Self-assembly and micellization of a dual thermoresponsive supramolecular pseudo-block copolymer

Article abstract of DOI:10.1021/ma102196q

This paper reports the studies on self-assembly and thermosensitive micellization phenomena of a supramolecular polymeric host-guest system consisting of star-shaped poly(N-isopropylacrylamide) (PNIPAAm) with a β-cyclodextrin (β-CD) core (the host polymer

Full text of DOI:10.1021/ma102196q

1182 Macromolecules 2011, 44, 1182–1193
DOI: 10.1021/ma102196q
Self-Assembly and Micellization of a Dual Thermoresponsive
Supramolecular Pseudo-Block Copolymer
Zhong-Xing Zhang,†,‡ Kerh Li Liu,‡ and Jun Li*,†,‡
†Division of Bioengineering, Faculty of Engineering, National University of Singapore, 7 Engineering Drive 1,
Singapore 117574, and ‡Institute of Materials Research and Engineering, A*STAR (Agency for Science,
Technology and Research), 3 Research Link, Singapore 117602, Republic of Singapore
Received September 22, 2010; Revised Manuscript Received January 5, 2011
ABSTRACT: This paper reports the studies on self-assembly and thermosensitive micellization phenomena
of a supramolecular polymeric host-guest system consisting of star-shaped poly(N-isopropylacrylamide)
(PNIPAAm) with a β-cyclodextrin (β-CD) core (the host polymer) and bis(adamantyl)-terminated poly-
(propylene glycol) (PPG) (the guest polymer) in aqueous solution. This interesting host-guest system
exhibited dual thermoresponse in aqueous solution because of the existence of two kinds of thermoresponsive
segments, PPG and PNIPAAm, in the polymeric guest and host components, respectively. This unique
thermoresponsive behavior was completely tunable by the ratio of host/guest up to 1.0. Beyond this range, the
effect of the host was saturated, indicating that the host-guest system involved a 1:1 complexation between
adamantyl moiety and β-CD core. In other words, this polymeric host-guest system was able to form ABA-
type supramolecular pseudo-block copolymer via inclusion complexation in aqueous solution. This pseudo-
block copolymer underwent a reversible temperature-induced transition from solution to micelle and further
to aggregate under suitable conditions. However, different from conventional polymeric micelles, in the
micelles formed from this pseudo-block copolymer, the shell (composed of host component) and the core
(composed of guest component) were connected by physical interactions rather than chemical bonding. The
micellization phenomena of the host-guest system were extensively studied by a combination of 1H NMR,
fluorescence probe technique, dynamic light scattering (DLS), transmission electron microscopy (TEM), and
atomic force microscopy (AFM). The critical micelle temperature (CMT) for this thermoresponsive
host-guest system was dependent on the composition and concentration of the components. The size of
the resultant noncovalently connected micelle could be easily tuned not only by adjusting the temperature and
the concentration of the components but also by the ratio of host/guest and the length of the PPG block in the
guest polymer.
1. Introduction
It was well-known that cyclodextrins (CDs) are a series of
natural cyclic oligosaccharides composed of 6, 7, or 8
-(þ)-
D
Conventional amphiphilic di- or triblock copolymers can self-
assemble to form nanostructures such as micelles in aqueous
medium, containing dense cores of the insoluble blocks, sur-
rounded by diffuse outer shells (coronas) formed by the soluble
blocks. These polymeric micelles may have promising biomedical
applications in drug and gene delivery, protein encapsulation,
and so on.1-3
Recently, construction of micelles by using the nonconven-
tional method is arousing people’s interest.4,5 Different from con-
ventional polymeric micelles, in these micelles, which can be called
as “noncovalently connected micelles (NCCMs)”, only physical
interactions rather than chemical bonds exist between shell and
core. To construct such micelles, hydrophilic component A and
hydrophobic component B are usually needed, which can interact
with each other via H-bonding,4,5 host-guest inclusion com-
plexation,6,7 and ligand-metal coordination interactions8 in
aqueous solution. The micelle formation in the aqueous mixture
of A and B is usually induced by means of intrinsic hydrophobic
interactions between chains of component B or external stimuli
such as temperature, pH, and UV/vis light irradiation, etc. To
date, NCCMs constructed with other noncovalent interactions
except H-bonding as driving force are relatively seldom reported.
glucose units linked by R-1,4-linkages and named R-, β-, or γ-CD,
respectively.9 Because of their unique capability of forming
inclusion complexes in the inner cavities and many other favor-
able physicochemical and biological properties, natural CDs and
their derivatives have been recognized as important natural host
molecules in the field of supramolecular chemistry.10-16 Investi-
gations by us and others in recent years have shown that the CD-
based inclusion complexes have many promising applications,
particularly in the areas of biotechnology and biomedicine.17-19
Recently, we demonstrated a novel supramolecular approach
controlling the thermoresponsive property of a poly(N-isopro-
pylacrylamide) (PNIPAAm) star polymer through inclusion
complexation between β-CD moiety and adamantyl groups.20
PNIPAAm is one of the most popular thermoresponsive poly-
mers, which shows dramatic and reversible phase transition
behavior in water with the lower critical solution temperature
(LCST) at 32 °C.21 This unique property makes PNIPAAm and
its copolymers “intelligent” systems for many promising applica-
tions, particularly in the areas of biotechnology, biomedicine, and
nanotechnology, e.g., for controlled drug delivery, gene delivery,
cell and enzyme immobilization, biosensor, and bioaffinity
separation.22-33
Herein, we presented a design of a dual thermoresponsive
supramolecular pseudo-block copolymer and its formation of
*Corresponding author: e-mail bielj@nus.edu.sg; Tel þ65-6516-
7273; Fax þ65-6872-3069.
pubs.acs.org/Macromolecules
Published on Web 02/04/2011
r 2011 American Chemical Society

Article
Macromolecules, Vol. 44, No. 5, 2011 1183
Scheme 1. Synthetic Procedures for (a) the β-CD-Based Macroinitiator (4Br-β-CD) and β-CD-Core Star-Shaped PNIPAAm (β-CD-(PNIPAAm)4)
via ATRP (Both Have an Average Degree of Substitution of 4) and (b) the Adamantyl-Containing PPGs
noncovalently connected micelles with PNIPAAm-grafted β-CD
as the shell and poly(propylene glycol) (PPG) as the core. The
supramolecular pseudo-block copolymer was formed from a
polymeric host-guest pair, i.e., star-shaped PNIPAAm with a
β-CD core (host polymer) and bis(adamantyl)-ended PPG (guest
polymer) via host-guest inclusion complexation in aqueous
solution. Upon the host-guest complexation, the resultant
pseudo-block copolymer exhibited interesting dual thermore-
sponse in aqueous solution because of the existence of two kinds
of thermoresponsive segments, PPG and PNIPAAm, in poly-
meric guest and host components, respectively, and this unique
thermoresponsive behavior was completely tunable by the ratio
of host/guest.
