Synthesis and self-assembly of well-defined cyclodextrin-centered amphiphilic A14B7 multimiktoarm star copolymers based on poly(ε-caprolactone) and poly(acrylic acid)

Article abstract of DOI:10.1002/pola.24066

Novel amphiphilic A₁₄B₇ multimiktoarm star copolymers composed of 14 poly(ε-caprolactone) (PCL) arms and 7 poly(acrylic acid) (PAA) arms with β-cyclodextrin (β-CD) as core moiety were synthesized by the combination of controlled ring-opening polymerization (CROP) and atom transfer radical polymerization (ATRP). 14-Arm star PCL homopolymers (CDSi-SPCL) were first synthesized by the CROP of CL using per-6-(tert-butyl-dimethylsilyl) -β-CD as the multifunctional initiator in the presence of Sn(Oct) ₂ at 125 °C. Subsequently, the hydroxyl end groups of CDSi-SPCL were blocked by acetyl chloride. After desilylation of the tert- butyldimethylsilyl ether groups from the β-CD core, 7 ATRP initiating sites were introduced by treating with 2-bromoisobutyryl bromide, which further initiated ATRP of tertbutyl acrylate (tBA) to prepare well-defined A ₁₄B₇ multimiktoarm star copolymers [CDS(PCL-PtBA)]. Their molecular structures and physical properties were in detail characterized by ¹H NMR, SEC-MALLS, and DSC. The selective hydrolysis of tert-butyl ester groups of the PtBA block gave the amphiphilic A₁₄B₇ multimiktoarm star copolymers [CDS(PCL-PAA)]. These amphiphilic copolymers could self-assemble into multimorphological aggregates in aqueous solution, which were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM).

Full text of DOI:10.1002/pola.24066

Synthesis and Self-Assembly of Well-Defined Cyclodextrin-Centered
Amphiphilic A14B7 Multimiktoarm Star Copolymers Based on
Poly(e-caprolactone) and Poly(acrylic acid)
PENG-FEI GOU, WEI-PU ZHU, NING XU, ZHI-QUAN SHEN
Department of Polymer Science and Engineering, MOE Key Laboratory of Macromolecule Synthesis and Functionalization,
Zhejiang University, Hangzhou 310027, China
Received 24 December 2009; accepted 9 April 2010
DOI: 10.1002/pola.24066
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Novel amphiphilic A14B7 multimiktoarm star copoly-
mers composed of 14 poly(e-caprolactone) (PCL) arms and 7
poly(acrylic acid) (PAA) arms with b-cyclodextrin (b-CD) as core
moiety were synthesized by the combination of controlled ring-
opening polymerization (CROP) and atom transfer radical poly-
merization (ATRP). 14-Arm star PCL homopolymers (CDSi-SPCL)
were first synthesized by the CROP of CL using per-6-(tert-butyl-
dimethylsilyl)-b-CD as the multifunctional initiator in the pres-
ence of Sn(Oct)2 at 125 ꢀC. Subsequently, the hydroxyl end
groups of CDSi-SPCL were blocked by acetyl chloride. After desi-
lylation of the tert-butyldimethylsilyl ether groups from the b-CD
core, 7 ATRP initiating sites were introduced by treating with
2-bromoisobutyryl bromide, which further initiated ATRP of tert-
butyl acrylate (tBA) to prepare well-defined A14B7 multimiktoarm
star copolymers [CDS(PCL-PtBA)]. Their molecular structures
and physical properties were in detail characterized by 1H NMR,
SEC-MALLS, and DSC. The selective hydrolysis of tert-butyl ester
groups of the PtBA block gave the amphiphilic A14B7 multimik-
toarm star copolymers [CDS(PCL-PAA)]. These amphiphilic
copolymers could self-assemble into multimorphological aggre-
gates in aqueous solution, which were characterized by dynamic
light scattering (DLS), transmission electron microscopy (TEM)
C
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and atomic force microscopy (AFM). 2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 2961–2974, 2010
KEYWORDS: amphiphiles; atom transfer radical polymerization
(ATRP); controlled ring-opening polymerization (CROP);
micelles; miktoarm star copolymer; poly(e-caprolactone); poly
(acrylic acid); poly(tert-butyl acrylate); ring-opening polymeriza-
tion; self-assembly; self-organization; star polymers
INTRODUCTION Miktoarm star copolymers consisting of
arms with different chemical nature emanating from one
central core have evoked considerable attention because of
their unique morphological and physical properties in the
solid state as well as in solution.1,2 Originally, miktoarm star
copolymers were synthesized from diphenylethylene deriva-
tives or chlorosilane via living anionic polymerization.3–8
However, these methods are restricted to few monomer and
involving strict polymerization conditions. Until recently, con-
trolled/living polymerization methods (CLP), such as con-
trolled ring-opening polymerization (CROP),9 atom transfer
radical polymerization (ATRP),10–12 nitroxide mediated radi-
cal polymerization (NMP),13 and radical addition fragmenta-
tion polymerization (RAFT)14 have proved to be efficient
methods to design well-defined and complex macromolecular
architectures because of the broad variety of applicable
monomers and benign polymerization conditions. Moreover,
the appearance of ‘‘click reactions’’15 also provides a power-
ful method for the synthesis and functionalization of poly-
mers with unique structures.16–18 Therefore, by the combina-
tion of CLP techniques or ‘‘click reactions,’’ various types of
miktoarm star copolymers with varying chemical composi-
tion and molecular topology have been synthesized.19–34
Typically, Gnanou and coworkers demonstrated that AB2
miktoarm star polymers can be prepared by ATRP and
chemical modification of ATRP-derived x-halogeno precur-
sors.35 Hadjichristidis and Dumas prepared A2B2 miktoarm
star polymers by the combination of CROP of e-caprolactone
(CL) and ATRP of styrene (St) or tert-butyl methacrylate
(tBMA) based on a heterotetra functional initiator.36–38
Hedrick and coworkers prepared A3B3 miktoarm star copoly-
mers containing poly(e-caprolactone) (PCL) and poly(methyl
methacrylate) (PMMA) arms using tandem CROP and ATRP
with a miktofunctional initiator.39 Zhao’s group reported ABC
miktoarm star copolymers by a three-step synthesis consist-
ing of CROP of CL, ATRP of MMA and NMP of St from a tri-
functional initiator.40 Very recently, Wang and coworkers
also described the synthesis of amphiphilic ABC miktoarm
star copolymers by the combination of CROP and ‘‘click
reactions.’’41
Additional Supporting Information may be found in the online version of this article. Correspondence to: W.-P. Zhu (E-mail: carrols@163.com) or
Z.-Q. Shen (E-mail: zhiquan_shen@163.com)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2961–2974 (2010) 2010 Wiley Periodicals, Inc.
