Transition-metal-free synthesis of alternating thiophene-perfluoroarene copolymers

Article abstract of DOI:10.1021/ja056074y

The fluoride-activated coupling between silyl-functionalized thiophene monomers and perfluoroarenes leads to moderately high-molecular-weight alternating copolymers in excellent yields and high chemical purity. The method bypasses transition-metal catalys

Full text of DOI:10.1021/ja056074y

Published on Web 02/03/2006
Transition-Metal-Free Synthesis of Alternating Thiophene-Perfluoroarene
Copolymers
Yongfeng Wang and Mark D. Watson*
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055
Received September 2, 2005; Revised Manuscript Received January 8, 2006; E-mail: mdwatson@uky.edu
Scheme 1. Synthesis of Alternating Thiophene-πF Copolymers.
Thiophene-containing polymers, copolymers, and small mol-
ecules are heavily pursued as organic electronic materials.1
Thiophenes are covalently entrained at their 2,5-positions within
these materials most commonly by transition-metal-catalyzed
coupling2 or (electro)chemical oxidative polymerization. The limited
means by which conjugated polymers can be purified may leave
behind residues from catalysts and reactive functionalities, as well
as intrinsic chemical defects arising from side reactions. If all other
factors were equal, selection of synthetic methodology could be
reduced to choosing which associated trace defects/impurities
minimally impact targeted properties.
Regioselective fluoride-displacement reactions3 undergone by
perfluorinated π-systems (πF) provide unique opportunities to
prepare conjugated polymers with minimal reagents/catalysts.
Reactions between chalcogenide nucleophiles and πF’s have been
exploited to make alternating copolymers,4 but few carbon-carbon
bond-forming polymerizations are reported. Notable examples are
oligomers and polymers obtained via reaction of πF’s with
metalated ferrocenes5a or indene,5b and small molecules from
lithiated thiophenes.1i We prepared poly(phenylene ethynylene)s5c
and thiophene copolymers from πF’s and bis-lithiated diethynyl-
benzenes or thiophenes, but metalated monomers which are
environmentally stable would be preferred.
We present preliminary investigations into exploiting the known6
fluoride activation of silicon-carbon bonds toward electrophiles
to prepare π-πF copolymers. Here, the electrophiles are perfluoro-
arenes and the silyl-functionalized, masked nucleophiles are 2,5-
bis(trimethylsilyl)thiophenes. These studies are further motivated
by the possibility to control supramolecular behavior via π-πF
interactions and manipulation of (opto)electronic properties7 at the
molecular level by incorporation of highly electronegative fluorides.
The synthetic pathway and representative polymers are illustrated
in Scheme 1. Facile purification of the monomers and resulting
polymers makes this an attractive alternative to Stille coupling. In
addition, πF’s are substantially less expensive than their counterparts
carrying additional functionality needed for transition-metal-
catalyzed couplings. Copolymerization with various πF’s is initiated
here with catalytic fluoride ion, which is regenerated with each
C-C bond formed. In principle, the only constituents of the reaction
mixture should be target polymer, solvent, fluorotrimethylsilane
(bp ) 16 °C), and the water-extractable initiator system.
a GPC vs polystyrene standards, toluene. b 10-5 M THF. c Differential
scanning calorimetry (midpoint). Reaction conditions not optimized.
significantly less regiopure. 19F NMR shows three major signals
corresponding to the symmetry depicted in Scheme 1, along with
seven smaller signals arising from end-groups. There are three
additional small signals, most likely from 2,7-substitution about
naphthalene (∼7% based on relative integrals). NMR indicates that
the end-groups are almost exclusively monosubstituted πF residues
for polymers 1-3. These polymers are therefore telechelic macro-
monomers, which might be end-functionalized via fluoride dis-
placement by other nucleophiles.
The end-groups are defined because the πF monomer was added
in excess. This stoichiometric imbalance should have led to lower
than observed degrees of polymerization according to the Carothers
equation [Pn ) (1 + r)/(1 - r), where r is the ratio of limiting to
excess monomer]. For polymer 1a, the employed 20% molar excess
of C6F6 should provide Pn ≈ 11 (Mn ≈ 4 kDa). The difference
between measured (28 kDa) and predicted Mn far exceeds the typical
2-fold overestimation by gel permeation chromatography for
semirigid polymers. Further, exact stoichiometry led to swollen but
insoluble gels. This indicates that the average functionality of C6F6
under these conditions is >2, i.e., on average, slightly more than
two reactive C-F bonds per molecule. This should lead to higher
than predicted Pn, branching, and gelation, the last of which can
be suppressed via stoichiometric imbalance.
1H, 13C, and 19F NMR spectra for 1a and 2 indicate high chemical
purity, in accordance with the structures in Scheme 1 (see
Supporting Information for full spectra). Polymer 2 is defect-free
within detection limits. The absence of detectable defects for
polymer 2 indicates that perfluorobiphenyl is reactive only at the
