Bioinspired core-crosslinked micelles from thymine-functionalized amphiphilic block copolymers: Hydrogen bonding and photo-crosslinking study

Article abstract of DOI:10.1002/pola.24853

Bioinspired core-bound polymeric micelles, based on hydrogen bonding and photo-crosslinking, of thymine have been prepared from poly(vinylbenzylthymine)- b-poly(vinylbenzyltriethylammonium chloride). The amphiphilic block copolymer was synthesized by 2,2-

Full text of DOI:10.1002/pola.24853

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ARTICLE
Bioinspired Core-Crosslinked Micelles from Thymine-Functionalized
Amphiphilic Block Copolymers: Hydrogen Bonding and
Photo-Crosslinking Study
1
2
3
1
1
Gagan Kaur, Shery L. Y. Chang, Toby D. M. Bell, Milton T. W. Hearn, Kei Saito
1
Centre for Green Chemistry, Monash University, Clayton, Victoria 3800, Australia
2
Monash Centre for Electron Microscopy and School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
3
Correspondence to: K. Saito (E-mail: kei.saito@monash.edu)
Received 10 February 2011; accepted 17 June 2011; published online 12 July 2011
DOI: 10.1002/pola.24853
ABSTRACT: Bioinspired core-bound polymeric micelles, based
on hydrogen bonding and photo-crosslinking, of thymine have
been prepared from poly(vinylbenzylthymine)-b-poly(vinylben-
zyltriethylammonium chloride). The amphiphilic block copoly-
mer was synthesized by 2,2-tetramethylpiperidin-1-oxyl-
mediated living radical polymerization in water/ethylene glycol
solution. Micelle characterization and critical micelle concentra-
tion measurements demonstrated that the hydrogen bonding
of the attached thymine units stabilizes the micelles. Further,
core-crosslinked polymeric micelles were formed by ultraviolet
(UV) radiation showing that the stability of the micelle could
be controlled by the UV crosslinking of the attached thymines.
VC 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
49: 4121–4128, 2011
KEYWORDS: biomimetic; block copolymer; core-shell polymers
INTRODUCTION Bioinspired mechanisms are used to create
stabilizes the hydrophobic core by acting as an interface
between the aqueous phase and the hydrophobic region.
This unique assembly makes polymeric micelles suitable can-
didates as chemical capsules for various applications such as
industrial surfactants, nanocapsules for biomedical usage,
electronic pollution control devices, removal agents for or-
ganic contaminates in water, and in other nanofabricated
alternative materials based on the principles of green chem-
1
,2
istry. Nature provides abundant and elegant examples of
the synthesis of molecules in terms of both atom economy
and energy required. Thymine, one of the nucleic bases in
DNA, has the ability to form both relatively strong hydrogen
3
–5
bonds and to be photo-crosslinked.
The hydrogen bonding
1
1–17
capabilities of thymine are well known to stabilize the double
helix structure of DNA. The photodimerization of thymine, a
ultraviolet (UV)-induced 2p þ 2p photocyclization, results in
the covalent dimerization of adjacent thymines, leading to the
disruption of the helical structure of DNA. This process has
been linked with the development of certain forms of skin
cancer. This photo-crosslinking of thymine occurs when irradi-
ated at wavelengths >270 nm UV. The 2p electron system of
the 5 and 6 carbons of two adjacent thymines form a cyclobu-
tane crosslink. Crosslinking can be reversed either by irradia-
tion at <249 nm or enzymatically. By using these mecha-
nisms, thymine-functionalized chemical compounds can be
materials.
Chemicals may be loaded into the core by
1
5,18–23
physical entrapment, conjugation, or complexation.
Fur-
thermore, the controlled release of encapsulated guest materi-
als from the polymeric micelles can be triggered by changes
in temperature and pH, by chemical stimuli, by enzymes, or
2
4–28
by application of light, ultrasound or magnetic field.
For polymeric micelles to have broad application use, it is
critical that the amphiphilic block copolymer micelles must
be stable. Critical micelle concentration (CMC) provides a
measure of stability of polymeric micelles. CMC is the thresh-
old concentration at or above which the micelles form.
Lower CMC values favor stable micelles even in dilute solu-
tions. Stability of the polymeric micelles is affected by the
6
–10
photo-crosslinked and photodecrosslinked.
2
9,30
Amphiphilic block copolymers form nanoscopic self-assem-
bling core/shell structures called polymeric micelles in the
bulk aqueous medium. During the formation of these
micelles, the hydrophobic segment forms the core while the
hydrophilic segment forms the shell. The hydrophilic shell
nature of interactions between chains in the core.
