Synthesis and direct visualization of block copolymers composed of different macromolecular architectures

Article abstract of DOI:10.1021/ma047375f

A novel approach toward the synthesis of block copolymers composed of architecturally different components, in this case, a nanoparticle covalently attached to a single linear coil is presented. By a synergistic combination of controlled radical polymeriz

Full text of DOI:10.1021/ma047375f

2674
Macromolecules 2005, 38, 2674-2685
Synthesis and Direct Visualization of Block Copolymers Composed of
Different Macromolecular Architectures
Jeffrey Pyun,†,‡ Chuanbing Tang,§ Tomasz Kowalewski,§ Jean M. J. Fre´chet,*,‡ and
Craig J. Hawker*,†
IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, Department of
Chemistry, University of California, Berkeley, California 94720, and Department of Chemistry,
Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213
Received December 20, 2004
ABSTRACT: A novel approach toward the synthesis of block copolymers composed of architecturally
different components, in this case, a nanoparticle covalently attached to a single linear coil is presented.
By a synergistic combination of controlled radical polymerization, convergent dendrimer synthesis, and
benzocyclobutene (BCB) cross-linking chemistry, strategies for the preparation of a variety of nanopar-
ticle-coil copolymers were developed. Atomic force microscopy (AFM) was used to confirm the formation
of architecturally differentiated block copolymers and enabled visualization of individual nanoparticles
and their linear chain components for unambiguous characterization of the nanoparticle-coil structures.
This confirmed the synthesis of the targeted nanostructure and revealed the dramatic effect that changes
in macromolecular architecture can have on the morphology and assembly of these hybrid nanoparticle
systems.
Introduction
that the morphology and properties of polymeric mate-
rials can be dictated by composition, molar mass and
architecture of the copolymer segments. Of these struc-
tural aspects, architecture is the most poorly studied,
yet it has the potential to be an extremely powerful tool
for controlling material properties.
To this end, hybrid materials composed of dendritic
and linear segments are an interesting class of block
copolymer that combine components of very different
architecture and composition into a single macromol-
ecule.4 Although still relatively unexplored, hybrid
dendron-coil copolymers have been shown to possess
unusual solid-state morphologies5 and have generated
interest as potential carriers for targeted drug delivery.6
The synthesis of dendron-coil copolymers possessing
dendritic, or branched, segments of high molar mass
however still remains a difficult challenge due to the
multistep synthesis required to prepare large dendrim-
ers, limiting the molecular weight of the dendron
fragment to less than 5000 g/mol.
In related studies, developments in living anionic
polymerization have enabled the synthesis of asym-
metric stars containing branched and linear segments
in the same macromolecules.7 These routes afford well-
defined block copolymers of precise architecture and
composition, though inherent difficulties associated with
sensitivity toward various functional groups have lim-
ited the application of this approach.
To overcome issues of molecular size and availability
of materials, an accelerated route to hybrid block
copolymers composed of nanoparticles and linear poly-
mers has recently been demonstrated using controlled
radical polymerization3d,g,h in which the 3-dimensional,
dendron-like block segments have much higher molec-
ular weights (e.g., MW greater than 50 000 g/mol).8
Herein, we report the synergistic use of dendrimer
synthesis and controlled radical polymerization for the
preparation of functional linear macromolecular archi-
tectures, which serve as useful precursors to hybrid
nanoparticle-coil copolymers. By the combination of
The synthesis of novel polymeric materials is an
evolving field of research with the preparation of
macromolecules possessing precise structures coupled
to improved properties being an area of particular
interest. In this respect, synthetic polymers currently
lag behind biological systems, as mimicry of the intricate
topology, 3-dimensional structure, and functionality of
biological polymers remains a difficult challenge. Natu-
rally occurring polymers, namely, enzymes and proteins,
often incorporate different architectures (i.e., globular,
linear) into a single macromolecule enabling the cre-
ation of discrete nanoenvironments for catalysis and
self-assembly.1a,b Recent developments in polymer thera-
peutics have seen the launching of several highly
successful drugs with mixed globular-linear architecture
in which a globular protein is attached to a synthetic
linear polymer such as poly(ethylene glycol).1c In these
systems the synthetic linear-globular assembly shows
vast performance enhancements over the natural globu-
lar protein on its own.
Despite these advances, biological mimicry in the
world of synthetic polymers is still in its infancy and it
is important to develop our abilities to prepare and
characterize macromolecular isomers and large archi-
tecturally defined assemblies through nanoscale control
of both the structure and the properties of the synthetic
constructs.2 The ability to tune properties of bulk
materials has been extensively pursued by directed self-
assembly of block copolymers, in both solution and in
the solid state,2a-f to form well-defined phase separated
structures, or nano-objects.2g-i Recent advances in
synthetic polymer chemistry have enabled the prepara-
tion of a much wider variety of well-defined di- and
multiblock copolymers3 and these systems demonstrate
* Corresponding author. E-mail: hawker@almaden.ibm.com.
