Synthesis and characterization of tris(2,2′-bipyridine)ruthenium(II)- centered polystyrenes via reversible addition - Fragmentation chain transfer (RAFT) polymerization

Article abstract of DOI:10.1021/ma049560z

A novel bipyridine-functionalized dithioester, 5,5′- bis(thiobenzoylthiomethyl)-2,2′-bipyridine (3), was first synthesized through a simple ester exchange reaction with the commercially available dithioester, carboxymethyl dithiobenzoate (CMDB), and furth

Full text of DOI:10.1021/ma049560z

7132
Macromolecules 2004, 37, 7132-7139
Synthesis and Characterization of
Tris(2,2′-bipyridine)ruthenium(II)-Centered Polystyrenes via Reversible
Addition-Fragmentation Chain Transfer (RAFT) Polymerization
Gu a n gch a n g Zh ou a n d Issifu I. Ha r r u n a *
Department of Chemistry, Clark Atlanta University, 223 J ames P. Brawley Drive SW,
Atlanta, Georgia 30314
Received March 4, 2004; Revised Manuscript Received J une 22, 2004
ABSTRACT: A novel bipyridine-functionalized dithioester, 5,5′-bis(thiobenzoylthiomethyl)-2,2′-bipyridine
(3), was first synthesized through a simple ester exchange reaction with the commercially available
dithioester, carboxymethyl dithiobenzoate (CMDB), and further used as a RAFT agent in bulk
polymerization of styrene for the synthesis of well-defined bipyridine-centered polystyrene polymers. The
molecular weight is well-controlled, and also the molecular weight distribution remains quite narrow.
This shows that the novel bipyridine-functionalized dithioester 3 is an efficient RAFT agent. More
importantly, the good agreement between theoretical and 1H NMR-determined molecular weights
indirectly indicates that almost all of the polymer chains are centered by the bipyridine functional groups
derived from the new RAFT agent. Two of the bipyridine-centered polystyrene polymers were further
complexed with ruthenium(II) ions to produce tris(bipyridine)ruthenium(II)-centered polystyrene met-
allopolymers. The metallopolymers were characterized by UV-vis, fluorescence, size exclusion chroma-
tography (SEC), and differential scanning calorimetry (DSC) techniques. The characterization data show
that tris(bipypridine)ruthenium(II) ions have been successfully incorporated into the center of polymer
chains.
Sch em e 1. Mech a n ism of RAF T P olym er iza tion
In tr od u ction
In recent years, there has been increased interest in
metal-containing macromolecules due to their unique
photochemical and electrochemical properties.1-3 Such
materials have found many practical applications as
supported catalysts,4 sensors,5 luminescent films, and
devices6 and for a wide range of other purposes. There-
fore, a number of different synthetic approaches have
been explored to prepare a variety of polymeric metal
complexes. Specifically, these can be divided into four
main types. First, conventional free radical polymeri-
zation, in which ligand7-11 or metal complex-function-
alized12,13 monomers are copolymerized with other
comonomers or homopolymerized. For example, employ-
ing this approach, Schubert has synthesized a series of
vinyl polymers bearing bipyridine or terpyridine func-
tional side chains.8,13 Also, Tew prepared random co-
polymers containing terpyridine functional group by this
means,11 and Fre´chet et al prepared copolymers con-
taining tris(bipyridine)ruthenium(II) (Ru(bpy)3(II)2+).9
More interestingly, thermosensitive hydrogels contain-
ing Ru(bpy)3(II)2+ have been prepared via conventional
radical copolymerization.12 Second, condensation
polymerization/ coupling reaction by which bipyridine-
backboned polymers are generally produced.14 Third,
end-group functionalization in which terpyridine or
bipyridine mono- or difunctionalized oligomers first
prepared by general chemical reactions and then further
interacted with metal ions to form supramolecular
metallopolymers.15-17 Fourth, living or controlled
polymerization in which bipyridine- or terpyridine-
functionalized initiators with or without metal ions are
used for atom transfer radical polymerization (ATRP)18-20
or ring-opening polymerization (ROP).19,21 Living poly-
merizations allow for exquisite control over molecular
weight and architecture in organic polymers. Moreover,
the properties of the final metallopolymers depend
mainly on several variable components: the metal
complex, its ligand set, as well as molecular weight and
architecture of the polymer itself. Thus, living polym-
erization is a powerful tool for synthesis of metal-
containing polymers with well-defined binding sites and
architecture.
