Anionic polymerization of p-pentamethyldisilyl-, p-heptamethyltrisilyl-, and p-nonamethyltetrasilylstyrenes

Article abstract of DOI:10.1021/ma021742a

The anionic polymerizations of p-pentamethyldisilylstyrene (1), p-heptamethyltrisilylstyrene (2), and p-nonamethyltetrasilylstyrene (3) were carried out under various conditions. The polymerizations of 1 and 2 proceeded in a living manner in THF at -78 °C to quantitatively afford polymers with predictable molecular weights and narrow molecular weight distributions. Suitable initiators for these living anionic polymerizations were sec-BuLi, lithium, sodium, and potassium naphthalenides, cumylpotassium, and living oligomers of α-methylstyrene lithium, sodium, and potassium salts. Well-defined diblock copolymers, poly(1)-block-polystyrene, poly(2)-block-polystyrene, polystyrene-block-poly(1), and polystyrene-block-poly(2) were successfully synthesized under similar conditions by a two-step sequential monomer addition, namely, styrene followed by 1 or 2, or vice versa. In contrast, the polymerization of 3 was problematic. No polymer was obtained in the polymerization of 3 in either THF at -78 °C or benzene at 40 °C with sec-BuLi and oligo(α-methylstyryl)lithium. Using oligo(α-methylstyryl)potassium as an initiator, 3 was polymerized quantitatively in THF at -78 °C for 0.5 h. Unfortunately, the side reaction, presumably attack of the Si-Si-Si-Si bond by the growing chain-end anion, could not be completely suppressed during the polymerization, which resulted in the formation of high molecular weight polymers by coupling between polymer chains.

Full text of DOI:10.1021/ma021742a

Macromolecules 2003, 36, 5081-5087
5081
Anionic Polymerization of p-Pentamethyldisilyl-, p-Heptamethyltrisilyl-,
and p-nonamethyltetrasilylstyrenes1
Ak ir a Hir a o,* Yu sh i An d o, Ta k a sh i Ish izon e, a n d Seiich i Na k a h a m a
Polymeric and Organic Materials Department, Graduate School of Science and Technology,
Tokyo Institute of Technology, 2-12-1, Ohokayama, Meguro-ku, Tokyo 152-8552, J apan
Received December 4, 2002; Revised Manuscript Received May 9, 2003
ABSTRACT: The anionic polymerizations of p-pentamethyldisilylstyrene (1), p-heptamethyltrisilylstyrene
(2), and p-nonamethyltetrasilylstyrene (3) were carried out under various conditions. The polymerizations
of 1 and 2 proceeded in a living manner in THF at -78 °C to quantitatively afford polymers with
predictable molecular weights and narrow molecular weight distributions. Suitable initiators for these
living anionic polymerizations were sec-BuLi, lithium, sodium, and potassium naphthalenides, cumylpo-
tassium, and living oligomers of R-methylstyrene lithium, sodium, and potassium salts. Well-defined
diblock copolymers, poly(1)-block-polystyrene, poly(2)-block-polystyrene, polystyrene-block-poly(1), and
polystyrene-block-poly(2) were successfully synthesized under similar conditions by a two-step sequential
monomer addition, namely, styrene followed by 1 or 2, or vice versa. In contrast, the polymerization of
3 was problematic. No polymer was obtained in the polymerization of 3 in either THF at -78 °C or benzene
at 40 °C with sec-BuLi and oligo(R-methylstyryl)lithium. Using oligo(R-methylstyryl)potassium as an
initiator, 3 was polymerized quantitatively in THF at -78 °C for 0.5 h. Unfortunately, the side reaction,
presumably attack of the Si-Si-Si-Si bond by the growing chain-end anion, could not be completely
suppressed during the polymerization, which resulted in the formation of high molecular weight polymers
by coupling between polymer chains.
In tr od u ction
Matyjaszewski and co-workers reported that poly(meth-
ylphenylsilylene) and poly(dihexylsilylene) were de-
graded by alkali metals and their naphthalenides to
exclusively afford cyclic oligomers via the cleavage of
their Si-Si bonds.8 Saegusa et al. also suggested that
the same cleavage was occurring during the anionic
ring-opening polymerization of phenylnonamethylcy-
clopentasilane.9 We therefore studied the anionic po-
lymerization of monomers possessing Si-Si and Si-Si-
Si bonds in further detail.
Herein, we present full details of the anionic polym-
erization of 1, 2, and p-nonamethyltetrasilylstyrene (3),
a related new styrene derivative possessing a Si-Si-
Si-Si bond as shown below:
Silicon-silicon (Si-Si) bonds, unlike their C-C coun-
terparts, exhibit unique and unusual electronic, photo-
chemical, and photophysical properties.2 Their chemis-
try is also unique. For example, Si-Si bonds are cleaved
with several electrophiles as well as nucleophiles.3 They
are readily oxidized to Si-O-Si bonds because of a
strong affinity for oxygen.4 These reactivities make
polymers containing Si-Si bonds attractive as func-
tional polymers, since their Si-Si bonds can participate
in a variety of useful and interesting reactions. How-
ever, such reactivities are not favorable in radical and
ionic polymerization of monomers with Si-Si bonds.
