Successive synthesis of well-defined star-branched polymers by a new iterative approach involving coupling and transformation reactions

Article abstract of DOI:10.1021/ma050248e

Successive synthesis of well-defined star-branched polymers has been successfully achieved by a new iterative methodology based on living anionic polymerization. The methodology involves only two sets of reaction conditions for the entire iterative reacti

Full text of DOI:10.1021/ma050248e

Macromolecules 2005, 38, 4577-4587
4577
Successive Synthesis of Well-Defined Star-Branched Polymers by a New
Iterative Approach Involving Coupling and Transformation Reactions
Tomoya Higashihara, Masato Nagura, Kyoichi Inoue, Naoki Haraguchi, and
Akira Hirao*
Polymeric and Organic Materials Department, Graduate School of Science and Engineering, Tokyo
Institute of Technology, H-127, 2-12-1, Ohokayama, Meguro-ku, Tokyo, 152-8552, Japan
Received February 3, 2005; Revised Manuscript Received March 23, 2005
ABSTRACT: Successive synthesis of well-defined star-branched polymers has been successfully achieved
by a new iterative methodology based on living anionic polymerization. The methodology involves only
two sets of reaction conditions for the entire iterative reaction sequence: (a) a coupling reaction of
the benzyl bromide-functionalized polymer with poly(substituted styryl)lithium end-capped with
1-(3-tert-butyldimethylsilyloxymethylphenyl)-1-phenylethylene to link two polymer chains and introduce
3-tert-butyldimethylsilyoxymethylphenyl group(s) of benzyl bromide precursor(s) and (b) a transformation
reaction of the introduced precursor(s) into benzyl bromide functionality (functionalities) by treatment
with (CH3)3SiCl-LiBr. By repeating these two reactions, an array of asymmetric star-branched polymers
were successively synthesized. They involved 3-arm ABC, 4-arm ABCD and A2B2, and 6-arm A2B2C2
stars whose A, B, C, and D segments are polystyrene, poly(R-methylstyrene), poly(4-methylstyrene), poly-
(methyl methacrylate), or poly(tert-butyl methacrylate), respectively. Their high degrees of compositional,
molecular weight, and architectural homogeneity as well as narrow molecular weight distributions
(Mw/Mn < 1.05) were confirmed by the analytical results of SEC, 1H NMR, VPO, and SLS. The poly(tert-
butyl methacrylate) segments of the A2B2C2 star-branched polymer were readily and quantitatively
hydrolyzed under the acidic conditions. As a result, a new ionic A2B2C2 star having poly(methacrylic
acid) segments was obtained.
Introduction
1,1-diphenylalkyl anions with 1-(4-(4-bromobutyl)phe-
nyl)-1-phenylethylene to introduce DPE functionalities.
A typical synthetic outline is illustrated in Scheme 1.
The first addition reaction was carried out between two
equiv of polystyryllithium (PSLi) and 1,3-bis(1-phe-
nylethenyl)benzene consisting of a double DPE skeleton
to link two polystyrene chains accompanying with two
1,1-diphenylalkyl anions newly generated at the linking
points. Second, the generated two anions were in situ
reacted with 1-(4-(4-bromobutyl)phenyl)-1-phenylethyl-
ene to introduce two DPE functionalities to afford an
in-chain-functionalized polystyrene with two DPE moi-
eties placed at the middle of the polymer chain. Because
this DPE in-chain-functionalized polystyrene also had
the DPE functionalities, the same process could be
repeated. Actually, a core-functionalized 4-arm star-
branched polystyrene with two DPE moieties, followed
by a 6-arm star-branched polystyrene with two DPE
moieties was successively synthesized by repeating the
iterative process two more times. Fortunately, the two
reactions in each iterative process proceeded virtually
quantitatively to result in the formation of such stars
in almost 100% yield. More importantly, the methodol-
ogy can be readily applied to the synthesis of asym-
metric star-branched polymers by using living anionic
polymers different in composition. In fact, we were
successful in synthesizing asymmetric 4-arm A2B2 and
6-arm A2B2C2 star-branched polymers by using poly-
styryllithium (A), poly(R-methylstyryl)lithium (B), and
polyisoprenyllithium (C), respectively. With 1,1-bis(3-
(1-phenylethenyl)phenyl)ethylene consisting of a triple
DPE skeleton in the same iterative methodology, regu-
lar A3, A6, A9, A12, and even A15 star-branched polysty-
renes as well as asymmetric 6-arm A3B3 and 9-arm
A3B3C3 star-branched polymers could be successively
synthesized in a similar manner.42 The key point of this
Star-branched polymers have been widely investi-
gated from both experimental and theoretical view-
points because their behaviors and properties in solu-
tion, melt, and solid state are quite distinct from those
of the corresponding linear analogues.1-8 Recently,
asymmetric star-branched polymers having arms dif-
ferent in composition have received much attention.
Because of their specific heterophase structures, in
addition to branched architectures, asymmetric star-
branched polymers may possibly induce microdomain
arrangements to form novel and interesting nanoscopic
objects with suprastructures.9-14 However, well-defined
asymmetric star-branched polymers are generally much
more difficult in synthesis than the corresponding
regular stars with the same arms because two or more
quantitative nature of reactions and the isolation of
intermediate polymers during the synthesis are often
required. A number of asymmetric star-branched poly-
mers have been synthesized so far, but the structural
variation of well-defined asymmetric stars is still limited
even at the present time.13,15-40 Therefore, the develop-
ment of new methodology is still an important challenge.
We have recently proposed the successive synthesis
of star-branched polymers by an iterative methodology
as a new concept.41 The methodology involves only two
sets of reaction conditions for the entire iterative
synthetic sequence: an addition reaction of living
anionic polymer to the 1,1-diphenylethylene (DPE)
derivative or the produced DPE-chain-functionalized
polymer and a coupling reaction of the newly generated
* Author
to
whom
correspondence
should
be
addressed. E-mail: ahirao@polymer.titech.ac.jp. Telephone:
+81-3-5734-2131. Fax: +81-3-5734-2887.
