Synthesis of poly(N-isopropylacrylamide)-poly(ethylene glycol) miktoarm star copolymers via RAFT polymerization and aldehyde-aminooxy click reaction and their thermoinduced micellization

Article abstract of DOI:10.1021/ma100800b

A facile synthetic pathway to poly(N-isopropylacrylamide) (PNIPAM)-poly(ethylene glycol) (PEG) miktoarm star copolymers with multiple arms has been developed by combining reversible addition-fragmentation chain transfer (RAFT) polymerization and aldehyde-

Full text of DOI:10.1021/ma100800b

Macromolecules 2010, 43, 5699–5705 5699
DOI: 10.1021/ma100800b
Synthesis of Poly(N-isopropylacrylamide)-Poly(ethylene glycol)
Miktoarm Star Copolymers via RAFT Polymerization and
Aldehyde-Aminooxy Click Reaction and Their Thermoinduced
Micellization
Zhaomian Wu, Hui Liang, and Jiang Lu*
Key Laboratory of Designed Synthesis and Application of Polymer Material, Key Laboratory for Polymeric
Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering,
Sun Yat-sen University, Guangzhou 510275, P. R. China
Received April 13, 2010; Revised Manuscript Received May 17, 2010
ABSTRACT: A facile synthetic pathway to poly(N-isopropylacrylamide) (PNIPAM)-poly(ethylene
glycol) (PEG) miktoarm star copolymers with multiple arms has been developed by combining reversible
addition-fragmentation chain transfer (RAFT) polymerization and aldehyde-aminooxy “click” coupling
reaction. Star PNIPAM with aldehyde functionalized core was initially prepared by the RAFT arm-first
technique via cross-linking of the preformed linear macro-RAFT agents using a newly designed aldehyde-
containing divinyl compound 6,60-(ethane-1,2-diylbis(oxy))bis(3-vinylbenzaldehyde) (EVBA). It was then
used as a multifunctional coupling agent for the subsequent formation of the second-generation PEG arms
via the click coupling reaction between its aldehyde groups and aminooxy-terminated PEGs. The thermo-
responsive micellization behavior of PNIPAM-PEG miktoarm star copolymer with different PEG arm
numbers in water was also investigated. Opportunities are open for thermoinduced intermolecular or
intramolecular micellization of PNIPAM-PEG miktoarm star copolymers via controlling the content ratio
of PNIPAM and PEG, forming multimolecular micelles and unimolecular micelles, respectively.
Introduction
Miktoarm star (also referred to as heteroarm star) copolymers
contain two or more chemically different arms emanated from the
Double-hydrophilic copolymers, comprised of two different hy-
drophilic segments, can self-assemble into micelle-like aggregates in
water if one segment becomes hydrophobic upon external stimulus
such as a change in solution pH, temperature, and ionic strength.1-4
This kind of copolymer has emerged as an important class of
polymers due to their potential applications in drug delivery and as
intelligent materials.5 Poly(N-isopropylacrylamide) (PNIPAM) is
one of the most widely studied thermoresponsive polymers and
undergoes a reversible phase transition at low critical solution
temperature (LCST) around 32 °C in water.6 At temperature below
the LCST, the intermolecular hydrogen bonding between PNIPAM
chains and water molecules is dominant, and therefore PNIPAM is
hydrophilic and soluble in water. At temperatures above the LCST,
intramolecular hydrogen bonding between the amide groups in
PNIPAM results in transition into compact and collapsed confor-
mations of PNIPAM chains, and the polymeric chain becomes
hydrophobic and insoluble in water. Poly(ethylene glycol) (PEG), as
a typical hydrophilic segment, has several biomedical and pharma-
ceutical applications due to its specific properties such as nontoxi-
city, biodegradability, biocompatibility, and resistance to recogni-
tion by the immune system.7 Therefore, the double-hydrophilic
copolymers combining PNIPAM and PEG should be of popular
interest. Compared to the numerous studies on block8 and graft9
copolymers, little work has focused on the double-hydrophilic star-
shaped copolymers composed of PNIPAM and PEG, and the
related investigations have been limited to the miktoarm star
copolymers with few arms, mainly containing 3 and 4 arms.10-12
same core. Compared to the synthesis of homoarm star polymers,
the synthesis of miktoarm star copolymers is more difficult, and
consequently their preparation is described in fewer reports.13,14
Previously, the synthesis of miktoarm star copolymers was mainly
accomplished by living anionic polymerization.15-17 However,
recent developments in living radical polymerization techniques,
such as atom transfer radical polymerization (ATRP),18 nitroxide-
mediated radical polymerization (NMP),19 and reversible addition-
fragmentation chain transfer (RAFT) polymerization,20 have ex-
panded the ability to generate miktoarm star copolymers because
of the wide range of applicable monomers and more facile opera-
tion conditions in comparison with living anionic polymerization.