4,4′-positions under these conditions. Small signals in the 19F
spectrum of 1a indicate ortho/meta linkages or branching with
estimated concentration e1/53 repeat units (1 defect/106 rings).
Similar to small-molecule products resulting from reaction of
other nucleophiles with perfluoronaphthalene,3 polymer 3 is
In any case, the Carothers equation assumes equal reactivity of
all functional groups, and therefore it cannot apply here. Most
substituents activate the para position of pentafluorobenzenes toward
further substitution.3 The first and second attacks on C6F6 should
occur with different rates.
The 2- and 5-positions of the thiophenes should also have
variable reactivity in this scheme. These polymerizations must
proceed through anionic intermediate(s), i.e., pentacoordinate silicate
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C O M M U N I C A T I O N S
Scheme 2. Model Reactions To Elucidate Reactivitya
a Conditions: (i) 0.1 equiv of CsF, 0.2 equiv of 18-C-6, toluene, 80 °C. (%) ) isolated yields. *Based on 13.
anions,6b whether or not desilylation to a formal carbanion
intermediate precedes C-C bond formation. Therefore, reactivity
at the 5-position should change significantly after an electron-
donating TMS group at the 2-position is replaced by an electron-
withdrawing πF group.
side chains increases the Tg by 30-40 °C in all cases. Unlike deeply
colored thiophene (co)polymers, which may attain ground-state
main-chain coplanarity, 1-3 are colorless solids and emit in the
UV/visible blue region. The λmax values for absorption/emission
correlate well with the polymer structures: replacement of benzene
by biphenyl led to a moderate blue shift, while naphthalene led to
a significant red shift (∼30-40 nm, see Supporting Information).
All reaction conditions reported here are not optimized. Opti-
mization is underway to reduce the required reaction temperatures
via alternate solvents and fluoride sources. Lower temperatures
should lead to even higher selectivity during bond formation. Our
findings with other silyl-functionalized π-systems suggest that this
pathway will find a broad scope similar to the related Hiyama
coupling reaction.
The model studies in Scheme 2 delineate functional group
reactivity during the polymerizations. The well-defined nature of
this chemistry is also supported by the mass balance for each model
reaction, which is excellent, given that the products were isolated
by chromatography. Compound 6 provided details beyond those
involved in the related polymerization. Nearly 50% conversion to
oligomers 8-10 requires that the thiophenes gain a second
nucleophilic site. A reasonable scenario is proton transfer to
fluoride-activated 6 from 7 (C6F5 lowers pKa), also accounting for
approximately eqimolar production of 4. Fluoride is lost in this
step but can be regenerated when the new anion of 7 reacts with
other πF’s to form the higher oligomers. The low Pn of the
oligomers results from gross imbalance in stoichiometry between
C6F6 and bifunctional thiophenes.
To avoid proton-transfer chemistry, the 5-position was blocked
with a methyl group (12). The ratio of isolated 13 and 14 indicates
that the first and second attacks on C6F6 proceed with essentially
the same rate. On the other hand, the high isolated yield of 16 upon
reaction of equimolar amounts of 5 with 13 shows that conversion
of the second TMS group of 5 is markedly more rapid than that of
the first. For that reason, ∼43% of 5 was recovered, separately, as
unreacted starting material and monodesilylated product 6. Since
the amount of desilylation was approximated by the amount of CsF
employed (∼10%) and desilylation is minimal during the related
polymerizations, 6 was likely formed during workup. The remaining
5 and 13 were converted to an inseparable mixture of 15a,b, thereby
accounting for nearly complete mass balance.
These reactions contrast the reported behavior of silyl thiophenes
during fluoride-activated palladium-catalyzed Hiyama coupling,8
during which the thienyl groups are expelled from the catalytic
cycle following proteodesilylation. We have, however, found that
stoichiometric CsF and a palladium catalyst can indeed lead to
moderately successful Hiyama coupling between silyl thiophenes
and iodobenzene (∼40% conversion, not shown). The fluoride
source may be crucial.
In regard to physical properties of the polymers 1-3, steric
repulsion between pendant alkoxy groups and fluorides should cause
significant twisting along the backbone, leading to amorphous
polymers. Accordingly, differential scanning calorimetry revealed
only second-order transitions, and no birefringence was seen from
thin films between cross-polars. Higher glass transition temperatures
(Tg) for related amorphous materials seems to improve LED
performance.9 Substituting perfluorinated biphenyl (2) or naphtha-
lene (3) for benzene as well as shorter methoxy (1b) for butoxy
Acknowledgment. This work was supported by the University
of Kentucky Research Foundation.
Supporting Information Available: Experimental procedures;
spectroscopic and analytical data; complete ref 1i. This material is
available free of charge via the Internet at http://pubs.acs.org.
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