10.1021/ma050248e CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/23/2005

4578 Higashihara et al.
Macromolecules, Vol. 38, No. 11, 2005
Scheme 1. Successive Synthesis of In-Chain-Functionalized Polystyrene with Two DPE Moieties and
Core-Functionalized 4- and 6-Arm Star-Branched Polystyrenes with Two DPE Moieties by an Iterative
Methodology Using 1,3-Bis(1-phenylethenyl)Benzene
CaH2 under a nitrogen atmosphere. Both LiCl and LiBr
(99%, Koso Chemical Co., Ltd.) were dried under the vacuum
line at 100 °C overnight. 1,1-Bis(3-tert-butyldimethylsily-
loxymethylphenyl)ethylene was synthesized according to our
procedures previously reported.44
methodology is that reaction sites can be always regen-
erated after each of the iterative processes and hence
the same reaction sequence can be, in principle, limit-
lessly repeated to successively synthesize an array of
star-branched polymers.
Measurements. Size-exclusion chromatography (SEC) was
performed on a TOSOH HLC 8020 instrument with UV
(254 nm) and refractive index detection. THF was used as a
carrier solvent at a flow rate of 1.0 mL/min at 40 °C. Three
polystyrene gel columns (pore size (bead size): 650 Å (9 µm),
200 Å (5 µm), and 75 Å (5 µm)) were used. Measurable
molecular weight ranges are 103-4 × 105. Calibration curves
were made with polystyrene, poly(methyl methacrylate), and
poly(tert-butyl methacrylate) samples synthesized by us for
Herein, we report on an alternative iterative meth-
odology for the successive synthesis of star-branched
polymers. The methodology involves a coupling reaction
of benzyl bromide-functionalized polymer with ω-func-
tionalized living anionic polymer with benzyl bromide
precursor(s) and a transformation reaction of the intro-
duced precursor(s) into benzyl bromide functionality
(functionalities). The two reactions are repeated in each
iterative reaction sequence to successively synthesize
star-branched polymers. The objective of the present
study is to examine the synthetic possibility and poten-
tial of this new alternative methodology, especially for
the synthesis of asymmetric star-branched polymers.
1
determining both Mn and Mw/Mn values. Both H (300 MHz)
and 13C (75 MHz) NMR spectra were measured in CDCl3 using
a BRUKER DPX spectrometer. Vapor pressure osmometry
(VPO) measurements were made with a Corona 117 instru-
ment in benzene at 40 °C with a highly sensitive thermoelectric
couple (TM-32K, sensitivity: 35 000 µV ( 10%/1 M) and with
equipment of very exact temperature control. Molecular weight
can be measured up to 100 kg/mol with an error of less than
10%. The apparatus constant was obtained by measuring five
standard polystyrenes samples and calibrating their values
against Mn values. Static light scattering (SLS) measurements
were performed with an Ohotsuka Electronics DLS-7000
instrument equipped with a He-Ne laser (633 nm) in THF at
25 °C. The refractive index increment (dn/dc) in THF at
25 °C was determined for each star-branched polymer with
an Ohotsuka Electronics DRM-1020 refractometer operating
at 633 nm. Intrinsic viscosities were measured with an
Ubbelohde viscometer in toluene at 35 °C.
Experimental Section
Materials. All chemicals (>98% purities) were purchased
from Aldrich, Japan and used as received unless otherwise
stated. Tetrahydrofuran (THF) (99%, Mitsubishi Chemical Co.,
Ltd.) was refluxed over Na wire for 12 h and distilled over
LiAlH4 under nitrogen. It was finally distilled from its sodium
naphthalenide solution on a high vacuum line (10-6 Torr).
Diphenylmethylpotassium was prepared from diphenylmethane
and potassium naphthalenide in THF according to the method
previously reported.43 Styrene, R-methylstyrene, and 4-methyl-
styrene (99%, Tokyo Kasei Kogyo Co., Ltd. Japan) were
washed with 10% NaOH aq, dried over MgSO4, and distilled
over CaH2 under reduced pressures. They were finally distilled
over Bu2Mg (∼5 mol %) on the vacuum line into ampules
equipped with break seals that were prewashed with potas-
sium naphthalenide in THF. Methyl and tert-butyl methacry-
lates were washed with 5% NaOH aq and H2O, dried over
MgSO4, and distilled over CaH2 under reduced pressures. They
were finally distilled over trioctylaluminum (>99%, Sumitomo
Chemical Co., Ltd.) (∼2 mol %) on the vacuum line into
ampules equipped with break seals that were prewashed with
1,1-diphenylhexyllithium in heptane. R,R′-Dichloro-p-xylene
(>99.5%) was recrystallized in hexane and dried under the
vacuum line overnight. Acetophenone and N,N-dimethyl-
formamide (DMF) were distilled over CaH2 under reduced
pressures. tert-Butyldimethylsilyl chloride (99%, Shinetsu
Chemical Co., Ltd.) was used as received. Chloroform
(99%, Nakarai Tesque Co., Ltd. Japan), acetonitrile
(99%, Nakarai Tesque Co., Ltd. Japan), and trimethylsilyl
chloride (98%, Tokyo Kasei Kogyo Co., Ltd.) were distilled over
1-(3-tert-Butyldimethylsilyloxymethylphenyl)-1-phe-
nylethylene (1). Under a nitrogen atmosphere, acetophenone
(6.60 g, 54.9 mmol) in THF (5.0 mL) was added dropwise at
0 °C to a stirred solution of the Grignard reagent, prepared
from 2-(3-bromophenyl)-1,3-dioxane (10.1 g, 44.1 mmol) and
Mg (1.48 g, 60.9 mmol) in THF (80 mL), and the reaction
mixture was stirred at 25 °C for 12 h. It was then acidified
with 2 N HCl (100 mL), extracted with ether thrice, and dried
over MgSO4. The usual workup afforded crude 1-(3-formylphe-
nyl)-1-phenylethanol as a yellow oil (9.78 g, 43.2 mmol, 98%)
and was followed by dehydration with a catalytic amount of
p-toluenesulfonic acid in benzene (50 mL) at 70 °C for 3 h.
