Precise synthesis of miktoarm star polymers by using a new dual-functionalized 1,1-diphenylethylene derivative in conjunction with living anionic polymerization system

Article abstract of DOI:10.1021/ma300699m

The general utility of a 1,1-diphenylethylene (DPE) derivative substituted with trimethylsilyl- and tert-butyldimethylsilyl-protected hydroxyl functionalities, as a new dual-functionalized core agent in conjunction with a living anionic polymerization system, has been demonstrated by the successful synthesis of various well-defined 3-arm ABC and 4-arm ABCD μ-star polymers. Two different protected hydroxyl functionalities were progressively deprotected to generate hydroxyl groups, followed by conversion to α-phenyl acrylate (PA) functions at separate stages, and the PA functions were reacted with appropriate living anionic polymers to result in the above μ-stars. In order to further synthesize μ-star polymers with five or more arms, a new iterative methodology using a functional DPE anion derived from the above DPE derivative has been developed. The reaction system of this methodology is designed in such a way that the PA function used as the reaction site is regenerated after the introduction of an arm segment in each reaction sequence, and this sequence, consisting of arm introduction and regeneration of the PA reaction site , is repeatable. With this methodology, a series of new well-defined μ-star polymers up to a 5-arm ABCDE type, composed of all different methacrylate-based polymer segments, were successfully synthesized for the first time.

Full text of DOI:10.1021/ma300699m

Article
pubs.acs.org/Macromolecules
Precise Synthesis of Miktoarm Star Polymers by Using a New Dual-
Functionalized 1,1-Diphenylethylene Derivative in Conjunction with
Living Anionic Polymerization System
Shotaro Ito, Raita Goseki, Saeko Senda, and Akira Hirao*
Polymeric and Organic Materials Department, Graduate School of Science and Engineering, Tokyo Institute of Technology, S1-6,
2-12-1, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: The general utility of a 1,1-diphenylethylene (DPE)
derivative substituted with trimethylsilyl- and tert-butyldimethylsilyl-
protected hydroxyl functionalities, as a new dual-functionalized core
agent in conjunction with a living anionic polymerization system, has
been demonstrated by the successful synthesis of various well-defined 3-
arm ABC and 4-arm ABCD μ-star polymers. Two different protected
hydroxyl functionalities were progressively deprotected to generate
hydroxyl groups, followed by conversion to α-phenyl acrylate (PA)
functions at separate stages, and the PA functions were reacted with
appropriate living anionic polymers to result in the above μ-stars. In
order to further synthesize μ-star polymers with five or more arms, a
new iterative methodology using a functional DPE anion derived from the above DPE derivative has been developed. The
reaction system of this methodology is designed in such a way that the PA function used as the reaction site is regenerated after
the introduction of an arm segment in each reaction sequence, and this sequence, consisting of “arm introduction and
regeneration of the PA reaction site”, is repeatable. With this methodology, a series of new well-defined μ-star polymers up to a 5-
arm ABCDE type, composed of all different methacrylate-based polymer segments, were successfully synthesized for the first
time.
In general, μ-star polymers are much more difficult to
synthesize than corresponding regular stars having the same
number of arm segments, since the synthesis always requires
several nearly quantitative reactions for the introduction of a
plural number of different arm segments into μ-stars. The
currently developed methodologies using the living anionic
polymerization system can cover the synthesis of a two-
component AxBy type. On the other hand, synthetic examples
of 3-arm three-component ABC μ-star polymers,15,33−37 three-
component stars with more than 3 arms,38−42 and 4-arm four-
component ABCD μ-stars43−46 have been very limited so far.
Although the success of the above-mentioned methodologies
can be evidenced to a certain extent by possible variation of
synthesized μ-star polymers, the reaction site(s) always
disappear after the synthesis of target μ-star polymers. None
of the methodologies allows for continued further synthesis of
μ-star polymers with more arms and compositions. Since 2001,
we have developed a novel and versatile methodology, which is
based on a new conceptual “iterative approach”. In this
approach, the reaction system is designed in such a way that the
same reaction site is always regenerated after the introduction
of an arm segment in each reaction sequence, and this “arm
INTRODUCTION
■
Star-branched polymers have been widely studied for a long
time because of synthetic challenges associated with preparing
them and because they offer different properties and behavior
in solution and bulk from those of the corresponding linear
polymers.1−6 Among the star-branched polymers, asymmetric
ones having different arm segments, the so-called mixed arm or
́
miktoarm (from the Greek word μικτοζ meaning mixed) star-
branched polymers (μ-star polymers),7−13 have received much
attention in recent years because their arm segments are
generally phase-separated at the molecular level, followed by
self-organizing, to produce new periodical suprastructures and
supramolecular assemblies on a nanosize scale which are quite
different from those formed by linear block polymers due to
their star-branched architectures.14−27 Such μ-star polymers are
expected to play an important role in the very near future as
next-generation functional polymers with many potential
applications in the fields of nanoscience and nanotechnology.
In order to synthesize well-defined μ-star polymers, the use
of living polymerization systems is essential. Among them, the
living anionic polymerization is undoubtedly the best system at
the present time28−32 for the synthesis of such well-defined
stars possessing precisely controlled and narrowly distributed
arm segments, compared to other living/controlled polymer-
ization systems.
Received: April 5, 2012
Revised: May 28, 2012
Published: June 8, 2012
© 2012 American Chemical Society
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Scheme 1. Iterative Methodology Using DPE Chemistry in Conjunction with Living Anionic Polymerization System
introduction−regeneration of the same reaction site” sequence
is repeatable.11−13,47−49 With this methodology, arm segments
can be successively and, in principle, limitlessly introduced by
repeating the reaction sequence to afford a series of μ-star
polymers with many arms and compositions. Scheme 1 shows a
representative iterative methodology using 1,1-diphenylethy-
lene (DPE) chemistry in conjunction with the above living
anionic polymerization system.50 The DPE function is capable
of not only reacting with living anionic polymers to link a
polymer chain but also of offering a DPE anion for regeneration
of the DPE function. The methodology involves only two
reaction steps in each reaction sequence: (1) a linking reaction
of a living anionic polymer with a chain-DPE-functionalized
polymer and (2) regeneration of the DPE function by reacting
a specially designed DPE-functionalized agent, 1-(4-(3-
bromopropyl)phenyl)-1-phenylethylene (1) with the DPE
anion produced by the linking reaction. The reaction sequence
involving the two reaction steps is repeated several times to
successively synthesize a series of μ-star polymers. As can be
seen in Scheme 1, the star polymer synthesized in the preceding
stage always corresponds to the starting material of the next
synthesis.
Synthesis was initiated by the reaction of 1 with a living
polymer end-capped with DPE to introduce the DPE function
at the chain-end. In the first reaction sequence, another living
polymer reacted with the resulting chain-end-DPE-function-
alized polymer to link the two polymer chains with the
production of a new DPE anion at the linking point. The DPE
derivative, 1, was reacted in situ with the DPE anion to
regenerate the same DPE function to be used as the next
reaction site, resulting in an in-chain-DPE-functionalized AB
diblock copolymer. The repetition of the reaction sequence
yielded a 3-arm ABC μ-star polymer with the DPE function
regenerated at the core. The reaction sequence was repeated
two more times to successively synthesize a 4-arm ABCD,
followed by a 5-arm ABCDE μ-star polymer. Similar iterative
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weights were determined by SEC with RI detection using standard
polystyrene calibration curve. The combination of viscometer, right
angle laser light scattering detection (RALLS), and RI detection was
applied for the online SEC system in order to determine the absolute
molecular weights of homopolymers, diblock copolymers, and star-
branched polymers.
methodologies have been further developed to allow access to a
wide variety of well-defined μ-star polymers with many arms
and components, such as ABC2, AB2C2, A2B2C2, A3B3C3,
AB2C4, ABCD2, ABC2D2, AB2C4D8, ABCD2E2, and
AB2C4D8E16 μ-star polymers.51−61
Since the living polymer used in the iterative methodology is
always required to react with the DPE function in each reaction
sequence, only highly reactive living anionic polymers, such as
the living polymers of styrene, 1,3-butadiene, isoprene, and
their derivatives, can be used. On the other hand, less reactive
living polymers of 2-vinylpyridine and alkyl methacrylates,
incapable of reacting with the DPE function, are not used. In
order to synthesize μ-star polymers consisting of poly(2-
vinylpyridine) and/or poly(alkyl methacrylate) segments, the
development of a new iterative methodology, in which living
anionic polymers of 2-vinylpyridine and alkyl methacrylates are
usable, is strongly demanded.
Herein, we report on the general utility of a DPE derivative,
1-(3-tert-butyldimethylsilyloxymethylphenyl)-1-(3-
trimethylsilyloxymethylphenyl)ethylene (2), as a new dual-
functionalized core agent in conjunction with the living anionic
polymerization system for the synthesis of various well-defined
3-arm ABC and 4-arm ABCD μ-star polymers. Moreover, we
also report on the development of a new iterative methodology
using a DPE anion derived from 2 for the successive synthesis
of a series of μ-star polymers up to a 5-arm ABCDE star
composed of all different methacrylate-based polymer seg-
ments.