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Furthermore, to obtain new miktoarm star copolymer mod-
els for the investigation of architecture-property relation-
ships and seek for new applications in biomedical materials,
nanotechnology and supermolecular science, the design and
synthesis of miktoarm star copolymers with novel and com-
plex architectures is of great importance. AnBm multimik-
toarm star copolymer with more than 10 arms is one of the
most complicated miktoarm star copolymers which are
rarely synthesized by simply utilizing CLP techniques.
Recently, Matyjaszewski and coworkers introduced a new
method to synthesize AnBm multimiktoarm star copolymers
by one-pot ATRP crosslinking reaction of several different
linear macroinitiators and a divinyl crosslinker. The average
arm numbers of these synthesized miktoarm star copolymers
can be extended to 50.42–45 Likewise, Wiltshire and Qiao
also prepared AnBm multimiktoarm star copolymers by
crosslinking of x-halogeno PCL and PMMA using ethylene
glycol dimethacrylate (EGDMA) as crosslinker.46 However, in
these cases, neither the exact arm numbers nor the precise
architecture of the miktoarm star copolymers could be con-
trolled. An alternate method to construct well-defined AnBm
multimiktoarm star copolymers with precise arm numbers
and lengths would resort to designing a heteromultifunc-
tional AnBm initiator which is able to initiate different poly-
merizations independently and sequentially. Nevertheless, it
is inconceivable to synthesize an AnBm initiator with more
than 10 functionalities from small molecules. Fortunately,
naturally occurring cyclodextrins (including a-CD, b-CD, and
c-CD), which have fixed numbers of primary and secondary
hydroxyl groups with different activities, provide excellent
platforms to design AnBm heteromultifunctional initiators for
the synthesis of well-defined multimiktoarm star copolymers.
During the past few years, several research groups have
focused on the synthesis of CD-centered star polymers.47–52
For example, Kakuchi and coworkers reported the umbrella-
like multimiktoarm star copolymers (AB20) synthesized from
a b-CD derivative by the combination of NMP and ATRP.53
However, the controllability of the polymerization is lost to
some extent in the ATRP step, well-defined AB20 miktoarm
star copolymers must be fractionated by preparative size
exclusion chromatography.
mers with complex structures such as graft,56 star block,57 and
dendrimer58 have been synthesized and used for self-assembly.
However, the study on the self-assembly originating from mik-
toarm star copolymers is still quite limited.59–61 Some unique
self-assembly behaviors of ABC miktoarm star copolymer have
been reported by Lodge and coworkers.62–64 It is expected that
complex multimiktoarm star copolymers will self-assemble
into novel aggregates in selective solvent. Thus, in this article,
the self-assembly behaviors of amphiphilic A14B7 multimik-
toarm star copolymers in aqueous solution were investigated
by dynamic light scattering (DLS), transmission electron mi-
croscopy (TEM), and atomic force microscopy (AFM).
EXPERIMENTAL
Materials
ꢀ
b-Cyclodextrin hydrate (b-CD) was dried over P2O5 at 80 C
under reduced pressure overnight before use. Pyridine, tolu-
ene, triethylamine (TEA) were dried over CaH2 and distilled
just before use. THF was distilled from the ketyl prepared
from sodium and benzophenone. e-Caprolactone (e-CL, 99%;
Acros) and tert-butyl acrylate (tBA; 99%; Aldrich) were dis-
tilled under reduced pressure before use. Acetic chloride
(AcCl; 99%; Aldrich) was distilled just before use.
N,N,N0,N00,N00-Pentamethyldiethylenetriamine (PMDETA; 99%;
Aldrich), 2-bromisobutyryl bromide (97%; Aldrich), and tert-
butylchlorodimethylsilane (TBDMSCl; 98%; Acros) were used
as received. CuBr (99%; Acros) was purified by stirring over-
night in acetic acid, after filtration, it was washed with etha-
nol and then dried in vacuum. Boron trifluoride diethyl
etherate (BF3ꢁEt2O; 48% BF3), stannous octoate [Sn(Oct)2;
97%] and other reagents were purchased from Sinopharm
Chemical Reagent, (China) and used as received.
Measurements
1H NMR spectra were recorded on a Bruker Avance DMX500
spectrometer in CDCl3 with tetramethylsilane as internal
standard.
ESI-positive MS was performed using Bruker Esquire 3000
plus spectrometer with an ESI interface.
The molecular weight and molecular weight distribution were
determined by size-exclusion chromatography/multiangle laser
light scattering (SEC-MALLS). The size-exclusion chromatogra-
phy (SEC) system consisted of a Waters degasser, a Waters
515 HPLC pump, Waters 2414 RI detector and columns: Styra-
gel, HT 3, HT 4. The calibration was performed with commer-
cial polystyrene standards. Tetrahydrofuran (THF) was used as
Considering the biocompatibility and biodegradability of
cyclodextrins, fabricating CD-based biocompatible multimik-
toarm star copolymers in relation to potential biomimetic
materials seems to be very attractive. Controlled ring-open-
ing polymerization (CROP) of cyclic lactones, lactides and
carbonates is one of the most convenient methods to synthe-
size biocompatible polymers. Thereby, in this article, well-
defined biocompatible amphiphilic A14B7 multimiktoarm star
copolymers based on b-CD have been synthesized by utiliz-
ing CROP of e-caprolactone (CL), ATRP of tert-butyl acrylate
(tBA) and selective hydrolysis from a b-CD derivative [per-6-
(tert-butyldimethylsilyl)-b-CD] (Scheme 1).
the mobile phase at a flow rate of 0.8 mL minꢂ1 at 30 C. The
ꢀ
refractive-index increment (dn/dc) was determined with a
Wyatt Optilab DSP differential refractometer at 690 nm.