After usual workup, 1-(3-formylphenyl)-1-phenylethylene was
obtained as an yellow oil (6.93 g, 33.3 mmol, 77%). It was
reduced with NaBH4 (2.52 g, 66.6 mmol) in ethanol (150 mL)
at 25 °C for 12 h to afford 1-(3-hydroxymethylphenyl)-
1-phenylethylene (6.73 g, 32.0 mmol, 96%). Under nitrogen,
tert-butyldimethylsilyl chloride (5.31 g, 35.2 mmol) in DMF
(20 mL) was added at 0 °C to a solution of 1-(3-hydroxymeth-

Macromolecules, Vol. 38, No. 11, 2005
Synthesis of Star-branched Polymers 4579
Table 1. Synthesis of BnBr Chain-End-Functionalized and In-Chain-Functionalized Polystyrenes, 3- and 4-Arm
Star-Branched Polystyrenes Using 1 in THF at -78 °Ca
Mn (kg/mol)
Mw (kg/mol)
Mw/Mn
functionalityd
calcd
1H NMR
g′ ) [η]star/[η]lineare
type
calcd
SEC
VPO
calcdb
SLSc
SEC
calcdf
exptl
chain-end
in-chain
3-arm
10.4
22.4
34.7
45.2
11.0
23.6
31.2
38.2
9.62
20.4
34.2
44.9
10.6
22.8
35.4
46.1
11.2g
24.1g
36.2
45.7
1.02
1.02
1.02
1.02
1
1
1
1
1.05
1.02
1.00
1.02
0.83
0.71
0.83
0.72
4-arm
a Yields of the isolated polymers were always more than 80%. b Calculated from Mn (calcd) and Mw/Mn (SEC) values. c Measured in
toluene at 25 °C. dn/dc ) 0.105-0.112 (mL/g). d Functionality of BnBr moiety. e Intrinsic viscosities of star-branched polystyrenes ([η]star
)
were measured in toluene at 35 °C. Intrinsic viscosities of linear polystyrenes with the same molecular weights ([η]linear) were calculated
from the equation: [η] ) 1.29‚10-4Mw(SLS)0.71 values (ref 47). f Calculated from eq 1: g′ ) [(3f - 2)/f 2]0.58[0.724 - 0.015(f - 1)]/0.724.
f ) arm number (3 e f e 18) (ref 46). g Measured by SEC.
ylphenyl)-1-phenylethylene (6.73 g, 32.0 mmol) and imidazole
(6.54 g, 96.0 mmol) in dry DMF (50 mL). The mixture was
allowed to stand with stirring at 25 °C for 6 h. After usual
workup, flash column chromatography (hexane/benzene )
2:1, v/v) afforded 9.15 g (28.2 mmol, 88%) of the title compound,
1, as a colorless syrup. A trace amount of water was removed
completely by azeotropic distillation from its absolute benzene
solution thrice.
First Iterative Process. PSLi (1.12 g, 0.112 mmol,
Mn (SEC) ) 10.0 kg/mol) in THF (11.5 mL) was end-capped
with 1 (0.164 mmol) in THF (2.90 mL) at -78 °C for 30 min
and in situ coupled with the chain-functionalized polystyrene
with BnBr moiety (0.983 g, BnBr functionality ) 0.0878 mmol)
in THF (18.2 mL) at -78 °C for 12 h. The polymer mixture
was quenched with degassed MeOH (5.0 mL) and poured into
a large amount of MeOH (400 mL) to precipitate the polymers.
The coupled polymer was isolated in 86% yield (1.81 g) by
fractional precipitation where hexane was slowly added into
cyclohexane solution of the polymer at 25 °C and the mixture
was allowed to stand for 24 h at 5 °C to precipitate the
polymer. The introduced 3-tert-butyldimethylsilyloxymethyl-
phenyl (SMP) group at the middle of the polymer chain was
quantitatively transformed into BnBr moiety under the condi-
tion identical with that mentioned above. The expected in-
chain-functionalized polystyrene with BnBr moiety was ob-
tained in 95% yield (1.71 g).
Second and Third Iterative Processes. Similarly, the
second and third iterations were carried out to successively
synthesize 3-, followed by 4-arm star-branched polystyrenes,
respectively. A 1.2-fold excess of PSLi end-capped with 1
toward BnBr functionality was always employed in each
coupling reaction. The resulting star-branched polystyrenes
were isolated by fractional precipitation. Both 3- and 4-arm
star-branched polystyrenes were obtained in 90 and 88%
yields, respectively. Their detailed results are listed in
Table 1.
Similarly, 3-arm ABC, followed by 4-arm ABCD asymmetric
star-branched polymers where A, B, C, and D segments were
polystyrene, poly (R-methylstyrene), poly(4-methystyrene), and
poly(methyl methacrylate) segments, respectively, were suc-
cessively synthesized starting from the same chain-end-
functionalized polystyrene with BnBr moiety synthesized
above. Detailed procedures and conditions are as follows:
First Iterative Process. Under the high vacuum condi-
tions (<10-6 Torr), PRMSLi (1.02 g, 0.108 mmol, Mn (1H NMR)
) 9.45 kg/mol) in THF (10.8 mL) was end-capped with 1
(0.139 mmol) in THF (2.35 mL) at -78 °C for 30 min and in
situ coupled with the chain-functionalized polystyrene with
BnBr moiety (0.720 g, BnBr functionality ) 0.0720 mmol) in
THF (7.85 mL) at -78 °C for 1 h. The polymer mixture was
quenched with degassed MeOH (5.0 mL) and poured into a
large amount of MeOH (200 mL) to precipitate the polymers.
The coupled polymer was isolated in 82% yield (1.17 g) by
fractional precipitation using a hexane/cyclohexane system.
