From small building blocks to complex molecular architecture

Article abstract of DOI:10.1021/ol060080x

We describe a synthesis of a dendrimer-like amphiphile containing a flat rigid core and 12 hydrophobic and hydrophilic arms. We employ a modular approach based on stepwise protection chemistry starting from simple building blocks. The key feature of this approach is the absence of a polymerization step, which makes it applicable for linear monofunctionalized precursors of any kind. This strategy also allows for precise control of the number of arms and ensures their alternating arrangement.

Full text of DOI:10.1021/ol060080x

ORGANIC
LETTERS
2006
Vol. 8, No. 7
1367-1370
From Small Building Blocks to Complex
Molecular Architecture
Eugene R. Zubarev,*,† Jun Xu,‡ Jacob D. Gibson,† and Arshad Sayyad†
Department of Chemistry, Rice UniVersity, Houston, Texas 77005, and Department of
Materials Science, Iowa State UniVersity, Ames, Iowa 50011
zubareV@rice.edu
Received January 12, 2006
ABSTRACT
We describe a synthesis of a dendrimer-like amphiphile containing a flat rigid core and 12 hydrophobic and hydrophilic arms. We employ a
modular approach based on stepwise protection chemistry starting from simple building blocks. The key feature of this approach is the
absence of a polymerization step, which makes it applicable for linear monofunctionalized precursors of any kind. This strategy also allows
for precise control of the number of arms and ensures their alternating arrangement.
The synthesis of dendritic1 and starlike2 molecules is more
challenging than the preparation of linear structures, yet these
compounds attract greater attention due to their unusual
properties. The availability of well-defined branched mol-
ecules is still limited, and new synthetic strategies should
be found. In principle, any living anionic polymerization from
the surface of poly(divinylbenzene) nodules offers a quick
one-step synthesis of star-shaped molecules.3 However, the
number and the spatial distribution of arms cannot be
controlled in this case, and only indirect methods can estimate
these critically important parameters. The use of well-defined
cores, such as carbosilane dendrimers,4 calixarenes,5 and
dendritic-linear hybrid structures,6 alleviates this limitation
but immediately brings another one. The initiator must not
have any electrophilic sites in order to survive highly basic
anionic conditions. Only a handful of multifunctional initia-
tors can meet this requirement.7 An alternative is given by
the atom transfer radical polymerization (ATRP) and nitrox-
ide-mediated polymerization (NMP), which exhibit much
greater functional group tolerance.8,9 Nonetheless, many
† Rice University.
‡ Iowa State University.
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10.1021/ol060080x CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/07/2006

Scheme 1. Synthesis of Starlike Amphiphile 1
important monomers, such as dienes, oxiranes, and cyclosi-
demonstrate that much larger structures can be constructed
using a modular stepwise approach. We employ protection
chemistry starting from commercially available linear mol-
ecules. In addition, various multifunctional cores can be used
in this route because coupling between the presynthesized
hairpin-like molecules and the multifunctional core proceeds
under mild esterification conditions at room temperature.
The first part of the overall synthesis required the
preparation of a hairpin-like PB-PEO diblock molecule 4
with the carboxyl group at the focal point (Scheme 1). Silyl-
protected 3,5-dihydroxybenzoic acid (DHBA) can be used
as a junction point when COOH-terminated linear polymers
are coupled to it using 1,3-diisopropylcarbodiimide and
DPTS.12 However, our attempts to couple commercially
available carboxyl-terminated PB resulted in a very poor
yield (∼20%), which was due to a low degree of carboxy-
lation, and the presence of a significant amount of dead
chains. For that reason, we used OH-terminated polybuta-
diene (MW ) 5400 g/mol, PDI ) 1.12) and switched its
terminus to carboxyl group via attachment of silyl-protected
biphenyl dicarboxylic acid (Scheme 1). Incorporation of a
biphenyl moiety was also beneficial in terms of NMR
analysis because it introduced additional proton resonances
that served as highly reliable internal references. Deblocking
of the triisopropylsilyl (TIPS) group followed by esterifica-
tion of the resulting compound 2 with excess carboxyl-
protected DHBA (10 equiv) produced functional precursor
3, which was purified by flash chromatography and char-
loxanes, do not polymerize under ATRP conditions. Here,
we demonstrate how complex macromolecular architectures
can be constructed from small building blocks and com-
mercially available linear precursors. This approach avoids
the polymerization and all of the limitations assosiated with
it and allows for the synthesis of macromolecular branched
structures with a broad range of monomeric units. As an
example, we used this approach to prepare high molecular
weight (MW ∼50000 g/mol) starlike amphiphiles containing
poly(ethylene oxide) and polybutadiene chains. This is an
important synthetic target because heteroarm star-shaped PB-
PEO structures prepared by polymerization remain unknown,
and only one example of a core-shell PB-PEO star (four
arms) synthesized by polymerization has been reported to
date.10
We previously described the synthesis of starlike molecules
containing short hydrophobic chains (20 repeat units, MW
) 1000 g/mol).11 The purpose of this communication is to
(7) (a) Bender, J. T.; Knauss, D. M. J. Polym. Sci., Part A: Polym. Chem.
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N.; Vlassopoulos, D. Macromolecules 2002, 35, 6592. (d) Adams, C. H.;
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J. P. A. Polymer 2004, 45, 4121.
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2001, 26, 337. (b) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. ReV.
2001, 101, 3661, (c) Ueda, J.; Kamigaito, M.; Sawamoto, M. Macromol-
ecules 1998, 31, 6762. (d) Miura, Y.; Narumi, A.; Matsuya, S.; Satoh, T.;
Duan, Q.; Kaga, H.; Kakuchi, T. J. Polym. Sci., Part A: Polym. Chem.
2005, 43, 4271. (e) Aliferis, T.; Iatrou, H.; Hadjichristidis, N. J. Polym.
Sci., Part A: Polym. Chem. 2005, 43, 4670.
1
acterized by H NMR and size-exclusion chromatography
(SEC).
The next step involved the reaction with glutaric acid-
terminated PEO (MW ) 2200, PDI ) 1.15), which was
(9) (a) Heise, A.; Hedrick, J. L.; Frank, C. W.; Miller, R. D. J. Am.
Chem. Soc. 1999, 121, 8647. (b) Heise, A.; Trollsas, M.; Magbitang, T.;
Hedrick, J. L.; Frank, C. W.; Miller, R. D. Macromolecules 2001, 34, 2798.
(10) Matmour, R.; Francis, R.; Duran, R. S.; Gnanou, Y. Macromolecules
2005, 38, 7754-7767.
(11) Xu, J.; Zubarev, E. R. Angew. Chem., Int. Ed. 2004, 43, 5491.
(12) Zubarev, E. R.; Stupp, S. I. J. Am. Chem. Soc. 2002, 124, 5762.
1368
Org. Lett., Vol. 8, No. 7, 2006

