Controlled Synthesis of Thiazole-Based Polymers and Block Copolymers by RAFT Polymerization of Azolyl S-Vinyl Sulfides and Metal Complexation

Article abstract of DOI:10.1021/acs.macromol.5b02573

We studied the RAFT polymerization of five S-vinyl sulfide derivatives containing azole units, including 2-benzothiazolyl vinyl sulfide (BTVS), 2-thiazolyl vinyl sulfide (2-TVS), 1,3,4-thiadiazolyl vinyl sulfide (1,3,4-TVS), 5-methyl-1,3,4-thiadiazolyl vi

Full text of DOI:10.1021/acs.macromol.5b02573

Article
pubs.acs.org/Macromolecules
Controlled Synthesis of Thiazole-Based Polymers and Block
Copolymers by RAFT Polymerization of Azolyl S‑Vinyl Sulfides and
Metal Complexation
Kazuhiro Nakabayashi,† Aiko Matsumura,† Yohei Abiko,‡ and Hideharu Mori*,†,‡
†Department of Polymer Science and Engineering and ‡Department of Organic Device Engineering, Graduate School of Science and
Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan
S
* Supporting Information
ABSTRACT: We studied the RAFT polymerization of five S-
vinyl sulfide derivatives containing azole units, including 2-
benzothiazolyl vinyl sulfide (BTVS), 2-thiazolyl vinyl sulfide
(2-TVS), 1,3,4-thiadiazolyl vinyl sulfide (1,3,4-TVS), 5-methyl-
1,3,4-thiadiazolyl vinyl sulfide (M-1,3,4-TVS), and 5-phenyl-
1,3,4-oxadiazolyl vinyl sulfide (P-1,3,4-OVS). The polymer-
ization of BTVS using dithiocarbamate-type and trithiocar-
bonate-type chain transfer agents (CTAs) was controlled, as shown by the relatively low polydispersities of the polymers and the
linear increase in molecular weight with increasing conversion. In addition, the molecular weight could be controlled by adjusting
the monomer-to-CTA ratio. The thiazole-, thiadiazole-, and oxadiazole-based polymers with relatively narrow molecular weight
distributions and predetermined molecular weights were obtained by RAFT polymerization of all monomers using the
dithiocarbamate-type CTA. The formation of metal complexes between the azole-containing polymers and various metal species
was investigated, concerning their optical and electrochemical behaviors, and the assembled structures of the metal−azole
complexes. Among the combinations of the five polymers and three metal species (Cu, Zn, and Ni), the complexation between
poly(BTVS) and copper species was the most strongly coordinating, affording a stable metal complex, poly(BTVS)-Cu. The
fluorescence behavior of poly(BTVS)-Cu, which is induced by increasing the length of the conjugated structure between the
benzothiazole rings and copper species, was controlled by the metal content in poly(BTVS)-Cu. The complexation of
benzothiazole-based block copolymers, in which only the BTVS segment interacts with the copper species, whereas another
styrene segment showed no specific interactions, exhibits characteristics of assembled structures and fluorescent properties.
tions in organic light-emitting diodes.14−16 Metal complexes
bearing pyridyl azolates, or a similar relevant functionality, have
been developed as promising candidates for organic electronic
INTRODUCTION
■
Azole compounds, such as imidazole, triazole, thiazole, and
oxazole, are important and versatile five-membered nitrogen
heterocyclic compounds containing at least one other non-
and optoelectronic applications including organic light-emitting
diodes and dye-sensitized solar cells.17 Nitrogen-rich azole-
carbon atom, that is, nitrogen, sulfur, or oxygen. A large
based ligands have been employed as versatile building blocks
number of azole-based derivatives have been developed, and
for coordination-oligomer and polymer assemblies, including
these are used as antifungal drugs and agents, inhibiting fungal
metal−organic frameworks.18
enzymes.1−3 An attractive feature of azole compounds is that
they bind easily with enzymes and receptors in organisms
through noncovalent interactions such as hydrogen bonds,
coordination bonds, π−π stacking, and van der Waals forces,
which are often used in medical chemistry for drug design.4
Benzothiazole, which is a privileged heterocyclic compound
that has a benzene ring fused to a thiazole ring, and its
derivatives are common and integral features of a variety of
natural products.5 The benzothiazole unit has attracted a great
deal of interest due to its wide range of pharmacological
activities,5,6 for example, its anticancer,7 antitumor,8 antimicro-
bial,9 and antiallergic activities.10 In addition to the wide variety
of medical and biological applications, much attention has been
paid to the complexation of azoles with various metal
species.11−13 For example, benzothiazole-derived Zn(II)
complexes have been extensively studied, concerning their
photophysical and electroluminescent properties, for applica-
An intriguing feature of azole heterocycles is that the
heteroatom participates in the π system, and this plays a crucial
role in their characteristic optoelectronic properties. In
particular, thiazole is a widely used electron-accepting hetero-
cycle due to the electron-withdrawing nitrogen in the imine
(CN) function; consequently, various thiazole-based mole-
cules have been introduced into organic semiconductors.19
A
wide variety of polymers containing thiazole, thiadiazole, and
benzothiazole moieties in the side chains have been synthesized
by conventional radical20−26 and controlled radical polymer-
izations.27−33 For example, Schubert et al. recently reported the
controlled syntheses of methacrylate-based copolymers con-
Received: November 27, 2015
Revised: February 4, 2016
© XXXX American Chemical Society
A
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taining donor−acceptor thiazole dyes,27 polystyrenes,28 and
polymethacrylates29 containing thiazole-based, blue-emitting
side-chains by reversible addition−fragmentation chain transfer
(RAFT) polymerization. They also demonstrated the RAFT
syntheses and investigated the energy transfer systems of
polymethacrylates composed of thiazole-based chromophores
and Os(II) metal complexes30 and a statistical terpolymer
containing a thiazole-based donor and an acceptor-type
chromophore derived from Ru complex.31 Atom transfer
radical polymerization was also used for the production of
well-defined polymethacrylates with pendant π-conjugated
benzothiazole32 and triblock copolymers composed of
thiadiazole-containing oligoester.33 Conventional radical poly-
merizations of acrylamides containing benzothiazole units were
used to develop fluorescent polymers involving an intra-
molecular proton transfer.20,21 Other examples involve the
syntheses, by conventional radical polymerization, of anti-
microbial polymethacrylates based on quaternized thiazole and
triazole side-chain groups,22 biologically active copolymers
composed of thiazole-containing methacrylamide and oxazole-
containing methacrylate,23 and an antimicrobial polymethacry-
late with pendant thiadiazole units.24 Because of scientific and
industrial interest, several investigations have been made on
thiazole-based polymer−metal complexes, which include
thiazole-based polymethacrylamide-Cu(II) complexes having
biological activity25 and thiazole-based copolymer−metal (Ni2+
and Nd3+) complexes having characteristic magnetic proper-
ties.26
Scheme 1. Reversible Addition−Fragmentation Chain
Transfer (RAFT) Polymerization of Azolyl S-Vinyl Sulfides
Using Different Chain Transfer Agents (CTAs)
Oxadiazole-containing polymers have also attracted signifi-
cant attention because oxadiazole compounds are electron
deficient and have versatile electron transporting proper-
ties.34,35 For example, random copolymers containing carbazole
donor−oxadiazole acceptor side chains have been used as
ambipolar hosts for phosphorescent light-emitting diodes36 and
for applications in high-performance-memory devices.37 Sim-
ilarly, RAFT polymerization of methacrylates containing hole-
or electron-transporting groups38 and nitroxide-mediated
radical polymerizations of styrene derivatives with hole/
electron transporting units39 and luminescent/charge transport
units40 were used to synthesize oxadiazole-based polymers and
diblock copolymers with well-defined structures. Conventional
radical polymerization of oxadiazole-containing styrene deriv-
atives resulted in the formation of a series of mesogen-jacketed
liquid crystalline polymers.41 Although these polymers
containing thiazole, thiadiazole, and oxadiazole side chains
have interesting properties, the monomers were limited to
classical monomers, such as styrene, (meth)acrylate, and
(meth)acrylamide derivatives.