Despite the reactivity of Si-Si bonds, Saigo and
Watanabe demonstrated that p-(2-allyl-1,1,2,2-tetra-
methyl)disilyl-R-methylstyrene could be quantitatively
polymerized with n-BuLi in THF at -78 °C without
difficulty.5 In the meantime, we have found for the first
time that p-pentamethyldisilylstyrene (1) and p-hep-
tamethyltrisilylstyrene (2) successfully undergo living
anionic polymerization with sec-BuLi and oligo(R-meth-
ylstyryl)lithium in THF at -78 °C.1 The success of the
living polymerization clearly demonstrates that the Si-
Si and even Si-Si-Si bonds are sufficiently stable
toward highly reactive anionic initiators and growing
chain-end carbanions derived from 1 and 2. This is in
sharp contrast to the fact that Si-Si linkages are readily
cleaved by nucleophiles under similar conditions. For
example, the Si-Si bond cleavage of disilanes by lithium
and sodium-potassium alloy has been known for a long
time as one of the most convenient methods for the
preparation of R3SiM (M ) Li, K).3,6 It has been
observed that alkyllithiums and trialkylsilyllithiums are
also useful in the cleavage of disilanes.3,7 Furthermore,
The overall objective of this study is to explore the
generality and limitation of living anionic polymeriza-
tion of the substituted styrenes with permethylated
oligosilane moieties, as part of our program to study the
anionic polymerization of monomers containing func-
tional silyl groups.10
Exp er im en ta l Section
Ma ter ia ls. Reagents were purchased from Tokyo Kasei,
Tokyo, J apan (reagent grade), unless otherwise stated. Both
styrene and R-methylstyrene were washed with 10% NaOH
and water successively and dried over CaCl2. After filtration,
they were distilled twice from CaH2 under reduced pressure
and then distilled from dibutylmagnesium (ca. 5 mol %) on a
vacuum line into ampules with break seals that were pre-
* Corresponding author. Telephone: 81-3-5734-2131. Fax: 81-
3-5734-2888. E-mail: ahirao@polymer.titech.ac.jp.
10.1021/ma021742a CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/12/2003

5082 Hirao et al.
Macromolecules, Vol. 36, No. 14, 2003
washed with 1,1-diphenylhexyllithium in heptane. Both ben-
zene and heptane were washed successively with H2SO4 and
water and dried over CaCl2. After filtration, they were distilled
from P2O5 under nitrogen and then distilled from sec-BuLi (ca.
5 mol %) on a vacuum line into ampules with break seals that
were prewashed with 1,1-diphenylhexyllithium in heptane.
THF was refluxed over sodium wire for 5 h and, after filtration,
distilled from LiAlH4 under nitrogen. It was finally distilled
from its sodium naphthalenide solution on a vacuum line.
Alkali metal naphthalenides were prepared in sealed reactors
with break seals under high vacuum conditions by reactions
of the corresponding alkali metals with a 1.1-fold excess of
naphthalene in THF for a few hours at 25 °C. After filtration,
their concentrations were determined by colorimetric titration
using absolute 1-octanol in THF in sealed reactors under high
vacuum conditions. Living oligomers of R-methylstyrene were
prepared from sec-BuLi or alkali-metal naphthalenides and
ca. a 3-fold excess of R-methylstyrene in THF at 25 °C for 1
min and at -78 °C for 1 h. 4-Chlorostyrene (99%, Hokko
Chemical Industry Co., Ltd) was distilled over CaH2 two times
under reduced pressures. Pentamethyldisilyl chloride (98%)
was purchased from Shinnetsu Chemical. Co., Ltd. Both
heptamethyltrisilyl chloride and nonamethyltetrasilyl chloride
were prepared according to the procedures previously re-
ported.11
colorless liquid: 1H NMR (CDCl3) δ 7.30-7.20 (m, 4H, HAr),
6.47 (dd, 1H, dCH), 5.70, 5.10 (2d, 2H, -CdCH2, J ) 10.8,
18.0 Hz), 0.26 (s, 6H, (CH3)3Si(CH3)2Si(CH3)2Si), 0.03 (s, 6H,
(CH3)3Si(CH3)2Si(CH3)2Si), -0.11 (s, 9H, Si(CH3)3); 13C NMR
(75 MHz, CDCl3) δ 139.7 (C4, Ar), 137.2 (CH2 ) CH), 134.8
(C1, Ar), 134.0 (C3, Ar), 125.7 (C2, Ar), 113.8 (CH2dCH), -1.34
((CH3)3Si(CH3)2Si(CH3)2Si), -2.95 ((CH3)3Si(CH3)2Si(CH3)2Si),
-6.66 ((CH3)3Si(CH3)2Si(CH3)2Si); IR (neat, cm-1) 779 (Si-C
stretch), 813 (Si-C stretch), 1245 (Si-CH3 deformation), 2952
(Si-C stretch).
p-Non a m eth ytetr a silylstyr en e (3). The title styrene, 3,
was synthesized by the reaction of nonamethyltetrasilyl
chloride (18.6 g, 65 mmol) with the Grignard reagent of
p-chlorostyrene, prepared from p-chlorostyrene (8.82 g, 64
mmol) and magnesium (1.81 g, 67 mmol), similar to the
synthetic procedure employed for 1. Styrene, 3, was isolated
by column chromatography on silica gel using hexanes as
eluents and dried by fractional distillation from CaH2 under
reduced pressure at 110 - 120 °C (0.16 Torr) to yield 3 (4.03
g, 12 mmol, 18%) as a colorless liquid: 1H NMR (CDCl3) δ
7.33-7.17 (m, 4H, HAr), 6.62 (dd, 1H, dCH), 5.60, 5.05 (2d,
2H, -CdCH2, J ) 11.0, 17.6 Hz), 0.26 (s, 6H, (CH3)3Si(CH3)2-
Si(CH3)2Si(CH3)2Si), 0.03 (s, 21H, (CH3)3Si(CH3)2Si(CH3)2Si-
(CH3)2Si); 13C NMR (75 MHz, CDCl3) δ 140.2 (C4, Ar),
137.7(C1, Ar), 137.1(CH2 ) CH), 134.0 (C3, Ar), 125.6(C2, Ar),
113.7 (CH2dCH), 0.49 ((CH3)3Si(CH3)2Si(CH3)2Si(CH3)2Si),
-1.20 ((CH3)3Si(CH3)2Si(CH3)2Si(CH3)2Si), -2.81((CH3)3Si-
(CH3)2Si(CH3)2Si(CH3)2Si), -5.44 ((CH3)3Si(CH3)2Si(CH3)2Si-
(CH3)2Si); IR (neat, cm-1) 777 (Si-C stretch), 812 (Si-C
stretch), 1245 (Si-CH3 deformation), 2947(Si-C stretch).
