Synthesis and self-assembly of a heteroarm star amphiphile with 12 alternating arms and a well-defined core

Article abstract of DOI:10.1021/ja036705t

We report on a stepwise synthesis of a heteroarm starlike amphiphile containing 12 alternating arms (six polystyrene and six poly(acrylic acid)) connected to a hexabiphenyl aromatic core. The synthesis does not involve polymerization, and only commercially available precursors are used. Most importantly, this amphiphile undergoes self-assembly into spherical and wormlike cylindrical micelles in aqueous and methanol solutions, and forms reverse 1D micellar structures in chloroform. This remarkable morphological diversity of the reported amphiphile 1 is believed to be a direct consequence of its well-defined molecular architecture. Copyright

Full text of DOI:10.1021/ja036705t

Published on Web 09/09/2003
Synthesis and Self-Assembly of a Heteroarm Star Amphiphile with 12
Alternating Arms and a Well-Defined Core
Jing Teng and Eugene R. Zubarev*
Department of Materials Science and Engineering, Iowa State UniVersity, Ames, Iowa 50011
Received June 16, 2003; E-mail: zubarev@iastate.edu
Although star-shaped molecules have been know since the late
1
940s, only recently did they become synthetically available. The
1
progress was primarily due to recent advances in living anionic
and radical polymerization. However, a number of challenges
2
remain, including control over the structure of a core and the number
3
of arms in such molecules. In addition, living polymerization
conditions are very demanding, and that necessitates development
4
of alternative synthetic approaches. Here, we demonstrate that a
stepwise synthesis starting from commercially available linear
precursors can produce well-defined starlike molecules with a
precisely controlled number and position of arms. The amphiphile
described here self-assembles into spherical and wormlike super-
Figure 1. Molecular graphics representation of starlike amphiphile 1 (a)
and a spherical supermicelle (b) formed via a closed association of 30
molecules of 1.
5
micelles in aqueous and methanol solutions, and forms reverse
micelles in chloroform. This morphological diversity has not been
previously observed in star-shaped amphiphiles and is believed to
be a direct consequence of the well-defined molecular architecture.
The synthesis of amphiphile 1 consists of two parts: the
preparation of a Y-shaped copolymer 3 and the synthesis of a
hexafunctional core 4 (Scheme 1). Carboxyl-terminated polystyrene
Scheme 1. Stepwise Synthesis of Starlike Amphiphile 1a
w
(M ) 2400, PDI ) 1.1, Polymer Source, Inc.) was coupled with
an excess of silyl-protected 3,5-dihydroxybenzoic acid under mild
esterification conditions. Analogous coupling of 2 with poly(tert-
butyl acrylate) followed by cleavage of the silyl group with DMAP
afforded functional polymer 3 (M ) 7300, PDI ) 1.07). All
n
products were obtained in high yields and were purified by flash
chromatography. The second part of the synthesis involved prepara-
tion of an aromatic core which could offer a reduced steric hindrance
and higher accessibility of the functional hydroxyl groups. The
esterification reaction between silyl-protected 4′-hydroxy-biphenyl-
4-carboxylic acid and hexahydroxybenzene, followed by deblocking
of DTS under acidic conditions, produced hexafunctional core 4 in
high yield and purity, as confirmed by MALDI mass spectrometry,
1
13
6
H NMR, and C NMR. The key step of the overall synthesis
was the coupling reaction between the core 4 and the Y-shaped
copolymer 3. Remarkably, the esterification was found to proceed
in nearly quantitative yield within just 1 h, and the product was easy
to purify by conventional flash chromatography. The reaction was
monitored by GPC, which demonstrated formation of a high molar
mass product with a molecular weight of 35 100 and a very low
w n
polydispersity (M /M ) 1.07). The final step involved deprotection
a
Reagents: (a) DPTS/DIPC, CH2Cl2, room temperature, 1 h; (b) DMAP,
of polyacrylate arms under acidic conditions (TFA). Interestingly,
the reaction can be monitored most effectively by 13C NMR because
the resonance generated by the tertiary carbon of tert-butyl groups
room temperature, 12 h; (c) HF, THF, room temperature, 24 h; (d) TFA
70% vol.), CH2Cl2, room temperature, 24 h.
(
(
80.9 ppm) does not overlap with other signals in the spectrum.
of poly(acrylic acid). The arms are fairly short, and each type
contains on average 25 monomer units. Thus, the starlike structure
1 has a rigid core and a flexible oligomeric shell. However, the
shell is not compositionally uniform, and it is the shell itself which
is amphiphilic (Figure 1a). Most importantly, all molecules have
an identical hexafunctional core, and its size (4 nm) is comparable
to the length of the arms (6 nm). This is in great contrast to the
The average length of polystyrene and poly(acrylic acid) arms in
the resulting heteroarm amphiphile 1 was calculated from NMR
spectra using biphenyl protons as the internal reference. The average
molecular weight of 1 was determined by GPC in a 0.1 M solution
w n
of LiBr in DMF and was found to be 31 300 (M /M
) 1.14).6
Compound 1 (Figure 1a) contains a well-defined hexabiphenyl
7
core, six hydrophobic arms of polystyrene, and six hydrophilic arms
heteroarm high polymers, where the average size of the central
11840