This pseudo-block copolymer showed thermosensitive micelle
formation behavior in aqueous solution. That is, under lower
temperature, the PPG chain of the guest polymer kept water-
soluble, and no micelle was formed. As temperature increased,
the PPG chains dehydrated and aggregated through hydropho-
bic-hydrophobic interactions, thus acting as a hydrophobic core
for the micelle formation. When temperature further increased,
the phase transition related to PNIPAAm on the corona of the
noncovalently connected micelles occurred. Thus, the micelles
were destabilized, resulting in micelle aggregation and precipita-
tion. This solution-micellization-micelle destabilization process
was totally temperature-reversible. The supramolecular system
presented here is very different from the reported systems,6,7 in
the system components design, the tunability to respond to
stimuli, the mechanism of noncovalently connected micelle for-
mation, and the structure of the resultant micelles. Through
forming the dual thermoresponsive pseudo-block copolymer, this
work has demonstrated a novel supramolecular approach in
designing and constructing noncovalently connected polymeric
micelles with tunable properties.
using copper(I)-mediated ATRP according to our previous
report,20 which was featured with a β-CD core as a molecular
recognition moiety and multiple PNIPAAm arms as actuation
moieties. Scheme 1a shows the synthetic route of star-shaped
PNIPAAm. First, a β-CD-based macroinitiator (4Br-β-CD)
was synthesized by treating native β-CD with 4 equiv of
2-bromoisobutyric bromide. In our experimental conditions,
the introduction of the initiation group, -OCO-C(CH3)2Br,
to β-CD ring via esterification reaction occurred mainly at the
6-positioned primary hydroxyl group of the β-CD ring. In
other words, the initiation groups were mainly grafted onto the
smaller rim of the β-CD ring in the macroinitiator, which was
confirmed by 13C NMR analysis of the macroinitiator. A
method for purifying this macroinitiator was based on the diff-
erence in solubility of initiator-modified β-CDs with different
degree of substitution (DS) in water and acetone (DS is defined
as the number of grafted initiation group per CD molecule). It
was found that the solubility of these β-CD based macroini-
tiators in H2O decreased dramatically with increasing DS. By
contrast, their solubility in acetone increased distinctly with the
increase of DS. Among them, the β-CD-based macroinitiator
with DS of 4, was slightly soluble in both water and acetone.
Thus, washing the crude product with water removed the
macroinitiators with DS < 4, while washing the crude product
with acetone removed the macroinitiators with DS > 4. By
using this purification method, the β-CD-based macroinitiator
with DS of 4 could be ensured a high level of purity. The DS of
1
the purified macroinitiator was determined by means of H
NMR spectroscopy (DS ca. 3.9) and X-ray photoelectron
spectroscopy (XPS) (DS ca. 4.3), respectively. Both methods
indicated that the DS was very close to 4.
Second, ATRP of NIPAAm using 4Br-β-CD as macro-
initiator was carried out under a nitrogen environment in the
presence of copper(I) bromide as catalyst and Me6-TREN as
ligand. Referenced to literature methods of ATRP,34,35
various experimental conditions were tested in order to
synthesize well-defined star polymers. It was found that the
best result could be obtained when methanol was chosen as
reaction medium to give a homogeneous reaction system, the
2. Results and Discussion
2.1. Synthesis and Characterization of Host and Guest
Polymers. 2.1.1. Star-Shaped PNIPAAm with a β-CD Core
(Host Polymer). This macromolecular host was synthesized

1184 Macromolecules, Vol. 44, No. 5, 2011
Zhang et al.
initial molar ratio of [monomer]/[initiation site]/[CuBr]/
[Me6-TREN] ([M]0/[I]0/[Cu]0/[L]0) was 30/1/1/1.2, and the
reaction temperature was at 70 °C. During the ATRP of
NIPAAm in methanol using 4Br-β-CD as initiator, the
monomer conversion was controlled to be less than 50% to
limit possible side reactions such as bimolecular termination
or thermal initiation. The number-average molecular weight
(Mn) of the final star polymer calculated based on 1H NMR,
XPS, and GPC measurements was 8166, 9169, and 4101 Da
(Mw/Mn = 1.24), respectively. These values based on differ-
ent measurements were found to correlate well with the
predicted molecular weight (7851 Da) calculated from the
feed initiator concentration and the consumption of mono-
mer, showing that the above-mentioned side reactions were
negligible under our polymerization conditions.20
2.1.2. Bis(adamantyl)-Terminated PPGs (Guest Polymers).
Theses macromolecular guests were obtained in high yield by
mixing the corresponding bis(amino)-ended PPGs with ada-
mantaneacetic acid in the presence of DCC and DMAP
(Scheme 1b). The bifunctional PPGs were attached with
adamantyl groups quantitatively. The attachment of two
adamantyl groups to PPGs was confirmed by comparing the
integral intensities of the peaks from the PPG segments
(1.00-1.20 ppm) and the adamantyl groups (1.85-2.05 ppm)
Figure 1. (left) Turbidity variations of aqueous solutions of PPG-2K-
(Ad)2 (guest polymer) as a function of temperature upon the addition of
β-CD-(PNIPAAm)4 (host polymer). Cguest polymer = 0.26 mg/mL (for
solutions a-f); Chostpolymer = 2.5 mg/mL (for solution g); host/guest
represents the molar ratio of β-CD core/adamantyl moiety. (right)
Representative photos of the 1:1 host-guest solution (e) at three typical
temperatures (8, 18, and 38 °C).
(solution a) and host polymer (solution g) exhibited only
single thermoresponsive profile, while host-guest systems
(solutions b-f) exhibited interesting double thermorespon-
sive profiles, showing these systems underwent two stages of
phase transition. This phenomenon is similar to those found
in aqueous solutions of conventional copolymers containing
two kinds of thermoresponsive blocks.38-44 For these host-
guest systems, stage I (below 31 °C) was closely related to the
phase transition of PPG segment in the guest polymer, while
stage II (above 31 °C) was due to the phase transition of
PNIPAAm arms grafted onto the β-CD core of the host
polymer. Representative photos of the 1:1 host-guest solu-
tion (e) at three typical temperatures (8, 18, and 38 °C)
(Figure 1, right) clearly showed a turbidity change upon in-
creasing the solution temperature.
Curve a in Figure 1 shows that the LCST of guest polymer
PPG-2K-(Ad)2 was ca. 5.6 °C. Herein, the LCST of the sample
solution, which is equivalent to the cloud point of this solution,
is defined as the temperature exhibiting a 34% decrease in
optical transmittance of this solution at 500 nm in stage I due to
PPG phase transition (i.e., half of the total decrease in stage I).
The LCST of this guest polymer solution shifted to a higher
temperature upon the addition of the macromolecular host, β-
CD-(PNIPAAm)4 (Figure 1, curves b-f). The LCST signifi-
cantly changed with a LCST increase (ΔLCST, referenced to
pure guest polymer) of 4.3 °C when the host/guest ratio, i.e., β-
CD core/adamantyl moiety molar ratio, was 0.25. The ΔLCST
further increased another 4.9 °C, resulting in an LCST of ca.
14.8 °C when the host/guest ratio reached 0.5. The LCST of
this self-assembling system was ca. 20.0 and 24.2 °C when the
host/guest ratio reached 0.75 and 1.0, respectively. However,
the LCST became almost saturated upon further increase in
the host/guest ratio beyond 1.0, indicating that the self-
assembling system involved a 1:1 complexation between ada-
mantyl moiety and β-CD core. The bis(adamantyl)-termi-
nated PPG-2K could be incorporated to the β-CD core of the
star polymer via inclusion complexation to form a supra-
molecular block copolymer (pseudo-block copolymer).