There has recently been a surge in interest in revers-
ible addition-fragmentation chain transfer (RAFT)
polymerization due to its applicability to a wide range
of monomers under a broad range of experimental
conditions as compared with ATRP and ROP tech-
niques.22-27 Since its invention by the CSIRO group,22
numerous investigations have been performed to elu-
cidate RAFT mechanism and kinetics. Rizzardo et al.28
proposed the RAFT mechanism, in which a propagating
•
radical Pm is added to a dormant species SdC(Z)-S-
Pn to form an adduct radical Pm-S-C•(Z)-S-Pn, which
can be fragmented from either side into a dormant
•
species Pm-S-C(Z)dS and a propagating radical Pn
(where m and n are interchangeable and are equal to
0, 1, 2, ...) (eq 1 in Scheme 1). However, there has been
an ongoing debate regarding the mechanism that causes
the rate retardation phenomena observed in some RAFT
polymerization systems.29 Two opposing hypotheses
have been proposed for the retardation. One is slow
fragmentation in which the adduct radicals are sug-
gested to be rather stable and have long lifetime,30 and
* To whom correspondence should be addressed: Tel 404-880-
6883; Fax 404-880-6890; e-mail iharruna@cau.edu.
10.1021/ma049560z CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/17/2004

Macromolecules, Vol. 37, No. 19, 2004
Tris(2,2′-bipyridine)ruthenium(II)-Centered Polystyrenes 7133
3.0 mg/mL. The SEC system was calibrated using a narrow
polystyrene standard (Mn ) 92 600, Mw ) 95 050, Mp ) 94 400,
Mw/Mn ) 1.03). Differential scanning calorimetry (DSC) mea-
surements were performed using a Seiko DSC 220C, which
was calibrated for both temperature and enthalpy using an
indium standard, under a nitrogen gas atmosphere with
powder samples, heating from 30 to 250 °C at a heating rate
of 10 °C/min. The glass transition temperature, Tg, was defined
as the midpoint of the change in heat capacity.
the other is cross-termination in which the adduct
radicals terminate with themselves or with the propa-
gating radicals.31 No matter how the RAFT mechanism
actually operates, the overall process results in mono-
mer units being inserted into the RAFT agent structure
(eq 2 in Scheme 1). The presence of RAFT agent 1
fragments at chain ends of the resulting polymer 2 has
been confirmed by several research groups using matrix-
assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF-MS).32
Syn th esis of 5,5′-Bis(th ioben zoylth iom eth yl)-2,2′-bi-
p yr id in e (3). A solution of 1.539 g (4.50 mmol) of 5,5′-bis-
(bromomethyl)-2,2′-bipyridine (3a ) and 1.370 g (18.00 mmol)
of thiourea in 100 mL of ethanol-acetone mixture solvents
(1:1, v/v) was heated at reflux for 20 h. After cooling, the white
precipitate was filtered, washed with acetone, and dried to give
The structure of RAFT agent 1 is crucial in controlling
the radical polymerization, and its effectiveness strongly
depends on the nature of the groups Z and R. The choice
of the Z group should favor not only the formation but
also fragmentation of adduct radicals and can avoid
inducing severe rate retardation as well.23 In light of
the ongoing mechanism debate, it is best that the Z
group is a nonstabilizing group such as benzyl.33 The
selection of the R group is also important in RAFT
polymerization. The R group should be a good homolyti-
cally leaving radical group that should also efficiently
reinitiate polymerization. For this purpose, a variety of
RAFT agents34 including those that are water-soluble,35
multiarmed,36 and polymeric37 have been reported in the
literature for special systems. However, there are very
few reports regarding the synthesis and utilization of
ligand-functionalized RAFT agents.38 In this paper, a
novel bipyridine-functionalized dithioester was prepared
and used as a RAFT agent in the polymerization of
styrene to produce bipyridine-centered polystyrene poly-
mers with different molecular weights and low polydis-
persities. The bipyridine-centered polymers were further
complexed with ruthenium metal ions to form Ru(bpy)3-
(II)2+-centered polystyrene metallopolymers. The met-
allopolymers were further characterized by UV-vis,
fluorescence, size exclusion chromatography (SEC), and
differential scanning calorimetry (DSC) techniques.
1
2.084 g (94%) of 3b; mp 239 °C (dec). H NMR (DMSO-d6): δ
) 9.31 (s, 3H, NH), 9.07 (s, 3H, NH), 8.75 (s, 2H, bpy H6,6′),
8.39 (d, 2H, J ) 8.00 Hz, bpy H3,3′), 8.01 (d, 2H, J ) 8.4 Hz,
bpy H4,4′), 4.63 (s, 4H, CH2S). Anal. Calcd for C14H18N6S2Br2:
C, 34.02; H, 3.67; N, 17.00; S, 12.98; Br, 32.33. Found: C,
34.25; H, 3.78; N, 16.76; S, 12.77; Br, 32.26.