Synthesis of 1-(3-tert-Butyldimethylsilyloxymethylphenyl)-
1-(3-trimethylsilyloxymethylphenyl)ethylene (2). To the sol-
ution of 1,1-bis(3-hydroxymethylphenyl)ethylene (7.15 g, 29.8 mmol)
and imidazole (2.03 g, 29.8 mmol) dissolved in dry DMF (40 mL) was
added tert-butyldimethylsilyl chloride (4.49 g, 29.8 mmol) in dry DMF
(20 mL), and the mixture was stirred at 25 °C for 4 h. The mixture
was quenched with saturated aqueous NaHCO3 (50 mL), extracted
with diethyl ether, washed with water, and dried over anhydrous
MgSO4. After the removal of solvent under reduced pressure, silica gel
column chromatography treated with triethylamine (hexane/dichloro-
methane = 5/1 (v/v)) afforded 1-(3-tert-butyldimethylsilyloxymethyl-
phenyl)-1-(3-hydroxymethylphenyl)ethylene in 20% yield (3.61 g,
10.2 mmol). 1-(3-tert-Butyldimethylsilyloxymethylphenyl)-1-(3-
hydroxymethylphenyl)ethylene thus prepared (3.59 g, 10.1 mmol)
was mixed with hexamethyldisilazane (3.25 g, 20.1 mmol) in
nitromethane (25 mL), and the mixture was allowed to react at 25
°C for 20 min under nitrogen. Removal of all volatile compounds by
vacuum pump (10−1 Torr), followed by the high-vacuum conditions
(10−6 Torr), yielded 1-(3-tert-butyldimethylsilyloxymethylphenyl)-1-
(3-trimethylsilyloxymethylphenyl)ethylene (2) (4.31 g, 10.1 mmol,
∼100%). In order to remove trace amounts of impurities that can react
with anionic species, 2 was finally purified under a high-vacuum line by
carefully adding sec-BuLi in heptane (0.05 M) until the color change
from colorless to faint yellow: 300 MHz 1H NMR (CDCl3) δ = 7.30−
7.19 (m, 8H, Ar), 5.45, 5.44 (s, 2H, =CH2), 4.73, 4.68 (s, 4H, −CH2−
O−), 0.91 (s, 9H, −C−(CH3)3), 0.14 (s, 9H, −Si−(CH3)3), 0.08 (s,
6H, −Si−(CH3)2). 75 MHz 13C NMR (CDCl3) δ = 141.6, 141.5,
141.1, 140.9, 128.3, 128.2, 127.3, 127.0, 126.6, 126.1, 126.0, 125.5
(Ar), 150.2 (CCH2), 114.3 (CCH2), 65.0, 64.7 (−CH2−O−),
26.3 (−C−(CH3)3), 18.5 (−C−(CH3)3), −0.2 (−Si−(CH3)3), −5.1
(−Si−(CH3)2).
EXPERIMENTAL SECTION
■
Materials. The reagents (>98% purities) were purchased from
Aldrich Japan and used as received unless otherwise stated. Styrene, α-
methylstyrene (αMS), 4-methylstyrene (4MS), 4-methoxystyrene
(MOS), and 1,1-diphenylethylene (DPE) were washed with 5%
NaOH solution and water and dried over MgSO4. After filtration of
MgSO4, they were distilled over CaH2 twice under reduced pressures.
Finally, styrene, αMS, 4MS, and MOS were distilled from their Bu2Mg
solutions (ca. 3 mol %) on the vacuum line (10−6 Torr). DPE was
finally distilled from its 1,1-diphenylhexyllithium (ca. 3 mol %)
solution on the vacuum line. 2-Vinylpyridine (2VP) was stirred over
KOH overnight. After filtration of KOH, 2VP was finally distilled over
fine powder CaH2 twice on the vacuum line. Methyl methacrylate
(MMA), ethyl methacrylate (EMA), tert-butyl methacrylate (tBMA),
benzyl methacrylate (BnMA), and 2-methoxyethyl methacrylate
(MOEMA) were washed with 5% NaOH and water and dried over
MgSO4. After filtration of MgSO4, they were finally distilled from their
(C8H17)3Al solutions (ca. 3 mol %) on the vacuum line. 2-tert-
Butyldimethylsilyloxyethyl methacrylate (Si-HEMA), ferrocenylmethyl
methacrylate (PFMMA), and (2,2-dimethyl-1,3-dioxolan-4-yl)methyl
methacrylate (acetal-DIMA) were synthesized and purified according
to the reported procedures.62−64 LiCl was dried with stirring at 120 °C
for 72 h. All monomers and LiCl were diluted with dry THF and then
divided into ampules equipped with break-seals under high-vacuum
conditions. α-Phenylacrylic acid was synthesized according to the
procedure previously reported.65 Both 1-(3-hydroxymethylphenyl)-1-
phenylethylene and 1,1-bis(3-hydroxymethylphenyl)ethylene were
synthesized according to the procedure previously reported.66
General Procedure of Postpolymerization, Model Linking
Reaction, and μ-Star Polymer Syntheses. Except for deprotection
and the Mitsunobu esterification reaction, all of the polymerizations
and linking reactions were carried out under high-vacuum conditions
(10−6 Torr) in sealed handmade glass reactors equipped with break-
seals. The reactor was sealed off from the vacuum line and prewashed
with a red colored 1,1-diphenylhexyllithium (ca. 0.05 M) in heptane
solution prior to the polymerization and linking reaction. All
operations were performed according to the usual high-vacuum
technique with break-seals.
Stability Evaluation of Living PMMA by Postpolymerization.
A typical procedure is as follows: MMA (5.90 mmol) in THF solution
(4.80 mL) was first polymerized with the initiator prepared from sec-
BuLi (0.0797 mmol) and DPE (0.160 mmol) in the presence of LiCl
(0.430 mmol) in THF solution (3.45 mL) at −78 °C for 20 min.
Then, the polymerization mixture was allowed to stand at −40 °C for
2 h. It was again cooled to −78 °C, and the postpolymerization was
subsequently carried out at −78 °C for 30 min by adding MMA (5.78
mmol) in THF solution (4.70 mL) precooled at −78 °C. The
polymerization was quenched with degassed methanol. Polymers were
recovered by the precipitation in hexane and analyzed by SEC and
t
RALLS. The postpolymerization of BMA, BnMA, or Si-HEMA was
carried out in similar manners.
Model Linking Reaction of Living PMMA with Chain-End-
PA-Functionalized PS. A typical procedure is as follows: A chain-
end-PA-functionalized PS was prepared by the procedure reported in
the Supporting Information. MMA (5.21 mmol) in THF solution
(5.18 mL) was polymerized at −78 °C for 20 min with the initiator
prepared from sec-BuLi (0.0444 mmol) and DPE (0.0683 mmol) in
the presence of LiCl (0.149 mmol) in THF solution (3.42 mL). Then,
the chain-end-PA-functionalized PS (Mn,SEC = 10.7 kg/mol, 0.0154
mmol for the PA function) in THF solution (2.07 mL) was added to
the living PMMA solution, and the mixture was allowed to react at
Measurements. Both 1H and 13C NMR spectra were measured on
a Bruker DPX300 in CDCl3. Chemical shifts were recorded in ppm
1
downfield relative to CHCl3 (δ = 7.26) and CDCl3 (δ = 77.1) for H
and 13C NMR as standard, respectively. Molecular weight and
polydispersity indices were measured on an Asahi Techneion AT-
2002 equipped with a Viscotek TDA model 302 triple detector array
using THF as a carrier solvent at a flow rate of 1.0 mL/min at 40 °C.
Three polystyrene (PS) gel columns (pore size (bead size)) were used:
650 Å (9 μm), 200 Å (5 μm), 75 Å (5 μm). The relative molecular
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−78 °C for 20 h. After quenching with degassed methanol, the
polymer mixture was precipitated in hexane and analyzed by SEC and
RALLS. The reaction efficiency was estimated by comparing SEC peak
areas corresponding to those of the linked polymer, deactivated
PMMA, and unreacted PS. Similarly, the linking reaction of living
PMMA or PtBMA with the same chain-end-PA-functionalized PS was
carried out under various conditions listed in Table 2. The syntheses of
chain-end-PA-functionalized PS and other chain-end-functionalized
(PS)s are reported in the Supporting Information.
methanol and freeze-dried from its absolute benzene solution for 24 h
(0.500 g, 90%). H NMR (CDCl3) (300 MHz): δ = 7.24−6.93 (m,
Ar), 6.31 (s, CCH2), 5.87 (s, CCH2), 5.19 (s, Ar−CH2−OCO−
CH(C6H5)CH2), 4.96 (s, Ar−CH2−OCO−CH(C6H5)−CH2−),
3.59 (s, −O−CH3), 2.07−0.70 (broad, backbone).
