Synthesis and characterization of bifunctional polymers carrying tris(bipyridyl)ruthenium(II) and triphenylamine units

Article abstract of DOI:10.1021/ma025824a

The synthesis, characterization, and properties of a highly soluble bifunctional polymer are described in which a tris(bipyridyl)Ru(II) unit acts as dye and triphenylamine units act as charge transport moieties. First a macroligand, a bipyridine carrying two poly(4-bromostyrene) chains, was synthesized by atom transfer radical polymerization (ATRP) of 4- bromostyrene in bulk using CuCl/PMDETA as the catalytic system and bis(chloromethyl) bipyridine as the initiator. The target polymer was then obtained via a polymer amination reaction in which the bromophenyl group was converted into a triphenylamine followed by metallation of the bipyridine unit of the macroligand with Ru(II) bis(bipyridine). The reaction conditions of ATRP and polymer amination reaction were optimized, and the degree of conversion for both steps was determined by gas chromatography (GC) analysis of rest monomer content and elemental analysis of unreacted bromine, respectively. The control in molecular weight was achieved maintaining a narrow distribution in the desired low molecular weight range of bulk polymerization of 4-bromostyrene. The polymer amination reaction using the Pd(OAc)₂ and P(t-Bu)₃ system was found to be very efficient, and the reaction was complete within 2 h. The metallation reaction could be followed by UV/vis spectroscopy. MALDI-TOF MS of the three polymers was carried out to obtain absolute molecular weights and their distribution. A comparison of these molecular weights gave additional information about the degree of polymer amination and metallation reaction. The thermal properties of the different polymers suggest that the thermal stability as well as the glass transition temperature increases from the starting macroligand which carries poly(4-bromostyrene) chains to the intermediate polymer having poly(vinyltriphenylamine) chains and finally to the bifunctional Ru(II) polymer complex.

Full text of DOI:10.1021/ma025824a

Macromolecules 2003, 36, 1779-1785
1779
Articles
Synthesis and Characterization of Bifunctional Polymers Carrying
Tris(bipyridyl)ruthenium(II) and Triphenylamine Units
Ka tja P eter a n d Mu k u n d a n Th ela k k a t*
Macromolecular Chemistry I, University of Bayreuth, D-95447 Bayreuth, Germany
Received November 11, 2002; Revised Manuscript Received J anuary 17, 2003
ABSTRACT: The synthesis, characterization, and properties of a highly soluble bifunctional polymer
are described in which a tris(bipyridyl)Ru(II) unit acts as dye and triphenylamine units act as charge
transport moieties. First a macroligand, a bipyridine carrying two poly(4-bromostyrene) chains, was
synthesized by atom transfer radical polymerization (ATRP) of 4-bromostyrene in bulk using CuCl/
PMDETA as the catalytic system and bis(chloromethyl) bipyridine as the initiator. The target polymer
was then obtained via a polymer amination reaction in which the bromophenyl group was converted into
a triphenylamine followed by metallation of the bipyridine unit of the macroligand with Ru(II)
bis(bipyridine). The reaction conditions of ATRP and polymer amination reaction were optimized, and
the degree of conversion for both steps was determined by gas chromatography (GC) analysis of rest
monomer content and elemental analysis of unreacted bromine, respectively. The control in molecular
weight was achieved maintaining a narrow distribution in the desired low molecular weight range of
bulk polymerization of 4-bromostyrene. The polymer amination reaction using the Pd(OAc)2 and P(t-
Bu)3 system was found to be very efficient, and the reaction was complete within 2 h. The metallation
reaction could be followed by UV/vis spectroscopy. MALDI-TOF MS of the three polymers was carried
out to obtain absolute molecular weights and their distribution. A comparison of these molecular weights
gave additional information about the degree of polymer amination and metallation reaction. The thermal
properties of the different polymers suggest that the thermal stability as well as the glass transition
temperature increases from the starting macroligand which carries poly(4-bromostyrene) chains to the
intermediate polymer having poly(vinyltriphenylamine) chains and finally to the bifunctional Ru(II)
polymer complex.
