Synthesis and characterization of silicon-containing block copolymers from nitroxide-mediated living free radical polymerization

Article abstract of DOI:10.1021/ma049217u

High etch resistance to oxygen plasma for silicon-containing polymers, and the high thermal and mechanical robustness of the etching product, silicon oxide, make it attractive to design novel silicon-containing block copolymers for direct patterning of na

Full text of DOI:10.1021/ma049217u

Macromolecules 2005, 38, 263-270
263
Synthesis and Characterization of Silicon-Containing Block Copolymers
from Nitroxide-Mediated Living Free Radical Polymerization
Ken-ichi Fukukawa,† Lei Zhu,‡ Padma Gopalan,§ Mitsuru Ueda,† and Shu Yang*,
Department of Organic & Polymeric Materials, Graduate School of Science and Engineering, Tokyo
Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan; Polymer Program,
Institute of Materials Science and Department of Chemical Engineering, The University of Connecticut,
Storrs, Connecticut 06269-3136; Department of Materials Science and Engineering,
University of WisconsinsMadison, 1117 Engineering Research Building, 1500 Engineering Drive,
Madison, Wisconsin 53706; and Department of Materials Science and Engineering,
University of Pennsylvania, Philadelphia, Pennsylvania 19104
Received April 22, 2004; Revised Manuscript Received October 14, 2004
ABSTRACT: High etch resistance to oxygen plasma for silicon-containing polymers, and the high thermal
and mechanical robustness of the etching product, silicon oxide, make it attractive to design novel silicon-
containing block copolymers for direct patterning of nanostructures on a desired substrate. Here, we
report the synthesis of a series of block copolymers from silicon-containing styrenic monomers and styrene
(St) or 4-acetoxystyrene (AcOSt) using living free radical polymerization via a R-hydride nitroxide-mediated
unimer (R-H unimer). Controlled polymerization with narrow polydispersity (PDI < 1.25) and high yield
(up to 80%) were achieved by optimizing polymerization time and temperature, addition of solvents, use
of rate accelerants, monomer addition sequence, and solvent polarity. Block copolymer morphologies before
and after O2 plasma were studied using small-angle X-ray scattering (SAXS) and transmission electron
microscopy (TEM). When silicon-containing block formed the major phase and silicon concentration was
greater than 12 wt %, the morphology and domain size were maintained after O2 plasma.
Introduction
variety of morphologies.10,11 The use of silicon-containing
thin film as a template may provide an alternative
approach to broaden the application of block copolymers
for nanopatterning. Block copolymers of polystyrene and
hydrosiloxane-modified poly(diene) have been demon-
strated as bilayer photoresists that provide high sensi-
tivity and etch selectivity.7,12,13 Thus, the possibility of
combining photolithographic groups and silicon-contain-
ing groups in block copolymers offers a new route to
pattern hierarchical nanostructures.
The spontaneous self-assembly of block copolymers
into nanometer domains is simple, fast, and low cost
compared to conventional lithography. The accessibility
to a wide range of periodic structures with feature size
less than 30 nm makes block copolymers especially
attractive as templates for nanopatterning.1-5 However,
steps such as selective ozonolysis or staining with heavy
metals of a particular block1,2 to increase etch selectivity
between blocks is necessary for pattern transfer of the
nanostructures from the block copolymers. In addition,
an intermediate layer, such as SiN, is always required
for successful pattern transfer to a substrate of interest.
In general, the use of organic block copolymers limited
at high temperatures because of low thermal/mechanical
stabilities. Thus, direct pattering of III-V semiconduc-
tors on block copolymer templates is nearly impossible
due to the high crystal growth temperatures (>600 °C).
In comparison to other block copolymers, silicon-
containing block copolymers may be suitable as a
template for direct patterning of nanostructures without
a complicated multiple-step process. When exposed to
oxygen plasma, the silicon-containing polymers are
oxidized to silicon oxide, whose high thermal and
mechanical stability has made it a long-time dielectric
insulator in microchip fabrication. The high etch resis-
tance to oxygen plasma compared to organic polymers
makes silicon-containing polymers favorable as bilayer
resists to pattern high aspect ratio structures6-9 as well
as to create nanoporous ceramic thin films with a
The recent advances in controlled living free radical
polymerization (LFRP),14 including nitroxide-mediated
radical polymerization (NMRP),15-17 atom transfer radi-
cal polymerization (ATRP),18,19 and reversible addition-
fragmentation chain transfer (RAFT),20-23 make it
possible to design and synthesize a variety of block
copolymers with novel functionalities. Here we report
(i) the synthesis of narrow dispersed silicon-containing
homopolymers from three kinds of silicon-containing
styrenic monomers, including 4-(pentamethyldisilyl)-
styrene (Si2St), 4-(bis(trimethylsilyl)methyl)styrene (Si2-
CSt), and 4-(pentamethyldisiloxymethyl)styrene (OSi2-
St), each containing two silicon atoms to enhance the
etch selectivity, and (ii) the synthesis of block copoly-
mers from silicon-containing styrenic monomers with
styrene and photosensitive acetoxystyene by sequential
monomer addition using an R-hydride nitroxide unimer.