The introduced SMP group at the junction between polystyrene
(A) and poly(R-methylstyrene) (B) segments was quantitatively
transformed into BnBr moiety under the condition identical
with that mentioned above. The expected in-chain-function-
alized AB diblock copolymer with BnBr moiety was obtained
in 95% yield (1.11 g).
1H NMR (CDCl3): δ 7.37-7.25 (9H, m, ArH), 5.49 and 5.47
(2H, ss, CH2)), 4.76 (2H, s, Ar-CH2-OSi), 0.940 (9H, s,
Si-C(CH3)3), 0.103 (6H, s, Si(CH3)2).
13C NMR (CDCl3): δ 150.2 (C)CH2), 141.6 (CH2dC-C(Ar)-
CH2-), 141.4 (CH2dC-C(Ar)), 128.4, 128.3, 128.2, 128.2,
127.7, 126.9, 126.1, and 125.5 (Ar), 114.3 (C)CH2), 65.0 (Ar-
CH2-OSi), 26.0 (Si-C(CH3)3), 18.5 (Si-C(CH3)3), -5.2
(Si(CH3)2).
Preparation of Living Anionic Polymers. All polymer-
izations were carried out under a high vacuum condition
(<10-6 Torr) in sealed glass reactors with break seals. The
reactors were always prewashed with the initiator solutions
after being sealed off from the vacuum line. Polystyryllithium
(PSLi), poly(R-methylstyryl)lithium (PRMSLi), and poly(4-
methylstyryl)lithium (P4MSLi) were prepared by the sec-BuLi-
initiated polymerization of the corresponding styrene deriva-
tives in THF at -78 °C for 20 min, 4 h, and 1 h, respectively.
The living polymer of methyl methacrylate was obtained by
the polymerization of methyl methacrylate in THF at -78 °C
for 0.5 h initiated with a 1:1 adduct anion prepared from sec-
BuLi and a 1.5-fold excess of DPE in the presence of five equiv
of LiCl (toward sec-BuLi). The living polymer of tert-butyl
methacrylate was obtained by the polymerization of tert-butyl
methacrylate with diphenylmethylpotassium in THF at
-78 °C for 3 h. Monomers and initiators were used in the
concentration ranges of ∼0.5-0.8 M and ∼0.04-0.06 M,
respectively.
Successive Synthesis of Star-Branched Polymers by
Iterative Methodology Using 1. Synthesis of Chain-End-
Functionalized Polystyrene with BnBr Moiety: Under the
high vacuum conditions (<10-6 Torr), PSLi (2.16 g,
0.196 mmol, Mn (SEC) ) 11.0 kg/mol) in THF (22.1 mL) was
end-capped with 1 (0.266 mmol) in THF (4.70 mL) at -78 °C,
and the mixture was allowed to stir for an additional 30 min
at -78 °C. The mixture was quenched with degassed MeOH
(5.0 mL) and poured into a large amount of MeOH (300 mL)
to precipitate the polymer. It was reprecipitated two times
from THF to MeOH and freeze-dried from its benzene solution
for 24 h. The resulting polymer (2.12 g, 0.193 mmol) was then
treated with trimethylsilyl chloride (1.50 g, 13.8 mmol) and
LiBr (0.894 g, 10.3 mmol) in a chloroform/acetonitrile (80 mL/
20 mL) mixed solution at 30 °C for 12 h under nitrogen. Then
the reaction mixture was poured into water (80 mL), extracted
with chloroform (30 mL) four times, and the combined organic
layer was dried over MgSO4. Removal of solvent under reduced
pressure followed by precipitation into a large amount of
MeOH (200 mL) and reprecipitating twice afforded the ex-
pected chain-end-functionalized polystyrene with BnBr moiety
(1.89 g, 98%, Mn (SEC) ) 11.2 kg/mol, Mw/Mn (SEC) )
1.02, and the degree of functionalization determined by
1H NMR ) 1.05).
Second Iterative Process. P4MSLi end-capped with 1
(0.793 g, 0.0714 mmol, Mn (1H NMR) ) 11.1 kg/mol) was
coupled with the BnBr in-chain-functionalized AB diblock
copolymer (0.987 g, BnBr functionality ) 0.0476 mmol) in THF
(22.5 mL) at -78 °C for 6 h. After quenching with MeOH, the
coupled polymer was isolated in 88% yield (1.33 g) by fractional
precipitation using a hexane/cyclohexane system. The intro-

4580 Higashihara et al.
Macromolecules, Vol. 38, No. 11, 2005
Removal of solvent under reduced pressure followed by
precipitation into a large amount of MeOH (300 mL) afforded
the expected in-chain-functionalized polystyrene with two
BnBr moieties (2.78 g, 98%, Mn (SEC) ) 24.7 kg/mol, Mw/Mn
1
(SEC) ) 1.02 and the degree of functionalization by H NMR
) 2.05).
Second Iterative Process. PRMSLi end-capped with 1
(1.78 g, 0.189 mmol, Mn (SEC) ) 9.43 kg/mol) was reacted with
the in-chain-functionalized polystyrene with two BnBr moi-
eties (2.22 g, 0.0899 mmol, BnBr functionality ) 0.0899 × 2
) 0.180 mmol) in THF (23.9 mL) at -78 °C for 1 h. The
reaction mixture was quenched with degassed MeOH (5.0 mL)
and poured into a large amount of MeOH (200 mL) to
precipitate the polymers. After isolation by fractional precipi-
tation using hexane/cyclohexane, the 4-arm A2B2 asymmetric
star-branched polymer with two SMP groups at the core where
A and B segments were polystyrene and poly(R-methylstyrene)
segments, respectively, was obtained in 91% yield (3.53 g). The
two SMP groups were quantitatively transformed into two
BnBr moieties under the similar condition to that described
above, resulting in the formation of the expected 4-arm A2B2
asymmetric star-branched polymer with two BnBr moieties
at the core in 95% yield (3.33 g).
