Synthesis and characterization of bis(2,2′:6′,2″- terpyridine)ruthenium(II)-connected diblock polymers via RAFT polymerization

Article abstract of DOI:10.1021/ma047955c

We report the synthesis and uses of a novel terpyridine-functionalized chain transfer agent (CTA) that produces well-controlled macromolecular architectures with terpyridine functionalities at one chain end via reversible addition-fragmentation chain tran

Full text of DOI:10.1021/ma047955c

4114
Macromolecules 2005, 38, 4114-4123
Synthesis and Characterization of
Bis(2,2′:6′,2′′-terpyridine)ruthenium(II)-Connected Diblock Polymers via
RAFT Polymerization
Guangchang Zhou and Issifu I. Harruna*
Department of Chemistry, Clark Atlanta University, 223 James P. Brawley Drive SW,
Atlanta, Georgia 30314
Received October 4, 2004; Revised Manuscript Received March 7, 2005
ABSTRACT: We report the synthesis and uses of a novel terpyridine-functionalized chain transfer agent
(CTA) that produces well-controlled macromolecular architectures with terpyridine functionalities at one
chain end via reversible addition-fragmentation chain transfer (RAFT) polymerization. The terpyridine-
terminated macromolecules with well-defined structures were further used for preparation of supra-
molecular diblock metallomacromolecules by bis(2,2′:6′,2′′-terpyridine)ruthenium(II) complex connectivity.
The successful connectivity between two macromolecular blocks via the metallocomplex was confirmed
by UV-vis, size exclusion chromatography (SEC), and differential scanning calorimetry (DSC) as well
as atomic force microscopy (AFM) techniques.
Introduction
erization is accessible to control molecular weight and
molecular weight distribution19 as well as advanced
architectures such as block,20 comb,21 and star22 copoly-
mers. In contrast, the RAFT process is distinguished
by its versatility with respect to both monomer choice
and polymerization conditions. The key to successful
operation of RAFT polymerization lies on the use of a
highly efficient chain transfer agent (CTA), which is
typically a thiocarbonylthio compound.23 In general, the
vast majority of RAFT-prepared (co)polymer chains bear
CTA fragment groups at both ends.24 This point has
been confirmed by NMR and matrix-assisted laser
desorption ionization time-of-flight mass spectrometry
(MALDI-TOF-MS) techniques.25,26 For utilizing the
feature of RAFT technique, few bipyridine-functional-
ized RAFT agents have recently been synthesized and
used to prepare metal-containing polymers.27,28 For
years, the 2,2′:6′,2′′-terpyridine ligand has been of
interest in the assembly of metallomacromolecules and
metallosupramolecules because of its chelating effect for
a wide range of transition metal ions.29 In this paper, a
new terpyridine-functionalized dithioester was first
synthesized and further used as a RAFT agent in the
polymerization of styrene and N-isopropylacrylamide
(NIPAM) to generate terpyridine end-functionalized
well-defined low-polydispersity polystyrene and poly(N-
isopropylacrylamide) (PNIPAM) polymers. The RAFT-
prepared terpyridine-terminated polymers were further
used for preparation of supramolecular diblock metal-
lopolymers by Ru(tpy)2(II)2+ complex connectivity. The
prepared supramolecular metallopolymers were further
characterized by UV-vis, size exclusion chromatogra-
phy (SEC), and differential scanning calorimetry (DSC)
as well as atomic force microscopy (AFM) techniques.
Metal-containing polymers are attracting significant
interest as these materials may combine the process-
ability and mechanical properties of polymers with the
unique optoelectronic properties of metal complexes.1,2
The development of metal-containing polymers with
unique property profiles has been propelled by their
(potential) use in diverse areas such as solar energy
conversion,3 luminescent sensing,4 electroluminescence
display,5 biotechnology,6 molecular machines,7 and mo-
lecular electronics.8 Thus, a number of metal-containing
polymers, whose metal moiety is in either the polymer
backbone or the side chain, have been reported in the
literature.9-11 In particular, metal-containing polymers
with well-defined architectures are currently under
extensive investigation.12-15 However, most synthetic
approaches to date have involved the construction of
systems with broad molecular weight distributions and
lack of control over polymer architecture. To overcome
these drawbacks, living and controlled polymerization
techniques have been employed.12-15 For example,
Fraser et al.14 synthesized a variety of metal-centered
linear and star-shaped metallopolymers using metallo-
supramolecular or metal-free bipyridine-functionalized
initiators by atom transfer radical polymerization (ATRP)
and ring-opening polymerization (ROP); Schubert et
al.15 prepared terpyridine end-group-functionalized poly-
styrene polymers using a terpyridine-functionalized
initiator via nitroxide-mediated polymerization (NMP).
Another approach to well-defined metal-containing poly-
mers involves the preparation of ligand-terminated
monochelic or telechelic polymers or oligomers with
well-defined structures by simple organic reactions,
followed by their chelation to metal ions.16-18 For
example, Schubert et al.16-18 prepared some bis(2,2′:
6′,2′′-terpyridine)ruthenium(II) (Ru(tpy)2(II)2+) complex-
connected supramolecular block (co)polymers.
Experimental Section
Materials. Thiourea (99%, Aldrich), carboxymethyl dithio-
benzoate (CMDB, 99%, Aldrich), ammonium hexafluorophos-
phate (NH4PF6, 99.99%, Aldrich), ruthenium(III) chloride
hydrate (RuCl3‚xH2O, 35-40% Ru, ACROS), N-ethylmorpho-
line (99%, ACROS), and 1,4-dioxane (99+%, spectrophotomet-
ric grade, Aldrich) were used as received. 2,2′-Azobis(isobu-
tyronitrile) (AIBN, 97%, Aldrich) was purified by recrystal-
In addition to ATRP, ROP, and NMP, reversible
addition-fragmentation chain transfer (RAFT) polym-
* Author to whom correspondence should be addressed. E-
mail: iharruna@cau.edu. Telephone: 404-880-6883. Fax: 404-880-
6890.