† IBM Almaden Research Center.
‡ University of California.
§ Carnegie Mellon University.
10.1021/ma047375f CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/01/2005

Macromolecules, Vol. 38, No. 7, 2005
Block Copolymers with Different Architectures 2675
Scheme 1. Synthesis of Nanoparticle-Coil Copolymer
(2) from Linear Diblock Copolymer Precursor (1)
Figure 1. SEC plots of (a) pBA macroinitiator (Mn ) 56 000
g/mol; Mw/Mn ) 1.18), (b) pBA-b-p(S-r-BCB) diblock copolymer
precursor 1 (Mn ) 89 000 g/mol; Mw/Mn ) 1.41), and (c)
particle-coil copolymer (2) (Mn apparent ) 55 000 g/mol; Mw/Mn
) 1.56) prepared from thermal induced cross-linking of 1.
were anticipated to yield optimal structures for AFM
visualization as previous reports on pBA containing
polymer brushes have demonstrated that the spreading
of pBA segments on polar surfaces facilitated the
imaging of individual macromolecules with single chain
resolution.12 Furthermore, the enhanced rigidity of the
intramolecularly cross-linked nanoparticle was expected
to impart both sharp height and phase contrast relative
to the linear segment during visualization of the nano-
particle-coil structures. Linear copolymers of poly(n-
butyl acrylate)-block-poly(styrene-random-(4-vinylben-
zocyclobutene) (pBA-b-p(S-r-BCB)) were therefore pre-
pared by sequential polymerization of n-butyl acrylate
(BA) followed by a mixture of styrene (Sty) and 4-vi-
nylbenzocyclobutene (BCB).17 Copolymers possessing
radii of gyration (Rg) greater than 5 nm were targeted
to facilitate characterization of the particle-coil copoly-
mer using AFM.
these controlled polymerizations with the benzocy-
clobutene (BCB) cross-linking process,8 well-defined
molecular objects possessing a polymeric nanoparticle
and a single linear chain were prepared.
The imaging of these molecular objects with molecular
resolution using atomic force microscopy (AFM) is an
extremely powerful technique for confirming the prepa-
ration and structure of complex polymeric nanostruc-
tures.9 Recent success in AFM imaging of various
polymeric structures has been reported with dendrim-
ers,10 graft and dendrigraft copolymers,11 dendronized
polymers,12 and molecular polymer brushes.13
Compared to other techniques, the real strength of
AFM is in directly visualizing the shape and structure
of single macromolecules or molecular assemblies and
when applied to complex polymer architectures have
permitted the unprecedented characterization of these
systems. For molecular brushes with both linear and
starlike architectures, Sheiko and Matyjaszewski were
able to determine and quantify arm length, defect
structures, and molecular organization by high-resolu-
tion AFM.14 Cylindrical macromolecular architectures
such as dendronized linear polymers have also been
visualized by both the groups of Mo¨ller and Percec15 and
their adsorption and organization on highly orientated
substrates examined. In perhaps the most far reaching
application of molecular visualization and polymer
architecture, Schlu¨ter has employed AFM, to not only
visualize these 3-dimensional macromolecules, but to
manipulate the position of single molecules.16 By using
photochemically active chain end functionalized den-
dronized linear polymers, an elementary step toward
molecular nanoconstruction was taken in subsequently
covalently connecting two individual polymer chains.
By combining new synthetic strategies for nanopar-
ticle-linear macromolecules with structural analysis by
AFM, the preparation and visualization of novel hybrid
architecture is reported.
In the first step of the synthesis, a pBA macroinitiator
(Mn ) 50 000; Mw/Mn ) 1.18) was prepared by bulk
polymerization of BA (7 M) at 125 °C in the presence of
the alkoxyamine, 2,2,5-trimethyl-3-(1-phenoxyethyl)-4-
phenyl-3-azahexane (7 × 10-3 M) and the corresponding
free nitroxide (5 mol % relative to alkoxyamine), reach-
ing a conversion of 54% in 20 h. Chain-extension of the
formed pBA macroinitiator was then performed in
solution with Sty/BCB (4:1 molar feed ratio, in 50 vol
% o-xylene) at 125 °C, where monomer conversions of
60% Sty and 59% BCB were obtained after 18 h.
Characterization of the crude polymerization mixture
using size exclusion chromatography (SEC) and 1H
NMR confirmed efficient blocking to the pBA segment
as higher molar mass (Mn ) 89 000; Mw/Mn ) 1.41,
Figure 1) and incorporation of p(S-r-BCB) (45 mol %
1
pBA, 45 mol % pS, 10 mol % pBCB measured from H
NMR).
