RAFT polymerization of s -vinyl sulfide derivatives and synthesis of block copolymers having two distinct optoelectronic functionalities

Article abstract of DOI:10.1021/ma400813e

Well-defined sulfur-containing polymers and block copolymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of S-vinyl sulfide derivatives, in which the thioether moiety is directly connected to the vinyl group. We initially investigated RAFT polymerization of four different S-vinyl sulfide derivatives, phenyl vinyl sulfide (PVS), 4-chlorophenyl vinyl sulfide (CPVS), 2,4-dichlorophenyl vinyl sulfide (DCPVS), and 4-bromophenyl vinyl sulfide (BPVS). Three xanthate-type chain transfer agents (CTAs), a dithiocarbamate-type CTA, and a dithioester-type CTA were compared for these RAFT polymerizations. Reasonable control of the polymerization of PVS was confirmed by an observed linear increase in the molecular weight with the conversion, feasibility to control molecular weight based on the ratio of monomer consumed to the amount of CTA used, chain end structure determined by ¹H NMR, and chain extension experiment. The RAFT polymerization of the bromo-substituted monomer (BPVS) also proceeded in the controlled fashion under the same conditions. Incorporation of optoelectronic groups, including anthracene, fluorene, diphenylamine, and phenothiazine on the bromophenyl pendant group of poly(BPVS) were accomplished by palladium-catalyzed postmodifications. We also investigated characteristic optoelectronic properties of modified polymers and block copolymers with two distinct electronic functionalities.

Full text of DOI:10.1021/ma400813e

Article
pubs.acs.org/Macromolecules
RAFT Polymerization of S‑Vinyl Sulfide Derivatives and Synthesis of
Block Copolymers Having Two Distinct Optoelectronic
Functionalities
Kazuhiro Nakabayashi,† 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: Well-defined sulfur-containing polymers and
block copolymers were synthesized by reversible addition−
fragmentation chain transfer (RAFT) polymerization of S-vinyl
sulfide derivatives, in which the thioether moiety is directly
connected to the vinyl group. We initially investigated RAFT
polymerization of four different S-vinyl sulfide derivatives,
phenyl vinyl sulfide (PVS), 4-chlorophenyl vinyl sulfide
(CPVS), 2,4-dichlorophenyl vinyl sulfide (DCPVS), and 4-
bromophenyl vinyl sulfide (BPVS). Three xanthate-type chain
transfer agents (CTAs), a dithiocarbamate-type CTA, and a dithioester-type CTA were compared for these RAFT
polymerizations. Reasonable control of the polymerization of PVS was confirmed by an observed linear increase in the molecular
weight with the conversion, feasibility to control molecular weight based on the ratio of monomer consumed to the amount of
1
CTA used, chain end structure determined by H NMR, and chain extension experiment. The RAFT polymerization of the
bromo-substituted monomer (BPVS) also proceeded in the controlled fashion under the same conditions. Incorporation of
optoelectronic groups, including anthracene, fluorene, diphenylamine, and phenothiazine on the bromophenyl pendant group of
poly(BPVS) were accomplished by palladium-catalyzed postmodifications. We also investigated characteristic optoelectronic
properties of modified polymers and block copolymers with two distinct electronic functionalities.
tionally, so-called iniferter system using styrene derivatives with
a dithiocarbamate moiety, which can also act as a photoiniferter
(photoinitiator−transfer agent−terminator),4 have been em-
ployed for the synthesis of graft copolymers5 and hyper-
branched polymers.6 A sulfur−sulfur cross-linked polymer was
previously synthesized from a similar styrene derivative with a
dithiocarbamate as a source of dormant radicals, in which the
dithiocarbamyl residue is the polymerizable part of the
molecule.7 Similar methacrylate-type monomers bearing the
dithiocarbamate photoiniferter moiety have been also re-
ported.8,9 Reversible addition−fragmentation chain transfer
(RAFT) polymerization and atom transfer radical polymer-
ization (ATRP) of pyridyl disulfide ethyl methacrylate afforded
functional polymers having pendant pyridyl disulfide reactive
groups, which were employed as simultaneous and reversible
functionalization10 to generate functional nano-objects, such as
reversibly cross-linked micelles,11 protein−polymer particles,12
and nanoparticles with biocompatible shells.13 ATRP and
RAFT polymerization of disulfide−based di(meth)acrylates,
which can be regarded as cross-linking agents, have been
employed for the synthesis of branched acrylic copolymers,14
cross-linked (bio)degradable nanogels,15 thiol-functionalized
INTRODUCTION
■
Sulfur-containing polymers derived from vinyl monomers have
attracted a great deal of attention, due to their potential
applications in numerous areas in polymer ressearch. For
example, sulfur-containing substituents, including linear thio-
ether and cyclic thiophene, are the most commonly used
groups for increasing a materials refractive index. Sulfur-
containing (meth)acrylates, a prevalent material for this
purpose, have a wide variety of applications in plastic lenses,
optical disc substrates, optical adhesives or encapsulants,
antireflective coatings, and photoresists.1 Thioether-containing
styrene derivatives have been previously prepared for surface-
enhanced Raman scattering-encoded polymers for combinato-
rial screening and biomedical diagnostics.2 Thioether-contain-
ing styrene has also been employed for the synthesis of gold
nanocomposites via chemisorption of gold nanoparticles onto
the thioether groups on the polymer side chains.3 So far, a
variety of polystyrene and poly(meth)acrylate derivatives with
sulfur-containing side chains have been reported, and most of
these sulfur-containing polymers were traditionally prepared by
conventional radical polymerizations.
Recently, the controlled radical polymerization (also known
as controlled/“living” radical polymerization or reversible
deactivation radical polymerization) of sulfur-containing vinyl
monomers has gained considerable interest, driven by the
remarkable development of thiol−disulfide chemistry. Tradi-
Received: April 20, 2013
Revised: July 3, 2013
© XXXX American Chemical Society
A
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block copolymer vesicles,16 and thiol/disulfide-functionalized
star polymers.17 A variety of block copolymers having a
polythiol segment were synthesized by RAFT polymerization of
vinyl monomers with protected thiol groups, involving a
methacrylate carrying a xanthate moiety,18 (meth)acrylamides
and (meth)acrylates with a 2-nitrobenzylthioether moiety,19,20
and (meth)acrylates with an acetyl thioalkyl group.21 Other
interesting systems involve the radical ring-opening polymer-
ization of sulfur-containing cyclic vinyl monomers, such as
cyclic allyl sulfide derivatives,22,23 which can afford functional
polymers with sulfur atom in the polymer backbones.
that have thermal and chemical stability as well as the structural
versatility. Differing from styrene, (meth)acrylate, and (meth)-
acrylamide-type monomers, the direct linkage of the sulfur
atom to the vinyl group is an important feature of these
monomers, because it may affect not only the polymerization
behavior but also various properties of the resulting polymers.
The development of a new S-vinyl monomer family, which is
applicable for controlled radical polymerization and can be
regarded as an analogous of vinyl ether, would represent a
significant advance in the field of functional polymeric
materials. An attractive feature of PVS derivatives is the
structural diversity through the reaction of 1,2-dibromoethane
with thiol compounds having various substituted groups, and
the feasibility to act as building blocks and functional units for
the construction of new macromolecules.