10.1021/ma047955c CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/12/2005

Macromolecules, Vol. 38, No. 10, 2005
Bis(2,2′6′:2′′-terpyridine)ruthenium(II)-Connected Polymers 4115
Table 1. Experimental Conditions and Characterization Data for RAFT Polymerization of Styrene and
N-isopropylacrylamide (NIPAM) with the Terpyridine-Functionalized Dithioester 1 as a CTA
c
d
e
monomer
CTA 1
initiator
time
(hr)
conv
(%)
Mn
Mn
Mn
PDIe
sample
(concn in 1,4-dioxane, °C)
(M × 10-2
)
(M × 10-2
)
(theory)
(NMR)
(SEC)
(SEC)
PS-L1-1
styrene (bulk, 100)
4.00
thermala
4
6
12.5
23.4
33.7
40.1
54.7
18.9
34.3
43.2
52.4
3330
5740
8180
9630
12 930
4700
8150
10 140
12 270
3440
5880
8370
9840
13 180
4770
8270
10 330
12 450
4500
7010
9460
1.06
1.04
1.06
1.18
1.13
1.06
1.08
1.08
1.11
PS-L1-2
PS-L1-3
11
13
20
1.5
2.5
3.3
4.0
PS-L1-4
10 300
13 900
1910
PS-L1-5
PNIPAM-L1-1
PNIPAM-L1-2
PNIPAM-L1-3
PNIPAM-L1-4
NIPAM (4.00 M, 75)
2.02
AIBNb(1.22)
2300
2600
2860
a No added initiator. b Abbreviations: AIBN, 2,2′-azobis(isobutyronitrile). c The theoretical molecular weight was calculated from the
expression: Mn(theory) ) ([M]i/[CTA]i)CM0 + MCTA, where [M]i and [CTA]i are the initial concentrations of the monomer and the chain
transfer agent 1 respectively, C is the fractional conversion, and M0, MCTA are the molecular weights of the monomer and the used RAFT
1
agent 1. d Determined by H NMR spectroscopy, according to two equations: For polystyrene series, Mn,NMR(PS) ) [(I1.0-2.5 ÷ 3)/(I8.35-8.70
÷ 6)] × 104.15 + 475.63; for PNIPAM series, Mn,NMR(PNIPAM) ) [(I4.00/(I8.82 ÷ 6)] × 113.16 + 475.63, where I1.0-2.5, I4.00, I8.35-8.70, and I8.82
are integral values of the peaks at δ ) 1.0-2.5, 4.00, 8.35-8.70, and 8.82, 104.15, 113.16, and 475.63 are the molecular weights of
styrene, NIPAM, and 1, respectively (cf. Figure 4A and B). e Determined by SEC using THF as eluent, and molecular weights were
reported as polystyrene equivalents; PDI ) Mw/Mn.
lization from methanol and stored in a freezer. 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. N-Isopropylacrylamide (NIPAM, 97%,
Aldrich) was purified by recrystallization from a mixture (60:
40, v/v) of toluene and hexane. 4′-(4-Bromomethylphenyl)-2,2′:
6′,2′′-terpyridine (1a) was synthesized according to literature
procedures.30 All other reagents and solvents were used as
received unless indicated otherwise.
dine (1c) (1.324 g, 3.73 mmol) was added to the solution. The
resulting mixture was stirred at 60 °C for 18 h. The red
precipitate was filtered, washed with water, dried under
vacuum, and further extracted with toluene to give crude
product 1 (1.325 g, 75%). The crude product was further
purified by column chromatography on aluminum oxide using
hexanes-ethyl acetate (2:1, v/v) as an eluent to afford pure 1:
mp 166-167 °C. 1H NMR (CDCl3): δ ) 8.75-8.67 (m, 4H,
tpyH3′,5′ and tpyH6,6′′), 8.64 (d, 2H, J ) 8.00 Hz, tpyH3,3′′), 8.01
(dd, 2H, J ) 7.2, 1.2 Hz, SdC-PhH2,6), 7.90-7.78 (m, 4H,
tpyH4,4′′ and tpy-PhH2,6), 7.54-7.47 (m, 3H, tpy-PhH3,5 and Sd
C-PhH4), 7.40-7.28 (m, 4H, tpyH5,5′′ and SdC-PhH3,5), 4.64
ppm (s, 2H, CH2S). 13C NMR (CDCl3): δ ) 227.3 (CdS), 156.1
(tpyC2′,6′), 155.9 (tpyC2,2′′), 149.5 (tpyC4′), 149.1 (tpyC6,6′′), 144.7
(SdC-PhC1), 137.9 (tpy-PhC4), 136.9 (tpyC4,4′′), 136.0 (tpy-
PhC1), 132.4 (SdC-PhC4), 129.8 (tpy-PhC3,5), 128.4 (SdC-
PhC3,5), 127.8 (tpy-PhC2,6), 126.9 (SdC-PhC2,6), 123.9 (tpyC5,5′′),
121.3 (tpyC3,3′′), 118.7 (tpyC3′,5′), 41.9 ppm (CH2S). HRMS
(EI): calcd for C29H21N3S2 m/z ) 475.1177, found m/z )
475.1128.
Characterization. 1H and 13C NMR spectra were recorded
on a Bruker ARX400 spectrometer at 400 and 100 MHz,
respectively. UV-vis spectra were recorded using a Beckman
DU640 spectrophotometer. High-resolution mass spectrometry
(HRMS) was performed on a VG Instruments 70SE using
electron impact (EI) ionization. Melting points were measured
with a Thomas-Hoover capillary melting point apparatus
without calibration. SEC was carried out on a Viscotek SEC
assembly consisting of a model P1000 pump, a model T60
refractive index-viscometer dual detector, a model LR40 laser
refractometer, and three mixed bed columns (bead size: 10
µm, the molecular weight range for those columns is 1000-
5 000 000) from American Polymer Standards Corporation
using THF as an eluent with a flow rate of 1.0 mL min-1 at
ambient temperature. Polymer concentrations for SEC experi-
ments were prepared in a concentration of about 3.0 mg/mL.