Differential scanning calorimetry (DSC) measurements were
performed on a TA Q100 apparatus. The samples were
ꢀ
heated from 0 to 100 C, held for 2 min to eraꢀse the thermal
history, then cooled to 0 ꢀC at a rate of 20 ꢂC1 minꢂ1, and
Meanwhile, it is well-known that the architecture of amphi-
philic copolymer is one of the primary factors that influences
the morphology and dimension of the polymeric self-assem-
bly aggregates.54,55 Up to date, various amphiphilic copoly-
ꢀ
ꢀ
finally heated to 100 C at a rate of 10 C min
.
The hydrodynamic diameter and size distribution of micelles
were determined by dynamic light scattering (DLS) at 90ꢀ
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SCHEME 1 Synthetic route of amphiphilic A14B7 multimiktoarm star copolymers base on b-CD.
ꢀ
angle to the incident beam and at 25 C on a Brookhaven 90
Plus particle size analyzer. All micelle solutions measured by
DLS had a polymer concentration of 0.3 mg/mL.
AFM samples were prepared through drop-casting of the mi-
celle solution (0.3 mg/mL) onto the freshly cleaved mica and
dried at room temperature for 48 h. AFM (tapping mode)
was operated on a Seiko SPI3800N station (Seiko Instru-
ments). Silicon tips (NSG10, NT-MDT) with a resonance fre-
quency of ꢃ330 KHz were used.
The critical micelle concentration (CMC) was determined by
fluorescence measurement using pyrene as a fluorescent
probe. Fluorescence excitation spectra were recorded on HI-
TEM images were obtained using JEM-1230 operating at an
acceleration voltage of 60 kV. A drop of 0.3 mg/mL micelle
solution was placed onto the surface of Formvar-carbon film-
coated copper grids. Excess solution was quickly wicked
away with a filter paper. All grids were finally negatively
stained by 2 wt % phosphotungstic acid.
A14B7 MULTIMIKTOARM STAR COPOLYMERS, GOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TACHI F-4500 fluorescence spectrometer at 390 nm emis-
sion wavelength and 2.5 nm slit width. The pyrene concen-
tration in the solution was chosen to be 6.0 ꢄ 10ꢂ7 M.
Synthesis of Per-6-(tert-butyldimethylsilyl)-b-CD (Scheme
1; CD-Core)
ature for 24 h. The precipitated byproduct was removed by
filtration, and the solution was evaporated to dryness. The
crude product was dissolved in 50 mL methylene chloride,
and then the organic phase was washed successively with 1
M NaHCO3 aqueous solution and 80 mL water and finally
dried over anhydrous MgSO4. The concentrated solution was
poured into cold diethyl ether to precipitate the product.
The resulting white solid was dried in vacuum at room tem-
perature for 24 h. Yield: 2.4 g (80% for polymer weight).
Per-6-(tert-butyldimethylsilyl)-b-CD was synthesized accord-
ing to the procedure described in the literature65 as follows:
dry b-CD (2.27 g, 2 mmol) was dissolved under vigorous
stirring in 50 mL anhydrous pyridine cooled in an ice bath.
tert-Butylchlorodimethylsilane (TBDMSCl) (2.40 g, 16 mmol)
dissolved in 20 mL anhydrous pyridine was then added
dropwise into the cooled reaction vessel over 30 min. The
reaction mixture was stirred for 18 h under argon atmos-
phere at room temperature, and then the solvent was
removed under reduced pressure to give a white solid, which
was taken up in chloroform and washed successively with
KHSO4 (100 mL, 1 M) and water. The organic phase was
dried over magnesium sulfate, filtered, and evaporated to
dryness. Yield: 3.10g (90%). MS for C84H168O35Si7: m/z ¼
1958 (MþNa)þ.
Desilylation of CDSi-ACSPCL (Scheme 1)
The primary hydroxyl groups on the cyclodextrin-core were
recovered by desilylation of the tert-butyldimethylsilyl ether
groups in the presence of BF3ꢁEt2O. A typical procedure was
as follows: BF3ꢁEt2O (0.5 mL, 2 mmol) in 10 mL anhydrous
CH2Cl2 was added dropwise to a solution of CDSi-ACSPCL95
(2.0 g, 1.04 mmol of tert-butyldimethylsilyl group) in 80 mL
anhydrous CH2Cl2 under stirring. The reaction mixture was
stirred for 12 h under argon atmosphere at room tempera-
ture and then washed successively with NaHCO3 (100 mL, 1
M) and water. The organic phase was dried over anhydrous
MgSO4. The concentrated solution was poured into cold
methanol to precipitate the product. The resulting white
solid (denoted as CD-ACSPCL, Scheme 1) was dried in vac-
uum at room temperature for 24 h. Yield: 1.7 g (90% for
polymer weight).
1H NMR (CDCl3, 500MHz): 0.03 (s, 21H, CH3-Si), 0.04 (s,
21H, CH3-Si), 0.89 (s, 63H, (CH3)3C), 3.55 (d, 7H, H-6a)03.62
(m, 14H, H-2; H-5), 3.72 (d, 7H, H-3), 3.89 (d, 7H, H-6b) 4.03
(d, 7H, H-4), 4.89(d, 7H, H-1).
Synthesis of Cyclodextrin-Centered 14-Arm Star Poly(e-
caprolactone)s (CDSi-SPCL) by Controlled Ring-Opening
Polymerization (Scheme 1)
Synthesis of CD-ACSPCL-Br Macroinitiator for ATRP
(Scheme 1)
A typical procedure was as follows: CD-ACSPCL95 (1.4 g,
0.78 mmol of OH group) was dissolved in 40 mL anhydrous
THF, followed by the introduction of triethylamine (TEA)
(0.79 g, 7.8 mmol, 1.1 mL) via a syringe. 2-Bromisobutyryl
bromide (1.80 g, 7.8 mmol, 1.0 mL) in 5 mL anhydrous THF
was added dropwise to the vigorously stirred solution at 0
ꢀC over 30 min. The reaction mixture was stirred at room
temperature overnight. The precipitated byproduct was
removed by filtration, and the solution was evaporated to
dryness. The crude product was dissolved in 40 mL methyl-
ene chloride, and then washed successively with 1 M
NaHCO3 aqueous solution and 40 mL water, and finally dried
over anhydrous MgSO4. The concentrated solution was
poured into methanol to precipitate the product. The result-
ing white solid was dried in vacuum at room temperature
for 24 h. Yield: 1.14 g (75% for polymer weight).