Here, we present the controlled synthesis of azole-based
polymers by RAFT polymerization of azolyl S-vinyl sulfides,
including 2-benzothiazolyl vinyl sulfide (BTVS), 2-thiazolyl
vinyl sulfide (2-TVS), 1,3,4-thiadiazolyl vinyl sulfide (1,3,4-
TVS), 5-methyl-1,3,4-thiadiazolyl vinyl sulfide (M-1,3,4-TVS),
and 5-phenyl-1,3,4-oxadiazolyl vinyl sulfide (P-1,3,4-OVS)
(Scheme 1). Among them, BTVS and 2-TVS are thiazole
derivatives, 1,3,4-TVS and M-1,3,4-TVS are categorized as
thiadiazole derivatives, and P-1,3,4-OVS is an oxadiazole
derivative. These azolyl S-vinyl sulfides have many attractive
features, including an aromatic nucleus, high dipole moment,
electron-accepting heterocycles, roles as ligands and coordina-
tion sites for metal species, and access to structurally diverse
materials. Another attractive point is that the thioether group in
the monomer unit can be readily converted to other functional
groups.42−44 In addition, the binding of thioethers to transition
metals, particularly concerning the coordination chemistry of
Cu(II)-S species, has attracted significant interest because of
their analogy to various class of metalloproteins, such as blue
copper proteins.45−48
Recently, we investigated the controlled synthesis of sulfur-
containing polymers by RAFT polymerization of phenyl S-vinyl
sulfide derivatives,49−52 which belong to a family of S-vinyl
monomers. Generally, S-vinyl sulfides are donor monomers
with π−ρ conjugation,53 which are distinctly different form
conventional styrene, (meth)acrylate, and (meth)acrylamide-
type monomers. In the case of phenyl S-vinyl sulfide (Q = 0.32,
e = −1.4), in which the sulfur atom is directly linked to the
vinyl group, the 3d orbital resonance between the sulfur atom
and the adjacent carbon atom is important in the copoly-
merization transition state formed with various comonomers.54
Similar to phenyl S-vinyl sulfide, in azolyl S-vinyl sulfides, a
connection between the azole groups and the vinyl group
through the thioether linkage, corresponding to π−ρ−π
conjugation, exists. Consequently, the structure of the
electron-accepting azole units in these monomers may affect
the polymerization behavior, in addition to characteristic
properties of the resulting S-vinyl heterocyclic polymers.
In this study, we initially investigated RAFT polymerization
of five S-vinyl sulfide derivatives to afford well-defined poly(S-
vinyl sulfide)s with thiazole, thiadiazole, and oxadiazole side
chains. Then, the formation of metal complexes between the
azole-containing polymers with three metal species (Cu, Zn,
B
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thiadiazolyl vinyl sulfide (M-1,3,4-TVS), and 5-phenyl-1,3,4-oxadia-
zolyl vinyl sulfide (P-1,3,4-OVS) were synthesized by the reaction of
1,2-dibromoethane with the corresponding thiol compounds, accord-
ing to a slightly modified reported method (Scheme S1, Supporting
Information).49,63 A typical experiment for the synthesis of BTVS is
detailed here: 2-mercaptobenzothiazole (25.0 g, 0.15 mol) and 1,2-
dibromoethane (56 g, 0.30 mol) were added to tetrahydrofuran (THF,
250 mL) at room temperature under a nitrogen atmosphere. To this
mixture, 1,8-diazabicyclo[5.4.0]undec-7-ene (45 mL, 0.30 mol) was
added dropwise with stirring at 0 °C. The reaction mixture was
refluxed with stirring for 24 h, cooled, and treated with chloroform
(300 mL) and water (300 mL). The organic layer was separated,
washed with water, and dried over MgSO4. After filtration, the solvent
was removed in vacuo, and the residue was distilled under reduced
pressure to give pure BTVS as a pale yellow liquid (9.8 g, 34%).
BTVS: yield = 34%, bp 140 °C/0.05 mmHg (lit.64 163 °C/10
and Ni) was investigated in terms of their optical and
electrochemical behaviors and the structures of the metal
complexes (Scheme 2). A key feature of this strategy is the
Scheme 2. Complexations of (a) Poly(BTVS) with Zn, Ni,
and Cu Species and (b) Poly(S-vinyl sulfide) Derivatives
with CuCl2
1
mmHg). H NMR (CDCl3): δ 8.0−7.2 (m, 4H, ArH), 7.1−7.0 (dd,
1H, CH2CH−), 5.7 (dd, 2H, CH2CH−) ppm. 13C NMR
(CDCl3): δ 163.8 (SCS), 152.5 (NC), 133.4 (SC), 125.5, 123.7, 121.0,
120.2, 119.6 (CH2CH−, ArC), ppm. Anal. Calcd for C9H7NS2: C,
55.92; H, 3.65; N, 7.25; S, 33.18. Found: C, 56.18; H, 3.75; N, 7.11; S,
32.87.
1
2-TVS: yield = 31%, bp 60 °C/0.05 mmHg. H NMR (CDCl3): δ
7.7 (s, 1H, NCH), 7.2 (s, 1H, SCH), 6.8−6.7 (dd, 1H, CH2
CH−), 5.6−5.5 (dd, 2H, CH2CH−) ppm. 13C NMR (CDCl3): δ
162.4 (SCS), 143.1 (NCH), 128.1 (CH2CH−), 119.8, 119.3
(SCH, CH2CH−) ppm. Anal. Calcd for C5H5NS2: C, 41.93; H,
3.52; N, 9.78; S, 44.77. Found: C, 42.01; H, 3.65; N, 9.56; S, 44.24.
1,3,4-TVS: yield = 21%; bp 100 °C at 0.05 mmHg. 1H NMR
(DMSO-d6): δ 9.6 (s, 1H, SCHN), 7.0−6.9 (dd, 1H, CH2CH−),
5.9−5.8 (dd, 2H, CH2CH−) ppm. 13C NMR (DMSO-d6): δ 163.6
(SCS), 154.8 (SCHN), 126.9 (CH2CH−), 121.4 (CH2CH−)
ppm.
1
M-1,3,4-TVS: yield = 20.9%, bp 130 °C/0.05 mmHg. H NMR
(CDCl3): δ 6.7 (dd, 1H, CH2CH−), 5.5 (dd, 2H, CH2CH−), 2.6
(s, 3H, CH3) ppm. 13C NMR (CDCl3): δ 160.4 (SCS), 162.6 (CH3C),
127.0 (CH2CH−), 120.6 (CH2CH−), 15.9 (CH3) ppm.
incorporation of electron-deficient azole units into the S-vinyl
monomers; this technique allows the controlled synthesis of
azole-based polymers by RAFT polymerization and complex-
ation with metal species. We also extended this methodology to
develop novel metalloblock copolymers by the selective
complexation of metal species with azole-based block
copolymers. This is the first report on the controlled radical
polymerization of azolyl S-vinyl sulfides, affording thiazole,
thiadiazole, and oxadiazole-based homopolymers and block
copolymers exhibiting characteristic complexation with metal
species.
1
P-1,3,4-OVS: yield = 17.4%, bp 170 °C/0.05 mmHg. H NMR
(CDCl3): δ 8.0−7.7, 7.5 (m, 5H, ArH), 6.9−6.8 (dd, 1H, CH2
CH−), 5.7−5.6 (dd, 2H, CH2CH−) ppm. 13C NMR (CDCl3): δ
165.9 (SCS), 162.0 (OCAr), 132.7, 128.9, 126.6, 123.8, 123.3, 121.0
(CH2CH−, ArC) ppm. Anal. Calcd for C5H6N2S2: C, 58.80; H,
3.95; N, 13.72; S, 15.70. Found: C, 59.15; H, 3.94; N, 13.86; S, 15.90.
1H and 13C NMR spectra and the solubility of these monomers are
shown in Figures S1 and S2 and Table S1, respectively (see Supporting
Information). It was necessary to store the monomers under nitrogen
in a freezer. In air at room temperature, pure BTVS existed as a pale
yellow liquid was changed gradually into yellow liquid, whereas 1,3,4-
TVS and M-1,3,4-TVS, which were transparent liquids initially,
changed into dark red liquids after several weeks. The relative
stabilities of these monomers were monitored by visual observation
and decreased in stability in the following order: BTVS and P-1,3,4-
OVS (more stable) > 2-TVS > 1,3,4-TVS and M-1,3,4-TVS (less
stable).
