Solvent replacement to thermo-responsive nanoparticles from long-subchain hyperbranched PSt grafted with PNIPAM for encapsulation

Article abstract of DOI:10.1002/pola.26609

Long-subchain hyperbranched polystyrene (lsc-hp PSt) with uniform subchain length was obtained through copper-catalyzed azide-alkyne cycloaddition click chemistry from seesaw macromonomer of PSt having one alkynyl group anchored at the chain centre and two azido group attached to both chain ends [alkynyl-(PSt-N₃)₂]. After precipitation fraction, different portions of lsc-hp PSt having narrow overall molecular weight distribution were obtained for further grafting with alkynyl-capped poly(N-isopropylacrylamide) (alkynyl-PNIPAM), which was obtained via single-electron transfer living radical polymerization of NIPAM with propargyl 2-bromoisobutyrate as the initiator and grafted onto the peripheral azido groups of lsc-hp PSt via click chemistry. Thus, amphiphilic lsc-hp PSt grafted with PNIPAM chains (lsc-hp PSt-g-PNIPAM) was obtained and would have star-like conformation in tetrahydrofuran (THF). By replacing THF with water, lsc-hp PSt-g-PNIPAM was dissolved at molecular level in aqueous solution due to the hydrophilicity of PNIPAM and exhibited thermal induced shrinkage of PNIPAM arms. The water-insoluble lsc-hp PSt would collapse densely and could be served as a reservoir to absorb hydrophobic chemicals in aqueous solution. The influence of overall molecular weight of lsc-hp PSt on the absorption of pyrene was studied.

Full text of DOI:10.1002/pola.26609

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Solvent Replacement to Thermo-Responsive Nanoparticles from Long-
Subchain Hyperbranched PSt Grafted with PNIPAM for Encapsulation
Chen He, Ban-Kun Jin, Wei-Dong He, Xue-Song Ge, Jing Tao, Jing Yang, Sheng-Qi Chen
Department of Polymer Science and Engineering, CAS Key Laboratory of Soft Matter Chemistry, University of Science and
Technology of China, Hefei, Anhui 230026, China
Correspondence to: W.-D. He (E-mail: wdhe@ustc.edu.cn)
Received 24 November 2012; accepted 25 January 2013; published online 14 February 2013
DOI: 10.1002/pola.26609
ABSTRACT: Long-subchain hyperbranched polystyrene (lsc-hp
PSt) with uniform subchain length was obtained through
copper-catalyzed azide-alkyne cycloaddition click chemistry
from seesaw macromonomer of PSt having one alkynyl
group anchored at the chain centre and two azido group
attached to both chain ends [alkynyl-(PSt-N3)2]. After precipi-
tation fraction, different portions of lsc-hp PSt having narrow
overall molecular weight distribution were obtained for fur-
ther grafting with alkynyl-capped poly(N-isopropylacrylamide)
(alkynyl-PNIPAM), which was obtained via single-electron
transfer living radical polymerization of NIPAM with propar-
gyl 2-bromoisobutyrate as the initiator and grafted onto the
peripheral azido groups of lsc-hp PSt via click chemistry.
Thus, amphiphilic lsc-hp PSt grafted with PNIPAM chains
(lsc-hp PSt-g-PNIPAM) was obtained and would have star-like
conformation in tetrahydrofuran (THF). By replacing THF with
water, lsc-hp PSt-g-PNIPAM was dissolved at molecular level
in aqueous solution due to the hydrophilicity of PNIPAM and
exhibited thermal induced shrinkage of PNIPAM arms. The
water-insoluble lsc-hp PSt would collapse densely and could
be served as a reservoir to absorb hydrophobic chemicals in
aqueous solution. The influence of overall molecular weight
of lsc-hp PSt on the absorption of pyrene was studied.