Through forming the pseudo-block copolymer, the hydro-
philic PNIPAAm arms on host polymer contributed to
increase the water solubility of the guest polymer. In stage
I, the LCST of the self-assembling system could be easily
tuned through changing the host/guest ratio, i.e., the com-
position ratio of hydrophilic PNIPAAm/hydrophobic PPG
like in conventional amphiphilic block copolymer systems. It
1
on their H NMR spectra (CDCl3). After modification of
these PPGs with adamantaneacetic acid, a slight increase in
molecular weight was observed by using GPC technique.
To further confirm the end-functionality of these adaman-
tyl-containing PPGs, a derivatization strategy was employed
to simplify the analysis of the polymer end group by NMR.
Herein the trichloroacetyl isocyanate (TCAI) was used as
derivatization reagent. TCAI was known to react quantita-
tively with amino groups, which causes a significant down-
field shift of the initial protons adjacent to these amino end
groups.36,37 Upon the addition of an excess of TCAI to the
NMR solutions (DMSO-d6) of these adamantane-modified
PPGs, no such a downfield shift was observed, showing there
was no free amino group in the modified PPG molecules, and
the terminal amino groups of the starting PPGs were quan-
titatively derivatized with adamantyl groups. The selected
characterization data including number-average molecular
weight (Mn) and the polydispersity index (PDI: Mw/Mn) for
the final adamantyl-containing PPGs as well as the corre-
sponding starting PPGs are summarized in Table S1 (see
Supporting Information).
2.2. Host-Guest Inclusion Complexation between β-CD-Core
Star Polymer and Bis(adamantyl)-Terminated PPGs. 2.2.1.
Thermoresponsive Behavior of the Host-Guest Self-Assembling
System. In this study, the interesting dual thermoresponsive
behavior was investigated for the host-guest systems compris-
ing of polymeric host β-CD-(PNIPAAm)4 and polymeric guest
PPG-(Ad)2 in aqueous solution. Regarding the solubility in
water at room temperature, the host polymer was very soluble,
while the guest polymers used in this study were poorly soluble,
except for PPG-400-(Ad)2, which was moderately soluble.
When the two components were mixed together, the LCST of
guest polymer increased dramatically, while the LCST of host
polymer decreased upon the host-guest inclusion complexa-
tion between adamantyl moiety and β-CD core. The detailed
experimental results for the aqueous mixtures of β-CD-
(PNIPAAm)4 and PPG-2K-(Ad)2 are given below as a typical
example.
Figure 1 shows the information on the thermoresponsive
behavior of a series of aqueous mixtures of β-CD-
(PNIPAAm)4 and PPG-2K-(Ad)2 with varied host/guest
ratio, which was investigated by the cloud point technique.
It can be seen from this figure that individual aqueous guest

Article
Macromolecules, Vol. 44, No. 5, 2011 1185
Figure 3. Turbidity variations of 1:1 host-guest systems comprising of
β-CD-(PNIPAAm)4 (host polymer) and different macromolecular
guests as a function of temperature. Concentration of the mixtures,
Chost-guest = 2.0 mg/mL.
Figure 2. Turbidity variations of the host-guest system comprising
PPG-2K-(Ad)2 and β-CD-(PNIPAAm)4 with the host/guest ratio of 0.5
and 1.0 during the temperature heating-up (solid symbol) and cooling-
down (open symbol) processes. Cguest polymer = 0.26 mg/mL.
macromolecular guest, respectively (in all cases, the total
concentration was fixed to be 2.0 mg/mL). It can be con-
cluded that the longer the PPG chain was, the lower the
LCST was, as well as the more obvious the dual thermo-
responsive behavior of the host-guest system was.
was also found that, by increasing the host/guest ratio, the
transition range of PPG for the self-assembling systems
(Figure 1, curves b-f) became less sharp than that for pure
PPG guest solution (Figure 1, curve a), indicating that the
aggregation process of PPG in these self-assembling systems
was different than that in pure PPG guest solution. This was
related to the formation of noncovalently connected micelles
described in section 2.3. A similar phenomenon was also
observed in conventional thermosensitive copolymer sys-
tems. For example, for narrow-dispersed PNIPAAm-based
(co)polymers, incorporation of a hydrophobic moiety or
block usually resulted in a lower LCST and a broader
transition range of PNIPAAm.45-47 By contrast, incorpora-
tion of a hydrophilic moiety or block usually resulted
in a higher LCST and a broader transition range of
PNIPAAm.48,49
In stage II, all host-guest solutions (b-f) exhibited a
similar LCST around 34.7 °C due to the phase transition
of PNIPAAm arm on host polymer. This value was a little
lower than that of pure aqueous host polymer (35.9 °C, g)
with the concentration of 2.5 mg/mL because of the decrease
in water solubility of the host polymer upon inclusion
complexation with PPG-2K-(Ad)2. The two-stage transition
was complete thermo-reversible for the host-guest systems.
Two representative examples are shown in Figure 2 when the
host/guest molar ratio was 0.5 and 1.0. It can be seen that the
heating-up curves matched well with cooling-down curves in
the turbidity test.
Subsequently, we investigated the effect of the PPG chain
length of the adamantyl-containing PPGs on the LCST
behaviors of the host-guest systems. In the case of PPG-
4K-(Ad)2, the trend of change in LCST of the systems with
varying the host/guest ratio was quite similar to the case
when PPG-2K-(Ad)2 was used as guest polymer. A more
clearly dual thermoresponsive behavior of these systems
could be observed (see Supporting Information, Figure
S1). By contrast, when PPG-400-(Ad)2 was used as macro-
molecular guest, the systems did not show any dual thermo-
responsive behavior (see Supporting Information, Figure
S2), indicating that the effect of PPG chain length is sig-
nificant.
2.2.2. Two-Dimensional NOESY NMR Measurement. To
further confirm the host-guest complexation of the β-CD-
core-containing star-shaped PNIPAAm with the adamantyl-
containing PPGs, 2D-NOESY NMR measurements were
carried out. Herein, the 1:1 mixture of β-CD-(PNIPAAm)4
and PPG-2K-(Ad)2 in deuterated water was used as a
representative sample (Figure 4). It was clearly found that
the methine protons (Hb) and the methylene protons (Hc) of
adamantyl moiety correlated well with both of the inner
protons C(3)H and C(5)H of β-CD core, which were located
on the wider and narrower side of β-CD core, respectively.
However, the correlation peaks between methylene protons
of adamantyl moiety, Ha, and the inner protons C(3)H and
C(5)H of β-CD core were not clear. All these results indicated
that in this case, where the guest molecule possessed two
adamantyl end groups, the adamantyl end formed complex
from the wider side of the β-CD core.20
2.3. Formation of Noncovalently Connected Mi-
celles. 2.3.1. NMR Spectroscopy Investigation. In this
study, the special micelles formed in host-guest self-assem-
bling systems were investigated by using the NMR techni-
que, which is a very popular method to study the effect of
solvent on the structure of micelle formed from conventional
amphiphilic diblock/triblock copolymers.50-53 It was found
that CDCl3 was a good nonselective solvent for both the
macromolecular host (star-shaped PNIPAAm with a β-CD
core) and macromolecular guests (bis(adamantyl)-termi-
nated PPGs), while D2O was a good selective solvent for
hydrophilic host polymer but poor solvent for hydrophobic
guest polymers.