Here, we present the first example of controlled synthesis of
sulfur-containing polymers by RAFT polymerization of S-vinyl
monomers, phenyl vinyl sulfide (PVS) derivatives (Scheme 1)
Scheme 1. Reversible Addition−Fragmentation Chain
Transfer (RAFT) Polymerization of S-Vinyl Sulfide
Derivatives Using Different Chain Transfer Agents (CTAs)
Several reports have been published on conventional radical
polymerization of S-vinyl sulfides.24−28 Price and Morita
reported radical copolymerization of PVS with various
comonomers including styrene and methyl acrylate, and
indicated the importance of the 3d orbital resonance between
the sulfur atom and the adjacent carbon atom in the transition
state of copolymerization reactions.24 They demonstrated that
PVS was comparable in reactivity to methyl vinyl sulfide, with
Q and e values of PVS measured as Q = 0.32, e = −1.4. Otsu et
al. reported radical polymerization of p-substituted PVS
derivatives to clarify the effect of the sulfur atom and
substituent group.25 They indicated that the higher reactivity
of vinyl sulfide relative to vinyl ether might be explained by the
3d orbital resonance between the propagating radical and the
adjacent sulfur atom in vinyl sulfide.29 Since vinyl sulfides are
typical electron donating monomers, alternating copolymeriza-
tions with electron accepting monomers, such as maleic
anhydride,30,31 acrylonitrile,32 and maleimide,33 have been
reported as taking place on the basis of donor−acceptor
interactions. Poly(PVS) was also employed as a precursor for
the synthesis of poly(vinylsulfonium salts).34
In this study, RAFT polymerization of S-vinyl sulfide
derivatives, regarded as an important class of S-vinyl monomers,
were employed for the synthesis of sulfur-containing polymers
and block copolymers with an as-designed chain structure and
narrow molecular weight distribution. In the first part, we
Scheme 2. (a) Synthesis of Block Copolymers by RAFT Polymerization of NVC Using Poly(S-vinyl sulfide) Macro-CTAs and
(b) Postmodification of Poly(BPVS-b-NVC)
B
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Scheme 3. Postmodifications of Poly(BPVS) by (a) Suzuki Coupling Reaction and (b) Buchwald−Hartwig Coupling Reaction
briefly compared RAFT polymerization behavior of four S-vinyl
sulfide derivatives: phenyl vinyl sulfide (PVS), 4-chlorophenyl
vinyl sulfide (CPVS), 2,4-dichlorophenyl vinyl sulfide
(DCPVS), and 4-bromophenyl vinyl sulfide (BPVS), as
shown in Scheme 1. Recent progress in RAFT polymerization
processes has allowed for the synthesis of well-defined
polymers by radical polymerization of nonconjugated mono-
mers. In particular, dithiocarbonates (xanthates) are known to
be useful for controlling the radical polymerization of
nonconjugated O-vinyl and N-vinyl monomers. On the basis
of the Q and e values of PVS (Q = 0.32, e = −1.4), the
reactivities of S-vinyl sulfide derivatives are considered to be
comparable to that of N-vinylcarbazole (NVC, Q = 0.26, e =
−1.29), which is a representative nonconjugated N-vinyl
monomer having a pendant electron-donating carbazolyl
moiety. With this in mind, we attempted to find a suitable
CTA and conditions to provide the controlled radical
polymerization of S-vinyl sulfide derivatives. The controlled
character of RAFT polymerization of the selected monomers,
PVS and BPVS, was investigated using the optimized
conditions. Then, we conducted the synthesis of block
copolymers by RAFT polymerization of NVC using poly(PVS)
as a macro-CTA (Scheme 2). Regardless of the potentially
interesting properties of the resulting products, the synthesis of
a well-defined block copolymer composed of poly(PVS) and
poly(NVC) is challenging, owing to the low reactivity of both
nonconjugated monomers and the low stability of the
propagating radicals. The effects of the order of block
construction and composition of the second monomer to the
macro-CTA were studied in detail, in terms of the controlled
character of the polymerization and feasibility to manipulate the
structures of the resulting block copolymers. Next, the synthesis
of well-defined block copolymers was conducted by RAFT
polymerization of BPVS using macro-CTA. The incorporation
of optoelectronic groups on the bromophenyl unit were
conducted by palladium-catalyzed postmodifications to afford
functional polymers with additional optoelectronic moieties,
such as anthracene (An), fluorene (Flu), diphenylamine
(DPA), and phenothiazine (PTZ), in the side chain (Scheme
3). The postmodification of poly(NVC)-b-poly(BPVS) also
afforded a block copolymer incorporating two distinct
electronic functionalities (Scheme 2b). We believe that this
synthetic approach and resulting polymers represent a new
paradigm, which will significantly extend to range of functional
materials composed of nonconjugated S-vinyl and N-vinyl
monomers with characteristic optoelectronic properties.
EXPERIMENTAL SECTION
■
Materials. 2,2-Azobis(isobutyronitrile) (AIBN, Kanto Chemical,
97%) was purified by recrystallization from methanol. 4-Chlorobenze-
nethiol (TCI, >98%), 2,4-dichlorobenzenethiol (TCI, >97%), 4-
bromobenzenethiol (TCI, >97%), sodium ethoxide (ca. 20% in
ethanol, TCI), 1,2-dibromoethane (TCI, >99%), 9-anthraceneboronic
acid (TCI), 9,9-dimethylfluoren-2-boronic acid (TCI) diphenylamine
(TCI, >99%), phenothiazine (TCI, >98%), tetrakis-
(triphenylphosphine)palladium(0) (Pd(PPh3)4, TCI, >97%), tris-
(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3, TCI, >75%), and
sodium carbonate (Wako, >99%), were used as received. Phenyl vinyl
sulfide (PVS, TCI, >98%) used as a monomer was purified by
distillation under reduced pressure. N-vinylcarbazole (NVC, TCI,
>98%) was recrystallized two times from methanol. Other materials
were used without further purification.
Five different chain transfer agents (CTAs) were employed in this
study, as shown in Scheme 1. Benzyl dithiobenzoate (CTA 1)45,47 and
benzyl 1-pyrrolecarbodithioate (CTA 2)48,49 were synthesized
according to the procedures reported previously. S-Benzyl-O-ethyl
dithiocarbonate (CTA 3),50−52 O-ethyl-S-(1-ethoxycarbonyl) ethyl-
dithiocarbonate (CTA 4),53,54 and O-ethyl-S-(1-phenylethyl) dithio-
carbonate (CTA 5)50,52,55 were synthesized by the reaction of
potassium ethyl xanthogenate with corresponding bromide (benzyl
C
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bromide for CTA 3, methyl 2-bromopropionate for CTA 4, and 1-
bromoethylbenzene for CTA 5, respectively) according to a procedure
reported in the literature. These xanthate-type CTAs were finally
purified by column chromatography on silica with n-hexane/ethyl
acetate (10/1 vol %) as the eluent.
Similarly, poly(CPVS), poly(DCPVS), and poly(BPVS) were
prepared by RAFT polymerization of the corresponding S-vinyl
sulfide derivatives (CPVS, DCPVS, and BPVS). The 1H and 13C 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:
Synthesis of S-Vinyl Sulfides. Three vinyl sulfide derivatives
having different substituent groups, 4-chlorophenyl vinyl sulfide
(CPVS), 2,4-dichlorophenyl vinyl sulfide (DCPVS), and 4-bromo-
phenyl vinyl sulfide (BPVS) were synthesized by the reaction of 1,2-
dibromoethane with corresponding thiol compounds according to a
previously reported method with slight modifications.45 In a typical
experiment for the synthesis of CPVS, to a solution of 13.6 g (0.20
mol) sodium ethoxide in 80 mL of ethanol was added 4-
chlorobenzenethiol (28.9 g, 0.20 mol) under nitrogen. After stirring
at room temperature for additional 30 min, the resulting 4-
chlorobenzenethiolate was added to a solution of 56.4 g (0.30 mol)
of 1,2-dibromoethane in 20 mL of ethanol at 0 °C within 1 h. Stirring
was continued for 1 h, and a solution of 34.0 g (0.50 mol) of sodium
ethoxide in ethanol was added. The reaction mixture was stirred at
reflux for 8 h, cooled, and treated with 300 mL of benzene and 300 mL
of water. The organic layer was separated, washed with water and
saturated aqueous NaCl solution, and dried over MgSO4. After
filtration, the solvent was removed in vacuo, and the residue was
distilled under reduced pressure to give pure CPVS as a transparent
liquid (14.7 g, 51%).
