Molecular brushes with extreme grafted side chain densities

Article abstract of DOI:10.1016/j.polymer.2012.06.008

A series of densely grafted poly(n-butyl acrylate) (PBA) molecular brushes with four different grafting densities were synthesized by the "grafting-from" approach using atom transfer radical polymerization (ATRP). A novel monomer, isopropylidene-2,2-Bis(methoxy)propionic hydroxyethylmethacrylate (IMPHMA), was synthesized and copolymerized with methyl methacrylate (MMA) under different monomer feed ratios to yield a series of linear poly(methyl methacrylate-stat-IMAPA), [PMMA-s-(PIMPHMA)]. The resulting copolymers were deprotected and transformed to macroinitiators, [PMMA-s-(PHEMA-IMPHMA-Br)]. n-Butyl acrylate (BA) was grafted from these macroinitiators to yield a series of molecular brushes, [PMMA-s-{(PIMPHMA)-g- PBA}], with various side chain lengths. Molecular brushes were characterized by gel permeation chromatography (GPC) and ¹H NMR. PBA side chains were cleaved by acid hydrolysis, and the resulting linear PBA polymers were characterized by GPC to study initiation efficiency during the synthesis of molecular brushes. The initiation efficiency increased with polymerization time and decreased with macroinitiators that had more initiation sites. Atomic force microscopy (AFM) measurements demonstrated the characteristic molecular structure by resolving individual brush molecules.

Full text of DOI:10.1016/j.polymer.2012.06.008

Polymer 53 (2012) 3462e3468
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Molecular brushes with extreme grafted side chain densities
Jae Min Bak a, Gourishanker Jha a, Eungjin Ahn b, Seo-Hyun Jung c, Han Mo Jeong a, Byeong-Su Kim b,
,
Hyung-il Lee a
*
a Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea
b Interdisciplinary School of Green Energy and School of NanoBioscience and Chemical Engineering, UNIST (Ulsan National Institute of Science and Technology),
Ulsan 689-798, Republic of Korea
c Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology, Ulsan 681-802, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of densely grafted poly(n-butyl acrylate) (PBA) molecular brushes with four different grafting
densities were synthesized by the “grafting-from” approach using atom transfer radical polymerization
Received 30 March 2012
Received in revised form
30 May 2012
(ATRP).
A
novel monomer, isopropylidene-2,2-Bis(methoxy)propionic hydroxyethylmethacrylate
(IMPHMA), was synthesized and copolymerized with methyl methacrylate (MMA) under different
monomer feed ratios to yield a series of linear poly(methyl methacrylate-stat-IMAPA), [PMMA-s-
(PIMPHMA)]. The resulting copolymers were deprotected and transformed to macroinitiators, [PMMA-s-
(PHEMA-IMPHMA-Br)]. n-Butyl acrylate (BA) was grafted from these macroinitiators to yield a series of
molecular brushes, [PMMA-s-{(PIMPHMA)-g-PBA}], with various side chain lengths. Molecular brushes
were characterized by gel permeation chromatography (GPC) and 1H NMR. PBA side chains were cleaved
by acid hydrolysis, and the resulting linear PBA polymers were characterized by GPC to study initiation
efficiency during the synthesis of molecular brushes. The initiation efficiency increased with polymeri-
zation time and decreased with macroinitiators that had more initiation sites. Atomic force microscopy
(AFM) measurements demonstrated the characteristic molecular structure by resolving individual brush
molecules.
Accepted 4 June 2012
Available online 15 June 2012
Keywords:
ATRP
Molecular brushes
Initiation efficiency
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
substrate [12,16,17]. This tension governs the extent and rate of
backbone scission. Tension depends on grafting density, length of
It is well known that the shape of polymer molecules is an
important factor that determines their properties [1]. During the
last decade, polymer scientists have introduced a new philosophy
of “molecular brushes” and synthesized more densely grafted
molecular brushes [2e5]. The unique physical and chemical prop-
erties of molecular brushes offer potential applications in various
fields including supersoft elastomers [6], precursors to nanowires
or nanotubes [7e9], photonic crystals [10,11], and molecular tensile
machines [12].
