Synthesis and direct topology visualization of high-molecular-weight star PMMA

Article abstract of DOI:10.1021/ma0484936

We report synthesis and direct topology visualization of single molecules of six-arm star poly(methyl methacrylate) (PMMA). We synthesized two hexafunctional initiators, 2,3,6,7,10,11-triph-enylene hexa-2-bromo-2- methylpropionate (fused-core initiator) a

Full text of DOI:10.1021/ma0484936

Macromolecules 2005, 38, 2093-2100
2093
Synthesis and Direct Topology Visualization of High-Molecular-Weight
Star PMMA
†
,†
‡
†
‡
L. Xue, U. S. Agarwal,* M. Zhang, B. B. P. Staal, A. H. E. M u1 ller,
§
†
C. M. E. Bailly, and P. J. Lemstra
Faculty of Chemical Engineering, Eindhoven University of Technology, 5600 MB, Eindhoven,
The Netherlands, Makromolekulare Chemie II, Universit a¨ t Bayreuth, D-95440 Bayreuth, Germany,
and Laboratory of Macromolecular Structureal Chemistry, Unit e´ de Physique et de Chimie des Hauts
Polym e` res, Universit e´ Catholique de Louvain, roix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium
Received July 23, 2004; Revised Manuscript Received December 15, 2004
ABSTRACT: We report synthesis and direct topology visualization of single molecules of six-arm star
poly(methyl methacrylate) (PMMA). We synthesized two hexafunctional initiators, 2,3,6,7,10,11-triph-
enylene hexa-2-bromo-2-methylpropionate (fused-core initiator) and ethylene glycol bis[3,4,5-tri(2-bromo-
2
-methylpropionate)] benzoylate (linear-core initiator). Their structures differ in the linking arrangement
of the initiating sites. Atom transfer radical polymerizations (ATRP) of methyl methacrylate (MMA) from
these initiators using CuCl as the catalyst and 4,4′-di-n-nonyl-2,2′-bipyridine as the ligand at 90 °C
w
produced six-arm star PMMAs with very high molecular weight ( Mh > 2 million) and narrow molecular
weight distribution (polydispersity index ≈ 1.3). A high initiation efficiency (>90%) of the initiating sites
is inferred from a good match of the Mh of the star PMMA with the expected value based on Mh of the
w
w
cleaved arms. SEC-LS analysis confirmed the compact structure of the star PMMA compared to linear
PMMA and the absence of star-star coupling. Tapping mode scanning force microscopy of a dip-coated
sample on mica enabled resolution of the individual arms linked to the initiating core and allowed clear
identification of predominantly six-arm star molecules.
Introduction
methacrylate) (PMMA) of high molecular weight ( Mh n ≈
16
3
67 000) and low polydispersity index (PDI ≈ 1.2). In
High-molecular-weight polymers with controlled ar-
chitectures are desirable for applications such as rhe-
ology modifications,1-5 as well as control of crystalliza-
tion characteristics, morphology development, and
this paper, we first report the synthesis of two hexafunc-
tional initiators: 2,3,6,7,10,11-triphenylene hexa-2-
bromo-2-methylpropionate (fused-core initiator, TP6Br)
and ethylene glycol bis[3,4,5-tri(2-bromo-2-methylpro-
pionate)] benzoylate (linear-core initiator, EG(Bz3Br)2).
We then use the well-optimized ATRP conditions to
synthesize high-molecular-weight fused-core and linear-
core star PMMAs by growing arms from these two
hexafunctional initiators. The control of the ATRP
processes is evaluated by following the growth of the
molecular weight of the stars. In addition, the growth
characteristics of the arms is independently evaluated
by size exclusion chromatography (SEC) and matrix
assisted laser desorption ionization time-of-flight mass
spectrometry (MALDI-TOF-MS) of the linear arms
cleaved from the star PMMA at the core. The efficiency
of initiation from the hexafunctional cores is character-
ized by relating the weight-average molecular weight
2,6-8
mechanical performance.
Star polymers are of inter-
est because they have an elementary branching topol-
ogy, with multiple linear chains linked to a single core.
To investigate the effect of the structure of the core, it
is desired to synthesize star polymers with core struc-
tures differing in linking arrangement of the arms, with
uniform arm lengths and number of arms. Strategies
toward making well-defined star polymers follow one
of the two routes. In the “arm-first” approach, mono-
functional linear chains are first synthesized by “living”/
controlled polymerization methods and then coupled to
multifunctional cross-linking agents to form the star
polymers. In the “core-first” approach, plurifunctional
initiators are used to grow arms by “living”/controlled
polymerization methods. Several researchers have de-
scribed the use of atom transfer radical polymerization
( Mh w) of the cleaved arms with the Mh w of the stars
determined by light scattering (SEC-LS). Tapping mode
scanning force microscopy (SFM) is used to directly
examine the topology of the star PMMA.