Hybrid nanoparticle-coil copolymer 2 was then pre-
pared by the slow addition of the pBA-b-p(S-r-BCB)
linear copolymer (1) to a benzyl ether solution at 250
°C (Scheme 1). Thermally induced intramolecular cross-
linking of p(S-r-BCB) segments yielded the target
copolymer (2), possessing a polystyrenic nanoparticle
and a single linear coil of pBA. SEC confirmed a
decrease in apparent molar mass and hydrodynamic
volume consistent with nanoparticle formation (Mn )
55 000; Mw/Mn ) 1.56) due to intramolecular cross-
Results and Discussion
A. Nanoparticle)Coil Copolymers from Linear
Diblock Copolymers. The initial approach for the
preparation of nanoparticle-coil diblock copolymers
involved the synthesis of linear diblock copolymers,
followed by selective intramolecular cross-linking of the
linear block containing pBCB groups (Scheme 1).17
Nanoparticle-coil copolymers composed of a linear poly-
(n-butyl acrylate) (pBA) and a polystyrenic nanoparticle
1
linking and chain collapse (Figure 1c). H NMR con-
firmed intramolecular cross-linking of pBCB groups, by
the loss of methylene protons from the pBCB groups at

2676 Pyun et al.
Macromolecules, Vol. 38, No. 7, 2005
Scheme 2. Limitations of Controlled Radical
firmed the synthesis of hybrid linear nanoparticle
diblock copolymers. For the linear pBA-b-p(S-r-BCB) (1)-
two thermal transitions were observed corresponding
to the glass transitions (Tg) of pBA (Tg ) -45 °C) and
p(S-r-BCB) (Tg ) 110 °C). Nanoparticle-coil copolymers
(2) also exhibited two transitions assigned to the Tg of
pBA (-45 °C) and a transition at 55 °C attributed to
the nanoparticle segment.18
Polymerization Approach in the Synthesis of
Nanoparticle-Coil Copolymer (2) from Linear
Diblock Precursor (1)
B. Nanoparticle-Coil Copolymers from Dendrit-
ic Initiators. To overcome the difficulties in preparing
hybrid nanoparticle-coil structures using linear diblock
copolymers, an alternative strategy was designed using
dendritic initiators to prepare asymmetric star copoly-
mers containing pBCB groups, which could then be
collapsed to give the desired products. It was envisaged
that the star copolymer strategy would greatly facilitate
the purification of these materials from homopolymer
impurities. In this approach, poly(benzyl ether) (pBE)
dendrons possessing latent initiator groups for atom
transfer radical polymerization (ATRP) at the periphery
and an alkoxyamine at the focal point were prepared
with typical ATRP initiating groups, benzylic halides
and R-haloesters, as the chain ends.3g This enables the
preparation of an asymmetric star with multiple short
arms of p(S-r-BCB) and a single long pBA chain.
Subsequent intramolecular cross-linking of p(S-r-BCB)
chains would give a nanoparticle-coil copolymer pos-
sessing a polystyrenic nanoparticle and a single pBA
linear coil (Scheme 3). The use of a dendritic initiator
circumvents many of the problems encountered using
the linear diblock copolymer precursor. In particular,
the functional dendritic initiator ensures high retention
of initiating sites and allows reactions to be performed
in an orthogonal fashion. Additionally, the use of ATRP
to grow p(S-r-BCB) chains at lower temperatures suf-
ficiently suppressed the contribution of thermal self-
initiation and any chains that were prepared would be
of a much lower molecular weight and therefore easily
removed by precipitation.
Synthesis of Fourth Generation Dendritic Ini-
tiator. Because of difficulties in carrying R-haloester
and benzyl bromide chain ends through a traditional
convergent growth approach,19 a protected Fre´chet-type
poly(benzyl ether) fourth generation dendron ((THP)16-
[G-4]) possessing 16-peripheral tetrahydropyranyl (THP)
protecting groups and an alkoxyamine focal point was
initially prepared.20 The resulting hexadeca THP de-
rivative, 3, was then deprotected with para-toluene-
sulfonic acid to give the phenolic derivative, 4, in greater
than 95% conversion, however difficulties in purifying
4 resulted in a decreased yield of 54% after purification.
Esterification of the terminal phenolic groups with
2-bromoisobutyryl bromide then yielded the multifunc-
tional dendritic initiator 5 in 70% yield after purification
by column chromatography.