CDSi-SPCL was synthesized by controlled ring-opening poly-
merization of e-CL using per-6-(tert-butyldimethylsilyl)-b-CD
(CD-Core) as the multifunctional initiator in the presence of
Sn(Oct)2. A typical polymerization procedure was as follows:
per-6-(tert-butyldimethylsilyꢀl)-b-CD (0.773 g, 5.6 mmol of
OH group) was dried at 70 C under reduced pressure over-
night and transferred into a flamed-dried ampoule. Then, CL
(3.88 g, 34 mmol) and a magnetic stirring bar were added to
the ampoule under argon atmosphere. After CD-Core was
dissolved in CL completely, the tube was then connected to a
schlenkline, where an exhausting-refilling process was
repeated three times. 20 mg Sn(Oct)2 in 1 mL dry toluene
was added to the mixture, and the exhausting-refilling pro-
cess was carried out again to removeꢀthe toluene. The am-
poule was put into an oil bath at 125 C for 24 h bulk poly-
merization. The crude polymer was dissolved in THF and
poured into excess methanol to precipitate the product,
which was dried in vacuum to constant weight. Yield: 4.58g
(98%).
Synthesis of A14B7 Multimiktoarm Star Copolymers of
CDS(PCL-PtBA) by ATRP (Scheme 1)
In a typical produce, CD-ACSPCL95-Br (400 mg, 0.21 mmol
of Br group) and CuBr (30 mg, 0.21 mmol) were charged
into a previously flame-dried ampoule filled with argon. The
ampoule was degassed and backfilled with argon three
times. Deoxygenated toluene (18 mL) and PMDETA (36 mg,
0.21 mmol) were added via a syringe, and the solution was
stirred for 5 min. Finally, tBA (2.56 g, 21 mmol) was added.
The reaction mixture was degassed with three freeze-pumꢀp-
thaw cycles, left in argon, and placed in an oil bath at 70 C
for 240 min. Subsequently, the polymerization mixture was
diluted with THF, passed through an alumina column to
Synthesis of Cyclodextrin-Centered 14-Arm Star
x-Acetyl-Poly(e-caprolactone)s (Scheme 1, CDSi-ACSPCL)
The hydroxyl end groups of the CDSi-SPCL were blocked by
acetic chloride. A typical procedure was as follows: CDSi-
SPCL95 (3.0 g, 3.28 mmol of OH group) and triethylamine
(TEA) (0.98 g, 9.8 mmol, 1.34 mL) werꢀe dissolved in 100 mL
dry THF. The solution was cooled to 0 C, and acetic chloride
(0.77 g, 9.8 mmol, 0.7 mL) in 10 mL dry THF was added
dropwise to the vigorously stirred solution over 30 min.
Thereafter, the reaction mixture was stirred at room temper-
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ARTICLE
TABLE 1 Characterization of Cyclodextrin-Centered 14-Arm Star PCL Homopolymersa
[CL]0/[CD-Core]
(Molar Ratio)
Conv
(%)c
Mn,Cal
Mn,NMR
Mn,MALLS
Mn,SEC
Sampleb
(g molꢂ1 d
)
(g molꢂ1 e
)
(g molꢂ1 f
)
(g molꢂ1 g
)
PDIg
NPCL
h
CDSi-SPCL95
CDSi-SPCL185
CDSi-SPCL265
a
85
185
255
98.2
97.5
98.9
11,500
22,500
30,700
12,800
23,000
32,100
13,800
24,400
31,500
14,200
25,300
36,300
1.13
1.26
1.15
6.8
13.2
18.9
f
Polymerization conditions: bulk, 125 ꢀC, 24 h.
Measured by connecting a multi-angle laser light scattering detector to
the SEC.
b
CDSi-SPCL95, 95 represents the average CL repeat units of one CDSi-
SPCL macromolecule determined by 1H NMR.
c
Determined by SEC analysis with polystyrene standards.
g
h
Conversion was calculated on the basis of gravimetric method.
The average number of CL units in each arm of the 14-arm star PCL
d
Calculated from the equation: Mn,Cal ¼ ([CL]0/[CD-Core]) ꢄ 114 ꢄ Con-
homopolymers, calculated from the 1H NMR.
version (%) þ MCD-Core
.
e
This calculation is based on the integration of the CL units as com-
pared to that of the tert-butyl protons of the CD-Core: Mn,NMR
¼
If
9ꢄ7
ð2Þ ꢄ ð Þ ꢄ 114 þ MCDꢂcore (see Figure 1).
Ia
remove the catalyst. The polymer was recovered by precipi-
tation into cold hexane and dried in vacuum. Yield: 1.22 g
(31.9%).
hydroxyl groups for the controlled ring-opening polymeriza-
tion of CL.
The structure of per-6-(tert-butyldimethylsilyl)-b-CD was
confirmed by 1H NMR and MS (see Supporting Information
Fig. S1).
Synthesis of Amphiphilic A14B7 Multimiktoarm Star
Copolymers of CDS(PCL-PAA) by Selective Hydrolysis
(Scheme 1)s
A typical example is given below. To a methylene chloride
solution of CDS(PCL95-PtBA248) (200 mg), trifluoroacetic
acid (1 mL) was added dropwise under stirring. The mixture
Synthesis of Cyclodextrin-Centered 14-Arm Star Poly(e-
caprolactone)s (CDSi-SPCL) by Controlled Ring-Opening
Polymerization
ꢀ
was kept at 30 C for 72 h. After the solvent was evaporated
The synthesis of CDSi-SPCL was carried out by the controlled
ring-opening bulk polymerization of CL using per-6-(tert-
butyldimethylsilyl)-b-CD as a multifunctional initiator and
Sn(Oct)2 as the catalyst at 125 ꢀC as shown in Scheme 1.