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KEYWORDS: amphiphilic copolymers; hyperbranched; hyper-
branched polymers; nanocarrier; nanoparticles; star polymers;
star-like polymer; stimuli-sensitive polymers; thermal response;
vesicles
INTRODUCTION Amphiphilic star-like copolymer, composed
of a solvent-dislike branched polymer core and solvent-like
linear polymer arms, has received much research attention
due to its unique spatial shape, lower viscosity, single-molec-
ular micellization in selective solvent and possible applica-
tions in encapsulation of active substances.1–4
With the improvement of living radical polymerization tech-
nique, a variety of macromolecules with hyperbranched/
dendritic structure could be synthesize by different facile
approaches.14–19 Single-electron transfer living radical poly-
merization (SET-LRP) is highly efficient to synthesize hyper-
branched/dendritic polymers with well-defined structure
and high molecular weight,14,20–24 by the combination with
TERminator Multifunctional INItiator (TERMINI) or thio-
bromo click methods.14–19 However, hyperbranched/dendri-
tic polymers used in the related literatures are mainly those
prepared from small molecules and the core size is limited.
Hyperbranched and dendritic polymers are mostly used as
the core of star-like copolymers.1,2 They have special physi-
cal and chemical properties because of their highly branched
architecture and the high numbers of functional end groups,1
and can be applied as coatings and resin additives,5 viscosity
modifiers,6 and nano-sized carriers.7 To improve the per-
formances of hyperbranched/dendritic polymers, grafting
linear polymeric chains onto the end groups has been inves-
tigated.1–4,7–12 The obtained star-like copolymers have the
combined architectures of both star polymers and hyper-
branched/dendritic polymers. As well, if a star-like copoly-
mer is made of hydrophobic hyperbranched/dendritic core
and hydrophilic grafting chains, it will have core-shell mor-
phology1,2,7,8 and some special behaviors can be observed,
like single-molecular micellization and supermolecular self-
assembly.8,11–13
Long-subchain hyperbranched (lsc-hb) polymers25–27 have
been frequently used as star-like copolymers.28–32 In 2002,
Dworak et al. synthesized star-like copolymers with a hyper-
branched poly[p- (chloromethyl)styrene] core and poly(ethyl-
ene oxide) (PEO) arms by the core-first approach. The hyper-
branched core was synthesized by p- (chloromethyl)styrene
via atom transfer radical polymerization (ATRP), and the
PEO arms was grafted onto the core via Williamson reac-
tion.28 After that, Kali et al. synthesized star-like copolymers
with a hyperbranched polystyrene (PSt) core and polyisobu-
tylene (PIB) arms by arm-first approach.29 Recently, Zhang
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et al. have synthesized different star-like copolymers through
self-condensing vinyl polymerization based on reversible
addition fragmentation transfer (RAFT) polymerization and
the second RAFT polymerization of other monomers.32 How-
ever, the subchain length of hyperbranched cores above are
not uniform, leading to the difficulty in the investigation of
structure-property correlation. Hence, star-like copolymers
with the defined structures of both hyperbrached core poly-
mer and linear grafting polymer should be synthesized.
Synthesis of Propargyl 2-Bromoisobutyrate (PBIB)
After propargyl alcohol (2.8 g, 0.05 mol), triethylamine
(5.0 g) and CH2Cl2 (30 mL) were added into a 250 mL flask,
the mixture was kept in ice bath. After that, 2-bromoisobu-
tyryl bromide (11.6 g, 0.05 mol) dissolved in CH2Cl2 (20 mL)
was charged dropwise. The reaction stood in ice bath for 1
h, and at 25 ꢀC for additional 7 h. Then, the mixture was
washed by distillated water (20 mL) trice and the organic
layer was dried by anhydrous MgSO4. After the removal of
solvent by rotary evaporation, propargyl 2-bromoisobutyrate
(PBIB) was obtained by vacuum distillation (yield: 65 %).
Thermo-responsive polymers are widely researched and can
be used in many fields like drug uploading,33–41 resulting
from their responsiveness in a control and reversible way to
the temperature.33–45 Poly(N-isopropylacrylamide) (PNIPAM)
is among the most concerning ones.43–47 Lower critical solu-
tion temperature (LCST) of PNIPAM is attributed to the alter-
ation of hydrogen bonding between its amide groups and
water.48–50 Molecular weight of PNIPAM,51 concentration of
PNIPAM aqueous solution,51 and introduction of other com-
ponents52 have the influence on PNIPAM LCST. Up to now,
lots copolymers having PNIPAM segments have been synthe-
sized to study the architecture-dependent thermo-responsive
properties.33–38,40,42–43,45,51–60 As for those containing PSt and
PNIPAM, block copolymers,43,53–54,57 graft copolymers,58,59 and
PSt colloid particles grafted with PNIPAM45,56,60 were studied.