General Polymerization Procedure. All polymerizations were
carried out in a degassed sealed tube. A representative example is as
follows: BTVS (0.39 g, 2.0 mmol), CTA 3 (4.7 mg, 0.02 mmol), and
AIBN (1.6 mg, 0.01 mmol) were placed in a dry glass ampule
equipped with a magnetic stirr bar, and then the solution was degassed
by three freeze−evacuate−thaw cycles. After the ampule was flame-
sealed under vacuum, it was stirred at 70 °C for 24 h. Since the
reaction was performed in bulk, the content of the flask was gradually
changed from the light yellow liquid into viscous material with the
polymerization time. The reaction was stopped by rapid cooling with
liquid nitrogen. For the determination of the monomer conversion, the
1H NMR spectrum of the polymerization mixture collected just after
the polymerization was measured in CDCl3 at room temperature. The
integration of the benzene ring protons of the side chain in polymer
(4H) at 8.3−6.8 ppm was compared with the monomer CH2CH−
resonance (2H) at around 5.7 ppm. Conversion determined by this
EXPERIMENTAL SECTION
■
Materials. 2,2′-Azobis(isobutyronitrile) (AIBN, Kanto Chemical,
97%) was purified by recrystallization from methanol. Styrene (St,
TCI, >99%) was purified by distillation under reduced pressure. Other
materials were used without further purification.
Eight different chain transfer agents (CTAs) were employed in this
study, as shown in Scheme 1. O-Ethyl-S-(1-ethoxycarbonyl)ethyl
dithiocarbonate (CTA 1) and O-ethyl-S-(1-phenylethyl) dithiocar-
bonate (CTA 2)55,56 were synthesized by the reaction of potassium
ethyl xanthogenate with corresponding bromide (ethyl 2-bromopro-
pionate for CTA 1 and 1-bromoethylbenzene for CTA 2) according to
a procedure reported in the literature. Benzyl 1-pyrrolecarbodithioate
(CTA 3),57,58 benzyl dithiobenzoate (CTA 4),59,60 and cumyl
dithiobenzoate (CTA 5)61,62 were synthesized according to the
procedures reported previously. 2-Cyano-2-propyldodecyl trithiocar-
bonate (CTA 6, Aldrich, >97%), 4-cyano-4-[(dodecylsulfanyl-
thiocarbonyl)sulfanyl]pentanoic acid (CTA 7, Aldrich, >97%), and
2-(dodecylthiocarbonothioylthio)-2-methylpropanoic acid (CTA 8,
Aldrich, >98%) were used as received.
Synthesis of Azolyl S-Vinyl Sulfides. Five azolyl S-vinyl sulfides,
2-benzothiazolyl vinyl sulfide (BTVS), 2-thiazolyl vinyl sulfide (2-
TVS), 1,3,4-thiadiazolyl vinyl sulfide (1,3,4-TVS), 5-methyl-1,3,4-
C
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method was 60%. After the crude sample was dissolved in a small
amount of chloroform, it was purified by reprecipitation into a large
excess of methanol, and the resulting product was dried under vacuum
at room temperature (0.176 g, 45%). The resulting poly(BTVS) was
soluble in THF, 1,4-dioxane, chloroform, and DMF and insoluble in n-
hexane, methanol, and water.
mL/min, 1.1 kW, 7.4 V, 900 V, 60 ns, 3 times, 3 times, and 0.96 mL/
min. Dynamic light scattering (DLS) was performed using a Zetasizer
Nano (Sysmex) with a He−Ne laser. Thermogravimetric analysis
(TGA) was performed on a SEIKO TGA/6200 at a heating rate of 10
°C/min under a nitrogen atmosphere. Cyclic voltammetry (CV)
experiments were performed on a BAS electrochemical analyzer
(model 660C) in the DMSO solutions with 0.1 M tetrabutyl-
ammonium hexafluorophosphate as a supporting electrolyte (scan
rate: 0.1 V/s). A three-electrode cell was used with platinum electrodes
as both the counter and working electrodes. Silver/silver ion (Ag in 0.1
M AgNO3 solution) was used as the reference electrode. Ferrocene/
ferrocenium (Fc/Fc+) was used as an internal standard. The potential
values obtained in reference to Ag/Ag+ were converted to the values
relative to the saturated calomel electrode (SCE).
Similarly, poly(2-TVS), poly(1,3,4-TVS), poly(M-1,3,4-TVS), and
poly(P-1,3,4-OVS) were prepared by RAFT polymerization of the
corresponding S-vinyl sulfide derivatives (2-TVS, 1,3,4-TVS, M-1,3,4-
1
TVS, and P-1,3,4-OVS). The H NMR spectra and the solubilities of
the resulting polymers are shown in Figures S3 and S4 and Table S1
(see Supporting Information).
The theoretical number-average molecular weight upon conversion
is defined as follows:
[Monomer]0
[CTA]0
RESULTS AND DISCUSSION
Mn(theory) =
× MMonomer × conv + MCTA
■
(1)
Monomer Synthesis and Screening of Polymerization
Conditions. Recently, we reported a series of investigations
concerning the developments of vinyl heterocyclic compounds
as building blocks for advanced functional materials, particularly
N-vinyl65 and S-vinyl49 monomers applicable for controlled
radical polymerizations. Key issues for further advances in this
field include the development of effective synthetic methods,
fine-tuning of the chemical structures of the heterocyclic
compounds, their location and distribution, and manipulation
of three-dimensional assembled structures under various
environments. In this study, we synthesized a variety of S-
vinyl heterocyclic compounds, which were then used in RAFT
polymerization to afford well-defined polymeric materials with
various architectures and characteristic properties. Five S-vinyl
sulfide derivatives having azole units (BTVS, 2-TVS, 1,3,4-TVS,
M-1,3,4-TVS, and P-1,3,4-OVS, see Scheme 1) were synthe-
sized by the reaction of 1,2-dibromoethane with corresponding
thiol compounds. The synthesis was carried out following a
previously reported, but slightly modified, method (Scheme S1,
Supporting Information).49,63 BTVS, 2-TVS, 1,3,4-TVS, and M-
1,3,4-TVS are liquid at room temperature, whereas P-1,3,4-
OVS was used as a solid monomer. This synthetic method is
advantageous because it involves a simple one-step reaction and
allows synthesis of structurally diverse polymers through the
employment a variety of heterocyclic thiol compounds.
Another attractive feature is their feasibility for use as azole-
based heterocyclic building blocks and functional units for the
construction of macromolecules, in which various properties
can be manipulated by the structures of the heterocyclic units.
Initially, free radical polymerization of BTVS was investigated
to find conditions for obtaining thiazole-based polymers having
relatively high molecular weights with sufficient yields. Effects
of the polymerization temperature and solvent on free radical
polymerizations were examined with AIBN as an initiator at
[M]0/[I]0 = 200 under various conditions (see Tables S2, S3
and Figures S5, S6, Supporting Information). Bulk polymer-
ization of BPVS at 70 °C afforded poly(BTVS) with reasonable
monomer conversion (80%) and high molecular weights (Mn =
19 200, Mw/Mn = 1.97). Free radical polymerization of BTVS
was also conducted at 100 °C using different initiators (Table
S4). However, higher polymerization temperature led to the
formation of the product having lower molecular weights,
which could account for an inherent tendency to undergo chain
transfer reactions at higher temperatures. We also investigated
free radical polymerization of other azolyl S-vinyl sulfides (2-
TVS, 1,3,4-TVS, M-1,3,4-TVS, and P-1,3,4-OVS; see Table S5
and Figure S7), indicating that similar polymerization behaviors
were obtained, depending on the structure of the azole units.
where MCTA and MMonomer are the molecular weights of CTA and
monomer, and [Monomer]0 and [CTA]0 are the initial concentrations
of monomer and CTA, respectively.