1
Finally, the in-chain-PA-functionalized PMMA (0.0152 mmol for
the PA function) thus prepared was reacted at −40 °C for 20 h with
living PMMA (0.0511 mmol, Mn(RALLS) = 10.4 kg/mol) prepared in
the same manner mentioned above. The target 3-arm AA′A″ star-
branched PMMA was isolated in 80% yield by fractional precipitation
using benzene/methanol (1/5, v/v) (0.622 g). Mn(RALLS) = 36.8 kg/
mol, Mw/Mn = 1.03 (SEC). 1H NMR (CDCl3) (300 MHz): δ = 7.24−
6.93 (m, Ar), 4.96 (s, Ar−CH2-O−), 3.59 (s, −O−CH3), 2.07−0.70
(broad, backbone).
The 3-arm ABC μ-star polymers were synthesized by using the
similar procedures mentioned above, and details are reported in the
Supporting Information.
Synthesis of 4-Arm ABCD μ-Star Polymers. The synthesis of 4-
arm ABCD μ-star polymers was also carried out by using the above-
mentioned similar procedures except for the use of in-chain-
functionalized PS-block-PMMA, and details are reported in the
Supporting Information.
Synthesis of 3-Arm AA′A″ Star PMMA and 3-Arm ABC μ-Star
Polymers. MMA (12.3 mmol) in THF solution (11.6 mL) was
polymerized at −78 °C for 20 min with the initiator prepared from
oligo(α-methylstyryl)lithium (0.103 mmol) (sec-BuLi (0.103 mmol)
and α-methylstyrene (0.433 mmol)) and 2 (0.153 mmol) in the
presence of LiCl (0.449 mmol) in THF solution (9.42 mL). The
polymerization was terminated with degassed methanol, and the
polymer was precipitated in methanol containing a few drops of acetic
acid. It was reprecipitated from THF to methanol and freeze-dried
from its absolute benzene solution. In the precipitation step, the TMS
functionality of 2 was selectively deprotected to regenerate the
hydroxyl group. Thus, a chain-end-(hydroxyl and TBDMS-protected
hydroxyl)-functionalized PMMA was obtained in 98% yield (1.26 g)
and characterized by SEC, RALLS, and 1H NMR. Mn(RALLS) = 13.6
Successive Synthesis of μ-Star Polymers by a New Iterative
Methodology. A chain-end-(TMS- and TBDMS-protected hydrox-
yl)-functionalized-PMMA was newly prepared in the same manner as
that mentioned above and used as the starting material in the
methodology.
1
kg/mol, Mw/Mn = 1.05 (SEC). H NMR (CDCl3) (300 MHz): δ =
7.24−6.93 (m, aromatic), 4.66 (s, Ar−CH2−OSi), 4.62 (s, Ar−CH2−
OH), 3.59 (s, −O−CH3), 2.07−0.70 (broad, backbone), 0.05 (s, Si−
(CH3)2).
Under an atmosphere of nitrogen, the chain-end-functionalized
PMMA (0.0926 mmol for the hydroxyl functionality) dissolved in dry
THF (15 mL) was mixed with PPh3 (5.76 mmol), α-phenylacrylic acid
(5.76 mmol), and diisopropyl azodicarboxylate (DIAD) (5.74 mmol)
at 0 °C. The reaction mixture was allowed to react at 25 °C for 16 h
and poured into methanol to precipitate the polymer. The polymer
was purified by reprecipitation twice from THF solution to methanol
and freeze-dried from its absolute benzene solution for 24 h. The
The First Iterative Process. The TMS-protected hydroxyl
functionality of the above-mentioned starting polymer was selectively
deprotected in methanol containing a few drops of acetic acid, and the
generated hydroxyl group was converted to the PA function, resulting
in a chain-end-(PA and TBDMS-protected hydroxyl)-functionalized
PMMA in 92% yield. Mn(RALLS) = 13.2 kg/mol, Mw/Mn = 1.03
(SEC). 1H NMR (CDCl3) (300 MHz): δ = 7.24−6.93 (m, aromatic),
4.66 (s, Ar−CH2−OSi), 4.62 (s, Ar−CH2−OH), 3.59 (s, −O−CH3),
2.07−0.70 (broad, backbone), 0.05 (s, Si−(CH3)2).
Living PEMA was prepared at −78 °C for 30 min by the
polymerization of EMA (16.3 mmol) in THF solution (14.5 mL) with
the initiator prepared from sec-BuLi (0.183 mmol) and DPE (0.229
mmol) in the presence of LiCl (0.585 mmol) in THF solution (11.1
mL) and in situ reacted with the above chain-end-(PA and TBDMS-
protected hydroxyl)-functionalized PMMA (0.0592 mmol for the PA
function) in THF solution (10.5 mL) at −78 °C. The reaction mixture
was allowed to further react at −40 °C for 20 h. After the usual work-
up, an in-chain-(TBDMS-protected hydroxyl)-functionalized PMMA-
b-PEMA was isolated in 68% yield by SEC fractionation (0.930 g).
Mn(RALLS) = 23.1 kg/mol, Mw/Mn = 1.03 (SEC). 1H NMR (CDCl3)
(300 MHz): δ = 7.24−6.93 (m, aromatic), 4.96 (s, Ar−CH2−OCO−),
4.66 (s, Ar−CH2−OSi), 4.05 (s, −O−CH2−CH3), 3.59 (s, −O−
CH3), 2.07−0.70 (broad, backbone), 0.05 (s, Si−(CH3)2).
The TBDMS-protected hydroxyl functionality was deprotected with
(C4H9)4NF at −10 °C for 10 h, and the generated hydroxyl group was
converted to the PA function. The functional DPE anion was prepared
from 2 (0.113 mmol) and oligo(α-methylstyryl)lithium (0.0754
mmol) (sec-BuLi (0.0754 mmol) and α-methylstyrene (0.263
mmol)) in THF solution (5.60 mL) at −78 °C for 30 min and in
situ reacted with the above in-chain-PA-functionalized PMMA-b-
PEMA (0.0565 mmol for the PA function) at −78 °C for 10 h. The
reaction was quenched with degassed methanol containing a few drops
of acetic acid. The polymer, once precipitated in a mixture of methanol
and water (5/1, v/v), was reprecipitated from its THF solution to a
mixture of methanol and water (5/1, v/v) and freeze-dried twice from
its absolute benzene solution. An in-chain-(hydroxyl and TBDMS-
protected hydroxyl)-functionalized PMMA-b-PEMA was obtained in
93% yield (0.727 g). Mn(RALLS) = 23.0 kg/mol, Mw/Mn = 1.03
(SEC). 1H NMR (CDCl3) (300 MHz): δ = 7.24−6.93 (m, aromatic),
4.96 (s, Ar−CH2−OCO−), 4.66 (s, Ar−CH2−OSi), 4.62 (s, Ar−
CH2−OH), 4.05 (s, −O−CH2−CH3), 3.59 (s, −O−CH3), 2.07−0.70
(broad, backbone), 0.05 (s, Si−(CH3)2).
1
polymer was obtained in 89% yield (1.12 g). H NMR (CDCl3) (300
MHz): δ = 7.24−6.93 (m, aromatic), 6.31 and 5.87 (s, CCH2), 5.19
(s, Ar−CH2−OCO), 4.66 (s, Ar−CH2−Si), 3.59 (s, −O−CH3), 2.07−
0.70 (broad, backbone), 0.05 (s, Si−(CH3)2).
MMA (10.4 mmol) was polymerized at −78 °C for 20 min with the
initiator prepared from sec-BuLi (0.104 mmol) and DPE (0.135
mmol) in the presence of LiCl (0.339 mmol) in THF solution (17.2
mL). The resulting living PMMA (0.104 mmol) was in situ reacted
with the above chain-end-(PA and TBDMS-protected hydroxyl)-
functionalized PMMA (0.0298 mmol for PA functionality) in THF
solution (5.47 mL), precooled at −78 °C. The reaction mixture was
allowed to further react at −40 °C for 20 h. The reaction was
quenched with degassed methanol, and polymers were precipitated in
hexane. The target in-chain-(TBDMS-protected hydroxyl)-function-
alized PMMA was isolated in 80% yield by fractional precipitation
using benzene/methanol (1/5 (v/v)) and reprecipitated in methanol
and freeze-dried from its absolute benzene solution for 24 h (0.579 g,
1
74%). Mn(RALLS) = 26.2 kg/mol, Mw/Mn = 1.03 (SEC). H NMR
(CDCl3) (300 MHz): δ = 7.24−6.93 (m, aromatic), 4.96 (s, Ar−
CH2−OCO−), 4.66 (s, Ar−CH2−OSi), 3.59 (s, −O−CH3), 2.07−
0.70 (broad, backbone), 0.05 (s, Si−(CH3)2).