(
ATRP) for synthesis of star polymers by this core-first
9-14
approach.
In addition to the necessary high initiation
15
efficiency required for the core-first approach, synthe-
sis of high-molecular-weight star polymers demands low
extent of chain transfer and termination reactions12
while maintaining a moderate reaction rate. We recently
reported good control during a one-pot ATRP from a
model unifunctional initiator, 2-bromo-2-methylpropi-
onate, in a system with high monomer-to-initiating site
ratio ([M]o/[I]o ) 6400), leading to linear poly(methyl
Experimental Section
1
1
H NMR Measurements. H NMR spectra were recorded
on a Varian Mercury Vx 400 spectrometer at 400 MHz using
CDCl as solvent.
3
Size Exclusion Chromatography (SEC) and Coupling
with Light Scattering (SEC-LS). Average molecular weights
and radii of gyration were measured by SEC and SEC-LS
coupling using a Waters GPC equipped with a Waters 510
(SEC) or Waters 6000 A (SEC-LS coupling) pump, Waters 410
*
Author to whom correspondence should be addressed.
differential refractometer (at 40 °C, wavelength ) 930 nm),
and Waters WISP 712 autoinjector with injection volume of
50 (SEC) or 150 µL (SEC-LS). Columns for SEC measurements
were one PLgel (5 µm particle size) 50 mm × 7.5 mm guard
E-mail: u.s.agarwal@tue.nl.
†
‡
§
Eindhoven University of Technology.
Universit a¨ t Bayreuth.
Universit e´ Catholique de Louvain.
1
0.1021/ma0484936 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/12/2005

2094 Xue et al.
Macromolecules, Vol. 38, No. 6, 2005
column and two PLgel mixed-C (5 µm particle size, for 200 to
the literature method;19 the product was recrystallized from
6
2
× 10 g/mol) 300 mm × 7.5 mm columns (40 °C). Columns
2
deionized water and dried in a vacuum oven under N . TP6OH
for SEC-LS coupling were a Waters Filter guard column and
(0.56 g, 1.7 mmol), anhydrous tetrahydrofuran (30 mL), and
triethylamine (2.4 mL, 17 mmol) were charged into a dry 100
mL three-neck round-bottom reaction flask equipped with a
stirring bar. 2-Bromoisobutyryl bromide (2.0 mL, 16 mmol)
was then added dropwise through a dropping funnel with
stirring at 0 °C. The solution was stirred overnight, slowly
warmed to room temperature, filtered and extracted three
four Styragel 300 mm × 7.8 mm columns (HR5, HR4, HR3
4
6
3
5
2
and HR2 for 5 × 10 -4 × 10 , 5 × 10 -6 × 10 , 5 × 10 -3 ×
4
2
4
1
0 , and 5 × 10 -2 × 10 g/mol, respectively). Data acquisition
and processing were performed using the Waters Millennium
3
2 (v3.2) software. Tetrahydrofuran (THF, Biosolve, stabilized
with BHT) was used as eluent at a flow rate of 1.0 mL/min.
SEC calibration was done using polystyrene (PS) standards
times using saturated aq NaHCO
4
washed with deionized water, dried over anhydrous MgSO ,
3
/diethyl ether ) 3, v/v, and
6
(
Polymer Laboratories, 580 to 7.1 × 10 g/mol), and molecular
1
weights were recalculated using the universal calibration
and vacuum-dried at 40 °C before use. H NMR: δ 8.31 (s, 6
H, Ar), 2.16 (s, 36 H, -CH ) ppm.
principle and Mark-Houwink parameters (PS: K ) 1.14 ×
3
-4
-4
1
0
dL/g, R ) 0.716; PMMA: K ) 0.944 × 10 dL/g, R )
Synthesis of 3,4,5-Tri(2-bromo-2-methylpropionate)]
Benzoic Acid (Bz3Br). 2-Bromoisobutyryl bromide (20 g,
0.087 mol) and anhydrous THF (20 mL) were added into a dry
three-neck round-bottom reaction flask equipped with a stir-
ring bar and an ice bath. Triethylamide (5.9 g, 0.059 mol) was
added with stirring. After the reaction was stirred for 0.5 h,
3,4,5-trihydroxybenzoic acid (1.0 g, 5.9 mmol) solution in
anhydrous THF (20 g) was added dropwise through a dropping
funnel. The solution was stirred overnight and slowly warmed
to room temperature. The product was filtered and extracted
1
7
0
.719). For SEC-LS coupling, a Dawn DSP-F multi-angle
laser light scattering detector (source is a 5 mV linearly
polarized He-Ne laser at 632.8 nm) from Wyatt was used and
data processed with the Wyatt Astra (v4.72.03) software.