Synthesis of Metal Complex. The preparation of metal
complexes was carried out according to an existing method.26 The
typical procedure is as follows; poly(BTVS) (0.10 g, 0.52 unit mmol
based on the repeating unit) and CuCl2 (0.070g) (molar ratio of
BTVS unit:CuCl2 = 1:1) were dissolved in DMF at 5% concentration
and stirred at 80 °C for 24 h under a nitrogen atmosphere. The final
product was obtained by precipitating the resulting solution in
methanol and filtering. The resulting product was dried under vacuum
at 60 °C to yield poly(BTVS)-Cu as dark brown solid (0.073 g, 43%).
Synthesis of Block Copolymers. A representative example of the
synthesis of the block copolymer composed of BTVS and St is as
follows: BTVS (1.93 g, 10 mmol), CTA 6 (34.5 mg, 0.1 mmol), and
AIBN (8 mg, 0.05 mmol) were placed in a dry ampule, and then the
mixture was degassed by three freeze−evacuate−thaw cycles. After the
ampule was flame-sealed under vacuum, it was stirred at 60 °C for 6 h.
After the product was purified by precipitation from a chloroform
solution into a large excess of methanol, the product was dried under
vacuum at room temperature to afford poly(BTVS) as a yellow solid
(0.65 g, 34%, Mn = 3500 g/mol, Mw/Mn = 1.21), which was employed
as a macro-CTA. The trithiocarbonate-terminated poly(BTVS) (0.069
g, 0.02 mmol), AIBN (1.6 mg, 0.01 mmol), and St (0.208 g, 2.00
mmol) were placed in a dry ampule. After the solution was degassed
by three freeze−evacuate−thaw cycles, the polymerization was
conducted at 70 °C for 24 h. The reaction mixture was purified by
reprecipitation from a chloroform solution into a large excess of
hexane and isolated by filtration to give a block copolymer,
poly(BTVS)-b-poly(St), as a pale yellow solid.
Instrumentation. 1H (400 MHz) and 13C NMR (100 MHz)
spectra were recorded with a JEOL JNM-ECX400. Elemental analysis
was carried out on a PerkinElmer 2400 II CHNS/O analyzer. The
number-average molecular weight (Mn) and molecular weight
distribution (Mw/Mn) were estimated by size-exclusion chromatog-
raphy (SEC) using a Tosoh HPLC HLC-8220 system equipped with
refractive index and ultraviolet detectors at 40 °C. The column set was
as follows: four consecutive hydrophilic vinyl polymer-based gel
columns [TSK-GELs (bead size, exclusion limited molecular weight):
α-M (13 μm, >1 × 107), α-4000 (10 μm, 4 × 105), α-3000 (7 μm, 9 ×
104), α-2500 (7 μm, 5 × 103), 30 cm each] and a guard column [TSK-
guard column α, 4.0 cm]. The system was operated at the flow rate of
1.0 mL/min using DMF containing 10 mM LiBr as the eluent.
Polystyrene standards (Tosoh) ranging from 1050 to 1 090 000 were
employed for calibration.
The UV−vis spectra were recorded using a JASCO V-630BIO UV−
vis spectrophotometer. The fluorescence spectra were obtained from a
JASCO FP-6100 spectrofluorophotometer. Inductive-coupled plasma
mass (ICP-MS) spectrometry measurements were performed on a
PerkinElmer ELAN DRC II spectrometer. Nebulizer gas flow, ICP RF
power, lens voltage, pulse stage voltage, dwell time, sweeps, readings
per replicate, and flow rate were respectively as follows: 0.91−1.01
D
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On the basis of these preliminary results, we selected bulk
polymerization of BPVS at 70 °C for further investigations
toward the synthesis of well-defined poly(azolyl S-vinyl
sulfide)s.
a low-molecular-weight polymer was obtained when using the
dithiobenzoate-type CTA 4. RAFT polymerization of BTVS at
100 °C using V-40, 1,1′-azobis(cyclohexane-1-carbonitrile),
with CTA 6 could not be controlled compared to that using
AIBN at 60 °C (Table S4). Therefore, we concluded that the
RAFT polymerization with CTAs 3, 6, and 8 at moderate
temperature was the suitable conditions in the case of azolyl S-
vinyl sulfide monomers.
RAFT Polymerization of 2-Benzothiazolyl Vinyl Sul-
fide (BTVS). The effect of the nature of the CTA on the radical
polymerization of BTVS was investigated in the presence of
AIBN as an initiator (Scheme 1). Previously, we reported that
three mediating agents with different Z groupsO-ethyl-S-(1-
ethoxycarbonyl) ethyldithiocarbonate (CTA 1), benzyl 1-
pyrrolecarbodithioate (CTA 3), and 2-cyano-2-propyldodecyl
trithiocarbonate (CTA 6)were efficient to achieve controlled
RAFT polymerization of phenyl S-vinyl sulfide derivatives,49−52
in spite of the significant difference in the chain transfer
constants. This may be due to the fact that S-vinyl sulfides have
intermediate reactivity between the conjugated and non-
conjugated monomers. On the basis of this, the bulk
polymerization of BTVS was conducted with different CTAs
at 70 °C for 24 h at [BTVS]0/[CTA]0/[AIBN]0 = 200/2/1.
The results are summarized in Table 1. Lowest polydispersities
In the next stage, the effect of the [CTA]0/[AIBN]0 ratio on
monomer conversion, molecular weights, and polydispersity of
the resulting poly(BTVS)s was examined. RAFT polymer-
ization of BTVS was conducted with CTAs 1−4, 6, and 8 at 70
°C for 24 h. The [CTA]0/[AIBN]0 ratio was adjusted to be
between 2 and 10, keeping the monomer/CTA molar ratio at
100/1 (Table S6). As expected, the monomer conversion
decreased as the [CTA]0/[AIBN]0 ratio increased from 2 to 10,
which is consistent with the general tendency for higher CTA/
initiator ratio (lower concentration of the initiator) to lead to
lower polymerization rates. In contrast, the effect of the
[CTA]0/[AIBN]0 ratio on the molecular weight distributions of
the resulting poly(BTVS) was not as remarkable (Figure S9).
In RAFT polymerization, the number of CTAs that have
effectively fragmented and reinitiated polymerization plus the
number of initiator-derived chains can contribute to the total
number of polymer chains. If the initiator-derived chains are
taken into account, the molecular weights of resulting polymers
decrease with decreasing [CTA]0/[Initiator]0 ratio when the
monomer-to-CTA ratio and monomer conversion are constant.
In an ideal RAFT process, however, the number of polymer
chains originated from initiators should be minimal, and the
number of CTA-derived chains is much larger than that of the
initiator-derived chains.66,67 In our system, as can be seen in
Table S6, both the conversion and molecular weights decreased
with increasing [CTA]0/[AIBN]0 ratio, whereas the poly-
dispersity remained narrow (Mw/Mn = 1.12−1.18 for CTAs 6
and 8 and Mw/Mn = 1.18−1.33 for CTA3).These results
indicate that reasonable control of the polymerization of BTVS
can be achieved in bulk RAFT polymerization using CTAs 3, 6,
and 8 at lower CTA/initiator ratio ([CTA]0/[AIBN]0 ratio =
2) at 70 °C, which are optimal conditions adjusting the trade-
off between the polymerization rate and controllability of the
radical polymerization.
Table 1. RAFT Polymerization of BTVS with AIBN Using
a
Different CTAs in Bulk at 70 °C for 24 h
b
c
b
d
d
conv
Mn
Mn (1H
Mn
Mw/Mn
entry
CTA
(%)
(theory)
NMR)
(SEC)
(SEC)
e
1
2
3
4
5
6
7
8
CTA 1
CTA 2
CTA 3
CTA 4
CTA 5
CTA 6
CTA 7
CTA 8
70
70
60
40
<5
75
<5
79
14000
14000
11000
8000
−
−
9400
10000
6100
1400
−
1.57
1.51
1.18
1.54
−
15000
14000
e
−
−
18000
15000
6000
1.15
−
16000
−
24000
−
5400
-
1.17
a
b
1
[BTVS]0/[CTA]0/[AIBN]0 = 200/2/1. Calculated by H NMR in
c
CDCl3. The theoretical molecular weight (Mn,theory) = (MW of
BTVS) × [BTVS]0/[CTA]0 × conv + (Mw of CTA). Number-
d
average molecular weight (Mn) and molecular weight distribution
(Mw/Mn) were measured by size-exclusion chromatography (SEC)
e
using polystyrene standards in DMF (10 mM LiBr). Undetectable.