In tr od u ction
a narrow polydispersity which favor nanoscale phase
separation between two functional domains. Thus, it is
advisable to define the interface between the two
functionalities exactly by attaching the two different
functional moieties by a covalent bond and if possible
to manipulate the interface formation between these two
moieties using intrinsic polymeric properties like phase
separation and aggregation in block copolymers.5,6 Some
synthetic attempts in this regard have been successfully
tried out by Hadziioannou et al. by developing bifunc-
tional diblock copolymers carrying p-phenylenevinylene
and fullerene units.7 For successful exploitation of the
desired morphology which creates the internal interface
and which in turn guarantees charge separation at
these internal interfaces, as for example in a solar cell,
a good control of the building up of phase domains of
the block copolymer is required. In the specific case of
a dye-sensitized TiO2 solar cell in which ruthenium(II)-
(bipyridyl) dye is chemisorbed onto TiO2 surface and a
triphenylamine derivative functions as the hole conduc-
tor, the interface between the chemisorbed mono-
molecular dye layer and hole conductor is very crucial
for charge separation and transport.8 The state-of-the-
art preparation of this interface is just by spin-coating
of a solution of the hole conductor onto the dye-coated
TiO2 surface. This results in insufficient contact between
dye and hole conductor which leads to low performance
Polymer technology involving optical and electronical
functional materials demand the design and synthesis
of new polymers with well-defined structure, control in
molecular weight, and low polydispersity. As most of the
electrooptical applications demand versatile functions
like light absorption or emission and charge injection/
transport properties, the conventional way of realizing
this is to design multicomponent single-layer devices or
multilayer devices. In most cases, these devices are very
thin; therefore, the various interfaces play a consider-
able role in the overall performance of the device, and
in certain cases, as for example in solar cells, they even
decide the degree of charge separation and hence the
efficiency of the cell. It has been shown that the concept
of copolymerization of different functional monomers is
a simple way of incorporating two different material
functions into a polymer chain.1-4 But in random
copolymerization there is no control of polydispersity or
molecular weight, and the interface between the two
functional moieties is also not present in the bulk due
to lack of phase separation of the functional domains.
On the other hand, living polymerizations and controlled
radical polymerizations deliver block copolymers with
* Corresponding author: e-mail Mukundan.Thelakkat@
uni-bayreuth.de.
10.1021/ma025824a CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/27/2003

1780 Peter and Thelakkat
Macromolecules, Vol. 36, No. 6, 2003
Sch em e 1. Sch em e of P olym er iza tion of
4-Br om ostyr en e via ATRP in Bu lk Usin g
of the cell. One way to overcome this problem is to use
additional bifunctional/bipolar compatibilizers which
bridge the interface between dye and hole conductor. A
chemical approach to solve this problem is to bind the
dye with hole transport polymers and use such bifunc-
tional systems alone or in combination with the con-
ventional dyes and hole conductors.
4,4′-Bis(ch lor om eth yl)-2,2′-bip yr id in e a s In itia tor a n d
Cu Cl/P MDETA a s Ca ta lytic System a t 60 °Ca
Keeping this in mind, we wanted to realize a bifunc-
tional polymer carrying a strongly absorbing polar dye
unit attached to well-defined polymer chains carrying
charge transport moieties based on triarylamines. The
controlled polymerization of the monomer, vinyltri-
phenylamine (VTPA), using anionic polymerization9,10
as well as by TEMPO (2,2,6,6-tetramethylpiperidine
oxide)-mediated radical polymerization11 is reported in
the literature. Here we report the successful synthesis
of “poly(vinyltriphenylamine)” using a different route
as the above-reported methods in the literature. Fraser
et al. have reported the atom transfer radical polymer-
ization (ATRP) of styrene starting from bis(chlorometh-
yl)bipyridine or from the corresponding ruthenium
complex as initiator to obtain poly(styrene) attached to
a ruthenium core.12 It is also possible to carry out ATRP
of other substituted styrenes like 4-bromostyrene using
conventional initiators like 1-phenylethyl bromide as
reported by Matyjaszewski et al.13
We made use of a combination of the above two
concepts to polymerize 4-bromostyrene in bulk using bis-
(chloromethyl)bipyridine as initiator for the first time
in order to obtain two poly(4-bromostyrene) chains with
well-defined molecular weights and low polydispersity
attached to a bipyridyl unit. Then we adopted a polymer
analogous amination reaction on this macroligand to
convert the bromophenyl group into a triphenylamine
moiety. In a final step, the bipyridine moiety was
complexed with bis(bipyridyl)Ru(II) precursors to obtain
a highly polar dye functionality covalently bound to two
hole transport polymer chains of vinyltriphenylamine.
Although the direct polymerization of the vinyltri-
phenylamine monomer using bis(chloromethyl)bipyri-
dine as initiator was feasible, this route was not favored
due to lack of reproducibility in the control of polymer-
ization under the conditions that we used.
