SuFEx-based synthesis of polysulfates

Article abstract of DOI:10.1002/anie.201403758

High-molecular-weight polysulfates are readily formed from aromatic bis(silyl ethers) and bis(fluorosulfates) in the presence of a base catalyst. The reaction is fast and proceeds well under neat conditions or in solvents, such as dimethyl formamide or N-methylpyrrolidone, to provide the desired polymers in nearly quantitative yield. These polymers are more resistant to chemical degradation than their polycarbonate analogues and exhibit excellent mechanical, optical, and oxygen-barrier properties. High-molecular-weight polysulfates are readily formed from aromatic bis(silyl ethers) and bis(fluorosulfates) in the presence of a base catalyst. The polymers were obtained in nearly quantitative yield under neat conditions, they are more resistant to chemical degradation than their polycarbonate analogues and exhibit excellent mechanical, optical, and oxygen-barrier properties. BPA=bisphenol A.

Full text of DOI:10.1002/anie.201403758

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DOI: 10.1002/anie.201403758
Sulfuryl-Based Click Chemistry
Hot Paper
SuFEx-Based Synthesis of Polysulfates**
Jiajia Dong, K. Barry Sharpless,* Luke Kwisnek, James S. Oakdale, and Valery V. Fokin*
Abstract: High-molecular-weight polysulfates are readily
formed from aromatic bis(silyl ethers) and bis(fluorosulfates)
in the presence of a base catalyst. The reaction is fast and
proceeds well under neat conditions or in solvents, such as
dimethyl formamide or N-methylpyrrolidone, to provide the
desired polymers in nearly quantitative yield. These polymers
are more resistant to chemical degradation than their poly-
carbonate analogues and exhibit excellent mechanical, optical,
and oxygen-barrier properties.
Sulfur(VI) fluorides, in particular sulfuryl fluoride
(SO2F2) and its monofluorinated derivatives, sulfonyl fluo-
ꢀ
ꢀ
rides (RSO2 F), sulfamoyl fluorides (R2NSO2 F), and fluo-
ꢀ
rosulfates (ROSO2 F), stand in stark contrast to other
sulfur(VI) halides. These sulfur oxofluorides are much more
hydrolytically stable, redox-silent, and do not act as halogen-
ating agents. Nevertheless, their selective reactivity can be
revealed when an appropriate nucleophile is present under
conditions in which fluoride ejection is assisted by the solvent,
the pH value, or an additive. These parameters define
conditions under which sulfur(VI) fluoride exchange
(SuFEx) reactivity achieves click chemistry status.[4] The
formation of sulfur(VI)–heteroatom bonds is described in
detail in an accompanying article in this issue.[5]
A
handful of high-fidelity reactions are at the core of
industrial processes producing polymers in multimillion ton
quantities. Most commodity polymers are synthesized from
olefins by forming carbon–carbon backbones, whereas engi-
neering polymers are commonly prepared through condensa-
tion reactions of monomers that often contain an activated
carbonyl group or its equivalent and a suitable nucleophile,
thus forming carbon–heteroatom linkages. Polyesters, poly-
amides, polyurethanes, and polyimides are produced in this
manner. Despite the variety of backbone structures, polymers
containing sulfur(VI) (“-SO2-”) connectors are virtually
absent from the literature and are barely used in industrial
applications (with the exception of polysulfones, in which the
sulfone group is already present in the monomers).[1]
In the early 1970s, Firth pioneered the synthesis of
poly(aryl sulfate) polymers from fluorosulfates of bisphe-
nol A (BPA), which he obtained from SO2F2 and disodium
salts of the bisphenol.[6] Preparation of these monomers
required prolonged heating, and the high polymer was
obtained in its pure form only after repeated precipitation.
Herein, we report a simple and straightforward SuFEx-based
method for the synthesis of high-molecular-weight polysulfate
polymers from aryl fluorosulfates and aryl silyl ethers under
simple and mild reaction conditions.