(Mw/Mn = 1.15−1.17) and reasonable monomer conversions
1
(75−79%, as determined by H NMR spectroscopy) were
obtained by the polymerization using the trithiocarbonate-type
CTAs 6 and 8. RAFT polymerizations of BTVS with the
dithiocarbamate-type CTA 3 also afforded a poly(BTVS) with a
narrow molecular weight distribution (Mw/Mn = 1.18). In those
cases, the molecular weights of the polymer chains calculated by
comparing the integrals of the peaks for the chain-end protons
to those of the main-chain protons were comparable to the
theoretical values, which were higher than those obtained from
size exclusion chromatography (SEC) in DMF (10 mM LiBr)
using polystyrene standards. Note that the thiocarbonylthio end
groups (dithiocarbamate moiety for CTA 3 and dithiocarbonate
moiety for CTAs 6 and 8), which correspond to the ω-end
groups derived from the Z group of CTA, were employed for
the evaluation (Figures S3 and S4). Unimodal SEC peaks were
detected in the SEC profiles of the poly(BTVS)s obtained with
both the dithiocarbamate-type and trithiocarbonate-type
mediating agents (CTAs 3, 6, and 8), as shown in Figure S8.
In contrast, poly(BTVS) obtained via polymerization with the
xanthate-type mediating agents (CTAs 1 and 2) had relatively
broad SEC peaks (Mw/Mn > 1.5). Using CTAs 5 and 7,
extremely low monomer conversions (<5%) were obtained, and
To manipulate the molecular weights of the resulting
poly(BTVS) polymers, the polymerization of BTVS was
conducted at [M]0/[CTA]0 ratios between 100 and 600,
using a ratio of CTA to AIBN of 1:2 (Figure 1 and Table S7).
When the polymerization of BTVS was conducted using CTA 3
in bulk at 70 °C for 24 h, a linear increase in the number-
average molecular weights with increasing [M]0/[CTA]0 ratio
was observed, and the molecular weight distributions remained
relatively narrow (Mw/Mn = 1.18−1.29) until [M]0/[CTA]0 =
600. A shift in the SEC trace toward higher-molecular-weight
regions with a unimodal peak was seen at higher [M]0/[CTA]0
ratios (Figure 1), demonstrating the efficient control over
polymer molecular weight. A similar tendency was also
observed in RAFT polymerizations of BTVS with trithiocar-
bonate-type mediating agents (CTAs 6 and 8). In contrast, the
polymerizations with CTAs 1, 2, and 4 afforded the polymers
with broad molecular weight distributions or uncontrolled
molecular weights (Figure S10). Nevertheless, these results
confirm the feasibility to manipulate the molecular weights of
poly(BTVS)s by adjusting the monomer-to-CTA ratio in
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poly(BTVS)s, as reaction proceeds up to about 60%
conversion. Furthermore, the number-average molecular
weight, Mn, increases in direct proportion to the conversion,
as shown in Figure 2b,c. These kinetic results suggest that the
polymerization of the benzothiazole-based monomer, BTVS,
mediated by the CTAs 3 and 6 proceeds in a controlled manner
at 70 °C under the conditions used in this study, in which
unfavorable chain transfer reactions can be suppressed by the
RAFT process.
RAFT Polymerization of S-Vinyl Sulfides Containing
Different Azole Units. The RAFT polymerization of four S-
vinyl sulfide derivatives containing different azole units was also
conducted at a ratio [M]0/[CTA]0/[AIBN]0 of 200/2/1 at 60
°C in bulk, and the results are summarized in Table 2.
Depending on the structure of the azole units, the S-vinyl
sulfide derivatives showed different reactivities for RAFT
polymerizations. When the polymerization of 2-TVS, which is
a thiazole derivative, was carried out using with CTAs 3 and 6,
poly(2-TVS)s with relatively low polydispersities (Mw/Mn =
1.23−1.25) were obtained with reasonable conversions degree
(56−68%, as determined by 1H NMR spectroscopy). The same
tendency was observed in RAFT polymerization of P-1,3,4-
OVS, which is an oxadiazole derivative. In both cases,
symmetrical unimodal SEC peaks without shoulders or tails
were seen for the poly(azolyl S-vinyl sulfide) polymers obtained
using CTAs 3 and 6 (Figure S11). These results agree well with
those of polymerization with BTVS, in which the dithiocarba-
mate-type CTA 3 and the trithiocarbonate-type CTA 6 were
the most efficient mediating agents to achieve controlled
character of the polymerization of the azolyl S-vinyl sulfide
derivatives.
In the cases of 1,3,4-TVS and M-1,3,4-TVS, which are
thiadiazole derivatives, free radical polymerizations at 60 °C
afforded polymers with high molecular weights and broad
molecular weight distributions (Mn = 19 000−43 000, Mw/Mn =
1.96−2.26); however, the monomer conversions were relatively
low (<35%). RAFT polymerization of 1,3,4-TVS and M-1,3,4-
TVS with CTA 3 led the formation of polymers with narrow
molecular weight distributions (Mw/Mn = 1.21−1.25), while a
negligible quantity of polymer was obtained when using CTA 6.
In both cases, the monomer conversions in the RAFT
polymerization with CTA 3 were comparable to those in free
radical polymerizations, which is believed to be related to
diffusion-controlled termination. Nevertheless, polymers with
relatively narrow molecular weight distributions were obtained
by RAFT polymerization of all azolyl S-vinyl monomers using
the dithiocarbamate-type CTA 3.
The polymerization of 1,3,4-TVS was investigated kinetically
with the dithiocarbamate-type CTA 3 in bulk at 60 °C and
[M]0/[CTA]0/[I]0 = 200/2/1. As can be seen in the time−
conversion and the pseudo-first-order kinetic plots (Figure
S12), only 25% conversion was reached after 24 h, the plot
having a curved semilogarithmic plot; this may be due relatively
high viscosity of the reaction mixture during bulk polymer-
ization. A similar conversion “plateau” was also detected for
RAFT polymerization of BTVS with CTA 3 (Figure 2a).
Figure 1. SEC traces of poly(BTVS)s obtained by RAFT polymer-
ization at different [AIBN]0/[CTA]0 ratios with (a) CTA 3, (b) CTA
6, and (c) CTA 8.
RAFT polymerization with appropriate mediating agents
(CTAs 3, 6, and 8) under suitable conditions.
The kinetic study of the BTVS polymerization was
conducted in the presence of CTAs 3 and 6 at [BTVS]0/
[CTA]0/[AIBN]0 = 200/2/1 in bulk at 70 °C (Figure 2). On
polymerization with CTA 6, a relatively high conversion
(>70%) occurred after 24 h (Figure 2d). Figure 2e shows a
linear increase in the number-average molecular weight, as
1
determined by H NMR measurements, with conversion. The
theoretical molecular weight, however, was inconsistent with
the experimental values determined by NMR and SEC.
Nevertheless, the SEC traces of poly(BTVS) polymers obtained
at different reaction times indicate a progressive increase in the
molar mass with the monomer conversion and narrow
unimodal SEC peaks (Mw/Mn = 1.12−1.15). In the case of
the polymerization of BTVS with CTA 3, the polymerization
rate was slower than that with CTA 6 (Figure 2a). Because the
polymerization was conducted in bulk, a conversion plateau was
present, and this was dependent on the viscosity of the reaction
mixtures and the molecular weights of the resulting polymers.
Nevertheless, symmetrical unimodal SEC peaks (Mw/Mn
=
1.24−1.30) were detected for the poly(1,3,4-TVS)s, suggesting
that the thiadiazole-based polymers with relatively low
polydispersities can be obtained by RAFT polymerization of
1,3,4-TVS under suitable conditions.