A mixture of 2.084 g (4.22 mmol) of 3b and 1.350 g (33.75
mmol) of NaOH in 100 mL of water was heated at 70 °C for
1.0 h. After cooling, 2.060 g (9.70 mmol) of carboxymethyl
dithiobenzoate (CMDB) was added. The mixture was stirred
at room temperature for 21 h. The red precipitate was filtered
and washed with water and dried under vacuum to give 1.857
g (90%) of 3. The crude product was further purified by
1
extraction using toluene to afford pure 3; mp 183-184 °C. H
NMR (CDCl3): δ ) 8.70 (s, 2H, bpy H6,6′), 8.35 (d, 2H, J )
8.40 Hz, bpy H3,3′), 8.00 (d, 4H, J ) 8.40 Hz, aryl H2,2′ and
H6,6′), 7.82 (d, 2H, J ) 8.0 Hz, bpy H4,4′), 7.53 (m, 2H, aryl H4,4′),
7.38 (q, 4H, J ) 7.6 Hz, aryl H3,3′ and H5,5′), 4.65 (s, 4H, CH2S).
13C NMR (CDCl3): δ ) 226.7 (CdS), 155.10 (bpy C2,2′), 149.8
(bpy C6,6′), 144.6 (aryl C1,1′), 137.6 (bpy C4,4′), 132.7 (aryl C4,4′),
131.6 (bpy C5,5′), 128.5 (aryl C2,2′), 127.0 (aryl C3,3′), 120.9 (bpy
C3,3′), 38.7 (CH2S). HRMS (EI): calcd for C26H20N2S4 m/z )
488.0509; found 488.0508.
P olym er iza tion P r oced u r e a n d Kin etics. A series of
parallel polymerizations were performed under argon. Polym-
erization mixtures were prepared according to the recipes
shown in Table 1. The conversions of the styrene monomer
were determined by the gravimetric method. The following
procedure is typical. RAFT agent 3 (58.0 mg, 11.9 ×10-2 mmol)
and styrene (2.670 g, 25.6 mmol) were added into a 10 mL
Schlenk flask. The polymer mixture was degassed through four
freeze-thaw-evacuate cycles, sealed under argon, and heated
at 100 °C for 6 h. The resulting polymerization mixture was
poured into a large excess of methanol to precipitate the
resulting polymer. The polymer was collected by filtration and
dried under vacuum at 50 °C overnight to yield 0.711 g of a
pink polymer P S-L3-1. The conversion of the monomer styrene
was determined to be 24.5%. 1H NMR (400 MHz, CDCl3): δ )
8.40-8.10 (br, m, bpy), 8.00-7.75 (br, m, Ph-CdS), 7.50-6.20
(br, m, Ph), 2.40-1.10 (br, m, CH2CH). SEC: Mn ) 8490, Mw
) 8560, Mw/Mn ) 1.01.
Exp er im en ta l Section
Ma ter ia ls. Thiourea (99%), carboxymethyl dithiobenzoate
(CMDB, 99%), 1,2-dimethoxyethane (DME, 99%), cis-bis(2,2′-
bipyridine)dichlororuthenium(II) hydrate [Ru(bpy)2Cl2‚xH2O],
and silver hexafluorophosphate (AgPF6, 99.99%) were pur-
chased from Aldrich Co. and used as received. Styrene (99%,
Aldrich) was purified by washing with an aqueous solution of
NaOH (5 wt %) to remove inhibitor, followed by distilled water
until the washings were neutral to litmus and fractionally
distilled under vacuum. 5,5′-Bis(bromomethyl)-2,2′-bipyridine
(3a ) was synthesized from 5,5′-dimethyl-2,2′-bipyridine as
described by Vogtle et al.39 All other reagents and solvents
were used as received unless indicated otherwise.
Ch a r a cter iza tion . 1H and 13C nuclear magnetic resonance
(NMR) spectra were recorded on a Bruker ARX400 spectrom-
eter at 400 Hz and 100 MHz, respectively. UV-vis spectra
were recorded using a Beckman DU640 spectrophotometer.
Fluorescence spectra were recorded on a Photon Technology
International (PTI) spectrofluorimeter. High-resolution mass
spectrometry (HRMS) was performed on a VG Instruments
70SE using electron impact (EI) ionization. Elemental analysis
was performed by Atlantic Microlab, Inc., Norcross, GA.