Since PNIPAM segments above are blocked on its chain end(s),
the thermo-responsive behavior is different from that of
PNIPAM homopolymer.
1H-NMR (300 MHz, CDCl3), d (TMS, ppm): 1.95 (s, CH3, 6H),
2.54 (t, CH, 1H), 4.46 (d, CH2, 2H).
Synthesis of Alkynyl-PNIPAM
Alkynyl-capped poly(N-isopropylacrylamide) (alkynyl-PNI-
PAM) was prepared through single-electron transfer living
radical polymerization (SET-LRP) of NIPAM with propargyl
2-bromoisobutyrate as the initiator. Into a 20 mL dry glass
tube with a magnetic stirring bar, PBIB (206 mg, 1 mmol),
CuCl (100 mg, 1 mmol), Me6TREN (950 mg, 0.6 mmol), iso-
propanol (10 mL), and NIPAM (10.0 g) were added. After
mixing thoroughly, the glass tube was degassed by three
freeze-vacuum-thaw cycles, and then sealed under vacuum.
After polymerization was carried out at 25 ꢀC for 3.5 h, the
reaction mixture was taken out of the tube and diluted with
THF. The resulting dispersion was passed through a short
column of neutral alumina to remove metal salt and crude
product was purified by twice THF/ether precipitation. After
being dried under vacuum at room temperature overnight,
alkynyl-PNIPAM was obtained (yield: 54%). Proton nuclear
magnetic resonance (1H NMR) spectroscopy and gel permea-
tion chromatograph (GPC) confirmed the successful synthesis
(Mn,NMR ¼ 5600, Mn,GPC ¼ 5200, Mw/Mn ¼ 1.07).
In this work, star-like copolymers with lsc-hb PSt as the core
and PNIPAM as the grafting chains were prepared and their
thermo-responsive behavior was studied in aqueous solution.
As well, the influence of overall molecular weight of lsc-hb
PSt on the absorption of pyrene was studied.
EXPERIMENTAL
Preparation of Narrowly Distributed lsc-hp PSt with
Peripheral Azido Groups
Materials
N-isopropylacrylamide (NIPAM, 97%, Kohjin Co., Japan) was
recrystallized from benzene/hexane (2:1 v/v) prior to use.
CuBr (Sinopharm Chemical Reagents Co.) was purified as
follows. After crude CuBr was reduced by 0.01 M Na2SO3
aqueous solution, the solid was filtered and washed with 1
wt % HBr aqueous solution. Finally, pure CuBr was obtained
by washing with acetic acid and alcohol trice. Seesaw macro-
monomer of PSt having one alkynyl group anchored at the
chain centre and two azido group attached to both chain
ends [alkynyl-(PSt-N3)2] with Mn ¼ 8800 and long-subchain
hyperbranched PSt (lsc-hp PSt) were synthesized according
to our previous literatures.27 THF was refluxed over CaH2
for 6 h, dried with sodium/benzophenone, and distilled
under reduce pressure before use. Tris(2-(dimethylamino)
ethyl)amine (Me6TREN) was synthesized according to the
literature.61 Triethylamine was stirred with KOH for 12 h at
room temperature, refluxed with toluene-4-sulfonyl-chloride
and distillated before use. Propargyl alcohol (Sinopharm
Chemical Reagents Co.), 2-bromoisobutyryl bromide (Sigma-
Aldrich), N,N,N’,N00,N00-pentamethyldiethylenetriamine (Sigma-
Aldrich), pyrene (Sigma-Aldrich) and other chemical agents
(Aladdin) were used as received.