Nonetheless, symmetrical unimodal SEC peaks (Mw/Mn
1.18−1.23) without shoulders or tails were seen for the
=
Formation of Metal Complexes with Azole-Based
Polymers. Synthetic polymers containing metal species are
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Figure 2. (a, d) Time−conversion (circles) and first-order kinetic (squares) plots for polymerization of BTVS with AIBN in the presence of (a−c)
CTAs 3 and (d−f) 6 ([BTVS]0/[CTA]0/[AIBN]0 = 200/2/1) in bulk at 70 °C. (b, e) Number-average molecular weights (NMR: triangles; SEC:
circles) and polydispersity (squares) as a function of conversion. (c, f) SEC traces of poly(BTVS)s.
a
Table 2. RAFT Polymerization of S-Vinyl Sulfide Derivatives Using Different CTAs in Bulk at 60 °C for 24 h
b
c
b
d
d
entry
monomer
2-TVS
CTA
conv (%)
Mn (theory)
Mn (1H NMR)
Mn (SEC)
Mw/Mn (SEC)
1
−
73
68
56
30
28
<5
−
−
40000
4200
3000
19000
3400
−
43000
7800
−
3.99
1.25
1.23
1.96
1.25
−
2.26
1.21
−
2
CTA 3
CTA 6
−
CTA 3
CTA 6
−
CTA 3
CTA 6
−
10000
8400
−
11000
10000
−
3
4
1,3,4-TVS
5
3600
2800
6
−
−
5300
−
f
7
M-1,3,4-TVS
34
−
−
f
g
8
32
f
9
0
−
−
11000
−
−
22000
e
10
11
12
P-1,3,4-OVS
60
51
13000
4900
5600
1.53
1.31
1.23
CTA 3
CTA 6
46
9700
13000
a
b
c
1
[M]0/[CTA]0/[AIBN]0 = 200/2/1. Calculated by H NMR in CDCl3. The theoretical molecular weight (Mn,theory) = (MW of monomer) ×
d
[M]0/[CTA]0 × (conv or yield) + (Mw of CTA). Number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were
e
measured by size-exclusion chromatography (SEC) using polystyrene standards in DMF (10 mM LiBr). Polymerization in DMF (10 mmol/mL).
f
g
Yield: methanol-insoluble part. Undetectable.
functional materials with a wide range of applications.68−70 In
this study, the preparation of metal complexes was carried out
between the resulting azole-based polymers and MCl2 (M =
Cu, Ni, and Zn) in DMF at 80 °C for 24 h (Scheme 2)
according to the previous literature.26 The results are
summarized in Table 3. When complex formation was carried
out at a 1:1 molar ratio of the BTVS unit and metal species
(MCl2), the poly(BTVS)−metal complexes were obtained with
reasonable yields (43−57%), and they showed various colors,
depending on the nature of the metal (Figure S13). The metal
contents were estimated to be 11, 0.16, and 0.50 wt % in
poly(BTVS)-Cu, poly(BTVS)-Ni, and poly(BTVS)-Zn, respec-
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nature of the metal species significantly affects the progress of
the coordination reactions. Furthermore, the solubility of the
obtained complexes changed compared to pristine poly(BTVS)
(Table S8), which should result from the formation of the
complexes (poly(BTVS)−Cu, poly(BTVS)−Ni, and poly-
(BTVS)−Zn, respectively).
Table 3. Formation of Poly(BTVS)−Metal Complexes at 80
a
°C for 24 h
b
c
thiazole
unit:M
yield
M cont
M cont
(mol %) appearance
a
entry metal
(%)
(wt %)
1
Zn
1:1
51
0.50
1.5
light
yellow
From dynamic light scattering (DLS) analysis, the hydro-
dynamic diameter distribution (Dh) of the pristine poly-
(BTVS), poly(BTVS)−Cu, poly(BTVS)−Ni, and poly-
(BTVS)−Zn complexes prepared at BTVS unit/MCl2 = 1/1
molar ratio were estimated to be 9.7, 730, 8.0, and 4.5 nm in
DMF, respectively (Figure S14a). The much larger Dh value of
poly(BTVS)−Cu should result from aggregation due to the
stronger coordination, consistent with the ICP analysis. In the
poly(BTVS)−Cu system, many kind of complexes may exist
because of a variety of coordination (i.e., inter- and intrachain
coordination). Even though DLS has the single peak, the peak
has dispersibility; in that regard, the aggregate is a mixture of
several regimes with ca. 200−1100 nm. When complex
formation was conducted at higher metal content (BTVS
unit/MCl2 = 1/5 molar ratio), the yields of the products were
relatively low (yield <25%), and the metal content increased
slightly for the complexes of poly(BTVS)−Ni and poly-
(BTVS)−Zn and decreased for the poly(BTVS)−Cu. The
DLS result of the complex prepared at BTVS unit/CuCl2 = 1/5
exhibited bimodal distributions (Figure S14b), suggesting that
the composition in the feed affects size and size distributions of
the polymer−Cu complexes. As can be seen in Table 4, the
2
3
Ni
57
43
0.16
11
0.54
brown
Cu
28
dark
brown
4
5
Zn
Ni
1:5
22
22
1.8
5.2
2.9
yellow
dark
brown
dark
brown
0.90
6
Cu
16
2.3
6.5
a
b
c
Zn:ZnCl2, Ni:NiCl2, Cu:CuCl2. Methanol-insoluble part. Deter-
mined by ICP.
tively, determined by inductive-coupled plasma mass (ICP-MS)
measurements. The obtained ICP analysis indicated that the
benzothiazole rings were coordinated more strongly to copper
ions than to either nickel or zinc species.
Figure 3 shows FT-IR spectra of the pristine poly(BTVS),
poly(BTVS)-Cu, poly(BTVS)-Ni, and poly(BTVS)-Zn com-
Table 4. Formation of Poly(S-vinyl sulfide)−Cu Complexes
a
at 80 °C for 24 h
b
c
yield
M cont
M cont
(mol %)
entry
polymer
(%)
(wt %)
1
2
3
4
5
poly(BTVS)
43
23
58
62
11
11
8.4
19
8.6
28
22
17
38
23
poly(2-TVS)
poly(1,3,4-TVS)
poly(M-1,3,4-TVS)
poly(P-1,3,4-OVS)
46
a
b
Azole unit:M = 1:1, M:CuCl2. Methanol-insoluble part. Dark brown
c
product was obtained in all cases. Determined by ICP.
metal contents of poly(2-TVS)−Cu, poly(1,3,4-TVS)−Cu,
poly(M-1,3,4-TVS)−Cu, and poly(P-1,3,4-OVS)−Cu (8.4−19
wt %) prepared at the monomer unit/CuCl2 = 1/1 molar ratio
were comparable to that of poly(BTVS)−Cu (11 wt %),
suggesting that these azole units have ability to coordinate to
copper species.
Figure 3. FT-IR spectra of poly(BTVS) and metal complexes. Lower
figure: magnification in the range of 1800−500 cm−1.
plexes. Characteristic peaks between 725 and 755 and 1502
cm−1 are seen for poly(BTVS), which may be attributed to the
thioether C−S bond and the thiazole ring, respectively.
Characteristic vibrations have been reported previously; for
example, the C−S bond71 and thiazole ring26 are indicated by
peaks at 750 and 1535 cm−1. In the spectrum of poly(BTVS)
obtained after Cu complexation, the peak attributed to the
thiazole ring at 1502 cm−1 almost disappeared, suggesting that
the thiazole ring contributes to the complexation with copper
species. In addition, a new peak at around 1670 cm−1 is also
detected in the poly(BTVS)−Cu complex, which may
correspond to the CN stretch in the benzothiazole unit
shifting after complexation with the metal atom. In contrast,
both poly(BTVS)−Ni and poly(BTVS)−Zn showed no
significant changes in their FT-IR spectra, suggesting that the
Optical and electrochemical behaviors of the metal
complexes were investigated by UV−vis absorption, fluores-
cence, and cyclic voltammetry (CV) measurements. The UV−
vis absorption and fluorescence measurements were carried out
in DMF, and we confirmed, visually, that all the complexes
were soluble, forming homogeneous solutions. In UV−vis
spectra of poly(BTVS) and the metal complexes (Figure 4a),
poly(BTVS) had an absorption band due to the benzothiazole
units below 300 nm; in contrast, the absorption peaks of
poly(BTVS)−Cu, poly(BTVS)−Ni, and poly(BTVS)−Zn
complexes were red-shifted compared to that of poly(BTVS).
