p-Phenylene sulfide oligomers and their properties. Ar-S couplings mediated by copper or by fluorine substitutions

Article abstract of DOI:10.1016/j.tetlet.2009.02.068

A series of monodisperse p-phenylene sulfide oligomers were efficiently synthesized by using a bidirectional growth. A strategy combining Cu-catalyzed Ar-S couplings for small oligomers and fluorine aromatic substitutions by aryl thiolates for longer ones was put forward. The latter method is superior to Cu-catalyzed reactions for longer oligomers. Fluorine chemistry brings some new advantages such as solubility and reactivity. Qualitative crystallinity studies were reported according to the oligomer size and the functional series, by using a microscope equipped with a heating stage and a camera.

Full text of DOI:10.1016/j.tetlet.2009.02.068

Tetrahedron Letters 50 (2009) 1977–1981
Contents lists available at ScienceDirect
Tetrahedron Letters
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p-Phenylene sulfide oligomers and their properties. Ar–S couplings mediated
by copper or by fluorine substitutions
Olivier Goyot b, Marc Gingras a,b,
*
a CNRS, Aix-Marseille University, UPR CNRS 3118 Interdisciplinary Center on Nanoscience of Marseille (CINaM), 163 Avenue de Luminy, Case 913, 13288 Marseille Cedex 9, France
b Former address: Faculté des Sciences, Université de Nice-Sophia Antipolis, 28 Ave. Valrose, 06108 Nice Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of monodisperse p-phenylene sulfide oligomers were efficiently synthesized by using a bidirec-
tional growth. A strategy combining Cu-catalyzed Ar–S couplings for small oligomers and fluorine aro-
matic substitutions by aryl thiolates for longer ones was put forward. The latter method is superior to
Cu-catalyzed reactions for longer oligomers. Fluorine chemistry brings some new advantages such as sol-
ubility and reactivity. Qualitative crystallinity studies were reported according to the oligomer size and
the functional series, by using a microscope equipped with a heating stage and a camera.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 31 December 2008
Revised 8 February 2009
Accepted 10 February 2009
Available online 14 February 2009
Keywords:
Poly(p-phenylene sulfide)
Fluorine
Ar–S coupling
Copper catalysis
Nucleophilic aromatic substitution
Poly(p-phenylene sulfide) polymers, which are commonly
called PPS (RytonTM), have a long history as commercial thermo-
plastics and as classic sulfur-based polymers. They are described
in many textbooks1 because of their useful properties (mechanical,
hydrophobicity, fire retardant, high refractive index, etc.). Addi-
tionally, they were the first conductive polymers (when doped),
not only incorporating carbon and hydrogen as elements.2 A patent
from Philipps Petroleum in 19673 and older syntheses4 promoted
their mass production and uses in several fields.
In spite of this industrial success, some systematic studies on
PPS oligomers with various chain lengths have been less com-
mon.5–7 In modern materials science, monodisperse oligomers
are used to correlate their properties as ‘‘perfect” macromolecules
to those of the parent polymers with higher polydispersities and
defects.8 It helps to extrapolate the limiting properties of a poly-
mer, such as its crystallinity and its molecular ordering.
The syntheses of PPS oligomers rely on a plethora of methods
for making Ar–S bonds. Common metal-catalyzed couplings in-
volve Cu, Ni, Pd, or Co catalysts with iodinated or brominated
aryl-type substrates and aromatic thiols, under basic conditions.9
Classic aromatic sulfurations with sulfur transfer agents are also
used.10 Another method is a nucleophilic aromatic substitution
by a thiolate anion on chlorinated or brominated substrates,11
but fluorinated compounds have been neglected in spite of their
higher reactivity and solubility.
The syntheses of higher PPS oligomers encountered many diffi-
culties coming from their solubility, their purification, and their
characterization. Strategies for making oligomers containing more
than three phenyl units have often been constrained to the uses of
metal-catalyzed reactions. Difficulties arising from such methods
are twofolds: (1) metal-mediated Ar–S couplings become ineffi-
cient as the oligomer size increases, even under oxygen-free condi-
tions (Scheme 1); (2) Ar–S couplings can be partially improved by
using iodinated substrates instead of brominated ones (Scheme 1).