The fraction of lsc-hp PSt was performed in a thermostatic
ꢀ
water bath (60.2 C). After lsc-hp PSt (5.0 g) was completely
dissolved in toluene (300 mL), methanol was added drop-
wise until the solution became a little white. Then, the solu-
tion temperature was raised to 50 ꢀC so that it turn clear
again. The temperature was slightly lowered to allow a small
fraction of lsc-hp PSt to precipitate and kept at this tempera-
ture for 12 h. The precipitate was collected, filtered and
dried under vacuum overnight. The procedure above was
repeated to obtain the desired fractions with different overall
molecular weight and narrow molecular weight distribution.
Synthesis of lsc-hp PSt Grafted with PNIPAM
by Click Chemistry
Into a 10 mL dry glass tube with a magnetic stirring bar,
narrowly distributed lsc-hp PSt (100 mg), CuBr (33 mg, 0.02
mmol), PMDETA (34 mg, 0.02 mmol), THF (2 mL), and
alkynyl-PNIPAM (100 mg) were added. After thorough mix-
ing, the glass tube was degassed by three freeze-vacuum-
thaw cycles, and then sealed under vacuum. The sealed tube
ꢀ
was immersed into a water bath at 40 C. After polymeriza-
ꢀ
tion stood for 24 h, the tube was rapidly cooled to 0 C. The
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polymer solution was dialyzed at room temperature against
distillated water for 2 days using dialysis tube with cutoff
molecular weight of 8000 to 14,000 g/mol. After vacuum
lyophilization and further dryness under vacuum at room
temperature overnight, lsc-hp PSt grafted with PNIPAM (lsc-
hp PSt-g-PNIPAM) was achieved (yield: 90%).
Absorption of Pyrene by lsc-hp PSt-g-PNIPAM
Pyrene (20 mg) in acetone (1.0 mL) was added into a 50 mL
flask and stood for the complete evaporation of acetone.
After that, lsc-hp PSt-g-PNIPAM (5.0 mg) micelle dispersion
(20.0 mL) was added. The dispersion was kept at room tem-
perature under stirring. After certain durations (2 h, 4 h, 6
h, 8 h, 24 h, and 72 h), the up-layer clear solution (2.0 mL)
was took out to determine the amount of pyrene absorbed
by lsc-hp PSt-g-PNIPAM micelles on a Unico UV/vis 2802PCS
spectrophotometer. After each measurement, the taken-out
micelle dispersion was put back.
Preparation of lsc-hp PSt-g-PNIPAM Micelle Dispersion
Under the room temperature, the deionized water (19.0 mL)
was added into the solution of lsc-hp PSt-g-PNIPAM (5.0 mg)
in THF (1.0 mL) under vigorous stirring by a syringe pump
at 0.1 mL/min. The mixture was stirred for another 6 h and
dialyzed at room temperature against distillated water for
24 h using dialysis tube with cutoff molecular weight of
8000 to 14,000 g/mol to remove THF. The final concentra-
tion of lsc-hp PSt-g-PNIPAM micelle dispersion was adjusted
to 0.25 mg/mL.
RESULT AND DISCUSSION
Star-like amphphilic copolymers with lsc-hp PSt core and
PNIPAM grafting arms were prepared as illustrated in
Scheme 1. Due to PNIPAM grafting chains, lsc-hp PSt-g-PNI-
PAM can be molecularly dissolved in water, that is, form
single-molecular micelles in aqueous solution. As well, it can
exhibit thermal response.
Characterization
1H NMR spectra were recorded on a Bruker DRX-300 NMR
(300 MHz) instrument in CDCl3 at room temperature with
tetramethylsilane as internal standard. Molecular weight and
molecular weight distribution were determined at 30 ꢀC on
a Waters 150C GPC equipped with Styragel columns (103,
104, and 105 Å) and a Waters 2414 refractive index detector.
THF was used as eluent at a flow rate of 1 mL/min. Nar-
rowly distributed PSt standards were used in the calibration
of molecular weight.