δ ) 3.0 and the appearance of resonances for cyclooctyl
groups at δ ) 2.6-2.8 which are the primary product
of the cross-linking reaction.8
The fidelity of the series of reactions resulting in the
hybrid nanoparticle-linear structure was questioned by
SEC which showed a lower molar mass shoulder in the
chromatogram of nanoparticle-coil copolymer 2 pre-
pared from pBA-b-p(S-r-BCB) precursor 1. This was
shown to be nanoparticles formed from p(S-r-BCB)
contaminants and characterized as homopolymer p(S-
r-BCB) impurities formed during the chain extension
of the pBA macroinitaitor from thermal self-initiation
of styrenic monomers (Scheme 2). The presence of the
p(S-r-BCB) impurities was not evident in the SEC
chromatograms of the crude product (Figure 1b) due to
coincidental overlap of the pBA-b-p(S-r-BCB) block
copolymer and p(S-r-BCB) contaminant, as well as, the
polydispersity of the crude mixture. However, after
addition to benzyl ether at 250 °C, the presence of both
nanoparticle-coil copolymers from pBA-b-p(S-r-BCB)
and nanoparticle impurities derived from p(S-r-BCB)
contaminants could now be resolved due to the greater
relative decrease in hydrodynamic volume for the ho-
mopolymer contaminant on collapse when compared to
the block copolymer. This ability to distinguish the
homopolymer contamination by chemistry in situations
where it is difficult using one-dimensional SEC and 1H
NMR is an interesting feature of different macromo-
lecular architectures.
Initially, the strategy to prepare hybrid nanoparticle-
coil copolymers using dendritic initiator 5 involved
copolymerization of styrene and 4-vinylbenzocyclobutene
(BCB) under ATRP conditions from the peripheral
2-bromoisobutyrate groups, followed by nitroxide medi-
ated polymerization of butyl acrylate (BA) from the focal
point. However, ATRP experiments using the dendritic
initiator 5 proved to be difficult due to the temperature
sensitivity of the alkoxyamine. Previously reported
conditions for the ATRP of styrenic monomers from
multiple functional initiators employed high tempera-
tures (T ) 90-130 °C) with less reactive catalysts, such
By rigorous fractional precipitation and preparatory
GPC, small amounts of pure diblock, 1, and hybrid
nanoparticle, 2, could be obtained for AFM and DSC
analysis. Differential scanning calorimetry (DSC), before
and after thermal treatment at 250 °C, further con-

Macromolecules, Vol. 38, No. 7, 2005
Block Copolymers with Different Architectures 2677
Scheme 3. General Approach to Synthesize Nanoparticle-Coil Copolymers Using a Dendritic initiator
Scheme 4. Synthesis of Dendritic Initiator (5) Composed of Fourth Generation Fre´chet-Type Dendron
Containing 16 Initiation Sites for ATRP at the Periphery and a Single Alkoxyamine Focal Pointa
a Conditions: (a) TsOH, acetone; (b) 2-bromoisobutyryl bromide, triethylamine, THF.
as copper(I) bromide-bipyridine (CuIBr/bpy) based
systems and at these temperatures significant homoly-
sis of the alkoxyamine group would be expected.21 Thus,
initial attempts to grow p(S-r-BCB) arms from 5 utilized
more reactive catalysts based on CuIBr and aliphatic
amines at 60-70 °C.22 However, only limited monomer
conversions (<10%) and low yields of p(S-r-BCB) star
copolymers were obtained. Polymerization of Sty/BCB
from 5 with CuIBr/bpy based catalysts were also at-
tempted at 90 °C but only low conversions (<5%) were
reached after long reaction times (t > 24 h). Attempts
to first perform the nitroxide mediated growth of the
linear poly(butyl acrylate) chain before ATRP from the
dendritic chain ends also proved to be difficult and
numerous problems were encountered when bulk ni-
troxide mediated polymerizations of BA from the focal

2678 Pyun et al.
Macromolecules, Vol. 38, No. 7, 2005
Scheme 5. Synthesis of Dendritic Initiator (14) Using the Convergent Growth Approacha
a Conditions: (a) 3,4-dihydro-2H-pyran, HCl(aq), CH2Cl2; (b) K2CO3, 18-crown-6, KI, 3,5-dihydroxybenzoate, acetone; (c) LiAlH4,
THF; (d) CBr4, PPh3, Hu¨nig’s base, THF; (e) K2CO3, 18-crown-6, 3,5-dihydroxybenzoate; (f) K2CO3, 18-crown-6, 13, acetone.
point of 5 were attempted using standard conditions at
125 °C. Despite long reaction times, polymerization of
BA was not observed, presumably due to chain transfer
reactions of formed radicals with terminal alkyl halide
groups of the dendrimer.
Synthesis of Third Generation Dendritic Initia-
tor and Dendron-Coil Macroinitiator. In light of
these difficulties, an alternate dendritic initiator was
designed to allow the sequential polymerization of butyl
acrylate followed by styrene/BCB using nitroxide medi-
ated and ATRP polymerizations, respectively. A novel
Frechet-type dendritic initiator was synthesized using
the convergent approach replacing the peripheral THP
protected phenolic groups from 3-5 with benzylic THP-
protecting groups (Scheme 5). The use of a benzylic
THP-protected dendrimer was particularly advanta-
geous as the benzylic-THP groups were shown not to
interfere with the nitroxide mediated process and could
be converted to ATRP initiating groups in a single step.