In spite of this elegant thought, the inconvenience is often shifted
toward an extra step for converting an aromatic bromide into an
iodide, of lower solubility. If this reaction is incomplete at high
temperatures, the residual bromide becomes an impurity that
might be difficult to get rid of.
Here, we propose an original synthetic strategy for making
functionalized PPS oligomers with many interesting features: (a)
a double-growth process; (b) a transition metal-free nucleophilic
aromatic substitutions with fluorinated intermediates (Scheme 1,
Eq. 3); (c) the use of fluorine chemistry for making Ar–S bond in
the synthesis of higher PPS oligomers; (d) a direct bromination of
higher PPS oligomers; (e) a higher solubility of para-methylated
or para-fluorinated PPS oligomers and their easier characterization
(by 19F/13C NMR); (f) Using fluorine or methyl end groups could
minimize the disturbance of PPS molecular ordering instead of
using solubilizing side chains; and (g) the melting points and the
qualitative crystallinity of many oligomers were determined
according to the chain length, by using a microscope equipped
with a heating stage and a camera.
* Corresponding author. Tel.: +33 491829155; fax: +33 491829155.
E-mail address: marc.gingras@univmed.fr (M. Gingras).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.068

1978
O. Goyot, M. Gingras / Tetrahedron Letters 50 (2009) 1977–1981
S
Br
Pd
(a)
+
S
S
HS
or Cu
n
n
Pd or
Cu
Br
S
I
CuI
KI
S
S
S
n
n
n
HS
(b)
(c)
Base
F
S
+
HS
S
S
Transition
metal-free
n
n
Scheme 1. Common synthetic strategies for making monodisperse PPS oligomers: (a) Pd- or Cu-catalyzed couplings with PPS-Br; (b) Bromide-iodide activation of PPS prior to
Pd- or Cu-catalyzed couplings and (c) SNAr couplings by a fluorine strategy.
The bidirectional growth of PPS oligomers was previously inves-
tigated, but in a sporadic manner.5–7 There is a need to improve
such studies because longer chains are quickly built in small num-
ber of steps (Scheme 2). As shown in Table 1, we promoted this
strategy by using bifunctional substrates 5, 7, 9, 10, 12, 14, 15,
and 17 having one to four phenyl units.
As a complementary approach to fluorine chemistry, para-
substituted methyl end groups were also tested. Similarly, they
helped in the 1H/13C NMR characterization and offered a better sol-
ubility. Complex syntheses of PPS with solubilizing side chains are
avoided.
Finally, we report a double bromination of PPS chains from
three to four phenyl units in a regioselective manner in yields
ranging from 83% to 89%, respectively (Table 1, entries 3 and 7).
This route was efficient on a gram-scale and avoided costly bromi-
nated or sulfanylated substrates. Overall, a few series of oligomers
of various sizes were prepared with para-terminating end groups
such as Me, Br, F, or H. In the same series, the melting points in-
crease according to the number of phenyl units. The same trend
was found in both halogen series, but brominated oligomers usu-
ally have much higher melting points. It is interesting to point
out that fluorine- and methyl-substituted oligomers of the same
size have similar melting points. A correlation between the proper-
ties of monodisperse PPS oligomers and PPS polymers will be part
of future studies.