Among the methodologies developed for the synthesis of miktoarm
star copolymers using living radical polymerization techniques, the
“in-out” method13-16,21,22 represents an important strategy for the
synthesis of miktoarm star copolymers with a large number of arms
(>20). In this method, a star polymer having the preserved dor-
mant initiating sites in the cross-linked core, synthesized by the arm-
first method, can be used as multifunctional initiators for the
subsequent growth of the second generation of arms. As a result
of sterically congested core, not all of the preserved initiating sites
participate in the generation of the second arms, and therefore the
number of the second generation of arms is always lower than the
number of the first generation of arms.23
Continuing efforts have been made over the years to develop
facile and efficient routes to synthesize miktoarm star copolymers
with tunable arm number. Matyjaszewski and co-workers23 have
prepared a series of miktoarm star copolymers containing two or
more types of arms with various arm number via one-pot cross-
linking the mixture of different linear macroinitiators with a
*Corresponding author: e-mail gaofenzi@mail.sysu.edu.cn; Tel þ86
20 8403 7562; Fax þ86 20 8411 2245.
r 2010 American Chemical Society
Published on Web 05/26/2010
pubs.acs.org/Macromolecules

5700 Macromolecules, Vol. 43, No. 13, 2010
Wu et al.
Scheme 1. Schematic Representation of the Synthesis of PNI-
PAM-PEG Miktoarm Star Copolymers via Combination of RAFT
Polymerization and Aldehyde-Aminooxy Click Reaction
postmodification, this simple methodology can be extended to other
miktoarm star structures. Except for the synthesis, the unique ther-
moresponsive micellization behaviors of the resulting new double-
hydrophilic miktoarm star copolymer were also investigated.
Experimental Section
Materials. N-Isopropylacrylamide (NIPAM) (TCI, 98%) was
recrystallized twice from n-hexane before use. 2,20-Azobis-
(isobutyronitrile) (AIBN) (Shanghai Chemical Reagent Co.,
99%) was purified by recrystallization from ethanol. Tetrahy-
drofuran (THF) (Shanghai Chemical Reagent Co., 99%) was
refluxedwithsodium chips under N2 until dry andfreshly distilled
before use. N,N-Dimethylformamide (DMF) (Shanghai Chemi-
cal Reagent Co., 99%) was distilled under reduced pressure over
CaH2 beforeuse. K2CO3 (ShanghaiChemicalReagent Co., 99%)
and 1,2-dibromoethane (Aldrich, 99%) were used as received.
2-(Dodecylsulfanylthiocarbonylsulfanyl)-2-methylpropionic
acid (DMPA),30 2-hydroxy-5-vinylbenzaldehyde (HVB),31 and
monoaminooxy end-functionalized poly(ethylene glycol) (PEG-
ONH2, molecular weight 2000)32 were synthesized according to
literature procedures.
Measurements. The number-average molecular weight (Mn)
and molecular weight distribution (Mw/Mn) were measured by gel
permeation chromatography (GPC) against PS standard in THF
at a flow rate of 1.0 mL/min at 35 °C on three Waters Styragel
columns (measurable molecular weight range: 100-5000, 500-
30 000, and 5000-600 000) connected to a Waters 1515 pump and
a Waters 2414 refractive index detector. The absolute weight-
average molecular weight (Mw) of the polymers was determined
by multiangle laser light scattering (MALLS) in THF at 40 °C on
the chromatograph system equipped with a Waters 1515 pump,
two PLgel 10 μm MIXED-B columns, a Waters 2414 refractive
index detector, and a Viscotek 270 dual detector (dual laser light
scattering, λ = 670 nm).
divinyl compound. This approach issimpleand general; however,
the synthesis of miktoarm star copolymers is challenging when
macroinitiators with different activity are used, owing to the
different incorporation rates of the macroinitiators. Highly
efficient “click” reactions24 were also used for the synthesis of
miktoarm star copolymers.14,22 The most popular “click” reac-
tion is the copper-catalyzed alkyne-azide cycloaddition reaction,
and various miktoarm star copolymers containing few arms with
different chemical compositions have been synthesized by com-
bining this click reaction with living radical polymerizations.25,26
Reaction between an aminooxyand an aldehyde orketone via the
formation of an oxime linkage is another highly efficient “click”
reaction.27 This “click” reaction has the advantage that, in
comparison to the most popular copper-catalyzed alkyne-azide
click reaction, other than the reacting reagents no other auxili-
aries including toxic metallic catalyst are required. Nevertheless,
the potential of oxime click chemistry in the synthesis of polymers
with complex topologies has yet to be fully explored although it
has proven to be a versatile approach for synthesis of polymer-
protein or drug conjugations.28 So far nothing has been done on
the application of this technique to prepare star polymers.
Herein, we report a facile synthetic pathway to double-hydro-
philic miktoarm star copolymers composed of PNIPAM and PEG
with multiple arms by combining RAFT arm-first technique and
aldehyde-aminooxy click reaction (Scheme 1). Star PNIPAM with
aldehyde-functionalized core was initially prepared by RAFT arm-
first technique via cross-linking of the preformed linear PNIPAM
macro-RAFT agent using an aldehyde-containing divinyl com-
pound. It was then used as a multifunctional coupling agent for the
subsequent formation of the second-generation PEG arms via the
click reaction between its aldehyde groups and the aminooxy-
terminated PEGs. The so far reported routes to synthesize mik-
toarm star copolymers using click chemistry usually involve a
synthesis of multifunctional miktoinitiator and/or coupling agent,
and thus only miktoarm star copolymers with a low number of arms
have been synthesized.26,29 Like the previous “in-out” method, our
method also enables the facile preparation of miktoarm star
copolymers having multiple arm numbers without the cumbersome
synthesis of multifunctional miktoinitiator and/or coupling agents.