The SEC system was calibrated using a narrow polystyrene
standard (Mn ) 9870, Mw ) 10 300, Mp ) 10 300, Mw/Mn
)1.04). DSC measurements 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, at a heating rate of 10 °C/min. The glass
transition temperature, Tg, was defined as the midpoint of the
change in heat capacity. AFM was performed using a Ther-
momicroscope CP Research AFM instrument in tapping mode
with a silicon tip. The AFM image was obtained at room
temperature in air.
Synthesis of 4-[2,2′:6′,2′′]Terpyridin-4′-yl-benzyl Dithio-
benzoate (1). A solution of 4′-(4-bromomethylphenyl)-2,2′:
6′,2′′-terpyridine (1a) (1.609 g, 4.00 mmol) and 0.381 g (5.01
mmol) of thiourea in 130 mL of ethanol-acetone mixture
solvents (1:1, v/v) was heated at reflux for 22.5 h. After cooling,
the precipitate was filtered, washed with acetone, and dried
to give 1.739 g (91%) of 1b. A mixture of 1b (1.739 g, 3.64
mmol) and NaOH (0.160 g, 4.00 mmol) in 40 mL of water was
heated at 70 °C for 4.5 h. After cooling, the precipitate was
filtered and washed with water, and then dried under vacuum
at room temperature overnight to give 4′-(4-mercaptometh-
ylphenyl)-2,2′:6′,2′′-terpyridine (1c) (1.324 g). Carboxymethyl
dithiobenzoate (CMDB) (0.795 g, 3.74 mmol) and NaOH (0.315
g, 7.88 mmol) were dissolved in 95 mL of water to give a clear
solution, then 4′-(4-mercaptomethylphenyl)-2,2′:6′,2′′-terpyri-
Polymerization Procedure and Kinetics. Two series of
parallel polymerizations were performed under argon. Polym-
erization mixtures were prepared according to the recipes
shown in Table 1.
A typical procedure for bulk polymerization of styrene was
as follows. RAFT agent 1 (28.0 mg, 5.89 × 10-2 mmol) and
styrene (1.334 g, 12.8 mmol) were added into a 10 mL Schlenk
flask. The polymer mixture was degassed through four freeze-
evacuate-thaw cycles, sealed under argon, and heated at 100
°C for 4 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 0.195 g of a pink
polymer, PS-L1-1. The conversion of the monomer, styrene,
was gravimetrically determined to be 12.5%. 1H NMR (400
MHz, CDCl3): δ ) 8.84-8.50 (br, m, tpy), 7.94-7.54 (br, m,
Ph-CdS and tpy), 7.50-6.10 (br, m, Ph), 2.50-1.05 ppm (br,
m, CH2CH). SEC: Mn ) 4500, Mw ) 4780, Mw/Mn ) 1.06.
The following is a typical procedure for RAFT polymeriza-
tion of N-isopropylacrylamide: NIPAM (1.132 g, 10.0 mmol),
1 (24.0 mg, 5.0×10-2 mmol), AIBN (5.0 mg, 3.0×10-2 mmol),
and 2.5 mL of 1,4-dioxane were charged into a Schlenk flask
equipped with a magnetic stirrer. The mixture was degassed
by five freeze-evacuate-thaw cycles and then heated at 75
°C under argon in a thermostated oil bath for 1.5 h. The
polymer mixture was precipitated in a large excess of diethyl
ether, isolated by filtration and dried at 50 °C under vacuum
to give 0.238 g of PNIPAM polymer, PNIPAM-L1-1. The
conversions of the monomer NIPAM were determined to be
18.9% by gravimetrical method. 1H NMR (CDCl3): δ ) 9.12-
8.57 (br, m, tpy), 8.38-7.75 (br, m, Ph-CdS, and tpy), 7.70-
7.15 (br, m, tpy and Ph-CdS), 7.15-5.75 (br, s, NH), 3.99 (br,

4116 Zhou and Harruna
Macromolecules, Vol. 38, No. 10, 2005
s, N-CH), 2.50-1.25 (br, m, CH2CH), 1.13 ppm (br, s,
C(CH3)2). SEC: Mn ) 1910, Mw ) 2030, Mw/Mn ) 1.06.
Preparation of Bis(terpyridine)ruthenium(II) Complex-
Centered Polystyrene. Terpyridine-terminated polystyrene,
PS-L1-5, (200.0 mg, 1.55 × 10-2 mmol based on Mn, theory )
12930 g/mol) and RuCl3‚xH2O (61.0 mg, 24.4 × 10-2 mmol)
were charged into a 50-ml Schlenk flask with 10 mL of EtOH
and 10 mL of THF. The mixture was deoxygenated by purging
argon for half hour and then heated at 85 °C under argon for
25 h. The precipitate was filtered and washed with water,
followed by diethyl ether, and finally dried under reduced
pressure to give RuIII(terpyridine)Cl3-terminated polystyrene,
PS-L1-5-RuCl3. The RuIII(terpyridine)Cl3-terminated polysty-
rene, PS-L1-5-RuCl3, and terpyridine-terminated polystyrene,
PS-L1-5, (250.0 mg) were charged into a 50-ml Schlenk flask
with 20 mL of THF and 10 mL of EtOH. The mixture was
heated at 85 °C under argon for 24 h, followed by the addition
of N-ethylmorpholine (0.10 mL), and then continued stirring
at the same temperature under argon for an additional 24 h.
After the mixture was cooled to room temperature, an excess
of NH4PF6 (45 mg, 0.27 mmol) was added. Stirring was
continued for 1 h, and subsequently, the solvents were removed
in vacuo. The crude product was obtained by the addition of
methanol to the residue, washed with water, and dried under
vacuum. The crude complex was stirred in diethyl ether at
room temperature for 2 days, then filtered and dried to give
target complex. The complex was further dissolved in THF (20
mL) and filtered to give a clear solution, and then hexane (60
mL) was added dropwise into the clear polymer solution. As a
result, the complex was precipitated out, whereas the trace
uncomplexed polystyrene was left in solution. The pure su-
pramolecular dimeric polystyrene ruthenium complex, PS-Ru-
(tpy)2-PS, was obtained after the removal of the clear solution
by pipet and dried in an oven under reduced pressure. Yield
was 365.0 mg (91%). 1H NMR (CDCl3): δ ) 7.50-6.12 (br, m,
Scheme 1. Synthetic Route of the
Terpyridine-functionalized CTA
Reagents and Conditions: (i) Thiourea, Ethanol-acetone (1:
1, v/v), Reflux, 22.5 H, 91%; (ii) NaOH, H2O, 70 °C, 4.5 h; (iii)
Ph-C(dS)-S-CH2COOH, NaOH, H2O, 60 °C, 18 H, 75%.