The styrene derivatives, 1-3, thus synthesized were finally
purified by distillation from dibutylmagnesium (ca. 5 mol %)
under high vacuum conditions. They were diluted with THF
or benzene to about 1 M solutions and stored at -30 °C.
An ion ic P olym er iza tion of 1)3. The anionic polymeri-
zation was 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. A typical polymerization
experiment was as follows. A THF (2.44 mL, 0.888 M) solution
of 1 (0.508 g, 2.17 mmol) kept at -78 °C was added to a THF
(3.67 mL, 0.0244 M) solution of potassium naphthalenide
(0.0895 mmol) through the break-seal with vigorous stirring
at -78 °C. The reaction mixture was allowed to stand for an
additional 0.5 h at -78 °C and quenched with degassed
methanol. The polymer was precipitated in methanol, repre-
cipitated twice from THF into methanol, and freeze-dried from
its benzene solution for 24 h. A polymer yield was quantitative.
The SEC showed a sharp monomodal distribution: Mn ) 12
kg/mol, Mw/Mn ) 1.05. The Mn value determined by VPO was
1
Mea su r em en ts. Both H (300 MHz) and 13C (75 MHz) NMR
spectra were measured in CDCl3 using a Bruker DPX spec-
trometer. Chemical shifts were reported in ppm downfield
1
relative to tetramethylsilane (δ 0.00) for H NMR and to CDCl3
(δ 77.1) for 13C NMR. Size-exclusion chromatography (SEC)
was performed on a TOSOH HLC 8020 instrument with
UV (254 nm) or 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 (TSKgel G4000HXL
, G3000HXL,
G2000HXL or TSKgel G5000HXL, G4000HXL, G3000HXL) were
used. Measurable molecular weight ranges are 103 to 5 × 104
and 104 to 5 × 105, respectively. A calibration curve was made
to determine Mn and Mw/Mn values with standard polysty-
renes. Vapor pressure osmometry (VPO) measurements were
made with a Corona 117 instrument in benzene at 40 °C with
a highly sensitive thermoelectric couple and with equipment
of very exact temperature control. Therefore, molecular weight
can be measured up to 100 ( 5 kg/mol.
p-P en ta m eth yld isilylstyr en e (1). The title styrene, 1, was
synthesized by the reaction of pentamethyldisilyl chloride with
the Grignard reagent of p-chlorostyrene. The Grignard re-
agent, prepared from p-chlorostyrene (14.7 g, 106 mmol) and
magnesium (3.01 g, 111 mmol) in THF (100 mL) was added
dropwise to a dry THF solution (50 mL) containing pentameth-
yldisilyl chloride (15.4 g, 92 mmol). The resulting mixture was
stirred at reflux for 5 h. It was acidified with 1 N HCl,
extracted with ether, and dried over MgSO4. The solvent was
removed under reduced pressure, and the residue was purified
by column chromatography on silica gel using hexanes as
eluents. The eluted portions showing one spot were collected
and the solvent was evaporated to afford a colorless oil. It was
distilled from CaH2 under reduced pressure at 63-65 °C (0.15
Torr) to yield 1 (9.34 g, 40 mmol, 44%) as a colorless liquid:
1H NMR (CDCl3) δ 7.30-7.20 (m, 4H, HAr), 6.53 (dd, 1H,
dCH), 5.65, 5.05 (2d, 2H, -CdCH2, J ) 11.0, 17.7 Hz), 0.18
(s, 6H, Si(CH3)2), 0.09 (s, 9H, Si(CH3)3); 13C NMR (75 MHz,
CDCl3) δ 139.5 (C4, Ar), 137.7 (C1, Ar), 137.1 (CH2 ) CH),
134.1 (C3, Ar), 125.7 (C2, Ar), 113.8 (CH2dCH), -2.13 ((CH3)3-
Si(CH3)2Si), -3.88 ((CH3)3Si(CH3)2Si); IR (neat, cm-1) 781
(Si-C stretch), 802 (Si-C stretch), 1246 (Si-CH3 deformation),
2952 (Si-C stretch).
1
15 kg/mol and closed to that predicted (Mn ) 13 kg/mol). H
NMR (CDCl3): δ 7.3-6.2 (m, 4H, HAr), 2.0-1.0 (m, 3H, CH2-
CH), 0.29 (s, 6H, Si(CH3)2), 0.01 (s, 9H, Si(CH3)3). Similarly,
the anionic polymerizations of 1-3 were carried out in either
THF at -78 °C or in benzene at 40 °C.