This paper describes the details of synthesis, charac-
terization, and different physical properties of a series
of metal-free macroligands, poly(4-bromostyrene)s or
poly(vinyltriphenylamine)s, and the final Ru(II) polymer
complexes having variable chain lengths. The main aim
here was to find out a synthetic route for bifunctional
systems which can be synthesized over a wide range of
molecular weights. It is usually difficult to obtain high
control of molecular weight distribution in the oligo-
meric regime in bulk polymerization. For preparing a
series of polymers with two different material functions
varying in their weight fractions, the method adopted
here is seemly efficient. Moreover, this method is also
of general nature and is applicable for the incorporation
of other organo-metal complex dyes and other charge
transport units like carbazoles and phenothiazines as
well.
a
Molar ratio of reagents is [monomer]:[initiator]:[CuCl]:
[PMDETA] ) 150:1:1:2.
unit, and the charge transport function is taken care of
by a triphenylamine moiety. All the analytical data for
the polymers from NMR, FT-IR, MALDI-TOF MS, and
UV/vis spectroscopy are given in the Experimental
Section.
The first step of the synthesis as shown in Scheme 1
involves the controlled free radical bulk polymerization
of 4-bromostyrene using CuCl/PMDETA (1,1,4,7,7-pen-
tamethyldiethylenetriamine) as the catalytic system.
PMDETA acts as complexing ligand for the CuCl/CuCl2
system which controls the equilibrium between active
radical and dormant bromide species in the reaction
mixture. In ATRP usually bipyridine derivatives are
used as ligands and phenylethyl bromide or ethyl
2-bromopropionate as initiators.13 But Fraser and co-
workers have shown that an initiator, bis(chloromethyl)-
bipyridine (1) in combination with a conventional
catalytic system like CuCl/PMDETA can be used for the
controlled radical polymerization of styrene.12 Therefore,
4,4′-bis(chloromethyl)-2,2′-bipyridine was used here as
initiator in combination with PMDETA/CuCl in order
to polymerize 4-bromostyrene in bulk. A series of
polymerization reactions were performed with the molar
ratio [monomer]:[initiator]:[CuCl]:[PMDETA] ) 150:1:
1:2 in order to optimize the reaction. The initiator 1 was
synthesized from the corresponding 4,4′-(trimethyl-
silylmethyl) derivative by a modified procedure of the
CsF method described in the literature, and it is given
in the Experimental Section.14
To obtain well-defined low molecular weight polymers,
the reaction was quenched by cooling and exposing the
reaction mixture to air at desired time intervals. Since
the polymerization was terminated at low degrees of
conversion, there is always rest monomer left in the
reaction mixture which makes a proper precipitation of
2 either in methanol or in n-hexane difficult. For this
reason, it is important to remove the remaining mono-
mer in high vacuum to obtain reasonable precipitation
of the polymer 2. The rest of the Cu salts in polymer 2
was removed by repeated washing of the polymer with
aqueous NH4Cl solution (see Experimental Section).
The molecular weight and polydispersity obtained
from GPC (THF and 0.25 wt % tert-butyl bromide as
eluent, polystyrene standards) for two different batches
of polymerization are shown in Table 1. As expected,
the polydispersity becomes lower with higher degree of
polymerization, and depending on the conditions used,
Resu lts a n d Discu ssion
A multistep synthetic route was selected to obtain
functional polymers with defined chain length and
narrow molecular weight distribution for tuning the
electrooptical properties of the resulting material. The
function of the dye is fulfilled by a tris(bipyridyl)Ru(II)

Macromolecules, Vol. 36, No. 6, 2003
Bifunctional Polymers 1781
F igu r e 2. Dependence of number-average molecular weight
Mn and polydispersity Mw/Mn on degree of monomer conversion
in bulk-ATRP of 4-bromostyrene; conversion determined by
GC measurements with anisole as external standard.
It can be clearly seen that the monodisperse eluograms
show constant increase in molecular weight. Both
number-average molecular weight Mn and weight-
average molecular weight Mw show a linear dependence
on reaction time, which is a measure of conversion.
The conversion at different time intervals was deter-
mined by gas chromatography (GC) analysis of the
amount of unreacted monomer. For this purpose, samples
were taken from the reaction mixture at designated time
intervals and they were diluted with THF, and a defined
amount of anisole was added as external GC standard.
The data obtained from GC analysis plotted vs the
corresponding molecular weights and polydispersities
obtained from GPC are shown in Figure 2. The attained
polydispersity of 1.3 for a conversion of about 40% in
the low molecular weight range of about 10 000 g/mol
is appreciably good for ATRP in bulk. Since we are
interested in synthesizing materials with low molecular
weights in order to use these materials as bifunctional
compatibilizers, this method offers an elegant way of
achieving this goal. Further lowering of polydispersity
is possible by optimizing the conditions of polymeriza-
tion which affect the concentration of active species and
rate of polymerization, as for example, to carry out the
polymerization in suitable solvents.