Preparation of Narrowly Distributed lsc-hp PSt with
Uniform Subchains
Through click chemistry of seesaw macromonomers, alkynyl-
(PSt-N3)2, lsc-hp PSt with uniform subchain length was
obtained. However, molecular weight distribution of the
products is extremely broad, as shown in Figure 1. After the
precipitation fraction, the narrowly distributed fractions with
different overall molecular weight but uniform subchain
length were obtained. GPC diagrams of the first five fractions
(Fig. 1) are symmetrically mono-modal and polydispersity
index (PDI ¼ Mw/Mn) ranges from 1.20 to 1.30 (Table 1),
suggesting that narrowly distributed lsc-hp PSt with uniform
subchains has been prepared through precipitation fraction.
Thus, Fraction 1 (lsc-hp PSt400) and Fraction 4 (lsc-hp
PSt150) were used to graft with alkynyl-PNIPAM through
alkynyl-azido click reaction.
Laser Light Scattering (LLS) Study
A commercial spectrometer (ALV/DLS/SLS-5022F) equipped
with multi-s digital time correlation (ALV5000) and a cylin-
drical 22 mW UNIPHASE He-Ne laser (k0¼ 632 nm) as the
light source was used. The incident beam was vertically
polarized with respect to the scattering plane. In static LLS
(SLS), the angular dependence of the absolute excess time-
average scattering intensity can lead to the weight-average
molecular weight (Mw,SLS) and the gyration radius (Rg). The
specific refractive index increment (dn/dC) of different poly-
mers was determined by a self-made differential refractome-
ter.62 In dynamic LLS (DLS), Laplace inversion of each meas-
ured intensity-intensity time correlation function G(2)(q,t) in
the self-beating mode can lead to a line-width distribution
G(C), where q is the scattering vector. For dilute solutions,
C is related to the translational diffusion coefficient D by
(C/q2)q!0,C!0 ! D, so that G(C) can be converted into a
hydrodynamic radius distribution f(Rh) via the Stokes-Ein-
stein equation, Rh ¼ (kBT/6pg0)/D, where kB, T, and g0 are
Boltzmann constant, absolute temperature and solvent
viscosity, respectively. The time correlation functions were
analyzed by CONTIN analysis.
Synthesis of lsc-hp PSt Grafted with PNIPAM
by Click Chemistry
Alkynyl-PNIPAM arms were separately synthesized through
SET-LRP of NIPAM with PBIB as the initiator in the presence
of CuCl/Me6-TREN.63 Figure 2(A) shows the IR spectrum of
the PNIPAM arms and their characteristic absorbance of tC¼O
at 1653 cmꢁ1 is obviously present. Figure 2(B) shows 1H-
NMR spectrum of alkynyl-PNIPAM. The signals at 4.01 (f),
2.05 (e), 1.60 (d), and 1.18 (c) ppm can be clearly observed
and are characteristic of PNIPAM block. The signal of alkynyl
proton (a) is overlapped with those from PNIPAM but the
signal (b) of methylene protons from PBIB appears at 4.67
ppm. From the integrate heights of b and c, number-average
molecular weight of alkynyl-PNIPAM by NMR (Mn,NMR) is cal-
culated to be 5600. GPC diagram of alkynyl-PNIPAM suggests
narrow distribution (PDI ¼ 1.07) and Mn,GPC of 5200.
For the structural characterization of ꢀdifferent polymers, the
measurements were performed at 25 C with THF as the sol-
vent. For the study of thermal response, lsc-hp PSt-g-PNIPAM
aqueous dispersions were tested in the temperature range of
25 to 40 ꢀC. All the LLS measurements were carried out
after the temperature reached its equilibrium.
Grafting alkynyl-PNIPAM onto peripheral azido groups of
lsc-hp PSt results in amphiphilic star-like copolymers with
long-subchain hyperbranched PSt as core and PNIPAM chains
as arms. All the overall molecular weight, subchain length of
lsc-hp PSt and PNIPAM arm length are well defined.
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SCHEME 1 Synthesis strategy of star-like amphphilic copolymers with lsc-hp PSt core and PNIPAM grafting chains.