Third Iterative Process. The living polymer of tert-butyl
methacrylate initiated with diphenylmethylpotassium
(0.653 g, 0.0634 mmol, Mn (1H NMR) ) 10.4 kg/mol) was
coupled with the 4-arm A2B2 asymmetric star-branched poly-
mer thus synthesized (1.07 g, 0.0244 mmol, BnBr functionality
) 0.0244 × 2 ) 0.0488 mmol) in THF (12.2 mL) at -40 °C for
12 h. Quenching with MeOH (5.0 mL) followed by fractionally
precipitating with a large amount of MeOH (200 mL) afforded
the expected 6-arm A2B2C2 asymmetric star-branched polymer
in 85% yield (1.34 g).
Hydrolysis of 6-Arm A2B2C2 Star-Branched Polymer.
The 6-arm A2B2C2 star polymer (0.504 g) synthesized herein
was dissolved in a mixture of 12 N HCl (1.0 mL) and
1,4-dioxane (50 mL), and the mixture was allowed to stir at
80 °C for 24 h. It was then concentrated to ∼5 mL and poured
into a large amount of MeOH to precipitate the polymer
(0.451 g, 100% yield). The polymer was reprecipitated twice
from the THF solution to MeOH and freeze-dried from its
benzene solution for 24 h. Complete hydrolysis of the tert-butyl
Figure 1. SEC profiles of the resulting polymers obtained
by coupling reaction of BnBr chain-end-functionalized poly-
styrene with PSLi end-capped with 1 and by transformation
reaction: (A) before fractionation, (B) after fractionation, and
(C) after transformation reaction.
1
ester function was confirmed by H NMR and IR analyses.
duced SMP group at the core of the ABC star-branched
polymer was quantitatively transformed into BnBr moiety. The
expected 3-arm ABC asymmetric star-branched polymer with
BnBr moiety at the core was obtained in 95% yield (1.11 g).
Third Iterative Process. The living anionic polymer of
methyl methacrylate initiated with 1,1-diphenyl-3-methyl-
pentyllithium (0.438 g, 0.0381 mmol, Mn (1H NMR) )
11.5 kg/mol) was coupled with the 3-arm ABC asymmetric
star-branched polymer thus synthesized (0.820 g, BnBr func-
tionality ) 0.0238 mmol) in THF (15.2 mL) at -40 °C for
12 h. Quenching with MeOH (5.0 mL) followed by fractionally
precipitating with a CH3CN/MeOH (4:1, v/v) mixed solvent
afforded the expected 4-arm ABCD asymmetric star-branched
polymer in 91% yield (0.992 g).
Successive Synthesis of Star-Branched Polymers by
the Same Iterative Methodology Starting from In-Chain-
Functionalized Polystyrene with Two BnBr Moieties.
First Iterative Process. PSLi (3.31 g, 0.288 mmol, Mn (SEC)
) 11.5 kg/mol) in THF (35.0 mL) was end-capped with 1
(0.338 mmol) in THF (2.28 mL) at -78 °C for 30 min, followed
by coupling with R,R′-dichloro-p-xylene (0.120 mmol) in THF
(3.85 mL) at -78 °C. After 1 h, the reaction mixture was
quenched with degassed MeOH (5.0 mL) and poured into a
large amount of MeOH (400 mL) to precipitate the polymers.
The coupled polymer was isolated in 90% yield by fractional
precipitation using hexane/cyclohexane. Under nitrogen, the
resulting polymer (2.83 g) was treated with trimethylsilyl
chloride (1.42 g, 13.1 mmol) and LiBr (0.949 g, 10.9 mmol) in
a chloroform/acetonitrile (80 mL/20 mL) mixed solution at
30 °C for 12 h. Then the reaction mixture was poured into
water (80 mL), extracted with chloroform (30 mL) four times,
and the combined organic layer was dried over MgSO4.
1H NMR: δ 7.20-6.20 (m, 1990 H, aromatic), 2.20-0.80
(m, 1246 H, -CH2CH(Ph)- and -CH2C(CH3)(CO)-), 0.80-
0.60 (m, 24H, -CH(CH3)CH2CH3), 0.60 to -0.90 (m, 551 H,
-CH2C(CH3)(Ph)-).
FT-IR (KBr, cm-3): 3420 (O-H), 1720 (CdO).
Isolation and Purification of Polymers. The objective
star-branched polystyrenes were effectively isolated from the
resulting polymer mixtures by fractional precipitation using
hexane/cyclohexane (2:3, v/v) at 5 °C. Hexane was added slowly
to a cyclohexane solution of polystyrene (∼1 g/150 mL) at
25 °C until the mixture became turbid, and the mixture was
allowed to stand at 5 °C for 1 h. The precipitated polymer was
recovered by carefully decanting the supernatant solution. The
AB block copolymer, ABC, and A2B2 stars were also isolated
in >80% yields in similar ways. On the other hand, ABCD
and A2B2C2 star-branched polymers were precipitated by
pouring their THF solutions into acetonitrile/MeOH (4:1, v/v)
and MeOH, respectively. The polymers all were purified by
reprecipitating twice from THF into MeOH and then freeze-
drying from their absolute benzene solutions for 6 h thrice.