Block Cop olym er iza tion . The synthesis of an AB diblock
copolymer of 1 and styrene was carried out by the sequential
addition of 1 followed by styrene in a manner similar to the
homopolymerization. The first stage polymerization was car-
ried out by mixing a THF (0.808 M, 8.25 mL) solution of 1
(1.56 g, 6.67 mmol) with oligo(R-methylstyryl)lithium (0.181
mmol) in THF (6.25 mL) at -78 °C. The reaction mixture was
allowed to stir for an additional 0.5 h at -78 °C. A small
portion (4.35 mL) was sampled to determine the molecular
weight and molecular weight distribution. The second block
was prepared by adding a THF (0.938 M, 19.5 mL) solution of
styrene (1.90 g, 18.3 mmol) to the living polymer solution
produced in the first-stage polymerization. After 0.5 h, the
reaction mixture was quenched with degassed methanol, and
the polymer (2.95 g, 99% yield) was precipitated in methanol.
It was purified by reprecipitation twice and freeze-dried. SEC
profiles of the polymers obtained at the first- and second-stage
polymerizations showed sharp monomodal distributions. Their
Mn and Mw/Mn values were 8.6 and 19 kg/mol and 1.05 and
1.09, respectively. The observed composition of [1] to [styrene]
p-Hep ta m eth ytr isilylstyr en e (2). The title styrene, 2, was
synthesized by the reaction of heptamethyltrisilyl chloride
(20.1 g, 90 mmol) with the Grignard reagent of p-chlorostyrene,
prepared from p-chlorostyrene (13.2 g, 95 mmol) and magne-
sium (2.71 g, 100 mmol), similar to the synthetic procedure
employed for 1. Styrene, 2, was isolated by column chroma-
tography on silica gel using hexanes as eluents and dried by
fractional distillation from CaH2 under reduced pressure at
96 - 101 °C (0.16 Torr) to yield 2 (4.99 g, 17 mmol, 19%) as a

Macromolecules, Vol. 36, No. 14, 2003
Anionic Polymerization of Styrenes 5083
Ta ble 1. An ion ic P olym er iza tion of 1 in THF a t -78 °C
for 0.5 h a
Mn × 10-3
[1]0/
b
initiator
[initiator]0 calcd
SECb VPO Mw/Mn
sec-BuLi/RMSc
sec-BuLi/RMS
sec-BuLi/RMS
sec-BuLi
64.1
124
323
34.2
39.4
28.0
32.8
36.0
22.0
24.2
66.0
15
31
76
12
29
60
16
38
81
1.03
1.03
1.03
1.04
1.06
1.11
1.13
1.08
1.03
1.05
1.05
8.3
6.1
8.2
Li-Napd
14
14
17
12
11
13
33
12
10
15
7.5
7.6
12
27
16
13
19
10
10
15
36
Na-Nape
Na-Nap/RMS
Cumyl-Kf
K-Napg
K-Nap
K-Kap/RMS
a
b
Yields of polymers were quantitative in all cases. Both Mn
and Mw/ Mn values were estimated by SEC using a standard
polystyrene calibration curve. c R-Methylstyrene. Lithium naph-
d
thalenide. e Sodium naphthalenide. f Cumylpotassium. Potassi-
g
um naphthalenide.
Ta ble 2. Block Cop olym er iza tion of Eith er 1, 2, or 3 a n d
Styr en e w ith Oligo(r-m eth ylstyr yl)lith iu m in THF
a t -78 °Ca
F igu r e 1. 1H NMR spectra of 1 (A) and the resulting polymer
(B).
Mn × 10-3
first
second
[M1]0/ [M2]0/
[I]0
1
obsdb Mw/Mn
by H NMR was 26/74 close to the calculated value of 20/80.
monomer monomer
[I]0
68.3 6.8-16 6.4-13
8.6-15 8.6-19
calcd
The anionic block copolymerization of 1, 2, or 3 with styrene
was carried out in a similar way.
styrene
1
65.4
36.7 144
80.5
1.06
1.09
1.06
1.05
1.09
1
styrene
2
styrene
68.3 8.3-20 8.8-19
Resu lts a n d Discu ssion
2
3
styrene
styrene
24.0 192
14.0 115
7.0-20 6.2-19
4.9-12 5.2-12
An ion ic P olym er iza tion of p-P en ta m eth yld isi-
lylstyr en e (1). The anionic polymerization of 1 was first
carried out in THF at -78 °C for 0.5 h with various
anionic initiators such as sec-BuLi, cumylpotassium,
and lithium, sodium, and potassium naphthalenides. In
addition, living oligomers of R-methylstyrene prepared
from sec-BuLi, lithium, sodium, or potassium naphtha-
lenides, and R-methylstyrene were used. There was an
instantaneous change in the color of the solution to
either orange red (in the case of Li+ as a countercation)
or dark red (in the cases of Na+ and K+ as countercat-
ions) where the monomer solution was added to each of
the initiators. The intense colors remained unchanged
at -78 °C even after 24 h, but disappeared immediately
by quenching with degassed methanol. Yields of poly-
a
b
Yields of polymers were quantitative in all cases. Molecular
weights of block copolymers were determined by Mn values of
homopolymers by VPO and compositions of block copolymers
determined by 1H NMR. Compositions calculated and observed of
block copolymers (first/second) are as follows: 49/51 and 52/48,
20/80 and 26/74, 54/46 and 59/41, 11/89 and 11/89, and 11/89 and
12/88. c Mw/ Mn values were estimated by SEC using polystyrene
calibration curve.
of polymers (quantitative in all cases), although molec-
ular weight distributions of the polymers obtained with
the initiators with Na+ were relatively broader than
those of the polymers obtained with initiators with Li+
and K+. These results and the characteristic coloration
in the polymerization systems strongly indicate that the
anionic polymerization of 1 is living. Thus, the Si-Si
bond was sufficiently stable toward the highly reactive
anionic initiators and the growing chain-end anions
derived from 1 in THF at -78 °C.
