Synthesis and encapsulation of an amphiphilic thermoresponsive star polymer with β-cyclodextrin and hyperbranched poly(oligo(ethylene glycol)methacrylate) as building blocks

Article abstract of DOI:10.1039/c4ra10407b

Novel macromolecular star polymers with triazole and cyclodextrin (CD) segments as branch points and poly(oligo(ethylene glycol)methacrylate) (POEGMAs) as dense hydrophilic branches were synthesized via a combination of azide-alkyne click chemistry and at

Full text of DOI:10.1039/c4ra10407b

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PAPER
Synthesis and encapsulation of an amphiphilic
thermoresponsive star polymer with b-cyclodextrin
and hyperbranched poly(oligo(ethylene glycol)-
methacrylate) as building blocks†
Cite this: RSC Adv., 2014, 4, 54268
Yinwen Li,ab Huilong Guo,ab Jian Zheng,ab Jianqun Gan,ab Yan Zhang,ab
a
a
Xiaoxiao Guan,ab Kun Wu and Mangeng Lu
*
Novel macromolecular star polymers with triazole and cyclodextrin (CD) segments as branch points and
poly(oligo(ethylene glycol)methacrylate) (POEGMAs) as dense hydrophilic branches were synthesized via
a combination of azide–alkyne click chemistry and atom transfer radical polymerization (ATRP). Firstly, a
tetrafunctional linking agent (PETP) was prepared by the reaction of pentaerythritol (PE) and propargyl
bromide, and then a four arm b-CD terminated star polymer (PE-CD) was obtained through the click
chemistry reaction. Finally, thermally-responsive star polymers (PE-CD–POEGMAs) with PE as the central
core, triazole and CD segments as branch points, and POEGMAs as side branches were synthesized by
the ATRP of MEO2MA and OEGMA using PE-CD terminated with bromines as macroinitiators. A study on
the thermoresponsivity and morphology of PE-CD–POEGMAs indicated that polymeric nano-aggregates
existed as multimolecular micelles and behaved with tunable thermosensitivity, which were driven by the
strong hydrophobic–hydrophilic interactions in the inner core and outer shell. The encapsulation
capacities towards multi-guest molecules were investigated and the results indicated that water soluble
guests could be encapsulated by PE-CD–POEGMAs, and the guest encapsulation capacities were
derived from the special star molecular structure properties of PE-CD–POEGMAs and the synergistic
encapsulation phenomenon of different guest molecules. This unique amphiphilic star polymer
illustrated the potential applications in supramolecular science, drug delivery and other nanotechnology
applications.
Received 14th September 2014
Accepted 15th October 2014
DOI: 10.1039/c4ra10407b
www.rsc.org/advances
changes of external conditions such as pH, temperature, ionic
strength and light irradiation, among them, thermoresponsive
Introduction
polymers are potentially interesting for a wide range of appli-
cations such as enzyme recycling, protein chromatography,
drug delivery, or tissue engineering.14–22 Classic thermores-
ponsive polymers exhibit aqueous LCSTs include poly(N-iso-
propylacrylamide) (PNIPAAm), poly(N-acryloylpyrrolidine),
poly(vinyl methyl ether), polypeptides, poly(dimethylaminoethyl
methacrylate), which have been by far the most studied and
applied.23–25 Recently, poly(oligo(ethylene glycol)methacrylate)s
(POEGMAs) were reported to possess similar or even superior
thermosensitivity than PNIPAAms. POEGMAs have good phase
transition reversibility and tunable lower critical solution
temperature (LCSTs) in aqueous medium, the biocompatibility
of POEGMAs is also excellent and attractive. To date, POEGMAs
have been widely incorporated into block or star copolymers,
polymeric brushes and hydrogels, which endow these materials
with more interesting properties.26–35 However, to endow star
polymers with thermoresponsivity, thermo-responsive mono-
mers or polymeric parts are usually introduced as building
blocks; PNIPAAms have been successfully incorporated to
Star polymers possess a three-dimensional branched architec-
ture in which chemically different building blocks are linked to
a single junction point. Due to their unique structures, star
polymers usually exhibit specic properties and potential
applications compared with the corresponding linear block
analogues.1–4 Therefore, star polymers have attracted the ever-
increasing attention of both scientists and engineers over the
past several decades and this eld has become a cutting-edge
area of polymer and materials research.5–13
Stimuli-responsive polymers are oen referred to as intelli-
gent polymeric systems because they exhibit reversible property
changes between micellar and unimer states in response to
aKey Laboratory of Cellulose and Lignocellulosics Chemistry, Chinese Academy of
Sciences, Key Laboratory of Polymer Materials for Electronics, Guangzhou Institute
of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, PR China. E-mail:
mglu@gic.ac.cn; liyinwen06@126.com
bUniversity of Chinese Academy of Sciences, Beijing 100039, PR China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra10407b
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various dendrimers or star polymers to form thermo-responsive methacrylate (OEGMA, Mn z 475–500 g molꢀ1) were all acquired
core–shell nanostructures,36–39 but only few works have been from TCI, Japan, and passed through short basic alumina
done on introducing POEGMAs into star polymers especially column in order to remove inhibitor before use. Rose bengal
with cyclodextrins (CDs).
