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Published on the web May 22, 2010
Photochemical Formation of a Core-crosslinked Micelle
using an Anthracene-containing Amphiphilic Copolymer
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Hiroshi Morikawa,* Yasuharu Kotaki, Ryota Mihara, Yuki Kiraku, Shigetoshi Ichimura, and Suguru Motokucho
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Department of Applied Chemistry, Faculty of Engineering, Kanagawa Institute of Technology,
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030 Shimo-ogino, Atsugi, Kanagawa 243-0292
Department of Applied Bioscience, Faculty of Applied Bioscience, Kanagawa Institute of Technology,
030 Shimo-ogino, Atsugi, Kanagawa 243-0292
Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University,
-14 Bunkyo-machi, Nagasaki 852-8521
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(
Received March 25, 2010; CL-100290; E-mail: morikawa@chem.kanagawa-it.ac.jp)
In order to develop photoresponsive polymeric micellar
O
C
CH3
CH3
C
CH3
C
systems, an amphiphilic block copolymer consisting of
anthracene moieties was successfully prepared by atom transfer
radical polymerization. In an aqueous solution, the photo-
dimerization of the anthracene moieties occurred in the micellar
core upon irradiation ( > 360 nm), keeping the mean diameter
constant. The photodimerization caused the interpolymer
reaction to form a core-crosslinked micelle.
CH3
O
CH2 CH2
O
113
C
CH2
CH2
O 41.7
O 3.8
CH3
C
C
O
O
CH3
CH2
Scheme 1. Structure of PEG113-b-P(MMA41.7-AM3.8).
Stimulus-responsive micelles, whose functions such as
formation, collapse, and shrinking behavior are triggered by
various external factors, have attracted considerable interest in
industry and academia because of their potential use as sensors,
methacrylate6 units (AM) (Scheme 1). The copolymer is
expected to form micellar aggregates in an aqueous solution
through self-assembly.
At first, a copolymer, PEG113-AM36.9 (absent MMA units)
was prepared, but insoluble in water, resulting in difficulty in
studying the copolymer. Thus, we introduced MMA as a
comonomer to the copolymer, changing AM contents.
The copolymer, PEG113-b-P(MMA41.7-AM3.8), was prepared
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nanoreactors, and encapsulation-releasing boosters. Photoirra-
diation is a fairly versatile stimulus for controlling micelle
aggregation behavior because the irradiation process does not
require any contact with the target systems or additives like acid
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and base. Another advantage of irradiation is the ability to
control irradiation area and power to induce a response from the
system.
from MMA, AM, and a PEG-based macroinitiator through a
modified procedure of atom transfer radical polymerization
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,8 1
(ATRP) from previous reports.
H NMR analysis revealed
Some functional molecules are known to exhibit photo-
active behavior, including cleavage, isomerization, dimerization,
and coupling. Amphiphilic copolymers containing cinnamoyl,
coumarin, or thymine photoactive moieties, which are respon-
sive to photoirradiation at ca. 300 nm, have been prepared to
MMA and AM units in the copolymer were 41.7 and 3.8,
9
¹1
respectively. Furthermore, the concentration of 0.5 g L co-
polymer solution in CHCl3 corresponds to AM unit concen-
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tration of 0.028 mM compared with 9-anthrylmethyl acetate in
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CHCl . The copolymer dissolved in water (10 g L ), presum-
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form crosslinked micelles in an aqueous solution. However,
when the photoresponsive system is applied to biomedical
applications, the ultraviolet irradiation can lead to serious
damage to the body and to substrate, which arises from high-
energy, low-wavelength UV light. Therefore, the photocontrol-
led architectures need to be responsive to visible or near-infrared
ably due to low AM units of 3.8. The apparent critical micelle
concentration (cmc) of the copolymer in water was estimated
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¹1
using Nile red as a fluorescence probe to be 0.036 g L
,
meaning that the interpolymer micellar aggregation occurred
above the concentration.
Photoirradiation at > 360 nm of the aqueous solution
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(10 g L¹1) was conducted after bubbling argon gas to avoid
light irradiation. We believe that one promising approach to
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solve this wavelength problem is to use ³-extended molecules
which exhibit photoactivity. Anthracene derivatives are photo-
active molecules that display absorption bands with a large
reactions between anthracene moieties and oxygen. Portions of
the aqueous solution were taken before irradiation and at
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predetermined irradiation times, and diluted to 0.5 g L with
water for UVvis measurements. Before irradiation, the absorp-
tion maxima corresponding to ³³* transitions of the anthra-
cene moieties in water were observed at 388, 368, 350, and
334 nm (Figure 1). Upon irradiation, the absorption peak
intensities decreased. This decrease is due to the photodimeri-
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molar absorption coefficient of ca. 10 , assigned to ³³*
transitions. They can be dimerized via [4,4]-cycloaddition by
photoirradiation at 300400 nm.5 The resulting photodimers
undergo dissociation upon irradiation or heating. Here, we report
the photochemical formation of crosslinked micelles in water
through the photodimerization of anthracene units tethered to an
amphiphilic copolymer as a photosensitive micellar system.
PEG113-b-P(MMA41.7-AM3.8) is an amphiphilic block co-
polymer composed of poly(ethylene glycol) monomethyl ether
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zation of the anthracene moieties, as reported previously. An
isosbestic point was observed at 311 nm in the early stages. The
peak intensities were 69, 48, 30, and 21% of the initial intensity
after 270, 540, 900, and 1200 min of irradiation, respectively,
for the peak at 388 nm. The photodimerization degree of the
(PEG), methyl methacrylate (MMA), and 9-anthrylmethyl
Chem. Lett. 2010, 39, 682683
© 2010 The Chemical Society of Japan