Under an atmosphere of nitrogen, the resulting in-chain-(TBDMS-
protected hydroxyl)-functionalized PMMA (0.0221 mmol), dissolved
in THF (8.0 mL), was mixed with (C4H9)4NF (0.663 mmol) in THF
solution (0.663 mL) at −10 °C. The reaction mixture was allowed to
stir at −10 °C for 10 h and then poured into a large amount of
methanol to precipitate the polymer. The polymer was purified by
reprecipitation from THF to methanol and freeze-dried from its
1
absolute benzene solution (0.556 g, 96%). H NMR (CDCl3) (300
MHz): δ = 7.24−6.93 (m, aromatic), 4.96 (s, Ar−CH2−OCO−), 4.62
(s, Ar−CH2−OH), 3.59 (s, −O−CH3), 2.07−0.70 (broad, backbone).
The resulting in-chain-hydroxyl-functionalized PMMA (0.0212
mmol for the hydroxyl group) was esterified with α-phenylacrylic
acid in the same manner as that mentioned above. The polymer was
purified by the reprecipitation three times from THF solution to
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The Second Iterative Process. The hydroxyl group of the above in-
chain-functionalized PMMA-b-PEMA was converted to the PA
function in the same manner as that mentioned above. Thus, an in-
chain-(PA and TBDMS-protected hydroxyl)-functionalized PMMA-b-
PEMA was prepared in 92% yield.
The successive synthesis of μ-stars by using living P4MS chain-end-
functionalized with 2 is reported in the Supporting Information.
RESULTS AND DISCUSSION
■
Stability of Living Poly(methyl methacrylate) (PMMA)
and Poly(tert-butyl methacrylate) (PtBMA). In this study,
we focus on the synthesis of new μ-star polymers mainly
composed of methacrylate-based polymer segments. However,
there is so far little information on the stability of living anionic
polymers of MMA and other alkyl methacrylate monomers,
although it is extremely important in the synthesis of such star-
branched polymers. For this reason, living polymers of both
MMA and tBMA were selected as representative examples, and
the stability was examined by postpolymerization, prior to star
polymer synthesis. Stability is evaluated by comparing two SEC
peak areas of the first polymer with the postpolymer and by
comparing the observed Mn value of the postpolymer with that
calculated from the feed ratio. For example, living MMA was
prepared by the polymerization with the initiator prepared from
sec-BuLi and DPE in the presence of a 3-fold excess of LiCl in
THF at −78 °C for 20 min and then allowed to stand at −40
°C for 2 h. The living PMMA was again cooled to −78 °C, and
the postpolymerization was carried out at −78 °C for 30 min by
adding a second MMA to the mixture. It has already been
ascertained that living PMMA is obtained within 10 min and
completely stable at −78 °C for 1 h. The SEC profile, as shown
in Figure 1, clearly exhibits two distinct peaks, which
Living PtBMA (0.103 mmol, Mn(RALLS) = 10.7 kg/mol) was
reacted with the in-chain-(PA and TBDMS-protected hydroxyl)-
functionalized PMMA-b-PEMA (0.0299 mmol for the PA function) at
−25 °C for 20 h to yield a core-(TBDMS-protected hydroxyl)-
functionalized 3-arm ABC μ-star polymer, composed of PMMA,
PEMA, and PtBMA. The polymer was isolated in 87% yield by SEC
fractionation (0.881 g). Mn(RALLS) = 33.8 kg/mol, Mw/Mn = 1.03
(SEC). 1H NMR (CDCl3) (300 MHz): δ = 7.24−6.93 (m, aromatic),
4.96 (s, Ar−CH2−OCO−), 4.66 (s, Ar−CH2−OSi), 4.05 (s, −O−
CH2−CH3), 3.59 (s, −O−CH3), 2.07−0.70 (broad, backbone), 1.44
(s, −C−(CH3)3), 0.05 (s, Si−(CH3)2).
In the resulting star, the TBDMS-protected hydroxyl functionality
was deprotected to generate the hydroxyl group, followed by
conversion to the PA function, and the PA function thus regenerated
was reacted with the DPE anion prepared in the same manner
mentioned above to introduce both the TMS- and the TBDMS-
protected hydroxyl functionalities.
The Third Iterative Process. The TMS-protected functionality of
the above core-functionalized μ-star polymer was selectively
deprotected in methanol containing a few drops of acetic acid, and
the generated hydroxyl group was converted to the PA function. Thus,
a core-(PA and TBDMS-protected hydroxyl)-functionalized 3-arm
ABC μ-star polymer was obtained in 96% yield.
Living PBnMA was prepared at −78 °C for 1 h by the
polymerization of BnMA (4.12 mmol) with the initiator prepared
from sec-BuLi (0.0788 mmol) and DPE (0.108 mmol) in the presence
of LiCl (0.272 mmol) and in situ reacted with the core-(PA and
TBDMS-protected hydroxyl)-functionalized ABC μ-star polymer
(0.0213 mmol for the PA function) at −78 °C. The reaction mixture
was then allowed to react at −40 °C for 20 h. After the usual work-up,
a core-(TBDMS-protected hydroxyl)-functionalized 4-arm ABCD μ-
star, composed of PMMA, PEMA, PtBMA, and PBnMA, was isolated
in 85% yield by SEC fractionation (0.779 g). Mn(RALLS) = 43.0 kg/
mol, Mw/Mn = 1.03 (SEC). 1H NMR (CDCl3) (300 MHz): δ = 7.24−
6.93 (m, aromatic), 4.88 (s, −CH2−Ph) 4.66 (s, Ar−CH2−OSi), 4.05
(s, −O−CH2−CH3), 3.59 (s, −O−CH3), 2.07−0.70 (broad, back-
bone), 1.44 (s, −C−(CH3)3), 0.05 (s, Si−(CH3)2).
In the resulting star, the TBDMS-protected hydroxyl functionality
was deprotected to generate the hydroxyl group, followed by
conversion to the PA function, and the PA function thus regenerated
was reacted with the DPE anion prepared in the same manner
mentioned above to introduce both the TMS- and the TBDMS-
protected hydroxyl functionalities.
The Fourth Iterative Process. The TMS-protected functionality of
the above core-functionalized μ-star polymer was selectively
deprotected in methanol containing a few drops of acetic acid, and
the generated hydroxyl group was converted to the PA function. Thus,
a core-(PA and TBDMS-protected hydroxyl)-functionalized 4-arm
ABCD μ-star polymer was obtained in 87% yield.
Figure 1. SEC profiles of deactivated PMMA and postpolymer
obtained by the postpolymerization.
correspond to those for PMMA deactivated after the first
polymerization and the postpolymer. The stability of living
PMMA at −40 °C for 2 h is calculated to be 41% by comparing
the two peak areas. It is also calculated to be 38% by comparing
the Mn value (26.7 kg/mol) of the postpolymer with the
predictable value (14.9 kg/mol) by assuming that the first living
PMMA is completely stable. An average value ((41 + 38)/2 =
40%) was adopted as the stability under the condition at −40
°C for 2 h. Additional results obtained under various conditions
are summarized in Table 1.
Living PMMA was stable at −78 °C for 1 h, but slowly
deactivated with time. It survived 90% and 85% after 10 and 20
h, respectively. It became unstable by raising the temperature to
−40 °C and only 40% and 27% of the original chain-end anion
Living PMOEMA was prepared at −78 °C for 2 h by the
polymerization of MOEMA (4.95 mmol) with the initiator prepared
from sec-BuLi (0.0502 mmol) and DPE (0.0864 mmol) in the
presence of LiCl (0.183 mmol) and in situ reacted with the core-(PA
and TBDMS-protected hydroxyl)-functionalized ABCD μ-star poly-
mer (0.0127 mmol for the PA function) at −78 °C. Then, the reaction
mixture was allowed to react with at −40 °C for 20 h. After the usual
work-up, a 5-arm ABCDE μ-star polymer, composed of PMMA,
PEMA, PtBMA, PBnMA, and PMOEMA, was isolated in 54% yield by
fractional precipitation using methanol/water (5/1, v/v) (0.353 g).
Mn(RALLS) = 51.5 kg/mol, Mw/Mn = 1.03 (SEC). 1H NMR (CDCl3)
(300 MHz): δ = 7.24−6.93 (m, aromatic), 4.88 (s, −CH2−Ph) 4.66 (s,
Ar−CH2−OSi), 4.24−3.96 (m, −O−CH2−CH3, −O−CH2−CH2−
O−), 3.59 (s, COO−CH3), 3.38 (s, −CH2−CH2−O−CH3) 2.07−
0.70 (broad, backbone), 1.44 (s, −C−(CH3)3), 0.05 (s, Si−(CH3)2).
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Table 1. Stability of Living PMMA, PtBMA, PBnMA, and
P(Si-HEMA) in THF
should once again be mentioned that neither polymerization
nor even oligomerization of the PA function occurred under
such conditions.