Three different monomer/initiator ratios were utilized to
achieve CDSi-SPCL with various polymerization degrees.
Detailed characterization results are summarized in Table 1.
in vacuum, the product was dissolved in THF and precipi-
tated in cold hexane. Yield: 110 mg (79% for polymer
weight).
Micellization of CDS(PCL-PAA)
At ambient temperature around 25 ꢀC, 25 mg of CDS(PCL-
PAA) sample was dissolved in 10 mL DMF, and then distilled
water (ꢃ10 mL) was added dropwise to the solution under
vigorous stirring. The solution was stirred for 2 h and dia-
lyzed against distilled water over 48 h to remove DMF. The
final concentration of the micelle solution was 1.0 mg/mL.
2
NPtBA ¼ ðIc þ Ig ꢂ IfÞ ꢄ ꢄ NPCL
(1)
If
where NPtBA is the average number of tBA units in each arm
(see Table 2), and If, Ic, Ig represent the integral area of the
signals at 4.0 ppm (Hf), 2.29 ppm (Hc) and 2.2 ppm (Hg)
due to the protons of the PCL and PtBA block.
RESULTS AND DISCUSSION
Synthesis of Per-6-(tert-butyldimethylsilyl)-b-CD
(CD-Core)
1
The synthesis of per-6-(tert-butyldimethylsilyl)-b-CD as a
multifunctional initiator was based on the following two
facts: first, only the 7 primary hydroxyl groups of b-CD can
be silylated by tert-butylchlorodimethylsilane due to the
inactivity of the hindered 14 secondary hydroxyl groups.
tert-Butyldimethylsilyl ether group is stable to most reaction
conditions and can be removed under specific mild condi-
tions, which provides a convenient way to design the initia-
tor from b-CD for the synthesis of A14B7 multimiktoarm star
copolymers in the next steps; second, because of the strong
intra and inter molecular hydrogen bonds, b-CD shows poor
solubility in e-caprolactone (CL), the incorporating of seven
tert-butyldimethylsilyl ether groups onto b-CD would reduce
the hydrogen bonds and increase its solubility in CL, which
permits the simultaneous initiating of all the 14 secondary
Figure 1 exhibits a representative H NMR spectrum of CDSi-
SPCL (CDSi-SPCL265). It clearly shows that besides the
major proton signals of PCL chains (Hc–f,), there are addi-
tional signals of the per-6-(tert-butyldimethylsilyl)-b-CD core
moiety, that is, signals corresponding to methyl protons (Hb)
and tert-butyl protons (Ha) of the tert-butyldimethylsilyl
ether groups. The triplet at 3.65 ppm (Hg) is assigned to the
PCL methylene protons conjoint with the hydroxyl end
groups. The number average molecular weight of the CDSi-
SPCL (Mn,NMR) can be calculated by the integration of signals
at 4.05 ppm (Hf) of the CL repeat units and that of signals at
0.89 ppm (Ha) of the tert-butyldimethylsilyl ether groups
(see Table 1). Notably, the actual molecular weights of CDSi-
SPCL obtained from 1H NMR spectra are close to the values
calculated by observed conversion and [CL]0/[CD-core],
A14B7 MULTIMIKTOARM STAR COPOLYMERS, GOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 1H NMR spectrum (500MHz) of CDSi-SPCL265 in
CDCl3. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
FIGURE 3 1H NMR spectrum (500MHz) of CDSi-ACSPCL265 in
CDCl3. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
which verifies the well-defined architectures of these
samples.
star copolymers. Thus, acetyl chloride was used to block the
hydroxyl end groups of CDSi-SPCL. From the 1H NMR spec-
trum shown in Figure 3, it is obvious that the signal of the
methylene protons of PCL conjoint with hydroxyl end groups
[Fig. 1 (Hg)] disappeared completely, whereas a new signal
corresponding to methyl protons of acetyl group appeared at
2.03 ppm. Moreover, integral ratio of Hf and Hh is in well
agreement with the theoretical value, suggesting that the
hydroxyl groups of CDSi-SPCL have been completely blocked.
SEC traces (Fig. 2) reveal unimodal elution peaks with rea-
sonably narrow polydispersity (ꢅ1.26), although slight tail-
ing was detected in the low-molecular-weight region, which
was attributed to unavoidable chain transfer side reactions
during the polymerization. The results of the actual molar
masses of CDSi-SPCL measured by SEC-MALLS have con-
firmed the values obtained from 1H NMR (Table 1), which
demonstrates that CDSi-SPCL with designed architectures
have been successfully prepared.
Desilylation of the Cyclodextrin-Centered 14-Arm Star
x-Acetyl-Poly(e-caprolactone)s
Synthesis of Cyclodextrin-Centered 14-Arm Star
x-Acetyl-Poly(e-caprolactone)s (CDSi-ACSPCL)
The tert-butyldimethylsilyl ether group is widely used for
hydroxyl protection which can be completely removed in the
presence of fluoride ion or aqueous acid. Here, boron tri-
fluoride diethyl etherate (BF3ꢁEt2O) was used to desilylate
the tert-butyldimethylsilyl ether groups. The 1H NMR spec-
trum of a representative desilylated CDSi-SPCL (denoted as
CD-ACSPCL265) is shown in Figure 4. After desilylation, only
the signals at 0.04 ppm (Hb) and 0.89 ppm (Ha) assigned to
the protons of the tert-butyldimethylsilyl ether groups disap-
peared, other signals assigned to the protons of PCL chains
remained intact, indicating that the seven primary hydroxyl
groups on the b-CD core moiety have been recovered, and
there were no degradation of the PCL chains occurred.