Since many silicon-containing monomers are known to
be temperature sensitive, we kept the reaction at 100
°C to obtain narrow dispersed silicon-containing ho-
mopolymers or block copolymers. To increase the po-
lymerization conversion and shorten the reaction time,
we introduced acetic anhydride as a rate accelerating
agent. The solvent polarity and monomer addition
sequence were found critical in controlling the copolym-
erization. The synthesized block copolymers showed
cylindrical and lamellae structures as revealed by small-
† Tokyo Institute of Technology.
‡ The University of Connecticut.
§ University of WisconsinsMadison.
University of Pennsylvania.
* To whom correspondence should be addressed. E-mail:
shuyang@seas.upenn.edu.
10.1021/ma049217u CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/30/2004

264 Fukukawa et al.
Macromolecules, Vol. 38, No. 2, 2005
Table 1. Homopolymerization Conditions of Silicon-Containing Monomers
polymerization
time (h)
GPCc
Mn × 10-3
entry
monomera
feed ratiob ([M]0/[I]0)
temp (°C)
yield (%)
PDI
NAf
1
Si2St
50
50
60
60
60
60
60
60
60
60
16
16
130
100
100
100
100
100
128
100
100
100
86
47
74
76
44
61
78
57
77
81
gelf
2
Si2St
5.6
1.13
1.15
1.14
1.09
1.07
1.15g
1.08
1.08
1.07
3
Si2St
60
8.7
9.7
5.6
7.8
11.9
7.5
10.4
12.1
4d
5
Si2St
64
Si2CSt
Si2CSt
OSi2St
OSi2St
OSi2St
OSi2St
64
6
128
34
7
8
64
128
64
9
10e
a Abbreviations of monomers: Si2St, 4-(pentamethyldisilyl)styrene; Si2CSt, 4-(bis(trimethylsilyl)methyl)styrene; OSi2St, 4-(pentam-
ethyldisiloxymethyl)styrene. b Feed ratio for the homopolymerization; [M]0/[I]0 ) [monomer]0/[initiator]0 (mol/mol). c The number- and
weight-average molecular weights (Mn and Mw) and polydispersity (PDI ) Mw/Mn) were determined by GPC from refractive index (RI)
detector in a flow rate of 1.0 mL/min in THF at 40 °C. Narrow dispersed polystyrenes were used as standards for universal calibration.
d Dichlorobenzene (DCB) was added as a solvent (10 wt % of monomer). Otherwise, polymerization was carried out in bulk. e Acetic
anhydride (AA) was added (1.5 equiv to initiator) to accelerate the reaction. f GPC was not measured due to gel formation. g GPC trace
indicated a shoulder at higher molecular weight region.
4-(Pentamethyldisiloxymethyl)styrene (OSi2St). The
Grignard reagent was prepared in the way similar to that of
Si2CSt, using magnesium turnings (21.9 g, 900 mmol) in THF
(40 mL) and chloromethylpentadisiloxane (39.3 g, 200 mmol).
It was then added dropwise to the solution of p-bromostyrene
(30.2 g, 165 mmol) and tetrakis(triphenylphosphine)palladium
(1.04 g, 0.900 mmol) in THF (120 mL) over 30 min, followed
by additional reflux for 12 h and decomposition by water. The
product was extracted with ether, dried with magnesium
sulfate, and concentrated in vacuo. The crude product was
purified by column chromatography (eluent: hexane) and
vacuum distillation (bp 52-54 °C at 3.1 × 10-4 Torr). The
overall yield was 18.1 g (41.5%). IR (NaCl, ν, cm-1): 2958 (C-
H, vinyl), 1627 (CdC, vinyl), 1608 (CdC, Ar), 1253 (Si-CH3),
angle X-ray scattering (SAXS) and transmission electron
microscopy (TEM). When silicon-containing block formed
the major phase and silicon concentration was greater
than 18 wt %, the morphology and domain size were
maintained after exposure to O2 plasma for more than
10 min.
Experimental Section
Chemicals. Unless noted, all chemicals were purchased
from Aldrich, Inc. and used as received. Anhydrous tetra-
hydrofuran (THF, 99.9%), 1,2-dichlorobenzene (DCB, 99%),
and N,N-dimethylformamide (DMF, 99.8%) were used in
experiments. N-(1′-Methylbenzyloxy)-2′,2′,6′,6′-tetramethylpi-
peridine (TEMPO unimer) was purchased from Binrad Indus-
tries, Inc., and vacuum-dried before use.
2,2,5-Trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahex-
ane (r-H Unimer). An R-hydride nitroxide-mediated unimo-
lecular initiator (denoted as R-H unimer) was synthesized
following the literature24 with minor modification. The cou-
pling reaction between 2,2,5-trimethyl-3-(1-phenylethoxy)-4-
phenyl-3-azahexane-3-nitroxide (R-H radical) and styrene in
the presence of the Jacobsen’s catalyst [N,N′-bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanediaminemanganese(III) chlo-
ride] was carried out under oxygen bubbling.25
1
1060 (Si-O). H NMR (δ, ppm): 7.27 (d, 2H, Ar-H), 6.99 (d,
2H, Ar-H), 6.67 (dd, 1H, CH2dCH-Ar), 5.67 (d, 1H, CH2d
CH), 5.14 (d, 1H, CH2dCH), 2.11 (s, 2H, ArCH2Si), 0.05 (s,
6H, CH2Si(CH3)2O), 0.04 (s, 9H, OSi(CH3)3). 13C NMR (δ,
ppm): 139.44, 136.88, 133.59, 128.42, 126.03, 111.92, 28.52,
1.84, -0.06.