Additionally, third generation dendritic initiators were
targeted, as larger dendrons yielded more compact stars,
which required higher degrees of polymerization per
arm to enable visualization by AFM. As a result, the
reaction sequence was nitroxide mediated growth of
pBA from the focal point, followed by deprotection/
functionalization of the THP chain ends and growth by
ATRP of the Sty/BCB arms from the dendron periphery.
This revised strategy proved to be versatile as polymer-

Macromolecules, Vol. 38, No. 7, 2005
Block Copolymers with Different Architectures 2679
Scheme 6. Synthesis of the Hybrid Particle-Coil Copolymer (18) from the Dendritic Initiator (14)a
a Conditions: (a) n-butyl acrylate, 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide, 125 °C; (b) Br2, PPh3, CH2Cl2, 0 °C; (c)
styrene, 4-vinylbenzocyclobutene Cu(I)Br, 4,4′-dimethyl-2,2′-bipyridine, benzyl ether, 100 °C; (d) thermal cross-linking in benzyl
ether at 250 °C.
izations from the focal point and the dendron periphery
were performed in an orthogonal fashion, giving asym-
metric stars and nanoparticle-coil copolymers of precise
molar mass and low polydispersity.
bromide 9 ((THPOCH2)2-[G-1]-CH2Br) in 87% yield. All
attempts to brominate 8 under a variety of conditions
in the absence of a base, such as Hu¨nig’s base, proved
unsuccessful and resulted in significant deprotection of
the THP protecting groups and decomposition. Genera-
tion growth was then accomplished by alkylation of 9
with methyl 3,5 dihydroxybenzoate to form 10, reduc-
tion of the methyl ester with LAH ((THPOCH2)4-[G-2]-
CH2OH, 11) and bromination of 11 gave the second
generation dendritic bromide 12 ((THPOCH2)4-[G-2]-
CH2Br) in 92% yield. The final third generation den-
dritic initiator 14 was then obtained by coupling of 12
with the diphenolic alkoxyamine 135a and fully charac-
terized by 1H NMR, 13C NMR, SEC and elemental
analysis (Scheme 5).
The synthesis of the THP-protected dendrimers start-
ing from the commercially available 4-(chloromethyl)-
benzyl alcohol, which was protected by the acid-
catalyzed addition of 3,4-dihydro-2H-pyran to give the
benzylic-THP derivative, 6. Alkylation of 6 with methyl
3,5 dihydroxybenzoate under phase transfer conditions
afforded the first-generation ((THPOCH2)2-[G-1]-CO2-
CH3) methyl ester, 7, in high yield. Subsequent reduc-
tion of 7 to 8 with lithium aluminum hydride (LAH),
followed by bromination of the benzyl alcohol with CBr4/
PPh3 in the presence of Hu¨nig’s base gave the benzyl

2680 Pyun et al.
Macromolecules, Vol. 38, No. 7, 2005
block copolymers possessing a single pBA chain bonded
to a dendron with eight peripheral THP groups. The
protected block copolymer 15 was converted in a single
step to a dendron-coil macroinitiator for ATRP via
bromination of peripheral benzylic-THP groups (Scheme
6). Conversion of the chain end THP groups to benzyl
bromides in the presence of Br2/PPh3 proved to be
quantitative, as determined from 1H NMR, yielding the
macroinitiator 16 in 90% yield after purification. This
facile transformation of THP protected benzyl alcohols
to ATRP initiating sites is a powerful synthetic tech-
nique and in this case enabled a wide range of styrenic
and acrylate monomers to be polymerized from the
dendron-coil macroinitiator to give asymmetric star
copolymers.
Growth of the asymmetric star from the macroinitia-
tor, 16, involved ATRP polymerization of a mixture of
styrene and 4-vinylbenzocyclobutene in the presence of
a copper(I) bromide 4,4′-dimethyl-2,2′-bipyridine cata-
lyst system at 100 °C. This afforded the [p(S-r-BCB)45]8-
b-[G-3]-b-pBA250 copolymer 17 (Mn ) 77 000; Mw/Mn )
1.22) which contains eight p(S-r-BCB) arms and a single
pBA linear coil connected by a poly(benzyl ether) den-
dritic fragment (Scheme 5). Comparison of SEC chro-
matograms from the dendritic initiator 14, dendron-
coil copolymer 15 and asymmetric star 17 showed a
progressive increase in hydrodynamic size with each
polymerization step while maintaining narrow molec-
ular weight distributions with only minor amounts of
chain-chain coupling being observed which leads to a
small <5% higher molecular weight shoulder (Figure
2). When compared to the original sequential nitroxide
mediated polymerization approach, where the linear
diblock copolymer (1) contained p(S-r-BCB) contami-
nants of comparable size, requiring rigorous fraction-
ation to isolate the pure pBA-b-p(S-r-BCB) block co-
polymer, the ATRP of Sty/BCB from the dendron coil
macroinitiator was a significant improvement. As the
p(S-r-BCB) arms were incorporated at lower tempera-
tures, the formation of linear p(S-r-BCB) impurities was
suppressed. Furthermore, any low molar mass linear
p(S-r-BCB) contaminants that were formed could be
easily removed from the larger asymmetric star 17 by
simple precipitation. As expected, analysis of the [p(S-
r-BCB)45]8-b-[G-3]-b-pBA250 star copolymer 17 by DSC
showed two thermal transitions at -39 °C and 93 °C
which were assigned to the glass transitions (Tg) of pBA
and p(S-r-BCB), respectively.
