Synthesizing amphiphilic block copolymers through macromolecular azo-coupling reaction

Article abstract of DOI:10.1039/c1cc16362k

This communication reports a new approach to synthesize amphiphilic block copolymers. The copolymers with well-defined structures were synthesized by macromolecular azo-coupling reaction between the diazonium salt of aniline-functionalized PEG and the pol

Full text of DOI:10.1039/c1cc16362k

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COMMUNICATION
Synthesizing amphiphilic block copolymers through macromolecular
azo-coupling reactionw
Yaning He,* Wei He, Renbo Wei, Zhen Chen and Xiaogong Wang*
Received 13th October 2011, Accepted 14th November 2011
DOI: 10.1039/c1cc16362k
This communication reports a new approach to synthesize
amphiphilic block copolymers. The copolymers with well-defined
structures were synthesized by macromolecular azo-coupling
reaction between the diazonium salt of aniline-functionalized
PEG and the polymeric blocks with a terminal suitable for the
azo-coupling reaction.
cycloaddition between polymeric blocks bearing functional
terminal azide and alkyne functionalities using copper(I) as
the catalyst, which has attracted considerable attention and
been applied for linking different polymeric blocks.4,5 Finding
other high-efficient coupling reactions is a necessary step to
further develop this synthetic strategy. Azo-coupling reaction
is widely used in the synthesis of dyes and pigments. In these
reactions, aromatic diazonium cations can efficiently react
with anilines or phenols with high yield. Recently, the post-
polymerization azo-coupling scheme has been developed to
introduce various azo chromophores through the azo-coupling
reactions between precursor polymers with anilino moieties and
diazonium salts in polar organic solvents.6 The reaction scheme
shows high efficiency and can produce azo polymers with high
degree of functionalization. For the azo-coupling reaction, no
catalyst and particular protection are needed. Although similar
reaction can also be expected for the synthesis of various
amphiphilic block copolymers, to our knowledge, no successful
example has been reported in the literature yet.
Block copolymers, which are composed of two or more
homopolymer chains linked by covalent bonds, have been
extensively investigated in recent years.1–3 The unique archi-
tecture and properties of block polymers promise the realization
of many potential nanotechnological applications such as
semiconductor arrays, nanoscale templating and nanoscale
separations.1 The amphiphilic block copolymers, consisting
of hydrophilic and hydrophobic blocks, can self-assemble in
selected solvents to form various polymeric aggregates such as
spherical micelles, vesicles and nanotubes, which can be used in
drug delivery systems, nanoreactors and others.2 In recent years,
block copolymers have been prepared by various living/
controlled polymerization methods, such as living cationic or
anionic polymerizations, atom transfer free radical polymerization
(ATRP), reversible addition fragmentation chain transfer (RAFT)
polymerization, and ring-opening metathesis polymerization
(ROMP).3 The use of macroinitiators is a typical synthetic
strategy to obtain block copolymers through the controlled radical
polymerization. However, in some cases, the macroinitiators show
less reactivity and are hard to completely form block copolymers.
Therefore, new synthetic strategies are needed to obtain block
copolymers, especially amphiphilic block copolymers, through
other feasible approaches.
In this communication, we report a novel modular strategy
for the synthesis of amphiphilic block copolymers by macro-
molecular azo-coupling reaction between the diazonium salt of
aniline-functionalized poly(ethylene glycol) (PEG) and another
polymer block with a terminal suitable for the azo-coupling
reaction. PEG has good water solubility and can be commer-
cially purchased with exact molecular weight. It has been widely
used as a hydrophilic block in preparing amphiphilic block
copolymers.
The synthetic route of aniline-functionalized PEG is shown
in Scheme 1. In the synthetic route, the tosylate ended PEG
was firstly synthesized by the esterification between poly-
(ethylene glycol) monomethyl ether and 4-toluenesulfonyl
chloride. Aniline-functionalized PEG was then obtained by
nucleophilic substitution reaction between the synthesized
PEG tosylates and p-aminobenzoic acid. The synthesis details
are given in ESI.w Another block with a terminal suitable for
azo-coupling reaction was obtained from ATRP initiator 1,
which was synthesized by esterification between 2-(N-ethyl-
anilino)ethanol and 2-bromoisobutyryl bromide with high
yield. Through ATRP with the initiator, hydrophobic blocks
with the terminal functional group could be easily obtained. In
this study, both polystyrene (PS) and poly(methyl methacrylate)
(PMMA) were used to demonstrate the feasibility of this
approach (Scheme 1).
Recently, it has been reported that polymeric building
blocks can be modularly synthesized via ‘‘click’’ chemistry.4
One obvious advantage of this approach is that the blocks
before the coupling reaction can be well characterized. On the
other hand, this approach needs reactive end groups to
guarantee the high yield coupling reaction and avoid possible
side reactions. A typical ‘‘click’’ reaction is 1,3-dipolar
Department of Chemical Engineering, Laboratory for Advanced
Materials, Tsinghua University, Beijing 100084, P. R. China.
E-mail: heyaning@mail.tsinghua.edu.cn, wxg-dce@mail.tisnghua.edu.cn
w Electronic supplementary information (ESI) available: Synthetic
procedures, experimental details and GPC curves. See DOI: 10.1039/
c1cc16362k
c
1036 Chem. Commun., 2012, 48, 1036–1038
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Scheme 1 Preparation of aniline-functionalized poly(ethylene glycol)
and polymers with a terminal suitable for the azo-coupling reaction,
which was obtained by utilizing functionalized ATRP initiator 1.
Fig. 1 Typical GPC traces of aniline functionalized PEG
(PEG–NH2) (PDI = 1.08, Mn = 3700), terminal-functionalized PS
(PS–N–Ph) (PDI = 1.29, Mn = 8800) and the coupled diblock
copolymer PEG-b-PS (PDI = 1.27, Mn = 10 000).
Azo coupling reactions were carried out between the diazonium
salt of aniline-functionalized PEG (PEG–NH2) and PS or PMMA
block with terminal functional groups in organic solvents. The
diazonium salt of aniline-functionalized PEG was prepared by
adding NaNO2 aqueous solution into the mixture of PEG–NH2
and HCl in water cooling with ice bath. Then azo-coupling
reaction was carried out in DMF for the PS block as a typical
example (Scheme 2). A similar reaction for the PMMA block is
given in ESI.w In order to drive the azo-coupling reaction to
completion, excess of aniline functionalized PEG was used. The
excess of PEG could be easily removed by washing with methanol
in this case. With this simple separation method, the applicability
of the methodology could be limited to asymmetric diblock
copolymers with high hydrophobic block content. To extend this
methodology to the synthesis of other diblock copolymers, more
efficient methods to remove the unreacted PEG component
might be required. On the other hand, for blocks with less
hydrophobicity, the azo-coupling reaction could be more efficient.
In this case, using slight excess of the diazonium salt of aniline-
functionalized PEG could be the alternative option.
was obtained after macromolecular azo-coupling reaction
(Fig. S3, ESIw). The typical molecular weights of PEG–NH2,
PMMA–N–Ph, and resulted PEG-b-PMMA from GPC are
3700, 14 000, and 15 400, respectively. The PDI of the block
copolymers is almost the same as the terminal-functionalized PS
or PMMA block. For example, the PDI of the functionalized
PS is 1.29 and the resulted block copolymer PEG-b-PS is 1.27.
The formation of the azobenzene linkage can be confirmed by
the UV-Vis spectra of the diblock copolymer. Fig. 2 shows the
typical UV-Vis spectra of the coupled diblock copolymer
PEG-b-PS and a corresponding azobenzene derivative in THF.
The synthesis of the azobenzene derivative is also given in
Scheme 2. It can be seen that they have a similar absorption
band position. The curves showed typical absorption behavior of
the pseudo-stilbene type of azo chromophores, e.g. bands corres-
ponding to the p–p* transition appear at longest wavelength and
chromophores exhibit absorption in the visible region. Above
results all indicate that the coupling reactions occur between the
two blocks with the high conversions for both PS and PMMA.
Above reaction scheme can be extended to prepare polymers
with different topographic shapes. A ‘‘Y’’ shaped amphiphilic
block copolymer was prepared by using this macromolecular
azo-coupling reaction scheme to demonstrate the concept. As
shown in Scheme 3, the terminal functionalized polymer with
two PS blocks (PS–N(Ph)–PS) was prepared by ATRP utilizing
the functionalized initiator 2, which was synthesized by
Fig. 1 shows the typical traces of aniline-functionalized PEG
(PEG–NH2), functionalized PS block (PS–N–Ph) and the
diblock copolymer (PEG-b-PS). Here, the calculated molecular
weights from GPC of PEG–NH2, PS–N–Ph, and resulted PEG-
b-PS are 3700, 8800, and 10 000, respectively. From the GPC
traces, it can be seen that no residual mono-block remains after
the macromolecular azo-coupling reaction. The significant shift
in the GPC trace towards higher molecular weight indicates the
formation of block copolymers linked by the azobenzene bridge.
For the terminal-functionalized PMMA, the similar GPC result
Scheme 2 Preparation of block copolymer PEG-b-PS and corres-
ponding azobenzene derivative via azo-coupling reaction.
Fig. 2 Typical UV-Vis spectra of the diblock copolymer PEG-b-PS
and a corresponding azobenzene derivative (AZO-M) in THF.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1036–1038 1037