Cross-
linking has been used by researchers to increase the struc-
tural stability of polymeric micelles in dilute solutions and
even in organic solvents. Crosslinking may be induced in the
shell or in the core. Shell crosslinked micelles can be
Additional Supporting Information may be found in the online version of this article.
V
C
2011 Wiley Periodicals, Inc.
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FIGURE 1 Core bound thymine-functionalized polymeric micelle.
ꢀ
prepared by UV irradiation, by addition of multifunctional
crosslinking reagents, or by the direct reaction between the
ature (60 C) and also from in situ TEM observations of mor-
phological changes of micelles while heating the micelle sam-
ples (from room temperature to 200 C). These higher
3
1–36
ꢀ
polymer chains.
Core crosslinked micelles have been
prepared by photo-crosslinking, free radical polymerization
within the core, sol–gel chemistry, and condensation reac-
temperature studies have been used to examine the effect of
hydrogen bonding and photo-crosslinking on the thermo-
stability of the micelles.
1
6,37–39
tions.
Photo-crosslinking is the method of choice as it
is non toxic, economical, and does not produce any by-prod-
ucts. Furthermore, suitable photo-crosslinking does not affect
EXPERIMENTAL
4
0
the micellar shape and size. Recently, our group has inves-
tigated core-crosslinked micelles containing thymine groups
Materials
All reagents and materials were purchased from Sigma-
Aldrich in the purest form available and used as received.
VBT and methylated vinylbenzylthymine (VMT) were synthe-
sized from thymine and vinylbenzyl chloride as described
4
1,42
as photo-crosslinkable units.
We have reported that
enhanced stability can be achieved with nano polymeric
micellar aggregates derived from amphiphilic block copoly-
mers of vinylbenzylthymine (VBT) and styrenesulfonate
41
previously.
4
1
(
VPS). In this previous work, we reported the synthesis of
Synthesis of VBA
In a round bottom flask, vinylbenzylchloride (12.5 mL, 0.08
mol) and triethylamine (12.5 g, 0.125 mol) were dissolved in
a block copolymer of VBT and VPS and characterization of
4
1
the micelles formed from it. However, the hydrogen bond-
ing in the core of the micelles and their thermostability were
not investigated in detail in our previous work.
ꢀ
methanol (15 mL). The reaction mixture was stirred at 30 C
overnight. The reaction mixture was then poured into excess
of dry ether, which resulted in formation of white solid. The
solid was recovered by filtration and the white product is
dried under vacuum. The white solid obtained was then
recrystallized in 2-propanol.
In this article, we report the synthesis of a related bioins-
pired core-shell thymine-containing polymeric micellar sys-
tem derived from the amphiphilic block copolymerization of
VBT and vinylbenzyltriethylammonium chloride (VBA). The
hydrogen bonding and photo-crosslinking attributes (Fig. 1)
of these new polymer micelles are documented focusing par-
ticularly on hydrogen bonding in the micelle core at higher
temperature. Copolymers of VBA with 1-pyrenylmethyl meth-
acrylates, (4-vinylbenzyl)-cinnamate, and VBT have been
1
Yield: 99%. H NMR: (400 MHz, CDCl ) d1.30 (t, 9H, ACH ),
3
3
3
5
1
.16 (q, J ¼ 7.2 Hz, 6H, ANACH A), 4.46 (s, 2H, ACH A),
2
2
.38 (d, J ¼ 10.8 Hz, 1H, vinyl CAH), 5.95 (d, J ¼ 17.2 Hz,
H, vinyl CAH), 6.798 (dd, J ¼ 6.8 Hz, 1H, vinyl CAH), 7.49
4
3–47
(d, J ¼ 8 Hz, 2H, aromatic CAH), 7.60 (d, J ¼ 8 Hz, 2H,
reported in literature,
but amphiphilic block copolymers
aromatic CAH).
of VBT and VBA have never been synthesized before. We
chose VBA as the hydrophilic block because polymers with
quaternary ammonium groups are known to possess high
Synthesis of Poly(VBT-b-VBA)
VBA (1.0 g, 3.94 mmol) was dissolved in 50% (v/v) ethylene
glycol/water (10 mL) and TEMPO (62 mg, 0.39 mmol) was
added to the solution. The reaction mixture was stirred at
4
8–53
antimicrobial activities.