Figure 3(A) shows FTIR spectra of lsc-hp PSt-g-PNIPAM and
its precursors. Before grafting, the spectrum of lsc-hp PSt
shows the absorbance signals at 2094 and 756 cmꢁ1, which
represent tN¼N¼N of the peripheral azido groups of lsc-hp
2(B). NMR spectrum of lsc-hp PSt-g-PNIPAM exhibits the
main signals from both St and NIPAM units, such as methine
proton of NIPAM at 4.01 ppm and aromatic protons of St
around 6.61 ppm. As well, the signal of methylene protons
next to alkynyl group disappears in lsc-hp PSt-g-PNIPAM
spectrum, indicating the removal of probably unreacted
alkynyl-PNIPAM during the purification.
PSt and c
of the phenyl of lsc-hp PSt, respectively. After
¼CH
grafting onto the PNIPAM arms, the signal at 2094 cmꢁ1
disappeared, indicating that azido groups have completely
reacted. Besides, FTIR spectrum of lsc-hp PSt-g-PNIPAM
shows the absorbance at 1653 cmꢁ1, attributed to tC¼O of
PNIPAM, while PSt absorbance bands at 3151, 1494 and 756
cmꢁ1 are still present, indicating that PNIPAM arms have
successfully grafted onto the lsc-hp.
Based on the integrate heights of the signals at 4.01 and
6.61 ppm, the copolymer composition of lsc-hp PSt-g-PNI-
PAM can be calculated. Furthermore, the number of PNIPAM
grafting chains on each lsc-hp PSt molecule (Np) could be
estimated from Mn,GPC of fractioned lsc-hp PSt and Mn,NMR of
PNIPAM. The values are 64 and 24 for lsc-hp PSt400-g-PNI-
PAM and lsc-hp PSt150-g-PNIPAM, respectively.
1H-NMR results also confirm the successful grafting of
alkynyl-PNIPAM arms onto lsc-hp PSt, as shown in Figure
TABLE 1 GPC Characterization of t lsc-hp PSt Before and After
Fraction (Fraction 1–5)
a
a
b
c
sample
Mn,GPC
Mw/Mn
DPw
Mw,SLS
lsc-hp PSt400
lsc-hp PSt310
lsc-hp PSt180
lsc-hp PSt150
lsc-hp PSt90
400 k
310 k
180 k
150 k
90 k
1.30
1.25
1.30
1.20
1.25
83
65
38
31
19
1 M
–
–
390 k
–
a
Mn,GPC and
Mw/Mn were obtained from GPC with refractive index
detector.
b
DPw is the number of alkyny-(PSt-N3)2 seesaw macromonomer in the
FIGURE 1 GPC diagrams of lsc-hp PSt with uniform subchains
fractions and was obtained by Mw,GPC/Mw,macromonomer
Mw,SLS was determined by static light scattering.
.
c
before and after fraction (Fraction 1–5).
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FIGURE 2 FT-IR (A) and 1H-NMR (B, in CDCl3) spectra of different polymers.
polymer concentration, Avogadro’s number and wavelength
of laser light, respectively. Berry plot was used in calculation.
ðMn;GPC
Þ
2A4:01 ꢂ 113
A6:61 ꢂ 104 ðMn;NMRÞhpꢁPSt
Np ¼
ꢂ
(1)
PNIPAM
As summarized in Table 2, Mw,SLS, Rg or hydrodynamic radius
(Rh) for lsc-hp PSt-g-PNIPAM is larger correspondingly than
that for lsc-hp PSt, indicating PNIPAM arms have been
grafted onto the core. As it is well-known, the value of Rg/Rh
reflects the polymer conformation and architecture.64,65 For
hyperbranched polymer, Rg/Rh is between 1.10 and 1.40,66
but it ranges from 1.5 to 1.8 for star polymers.67 The data in
Table 2 demonstrated clear difference in Rg/Rh between lsc-
hp PSt and lsc-hp PSt-g-PNIPAM, suggesting the variation of
hyperbranched architecture to star-like architecture.
where A6.61 and A4.01 are the integrate heights of the corre-
sponding signals, (Mn,GPC hp-PSt and (Mn,NMR PNIPAM are the
related number-averaged molecular weights, respectively.