Absolute molecular weights were calculated with the help of
18
Zimm plots (refractive index increment dn/dc ) 0.086 mL/g).
An anionically polymerized linear PMMA sample (Mh
30 000, Polymer Laboratories) was used as reference.
Matrix Assisted Laser Desorption Ionization Time-of-
w
≈
8
Flight Mass Spectrometry (MALDI-TOF-MS). The poly-
mer sample solution was prepared in THF at a concentration
of 1 mg/mL. The matrix used was trans-2-[3-(4-tert-butylphe-
nyl)-2-methyl-2-propenylidene] malononitrile. The matrix solu-
tion in THF was prepared at a concentration of 40 mg/mL.
Silver trifluroacetate (cationic ionization agent, Aldrich, 98%)
solution in THF was prepared at a concentration of 1 mg/mL.
In a typical MALDI experiment, solutions of the polymer
sample, the matrix, and the salt were premixed in the ratio:
with diethyl ether/water, rotary evaporated, and then vacuum-
1
dried for 48 h. H NMR: δ 7.91 (s, 2H, Ar), 2.08 (s, 18H, -CH
3
)
ppm.
Synthesis of Ethylene Glycol Bis[3,4,5-tri(2-bromo-2-
). Bz3Br (2.0
methylpropionate)] Benzoylate (EG(Bz3Br)
2
g, 3.2 mmol), ethylene glycol (94 mg, 1.5 mmol), and 4-pyrro-
lidinopyridine (45 mg, 0.30 mmol) were dissolved in dry
chloroform (10 g), and DCC (0.64 g, 3.2 mmol) solution in
chloroform (10 mL) was added while stirring. After being
stirred for 2 h, the solution was filtered and then purified by
column chromatography over silica gel (0.040-0.063 mm),
5
µL sample: 5 µL matrix:0.5 µL salt. Approximately 0.5 µL of
the obtained mixture was hand-spotted on the target plate.
All of the spectra were acquired on the Voyager DE-STR
using (hexane/diethyl ether ) 3:2, v/v) as eluent, and subse-
1
(
(
Applied Biosystems) in the linear mode under high vacuum
quent crystallization from this eluent gave EG(Bz3Br)
2
. H
-
6
<1.37 × 10 Pa). The frequency of the laser is 20 Hz, and
NMR: δ 7.85 (s, 4H, Ar), 4.69 (s, 4H, -CH
36H, -CH ) ppm.
Typical ATRP Procedure. For ATRP of fused-core star
PMMA with monomer-to-initiating site ratio ([M] /[I]
2 2
CH -), 2.06 (s,
the voltage is 25 000 V. For each spectrum, 1000 laser shots
were accumulated.
3
Scanning Force Microscopy (SFM). SFM images were
recorded on a Digital Instruments Dimension 3100 microscopy
operated in Tapping Mode (free amplitude of the cantilever ≈
o
)
o
16 333), MMA (98 g, 0.98 mol), dnNbpy (0.36 g, 0.87 mmol)
and CuCl (0.046 g, 0.46 mmol) were charged into a reaction
flask and purged with dry argon for 1 h. TP6Br (0.01 g, 0.01
mmol) was charged into another flask, diluted with xylene (10
mL), purged with dry argon for 1 h, and transferred dropwise
over 3 min to the reaction flask. The reaction flask was then
immersed into an oil bath preheated to 90 °C. The reaction
mixture was stirred for 265 min. Samples were withdrawn
with a syringe at desired times. Each sample was dissolved
in THF, and the molecular weight distribution was determined
by using SEC without further purification. With increasing
viscosity, the stirring bar did not mix the reaction mixture
efficiently after 125 min. However, the reaction continued, and
the last sample was collected at 265 min with a spatula.
Precipitation Fractionation. When mentioned, the star
polymer was purified by fractional precipitation. The fused-
2
0 nm, spring constant ) 42 N/m, drive amplitude ) 277.4
mV, amplitude set point ≈ 0.98). The standard silicon nitride
probes (Olympus, OMCL-AC160TS) were driven at 3% offset
below their resonance frequencies in the range of 250-350
kHz. Height and phase images were taken at tip velocity of 4
µm/sec. The SFM measurements were performed in the
common laboratory conditions. The sample was prepared by
dip-coating from a dilute THF solution of the star PMMA onto
freshly cleaved mica. To prevent polymer from aggregating
during the solvent evaporation, a very dilute solution (4 mg/
L, lower than the overlapping concentration by orders of
magnitude) was used.