Melting points were measured with a Thomas-Hoover capillary
melting point apparatus without calibration. Size exclusion
chromatography (SEC) was carried out on a Viscotek SEC
assembly consisting of a model P1000 pump, a model T60 dual
detector, a model LR40 laser refractometer, and three mixed
bed columns (pore size: 10 µm; the molecular weight range
for those columns is 1000-5 000 000) from America Polymer
Standards Corp. using THF as an eluent with a flow rate of
1.0 mL min-1 at ambient temperature. Polymer concentrations
for SEC experiments were prepared in a concentration of about
Syn th esis of Ru (bp y)3(II)2+-Cen ter ed P olystyr en e.40
Ru(bpy)2Cl2‚xH2O (55.0 mg, 108.6 µmol) was dissolved in
methanol (15 mL), and then AgPF6 (170.5 mg, 674.4 mmol)
was added. The purple-red solution was heated at reflux under
argon for 19 h. The resultant reddish methanol solution
containing a white precipitate was filtered, and then the
filtrate was transferred into a 25 mL Schlenk flask and
evaporated to almost dryness. Bipyridine-centered polystyrene
P S-L3-1 (200.0 mg, 33.4 µmol based on the molecular weight
of Mn,theory ) 5992 g/mol, Mw/Mn,SEC ) 1.01), DME (10 mL),
and methanol (5 mL) were added into the flask. The reaction
mixture was deoxygenated by purging with argon for half hour
and then heated at reflux for 3 days. The reaction was cooled
to room temperature and then evaporated to almost dryness,
followed by the addition of methanol. The precipitated complex
was filtered and washed with methanol to remove unreacted
ruthenium complex to give 233.0 mg of a brown product. The
crude complex was dissolved in THF (10 mL) to give a clear
solution, and then hexane (35 mL) was added dropwise into

7134 Zhou and Harruna
Macromolecules, Vol. 37, No. 19, 2004
Ta ble 1. Exp er im en ta l Con d ition s a n d Ch a r a cter iza tion Da ta for Bu lk P olym er iza tion of Styr en e a t 100 °C w ith th e
Bip yr id in e-F u n ction a lized Dith ioester 3 a s a Ch a in Tr a n sfer Agen t (CTA) via Th er m a l Au toin itia tion
b
b
b
c
d
styrene
(mmol)
3a
time
(h)
convn
(wt %)
Mn
Mw
Mw/Mn
Mn
Mn
sample
(mmol × 10-2
)
(SEC)
(SEC)
(SEC)
(theory)
(NMR)
P S-L3-1
P S-L3-2
P S-L3-3
P S-L3-4
P S-L3-5
P S-L3-6
25.6
12.8
12.9
13.0
12.9
13.1
11.9
5.9
6.0
6.0
6.0
6.0
6
15
10
12
7
24.5
48.6
35.7
42.9
27.4
60.5
8 490
16 800
12 300
15 000
9 220
8 560
17 400
14 800
19 000
9 390
1.01
1.04
1.20
1.27
1.02
1.15
5 992
11 426
8 532
10 175
6 653
6 157
11 622
8 657
10 241
6 807
20
21 500
24 800
14 293
14 595
a
b
The molar amount of dithioester end group is 2 times the value of the CTA 3 shown in Table 1. Determined by SEC using THF as
eluent on the basis of a polystyrene calibration curve. c The theoretical molecular weight was calculated from the expression ([M]i/[CTA]i)CM0
+ MCTA, where [M]i and [CTA]i are the initial concentrations of the monomer styrene and the CTA 3, respectively, C is the fractional
conversion, and M0 and MCTA are the molecular weights of the styrene monomer and the used CTA 3, respectively. Determined by 1H
d
NMR spectroscopy according to the equation Mn,NMR ) [(I1.10-2.40 ÷ 3)/(I8.10-8.40 ÷ 4)] × 104.15 + 488.71, where I1.10-2.40 and I8.10-8.40 are
integral values of the peaks at δ ) 1.10-2.40 and 8.10-8.40; 104.15 and 488.71 are the molecular weights of styrene and 3, respectively
(cf. Figure 4).
Sch em e 2. Syn th etic Rou te for
Bip yr id in e-F u n ction a lized RAF T Agen t
the clear polymer solution. As a result, the complex was
precipitated out and the uncomplexed polystyrene was left in
solution. After the removal of the clear solution by pipet, the
precipitate was washed with diethyl ether twice and then
stirred in methanol for 2 days. The pure supramolecular
polystyrene-ruthenium complex P S-L3-1-Ru was obtained by
filtration, washed in turn with methanol and water, and dried
in an oven under vacuum. Yield: 179.2 mg, 80%. 1H NMR
(CDCl3): δ ) 8.64-8.09 (br, m, bpy), 8.09-7.8 (br, m, bpy),
7.80-7.34(br, m, bpy and Ph-CdS) 7.31-6.25 (br, m, Ph),
2.45-1.15 (br, m, CH2CH). UV-vis (CHCl3): λmax MLCT (ꢀ)
) 451 nm (15 286 M-1 cm-1). UV-vis (CH2Cl2): λmax MLCT
(ꢀ) ) 452 nm (14 640 M-1 cm-1).
P S-L3-2-Ru was similarly prepared as described above
using the following reagent loadings and purification condi-
tions. For formation of P S-L3-2-Ru : Ru(bpy)2Cl2‚xH2O (55.0
mg, 108.6 µmol), AgPF6 (170.5 mg, 674.4 µmol), and P S-L3-2
(300.0 mg, 26.3 µmol based on the molecular weight of Mn,theory
) 11 426 g/mol, Mw/Mn,SEC ) 1.04). For purification of P S-L3-
2-Ru : 12 mL of tetrahydrofuran (THF) vs 35 mL of hexane.
