Ruthenium-catalyzed knoevenagel condensation: A new route toward cyano-substituted poly(p-phenylenevinylene)s

Article abstract of DOI:10.1021/ma049243s

The ruthenium-catalyzed Knoevenagel reaction was developed for the preparation of cyano-substituted conjugated polymers. The polymerization was quenched by pouring the reaction mixture into methanol. The structures of polymers were confirmed by spectroscopic studies and elemental analysis. The results show that Knoevenagel polycondensation mediated by transition metal complexes enjoys the advantages of neutral and mild reaction conditions.

Full text of DOI:10.1021/ma049243s

Macromolecules 2004, 37, 7061-7063
7061
Notes
Ru th en iu m -Ca ta lyzed Kn oeven a gel
Con d en sa tion : A New Rou te tow a r d
Cya n o-Su bstitu ted
P oly(p-p h en ylen evin ylen e)s
J ia n Lia o a n d Qin g Wa n g*
Department of Materials Science and Engineering,
The Pennsylvania State University,
University Park, Pennsylvania 16802
Received April 19, 2004
Revised Manuscript Received J uly 5, 2004
CN-PPV, a poly(p-phenylenevinylene) (PPV) deriva-
tive with cyano groups on the vinyl units, is one of the
most important conducting polymers for use as electron-
transporting materials in highly efficient organic light-
emitting diodes (LEDs).1-5 The introduction of an
electron-withdrawing CN group to π-conjugated polymer
backbone reduces the lowest unoccupied molecular
orbital (LUMO) energy levels and increases the electron
affinity of the polymer. As a result, the electrolumines-
cence efficiency (photons emitted/charge injected) is
greatly improved due to the enhanced electron injection
and transport. Up to now, the only method available
for the preparation of CN-PPV is base-catalyzed Kno-
evenagel condensation between equimolar amounts of
terephthaldehyde and benzene-1,4-diacetonitrile deriva-
tives.6,7 The condensation polymerization takes place
upon addition of an excess strong base such as tetrabu-
tylammonium hydroxide, potassium tert-butoxide, and
sodium methoxide in a 1:1 v/v mixture of tert-butyl
alcohol and tetrahydrofuran. Under these conditions,
crossing-linking caused by Michael addition side reac-
tion occurs during the reaction and/or purification. This
reaction condition is also not compatible with the
synthesis of the polymers containing functional groups
that are sensitive to basic conditions. Furthermore, the
resulting polymers precipitate out quickly from the
reaction medium due to poor solubility of the polymers
in alcohols, which makes it difficult to control the
polymer structures and molecular weights. These find-
ings prompted us to explore milder conditions to prepare
CN-PPV. As part of our ongoing projects aimed at the
development of new strategies for the synthesis and
assembly of conducting polymers, we have been study-
ing transition-metal complex-mediated polycondensa-
tion for the preparation of multifunctional conjugated
polymers.8
F igu r e 1. Structures of the monomers for the ruthenium-
catalyzed Knoevenagel reactions.
cent to nitriles to afford R-cyanoalkylmetal hydride
complex, which can be trapped with electrophiles such
as carbonyl compounds to give a carbon-carbon bond
at the R-position of nitriles. The general reaction condi-
tions in the synthesis of small molecules via the
ruthenium-catalyzed aldol reaction have been well
documented. This catalytic transformation proceeds
highly efficiently under neutral and mild conditions.11
Herein, we demonstrate the feasibility of the ruthenium
complex-mediated Knoevenagel coupling in polyconden-
sation. This approach provides us an alternative method
to the synthesis of cyano-substituted conjugated poly-
mers.
Figure 1 lists the monomers involved in this study.
The monomers were synthesized in good yields, and the
synthetic details are given in the Supporting Informa-
tion.12-14 The syntheses of polymers are outlined in
Scheme 1. The polymerization was carried out in dry
tetrahydrofuran (THF) in the presence of ruthenium
dihydride complex RuH2(PPh3)4 (3 mol %) and 1,2-bis-
(diphenylphosphino)ethane (dppe) (6 mol %) at 60 °C
under an argon atmosphere. The typical reaction time
was around 30 min when the polymer started to
precipitate out. The polymerization was quenched by
pouring the reaction mixture into methanol. After
purification, the polymers were obtained in over 90%
yields. The structures of polymers were confirmed by
The C-H activation with low-valent ruthenium hy-
dride complexes for the formation of R,â-unsaturated
nitriles has been recently studied by Murahashi et al.9,10
The reaction mechanism involves an oxidative addition
of hydridoruthenium species to the R-C-H bond adja-
1
spectroscopic studies and elemental analysis. In the H
NMR spectra of polymers, the chemical shifts of two
vinyl protons appear at around 8.10 ppm and those of
aromatic protons appearing at around 7.95, 7.70, and
7.30 ppm. The features of the UV/vis and fluorescence
* To whom correspondence should be addressed. E-mail: wang@
matse.psu.edu.
10.1021/ma049243s CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/10/2004

