Amphiphilic polymer conetworks prepared by controlled radical polymerization using a nitroxide cross-linker

Article abstract of DOI:10.1002/pola.24156

Tandem atom transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization (NMRP) were used to synthesize a polystyrene-co- poly(acrylic acid) (poly(St-co-AA)) network, in which the two components were interconnected by covalent bond

Full text of DOI:10.1002/pola.24156

Amphiphilic Polymer Conetworks Prepared by Controlled Radical
Polymerization Using a Nitroxide Cross-linker
WEIJIE ZHAO,1,2 MING FANG,1,2 JUNPO HE,1,2 JUNYU CHEN,1,2 WEI TANG,1,2 YULIANG YANG1,2
1Department of Macromolecular Science, Fudan University, Shanghai 200433, China
2Key Laboratory of Molecular Engineering of Polymers at Fudan University, Ministry of Education, Shanghai 200433, China
Received 19 March 2010; accepted 24 May 2010
DOI: 10.1002/pola.24156
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Tandem atom transfer radical polymerization (ATRP)
and nitroxide-mediated radical polymerization (NMRP) were used
to synthesize a polystyrene-co-poly(acrylic acid) (poly(St-co-AA))
network, in which the two components were interconnected by
covalent bond. First, a specific cross-linker, 1,4-bis(10-(400-acryloy-
loxy-200,200,600,600-tetramethylpiperidinyloxy)ethyl)benzene (di-AET),
a bifunctional alkoxyamine possessing two acrylate groups, was
copolymerized with tert-butyl acrylate through ATRP to prepare a
precursor gel. The gel was then used to initiate the NMRP of sty-
rene to prepare poly(St-co-(t-BA)) conetwork, in which the cross-
linkages are composed of polystyrene segments. Finally, the
poly(St-co-(t-BA)) conetwork was hydrolyzed to produce amphi-
philic poly(St-co-AA) conetwork. The resulting gels show swelling
ability in both organic solvent and water, which is characteristic
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of amphiphilic conetworks. 2010 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 48: 4141–4149, 2010
KEYWORDS: amphiphiles; atom transfer radical polymerization;
conetworks; networks; nitroxide-mediated radical polymeri-
zation
INTRODUCTION In recent years, a number of macromolecu-
lar architectures which are otherwise difficult to control
were synthesized via controlled/‘‘living’’ free radical poly-
merization such as nitroxide-mediated radical polymerization
(NMRP),1–3 atom transfer radical polymerization (ATRP)4–6
or sulfonyl halides-initiated living radical polymerization,7
and reversible addition fragmentation chain transfer
(RAFT).8 One of the merits of these controlled systems is
that they allow initiator design to synthesize polymers with
desired structures, as originally demonstrated by the pioneer
work on unimolecular initiator alkoxyamine.9 The functional-
ity of the initiator is retained at the chain end, forming
telechelic polymers of which the end groups can be further
used, sometimes after transformation, to initiate new
segments through different polymerization mechanisms.10
Especially, initiator or mediating agents bearing a vinyl
group have been used to prepare hyperbranched/dendritic
polymers.11–16
mer chains.24 Our specific interest is amphiphilic conetwork,
in which one component is hydrophilic and the other is
hydrophobic. In this field, Kennedy and coworkers25 and
Patrickios et al.26 have synthesized amphiphilic graft/block
polymers with living chain ends and used sequential living
polymerization/cross-linking to prepare a number of amphi-
philic conetworks.
We hereby report the synthesis of polymer conetworks
of polystyrene and poly(acrylic acid) using
a bifunc-
tional alkoxyamine, 1,4-bis(10-(400-acryloyloxy-200,200,600,600-
tetramethylpiperidinyloxy)ethyl)benzene, hereafter coded as
di-AET. The synthetic process involves tandem ATRP of t-BA
and di-AET, and NMRP of styrene initiated by the alkoxy-
amine moiety in di-AET units (Scheme 1). The feature of this
method is that the second component of the conetwork is
formed by chain extension of the precursor gel. After hydro-
lysis of the P(t-BA) segments, the conetworks are
transformed into amphiphilic conetworks. In the product,
cross-linking point is formed by alkoxyamine moiety, which
is thermally reversible. It should be mentioned that the
reversibility of CAOAN bond in alkoxyamine has been used
in the synthesis of conetworks,27 block copolymers,28,29
shell–cross-linked nanospheres,30 and comb-like copoly-
mers.31 The labile bond is stable in the mild hydrolysis
reaction of tert-butyl esters developed by Wooley and
coworkers.30 The conetwork in the study by Otsuka and
Takahara27(a) is synthesized through a different way, in
Polymer conetworks are two-component networks in which
the polymer segments of different chemical nature are inter-
connected by covalent bond. Well-defined polymer conet-
works have been synthesized through controlled/‘‘living’’
radical copolymerization of vinyl and divinyl monomers.17–20
According to Kennedy’s classification,21 polymer conetworks
can be synthesized either by radical copolymerzation/cross-
linking of monomers and telechelic macromonomers22,23 or
by chemical combination/cross-linking of two kinds of poly-
Correspondence to: J. He (E-mail: jphe@fudan.edu.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4141–4149 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic route of amphiphilic conetworks via sequential ATRP and NMRP.