1
Yield: 235.0 mg, 74%. H NMR (CDCl3): δ ) 8.66-8.14 (br,
m, bpy), 8.14-7.82 (br, m, bpy) 7.82-7.33 (br, m, bpy and Ph-
CdS), 7.31-6.25 (br, m, Ph), 2.45-1.10 (br, m, CH2CH). UV-
vis (CHCl3): λmaxMLCT (ꢀ) ) 451 nm (14 963 M-1 cm-1). UV-
spectrum, the resonances labeled 1, 2, and 4 (chemical
shift δ ∼ 8.70, 8.35, and 7.83 ppm, respectively) repre-
sent the aromatic protons of the bipyridine functional
group, those labeled 3, 5, and 6 (δ ∼ 8.00, 7.52, and 7.38
ppm, respectively) represent aromatic protons of the
phenyl group from the starting reactant CMDB, and
those labeled 7 represent the methylene protons in the
junction of the bipyridine group. Similarly, the peaks
are assigned and labeled in the 13C NMR spectrum.
Moreover, the exact mass of the RAFT agent 3 was
determined to be 488.0508, which is in excellent agree-
ment with the calculated value m/z ) 488.0509 for
C26H20N2S4. (The HRMS spectrum is given in Figure
S1 in the Supporting Information.) These show that the
synthesized compound is the target RAFT agent 3.
vis (CH2Cl2): λ maxMLCT (ꢀ) ) 452 nm (14 547 M-1 cm-1).
Resu lts a n d Discu ssion
Syn t h esis of Bip yr id in e-F u n ct ion a lized R AF T
Agen t. The key to successful RAFT polymerization is
the presence of a suitable chain transfer agent (CTA),
which is typically a dithioester. These CTAs, with the
exception of carboxymethyl dithiobenzoate (CMDB), are
not commercially available. Using CMDB, ester ex-
change reactions between dithiocarboxylic acid and
thiols have been applied to the synthesis of dithioesters
such as tert-butyl dithiobenzoate.41 This synthetic route
is quite simple, sometimes consisting of one-step pro-
cedure. Therefore, the ester exchange approach was
used to synthesize a novel bipyridine-functionalized
RAFT agent. Scheme 2 shows a synthetic route for the
synthesis of bipyridine-functionalized RAFT agents.
First, the bromomethyl groups of 3a were reacted with
thiourea under reflux using ethanol/acetone (1:1 v/v) as
a reaction media to give the corresponding isothiourea
hydrobromide salts 3b. The isothiourea hydrobromide
salts of 3b were hydrolyzed into the corresponding thiols
in the presence of a base catalyst at 70 °C, and then
the dithiol 3c was reacted in situ with CMDB to afford
the desired RAFT agent 3. The crude RAFT agent 3 was
further purified by extraction with toluene to produce
pure 3 in good yield.
RAF T P olym er iza tion of Styr en e. To examine the
effectiveness of the novel dithioester 3 as a RAFT agent,
a series of styrene polymerizations were carried out in
bulk at 100 °C for different durations. All experiments
were conducted with ∼0.04 M RAFT agent 3. The
polymerization condition and results (loading amounts,
molecular weights, polydispersities, and conversion) of
these experiments are summarized in Table 1.
The evolution of molecular weight is clearly observed
as peak shifts to shorter elution times, as determined
by SEC (Figure 2). Furthermore, the pseudo-first-order
rate plot (Figure 3A) is linear; the molecular weights
determined by SEC and NMR respectively increased
linearly with conversion as shown in Figure 3B. The
Figure 1 shows the 1H and 13C NMR spectra, recorded
1
in CDCl3, of the purified RAFT agent 3. In the H NMR

Macromolecules, Vol. 37, No. 19, 2004
Tris(2,2′-bipyridine)ruthenium(II)-Centered Polystyrenes 7135
F igu r e 3. (A) Pseudo-first-order kinetic plot of ln([M]0/[M])
vs polymerization time and (B) number-average molecular
weight and polydispersity as a function of conversion for bulk
polymerization of styrene by RAFT using 3 as a transfer agent
at 100 °C via thermal autoinitiation (Table 1, sample P S-L3-1
to P S-L3-6). [Styrene] ) 8.70 M, [3] ) 0.04 M, [styrene]/[3] )
218/1.
F igu r e 1. 1H NMR (upper) and 13C NMR (lower) spectra in
CDCl3 of the bipyridine-functionalized RAFT agent 3.
polystyrenes with low polydispersity and controlled
molecular weight. In addition to styrene, RAFT polym-
erization of N-isopropylacrylamide was also controlled
using the RAFT agent 3 to produce bipyridine-centered
thermosensitive polymers.42
The new transfer agent 3 mediated RAFT polymeri-
zation of styrene produces polystyrene with a structure
in which the bipyridine functional group is located in
the center of polystyrene chains (see Scheme 3). These
results are similar to those obtained by using other
difunctional transfer agents with analogous structure
such as 1,4-bis(2-(thiobenzoylthio)prop-2-yl)benzene.43
To confirm the existence of the bipyridine functional
group derived from the transfer agent 3 in the polysty-
rene, the prepared polystyrene (P S-L3-1 in Table 1) was
F igu r e 2. SEC chromatograms (RI traces) of polystyrenes
prepared by bulk RAFT polymerization of styrene using the
bipyridine-functionalized dithioesters 3 as a chain transfer
agent at 100 °C via thermal autoinitiation for different reaction
times (Table 1, samples P S-L3-1 to P S-L3-6). [Styrene] ) 8.70
M, [3] ) 0.040 M, [styrene]/[3] ) 218/1.