The 14-arm star poly(e-caprolactone) (CDSi-SPCL) contains
reactive hydroxyl end groups which can also react with 2-
bromisobutyryl bromide in the next step, it will interfere
with the designed synthetic route of A14B7 multimiktoarm
Synthesis of CD-ACSPCL-Br Macroinitiator for ATRP
To introduce seven hetero arms onto the presynthesized 14-
arm star poly(e-caprolactone) (CD-ACSPCL), the hydroxyl
groups on the b-CD core moiety were functionalized with
2-bromoisobutyryl bromide, which is a convenient initiator
for the ATRP of acrylate monomers. Considering the steric
hindrance of the seven primary hydroxyl groups, large excess
of 2-bromoisobutyryl bromide was used to ensure all the
hydroxyl groups could be capped. The 1H NMR spectrum of
FIGURE 2 SEC traces of cyclodextrin-centered 14-arm star PCL
homopolymers. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
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FIGURE 5 1H NMR spectrum (500MHz) of CD-ACSPCL265-Br in
CDCl3. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
FIGURE 4 1H NMR spectrum (500MHz) of CD-ACSPCL265 in
CDCl3. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
FIGURE 6 1H NMR spectra (500MHz) of CDS(PCL265-PtBA133) (A) in CDCl3 and CDS(PCL265-PAA133) (B) in DMSO-D6. [Color figure
can be viewed in the online issue, which is available at www.interscience.wiley.com.]
A14B7 MULTIMIKTOARM STAR COPOLYMERS, GOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Characterization of the A14B7 Multimiktoarm Star Copolymers [CDS(PCL-PtBA)]a
Time
(min)
Conv
(%)c
Mn,Cal
Mn,NMR
Mn,SEC
Sampleb
Macroinitiator
(g molꢂ1 d
)
(g molꢂ1 e
)
(g molꢂ1 f
)
PDIf
NPtBA
g
CDS(PCL95-PtBA248)
CDS(PCL95-PtBA333)
CDS(PCL185-PtBA93)
CDS(PCL265-PtBA133)
CDS(PCL265-PtBA442)
a
CD-ACSPCL95-Br
CD-ACSPCL95-Br
CD-ACSPCL185-Br
CD-ACSPCL265-Br
CD-ACSPCL265-Br
240
300
180
180
360
31.9
44.3
12.1
16.4
58.5
41,400
52,500
33,800
46,800
84,500
44,600
55,400
34,900
49,100
88,700
31,800h
35,400
28,200
46,300
80,700
1.12
1.13
1.10
1.19
1.34
35.4
47.6
13.3
19.0
63.1
Polymerization conditions: toluene, [tBA]0 ¼1.0 mol/L, 70 ꢀC, [tBA]0/
[CD-ACSPCL-Br]0/CuBr/PMDETA ¼ 100:1:1:1, where [tBA]0, [CD-ACSPCL-
Br]0 represent initial concentrations of tBA and initiation sites on CD-
ACSPCL-Br macroinitiator, respectively.
version (%)þMCD-ACSPCL-Br, where [tBA]0/[CD-ACSPCL-Br]0 equals 100.
e
Calculated from 1H NMR spectra of the miktoarm star copolymers as
2
If
follows: Mn,NMR ¼ ðIc þ Ig ꢂ IfÞ ꢄ ꢄ NPCL ꢄ 128 þ MCDꢂACSPCLꢂBr (see Fig-
ure 6A).
b
f
CDS(PCL95-PtBA248), 95 and 248 represents the average CL and tBA
Determined by SEC analysis with polystyrene standards.
g
repeat units of one CDS(PCL-PtBA) macromolecule, respectively, deter-
The average number of tBA units in each arm of the A14B7 multimik-
toarm star copolymers, calculated from the 1H NMR.
mined by 1H NMR.
c
h
Conversion was calculated on the basis of gravimetric method.
Calculated from the equation: Mn,Cal ¼ ([tBA]0/[CD-ACSPCL-Br]0)ꢄ128ꢄCon-
Mw,MALLS ¼ 48,100, Mn,MALLS ¼ 42,900.
d
the functionalized macroinitiator CD-ACSPCL265-Br was pre-
sented in Figure 5. After esterification, new signals corre-
sponding to methyl protons (Ha) of the bromo ester group
appeared at 1.92 ppm, and the ratio of peak areas of Ha and
Hh (Ia/Ih) is 0.91, which is close to the theoretical value
(1.0). It is therefore demonstrated that nearly all the primary
hydroxyl groups on the b-CD core moiety of CD-ACSPCL have
been substituted with the bromo ester groups. In addition,
the integral values of the PCL arms are not changed, suggest-
ing that no hydrolysis of the PCL arms occurred.
the PCL and PtBA block can be clearly detected. The poly-
merization degree of PtBA arms can be calculated by eq 1:
Figure 7 shows the SEC traces of the A14B7 multimiktoarm
star copolymers [CDS(PCL-PtBA)] in comparison with those
of the original 14-arm star PCL homopolymer precursors. It
reveals that, after ATRP of tBA, all the SEC traces shift to the
higher molar mass region. When the tBA monomer conver-
sion was below 45%, symmetrical unimodal elution peaks
with narrow molecular weight distributions (<1.20) were
observed, indicating the successful synthesis of the CDS(PCL-
PtBA) with well-defined structure. However, when the mono-
mer conversion of tBA reached 58.5% [CDS(PCL265-
PtBA442)], the PDI value of the obtained miktoarm star co-
polymer increased to 1.34, which was tentatively attributed
to a small amount of coupling reactions between the star
copolymers in ATRP process. To further confirm the architec-
ture of the A14B7 multimiktoarm star copolymer, actual
molar mass of one sample [CDS(PCL95-PtBA248), Table 2]
was measured by SEC-MALLS. The Mn,MALLS of CDS(PCL95-
PtBA248) was 42,900, which was close to the value calcu-
lated from the 1H NMR (Mn,NMR ¼ 44,600), indicating the
sample prepared possesses the designed A14B7 structure. It
should be pointed out that some of the miktoarm star
copolymers showed more symmetrical SEC traces with lower
PDIs compared with those of the corresponding star homo-
polymer precursors. These observations may be attributed to
the fact that a small fraction of star PCL homopolymers with
low molecular weight have been removed by repeated pre-
cipitations during the synthesis process of CD-ACSPCL-Br
macroinitiator.