The properties of polymer brushes can be controlled by varia-
tion in grafting density and length of backbone and side chains
[13e15]. It has been recently reported that the carbonecarbon
covalent bonds of the backbone of molecular brushes can sponta-
neously break upon adsorption onto a substrate due to the tension
generated by attractive interaction between the side chains and the
side chains, and extent of substrate attraction [18]. This behavior
has been observed for brushes with a fairly long backbone (DP of
backbone
¼
2000e3000) and side chains (DP of side
chains ¼ 130e150). While the influences of side chain lengths
and substrate attraction on scission have been extensively
studied, fewer studies have focused on the effect of grafting density.
Molecular brushes have been synthesized by three methods:
grafting-onto [19], grafting-through [20e22], and grafting-from
[23e25]. The grafting-from method has been extensively studied
due to its ability to prepare brushes with high grafting density from
a backbone with a predetermined number of initiation sites.
Generally,
a backbone macroinitiator for the grafting-from
approach contains one initiation site per monomer repeating
unit. However, it is necessary to design a macroinitiator that has
more than one initiation site per repeating unit to investigate how
fast molecular brushes degrade with increasing grafting density. In
this paper, we report the synthesis of molecular brushes with
extreme grafted side chain density. In addition, we report on the
trend of initiation efficiency [26] with respect to the number of
* Corresponding author.
E-mail address: sims0904@ulsan.ac.kr (H.-i. Lee).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.06.008

J.M. Bak et al. / Polymer 53 (2012) 3462e3468
3463
initiation sites related to a steric interaction. For this purpose, we
synthesized a protected monomer that can potentially offer two
initiation sites. Polymerization of this monomer followed by
deprotection and esterification led to a macroinitiator with two
ATRP [27e32] initiation sites per repeating unit. The initiation
efficiency during brush polymerization from various macro-
initiators with different initiation site densities is discussed.
remove water-soluble urea salt. The crude product was purified
by liquid chromatography on silica gel, eluting with hexane and
ethyl acetate to 1:2 to give IMPHMA as a colorless viscous liquid:
3.96 g (82%). 1H NMR (300 MHz, CDCl3,
d in ppm): 1.14 (s, 3H, eCH3),
1.36 (s, 6H, eCH3), 1.88 (s, 3H, eCH3), 3.56 (d, 2H, eCH2O), 4.10 (d,
2H, eCH2O), 4.31 (t, 4H, eCH2O).
2.3.3. Typical copolymerization procedure: PMMA-s-PIMPHMA (A2)
2. Experimental
EBiB (4.5
m
L, 0.033 mmol), IMPHMA (4.29 g, 15 mmol), MMA
L,
(0.5 g, 5 mmol), CuBr2 (0.0002 g, 0.001 mmol), PMDETA (3.5
m
2.1. Materials
0.016 mmol), and anisole (2.5 mL) were added to a 25 mL Schlenk
flask. The flask was deoxygenated by five freezeepumpethaw
cycles. CuBr (2.3 mg, 0.016 mmol) was added to the frozen
mixture in the presence of argon. The flask was filled with argon
and heated in an oil bath at 70 ꢀC. Polymerization was stopped 5 h
later by exposing the solution to air. The resulting solution was
passed through neutral alumina to remove the copper complex.
The polymer was precipitated twice in hexane and dried under
vacuum at room temperature for 24 h.
All chemicals were purchased from Aldrich Chemical Co.
or Tokyo Chemical Industry Co. Ltd. (TCI) and used as received
unless otherwise stated. 2-Hydroxyethyl methacrylate (HEMA), N-
N-N0-N0-N0-pentamethyldiethylenetriamine (PMDETA), 1-Ethyl-3-
(3-dimethylaminopropyl)-carbodiimide (EDC), N-N0-dicyclohex-
ylcarbondimide (DCC), and anisole were purchased from TCI. 2,2-
Bis(hydroxymethyl) propanoic acid (2-Bis-MPA) (98%), p-Toluene
sulphonic acid (TsOH) (98.5%), 2-,2-dimethoxypropane (98%), 4-
Dimethylaminopyridine (DMAP) (99%), ethyl 2-bromoisobutyrate
(98%) (EBiB), 0.1 N Hydrochloric acid, 2-bromo-2-methyl pro-
pionic acid (98%), Methyl methacrylate (MMA), n-butyl acrylate
(BA), 1-butanol (99.8%), and conc. sulfuric acid were purchased
from Aldrich. Copper (I) Bromide (CuBr) was purified by washing
with glacial acetic acid, followed by washing with ethanol and
acetone, and then dried under vacuum.