)
)
To further confirm the formation of star-like architecture for
lsc-hp PSt-g-PNIPAM, static LLS (SLS) and dynamic LLS
(DLS) studies were carried out in THF. In SLS, the measure-
ment was done at each angle for three times with duration
of 30 s. The error was set under 5%. Weight-average molec-
ular weight by SLS (Mw,SLS) and gyration radius (Rg) were
obtained according to eq. 2.
Thermal Responsive Behavior of lsc-hp
PSt-g-PNIPAM Observed by LLS
ꢀ
ꢁ
KC
1
1
With solvent replacement from THF to water at room tem-
perature, aqueous solutions of lsc-hp PSt-g-PNIPAM were
obtained. In the solvent replacement procedure, no sediment
was produced and the final mixtures kept clear, indicating
that lsc-hp PSt-g-PNIPAM would be molecularly dissolved in
water, that is, single-molecular micelles would be formed.
ꢃ
1 þ Rgq2 þ 2A2C
(2)
RvvðqÞ Mw
3
where RVV(q), A2, and q are Rayleigh ratio, the second virial
coefficient, and scattering factor, respectively. The constant,
K, equals to 4p2 (dn/dC)2/(NAk04) with C, NA, and k0 being
FIGURE 3 Temperature dependence of Rh (A) and Rg/Rh (B) for lsc-hp PSt-g-PNIPAM after solvent replacement from THF to water
(Cpolymer ¼ 0.1 mg/mL for DLS; Cpolymer ¼ 0.01, 0.04, 0.07, and 0.1 mg/mL for SLS).
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TABLE 2 Structure Parameters of lsc-hp PSt and lsc-hp PSt-g-
PNIPAM because PNIPAM arms are attached onto hydropho-
bic PSt core.70 Moreover, LCST of lsc-hp PSt400-g-PNIPAM is
a little higher than that of lsc-hp PSt150-g-PNIPAM, probably
attributed to higher density of PNIPAM arms around PSt
core.71
PNIPAM in THF
dn/dC
(mL/g)a
Rg
(nm)b
Rh
(nm)c
b
b
Polymer
Mw,SLS
Rg/Rh
lsc-hp PSt400
0.108
0.104
1.0 M
2.1 M
34
28
1.21
1.53
Absorption of Pyrene by lsc-hp PSt-g-PNIPAM
in Aqueous Solution
lsc-hp PSt400-g-
153
102
PNIPAM
Because of hydrophobic PSt core, lsc-hp PSt-g-PNIPAM could
act a reservoir to encapsulate hydrophobic chemicals in
aqueous solution. Thus, pyrene was used as a model and lsc-
hp PSt-g-PNIPAM (5.0 mg) micelle dispersion (20.0 mL) was
used to absorb pyrene.
lsc-hp PSt150
0.108
0.104
390 k
800 k
18
14
60
1.28
1.63
lsc-hp PSt150-g-
100
PNIPAM
a
The dn/dC value of PSt was determined at 25 ꢀC (k ¼ 633 nm). The dn/
dC value of lsc-hp PSt-g-PNIPAM was estimated by: dn/dC
lsc-hp PSt-g-
Figure 4(A) shows the comparison of Uv-vis spectra of lsc-
hp PSt-g-PNIPAM aqueous solution before and after the
absorption of pyrene. Besides the absorbance band of phenyl
group at 202 nm, those of pyrene appears at 240, 273, and
335 nm, indicating that water insoluble pyrene has transfer
into the core of lsc-hp PSt-g-PNIPAM. Figure 4(B) demon-
strates the variation of absorbed pyrene amount with time.
For both lsc-hp PSt400-g-PNIPAM and lsc-hp PSt150-g-PNI-
PAM, the absorption of pyrene reaches nearly the equilib-
rium at 10 h. The absorption amount of pyrene for lsc-hp
PSt150-g-PNIPAM is a little faster and higher, because its
overall surface area and weight percent of PSt are larger,
which can be estimated from NMR analysis. The saturated
encapsulation amounts of pyrene are 12.5 and 8.5, respec-
tively for lsc-hp PSt150-g-PNIPAM and lsc-hp PSt400-g-PNI-
PAM, being 2.5 and 1.6 times of polymer weights.