Synthesis of A14B7 Multimiktoarm Star Copolymers of
CDS(PCL-PtBA) by ATRP
The macroinitiator CD-ACSPCL-Br was used for the cop-
per(I)-mediated ATRP of tert-butyl acrylate (tBA) to produce
A
14B7 multimiktoarm star copolymers in conjunction with
PMDETA as a ligand. Previously, Gnanou and coworkers66
reported the controlled polymerization of tert-butyl acrylate
(tBA) by ATRP from an octafunctional initiator and found
that although the star-star coupling was not obvious com-
pared with styrene polymerization, low monomer conver-
sions were still needed. Likewise, some other groups also
reported that well-defined star PtBA prepared by ATRP
should take care of the polymerization conditions such as
reaction temperature, reagent concentrations, and monomer
conversions to minimize intermolecular star-star coupling
reactions.67,68 Therefore, in this article, all polymerization
were conducted in toluene under optimized conditions:
[tBA]0 ¼ 1.0 mol/L, 70 ꢀC, [tBA]0/[CD-ACSPCL-Br]0/CuBr/
PMDETA ¼ 100:1:1:1. where [tBA]0,[CD-ACSPCL-Br]0 repre-
sent initial concentrations of tBA and initiation sites on CD-
ACSPCL-Br macroinitiator, respectively. Relatively low mono-
mer conversions were kept to decrease intermolecular cou-
pling reactions and the lengths of the PtBA blocks were var-
ied through terminating the polymerizations at different time
intervals. Table 2 summarizes the results of these polymer-
izations. The 1H NMR spectrum of a representative A14B7
multimiktoarm star copolymer [CDS(PCL265-PtBA133)] is
shown in Figure 6(A). All the expected peaks attributed to
Thermal and Crystallization Behaviors of 14-Arm Star
Poly(e-caprolactone)s (CDSi-SPCL) and A14B7
Multimiktoarm Star Copolymers [CDS(PCL-PtBA)]
The thermal and crystallization behaviors of CDSi-SPCL
homopolymers and CDS(PCL-PtBA) miktoarm star copoly-
mers were investigated by DSC as shown in Figure 8. In the
case of CDSi-SPCL homopolymers, both melting temperature
(Tm) and crystallization temperature (Tc) increase with the
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FIGURE 7 SEC traces of A14B7 multimiktoarm star copolymers
and their precursors. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
increasing molecular weight. As for miktoarm star copoly-
mers CDS(PCL-PtBA), Tm for PCL segment (Tm,PCL) could be
detected when the molecular weight of PCL segment is high
enough
[CDS(PCL185-PtBA93),
CDS(PCL265-PtBA133),
CDS(PCL265-PtBA442)] and the values of the Tm,PCL and
Tc,PCL are lower than that of the corresponding CDSi-SPCL
precursors. Moreover, not any Tg trace for the PtBA segment
is observed. For the samples with the same molecular weight
of PCL segment [CDS(PCL265-PtBA133) and CDS(PCL265-
PtBA442)], both Tm,PCL and Tc,PCL decrease with the increas-
ing arm length of PtBA segment. These observations clearly
indicate that in A14B7 multimiktoarm star copolymers, amor-
phous PtBA segment highly hinders the crystalline process
of the PCL segment and short PCL segment and PtBA seg-
ment are miscible. However, long PCL segment display strong
crystallization ability, leading to individual Tm and Tc.
FIGURE 8 DSC curves of CDSi-SPCL homopolymers and
CDS(PCL-PtBA) miktoarm star copolymers in the cooling run (a)
and the second heating run (b). [Color figure can be viewed in the
online issue, which is available at www.interscience.wiley.com.]
Synthesis of Amphiphilic A14B7 Miktoarm Star
Copolymers of CDS(PCL-PAA) by Selective Hydrolysis
The tert-butyl groups of the PtBA block were converted into
carboxyl groups by selective hydrolysis with excess of tri-
fluoroacetic acid in dichloromethane to form PAA block69
(Scheme 1). It has earlier been shown that to avoid degrada-
tion of the polyester, short reaction time at ambient tempera-
ture can cleave off the isobutene moieties from PtBA block
completely.70 Nevertheless, in our work, it is observed that
only 70% of the tert-butyl groups had been converted to car-
boxyl form at room temperature for 12 h. This may due to
the fact that the decreasing solubility of the amphiphilic
copolymers in dichloromethane led to some precipitation in
the course of the reaction, making the remained tert-butyl
groups inaccessible for hydrolysis. Therefore, complete hy-
drolysis of the tert-butyl groups had been achieved by
TABLE 3 Characterization of the Amphiphilic A14B7 Miktoarm
Star Copolymers [CDS(PCL-PAA)] Micelles by DLS
Weight
Particle
Size
Fraction
of PAA (%) (nm)a
CMC
(mg Lꢂ1
)
Sample
PDIb
CDS(PCL95-PAA248)
CDS(PCL95-PAA333)
CDS(PCL185-PAA93)
58.3
65.3
22.5
15.6
11.2
0.254 1.73
0.218 2.34
0.238 0.54
0.205 0.56
0.265 0.74
123.7
117.3
40.2
CDS(PCL265-PAA133) 22.9
ꢀ
increasing the temperature to 30 C and prolonging the reac-
CDS(PCL265-PAA442) 49.8
tion time to 72 h. Figure 6(B) shows the 1H NMR spectrum
a
Number-average mean diameters measured by dynamic light scatter-
ing (DLS).
of the amphiphilic A14B7 multimiktoarm star copolymer
1
b
CDS(PCL265-PAA133). Compared with the H NMR spectrum
PDI denotes the polydispersities of nanoparticles in aqueous solution.
A14B7 MULTIMIKTOARM STAR COPOLYMERS, GOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 9 TEM micrography of aggregates from (a) CDS(PCL95-PAA333), (b) CDS(PCL95-PAA248), (c) CDS(PCL265-PAA442), (d)
CDS(PCL265-PAA133), (e) CDS(PCL185-PAA93). The scale bar in (a) (b) (c) represents 100 nm and in (d) (e) represents 200 nm.
of CDS(PCL265-PtBA133) in Figure 6(A), signal at 1.4 ppm
assigned to tert-butyl protons (Ha) has completely disap-
peared and a new signal corresponding to acid proton (Hi)
appears at 12.7 ppm, indicating the removal of the tert-butyl
groups. In addition, the major proton signals of PCL chains
(Hc,d,e,f) remained essentially intact, which suggests the suc-
cessful synthesis of the amphiphilic A14B7 multimiktoarm
star copolymer without any hydrolysis of PCL segments.