We qualitatively found that most oligomers up to four phenyl
units are crystalline solids under a microscope lens. However,
recrystallization becomes less easy, and some amorphous domains
are observed for oligomers with five or six phenyl units. It seems
that molecular ordering and the degree of crystallinity decrease
with the oligomer size. Such observations are in agreement to a
rare study with a single series of oligomers (Fig. 1).5a
In our case, the use of iodide or bromide substituents did not
significantly change the yields for Cu(I)-catalyzed Ar–S couplings
for short oligomers having 2–3 phenyl units (Table 1, entries 1–
5). Yields are generally good to excellent. The efficiency slightly
diminishes for oligomers with four phenyl units when using the
classic method with Cu2O.12 The use of expensive Pd catalysts is
unnecessary in most cases. A thorough freeze-thaw deoxygenation
was not needed and a several-gram scale synthesis can be
achieved. As for higher PPS oligomers with five or six phenyl units,
it was unambiguously established that a nucleophilic aromatic
fluorine substitution under transition metal-free conditions was a
better approach compared to the method with Cu2O.13 Entries 10
and 11 in Table 1 provided a clear-cut evidence because 18 could
not be produced in a significant amount with Cu2O (entry 11),
and this was accompanied by a lot of side-products. To the con-
trary, a 47% yield (unoptimized) was reported in entry 10. These
poor results with Cu2O are in line to previous ones for longer
chains (>4 phenyl units), where oxygen-free conditions are neces-
sary but not always sufficient for obtaining low to moderate
yields.6 The use of fluorine substitution is novel as a systematic
strategy for generating PPS oligomers. Fluorine chemistry allowed
easy preparations of the fluorinated PPS chains, and their charac-
terization was facilitated by sensitive 19F NMR. Furthermore, the
purifications and 1H/13C NMR are easier because of a greater solu-
bility in common solvents compared to brominated or iodinated
analogs.
In summary, we reported new studies on a series of symmetri-
cal PPS oligomers terminated by F, Br, and Me end groups. We put
forward a successful synthetic strategy for PPS oligomers based on
novel features, including some fluorinated intermediates for mak-
ing Ar–S bonds under transition metal-free conditions. It permitted
to obtain higher p-methylated oligomers with a greater solubility.
S
X
Z
+
S
X
SH
n
X = I, Br, F
Z = H, Me, F
n = 0,1,2
Z
S
S
S
S
Z
n
Scheme 2. Examples of bidirectional growth of PPS oligomers.

Table 1
Synthesis of PPS oligomers
Entry
1
Method (h,°C)a
Aromatic thiol or bromine
Aromatic halide
PPS oligomers
Mp (°C)
Yield (%)
63
1
2
3
S
SMe
SH
A (24, 170)
40.6
SMe
Br
6
7
4
5
Br
S
S
S
SH
2
3
B (70, 160)
C (45, 90)
81.3 Lit5a 82
86
83
Br
S
S
6
9
9
Br
Br2
154.7 Lit5a 154
S
Br
F
8
10
11
I
I
F
S
S
SH
4
5
6
D (24, 160)
E (25, 175)
F (24, 165)
97.9 Lit5a 93
65
84
99
I
S
S
F
1
SH
95.4
I
4
13
14
12
13
15
S
S
S
SH
102.9
Br
S
Br
S
S
S
S
S
S
7
8
G (42, refl)
F (22, 165)
Br2
175.3
131.4
89
68
Br
S
S
Br
S
16
1
S
SH
I
I
I
I
8
15
10
17
S
S
SH
S
9
F (23, 165)
H (46, 180)
136.7
71
F
S
F
F
18
1
S
S
F
S
SH
10
167.2
47
S
S
S
F
(continued on next page)

1980
O. Goyot, M. Gingras / Tetrahedron Letters 50 (2009) 1977–1981
Figure 1. Qualitative crystallinity of p-methylated oligomers under a microscope
lens equipped with a camera at 20 °C: (a) 11; (b) 16; (c) 18; (d) 20.
The limit to the efficiency of the classic Cu2O method was evalu-
ated. The qualitative degree of crystallinity was investigated for a
series of PPS oligomers.
Acknowledgments
We acknowledge Professor Nicolas Sbirrazzuoli for a few DSC
measurements, Jean-Denis Sylvain for microscopy studies, CNRS
(French National Center for Scientific Research), the University of
Nice-Sophia Antipolis, and Jean-Marie Guigonis for MS spectra.
References and notes
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E.; Baughman, R. H. J. Chem. Phys. 1981, 75, 1919–1927; (b)Handbook of Organic
Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons: New
York, 1997; Vols. 1–4, p 3334.