Results and Discussion
Successive Synthesis of Star-Branched Poly-
mers by Iterative Methodology Using 1-(3-tert-
Butyldimethylsilyloxymethylphenyl)-1-phenyleth-
ylene (1). Scheme 2 shows the first proposed iterative
methodology using 1 for the successive synthesis of star-
branched polymers. As can be seen, the methodology
involves only two sets of reaction conditions for the
entire iterative reaction sequence: a coupling reaction

Macromolecules, Vol. 38, No. 11, 2005
Synthesis of Star-branched Polymers 4581
Scheme 2. Successive Synthesis of BnBr Chain-End- and In-Chain-Functionalized Polystyrenes and BnBr
Core-Functionalized 3- and 4-arm Star-Branched Polystyrenes by an Iterative Methodology Using
1-(3-tert-Butyldimethylsilyloxymethylphenyl)-1-phenylethylene (1)
and a transformation reaction. First of all, we have
intended to test the synthesis of regular star-branched
polystyrenes to make sure whether the proposal meth-
odology does work satisfactorily in terms of the succes-
sive star-branched polymer synthesis. Throughout the
synthesis, the arm segment was designated to be
∼10 kg/mol in Mn value. The starting chain-end-func-
tionalized polystyrene with benzyl bromide (BnBr)
moiety was prepared by the end-capping of PSLi with
1, followed by treatment with (CH3)3SiCl-LiBr.
showing that the signals at 0.08, 0.94, and 4.66 ppm
assigned to methyl protons of the Si-CH3 and Si-
C(CH3)3 groups and methylene protons of the benzyl
silyl ether (-C6H4-CH2-OSi≡) completely disappeared,
and alternatively, a new signal at 4.35 ppm character-
istic of methylene protons of the BnBr moiety appeared.
The degree of BnBr functionalization was confirmed to
be quantitative (f ) 1.02) by comparing two signal
intensities at 4.35 and 0.64 ppm characteristic to
methylene and methyl protons of the BnBr moiety and
the initiator fragment, respectively. SEC profiles of the
polymers before and after treatment are very similar
in shape and distribution as shown in Figure 1B and
1C. As was seen in Table 1, the Mn value of the resulting
polymer measured by either SEC or VPO agreed well
with that predicted. Thus, the objective in-chain-func-
tionalized polystyrene with BnBr moiety was obtained
by the first iteration.
The first iterative process involves the coupling reac-
tion of the PSLi end-capped with 1 with the BnBr chain-
end-functionalized polystyrene, followed by transforma-
tion into a BnBr moiety with (CH3)3SiCl-LiBr. The SEC
profile of the reaction mixture showed only two distinct
sharp peaks corresponding to the coupled product and
the deactivated PSLi end-capped with 1 used in excess
in the reaction (see Figure 1A). The coupling efficiency
was essentially quantitative on the basis of the two peak
areas. The coupled product was isolated in 86% yield
by fractional precipitation and then treated with (CH3)3-
SiCl-LiBr. The quantitative transformation of the tert-
butyldimethylsilyloxymethylphenyl (SMP) group into
Similarly, the second and third iterative processes
were carried out under identical conditions. The SEC
profile of the polymer isolated at each reaction stage
exhibited a sharp monomodal peak and shifted to a
higher molecular weight side as the iteration proceeded.
In each case, the coupling efficiency was virtually
1
BnBr functionality was confirmed by H NMR spectra

4582 Higashihara et al.
Macromolecules, Vol. 38, No. 11, 2005
Scheme 3. Synthesis of BnBr Chain-End-Functionalized Polystyrene (A), AB Diblock Copolymer, 3-Arm ABC, and
4-Arm ABCD Asymmetric Star-Branched Polymers by the Iterative Methodology Using 1
quantitative on the basis of the SEC profiles. The
characterization results are also summarized in Table
1. As expected from smaller hydrodynamic volumes
attributable to their star-branched structures,45 the Mn
values of the polymers obtained at the second and third
iterations estimated by SEC were somewhat smaller
than the predicted values. On the other hand, both Mn
and Mw values determined by VPO and SLS were in
good agreement with those predicted. The star-branched
architectures of the resulting stars could be supported
by good agreement between the g′ values defined as
[η]star/[η]linear, experimentally determined and predicted
from the established eq 1, where f is the arm number
for regular star-branched polymers.46
sive synthesis of asymmetric star-branched polymers
with different arms. As illustrated in Scheme 3, the
synthesis starts from the same chain-end-functionalized
polystyrene with BnBr moiety. Poly(R-methylstyryl)-
lithium and poly(4-methylstyryl)lithium end-capped
with 1 were used in the first and second iterations,
respectively. The living anionic polymer of methyl
methacrylate initiated with 1,1-diphenyl-3-methyl-
pentyllithium was employed in the coupling reaction of
the final iterative reaction sequence. Accordingly, the
A, B, C, and D segments were polystyrene, poly(R-
methylstyrene), poly(4-methylstyrene), and poly(methyl
methacrylate) chains, respectively. The coupling and
transformation reactions in each of the iterations were
carried out under conditions similar to those employed
in the synthesis of regular star-branched polystyrenes.
As shown in Figure 2, only two sharp peaks correspond-
ing to the coupled product and the excess living polymer
were observed in each of the iterations. No intermediate
polymers were present in either case. The coupling
efficiencies were virtually quantitative in all cases on
the basis of SEC profiles using an UV detector. The
coupled products were isolated by fractional precipita-
tion. The characterization results are summarized in
Table 2.
g′ ) [(3f - 2)/f 2]0.58[0.724 - 0.015(f - 1)]/0.724 (1)
Overall, the successive synthesis of 3-arm, followed
by 4-arm star-branched polystyrenes was successfully
achieved without difficulty by repeating the iterative
processes three times. Thus, the proposed methodology
works satisfactorily.
On the basis of the successive synthesis of regular
star-branched polystyrenes successfully achieved, the
proposed methodology has been applied to the succes-

Macromolecules, Vol. 38, No. 11, 2005
Synthesis of Star-branched Polymers 4583
Figure 2. SEC profiles of the resulting reaction mixtures and
the isolated polymers in the synthesis of A homopolymer, AB
diblock copolymer, 3-arm ABC, and 4-arm ABCD asymmetric
star-branched polymers.
Figure 3. 1H NMR spectra of the isolated (A) A homopolymer,
(B) AB diblock copolymer, (C) 3-arm ABC, and (D) 4-arm
ABCD asymmetric star-branched polymers.