The block copolymerization of 1 and styrene was
carried out in THF at -78 °C with oligo(R-methylstyryl)-
lithium by a two-step sequential monomer addition, 1
followed by styrene or vice versa. The first and second
polymerization times were 0.5 and 0.5 h, respectively.
Yields of polymers were quantitative in all cases. The
results are summarized in Table 2.
1
mers were quantitative in all cases. Figure 1 shows H
NMR spectra of 1 and the resulting polymer. A s can
be seen, the resonances corresponding to the vinyl
protons of 1 disappear completely. The results are
summarized in Table 1. Like most styrene monomers,
no polymerization occurred with Grignard reagents such
as benzylmagnesium chloride and dibutylmagnesium,
LiAlH4, and potassium tert-butoxide.
As previously reported,1 both sec-BuLi and oligo(R-
methylstyryl)lithium were effective initiators to quan-
titatively afford the polymers with predictable molecular
weights and narrow molecular weight distributions;
their Mw/Mn values were less than 1.04. Although the
estimated Mn values by SEC relative to polystyrene
were always somewhat smaller than those calculated
from [monomer] to [initiator] ratios, the observed Mn
values by VPO agreed well with those calculated. As
can be seen in Table 1, sodium and potassium naph-
thalenides, cumylpotassium, and living oligomers of
R-methylstyrene sodium and potassium salts are also
effective initiators, giving the polymers with Mn values
close to the calculated ones and narrow molecular
weight distributions in 100% yield. Differences among
counterions of such initiators were not observed in yields
Figure 2 shows SEC chromatograms of the resulting
polymer and its prepolymer of 1. The SEC peak moves
toward higher molecular weight after the addition of
styrene, whereas the molecular weight distribution
remains narrow. The peak corresponding to the pre-
polymer of 1 completely disappeared. All of the analyti-
1
cal results by VPO and H NMR indicate that the block
copolymerization proceeds to afford the expected AB
diblock copolymer of poly(1)-block-polystyrene. The suc-
cess of the block copolymerization further confirms the
living character of the polymerization of 1. Similarly, a
well-defined BA diblock copolymer, polystyrene-block-
poly(1), could be successfully synthesized by reversing

5084 Hirao et al.
Macromolecules, Vol. 36, No. 14, 2003
F igu r e 3. 1H NMR spectra of 2 (A) and the resulting polymer
(B).
F igu r e 2. SEC profiles of poly(1)-block-polystyrene (A) and
poly(1) (B) at the first-stage polymerization: peak A, Mn ) 19
kg/mol, Mw/Mn ) 1.06; peak B, Mn ) 5.0 kg/mol, Mw/Mn ) 1.04.
Ta ble 4. An ion ic P olym er iza tion of 2 in THF a t -78 °C
for 0.5 h a
Ta ble 3. An ion ic P olym er iza tion of 1 w ith sec-Bu Li in
Ben zen e a t 40 °C for 1 h a
Mn × 10-3
[1]0/
SECb VPO Mw/Mn
b
initiator
sec-BuLi
[initiator]0 calcd
Mn × 10-3
[1]0/
[sec-BuLi]0
29.8
23.6
41.1
89.0
8.7
7.6
12
7.3
5.5
9.0
21
39
10
8.7
11
7.9
12
22
10
8.3
1.03
1.08
1.04
1.03
1.04
1.11
1.10
1.05
1.03
1.05
1.04
b
calcd
SECb
VPOc
Mw/Mn
sec-BuLi/RMSc
sec-BuLi/RMS
sec-BuLi/RMS
sec-BuLi/RMS
Na-Napd
45.9
76.7
120
11
14
29
12
22
30
16
29
40
1.06
1.06
1.08
12
28
59
12
12
15
12
16
29
26
52
12
11
15
10
18
27
178
20.0
16.5
22.6
17.8
30.0
46.1
a
b
Yields of polymers were quantitative in all cases. Both Mn
and Mw/Mn values were estimated by SEC using a standard
polystyrene calibration curve. c In benzene at 40 °C.
Na-Nap/RMS
K-Nape
K-Nap/RMS
K-Nap/RMS
K-Kap/RMS
the sequence of monomer addition, namely, styrene
followed by 1. Both results indicate that the electrophi-
licities of styrene and 1 and the nucleophilicities of their
living anionic polymers are very similar. This is also
supported by the fact that the value (113.8 ppm) of the
vinyl â-carbon of 1 is the same as that of styrene (113.8
ppm) in 13C NMR chemical shift.
Next, the polymerization of 1 was conducted with sec-
BuLi in benzene at 40 °C for 1 h. Upon addition of 1 to
sec-BuLi, an immediate color change from almost color-
less to pale yellowish orange occurred and the color
remained unchanged for 1 h. Three polymerizations
were carried out at different ratios of monomer to
initiator. The polymer yields were quantitative in all
cases. The results are listed in Table 3.
The resulting polymers showed sharp symmetrical
monomodal SEC peaks without shoulders and tailings.
However, the observed Mn values by VPO were always
higher than those calculated. This is caused by the
partial deactivation of sec-BuLi prior to the polymeri-
zation, possibly due to the Si-Si bond attack by sec-
BuLi. The similar Si-Si bond attack by organolithium
compounds was reported by several research groups.3,7
However, the propagation step proceeds without prob-
lem, since the molecular weight distributions of the
resulting polymers are narrow.