(RB), levooxacin lactate (LL), rhodamine B (Rh) and congo red
Encapsulation is one of the most attractive areas in (CR) were all acquired from Aladdin, China, and their molecular
contemporary supramolecular chemistry and shows great structures are shown in Scheme 1. N,N-Dimethylformamide
potential in drug delivery, medical diagnostics and materials (DMF) was supplied by Sinopharm, China and reuxed over
science. Encapsulation materials include liposomes, chitosan, CaH2 and stored over 4 A0 molecular sieves. All other reagents
crown ethers, and cyclodextrins etc. can be dened as functional were also supplied by Sinopharm, China and used without
polymers which show superior encapsulation properties due to further purication.
their special macromolecule structures and unique physico-
chemical characteristics.40–50 amphiphilic linear block or Synthesis of mono-6-OTs-CD (TCD)
pseudo block polymers are known for their ability to self
b-CD (24 g) was suspended in 180 mL of water, NaOH (2.623 g)
assemble into polymeric nano-aggregates which have been
in 20 mL of water was added dropwise over 30 min, and reacted
considered as good candidates for encapsulation investigations
under vigorous agitation at 0 ꢁC for a period of 1 h. Then
and applications, however, the formed nano-aggregates are
p-toluenesulfonyl chloride (4.032 g) in 20 mL of acetonitrile was
added dropwise over 1 h, causing immediate formation of a
white precipitate. Aer 3 h of stirring at 20 ꢁC ltered off
usually unstable and easily dissociate with altered environ-
mental parameters.51–53 Compared with amphiphilic linear
polymers, amphiphilic nonlinear polymers such as dendrimers,
star polymers, and hyperbranched polymers with branched
architectures can stably exist as unimolecular micelles or
multimolecular micelles in aqueous solution because of their
unreacted toluenesulfonyl chloride, the solution was neutral-
ized by hydrochloric acid ꢁwith a pH of 7–8, and the ltrate
refrigerated overnight at 4 C. The white precipitate was recov-
ered by suction ltration and recrystallization in water for at
unique chemical structures, and their encapsulation behaviors
have been studied and mainly focused on single guest encap-
sulation.54–57 Most recently, the encapsulation phenomena of
multi-guest molecules using amphiphilic hyperbranched poly-
mers was reported by Tian et al.58 However, there are still few
attentions and researches on encapsulation of amphiphilic
thermal responsive star polymer with CDs and hyperbranched
POEGMAs as building blocks for multi-guest molecules.
Our research group previously investigated the synthesis and
supramolecular self-assembly of a variety of stimuli-responsive
POEGMAs with complex architectures. Motivated by recent
attempts in the most attractive areas of encapsulation in
contemporary supramolecular host–guest chemistry, herein the
synthesis and characterization of thermoresponsive star poly-
mers with PE as the central core, triazole and CD segments as
the branch points, and POEGMAs as branches were reported,
and the encapsulation behaviors of this newly synthesized star-
shaped polymers using rose bengal (RB), levooxacin lactate
(LL), rhodamine B (Rh) and congo red (CR) as guest molecules
were investigated in details.
ꢁ
least three times. The sample obtained was dried at 60 C for
48 h in a vacuum oven, and TCD was obtained as a white solid
(yield: z21.40%). FT-IR (KBr, cmꢀ1): 3388 (s, OH), 2928 (w,
CH2), 1597 (s, Ph). 1H NMR (400 MHz, DMSO-d6): d (ppm) 7.42
(2H), 7.74 (2H); 5.65–5.92 (2,3-OHs of b-CD); 4.75–4.83 (1-Hs of
b-CD); 3.28–3.64 (2,3,4,5,6-Hs of b-CD), 2.42 (CH3).
Synthesis of mono-6-deoxy-6-azido-b-cyclodextrin (CD-N3)
CD-N3 was prepared by the azidation of TCD according to
literature procedures with a few modications. A mixture of
b-CD-OTs (6.0 g), dry DMF (20 mL), KI (0.39 g), and NaN3 (3.05 g)
was stirred at 60 ꢁC for 24 h. Aer cooling to room temperature,
the mixture was precipitated into an excess of acetone, then
suction ltration washed with ethanol, cold water and acetone,
separately, last suction ltration and drying for 24 h at room
temperature in a vacuum oven yielded a white powder (yield:
z88%). FT-IR (KBr, cmꢀ1): 2108 (N3). 1H NMR (400 MHz,
DMSO-d6): d (ppm) 5.70–5.95 (2,3-OHs of b-CD); 4.80–4.90 (1-Hs
of b-CD); 4.4–4.6 (6-OH of b-CD); 3.20–4.0 (2,3,4,5,6-Hs of b-CD).
Synthesis of tetrakis(2-propynyloxymethyl)methane (PETP)
Experimental
Materials
Tetrakis(2-propynyloxymethyl)methane (PETP) was prepared as
follows. A round-bottomed ask, equipped with a magnetic stir
b-Cyclodextrin (b-CD) was acquired from Aladdin, China, and bar, was charged with pentaerythritol (2 g, 0.014 mmol) and
puried by recrystallization from water twice prior to use. p-Tol- NaH (12.5 g, 0.22 mmol). Anhydrous DMF (25 mL) was added by
uenesulfonyl chloride (p-TsCl), sodium azide (NaN3), pentaery- a syringe and the reaction mixture was stirred at 5 ꢁC for 30 min.