[monomer]0
[CTA]0
Mn(theory) =
× Mmonomer × convn+MCTA
(1)
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 Block Copolymers. A representative example of the
synthesis of the block copolymer composed of PVS and NVC is as
follows: PVS (2.72 g, 20 mmol), CTA 4 (44.4 mg, 0.20 mmol), and
AIBN (16.0 mg, 0.10 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
24 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(PVS) as a white solid
(1.66 g, 61%, Mn,NMR = 7 800 g/mol, Mw/Mn = 1.45), which was
employed as a macro-CTA. The xanthate-terminated poly(PVS) (0.16
g, 0.020 mmol), AIBN (1.6 mg, 0.010 mmol), NVC (0.39 g, 2.00
mmol), and 1,4-dioxane (1.0 mL) were placed in a dry ampule. After
the solution was degassed by three freeze−evacuate−thaw cycles, the
polymerization was conducted at 60 °C for 24 h. The reaction mixture
was purified by reprecipitation from a chloroform solution into a large
excess of n-hexane and isolated by filtration to give a block copolymer,
poly(PVS)-b-poly(NVC), as a white solid. The block copolymer was
soluble in THF, chloroform, DMF, and insoluble in diethyl ether, n-
hexane, methanol, and water. The copolymer composition was
determined using elementary analysis by nitrogen content of the
block copolymers.
CPVS: Yield = 51%, bp 90−92 °C/0.05 mmHg (lit.25 91 °C/5
1
mmHg). H NMR (CDCl3): δ 7.4−7.2 (m, 4H, ArH), 6.5 (dd, 1H,
CH2CH−), 5.4−5.3 (dd, 2H, CH2CH−) ppm. 13C NMR
(CDCl3): δ 133.3, 133.0, 132.0 (ArC), 131.6 (CH), 129.5 (ArC),
116.7 (CH2) ppm. Anal. Calcd for C8H7ClS: C, 56.30; H, 4.13; Cl,
20.77; S, 18.79. Found: C, 56.24; H, 4.29; S, 18.65.
1
DCPVS: Yield = 50%. bp 95−97 °C at 0.05 mmHg. H NMR
(CDCl3): δ 7.4−7.2 (m, 3H, ArH), 6.5−6.4 (dd, 1H, CH2CH−),
5.6−5.5 (dd, 2H, CH2CH−) ppm. 13C NMR (CDCl3): δ 134.5,
133.2, 133.1 (ArC), 131.0 (CH), 130.0, 129.0, 127.8 (ArC), 119.7
(CH2) ppm. Anal. Calcd for C8H6Cl2S: C, 46.85; H, 2.95; Cl, 34.57;
S, 15.63. Found: C, 46.55; H, 3.25 ; S, 15.87 .
Postmodifications of Poly(BPVS). Postmodifications of poly-
(BPVS) were conducted by a Suzuki coupling reaction according to a
previously reported procedure with slight modifications.46 The THF
solution (5 mL) of poly(BPVS) (0.050 g, 0.23 unit mmol based on the
repeating unit), 9-anthraceneboronic acid (0.23 g, 1.05 mmol), and
tetrakis(triphenylphosphine) palladium(0) (16.1 mg, 0.014 mmol) was
stirred at room temperature under nitrogen. After the mixture was
completely dissolved, aqueous Na2CO3 solution (20 wt % in water,
0.38 mL) was added, and the reaction mixture was refluxed for 48 h in
the dark. After the reaction, it was purified by reprecipitation into a
large excess of methanol, and the resulting product was dried under
vacuum at room temperature to give poly(BPVS-An) as a yellow solid
BPVS: Yield = 60%. bp 85−95 °C at 0.05 mmHg (lit.25 110 °C/7
1
mmHg). H NMR (CDCl3): δ 7.2−7.4 (m, 4H, ArH), 6.5 (dd, 1H,
CH2CH−), 5.3−5.4 (dd, 2H, CH2CH−) ppm. 13C NMR
(CDCl3): δ 133.7, 132.3, 131.9 (ArC), 131.2 (CH), 121.2 (ArC),
116.7 (CH2) ppm. Anal. Calcd for C8H7BrS: C, 44.67; H, 3.28; Br,
37.15; S, 14.91. Found: C, 44.20; H, 3.20; S, 15.06.
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)
General Polymerization Procedure. All polymerizations were
carried out in a degassed sealed tube. A representative example is as
follows:
1
(0.072 g, >99%). H NMR (CDCl3): δ 8.4−6.5 (13H, ArH), 4.6−3.3
(1H, −CH-S), 2.5−1.0 (2H, CH2-CH−) ppm.
PVS (0.27 g, 2.0 mmol), CTA 4 (4.4 mg, 0.02 mmol), and AIBN
(1.6 mg, 0.01 mmol) were placed in a dry glass ampule equipped with
a magnetic stir 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 60 °C for 24 h. Since the reaction was
performed in bulk, the content of the flask was gradually changed from
the transparent 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 methine protons of the main chain in polymer (1H)
at 3.4−4.2 ppm was compared with the monomer CH2CH-
resonance (1H) at around 6.6 ppm. Conversion determined by this
method was 71%. 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 (0.17 g, 64%). The resulting poly(PVS) was
soluble in acetone, THF, 1,4-dioxane, ethyl acetate, chloroform, DMF,
and DMSO, and insoluble in n-hexane, methanol, and water.
The Suzuki coupling reaction was also employed for the
incorporation of the fluorene unit using 9,9-dimethylfluoren-2-boronic
acid. Under the same procedure, poly(BPVS-Flu) was obtained as a
1
beige solid (>99%). H NMR (CDCl3): δ 7.9−6.7 (br, 11H, ArH),
4.4−3.3 (br, 1H, −CH−S), 2.4−0.9 (m, 8H, CH2−CH−, −CH3)
ppm.
Postmodifications of poly(BPVS) were conducted by the
Buchwald−Hartwig coupling reaction according to a previously
reported procedure with slight modifications.47−49 A typical reaction
procedure is as follows. Poly(BPVS) (0.050 g, 0.23 unit mmol based
on the repeating unit), phenothiazine (1.22 g, 6.12 mmol), sodium
tert-butoxide (NaO-t-Bu, 0.29 g, 3.06 mmol), and Pd2(dba)3 (0.047 g,
0.051 mmol) were placed in a 100 mL flask under nitrogen. Then, 1,4-
dioxane (25 mL) and tri-tert-butylphosphine (P(t-Bu)3, 62 mg, 0.31
mmol) were added to the solution at room temperature. The reaction
mixture was stirred at 100 °C for 24 h. After cooling to room
temperature, the mixture was quenched by the addition of aqueous
ammonia (100 mL), and the product was extracted three times with
CHCl3. The product was purified by two times reprecipitation into
diethyl ether. The resulting product was finally dried under vacuum at
D
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room temperature to give poly(BPVS-PTZ) as a brownish gray solid
(0.077 g, >99%). 1H NMR (CDCl3): δ 7.9−5.9 (m, 12H, ArH), 4.3−
3.1 (br, 1H, −CH−S), 2.3−0.9 (m, 2H, CH2−CH−) ppm.