Materials. 2,3,6,7,10,11-Hexamethoxytriphenylene (TP6O-
Me) (Aldrich, 99%), boron tribromide solution in dichlo-
romethane (Aldrich, 1.0 M, >99%), triethylamine (Merck,
6
core star PMMA ( Mh w ≈ 2.35 × 10 ) was dissolved in THF (400
>
99%), 2-bromoisobutyryl bromide (Merck, >98%), 3,4,5-
mL) at a concentration of ∼3 wt% and heated to 50 °C.
n-Heptane (200 mL) was added gradually to the hot-stirred
solution. High-molecular-weight fraction (∼12 g) precipitated
first upon slow cooling to room temperature.
trihydroxybenzoic acid (gallic acid, Aldrich, 97%), anhydrous
tetrahydrofuran (Aldrich, inhibitor free, 99.5+%), anhydrous
ethylene glycol (Aldrich, 99.8%), 4-pyrrolidinopyridine (Aldrich,
98%), dicyclohexylcarbodiimide (DCC) (Merck, >99%), and
Cleavage of the Arms of the Star PMMA. The star
PMMA (10 mg) was dissolved in THF (10 mL). Methanol (2
mL) and NaOH (1 mg) were added, and the solution was
refluxed overnight at 80 °C to trans-esterify the ester groups
at the core of the star PMMA. After evaporating to dryness,
the cleaved product was analyzed by using SEC, MALDI-TOF-
silica gel 60 (0.040-0.063 mm) (Merck) were used as received.
Methyl methacrylate (MMA) (Aldrich, 99%) was passed through
a basic activated alumina column, extracted with NaOH (5
wt% aqueous), washed with water, dried, and distilled just
before use. CuCl (Aldrich, 99.995%) was refluxed with glacial
acetic acid, washed with absolute ethanol, and dried in a
vacuum oven at 100 °C for 3 days. Xylene (Biosolve, 99%) was
1
MS, SEC-LS, and H NMR.
distilled from CaH
2
before use. 4,4′-Di-n-nonyl-2,2′-bipyridine
Results and Discussion
(
dnNbpy) (Aldrich, 97%) was recrystallized from absolute
1
6
Synthesis of the Hexafunctional Initiators.
Scheme 1 outlines the synthesis of the fused-core
hexafunctional initiator and its use to prepare the fused-
core six-arm star PMMA. The commercially available
ethanol and dried in a vacuum oven at 40 °C overnight.
Synthesis of 2,3,6,7,10,11-Triphenylene Hexa-2-bromo-
-methylpropionate (TP6Br). TP6OMe was hydrolyzed into
,3,6,7,10,11-hexahydroxytriphenylene (TP6OH) according to
2
2

Macromolecules, Vol. 38, No. 6, 2005
High-Molecular-Weight Star PMMA 2095
Scheme 1. Synthesis of Fused-core Hexafunctional Initiator, 2,3,6,7,10,11-Triphenylene
Hexa-2-bromo-2-methylpropionate (TP6Br) and the “Core-First” Atom Transfer Radical Polymerization (ATRP)
of the Fused-Core Star PMMAa
a
3 2 3 2 3
i. 6 BBr /H O; ii. 6 Br(CO)C(CH ) Br/N(Et) ; iii. MMA, Cu(I)Cl/dnNbpy.
TP6OMe containing six methoxy groups attached to
triphenylene were hydrolyzed to give TP6OH containing
six hydroxyl functional groups.19 The ATRP initiating
groups with -Br functionality were linked to each of
the hydroxyl groups of TP6OH by an esterification
reaction to form the hexafunctional fused-core initiator
as the catalyst and dnNbpy as the ligand. Figure 3a
shows that the SEC-determined Mh w of star PMMA
increases continuously with the time of reaction, while
maintaining a low PDI. SEC columns separate polymers
according to their hydrodynamic volumes.20 Since SEC
is calibrated with linear polymer standard samples, it
underestimates the molecular weight of the star poly-
(
TP6Br, named to indicate the linking structure be-
tween the ATRP initiating sites).
1
Figure 1 shows the H NMR spectrum of this esteri-
fication product: the signal at 8.31 ppm is characteristic
of the aromatic protons and the signal at 2.16 ppm of
the methyl protons located in the R-position to bromines.
There is no signal left at 7.7 or 7.75 ppm corresponding
to the aromatic protons in TP6OH or TP6OMe, respec-
1
9
tively, indicating quantitative esterification.