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This work was supported by Natural Science Foundation of
China (21074065 and 91027024).
Notes and references
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Scheme 3 Preparation of ‘‘Y’’ shaped block copolymer PEG-b-PS via
macromolecular azo-coupling reaction.
esterification between N,N-diethanolaniline and 2-bromoiso-
butyryl bromide. After the azo coupling reaction with diazonium
salts of PEG–NH2, the ‘‘Y’’ shaped amphiphilic block copolymer
was obtained. Similarly, the shift in the GPC trace towards
higher molecular weight was observed, which verified that the
macromolecular azo-coupling reaction occurred (Fig. S5, ESIw).
However, it can be seen from the figure that the unreacted
residue (PS–N(Ph)–PS) exists after the reaction. This result
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coupling reaction in the macromolecular chain has a significant
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In conclusion, we have demonstrated a new approach for the
synthesis of amphiphilic diblock copolymers via macromolecular
azo-coupling reaction between the diazonium salt of aniline-
functionalized PEG and another polymer block with a terminal
group suitable for the azo-coupling reaction. The coupling can be
carried in organic solvents under extremely mild conditions.
Amphiphilic block copolymers with well-defined structures can
be prepared through this approach. The reaction scheme can be
extended to the synthesis of amphiphilic copolymers with other
topographic shapes although the conversion can be affected by
the functional group position.
c
1038 Chem. Commun., 2012, 48, 1036–1038
This journal is The Royal Society of Chemistry 2012