Thus, it was possible to obtain a macroligand of
bipyridine carrying two poly(4-bromostyrene) chains as
arms which can be later complexed with Ru(II) bis-
(bipyridine)s for the incorporation of the dye function.
Moreover, the bromo substituents of the resulting poly-
(4-bromostyrene) provide an ideal opportunity to attach
a charge transport moiety like triphenylamines by
polymer analogous amination with diphenylamine. For
the latter we require a fast and efficient method of
amination of aryl halide.
The polymer amination reaction with diphenylamine
was carried out as shown in Scheme 2. Similar polymer
amination reactions of poly(4-bromostyrene) using amines
like carbazoles or phenazines were found to proceed
smoothly with a high degree of substitution.15 For the
conversion of macroligand 2 to macroligand 3, a general
method of Pd-catalyzed amination reaction of aryl
halides was used, which was developed and studied by
Hartwig et al. and Buchwald et al.16,17 It is known from
literature that side reactions involving incorporation of
phosphor and hydrodehalogenation are possible if aryl
halides containing electron-poor substituents are used.
To avoid any undesired product and to obtain complete
conversion in appreciably short periods, different com-
F igu r e 1. (A) GPC elution curves for a series of polymers 2
with increasing molecular weights. (B) Dependence of number-
average molecular weights Mn and weight-average molecular
weights Mw on time for ATRP of 4-bromostyrene in bulk at 60
°C (GPC data obtained with THF containing 0.25 wt % tert-
butylammonium bromide as eluent and calibration with
polystyrene standards).
Ta ble 1. GP C Da ta (Obta in ed w ith THF Con ta in in g 0.25
w t % ter t-Bu tyl Am m on iu m Br om id e a s Elu en t a n d
Ca libr a tion w ith P olystyr en e Sta n d a r d s) of Tw o Sets of
Ma cr oliga n d 2 Obta in ed a t Va r iou s Tim e In ter va ls via
ATRP in Bu lk Usin g 4,4′-Bis(ch lor om eth yl)bip yr id in e a s
In itia tor , Cu Cl, a n d P MDETA a s Ca ta lytic System
batch
1
time [min]
Mw [g/mol]
Mn [g/mol]
PDI
180
240
300
360
60
120
180
240
300
360
4348
5811
7535
9779
2764
3309
5130
6987
9159
11000
3060
4267
5756
7559
1803
2610
3540
4959
6558
8237
1.42
1.36
1.31
1.29
1.53
1.27
1.45
1.41
1.40
1.34
2
molecular weights can be tuned in the oligomeric range.
At higher conversion, high molecular weight polymer 2
with Mn in the range of 30 000 g/mol was obtained. It
was rather easy to obtain high molecular weight poly-
mers than low molecular weight polymers with narrow
molecular weight distribution using ATRP of 4-bromo-
styrene in bulk. This observation is similar to that
reported for bulk polymerization of styrene under
similar conditions.12 But our special interest was to
obtain polymer chains with less number of repeating
units in order to obtain high dye content as well as good
filling properties for the polymer in a nanoporous TiO2
layer. For a series of polymer 2, the individual GPC
curves and the dependence of the corresponding molec-
ular weights on time of reaction are shown in Figure 1.

1782 Peter and Thelakkat
Macromolecules, Vol. 36, No. 6, 2003
F igu r e 4. Plot of bromine content (weight percentage)
determined from elemental analysis for polymers 3 at different
time intervals of conversion along with the corresponding
triphenylamine content as calculated from the number of
repeating units taking into consideration the number-average
molecular weight of 30 481 g/mol for the starting polymer 2
as obtained from GPC. The lines are exponential fits of the
data points.
F igu r e 3. 1H NMR spectra of polymer 2 and polymer 3 (in
CHCl3, 250 MHz) which shows the disappearance of the two
doublets from polymer 2 and the appearance of multiplets
corresponding to polymer 3 aryl hydrogen signals after a
reaction for about 1 h.