The amphiphilic block copolymers, poly(vinylbenzylthy-
mine)-b-poly(vinylbenzyltriethylammonium chloride) (poly-
ꢀ
60 C until the TEMPO completely dissolved giving the solu-
(VBT-b-VBA)) were synthesized by 2,2-tetramethylpiperidin-
tion an orange color. Na S O (50 mg, 0.26 mmol) and
2
2 5
1
-oxyl (TEMPO)-mediated living radical polymerization in
K S O (60 mg, 0.22 mmol) were then added to the solution
2 2 8
ꢀ
water/ethylene glycol solution. Hydrogen bonded micelles,
core-photo-crosslinked micelles, and nonhydrogen bonded
micelles were prepared from these block copolymers and
their morphology and stability examined by dynamic light
scattering (DLS), transmission electron microscopy (TEM),
and CMC measurements. The stability of micelles was further
investigated by measuring the CMC values at higher temper-
and stirring continued at 60 C under nitrogen until the
reaction mixture became clear. The reaction mixture was
ꢀ
then heated at 125 C for 20 h under nitrogen to prepare
homo VBA polymer. VBT (0.95 g, 3.94 mmol) was then
added to the homo-VBA polymer solution and heating con-
ꢀ
tinued for further 24 h at 125 C under nitrogen with stir-
ring. The polymerized product was purified by adding the
4
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reaction mixture drop wise to acetone (300 mL) while stir-
ring. The resulting solids were collected by filtration and
dried under vacuum.
Photo-Crosslinking of Micelles
A solution of the block copolymer micelles (6.7 mg mL
ꢂ1
)
was crosslinked in a quartz cuvette by irradiation in a short
wave (254 nm) crosslinker (CL-1000 Crosslinker, UVP Ltd.)
1
Yield: 100%. H NMR: (400 MHz, DMSO-d6) d0.80–1.90 (br,
ꢂ2
with 4, 6.5, and 8 J cm . After photo-crosslinking, the UV
1
8H, ANACH ACH , ACH ), 4.65 (br, 2H, ACH A), 6.05–
2
3
3
2
absorption spectrum of each solution was obtained using
Hitachi U-1800 UV/Vis Spectrophotometer. The absorption
spectra were recorded for wavelength range of 220–350 nm.
7
.80 (br, 8H, aromatic CAH), 11.3 (br, 1H, NAH). IR: mNAH
¼
4
3421, m
¼ 1684. M ¼ 1.16 ꢁ 10 , M /M ¼ 1.8.
C ¼¼ O
n
w
n
Synthesis of Poly(VMT-b-VBA)
Poly(VMT-b-VBA) was synthesized by a similar method as
used for synthesis of poly(VBT-b-VBA).
Critical Micelle Concentration
The CMCs of the copolymers were estimated by fluorescence
spectroscopy using the dye Nile Red as a probe. The micellar
solutions were diluted to a concentration range of 1.00 ꢁ
1
Yield: 100%. H NMR: (400 MHz, DMSO-d6) d0.70–2.10 (br,
ꢂ5
ꢂ1
1
(
0
to 1.00 mg mL . An acetone solution of nile red
1
2
3
8H, ANACH ACH , ACH ), 3.16 (br, 3H, NACH ) 4.75 (br,
2 3 3 3
ꢂ1
1 mg mL ) was prepared. Approximately 30 lL of dye
H, ACH A), 6.10–8.30 (br, 8H, aromatic CAH). IR: m
¼
2
NAH
4
solution was added to 3 mL of each of the diluted solutions.
The acetone was then evaporated from the solutions. The
fluorescence was then measured for each solution using a
Cary Eclipse Fluorescence Spectrophotometer. The measure-
405, m
¼ 1678. M ¼ 1.01 ꢁ 10 , M /M ¼ 2.4.
C ¼¼ O
n
w
n
Polymer Characterization
NMR spectra were taken on a Bruker 400 MHz NMR spec-
trometer. Molecular weights were determined by gel permea-
tion chromatography (GPC) performed on a Tosoh Ecosec
HLC-8320GPC equipped with both refractive index and UV
detectors (UV-detection, k ¼ 280 nm) using Tosoh alpha
ꢀ
ments were carried out at 20 C using an excitation wave-
length of 550 nm and emission wavelengths in the range
570–800 nm. To measure the CMC of the crosslinked mi-
celle, the crosslinked micelle solutions were also diluted to
ꢂ5
ꢂ1
4000 and 2500 columns. Dimethylformamide (DMF) contain-
a concentration range of 1.0 ꢁ 10 to 1.0 mg mL and
fluorescence was measured by the same procedure as the
uncrosslinked micelles. The CMC measurements were also
ing LiBr (10 mM) was used as the solvent. Calibration curves
were obtained using polystyrene standards. To determine
molecular weight of the hydrophilic block poly(VBA), aque-
ous mobile phase was used (0.5 M aqueous NaBr [80% v/v]
containing acetonitrile [20% v/v]). Calibration curves were
obtained using polyethylene glycol standards.