Synthesis of Silicon-Containing Macroinitiators. Prepa-
ration of Poly(4-(pentamethyldisilyl)styrene) (Poly(Si2St), En-
try 4 in Table 1). As an example, a mixture of R-H unimer
(0.048 g, 0.15 mmol), Si2St (2.11 g, 9.0 mmol), and DCB (0.16
mL, 10 wt % of the total feed) was first degassed by three
freeze/thaw cycles and placed in a preheated oil bath set at
100 °C with stirring. After 64 h, the mixture in the reaction
flask became solidified and could no longer be stirred. Further
reaction did not seem to increase the molecular weight
substantially. Thus, the reaction was quenched with an ice
bath at 64 h. The mixture was dissolved in dichloromethane
and reprecipitated into methanol. The resulting precipitate
was filtered off and dried under vacuum at room temperature,
resulting in desired macroinitiator, poly(Si2St), as white solid
1.62 g (yield of 76.4%); Mn ) 9700 and PDI ) 1.14.
4-(Pentamethyl disilyl)styrene (Si2St). Si2St was syn-
thesized according to the literature.26 The purification was
carried out by column chromatography (eluent: hexane) and
vacuum distillation (bp 51-52 °C at 2.5 × 10-4 Torr). The
overall yield was 51.8%.
4-(Bis(trimethylsilyl)methyl)styrene (Si2CSt). Bis(tri-
methylsilyl)chloromethane (24.9 g, 128 mmol) was added to
activated magnesium tunings (9.72 g, 400 mmol) in THF (25.0
mL) with good agitation under reflux conditions in a nitrogen
atmosphere. After the completion of addition, the reaction
mixture was refluxed for 2 h and transferred via cannula into
an additional funnel attached to a second round-bottom flask.
The Grignard reagent was added dropwise over 20 min into
the solution of p-bromostyrene (21.0 g, 115 mmol) and tetrakis-
(triphenylphosphine)palladium27 (0.786 g, 0.680 mmol) as a
catalyst in THF (100 mL). After the addition, the reaction
mixture was refluxed for 12 h and decomposed with water.
The product was extracted with ether, dried with magnesium
sulfate, and concentrated in vacuo. The crude product was
purified by column chromatography (eluent: hexane) and
vacuum distillation (bp 64-65 °C at 3.6 × 10-4 Torr). The
overall yield was 20.0 g (66.3%). IR (NaCl, ν, cm-1): 2954 (C-
H, vinyl), 1627 (CdC, vinyl), 1604 (CdC, Ar), 1249 (Si-CH3).
1H NMR (δ, ppm): 7.23 (d, 2H, Ar-H), 6.87 (d, 2H, Ar-H),
6.65 (dd, 1H, CH2dCH-Ar), 5.64 (d, 1H, CH2dCH), 5.11 (d,
1H, CH2dCH), 1.50 (s, 1H, ArCHSi2), 0.01 (s, 18H, Si(CH3)3).
13C NMR (δ, ppm): 143.15, 136.86, 132.80, 128.79, 125.99,
111.57, 29.66, 0.11.
Preparation of Poly(4-(bis(trimethylsilyl)methyl)sty-
rene) (Poly(Si2CSt), Entry 6 in Table 1). The procedure
was similar to that of poly(Si2St) with yield of 60.9%, Mn
)
1
7800, and PDI ) 1.07. H NMR (δ, ppm): 6.53-6.26 (br, m,
4H), 1.79 (br, s, 1H), 1.31 (br, s, 3H), -0.01 (br, s, 18H). 13C
NMR (δ, ppm): 139.10, 127.89, 126.98, 39.82, 28.62, 0.30.
Preparation of Poly(4-(pentamethyl disiloxymethyl)-
styrene) (Poly(OSi2St), Entry 9 in Table 1). The procedure
was similar to that of poly(Si2St) with yield of 76.8%, Mn
)
10 400, and PDI ) 1.08. 1H NMR (δ, ppm): 6.71-6.27 (br, m,
4H), 2.01 (br, s, 2H), 1.79 (br, s, 1H), 1.34 (br, s, 2H), 0.03 (br,
s, 15H). 13C NMR (δ, ppm): 135.81, 127.64, 125.58, 40.42,
28.01, 1.94, -0.05.
Synthesis of Silicon-Containing Block Copolymers.
Preparation of Poly{styrene-b-[4-(pentamethyldisilyl)styrene]}
(Poly(St-b-Si2St), Entry 11 in Table 4). Polystyrene (PSt)
macroinitiator was synthesized in the way similar to that of
poly(Si2St). 0.340 g of PSt (0.097 mmol; Mn ) 3500, PDI )

Macromolecules, Vol. 38, No. 2, 2005
Silicon-Containing Block Copolymers 265
1.17) was first dissolved in DMF (0.52 g, 20 wt % of total
reactants) and Si2St (2.27 g, 9.71 mmol; ca. 3 equiv to PSt)
was added as second monomer with stirring under nitrogen
until a clear solution was formed. The mixture was degassed
by three freeze/thaw cycles before immersing into preheated
oil bath at 100 °C. After stirring for 64 h, the reaction was
quenched by an ice bath to stop the polymerization. The
mixture was dissolved in dichloromethane and reprecipated
into methanol. The precipitate was filtered off and dried under
the vacuum at room temperature to yield 1.89 g (72.6%) of
block copolymer, Mn ) 18 700 and PDI ) 1.07.