of NMR detection. To verify the above results obtained
from NMR spectroscopy, elemental analysis was carried
out to determine the content of bromine in the resulting
polymer 3. The conversion of polymer 2 to polymer 3
could be calculated from weight percentage of remaining
bromine as determined by elemental analysis for a
series of samples taken at different time intervals. For
polymer 2 with a number-average molecular weight Mn
(GPC) of 3107 g/mol, the number of repeating units was
calculated as 16. This gives a theoretical weight of
bromine as 41.2%. The elemental analysis gave exactly
the same value of 41.2% for bromine content, and after
2 h of polymer analogous reaction, the rest bromine
content in polymer 3 was 7.56%, which corresponds to
a conversion of 82%. We also estimated the degree of
conversion for the polymer analogous reaction for a
polymer 2 with higher molecular weight. For example,
a batch of polymer 2 with Mn ) 30 481 g/mol (repeating
units ) 135, theoretical Br content ) 43.2%) gave 43.9%
Br in elemental analysis, which is in agreement with
the theoretical value bearing in mind that the molecular
weight obtained from GPC is not absolute. After a
reaction time of 65 min the Br content falls to 7.20%,
corresponding to a conversion of 84% and after 2 h of
reaction time; the rest of the Br content is only 0.5%,
which corresponds to 99% conversion. Figure 4 depicts
the rest of the bromine content in polymer 3 for case
two (high molecular weight polymer) plotted against
time of conversion, and it follows an exponential behav-
ior as expected for a polycondensation reaction. Using
the number-average molecular weight obtained from
GPC for this series of polymer 3 and the bromine
content from elemental analysis, the triphenylamine
content can also be calculated by taking into consider-
ation the number of repeating units of 4-bromostyrene
and vinyltriphenylamine present at different degrees of
conversion. These values are also plotted in Figure 4
for a comparison. From the TPA content, the conversion
value at 65 min comes to 88 wt %. Thus, it is very
obvious that the polymer analogous reaction is complete
within 1-2 h.
Sch em e 2. Sch em a tic Rep r esen ta tion of P d -Ca ta lyzed
P olym er An a logou s Am in a tion of P olym er 2 w ith
Dip h en yla m in e Usin g P d (OAc)2 a n d P (t-Bu )3 a s
Ca ta lytic System
binations of Pd catalysts and ligands were tested to find
out the most efficient system, and it turned out that a
combination of Pd(OAc)2 and P(t-Bu)3 in the presence
of NaOt-Bu gave the best results. With the described
catalytic system it was possible to achieve high conver-
sion of the aryl bromide to triphenylamine unit in less
than 2 h. The working-up procedure for further purifi-
cation is also simple. After cooling the reaction mixture
to room temperature and filtration over alumina to
remove the residual Pd, half of the solvent was evapo-
rated, and polymer 3 was precipitated in methanol.
Further reprecipitation by adding the THF solution of
the polymer into MeOH yielded 90% of the desired
product 3.
The polymer amination reaction was followed via
NMR spectroscopy (see Figure 3). After about 1 h of
reaction, the aryl hydrogen signals of polymer 2 com-
pletely disappear (which usually appear as doublets at
6.29 and 7.17 ppm) and the multiplet signals corre-
sponding to triphenylamine protons appear, and there-
fore the reaction seems to be complete within the errors
The UV/vis spectroscopy of polymer 3 in CHCl3
solution shows additional strong absorption at about 305
nm due to the introduction of the triphenylamine unit;
the absorption maximum λmax of the simple tripheny-
lamine molecule is 305 nm whereas the starting polymer
2 has a maximum absorption at 276 nm. The FTIR
spectrum of polymer 3 shows no ν(C-Br) band at 1073

Macromolecules, Vol. 36, No. 6, 2003
Bifunctional Polymers 1783
Sch em e 3. Sch em a tic Rep r esen ta tion of Com p lexa tion of th e Bip yr id in e Un it in th e P olym er 3 w ith
Ru (bp y)2Cl2‚2H2O Usin g th e Activa ted Silver Sa lt Meth od in DMF u n d er Reflu x
cm-1 which was originally present in polymer 2 and
instead exhibits additional bands for C-N streching at
1312 and 1275 cm-1 which are evidently absent in
polymer 2. Thus, elemental analysis, FTIR, NMR, and
UV/vis absorption data indicate that the Pd-catalyzed
polymer reaction provided the desired product 3 in
which triphenylamine is the pendant group.
The final step of metallation of polymer 3 with Ru-
(bpy)2Cl22H2O was carried out according to Scheme 3
using the activated silver salt method.18 For this reac-
tion, Ru(bpy)2Cl22H2O together with the silver salt, CF3-
SO3Ag, was stirred at room temperature in acetone for
3 h, and the precipitated AgCl was removed by filtra-
tion. The deep red precursor complex Ru(bpy)2(CF3-
SO3)2(acetone)2 is much more active than Ru(bpy)2Cl2-
2H2O, and so it was reacted with polymer 3 by refluxing
F igu r e 5. UV/vis spectrum of polymer 4 (Mn ) 3940 g/mol
from MALDI-TOF MS) with a degree of metallation of 83%
a solution of 3 and precursor complex in DMF overnight.