thritol (PE), and propargyl bromide was purchased from Aladdin, Propargyl bromide (20 g, 0.17 mmol) was slowly added over a
China. N,N,N,N,N-Pentamethyldiethylenetriamine (PMDETA) and 120 min period. The reaction mixture was then heated at 40 ꢁC
2-bromoisobutyryl bromide (BIBB) were was purchased from TCI, for 40 h. Water (100 mL) was added aer cooling, and the
Japan. Copper(I) bromide (TCI, Japan) was washed with mixture was extracted with ethyl acetate. The organic layers
glacial acetic acid in order to remove any soluble oxidized species, were combined, washed with water and then with brine, and
then ltered and washed with ethanol, nally dried under dried over Na2SO4. Removal of solvent by evaporation under
vacuum. 2-(2-Methoxyethoxy)ethyl methacrylate (MEO2MA, reduced pressure le a residue that was puried by silica gel
Mn
¼
188.2 g
molꢀ1), oligo(ethylene glycol)methyl ether column eluting with ethyl acetate to give crude PETP as an
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Scheme 1 The molecular structures of rose bengal (RB), levofloxacin lactate (LL), rhodamine b (Rh) and congo red (CR).
orange liquid, then precipitate with cold hexanes an orange synthesized as follows: to a 25 mL round-bottomed ask in ice
solid was obtained (yield: z72%). FT-IR (cmꢀ1): n 3285, 2111. bath, PE-CD (0.493 g, 0.1 mmol), was dissolved in 30 mL DMF
1H NMR (400 MHz, CDCl3): d (ppm) 4.12 (8H, HCCCH2); 3.54 and cooled to 0 C. 2-Bromoisobutyryl bromide (BIBB, 2.76 g,
ꢁ
(8H, C(CH2)4); 2.4 (4H, HCCCH2). 13C NMR (CDCl3): d (ppm) 12 mmol) dissolved in 10 mL DMF was then added dropwise to
80.0, 74.0, 69.0, 58.7, 44.7.
the solution with magnetic stirring over a period of 1 h at 0 ꢁC.
The reaction temperature was maintained at 0 C for another
ꢁ
2 h and then allowed to rise slowly to ambient temperature aer
which the reaction was allowed to continue for 24 h. The nal
reaction mixture was precipitated in excess diethyl ether. The
white powder precipitate was washed with acetone and water
(v/v, 9 : 1) for at least three times. The puried PE-CD–Br48 was
collected, washed with acetone, and dried under reduced
pressure. Yield: z65%. FT-IR (KBr): 3385 cmꢀ1 (O–H);
1720 cmꢀ1 (C]O). 1H NMR (DMSO-d6, TMS): d (ppm) 7.95 (1H,
methine proton in 1,2,3-triazole); 5.70–5.95 (2,3-OHs of b-CD);
4.90–4.98 (1-Hs of b-CD); 3.31–3.92 (8H, C(CH2)4, overlaps with
2,3,4,5,6-Hs in b-CD); 1.85–1.90 (CH3).
Synthesis of four-arm CD-terminated macromonomer (PE-CD)
The synthesis of four-arm CD-terminated macromonomer was
accomplished by the click reaction between CD-N3 and
tetrakis(2-propynyloxymethyl)methane (PETP). A typical proce-
dure was as follows. CD-N3 (1.12 g), pentaerythritol tetrapro-
piolate (0.0655 g), and CuBr (0.2875 g) were rst dissolved in
20 mL of DMF. The solution was bubbled with nitrogen for 30
min. PMDETA (0.3475 g) was added to the mixture. The mixture
was bubbled with nitrogen again for 30 min and sealed under
ꢁ
nitrogen atmosphere. The reaction was conducted at 60 C for
24 h. The reaction mixture was exposed to air and then
precipitated into an excess of diethyl ether. The crude product
was dissolved in deionized water and dialyzed against deionized
water for 2 days to remove copper catalysts and excess CD-N3.
Aer freezing drying, a white powder was obtained. Yield:
z76%. FT-IR (KBr): 3385 cmꢀ1 (O–H); 2870 cmꢀ1 (C–H). 1H
NMR (DMSO-d6, TMS): d (ppm) 7.95 (1H, methine proton in
1,2,3-triazole); 5.70–5.95 (2,3-OHs of b-CD); 4.80–4.90 (1-Hs of
b-CD); 4.4–4.6 (6-OH of b-CD); 4.28(8H, HCCCH2), 3.54 (8H,
C(CH2)4, overlaps with 2,3,4,5,6-H in b-CD).
Synthesis of star polymer (PE-CD–POEGMAs)
PE-CD–POEGMAs (P1 and P2) were synthesized by the ATRP of
MEO2MA and OEGMA monomers using PE-CD–Brx as the
macroinitiator. A typical procedure for P2 is described as
follows: the Schlenk tube was purged with dry argon for 30
minutes,
a degassed mixture of 2-(2-methoxyethoxy)ethyl
methacrylate (1520 eq.), oligo(ethylene glycol)methyl ether
methacrylate (80 eq.), ethanol (monomers–ethanol ꢂ 1 : 1.5
v/v), PE-CD–Br48 (1 eq.) initiator and copper bromide (57 eq.)