2.3.4. Deprotection: PMMA-s-(PIMPHMA-OH) (B2)
1.0 g of PMMA-s-PIMPHMA was dissolved in 20 mL of THF and
methanol mixture (ratio ¼ 7:3). The solution pH was maintained
around 3 while 0.1 N HCl was added drop-wise to a flask. The
reaction mixture was stirred for 24 h at room temperature. The
resulting reaction mixture was precipitated into diethyl ether and
dried under high vacuum at room temperature for 24 h.
2.2. Instrumental
2.3.5. Functionalization with ATRP initiation groups: PMMA-s-
(PIMPHMA-Br) (C2)
The apparent molecular weight and molecular weight distri-
butions of polymers were measured on a GPC (Agilent technologies
1200 series) using polystyrene standard with DMF as the eluent at
30 ꢀC and a flow rate of 1.00 mL/min 1H NMR spectra were collected
in DMF-d7 and CDCl3 on a Bruker avance 300 MHz NMR spec-
trometer. The morphology of the samples was investigated using
atomic force microscopy (AFM, Nanoscope V, Veeco) via a tapping
mode.
PMMA-s-(PIMPHMA-OH) (0.97 g, 4 mmol of OH groups),
bromo-2-methyl propanoic acid (2.01 g, 12 mmol), DCC (2.47 g,
12 mmol), and DMF (8 mL) were placed in a 25 mL round-bottom
flask. The flask was sealed and kept in an ice bath. The reaction
mixture was stirred for 40 h at room temperature. The resulting
reaction mixture was filtered and precipitated into methanol/water
(80/20 w/w). The precipitate was redissolved in 10 mL of CHCl3,
precipitated into hexane, and dried under vacuum at room
temperature for 24 h.
2.3. Synthesis
2.3.6. Typical brush polymerization procedure of PMMA-s-
[(PIMPHMA-g-PBA)] (D2)
2.3.1. Isopropylidene-2,2-Bis(methoxy)propionic acid (I-Bis-MPA)
[33]
PMMA-s-(PIMPHMA-Br) (C2) (0.11 g, assumed to contain
0.5 mmol of initiating groups), n-BA (28.8 g, 200 mmol), anisole
(3.0 mL), CuBr2 (0.0005 g, 0.025 mmol), and dNbpy (0.20 mg,
0.5 mmol) were added to a 50-mL Schlenk flask. The reaction
mixture was degassed by three freezeepumpethaw cycles. After
stirring for 0.5 h at room temperature, CuBr (0.038 g, 0.25 mmol)
was added under nitrogen, and the flask was placed in a preheated
oil bath at 70 ꢀC. Polymerization was stopped after 40 h by cooling
the flask to room temperature and opening the flask to air. The
resulting polymer solution was purified by passing through
a column of neutral alumina. The solvent and remaining monomer
were removed under high vacuum (1 mmHg). The resulting
product was dried at room temperature for 12 h.
10.00 g (74.55 mmol) of 2,-2 Bis (hydroxyl methyl) propanoic
acid (Bis-MPA), 13.8 mL (111.83 mmol) of 2,2-dimethoxypropane,
and 0.71 g (3.73 mmol) of p-toluenesulfonic acid monohydrate
were dissolved in 50 mL of acetone. The reaction mixture was
stirred for 2 h at room temperature. Approximately 5 mL of an NH3/
EtOH (2 M) solution was then added to neutralize the catalyst, and
the solvent was evaporated at room temperature. The residue was
then dissolved in (250 mL) CH2Cl2 and extracted with two portions
of (20 mL) water. The organic phase was dried with MgSO4
and evaporated to give I-Bis-MPA as white crystals: 11.7 g (90%).
1H NMR (300 MHz, CDCl3,
d in ppm): 1.18 (s, 3H, eCH3), 1.40
(s, 3H, eCH3), 1.43 (s, 3H, eCH3), 3.64 (d, 2H, eCH2O), 4.14 (d,
2H, eCH2O).
2.3.7. Typical procedure for side chain hydrolysis
2.3.2. Isopropylidene-2,2-Bis(methoxy)propionic hydroxyethyl-
methacrylate (IMPHMA)
PMMA-s-[(PIMPHMA-g-PBA)] (D2) (25 mg) was dissolved in
THF (1.5 mL) and 1-butanol (8.5 mL). Concentrated sulfuric acid (4
drops) was added, and the solution was heated at 100 ꢀC for 7 days.