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Jr.; Hills, H. W., Jr. Chem. Abstr. 1968, 68, 13598e.
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Bull. Soc. Chim. Belg. 1989, 98, 635–641; (c) Genvresse, P. Bull. Soc. Chim. Fr.
1897, 17, 599–609; (d) Lenz, R. W.; Handlovis, C. E.; Smith, H. A. J. Polym. Sci.
1962, 58, 351–367; (e) MacCallum, A. D. J. Org. Chem. 1948, 13, 154–159.
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Risse, W.; Heitz, W. Makromol. Chem. Suppl. 1985, 12, 105–123; (c) Tsuchida, E.;
Yamamoto, K.; Oyaizu, K.; Suzuki, F.; Hay, A. S.; Wang, Z. Y. Macromolecules
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6. (a) Pinchart, A.; Dallaire, C.; Van Bierbeek, A.; Gingras, M. Tetrahedron Lett.
1999, 40, 5479–5482; (b) Pinchart, A.; Dallaire, C.; Gingras, M. Tetrahedron Lett.
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7. (a) Vicente, J.; Abad, J. A.; López-Nicolas, R. M. Tetrahedron 2008, 64, 6281–
6288; (b) Vicente, J.; Abad, J. A.; López-Nicolas, R. M. Tetrahedron Lett. 2005, 46,
5839–5840.
8. Electronic Materials: The Oligomer Approach; Müllen, K., Wegner, G., Eds.; John
Wiley, New & Sons: New York, 1998; p 628.
9. Among some reviews: (a) Barañano, D.; Mann, G.; Hartwig, J. F. Curr. Org.
Chem. 1997, 1, 287–305; (b) Kondo, T.; Mitsudo, T.-a. Chem. Rev. 2000, 100,
3205–3220. Original references: Cu-cat.; (c) Adam, R.; Reifschneider, W.;
Nair, D. Croatia Chim. Acta 1957, 29; (d) Adams, R.; Ferretti, A. J. Am. Chem.
Soc. 1959, 81, 4927–4931; (e) Lindley, J. Tetrahedron 1984, 9, 1433–1456. Pd
cat.; (f) Kosugi, M.; Shimizu, T.; Migita, T. Chem. Lett. 1978, 13–14; (g)
Migita, T.; Shimizu, T.; Asami, Y.; Shiobara, J.-I.; Kato, Y.; Kosugi, M. Bull.
Chem. Soc. Jpn. 1980, 53, 1385–1389; (h) Fernandez-Rodriguez, M. A.; Shen,
Q.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 2180–2181. Ni-cat.; (i) Cristau,
H. J.; Chabaud, B.; Christol, H. Synthesis 1981, 11, 892–894; (j) Takagi, K.
Chem. Lett. 1987, 2221–2224. Co-cat.; (k) Wong, Y.-C.; Jayanth, T. T.; Cheng,
C.-H. Org. Lett. 2006, 8, 5613–5616.
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1645–1648; (b) Chahma, M.; Hicks, R. G.; Myles, D. J. T. Macromolecules 2004,
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Trav. Chim. Pays-Bas. 1961, 80, 1057–1065.

O. Goyot, M. Gingras / Tetrahedron Letters 50 (2009) 1977–1981
1981
12. Typical procedure for 17: Into
iodophenyl)sulfide (600.0 mg; 1.37 mmol) and Cu2O (206.0 mg; 1.44 mmol).
Pyridine (800 L), quinoline (3.2 mL), and 4-fluorothiophenol (292 L;
a
50 mL flask were introduced bis(4-
1,3-dimethyl-2-imidazolidinone (1.2 mL) were injected via
a
syringe.
Powdered NaH (25.0 mg; 1.05 mmol) was slowly added while stirring
vigorously, and the mixture was heated in a silicone oil bath at 180 °C for
46.5 h. After cooling and stirring to rt, CH2Cl2 (45 mL) was added and filtration
was achieved. The organic phase was washed with H2O (30 mL) and separated.