which a thermodynamic network is formed by mixing two
kinds of linear polymers with pendent alkoxyamines.
as the eluent at a flow rate of 1 mL/min at 40 ꢀC. The
columns were calibrated by narrow polystyrene (Mw range:
200–3 ꢁ 106 g/mol) standards. Fourier transform-infrared
(FT-IR) spectroscopy was performed on a Magna-550 instru-
ment (KBr pellet). Element analysis of C, H, and N was
carried out on an ELEMENTAR VARIO EL instrument. 1H
NMR measurement was carried out on a Bruker DMX500
instrument, using CDCl3 as solvent and tetramethylsilane
as the reference. Mass spectra (EI) measurement was
performed on a HP5989A instrument. Differential scanning
calorimetry (DSC) analyses were performed on a TA DSC
Q2000 instrument over the temperature range ꢂ20 to
150 ꢀC under nitrogen with a scan rate of 10 ꢀC/min. Glass
transition temperatures (Tg) were obtained after a heating/
cooling cycle using the midpoint method. Monomer
EXPERIMENTAL
Materials
Styrene (Yonghua Special Chemicals, Shanghai, China, 99%) was
passed through Al2O3 and silica gel columns, respectively, dis-
tilled under reduced pressure and stored under argon. tert-Butyl
acrylate (t-BA; Lancaster, Heysham, UK, 99%) was distilled
under vacuum over CaH2. Tetrahydrofuran (THF; Shanghai Feida,
Shanghai, China, 99.5%) was refluxed over sodium and distilled.
Acryloyl chloride (AC; Haimen Fine Chemicals, Haimen, China,
99%), dichloromethane (CH2Cl2; Shanghai Dahe, Shanghai,
China), N,N-dimethyl formamide (DMF; Shanghai Chemical Rea-
gent Co., Shanghai, China, 99%), xylene (Shanghai Feida, Shang-
hai, China, >80%), and trifluoroacetic acid (TFA; Sinopharm,
Shanghai, China, 99%) were used as received. Copper(I) bromide
(Runjie Chemicals, Shanghai, China, 99%) was purified by wash-
ing with acetic acid, absolute ethanol, and ethyl ether and then
dried under vacuum. Triethylamine (Sinopharm, Shanghai, China,
99%),
4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxy
(HO-
TEMPO, 1, Aldrich, St. Louis, USA, CP), N,N,N0,N00,N00-pentamethyl-
diethylenetriamine (PMDETA; Aldrich, St. Louis, USA, 99%), 2-
bromopropionate (BMP; Aldrich, St. Louis, USA, 99%), n-bromo-
succinimide (NBS; Lancaster, Heysham, UK, 98%), and 1,4-dieth-
ylbenzene (2, ACROS, New Jersy, USA, 98%) were used as
received.
Measurements
Mn and Mw/Mn of all samples were measured by gel permea-
tion chromatography (GPC) through three Waters Styragel
columns (pore size: 102, 103, and 104 Å) in a Waters 2010
system equipped with a Waters 410 RI detector, using THF
SCHEME 2 Synthesis of di-AET.
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Al2O3 column (ethyl acetate/petroleum ether 1:5 v/v). The final
product was a white powder, yield: 60%. The structure was
confirmed by IR, MASS, Element Analysis, and 1H NMR (Fig. 1).