Unsurprisingly, most reported attempts to synthesize
sulfur(VI)-containing polymers relied on reactions mimicking
carbonyl-group-based condensations, that is, reactions of
sulfonyl chlorides with nucleophiles,[2] and, to a much lesser
extent, Friedel–Crafts sulfonylations.[3] Although polymers
obtained by those methods can have attractive properties,
such as good thermal and hydrolytic stability and mechanical
resilience,[2c–e] the unselective reactivity of sulfur(VI) chlo-
rides, which are susceptible to hydrolysis and participate in
redox transformations and radical chlorinations, significantly
limits the utility of these methods and materials.
Reactions of silylated and fluorinated compounds are, of
course, well known in organic synthesis[7] and in polymer
chemistry.[8] For example, in 1983, Kricheldorf et al. intro-
duced the “silyl method” for the synthesis of poly(aryl ethers)
ꢀ
taking advantage of the strength of the Si F bond and the
innocuous nature of the silyl fluoride byproducts.[9] In 2008,
Gembus and co-workers demonstrated that sulfonyl fluorides
react with silyl ethers in the presence of a catalytic amount of
1,8-diazabicyclo[5.4.0]undec-7-ene [DBU; Eq. (1)], produc-
ing aryl sulfonates.[10] We in turn found that fluorosulfates
react with aryl silyl ethers under similar conditions to form
diorganosulfates [Eq. (2)].
[*] J. Dong,[+] K. B. Sharpless
necessarily represent the official views of the National Institutes of
Health), and the National Science Foundation (CHE-0848982 and
CHE-1302043 to V.V.F.). J.S.O. acknowledges an NSF graduate
fellowship. We thank Prof. S. Nazarenko, Prof. J. S. Wiggins, J. Tu, J.
Goetz, and K. Meyers (University of Southern Mississippi) for help
with the processing and measuring of mechanical and transport
properties, Prof. P. Iovine (University of San Diego) for help with
DSC and TGA measurements, Prof. M. G. Finn and Dr. L. Krasnova
(TSRI) for helpful discussions, C. Higginson and Dr. K. Breitenkamp
(TSRI) for help with the GPC analyses, Wyatt Technology for
providing access to a demo MALS instrument, Dr. A. O. Meyer for
guidance with the analysis, and Dow Agro for the generous gift of
sulfuryl fluoride. SuFEx=sulfur(VI) fluoride exchange.
Department of Chemistry and The Skaggs Institute for Chemical
Biology, The Scripps Research Institute
La Jolla, CA 92037 (USA)
E-mail: sharples@scripps.edu
Homepage: http://www.scripps.edu/sharpless
L. Kwisnek,[+] J. S. Oakdale,[+] V. V. Fokin
Department of Chemistry, The Scripps Research Institute
La Jolla, CA 92037 (USA)
E-mail: fokin@scripps.edu
Homepage: http://www.scripps.edu/fokin
[+] These authors contributed equally to this work.
[**] This work was supported by the Skaggs Institute for Chemical
Biology (K.B.S.), the National Institute of General Medical Sciences,
the National Institutes of Health (R01GM087620 to V.V.F.; the
content is solely the responsibility of the authors and does not
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403758.
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(NMP) and dimethylformamide (DMF) as the optimal
solvents for the preparation of polysulfates (Scheme 1b).
Following precipitation with methanol, BPA-polysulfate
(BPA-PS) was recovered as white powder in 95% yield
(Mn = 30900 Da, referenced to polystyrene standards; see the
Supporting Information for details). The results were similar
when the TBS monomer 2c was used (Mn = 24600 Da); in the
latter case, liquid tert-butylfluorodimethylsilane (3c, TBSF)
was generated as a byproduct, which was removed by
distillation.
An investigation of different catalysts[12] revealed that
compounds containing amidine, guanidine, or phosphazene
moieties were active, along with fluoride [introduced through
the organic fluoride source tris(dimethylamino)sulfonium
difluorotrimethylsilicate (TASF) or the inorganic fluoride
source cesium fluoride] and potassium tert-butoxide
(tBuOK). Preliminary investigations point at the involvement
of hypervalent silicon intermediates, which can be formed by
activation of the silyl ethers, either directly with a basic
catalyst or with bifluoride [HF2]ꢀ released from the fluoro-
sulfate group and adventitious water. Activation of fluoro-
sulfates with DBU, in a similar fashion to the reported
interactions of sulfonyl fluorides with DBU,[13] may also
provide [HF2]ꢀ. Catalytic amounts of tBuOK and fluoride
salts gave polysulfates only with TBS monomer 2c, suggesting
that the greater stability of the TBS ethers under basic
conditions[14] was critical for efficient polymerization.