Figure 2. SEC traces of diblock precursor 4 (blue) and the star-
shaped PB-PEO amphiphile 1 (red).
diblock 4 from a target starlike structure can be rather
cumbersome since the affinity of PB-PEO to silica gel (Rf)
may not change appreciably upon such transformation. This
was indeed observed in our initial experiments. To avoid
the tedious separation, we used only a slight excess of starting
material (6.3 equiv) with respect to a hexabiphenyl core 5,
which was synthesized separately. Accurate estimation of
the reagents weight ratio was possible due to availability of
the absolute molecular weights of both PB-PEO amphiphile
4 and the hexafunctional core 5 as determined by MALDI-
TOF (Figure 1).
The esterification was found to proceed very rapidly,
and only a trace amount of starting compound 4 was
observed in the SEC trace of the reaction mixture after 1 h
(Figure 2). The appearance of a high molar mass peak
with MW ) 49.8 kDa and PDI ) 1.14 suggested that
the reaction was nearly complete and that mainly hexa-
substituted product formed because the polydispersity of
the new peak was as low as that of the starting material
(1.14 vs 1.12). The structure of the product was con-
Figure 1. MALDI-TOF spectra of hexabiphenyl core 5 (top) and
PB-PEO diblock 4 (bottom).
found to be a rapid conversion, although the isolation of the
amphiphilic product was quite challenging and resulted in
some losses upon purification on silica gel eluting with
methanol/chloroform (11:89 v/v) mixture. We also found that
partial reesterification takes place in the presence of excess
methanol at temperatures above 50 °C.
To activate the focal point of the amphiphile 4, we used
TBAF deprotection at subambient temperature (-78 °C),
which allows for a selective cleavage of the silyl group in
the presence of alkyl aryl esters that remain intact under these
mild basic conditions. The resulting hairpin-like PB-PEO
amphiphile 4 was purified by column chromatography, and
its structure was confirmed by 1H NMR, whereas its
molecular weight was determined by MALDI-TOF (MW )
7400) and SEC (MW ) 13800, PDI ) 1.12).
1
firmed by H NMR as the new resonances from the core
biphenyls appeared in the spectrum and their integration
was consistent with the expected numbers. Thus, there is no
steric limitation in this final step even though the degree of
polymerization of the diblock compound 4 is greater than
150.
Our preliminary observations show that the star-shaped
amphiphile 1 forms translucent solutions in hexane and water
which is indicative of large micellar aggregates present at
exceedingly low concentrations (∼0.3 wt %). Nearly com-
plete disappearance of the PEO signal at 3.6 ppm was
observed in deuterated hexane, whereas PB signals at 5.4
and 2.1 ppm did not change in comparison with spectra
obtained in a nonselective solvent (CD2Cl2). On the other
hand, PB resonances were considerably suppressed (∼80%
reduction) in NMR spectra collected from D2O solutions of
star-shaped amphiphile 1. Hexane and water are selective
The diblock PB-PEO molecule 4 offers an opportunity to
produce an amphiphilic heteroarm star in just one step
provided the coupling with a complementary multifunctional
core is highly efficient. On the other hand, the separation of
Org. Lett., Vol. 8, No. 7, 2006
1369

solvents for 1, and these experiments indicate the self-
assembling behavior and the formation of both regular and
reverse micelles.
serve as a complementary tool to construct complex
macromolecular architectures.
Acknowledgment. Financial support provided by Rice
University, the Welch Foundation, and the ACS Petroleum
Research Fund (40727 G-10) is gratefully acknowledged.
In conclusion, we developed an efficient route to high
molecular weight PB-PEO star-shaped amphiphiles. The
modular approach employed here is very versatile and may
allow for the synthesis of star-shaped molecules with various
types of arms as long as the monofunctional linear precursors
are available. This synthetic strategy may help overcome
classical limitations of living polymerization techniques and
Supporting Information Available: Experimental pro-
cedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL060080X
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Org. Lett., Vol. 8, No. 7, 2006