Scheme 1. Syntheses of
4-(Bis(trimethylsilyl)methyl)styrene (Si2CSt) and
4-(Pentamethyldisiloxymethyl)styrene (OSi2St)
Polymer Characterizations. The infrared (IR) measure-
ments were made using a Horiba FT-210 spectrophotometer.
Chemical compositions of the block copolymers and their
degree of polymerization (DPn) were calculated from 1H NMR
(360 MHz) and 13C NMR (90 MHz) spectra using a Bruker AM-
360 MHz NMR spectrometer. Deuterated chloroform (CDCl3)
was used as a solvent at room temperature. The number- and
weight-average molecular weights (Mn and Mw) and polydis-
persity (PDI ) Mw/Mn) were determined by gel permeation
chromatography (GPC) using a Viscotek TDA-300 with three
columns (Waters Styragel HR-3 (×2) and Viscotek GMH-H
(×1), molecular weight range of 30-500K and >50 M, respec-
tively, RI detector) at a flow rate of 1.0 mL/min in THF at 40
°C. Narrow dispersed polystyrenes were used as standards for
universal calibration. The glass-transition temperatures (Tgs)
were studied by a Perkin-Elmer DSC-7 at a heating rate of
10 °C/min under nitrogen and calibrated by indium. The
midpoint of the change in heat capacity during the second heat
was reported as Tg in our experiments. Monomer density was
estimated by measuring average weight in 1 mL at 25 °C.
Small-Angle X-ray Scattering (SAXS). A 2D SAXS
experiment was performed at synchrotron X-ray beamline
×3A2 in the National Synchrotron Light Source, Brookhaven
National Laboratory. The wavelength of the X-ray beam was
0.154 nm. The beam center was calibrated using silver
be achieved when 10 wt % or more of silicon is
introduced.26 Thus, three kinds of silicon-containing
styrenic monomers with each containing two silicon
atoms were selected and synthesized (Scheme 1). Si2St
was synthesized following the literature procedure. Both
Si2CSt and OSi2St were prepared by a cross-coupling
reaction in the presence of palladium catalyst27 with
p-bromostyrene and the corresponding Grignard re-
agents, which were prepared from silicon-containing
methyl chlorides. Although syntheses of Si2CSt has been
studied previously,28 the cross-coupling reaction we
report here is more straightforward, and it eliminates
possible side reaction, which only has one trimethylsilyl
group substituted on a benzene ring, resulting in
4-(trimethylsilylmethyl)styrene. The silicon contents of
Si2St, Si2CSt, and OSi2St are 24.0, 21.4, and 21.2 wt %,
respectively (Table 3). The structures of synthesized
monomers were characterized by 1H NMR spectroscopy.
As shown in Figures 1a and 2a, characteristic peaks
assigned to the CHdCH bond (5.1-6.7 ppm), CH2 (1.50
ppm), and C((CH3)2Si)2 (∼0 ppm) in Si2CSt and the
CHdCH bond (5.1-6.7 ppm), CH2 (2.11 ppm), and
(CH3)2Si-O-Si (CH3)2 (∼0 ppm) for OSi2St, respec-
tively, indicating formation of the desired monomers.
Homopolymerization of Silicon-Containing Sty-
renic Monomers. Controlling polymerization is crucial
to achieve well-defined morphologies with sharp phase
boundary between blocks. We first investigated homo-
polymerization conditions for Si2St, Si2CSt, and OSi2St
to obtain narrow dispersed silicon-containing macroini-
tiator (see Scheme 2 and Table 1). To create nanostruc-
tures with feature size less than 20 nm for future
nanopatterning of quantum dots, we kept the feed ratio
of monomer to initiator (mol/mol) no greater than 60 so
that the final molecular weight was in a range of
10 000-20 000.
behenate with the primary reflection peak at 1.076 nm-1
.
Typical data acquisition time was 1 min. 2D images were
obtained using Fuji imaging plates equipped with a Fuji BAS-
2500 scanner. One-dimensional (1D) SAXS curves were ob-
tained by integration of the corresponding 2D SAXS patterns.
Transmission Electron Microscopy (TEM). TEM ex-
periments were performed on a Philips EM300 at an ac-
celerating voltage of 80 kV. Thin sections with a thickness of
∼75 nm were obtained with a diamond knife at room temper-
ature on a Leica Ultracut UCT microtome. The thin sections,
floating on a water boat, were then collected onto 400 mesh
TEM grids and dried at room temperature for 3 days. Bright-
field TEM images were directly obtained without staining since
silicon-containing polymers provided enough electron density
contrast with respect to hydrocarbon chains. The TEM grids
with thin sections were treated with O2 plasma for various
amount of time for different samples.
Si2St was polymerized at a relatively mild tempera-
ture (100 °C) because this monomer is not stable at
normal polymerization condition for styrene (∼130 °C).