This “sulfate click reaction” is extremely efficient, and
when bis(aryl fluorosulfates) 2a and bis(aryl silyl ethers) 2b–
e are used, high-molecular-weight polymers are produced in
nearly quantitative yield (Scheme 1). The reaction is cata-
lyzed by organic bases, such as DBU or 2-tert-butylimino-2-
diethylamino-1,3-dimethylperhydro-1,3,2-diaza-phosphorine
(BEMP),[11] or fluoride salts, such as cesium fluoride. This
transformation provides the desired products in essentially
quantitative yields, it is compatible with many functional
groups and does not require special equipment or precau-
tions. The low exothermicity of this reaction facilitates scaling
up, as described below.
The molecular weight of the polymers was proportional to
the monomer concentration. Thus, polymerization of 2a and
2b under solvent-free conditions at 1508C produced the
polysulfates of the highest molecular weight, with decreasing
Mn values as monomer concentrations were lowered.[15] At
the same time, larger amounts of cyclic side products were
observed under more dilute conditions. These trends mirror
earlier findings for the synthesis of BPA-polycarbonate (BPA-
PC).[16] Other catalysts examined under the solvent-free
(“bulk”) conditions included BEMP, CsF, and tBuOK.
Similarly to the results described above, both BEMP and
CsF provided polysulfates with greater Mn values than DBU.
However, tBuOK was ineffective under the bulk conditions.
Further investigations showed that the molecular weight
of the polymers depended on the nature of the catalyst, its
loading, and the nature of the silyl group (Table 1 and
Figure 1). The TBS monomer 2c consistently produced the
largest polymers, with Mn values surpassing 100000 Da when
BEMP was used as the catalyst (Table 1, entries 4–6). DBU
generally resulted in polymers of less than 70000 Da and was
ineffective at low loadings (see entries 4 and 7). The
Mn values of polymers obtained from the TMS monomer
2b, in contrast to the TBS analogue, did not exceed 40000 Da
regardless of the polymerization conditions. TBDPS (2d) and
TIPS (2e) BPA ethers also successfully polymerized in the
bulk and produced polysulfates of variable Mn values
(entries 11 and 13), although higher loadings of the BEMP
catalyst were required (entry 10 vs. 11). Thus, bis-TBS ether
2c has emerged as the “Goldilocks” monomer, yielding large
polymers using low catalyst loadings. Finally, several samples
were subjected to multi-angle light scattering (MALS)
analysis for absolute-molecular-weight determination. As
Scheme 1. a) Preparation of fluorosulfate and silyl ether monomers
from bisphenol A. b) Synthesis of polysulfates. Mn values determined
in reference to polystyrene standards (by GPC). TBDPS=tert-butyldi-
phenylsilyl, TBS=tert-butyldimethylsilyl, TIPS=triisopropylsilyl,
TMS=trimethylsilyl.
Both the fluorosulfate and the silyl ether monomers were
readily obtained from BPA (Scheme 1a). Its treatment with
SO2F2 gas in the presence of triethylamine generated
bis(fluorosulfate) 2a, which was isolated as a shelf-stable
white crystalline solid in high yield on mole scale after
a simple work-up that did not require chromatographic
purification. The bis(silyl ether) monomers 2b–e are either
commercially available (2b) or were easily prepared on large
scale following standard procedures (2c–e).
The initial examination of the reaction between mono-
mers 2a and 2b in different solvents (1m in monomers) in the
presence of DBU (20 mol%) identified N-methylpyrrolidone
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Table 1: Comparison of solvent-free polymerization conditions.[a]
degradation than its polycarbonate analogue: Treating poly-
sulfate with sodium hydroxide (1.3m, 1:2 EtOH/H2O) at
ambient temperature or with a saturated solution of sodium
carbonate at 808C for 16 hours caused no observable change
in Mn, whereas the analogous polycarbonate was completely
hydrolyzed to low-molecular-weight materials, as indicated by
gel-permeation chromatography (GPC; Figure 2).