The block copolymerization was carried out under
similar conditions by a two-step sequential addition of
1 followed by styrene. The resulting polymer showed a
sharp monomodal SEC distribution (Mw/Mn ) 1.06). No
SEC peak corresponding to the prepolymer of 1 was
observed. The observed Mn value of the polymer was
higher than that calculated (Mn calcd ) 33 kg/mol vs. Mn
a
b
Yields of polymers were quantitative in all cases. Both Mn
and Mw/Mn values were estimated by SEC using a standard
polystyrene calibration curve. c R-Methylstyrene. Sodium naph-
d
thalenide. e Potassium naphthalenide.
obsd ) 43 kg/mol). Accordingly, sec-BuLi used as an initiator was
partly deactivated, but the growing chain-end anion derived from
1 could initiate quantitatively the polymerization of styrene in
benzene at 40 °C.
An ion ic P olym er iza tion of p-Hep ta m eth yltr isi-
lylstyr en e (2). For the anionic polymerization of 2 in
THF at -78 °C for 0.5 h, sec-BuLi, sodium and potas-
sium naphthalenides, living oligomers of R-methylsty-
rene prepared from sec-BuLi, sodium and potassium
naphthalenides and R-methylstyrene were used as
initiators. Upon addition of 2 to each initiator, the
intense red color appeared and remained unchanged at
-78 °C after 24 h. The polymer yields were quantitative
1
in all cases. Figure 3 shows H NMR spectra of 2 and
the resulting polymer. The resonances assigned to the
vinyl protons of 2 disappeared completely in the poly-
mer. The results are summarized in Table 4.
All initiators were effective, and the resulting poly-
mers showed sharp symmetrical monomodal SEC dis-
tributions. The Mn values determined by VPO agreed
well with those calculated from [monomer] to [initiator]
ratios in all cases. Similar to the polymerization of 1,
the polymers obtained with the initiators with Na+ were
always slightly broad in molecular weight distribution.
Both AB and BA diblock copolymers with well-defined
structures were successfully synthesized by the sequen-
tial addition of 2 followed by styrene and vice versa in

Macromolecules, Vol. 36, No. 14, 2003
Anionic Polymerization of Styrenes 5085
Ta ble 5. An ion ic P olym er iza tion of 2 w ith sec-Bu Li in
Ben zen e a t 40 °C for 1 h a
Mn x 10-3
[1]0/
[sec-BuLi]0
b
calcd
SECb
VPOc
Mw/Mn
19.5
28.1
50.0
5.7
8.2
15
7.8
14
22
13
21
30
1.10d
1.26d
∼1.5e
a
b
Yields of polymers were quantitative in all cases. Both Mn
and Mw/ Mn values were estimated by SEC using standard
polystyrene calibration curve. c In benzene at 40 °C. Bimodal
d
distribution. e Multimodal distribution.
THF at -78 °C with oligo(R-methylstyryl)lithium. The
results are also listed in Table 2. The SEC, VPO, and
1H NMR analyses confirmed their expected structures.
The results of both homopolymerization and block
copolymerization indicate that the anionic polymeriza-
tion of 2 could proceed by a living mechanism. Thus,
the Si-Si-Si bond of 2 was also sufficiently stable and
did not interfere with the anionic polymerization of 2
under the conditions in THF at -78 °C.
F igu r e 4. SEC profile of poly(3) obtained at [M]/[I] of 60.4 in
THF at -78 °C for 0.5 h with cumylpotassium capped with
DPE: Mn ) 27 kg/mol; Mw/Mn ) 1.24.
on the Si-Si cleavage of disilanes and polysilanes with
anionic species previously reported.3,6,7 Therefore, the
Si-Si-Si bond is not stable toward sec-BuLi and/or the
growing chain-anion derived from 2 in benzene at 40
°C.
However, the polymerization of 2 with sec-BuLi in
benzene at 40 °C was problematic. The polymerization
mixture exhibited a pale yellowish orange color, which
was similar to the color observed in the polymerization
of 1 under the same conditions. Three polymerizations
were carried out at different ratios of monomer to
initiator. The yields of polymers were quantitative in
all cases. The results are listed in Table 5.
The polymer obtained at the ratio of [M] to [I] of 20
showed a sharp monomodal SEC peak with a high
molecular weight shoulder whose Mn value was doubly
increased (ca. 10% relative to the main sharp peak). The
extent of the high molecular weight shoulder became
more significant (ca. 20%) at the ratio of 28. Moreover,
several high molecular weight peaks were observed in
the polymer obtained at the ratio of 50 as shown in
Figure 4. In every case, the Mn value of the polymer
determined by VPO was always higher than that
calculated. This is possibly due to initiator loss by the
reaction of sec-BuLi with the Si-Si-Si bond at the
initiation step, similar to the polymerization of 1 as
mentioned above. Furthermore, in the polymerization
of 2, the growing chain-end anion may attack on the
Si-Si-Si bond of another polymer side chain during
and after the polymerization, resulting in the formation
of high molecular weight polymer as shown in Scheme
1. Although we have not any direct evidence for this,
the mechanism shown above seems reasonable based
The formation of high molecular weight polymer could
not be suppressed even at the early stage of the
polymerization of 3 ([M]/[I] ) 50, conversion ) 20%),
indicating that the side reaction occurs competitively
at the early stage of the polymerization. Moreover, the
formation was not suppressed in the polymerization of
2 carried out in benzene at 10 °C.