Scheme 2 outlines the two-step preparation of the
linear-core ATRP initiator, where three initiating groups
with -Br functionality were first linked to gallic acid
to give Bz3Br. Two of the acid-functionalized Bz3Br
molecules were then coupled to ethylene glycol to form
thelinearcorehexafunctionalinitator,namedEG(Bz3Br)2
to indicate its central linear structure linking the two
sets of three initiating sites each.
1
The H NMR spectra of the coupling product
EG(Bz3Br)2 and its precursor Bz3Br are shown in
Figure 2. The signals at 7.91 and 2.08 ppm are assigned
to the aromatic and methyl protons in the precursor
Bz3Br. After the coupling reaction with ethylene glycol,
the signals shift to 7.85 and 2.06 ppm, respectively, with
the appearance of a new signal at 4.69 ppm correspond-
ing to the ethylene protons in the linear-core initiator
EG(Bz3Br)2. The disappearance of the signal at 7.91
ppm of the precursor Bz3Br indicates quantitative
coupling.
ATRP of Fused-Core Star PMMA. Well-defined
fused-core star PMMA were synthesized by ATRP from
the fused-core initiator (Scheme 1), using a high mono-
mer-to-initiating site ratio ([M]o/[I]o ) 3400), with CuCl
1
Figure 1. H NMR spectrum of the fused-core initiator
2
,3,6,7,10,11-triphenylene hexa-2-bromo-2-methylpropionate
(TP6Br) in CDCl .
3

2096 Xue et al.
Macromolecules, Vol. 38, No. 6, 2005
Scheme 2. Synthesis of Linear-Core Hexafunctional Initiator Ethylene Glycol
Bis[3,4,5-tri(2-bromo-2-methylpropionate)] Benzoylate (EG(Bz3Br)
2
) and the “Core-First” Atom Transfer Radical
a
Polymerization (ATRP) of the Linear-Core Star PMMA
a
i. 3 BrCOC(CH
3 2
) Br/N(Et)
3
; ii. 1/2 Ethylene glycol/dicyclohexylcarbodiimide/4-pyrrolidinopyridine; iii. MMA, Cu(I)Cl/dnNbpy.
mers due to their compact volumes compared to the
linear chains with the same molecular weight. However,
theoretical predictions of macromolecular volumes based
on their conformations allow us to estimate the true
molecular weight of a star polymer (Ms) of known
architecture (f arms) from the apparent molecular
weight (Ml) measured in SEC calibrated with linear
weight of a six-arm star polymer (Ms) should be 1.48
times the molecular weight values (Ml) shown in Figure
3a as values measured by SEC calibrated with linear
polymers. We therefore also used SEC-LS to determine
the absolute Mh w of the final sample (125 min) of the
fused-core star PMMA as 535 000. This is 1.41 times
the Mh w of the same sample as determined by SEC
(Figure 3a, Mh w ≈ 380 000) calibrated with linear
polymers, and thus in agreement with the theoretical
prediction (eq 1)
We also evaluated the progress of the ATRP by
carrying out cleavage of the arms by transesterification
of the ester groups at the core of the star PMMA
samples withdrawn during the polymerization at vari-
ous times. The Mh w of the cleaved arms was then
characterized by SEC and MALDI-TOF-MS. Figure 3b
shows that the Mh w of the cleaved arms also increased
during the reaction, while the PDI remained low, again
indicating that the ATRP proceeded in a controlled
manner. The Mh w values determined by MALDI-TOF-
MS are about 10% higher than the corresponding values
determined by SEC. This could be related to the limited
accuracy of the universal calibration employed in SEC
measurements or to some molecular weight dependence
of the ionization efficiency during MALDI-TOF-MS. The
arm Mh w values further permit us to estimate the Mh w of
the star polymers by considering that six arms make
up each of the star PMMA. These estimates are also
plotted in Figure 3a. When the so-estimated value of
the star PMMA (Figure 3a) from the MALDI-TOF-MS
data of the arms is extrapolated to higher reaction
times, the value at 125 min seems to correspond well
with the Mh w of the star PMMA directly determined by
SEC-LS. This indicates a high efficiency of initiation
from all six sites of the hexafunctional initiator.