1
1
analyzed by H NMR. Its H NMR spectrum is shown
in Figure 4. Peaks a and b at 8.10-8.40 ppm were
attributed to the central bipyridine group derived from
the CTA 3, and the peak c is assigned to the charac-
teristic peak of the dithiobenzoate end group derived
similarly from the CTA 3. The characteristic peaks a
and b can be used to calculate the molecular weight of
the RAFT-prepared polystyrene with the assumption
that each polystyrene chain was centered by a bipyri-
dine group derived from the RAFT agent 3. In addition,
the theoretical molecular weight can be calculated by
the conversion of styrene monomer. Both theoretical and
1H NMR determined molecular weights of all prepared
polystyrenes are given in Table 1. The good agreement
between Mn,theory and Mn,NMR shows that almost all the
polystyrene chains are centered by the bipyridine
functional group derived from the RAFT agent 3 used
for polymerization.
most notable observation from Figure 3B is the marked
deviation of the SEC molecular weight from the theo-
retical molecular weight. However, the NMR-deter-
mined molecular weight is in good agreement with the
theoretical molecular weight (Figure 3B and Table 1).
This indicates that the molecular weights determined
by SEC are not absolute molecular weights. The main
cause for this discrepancy is that a polystyrene standard
(Mn ) 92 600) of higher molecular weight than the
measured one (Mn < 15 000) was used for calibration.
The polydispersities are generally low, ranging from
1.01 to 1.27 at all polymerization times. In all cases,
the polydispersity index (PDI) remains well below the
theoretical limit of 1.5 for conventional free radical
polymerization. All these results indicate that the new
RAFT agent 3 is quite efficient for the preparation of

7136 Zhou and Harruna
Macromolecules, Vol. 37, No. 19, 2004
F igu r e 5. UV-vis spectra (CHCl3) of the supramolecular
polystyrene ruthenium complexes P S-L3-1-Ru and P S-L3-2-
Ru and their corresponding bipyridine-centered precursor
polymers P S-L3-1 and P S-L3-2 as well as the starting complex
Ru(bpy)2Cl2‚xH2O.
F igu r e 4. The 400 MHz 1H NMR spectrum (CDCl3) of the
RAFT-prepared polystyrene P S-L3-1 using 3 as a RAFT agent.
precipitating its corresponding precursor polymer
(øw ) 3.74 for P S-L3-1 and 2.48 for P S-L3-2), then the
crude polymer was dissolved in THF weighed in advance
to give a clear solution, and then a suitable amount of
hexane was added dropwise into the clear polymer
solution. (The amount of hexane added can be calculated
using øw and the amount of THF used.) As a result, the
complex precipitated and the uncomplexed polystyrene
was left in solution. The pure complex was obtained
after the removal of the clear solution by pipet. The
purification procedure was repeated to yield pure com-
plexed polystyrene. The purified complex was dried in
an oven under reduced pressure.
Sch em e 3. Sch em a tic Rep r esen ta tion for P r ep a r a tion
of Tr is(2,2′-bip yr id in e)r u th en iu m (II)-Cen ter ed
P olystyr en e Meta llop olym er s
The purified complexes were characterized by both
UV-vis spectroscopy and fluorescence techniques. Their
UV-vis spectra (Figure 5) exhibited a new absorption
band at 451 nm attributed to the characteristic MLCT
(metal-to-ligand charge transfer) absorption band of the
tris(bipyridine)Ru(II) unit as compared with their cor-
responding precursor polymers and the starting complex
Ru(bpy)2Cl2‚xH2O. Moreover, the fluorescence spectra
of the two complexes P S-L3-1-Ru and P S-L3-2-Ru are
different from that of their corresponding precursor
polymers P S-L3-1 and P S-L3-2 (Figure 6). The emis-
sion spectra of the complexes show only the well-known
[Ru(bpy)3]2+ band with λmax ) 625 nm, which can be
excited by multiband lights, λex ) 300 and 465 nm, as
observed from their excitation spectra. However, the
emission spectra of the precursor P S-L3-1 and P S-L3-2
show a band with λmax ) 350 nm. The optical properties
of the two complexes are summarized in Table 2. They
indicate the successful grafting of the ruthenium bis-
(bipyridine) onto the center of the polymer.