Herein, the 1:1 mixture of β-CD-(PNIPAAm)4 and PPG-
2K-(Ad)2 was used as a representative sample. In CDCl3, the
peaks due to the PPG segment (δ 3.30-3.70 ppm), PNI-
PAAm segment (δ 1.13, 3.99 ppm), and adamantyl moiety (δ
1.60-2.00 ppm) were well-resolved at room temperature,
and the resonance pattern did not change with temperature
ranging from 8 to 38 °C (Figure 5, upper left). By contrast, in
D2O at room temperature, PNIPAAm signals at δ 1.09 and
3.84 ppm were still sharp and well-resolved; however, the
signals due to the PPG segment at δ 3.20-3.60 ppm turned
into broad and poorly resolved peaks. The signals due to
As seen in Figure 3, for the 1:1 host-guest systems with
same β-CD-(PNIPAAm)4 host but different guest polymers,
the LCST values were 11.6, 24.4, and 37.2 °C when PPG-
4K-(Ad)2, PPG-2K-(Ad)2, and PPG-400-(Ad)2 were used as

1186 Macromolecules, Vol. 44, No. 5, 2011
Zhang et al.
Figure 4. The full-scale (left) and selected-scale (right) 2D-NOESY NMR spectra of a 1:1 host-guest mixture of β-CD-(PNIPAAm)4 and PPG-
2K-(Ad)2 in D2O at 18 °C (300 MHz).
Figure 5. (left) 1H NMR spectra of 1:1 mixture of β-CD-(PNIPAAm)4 and PPG-2K-(Ad)2 in CDCl3 (upper) and D2O (lower) at 25 °C, respectively.
(right) Temperature-variable 1H NMR spectra of 1:1 mixture of β-CD-(PNIPAAm)4 and PPG-2K-(Ad)2 in D2O at (a) 8, (b) 18, (c) 25, and (d) 38 °C.
Concentration of the mixtures, Chost-guest = 2.0 mg/mL.
adamantyl moiety were invisible under this condition, which
may be hidden by the relatively strong signals of the main-
chain methylene and methine protons of PNIPAAm at δ
1.30-2.40 ppm (Figure 5, lower left). Interestingly, a tem-
perature-dependent resonance pattern was observed for the
1:1 host-guest mixture in D2O, which was corresponding to
the formation of noncovalently connected micelles (Figure 5,
right).
reason that PPG segment became more and more hydro-
phobic in water at higher temperatures. This result also
indicated the formation of core-corona micelle with a
relatively hydrophobic core made up of guest polymer
PPG-2K-(Ad)2 and a hydrophilic outer corona made up of
star-shaped PNIPAAm in water under higher temperature
(>8 °C).50,51,53
2.3.2. Fluorescence Probe Investigation. The fluorescence
probe technique has been widely used as a powerful tool to
study micellar properties of conventional amphiphilic block
copolymers.54,55 It was known that both emission and
excitation spectra of pyrene undergo significant changes
upon micellization of block copolymers in an aqueous solu-
tion of pyrene (usually, 6 ꢀ 10-7 M). These changes are
caused by the transfer of pyrene molecules from the polar
aqueous environment to the hydrophobic micellar cores and
are related to the location of the pyrene molecules in the
solution. One important change should be noted is that the
fluorescence excitation spectrum of pyrene shows a shift of
the low-energy band of the La (S2 r S0) from ca. 333 to 338
nm. It has been reported that this (0,0) absorption band
change of pyrene is more sensitive to the true onset of
According to the LCST behavior of the 1:1 host-guest
mixture described in Figure 1, the mixture was clear at 8 °C.
From the H NMR spectrum (Figure 5, right, a), it was
1
found that the peaks ascribed to both the PNIPAAm and
PPG were sharp because the two segments could interact
freely with surrounding deuterated water molecules under
this condition. When the temperature was increased, the
mixture turned from clear to slightly turbid (18 °C) and then
turbid (25 and 38 °C) (Figure 1). During this process,
PNIPAAm was still shown as sharp peaks. However, the
peak intensity due to PPG segment decreased gradually with
increasing temperature (Figure 5, right, b-d). This observa-
tion result was attributed to the restriction of the molecular
motion of PPG segment in guest polymer, resulting from the

Article
Macromolecules, Vol. 44, No. 5, 2011 1187
Figure 7. Micelle size and scattered light intensity of the 1:1 host-guest
system of β-CD-(PNIPAAm)4 and PPG-2K-(Ad)2 as a function of
Figure 6. Plots of the I339/I334 ratio of pyrene excitation spectra as a
function of temperature in (a) 1:1 host-guest mixture of β-
CD-(PNIPAAm)4 and PPG-2K-(Ad)2 (Chost-guest = 2.0 mg/mL), (b)
aqueous solution of β-CD-core star-shaped PNIPAAm (control 1, Chost
= 1.74 mg/mL), (c) aqueous suspension of PPG-2K-(Ad)2 (control 2,
Cguest = 0.26 mg/mL), and (d) pyrene aqueous solution (control 3). The
steady-state fluorescence excitation spectra were monitored at 373 nm.
The concentration of pyrene is 6.0 ꢀ 10-7 M.
temperature in water. Total concentration of the system: Chost-guest
=
2.0 mg/mL.
microenvironment. The ratio of I339/I334 in this case was similar
to the reported results for pyrene in some triblock copolymers
solutions at low concentration range (i.e., before the micelles
were formed).45,56,58 However, a dramatic fall of I339/I334
occurred in the 1:1 host-guest solution when the temperature
was further increased, and this ratio dropped to the lowest
aggregation than either lifetime measurements or fluores-
cence emission changes.56,57 This change is described in
terms of the ratio of the intensities of the two bands at ca.
338 and 333 nm in the pyrene fluorescence excitation spec-
trum.
value when temperature reached ca. 32 °C. The decrease in I339
/
I334 (Δ(I339/I334), referenced to the initial value, 2.4, at 4 °C) was
ca. 1. The abrupt change of I339/I334 was indicative for the
formation of noncovalently connected micelle. The increased
temperature of the system resulted in the formation of hydro-
phobic micellar core composed of aggregated PPG chains.
During the micellization, pyrene molecules transferred from
the hydrophobic β-CD cavity of the host polymer to the
relatively less hydrophobic core of the noncovalently con-
nected micelle, which caused abrupt decrease in the intensity
In this study, the thermosensitive micellization behavior of
the 1:1 host-guest system of β-CD-(PNIPAAm)4 and PPG-
2K-(Ad)2 was determined as a representative example by
using the fluorescence excitation spectra of the pyrene probe.
The two excitation bands of pyrene were centered at 334 and
339 nm in our system. Figure 6a shows the intensity ratio of
I339/I334 of pyrene excitation spectra as a function of tem-
perature for this 1:1 host-guest system. Aqueous solution of
host polymer (Figure 6b), aqueous suspension of guest polymer
(Figure 6c), and pyrene aqueous solution (Figure 6d) were
used as controls. The intensity ratio of I339/I334 versus
temperature for the 1:1 host-guest system showed a clear
reverse sigmoid curve. By comparison, only a little change
in the ratio of I339/I334 was observed for the three control
systems.
According to the results of turbidity experiments described
in Figure 1, the curve for the 1:1 host-guest mixture in Figure 6
would be divided into two distinctive regions. Region I (4-32
°C) was relevant to the phase transition of PPG block in the
guest polymer. At the beginning of this region (below 8 °C), the
host-guest solution was clear and the ratio of I339/I334 was
stable at ca. 2.4. This value of I339/I334 ratio was slightly lower
than that in pure host polymer solution (ca. 2.5, Figure 6b) but
much higher than those in pure guest polymer suspension (ca.