MLCT(ꢀ) ) 491 nm (22 371 M-1 cm-1). Mn,theory ) 18 170,
33
32
Mn,UV-Vis ) 20 140.
Results and Discussion
Synthesis of Terpyridine-Functionalized CTA. A
CTA, which is typically a thiocarbonylthio compound,
Z-C(dS)-S-R, plays a key role in RAFT polymerization
and its molecular structural design needs to consider
two important factors.27 First, Z should not only activate
the CdS double bond toward radical addition to form
the intermediate radical, but also favor the fragmenta-
tion of the intermediate radical to free the reinitiating
radical (R•). Second, R should be a good leaving group
that efficiently reinitiates polymerization. In producing
a CTA to efficiently control polymerization, both the
activating substituent Z and the R leaving group should
be properly chosen. To date, numerous CTAs have been
designed and synthesized through different synthetic
approaches for different monomer systems.27 Among
these synthetic routes, the ester exchange reaction
between dithiocarboxylic acid and thiol is quite simple
and has been used to synthesize some dithioesters.27,34
We used the ester exchange approach to synthesize a
novel terpyridine-functionalized CTA from the only
commercially available carboxymethyl dithiobenzoate
according to the synthetic route shown in Scheme 1.
During the preparative course of the terpyridine-
functionalized CTA, a key terpyridine-functionalized
thiol intermediate, 1c, was formed by careful alkaline
hydrolysis of the isothiourea hydrobromide salt, 1b,
produced through the reaction of 4′-(4-bromomethyl-
phenyl)-2,2′:6′,2′′-terpyridine (1a) with thiourea at re-
flux using an ethanol-acetone mixture as a reaction
media. The formation of the thiol, 1c, was confirmed
by its accurate mass analysis (HRMS (EI): calcd for
C22H17N3S m/z ) 355.1143, found m/z ) 355.1104,
HRMS spectrum is shown in Figure S1 in the Support-
ing Information). The thiol, 1c, was in situ reacted with
carboxymethyl dithiobenzoate (CMDB) to produce terpy-
ridine-functionalized CTA, 1. The crude CTA, 1, was
purified by column chromatography on alumium oxide
using hexanes-ethyl acetate (2:1, v/v) as an eluent to
afford pure CTA, 1, in moderate yield. The observed
exact mass of the CTA, 1, (m/z ) 475.1128) is in
excellent agreement with the calculated value (m/z )
475.1177) for C29H21N3S2 (HRMS spectrum in Figure
S2). Both 1H NMR and 13C NMR spectra also elucidate
the structure of the CTA 1 (Figure 1).
Ph), 2.39-1.07 ppm (br, m, CH2CH). UV-vis (CH2Cl2): λmax
-
MLCT(ꢀ) ) 491 nm (23 069 M-1 cm-1). Mn,theory ) 26 240,
31
32
Mn,UV-vis ) 28 210.
Preparation of Thermosensitive Bis(terpyridine)ru-
thenium(II) Complex-Connected Diblock Copolymers.
Terpyridine-terminated polystyrene, PS-L1-3, (100.0 mg,
1.22×10-2 mmol based on Mn, theory ) 8180 g/mol) and RuCl3‚
xH2O (40.0 mg, 13.9 × 10-2 mmol) were charged into a 25-ml
Schlenk flask with 15 mL of EtOH-THF (2:1, v/v) mixture
solvents. The mixture was heated at 85 °C under argon for 25
h. The solvents were removed under reduced pressure, and
then methanol was added to precipitate the polymeric mono-
complex. The monocomplex was filtered and washed with
water, followed by diethyl ether, and finally dried under
reduced pressure to give RuIII(terpyridine)Cl3-terminated poly-
styrene, PS-L1-3-RuCl3, (104.0 mg). RuIII(terpyridine)Cl3-
terminated polystyrene, PS-L1-3-RuCl3, (104.0 mg) and terpy-
ridine-terminated PNIPAM polymer, PNIPAM-L1-3, (250.0
mg, 2.5 × 10-2 mmol based on Mn, theory ) 10 140 g/mol) were
charged into a 25-ml Schlenk flask with 10 mL of THF and
7.5 mL of EtOH. The mixture was heated at 85 °C under argon
for 1 h 10 min, followed by the addition of N-ethylmorpholine
(0.10 mL), and then stirring continued at the same tempera-
ture under argon for an additional 21.5 h. After the mixture
was cooled to room temperature, an excess of NH4PF6 (40.0
mg, 24.5 × 10-2 mmol) was added. Stirring was continued for
1 h, and subsequently, the solvents were removed in vacuo.
Methanol was added, and trace precipitate was filtered off.
The methanolic filtrate was evaporated to almost dryness
under vacuum, and then diethyl ether was added and stirred
at room temperature for 2 days. The precipitate was filtered
and water added. The aqueous mixture was stirred at room
temperature for 2 days. The aqueous mixture was separated
by centrifugation and further purified by repetitive centrifuga-
tion and redispersion with water and methanol in turn. The
pure complex, PS-Ru(tpy)2-PNIPAM, was recovered by the
addition of diethyl ether and dried under vacuum to give 148.0
RAFT Polymerization of Styrene. To test the
effectiveness of the terpyridine-functionalized dithioester,
1, as a RAFT agent, styrene was polymerized thermally
in the presence of the CTA 1 at 100 °C (Scheme 2). The
molar ratio between styrene and the CTA, 1, was kept
1
mg of product (67%). H NMR (400 MHz, CDCl3): δ ) 7.42-
5.59 (br, m, Ph and NH), 4.01 (br, s, CH-N), 2.69-1.25 (br,
m, CH2CH), 1.14 ppm (br, s, C(CH3)2). UV-vis (CH2Cl2): λmax
-

Macromolecules, Vol. 38, No. 10, 2005
Bis(2,2′6′:2′′-terpyridine)ruthenium(II)-Connected Polymers 4117
Figure 2. SEC chromatograms (RI traces) of polystyrenes
prepared by bulk RAFT polymerization of styrene using the
terpyridine-functionalized dithioesters 1 as a CTA at 100 °C
via thermal auto-initiation for different polymerization times
(Sample PS-L1-1 to PS-L1-5 in Table 1). [Styrene] ) 8.70 M,
[1] ) 0.04 M, [Styrene]/[1] ) 216/1.