In this section, we searched for more optimized reaction
conditions and other new reaction sites capable of linking with
living PMMA. A model reaction was carried out between living
PMMA (Mn = ca. 5.00−10.0 kg/mol) and chain-end-PA-
functionalized PS (Mn = 10.7 kg/mol) in THF under various
conditions, and the efficiency under each reaction condition is
listed in Table 2. As mentioned above, the reaction was not
living polymer
PMMA
temp (°C)
time (h)
stability (%)
−78
−78
−40
−40
−20
−78
−40
−25
−25
−78
−40
−78
−40
10
20
2
90
85
40
27
0
5
2
PtBMA
20
5
100
88
20
0
2
Table 2. Model Linking Reaction between Chain-End-PA-
Functionalized PS and Living PMMA or PtBMA in THF
5
PBnMA
20
2
90
50
70
60
living
polymer
temp
(°C)
[PΘ]/[PA]
time
(h)
reaction efficiency
(mol/mol)
(%)
P(Si-HEMA)
20
2
PMMA
−78
−40
−40
−20
−78
−40
−25
3.0
3.0
2.0
2.2
2.6
2.8
3.0
20
10
10
10
20
10
10
35
100
60
survived after 2 and 5 h, respectively. The chain-end anion was
completely deactivated at −20 °C after 2 h. In contrast, living
PtBMA appeared to be more stable.67 For instance, it was
completely stable at −78 °C even after 20 h. Of the original
anion, 88% remained active at −40 °C after 5 h. However,
living PtBMA was not stable at −25 °C (only 20% active after 2
h) and completely deactivated after 5 h.
65
PtBMA
48
70
100
a
Equivalent of living polymer for PA function.
It was also observed that chain-end anions of living P(Si-
HEMA) and PBnMA survived 70% and 90% at −78 °C after 20
h. As expected, their activities decreased 60% and 50%,
respectively, at −40 °C after 2 h. Thus, they are less stable in
activity than living PtBMA, but similar to living PMMA.
Although the stability of living polymers obtained from other
methacrylate monomers used in this study have not been
examined yet, we estimate from the results on the stability of
living P(Si-HEMA) and PBnMA that their living polymers may
be similar in stability to living PMMA because their ester
moieties are primary types like the case of MMA.
Model Linking Reaction of Either Living PMMA or
PtBMA with Chain-End-PA-Functionalized PS. As already
reported in our previous papers,38,68−72 the PA function
introduced at the polymer chain-end was quantitatively reacted
with highly reactive living polymers of styrene and 2VP to link
the polymer chains to each other at −78 °C within a few hours.
A 1.5-fold excess of each living polymer for the PA function was
enough to complete the reaction. As shown in Scheme 2, this
reaction is not a coupling reaction, but a 1:1 addition reaction
and no further addition of the PA function occurred at −78 °C
under the conditions using excess amounts of living polymers.
A new enolate anion was produced from the PA function after
the linking reaction but could not be used in the further linking
reaction or polymerization due to its low reactivity and stability.
Unlike the reaction of the PA function with living PS or
P2VP, living PMMA reacted very sluggishly with the PA
function at −78 °C and was far from complete even after 20 h.
The reaction proceeded quantitatively at −40 °C for 20 h by
using a 3-fold excess of living PMMA for the PA function. It
complete at −78 °C. The reaction efficiency was only 35% after
20 h using a 3-fold excess of living PMMA. It gradually
increased for longer reaction times but was far from complete
(∼50%) even after 72 h. A quantitative reaction was achieved
by raising the reaction temperature to −40 °C and using a 3-
fold of living PMMA for 10 h. The reaction efficiency decreased
to 60% by decreasing the amount of living PMMA to a 2-fold
excess. Because of the instability of living PMMA at −40 °C
listed in Table 1 and the sluggish reaction rate, a 3-fold of living
PMMA may be required to complete the reaction. The reaction
efficiency at −20 °C was observed to be 65% using a 2.2-fold
excess of living PMMA. This result was surprising, taking into
consideration that the living PMMA was completely
deactivated at −20 °C within 2 h (see Table 1).
Next, the linking reaction of living PtBMA (Mn = ca. 5.00−
10.0 kg/mol) with chain-end-PA-functionalized PS (Mn = 10.7
g/mol) was also performed under various conditions, and those
results are also summarized in Table 2. Again, the reaction was
sluggish at −78 °C and incomplete even after 20 h. As
expected, the reaction efficiency was increased by raising the
reaction temperature to −40 °C. However, the efficiencies
obtained by several repeated reactions were around 70% with
the use of a 2.8-fold excess of living PtBMA. The reaction was
observed to quantitatively proceed at −25 °C for 10 h with a 3-
fold excess of living PtBMA. The reaction seemed to be
complete within 1 h, as estimated from the stability at −25 °C
listed in Table 1. Based on these results, reaction conditions at
−40 and −25 °C were employed for living PMMA and PtBMA,
respectively. In each case, a 3-fold excess of living polymer was
used and the reaction time was set at 20 h for convenience of
Scheme 2. 1:1 Addition Reaction of Living Anionic Polymer with Chain-End-PA-Functionalized Polymer
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experimental operation. In the linking reaction of the PA
terminus with a living polymer of another methacrylate
monomer, the former condition at −40 °C for 20 h was first
used. If the reaction was not complete under such a condition,
the latter condition at −25 °C for 20 h necessary for
completion with the use of living PtBMA was employed. All
of the linking reactions carried out in this study were actually
observed to be complete at −40 °C for 20 h using a 3-fold
excess of living polymer for the PA function.
chain-end-(methacrylate)-functionalized PS at −40 °C for 20 h
(77% efficiency). Unfortunately, a further addition reaction of
the methacrylate terminus with the enolate anion produced
after the reaction occurred to some extent. The efficiency was
49% in the linking reaction between a chain-end-(4-vinyl-
benzoate)-functionalized PS and a 2.2-fold excess of living
PMMA. It was not improved by using a 3-fold excess of living
PMMA. No reaction occurred between living PMMA or with
either of the α, β-disubstituted acrylate termini at −40 °C,
possibly due to steric hindrance.
A methyl maleate terminus quantitatively reacted with living
PMMA (a 2.2-fold excess) at −40 °C for 20 h. On the other
hand, the linking reaction of the chain-end-(methyl maleate)-
functionalized PS with either the living PS end-capped with
DPE or living P2VP was always stopped at around 20−30%
yield at −78 °C for 10 h. This is probably due to proton
abstraction from the maleate vinylene group by living polymer
anions. Among the reaction sites examined, the PA function
was eventually the best reaction site and, therefore, used in the
linking reaction to synthesize star-branched polymers through-
out this study.
In order to explore other agents effective in the linking
reaction, the following chain-end-functionalized (PS)s (Mn =
10.7 kg/mol), as shown in Figure 2, were newly prepared and
Figure 2. Chain-end-functionalized (PS)s with various reaction sites.
General Utility of 2 in μ-Star Polymer Synthesis. 3-Arm
ABC μ-Star Polymers. We previously used benzyl bromide
function as a reaction site to connect two polymer chains.9−13
In fact, the benzyl bromide reaction site was found to
quantitatively react with a variety of living anionic polymers
ranging from highly reactive living PS to less reactive living
PMMA. However, one of the major problem was the
transformation stage from precursory benzyl alcohol to benzyl
bromide function, which required strong acidic conditions. It
was actually observed that PtBMA, PBnMA, and most
poly(alkyl methacrylate)s having protective functionalities
were not stable and partly or even completely decomposed
under such acidic conditions. In order to avoid this problem, we
have developed PA function and used it as a new reaction site
instead of benzyl bromide function in recent papers.38,68−72 As
reported previously, the transformation to PA function was
carried out under slightly basic conditions, in which the above
used in the linking reaction with living PMMA. The results are
summarized in Table 3. Living PMMA efficiently reacted with a
Table 3. Model Linking Reaction between Living PMMA
and Chain-End-Functionalized PS with Various Reaction
Sites in THF at −40 °C for 20 h
a
chain-end-functionalized PS reaction site
reaction efficiency (%)
methacrylate
77
49
0
4-vinylbenzoate
α-methylcinnamate
α-phenylcinnamate
methyl maleate
0
100
a
[Living PMMA]/[reaction site] = 2.2−3.0.
Scheme 3. Synthesis of 3-Arm AA′A″ Star PMMA
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polymers were stable. Therefore, the PA reaction site was used
throughout this study. In order to synthesize a variety of well-
defined μ-star polymers, we have developed a specially designed
DPE derivative substituted with trimethylsilyl (TMS)- and tert-
butyldimethylsilyl (TBDMS)-protected hydroxyl functional-
ities, 2, as a new dual-functionalized core agent. This DPE
derivative can progressively offer two reaction sites via two
different silyl-protected hydroxyl functionalities and, therefore,
can be used for the synthesis of both 3-arm ABC and 4-arm
ABCD μ-star polymers. Moreover, 2 is found to be effectively
used for the successive synthesis of a series of μ-star polymers
by a new iterative methodology, which will be introduced in the
next section.