However, in comparison to “in-out” method, the number of the
second generation arms is more independent of the number of the
first generation arms and can be tuned easily by varying the feed
ratio of the aldehyde-aminooxy click reaction. Because aminooxy
end groups can be easily incorporated into the polymers by living
radical polymerizations using aminooxy-containing initiator28 or by
1H NMR (300 MHz) spectra were recorded in CDCl3 at room
temperature on a Varian Unity Inova 300 spectrometer. Poly-
mer samples for H NMR and MALLS analysis were fractio-
1
4
˚
nated by preparative GPC (column: Ultrastyragel 10 A).
Elemental analysis was determined on a CHNS-Vario ele-
mental analyzer. FAB mass measurement was carried out on a
VG ZAB-HS spectrometer.
Dynamic light scattering (DLS) measurements were con-
ducted at 25 and 60 °C on a Brookhaven BI-200SM apparatus
with a BI-9000AT digital correlator and a He-Ne laser at 532 nm.
Prior to the measurement, the sample solutions were filtered
through hydrophilic poly(ether sulfone) syringe filters (SCAA-
201, Anpel, 0.45 μm pore size). The data were analyzed by the
CONTIN algorithm.
The light transmittance of the aqueous solution of the poly-
mers was monitored at a wavelength of 500 nm on a PGEN-
ERAL TU-1901 UV/vis spectrophotometer. The operation was
conducted from 25 to 60 °C at a heating rate of 0.2 °C/min.
Synthesis of Aldehyde Functionalized Divinyl Cross-Linker
EVBA. The aldehyde functioned cross-linker EVBA was pre-
pared by the reaction of 1,2-dibromoethane with HVB. A solution
of 1,2-dibromoethane (7.52 g, 40 mmol) in DMF (5 mL) was
added dropwise to a mixture of HVB (11.86 g, 80 mmol) and
K2CO3 (11.06 g, 80 mmol) in DMF (20 mL). The mixture was
stirred at 80 °C under N2 for 12 h. The reaction mixture was
poured into excess ice water and then extracted with ethyl acetate.
The organic extract was washed with water and then brine, dried
over anhydrous Na2SO4, and evaporated to dryness. The residue
was purified by silica gel chromatography eluted with acetate/
n-hexane (20:80 v/v) to give EVBA as a light yellow solid (8.38 g,
1
65%); mp = 131-132 °C. H NMR (CDCl3), δ (TMS, ppm):
4.54 (m, 4H, -CH2-CH2-), 5.24 (d, J=10.8 Hz, 2H, trans-CH2),
5.69 (d, J=17.4 Hz, 2H, cis-CH2), 6.63 (dd, J1=10.8 Hz, J2=
17.4 Hz, 2H,=CH-), 7.01 (d, J=8.7 Hz, 2H, 6-aromtic proton),
7.6 (dd, J1=2.4 Hz, J2=8.4 Hz, 2H, 4-aromtic proton), 7.88

Article
Macromolecules, Vol. 43, No. 13, 2010 5701
Scheme 2. Synthesis of Aldehyde Functionalized Divinyl Cross-Linker
(d, J=2.4 Hz, 2H, 3-aromtic proton), 10.43 (s, 2H, aldehyde
proton). Anal. Calcd for C20H18O4: C 74.52; H 5.63. Found: C
74.29; H 5.66. FAB MS: m/z calcd for C20H18O4 322.34; found
322 (Mþ).
Synthesis of PNIPAM Macro-RAFT Agent. For a typical
example is given below. A mixture of NIPAM (4.972 g, 44
mmol), DMPA (160 mg, 0.44 mmol), AIBN (7.2 mg, 0.044
mmol), and DMF (11 mL) was degassed with three freeze-
pump-thaw cycles, sealed under N2, and heated at 60 °C.
After 7 h at 55% conversion, the sealed ampule was cooled in
an ice bath. The reaction mixtures were diluted with THF and
then poured into a large amount of n-hexane. The precipitated
polymer was washed with n-hexane and dried in vacuo to
give the PNIPAM macro-RAFT agent with Mn,GPC=5000 and
Mw/Mn = 1.07.
Synthesis of (PNIPAM)n-polyEVBA Star Polymer. PNIPAM
macro-RAFT agent (0.5 g, 100 μmol), EVBA (80.5 mg, 250
μmol), AIBN (1.6 mg, 10 μmol), and THF (5 mL) was degassed
with three freeze-pump-thaw cycles, sealed under N2, and
heated at 60 °C. After the predetermined time, the sealed ampule
was cooled in an ice bath. The reaction mixtures were diluted
with THF and immediately analyzed by GPC. After 48 h, the
star polymer yield was ca. 80%, estimated by comparing peak
area of star polymer and unlinked linear polymer in GPC. Then
a large amount of n-hexane was added to precipitate the star
polymer. The star polymer was purified by fractional precipita-
tion in THF/n-hexane.