The bulk polymerization of PVS was conducted with
different CTAs and AIBN as an initiator at 60 °C for 24 h at
[M]0/[CTA]0 = 100, using the ratio of AIBN to CTA of 1:2
([M]0/[CTA]0/[AIBN]0 = 200/2/1). The results are summar-
ized in Table 1. When the polymerization was carried out using
Similarly, the palladium-catalyzed reaction of diphenylamine into
the bromophenyl moieties in poly(BPVS) afforded poly(BPVS-DPA)
1
as a brown solid (0.066 g, 94%). H NMR (CDCl3): δ 7.7−6.6 (m,
14H, ArH), 4.2−3.0 (m, 1H, −CH−S), 2.4−1.0 (m, 2H, CH2−CH−)
ppm. These resulting polymers were soluble in chloroform and THF,
while insoluble in acetone, diethyl ether, methanol, n-hexane, and
water (Table S2, Supporting Information).
Table 1. RAFT Polymerization of PVS with AIBN Using
Different CTAs in Bulk at 60 °C for 24 h
a
c
d
e
e
Instrumentation. 1H (400 MHz) and 13C NMR (100 MHz)
spectra were recorded with a JEOL JNM-ECX400. Elemental analysis
was carried out on a Perkin-Elmer 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-
guardcolumn 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 1090000 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 spectrofluoropho-
tometer. Thermogravimetric analysis (TGA) was performed on a
SEIKO TGA/6200 at a heating rate of 10 °C/min under nitrogen
atmosphere.
convn
(%)
Mn
Mn
Mw/Mn
(SEC)
b
entry CTA
(theory)
(SEC)
1
2
3
4
5
6
−
77
<5
60
63
71
56
−
−
8400
8800
9900
7900
18 400
−
3700
3600
5200
4700
2.03
−
1.64
2.29
1.44
1.60
CTA 1
CTA 2
CTA 3
CTA 4
CTA 5
a
b
[PVS]0/[CTA]0/[AIBN]0 = 200/2/1. CTA 1 = benzyl dithioben-
zoete, CTA 2 = benzyl 1-pyrrolecarbodithioate, CTA 3 = S-benzyl-O-
ethyl dithiocarbonate, CTA 4 = O-ethyl-S-(ethoxy carbonyl) ethyl-
dithiocarbonate, and CTA 5 = O-ethyl-S-(1-phenylethyl) dithiocar-
c
d
bonate. Calculated by 1H NMR in CDCl3. The theoretical molecular
weight (Mn,theory) = (MW of PVS) × [PVS]0/[CTA]0 × convn + (MW
e
of CTA). Measured by SEC using polystyrene standards in DMF (10
mM LiBr).
the xanthate-type CTA 4, a poly(PVS) having a relatively low
polydispersity (Mw/Mn = 1.44) was obtained with a relatively
1
high conversion (71%, as determined by H NMR spectros-
RESULTS AND DISCUSSION
copy). A unimodal SEC peak is detected in the SEC profiles of
the poly(PVS) obtained with CTA 4; analysis of the resulting
poly(PVS) via polymerization with the dithiocarbamate-type
CTA 2 showed a broad unimodal SEC peak (Mw/Mn = 1.64).
RAFT polymerizations of PVS with CTA 3 and CTA 5 afforded
poly(PVS)s having broad molecular weight distributions (Mw/
Mn = 2.29 and 1.60, respectively). Extremely low monomer
conversion (<5%) was obtained for the polymerization with
CTA 1 under the same conditions. These results suggest that
the xanthate-type CTA 4 is effective for RAFT polymerizations
of PVS, and unfavorable chain transfer and termination
reactions due to the high reactivity of the propagating radicals
derived from PVS could be suppressed by the RAFT process
using the xanthate-type CTA. Among xanthate-type CTAs,
CTA 4 showed a good balance between the leaving ability and
reinitiation ability from the electron-withdrawing ester moiety
that is stabilized by resonance effect. These results also support
our strategy for RAFT polymerization utilizing O-alkyl
xanthates as CTAs as a very efficient method to control the
polymerization of highly reactive propagating radicals derived
from PVS, even in the presence of a sulfur atom connected
directly to the vinyl group of the S-vinyl monomer.
In the next stage, we examined the influence of CTA, solvent,
and the [CTA]0/[AIBN]0 ratios in terms of monomer
conversion, molecular weights, and polydispersity of the
resulting poly(PVS)s. The RAFT polymerization of PVS was
examined with CTA 4 at 60 °C for 24 h. The [CTA 4]0/
[AIBN]0 ratio was adjusted between 2 and 10, while the
monomer/CTA molar ratio was held constant at 100/1 (Table
S3 and Figure S5, Supporting Information). When the
polymerization was carried out in bulk, the monomer
conversion decreased from 71% to 40% as the [CTA]0/
[AIBN]0 ratio increased from 2 to 10, indicating a significant
influence of the [CTA]0/[AIBN]0 ratio on monomer
conversion. In contrast, the molecular weight distributions of
■
RAFT Polymerization of S-Vinyl Sulfide Derivatives. In
this study, RAFT polymerization reactions of four S-vinyl
sulfide derivatives having different substituent groups were
investigated, in order to understand the polymerization
mechanism of this class of monomers. When selecting RAFT
agents for the polymerization of PVS, one must carefully
consider the characteristic of the propagating PVS radical;
accordingly, we initially studied the effect of the nature of the
CTA on the radical polymerization of PVS in the presence of
AIBN as an initiator (Scheme 1). Dithiocarbonates (xanthates)
are known to be useful for controlling the radical polymer-
ization of nonconjugated O-vinyl and N-vinyl monomers, such
as vinyl acetate,50−52 N-vinylpyrrolidone,52−56 N-vinylcarbazole
(NVC),39,57,58 N-vinylindole,40 and N-vinylphthalimide.43
Recently, we also reported that a xanthate-type CTA was the
most efficient for the synthesis of well-defined polymers by
RAFT polymerization of vinyl sulfonate esters,59,60 which can
be regarded as S-vinyl monomers with direct linkage of the
sulfonate moiety to the vinyl group. In this study, three
xanthate-type mediating agents having different leaving groups
(R-groups), S-benzyl-O-ethyl dithiocarbonate (CTA 3), O-
ethyl-S-(1-ethoxycarbonyl) ethyldithiocarbonate (CTA 4), O-
ethyl-S-(1-phenylethyl) dithiocarbonate (CTA 5), were se-
lected as CTAs. For comparison, we also employed
dithiocarbamate-type (CTA 2) and dithiobenzoate-type (CTA
1) as mediating agents. Dithiobenzoate-type CTA (CTA 1) has
been used for RAFT polymerization of conventional conjugated
monomers, such as styrene, methyl acrylate, and N-
isopropylacrylamide.61−64 Dithiocarbamate-type CTA 2, in
which a nitrogen lone pair is part of an aromatic ring system,
has been employed for RAFT polymerizations of conventional
conjugated monomers such as styrene,35,65,66 methyl acrylate,66
and N-isopropylacrylamide,67 as well as less active monomers
including N-vinylphthalimide43 and N-vinylnaphthalimide.68
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Figure 1. 1H NMR spectra (CDCl3) of (a) poly(PVS), (b) poly(CPVS), (c) poly(DCPVS), and (d) poly(BPVS) obtained by RAFT polymerization
with CTA 4.
the resulting poly(PVS)s increased slightly with increasing the
[CTA]0/[AIBN]0 ratio. This polymerization behavior is
inconsistent with the general trend that higher [CTA]0/
[AIBN]0 ratios (lower initiator concentration) may afford
better overall control of the polymerization, as the result of a
decrease in total radical species available for unfavorable side
reactions. This may be due to the relatively high viscosity
during the bulk polymerization of PVS; the content of the
reaction mixture was actually transformed from a transparent
liquid into viscous material upon reaching a certain level of
conversion. Hence, the effect of the [CTA]0/[AIBN]0 ratio on
the RAFT polymerization of PVS is not as remarkable.