Figure 1. 1H NMR (A) and 13C NMR (B) spectra (CDCl3) of
the terpyridine-functionalized CTA 1.
Figure 3. (a) 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 polym-
erization of styrene by RAFT using 1 as a transfer agent at
100 °C via thermal auto-initiation (Sample PS-L1-1 to PS-
L1-5 in Table 1). [Styrene] ) 8.70 M, [1] ) 0.04 M, [Styrene]/
[1] )216/1.
Scheme 2
Figure 2 shows the SEC curves of the polystyrene
samples prepared at different polymerization times. A
continuous shift of the curves toward shorter elution
times is observed, indicating continuously increasing
molar masses. Furthermore, first-order kinetics with
respect to monomer concentration showed a linear
relationship throughout the studied region (12-55%
conversion) (Figure 3a). Also, the molecular weights
determined by SEC and NMR increased linearly with
conversion as shown in Figure 3b. However, there are
deviations between SEC-determined and theoretical
molecular weights (Table 1 and Figure 3b). The possible
cause for this discrepancy is the effect of the chain end-
group-terpyridinyl. As the measured molecular weight
constant at 216 in all of the polymerizations. The
conversion and the molar mass evolution were studied
as a function of time. The molar mass characteristics
of the polymers were determined by SEC.

4118 Zhou and Harruna
Macromolecules, Vol. 38, No. 10, 2005
Figure 5. SEC chromatograms (RI traces) of poly(N-isopro-
pylacrylamide)s prepared by RAFT polymerization of NIPAM
at 75 °C using 1,4-dioxane as reaction media, 1 as a transfer
agent, AIBN as an initiator. [NIPAM] ) 4.00 M, [NIPAM]/
[1]/[AIBN] )328/1.66/1 (Sample PNIPAM-L1-1 to PNIPAM-
L1-4 in Table 1).
conformation, while above the LCST, the polymer is
dehydrated and adopts a globular conformation and the
globular polymer chains form intermolecular aggre-
gates. This interesting property has been applied in
various ways in organic chemistry, biotechnology, and
materials sciences.36 Recently, the RAFT polymerization
of NIPAM has received much attention.26,37 Few chain
transfer agents such as benzyl dithiobenzoate and
2-dodecylsulfanylthiocarbonyl-sulfanyl-2-methyl propi-
onic acid have been used and confirmed to be efficient
RAFT agents for polymerization of NIPAM.26,37
Although the novel terpyridine-functionalized dithio-
ester, 1, is an efficient RAFT agent for polymerization
of styrene, it is not certain whether it is efficient for
polymerization of NIPAM. To examine this point, a
series of parallel polymerizations of NIPAM were per-
formed at 75 °C using 1,4-dioxane as solvent and AIBN
as an initiator in the presence of the CTA 1 for different
reaction times. The polymerization conditions and char-
acterization data are summarized in Table 1.
Figure 5 shows an overlay of the RI traces from the
polymerization of NIPAM at different times. The traces
clearly shift to lower elution times with increasing
conversion, one indicator of a controlled polymerization.
However, these SEC traces show unobvious shoulders
on the low molecular weight side, which were caused
by progressive decomposition of the initiator AIBN
during the polymerization. Figure 6 shows the first-
order rate plot of ln([M]0/[M]) vs polymerization time.
There is an induction period of approximately 30 min,
after which the polymerization exhibits pseudo-first-
order kinetics, consistent with a controlled polymeriza-
tion. An induction period is often observed in RAFT
polymerization.38 The induction period was attributed
to the slow initiation by the more stable 4-[2,2′:6′,2′′]-
terpyridin-4′-yl-benzyl radical generated from the CTA
1. As observed from Figure 6b, the molecular weights
determined by NMR and SEC increased linearly with
conversion (the range 18-53%). Moreover, the NMR-
determined molecular weight agrees well with the
theoretical molecular weight calculated according to the
monomer conversion (Figure 6b and Table 1). However,
there is marked deviation between the SEC-determined
and theoretical molecular weights. The main cause for
this discrepancy is that a polystyrene standard (Mn )
9870 g/mol) was used for calibration. Also evident from
Table 1, and plotted in Figure 6b, are the low polydis-
persity indexes (PDI) observed for the PNIPAM poly-
Figure 4. 1H NMR (CDCl3) spectra of the representative
RAFT-prepared polystyrene sample PS-L1-2 (A) and poly(N-
isopropylacrylamide) sample PNIPAM-L1-1 (B).
is lowered, the effect of the terpyridine chain end group
becomes more obvious, and consequently results in a
relatively large deviation (Table 1). Additionally, the
NMR-determined molecular weights are quite close to
the theoretical molecular weight (Figure 3b and Table
1). The resulting polydispersities are generally low,
ranging from 1.04 to 1.18 at all polymerization times.
The linear increase in molecular weight and low poly-
dispersity demonstrates that the new terpyridine-func-
tionalized dithioester 1-mediated bulk polymerization
of styrene is a living radical polymerization.