Figure 2. SEC plots of (a) dendritic initiator 14 (Mn ) 2800
g/mol; Mw/Mn ) 1.01), (b) [G-3]-b-pBA diblock copolymer 15
(Mn ) 37 000 g/mol; Mw/Mn ) 1.19), and (c) p(S-r-BCB)-b-[G-
3]-b-pBA star copolymer 17 (Mn ) 77 000 g/mol; Mw/Mn ) 1.22).
Figure 3. 1H NMR spectra of p(S-r-BCB)-b-[G-3]-b-pBA star
copolymer 17 (top) and nanoparticle-coil copolymer 18 (bot-
tom) obtained after thermally induced cross-linking of p(S-r-
BCB) arms.
Synthesis of Nanoparticle)Coil Copolymers.
The hybrid nanoparticle-coil copolymer, 18, was then
prepared from the star copolymer precursor 17 using
similar conditions as for the synthesis of 2. On the basis
of the star copolymer structure, thermally induced
intramolecular cross-linking of the p(S-r-BCB) arms
results in formation of nanoparticle-coil copolymers
consisting of a glassy PSt nanoparticle covalent attached
to a single linear rubbery pBA coil. Comparison of the
1H NMR spectra of star copolymer 17 and nanoparticle-
coil copolymer 18 (Figure 3) verified consumption of
pBCB (resonance at δ ) 3.0) and the formation of
cyclooctyl cross-links at δ ) 2.6-2.8.8 Additionally,
resonances from the 3 different polymeric blocks; me-
thylene protons from pBA at δ ) 4.0, benzyl CH2 units
of the dendrimer at δ ) 4.85, and aromatic protons at
δ ) 6-7.5 from pS were clearly resolved and unchanged
after thermal treatment in benzyl ether at 250 °C. This
confirms the presence of all block copolymer segments
Figure 4. SEC plots of (a) p(S-r-BCB)-b-[G-3]-b-pBA star
copolymer 17 (Mn ) 77 000 g/mol; Mw/Mn ) 1.22) and (b)
nanoparticle-coil copolymer 18 (Mn apparent ) 62, 000 g/mol; Mw/
Mn ) 1.22) obtained after intramolecular cross-linking of 17.
A hybrid dendron-coil block copolymer, [G-3]-b-
pBA250 (15) (Mn ) 37 000; Mw/Mn ) 1.19) was then
synthesized by the bulk nitroxide mediated polymeri-
zation of BA in the presence of dendritic initiator 14
and 5 mol % of free nitroxide. The molar mass and
composition were determined from SEC (Figure 2) and
1H NMR integration of the unique resonances for the
dendron fragment and linear pBA chain and an initia-
tion efficiency of greater than 90% was obtained for the
polymerization of BA from the dendron focal point.23 The
copolymer could also be purified and trace amounts of
residual initiator removed by filtration of the crude
reaction mixture through a short plug of silica gel. This
synthetic strategy afforded well-defined dendron-coil

Macromolecules, Vol. 38, No. 7, 2005
Block Copolymers with Different Architectures 2681
Scheme 7. Synthesis of Nanoparticle-Coil Copolymer (22) with Inverted Compositiona
a A linear segment of PS is prepared in this system, along with a rubbery nanoparticle of pBA and pBCB. Conditions: (a)
styrene, 125 °C; (b) Br2, PPh3, CH2Cl2, 0 °C; (c) n-butyl acrylate, 4-vinylbenzocyclobutene CuIBr, 4,4′di(5-nonyl)-2,2′-bipyridine,
benzyl ether, 90 °C; (d) thermal cross-linking in benzyl ether at 250 °C.
in the final copolymer and the structural integrity of
the hybrid system.
nanoparticle contaminants derived from p(S-r-BCB)
impurities were not detected in the SEC of 18 which
further validates the synthetic strategy
SEC further confirmed the synthesis of a well-defined
nanoparticle-coil copolymer, as the chromatogram re-
vealed that the final product possessed a relatively
symmetrical molecular distribution and low polydisper-
sity (Mw/Mn ) 1.22, Figure 4). A slight decrease in
apparent molar mass and hydrodynamic volume was
observed due to intramolecular cross-linking of p(S-r-
BCB) arms which is consistent with previous studies.