1H NMR (500 MHz, CDCl3) d(ppm) ¼ 0.60, 1.11, 1.27, and 1.34
(each br s, 24H, CH3), 1.48 (complex m, 6H, CH3,d, 2H, CH2
piperdine ring), 1.74–1.88 (d, 6H, CH2), 4.75 (d, 2H, CH), 5.07
(q, 2H, CH), 5.79 (d, 2H, CHH¼¼CH), 6.08 (q, 2H, CH2¼¼CH), 6.37
(d, 2H, CHH¼¼CH), and 7.25 (s, 4H, ArH). MS (EI): m/e ¼ 387,
201, 124, 109, 77, 61, 55, 44, 41. IR (KBr): 1720 (C¼¼O) and
1372 (NAO) cmꢂ1. Anal. calcd for C34H52N2O6: C, 69.83;
H, 8.96; N, 4.79. Found: C, 70.02; H, 8.82; N, 4.45.
Synthesis of Precursor Gel by Atom Transfer
Copolymerization of t-BA and di-AET
t-BA (3.50 g, 27.3 mmol), di-AET (varying amount), and CuBr
(39.1 mg, 0.27 mmol) were added to a glass tube. The feed ratio
of [t-BA]0/[di-AET]0 was varied from 30 to 80 as shown in Ta-
ble 1. The tube was sealed with a rubber septum, deaerated,
and back-filled with nitrogen three times. PMDETA (50.1 mg,
0.29 mmol) was added, and the solution was stirred until the
FIGURE 1 1H NMR spectra of di-AET (Solvent: CDCl3).
conversions after gel point were determined by thermogravi-
metric analysis (TGA) on a NETZSCH TG209 instrument. The
Cu complex formed at r.t.34 The tube was heated at 70 C, and
ꢀ
ꢀ
temperature was elevated from 25 to 550 C at a rate of 20
BMP (10.0 mg, 0.55 mmol) was added. The reaction was kept at
ꢀ
ꢀC/min. The weight loss above 275 C gave the polymer con-
ꢀ
70 C for 60 h and stopped by immersing the tube into liquid
tent. The sol-gel was extracted twice by Soxhlet apparatus
using THF/H2O (90:10 v/v, for precursor gel),ꢀ or THF (for
polymer conetworks) at the temperature of 90 C, each time
72 h. The swelling ratio of gel was measured at different
temperatures. Dry gel samples (ꢃ0.05 g) were placed in 5
mL solvent (xylene, water, or DMF mixed with CH2Cl2/water)
in a 20 mL flask and kept for 72 h. The swollen gel was blot-
ted with filter paper to remove surface solvent and weighed.
N2. The gel point was defined as that no flow occurred when
the sample tube was inverted. The final conversions at 60 h and
the gel fraction (rgel) are shown in Table 1.
Determination of the Gel Fraction
The gel fraction (rgel) is calculated by Wdry/Wpolymer, in
which Wpolymer is the total weight of polymer determined by
TGA. The dry gel was obtained by twice extraction in Soxhlet
apparatus using THF/H2O (90:10 v/v) at the temperature of
The swelling ratio was defined as Wswollen/Wdry
.
ꢀ
90 C, each time 72 h.
Synthesis of di-AET
4-Acryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxy (5.50 g,
24 mmol) (3, synthesized according to the literature32), 1,4-
bis(10-bromoethyl)benzene (1.46 g, 5 mmol) (4, synthesized
according to the literature33), CuBr (1.32 g, 10 mmol), and
Cu(0) (310 mg, 5 mmol) were dissolved in THF (25 mL) in a 50
mL flask, and the solution was deaerated with three cycles of
freezing-pump-thawing. Then PMDETA (3.46 g, 20 mmol) was
added into the flask and the solution turned to dark green in
color. The mixture was kept at room temperature for 48 h. The
resulting compound was diluted with THF, passed through
Synthesis of Polymer Conetworks of Polystyrene (PS)
and P(t-BA) by NMRP
The obtained precursor gel (170 mg) was swollen in styrene
(3.65 g, 35 mmol) for 24 h. The mixture was deaerated
through three cycles of freezing-pump-thawing, then heated
at 120 ꢀC for different periods. The reaction was stopped
by liquid N2. After being extracted by THF, the gel was
dried under vacuum. The overall yields of gels are shown in
Figure 5. For GPC measurement, the polymer conetworks
14
ꢀ
were cleaved by phenyl hydrazine at 120 C for 24 h.