The Si-Si bond has a lower bond energy (80.5 kcal/mol)
than that of Si-C bond (89.4 kcal/mol).29 Gelation was
observed after Si2St was polymerized at 130 °C for 16
h (Table 1, entry 1). When the polymerization temper-
ature was lowered to 100 °C, a narrow dispersed poly-
(Si2St) was obtained (Table 1, entries 2 and 3). Addition
of a small amount of solvent, such as dichlorobenzene
(DCB, 10 wt % of a monomer), led to higher molecular
weight of poly(Si2St) while maintaining a narrow mo-
lecular weight distribution (Table 1, entry 4). This may
be attributed to improved solubility of the propagating
radical in DCB. Narrow dispersed poly(Si2CSt)s were
obtained at 100 °C (Table 1, entries 5, 6). Despite
polymerization of Si2CSt for a long time (128 h) at 100
°C, a narrow dispersed poly(Si2CSt) (PDI of 1.07) was
obtained. A similar temperature dependence was ob-
served in the polymerization of OSi2St (Table 1, entries
7-9). It has been suggested that Si-O bonds can be
Results and Discussion
The ability to transfer block copolymer morphologies
to the underlying substrate makes it attractive to
pattern nanometer-sized features beyond the capability
of conventional lithography. However, the poor etch
selectivity between blocks and low thermal/mechanical
stability at high temperatures has limited the use of
organic block copolymers for direct pattern transfer
nanostructures into semiconductors. Silicon-containing
polymers have been used in lithography due to their
high etching resistance under oxygen plasma compared
to organic moieties (C, H, O, N, etc.). To take advantage
of the high etch resistance and high thermal/mechanical
stability of silicon oxide, we synthesized a series of block
copolymers from silicon-containing styrenic monomers
and styrene (St) or 4-acetoxystyrene (AcOSt) using
nitroxide-mediated living free radical polymerization.
Synthesis of Silicon-Containing Styrenic Mono-
mers. Large etch selectivity in organic materials can

266 Fukukawa et al.
Macromolecules, Vol. 38, No. 2, 2005
Table 2. Degree of Polymerization (DPn) Calculated from
1H NMR Dataa
c
d
monomerb
Mn × 10-3
DPn
Tge (°C)
St
3.5
5.0
9.7
7.5
5.6
32
36
37
23
21
99
122
100
0.5
AcOSt
Si2St f
OSi2St
Si2CSt
122
a All polymerizations were carried out at 100°C for 64 h with
[M]0/[I]0 ) 60. b Abbreviations of monomers: St, styrene; AcOSt,
4-acetoxystyrene; Si2St, 4-(pentamethyldisilyl)styrene; Si2CSt,
4-(bis(trimethylsilyl)methyl)styrene; OSi2St, 4-(pentamethyldisi-
loxymethyl)styrene. c Estimated from GPC. d Calculated from alkyl
protons of R-H unimer and aromatic protons of each repeating unit
in a1H NMR. e Determined by DSC (10 °C/min under nitrogen).
f Dichlorobenzene (DCB) was added as a solvent (10 wt % of
monomers) in the reaction. Without solvent, a shoulder was
observed at the higher molecular weight region with DPn of 32.
Figure 1. 1H NMR spectra of (a) Si2CSt and (b) poly(Si2CSt)
(polymerized in bulk at 100 °C for 128 h using R-H unimer;
entry 6 in Table 1). (*) Impurities from solvents: acetone, 2.17
ppm; H2O, 1.56 ppm.
Scheme 2. Syntheses of Macroinitiators and Block
Copolymers by NMRP at 100 °C
Figure 2. 1H NMR spectra of (a) OSi2St and (b) poly(OSi2St)
(polymerized in bulk at 100 °C for 128 h using R-H unimer;
entry 9 in Table 1). (*) Impurities from H2O: 1.56 ppm.
cleaved off by organolithium through nucleophilic attack
in anionic polymerization.30 As expected, living free
radical polymerization is more tolerant to the functional
groups in the monomers, and no gel formation was
observed in the polymerization of OSi2St at 100 °C.
g Si2St > St > OSi2St, Si2CSt, which is consistent with
the studies done both in conventional and living free
radical polymerization of para-substituted styrenes31-33
using 2,2′-azobis(isobutyronitrile) (AIBN) or benzoyl
peroxide (BPO) as a radical initiator. These studies
suggest that electron-withdrawing groups can stabilize
the propagating radicals. Namely, silicon atom directly
attached to the aromatic group (R-position) acts as an
electron-withdrawing group, while silicon atom attached
though methylene (â-position) acts an electron-donating
group compared to styrene for radical polymerization.
When silicon atom is directly attached to the aromatic
group (R-position; e.g., Si2St), it acts as an electron-
withdrawing group due to the interaction of the dπ-pπ
orbital; when the silicon atom is attached through
methylene (â-position; e.g., Si2CSt, OSi2St) linkage, it
acts as an electron-donating group due to σ-π hyper-
To better understand the reactivity of monomer under
living free radical conditions, the homopolymerization
of para-substituted Sts, including Si2St, Si2CSt, Si2OSt,
and AcOSt, was studied, and their reactivities were
compared to that of St. The reactivity of each monomer
depends on a number of parameters such as polarity,
inductive effect, and steric effects of the para-substit-
uents. All polymerizations were carried out at 100 °C
for 64 h, and the degree of polymerization (DPn) was
estimated by comparing the peak areas of alkyl protons
in R-H unimer (0.7-1.1 ppm) and aromatic protons
1
(6.0-6.8 ppm) in the repeating units in the H NMR
spectrum (Figures 1b and 2b). The results shown in
Table 2 indicate the following reactivity order: AcOSt
conjugation of CH2δ--Siδ+ 32 Therefore, the propagating
.