To examine the general utility of 2 as a dual-functionalized
core agent in the synthesis of 3-arm μ-star polymers, a 3-arm
AA′A″ star-branched PMMA was first synthesized, as illustrated
in Scheme 3. The synthesis was initiated to prepare an α-
terminal-functionalized PMMA with TMS- and TBDMS-
protected hydroxyl functionalities. This polymer was prepared
by the living polymerization of MMA in THF at −78 °C for 20
min with the functional DPE anion prepared from 2 and
oligo(α-methylstyryl)lithium. In order avoid the attack on the
TMS-protected functionality of 2 by sec-BuLi, a less reactive
and bulkier oligo(α-methylstyryl)lithium was used. A 3-fold
excess of LiCl was added prior to the polymerization to narrow
the molecular weight distribution of the resulting PMMA. It is
common known that TMS-protected functionality is depro-
tected under milder conditions than TBDMS one. In practice,
the TMS-protected functionality was readily and completely
deprotected to generate a hydroxyl group by pouring the
polymer into methanol containing a few drops of acetic acid,
while the TBDMS-protected functionality was stable under
such conditions. The hydroxyl group was converted to a PA
function by the Mitsunobu esterification reaction with α-
phenylacrylic acid. The conversion was confirmed to be
Figure 3. SEC profiles of the polymer mixture obtained by the linking
reaction (a) and the linked polymer after fractional precipitation (b).
PA function in the same manner mentioned above. Then, a 3-
fold excess of living PMMA, separately prepared, was reacted
with the resulting in-chain-PA-functionalized PMMA. The SEC
profile of the reaction mixture exhibited only two peaks for the
linked product and the deactivated living PMMA used in
excess, and the reaction efficiency was ca. 100% based on the
two peak areas (see Figure 4a). Moreover, the peak for the in-
chain-PA-functionalized PMMA was not present, indicating
also the complete linking reaction. The linked polymer isolated
in 80% yield by fractional precipitation possessed an Mn value
(36.8 kg/mol), measured by RALLS, in agreement with that
predicted (34.8 kg/mol) and a narrow molecular weight
distribution (Mw/Mn = 1.03) (see Figure 4b). These results
clearly indicate the successful synthesis of the target 3-arm
AA′A″ star PMMA with well-defined structures. Thus, 2
satisfactorily worked as the dual-functionalized core agent in
the synthesis of 3-arm star-branched PMMA.
This method of using 2 was next applied to the synthesis of
3-arm ABC μ-stars. The same aforementioned PMMA α-
terminal-functionalized with two different silyl-protected
hydroxyl functionalities was used as the starting polymer. The
first reaction sequence (selective deprotection of the TMS-
protected functionality, conversion to the PA function used as
the reaction site, and the linking reaction with a 3-fold excess of
living poly(ethyl methacrylate) (PEMA)), followed by the
second reaction sequence (deprotection of the TBDMS-
protected functionality, conversion to the PA function, and
the linking reaction with a 3-fold excess of living poly(tert-butyl
methacrylate) (PtBMA)), was carried out under identical
conditions except for the linking reaction between living
PtBMA and in-chain-PA-functionalized PMMA-b-PEMA, which
was performed at −25 °C for 20 h. As expected, both reaction
sequences proceeded cleanly and quantitatively. The final
polymer isolated by SEC fractionation was observed to have a
sharp monomodal distribution (Mw/Mn = 1.07). The Mn values
1
virtually quantitative by the H NMR spectrum, where new
signals corresponding to vinylene protons of the PA function
appeared at 6.31 and 5.87 ppm with the expected integral
ratios. Moreover, the benzyl protons were completely shifted
from 4.62 to 5.19 ppm, also strongly indicating a quantitative
conversion. The resulting chain-end-(PA and TBDMS-
protected hydroxyl)-functionalized PMMA was reacted with a
3-fold excess of living PMMA, separately prepared, in THF at
−40 °C for 20 h.
The reaction was terminated with degassed methanol, and
the reaction mixture was precipitated in hexane. The SEC
profile exhibits only two peaks for the linked product and the
deactivated living PMMA used in excess in the reaction, as
shown in Figure 3a. The linking efficiency was estimated to be
almost quantitative by comparing these two peak areas. The
linked polymer was isolated in 80% yield by fractional
precipitation, reprecipitated twice, and freeze-dried from its
absolute benzene solution. The isolated polymer possessed a
sharp monomodal SEC distribution (see Figure 3b). The Mn
value (26.2 kg/mol) observed by SEC-RALLS was in good
agreement with that calculated (Mn = 25.3 kg/mol). Neither
oligomerization nor even dimerization of the PA function with
the produced enolate anion occurred under the conditions
employed.
The TBDMS-protected functionality which remained
between two PMMA segments was deprotected by treatment
with a 30-fold excess of (C4H9)4NF in THF at −10 °C for 10 h.
The hydroxyl group thus generated was also converted to the
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functional DPE anion prepared from 2 and oligo(α-
methylstyryl)lithium in the presence of a 3-fold excess of
LiCl in THF at −78 °C for 5 h. The TMS-protected
functionality was selectively deprotected in methanol contain-
ing a few drops of acetic acid, and the generated hydroxyl group
was converted to the PA function by the Mitsunobu
esterification reaction. Then, a 3-fold excess of living
P(acetal-DIMA) was reacted with the α-terminal-(PA and
TBDMS-protected hydroxyl)-functionalized PtBMA in THF at
−40 °C for 20 h. The resulting polymer was treated with a 30-
fold excess of (C4H9)4NF in THF at −10 °C for 20 h. With this
treatment, the TBDMS-protected functionality was deprotected
to generate the hydroxyl group, while the acetal-protected
functionality of P(acetal-DIMA) was stable and remained
completely intact under the conditions. The hydroxyl group
was converted to the PA function. Finally, a 3-fold excess of
living P(Si-HEMA) was reacted with the in-chain-PA-function-
alized PtBMA-b-P(acetal-DIMA) at −40 °C for 20 h.
The resulting polymer is of special interest in terms of
functionality because it can be converted to a new functional 3-
arm ABC μ-star polymer composed of poly(2-hydroxyethyl
methacrylate) (PHEMA), poly(2,3-dihydroxypropyl methacry-
late) (PDIMA), and poly(methacrylic acid) segments by
progressively treating it with (C4H9)4NF, 2 N HCl, followed
by (CH3)3SiCl/LiBr. It should herein be mentioned that both
of the above two polymers are the first successful well-defined
3-arm ABC μ-star polymers composed of three different
methacrylate-based polymer segments.
A 3-arm ABC μ-star polymer consisting of PMMA, PS, and
P2VP segments could also be synthesized by the method of
using 2. The synthesis started from the same PMMA α-
terminal-functionalized with two different silyl-protected
hydroxyl functionalities as that used above. A 1.5-fold excess
of living PS end-capped with DPE and a 2-fold excess of living
P2VP were reacted in the first and second linking reaction
steps. Because of the higher nucleophilicity of both living
polymers, the reactions were carried out in THF at −78 °C in
order to avoid undesirable side reactions, such as ester and C
N bond attack by their chain-end anions. The target 3-arm
ABC μ-star-branched polymer composed of PMMA, PS, and
P2VP segments was successfully synthesized without difficulty.
This successful synthesis clearly demonstrates that the PA
function is a very convenient reaction site, capable of reacting
with all of three living polymers having quite different chain-
end reactivity (or nucleophilicity).
Figure 4. SEC profiles of the polymer mixture obtained by the linking
reaction (a) and 3-arm AA′A″ star PMMA after fractional precipitation
(b).
1
observed by RALLS and H NMR agreed with that calculated,
as listed in Table 4. Furthermore, agreement between the
1
compositions observed by H NMR and that calculated was
satisfactory. Thus, a new 3-arm ABC μ-star polymer composed
of PMMA, PEMA, and PtBMA was successfully synthesized.
It was also possible to synthesize another 3-arm ABC μ-star
polymer consisting of PtBMA, P(Si-HEMA), and P(acetal-
DIMA) segments by carrying out similar reaction sequences.
The synthesis started from PtBMA α-terminal-functionalized
with TMS- and TBDMS-protected hydroxyl functionalities,
Like DPE, the DPE derivative, 2, reacted quantitatively with
living PS (poly(styryllithium) (PSLi)) in a 1:1 addition manner
t
which was prepared by the polymerization of BMA with the
Table 4. Characterization Results of 3-Arm ABC μ-Star Polymers
a
polymer segments
B
Mn (kg/mol)
Mw/Mn
composition (wt %)
b
c
A
C
calcd
RALLS
1H NMR
SEC
calcd
1H NMR
PMMA
PMMA
PtBMA
PMMA
PS
PMMA
PEMA
PDIMA
PS
PMMA
PtBMA
PHEMA
P2VP
34.8
30.8
48.5
34.9
33.6
31.0
31.5
30.5
30.1
36.8
32.4
51.3
41.3
35.2
31.2
31.7
32.5
1.03
1.07
1.04
1.05
1.04
1.03
1.04
1.04
1.04
28.4
49.4
35.6
33.8
31.7
31.1
28.1
30.1
38/30/32
49/17/34
24/35/41
37/34/29
34/36/30
34/37/29
41/28/31
42/28/30
39/28/33
48/15/37
22/34/44
37/37/26
32/37/31
33/37/30
41/29/30
43/30/27
PMMA
PMMA
PMMA
P2VP
PEMA
PtBMA
PDIMA
PtBMA
PMOS
PS
PS
PS
PS
P2VP
30.0
a
b
c
A/B/C polymer segments. Determined by SEC with triple detectors. Determined by SEC using PS standards.