ꢀ
ꢂ2
carried out at 60 C for poly(VBT-b-VBA) micelles (before
and after 8 J cm dose of UV radiation) and poly(VMT-b-
VBA) micelles.
RESULTS AND DISCUSSION
Micelle Formation
Poly(VBT-b-VBA) and Poly(VMT-b-VBA) Synthesis
A typical block copolymer (0.1 g) in DMSO (10 mL) was
transferred through a 0.45-lm filter into a preswollen semi-
permeable membrane (Spectra/Por, SPEC-TRUM, molecular
weight cutoff, 3500) and dialyzed against water (3 L) for
The thymine-functionalized monomer VBT was synthesized
from p-vinylbenzylchloride and thymine in aqueous ethanol
46
as reported previously. The amphiphilic block copolymer
synthesis was carried out by a TEMPO-mediated living radi-
cal polymerization system in a water/ethylene glycol mix-
ture (Scheme 1). It is worth noting that this living radical
polymerization system can be carried out as an aqueous
mixture, a principle of green chemistry. The use of water
also allows the polymerization of VBA without any addi-
tional treatment.
72 h to form a micelle. The dialysate water was exchanged
at 2, 5, and 8 h intervals.
Micelle Characterization
The polymeric micelles were characterized by DLS and TEM.
The DLS measurements were performed using a Brookhaven
particle size analyzer equipped with a laser operating at a
wavelength of 659 nm at 25 C. ZetaPlus particle sizing soft-
ware version 4.04 was used to calculate the average effective
ꢀ
To achieve the synthesis of the block copolymers, the homo-
polymer of VBA was prepared first and then VBT was added
to the reaction solution to prepare the block copolymer with
ꢂ1
diameter of the micelles. Approximately 1.0 mg mL block
copolymer micelle solution was filtered and used for the
measurement. For the TEM specimen preparations, block
copolymer micelle solutions were stained with 0.2% uranyl
acetate solution. All the measurements were done above the
CMCs. The sample solution was drop cast on a copper TEM
grid coated with holey amorphous carbon film. The TEM
images were taken using a JEOL 2100F TEM operated at 200
kV. For investigation of stability of micelles at elevated tem-
perature, the micelle samples were heated in situ in JEOL
equal mole ratios. In this manner, poly(VBT-b-VBA) (Mn
¼
4
1.16 ꢁ 10 , M /M ¼ 1.8) was prepared. In addition, 3-
w
n
methyl-1-(4-vinylbenzyl)thymine (VMT) was prepared by
methylation of VBT as a surrogate molecule to block the
hydrogen bond donating ability of the monomer. The amphi-
philic block copolymer of VMT and VBA, poly(VMT-b-VBA)
4
(M ¼ 1.01 ꢁ 10 , M /M ¼ 2.4), was then synthesized to
n
w
n
examine the effect of hydrogen bonding on the stability of
micelles. The polydispersity values of the block copolymers
are high when compared with other TEMPO-mediated living
radical polymerizations. However, it is worth noting that
the polydispersity values obtained are low when compared
with previously reported block copolymer synthesis using
2100F using a heating holder (Gatan Inc.). The specimens
5
4
were initially observed at room temperature and then heated
ꢀ
ꢀ
up to 200 C with 20 C intervals. At each step, the tempera-
ture remained constant for at least 10 min.
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SCHEME 1 Synthesis of amphiphilic block copolymers.
TEMPO-mediated living radical polymerization in aqueous
solvent systems.
Block Copolymer Micelle Formation, Characterization,
and UV Crosslinking
5
4,55
1
It is known that an amphiphilic block copolymer with a suit-
able hydrophilic/hydrophobic balance will form micelles in
aqueous solution. Dialysis method was used to form poly-
meric micelles and the size of the block copolymer micelles
was analyzed by DLS. Table 1 summarizes the size of the
formed block copolymer micelles.
Polymer structures were characterized by H NMR and IR.