was added to a Schlenk tube, degassed via three freeze–thaw–
Synthesis of four-arm CD-terminated macroinitiator
(PE-CD–Brx)
pump cycles and back-lled with argon. Then 2,2-bipyridyl
ꢁ
PE-CD–Brx (x z 16 and 48) were synthesized by PE-CD and (120 eq.) was added. The mixture was heated at 50 C in an oil
2-bromoisobutyryl bromide. PE-CD–Brx (x
z
48) was bath for six hours. The experiment was stopped by opening the
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ask and exposing the catalyst to air. The nal mixture was aqueous solutions (1 mg mLꢀ1) was recorded at temperatures
diluted in ethanol and passed through a short silica column ranging from 20 ꢁC to 70 ꢁC. The lower critical solution
(60–200 mesh) in order to remove copper catalyst. Then, the temperature (LCST) of the aqueous polymer solution at specic
ltered solution was diluted with deionized water and subse- concentration was determined as the temperature correspond-
quently puried by dialysis in water (Roth, ZelluTrans ing to 10% decrease in the optical transmittance. The hydro-
membrane, molecular weight cut-off: 7000). Last, freeze-drying dynamic diameters (Dh) of the capsules and their polydispersity
in vacuum PE-CD–POEGMAs was obtained as clear viscous indices (PDI) were determined by dynamic light scattering (DLS)
solid. Yield: z43%. The synthetic routes employed for the on a Malven Zetasizer Nano System (Nano-zs90). The emulsions
preparation of PE-CD–POEGMAs are shown in Scheme 2.
were passed through 1.0 mm lters before DLS measurements.
The measurements were conducted in a 3.0 mL quartz cuvette,
using a 670 nm diode laser, and the scattering angle used was
90ꢁ. Each set of Dh and PDI values was the average from ve
measurements. The morphology and architecture of nano-sized
aggregates were further visualized by transmission electron
microscopy (TEM), and the samples were prepared by placing
polymer aqueous solution on copper grids in a biochemical
incubator thermostatted at 25 ꢁC or 50 ꢁC, and stained with
phosphotungstic acid before observation on a JEM-100CX II
microscope operated at 80 kV.
Characterization
Fourier transform infrared (FT-IR) spectra were recorded on a
Nicolet 5100 spectrometer by KBr sample holder method in the
1
fundamental region of 400–4000 cmꢀ1. H NMR spectra were
obtained on a Bruker DMX-400 spectrometer. Deuterated
chloroform (CDCl3), deuterated water (D2O), or deuterated
dimethyl sulfoxide (DMSO-d6) was used as the solvent. The
number-average molecular weight (Mn) and polydispersity index
(Mw/Mn) of each polymer were determined at 35 ꢁC using a
Waters 1515 size exclusion chromatograph (SEC) equipped with
a Waters 2414 refractive index (RI) detector. DMF was used as
Single guest encapsulation
the eluant and the columns used were the styragel HR3 and HR4 The single guest encapsulation of PE-CD–POEGMAs was
columns calibrated by narrow PS standards.
measured by uorescence spectrophotometry. Firstly, RB (5 ꢃ
The LCSTs were determined by UV-vis spectroscopy (U-3010 10ꢀ4 mol Lꢀ1), LL (5 ꢃ 10ꢀ4 mol Lꢀ1), Rh (5 ꢃ 10ꢀ4 mol Lꢀ1) and
Spectrophotometer), and transmittance of the polymeric CR (5 ꢃ 10ꢀ4 mol Lꢀ1) as guest molecules were dissolved in PBS
Scheme 2 The synthetic routes employed for the preparation of four arm star-shaped PE-CD–POEGMAS.
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buffer solution with ionic strength equal to 0.1 mol Lꢀ1 and pH
¼ 7.4, respectively. Then, the PE-CD–POEGMAs solution with
various desired concentrations ranging from 1.0 ꢃ 10ꢀ1 mg
mLꢀ1 to 1.0 ꢃ 10ꢀ7 mg mLꢀ1 were mixed with the above guest
solution, respectively. All mixed solutions were maintained for
more than 24 h to ensure the binding equilibrium and then
stirred prior to measurement.
Multi-guest encapsulation
The multi-guest encapsulation of PE-CD–POEGMAs was
also measured by uorescence spectrophotometry. The
PE-CD–POEGMAs solution with various desired concentrations
ranging from 1.0 ꢃ 10ꢀ1 mg mLꢀ1 to 1.0 ꢃ 10ꢀ7 mg mLꢀ1 were
mixed together with ve multi-guest groups RB + Rh, LL + Rh,
CR + Rh, CR + LL + Rh, RB + LL + Rh, respectively. All mixed
solutions were maintained for more than 24 h to ensure the
binding equilibrium and then stirred prior to measurement. All
the solvent used for the solution preparation was PBS buffer
solution with ionic strength equal to 0.1 mol Lꢀ1 and pH ¼ 7.4.
Fig. 1 FT-IR spectra of CD-N3, PETP and PE-CD.
Next, PE-CD was synthesized by the click reaction between
PETP and an excess of CD-N3. In the 1H NMR spectrum of PE-CD
(Fig. 2(b)), the characteristic signals for b-CD at d ¼ 3.20–4.30,
4.40–4.90, 5.70–5.95 ppm and the proton peak of the 1,2,3-tri-
azole ring at d ¼ 7.95 ppm appeared, indicating the occurrence
of the 1,3-dipolar cycloaddition reaction. Moreover, in the FT-IR
spectrum of PE-CD, the stretching vibration peak of the
hydroxyl reemerged suggesting that CD terminals were bonded
to the PETP-end. Meanwhile, the characteristic absorption of
the azido group at 2104 cmꢀ1 and those of the alkynyls at
2111 cmꢀ1 disappeared, which further conrmed the complete
click and removal of the unreacted CD-N3. The MALDI-TOF-MS
result of PE-CD (Fig. S1 in the ESI†) also showed that the m/z of
PE-CD + xNa+ was 4878.23, which is quite agreement with the
theory molecular weight (4931). Moreover, we also used SEC
measurement to further conrm the molecular weight,
although the molecular weight was somewhat larger than the
actual molecular weight, but the PDI (Mw/Mn) was just 1.07.