Differential scanning calorimetry (DSC) analysis demon-
strated that Tg,1, the temperature at which the glass transition
is unaffected by an increase in molecular weight, was reached
when the Mn value approached approximately 20000 Da
(Figure 3).
MALS
PS
Entry
Monomers
Cat. (mol%)
Mn
Mn
Dispersity
1 (A)
2
3 (B)
2a and 2b
BEMP (1)
BEMP (10)
DBU (20)
2500
8800
10600
17000
34000
38000
1.2
1.4
1.5
4 (D)
5
6
7
2a and 2c
BEMP (1)
BEMP (10)
BEMP (20
DBU (1)
58000
n.d.
n.d.
120000
128000
143000
1.8
1.6
1.5
no polymer formation
8 (C)
9
DBU (20)
CsF (20)
19600
38300
66000
93000
1.5
1.7
Tg was in the range of 95 to 1008C for high-molecular-
weight polymers. No crystalline melting or crystallization
peaks were identified, indicating that synthesized BPA-PS
was amorphous. TGA analysis showed the excellent thermal
stability of the BPA-polysulfates.[17] The thermal decomposi-
tion temperature increased only slightly with Mn, but in all
cases, a weight loss of 5% occurred at approximately 3508C
or higher. These results are in good agreement with the data
obtained by Firth.[6b]
10
11
2a and 2d
2a and 2e
BEMP (1)
BEMP (10)
no polymer formation
n.d.
118000
1.5
12
13
BEMP (1)
BEMP (10)
no polymer formation
n.d.
51400
2.0
[a] Polymerization conditions: no solvent, 1508C, 2 h. Workup: dissolu-
tion in DMF followed by subsequent precipitation from methanol. GPC
traces of selected polymers (A–D) are shown in Figure 1. MnMALS values
determined by multi-angle light scattering. MnPS values in reference to
polystyrene standards. n.d.=not determined.
Samples for physical and mechanical analyses were
produced by pelletization and compression molding of the
polymer synthesized using bulk polymerization conditions.
When pressed thin, transparent and flexible, yet stiff films
were obtained. Pristine thin films were used for gas perme-
ability measurements (Figure 4). Thicker samples, like those
used for tensile measurements, had an opaque tan color.
Density, tensile properties, and oxygen permeability were
measured and compared to those of polycarbonate (commer-
cially available as Lexan). As summarized in Table 2, BPA-PS
was slightly more dense, had a higher tensile modulus, and
a similar yield stress when compared with BPA-PC. These
tensile properties, although not optimal because of compres-
Figure 1. GPC (UV detector) traces to accompany Table 1.
was also the case for BPA-polycar-
bonates,[16b] polystyrene standards
significantly overestimated the
molecular weights of the BPA-
polysulfates. This was especially
noticeable for lower polymers
(entries 1–3), with the error being
reduced to approximately twofold
for high polymers (cf. entries 4 and
9).
The bulk polymerization of 2a
and 2c was scaled up to 0.5 mol.
The reaction was performed at
1208C (internal
Figure 2. Hydrolytic stability of polysulfate compared to that of polycarbonate (as judged by GPC
temperature, analysis).
1358C oil bath temperature) for
two hours using BEMP (1 mol%)
as the catalyst. No significant elevation of the internal
temperature was observed during the course of the reaction.
BPA-PS with Mn = 58000 Da (MALS) was obtained in
quantitative yield (145 g). The polymer was mildly soluble
in a wide range of organic solvents, including chloroform,
dichloromethane, and acetone, whereas the best solubility
was observed in DMSO and DMF (ca. 1 g per 2 mL of DMF,
with heating). BPA-PS was much more resistant to hydrolytic
Table 2: Average measured properties of BPA-polysulfate and BPA-
polycarbonate (Lexan).