The molecular weights of the formed block copolymers were
measured by GPC in 10 mM LiBr–DMF as the mobile phase
using polystyrene standards for calibration curves. GPC also
confirmed the formation of block copolymer as it showed a
single main peak (Fig. 2). The molar ratios of VBT and VBA
in the poly(VBT-b-VBA) and those of VMT and VBA in the
Photo-crosslinked micelles were prepared by pouring the mi-
celle solution into a quartz cell and irradiating at 254 nm
1
poly(VMT-b-VBA) were determined by H NMR as 1:0.89 and
:1.17, respectively. Furthermore, the conversion of first
ꢂ2
1
with 4, 6.5, and 8 J cm . After each irradiation, the UV spec-
monomer VBA resulting in the formation of hydrophilic
block poly(VBA) was also characterized to confirm the quan-
titative consumption of a VBA in the first block segment of
block copolymers. A quantitative yield was obtained for the
conversion of VBA and molecular weight of the poly(VBA)
was determined by GPC using aqueous mobile phase (0.5M
aqueous NaBr [80% v/v] containing acetonitrile [20% v/v])
trum of the micelle solution was measured. The absorbance
was observed to reach a maximum value at 272 nm. With
increased time of UV exposure, the intensity of the absorp-
tion at 272 nm decreased (Fig. 3). It is well known that thy-
mine has an absorption at 270 nm and that this peak
decreases as a result of 2p þ 2p photodimerization. It is
clear that the amount of crosslinking is related to the UV
irradiation used. After irradiation of the micelle samples at
using polyethylene glycol standards for calibration curves
ꢂ3
ꢂ2
(
¼
M
n
¼ 4.07x 10 , M
w
/M
n
¼ 1.45, [Monomer]
0
/[Initiator]
0
power settings of 4, 6.5, and 8 J cm , UV spectra (Fig. 3)
15). Molecular weight of the hydrophilic block using aque-
showed that conversion of thymines to crosslinked product
is not complete but at least 17% of the thymines were pho-
todimerized. This suggests that the crosslinking has mostly
occurred between the neighboring polymer chains and hence
the micellar core is not completely crosslinked. However, the
crosslinking of polymers inside the micelle core was enough
to enhance the stability of the micelle.
ous mobile phase was found to be in the range as expected
from the molecular weight of the block copolymers obtained
using 10 mM LiBr–DMF mobile phase and mole ratios of two
1
monomers in the polymer obtained from H NMR.
DLS measurements were also carried out on crosslinked
micellar solutions. DLS results indicate a decrease in the size
of micelles upon photo-crosslinking. These results suggest
a
TABLE 1 Size of Block Copolymer Micelles by DLS
Irradiation
Number Average
Diameter (nm)
ꢂ2
Entry
Polymer
(J cm
)
1
2
3
4
5
a
poly(VBT-b-VBA)
poly(VBT- b-VBA)
poly(VBT- b-VBA)
poly(VBT-b-VBA)
poly(VMT-b-VBA)
0
103
95
4
6.5
8
87
82
0
122
FIGURE 2 GPC trace of poly(VBT-b-VBA) and poly(VMT-b-VBA)
ꢂ1
All of the measurement was carried out using 1.0 mg mL micelle
solution.
in 10 mM LiBr–DMF as the mobile phase.
4
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block copolymer micelles proceeds only inside the core and
did not change the spherical morphology of the micelles.
Furthermore, the in situ heating TEM results indicated differ-
ences between the micelle samples when heated up to
ꢀ
2
00 C (Fig. 5). It was observed that poly(VBT-b-VBA)
ꢀ
micelles were stable to heating to 60 C. However, the TEM
image of the micelle started to fade when the sample was
heated above 60 C. In comparison, the crosslinked poly(VBT-
ꢀ
ꢀ
b-VBA) micelles were stable even at 200 C, which supports
the stability enhancement of the micelles by crosslinking. The
higher temperature study indicated that the thermostable
property of the micelles can be controlled by the irradiation.
CMC Measurements
CMC measurements can be used to assess the stability of
micelles. A lower CMC value indicates higher stability. The
CMCs of the block copolymer micelles were determined by
fluorescence measurements in aqueous solution using Nile
Red as a probe (see Supporting Information). The CMC is
identified as the break point on a plot of fluorescence inten-
sity versus concentration (Fig. 6). Nile Red has a strong
fluorescence in a hydrophobic environment, but in a polar
environment such as water, fluorescence quenching is greatly
enhanced, significantly reducing fluorescence emission. Thus,
the CMC can be determined by measuring the concentration
point at which the fluorescence emission intensity of Nile
Red dye becomes constant. The CMCs of poly(VBT-b-VBA)
micelles after UV crosslinking (irradiation doses: 0, 4, 6.5,
FIGURE 3 The UV absorption spectra of poly(VBT-b-VBA) mi-
ꢂ2
celle solution upon UV exposure with 0, 4, 6.5, and 8 J cm
.
that photo-crosslinking because of photodimerization of thy-
mines between adjacent polymer chains brings the polymer
chains closer in the core causing the micelles to shrink in size.
Moreover, DLS results show a decrease in the size of micelles,
confirming that photo-crosslinking is occurring in the core of
the micelles and not between micelles which would have
resulted in increase in micelle size. Furthermore, the size
ꢂ2
remained constant after 8 J cm of UV irradiation. This result
might be due to the maximum limit of crosslinking reached.