With a view of preparing poly(PVS)s over a wide molar mass
range, different degrees of polymerization were targeted. For
this purpose, the polymerization of PVS was conducted at
different [M]0/[CTA 4]0 ratios between 100 and 600, while the
[CTA]0/[AIBN]0 molar ratio was held constant at 2 (see Table
S4, Supporting Information). When the polymerization of PVS
was conducted in bulk at 60 °C for 24 h, the number-average
molecular weights of the poly(PVS)s increased from 4900 to
7500 with the [M]0/[CTA]0 ratio. A shift in the SEC trace
toward higher molecular weight regions with a unimodal peak
was seen with increasing [M]0/[CTA]0 ratio (Figure S6,
Supporting Information), demonstrating the effective control of
the molecular weights. Yet, the molecular weight distributions
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tend to broaden slightly with increasing [M]0/[CTA]0 ratio.
This behavior can also be ascribed to the relatively high
viscosity during the bulk polymerization of PVS. Nevertheless,
the obtained results suggest that the molecular weights of
poly(PVS)s can be easily adjusted by the monomer-to-CTA
ratio, and polymers having relatively low polydispersities can be
easily obtained by RAFT polymerization of PVS under suitable
conditions.
In all cases, the experimental molecular weights determined
by conventional SEC in DMF were found to be lower than the
theoretical values. These discrepancies are believed to result
from the difference in hydrodynamic volume between poly-
(PVS)s and the linear polystyrene standards used for SEC
calibration. In order to clarify this point, NMR spectroscopy
was used to determine the chain-end structure and absolute
molecular weights of poly(PVS)s. The CTA-derived initiator
may produce a polymer with CTA fragments at the polymer
chain ends (α-chain end with the (1-ethoxycarbonyl) ethyl
group and ω-chain end with the dithiocarbonate group in the
1
case of CTA 4); the H NMR spectrum of the poly(PVS)
obtained with CTA 4 is presented in Figure 1a. The
characteristic peaks at 7.6−7.1 (aromatic ring), 4.1−3.3 (CH
in the polymer main chain), and 2.1−1.0 (CH2 in the polymer
main chain) ppm are clearly seen, which are attributed to the
poly(PVS) main chain. In addition to these peaks, methylene
protons are clearly visible at 4.7−4.4 ppm, which correspond to
the two methylene protons of the end groups. This is an
indication that the polymer chain end is capped with the
fragments of CTA as expected according to the general
mechanism of the RAFT process. The molecular weight of the
polymer chain can be calculated by comparing the integrals of
the peaks for the chain-end protons to those of the main-chain
protons; thus, the molecular weight of poly(PVS) prepared
with CTA 4 could be calculated by comparison of the area of
the peaks at 4.1−3.3 ppm corresponding to the main chain
protons (1H) with the peak at 4.7−4.4 ppm corresponding to
the end group (peak “d” in Figure 1a). As shown in Table S4
(Supporting Information), the molecular weights determined
by NMR were comparable to the theoretical values calculated
using eq 1, which are apparently higher than those obtained
from SEC in DMF (0.01 M LiBr) using polystyrene standards.
The controlled/living character of the polymerization of PVS
was studied by performing kinetics investigations in the
presence of CTA 4 with AIBN in bulk at 60 °C. When the
reaction was conducted at [PVS]0/[CTA]0/[AIBN]0 = 200/2/
1, a relatively high conversion (71%) was reached after 24 h. An
almost linear first-order kinetic plot is seen up to the
conversion end point (24 h) under the conditions (Figure
2a). As shown in Figure 2b, a linear increase in the number-
average molecular weights, Mn, with conversion is seen in the
polymerization of PVS, with a decreasing polydispersity index
observed with increasing monomer conversion: from Mw/Mn =
1.79 at 10% into 1.44 at 71%. The SEC traces of poly(PVS)
obtained at different polymerization times illustrate the increase
in molecular weight with time, as can be seen in Figure 2c.
Symmetrical unimodal SEC peaks without shoulders and
tailings are seen for the poly(PVS)s even at the last stages of
polymerization, suggesting the absence of a termination
reaction due to the combination of growing polymer chains
during a longer reaction time.
Figure 2. (a) Time−conversion (circles) and first-order kinetic
(squares) plots for polymerization of PVS with AIBN in the presence
of CTA 4 ([PVS]0/[CTA 4]0/[AIBN]0 = 200/2/1) in bulk at 60 °C.
(b) Number-average molecular weights (NMR, squares; SEC,
triangles) and polydispersity (circles) as a function of conversion.
(c) SEC traces of poly(PVS)s.
point, the dithiocarbonate-terminated poly(PVS) prepared by
the polymerization using CTA 4 was employed as a macro-
CTA for chain extension experiments. The chain extension was
performed in bulk at 60 °C for 24 h at different [M]0/[macro-
CTA]0 ratios between 100 and 400, while the [AIBN]0/
[macro-CTA]0 ratio was held constant at 1/2 (Scheme S3 and
Table S5, Supporting Information). After chain extension was
complete, the polymer was recovered by precipitation in
methanol. The resulting products were analyzed by SEC in
DMF (10 mM LiBr), and compared to the original macro-
CTA. As can be seen in Figure S7 (Supporting Information), a
clear shift in the SEC trace toward higher molecular ranges was
observed with an increase of the [PVS]0/[macro-CTA]0 ratio.
The SEC traces show unimodal peaks with no significant high
molecular weight species and unreacted macro-CTA impurities.
These results suggest that most of the chain ends of the
An important criterion of a controlled/living character of
polymerization is the successful extension of a chain from a
preformed polymer chain as a macro-CTA. To investigate this
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a
Table 2. RAFT Polymerization of Substituted Phenyl Vinyl Sulfides Using Different CTAs in Bulk at 60 °C for 24 h
b
c
b
d
d
entry
monomer
CPVS
CTA
convn (%)
Mn (theory)
Mn (1H NMR)
Mn (SEC)
Mw/Mn (SEC)
1
2
3
4
5
6
7
8
9
−
52
50
53
54
47
55
71
48
59
−
8800
9300
−
−
11 200
12 700
−
10 800
11 400
−
14 800
4100
4500
7200
3200
4300
17 700
4400
7700
1.61
1.43
1.58
1.58
1.50
1.66
1.56
1.35
1.36
CTA 2
CTA 4
−
CTA 2
CTA 4
−
DCPVS
BPVS
9900
11 500
−
CTA 2
10 600
22 700
CTA 4
12 900
12 200
a
b
c
[monomer]0/[CTA]0/[AIBN]0 = 200/2/1. Calculated by 1H NMR in CDCl3. The theoretical molecular weight (Mn, theory) = (MW of monomer)
d
× [monomer]0/[CTA]0 × convn + (MW of CTA). Measured by SEC using polystyrene standards in DMF (10 mM LiBr).
poly(PVS) are functionalized with the dithiocarbonate end
groups, which can be used as a macro-CTA for further chain
extension reactions.
The RAFT polymerization of three PVS derivatives having
halogen substituted groups was also conducted at the ratio
[M]0/[CTA]0/[AIBN]0 = 200/2/1 in bulk, and the results are
summarized in Table 2. When the polymerization of BPVS was
carried out with the xanthate-type CTA 4 and AIBN at 60 °C
for 24h, the poly(BPVS) having a relatively low polydispersity
(Mw/Mn = 1.36) was obtained with a reasonable molecular
weight and monomer conversion (Mn,NMR = 12200, Mn,SEC
=
7700, convn = 59%). The polymerization of BPVS with the
dithiocarbamate-type CTA 2 also afforded the bromide-
substituted polymer with relatively narrow molecular weight
distribution; the theoretical molecular weight, however, was
inconsistent with the experimental values determined by NMR
and SEC. In the cases of RAFT polymerizations of DCPVS and
CPVS under the same conditions, polymers having broader
polydispersities (1.43−1.66) were obtained, regardless of the
nature of CTA.