2
1-23
polymer:
1
/2
3/(1+R)
Ml
f
)
(1)
Ms
(
x
)
2(f - 1) - f + 2
1
7
where R ) 0.719 for PMMA in THF is the Mark-
Houwink constant. This suggests that the molecular
While past attempts to synthesize well-controlled star
1
Figure 2. H NMR spectra of linear-core initiator ethylene
polymers by ATRP are limited to Mh w lower than
glycol bis[3,4,5-tri(2-bromo-2-methylpropionate)] benzoylate
600 000,1
3,24
we desired star PMMA of much higher
(
EG(Bz3Br)
2
, solid line) and its precursor 3,4,5-tri(2-bromo-2-
methylpropionate)] benzoic acid (Bz3Br, dashed line) in CDCl
3
.
molecular weight for potential application as solution

Macromolecules, Vol. 38, No. 6, 2005
High-Molecular-Weight Star PMMA 2097
1
Figure 5. H NMR spectra of star PMMA (solid line, corre-
sponding to Figure 4 dashed line c) and the cleaved arms
(
dashed line, corresponding to Figure 4 dashed line d).
2
65 min ( Mh w ≈ 1.32 million, DPI ≈ 1.30 by SEC) was
also analyzed by SEC-LS and found to have Mh w ≈ 2.35
million. We do notice a small and broad peak at the low-
molecular-weight end. This peak appeared at an early
stage during the ATRP, as seen in Figure 4 a (t ) 61
min). Shifting of this minor peak also to higher molec-
ular weights with progress of reaction (Figure 4 b)
suggests that this may correspond to a small fraction
of linear PMMA chains formed by chain transfer reac-
tions. However, this apparent linear fraction could be
easily removed (Figure 4 c) by a subsequent purification
by precipitation fractionation, as described in the Ex-
perimental Section. This final star PMMA sample
(
Figure 4 c) was also subjected to cleavage of the arms,
and the arms were analyzed by SEC (Figure 4 d, Mh w ≈
3
39 700, PDI ≈ 1.16). Considering the possible limita-
tion of the universal calibration employed in SEC, the
arm molecular weight was further analyzed by SEC-
LS. The so-determined Mh w of 360 000 corresponds
reasonably well to the expected value 390 000 for a six-
arm star PMMA of Mh w ≈ 2.35 million and thus indicates
a well-controlled six-arm star architecture of this very
high-molecular-weight fused-core star PMMA.
Figure 3. Mh
symbols) of PMMA formed at different times during the
polymerization with monomer-to-initiating site ratio ([M] /[I]
3400), (a) fused-core star PMMA by SEC and SEC-LS and
w w n
(empty symbols) and PDI () Mh / Mh ) (filled
o
o
)
the comparison with six times the molecular weight of their
cleaved arms, (b) the cleaved arms by SEC and MALDI-TOF-
MS. The dashed line in (a) is the linear fit to the MALDI-TOF-
MS data.
As in our earlier work with linear PMMA,16 control
was achieved here by maximizing the initiation ef-
ficiency and minimizing the inter- and intramolecular
termination of the growing species by (a) using very high
monomer-to-initiating site ratio ([M]o/[I]o), (b) predilu-
tion of the initiator core before adding into the poly-
merization flask, (c) halogen exchange to speed up
initiation, and slow propagation, and (d) limiting our
polymerizations to low monomer conversion.
1
H NMR (400 MHz) spectra of the original star
PMMA and the cleaved arms are shown in Figure 5;
the chemical shifts at 0.84, 1.02, and 1.2 ppm correspond
to the syndiotactic, heterotactic, and isotactic methyl
protons (-CH3), respectively.25 The signal at 3.60 ppm
corresponds to the ester protons (-OCH3). As expected,
the area ratio of these two types of protons is 1 for both
the star PMMA and the cleaved-arm samples.
ATRP of Linear-Core Star PMMA. High-molecu-
lar-weight linear-core star PMMA were synthesized
from the hexafunctional initiator EG(Bz3Br)2 as shown
in Scheme 2 by ATRP of MMA with high [M]0/[I]0
()19 074). Similar to the case of fused-core star PMMA,
the SEC peaks of the linear-core star PMMA samples
shifted to increasing molecular weights with increasing
reaction times (Figure 6a), while retaining narrow
width, again indicating the controlled nature of the
ATRP reaction. The corresponding Mh w and PDI values
are shown in Figure 6b. The final sample at 240 min
( Mh w ≈ 1.19 million, PDI ≈ 1.26 by SEC) was analyzed
Figure 4. SEC traces of high-molecular-weight fused-core star
o o
PMMA formed at different times during the [M] /[I] ) 16 333
polymerization. Solid lines a and b represent samples with-
drawn during the polymerization at 61 and 265 min, respec-
tively. Dashed line c represents precipitation fractionation
product of b. Dashed line d represents the cleaved arm of c.