On the basis of the RAFT polymerization mechanism,
RAFT polymerization provides a means of introducing
specifically placed functional moieties into polymer
chains using appropriately functionalized RAFT agents.24
Therefore, the terpyridine-functionalized RAFT agent
1-mediated styrene polymerization can produce terpy-
ridine-terminated polystyrene polymers. To confirm this
point, NMR end group analysis was performed. Figure
1
4A shows the H NMR spectrum of a representative
sample, PS-L1-2. The characteristic peak of the terpy-
ridine-functional end groups derived from the RAFT
agent 1 was observed at 8.70 ppm, and was used to
calculate the molecular weight, assuming that each
polystyrene chain end bears a terpyridyl group derived
from the RAFT agent 1. The close agreement between
the NMR-determined and theoretical molecular weight
confirms that most of polystyrene chains are terminated
by the terpyridine-functional group derived from the
used RAFT agent 1 for polymerization, as expected in
a reversible addition-fragmentation chain transfer
mechanism. However, a small percentage of nonterpy-
ridine end-fuctionalized polymer chains are inevitably
formed by the thermal autoinitiation-produced radicals.
RAFT Polymerization of N-Isopropylacryla-
mide. PNIPAM is one of the most widely studied
temperature-responsive polymers that undergoes a
reversible, inverse phase transition at a temperature
known as the lower critical solution temperature (LCST)
of about 32 °C in pure water.35 Below the LCST, the
polymer chain is hydrated and adopts an extended coil

Macromolecules, Vol. 38, No. 10, 2005
Bis(2,2′6′:2′′-terpyridine)ruthenium(II)-Connected Polymers 4119
characteristic peak of terpyridine functional groups is
clearly observed at 8.82 ppm. The integral areas of the
characteristic peaks at 8.82 and 4.00 ppm corresponding
to the terpyridine and isopropyl groups, respectively,
were used to determine the molecular weight of the
RAFT-prepared PNIPAM polymers. The calculated
values are given in Tale 1. The good agreement between
the NMR-determined and theoretical molecular weights
indicates that most of polymer chains bear terpyridine-
functional end groups originating from the CTA, 1.
Undoubtedly, a few polymer chains do not bear a
terpyridine functional group at one end because AIBN-
decomposed radicals also participate in initiating po-
lymerization. The content of the polymers without
terpyridine functional end groups depends mainly on
the amount of the efficient radicals formed from initia-
tors.
Bis(terpyridine)ruthenium Complex-Connected
Diblock Copolymers. The purpose of synthesizing the
novel terpyridine-functionalized RAFT agent is to pre-
pare a variety of terpyridine-terminated polymers by
mediated RAFT polymerization. Terpyridine-terminated
polystyrene and PNIPAM polymers with controlled,
narrow molecular weight were successfully synthesized
by the RAFT process using 1 as a CTA. Terpyridine
ligands form mono- and biscomplexes with a wide
variety of transition metal ions.17 The stability constants
and the kinetics of formation of the different complexes
strongly depend on the nature of the metal ions used.17
In this respect, RuIII forms a very stable monocomplex
with one terpyridine ligand, while RuII only forms a
stable biscomplex with terpyridine ligands.17 Therefore,
Ru(tpy)2(II)2+ complex-connected diblock polymers were
synthesized using RAFT-prepared terpyridine-termi-
nated polymers.
To form ruthenium complexes, the representative
terpyridyl-terminated polystyrene, PS-L1-5, was treated
with an excess of RuIIICl3‚xH2O to give an intermediate
monocomplex, PS-L1-5-RuIIICl3. The monocomplex was
then further treated with a slight excess of terpyridyl-
terminated polystyrene, PS-L1-5 (Scheme 3). In this
approach, ethanol was utilized as the reducing agent
and N-ethylmorpholine as the catalyst to reduce the
RuIII ions to RuII ions. Subsequently, a biscomplex was
formed between the ruthenium-filled terpyridine unit
Figure 6. (a) Pseudo-first-order kinetic plot of ln([M]0/[M])
vs polymerization time and (b) number average molecular
weights and polydispersity as a function of conversion for
RAFT polymerization of NIPAM at 75 °C using 1,4-dioxane
as reaction media, 1 as a transfer agent, AIBN as an initiator.
[NIPAM] ) 4.00 M, [NIPAM]/[1]/[AIBN] ) 328/1.66/1 (Sample
PNIPAM-L1-1 to PNIPAM-L1-4 in Table 1).
mers synthesized using the terpyridine-functionalized
dithioester 1 as a RAFT agent. In all instances, the PDIs
are e 1.18. All the polymerization characteristics indi-
cate that the terpyridine-functionalized dithioester 1 is
a quite efficient RAFT agent for the polymerization of
NIPAM.
Like the styrene polymerization, the polymerization
of NIPAM in the presence of the CTA, 1, produces
terpyridine-terminated thermosensitive PNIPAM poly-
1
mers. Figure 4B shows the H NMR spectrum of the
representative sample, PNIPAM-L1-3, prepared by
RAFT polymerization of NIPAM using 1 as a CTA. The
Scheme 3

4120 Zhou and Harruna
Macromolecules, Vol. 38, No. 10, 2005
Figure 8. DSC curves of the prepared supramolecular dimeric
polystyrene ruthenium complex, PS-Ru(tpy)2-PS, and its
corresponding terpyridine-terminated precursor polymers, PS-
L1-5, after two time heating runs.
schematic structure in Figure S3). This indicates that
the degree of metallation is quite high.
Figure 7. UV-vis spectra (CH2Cl2) of the prepared supramo-
lecular dimeric polystyrene ruthenium complex, PS-Ru(tpy)2-
PS, and bis(terpyridine)ruthenium complex-connected poly-
styrene and poly(N-idopropylacrylamide) diblock copolymer,
PS-Ru(tpy)2-PNIPAM, as well as their corresponding terpy-
ridine-terminated precursor polymers, PS-L1-5 for PS-Ru-
(tpy)2-PS, and both PS-L1-3 and PNIPAM-L1-3 for PS-
Ru(tpy)2-PNIPAM. For comparison, a representative inter-
mediate monocomplex, PS-L1-5-RuIIICl3 was also added.