The polymerization of BPVS was kinetically investigated in
bulk at 60 °C and [BPVS]0/[CTA 4]0/[I]0 = 200/2/1. The
time−conversion and the pseudo first-order kinetic plots are
shown in Figure 3a. More than 55% conversion is reached after
24 h with a curved semilogarithmic plot observed, which may
be due relatively high viscosity during the bulk polymerization.
A similar conversion “plateau” was also detected for RAFT
polymerization of the nonsubstituted PVS. Nevertheless, a
linear relationship between Mn and conversion with maintain-
ing relatively low polydispersity is observed, as shown in Figure
3b, suggesting negligible irreversible transfer reactions. Sym-
metrical unimodal SEC peaks were observed for the polymers
obtained even at higher conversions (>55%, Figure 3c), and the
polydispersity indices for all samples ranged between 1.36 and
1.49. These results suggest that the polymerization of BPVS
mediated by CTA 4 proceeds in a controlled manner at 60 °C
under the conditions used in this study, even in the presence of
bromine substituted group.
Synthesis of Block Copolymers. Block copolymers
incorporating semiconducting segments and characteristic
optoelectronic functions have attracted significant research
interest,69−74 because of their ability to form a variety of
ordered structures via self-assembly processes and their possible
applications for organic light-emitting devices, photovoltaics,
and organic field-effect transistors. In particular, there is
growing interest in developing novel block copolymers having
two distinct electronic functionalities, such as donor and
acceptor chromophores.71,75 In this study, we selected poly(N-
Figure 3. (a) Time−conversion (circles) and first-order kinetic
(squares) plots for polymerization of BPVS with AIBN in the presence
of CTA 4 in bulk at 60 °C ([BPVS]0/[CTA 4]0/[AIBN]0 = 200/2/1).
(b) Number-average molecular weights (NMR, squares; SEC,
triangles) and polydispersity (circles) as a function of conversion.
(c) SEC traces of poly(BPVS)s.
vinyl carbazole), poly(NVC), as a hydrophobic segment for the
synthesis of sulfur-containing block copolymers, due to their
attractive features such as hole-transporting, high charge-carrier,
and electroluminescent properties.76 Initially, we conducted the
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Table 3. Synthesis of Block Copolymers by RAFT Polymerization of NVC Using Poly(S-vinyl sulfide) Macro-CTA at 60 °C for
24 h in 1,4-Dioxane
a
b
c
d
e
e
d
entry
macro-CTA
poly(PVS)
[NVC]0/ [macro-CTA]0
yield (%)
Mn (theory)
Mn (EA)
Mn (SEC)
Mw/Mn (SEC)
composition n:m
1
2
3
4
5
6
7
100
200
400
50
92
91
88
83
81
92
98
25 600
43 000
75 800
20 200
27 900
47 800
89 500
20 500
37 100
50 500
19 400
28 100
47 000
99 900
14 600
21 000
27 700
13 200
19 400
28 500
40 600
1.39
1.61
1.60
1.24
1.28
1.36
1.54
35:65
23:73
18:92
61:39
41:59
24:76
11:89
poly(BPVS)
100
200
400
a
[macro-CTA]0/[AIBN]0 = 2, [M] = 0.464 g/mL, Macro-CTA = poly(PVS): Mn, NMR =7,800, Mn, SEC = 4,900, Mw/Mn = 1.45, poly(BPVS): Mn, NMR
b
c
= 12,200, Mn, SEC = 7,700, Mw/Mn = 1.36. Hexane-insoluble part. The theoretical molecular weight (Mn,theory) = (MW of NVC) × [NVC]0/
[Macro-CTA]0 × yield + (MW of Macro-CTA). Calculated by elemental analysis (EA). Measured by SEC using polystyrene standards in DMF
(10 mM LiBr).
d
e
synthesis of a block copolymer by RAFT polymerization of
NVC using poly(PVS) as a macro-CTA, as shown in Scheme
2a. For the synthesis of well-defined block copolymers by the
RAFT process, the order of blocking is crucial. In this case, the
first xanthate-terminated poly(PVS) should have a high transfer
constant in the subsequent polymerization of the second NVC
monomer, to give the second block.77,78 Additionally, in order
to achieve block copolymer with low polydispersity, the rapid
conversion of macro-CTA to block copolymer is also required,
by which all of the second blocks can be initiated approximately
at the same time. It should be noted that RAFT agents suitable
for controlling the polymerization of nonconjugated monomers
(less activated monomers including O-vinyl and N-vinyl
monomers) tend to be ineffective with conjugated monomers
(more activated monomers including styrene and (meth)-
acrylates).79 Since the resonance stabilization of propagating
radicals derived from PVS (Q = 0.32, e = −1.4) is slightly higher
than that of NVC (Q = 0.26, e = −1.29), it is reasonable to
expect that the leaving ability of the propagating poly(PVS)
radical is higher than that of the second poly(NVC) radical, in
addition to the steric hindrance impaired by the macro-CTA.
The xanthate-terminated poly(PVS) having low molecular
weight was initially prepared by RAFT polymerization with
CTA 4. The polymerization of NVC was conducted in the
presence of the xanthate-terminated poly(PVS) as a macro-
CTA in dry 1,4-dioxane at 60 °C for 24 h, while maintaining
the [macro-CTA]0/[AIBN]0 ratio at a constant value of 2:1.
The [NVC]0/[macro-CTA]0 ratio was varied from 100 to 400
to control the comonomer content and molecular weight. The
Figure 4. (a) Poly(PVS) macro-CTA (red line, Mn,NMR = 7800, Mn,SEC
= 4900, Mw/Mn = 1.45) and the products obtained by the
reaction conditions, monomer conversion, molecular weights,
polydispersities, and comonomer compositions are summarized
in Table 3. Figure 4a shows the SEC traces of the parent
poly(PVS) and resulting block products. The SEC chromato-
grams of the starting macro-CTA and the growth polymers
show a clear shift of the molecular weight to the higher
molecular ranges. A slight tailing can be seen in the SEC traces
of the block copolymers, which could be due to some RAFT Z-
group loss in the macro-CTA. Nevertheless, the block
copolymers, poly(PVS)-b-poly(NVC)s, having relatively higher
molecular weights (Mn,EA = 20 500−50 500, Mw/Mn = 1.39−
1.61) were obtained with reasonable conversion (yield = 88−
92%). The 1H NMR spectrum of the block copolymer,
poly(PVS)-b-poly(NVC), is shown in Figure 5a. The peaks
corresponding to poly(NVC) and poly(PVS) are clearly
observed in the spectrum measured in CDCl3. These results
suggest that, under the conditions employed in this study,
fragmentation from the intermediate radical to the xanthate-
polymerization of NVC and (b) poly(BPVS) macro-CTA (red line,
Mn,NMR = 12 200, Mn,SEC = 7700, Mw/Mn = 1.36) and the products
obtained by the polymerization of NVC.
terminated poly(PVS), as well as the reinitiation process, are
sufficiently effective, resulting in the rapid conversion of the
macro-CTA into the block copolymer.