rheology modifier. For this, we carried out the ATRP
while using a much higher [M]o/[I]o () 16 333). As seen
in Figure 4, the SEC peaks of the samples withdrawn
during ATRP shifted to higher molecular weights while
retaining narrow width. This indicates that the ATRP
proceeded in a controlled manner. The final sample at

2098 Xue et al.
Macromolecules, Vol. 38, No. 6, 2005
Figure 7. SEC-LS characterization of PMMA samples. Tri-
angles, linear PMMA reference; squares, fused-core star
PMMA (curve c of Figure 4); circles, linear-core star PMMA
sample (curve g of Figure 6a). (a) Molecular weight (symbols)
and concentration distribution (lines) vs elution volume. The
solid line represents fused-core star PMMA, and the dashed
line represents linear-core star PMMA. (b) Radius of gyration
vs molecular weight. The lowest MW data for the star polymers
overlap with those of the linear reference sample and have
been removed for clarity. The solid lines represent power law
with 0.5 exponent.
Figure 6. SEC traces (a), weight-average molecular weight
(empty symbols) and polydispersity index (filled symbols), and
the comparison with six times the molecular weight of their
cleaved arms (b) of the linear-core star PMMA formed at
o
different times during the progress of ATRP using high ([M] /
o
[I] ) 19 074). Solid lines represent samples withdrawn during
the polymerization at following times: a, 10 min; b, 20 min;
c, 30 min; d, 80 min; e, 173 min; and f, 240 min. Dashed line
g represents the precipitation fractionation product of the 240
min sample (curve f). Dashed line h represents cleaved arms
of the fractionated star PMMA (curve g).
have higher MW compared to linear PMMA at the same
hydrodynamic volume (assumed to correspond to elution
volume, Ve) as shown in Figure 7a. The change in slope
of the MW/Ve curves for both the linear and the star
PMMA at Ve ≈ 24-25 mL is related to the column
characteristics. The concentration signals from both star
polymers are also shown in the same figure, and we
notice that, over the central part of the star polymer
distributions, the MW ratio at identical hydrodynamic
volume of linear and star molecules remains essentially
constant (∼1.7). This value is higher than the 1.48 figure
predicted by the Cassasa formula (eq 1). The discrep-
ancy can be due to one or more of the following. First,
the linear PMMA used as reference has been anionically
polymerized and therefore has different tacticity and,
hence, a different hydrodynamic radius vs MW relation-
ship than the PMMA synthesized by ATRP. Second,
excluded volume effects (THF is not a theta solvent for
PMMA), unaccounted for by eq 1, could also play a role.
Third, eq 1 may have additional dependence on pore size
and arm length near the exclusion limit of the SEC
columns.23
by SEC-LS and found to have Mh w ≈ 2.1 million. Similar
to the fused-core star PMMA case (Figure 4, samples a
and b), the last two samples in Figure 6a also show
minor peaks at low molecular weight, shifting to higher
molecular weights during the polymerization. Again,
this low-molecular-weight peak was easily removed by
precipitation fractionation (dashed line g of Figure 6a).
This linear-core star PMMA sample was then subjected
to cleavage into arms, which were analyzed by SEC
(
dashed line h of Figure 6a, Mh w ≈ 336 100, PDI ≈ 1.18)
and SEC-LS ( Mh w ≈ 370 000). These values compare well
with the value 350 000 expected on the basis of six-arm
star PMMA of Mh w ≈ 2.1 million.
Molecular Characteristics of the Star PMMAs by
SEC-LS. Shapes of the polymer molecules in solution
are best investigated by combined measurements of
radius of gyration (Rg, z-average) and absolute molec-
ular weight (MW, weight average) by light scattering
2
6
of fractions eluting during SEC. In Figure 7, we
compare these characteristics of our very high-molecu-
lar-weight fused-core and linear-core star PMMA samples
with a linear PMMA reference. Star PMMA samples
At the low end of the molecular weight distributions
corresponding to elution volume greater than 25.5 and

Macromolecules, Vol. 38, No. 6, 2005
High-Molecular-Weight Star PMMA 2099
Figure 8. SFM Tapping Mode images of star PMMA (Mh
w
≈ 535 000, by SEC-LS) on mica: (a) height image (z-range: 1.8 nm);
(
b) phase image (z-range: 15°); (c) the height profile of two PMMA arms along the solid line indicated in (a); (d) schematic
representation of the possible molecular conformations of labeled star molecules in (a) and (b).
2
6 mL for the fused-core and linear-core star PMMA,
the highest MW fraction for the linear-core star PMMA.