ꢂ2
Examination of the micelle solution by IR was carried out in
an attempt to demonstrate the hydrogen-bonding of thymine
units but this was inconclusive because of the many broad
and 8 J cm ) were also measured to determine the effect of
photo-crosslinking on the stability of the micelles. In addi-
tion, the CMC of poly(VMT-b-VBA) micelles was measured to
examine the effect of hydrogen bonding on the stability of
the micelles.
ꢂ1
peaks around 3500–3000 cm due to ANH bond in thymine.
The bright-field TEM images (Fig. 4) of the stained poly(VBT-
b-VBA) micelles were taken before and after crosslinking.
ꢀ
Data for the measured CMCs (at 20 C) of the block copoly-
TEM images of the poly(VBT-b-VBA) micelle samples,
mer micelles are summarized in Table 2. Most significantly,
the poly(VMT-b-VBA) micelles, which are incapable of form-
ing self-assembled hydrogen bond dimers in the core
because of methylation, have a much higher CMC value when
‘‘before’’ and ‘‘after’’ photo-crosslinking, demonstrated largely
spherical objects in the range of 30–60 nm. These results
also indicate that crosslinking of attached thymine in these
ꢂ2
FIGURE 4 Bright-field TEM images of poly(VBT-b-VBA) micelles before and after UV crosslinking (8 J cm ).
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ꢀ
FIGURE 5 Bright-field TEM images of (a) poly(VBT-b-VBA) micelle at room temperature, 60, 100, and 200 C as well as (b) core-
ꢀ
crosslinked poly(VBT-b-VBA) micelle at room temperature, 60, 100, and 200 C.
compared with poly(VBT-b-VBA) micelles that are capable of
hydrogen bonding in the core. The CMC of poly(VMT-b-VBA)
was found to be approximately 80% higher than poly(VBT-b-
VBA). These observed differences in the CMC values of the
poly(VMT-b-VBA) micelles and the poly(VBT-b-VBA) micelles
are consistent with a functional role for hydrogen bonding of
the thymine units. In addition, these observations confirm
that hydrogen bonding of the attached thymines enhances
the stability of the block copolymer micelles. The CMC of the
poly(VBT-b-VBA) micelles decreases with increased UV
value indicating the lowest stability, whereas the crosslinked
poly(VBT-b-VBA) micelles had the lowest CMC value indicat-
ing highest stability. The CMC of crosslinked poly(VBT-b-
ꢀ
ꢂ1
VBA) micelles measured at 60 C was still low (39 lg mL
)
when compared with the CMC of the uncrosslinked micelle
ꢂ1
(46 lg mL ) consistent with the stability enhancement of
the micelles due to crosslinking. Collectively, these results
clearly indicate that the stability of the micelles is enhanced
by the core-photo-crosslinking of the attached thymines and
the stability of the micelles can be modulated by the UV
irradiation.
ꢂ2
irradiation. With 8 J cm of UV irradiation, the CMC of the
poly(VBT-b-VBA) micelle decreased by approximately 45%
when compared with the uncrosslinked sample.
CONCLUSIONS
ꢀ
We have synthesized an amphiphilic block copolymer of VBT
and VBA by TEMPO-mediated living radical polymerization
and used this novel material to prepare polymeric micelles
in aqueous solution. DLS, TEM, and CMC measurements
demonstrated that the amphiphilic block copolymers of VBT
and VBA can form stability-enhanced self-assembling
micelles based on hydrogen bonding of the attached thy-
mines in the core and that this stability can be further
The CMC values were also measured at 60 C (Table 3). At
ꢀ
60
C, poly(VMT-b-VBA) micelles have the greatest CMC
TABLE 2 Critical Micelles Concentration (CMC) of Block
Copolymer Micelles at 20 8C
ꢀ
Irradiation
CMC at 20 C
ꢂ2
ꢂ1
Entry
Polymer
(J cm
)
(lg mL
)
1
2
3
4
5
poly(VMT-b-VBA)
poly(VBT-b-VBA)
poly(VBT-b-VBA)
poly(VBT-b-VBA)
poly(VBT-b-VBA)
0
62
35
27
25
19
0
4
6.5
8
FIGURE 6 Fluorescence intensity of Nile Red at its emission
maximum versus copolymer concentration.
4
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TABLE 3 Critical Micelles Concentration (CMC) of Block
Copolymer Micelles at 60 8C
15 Adams, M. L.; Lavasanifar, A.; Kwon, G. S. J Pharmaceut
Sci 2003, 92, 1343–1355.