However the higher solution density of star copolymer
17 relative to the linear diblock copolymer precursor did
lead to only a minor change (ca. 20%) in hydrodynamic
size being observed in the synthesis of 18. In sharp
contrast to the nanoparticle-coil copolymer 2, derived
from a linear diblock copolymer, lower molecular weight
In an effort to further demonstrate the synthetic
versatility of this system, the hybrid nanoparticle-coil
copolymer 20 with an inverted composition relative to
18, a glassy pS linear coil connected to a rubbery poly-
(butyl acrylate) nanoparticle was prepared (Scheme 7).
Using a similar methodology as shown in Scheme 5,
nitroxide mediated polymerization of styrene from the
dendritic initiator, 14, gave the hybrid dendritic-linear
copolymer, [G-3]-b-pS400 (Mn ) 42 000; Mw/Mn ) 1.14).
Activation of the chain ends followed by ATRP of BA/
BCB from the peripheral benzyl bromide groups gave
the asymmetric star 21 with a composition of [p(BA-r-
BCB)60)]8-b-[G-3]-b-pS400 (Mw/Mn ) 84 000; Mw/Mn
)

2682 Pyun et al.
Macromolecules, Vol. 38, No. 7, 2005
chain of glassy pS. Characterization of the nanopar-
ticle-coil copolymer with an inverted composition was
conducted using 1H NMR, SEC and AFM. A stack SEC
chromatogram of the pBEG3-b-pS400 dendron-coil, [p(BA-
r-BCB)60)]8-b-[G-3]-b-pS400 asymmetric star 21 and the
inverted nanoparticle-coil copolymer 22 confirmed an
increase in apparent molar mass after each polymeri-
zation step and a contraction of hydrodynamic size after
the intramolecular cross-linking of p(BA-r-BCB) arms,
while maintaining low polydispersities (Mw/Mn < 1.18)
at each stage of the synthesis were maintained (Figure
5).
Figure 5. SEC plots of (a) [G-3]-b-pS macroinitiator (Mn
)
Visualization of Nanoparticle)Coil Copoly-
mers. In previous studies,11-13 architecturally distinct
macromolecules were examined and visualized by AFM.
The ability to prepare block copolymer structures con-
sisting of more than one macromolecular architecture
opens up the possibility of using AFM to distinguish
individual blocks in a hybrid block copolymer visually
and to confirm the structural evolution. To address this
issue, purified nanoparticle-coil copolymers, 2, obtained
42 000 g/mol; Mw/Mn ) 1.18), (b) p(BA-r-BCB)-b-[G-3]-b-pS star
copolymer 21 (Mn ) 84 000 g/mol; Mw/Mn ) 1.13), and (c)
nanoparticle-coil copolymer 22 (Mn apparent ) 63 000 g/mol; Mw/
Mn ) 1.18) prepared from thermal induced cross-linking of 21.
1.13). The addition of this star copolymer to a dilute
benzyl ether solution at 250 °C yielded the final nano-
particle-coil copolymer 22 composed of a rubbery nano-
particle of cross-linked p(BA-r-BCB) bound to a single
Figure 6. TMAFM images of hybrid nanoparticle-coil copolymer 2 prepared from pBA-b-p(S-r-BCB) linear precursor 1. Left:
3-D rendering of 1 µm by 1 µm scan. Right: 250 nm by 250 nm zoomed-in portion of left image. Taller globular features correspond
to polystyrenic nanoparticle segments and wormlike threads correspond to individual chains of pBA.
Figure 7. TMAFM images of p(S-r-BCB)-b-[G-3]-b-pBA star copolymers 17. Copolymers were spin-coated onto a mica surface
from a very dilute chloroform solution (5 µg/mL). Left: 3-D rendering of 1 µm by 1 µm scan. Right: 250 nm by 250 nm zoomed-in
portion of left image.

Macromolecules, Vol. 38, No. 7, 2005
Block Copolymers with Different Architectures 2683
Figure 8. TMAFM images of nanoparticle-coil copolymer 18 prepared from p(S-r-BCB)-b-[G-3]-b-pBA star copolymers 17.
Copolymers were spin-coated onto a mica surface from a very dilute chloroform solution (5 µg/mL). Left: 3-D rendering of 1 µm
by 1 µm scan. Right: 250 nm by 250 nm zoomed-in portion of left image.
from the pBA-b-p(S-r-BCB) linear diblock copolymer
precursors were spin-coated onto mica substrates from
dilute chloroform solutions (5 µg/mL). By using ultradi-
lute conditions a submonolayer coverage of copolymer
chains was obtained which allowed the nanoparticle-
coil copolymer 2 to be successfully imaged with high
resolution TMAFM (Figure 6). AFM height images
revealed the presence of wormlike threads connected to
taller globular protrusions. Globular and wormlike
features were assigned respectively to 3-dimensional
polystyrene nanoparticles and linear pBA segments.