This changes in the UV−vis spectra resulted from the
elongation of the conjugated structure due to the coordination
between benzothiazole rings and metal species (N···M2+···N),
in addition to the thioether−metal interaction (S···M2+···S). In
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Figure 4. (a, c) Absorption and (b, d) fluorescence spectra of (a, b)
poly(BTVS), poly(BTVS)−Cu, poly(BTVS)−Ni, and poly(BTVS)−
Zn complexes (the excitation wavelengths were 302, 340, 365, and 365
nm, respectively) and (c, d) poly(BTVS)−Cu (Cu content = 11 and
2.9 wt % for blue and red lines, respectively; excitation wavelength was
340 nm) in DMF solutions (concentration = 0.1 mg/mL).
Figure 5. (a) Absorption and (b) fluorescence (excitation wavelength:
340 nm) spectra of complexes of the azole-based polymers with
copper in DMF (concentration = 0.1 mg/mL).
the fluorescence spectra, the metal complexes exhibited
fluorescence due to the elongation of the conjugated structure;
in particular, strong fluorescence was observed in poly-
(BTVS)−Cu (Figure 4b). Interestingly, the fluorescence
spectra of poly(BTVS)−Ni and poly(BTVS)−Zn changed
after 24 h, whereas no significant change was observed in the
fluorescence spectrum of poly(BTVS)−Cu (Figure S15). This
result suggests that complexes formed on coordination to
copper species are more stable than those formed on
coordination to zinc and nickel species in poly(BTVS). For
poly(BTVS)−Cu, the relationship between fluorescence
behavior and metal content in the complex was investigated
in detail. In the fluorescence spectra of two poly(BTVS)−Cu
(Cu content = 2.9 and 11 wt %), maximum emissions were
observed at 421 and 491 nm in poly(BTVS)−Cu (Cu content
= 2.9 wt %) and poly(BTVS)-Cu (Cu content = 11 wt %),
respectively (Figure 4d). The higher metal content led to peaks
being more red-shifted in the UV−vis spectra (Figure 4c). This
shift suggests that the metal content is the dominant driving
force that leads to a longer conjugated structure, resulting in
differences in the fluorescence behaviors of the complexes.
Therefore, the stability of the fluorescence signals, which occur
on complexation of poly(BTVS) with metal species, can be
controlled by selecting the correct metal species and the metal
content.
The electrochemical properties of the metal complexes with
copper species were investigated by the CV measurements
(Figure 6). Each experiment was carried out in the degassed
DMSO (or DMF) solutions with 0.1 M tetrabutylammonium
hexafluorophosphate under the nitrogen atomosphere. The
pristine polymers, poly(BTVS), poly(2-TVS), poly(1,3,4-TVS),
poly(M-1,3,4-TVS), and poly(P-1,3,4-OVS), showed clear
reduction waves due to the electron-deficient azole units. In
the previous work by Beloglazkina and co-workers, copper
complex with the benzothiazole-based molecule exhibited the
higher reduction potential in comparison to the pristine
benzothiazole-based molecule.72 As can be seen in Figure 6a,
the reduction potential of poly(BTVS)−Cu was observed at
0.04 V whereas the pristine poly(BTVS) showed the reduction
potential of −0.08 V. The reduction potential of poly(2-TVS)
was also shifted from −0.01 to 0.03 V after the coordination
with copper species (Figure 6b). The change of CV behaviors
was detected in poly(1,3,4-TVS), poly(M-1,3,4-TVS), and
poly(P-1,3,4-OVS). Their CV behaviors were different from
those of poly(BTVS) and poly(2-TVS), which should result
from the structural factor of the azole units. Consequently, the
obtained electrochemical results supported the formation of the
metal complexes with copper species. In these systems, each
redox event is attributed to a mixture of several complexes
formed through coordination between inter- and intrachains.
Then, DLS analysis was used to investigate the metal
complexes formed with copper species. As can be seen in
Figure 7a, the Dh value of poly(BTVS) shifted to the larger
values after complexation with copper species, which could be
due to aggregation caused by the strong coordination between
poly(BTVS) and copper species. Poly(2-TVS)−Cu and poly(P-
1,3,4-OVS)−Cu showed similar DLS behavior to that of
poly(BTVS) (Figure 7b,e), whereas poly(1,3,4-TVS)−Cu and
poly(M-1,3,4-TVS)−Cu had bimodal DLS profiles, indicating
We also investigated the UV−vis absorption and fluorescence
behaviors of metal complexes using poly(2-TVS), poly(1,3,4-
TVS), poly(M-1,3,4-TVS), and poly(P-1,3,4-OVS) with copper
species in DMF. As well as the metal complexes with
poly(BTVS), four metal complexes with copper species had
different spectra compared to the pristine polymers (Figure 5);
this may be due to the coordination of polymer to copper
species. Unfortunately, the complexes did not exhibit the same
strong fluorescence as poly(BTVS)−Cu.
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Figure 7. DLS traces of pristine polymers (blue line) and complexes
with copper (red line) of (a) poly(BTVS), (b) poly(2-TVS), (d)
poly(M-1,3,4-TVS), and (e) poly(P-1,3,4-OVS) in DMF and (c)
poly(1,3,4-TVS) in DMSO at 25 °C (concentration = 2.0 mg/mL).
Figure 6. CV profiles of pristine polymers (blue line) and complexes
with copper (red line) of (a) poly(BTVS), (b) poly(2-TVS), (d)
poly(M-1,3,4-TVS), and (e) poly(P-1,3,4-OVS) in DMF and (c)
poly(1,3,4-TVS) in DMSO at 25 °C (concentration = 2.0 mg/mL).
investigated under the same procedure as that between
poly(BTVS) and CuCl2. According to the job plot from the
experiments (Figures S17and S18), the formation of 1:1
complex was plausible. Furthermore, the intermolecular
complexation may contribute to the formation of aggregated,
network structures (Figure 8a); in contrast, intramolecular
complexation may lead to the production of metallopolymers
without significant changes in the morphology and assembled
structures.
Synthesis of Block Copolymers and Metal Complex-
ation. Block copolymers containing metal centers, so-called
metalloblock copolymers, have attracted much interest as new
functional materials with characteristic assembled architec-
tures.69,73 Here, we developed novel metalloblock copolymers
by complexing metal species with azole-based block copoly-
mers. For the purpose, we synthesized block copolymers by
RAFT polymerization of styrene (St) using poly(BTVS) as a
macro-CTA (Scheme 3a). As mentioned in a previous section,
the controlled radical polymerization of BTVS could be
achieved using the trithiocarbonate-type CTA 6, which is an
effective CTA with high chain transfer constant, in controlled
radical polymerization of conjugated monomers, such as St.
Therefore, trithiocarbonate-terminated poly(BTVS)s with low
molecular weights were initially prepared by RAFT polymer-
ization of BTVS with CTA 6. Then, the resulting macro-CTAs
were employed for block copolymer synthesis. As shown in
Table 5, the CTA-to-initiator ratio ([macro-CTA]0/[AIBN]0)
was held constant at 2, while the monomer-to-CTA ratio
([St]0/[poly(BTVS) macro-CTA]0) was varied from 50 to 400
that the Dh values correspond to pristine polymers and
aggregates (Figure 7c,d). These results demonstrate that the
structure of azole units in polymers was the most important
factor controlling the coordination ability to the metal species.
In the thermogravimetric analysis (Figure S16), decomposition
began at around from 240 °C in poly(BTVS). On the other
hand, the metal complexes poly(BTVS)−Cu, poly(BTVS)−Ni,
and poly(BTVS)−Zn exhibited high thermal stability, as did the
pristine polymer.