1
6 Henselwood, F.; Liu, G. Macromolecules 1997, 30, 488–493.
Irradiation
CMC
1
7 Savic, R.; Luo, L.; Eisenberg, A.; Maysinger, D. Science
ꢂ2
ꢂ1
)
Entry
Polymer
(J cm
)
(lg mL
2003, 300, 615–618.
8 Kataoka, K.; Matsumoto, T.; Yokoyama, M.; Okano, T.;
1
1
2
5
poly(VMT-b-VBA)
poly(VBT-b-VBA)
poly(VBT-b-VBA)
0
0
8
93
46
39
Sakurai, Y.; Fukushima, S.; Okamoto, K.; Kwon, G. S. J Contr
Release 2000, 64, 143–153.
1
9 Gao, Z.; Lukyanov, A. N.; Singhal, A.; Torchilin, V. P. Nano
Lett 2002, 2, 979–982.
2
2
0 Lukyanov, A. N.; Gao, Z.; Torchilin, V. P. J Contr Release
003, 91, 97–102.
controlled by core-photo-crosslinking of the attached thy-
mines. As such, polymeric micelles obtained from poly(VBT-
b-VBA) have the potential to allow the encapsulation of guest
materials by hydrogen bonding due to the attached thymines
in the core. Furthermore, the control of the CMC of the block
copolymer micelles by UV photo-crosslinking provides a new
opportunity for the controlled release of materials encapsu-
lated in the micelles. Thus, based on a bioinspired mecha-
nism, these investigations provide a novel approach for the
design and use of stable and controllable nano micelles.
2
1 Nishiyama, N.; Bae, Y.; Miyata, K.; Fukushima, S.; Kataoka,
K. Drug Discov Today Tech 2005, 2, 21–26.
2
2
2 Qiu, L.; Zheng, C.; Jin, Y.; Zhu, K. Expert Opin Ther Pat
007, 17, 819–830.
2
3 Nishiyama, N.; Kato, Y.; Sugiyama, Y.; Kataoka, K. Pharma-
ceut Res 2001, 18, 1035–1041.
2
6
4 Nishiyama, N.; Kataoka, K. Pharmacol Therapeut 2006, 112,
30–648.
2
5 Dimitrov, I.; Trzebicka, B.; M u¨ ller, A. H. E.; Dworak, A.; Tsve-
tanov, C. B. Prog Polym Sci 2007, 32, 1275–1343.
ACKNOWLEDGMENT
2
6 Oerlemans, C.; Bult, W.; Bos, M.; Storm, G.; Nijsen, J. F. W.;
This work was supported by Monash University Faculty of Sci-
ence Early Career Research Fund and Australian Research Coun-
cil. The authors thank Roy Jackson, Tony Patti, and Warwick
Raverty for fruitful discussions, and Monash Centre for Electron
Microscopy for providing access to the JEOL 2100F TEM.
Hennink, W. E. Pharmaceut Res 2010, 27, 2569–2589.
27 Bae, Y.; Kataoka, K. Adv Drug Deliv Rev 2009, 61, 768–784.
28 Alvarez-Lorenzo, C.; Concheiro, A. Mini Rev Med Chem
2008, 8, 1065–1074.
29 Jones, M. C.; Leroux, J. C. Eur J Pharmaceut Biopharmaceut
1
3
3
999, 48, 101–111.
0 Haag, R. Angew Chem Int Ed 2004, 43, 278–282.
REFERENCES AND NOTES
1 Zhang, Y.; Jiang, M.; Zhao, J.; Zhou, J.; Chen, D. Macromo-
lecules 2004, 37, 1537–1543.
1
Warner, J. C.; Cannon, A. S.; Dye, K. M. Environ Impact
Assess Rev 2004, 24, 775–799.
Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and
Practice; Oxford University Press: London, 1998.
3
2 Pilon, L. N.; Armes, S. P.; Findlay, P.; Rannard, S. P. Lang-
muir 2005, 21, 3808–3813.
2
33 Pilon, L. N.; Armes, S. P.; Findlay, P.; Rannard, S. P. Eur
Polym J 2006, 42, 1487–1498.
3
Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737–738.
3
4 Jiang, X.; Luo, S.; Armes, S. P.; Shi, W.; Liu, S. Macromole-
4
Boal, A. K.; Ilhan, F.; Derouchey, J. E.; Thurn-Albrecht, T.;
cules 2006, 39, 5987–5994.
Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746–748.
Viswanathan, K.; Ozhalici, H.; Elkins, C. L.; Heisey, C.; Ward,
T. C.; Long, T. E. Langmuir 2006, 22, 1099–1105.
Cheng, C. M.; Egbe, M. I.; Grasshoff, J. M.; Guarrera, D. J.;
3
9
5 Liu, S.; Armes, S. P. J Am Chem Soc 2001, 123, 9910–
911.