The biscomplex, PS-Ru(tpy)2-PS, was further char-
acterized by DSC. Figure 8 shows the DSC curves of
the biscomplex and its corresponding precursor PS-L1-5
with two heating runs. During the first heating run, the
glass transition of the biscomplex, PS-Ru(tpy)2-PS, is
not obvious because of the existence of polar solvents
in the complex. However, the glass transition of the
biscomplex becomes obvious during the second heating
run. The glass transition temperatures (Tgs) of the
biscomplex, PS-Ru(tpy)2-PS, and its corresponding
polystyrene precursor, PS-L1-5, are 108 and 102 °C,
respectively. The increase in Tg for the biscomplex arise
from the actual formation of Ru(tpy)2(II)2+ complexes
between two terpridine-terminated polystyrene macro-
molecules. All analysis data further indicate the suc-
cessful formation of the supramolecular dimeric poly-
styrene ruthenium complex, PS-Ru(tpy)2-PS.
As in the preparation of the supramolecular dimeric
polystyrene ruthenium complex, PS-Ru(tpy)2-PS, the
RAFT-prepared terpyridine-terminated polystyrene, PS-
L1-3, was reacted with an excess of RuCl3‚xH2O in THF-
EtOH (2:1, v/v) mixture solvents at 85 °C for 25 h to
give a monocomplex, PS-L1-3-RuIIICl3 (Scheme 3). The
excess of RuCl3‚xH2O was removed by washing with
MeOH. The monocomplex, PS-L1-3-RuIIICl3, was fur-
ther complexed with slightly excess terpyridine-termi-
nated PNIPAM-L1-3 in EtOH at 85 °C under catalysis
of N-ethylmorpholine for 22 h, and then an excess
amount of ammonium hexafluorophosphate (NH4PF6)
was added into the cooled mixture. As a result, a Ru-
(tpy)2(II)2+ complex-connected polystyrene and PNIPAM
diblock metallopolymer, PS-Ru(tpy)2-PNIPAM, was
formed (Scheme 3) and further purified by removing the
uncomplexed precursors, PS-L1-3 and PNIPAM-L1-3,
with diethyl ether and water, respectively.
and the uncomplexed terpyridine unit, which resulted
in symmetrical dimers. After exchange of the counter-
ions by the addition of an excess of NH4PF6, the
supramolecular dimeric polystyrene ruthenium com-
plex, PS-Ru(tpy)2-PS, was isolated by precipitation.
The uncomplexed polystyrene PS-L1-5, which is soluble
in diethyl ether, and trace inorganic salts were removed
using diethyl ether and THF, respectively. As a result,
the pure supramolecular dimeric polystyrene ruthenium
complex, PS-Ru(tpy)2-PS, was obtained.
The purified bis(terpyridine) ruthenium polystyrenic
complex PS-Ru(tpy)2-PS was analyzed by SEC. How-
ever, there is a remarkable difference between the SEC
curves of the complex and its precursor polystyrene
(Figure S4). Particularly, there is no RI signal corre-
sponding to the complex. The main cause is that the
bis(terpyridine)ruthenium(II) complex in the center of
dimeric polystyrene chain seem to adsorb more strongly
to the stationary phase than the precursor polystyrene
PS-L1-5. This further proved that the purification
procedure was quite successful.
As a powerful proof for the formation of biscomplex,
the UV-vis spectrum (Figure 7) of the polystyrene
ruthenium complex revealed clearly the typical metal-
ligand charge-transfer (MLCT) band of the Ru(tpy)2-
(II)2+ complex at λ ) 491 nm as well as the π-π*
transitions of the terpyridine ligand at λ ) 310 nm.18,39
Meanwhile, the characteristic peak at λ ) 418 nm,
attributed to the intermediate monocomplex PS-L1-5-
RuIIICl3, disappeared. To further elucidate and support
the content, the Ru(II) metal complex in the polystyrene
dimer, the UV-vis absorbance at λ ) 491 nm in CH2-
Cl2 was accurately measured and used to calculate the
molar absorptivity (ꢀ). On the basis of the theoretical
molecular weight of PS-Ru(tpy)2-PS (Mn ) 26 240
g/mol),31 ꢀ ) 20369 M-1 cm-1, which is close to the
literature value ꢀ ) 24 800 M-1 cm-1 for the standard
The purified biscomplex, PS-Ru(tpy)2-PNIPAM, was
characterized by UV-vis technique. Figure 7 shows the
UV-vis spectra of the biscomplex and its corresponding
precursor polymers, PS-L1-3 and PNIPAM-L1-3. As
observed from Figure 7, a new absorption band at λ )
491 nm attributed to the metal-ligand charge-transfer
(MLCT) band of the Ru(tpy)2(II)2+ complex appeared
through the comparison with the two precursor poly-
mers. Similarly, on the basis of the theoretical molecular
weight of the complex, PS-Ru(tpy)2-PNIPAM (Mn )
18 718 g/mol),33 the molar absorptivity (ꢀ) was deter-
mined by UV-vis technique to be 22 371 M-1 cm-1 at λ
complex bis[4′-(4-tolyl)-2,2′:6′,2′′-terpyridine]ruthenium
2-
)
40 (the
hexafluophosphate ([Ru(tpy-Ph-CH3)2](II)[PF6]2

Macromolecules, Vol. 38, No. 10, 2005
Bis(2,2′6′:2′′-terpyridine)ruthenium(II)-Connected Polymers 4121
Figure 9. 1H NMR spectrum (CDCl3) of the bis(terpyridine)-
ruthenium complex-connected polystyrene and poly(N-isopro-
pylacrylamide) diblock metallopolymer, PS-Ru(tpy)2-PNIPAM.
) 491 nm in CH2Cl2, which is close to the literature
value ꢀ ) 24 800 M-1 cm-1 for the standard complex
Figure 10. DSC curves of the supramolecular ruthenium
complex, PS-Ru(tpy)2-PNIPAM, and its corresponding two
terpyridine-terminated precursor polymers, PS-L1-3 and
PNIPAM-L1-3, during two heating runs.
2-
[Ru(tpy-Ph-CH3)2](II)[PF6]2 and the measured value
for the biscomplex, PS-Ru(tpy)2-PS. This indicates that
the degree of metallation is larger than 90%. To further
exclude the remaining two precursors, PS-L1-3 and
PNIPAM-L1-3, in the biscomplex, the purified complex
PS-Ru(tpy)2-PNIPAM was characterized by SEC.