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Scheme 4. Synthesis of 4-Arm ABCD μ-Star Polymer
in THF at −78 °C to introduce two different silyl-protected
hydroxyl functionalities at the ω-chain-end. In this reaction,
PSLi was end-capped with a few units of α-methylstyrene in
order to avoid the attack on the TMS-protected functionality of
2 by PSLi, similar to the case using sec-BuLi. The TMS-
protected functionality was deprotected by pouring the
polymer into methanol containing a few drops of acetic acid,
and the generated hydroxyl group was converted to the PA
function. The resulting ω-chain-end-(PA and TBDMS-
protected hydroxyl)-functionalized PS was reacted with a 3-
fold excess of living PMMA at −40 °C for 20 h to afford an in-
chain-(TBDMS-protected hydroxyl)-functionalized PS-b-
PMMA. Subsequently, the TBDMS-protected hydroxyl func-
tionality was deprotected with (C4H9)4NF at −10 °C, and the
hydroxyl group generated was converted to the PA function.
The in-chain-PA-functionalized PS-b-PMMA thus prepared was
reacted with either of a 3-fold excess of living PEMA, PtBMA,
or P(acetal-DIMA) at −40 °C for 20 h, resulting in three ABC
μ-star polymers, composed of PS, PMMA, and one of the above
methacrylate-based polymer segments. Their well-defined and
anionic polymerization system becomes a versatile synthetic
procedure, with which a variety of well-defined 3-arm ABC μ-
star polymers can be synthesized. Most of these polymers
herein synthesized, composed of all different methacrylate-
based polymers or PS, P2VP, and methacrylate-based polymers,
are new μ-star polymers which are difficult to synthesize by any
other method.
4-Arm ABCD μ-Star Polymers. As mentioned in the
Introduction, only a few successful examples of well-defined
4-arm ABCD μ-stars, synthesized by living anionic polymer-
ization, have so far been reported.43−46 With the above method
of using 2, the synthesis of 4-arm ABCD μ-star polymers is also
possible, as illustrated in Scheme 4. In this synthesis, the first
step involves the reaction of PSLi end-capped with a few units
of α-methylstyrene with 2, followed by the polymerization of
MMA, to prepare PS-b-PMMA in-chain-functionalized with
TMS- and TBDMS-protected hydroxyl functionalities. Similar
to the above method, the first reaction sequence (selective
deprotection of the TMS-protected functionality, conversion to
the PA function, and the linking reaction with a 3-fold excess of
living PtBMA at −25 °C for 20 h) was carried out to result in a
3-arm ABC μ-star polymer core-functionalized with a TBDMS-
protected functionality. Then, the second reaction sequence
(deprotection of the TBDMS-protected functionality, con-
version of the hydroxyl group to the PA reaction site, and the
linking reaction with a 3-fold excess of living poly-
(ferrocenylmethyl methacrylate) (PFMMA)) yielded a target
4-arm ABCD μ-star polymer. As can be seen in Figure 5a, the
SEC profile exhibits only two peaks corresponding to the linked
product and the deactivated living PFMMA used in excess in
the reaction, and the peak corresponding to the 3-arm ABC star
disappears. This clearly indicates that the linking reaction was
quantitative under such conditions. The linked polymer was
isolated by SEC fractionation (see Figure 5b) and characterized
by SEC, RALLS, and 1H NMR analyses. The results are
summarized in Table 5.
1
expected structures were confirmed by SEC, RALLS, and H
NMR analyses, as listed in Table 4. Similarly, two more 3-arm
ABC μ-stars composed of PS, P2VP, PtBMA, PS, and P2VP,
poly(4-methoxystyrene) (PMOS) could be synthesized by the
linking reaction of in-chain-PA-functionalized PS-b-P2VP with
a 3-fold excess of living PtBMA at −25 °C for 20 h and with a
1.5-fold excess of living PMOS at −78 °C for 10 h (see also
Table 4).
In the synthesis of 3-arm ABC μ-star polymers by the
method using 2, the addition order of living anionic polymer to
be reacted in the reaction sequence is not limited and any order
is acceptable. The only exception is the use of living P(Si-
HEMA), since selective deprotection of the TBDMS-protected
functionality of 2 is not possible in the presence of P(Si-
HEMA) segment having the same TBDMS-protected function-
alities in the side chain. Therefore, this living polymer must be
used in the final linking reaction step. Thus, the general utility
of 2 as a dual-functionalized core agent is obvious, and the
synthetic method of using 2 in conjunction with the living
It was observed that the isolated polymer possessed a sharp
monomodal SEC distribution (Mw/Mn = 1.03). The molecular
1
weights determined by RALLS and H NMR (Mn,RALLS = 42.7
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system of the iterative methodology is designed in such a way
that the same reaction site is always regenerated after the
introduction of an arm segment in each reaction sequence and
this reaction sequence consisting of “arm introduction and
regeneration of the same reaction site” is repeatable.
Accordingly, the arm segments can be successively and, in
principle, limitlessly introduced by repeating the reaction
sequence.
The proposed new iterative methodology is illustrated in
Scheme 5. The synthesis was initiated from the same α-terminal
PMMA functionalized with TMS- and TBDMS-protected
hydroxyl functionalities as that often used in the synthesis of
3-arm μ-star polymers. The first reaction sequence involves the
selective deprotection of the TMS-protected functionality,
conversion of the generated hydroxyl group to the PA function,
and the linking reaction with a 3-fold excess of living PEMA. An
in-chain-(TBDMS-protected hydroxyl)-functionalized PMMA-
block-PEMA was obtained at this stage. Then, the TBDMS-
protected functionality was deprotected with (C4H9)4NF to
generate the hydroxyl group, followed by conversion to the PA
function.
The PA function thus regenerated in the PMMA-b-PEMA
was reacted with a 2-fold excess of the functional DPE anion
prepared from 2 and oligo(α-methylstyryllithium) at −78 °C
for 30 min, used as the initiator in the polymerization of MMA.
1
The reaction was confirmed to be virtually quantitative by H
NMR analysis. These reaction steps are also involved in the first
reaction sequence (or the first iterative process).
Figure 5. SEC profiles of the polymer mixture obtained by the linking
reaction (a) and 4-arm ABCD μ-star polymer after SEC fractionation
(b).
A key point of the first sequence was the reaction between
the PA reaction site regenerated via the TBDMS-protected
functionality and the functional DPE anion to introduce the
TMS- and TBDMS-protected hydroxyl functionalities. Accord-
ingly, the resulting PMMA-b-PEMA is the same in chain-
functionality as the starting PMMA, except for the PEMA
segment. By using two different silyl-protected functionalities,
the same reaction sequence (selective deprotection of the
TMS-protected hydroxyl functionality, conversion of the
hydroxyl group to the PA function, the linking reaction of a
3-fold excess of living PtBMA at −25 °C for 20 h, deprotection
of the TBDMS-protected functionality, conversion of the
hydroxyl group to the PA function, and the reaction of the PA
reaction site with a 2.0-fold excess of the functional DPE anion)
could be repeated. As a result, a 3-arm ABC μ-star polymer
core-functionalized with TMS- and TBDMS-protected hydrox-
yl functionalities, composed of PMMA, PEMA, and PtBMA
segments, was synthesized. Once again, the final reaction step
between the PA reaction site and the functional DPE anion is
the key to further continuing the same reaction sequence. In
practice, a 4-arm ABCD μ-star polymer, followed by a 5-arm
ABCDE μ-star polymer, was successively synthesized by
repeating the same reaction sequence two more times. Living
poly(benzyl methacrylate) (PBnMA) and poly(2-methoxyethyl
methacrylate) (PMOEMA) were sequentially reacted to
introduce the D and E segments, respectively.
and Mn,NMR = 41.1 kg/mol) were in good agreement with that
calculated (Mn = 41.4 kg/mol). Moreover, the composition
observed by 1H NMR was consistent with that calculated from
the feed ratio. Thus, a new 4-arm ABCD μ-star polymer
composed of PS and three different methacrylate-based
polymer segments (PMMA, PtBMA, and PFMMA) was
successfully synthesized. One more 4-arm ABCD μ-star could
also be synthesized by the linking reaction of the above core-
PA-functionalized 3-arm ABC μ-star with a 2-fold excess of
living P2VP at −78 °C for 10 h. By analogy with the synthesis
of 3-arm ABC μ-stars, 2 also plays a key role as a dual-
functionalized core agent in the synthesis of 4-arm ABCD μ-
star polymers.