Particle-like features were 2-5 nm tall, and their
diameter (uncorrected for the finite size of AFM tip)
ranged from 10 to 20 nm. This difference indicated that
upon deposition on mica, the polystyrene nanoparticles
underwent flattening similar to one observed in other
similar systems (e.g., shell cross-linked micelles).24 The
overall dimensions of wormlike threads varied, since the
linear pBA chains would be expected to adopt both
extended and coiled conformations. The composition and
architecture of the copolymer was an important factor
in facilitating molecular visualization on mica sub-
strates. The presence of the pBA component enabled
adsorption and spreading of the linear segment on the
surface, while the glassy cross-linked nanoparticle
imparted sufficient shape-persistence to allow AFM
imaging. Of particular significance was the observation
that essentially all nanoparticles had a single pBA chain
attached which directly confirms the synthetic meth-
odology.25 “Particle-coil”-like shapes were not however
observed for the corresponding linear pBA-b-p(S-r-BCB)
block copolymers 1, presumably due to the lack of shape
persistence of the polystyrenic block. Instead only
globular structures were observed on mica surface for
this linear block copolymer.
circular aggregates was observed. The higher tendency
of the non-cross-linked star copolymer 17 to form such
clusters in comparison with copolymer 2 may be a result
of the branched nature of star blocks, which are more
likely to be subject to significant entanglement than the
compact, cross-linked polystyrenic nanoparticles present
in 2.
A direct comparison with the nanoparticle-coil co-
polymer 18 prepared from the star copolymer 17 adds
further evidence to the importance of macromolecular
architecture in these systems. In this case, conversion
of the star copolymer to the nanoparticle lead to
significantly different behavior and individual molecular
species could again be later visualized with TMAFM
(Figure 8). Both height and phase images revealed the
presence of taller spherical features connected to shorter
threadlike extensions corresponding to polystyrenic
nanoparticle and pBA coil segments.24 The apparent
lateral size (not corrected for AFM tip size) of particle-
like features was in the range of 10-20 nm, and was
similar to the size of their counterparts in copolymer
17 (Figure 7). In contrast with un-cross-linked copoly-
mer 17, a significant fraction of nanoobjects was not
associated with large round aggregates. This higher
percentage of individual nanoparticle-coil nanoobjects
in the AFM images of copolymer 18, when compared to
17, points to a reduced aggregation due to cross-linking
of the polystyrene block leading to nanoparticle forma-
tion.26
Conclusion
A novel class of hybrid copolymers, termed nanopar-
ticle-coil copolymers, has been prepared by using a
synergistic combination of controlled radical polymeri-
zation, benzocyclobutene chain collapse chemistry and
convergent dendrimer synthesis. The mixed architec-
ture, nanoparticle-coil copolymers were synthesized
from both linear and dendritic precursors, although the
use of a dendritic initiator overcame many of the
synthetic limitations associated with the linear diblock
copolymer approach. Direct visualization of single nano-
particle-coil copolymer architectures was possible using
high resolution AFM and the individual elements,
globular polystyrene nanoparticle and random coil,
This lack of shape persistence is alleviated to a certain
extent in the star copolymer structures due to the
3-dimensionality introduced via the polyether den-
drimer. AFM imaging of the star copolymer 17 was
therefore performed under similar conditions as for the
diblock copolymer, 1, and the resulting particle-coil
copolymer 2 (Figure 7). As with copolymer 2, particle-
coil objects similar to those shown in Figure 6 were
discernible, however individual structures were much
less resolved and a much higher tendency to form large

2684 Pyun et al.
Macromolecules, Vol. 38, No. 7, 2005
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served at molecular resolution. The success at visual-
izing these materials and their aggregation behavior
was found to be directly related to the architecture of
the hybrid block copolymer and their ability to undergo
entanglement and/or interdigitation. We are currently
evaluating these novel hybrids as intelligent nanopo-
rogens for low-k dielectric materials, as well-defined
building blocks to prepare higher order assemblies, and
as biological mimics for therapeutic applications.
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Acknowledgment. We thank the MRSEC Program
of the National Science Foundation under Award DMR-
0213618 for the Center for Polymeric Interfaces and
Macromolecular Assemblies, the NIRT Program of the
National Science Foundation Grant No. 0210247, IBM
Corporation, DOE-BES (LBNL Polymer Program) and
AFOSR for financial support.
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Supporting Information Available: Text giving the
experimental details27-31 on the synthesis of dendritic initia-
tors, block copolymers, and nanoparticle-coil copolymers. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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