Synthesis of (PNIPAM)n-polyEVBA-(PEG)m Miktoarm Star
Copolymers via Aldehyde-Aminooxy Click Reaction. The pur-
ified (PNIPAM)n-polyEVBA (Mn,GPC = 53 400 and Mw/Mn =
1.36; 100 mg, 0.086 mmol of aldehyde group, assuming 100%
conversion of EVBA) and PEG-ONH2 (molecular weight
2000, 260 mg, 0.130 mmol of aminooxy group) were dissolved
in 10 mL of THF and stirred at room temperature for 24 h.
The resulting miktoarm copolymer was purified by dialysis
against THF for 24 h using a semipermeable membrane
(cutoff molecular weight 5000) to remove the unreacted PEG-
ONH2.
Figure 1. GPC traces for the formation of (PNIPAM)n-polyEVBA star
polymer at various reaction times. Experimental conditions: Mn of
linearPNIPAM chains = 5000; [EVBA]/[PNIPAM-RAFT]/[AIBN] =
25/10/1, [EVBA] = 0.05 M, in THF at 60 °C. GPC conditions: RI
detector, linear polystyrene as standard. (A) 0 h; (B) 24 h; (C) 48 h.
Since the amount of cross-linking agent is a crucial factor in
the star polymer formation,18 several trials were conducted to
determine the suitable feed ratio of cross-linker EVBA to
PNIPAM macro-RAFT agent (PNIPAM-RAFT). A higher
ratio of [EVBA]/[PNIPAM-RAFT] > 6 led to insoluble gel,
while a large amount of linear precursor remained un-cross-
linked at a lower ratio of [EVBA]/[PNIPAM-RAFT] < 1.5.
Thus, a feed ratio of [EVBA]/[PNIPAM-RAFT]/[AIBN] =
25/10/1 ([EVBA] = 0.05 M) was employed for the star poly-
mer synthesis in THF at 60 °C. At a timed interval, the reac-
tion mixtures were diluted with THF and immediately moni-
tored by GPC.
The GPC curves in Figure 1 indicated that the amount of
the linear precursor incorporated into the star polymers
increased with the reaction time, which was confirmed by
the decreasing GPC signal corresponding to the linear poly-
mer and an increasing new GPC elution peaks shifting to
higher molecular weight. After 48 h, the star polymer became
the dominant population with a rather small amount of the
unreacted linear polymer. The star polymer yield was ca.
80%, calculated from the GPC area ratio of the star polymer
and the linear precursor.18 The cross-linker EVBA was
almost completely consumed, as evidenced by the disappear-
ance of EVBA peak in GPC (retention time of EVBA at 30 min,
not shown here).
Results and Discussion
Synthesis of (PNIPAM)n-polyEVBA Star Polymer with
Aldehyde Functionalized Core. For the synthesis of multiarm
star polymers with aldehyde functionalized core, we em-
ployed the arm-first technique, in which preformed linear
polymers with living dormant chain ends are joined together
using a newly designed difunctional vinyl linking agent
EVBA (Scheme 2) to form a microgel core carrying the linear
arm on its surface. The trithiocarbonate terminated linear
PNIPAM macro-RAFT agent was first prepared through
RAFT polymerization of NIPAM using DMPA as RAFT
agent and AIBN as initiator in DMF at 60 °C. After 7 h
at 55% conversion, a well-defined PNIPAM macro-RAFT
agent with Mn,GPC=5000 and Mw/Mn=1.07 was obtained.
The obtained linear PNIPAM macro-RAFT agent was
then allowed to react with the divinyl monomer EVBA in the
presence of AIBN to induce polymer linking for the forma-
tion of star PNIPAM with aldehyde functionalized core. The
RAFT approach was selected for preparation of star poly-
mer because of its great tolerance to a wide range of func-
tional groups including aldehyde groups.33-37
After fractional precipitation of the star polymer in THF/
n-hexane to remove the unreacted linear polymer, a mono-
modal GPC trace for (PNIPAM)n-polyEVBA star polymer
with narrow molecular weight distribution (Mw/Mn = 1.36)
was detected (Figure 1C, dashed line). The purified star
1
polymer was analyzed by H NMR spectroscopy (Figure
2A). In addition to the characteristic signals of the methine
proton (b, 3.8-4.2 ppm) from PNIPAM arms, the peak of
aldehyde protons (c, 9.8-10.8 ppm) from polyEVBA core
was distinct but with signal broadening and weakening due
to the restricted rotation of the highly cross-linked polyEV-
BA core.23 The 1H NMR along with GPC results indicate the
successful synthesis of the star PNIPAM with aldehyde
functionalized core.
The absolute weight-average molecular weights (Mw) and
the average number of arms per star polymer molecule
(Narm) were characterized by GPC with MALLS detector.

5702 Macromolecules, Vol. 43, No. 13, 2010
Wu et al.