The synthesis of block copolymers was also conducted by
RAFT polymerization of NVC using poly(BPVS) as the macro-
CTA, which was initially prepared by RAFT polymerization
with CTA 4. When the polymerization of NVC was conducted
in dry 1,4-dioxane using the xanthate-terminated poly(BPVS)
macro-CTA at [NVC]0/[macro-CTA]0 = 50−400, high
monomer conversions (83−98%) were reached at 60 °C after
24 h (Table 3). As can be seen in Figure 4b, the comparison of
the SEC traces of the starting macro-CTA with the
corresponding block copolymers shows a clear shift to higher
molecular weights for the block copolymers. The molecular
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1
Figure 5. H NMR spectra (CDCl3) of (a) poly(PVS-b-NVC), (b) poly(BPVS-b-NVC), and (c) poly(BPVS-An-b-NVC).
weight distributions displayed a narrow dispersity (1.24−1.36)
existence in the final product of a certain quantity of residual
low-molecular-weight chains, possibly corresponding to un-
reacted or dead chains. That is, the leaving ability of the
propagating poly(NVC) radical was lower than that of the
second poly(PVS) radical in this system, resulting in insufficient
block formation. A similar result was also observed for the
RAFT polymerization of BPVS using the xanthate-terminated
poly(NVC) as a macro-CTA (Table S7 and Figure S9,
Supporting Information). Overall, we confirmed that the
order of the blocks is crucial for the synthesis of well-defined
block copolymers composed of poly(NVC) and poly(S-vinyl
sulfide) derivatives; block copolymers could be obtained by
RAFT polymerization of NVC using poly(S-vinyl sulfide)
macro-CTA, whereas polymerization with an opposite order
afforded insufficient products.
Very recently, Keddie and Moad reported that controlled
character of the RAFT polymerization of NVC was achieved
using trithiocarbonates, allowing for the synthesis of homopol-
ymers and block copolymers with low polydispersities.80
Hence, we evaluated RAFT polymerization of S-vinyl sulfide
monomer using trithiocarbonates. Our preliminary experiments
indicated that the controlled character of the polymerization of
PVS with trithiocarbonates were comparable to that with
xanthate-type CTAs used in this study (Figure S10, Supporting
Information), suggesting that trithiocarbonate-type CTAs also
have a potential for the controlled synthesis of block
copolymers. Experiments aiming to synthesize various block
copolymers composed of conjugated monomers and non-
conjugated S-vinyl sulfides using various CTAs are now in
progress, which will be reported separately.
up to [NVC]/[macro-CTA] = 200, with unimodal SEC traces
1
and high-molecular-weight species being absent. In the H
NMR spectrum of the block copolymer measured in CDCl3,
peaks corresponding to both components are clearly detected
(Figure 5b). In all cases, the molecular weights of the block
copolymers evaluated from the composition determined by
elemental analysis and the molecular weight of the poly(BPVS)
macro-CTA were comparable to the theoretical values. These
results clearly demonstrate that the chain extension from the
poly(BPVS) macro-CTA to NVC is a controllable process and
provides block copolymer with as-designed chain structure and
relatively narrow molecular weight distribution. Note that there
was the tendency for the controlled polymerization to be
difficult with an increase of [NVC]0/[macro-CTA]0 ratio, that
is, the slight tailing in the lower molecular weight regions due
to the increase in the viscosity or the byproducts formed by the
unfavorable chain transfer and termination reactions was
observed at [NVC]/[macro-CTA] = 400 (Figure 4b).
The RAFT polymerization of PVS with a xanthate-
terminated poly(NVC), which was prepared by RAFT
polymerization of NVC in the presence of CTA 4, was also
conducted for the synthesis of the corresponding block
copolymer (Scheme S4 and Table S6, Supporting Information).
When the polymerization of PVS was conducted using the
xanthate-terminated poly(NVC) in bulk at 60 °C, the
polymerization was relatively slow (yield =69−33% after 24 h
at [PVS]0/[macro-CTA]0/[AIBN]0 = 800−200/2/1). As can
be seen in Figure S8 (Supporting Information), a shift in the
SEC trace toward a higher molecular weight region was seen
after the chain extension, while the resulting products showed
broad SEC traces with an apparent shoulder peak at low
molecular weight regions. The results indicated the possible
Postmodifications. The incorporations of photoactive
chromophores on the halide units of the poly(BPVS) was
conducted by postmodifications using Suzuki and Buchwald−
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Table 4. Postmodifications of Poly(BPVS) with Optoelectronic Compounds
a
b
c
d
e
e
DS (%) (1H NMR (EA))
Td10 (°C)
λmax (nm)
λmax (nm)
abs
ex
polymer
reactant
catal./base
yield
poly(BPVS)
−
An
−
−
>99
>99
94
−
>99
302
340
347
332
339
266
369
314
308
−
415
376
377
446
f
poly(BPVS-An)
poly(BPVS-Flu)
poly(BPVS-DPA)
poly(BPVS-PTZ)
Pd(PPh3)4/Na2CO3
Flu
DPA
PTZ
>99
g
Pd2(dba)3 /NaO-t-Bu
74 (64)
>99 (94)
>99
320
a
b
An = 9-anthraceneboronic acid, Flu = 9,9-dimethylfluoren-2-boronic acid, DPA = diphenylamine, and PTZ = phenothiazine. Poly(BPVS-An) and
c
poly(BPVS-Flu): methanol-insoluble part. Poly(BPVS-DPA) and poly(BPVS-PTZ): diethyl ether insoluble part. DS = degree of substitution
d
e
1
calculated by H NMR and elemental analysis. Determined by TGA; heating rate = 10 °C/min; N2 atmosphere. Measured by THF solution.
f
Reaction was conducted in the presence of Na2CO3 (10 equiv for the bromo unit) and Pd(PPh3)4 (6 mol % for the bromo unit) in THF under
g
reflux for 48 h. Reaction was conducted in the presence of NaO-t-Bu (15 equiv for the bromo unit), Pd2(dba)3 (25 mol % for the bromo unit), and
ligand (P(t-Bu)3, P/Pd ratio = 3/1) in 1,4-dioxane at 100 °C for 24 h.
Hartwig coupling reactions, as shown in Scheme 3. An
advantage of the postmodification methods is its versatility,
even if postmodification reactions in polymers can suffer
occasionally from lower conversion. When the optimal
postmodification route which afforded the quantitative
conversion was found, however the postmodification route
can be a useful method for the incorporation of the various
substituents. In contrast, the synthetic route with the monomer
synthesis with the optoelectronic functionality, followed by the
polymerization, may suffer from the low solubility and low
polymerization rate of the higher molecular weight monomer.
Table 4 summarizes the results of the palladium-catalyzed
coupling reactions of poly(BPVS). Initially, the Suzuki-coupling
reaction of poly(BPVS) using a monofunctional boronic acid
was conducted with tetrakis(triphenylphosphine)palladium(0)
as a catalyst under basic conditions, as shown in Scheme 3a.
The poly(BPVS) obtained by RAFT polymerization was
reacted with a large excess of 9-anthraceneboronic acid in the
presence of the Pd catalyst and Na2CO3 in THF/water.
Purification of the product was conducted by precipitation into
methanol, yielding a yellow product, which was soluble in a
number of organic solvents, including chloroform and THF
(Table S2, Supporting Information). Product yield was
calculated on the basis of the amount of aryl halide unit in
product, poly(BPVS-PTZ), shows all characteristic proton
signals corresponding to the methylene (0.9−2.3 ppm),
methine main chain (3.1−4.3 ppm), and phenyl groups
(5.9−7.9) ppm (Figure 6e). The conversion calculated by
comparison of the signals at 3.1−4.3 ppm corresponding to the
methine protons in the main chain to the signal at 5.9−7.9 ppm
corresponding to the aromatic protons was estimated to be
>99%, which is in fair agreement with the value determined by
the elemental analysis (nitrogen content). The introduction of
diphenylamine on the aryl halide unit proceeded smoothly with
a relatively high conversion to afford sulfur-containing polymers
with the triphenyl amine unit in the side chain (Figure 6d). In
all cases, the magnification of 1H NMR spectra of the products
in the range of 6.0−9.5 ppm indicated clearly the incorporation
of photoactive chromophores, which were quite different from
that of the poly(BPVS) (Figure S11, Supporting Information).