To estimate the extent of the metal complex forma-
tion, Fraser introduced a concept “chelation efficiency”
based on the findings that extinction coefficients for
polymeric metal complexes are comparable to that of
their corresponding nonpolymeric complex and vary
little with polymer composition.45,46 Here, we adopted
Fraser’s chelation efficiency method to estimate the
ruthenium content in the metallopolymers. Chelation
efficiency was defined as the ratio of the extinction
coefficiency determined from λmax for the polymeric
metal complex with that of [Ru(bpy)3](PF6)2. Extinction
coefficients (ꢀ) of the polymeric metal complexes were
measured by UV-vis analysis using theoretically cal-
culated number-average molecular weights to determine
polymer concentrations. The extinction coefficient (ꢀ) for
the standard complex [Ru(bpy)3](PF6)2 is reported to be
Tr is(b ip yr id in e)r u t h en iu m Com p lex Cen t er ed
P olyst yr en es. Ruthenium(II) tris(bipyridine) com-
plexes, [Ru(bpy)3]2+ and related derivatives are ubi-
,
quitous in chemistry.44 The stability and unique pho-
tophysical properties of these systems have been ex-
ploited for artificial photosynthesis, in sensor, in pho-
torefractive materials, and in the study of electron
transfer in proteins and DNA.44 To form ruthenium
complexes, the bipyridine-centered polystyrenes pre-
pared by RAFT polymerization using the dithioester 3
as a RAFT agent were further complexed with Ru(II)
metal ion to give [Ru(bpy)3]2+-centered polystyrenes
according to the procedure reported by Fraser and co-
workers40 (Scheme 3).
Excess starting complex Ru(bpy)2Cl2‚xH2O was re-
moved by washing using methanol. The uncomplexed
polystyrene can be removed according to the following
typical procedure. The specific weight ratio øw in the
hexane-THF mixture at room temperature, under
which the complex can be precipitated out, was first
determined (øw ) 2.50 for P S-L3-1-Ru and 2.20 for P S-
L3-2-Ru ). Meanwhile, øw was also determined for

Macromolecules, Vol. 37, No. 19, 2004
Tris(2,2′-bipyridine)ruthenium(II)-Centered Polystyrenes 7137
Ta ble 2. Absor p tion , Em ission , a n d Th er m a l P r op er ties for Tr is(bip yr id in e)r u th en iu m -Cen ter ed P olystyr en e
Meta llop olym er s a n d Th eir Cor r esp on d in g P r ecu r sor P olym er s
MLCT absorptionb
Mna (calcd)
λ
max (nm)
ꢀ (M-1 cm-1
)
chelation efficiencyd
emissione λmax (nm)
Tgf (°C)
c
sample
P S-L3-1
P S-L3-2
P S-L3-1-Ru
P S-L3-2-Ru
5 992
11 426
6 695
350
350
625
625
102.6
104.5
114.5
110.6
452
452
14 640
14 547
0.92
0.91
12 129
a
The calculated molecular weights of the precursor polymers are their theoretical molecular weights shown in Table 1. The molecular
weights of the ruthenium metallopolymers are calculated according to the equation Mn,calcd ) Mn,calcd(PS) + M[Ru(bpy) ][PF ] , where Mn,calcd(PS)
2
6 2
is the calculated molecular weights of the corresponding precursor polymer and M[Ru(bpy) ][PF is the molecular weight of [Ru(bpy)2][(PF6)2]
]
2
6 2
b
(bpy ) bipyridine) grafted on the center of the precursor polymer. Metal-to-ligand charge transfer (MLCT) absorption band of the
metallopolymer recorded in CH2Cl2 solution. c Molar absorptivity of the MLCT band of the metallopolymer. The estimated chelation
d
efficiency: ꢀMLCT[Ru(bpy)2(bpyPS2)]2+[PF6]22-/ꢀMLCT[Ru(bpy)3]2+[PF6]2 (bpyPS2 ) P S-L3-1 for P S-L3-1-Ru ; P S-L3-2 for P S-L3-2-Ru ).
2-
λmax [Ru(bpy)3]2+[PF6]2 in CH2Cl2 ) 452 nm, ꢀ ) 16 000 M-1 cm-1 e THF solution. f The glass transition temperature (defined as the
.
2-
midpoint of the change in heat capacity) measured by DSC under nitrogen.