Figure 3. GPC traces of PNIPAM-PEG miktoarm star copolymer
before (B) and after purification (C) prepared via click reaction between
(PNIPAM)n-polyEVBA star polymer and PEG-ONH2 precursors (A).
was fractionated by preparative GPC and then analyzed by
the 1H NMR spectrum (Figure 2B). Apart from the peaks of
the methine proton for PNIPAM, the characteristic reso-
nances of methylene protons (d, 3.5-3.8 ppm) for PEG
appeared, indicating the introducing of PEG segment into
the star. Notably, the signal of aldehyde protons for its
precursor (Figure 2A) completely disappeared, suggesting
all the aldehyde groups located near the core surface had
been almost reacted. On the basis of the integration areas of
methine proton for PNIPAM and methylene protons for
PEG and the degree polymerization of these two arms (44 for
PNIPAM and 45 for PEG), the ratio of PEG and PNIPAM
arm number NPEG/NPNIPAM was derived to be 2.92. Having
value of NPNIPAM = 13 determined previously by MALLS,
NPEG was thus calculated to be 38.
As expected, not all of aldehyde groups within the star core
can be clicked by PEG-ONH2 to form second-generation
PEG arms due to the cross-linked nature of the star core.
Comparing the observed NPEG value (∼38) with the total
amount of aldehyde groups in the star core (∼65 aldehyde
groups per star, the calculated value based on the EVBA
addition amount for the star PNIPAM formation) reveals
that around 58% fraction of aldehyde groups, the “reactive”
aldehyde groups located near the core surface, are accessible
to PEG-ONH2 and participate in the click reaction. It can be
expected that the added PEG-ONH2 should be clicked
quantitatively when the initial PEG-ONH2 amount added
to the reaction was lower than that of the “reactive” aldehyde
groups in the star core. In this case, the number of the second
PEG arm can be controlled by varying the aminooxy/alde-
hyde feed ratio. In a ratio of [aminooxy]/[aldehyde] = 0.25:1,
for example, PEG-ONH2 was almost consumed, and the
observed NPEG was 14, which was in good agreement with
the expected value (∼16) assuming 100% of click efficiency.
The characterization data on PNIPAM-PEG miktoarm
star polymers are given in Table 1. The molecular weights
determined by GPC were lower than that by MALLS, which
indicates that the star polymers were more compact than the
linear counterparts with the same molecular weight.39
Thermoinduced Micellization of PNIPAM-PEG Mik-
toarm Star Copolymers in Aqueous Solution. The obtained
new double-hydrophilic miktoarm star copolymer contains a
permanently hydrophilic PEG block and a thermorespon-
sive PNIPAM block which exhibits a lower critical solution
temperature (LCST) around 32 °C. At temperature below
the LCST, the intermolecular hydrogen bonding between
PNIPAM chains and water molecules is dominant, and
Figure 2. 1H NMR spectra in CDCl3 of (PNIPAM)n-polyEVBA star
polymer (A) and (PNIPAM)n-polyEVBA-(PEG)m miktoarm star co-
polymer (B).
Narm was calculated using the equation Narm= Mw,star-
Arm /Mw,PNIPAM
38
,
where Mw,star and Mw,PNIPAM are
wt %
the absolute weight-average molecular weights of star poly-
mer and linear PNIPAM precursor, respectively, measured
from MALLS; Armwt % is the weight fraction of PNIPAM
arm in the star polymer, determined on the basis of the feed
ratio for the preparation of the star polymer. Consequently,
the star polymer has 13 arms per molecule.
Synthesis of (PNIPAM)n-polyEVBA-(PEG)m Miktoarm
Star Copolymers via Aldehyde-Aminooxy Click Reaction.
Aldehydes can react readily with aminooxy groups via the
formation of a highly stable oxime linkage under quite mild
conditions in a near-quantitative yield.28 Thus, the obtained
(PNIPAM)n-polyEVBA star polymer bearing aldehyde groups
within the core was allowed to simply click with PEG-ONH2
(Mn = 2000) affording the synthesis of PNIPAM-PEG mik-
toarm star copolymer (Scheme 1). The click reaction was
carried out in THF at room temperature for 24 h with a feed
ratio of [PEG-ONH2]/[-CHO group]=1.5. GPC profile of the
reaction mixtures showed two peaks for the expected
(PNIPAM)n-polyEVBA-(PEG)m miktoarm star polymer and
PEG-ONH2 used in excess (Figure 3B). After purification of
the star polymer by dialysis against THF to remove the free
PEG-ONH2, the miktoarm star polymer displayed a narrow
and monomodal GPC distribution (Mw/Mn = 1.12) shifting to
higher molecular weight region compared with that of the
precursor (PNIPAM)n-polyEVBA (Figure 3C).