Macromolecules, Vol. 38, No. 2, 2005
Silicon-Containing Block Copolymers 267
Table 3. Density and Silicon Content of Each Monomer
monomera FW (g/mol) densityb, d (g/mL) Si content (wt %)
St
104.15
162.19
234.48
262.51
262.54
0.903
1.061
0.870
0.899
0.894
0
AcOSt
Si2St
OSi2St
Si2CSt
0
24.0
21.2
21.4
a Abbreviations of monomers: St, styrene; AcOSt, 4-acetoxy-
styrene; Si2St, 4-(pentamethyldisilyl)styrene; Si2CSt, 4-(bis(tri-
methylsilyl)methyl)styrene; OSi2St, 4-(pentamethyldisiloxymeth-
yl)styrene. b Estimated by measuring average mass of each mono-
mer in 1 mL at 25 °C.
site on a polymer chain bearing electron-withdrawing
groups, such as Si2St, can stabilize radicals more
strongly than that bearing electron-donating groups,
such as OSi2St and Si2CSt, resulting in a higher degree
of polymerization under similar conditions. In our
studies 10 wt % of DCB was added as solvent in Si2St
polymerization to obtain narrow dispersed polymers.
Hence, it is possible that the actual reactivity of Si2St
is higher than that of AcOSt.
Although polymerization at a low temperature (100
°C) produces narrow dispersed polymers, longer reaction
time is typically required to achieve higher yields of
silicon-containing homopolymers. To resolve this, a rate
accelerating agent, acetic anhydride (AA),34 was em-
ployed. Rate acceleration was clearly observed in the
polymerization of Si2OSt, which was the least compared
to other monomers (Table 2). The polymerization of
OSi2St in the presence of AA (1.5 equiv to unimer) at
100 °C for 64 h proceeded smoothly, resulting in narrow
dispersed macroinitiators with the highest yield and
number-average molecular weight in its category (Table
1, entry 10).
Figure 3. GPC profiles of (a) poly(OSi2St)a, (b) poly(Si2St)b,
(c) poly(OSi2St), and (d) poly(Si2CSt). All polymerization was
conducted in bulk at 100 °C for 64 h. a: Acetic anhydride (1.5
equiv to R-H unimer) was added into the reaction. b: N,N-
Dimethylformamide (DMF) (10 wt %) was added into the
reaction.
which clearly supports the formation of well-defined
homopolymers.
Synthesis of Silicon-Containing Block Copoly-
mers. Block copolymerization was carried out in a
similar way as the synthesis of homopolymers described
above (Scheme 2) and precipitated in methanol. The
molar ratio between blocks was calculated from the
integrated intensities of aromatic and trimethylsilyl
protons in 1H NMR spectrum. It was then converted to
volume ratio using measured monomer density (see
Table 3). The polymerization conditions and the result-
ing block copolymer characteristics are summarized in
Table 4.
Since the macroinitiators were not soluble in the
second monomers at 100 °C, a small amount of solvent
(20 wt % of the total feed of macroinitiator and second
monomer), such as DCB or DMF, was added to dissolve
both the macroinitiators and second monomers. All
block copolymerization proceeded smoothly, giving the
The polymers were characterized by1H NMR and
GPC. For example, in the 1H NMR spectrum of poly(Si2-
CSt), resonances at 0 ppm (18 H) and 6.0-6.8 ppm (4
H) corresponding to trimethylsilyl and aromatic protons,
respectively, were always observed while vinyl protons
in Si2CSt disappeared completely as the polymerization
proceeds (Figure 1b). As shown in Figure 3, all GPC
chromatographs showed symmetrical unimodal peaks,
Table 4. Synthesis Conditions of Silicon-Containing Block Copolymers from Various Macroinitiators and Their Block
Copolymer Characteristics
block copolymer
block copolymerization
feed ratio
block ratio:
first/second
d
first
Mn ×
second
blocka
[M2]/[MI],b time temp
yield Mn
×
mol
vol
Si content
(wt %)
e
f
entry
blocka
10-3
(w/w)
(h)
(°C)
solventc
(%)
10-3
PDId
%
%
11
12
13
14
15
16
17
18
19
20
21
PSt
3.5
7.6
7.0
5.5
5.5
7.5
7.5
7.5
5.0
5.0
4.1