After precipitation of the polymer into methanol, the
product was filtered out and washed with methanol
several times to remove any unreacted Ru(bpy)2(CF3-
SO3)2(acetone)2 or other low molecular Ru complexes
formed during the reaction. The metallation reaction
can be well-followed by observing the shift of MLCT
(metal-to-ligand charge transfer) absorption maximum
in UV/vis spectroscopy of the bis(bpy) system, Ru(bpy)2-
(CF3SO3)2(acetone)2, at 550 nm to the tris(bpy) system,
tris(bipyridyl)Ru(II) unit, at 456 nm. The UV/vis ab-
sorption spectrum of polymer 4 after 24 h of reaction is
given in Figure 5, and it shows a clear absorption band
at 456 nm and no absorption at 550 nm. With NMR
spectroscopy it was possible to detect the bipyridine
hydrogen signals in the low molecular weight polymers
3 and 4. The bipyridine ligands show broad signals in
the region from 7.4 to 8.3 ppm, which could not be
resolved in detail (see Experimental Section). For high
molecular weight polymers 3 and 4, the bipyridyl proton
signals became weaker due to the decreasing weight
percentage of bipyridyl unit in the polymer chain.
A comparison of the molecular weights of the metal-
free macroligands 2 and 3 with the polymer complex 4
using a common GPC was not possible due to the
difference in intrinsic viscosities and hydrodynamic
volumes of these polymers and also due to lack of
calibration standards for these new types of polymers.
The macroligand 3 and polymer complex 4 carrying
triphenylamine units seem to interact strongly with the
stationary phase, leading to delayed elution and tailing
and correspondingly 2.1% Ru content recorded in CHCl3
solution. The inset shows the magnified absorption maximum
λmax of hexacoordinated Ru(II) core at 456 nm present only in
polymer 4 and which is absent in polymer 3.
effect as compared with polymer 2. This results in low
values of molecular weights for polymer 3 compared to
its precursor polymer 2 using the polystyrene calibration
which is not the reality. To overcome this, elution was
also carried out in the presence of added salts in THF
or dimethylacetamide as solvent, but without any suc-
cess. The calibration of molecular weights for the
polymer 3 using light scattering has large margins of
error due to the low molecular weight nature of these
polymers. An alternative is to use MALDI-TOF MS
analysis, and it was carried out for all the three
polymers 2, 3, and 4 resulting from one reaction batch.
Under the used conditions of our measurement, we
observed that the flow of polymer 2 to the detector was
comparably better than those of polymers 3 and 4. For
the MALDI-TOF MS analysis we selected the starting
polymer 2 with an Mn value of 3107 g/mol as obtained
from GPC. In MALDI-TOF an Mn value of 2720 g/mol
was determined for polymer 2, which is in agreement
with the GPC value. The Mn values for the correspond-
ing polymers 3 and 4 are 3350 and 3940 g/mol, respec-
tively, giving proof of polymer amination and metalla-
tion in terms of absolute molecular weights. The peak
molecular weights for all the three polymers from
MALDI-TOF are 2570, 3340, and 3930 g/mol, respec-
tively. The MALDI-TOF curves are given in Figure 6.

1784 Peter and Thelakkat
Macromolecules, Vol. 36, No. 6, 2003
F igu r e 7. Comparison of second heating curves for the
polymers 2-4 as obtained from DSC measurement (heating
rate 10 K/min). Polymers 3 and 4 were obtained from polymer
2 with an Mn value of 3107 g/mol (GPC) and 2720 g/mol
(MALDI-TOF MS).
F igu r e 6. Comparison of MALDI-TOF mass spectra of
polymers 2-4 from one batch of reaction (recorded with
dithranol as matrix and silvertriflate as additive).
It can be obviously seen that all the three polymers are
monodisperse, and the difference in peak molecular
weights between polymer 2 and polymer 3 corresponds
to 92% conversion for polymer amination. The weight
difference between polymer 3 and polymer 4 is 590 mass
units, which exactly corresponds to 83% conversion for
the metallation reaction. Thus, the successful conversion
of the macroligand carrying poly(p-bromostyrene) chains
to poly(vinyltriphenylamine) chains and its final met-
allation could be verified, and the degree of conversion
for each step could be determined from the absolute
molecular weights obtained from MALDI-TOF MS
analysis.
Using the method described here, the weight fraction
of ruthenium dye in final polymer can be varied over a
wide range by varying the length of poly(4-bromo-
styrene) chains (from 5 wt % dye content for a molecular
weight of 12 300 g/mol up to 25 wt % dye content for a
molecular weight of 2670 g/mol). These polymers are
highly soluble in the metal-free form as well as in the
complexed form in usual solvents like CHCl3, toluene,
etc., and they form uniform and smooth films by spin-
coating.