Therefore, based on above analysis PE-CD was successfully
synthesized.
Results and discussion
Synthesis of star polymers (PE-CD–POEGMAs)
To obtain PE-CD–POEGMAs, TCD was rst synthesized by using
a b-CD monomer according to our previous work.34 CD-N3 and
tetrakis(2-propynyloxymethyl)methane (PETP) were synthesized
according to literatures with a few modications. Aerwards,
click chemistry was used as a versatile strategy to prepare four-
arm CD-terminated star monomer (PE-CD) because of its high
specicity, high yield, and near-perfect delity. PE-CD was then
sequentially modied using 2-bromoisobutyryl bromide to
obtain macroinhibitors (PE-CD–Brx, x z 16 and 48). Lastly,
PE-CD–POEGMAs (P1 and P2) were achieved by ATRP using
MEO2MA and OEGMA as the monomers and PE-CD–Brx as the
macroinitiators. Polymeric structures of TCD, CD-N3, PETP, PE-
CD, PE-CD–Brx and PE-CD–POEGMAs were characterized by FT-
1
IR, H NMR, SEC, and MALDI-TOF-MS measurements.
Firstly, as shown in Scheme 2, the synthesis of the four-arm
star polymers started from the synthesis of tetrakis-
(2-propynyloxymethyl)methane (PETP). The synthetic methods
of PETP reported in previous literatures are somewhat ambig-
uous and the results are even completely contradictory.60–62 In
order to improve the synthetic method and productivity of
PETP, treatment of pentaerythritol by our modied method
with propargyl bromide and NaH synthesized the target PETP in
good yield. The FT-IR and 1H NMR spectra of PETP are shown in
Fig. 1 and 2(a), separately. The chemical shis at 4.12 (8H,
–OCH2), 3.54 (8H, C(CH2)4), and 2.43 (4H, CH) were attributed to
protons originating from pentaerythritol and propargyl
segments. Moreover, in the FT-IR spectrum of PETP, the
disappearance of the stretching vibration peak of the hydroxyl
group at 3443 cmꢀ1, together with the appearance of the char-
To gra thermoresponsive POEGMAs chains to the CD
terminated PE-CD, selective esterication was rst conducted
on the CD moieties to introduce bromines into PE-CD. BIBB was
used to react with the hydroxyl groups of b-CD. As a result,
PE-CD based macroinitiators bearing several bromrines at the
CD moieties were obtained. The 1H NMR spectra of PE-CD–Brx
are shown in Fig. 3. Signals associated with the 2-bromoisobu-
tyryl residue (–C(CH3)2Br) in PE-CD–Brx were clearly discernible
at d ¼ 1.85–1.90 ppm, indicating that bromines have been
successfully introduced into PE-CD. According to previous
reports, due to the difference in activity among the 2,3,6-OHs
selective modication can be fullled by carefully controlling
the reaction conditions, normally the hydroxyls adjacent to C-2,
-3 and -6 of b-CD (2,3,6-OH) are all able to react with BIBB
during a typical esterication procedure. However, the actual
conversion ratio of this reaction was not up to 100%, because
bromoisobutyryl bromide is very active, and easily react with the
acteristic absorption peaks of alkynyl groups (2111 cmꢀ1
,
3285 cmꢀ1) suggested that the terminal hydroxyl groups of
pentaerythritol completely reacted with propargyl bromide.
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Fig. 2 1H NMR spectra of PETP (a), and PE-CD (b).
residual H2O in the reaction system, in order to maximum could, then added excessive amount of bromoisobutyryl
control the number of hydroxyls of CDs to react with bromo- bromide to PE-CD to ensure the proper ATRP macroinitiators
isobutyryl bromide, we rst reduced the H2O as best as we what we wanted and designed. The best mole ratio of PE-CD to
Fig. 3 1H NMR spectra of PE-CD–Br16 (a) and PE-CD–Br48 (b).
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ꢁ
BIBB was 1 : 1.5x for the synthesis of PE-CD–Brx (x z 16), and 43.71 C, respectively. In contrast to previous reports, the
1 : 2.5x for the synthesis of PE-CD–Brx (x z 48). The number difference for PE-CD–POEGMAs was that the LCSTs were higher
1
average x of Br was calculated to be 15.7 and 47.5 by H NMR than those linear POEGMA copolymers without CD core. This
based on integral ratios of resonance peaks from triazole rings behavior might attribute to star structures and CD cores with
at 7.95–8.0 ppm and –OCO–C(CH3)2Br groups at 1.85–1.90 ppm. multi-POEGMA branches affected the dehydration process of
The synthesis and characterization of PE-CD–Brx are summa- PE-CD–POEGMAs.