0.5-0.65, Figure 6c) and pure pyrene aqueous solution (ca.
0.28, Figure 6d). In the case of pure host polymer solution,
pyrene molecules were mainly encapsulated into the hydro-
phobic β-CD cavity of the host polymer, which resulted in the
highest ratio of I339/I334 among the four systems under study. In
the case of 1:1 host-guest mixture, the partition of pyrene
inside the hydrophobic β-CD cavity was lower than that in
pure host polymer solution because of the presence of ada-
mantane-containing guest polymer. Adamantyl moiety was
known to fit better into the β-CD cavity than pyrene. While in
the case of pure guest polymer suspension, the ratio of I339/I334
was very low (near the corresponding value in pure pyrene
aqueous solution) because pyrene molecules stayed in a polar
ratio of I339/I334. By comparison, no such an abrupt Δ(I339/I334
)
was observed with increasing temperature for pure host poly-
mer solution as well as pure guest polymer suspension. Accord-
ing to curve a in Figure 6, the critical micelle temperature
(CMT) for this thermoresponsive 1:1 host-guest mixture was
calculated to be 9.1 °C, which was defined as the temperature
point with an abrupt decrease in the ratio of I339/I334. A
slowdown of the decrease in I339/I334 was observed when the
temperature was raised to around 32 °C. The value of I339/I334
around this temperature was ca. 1.5, which was similar to some
reported results for pyrene in conventional triblock copolymers
solutions at high concentration range (i.e., after the micelles
were formed).45,56,58
Region II (32-45 °C) was relevant to the phase transition
of PNIPAAm block in the host polymer. The PNIPAAm
chains began to collapse and aggregate, and the micellar
system lacked the protection of hydrophilic shell was gradu-
ally destroyed. In this region, a little increase in I339/I334 was
found for both the 1:1 host-guest system and pure host
polymer solution, while a little decrease in I339/I334 was found
for pure guest polymer suspension. For the pyrene aqueous
solution, the ratio of I339/I334 was almost constant over the
temperature range of 4-45 °C.
2.4. Size and Morphology of Noncovalently Connected
Core-Shell Micelles. The size of micelles is an important
factor to determine the applications. The uptake character-
istics of any drug encapsulated in micelle will be affected by
the micellar size and morphology.3 In this study, the size
distribution and morphology of the noncovalently connected
core-shell micelles were investigated by DLS, TEM, and

1188 Macromolecules, Vol. 44, No. 5, 2011
Zhang et al.
Figure 8. (top) Representative TEM micrographs of the noncovalently connected micelles formed from 1:1 host-guest mixture of β-CD-core star-
shaped PNIPAAm and PPG-2K-(Ad)2 at 14, 22, and 30 °C (a-c). (bottom) Histogram showing the size distribution of the noncovalently connected
micelles by DLS at 14, 22, and 30 °C from three individual measurements (a0-c0). Concentrationof the1:1 host-guest mixture: Chost-guest = 2.0 mg/mL.
AFM observation, respectively. Meanwhile, DLS was proved
to be a sensitive method to trace the formation of noncova-
lently connected core-shell micelle according to the change
in particle size and scattered light intensity in the 1:1 host-
guest system. The result from DLS study was consistent with
that from fluorescence probe investigation.
22 °C, a rapid increase in particle size and a corresponding
rapid decrease in count rate were observed. Meanwhile, the
mixture became very turbid, showing clearly the destabiliza-
tion of the noncovalently connected micelles. In this tempera-
ture range, micelles aggregation and precipitation occurred.
Figure 8 shows the representative TEM micrographs of
noncovalently connected micelles formed from 1:1 host-
guest mixture of β-CD-core star-shaped PNIPAAm and
PPG-2K-(Ad)2 at 14, 22, and 30 °C, respectively. From TEM
micrographs, smaller spherical particles (ca. 110-160 nm in
diameter) were observed for sample solution prepared and
dried at 14 °C (Figure 8a), whereas bigger spherical particles
(ca. 200-300 nm in diameter) were observed for sample
solution prepared and dried at 22 °C (Figure 8b), showing
that the noncovalently connected micelles formed at higher
temperature had bigger size. These micelles were well-dis-
persed as individual nanoparticles. In comparison, very big
particles with diameter of ca. 1 μm were observed in the case
of 30 °C (Figure 8c). These results supported the observa-
tions made with DLS mentioned above (as seen in Figure 7).
From the TEM images, a dense dark core and a less dense
corona could be observed, which was especially clear for the
sample prepared and dried at 22 °C. This was due to the
action of the staining agent, phosphotungstic acid, which
stains the hydrophobic core (PPG) to a greater extent. The
TEM micrographs also showed a narrow size distribution of
the noncovalently connected micelles formed at 14 and 22
°C. This was consistent with the observation from DLS,
which gave very narrow polydispersity of the micelle size of
ca. 0.1 for the two samples (Figure 8, a0 and b0). However, for
the micelles formed at 30 °C, the particle size distribution was
very broad due to the micelle aggregation at this temperature
(Figure 8, c and c0). It was found that the diameters of the
micelles from the TEM micrographs were clearly smaller
than those from DLS measurements. This could be related to
The changes of micelle size and count rate with tempera-
ture (3-26 °C) for the 1:1 host-guest system of β-CD-
(PNIPAAm)4 and PPG-2K-(Ad)2 are shown in Figure 7.
The scattered light intensity was given here in kilocounts per
second (kcps). It can be seen that the values of both micelle
size and count rate were very small and kept unchanged
under lower temperature (<8 °C). However, the two values
increased dramatically when the temperature was increased
above 8 °C. The increase in count rate could be attributed to
a change in the refractive index of the guest molecule as its
PPG block underwent a phase transition from random coil
to condensed globule. The latter structure of PPG possessed
higher refractive index than that of the former one. This
change indicated clearly that the micelles began to form at
around 8 °C. According to the curve of count rate versus
temperature, the critical micelle temperature (CMT) was
calculated to be 8.7 °C, which was very close to that obtained
from fluorescence probe investigation earlier (9.1 °C).
In the temperature range from 10 to 20 °C, the count rate
increased rapidly from 4.4 ꢀ 103 to 2.6 ꢀ 104 kcps, while the
average micelle diameter increase slowly from 150 to 300 nm,
showing that the noncovalently connected micelles were rela-
tively stable in this temperature range. In this stage, the
host-guest solution was observed to be slightly turbid and
was very faint blue according to the turbidity experiments
described earlier. At each temperature, the size of the resul-
tant micelle with very narrow polydispersities (ca. 0.1-0.2)
could remain constant, and the system did not form aggre-
gates for at least 1 day. When temperature reached to above

Article
Macromolecules, Vol. 44, No. 5, 2011 1189
Figure 9. Representative AFM images and correspondingline profiles ofthe noncovalently connected micelles formed from 1:1 host-guest solution of
β-CD-core star-shaped PNIPAAm and PPG-2K-(Ad)2 at 14 °C (a) and 22 °C (b). Concentration of the 1:1 host-guest solution: Chost-guest
=
2.0 mg/mL.
Scheme 2. Schematic Drawing Showing the Formation of Pseudo-Block Copolymer and Noncovalently Connected Micelles as Well as Micelle
Destabilization in a 1:1 Host-Guest Mixture with Increasing Temperature
the fact that DLS measures the hydrodynamics diameter of
the micelles in an aqueous environment whereas the TEM
micrographs show the dehydrated solid state of the micelles.