2 and 3. This gives hints about a good thermal stability
of the final polymer containing a hexacoordinated tris-
(bipyridyl)Ru(II) complex. Moreover, the glass transition
temperature is also improved to 145 °C compared to 110
and 130 °C for polymers 2 and 3, respectively.
Con clu sion
ATRP in bulk was adapted to synthesize poly(4-
bromostyrene) with controlled molecular weight and low
polydispersity starting from bis(chloromethyl)bipyridine
as initiator for the first time. A series of polymers with
well-defined molecular weights and narrow distribution
could be obtained. The polymer amination reaction
using Pd(OAc)2 and P(tBu)3 offered a fast and efficient
method to introduce the charge transport moiety, with
triphenylamine as pendant groups. The final complex-
ation of the bipyridyl unit present in the polymer led to
a highly soluble bifunctional polymer in which charge
transport polymer chains are attached to a tris(bi-
pyridyl)Ru(II) core. MALDI-TOF MS analysis gave the
absolute molecular weights for all the three types of
polymers, and the degree of conversion of polymer
amination and metallation determined from these ab-
solute molecular weights agree well with those obtained
from elemental analysis. We expect that the Ru-
containing polymer shows aggregation behavior due to
its bipolar nature, and this can lead to interesting
morphologies in bulk as a result of dye aggregation and
phase separation. At present, the film formation proper-
ties of these polymers from different solvent combina-
tions and under different thermal treatments are under
investigation.
Th er m a l P r op er ties
Thermal characterization of all the polymers 2, 3, and
4 obtained from the same batch as the one used for
MALDI-TOF analysis was performed by TGA (thermo-
gravimetric analysis) and DSC (differential scanning
calorimetry). The starting polymer 2 has Mn value of
3107 g/mol as obtained from GPC and 2720 g/mol as
determined from the absolute method, MALDI-TOF MS
analysis. A comparison of second heating curves for the
polymers 2, 3, and 4 as obtained from DSC measure-
ment is shown in Figure 7. The absence of any melt peak
or recrystallization peaks points toward amorphous
nature of these polymers. Polymers with structure 2
show 4% weight loss at about 230 °C, which can be
attributed to the removal of chlorine end groups. In all
cases, glass transitions were evident in the DSC curves
at about 110 °C similar to any standard poly(styrene)
in the low molecular weight regime. Polymers 3 having
triphenylamine pendant groups exhibit 4.7% weight loss
at 238 °C and a glass transition temperature at about
130 °C, an increase of about 20 °C corresponding to the
substitution of bromine with diphenylamine. After met-
allation, an increase in thermal stability is observed for
polymer 4 compared to polymers 2 and 3. The onset of
weight loss in TGA for polymer 4 is at about 287 °C,
which is almost 50-60 °C higher than that for polymers
Exp er im en ta l Section
Ma ter ia ls. 4-Bromostyrene from Aldrich was vacuum-
distilled and stored in a refrigerator under an argon atmo-
sphere. Cu(I) chloride (Merck) was purified as described by
Keller.19 Sodium tert-butylate and CsF were dried by heating
under vacuum and stored under argon. Toluene was distilled
from Na prior to use; DMF was refluxed over CaH2 and
distilled under vacuum. 4,4′-Bis(trimethylsilyl)-2,2′-bipyridine
was synthesized as described by Fraser et al.14 PMDETA,
Ru(bpy)2Cl2‚2H2O (Lancaster), palladium acetate, tri-tert-
butylphosphine, and diphenylamine (Aldrich) were purchased
and used as received.
In str u m en ta tion . 1H NMR spectra were acquired on a
Bruker AC 250 spectrometer (250 MHz); the molecular weights
of polymers were determined by gel permeation chromatog-
raphy (GPC) in THF + 0.25 wt % tert-butylammonium bromide
with UV and RI detectors (Waters). UV/vis spectra were
recorded using a Hitachi U-3000 spectrometer, IR spectroscopy

Macromolecules, Vol. 36, No. 6, 2003
Bifunctional Polymers 1785
was carried out with a BioRad Digilab FTS-40 (FTIR), and
GC analysis was done in a GC 8000 series from Fisons
Instruments using anisole for calibration. The thermal deg-
radation of the polymers was studied using a Netzsch TGA
STR 409 with a standard heating rate of 10 K/min under N2
atmosphere, and the differential scanning calorimetry was
carried out with a Perkin-Elmer DSC 7. The MALDI-TOF MS
analysis was performed using a Bruker Reflex III equipped
with a 337 nm N2 laser, an acceleration voltage of 20 kV with
dithranol as matrix, and silver triflate as additive.