TABLE 1 Synthesis of Precursor Gel by Atom Transfer Radical Copolymerization of t-BA and di-AETa
[t-BA]0/
Gel Point
(min)
Final
Conversionb
rgel
Swelling Ratio
(50 ꢀC, g/g)
c
Tc (ꢀC)
Run
[di-AET]0
(gel fraction)
1
2
3
4
a
80
70
50
30
1140
710
280
240
90.0%
93.1%
91.6%
96.0%
0.50
0.47
0.80
0.98
/
40–45
55–60
65–70
85–90
15.92
8.38
3.37
Experimental conditions: [t-BA]0/[BMP]0/[CuBr]0/[PMDETA]0 ¼ 50/1.0/
0.50/0.53. All reactions were stopped after 60 h to reach the complete
conversion of monomers.
from 25 to 550 ꢀC at a rate of 20 ꢀC/min. The weight loss above 275 ꢀC
gave the polymer content.
c
Tc is defined as the critical temperature when the swelling system (in
xylene) becomes fluidic.
b
Final conversions were determined by thermogravimetric analysis
(TGA) on a NETZSCH TG209 instrument. The temperature was elevated
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 Synthesis of precursor gel by atom transfer radi-
cal copolymerization of di-AET and t-BA, before (a) and after
(b) gel point. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Synthesis of Amphiphilic Conetworks by Hydrolysis
of Polymer Conetworks, Poly(St-co-(t-BA))
Polymer conetwork poly(St-co-(t-BA)) (2.54 g) was swollen
in CH2Cl2 (50 mL) for 8 h. TFA (3.91 g, 34.3 mmol) was
added, and the mixture was stirred at room temperature for
8 h. Then, the solvent was evaporated. Tꢀhe gel was washed
with THF, and dried under vacuum at 30 C. Yield: 70.2%.
RESULTS AND DISCUSSION
The synthetic route is outlined in Scheme 1. First, precursor
gels are prepared by ATRP of t-BA using di-AET as the cross-
linking agent. It has been reported that the polymer co-
network prepared in living radical polymerization system is
more homogenous than that in the conventional polymeriza-
tion system.17(a) In this case, the distribution of di-AET units
among primary chains should be uniform because of living
feature of ATRP in the first step. Then the alkoxyamine
groups in the precursor gel initiate NMRP of styrene to
prepare the polymer conetworks, poly(St-co-(t-BA)). Finally,
the conetwork is hydrolyzed to prepare an amphiphilic
conetwork, poly(St-co-AA).
FIGURE 3 (a) Evolutions of GPC curves at the initial stage of syn-
thesizing precursor gel by atom transfer radical copolymeriza-
tion of di-AET and t-BA, and the reference signals of t-BA (5.90%,
w/w) and di-AET (1.25%, w/w) are also provided. (b) Dependence
of ln([M]0/[M]) on reaction time. (c) Conversions relationship
between t-BA and di-AET during the reaction process. Experi-
mental conditions: [t-BA]0/[di-AET]0/[BMP]0/[CuBr]0/[PMDETA]0
¼ 50/1.0/1.0/0.50/0.53, bulk, at 70 ꢀC. Narrow polystyrene stand-
ards for THF GPC calibration. The conversion was calculated by
the integration of area of the separate GPC peaks of the mono-
mer. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
di-AET was synthesized by atom transfer radical coupling re-
action according to the process developed by Matyjaszewski
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ARTICLE
et al.35 The reaction is performed at room temperature so
that the vinyl group does not polymerize to give polymer
species in this stage. The yield is rather high despite of the
bifunctionality. The characterization results of the product
The above experiment was conducted in the presence of air
where the oxygen captures the carbon-centered radical
efficiently, therefore the dissociation of alkoxyamine is irre-
versible and the resulting fluid cannot gel again through
recombination of alkoxyamine. The results of Otsuka and
Takahara27 indicate that the presence of an inert atmosphere
such as N2 is necessary to keep the alkoxyamine active. In
the next section, the network extension is carried out under
nitrogen atmosphere, therefore the network can hold through-
out the process despite much higher temperature than Tc.
1
are given in the experimental section. Figure 1 shows the H
NMR spectrum of di-AET. Both aromatic and vinylic protons
are clearly observed at chemical shift of 7.24 and 5.8–
6.4 ppm, respectively, with integration value 4:3. The
benzylic methine gives a quadruplet at 4.75 ppm, whereas
the methine of piperdine ring gives a pentat at 5.1 ppm. It
should be noted that the methyl signals from diethylbenzene
and one group of methylene of the piperidine ring overlap at
1.48 ppm.
Synthesis of Polymer Conetworks, the
Poly(St-co-(t-BA)) Gel
The cross-linking point in the precursor gel consists of
bifunctional alkoxyamine, an initiator for NMRP of styrene.