Successive Synthesis of μ-Star Polymers with up to 5
Arms by a New Iterative Methodology. In the preceding
section, we demonstrated the general utility of 2 in the
synthesis of both 3-arm ABC and 4-arm ABCD μ-star
polymers. However, μ-star polymers having more arms cannot
be synthesized by the above method. The reason is that the PA
reaction site disappears after the final linking reaction, and the
arm segment can no longer be introduced into the star. In order
to further synthesize μ-star polymers having five or more arms,
we have herein developed a new iterative methodology,
conceptually similar to those previously reported by our
group.11−13,47−49 As described in the Introduction, the reaction
Table 5. Characterization Results of 4-Arm ABCD μ-Star Polymers
a
polymer segments
A/B/C
Mn (kg/mol)
Mw/Mn
composition (wt %)
b
c
D
calcd
RALLS
1H NMR
SEC
calcd
1H NMR
PS/PMMA/PtBMA
PS/PMMA/PtBMA
PFMMA
41.4
41.7
42.7
42.9
41.1
39.9
1.03
1.04
26/28/23/23
25/26/23/26
27/30/21/22
26/27/23/24
P2VP
a
b
c
A/B/C/D polymer segments. Determined by SEC with triple detectors. Determined by SEC using PS standards.
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Scheme 5. Successive Synthesis of μ-Star Polymers by a New Iterative Methodology
Table 6. Synthesis of μ-Star Polymers by a New Iterative Methodology
a
Mn (kg/mol)
Mw/Mn
composition (wt %)
b
c
μ-star
calcd
RALLS
1H NMR
SEC
calcd
38/31/31
1H NMR
d
ABC
32.2
43.4
52.0
39.3
51.7
33.8
43.0
51.5
41.2
52.7
32.3
43.2
51.6
38.4
50.2
1.03
1.03
1.03
1.03
1.04
37/32/31
d
ABCD
30/25/22/23
30/26/20/24
d
e
ABCDE
24/21/18/19/18
28/30/22/20
26/20/16/20/18
26/29/23/22
e
ABCD
ABCDE
21/23/17/17/22
21/24/16/17/22
a
b
c
d
A/B/C/D/E polymer segments. Determined by SEC with triple detectors. Determined by SEC using PS standards. A/B/C/D/E = PMMA/
e
PEMA/PtBMA/PBnMA/PMOEMA. A/B/C/D/E = PS/PMMA/PtBMA/P4MS/PBnMA.
The characterization results are summarized in Table 6. In
the 3-arm ABC, 4-arm ABCD, and 5-arm ABCDE μ-star
polymers, their molecular weights determined by RALLS and
1H NMR are in good agreement with those calculated from
monomer to initiator ratios and narrow molecular weight
distributions are attained. Furthermore, their observed
compositions are consistent with those calculated. These
results clearly indicate the successful synthesis of target μ-star
polymers with well-defined and expected structures. For
comparison, all SEC peaks for A, AB, 3-arm ABC, 4-arm
ABCD, and 5-arm ABCDE μ-star polymers are shown in Figure
6. It should be mentioned that the resulting polymers are quite
new and the first successfully synthesized μ-star polymers
composed of all different methacrylate-based arm segments.
Since the final 5-arm μ-star still possesses the TBDMS-
protected functionality which is convertible to the PA reaction
site at the core, it may be possible to continue the same
reaction sequence.
regenerated via the TBDMS-protected functionality and the
functional DPE anion in order to introduce both the TMS- and
the TBDMS-protected hydroxyl functionalities. Interestingly
and importantly, another route exists for the introduction of
two such silyl-protected functionalities. As illustrated in Scheme
6, living anionic poly(4-methylstyrene) (P4MS), like living PS
mentioned in the preceding section, readily reacts with 2 in a
1:1 addition manner to introduce two silyl-protected hydroxyl
functionalities at the chain-end, and the chain-end anion is
changed to the DPE anion, which is exactly the same in
structure as the functional DPE anion. Accordingly, this chain-
end-functionalized living P4MS with 2 is expected to react with
a chain-PA-functionalized polymer to introduce two silyl-
protected hydroxyl functionalities. More importantly, the
iterative methodology can be started from the resulting
polymer having two silyl-protected functionalities.
A typical example is shown in Scheme 7, in which a 4-arm
ABCD, followed by a 5-arm ABCDE μ-star polymer, can be
synthesized by the method using a chain-end-functionalized
living P4MS with two silyl-protected functionalities and the
As often mentioned above, the key step of the iterative
methodology is the reaction between the PA reaction site
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at the chain-end. The resulting chain-end-functionalized
P4MSLi with 2 was in situ reacted with the core-PA-
functionalized 3-arm ABC μ-star at −78 °C for 10 h to link
the P4MS chain to the star, resulting in a 4-arm ABCD μ-star
polymer core-functionalized with two silyl-protected hydroxyl
functionalities. The TMS-protected functionality was selectively
deprotected by pouring the polymer into methanol containing a
few drops of acetic acid, and the generated hydroxyl group was
converted to the PA function. Then, a 3-fold excess of PBnMA
was reacted with the resulting core-(PA and TBDMS-protected
hydroxyl)-functionalized 4-arm ABCD μ-star at −40 °C for 20
h. Under such conditions, the reaction was complete and
yielded a 5-arm ABCDE μ-star composed of PS, PMMA,
PtBMA, P4MS, and PBnMA segments. The expected and well-
defined structure of the polymer was confirmed by the
analytical results listed in Table 6. Thus, the chain-end-
functionalized P4MSLi with 2 could also be utilized in the
methodology. The use of this functionalized living polymer is
advantageous to introduce one arm segment in addition to the
two silyl-protected functionalities, and therefore, one linking
reaction step can be skipped.
Figure 6. SEC profiles of A, AB, 3-arm ABC, 4-arm ABCD, and 5-arm
ABCDE μ-star polymers synthesized by a new iterative methodology.
In summary, the proposed iterative methodology proves
effective to successively synthesize a series of μ-star polymers
with many arms and can continue via the two different silyl-
protected functionalities introduced by reacting of the
regenerated PA reaction site with either the DPE anion
prepared from 2 or the living P4MS (and possibly living
polymers of styrene and its derivatives) end-functionalized with
2. One more important issue to be emphasized in this study is
to use the PA function as the reaction site, which is capable of
reacting with a variety of living anionic polymers from living PS,
P2VP, to PMMA or other poly(alkyl methacrylate)s with
different chain-end nucleophilicities. Accordingly, the iterative
methodology allows access to a wide variety of well-defined μ-
star polymers composed of not only methacrylate-based
polymers but also PS and P2VP segments.
Scheme 6. 1:1 Addition Reaction of Living P4MS with 2
core-PA-functionalized ABC μ-star polymer composed of PS,
PMMA, and PtBMA synthesized in the preceding section (see
Scheme 4). Poly(4-methylstyryl)lithium (P4MSLi) was first
prepared by the living anionic polymerization of 4-methylstyr-
ene with sec-BuLi in THF at −78 °C for 1 h, and a 2.0-fold
excess of 2 was subsequently reacted in situ to introduce both
the TMS- and the TBDMS-protected hydroxyl functionalities
Scheme 7. Successive Synthesis of 4-Arm ABCD and 5-Arm ABCDE μ-Star Polymers by an Iterative Methodology Using Living
P4MS Chain-End Functionalized with 2
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CONCLUSIONS
■
We have demonstrated the general utility of the DPE derivative
substituted with TMS- and TBDMS-protected hydroxyl
functionalities, 2, as a new dual-functionalized core agent in
conjunction with the living anionic polymerization system for
the synthesis of various well-defined 3-arm ABC and 4-arm
ABCD μ-star polymers. Two different protected hydroxyl
functionalities were progressively deprotected to generate
hydroxyl groups, followed by conversion to the PA functions
at separate stages, and the PA functions were reacted with
appropriate living anionic polymers to result in the above μ-
stars.
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A new iterative methodology using the functional DPE anion
prepared from 2 and oligo(α-methylstyryl)lithium has been
developed in order to synthesize a series of μ-stars with up to
five. The key step of the methodology is the reaction between
the PA reaction site regenerated from the TBDMS-protected
functionality and the functional DPE anion to introduce both
the TMS- and the TBDMS-protected hydroxyl functionalities.
With the use of two such different silyl-protected hydroxyl
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ASSOCIATED CONTENT
■
S
* Supporting Information
(22) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Nano Lett. 2006, 6, 1245−
1249.
Text giving the syntheses of chain-end-PA-functionalized PS
and other chain-end-functionalized (PS)s, the synthesis of 3-
arm ABC μ-star polymers, and the successive synthesis of μ-star
polymers by using living P4MS chain-end-functionalized with 2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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765−771.
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AUTHOR INFORMATION
(26) Junnila, S.; Houbenov, N.; Hanski, S.; Iatrou, H.; Hirao, A.;
Hadjichristidis, N.; Ikkala, O. Macromolecules 2010, 43, 9071−9076.
(27) Moughton, A. O.; Hillmyer, M. A.; Lodge, T. P. Macromolecules
2012, 45, 2−19.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
A.H. gratefully acknowledges partial support by a grant (B:
21350060) from a Grant-in-aid for Scientific Research from the
Ministry of Education, Science, Sports, and Culture of Japan.
A.H. is also thankful for financial support from Denki Chemical
Co. Ltd. in Japan.
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be completed at −78 °C. It was already observed that the living
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was carried out to check the stability of living PtBMA at −40 °C for 2
t
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dx.doi.org/10.1021/ma300699m | Macromolecules 2012, 45, 4997−5011