These results indicate that both the Suzuki and Buchwald−
Hartwig coupling reactions are effective methods for the
introduction of additional optoelectronic components into the
bromophenyl unit of the poly(BPVS), producing novel sulfur-
containing polymers having additional electronic functionalities
with high conversions (degrees of the substitution) and
recoveries.
Postmodification of the block copolymer, poly(BPVS-An-b-
NVC), was also attempted using the Suzuki coupling reaction.
Incorporation of the anthracene unit onto the poly(BPVS)
segment in the block copolymer was carried out with a large
excess of 9-anthraceneboronic acid in the presence of the Pd
catalyst and Na2CO3 in THF/water under the same conditions.
1
the feed and the conversion. The H NMR spectrum of the
product shows the characteristic broad peaks at 6.5−8.5 ppm
(Figure 6b), which are attributed to the phenyl and anthryl
1
groups. Comparison of the H NMR spectra of the resulting
poly(BPVS-An) and the poly(BPVS) used as a starting polymer
revealed an increase in peak intensity of the aromatic signals
after the postmodification. Conversion of the coupling reaction
determined by comparison of the aromatic peaks at 6.5−8.4
ppm (13H) to the peaks at 3.3−4.6 ppm attributed to the main
chain (1H) was estimated to be 99%. The Suzuki-coupling
reaction of poly(BPVS) using 9,9-dimethylfluoren-2-boronic
acid under the same conditions afforded poly(BPVS-Flu)
1
As can be seen in Figure 5c, the H NMR spectrum of the
resulting poly(BPVS-An-b-NVC) showed characteristic broad
peaks at 6.5−8.4 ppm attributed to the phenyl and anthryl
groups, suggesting the incorporation of the photoactive
chromophore. The resulting novel block copolymer has two
distinct electronic functionalities, in which the carbazole-
containing segment is covalently linked with the anthracene-
containing segment.
1
quantitatively, which was evaluated by H NMR (Figure 6c).
Buchwald−Hartwig coupling reactions between the pendant
bromophenyl group of poly(BPVS) with photoactive com-
pounds having an N−H bond were conducted using tris-
(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) as a cata-
lyst, sodium tert-butoxide (NaO-t-Bu) as a base, and tritert-
butylphosphine (P(t-Bu)3) as a ligand according to procedures
reported previously (Scheme 3b).47−49 Table 4 shows the
results of the Buchwald−Hartwig coupling reactions of
poly(BPVS-Br) using phenothiazine and diphenylamine. The
reaction of poly(BPVS) with phenothiazine was conducted in
Optical and Thermal Properties. For the investigation of
characteristic optoelectronic properties, we employed four
polymers having different substituent groups, poly(BPVS-An),
poly(BPVS-Flu), poly(BPVS-DPA), and poly(BPVS-PTZ),
which were modified with anthracene, fluorene, diphenylamine,
and phenothiazine, respectively. Figure 7a depicts the
absorbance spectra of the poly(BPVS) and modified polymers
in THF. Poly(BPVS) has a maximum absorption peak at ca.
270 nm due to the phenyl groups. After modification, the
characteristic absorption peaks corresponding to the incorpo-
rated optoelectronic moieties are clearly observed at ca. 330−
1
1.4-dioxane at 100 °C for 24 h. The H NMR spectrum of the
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1
Figure 6. H NMR spectra (CDCl3) of (a) poly(BPVS) used as a starting material, and (b-e) resulting polymers obtained after postmodifications.
400, 300−350, 280−350, and 300−350 nm in poly(BPVS-An),
poly(BPVS-Flu), poly(BPVS-DPA), and poly(BPVS-PTZ),
respectively. The fluorescence properties of poly(BPVS),
poly(BPVS-An), poly(BPVS-Flu), poly(BPVS-DPA), and poly-
(BPVS-PTZ) were also investigated. As can be seen in parts c
and d of Figure 7, poly(BPVS) shows no fluorescence whereas
the four modified polymers show the fluorescence due to the
incorporation of optoelectronic moieties. Poly(BPVS-An),
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Figure 7. Absorption (concentration = (a) 1.0 × 10−4 chromophore unit mol/L and (b) 1.0 × 10−5 chromophore unit mol/L in THF) and
fluorescence spectra (concentration = (c, d) 1.0 × 10−5 chromophore unit mol/L in THF) of the resulting polymers. Exciting wavelength:
poly(BPVS), 308 nm; poly(BPVS-An), 369 nm; poly(BPVS-Flu), 314 nm; poly(BPVS-DPA), 308 nm; poly(BPVS-PTZ), 320 nm.
poly(BPVS-Flu), poly(BPVS-DPA), and poly(BPVS-PTZ)
have the fluorescence in a range of ca. 400−600, 350−450,
350−550, and 370−530 nm, respectively. The strongest
fluorescence is, in particular, observed in poly(BPVS-Flu),
which resulted from the high quantum efficiency of the fluorene
moiety.
The absorption and fluorescence properties of the block
copolymers, poly(BPVS-b-NVC) and poly(BPVS-An-b-NVC)
obtained by postmodification, are shown in Figure 8. The
absorption due to the poly(NVC) segment is observed at ca.
300−350 nm in the parent block copolymer, poly(BPVS-b-
NVC). After postmodification, poly(BPVS-An-b-NVC) shows
the absorption due to the anthracene groups at ca. 350−400
nm, which indicates incorporation of the anthracene group; this
result was consistent with the 1H NMR analysis of poly(BPVS-
An-b-NVC). In the thin film state, the absorption spectrum is
slightly red-shifted compared to that of the THF solution state,
suggesting a specific conformation in the thin film.
Fluorescence spectra of poly(BPVS-b-NVC) shows a fluo-
rescence at ca. 350−500 nm, which corresponds to that of the
poly(NVC) segment. In the case of poly(BPVS-An-b-NVC), a
strong fluorescence due to the anthracene groups is also
observed in a range of 350−500 nm (excitation wavelength:
294 nm). The formation of an excimer (i.e., the carbazole−
carbazole and anthracene−anthracene dimer, respectively) can
be observed according to the previous literature;81−83 however,
fluorescence due to such excimer species was not observed in
poly(BPVS-An-b-NVC), which is likely due to the fact that the
Figure 8. Absorption spectra (concentration = 1.0 × 10−4
relatively bulky side chains (i.e., 9-phenylanthracene groups)
would prevent the formation of excimers. The obtained results
demonstrate that the control of the fluorescence behaviors of
poly(BPVS)-based homopolymer and block copolymer were
successfully achieved by the incorporation of various
chromophore unit mol/L in THF) and fluorescence spectra
(concentration = 1.0 × 10−5 chromophore unit mol/L in THF) of
the resulting block copolymers. Excited wavelength: 294 nm.
optoelectronic groups by efficient palladium-catalyzed coupling
reactions, and besides, the tuning of other properties such as
the electrochemical behaviors can be expected by using this
system.
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The thermal properties of the modified polymers were
evaluated by TG measurements. As shown in Figure 9, the
and polymerization results under various conditions, reaction
schemes, and GPC traces of the resulting polymers. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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 (24350053).
Figure 9. TGA thermograms of the polymers obtained by post-
modifications of poly(BPVS).
poly(BPVS) was stable up to 280 °C, and then the thermal
degradation started. The temperature for the 10% weight loss
of poly(BPVS-An) under a nitrogen atmosphere was 340 °C,
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