The size and shape of the noncovalently connected mi-
celles formed from the 1:1 host-guest solution of β-CD-core
star-shaped PNIPAAm and PPG-2K-(Ad)2 were further
measured in the solid state by AFM (Figure 9). The AFM
observation gave the mean micelles diameter around 180 and
220 nm for the samples prepared and dried at 14 and 22 °C,
respectively. These results were in accordance with those
found in TEM images.
On the basis of our extensive study on the 1:1 host-guest
mixture of β-CD-core star-shaped PNIPAAm and PPG-
2K-(Ad)2 by a combination of 1H NMR, 2D-NOESY NMR,
fluorescence probe technique, DLS, TEM, and AFM, a
mechanism for the self-assembly and thermosensitive micel-
lization of the pseudo-block copolymer under study was
proposed (Scheme 2). It can be seen from Scheme 2 that the
1:1 host-guest mixture containing two kinds of thermore-
sponsive blocks underwent a thermoreversible transition
from free pseudo-block copolymer to stable noncovalently
connected micelles and then to unstable aggregates by chan-
ging the temperature in the range of 2-45 °C. Considering
potential applications, the thermo-induced micellization
step (related to the phase transition of PPG) has more inte-
rest than the micelle aggregation step (related to the phase
transition of PNIPAAm) in this study.
We further studied the thermosensitive micellization be-
haviors of other host-guest systems with different host/
guest ratio and PPG chain length. Some selected character-
ization data for these host-guest systems are summarized in
Table 1. It was found from this table that LCST, CMT, mean
diameters of resultant micelles, and the size polydispersity
were greatly dependent on the PPG chain length of guest

1190 Macromolecules, Vol. 44, No. 5, 2011
Zhang et al.
Table 1. Solution Properties of Polymeric Host-Guest Systems Comprising of β-CD-Core Star-Shaped PNIPAAm and Bis(adamantyl)-
Terminated PPGs
guest polymer
host/guest
Chost-guest (mg/mL)
LCST (°C)a
CMT (°C)b
d, 14 °C (nm)c
d, 22 °C (nm)c
PPG-400-(Ad)2
PPG-2K-(Ad)2
PPG-2K-(Ad)2
PPG-2K-(Ad)2
PPG-2K-(Ad)2
PPG-4K-(Ad)2
1.0
1.0
1.0
0.5
1.5
1.0
2.0
2.0
0.6
1.1
2.9
2.0
37.2
24.4
33.0
15.6
24.9
11.6
31.0
8.7
18.0
solution
solution
210 ( 1 (0.10)
71 ( 4 (0.42)
790 ( 33 (0.64)
345 ( 17 (0.38)
1190 ( 55 (0.99)
367 ( 9 (0.10)
148 ( 1 (0.22)
1458 ( 90 (1.00)
1149 ( 33 (0.94)
aggregation
11.0
4.0
a Determined by means of the cloudy point technique. b Critical micelle temperature is experimentally described in the present study by an abrupt
increase in the mean count rate. c Mean Z-average diameters by dynamic light scattering from three individual measurements. Numbers in parentheses
represent the average polydispersity of the particle size.
polymer, host/guest ratio, and system concentration and
were tunable by easily changing the composition of the self-
assembling systems.
according to the literature.59 All other reagents were used as
received without further purification.
4.2. Synthesis of Star-Shaped PNIPAAm with a β-CD Core via
Atom Transfer Radical Polymerization (ATRP). a. Synthesis of
β-CD-Based Macroinitiator (4Br-β-CD). β-CD (5.11 g, 4.5
mmol) was dissolved in 30 mL of anhydrous N,N-dimethylace-
tamide with stirring and then was cooled to 0 °C. Subsequently,
a solution of 2-bromoisobutyric bromide (4.14 g, 18.0 mmol) in
anhydrous N,N-dimethylacetamide (10 mL) was added drop-
wise to the β-CD solution for a period of 1 h at 0 °C under a N2
atmosphere with rapid stirring. After the addition, the reaction
temperature was maintained at 0 °C for another 2 h and then
allowed to rise slowly to room temperature, after which the
reaction was allowed to continue for 24 h. The final reaction
mixture was precipitated with 900 mL of diethyl ether. The
resulting white powder was collected by centrifugation and
washed with acetone (2 ꢀ 30 mL). In the purification process,
the crude product was suspended in 100 mL of deionized water,
and the mixture was stirred at rt overnight. After that, the solid
was collected by centrifugation, washed well with deionized
water (2 ꢀ 30 mL) and acetone (2 ꢀ 30 mL), and then dried
under vacuum at 40 °C. Yield 4.25 g (54.6%, based on a
substitution degree of 4). FTIR (KBr): ν = 3383 (br, ν(O-
H)), 2928 (m, ν(C-H)), 1736 (s, ν(CdO)), 1636 (m), 1286 (m),
1157 (s), 1104 (s), 1079 (s), 1030 (s, ν(C-O-C)), 947 (w), 759
3. Conclusions
A polymeric host-guest system was designed using star-
shaped PNIPAAm with a β-CD core and bis(adamantyl)-termi-
nated PPGs, which was able to form ABA-type dual thermo-
responsive supramolecular pseudo-block copolymers in aqueous
solution via host-guest complexation between the β-CD core of
the host polymer and the adamantyl moieties of the guest
polymers. The pseudo-block copolymers showed tunable LCST
behavior inaqueoussolutionbyvarying the host/guest ratio upto
1.0. The pseudo-block copolymers could self-assemble into non-
covalently connected micelles with the PPG segment as the core
and hydrophilic PNIPAAm-grafted β-CD as the corona. Typi-
cally, a 1:1 host-guest mixture of β-CD-core star PNIPAAm and
PPG-2K-(Ad)2 reversibly experienced the change from clear
solution (below 8 °C, no micelle) to slightly turbid solution
(10-20 °C, micelle formation) and then to turbid mixture
(above 22 °C, micelle destabilization).
The critical micelle temperature (CMT) of the host-guest
system was dependent on the composition and concentration of
the components. The size of the resultant noncovalently con-
nected micelle could be easily tuned not only by changing the
temperature, the concentration of the system, but also through
changing the ratio of host/guest and the length of the PPG chain,
which provided more rooms to manipulate and control the
micelle formation as compared to that in conventional amphi-
philic copolymer.
1
(w), 704 (w), 649 (w, ν(C-Br)), 581 cm-1 (m). H NMR (400
MHz, DMSO-d6, δ): 1.88 (m, 24H, -OCO-C(CH3)2Br), 3.00-
6.00 (m, 66H, -OH and -CH- of β-CD). 13C NMR (100 MHz,
DMSO-d6, δ): 170.7, 170.5 (CdO), 102.5-101.3 (C(1) of β-CD),
81.9-80.5 (C(4) of β-CD), 73.1, 72.8 (C(3) of β-CD), 72.1 (C(2)
of β-CD), 72.0 (C(5) of β-CD), 69.1, 69.0 (C(50) of β-CD), 65.0,
64.4 (C(60) of β-CD), 59.4 (C(6) of β-CD), 57.1 (C-Br), 30.4, 30.3
(CH3). Anal. Calcd for C58H90Br4O39 2H2O: C 39.42, H 5.36,
Br 18.09. Found: C 37.96, H 5.46, Br 22.39.