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FIGURE 10 TM-AFM height images of (a) and (b) CDS(PCL95-PAA248), (c) and (d) CDS(PCL265-PAA133), (e) and (f) CDS(PCL185-
PAA93). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Self-Assembly Behaviors of Amphiphilic A14B7
Multimiktoarm Star Copolymers of CDS(PCL-PAA)
to the multimiktoarm star architecture. These results indi-
cate that the aggregates in the aqueous solution are thermo-
dynamically stable which can be further used as potential
nanocarriers.
These synthesized A14B7 multimiktoarm star copolymers
possess an amphiphilic structure consisting of hydrophobic
PCL segment and hydrophilic PAA segment, which can form
aggregates sin aqueous solution. The critical micelle concen-
tration (CMC) of amphiphilic copolymer was usually meas-
ured by fluorescence technique using pyrene as a probe.71
Pyrene will preferentially incorporate into hydrophobic
microdomains showing strong fluorescence intensity while
weak fluorescence intensity and a shift of excitation peak in
a polar environment (aqueous solution). In addition, the
sharp rise in intensity ratio of peaks at 338 and 333 nm of
pyrene in the excitation spectra indicates the on-set of micel-
lization (CMC) for amphiphilic copolymers (Supporting Infor-
mation, Fig. S2). The CMC value of these amphiphilic mik-
toarm star copolymers was relatively low (ranging from 0.54
mg/L to 2.34 mg/L), increasing as the weight fraction of
PAA increased, as shown in Table 3, which was probably due
The morphologies of the self-assembled aggregates from
these amphiphilic A14B7 multimiktoarm star copolymers
were investigated by DLS and TEM as shown in Figure 9 and
Table 3. Nearly spherical micelles with number average
mean diameters from 10 to 50 nm were observed when the
weight fraction of PAA segment was ranging from 65.3% to
49.8% for CDS(PCL95-PAA333), CDS(PCL95-PAA248) and
CDS(PCL265-PAA442). The micelle diameters increased as
the decreasing of the weight fraction of hydrophilic PAA
block. However, with further decreasing the weight fraction
of PAA block to 22.9% within CDS(PCL265-PAA133) sample,
large micelles with an average diameter more than 120 nm
were observed. These observations are attributed to the fact
that short hydrophilic PAA blocks could not evade the
A14B7 MULTIMIKTOARM STAR COPOLYMERS, GOU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
hydrophobic interaction and van der Waals interaction
between exposed hydrophobic PCL domains, and the copoly-
mers prefer to form large aggregates to balance these inter-
actions. Nevertheless, in the case of CDS(PCL185-PAA93)
having a similar PAA weight fraction (22.5%) compared with
CDS(PCL265-PAA133), wormlike aggregates were observed.
It should be noted that the sizes of these wormlike aggre-
gates measured by DLS are not in accordance with the TEM
results (Table 3), which is attributed to that the basic equa-
tions to calculate the hydrodynamic diameter in DLS mea-
surement are originated from the correlation function cumu-
lant analysis based on the assumption that the particles are
noninteracting spheres and not anisotropic objects.72 From
these results, we could conclude that both the architecture
of the copolymers and the hydrophilic PAA composition
influence the morphology and dimension of the self-
assembled aggregates. However, the influence of the A14B7
miktoarm star architecture on the morphology of the aggre-
gates is still unclear, which is deserved to be investigated in
the near future.
via the combination of CROP and ATRP from a b-CD deriva-
tive. 1H NMR and SEC-MALLS analyses confirmed the
designed structures. Selective hydrolysis of the PtBA block
by TFA gave the amphiphilic A14B7 multimiktoarm star
copolymers [CDS(PCL-PAA)]. These copolymers could self-
assemble into various morphologies in aqueous solution,
which can be controlled by both the macromolecular archi-
tecture and the composition of the copolymers. AFM mea-
surements implied that the aggregates self-assembled from
these miktoarm star copolymers had highly collapsed struc-
tures on mica surfaces probably owing to the driven force of
the PCL crystallization process. What roles did the A14B7
multimiktoarm star architecture play in the self-assembly
were still unclear, which is currently investigated in our
laboratory.
The authors are grateful to the financial supports from the
National Natural Science Foundation of China (Key program
20434020), special fund from the Major State Basic Research
Project (2005CB623802) and the Committee of Science and
Technology of Zhejiang Province.
To further study the self-assembled morphologies of these
amphiphilic miktoarm star copolymers, tapping-mode atomic
force microscopy (TM-AFM) was used to observe the dry
state of the aggregates on mica surfaces. The copolymers
were drop-cast from water with the concentration of 0.3
mg/mL and dried onto mica surfaces at room temperature
for 48 h. Figure 10 shows the TM-AFM height images of
CDS(PCL-PAA) with different compositions. Generally speak-
ing, the morphologies of these aggregates observed by TM-
AFM were comparable with TEM results, but the dimensions
were not. For CDS(PCL95-PAA248), flat circular structures
with an average diameter of 100 nm was visualized [Fig.
10(a,b)], which was significantly larger than the TEM and
DLS results (ꢃ15 nm). However, the measured average
height of these micelles in dry state was less than 2 nm,
indicating a highly collapsed structure. As for CDS(PCL265-
PAA133) and CDS(PCL185-PAA93), disklike structure [Fig.
10(c,d)] and wormlike structure [Fig. 10(e,f)] with signifi-
cantly greater diameters (widths) than heights (thicknesses)
were observed, which also strongly suggested these aggre-
gates collapsed on mica during the drying process. These
observations could be attributed to the following two factors:
first, the miktoarm star copolymers should probably spread
out on the surface due to similar polarity of the hydrophilic
PAA blocks and mica; on the other hand, it has been estab-
lished that crystallization of PCL occurs by the formation of
folded chain structures within a lamellar lattice, the forma-
tion of collapsed lamellar structures were probably driven
by the crystallization of the PCL blocks during the drying
process.73 Melting temperature of PCL (Tm,PCL) was clearly
detected in the DSC curves of these samples in solid state
obtained by lyophilization (Supporting Information Fig. S3),
which provides convincing evidence for this assumption.
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