Polymer
d
Oxygen
permeability
(Barrer)
Tensile modulus
(GPa)
Yield stress
(MPa)
[gcmꢀ3
]
BPA-PS
BPA-PC 1.210 1.4
1.310 0.24
2.0
1.7
50
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Table 3: Preparation of polysulfate copoloymers.[a]
PS
Structure
Monomers
Mn
Dispersity
1
2
R=OSO2F
(4a)
R=OTMS 4a+2b
(4b)
4a+4b
46100
36000
1.5
1.4
R=OSO2F
(5a)
R=OTMS
3
5a+5b
52000[b]
1.6
(5b)
R=OSO2F
6a+6c
(6a)
R=OTBS 6a+2c
(6c)
4
5
58700
67100
1.4
1.4
6
2a+6c
46600
1.4
R=OSO2F
7a+7c
(7a)
R=OTBS 7a+2c
(7c)
Figure 3. Relationship between Tg and Mn for representative BPA
polysulfates. The inset provides DSC second heating thermograms.
Line provided to guide the eye.
7
8
34700
37200
1.5
1.5
9
2a+7c
30600
1.5
R=OSO2F
8a+8c
(8a)
R=OTBS 8a+2c
(8c)
10
11
80100
52500
1.4
1.5
12
2a+8c
36900
1.4
R=OSO2F
9a+9c
(9a)
R=OTBS 9a+2c
(9c)
13
14
34100[c]
41100
1.6
1.6
Figure 4. Left: precipitated, unprocessed BPA-polysulfate. Right: com-
pression-molded BPA-polysulfate film.
15
2a+9c
21800[d]
1.4
R=OSO2F
(10a)
R=OTBS 10a+2c 48700
(10c)
16
17
10a+10c 81100
1.4
1.3
sion-molded samples, provide a preliminary, relative compar-
ison to the well-established structural analogue BPA-PC.
Importantly, polysulfate exhibited the uncommon ductile, yet
rigid mechanical behavior similar to polycarbonate. The
oxygen permeability of BPA-PS was significantly lower than
that of BPA-PC, suggesting that BPA-PS has a smaller free
volume at room temperature.
Finally, the compatibility of the polymerization reaction
with different functional groups was examined. Monomers 4–
13 were prepared as described above and included bisphen-
ol AF (4a/4b), naphthalene (5a/5b), ether (6a/6c), ester (9a/
9c and 12c), sulfide (8a/8c), ketone (9a/9c), amide (10a/10c
and 13c), and sulfone (11a/11c) derivatives. The selectivity of
the reaction was demonstrated by the successful formation of
co-polysulfates containing technologically useful blocks
found in other engineering polymers. The polymerizations
were conducted at room temperature in NMP (1m) with
20 mol% of DBU for 48 hours. As Table 3 illustrates, a variety
of homopolymers and BPA copolymers were obtained,
demonstrating the compatibility of the SuFEx reaction with
different functional groups.
18
2a+10c 75400
1.4
R=OSO2F
(11a)
19
20
11a+11c 22100[d]
1.3
1.3
R=OTBS 11a+2c 24800
(11c)
R=OTBS
21
2a+12c 53000
(12c)
1.5
R=OTBS
22
23
2a+13c 43900
(13c)
1.4
1.3
R=SO2F
(14)
14+2c
46500
[a] Polymerization conditions: DBU (20 mol%), NMP (1m), RT, 24 h.
Material isolated by precipitation from methanol and analyzed by GPC.
MnPS values in reference to polystyrene standards. [b] Polymerization per-
formed at 808C. [c] Increasing the temperature to 1008C gave an Mn value of
46100 Da, 1.5 PDI. [d] Oligomeric product. [e] Increasing the temperature to
1008C gave a polymer with Mn =43200 Da, dispersity 1.4.
Among BPA-copolymers of similar structure, the polymer
molecular weight was sensitive to the electronic properties of
the silyl monomer: Lower polymers were obtained when
either electron-donating (entry 5 vs. 6, entry 11 vs. 12) or
-withdrawing (entry 8 vs. 9, entry 14 vs. 15) groups were
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Aside from providing a practical route to polymers with
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sulfur(VI) oxofluorides. This new reaction should find
immediate applications across a variety of disciplines. Further
investigations of the mechanism of this process and its
applications are now underway and will be reported shortly.
Received: March 27, 2014
Revised: May 15, 2014
Published online: August 5, 2014
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Keywords: click chemistry · polymerization · SuFEx reaction
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