We can thus conclude that the star-star coupling was
virtually eliminated during our ATRP, perhaps due to
the termination of polymerization at low conversion (less
than 0.2).
respectively, the MW vs Ve curves of the star polymers
deviate toward the linear reference curve (Figure 7a).
This could be related to a small extent of incomplete
ATRP initiation resulting in fewer arms or a small
extent of chain-transfer reactions resulting in some
smaller arms and some linear chains, as was also
inferred from the SEC observations (Figures 4 and 6a).
Figure 7b shows that most of the star PMMA samples
have lower Rg than linear PMMA of same MW. The
close overlap of the linear core and fused core star
PMMA characteristics in Figure 7a and b indicates a
similar branching topology for these two samples. Over
the main part of the molecular weight distribution of
the star polymers, the Rg/MW relationship follows a
power law with an exponent very close 0.5 (Figure 7b).
The discrepancy below 1.4 million, for the star Rg/MW
relationship with an exponent lower than 0.5, may be
due to some star molecules of fewer arms or some linear
chains, leading to a less compact conformation. If
intermolecular coupling of star polymers takes place
during polymerization, the coupled product should be
discernible as a prominent high-molecular-weight shoul-
SFM Visualization of Branching Topology of
Star PMMA. In recent years, SFM has emerged as a
very useful tool for direct visualization of isolated linear
polymer molecules.27 The technique has also been
employed for visualization of star and branched poly-
mers but only after dense grafting to enhance the
2
7-30
backbone feature width
or metallization to enhance
the contrast.3
1,32
As shown in Figure 8, we managed to
visualize the branch architecture by directly fixing the
star-PMMA molecules ( Mh w ≈ 535 000 by SEC-LS, the
final sample in Figure 3a) on mica substrate in an
isolated form by dip-coating from a very dilute solution
in THF. We believe that the mobility of the PMMA
chains was limited by the interaction of the ester groups
of PMMA with the polar mica substrate. In addition,
fast evaporation of the solvent helped avoid the ag-
gregation of the arm-branches. The height profile in
Figure 8c permits the estimation of the thickness of the
arm-branches as about 0.2 nm. The SFM images in
Figure 8 allows us to count the numbers of arms in
individual star molecules, such as six arms for the
molecules labeled A1-A9, five arms for the molecule
labeled B1. As shown further in Figure 8d as the
schematic sketch of Figure 8a and b, the majority of the
star PMMA molecules have six arms. The molecules A4
1
2,23
der in the concentration signal.
However, the SEC
peaks (Figures 4, 6a, and 7a) are fairly symmetric and
devoid of any distinct shoulder. Moreover, the exponent
of the Rg vs MW power law must be affected by coupling,
if present. Only the slightest hint of such a crossover
can be observed at the highest MW in Figure 7b for the
fused-core star PMMA, while the power law extends to

2
100 Xue et al.
Macromolecules, Vol. 38, No. 6, 2005
(2) Gao, C.; Yan, D. Prog. Polym. Sci. 2004, 29, 183.
and A5 appear at first sight to be a case of intermo-
lecular coupling between arms of the two molecules, but
that would have resulted in detection of 11 independent
arms. However, a close examination reveals 12 arms for
the two molecules, indicating a simple overlapping
during the coating process as one arm of the molecule
A4 happens to reach the core of molecule A5. For some
cases such as D1, a judgment of the number of arms is
made ambiguous by the apparent overlapping of arms.
Intramolecular coupling, incomplete initiation, and
transfer reactions at the early stage of the polymeriza-
tion may result in the four-arm star, five-arm star, and
linear molecules. However, the extent of such reactions
is limited. For example, we find only one linear chain
(
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Six-arm star PMMAs with two different core struc-
tures were synthesized by core-first ATRP. While in the
linear core star PMMA, two sets of three arms each were
linked with a linear segment, all six arms in the fused-
core star PMMA were independently connected to a
central fused ring structure. Molecular weight charac-
terization of the star PMMA and the cleaved arms
indicated that the ATRP initiated from all the six
initiating sites of the specially synthesized hexafunc-
tional initiators and proceeded in a controlled manner.
Star PMMAs of molecular weight higher than 2 million,
and low polydispersity, were thus obtained while using
very high [M]o/[I]o and terminating the reaction at low
monomer conversion. The smaller Rg of the star PMMA
as compared to linear PMMA of same molecular weight
and a power law relationship between Rg and molecular
weight confirmed the compact and regular structure of
the star PMMA. Isolated molecules of ATRP product
were visualized by tapping mode SFM, and most of
these clearly showed the six-arm star PMMA architec-
ture.
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Acknowledgment. We thank Wieb Kingma for SEC
measurements and Pascal van Velthem for SEC-LS
measurements.
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