The isolated polymers all possessed sharp monomodal
SEC distributions without any shoulders and tailings
(see also Figure 2). Their Mn and Mw values calculated
from monomer-to-initiator ratios were in good agree-
ment with those determined by 1H NMR and SLS,
respectively. 1H NMR spectra of these polymers are
shown in Figure 3. The resonances characteristic to
polystyrene (A) (6.2-7.2 ppm for the phenyl protons),
poly(R-methylstyrene) (B) (-0.95-0.5 ppm for the
R-methyl protons), poly(4-methylstyrene) (C) (2.35 ppm
for 4-methyl protons), and poly(methyl methacrylate)
(D) (3.59 ppm for methoxy protons) segments are clearly
observed. The compositions measured by 1H NMR using
these resonances agreed quite well with those predicted
as listed in Table 2. Thus, we were able to successively
synthesize in-chain-functionalized AB block copolymer,
3-arm ABC and 4-arm ABCD asymmetric star-branched
polymers by repeating the iterative processes three
times. Obviously, the proposed methodology is very
effective for the successive synthesis of asymmetric star-
branched polymers. Incompatibility of the different
polymer segments caused by microphase separation
seemed not to be affected under the conditions employed
here. The successful synthesis of 4-arm ABCD asym-
metric star-branched polymer is especially attractive
because only two synthetic examples of similar ABCD
asymmetric stars have been reported so far.18b,40
BnBr moieties. The synthetic outline is illustrated in
Scheme 4. With this methodology, two polymer seg-
ments can be introduced via the two BnBr function-
alities in each iteration. The starting polymer was
synthesized by the coupling reaction of R,R′-dichloro-
p-xylene with two equiv of PSLi end-capped with 1 and
the subsequent treatment with (CH3)3SiCl-LiBr in the
first iteration. With use of the in-chain-functionalized
polystyrene with two BnBr moieties thus synthesized,
the same iterative reaction sequence involving the
coupling and transformation reactions was repeated two
more times. Living anionic polymers of R-methylstyrene
and tert-butyl methacrylate (∼10 kg/mol) were used in
the coupling reactions of the second and third iterative
processes, respectively. Accordingly, A, B, and C seg-
ments were polystyrene, poly(R-methylstyrene), and
poly(tert-butyl methacrylate) segments, respectively.
The successive synthesis of 4-arm A2B2 and 6-arm
A2B2C2 asymmetric star-branched polymers is thus
expected.
In the first iteration, R,R′-dichloro-p-xylene was coupled
with a 2.4-fold excess of PSLi end-capped with 1 in THF
at -78 °C for 1 h. Figure 4(A) shows the SEC profile of
the reaction mixture. The coupled product was eluted
as a major peak at a higher molecular weight and
isolated in 90% yield by fractional precipitation (see
Figure 4B). The isolated polymer was then treated with
(CH3)3SiCl-LiBr. The complete transformation from
two SMP groups into two BnBr functionalities was
Successive Synthesis of Star-Branched Poly-
mers by the Same Iterative Methodology Starting
with In-Chain-Functionalized Polystyrene with
Two BnBr Moieties. To further examine the proposed
methodology as a more general procedure, we have
intended to synthesize one more series of asymmetric
star-branched polymers starting from the newly syn-
thesized in-chain-functionalized polystyrene with two
1
confirmed by H NMR analysis. The Mn value deter-
mined by SEC agreed with that calculated, as was seen

4584 Higashihara et al.
Macromolecules, Vol. 38, No. 11, 2005
Scheme 4. Successive Synthesis of BnBr In-Chain-Functionalized Polymer, BnBr Core-Functionalized 4-arm
A2B2, and 6-Arm A2B2C2 Asymmetric Star-Branched Polymers by the Iterative Methodology Using 1
Table 2. Synthesis of BnBr Chain-End-Functionalized Polystyrene (A), AB Diblock Copolymer, 3-Arm ABC and 4-Arm
ABCD Asymmetric Star-Branched Polymers Using 1 in THF at -78 °Ca
Mn (kg/mol)
1H NMR
Mw (kg/mol)
Mw/Mn
composition (wt %)d
calcd
1H NMR
type
calcd
calcdb
SLSc
SEC
chain-end (A)
AB diblock
ABC star
10.4
19.8
31.7
45.8
10.0
20.8
34.4
46.5
10.6
20.2
32.3
46.7
11.2e
21.2f
35.7
48.8
1.02
1.02
1.02
1.02
100
100
50/50
48/52
32/34/34
26/24/25/25
34/32/34
27/23/25/25
ABCD star
a Yields of the isolated polymers were always more than 80%. A, B, C, and D segments were polystyrene, poly(R-methylstyrene),
poly(4-methylstyrene), and poly(methyl methacrylate), respectively. b Calculated from Mn (calcd) and Mw/Mn (SEC) values. c In THF at
25 °C. The dn/dc values were 0.182 and 0.167 mL/g for ABC and ABCD stars. d A/B, A/B/C, and A/B/C/D, respectively. e Measured by
SEC. f Calculated from Mn (1H NMR) and Mw/Mn (SEC) values.
Table 3. Synthesis of In-Chain-Functionalized Polystyrene (A2), 4-Arm A2B2 and 6-Arm A2B2C2 Asymmetric
Star-Branched Polymers in THF at -78 °Ca
Mn (kg/mol)
Mw (kg/mol)
Mw/Mn
composition (wt %)d
calcd
1H NMR
type
calcd
SEC
VPO
1H NMR
calcdb
SLSc
SEC
in-chain (A2)
A2B2 star
A2B2C2 star
23.6
43.0
64.4
24.7
30.5
46.0
24.1
44.3
65.7
25.2e
43.7
67.0
1.02
1.03
1.02
100
100
53/47
36/32/32
44.1
65.5
43.8
64.3
54/46
36/32/32
a Yields of the isolated polymers were always more than 85%. A, B, and C segments were polystyrene, poly(R-methylstyrene), and
poly(tert-butyl methacrylate), respectively. b Calculated from Mn (calcd) and Mw/Mn (SEC) values. c In THF at 25 °C. dn/dc ) 0.181 and
0.152 mL/g for types of A2B2 and A2B2C2 star, respectively. d A/B and A/B/C. e Measured by SEC.
in Table 3. Thus, the starting in-chain-functionalized
polystyrene with two BnBr moieties was obtained.
in Figure 4C exhibited only two sharp peaks corre-
sponding to the coupled product and the deactivated
poly(R-methylstyryl)lithium used in excess in the reac-
tion. The coupled product was isolated in 91% yield by
fractional precipitation (see Figure 4D).