The solvent was removed under vacuum, and the remaining
residue was dissolved in CHCl3 (5 mL). After extracting with water
(2 mL), the organic layer was isolated and vacuum-distilled at room
temperature. The resulting polymer was characterized by GPC,
which indicated nearly quantitative cleavage.
3.40 g (19.50 mmol) of I-Bis-MPA, 2.21 g (17 mmol) of 2-
hydroxyethyl methacrylate, and 0.244 g (2 mmol) of DMAP were
mixed in 50 mL of CH2Cl2. The reaction flask was kept in an ice bath
for 30 min and 3.74 g (19.50 mmol) EDC was then added. Stirring at
room temperature was continued for 45 h. Once the reaction was
complete, the product was washed with water three times to

3464
J.M. Bak et al. / Polymer 53 (2012) 3462e3468
m
nO
m
O
nO
O
O
O
O
O
O
O
EBiB, CuBr, PMDETA
Anisole
0.1 N HCl
O
O
O
O
O
O
O
O
O
MMA
O
OH OH
O
O
PMMA-s-(PIMPHMA-OH),
B1, B2, B3, and B4
IMPHMA
PMMA-s-PIMPHMA,
A1, A2, A3, and A4
nO
mO
O
O
nO
mO
O
O
O
DCC, DMAP, DMF
O
n-BA, CuBr, dNbpy
Anisole
O
O
OH
O
P
P
Br
O
O
O
O
O
O
O
O
O
O
Br
Br
O
O
PMMA-s-(PIMPHMA-Br),
C1, C2, C3, and C4
PMMA-s-[(PIMPHMA)-g-PBA)],
D1, D2, D3, and D4
Scheme 1. Synthesis of molecular brushes containing PBA in the side chains from a series of [PMMA-s-(PIMPHMA-Br)] backbones.
3. Results and discussion
with increasing conversion is observed, which is qualitatively
indicative of the molecular weight increasing throughout the
polymerization. The controlled nature of copolymerization is sup-
ported by the fact that polydispersity remained low during the
entire polymerization, as shown in Table 1. The resulting copoly-
mers were also analyzed by 1H NMR spectroscopy to observe their
copolymer composition in the backbone. The final composition of
copolymer A2 was calculated from the integral intensities of the
(a) eOCH3 group peak (3.58 ppm) in MMA and the (b) eOCH2 group
peak (4.36 ppm) in IMPHMA (Fig. 2). Results from four copoly-
merizations are summarized in Table 1.
Scheme 1 outlines the synthetic route for preparing densely
grafted brushes with PBA side chains. Esterification was carried out
with HEMA and I-Bis-MPA in the presence of EDC and DMAP to
yield a novel monomer, IMPHMA, which could be derivatized to
have an initiating moiety for brush synthesis. Copolymerization of
IMPHMA and MMA with different molar ratios of 50:50, 75:25,
90:10, and 100:0 was carried out to prepare a series of [PMMA-s-
(PIMPHMA)] copolymers (A1, A2, A3, and A4) with different
grafting densities along the backbone. Evidence for the controlled
nature of IMPHMA and MMA copolymerization was obtained by
GPC (Fig. 1 and supporting information). The representative
example of copolymerization (IMPHMA:MMA ¼ 75:25) that led to
copolymer A2 is shown in Fig. 1a. An increase in molecular weight
In the next step, the deprotection reaction was carried out in
a mixed solvent of THF and methanol (7:3 ratio). Then 0.1 N HCl
was added to the solution until solution pH reached 3. I-bis-MPA
groups present in a series of copolymer A were deprotected to
time (h)
a
A2
b
1 h
2 h
3 h
4 h
5 h
B2
C2
4
5
5
6
10
10
10
10
Molar Mass
Molar Mass
Fig. 1. GPC traces for a) the copolymerization of IMPHMA and MMA (A2), b) a backbone PMMA-s-PIMPHMA (75% of IMPHMA along the backbone) (A2), a deprotected product
PMMA-s-(PIMPHMA-OH) (B2), and a macroinitiator PMMA-s-(PIMPHMA-Br) (C2).