¼ w1 ꢂ dn/dClsc-hp
þ w2 ꢂ dn/dCPNIPAM, which w1 and w2 are
PNIPAM
PSt
weight percentage of lsc-hp PSt and PNIPAM, respectively.
b
Angle of SLS measurement ranged from 15ꢀ to 150ꢀ under Cpolymer
0.01, 0.04, 0.07, and 0.1 mg/mL.
DLS measurement was done at 30ꢀ under Cpolymer ¼ 0.1 mg/mL.
¼
c
Comparing the data listed in Table 2 and those shown in
Figure 3, Rh of star-like copolymers with two overall molecu-
lar weights of lsc-hp PSt sharply decreases from 102 and 60
nm in THF to 40 and 19 nm in water for lsc-hp PSt400-g-
PNIPAM and lsc-hp PSt150-g-PNIPAM, respectively. This result
suggests that the core of ls-hp PSt collapses seriously and
might cause the crowded package of PNIPAM arms around
the core. Indeed, Rg/Rh value of lsc-hp PSt-g-PNIPAM drops
to about 1.0 after the solvent replacement, supporting the
suggestion of dense conformation of lsc-hp PSt-g-PNI-
PAM.68,69 In other words, loose star-like conformation of lsc-
hp PSt-g-PNIPAM in THF would be converted into dense
spherical conformation with dense core and crowded arms
in water.
CONCLUSIONS
Through azide-alkyne cycloaddition click chemistry from see-
saw macromonomer of alkynyl-(PSt-N3)2 and precipitation
fraction, lsc-hp PSt with uniform subchain length and overall
molecular weight was obtained. SET-LCR of NIPAM in isopro-
panol mediated with PBIB/CuCl/Me6-TREN produced
alkynyl-PNIPAM, which was grafted onto the peripheral azido
groups of lsc-hp PSt via click chemistry. Thus, amphiphilic
lsc-hp PSt grafted with PNIPAM chains was successfully
prepared and confirmed by different techniques. With the
The thermal responsive behavior of lsc-hp PSt-g-PNIPAM
was studied by SLS and DLS. Due to conformation transition
of PNIPAM arms upon heating, obvious decrease in Rh
ꢀ
appears at 29.6 and 30.2 C for lsc-hp PSt400-g-PNIPAM and
lsc-hp PSt150-g-PNIPAM, respectively. Lower critical solution
temperature (LCST) of PNIPAM homopolymer in aqueous so-
lution is 32 ꢀC, being higher than the value of lsc-hp PSt-g-
FIGURE 4 Comparison of UV spectra of lsc-hp PSt-g-PNIPAM aqueous solution before and after pyrene absorption (A), and de-
pendence of pyrene absorption of lsc-hp PSt-g-PNIPAM on time.
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21 N. H. Nguyen, M. E. Levere, J. Kulis, M. J. Monteiro, V.
Percec, Marcomolecules, 2012, 45, 4606–4622.
solvent replacement, aqueous solutions of lsc-hp PSt-g-PNI-
PAM were obtained accompanying with the sharp decrease
in molecular size and the conformation change. Heating the
aqueous solution of lsc-hp PSt-g-PNIPAM led to the collapse
of PNIPAM arms, resulting in the decrease of macromolecule
size. LCST of lsc-hp PSt400-g-PNIPAM with higher overall mo-
lecular weight of PSt core was observed to be lower, due to
less grafting density of PNIAM arms and the hydrophilicty of
PSt core. This amphiphilc star-like copolymer can be served
as a reservoir to encapsulate hydrophobic chemicals such as
pyrene.
22 N. H. Nguyen, M. E. Levere, V. Percec, J. Polym. Sci. Part A:
Polym. Chem. 2012, 50, 860–873.
23 M. E. Levere, N. H. Nguyen, X. F. Leng, V. Percec, Polym.
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27 C. He, W. D. He, L. W. Li, W. X. Jiang, J. Tao, J. Yang, L.
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