The aqueous phase was further extracted with CH2Cl2 (15 mL). After combining
the organic phases, drying over anhydrous MgSO4, filtration and evaporation of
the solvent provided a greenish paste (528.0 mg). The crude product was
dissolved in CH2Cl2 (12.0 mL), filtrated over a short column (SiO2; CH2Cl2), and
the solvent was evaporated to afford a light green solid (295.0 mg; 100% yield)
which was recrystallized in boiling EtOH/toluene (10.0 mL/2.0 mL), to provide
a first crop (78.0 mg). Another recrystallization of the solid from the mother
l
l
351.0 mg; 2.74 mmol) were injected via a syringe while stirring. The mixture
was heated in a silicone oil bath at 165 °C for 23 h. A dark solid resulted and
toluene (25 mL) was added. After filtration, a solution of HCl (30 mL, 3 M) was
added. The organic phase was separated, washed with H2O (30 mL), dried over
anhydrous MgSO4, filtrated, and the solvent was evaporated. A brown solid
(682.0 mg) was recovered. Filtration on done on a short column (SiO2; eluent:
n-hex 100%; then increasing volume of toluene until 100%). Evaporation of
solvents and drying afforded a yellow solid (473.0 mg). It was stirred in ethanol
(20 mL)/toluene (3 mL), and filtration provided 17 (429.0 mg; 71%). Mp 97.9 °C
(EtOH, colorless solid); 1H NMR (200.13 MHz, CDCl3, 24 °C, TMS): d 7.40 (ddapp
,
liquor with boiling EtOH/toluene (5.0 mL/1.0 mL) afforded a second crop
J = 9.0, 5.3 Hz, 4H), 7.21 (dapp, J = 8.6 Hz, 4H), 7.13 (dapp, J = 8.8 Hz, 4H), 7.04
(ddapp, J = 8.9, 8.8 Hz, 4H); 13C NMR {F} (50.32 MHz, CDCl3, 24 °C, TMS): d
116.37, 116.80, 130.07, 131.62, 133.82, 134.59, 134.74, 135.50; 19F NMR
(188.31 MHz, CDCl3, 24 °C, CFCl3): d À113.58; MS (El, 70 eV) m/z = 438 (M+,
100.0%), 311 ([FC6H4SC6H4SC6H4]+, 36.6%), 235 ([FC6H4SC6H4S]+, 18.9%),
203([FC6H4SC6H4]+, 25.1%), 127 ([FC6H4S]+, 11.0%), 95 ([FC6H4]+, 5.2%). Note:
17 can be produced on a 3g scale (60% yield) by a similar procedure.
(29.0 mg). The crops were combined (107.0 mg; 36%). Mp 187.3 °C (colorless
solid); 1H NMR (200.13 MHz, CDCl3, 24 °C, TMS): d 2.36 (s, 6H, CH3), 7.12–7.27
(m, 20H), 7.33 (dapp, J = 8.1, 4H); 13C NMR (50.32 MHz,CDCl3,24 °C,TMS): d
21.17 (CH3), 129.78, 130.26, 130.96, 131.10, 131.62, 132.02, 132.21, 132.94,
133.02, 138.05, 138.97, 139.27; MS (El, 70 eV) m/z = 646 (M+, 100.0%),
523([M+ÀCH3PhS]+,
2.6%),
448
([M+ÀCH3PhSPh]+,
2.7%),
416
([M+ÀCH3PhSPhS]+, 3.1%), 340 ([CH3PhSPhSPhS]+, 4.3%), 307 ([CH3–C6H4S–
C6H4–S–C6H4]+, 7.2%), 199 ([CH3–C6H4–S–C6H4]+, 19.6%), 184 ([C6H4–S–C6H4]+,
36.9%), 123 ([CH3–C6H4–S]+, 15.0%), 91 ([CH3–C6H4]+, 27.9%).
13. Typical procedure for 20: Into a dry 25 mL flask under nitrogen was introduced
17 (200.0 mg; 456.0 lmol). 4-methylthiophenol (119.0 mg; 958.0 lmol) and