In order to confirm the formation of the PNIPAM-PEG
miktoarm star copolymers, the miktoarm star copolymer

Article
Macromolecules, Vol. 43, No. 13, 2010 5703
Table 1. Characterization of (PNIPAM)n-polyEVBA Star Polymer and (PNIPAM)n-polyEVBA-(PEG)m Miktoarm Star Copolymer
c
f
g
sample
Mn (GPC)
Mw/Mn (GPC)
Mw
NPNIPAM
NPEG
(PNIPAM)13-polyEVBAa
(PNIPAM)13-polyEVBA-(PEG)38
(PNIPAM)13-polyEVBA-(PEG)14
53 400
65 300
61 100
1.36
1.12
1.17
90 600d
166 600e
118 600e
13
13
13
b
b
38
14
a Prepared by arm-first technique via cross-linking of the linear PNIPAM RAFT agent (Mn,GPC = 5000, Mw/Mn = 1.07, Mw by MALLS = 5800)
using aldehyde-containing divinyl compound EVBA. b Prepared by click reaction of aldehyde core-functionalized star PNIPAM and PEG-ONH2, the
feed ratio of [aminooxy]/[aldehyde] was 1.5:1 for (PNIPAM)13-polyEVBA-(PEG)38 and 0.25:1 for (PNIPAM)13-polyEVBA-(PEG)14
weight-average molecular weight. d By GPC with MALLS detector. e Determined by combination of MALLS and 1H NMR. f The average number of
PNIPAM arms per star molecule: NPNIPAM = Mw,starArmwt %/Mw,PNIPAM
g Average number of PEG arms per star molecule: NPEG
(DPPNIPAM PEG,3.5-3.8NPNIPAM)/(4DPPEG PNIPAM,3.8-4.2
), IPNIPAM,3.8-4.2 and IPEG,3.5-3.8 are the 1H NMR in CDCl3 integration areas of methine
protons for PNIPAM and methylene protons for PEG, respectively; DP is the degree of polymerization.
.
c Absolute
.
=
I
I
Scheme 3. Illustration of the Thermoinduced Micellization Behavior of
the (PNIPAM)13-polyEVBA-(PEG)m Miktoarm Star Copolymers with
Different PEG Content
Figure 4. Temperature dependences of optical transmittance at 500 nm
obtained for 10 mg/mL aqueous solutions of (PNIPAM)13-poly-
EVBA-(PEG)m miktoarm star copolymers with different PEG arms.
(A) m = 14; (B) m = 38.
that PNIPAM arms in the miktoarm star copolymer col-
lapsed into hydrophobic segments upon a temperature in-
crease, resulting in the formation of amphiphilic miktoarm
star copolymer which subsequently assembled via intermole-
cular hydrophobic interactions (intermolecular micellization)
to multimolecular micelles with collapsed PNIPAM core and
stretched PEG corona (Scheme 3). To further confirm the
formation of micelles, the size distributions of the miktoarm
star copolymer aqueous solutions at 25 and 60 °C were
measured by dynamic light scattering (Figure 5A). At 25 °C
(below the phase transition temperature), the miktoarm star
copolymer molecularly dissolves in water with a hydrody-
namic radius of ∼9.8 nm. At 60 °C (above the phase transition
temperature), intermolecular micellization occurs, accompa-
nied by an increase of hydrodynamic radius (∼33.6 nm).
Because the micelles formed here are small (size in nanoscale)
and thus are not able to scatter light efficiently, the remaining
transmittance at elevated temperature for anaqueoussolution
of (PNIPAM)13-polyEVBA-(PEG)14 is still high (∼80%,
Figure 4, curve A). It is also necessary to point out that the
thermal transition process of the miktoarm star copolymer is
very broad and covers a temperature range from 31 to 55 °C,
which is quite different from the sharp phase transition of
PNIPAM homopolymers.
When more PEG arms were incorporated as in the case of
(PNIPAM)13-polyEVBA-(PEG)38 (molar ratio of PEG to
PNIPAM units ∼3), the transmittance did not change with
temperature and was higher than 98% even at the highest
temperature investigated (60 °C, Figure 4, curve B); thus, the
solution was transparent over the experimental temperature
range. Dynamic light scattering results (Figure 5B) showed
that the hydrodynamic radius of the miktoarm star copolymer
in aqueous solution at 60 °C (∼8.1 nm) was almost consis-
tent with that at 25 °C (∼9.2 nm). These results suggested
that upon heating of the aqueous solution of (PNIPAM)13-
polyEVBA-(PEG)38 no intermolecular aggregation occurred
therefore PNIPAM is hydrophilic and soluble in water. At
temperature above the LCST, intramolecular hydrogen
bonding between the amide groups in PNIPAM results in
transition into compact and collapsed conformations of
PNIPAM chains, and the polymeric chain becomes hydro-
phobic and insoluble in water. Thus, PNIPAM-PEG mik-
toarm star copolymer might exhibit thermoresponsive
micellization behavior in aqueous solution. Compared to
linear counterparts (block copolymers), miktoarm star co-
polymers exhibit more complex phase-separation behavior
in solution due to that two different kinds of arms are linked
to a single junction point.40 Thus, it may be interesting to
investigate the micellization behavior of the synthesized
PNIPAM-PEG miktoarm star copolymers. Miktoarm star
copolymers (PNIPAM)n-polyEVBA-(PEG)m with equal
PNIPAM arm number (n = 13; Mn of PNIPAM ∼5000)
but different PEG arm number (m = 38 and 14, respectively;
Mn of PEG ∼ 2000) were used in order to check the effect of
the miktoarm star copolymer composition on the thermo-
induced micellization mechanism. The temperature sensiti-
vity of the miktoarm star copolymers which should be
strongly correlated with their micellization behavior in water
was examined by turbidity measurements.