An ion ic P olym er iza tion of p-Non a m eth yltetr a -
silylstyr en e (3). In contrast to the successful polymer-
izations of 1 and 2, 3 did not polymerize with sec-BuLi
or oligo(R-methylstyryl)lithium in THF at -78 °C. The
characteristic red color of oligo(R-methylstyryl)lithium
disappeared immediately on mixing with 3, and no
polymer was obtained. Although the polymerization of
3 proceeded to a small extent with oligo(styryl)lithium,
the SEC of the resulting product exhibited a multimodal
distribution. The 1H NMR spectrum of the product
corresponded to a few units of 3 attached to the
oligostyrene. Obviously, the anionic initiators used and
the growing chain-end anion, even if produced, reacted
with the Si-Si-Si-Si bond of 3 at either the initial or
early stage of the polymerization under such conditions.
Monomer, 3, polymerized using oligo(R-methylstyryl)-
potassium as an initiator in THF at -78 °C. The
polymers were obtained quantitatively after 0.5 h at the
ratios of [3] to [initiator] in the range of 11-60. The
results are summarized in Table 6. Polymers possessing
Mn values of less than 10 kg/mol showed sharp mono-
Sch em e 1

5086 Hirao et al.
Macromolecules, Vol. 36, No. 14, 2003
Ta ble 6. An ion ic P olym er iza tion of 3 in THF a t -78 °C
as well as carbanion chemistry, since it had been
believed that these bonds are readily cleaved by reactive
anionic species. Thus, the range of functionalized sty-
renes amenable to living anionic polymerization is
broadened. However, the anionic polymerization of 3
had only a limited success, since the Si-Si-Si-Si bond
was not completely stable even in THF at -78 °C.
The difference between 1, 2, and 3 is thus critical in
the anionic polymerization behavior, although the rea-
son has not been understood at the present time. We
tentatively consider that the number of Si-Si linkages
(one, two, and three for 1, 2, and 3) and stability of the
leaving group produced by the attack of anionic species
for 0.5 h a
Mn x 10-3
[1]0/
b
initiator
sec-BuLi
[initiator]0 calcd SECb VPO Mw/Mn
15.0
16.7
20.1
40.0
11.0
26.0
34.1
42.5
60.4
5.3
5.8
7.0
14
4.9
9.1
12
15
22
sec-BuLi/RMSc
sec-BuLi/RMS
sec-BuLi/RMS
Cumyl-Kd/RMS
Cumyl-K/RMS
Cumyl-K/RMS
Cumyl-K/RMS
Cumyl-K/RMS
5.2
9.3
10
14
15
5.8
10
14
18
27
1.04
1.10
1.18e
1.19e
1.24e
a
-
such as Si -, Si-Si -, or Si-Si-Si may possibly be
Yields of polymers were quantitative with oligo(R-methyl-
b
styryl)potassium (Cumyl-K/RMS). Both Mn and Mw/ Mn values
key factors for occurring the side reactions leading to
high molecular weight polymer formation and loss of
initiator.
were estimated by SEC using standard polystyrene calibration
curve. c R-Methylstyrene. Cumylpotassium. e Multimodal ditri-
d
bution.
Refer en ces a n d Notes
modal SEC distributions, Mw/Mn values being less than
1.1. Their Mn values by VPO were close to those
calculated. However, additional high molecular weight
polymers were always formed when the molecular
weight of the polymer exceeded over ca. 12 kg/mol. The
extent of the formation became more significant with
increasing molecular weight and the shoulder tended
to be multimodal. Similar to the case in the polymeri-
zation of 2 in benzene as mentioned above, the forma-
tion of the high molecular weight polymers may be due
to the growing chain-end anion attack on the Si-Si-
Si-Si bond of another polymer chain probably after the
conclusion of polymerization. In the case where the Mn
value of poly(3) segment is limited to less than 10 kg/
mol, it is feasible to synthesize a well-defined diblock
copolymer, poly(3)-block-polystyrene, by the sequence
of monomer addition, 3 followed by styrene. The result-
ing polymer exhibited a sharp monomodal SEC distri-
bution (see Table 2). As expected, the formation of high
molecular weight polymers was always observed in the
block copolymers whose poly(3) segments were 12 kg/
mol or more in molecular weight.
In practice, the polymerization of 3 could be controlled
under limited conditions with use of the initiators
possessing K+. On the other hand, no polymerization
occurred with the initiators with Li+. The effect of
countercation is critical in the polymerization of 3.
However, the Si-Si-Si-Si bond was not stable, and the
above-mentioned side reaction could not be completely
suppressed even in THF at -78 °C. Thus, there is a
crucial difference between Si-Si-Si and Si-Si-Si-Si
bonds in stability under the condition of living anionic
polymerization.