This kind of supramolecular pseudoblock copolymers formed
from host-guest complexation has great potential for drug
delivery applications owing to their tunability of thermorespon-
sive behavior, noncovalent connected micelle-forming ability,
and their potential biocompatibility.
3
b. ATRP of NIPAAm Using 4Br-β-CD as Macroinitiator. 4Br-
β-CD (176.8 mg, 0.103 mmol) and NIPAAm (1.358 g, 12.0
mmol) were added into a dry flask. The flask was sealed with a
septum and subsequently purged with dry nitrogen for several
minutes. Then, Me6-TREN (111.0 mg, 0.48 mmol) in 3.5 mL of
thoroughly degassed methanol was added with a degassed
syringe. The mixture was stirred at rt and purged with dry
nitrogen for 10 min. Last, CuBr (57.4 mg, 0.4 mmol) was added,
and the mixture was purged with dry nitrogen for another 10
min at rt. The mixture was then heated at 70 °C in an oil bath.
After 4 h, the experiment was stopped by opening the flask and
exposing the catalyst to air. The final green mixture was diluted
with THF and passed through a short Al2O3 column to remove
copper catalyst. The resulting eluate solution was concentrated
to ca. 10 mL and precipitated with 500 mL of diethyl ether. The
white product was collected by centrifugation, washed with
diethyl ether, and dried under vacuum at 40 °C. Yield 672.8
mg (43.8%). FTIR (KBr): ν = 3394 (br, ν(O-H)), 3300 (br,
ν(N-H)), 2975, 2936, 2878 (m, νs(C-H) and νas(C-H)), 1647
(s, ν(CdO), amide I), 1550 (s, ν(C-N) and δ(N-H), amide II),
1459 (m), 1388, 1369 (m, ν(C-N)), 1155 (m), 1081 (m), 1036
cm-1 (m, ν(C-O-C)). 1H NMR (400 MHz, DMSO-d6, δ):
1.04 (b, -CH(CH3)2), 1.44 (b, -CH2-CH(CONH-)-), 1.96
4. Experimental Section
4.1. Materials. β-Cyclodextrin (98%) was purchased from
Sigma and used after recrystallization from water and drying at
100 °C under vacuum. N-Isopropylacrylamide (Aldrich, 99%)
was recrystallized twice from hexane before use. Copper(I)
bromide (Fluka, 98%) was washed with glacial acetic acid in
order to remove any soluble oxidized species, filtered, washed
with ethanol, and dried. 2-Bromoisobutyric bromide (99%),
anhydrous N,N-dimethylacetamide (99%), adamantaneacetic
acid (98%), N,N0-dicyclohexylcarbodiimide (DCC, 1.0 M in
methylene chloride), and trichloroacetyl isocyanate (TCAI)
(96%) were all purchased from Aldrich. 4-(Dimethylamino)-
pyridine (DMAP, >99%) was purchased from Fluka. Poly-
(propylene glycol) bis(2-aminopropyl ether)s with average mo-
lecular weight of 400 Da (NH2-PPG-400-NH2), 2000 Da (NH2-
PPG-2K-NH2), and 4000 Da (NH2-PPG-4K-NH2) were obtained
from Aldrich. Methylene chloride was distilled over CaH2 before
use. Tris[2-(dimethylamino)ethyl]amine (Me6-TREN) was prepared

Article
Macromolecules, Vol. 44, No. 5, 2011 1191
(b, -CH2-CH(CONH-)-), 3.50-4.00 (m, C(2)H-C(6)H of
β-CD, overlapped with methine proton of isopropyl group),
3.84 (b, -CH(CH3)2), 4.30-4.90 (m, -O(6)H and C(1)H of β-
CD), 5.60-5.90 (b, -O(2)H and -O(3)H of β-CD), 6.90-7.60
(b, -CONH-). 13C NMR (100 MHz, D2O, δ): 177.9 (CdO of
NIPAAm unit), 105.0 (C(1) of β-CD), 83.6 (C(4) of β-CD),
71.1-75.9 (C(2), C(3) and C(5) of β-CD), 62.6 (C(6) of β-CD),
44.4-45.8 (CH of NIPAAm unit), 34.1-39.6 (CH2 of NI-
PAAm unit), 24.3 (CH3 of NIPAAm unit). Anal. Calcd for
3.80-3.45 (m, 133H, -CH2- in PPG chain), 3.45-3.30 (m,
65H, -CH(CH3)- in PPG chain), 1.96 (br, 6H, methine protons
of adamantyl group), 1.90 (s, 4H, -NHCO-CH2-adamantyl),
1.75-1.58 (m, 24H, methylene protons of adamantyl group),
1.20-1.05 (200H, -CH3- in PPG chain). 13C NMR (100 MHz,
CDCl3, δ): 170.37 (-CO-NH-), 76.84-75.08 (-CH2- in PPG
chain), 73.52-72.18 (-CH(CH3)- in PPG chain), 52.06
(-CH2-adamantyl), 45.27 (-CH(CH3)-NHCO-), 42.78,
36.96, 32.88, 28.85 (adamantyl group), 18.08-17.16 (-CH3 in
PPG chain).
C
394H706Br4N56O95 (based on the number of arm n = 4 and
number of repeating unit per arm m = 14): C 58.66, H 8.82, N
9.72. Found: C 58.33, H 9.52, N 10.10.
4.4. Measurements and Characterization Methods. General.
Gel permeation chromatography (GPC) analysis was carried
out with a Shimadzu SCL-10A and LC-8A system equipped
with two Phenogel 5 μ 50 and 1000 A columns (size: 300 ꢀ 4.6
4.3. Synthesis of Adamantyl-Containing Poly(propylene
glycol)s. These polymers were synthesized by coupling ada-
mantaneacetic acid with corresponding PPGs. The procedure
for the preparation of PPG-2K-(Ad)2 from commercial (NH2-
PPG-2K-NH2) and adamantaneacetic acid is given below as a
typical example.
˚
mm) in series and a Shimadzu RID-10A refractive index detec-
tor. Tetrahydrofuran (THF) was used as eluent at a flow rate of
0.20 mL/min at 45 °C. Monodispersed poly(ethylene glycol)
(PEG) standards were used to obtain a calibration curve. Proton
and carbon nuclear magnetic resonance (1H and 13C NMR)
spectra were recorded at room temperature on a Bruker Avance
DRX 400 MHz NMR spectrometer operating at 400.1 and
100.6 MHz, respectively. Chemical shifts were reported in parts
per million (ppm) on the δ scale and were referenced to residual
protonated solvent peaks: CDCl3 spectra were referenced to
CHCl3 at δH 7.26 and CDCl3 at δC 77.36; D2O spectra were
referenced to HDO at δH 4.70; DMSO-d6 spectra were refer-
enced to (CHD2) (CD3)SO at δH 2.50. The 2D-NOESY (two-
dimensional nuclear Overhauser effect spectroscopy) NMR
experiments were performed at 300 MHz in D2O on a Bruker
Avance DRX 300 MHz NMR spectrometer with sodium 3--
(trimethylsilyl)propane-1-sulfonate as reference. Fourier trans-
form infrared (FTIR) spectra of samples in KBr pellet or
deposited on surface of NaCl plate were recorded on a Perkin-
A mixture of NH2-PPG-2K-NH2 (2.0 g, 1.0 mmol) and
DMAP (488.7 mg, 4.0 mmol) was dried under vacuum at
50 °C overnight. After being cooled to room temperature, a
solution of adamantaneacetic acid (582.7 mg, 3.0 mmol) in
60 mL of anhydrous dichloromethane (DCM) was added with
stirring. The reaction mixture was then heated to reflux, and the
trace amount of water in the reaction mixture was removed by
azeotropic distillation under a nitrogen atmosphere. When the
residual solution in the reaction flask was about 8 mL, it was
cooled to room temperature. Subsequently, a solution of DCC
(5 mL, 1 mol/L) in DCM was added to the reaction mixture
dropwise under stirring over a period of 5 min. Several minutes
later, the byproduct dicyclohexylurea (DCU) precipitated as a
white powder. After being stirred for another 48 h at room
temperature, the final reaction mixture was filtered to remove
insoluble DCU. The filtrate was concentrated to ca. 5 mL and
then purified by passing through a Sephadex LH-20 column
(2.5 ꢀ 40 cm) with methanol as eluent. Yield 1.5 g (75%). FTIR
(film on NaCl plate): ν = 3319 (m, ν(N-H), amide), 2971, 2901
(s, νs(C-H) and νas(C-H)), 1650 (m, ν(CdO), amide I), 1536
(m, ν(C-N) and δ(N-H), amide II), 1109 cm-1 (s, ν(C-O-C)).