7062 Notes
Macromolecules, Vol. 37, No. 18, 2004
Sch em e 1. Syn th esis of Cya n o-Su bstitu ted Con ju ga ted P olym er s
spectra are consistent with the results reported in the
literature.1,7 The thienylene-phenylene copolymer VII
possesses a lower band gap than polymers I-VI. As
shown in Figure 2, the maximum absorption peaks
corresponding to π-π* transition appear at 510 nm for
polymer VII and at around 470 nm for CN-PPVs. The
fluorescence spectrum of polymer VII in 1,1,2,2-tetra-
chloroethane gives an emission maximum at 700 nm.
The synthesized polymers are only partially soluble in
THF, the solvent used in our gel permeation chroma-
tography (GPC) measurements. The typical weight-
average molecular weight (Mw) measured by GPC is
around 7 kDa, and the polydispersity (PD) is about 1.3
using polystyrene as standards, which reflects only the
THF-soluble part of the polymers. Robust free-standing
films can easily be cast from polymer solutions. These
results are comparable to the polymers synthesized by
KOBut-catalyzed Knoevenagel condensation in our par-
allel experiments.
We next investigated the scope and optimized reaction
conditions of this ruthenium-catalyzed Knoevenagel
polycondensation. Because of low reactivity of aromatic
cyanide, addition of a catalytic amount of an electron-
donating bidentate phosphine such as dppe was found
to be necessary for the polymerization. Without dppe
no reaction was observed, and only monomers were
recovered. Among bidentate phosphine ligands such as
1,3-bis(diphenylphosphino)propane (dppp), 1,1-bis(di-
phenylphosphino)methane (dppm), 1,4-bis(diphenylphos-
phino)butane (dppb), and 1,4-bis(diphenylphosphino)-
butane, dppe gives the best results. RuH2(PPh3)4 is the
best catalyst among the catalysts examined which
include RuH(C2H4)(PPh3)2, RuH2(CO)(PPh3)3, ReH-
(N2)(PMe2Ph)4, and Pd2(dba)3CHCl3. The amount of
catalysts was also found crucial to the polymerization
process. In dilute solution, when the concentrations of
RuH2(PPh3)4 and dppe were lower than 6 and 12 mM,
respectively, no polymerization occurred, and only dimers
or trimers were obtained. The optimized concentrations
of RuH2(PPh3)4 and dppe are 12 and 24 mM, respec-
tively. It is worth mentioning that the polymerization
also proceeds efficiently at room temperature, although
longer reaction time (about 12 h) is needed for the
completion of reaction. The solvent is known to play an
important role in polymerization, which affects both the
stability of catalyst and the molecular weights of the
resulting polymers.15 It was found that this ruthenium-
catalyzed polycondensation can be operated in a variety
of organic solvents, such as benzene, toluene, chloro-
form, DMF, NMP, and CH3CN. This provides us flex-
ibility in selecting solvents to match the solubility of
the resulting polymers.
As expected, the monomer structure drastically affects
the polymerization process as well. Benzene-1,4-diac-
etonitrile with 2,5-substituted groups including diphen-
yl, dichloro, dialkyl, and dialkoxyl with different chain
lengths have been tested. None of the substituted ones
(monomers 10-15) produced polymers. In most cases,
no reaction occurred and the starting monomers were
recovered. These results indicate that the oxidative
addition and/or the transmetalation steps in the ruthe-
nium-catalyzed Knoevenagel reaction are extremely
sensitive to the steric bulkiness of the substituent on
the reactants. Similar results have been observed in
palladium-catalyzed Stille coupling polycondensation.16
On the other hand, terephthaldehyde bearing dialkoxy-
substituted side chains (monomers 2-6) exhibits much
higher reactivity than the similar dialkyl-substituted
terephthaldehyde. For dialkyl-substituted terephthal-
dehyde (monomers 7 and 8), the polymerization could
not proceed further after forming dimers or trimers. We
reason that electron-donating alkoxy side chains stabi-
lize hydrido(aldolato)-ruthenium(II) intermediate formed
by the interaction of hydrido(enolate)-ruthenium with
an aldehyde. The enolate ruthenium is an oxidative
addition product of ruthenium dihydride complex with
nitrile.11 This result is consistent with the observed
remarkable effect of electron-donating phosphine ligands
in the polymerization. Similarly, monomer 16, 3-hexyl-
2,5-thiophenedicarboxaldehyde, can be polymerized
smoothly to yield the copolymer VII. The electron-
donating property of sulfur atom is believed to facilitate
the reaction.
In conclusion, the ruthenium-catalyzed Knoevenagel
reaction has been developed for the preparation of
cyano-substituted conjugated polymers. Compared with
the conventional inorganic base-catalyzed reaction, the
Knoevenagel polycondensation mediated by transition-
F igu r e 2. UV/vis and fluorescence spectra of polymers II, VI,
and VII in tetrachloroethane.

Macromolecules, Vol. 37, No. 18, 2004
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Ack n ow led gm en t. This work was supported by the
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