1
rized in Table 1. Combined the FT-IR, H NMR, SEC, and the
The potential applications of polymers in biomaterials
MALDI-TOF-MS (Fig. S2 and S3 in the ESI†) results of PE-CD–Brx related elds are known to be generally related to the aqueous
the basic ideal ATRP macroinitiators were synthesized. media. Therefore, the thermoresponsivity and morphology of
Using PE-CD–Brx with 2-bromoisobutyryl active initiations as the synthesized PE-CD–POEGMAs in an aqueous solution were
the macroinitiators, and MEO2MA and OEGMA as comono- observed by DLS and TEM. The typical intensity diameter
ꢁ
ꢁ
mers, star polymers, PE-CD–POEGMAs (P1 and P2) were distributions of P2 at 25 C and 50 C are shown in Fig. 6. At
synthesized via ATRP, and the synthesis and characterization 25 ꢁC, these particles with Z-average diameters of 156.8 nm with
1
are summarized in Table 2. The FT-IR and H NMR spectra of PDI ¼ 0.23 for P2, suggested that the polymer might exist as
PE-CD–POEGMAs are shown in Fig. 4 and 5. In the FT-IR polymeric aggregates rather than unimolecular micelles in
spectrum of PE-CD–POEGMAs (P2), the stretching vibration aqueous solution, which is driven by the strong hydrophobic–
peak of the carbonyl group (C]O) increased compared hydrophilic interactions in the inner core and outer shell. While
with PE-CD–Br48. Moreover, in the 1H NMR spectrum of when the temperature rose above the LCST, it was found obvi-
PE-CD–POEGMAs (P2), although the signals of 2,3,4,5,6-Hs ously that PE-CD–POEGMAs could self-assemble into more
(protons on C-2, C-3, C-4, C-5 and C-6 position of b-CD) overlapped evenly nano-sized aggregates, the basically uniform aggregates
with the Hs of the methylene protons and the methoxyl protons in formed with a Z-average diameter (DZ) of 104.3 nm with a
PEG chains in the region of d ¼ 3.40–4.10. The characteristic narrow PDI of 0.072 for P2.
signals of b-CD, together with that of the 1,2,3-triazole ring, can
To examine visually the size and morphology of
also be observed at d ¼ 4.40–4.85, 5.70–5.95 and 7.95, indicating PE-CD–POEGMAs, the typical TEM image of P2 is presented in
that PE-CD–POEGMAs was successfully synthesized.
Fig. 7. Spherical micelles with a concentration of 0.2 mg mLꢀ1
were found to be uniformly dispersed in the aqueous solution.
These micelles, constructed from PE-CD–POEGMAs, showed
light core surrounded by a black, thin corona, presenting a
Thermoresponsivity and morphology of PE-CD–POEGMAs
Temperature-dependent phase transition behaviors of the
PE-CD–POEGMAs aqueous solutions (1.0 mg mLꢀ1) were
investigated by UV-vis spectroscopy to determine their LCSTs.
PE-CD–POEGMAs were transparent with a characteristic light
bluish tinge at room temperature, and upon heating gradually
turned cloudy, and then the solution became white wholly but
without precipitated, and the LCSTs for P1 and P2 were 41.03 ꢁC
Table 1 Synthesis and characterization of PE-CD and PE-CD–Brx
m/zc
Mn,SEC
PDId
a
b
d
Samples
Mn,theory
Mn,NMR
PE-CD
PE-CD–Br16
PE-CD–Br48
4931
7315
12 083
—
7136
11 306
4878.23
7419.03
12 560.67
5904
8740
19 320
1.07
1.12
1.15
a
b
Calculated in accordance with the theory feed ratio. Calculated in
accordance with the 1H NMR results. The MALDI-TOF-MS results.
c
d
Molecular weights and molecular distributions (Mw/Mn, PDI) were
determined by SEC using DMF as eluent relative to polystyrene
standards.
Fig. 4 FT-IR spectra of PE-CD–Br48 and PE-CD–POEGMAS (P2).
Table 2 Synthesis and characterization of PE-CD–POEGMAs (P1 and P2)
PDIb
LCSTc (ꢁC)
b
Samples
Macroinitiator
nMEO MA : nOEGMA
Mn,SEC
2
a
P1
P2
PE-CD–Br16
PE-CD–Br48
95 : 5
95 : 5
82 747
155 135
1.24
1.28
41.03
43.71
a
a
Synthesized by the ATRP of MEO2MA and OEGMA using PE-CD–Brx as macroinitiators. b Molecular weights and molecular distributions (Mw/Mn,
PDI) were determined by SEC using DMF as eluent relative to polystyrene standards. c Measured with a concentration of 1.0 mg mLꢀ1, temperature
increase at 1 ꢁC minꢀ1 by UV-vis spectroscopy.
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Fig. 5 1H NMR spectrum of PE-CD–POEGMAS (P2).
typical micellar characteristic, and it was apparent that the thermoresponsiveness of the POEGMAs in the shell of micelles
polymers appeared as uniform particles with the diameter of allowed a controlled phase transition into aggregates above
70–120 nm, which was consistent with the DLS analysis. This LCST.63–66
result indicated that these micelles consist of hydrophobic PE
Generally, polymeric micelles can be prepared by adding
and triazole rings as the inner core and hydrophilic POEGMA water to an organic solution of polymers until the required
chains as the external shell. Scheme 3 summarizes the amount of water for micelle formation is obtained. However,
thermally-induced self-assembly behaviors for PE-CD–POEG- previous reports have demonstrated that micelles can also be
MAs. It existed as aggregates in aqueous solutions and the formed by direct dissolved amphiphilic polymers in water.58,59
Fig. 6 Size distributions of the aggregates formed by P2 in aqueous solution (0.2 mg mLꢀ1) at 25 (a) and 50 ꢁC (b), separately.