1H NMR (400 MHz, CDCl3, δ): 6.10-5.65 (br, 2H, -NH-
CO-), 4.20-4.00 (br, 2H, -CH(CH3)-NHCO-), 3.80-3.45
(m, 68H, -CH2- in PPG chain), 3.45-3.30 (m, 32H, -CH-
(CH3)- in PPG chain), 1.96 (br, 6H, methine protons of ada-
mantyl group), 1.89 (s, 4H, -NHCO-CH2-adamantyl), 1.75-
1.60 (m, 24H, methylene protons of adamantyl group),
1.20-1.05 (102H, -CH3- in PPG chain). 13C NMR (100 MHz,
CDCl3, δ): 170.36 (-CO-NH-), 76.84-75.07 (-CH2- in PPG
chain), 73.59-72.18 (-CH(CH3)- in PPG chain), 51.87
(-CH2-adamantyl), 45.26 (-CH(CH3)-NHCO-), 42.76,
36.96, 32.87, 28.81 (adamantyl group), 18.08-17.17 (-CH3 in
PPG chain).
PPG-400-(Ad)2. Yield, 44.1%. FTIR (film on NaCl plate): ν
= 3300 (m, ν(N-H), amide), 2970, 2902, 2849 (s, νs(C-H) and
νas(C-H)), 1641 (s, ν(CdO), amide I), 1544 (m, ν(C-N) and
δ(N-H), amide II), 1104 cm-1 (s, ν(C-O-C)). 1H NMR (400
MHz, CDCl3, δ): 6.10-5.65 (br, 2H, -NH-CO-), 4.20-4.00
(br, 2H, -CH(CH3)-NHCO-), 3.80-3.25 (m, 17H, -CH2-
CH(CH3)- in PPG chain), 1.96 (br, 6H, methine protons of
adamantyl group), 1.90 (s, 4H, -NHCO-CH2-adamantyl),
1.75-1.59 (m, 24H, methylene protons of adamantyl group),
1.20-1.05 (19H, -CH3- in PPG chain). 13C NMR (100 MHz,
CDCl3, δ): 170.37 (-CO-NH-), 76.84-75.30 (-CH2- in PPG
chain), 73.50-72.17 (-CH(CH3)- in PPG chain), 52.06 (-CH2-
adamantyl), 45.26 (-CH(CH3)-NHCO-), 42.79, 36.97, 32.87,
28.86 (adamantyl group), 18.08-17.19 (-CH3 in PPG chain).
PPG-4K-(Ad)2. Yield, 85.6%. FTIR (film on NaCl plate): ν
= 2971, 2900 (s, νs(C-H) and νas(C-H)), 1651 (m, ν(CdO),
amide I), 1528 (m, ν(C-N) and δ(N-H), amide II), 1108 cm-1
(s, ν(C-O-C)). 1H NMR (400 MHz, CDCl3, δ): 6.10-5.50 (br,
2H, -NH-CO-), 4.20-4.00 (br, 2H, -CH(CH3)-NHCO-),
Elmer FTIR 2000 spectrometer in the region of 4000-400 cm-1
;
64 scans were signal-averaged with a resolution of 2 cm-1
at room temperature. The X-ray photoelectron spectroscopy
(XPS) measurements were performed on a Kratos AXIS HSi
spectrometer equipped with a monochromatized Al KR X-ray
source (1486.6 eV photons). Elemental analyses were carried out
using a Perkin-Elmer 2400 CHN/CHNS elemental analyzer.
Lower Critical Solution Temperature (LCST) of Host-Guest
Self-Assembling Systems. A predetermined amount of bis(ada-
mantyl)-terminated PPGs (guest polymers) and a corresponding
amount of β-CD-core star-shaped PNIPAAm (host polymer)
were dissolved in 1 mL of distilled water, and the transmission
changes of the self-assembling system (in a 1 cm sample cell
referenced against distilled water) at 500 nm were followed by
using a UV-vis spectrophotometer (Shimadazu UV-2501PC)
equipped with a Fisher Scientific Isotemp 1013P circulating
water bath in the temperature range of 2-50 °C at various
temperatures (heating rate: 1.2 °C/min; cooling rate: 0.5 °C/min;
however, the heating and cooling rate was 0.12 °C/min at the
temperature range between 3 °C below and above the LCST).
The LCST of host-guest solution, which is equivalent to its
cloud point, is defined as the temperature exhibiting half of the
total decrease in optical transmittance of the solution at 500 nm
in stage I due to PPG phase transition.
Critical Micelle Temperature Determination of Host-Guest
Self-Assembling Systems by Fluorescence Spectroscopy. Steady-
state fluorescence spectra were recorded on a Shimadzu RF-
5301PC spectrofluorophotometer in the temperature range of
4-45 °C, and the sample was allowed to equilibrate for 12 min at
each temperature before the measurements were taken. Excita-
tion spectra were monitored at λem = 373 nm. Slit widths for
both excitation and emission sides were maintained at 3.0 nm.
Sample solutions were prepared by dissolving a predetermined
amount of bis(adamantyl)-terminated PPGs (guest polymers)
and corresponding amount of β-CD-core star polymer (host
polymer) in an aqueous pyrene solution (1 mL), and the solu-
tions were allowed to stand for 1 day for equilibration at 4 °C.

1192 Macromolecules, Vol. 44, No. 5, 2011
Zhang et al.
The concentration of the 1:1 host-guest mixtures was fixed
at 2.0 mg/mL, and the concentration of pyrene was kept at
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Acknowledgment. This work was jointly supported by MIND-
EF-NUS Joint Applied Cooperation Program (MINDEF/NUS/
JPP/10/02), Singapore Ministry of Education Academic Research
Fund Tier 2 Grant (R-397-000-031-112), Incentive Funding from
Faculty of Engineering, National University of Singapore (R-397-
000-031-731), and Institute of Materials Research and Engineering
(IMRE), A*STAR (Agency for Science, Technology and Re-
search), Singapore.
Supporting Information Available: A table of selected
characterization data for adamantyl-containing PPGs and star-
ing PPGs and figures showing the turbidity variations with
temperature for a series of host-guest mixtures with PPG-
4K-(Ad)2 and PPG-400-(Ad)2 as guest polymer, respectively.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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