(1) Part of this work was presented at the 208th National
Meeting of the American Chemical Society, Division of
Polymer Chemistry, Special Symposium on “Synthesis of
Controlled Polymeric Structures through Living Polymeri-
zation and Related Processes”, Washington DC, August 21-
25, 1994 and reported in: Hirao, A.; Ando, Y.; Nakahama,
S. Macromol. Symp. 1995, 95, 293-301.
(2) (a) Ishikawa, M. Pure Appl. Chem. 1978, 50, 11-18. (b)
Sakurai, H. J . Organomet. Chem. 1980, 200, 261-286. (c)
Barton, D.; Ollis, D. Comprehensive Organic Chemistry. The
Synthesis and Reactions of Organic Compounds. Sulphur,
Selenium, Silicon, Boron, Organometallic Compounds; Flem-
ing, I., Ed; Pergamon Press: Oxford, England, 1979; Vol 3,
pp541-686. (d) West, R. In Comprehensive Organometallics
Chemistry; Stone, F. G. A., Abel. E. W., Eds; Pergamon
Press: Oxford, 1983; Vol 9, pp365-397. (e) Brook, A. G. In
the Chemistry of Organic Silicon Compounds; Patai, S.,
Rappoport, Z., Eds; Wiley: New York, 1989; Chapter 15. (f)
Miller, R. D.; Michl, J . Chem. Rev. 1989, 89, 1359-1410. (g)
Zeigler, J . M.; Fearon, F. W. G. Silicon-Based Polymer
Science; Advances in Chemistry Series 224; American Chemi-
cal Society: Washington, DC, 1990. (h) Kira, M.; Miyazawa,
T.; Sugiyama, H.; Yamaguchi, M.; Sakura, H. J . Am. Chem.
Soc. 1993, 115, 3116-3124.
(3) (a) Wittenberg, D.; Gilman, H. Q. Rev. 1959, 13, 116-145.
(b) Davis, D. D.; Gray, C. E. Organomet. Chem. Rev. (A) 1970,
6, 283-318.
(4) (a) Sakurai, H.; Imoto, T.; Hayashi, N.; Kumada, M. J . Am.
Chem. Soc. 1965, 87, 4001-4010. (b) Tamao, K.; Kumada,
M.; Ishikawa, M. J . Organomet. Chem. 1971, 31, 17-34. (a)
Dixon, T. A.; Steele, K. P.; Weber, W. P. J . Organomet. Chem.
1982, 231, 299-305. (b) Trefonas, P., III; West, R. J . Polym.
Sci., Polym. Lett. Ed. 1985, 23, 469-473.
(5) Saigo, K.; Watanabe, F. J . Polym. Sci., Part A: Polym. Chem.
1989, 27, 2611-2624.
(6) (a) Gilman, H.; Wu, T. C. J . Am. Chem. Soc. 1951, 73, 4031-
4033. (b) Gilman, H.; Wu, T. C. J . Org. Chem. 1953, 18, 753-
764. (c) Gilman, H.; Ingham, R. K.; Smith, A. G. J . Org. Chem.
1953, 18, 1743-1749. (d) Gilman, H.; Brook, A. G. J . Am.
Chem. Soc. 1954, 76, 278-279 (e) Gilman, H.; Lichtenwalter,
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1511-1514.
Con clu sion s
We have investigated the anionic polymerizations of
three para-substituted styrene derivatives possessing
Si-Si, Si-Si-Si, and Si-Si-Si-Si bonds, 1-3, under
various conditions. Both 1 and 2 were successful in
undergoing living anionic polymerization in THF at -78
°C. Well-defined diblock copolymers, poly(1)-block-
polystyrene, poly(2)-block-polystyrene, polystyrene-block-
poly(1), and polystyrene-block-poly(2) were synthesized
under similar conditions by a two-step sequential
monomer addition, namely, styrene followed by 1 or 2
or vice versa. The finding that the Si-Si and Si-Si-Si
bonds are sufficiently stable under the conditions of
living anionic polymerization is of particular interest
and significance in the fields of anionic polymerization
(7) (a) Gilman, H.; Smith, C. L. J . Organomet. Chem. 1966, 6,
665-668. (b) Gilman, H.; Smith, C. L. J . Organomet. Chem.
1967, 8, 245-253. (c) Gilman, H.; Smith, C. L. J . Organomet.
Chem. 1968, 14, 91-107. (d) Sakurai, H.; Kondo, F. J .
Organomet. Chem. 1975, 92, C46-C48. (e) Mehrotra, S. K.;
Kawa, H.; Baran, J . R., J r.; Ludvig, M. M.; Lagow, R. J . J .
Am. Chem. Soc. 1990, 112, 9003-9004.
(8) Kim, H. K.; Matjaszewski, K. J . Polym. Sci., Part A: Polym.
Chem. 1993, 31, 299-307.
(9) Suzuki, M.; Kotani, J .; Gyobu, S.; Kaneko, T.; Saegusa, T.
Macromolecules 1994, 27, 2360-2363.
(10) For reviews: (a) Hirao, A.; Nakahama, S. Prog. Polym. Sci.
1992, 17, 283-317. (b) Hirao, A.; Loykulnant, S.; Ishizone,
T. Prog. Polym. Sci. 2002, 27, 1399-1471.
(11) Ishikawa, M.; Kumada, M.; Sakurai, H. J . Organomet. Chem.
1970, 23, 63-69.

Macromolecules, Vol. 36, No. 14, 2003
Anionic Polymerization of Styrenes 5087
(12) We carried out the model reaction of sec-BuLi with 1,1,2,2,2-
tetramethyldisilylbenzene in benzene at 40 °C for 1 h in order
to examine and minimize the effect the side reaction that
seemed to be occurring at the early stage of the polymeriza-
tion of 1. Unexpectedly, no reaction occurred at all, and
1,1,2,2,2-tetramethyldisilylbenzene was quantitatively re-
covered under such conditions. No direct evidence for the
cleavage of Si-Si bond of 1 by sec-BuLi was obtained by this
reaction. However, we still consider that the cleavage of 1
by sec-BuLi seems to be occurring, since the Si-Si bond of 1
is more reactive than that of 1,1,2,2,2-tetramethyldisilylben-
zene. In the polymerization of 1 with sec-BuLi at 1:1 ratio,
only oligomers of 1 were produced. The product by the
cleavage of 1 by sec-BuLi was not obtained.
MA021742A