After the discovery of the relevance of Cu−thioether bonds
in blue copper proteins, a large number of compounds
containing thioether groups have been developed to investigate
complexation with Cu species.45−48 For example, various
imidazole compounds containing the thioether group were
investigated, in which both the imidazole nitrogen atom and the
thioether sulfur atom act as donor components to afford copper
complexes.45,48 In mainly blue copper proteins, the orbital
containing an unpaired electron of Cu(II) overlaps with
histidine and cysteine moieties.45,46 Concerning color, the
deep blue color is reported to be caused by charge transfer in
the Cu−thiolate bond in the blue copper proteins.46 In our
system, both nitrogen atoms of the benzothiazole ring and
sulfur atom of the thioether moiety in the monomer unit
(BTVS) may chelate with Cu species via intramolecular and
intermolecular complexation. To understand the complex
formation between poly(BTVS) and Cu species, the behavior
of complex formation between BTVS monomer and CuCl2
with the several different [BTVS]/[CuCl2] ratios was
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Figure 8. Postulated structures of (a) poly(BTVS)−Cu complexes and (b) assembled structures of the benzothiazole-based block copolymer and
metalloblock copolymer in ethyl acetate (EtOAc) and THF.
Scheme 3. Synthesis of Block Copolymers by RAFT Polymerization of Styrene Using Poly(azoyl S-vinyl sulfide) Macro-CTAs
to control the comonomer content and molecular weight.
Under these conditions, the targeted block copolymers were
obtained in high yields (yield >85%) with relatively narrow
molecular weight distributions (Mw/Mn = 1.26−1.44). The
molecular weights of the benzothiazole-based block copolymers
evaluated from the compositions determined by elemental
analysis and the molecular weights of the poly(BTVS) macro-
CTAs were comparable to the theoretical values. The SEC
chromatograms of the starting macro-CTA and growth
polymers showed that the molecular weight clearly shifted to
higher molecular weights at higher [St]0/[macro-CTA]0 ratios
(Figure 9a). These results clearly demonstrate the successful
synthesis of poly(BTVS)-b-poly(St) block polymers, in which
one segment has the ability to coordinate to various metal
species.
The same synthetic strategy was used for the synthesis of
block copolymers composed of 2-TVS and St, in which RAFT
polymerization of St was conducted using poly(2-TVS) as a
macro-CTA, as shown in Scheme 3b. The thiazole-based block
copolymers, poly(2-TVS)-b-poly(St)s, with predetermined
molecular weights and relatively low polydispersities (Mn,SEC
= 8400−35 000, Mw/Mn = 1.16−1.30) were obtained with
reasonable yields (60−82%, Table 5). Increasing the monomer-
to-macro-CTA ratio leads to a shift in the SEC trace toward
higher molecular weights with no significant homopolymer
impurity (Figure 9b), clearly demonstrating efficient block
formation. Thus, chain extension from the trithiocarbonate-
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Table 5. Synthesis of Block Copolymers by RAFT Polymerization of Styrene Using Poly(azoyl S-vinyl sulfide) Macro-CTAs for
a
24 h in Bulk
b
c
d
e
e
d
entry
macro-CTA
[M]0/[Macro-CTA]0
yield (%)
Mn (theory)
Mn (EA)
Mn (SEC)
Mw/Mn (SEC)
composition BTVS:St
f
h
1
poly(BTVS)
50
100
200
400
100
200
50
87
96
94
92
91
95
70
81
82
8000
13000
23000
42000
16000
26000
7100
5800
9100
9000
16000
28000
52000
17000
28000
8400
1.36
1.30
1.29
1.26
1.39
1.44
1.30
1.21
1.16
1.22
45:55
25:75
14:86
9:91
f
2
f
3
15000
22000
12000
17000
4000
f
4
f
i
5
poly(BTVS)
42:58
26:74
77:23
49:51
30:70
19:81
f
6
g
j
7
poly(2-TVS)
g
8
100
200
12000
20000
28000
5300
14000
27000
35000
g
9
7700
g
10
400
60
11000
a
b
c
[Macro-CTA]0/[AIBN]0 = 2. Hexane-insoluble part. The theoretical molecular weight (Mn,theory) = (MW of St) × [St]0/[Macro-CTA]0 × yield +
d
e
(MW of Macro-CTA). Calculated by elemental analysis (EA). Measured by size-exclusion chromatography (SEC) using polystyrene standards in
DMF (10 mM LiBr). Polymerization at 70 °C. Polymerization at 60 °C. Poly(BTVS) macro-CTA. Mn,SEC = 3500, Mw/Mn = 1.21. Mn,SEC = 6800,
Mw/Mn = 1.13. Poly(2-TVS) macro-CTA: Mn,SEC = 3400, Mw/Mn = 1.20.
f
g
h
i
j
contrast, the poly(BTVS)/Cu complex did not dissolve well in
either solvent. As can be seen in Figure 10a, a relatively narrow
Figure 9. SEC traces (DMF + 10 mM LiBr) of (a) poly(BTVS)-b-
Figure 10. (a, b) DLS traces and (c, d) fluorescence (excitation
poly(St)s and poly(BTVS) macro-CTA (Mn,SEC = 3500, Mw/Mn
=
wavelength: 340 nm) spectra of poly(BTVS)-b-poly(St) and poly-
(BTVS)/Cu-b-poly(St) (BTVS: St = 42:58) in (a, c) ethyl acetate and
(b, d) THF at 25 °C (concentration = 2.0 mg/mL).
1.21) and (b) poly(2-TVS)-b-poly(St)s and poly(2-TVS) macro-CTA
(Mn,SEC = 3400, Mw/Mn = 1.20). See Table 5 for detailed
polymerization conditions.
hydrodynamic diameter distribution (Dh = 177 nm) of the
benzothiazole-based block copolymer was observed in ethyl
acetate, indicating the formation of micelles consisting of an
insoluble poly(BTVS) core and soluble poly(St) shell. After
complexation with Cu species, the metalloblock copolymer,
poly(BTVS)/Cu-b-poly(St), showed a remarkable decrease in
the hydrodynamic diameter (Dh = 109 nm), which may be due
to core shrinkage that occurs on complexation. Figure 10c
shows the fluorescence spectra of the block copolymer before
and after complexation in ethyl acetate. In the case of
poly(BTVS)/Cu-b-poly(St), the maximum emission occurs at
421 nm, suggesting that the elongation of the conjugated
structure by the complexation with Cu species occurs even in
the core part of the self-assembled structures. In the DLS
spectrum of poly(BTVS)-b-poly(St) in THF, as expected,
smaller species (Dh < 10 nm) were detected (Figure 10b),
terminated poly(azolyl S-vinyl sulfide) macro-CTAs to St can
be successfully achieved under the conditions, providing azole-
based block copolymers with an as-designed chain structure
and narrow molecular weight distributions.
The selective complexation of metal species (CuCl2) with the
benzothiazole-based block copolymer (BTVS:St = 42:58) was
carried out at a molar ratio of BTVS unit/CuCl2 = 1:1 in DMF
at 80 °C for 24 h. In this study, we focused on how selective
complexation of the metal species with the poly(BTVS)
segments in the block copolymers lead to changes in self-
assembled structures and characteristic fluorescent properties.
Complexation-responsive assembled structures of the block
copolymers were characterized using DLS in ethyl acetate and
THF. Note that poly(BTVS) was soluble in THF and insoluble
in ethyl acetate. Poly(St) was soluble in both solvents. In
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indicating that pristine block copolymer exists in the
unimolecularly dissolved state, whereas the poly(BTVS)/Cu-
b-poly(St) shows bimodal DLS peaks, corresponding to the
presence of unimers and larger aggregates. In the fluorescence
spectra, there was no significant effect of the solvent on the
peak intensity and position (Figure 10d). A similar tendency
was observed in the metalloblock copolymer prepared from the
benzothiazole-based block copolymer with different comono-
mer compositions (BTVS:St = 26:74) (Figure S19). These
results suggest that the poly(BTVS)/Cu-b-poly(St) are
composed of two components (Figure 8b); the small species
observed by DLS in THF is interpreted as a loose
poly(BTVS)/Cu complex, in which the complexation between
the Cu and poly(BTVS) segment takes place around single
poly(BTVS) chain to afford an intramolecular complex. The
larger species may correspond to a tight poly(BTVS)/Cu
complex, in which Cu species interacts tightly with the
poly(BTVS) segment via intermolecular complexation.
graphs, DLS traces, and fluorescence spectra of
complexes, and tables summarizing the solubility of the
products, polymerization results under various con-
ditions, metal contents and solubility of the complexes
(PDF)
AUTHOR INFORMATION
Corresponding Author
*(H.M.) E-mail: h.mori@yz.yamagata-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan (B, 24350053).
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CONCLUSION
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