PSi2St
PSi2St
PSi2CSt
POSi2St
POSi2St
PSt
3
1
1
2
2
2
3
2
3
1
4
64
20
40
40
80
40
41
40
64
94
64
100
100
130
100
100
100
128
100
100
100
130
DMF (20)
DCB (30)
N/A
72.6
26.3
78.1
17.4
20.6
43.2
77.4
28.6
58.7
31.5
14.3
18.7
10.8
16.4
7.7
1.07
1.10
1.15
1.10
1.11
1.07
1.20
1.07
1.08
1.10
1.24
30/70 15/85
81/19 65/35
61/39 38/62
87/13 73/27
86/14 70/30
51/49 73/27
25/75 46/54
45/55 61/39
31/69 21/79
25/75 46/54
62/38 33/67
20.2
8.3
13.2
5.8
6.3
15.4
9.8
12.2
18.2
10.6
13.6
PSt
PSt
PSt
DCB (20)
DCB (20)
DCB (20)
DCB (28)
DCB (20)
DMF (20)
DMF (20)
DMF (20)
PSt
8.1
POSi2St
POSi2St
POSi2St
PAcOSt-R
PAcOSt-a
PAcOSt-T
11.1
12.7
13.1
21.3
10.2
9.0
PSt
PAcOSt
PSi2St
PSi2CSt
PSi2CSt
a Abbreviations of polymers: PSt, polystyrene; PAcOSt, poly(4-acetoxystyrene; PSi2St, poly(4-(pentamethyldisilyl)styrene); PSi2CSt,
4-(bis(trimethylsilyl)methyl)styrene; POSi2St, poly(4-(pentamethyldisiloxymethyl)styrene); PAcOSt-R (entries 19, 20), 2,2,5-trimethyl-3-
(1-phenylethoxy)-4-phenyl-3-azahexane-3-nitroxide (R-H radical) terminated macroinitiator of PAcOSt; PAcOSt-T (entry 21), 2,2,6,6-
tetramethylpiperidinyloxy (TEMPO) terminated macroinitiator of PAcOSt. b Feed ratio for the block copolymerization; [M2]/[MI] ) [second
monomer]/[macroinitiator] (w/w). c Amount of solvent is indicated as wt % of total reactants. d The number- and weight-average molecular
weights (Mn and Mw) and polydispersity (PDI ) Mw/Mn) were determined by GPC from refractive index (RI) detector in a flow rate of 1.0
1
mL/min in THF at 40 °C. Narrow dispersed poly(St)s were used as standards for universal calibration. e Calculated from H NMR (e.g.,
see Figure 5 for poly(AcOSt-b-Si2CSt), entry 20). f Calculated from molar ratio and monomer density from Table 3.

268 Fukukawa et al.
Macromolecules, Vol. 38, No. 2, 2005
Table 5. Characteristics of Silicon-Containing Block Copolymers
d(100)
d(100) (nm)
after O2
entry in
Table 4
(nm) before
block copolymers
Mn
PDI
vol ratio
Si (wt %)
morphologya
O2 plasmab
plasmac
12
16
17
18
19
20
P(St-b-Si2St)
10 840
11 110
12 720
13 070
21 270
10 190
1.10
1.07
1.20
1.14
1.08
1.10
65/35
73/27
46/54
61/39
21/79
46/54
8.3
15.4
9.8
12.2
18.2
10.6
disordered
disordered
lamellae
cylinder
P(OSi2St-b-St)
P(OSi2St-b-St)
14.1
16.7
19.3
13.8
P(OSi2St-b-AcOSt)
P(AcOSt-b-Si2St)
P(AcOSt-b-Si2CSt)
10.6
18.0
d
cylinder
lamellae
a Confirmed from both SAXS and TEM. b Measured from SAXS. c Measured from TEM and normalized based on the SAXS data before
exposed to oxygen plasma for 10 min. d The lamellae delaminated after oxygen plasma treatments.
desired block copolymers with narrow PDIs at 100 °C.
Block copolymerization of polystyrene macroinitiator
with Si2CSt under normal conditions (130 °C) without
solvent yielded a narrow PDI. Unlike monomers with
Si-Si bond, lower polymerization temperature was not
necessary for Si2CSt as Si-C bond is more robust (Table
4, entry 13).
Further, the effect of sequence of monomer addition
on the synthesis of block copolymers was studied. The
poly(OSi2St) macroinitiator was more effective in initi-
ating St polymerization (Table 4, entry 16) than the
reverse addition sequence (Table 4, entries 13), resulting
in shorter reaction time at 100 °C in DCB and higher
yield. These results can be explained by the higher
reactivity of second monomer (St > OSi2St as discussed
earlier) and higher solubility of the propagating chain
in DCB. The synthesis of poly(OSi2St-b-AcOSt) block
copolymer follows a similar trend since AcOSt is more
reactive than OSi2St (Table 4, entry 18).
Figure 4. GPC traces of poly(AcOSt-b-Si2CSt) from different
initiators using (a) poly(AcOSt-T) macroinitiator, (b) TEMPO-
unimer as initiator (see entry 21 in Table 4), (c) poly(AcOSt-
R) macroinitiator, and (d) R-H unimer as initiator (see entry
20 in Table 4).