2850, 1588, 1492, 1312, 1275, 826, 751, 694. UV/vis (CHCl3):
λmax ) 303 nm. Mn (from MALDI-TOF) ) 3350 g/mol.
Com p lexa t ion of P olym er 3 w it h R u (bp y)2Cl2‚2H 2O.
CF3SO3Ag (0.375 mmol, 96.3 mg) and Ru(bpy)2Cl2‚2H2O (0.187
mmol, 97.5 mg) were dissolved in 30 mL of dry acetone and
stirred under argon for 3 h. After filtration of AgCl over a filter
paper, acetone was removed under vacuum at room temper-
ature. The dark red residue was dissolved in 30 mL of dry
DMF, and polymer 3 (2.6 mmol of monomer unit, 0.7 g) was
added. The reaction mixture was refluxed for 24 h. At the end
of reaction about 75% of the DMF was removed, and the rest
was dissolved in a small amount of CHCl3. The product (4)
was precipitated out by adding the CHCl3 solution into MeOH,
filtered, and washed repeatedly with MeOH until the washings
are colorless. After drying in a vacuum an orange powder was
Syn th esis. 4,4′-Bis(ch lor om eth yl)-2,2′-bip yr id in e (1).
To a solution of 4,4′-bis(trimethylsilylmethyl)-2,2′-bipyridine
(43 mmol, 14.13 g) and Cl3CCCl3 (86 mmol, 20.26 g) in 150
mL of dry DMF under an inert atmosphere anhydrous CsF
(86 mmol, 13.06 g) was added. The reaction mixture was
stirred at room temperature until the starting TMS compound
was fully used up. The reaction mixture was poured into a
mixture of EtOAc and H2O (200 mL each), the organic layer
was separated, the aqueous layer was extracted three times
with 100 mL of EtOAc each, and the combined organic
fractions were dried over Na2SO4. Filtration and evaporation
of the solvent gave a yellowish product which can be purified
by washing with cold toluene cooled using liquid N2 to yield a
1
obtained in ca. 83% yield. H NMR (CDCl3, 250 MHz) δ: 1.57
(m, 2 H), 2.00 (m, 1 H), 6.59-6.89 (m, 9 H), 7.41 (m, 0.06 H),
7.54 (m, 0.06 H), 7.74 (m, 0.08 H), 7.91 (m, 0.12 H), 8.33 (m,
0.08 H). IR (KBr, ν, cm-1): 2940, 2899, 1567, 1492, 1437, 1318,
1235, 1063, 801. UV/vis (CHCl3): λmax ) 456 nm; Mn (from
MALDI-TOF) ) 3940 g/mol.
Ack n ow led gm en t . The financial support for this
research work from German Research Council (DFG/
SFB 481) is kindly acknowledged. We also thank Dr.
M. G. Lanzendo¨rfer, University of Bayreuth, for the
MALDI-TOF MS analysis.
1
white solid: 5.00 g (46%). H NMR (CDCl3, 250 MHz) δ: 4.58
(s, 2 H), 7.35 (d, 1 H), 8.39 (s, 1 H), 8.66 (d, 1H). IR (KBr, ν,
cm-1): 3001, 1594, 1558, 1457, 1376, 1279, 1216, 855. m/z 252
(M+).
4,4′-Bis[poly(4-br om ostyr yl)m eth yl]-2,2′-bip yr id in e (2).
A solution of 4-bromostyrene (115 mmol, 21 g) and 4,4′-bis-
(chloromethyl)-2,2′-bipyridine (1) (0.77 mmol, 129.1 mg) was
placed in an argon-flushed three-neck flask. The mixture was
degassed in three cycles by the freeze-pump-thaw method13
followed by the addition of CuCl (0.77 mmol, 75.8 mg). The
mixture turns first deep brown and then becomes light brown
when the ligand PMDETA (1.53 mmol, 265.1 mg) was added.
Equipped with a septum and a condenser, the flask was
immersed in an oil bath maintained at 60 °C, and the initial
time was recorded. Samples (2 mL) were taken out of the
reaction mixture at certain time intervals through the septum
with an argon-flushed syringe. They were cooled and exposed
to air to terminate the polymerization and were used for GC
analysis after dilution with THF. After precipitation in n-
hexane the slightly bluish polymers were filtered, dried, and
washed first with methanol and then with an aqueous solution
of NH4Cl to remove the residual copper catalyst. The polymers
were reprecipitated from THF into methanol to give a white
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1
solid. H NMR (CDCl3, 250 MHz) δ: 1.25 (m, 2 H), 1.56 (m, 1
H), 6.29 (d, 2 H), 7.16 (d, 2 H). IR (KBr, ν, cm-1): 2924, 1592,
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