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Fig. 7 Typical tem images obtained by aqueous solutions of P2 (0.2 mg mLꢀ1) at 25 (a) and 50 ꢁC (b).
In this work, the stable multimolecular micelles could be shown in Fig. 8(b). The CMC was obtained from the intersection
formed by direct dissolved PE-CD–POEGMAs into water. The of the baseline and tangent of the rapidly rising I385/I373, and the
self-assembly process was fast and did not require other special value was determined as 0.0043 mg mLꢀ1 which further
conditions. Therefore, this multimolecular micelle system conrmed as multimolecular micelles in an aqueous solution
prepared by the newly synthesized PE-CD–POEGMAs might be for PE-CD–POEGMAs.
more fascinating. To conrm quantitatively the formation of
multimolecular micelles, the CMC was estimated by uores- Single guest encapsulation capacities and mechanism
cence spectrophotometry using pyrene as a hydrophobic probe.
For PE-CD–POEGMAs, multimolecular micelles consisting of
The excitation spectra of pyrene in the P2 solutions with various
hydrophobic PE and triazole cores and hydrophilic POEGMAs
concentrations are shown in Fig. 8(a). The peak
shells could be easily formed in aqueous solution. Subse-
intensity increased and red shi was all observed with increased
quently, RB, LL, Rh and CR, common hydrophilic guest mole-
cules were chosen for investigating the encapsulation
PE-CD–POEGMAs concentration, indicating the formation of
micelles. The intensity ratio (I385/I373) of the pyrene excitation
properties of PE-CD–POEGMAs in aqueous solution. They are
spectra versus the logarithm of the copolymer concentration is
soluble and do uoresce in water, and once encapsulate inside
Scheme 3 Schematic illustrations of the thermally-induced self-assembly and possible encapsulation behaviors with single or multi-guests for
PE-CD–POEGMAS.
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Fig. 8 (a) Excitation spectra of pyrene as a function of P2 concentrations in PBS buffer (pH ¼ 7.4); (b) plot of I385/I373 against logarithm of P2
concentration.
micelles, their aqueous solutions start to behave uoresce the above uorescence enhancement results indicated the
enhancement or decrease. Except RB and CR have similar interactions of water soluble guests (LL, RB, Rh and CR) with
emission wavelengths, other molecules were selected because the hydrophilic shell of the PE-CD–POEGMAs micelles, and also
their main absorption peaks do not overlap with one another in suggested that LL, RB, Rh and CR could not diffuse through the
mixed aqueous solution. Thus, their absorption intensities can hydrophilic shell and interact with the hydrophobic micellar
be detected by uorescence emission spectroscopy in an core. PE-CD–POEGMAs have large hydrophilic POEGMAs
aqueous solution. The emission wavelengths for RB, LL, Rh and chains and small hydrophobic core, water soluble guest mole-
CR are 414 nm, 450 nm, 581 nm, and 417 nm, separately (Fig. S4 cules were mainly encapsulated in the hydrophilic shell, which
in the ESI†).
resulted in the uorescence enhancement effect.
Fig. 9 represents changes of uorescence intensities of LL
The encapsulation results of P1 with LL, RB, Rh and CR are
and Rh depending on different concentrations of P2, separately. shown in Fig. S6 in the ESI,† It was interesting to observe that
The enhanced uorescence emission intensities of LL or Rh the P1 also showed guest encapsulation capacities, but the
with increase of the polymer concentration indicated that increases of the uorescence intensities with increasing of the
PE-CD–POEGMAs encapsulated LL or Rh in the hydrophilic polymer concentration were obviously lower than that of P2.
shell of PE-CD–POEGMAs micelles. Similar tendency was also While for P2, it showed more obvious increase in the uores-
observed for other guest molecules (RB and CR with P2 in Fig. S5 cence intensity with increasing of its concentration, which
in the ESI†). All the polymeric guests (LL, RB, Rh and CR) indicated that P2 can encapsulate guest molecules more effi-
investigated in this study were revealed a steady increase of the ciently. Although PE-CD–POEGMs had similar molecular
amount of encapsulated even at very low concentration without structure, there were indeed existed differences for P1 and P2
the presence of CMC. However, different from previous report, with 16 and 48 branched POEGMAs. These results indicated
Fig. 9 Fluorescence emission spectra of LL (a) and Rh (b) at pH ¼ 7.4 in the presence of different P2 concentrations.
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Fig. 10 Fluorescence emission spectra of LL + Rh (a), and CR + Rh (b) at pH ¼ 7.4 in the presence of different P2 concentrations.
that the encapsulation capacities were not only dependent on intensities obviously increased with increased polymer
the polymer concentration but also dependent on the polymer concentration. This nding indicated that all guest molecules
molecular structures of PE-CD–POEGMAs.67
were simultaneously encapsulated into PE-CD–POEGMAs.
Thus, the encapsulation property of PE-CD–POEGMAs could be
applied to multi-guest molecule systems.