J.M. Bak et al. / Polymer 53 (2012) 3462e3468
3465
Table 1
Reaction conditions and results for the copolymerization of MMA and IMPHMA.
Entries
[M1]:[M2]:[I]:[Cuþ]:[Cu2þ]:[L]a
Convb (%)
M1
Incorporation
ratio (M1:M2)c
DPb
M1
Mn, theo ( ꢁ 10ꢂ4
)
Mn, appd( ꢁ 10ꢂ4
)
PDId
M2
M2
A1
A2
A3
A4
300:300:1:0.5:0.025:1
450:150:1:0.5:0.025:1
540:60:1:0.5:0.025:1
600:0:1:0.5:0.025:1
62
70
75
48
65
75
72
e
47:53
74:26
88:12
100:0
186
315
405
288
195
113
43
7.3
10.1
12.0
11.2
8.4
13.1
14.0
21.3
1.08
1.11
1.12
1.20
a
M1 ¼ IMPHMA, M2 ¼ MMA, I ¼ EBiB, Cuþ ¼ CuBr, Cu2þ ¼ CuBr2, L ¼ PMDETA.
Obtained from gas chromatography.
Calculated from NMR results.
b
c
d
Determined by GPC in DMF with PMMA calibration.
m O
g
Br
g
O
O
n O
O
O
O
O
O
O
g
Br
C2
m O
n O
O
O
OH
OH
O
O
B2
m O
n O
c
a
O
O
f
f
d, e
O
c
b
d, e
d, e
O
O
b
a
d, e
b
O
A2
8
7
6
5
4
ppm
3
2
1
0
Fig. 2. 1H NMR spectra of a backbone PMMA-s-PIMPHMA (75% of IMPHMA along the backbone) (A2), a deprotected product PMMA-s-(PIMPHMA-OH) (B2), and a macroinitiator
PMMA-s-(PIMPHMA-Br) (C2).
0 h
form hydroxyl side groups, yielding a series of copolymer B. The
resulting copolymers were not soluble in THF or chloroform,
which indicates successful deprotection. Esterification reactions
between the resulting copolymers and 2-bromo-2methyl pro-
pionic acid were carried out to introduce an ATRP initiating group
at the end of the repeating unit. 1H NMR and GPC were used to
analyze the deprotection and functionalization processes. The 1H
NMR spectrum provided evidence of the complete transformation
of the terminal hydroxyl groups to ATRP initiation groups. As
shown in Fig. 2, the disappearance of the peak (f) of a I-Bis-MPA
methyl group at 1.36 ppm and the complete change in solubility
supported the transformation into hydroxyl groups. After esteri-
fication, a series of copolymer C became soluble again in THF or
chloroform and the peak (g) appeared in the NMR, demonstrating
successful loading of the ATRP initiation groups. After depro-
tection, the apparent molecular weight of B2 increased about two-
fold compared to that of B2. This discrepancy could be attributed
to the difference in hydrodynamic volumes between two polymers
with respect to PMMA in DMF (Fig. 1b) [34]. After esterification,
the apparent molecular weight of C2 became similar to that of A2,
5 h
15 h
25 h
40 h
48 h
5
6
10
10
Molar Mass
indicating successful transformation (Fig.
information).
1 and supporting
Fig. 3. GPC traces during the syntheses of PMMA-s-[(PIMPHMA-g-PBA)] brushes (D2)
(5e48 h) from a macroinitiator PMMA-s-(PIMPHMA-Br) (C2) (0 h).

3466
J.M. Bak et al. / Polymer 53 (2012) 3462e3468
Table 2
Results of n-Butyl Acrylate (BA) Polymerization from a series of [PMMA-s-(PIMPHMA-Br)] backbones.
b
c
d
Brushes
D1
Macroinitiators
C1
Time
(h)
Conv
(%)a
DPn,SC-theory
Mn, theory
Mn,
app
PDId
5
15
25
40
5
15
25
40
5
15
25
40
5
15
25
40
1.8
2.8
5.5
8.3
1.3
2.0
5.0
7.5
1.0
2.8
4.0
5.8
1.3
4
7
11
22
33
5
333,000
524,000
1,048,000
1,571,000
403,000
192,000
241,000
385,000
528,000
221,000
382,000
723,000
920,000
320,000
611,000
796,000
1,130,000
239,000
510,000
630,000
1,310,000
1.18
1.15
1.19
1.19
1.21
1.25
1.24
1.20
1.25
1.28
1.26
1.26
1.31
1.35
1.34
1.37
D2
D3
D4
C2
C3
C4
8
645,000
20
30
4
11
16
23
5
16
21
35
1,613,000
2,420,000
415,000
1,140,000
1,659,000
2,385,000
369,000
1,180,000
1,548,000
2,580,000
5.3
8.8
a
Obtained from gas chromatography.