In the second iteration, the in-chain-functionalized
polystyrene with two BnBr moieties was reacted with
a 2.1-fold excess of poly(R-methylstyryl)lithium in THF
at -78 °C for 1 h, followed by treatment with (CH3)3-
SiCl-LiBr under the same conditions as the first
iteration. The SEC profile of the reaction mixture shown
The Mn and Mw values of the resulting polymer
determined by 1H NMR, VPO, and SLS agreed well with
1
those predicted. As shown in Figure 5B, the H NMR

Macromolecules, Vol. 38, No. 11, 2005
Synthesis of Star-branched Polymers 4585
Figure 4. SEC profiles of the starting in-chain-functionalized
polystyrene (A and B) and the resulting 4-arm A2B2 (C and
D), and 6-arm A2B2C2 asymmetric star-branched polymers
(E and F) before and after fractionation, respectively.
Figure 5. 1H NMR spectra of the isolated (A) in-chain-
functionalized PS, (B) 4-arm A2B2, and (C) 6-arm A2B2C2
asymmetric star-branched polymers.
The iterative methodology proposed in this section
works very satisfactorily to be able to successively
synthesize 4-arm A2B2 and 6-arm A2B2C2 asymmetric
star-branched polymers. Importantly, the coupling ef-
ficiency was always virtually quantitative in each of the
reactions, and the resulting polymers all were well-
defined in architecture and precisely controlled in chain
length.
Interestingly, the asymmetric A2B2C2 star-branched
polymer thus synthesized could be converted to a new
ionic star-branched polymer by hydrolyzing the poly-
(tert-butyl methacrylate) segments to poly(methacrylic
acid) segments with 0.2 N HCl in 1,4-dioxane. Table 4
lists solubilities of such A2B2C2 stars before and after
hydrolysis in usual solvents.
spectrum shows the characteristic resonances at
-0.95-0.5 ppm assigned to R-methyl protons of the B
segment. The composition measured by the same spec-
trum was in good agreement with that calculated. Thus,
obviously, the resulting polymer proved to be the
expected 4-arm A2B2 asymmetric star-branched poly-
mer.
Finally, a 6-arm A2B2C2 asymmetric star was syn-
thesized by the coupling of the BnBr core-functionalized
A2B2 star with a 2.6-fold excess of living anionic polymer
of tert-butyl methacrylate in THF at -40 °C for 12 h.
As was observed in Figure 4E, the coupling reaction
proceeded satisfactorily. The excess poly(tert-butyl meth-
acrylate) was readily removed by washing the polymer
mixture with methanol because it was soluble in metha-
nol. The isolated polymer exhibits a sharp monomodal
SEC distribution as shown in Figure 4F. As expected,
overall SEC profiles shown in Figure 4 clearly demon-
strate that each peak of the coupled polymer is narrow
and monomodal and shifts to a higher molecular side
as the iteration process proceeds. The Mn and Mw values
determined by 1H NMR, VPO, and SLS agreed well with
those calculated from the feed ratios. As shown in
Figure 5C, there are characteristic resonances at 6.2-
7.2, -0.95-0.5 and 1.43 ppm assignable to the phenyl
protons (polystyrene (A)), the R-methyl protons (poly-
(R-methylstyrene) (B)), and the methyl protons of the
tert-butyl group (poly(tert-butyl methacrylate) (C)), re-
spectively. The observed composition by 1H NMR using
such resonances was almost consistent with that pre-
dicted. Accordingly, all of the analytical results clearly
indicate that the resulting polymer is the expected
6-arm A2B2C2 asymmetric star-branched polymer with
well-defined architecture. Again, incompatibility of the
different segments was not affected in the synthesis.
Conclusions
We have proposed a new alternative methodology on
the basis of an iterative divergent approach for the
successive synthesis of star-branched polymers. The
methodology involves only two sets of reaction condi-
tions: (a) a coupling reaction of the BnBr-functionalized
polymer with living anionic polymer end-capped with 1
for linking polymer segments and the introduction of
BnBr precursor(s) and (b) a transformation reaction of
the introduced precursors into BnBr functionalities by
treatment with (CH3)3SiCl-LiBr. The methodology
works satisfactorily for the successive synthesis of a
series of asymmetric star-branched polymers. In fact,
3-arm ABC, 4-arm ABCD, 4-arm A2B2, and 6-arm
A2B2C2 asymmetric stars were successfully synthesized
by employing living anionic polymers of styrene,
R-methylstyrene, 4-methylstyrene, methyl methacry-
late, and tert-butyl methacrylate. One of the successful
reasons for the successive star-branched polymer syn-

4586 Higashihara et al.
Macromolecules, Vol. 38, No. 11, 2005
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theses may be due to the fact that the arm segments
are not emanating from a single point core, but always
separated at a regular interval of a connecting alkyl-
phenyl group in the stars synthesized herein as il-
lustrated in Scheme 5. Accordingly, the BnBr reaction
sites may possibly be less sterically hindered even after
repeating the iteration process and thereby make it
possible to continue the iteration. Strictly defining the
structures of the resulting polymers, they are more
comparable with homopolymers and co-oligomers of
macromonomers rather than star-branched polymers.
Acknowledgment. The financial support of a Grant-
in-Aid for Scientific Research (no. 00164351) from the
Ministry of Education, Science, Sports, and Culture of
Japan is gratefully acknowledged. This research was
supported in part by Sumitomo Chemical Co. Ltd and
Nippon Soda Co. Ltd. Both N.H. and T.H. are also
thankful for support from the Japan Society for the
Promotion of Science Research Fellowships for Young
Scientists.
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