F igu r e 7. DSC curves of the supramolecular polystyrene
ruthenium complexes P S-L3-1-Ru and P S-L3-2-Ru as well
as their corresponding bipyridine-centered polystyrene P S-
L3-1 and P S-L3-2.
proving that the purification procedure for the polymeric
complexes is quite efficient. On the other hand, this
shows that standard size exclusion chromatography
(SEC) is rather difficult to analyze bipyridine metal
complex containing polymers because of strong interac-
tions (caused by the nitrogen atoms and the charge of
the metal ions) of the compounds with the SEC column
materials (most polystyrene/divinylbenzene cross-linked
beads). Such a SEC problem had been met by the other
authors.20,48 Nevertheless, some examples of successful
analysis are reported in the literature.45,49 However, the
SEC conditions for successful analyses are different
from that used here. Furthermore, the molecular weights
of the prepared polymeric complexes are relatively low;
thus, the interactions of the charged compounds with
SEC columns are stronger. These are probable causes
why no SEC signal is observed in the SEC analysis of
the polymeric complexes. However, the difference be-
tween the SEC curves of the complex and its corre-
sponding bipyridine-centered polymer indicated that the
complex was formed.
Since the change of the microstructure of the polymer
chains can affect the flexibility of the polymer chains,
which is only one contributing factor to glass transition
temperature, it is possible to observe the change in the
chain structure through the measurement of the change
of glass transition temperature. Therefore, thermal
characterizations of the two polystyrenic complexes P S-
L3-1-Ru and P S-L3-2-Ru as well as their correspond-
ing precursor polymers P S-L3-1 and P S-L3-2 were
performed by DSC. The DSC curves are displayed in
Figure 7. In all cases, glass transitions were evident in
the DSC curves at higher than 100 °C, corresponding
to standard polystyrenes in the low molecular weight
F igu r e 6. Fluorescence emission spectra (THF) (excitation
wavelength λex ) 300 and 465 nm, respectively) and excitation
spectra (emission wavelength λem ) 625 nm for complex, 350
nm for precursor polystyrene) of the supramolecular polysty-
rene-ruthenium complexes P S-L3-1-Ru (A) and P S-L3-2-Ru
(B) as well as their corresponding bipyridine-centered poly-
styrene P S-L3-1 (A) and P S-L3-2 (B).
16 000 M-1 cm-1 in CH2Cl2 (λmax ) 452 nm).47 The
literature value was used to calculate the chelation
efficiencies of the two polymeric complexes. These data
are summarized in Table 2. For each polymeric complex,
the extent of the metal complex formation reached above
90% but less than 100%. To further exclude the residue
of the uncomplexed precursor polystyrene in the com-
plexes, the purified complexes were further character-
ized using the SEC technique. Their SEC curves are
different from those of the corresponding precursor
polymers (Figure S2 in the Supporting Information).
There are no signals corresponding to either the complex
or the precursor polymer in their SEC curves, which
were although recorded by three different detectors. On
one hand, this indicates that there is no precursor
polymer remaining in the purified complex, thus further

7138 Zhou and Harruna
Macromolecules, Vol. 37, No. 19, 2004
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range. The glass transition temperature (Tgs) of these
samples are listed in Table 2. As can be seen from Table
2, the glass transition temperatures of all polymer
complexes are higher than their corresponding precur-
sor polymers due to the effect of the hexacoordinated
tris(bipyridine)ruthenium(II) complex incorporated into
the center of the polymer chain. Similar findings have
been previously reported.20 The observed elevation of
Tg due to metalation is larger for lower molecular weight
systems.
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Con clu sion s
A novel bipyridine-functionalized chain transfer agent
3 has synthesized by a convenient multistep approach,
which includes the reaction with thiourea, hydrolysis,
and thioester exchange, starting from a commercially
available ligand. The structure of the new chain transfer
agent 3 was confirmed by 1H NMR, 13C NMR, and
HRMS techniques. The new bipyridine-functionalized
dithioester 3 was further used as a RAFT agent in the
RAFT polymerization of styrene. Kinetics studies show
that it is a very efficient RAFT agent for polymerization
of styrene. Excellent control of the molecular weight and
very narrow molecular weight distribution were at-
tained. The 1H NMR spectrum confirmed the RAFT-
prepared polymer is centered by bipyridine functional
groups. The bipyridine-centered polystyrene polymers
were further reacted with Ru(bpy)2(II) ion to produce
metallopolymers. Successful metalation was confirmed
by UV-vis, fluorescence, SEC, and DSC techniques.
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Ack n ow led gm en t . The financial support for this
research work from the Office of Naval Research (ONR
Grant N00014-01-1-1042) is gratefully acknowledged.
We thank Dr. Airan Perez of the Office of Naval
Research, Arlington, VA, for helpful discussions. Also,
we thank Dr. David Bostwick, Georgia Institute of
Technology, for HRMS analysis.
Su p p or tin g In for m a tion Ava ila ble: HRMS spectrum of
the new bipyridine-functionalized RAFT agent 3 and SEC
curves, recorded on three different detectors (refractive index
(RI) detector, viscometer (DP) detector, and light scattering
(LS) detector), of the supramolecular polystyrene-ruthenium
complexes P S-L3-1-Ru and P S-L3-2-Ru as well as their
corresponding bipyridine-centered polystyrene P S-L3-1 and
P S-L3-2. This material is available free of charge via the
Internet at http://pubs.acs.org.
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