b
c
Calculated from DPn,SC-theory ¼ ([BA]/[PMMA-s-(PIMPHMA-Br)]) ꢁ conversion.
Calculated from Mn,
Determined by GPC in DMF with PMMA calibration.
¼ ([BA]/[PMMA-s-(PIMPHMA-Br)]) ꢁ (molecular weight)BA ꢁ conversion.
theory
d
Table 3
Results of side chains cleaved from PMMA-s-[(PIMPHMA-g-PBA)] brushes.
b
c
d
e
Brushes
D1
Time (h)
Conv (%)a
Mn,SC-theory
Mn,SC-cleaved
DPn,SC-theory
DPn,SC-cleaved
PDIc
ff
5
15
25
40
5
15
25
40
5
15
25
40
5
15
25
40
1.8
2.8
5.5
8.3
1.5
2.0
5.0
7.5
1.0
2.8
4.0
5.8
1.3
4
900
1400
2800
4200
640
1020
2560
3840
510
1400
2050
2950
640
2048
2680
4480
1730
2140
3700
4580
2000
2340
3920
4520
1960
3270
3690
4900
2790
6440
7680
8600
7
11
22
33
5
14
17
29
36
16
18
31
35
15
26
29
38
22
50
60
67
1.18
1.15
1.19
1.19
1.21
1.25
1.24
1.20
1.25
1.28
1.26
1.26
1.31
1.35
1.34
1.37
52
69
76
92
32
44
65
85
26
43
56
60
23
32
35
52
D2
D3
D4
8
20
30
4
11
16
23
5
16
21
35
5.3
8.8
a
Obtained from gas chromatography.
b
c
d
e
f
Mn,SC ¼ Mn of the side chain, Mn,SC-theory ¼ (Mn,brush ꢂ Mn,[PMMA-sꢂ(PIMPHMAꢂBr)])/DPn,[PMMA-sꢂ(PIMPHMAꢂBr)]
.
Determined by GPC in DMF with PMMA calibration.
Calculated from DPn,SC-theory ¼ ([BA]/[PMMA-s-(PIMPHMA-Br)]) ꢁ conversion.
Calculated from DPn,SC-cleaved ¼ Mn,SC-cleaved/(molecular weight)BA
.
Initiation efficiency calculated from f ¼ Mn,SC-theory/Mn,SC-cleaved
.
A series of molecular brushes (D1, D2, D3, and D4) were
synthesized by grafting n-BA from a series of backbones (C1, C2, C3,
and C4) (Scheme 1). As shown in Fig. 3, the GPC traces for D2
synthesis shifted continuously toward higher molecular weight
throughout the polymerization due to both the propagation from
the main chain and the side chain. No high molecular weight
shoulder was observed when compared with that of the macro-
initiator (C2), indicating no evidence for intermolecular
brushebrush coupling. The brush syntheses appeared to proceed in
a controlled fashion because the polydispersities for all cases
remained low. Results from the brush synthesis are summarized in
Table 2.
To investigate the influence of the steric congestion of the
initiation sites on the initiation efficiency during molecular brush
synthesis, the side chains of a series of molecular brushes (D1, D2,
D3, and D4) were cleaved by acid hydrolysis in 1-butanol at 100 ꢀC.