Previously, silicon-containing block copolymers have
been synthesized as photoresists for higher sensitivity
and etch resistance.7,12 By combining bottom-up (self-
assembly) and top-down (lithography) approaches in
photosensitive silicon-containing block copolymers, com-
plex hierarchical hybrid structures can be created. Here
we investigate the block copolymerization of 4-acetoxy-
styrene (AcOSt) with silicon-containing monomers. Poly-
(AcOSt) has been studied as chemically amplified
photoresist, which can be hydrolyzed to poly(hydroxy-
styrene) in the presence of acid and dissolved away by
aqueous base solution to generate patterns when ex-
posed to UV light. Narrow dispersed poly(AcOSt) has
been synthesized using TEMPO unimer by Barclay et
al.,35 and the PDI was found to be important in control-
ling the dissolution rate of poly(4-hydroxystyrene). In
this study we used both nitroxide unimers, R-H unimer,
and TEMPO-unimer to evaluate the effect of initiator
reactivity on block copolymerization. Two types of poly-
(AcOSt) macroinitiators (Table 4, entries 19-21) were
synthesized: poly(AcOSt)-R from R-H unimer and poly-
(AcOSt)-T from TEMPO unimer. Both the macroinitia-
tors show similar GPC profiles with Mn (PDI) as 5000
(1.13) and 4100 (1.16), respectively. Since both AcOSt
and Si2CSt are relatively stable, polymerization of the
second block could be conducted at 130 °C using poly-
(AcOSt)-T as the macroinitiator. GPC traces suggest
that the efficiency of propagation of the second block
depends on the nature of the capping group on the
macroinitiator (see Figure 4). Poly(AcOSt-b-Si2CSt)-R
shows narrow distribution in GPC even with smaller
feed of second monomer (i.e., poly(AcOSt-R)/Si2CSt )
1/1 vs poly(AcOSt-T)/Si2CSt ) 1/4), while poly(AcOSt-
b-Si2CSt)-T has a shoulder in a larger retention volume
region, which can be assigned to unreacted poly(AcOSt-
T). This behavior can be explained by the lower reactiv-
Figure 5. 1H NMR spectrum of poly(AcOSt-b-Si2CSt) corre-
sponding to entry 20 in Table 4. Block ratio of poly(AcOSt)
and poly(Si2CSt) was estimated from the integrations of a
(assigned to acetyl groups) and b (assigned to trimethylsilyl
groups). (*) Impurities from solvent and H2O.
ity of the terminal TEMPO-radicals compared to that
of the R-H radicals to initiate Si2CSt monomers, which
is consistent to the results reported by Hawker et al.,15
and the nitroxide structure plays an important role in
1
the control of polymerization. The H NMR spectrum
of poly(AcOSt-b-Si2CSt)-R showed five broad signals at
0, 1.3, 1.7, 2.2, and 6.0-6.8 ppm for the trimethylsilyl,
methylene, methyne, acetyl, and aromatic protons,
respectively (Figure 5). These GPC traces and 1H NMR
spectra confirmed the synthesis of the desired block
copolymers.
Morphology Characterization of the Silicon-
Containing Block Copolymers. To understand the

Macromolecules, Vol. 38, No. 2, 2005
Silicon-Containing Block Copolymers 269
Figure 6. Small-angle X-ray scattering (SAXS) results of
silicon-containing block copolymers. The entry numbers are
the same as those in Table 4.
Figure 8. Transmission electron micrograph (TEM) images
of PAcOSt-PSi2St (21/79 v/v, entry 19 in Tables 4 and 5) before
and after O2 plasma for 10 min.
containing block such that we are able to obtain the
bright-field TEM images without staining. SAXS pro-
files seen in Figure 6 suggest that these block copoly-
mers form disordered (sample 12 and 16), cylinder
(sample 18 and 19), and lamellae (sample 17 and 20)
structures depending on their volume ratios between
two blocks and their molecular weight, which agrees
well with TEM results. The microtomed thin films (∼75
nm) on TEM grids were subject to O2 plasma for 1.5, 5,
and 10 min for samples 18, 20, and 19, respectively. The
film with lamellar structures fell apart after O2 plasma
(see Figure 7) due to the loss of more than half of the
mass. In contrast, when silicon-containing block is the
major phase (sample 18 and 19), the film morphologies
are preserved after exposure to O2 for more than 10 min
(see Figure 8).
The domain size before and after O2 plasma is found
highly dependent on the silicon content in the block
copolymers. For example, the d spacing of sample 19
(18.2 wt % Si) almost did not change, from 19.38 to 18.0
nm, while that of sample 18 (12.2 wt % Si) decreased
from 16.7 to 10.6 nm, based on TEM observations. Our
results are consistent with the previous study, which
suggests that large etch resistance to O2 plasma can be
achieved when polymers have greater than 10 wt % of
silicon.26
Figure 7. Transmission electron micrograph (TEM) images
of PAcOSt-PSi2CSt (46/54 v/v, entry 20 in Tables 4 and 5)
before and after O2 plasma for 5 min.
structures of these newly synthesized block copolymers
and their response to oxygen plasma, we have used
TEM and small-angle X-ray scattering (SAXS) to char-
acterize their morphologies before and after oxygen
plasma (see Table 5). Since silicon has larger mass than
carbon, silicon-containing block provides enough elec-
tron density contrast with respect to the non-silicon-
Conclusion
Well-defined, narrow dispersed silicon-containing block
copolymers were synthesized from the corresponding
silicon-containing monomers and St or AcOSt using

270 Fukukawa et al.
Macromolecules, Vol. 38, No. 2, 2005
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Acknowledgment. K.F. is grateful for Bell Labs
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