. S. J. Am. Chem.
Soc. 2012, 134, 12446−12449. (b) Piermattei, A.; Karthikeyan, S.;
Sijbesma, R. P. Nat. Chem. 2009, 1, 133−137.
(7) (a) Robb, M. J.; Moore, J. S. J. Am. Chem. Soc. 2015, 137, 10946−
10949. (b) Kryger, M. J.; Ong, M. T.; Odom, S. A.; Sottos, N. R.;
White, S. R.; Martinez, T. J.; Moore, J. S. J. Am. Chem. Soc. 2010, 132,
4558−4559. (c) Shiraki, T.; Diesendruck, C. E.; Moore, J. S. Faraday
Discuss. 2014, 170, 385−394.
(8) (a) Lawson, R. A.; Noga, D. E.; Tolbert, L. M.; Henderson, C. L.
J. Micro/Nanolithogr., MEMS, MOEMS 2009, 8, 043010. (b) Shirai,
M.; Tsunooka, M. Prog. Polym. Sci. 1996, 21, 1−45. (c) Shirai, M.;
Tsunooka, M. Bull. Chem. Soc. Jpn. 1998, 71, 2483−2507.
(d) Higashihara, T.; Saito, Y.; Mizoguchi, K.; Ueda, M. React. Funct.
Polym. 2013, 73, 303−315. (e) Aoai, T.; Kodama, K.; Yamanaka, T.;
Yagihara, M. J. Photopolym. Sci. Technol. 1998, 11, 409−418.
(9) Barclay, G. G.; Medeiros, D. R.; Sinta, R. F. Chem. Mater. 1995, 7,
1315−1324.
(10) Lide, D. R.; CRC Handbook of Chemistry and Physics, 79th ed.;
CRC Press: Boca Raton, FL, 1998/1999.
(11) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.;
Baudry, J.; Wilson, S. R. Nature 2007, 446, 423−427.
(12) Beyer, M. K. J. Chem. Phys. 2000, 112, 7307−7312.
(13) Diesendruck, C. E.; Zhu, L.; Moore, J. S. Chem. Commun. 2014,
50, 13235−13238.
D
DOI: 10.1021/jacs.6b00097
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