It has been previously demonstrated that complete side chain
cleavage occurs under these conditions while retaining the ester
functionalities of each monomer unit. A series of cleaved side
chains were then characterized by GPC to examine initiation
90
60
30
0
D1
D2
D3
D4
0
2
4
6
8
10
Conversion (%)
Fig. 4. Initiation efficiency as
(D1 w D4).
a
function of conversion for
a
series of brushes

J.M. Bak et al. / Polymer 53 (2012) 3462e3468
3467
Fig. 5. (a) Height and (b) amplitude-mode AFM images of the D4 brushes adsorbed onto a silicon wafer. The z-scale of the height is 30 nm and phase is 50ꢀ, respectively.
efficiency during the growth of side chains. Molecular weights for
the cleaved side chains were calculated using a conventional cali-
bration based on linear PMMA standards (Table 3). Initiation effi-
ciency is, in principle, defined as the ratio of the actual initiating
sites to the total number of Br initiation groups. The initiation
efficiency (f) can be also obtained by the ratio between Mn,SC-theory
and Mn,SC-cleaved (f ¼ Mn,SC-theory/Mn,SC-cleaved) [26]. For all cases, f is
low in the early stage of polymerization (23e52 %). It usually
increased with the progress of polymerization and reached
52e92 %. The side chains that begin growing early from a fraction
of the initiation sites sterically hinder the initiation sites of
adjacent repeating units of the backbone. This causes
noncomplete initiation at the initial time. Fig. 4 shows that f
gradually increases with monomer conversion. It should be noted
that as the steric hindrance of the initiation sites increases, f
decreases. For example, f of D4 with the highest steric hindrance of
the initiation sites is initially similar to those of D2 and D3.
However, it becomes the least sterically hindered among the four
brushes as polymerization progresses. A comparison between the
initiation efficiencies of four brushes demonstrated that increases
in steric hindrance of the initiation sites during brush synthesis
prevented the side chains from growing efficiently and decreased
the overall initiation efficiency.
Atomic force microscopy (AFM) offers a facile access to the
structural information of polymer brushes prepared through direct
visualization of molecular structures [35,36]. As a representative
brush, we have measured the D4 brushes of [PMMA-s-(PIMPHMA-
g-PBA)] with a highest degree of branches. As shown in Fig. 5, the
diluted solution of D4 brush deposited on a silicon wafer exhibits
a wormlike morphology similar to that of previous report. Unlike
the crystalline brushes, the amorphous brushes of PBA chain serve
to the adsorption of the side chains on the surface without altering
the molecular conformation, resulting in the clear separation of
individual macromolecular brushes. In addition, we found that the
average diameter and contour length of the D4 brush correspond to
35 ꢃ 2.5 nm and 166 ꢃ 17 nm (averaged over 50 samples), which
are in good agreement with the theoretical calculations based on
the molecular weights and degree of branches measured in GPC.
spacing of ATRP initiating groups along the backbone ranging from
50 to 100 %. From these macroinitiators, molecular brushes with
densely grafted PBA side chains were synthesized using the
grafting-from strategy by ATRP. The synthesized molecular brushes
were of high molecular weight and relatively low polydispersities.
The initiation efficiencies of the ATRP processes were determined
by cleaving the side chains and performing GPC analysis. In the
early stage of polymerization, the initiating efficiencies of four
brushes ranged from 23 to 52 % but reached around 52e92 % in the
final stage of polymerization. Initiation efficiency decreased with
increasing density of initiation groups along the backbone. This
result demonstrated that slow initiation was caused by steric
hindrance. AFM demonstrated that the molecular brushes
adsorbed on the mica surface have a wormlike structure. Ongoing
experiments will focus on the rate of adsorption-induced degra-
dation of molecular brushes as a function of grafting density,
offered by the different density of the side chains.
Acknowledgments
This work was supported by the WCU research program (R31-
2008-000-20012-0), Priority Research Center Program (2009-
0093818), and Basic Science Research Program (2010-0008642)
through the National Research Foundation of Korea funded by the
Ministry of Education, Science, and Technology of Korea.
Appendix A. Supporting information
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.polymer.2012.06.
008.
References
[1] Matyjaszewski K. Prog Polym Sci 2005;30(8e9):858e75.
[2] Lee H-i, Pietrasik J, Sheiko SS, Matyjaszewski K. Prog Polym Sci 2010;35(1e2):
24e44.
[3] Sheiko SS, Sumerlin BS, Matyjaszewski K. Prog Polym Sci 2008;33(7):759e85.
[4] Zhang M, Mueller AHE. J Polym Sci Part A Polym Chem 2005;43(16):3461e81.
[5] Dziezok P, Sheiko SS, Fischer K, Schmidt M, Moller M. Angew Chem Int Ed
1997;36(24):2812e5.
4. Conclusions
[6] Pakula T, Zhang Y, Matyjaszewski K, Lee H-i, Boerner H, Qin S, et al. Polymer
2006;47(20):7198e206.