Multi-guest encapsulation capacities and mechanism
To further investigate the encapsulation capacity and
mechanism of PE-CD–POEGMAs towards multi-guest mole-
cules, consequently, gradual multi-guest encapsulation experi-
ments were conducted.58 Rh was rstly added into P2 solution,
and then LL or CR or RB was added with different amount,
separately. The results of LL and CR are shown in Fig. 12 and RB
is shown in Fig. S8 in ESI.† The results showed that the peak
intensities decreased aer the addition of the other guests LL,
CR, RB, separately. Obviously, the encapsulation for P2 to Rh
was far to the encapsulation balance and saturation, in other
words, when go on adding LL or CR or RB, P2 would go on
encapsulating the new added guest molecules. Therefore, Rh
was encapsulated in the inner layer and behaved uorescence
decrease effect.
Although PE-CD–POEGMAs had excellent single guest encap-
sulation capacities, and as the hydrophilic shell density
increased the encapsulation capacities also increased. Subse-
quently, using the LL, RB, Rh and CR as multi-guest molecules,
one-step double-guests encapsulation experiments were con-
ducted for P2, and the results of LL + Rh, CR + Rh are shown in
Fig. 10 and that of RB + Rh are shown in Fig. S7 in ESI.† The
results indicated that with increased polymer concentration,
the peak intensities of LL + Rh, CR + Rh, RB + Rh solutions
obviously increased. Given the strong peak intensities of LL, the
peak of CR or RB were wholly overlapped, therefore, only using
CR + Rh + LL and CR + Rh + LL as triple-guests, the triple-guests
encapsulation experiments were further conducted and the
results are shown in Fig. 11. Although the peaks of CR or RB
were wholly overlapped with that of LL, and the corresponding
value at the top of the peaks was not observed, the peak
When two guest molecules were simultaneously encapsu-
lated into the P2 solution, then the third one was gradually
added to the double guests system, and the results are shown in
Fig. 11 Fluorescence emission spectra of Rh + LL + RB (a), and RB + LL + Rh (b) at pH ¼ 7.4 in the presence of different P2 concentrations.
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Fig. 12 Fluorescence emission spectra of Rh + P2 solutions with gradual adding of LL (a) or CR (b) solutions at pH ¼ 7.4, respectively.
Fig. 13 Fluorescence emission spectra of CR + Rh + P2 (a), LL + Rh + P2 (b) solutions with gradual adding of LL (a), CR (b) solutions at pH ¼ 7.4,
respectively.
Fig. 13 and in Fig. S9 in ESI.† Taking Rh for example, when Rh + possible encapsulation mechanism of PE-CD–POEGMAs with
CR were simultaneously encapsulated into P2 solution, and single or multi-guests are shown in Scheme 3.
then LL was gradually added, the results showed that the peak
intensities of Rh increased, similar result was obtained for Rh +
LL with gradually added CR. While for CR + LL with gradually
Conclusion
added Rh, the peak intensities of CR + LL behaved puzzled
which was resulted from the overlapped emission wavelengths
of CR and LL.
The possible explanation was that during the guest encap-
sulation process, both guest and host molecules would rear-
In summary, thermoresponsive amphiphilic star polymers,
PE-CD–POEGMAs with PE as the central core, triazole and CD
segments as the branch points, and POEGMAs as branches
were synthesized by the azide–alkyne click chemistry and
ATRP. The thermoresponsivity and morphology of
range to maximize the interactions, and different guest
PE-CD–POEGMAs were studied in details. The thermores-
molecules showed different synergistic encapsulation abilities
ponsive PE-CD–POEGMAs showed tunable LCST behaviors in
based on their molecular properties.36,68 These results
aqueous solution, and stable multimolecular micelles with
further indicated that besides the molecular structures of
the PE and triazole segments as the core and hydrophilic
PE-CD–POEGMAs, the synergistic encapsulation phenomenon
and molecular recognition property were also the driving force.
Thus, the encapsulation capacity and interaction mechanism of
PE-CD–POEGMAs towards multi-guest molecules are derived
from the synergistic encapsulation phenomena of different
guest molecules and the molecular recognition property of the
hyperbranched structure, and the schematic representation for
POEGMAs as the corona could be formed by direct dissolved
PE-CD–POEGMAs into water. The encapsulation capacities of
PE-CD–POEGMAs towards single and multi-guest molecules
were investigated and the results indicated that water-soluble
guests (RB, Rh, LL and CR) could be encapsulated by
PE-CD–POEGMAs. The multi-guest encapsulation capacities
of PE-CD–POEGMAs towards multi-guest molecules
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were derived from the molecular structure properties of 22 S. Yamamoto, J. Pietrasik and K. Matyjaszewski,
PE-CD–POEGMAs and the synergistic encapsulation Macromolecules, 2008, 41, 7013.
phenomenon of different guest molecules. As a result of their 23 F. Sakai, G. S. Chen and M. Jiang, Polym. Chem., 2012, 3, 954.
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Acknowledgements
The authors appreciate the Intergration of Industry, Education
and Research of Guangdong Province project (project no.
2011A091000007), Guangdong-Hongkong Technology Cooper-
ation Finding (project no. 2009A091300012) and National
Natural Science Foundation of China (project no. 20974121).
They also wish to thank professor Yu qijun (South China
University of Technology) for his support and collaboration in
the test involved in this study.
28 Z. X. Zhang, K. L. Liu and J. Li, Macromolecules, 2011, 44,
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