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J. AM. CHEM. SOC. 2003, 125, 11840-11841
10.1021/ja036705t CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Most importantly, a TEM investigation demonstrated that an
increase in concentration results in a morphological transition from
spherical to cylindrical wormlike micelles (Figure 2c). When the
concentration of the aqueous solution of 1 is increased 10 times, a
small amount of spheres is still present, but the majority of the
sample is in the form of one-dimensional winding objects with a
fairly uniform diameter (∼18 nm) and the length ranging from 40
to 200 nm. Given the overall contrast and the diameter of these
structures, it is reasonable to believe that PS chains are confined
in the center of cylinders and PAA arms are localized on the
periphery. In addition, very similar structures were found in
methanol solutions.6
Our recent investigation revealed that amphiphile 1 also forms
reverse micelles in chloroform. Figure 2d shows a TEM image of
one-dimensional structures which measure 17 ( 2 nm in width
6
and up to 1 µm in length. NMR analysis of chloroform solutions
demonstrated that the signals of PAA are significantly suppressed,
unlike those from PS arms, which is indicative of reverse micellar
aggregates. Static light-scattering experiments also confirmed the
presence of micelles in chloroform solutions, and their radius of
Figure 2. TEM micrographs of structures formed by amphiphile 1. (a)
-
6
Negatively stained sample cast from 3.3 × 10 M aqueous solution. (b)
-6
Positively stained (RuO4) sample cast from 3.3 × 10 M aqueous solution.
-
5
(
3
c) Sample cast from 3.3 × 10 M aqueous solution. (d) Sample cast from
-
6
gyration (∼120 nm), apparent molecular weight (1.1 × 10 g/mol),
8
.3 × 10 M chloroform solution.
-
4
2
and the second virial coefficient (4.6 × 10 mol mL/g ) were
6
core is negligible in comparison with the arms. Therefore, interac-
tions of molecules 1 with selective solvents will be defined by the
interplay of three structural elements, that is, a rigid hydrophobic
core and two flexible subshells: hydrophilic (6 PAA arms) and
hydrophobic (6 PS arms), respectively.
determined from the Zimm plot. In contrast, micelles were not
observed in chloroform solutions of the star-shaped precursor PS
6
-
PtBA -core. Therefore, the self-assembly requires not only a well-
6
defined molecular architecture but also the amphiphilicity of the
shell.
It is worth noting that, regardless of solvent and the type of
micelles, that is, spherical or cylindrical, regular or reverse, the
hexabiphenyl cores of molecules 1 will always be confined at the
interface between the solvophilic corona and the solvophobic core
We conceived structure 1 on the basis of several simple
assumptions. Interactions of molecule 1 (Figure 1a) with water
should cause solvation of PAA arms, whereas PS chains would try
to minimize their exposure to such a polar environment. However,
the presence of a relatively large and rigid core would keep PS
arms spatially separated, and their mere collapse on the scale of
one molecule may not be sufficient to avoid unfavorable contacts
with water. Instead, the free energy of the system could be
minimized in a much more efficient way if PS arms of many
molecules 1 were brought together to form a large hydrophobic
core of a supermicelle as illustrated in Figure 1b.
(Figure 1b). This brings to mind one potential application of such
systems. If the cores, connecting the arms, were carriers of some
function, that is, nanoparticles or catalytic centers, this micellization
process would provide an effective way to organize them into well-
defined zero- and one-dimensional arrays.
1
Supporting Information Available: Experimental details, H and
C NMR spectra, GPC, SLS data, and TEM images (PDF). This
13
material is available free of charge via the Internet at http://pubs.acs.org.
Transmission electron microscopy (TEM) studies of samples cast
-
6
from very dilute (3.3 × 10 M) aqueous solutions of 1 revealed
the presence of round-shaped objects that are fairly uniform and
measure 17 ( 4 nm in diameter (Figure 2a). To confirm that these
objects are spheres and not flat disks, we obtained height profiles
that showed that the size of these structures in vertical dimension
is also about 17 nm. A more important question, however, is
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association mechanism.8
Ruthenium tetroxide is a selective staining agent because it reacts
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(
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(
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g
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6) See Supporting Information.
30 amphiphiles of 1 via a closed-association mechanism would form
(
supermicelles with a diameter ranging from 15 to 22 nm depending
on the conformation of PAA arms. That model also assumes that
self-assembly is preceded by a spatial separation of hydrophilic (6
PAA arms) and hydrophobic (6 PS arms) subshells, placing them
above and below the plane of the aromatic core (Figure 1b).
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3
284-3292. (b) Kanaoka, S.; Nakata, S.; Yamaoka, H. Macromolecules
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002, 35, 4564-4566.
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