[7] Zhang M, Estournes C, Bietsch W, Mueller AHE. Adv Funct Mat 2004;14(9):
871e82.
[8] Huang J, Tang C, Lee H, Kowalewski T, Matyjaszewski K. Macromol Chem Phys
2007;208(21):2312e20.
Well-defined macroinitiators with different initiation site
densities were synthesized by the copolymerization of IMPHMA
and MMA followed by deprotection and transformation into
PMMA-PIMPHMA-Br. These macroinitiators exhibited a statistical

3468
J.M. Bak et al. / Polymer 53 (2012) 3462e3468
[9] Tang C, Dufour B, Kowalewski T, Matyjaszewski K. Macromolecules 2007;
40(17):6199e205.
[22] Ishizu K, Tsubaki S, Uchida SJ. Macromol Sci. Pure Appl Chem 1995;A32(7):
1227e34.
[10] Rzayev J. Macromolecules 2009;42(6):2135e41.
[11] Xia Y, Olsen BD, Kornfield JA, Grubbs RH. J Am Chem Soc 2009;131(51):
18525e32.
[12] Park I, Sheiko SS, Nese A, Matyjaszewski K. Macromolecules 2009;42(6):
1805e7.
[13] Sumerlin BS, Matyjaszewski K. Macromol Eng 2007;2:1103e35.
[14] Rathgeber S, Pakula T, Wilk A, Matyjaszewski K, Beers KL. J Chem Phys 2005;
122(12). 124904/124901-124904/124913.
[23] Beers KL, Gaynor SG, Matyjaszewski K, Sheiko SS, Moeller M. Macromolecules
1998;31(26):9413e5.
[24] Lee H, Jakubowski W, Matyjaszewski K, Yu S, Sheiko SS. Macromolecules
2006;39(15):4983e9.
[25] Lee H-i, Matyjaszewski K, Yu-Su S, Sheiko SS. Macromolecules 2008;41(16):
6073e80.
[26] Sumerlin BS, Neugebauer D, Matyjaszewski K. Macromolecules 2005;38(3):
702e8.
[15] Rathgeber S, Lee H-I, Matyjaszewski K, Di Cola E. Macromolecules 2007;
40(21):7680e8.
[16] Sheiko SS, Sun FC, Randall A, Shirvanyants D, Rubinstein M, Lee H-i, et al.
Nature 2006;440(7081):191e4.
[17] Park I, Nese A, Pietrasik J, Matyjaszewski K, Sheiko SS. J Mater Chem 2011;
21(23):8448e53.
[18] Lebedeva NV, Sun FC, Lee H-i, Matyjaszewski K, Sheiko SS. J Am Chem Soc
2008;130(13):4228e9.
[19] Gao H, Matyjaszewski K. J Am Chem Soc 2007;129(20):6633e9.
[20] Vogt AP, Sumerlin BS. Macromolecules 2006;39(16):5286e92.
[21] Neugebauer D, Zhang Y, Pakula T, Sheiko SS, Matyjaszewski K. Macromole-
cules 2003;36(18):6746e55.
[27] Matyjaszewski K, Xia J. Chem Rev 2001;101(9):2921e90.
[28] Wang J-S, Matyjaszewski K. J Am Chem Soc 1995;117(20):5614e5.
[29] Braunecker WA, Matyjaszewski K. Prog Polym Sci 2007;32(1):93e146.
[30] di Lena F, Matyjaszewski K. Prog Polym Sci 2010;35(8):959e1021.
[31] Kamigaito M, Ando T, Sawamoto M. Chem Rev 2001;101(12):3689e746.
[32] Tsarevsky Nicolay V, Matyjaszewski K. Chem Rev 2007;107(6):2270e99.
[33] Malkoch M, Malmstroem E, Hult A. Macromolecules 2002;35(22):8307e14.
[34] Jung S-H, Song H-Y, Lee Y, Jeong HM, Lee H-i. Macromolecules 2011;44(6):
1628e34.
[35] Sheiko SS, da Silva M, Shirvaniants D, LaRue I, Prokhorova S, Moeller M, et al.
J Am Chem Soc 2003;125(22):6725e8.
[36] Sheiko SS, Moeller M. Chem Rev 2001;101(12):4099e123.