The SEC curve (Figure S5) of the purified biscomplex
is different from its corresponding two precursor poly-
mers. The main cause for the absence of RI signal
corresponding to the purified complex is that the Ru-
(tpy)2(II)2+ complex in the connection of polystyrene and
PNIPAM chains interact strongly with the stationary
phase, leading to serious adsorption compared with the
two precursor polymers, PS-L1-3 and PNIPAM-L1-3.
This further shows that the purification procedure is
efficient. Figure 9 shows the 1H NMR spectrum (CDCl3)
of the Ru(tpy)2(II)2+ complex-connected polystyrene
and PNIPAM diblock metallopolymer, PS-Ru(tpy)2-
PNIPAM. The characteristic peaks at 6.00-7.40 and
4.00 ppm corresponding to the polystyrene and PNIPAM
segments, respectively, were observed. This further
confirmed that the polystyrene and PNIPAM were
connected by the Ru(tpy)2(II)2+ complex. To further show
the existence of polystyrene and PNIPAM segments in
the supramolecular complex, PS-Ru(tpy)2-PNIPAM,
thermal characterization was performed by DSC. Figure
10 shows the DSC curves of the biscomplex and its
corresponding two precursors, PS-L1-3 and PNIPAM-
L1-3, during two heating runs. During the first heating
run, the glass transition of the biscomplex, PS-Ru-
(tpy)2-PNIPAM, is not clear for the polystyrene seg-
ment because of hydrogen bonding in the PNIPAM
segment in the biscomplex. However, two glass transi-
tions corresponding to polystyrene and PNIPAM seg-
ments were clearly observed for the biscomplex, PS-
Ru(tpy)2-PNIPAM, during the second heating run.
The two glass transition temperatures (Tgs) of the
biscomplex and its corresponding two precursors, PS-
L1-3 and PNIPAM-L1-3, were determined to be 106,
142, 92, and 136 °C, respectively. The increase in Tg
for both polystyrene and PNIPAM segments of the
biscomplex arise from the formation of Ru(tpy)2(II)2+
complexes between terpridine-terminated polystyrene
and PNIPAM macromolecular chains. All analysis
data further indicate the successful formation of the
supramolecular ruthenium complex, PS-Ru(tpy)2-
PNIPAM. Interestingly, the transition corresponding
to the phase transition temperature, namely, LCST of
Figure 11. AFM image of the bis(terpyridine)ruthenium
complex-connected polystyrene(PS) and poly(N-isopropylacryl-
amide) (PNIPAM) diblock metallopolymer, PS-Ru(tpy)2-
PNIPAM (Mn,theory ) 18 170 g/mol, PS mol % (theory) ) 46.7%).
AFM image size: 2 µm × 2 µm. Data scale: Height 41.57 nm.
the PNIPAM segment of the biscomplex and the precur-
sor PNIPAM-L1-3 disappeared during the second heat-
ing run. Hydrogen bonding from PNIPAM was de-
stroyed before the second heating run. On the basis of
the DSC results, it can be deduced that the LCST
phenomena of PNIPAM actually originates from the
hydrogen bonds existing in the PNIPAM chains.
The diblock metallopolymer, PS-Ru(tpy)2-PNIPAM,
has a strong tendency to form micelles because of its
structural amphiphilic property. AFM was used on solid
substrates to observe the surface morphology. Figure
11 shows the AFM image of the diblock metallopolymer,
PS-Ru(tpy)2-PNIPAM. Spherical particles from mi-
celle aggregation were observed, some formed self-
assembled arrays. This further demonstrates the for-
mation of amphiphilic diblock metallopolymer via the
connectivity of Ru(tpy)2(II)2+ complex.
Conclusions
A novel terpyridine-functionalized dithioester was
synthesized by a simple thioester exchange reaction and
further used as a RAFT agent in the RAFT polymeriza-
tions of styrene and NIPAM. For each monomer, the
pseudo-first-order kinetic plot of ln([M]0/[M]) vs polym-
erization time is linear; moreover, the molecular weights

4122 Zhou and Harruna
Macromolecules, Vol. 38, No. 10, 2005
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determined by SEC and NMR increased linearly with
conversion. The polydispersities of these RAFT-prepared
polymers are generally low (Mw/Mn e 1.20). The results
indicate that the new terpyridine-functionalized RAFT
agent is quite efficient for the preparation of polystyrene
and PNIPAM polymers with low polydispersity and
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RAFT-prepared polymers indicate that they were func-
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most of polymer chains were functionalized at one chain
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end-functionalized polystyrene and PNIPAM polymers
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clearly revealed the typical metal-ligand charge-
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λ ) 491 nm as well as the π-π* transitions of the
terpyridine ligand at λ ) 310 nm. Similarly, the glass
transition temperatures of the metallopolymers are
higher than that of their corresponding precursor
polymers. The results indicate the formation of bis-
(terpyridine)ruthenium complex at the conjunction of
two blocks. In addition, surface observation by AFM
shows amphiphilic diblock metallopolymers are able to
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Acknowledgment. The financial support for this
research work from the Office of Naval Research (ONR
Grant No. N00014-01-1-1042) is gratefully acknowl-
edged. 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.
Supporting Information Available: The HRMS spectra
of the thiol intermediate 1c and the new terpyridine-function-
alized CTA 1, the schematic structure of the standard complex
bis[4′-(4-tolyl)-2,2′:6′,2′′-terpyridine]ruthenium hexafluophos-
phate, and SEC curves of the bis(terpyridine)ruthenium
complex-connected diblock metallopolymers PS-Ru(tpy)2-PS
and PS-Ru(tpy)2-PNIPAM. This material is available free
of charge via the Internet at http://pubs.acs.org.
References and Notes
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.
6
2
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(33) The theoretical molecular weight of PS-Ru(tpy)2-
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Mn,theory(PS-Ru(tpy) -PNIPAM) )Mn,theory(PS-L1-3) +Mn,theory(PNIPAM-L1-3)
2
+ MRu(PF )
.
6
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MA047955C