Figure 4 shows the temperature-dependent transmittance
at 500 nm of the miktoarm star copolymer aqueous solutions
(10 mg/mL). For the miktoarm star copolymer (PNIP-
AM)13-polyEVBA-(PEG)14 having a small number of PEG
arm and therefore a small PEG fraction (molar ratio of PEG
to PNIPAM units ∼1.1), the optical transmittance began to
decrease around 31 °C (curve A), accompanied by a color
change of the solution upon a temperature increase, from
completely colorless and transparent to a blue tinge, indicat-
ing the formation of micelles. These results demonstrated

5704 Macromolecules, Vol. 43, No. 13, 2010
Wu et al.
Figure 5. Size distributions (measure by dynamic light scattering at scattering angle 90°) of the (PNIPAM)13-polyEVBA-(PEG)m miktoarm star
copolymers at 25 and 60 °C. (A) m = 14; (B) m = 38.
Figure 7. Plots of temperature vs the ratio of proton signal intensity of
methyl (a) fromPNIPAM arms to that of methylene(b) fromPEG arms.
Detailed temperature-dependent 1H NMR spectra of (PNIPAM)13-
polyEVBA-(PEG)38 miktoarm star copolymers are shown in Figure 6.
scattering results have demonstrated that no intermolecular
aggregation occurred for (PNIPAM)13-polyEVBA-(PEG)38
1
Figure 6. Temperature-dependent H NMR spectra of (PNIPAM)13
-
in aqueous solution upon heating, it must undergo intra-
molecular micellization at elevated temperature to form
unimolecular micelles consisting of collapsed and compact
PNIPAM core and soluble PEG corona (Scheme 3). In
contrast to the case for (PNIPAM)13-polyEVBA-(PEG)14
having smaller number of PEG arms, the more PEG arms in
(PNIPAM)13-polyEVBA-(PEG)38 may have a shielding ef-
fect to protect the collapsed PNIPAM segments against the
intermolecular aggregation. Thus, for the PNIPAM-PEG
miktoarm star copolymers, opportunities are open for ther-
moinduced intermolecular or intramolecular micellizations
to form multimolecular micelles and unimolecular micelles,
respectively, via controlling the content ratio of PNIPAM
and PEG.
polyEVBA-(PEG)38 miktoarm star copolymers solution in D2O.
although PNIPAM arms may collapse in the miktoarm star
copolymer.
In order to find out whether the PNIPAM arms collapse at
elevated temperature, 1H NMR was employed to check the
thermoresponsive behavior of the miktoarm star copolymer.
1H NMR is a powerful and qualitative technique of reflecting
the conformational change of polymer chain because the
proton signals of collapsed polymer chain may become
strongly attenuated or even vanish. Figure 6 shows repre-
sentative temperature-dependent 1H NMR spectra of
(PNIPAM)13-polyEVBA-(PEG)38 in D2O solution (10 mg/
mL) in the temperature range from 28 to 60 °C. The signal
intensity due to PEG methylene protons (b) remained over
the experimental temperature range except for slight chemi-
cal shift changes, whereas the signal intensities due to
PNIPAM methine (c) and methyl (a) protons became atte-
nuated at elevated temperatures, indicating thermoinduced
collapse of PNIPAM chains did occur. By plotting the peak
intensity ratio of a/b vs temperature, it was observed that the
signal intensity from PNIPAM arms started to decrease
around 33 °C and almost disappeared at 60 °C (Figure 7),
which should correspond to the PNIPAM chain collapse
process. Since the turbidity measurement and dynamic light
Conclusion
In summary, we have successfully demonstrated a facile
synthetic pathway to PNIPAM-PEG miktoarm star copolymers
with multiple arms using a combination of RAFT arm-first
technique and aldehyde-aminooxy “click” coupling reaction.
Star PNIPAM bearing multiple aldehyde functionalities in the
core was initially prepared by RAFT arm-first technique via
cross-linking of the preformed linear macro-RAFT agents using
a newly designed aldehyde-containing divinyl compound EVBA.
The aldehyde groups preserved in the star core was then allowed

Article
Macromolecules, Vol. 43, No. 13, 2010 5705
to simply click with aminooxy-terminated PEGs to form
PNIPAM-PEG miktoarm star copolymers. By controlling the
content ratio of PNIPAM and PEG, the obtained double-
hydrophilic miktoarm star copolymer may form multimolecular
micelles and unimolecular micelles via micellization of intermo-
lecular and intramolecular, respectively.
(18) Gao, H.; Matyjaszewski, K. Macromolecules 2006, 39, 3154–3160.
(19) Bosman, A. W.; Heumann, A.; Klaerner, G.; Benoit, D.; Frechet,
J. M. J.; Hawker, C. J. J. Am. Chem. Soc. 2001, 123, 6461–6462.
(20) Liu, J. Q.; Liu, H. Y.; Jia, Z. F.; Bulmus, V.; Davis, T. P. Chem.