5
3
6 Butun, V.; Wang, X. S.; de Paz Banez, M. V.; Robinson, K. L.;
6
Billingham, N. C.; Armes, S. P.; Tuzar, Z. Macromolecules 1999,
3
Pai, R. P.; Warner, J. C.; Taylor, L. D. J Polym Sci Part A: Polym
Chem 1995, 33, 2515–2519.
3, 1–3.
3
7 Bronich, T. K.; Keifer, P. A.; Shlyakhtenko, L. S.; Kabanov, A.
7
Grasshoff, J. M.; Taylor, L. D.; Warner, J. C. U.S. Patent
V. J Am Chem Soc 2005, 127, 8236–8237.
5,708,106, January 13, 1998.
3
1
8 Wang, G.; Henselwood, F.; Liu, G. Langmuir 1998, 14,
554–1559.
8
Kiarie, C.; Bianchini, J.; Trakhtenberg, S.; Warner, J. C.
J Macromol Sci Pure Appl Chem 2005, 42A, 1489–1496.
Whitfield, J. R.; Morelli, A.; Warner, J. C. J Macromol Sci
Pure Appl Chem 2005, 42A, 1541–1546.
0 Trakhtenberg, S.; Hangun-Balkir, Y.; Warner, J. C.; Bruno, F.
F.; Kumar, J.; Nagarajan, R.; Samuelson, L. A. J Am Chem Soc
3
3
9 Tao, J.; Stewart, S.; Liu, G.; Yang, M. Macromolecules 1997,
0, 2738–2745.
9
4
0 Chen, Y.; Tavakley, A. E.; Mathiason, T. M.; Taton, T. A.
1
J Polym Sci Part A Polym Chem 2006, 44, 2604–2614.
2005, 127, 9100–9104.
41 Saito, K.; Ingalls, L. R.; Lee, J.; Warner, J. C. Chem Commun
2
007, 2503–2505.
11 Hamley, I. W. In Encyclopedia of Polymer Science and Tech-
nology; Mark, H. F., Ed.; Wiley: New Jersey, 2003, pp 457–482.
2 Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers;
Wiley: New Jersey, 2003.
42 Saito, K.; Warner, J. C. Green Chem Lett Rev 2009, 2, 71–76.
1
43 Tazuke, S.; Takasaki, R. J Polym Sci Polym Chem Ed 1983,
21, 1529–1534.
13 Kataoka, K.; Harada, A.; Nagasaki, Y. Adv Drug Deliv Rev
44 Cannon, A. S.; Warner, J. C. J Macromol Sci Pure Appl
2001, 47, 113–131.
Chem 2005, 42A, 1507–1514.
14 R o¨ sler, A.; Vandermeulen, G. W. M.; Klok, H. A. Adv Drug
45 Martino, D. M.; Reyna, D.; Estenoz, D. A.; Trakhtenberg, S.;
Deliv Rev 2001, 53, 95–108.
Warner, J. C. J Phys Chem A 2008, 112, 4786–4792.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4121–4128
4127

ARTICLE
WWW.POLYMERCHEMISTRY.ORG
4
6 Bianchini, J. R.; Saito, K.; Balin, T. B.; Dua, V.; Warner, J. C.
J Polym Sci Part A Polym Chem 2007, 45, 1296–1303.
7 Barbarini, A. L.; Estenoz, D. A.; Martino, D. M. Macromol
React Eng 2010, 4, 453–459.
8 El-Hayek, R. F.; Dye, K.; Warner, J. C. J Biomed Mater Res
Part A 2006, 79A, 874–881.
9 Senuma, M.; Tashiro, T.; Iwakura, M.; Kaeriyama, K.; Shi-
mura, Y. J Appl Polym Sci 1989, 37, 2837–2843.
51 Lu, G.; Wu, D.; Fu, R. React Funct Polym 2007, 67,
355–366.
4
52 Kenawy, E. R.; Worley, S. D.; Broughton, R. Biomacromole-
cules 2007, 8, 1359–1384.
4
53 Ikeda, T.; Tazuke, S.; Suzuki, Y. Die Makromol Chem 1984,
185, 869–876.
4
54 Shim, S. E.; Oh, S.; Chang, Y. H.; Jin, M.; Choe, S. Polymer
2004, 45, 4731–4739.
50 Dizman, B.; Elasri, M. O.; Mathias, L. J. J Appl Polym Sci
55 Desjardins, A.; Eisenberg, A. Macromolecules 1991, 24,
2004, 94, 635–642.
5779–5790.
4
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4121–4128