Commun. 2008, 6582–6584.
(21) Blencowe, A.; Tan, J. F.; Goh, T. K.; Qiao, G. G. Polymer 2009, 50,
5–32.
(22) Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev. 2009, 109,
4963–5050.
(23) Gao, H.; Matyjaszewski, K. J. Am. Chem. Soc. 2007, 129, 11828–
11834.
(24) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed.
Acknowledgment. Financial support by Natural Science
Foundation of China (project no. 20874115) is greatly acknowl-
edged.
2001, 40, 2004–2021.
References and Notes
(25) Zhu, J.; Zhu, X. L.; Kang, E. T.; Neoh, K. G. Polymer 2007, 48,
6992–6999.
(26) Altintas, O.; Hizal, G.; Tunca, U. J. Polym. Sci., Part A: Polym.
(1) Sfika, V.; Tsitsilianis, C.; Kiriy, A.; Gorodyska, G.; Stamm, M.
Macromolecules 2004, 37, 9551–9560.
(2) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science
Chem. 2006, 44, 5699–5707.
1996, 274, 959–963.
(27) Heredia, K. L.; Tolstyka, Z. P.; Maynard, H. D. Macromolecules
2007, 40, 4772–4779.
(28) Heredia, K. L.; Maynard, H. D. Org. Biomol. Chem. 2007, 5, 45–53.
(29) Altintas, O.; Hizal, G.; Tunca, U. J. Polym. Sci., Part A: Polym.
Chem. 2008, 46, 1218–1228.
(30) Lai, J. T.; Filla, D.; Shea, R. Macromolecules 2002, 35, 6754–6756.
(31) Shi, M.; Li, A. L.; Liang, H.; Lu, J. Macromolecules 2007, 40, 1891–
1896.
(32) Schlick, T. L.; Ding, Z.; Kovacs, E. W.; Francis, M. B. J. Am.
Chem. Soc. 2005, 127, 3718–3723.
(3) Pispas, S. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 606–613.
(4) Xu, J.; Ge, Z.; Zhu, Z.; Luo, S.; Liu, H.; Liu, S. Macromolecules
2006, 39, 8178–8185.
(5) Yan, J. J.; Ji, W. X.; Chen, E. Q.; Li, Z. C.; Liang, D. H.
Macromolecules 2008, 41, 4908–4913.
(6) Gil, E. S.; Hudson, S. M. Prog. Polym. Sci. 2004, 29, 1173–1222.
(7) Lapienis, G. Prog. Polym. Sci. 2009, 34, 852–892.
(8) Motokawa, R.; Morishita, K.; Koizumi, S.; Nakahira, T.; Annaka,
M. Macromolecules 2005, 38, 5748–5760.
(9) Chen, H.; Li, J.; Ding, Y.; Zhang, G.; Zhang, Q.; Wu, C. Macro-
molecules 2005, 38, 4403–4408.
(33) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2009, 62,
1402–1472.
(10) Tang, X. D.; Liang, X. C.; Yang, Q.; Fan, X. H.; Shen, Z. H.; Zhou,
Q. F. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4420–4427.
(11) Li, J.; He, W. D.; He, N.; Han, S. C.; Sun, X. L.; Li, L. Y.; Zhang,
B. Y. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 1450–1462.
(12) Lin, H. H.; Cheng, Y. L. Macromolecules 2001, 34, 3710–3715.
(13) Gao, H.; Matyjaszewski, K. Prog. Polym. Sci. 2009, 34, 317–350.
(14) Hirao, A.; Hayashi, M.; Loykulnant, S.; Sugiyama, K. Prog.
Polym. Sci. 2005, 30, 111–182.
(34) Moad, G.; Rizzardo, E.; Thang, S. H. Polymer 2008, 49, 1079–
1131.
(35) Sun, G. R.; Cheng, C.; Wooley, K. L. Macromolecules 2007, 40,
793–795.
(36) Xiao, N. Y.; Li, A. L.; Liang, H.; Lu, J. Macromolecules 2008, 41,
2374–2380.
(37) Xiao, Z. P.; Yang, K. M.; Liang, H.; Lu, J. J. Polym. Sci., Part A:
Polym. Chem. 2010, 48, 542–550.
(15) Tsitsilianis, C.; Chaumont, P.; Rempp, P. Makromol. Chem. 1990,
191, 2319–2328.
(16) Tsitsilianis, C.; Graff, S.; Rempp, P. Eur. Polym. J. 1991, 27, 243–
246.
(38) Baek, K. Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2001,
34, 215–221.
(39) Terashima, T.; Ouchi, M.; Ando, T.; Kamigaito, M.; Sawamoto,
M. Macromolecules 2007, 40, 3581–3588.
(17) Hadjichristidis, N. J. Polym. Sci., Part A: Polym. Chem. 1999, 37,
857–871.
(40) Kiriy, A.; Gorodyska, G.; Minko, S.; Stamm, M.; Tsitsilianis, C.
Macromolecules 2003, 36, 8704–8711.