Therefore, the extracted gel is swollen in styrene and then
heated at 120 ꢀC. The polymerization readily starts as indi-
cated by the yield of the gel for different periods. The gel
can be analyzed after becoming fluidic by reaction with
phenyl hydrazine, a reducing agent that breaks the alkoxy-
amine cross-linkage.10 Figure 5 shows the GPC traces for
cleaved samples taken at different polymerization reaction
times. The molecular weight increases along with reaction
time, a feature of controlled radical polymerization (Samples
a, c, d, e and g). The traces also indicate the influence of
alkoxyamine contents in precursor gel on the NMRP of
styrene (Samples a, b, e and f). The lower Mn of Sample f is
because of the higher amount of alkoxyamine than Sample e
which serves as an initiator in NMRP of styrene in precursor
gel (Sample b). However, the molecular-weight distribution
is notably broad. This is not only because the polymerization
is heterogeneous but also because the nitroxyl radical is
attached to polymer chains and thus exhibits lower
Preparation and Characterization of the Precursor Gel
The precursor gel was synthesized by ATRP. The reaction
temperature is 70 ꢀC, at which alkoxyamine is stable. The
copolymerization is performed under normal ATRP condi-
tions. Initially the reaction proceeds steadily in a fluidic mix-
ture, then gelation occurs suddenly, as shown in Figure 2.
The polymerization at the initial stage is monitored by GPC.
Figure 3(a) shows broad molecular-weight distribution
because of the branching reaction caused by polymerization
of di-AET. Fortunately, the signals for di-AET and t-BA can be
observed separately in GPC profiles, with only slight overlap
of t-BA with another unknown side species. Therefore, we
obtain the conversion-time plots [Fig. 3(b,c)] by the integra-
tion of peak area calibrated by the corresponding pure
compounds (monomer and di-AET). The larger consumption
rate of di-AET in relative to monomer is simply because of
to the bifunctionality. Along with the reaction time, t-BA
exhibits a marginal increase in conversion rate [Fig. 3(c)].
The above obtained gel is dark green in color. It is then
extracted by THF/H2O (90:10 v/v) to obtain a colorless
sample. The extracted gel is used to perform swelling study.
The results are listed in Table 1 for polymerization systems
with varying amount of di-AET. Clearly, with larger amount
of cross-linker, the time for gelation is shorter, gel fraction in
the final product is higher, and the swelling ratio is lower,
indicating higher density of cross-linking. All these features
agree with traditional Flory-Stockmayer gel theory.36
The unique character of the gel is that di-AET forms dynamic
cross-linkages because of the ability of dissociation of alkoxy-
amine. A gel–sol transition is observed in swelling study as
shown in Figure 4. At lower temperatures the swelling ratio
is stable. When the gel is swollen at rising temperature, the
swelling ratio slightly increases because of partial dissocia-
tion of alkoxyamine. At this point, the structure of the
network is not destroyed, but the mesh increases in size.
When temperature reaches a critical value (Tc), large part of
the cross-linking alkoxyamine is broken. The network cannot
hold and the system becomes fluidic. Table 1 shows clearly
the dependence of critical temperature on the feed ratio of
di-AET to monomer. In general, larger amounts of di-AET
lead to higher critical temperature because more cross-link-
ings are formed.
FIGURE 4 The swelling curves in xylene of different precursor
gels and polymer conetworks at different temperatures. Sam-
ple: (n) a, precursor gel 4, feed ratio: [t-BA]0/[di-AET]0 ¼ 30. (D)
b, precursor gel 3, feed ratio: [t-BA]0/[di-AET]0 ¼ 50. (l) c, pre-
cursor gel 2, feed ratio: [t-BA]0/[di-AET]0 ¼ 70. (~) d, polymer
conetworks, precursor gel 3 polymerized with styrene for 12 h.
(!) e, polymer conetworks, precursor gel 3 polymerized with
styrene for 60 h.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
work, nevertheless, TFA is used as hydrolyzing agent, a mild
method developed by Wooley and coworkers.38 The polymer
conetwork is hydrolyzed for 8 h in a swollen state (CH2Cl2),
and the product (amphiphilic conetworks) is washed by THF
and dried under vacuum. For GPC measurement, the amphi-
ꢀ
philic conetwork was cleaved by phenyl hydrazine at 120 C
for 24 h. Figure 5 shows a slight decrease in molecular
weight after hydrolysis (h, Mn ¼ 84,900 g/mol, Mw/Mn
¼
1.57) than the original sample (g, Mn ¼ 92,600 g/mol, Mw/
Mn ¼ 1.49).
DSC analyses have been carried out for the precursor gel,
the polymer conetworks poly(St-co-(t-BA)) and the amphi-
philic conetworks poly(St-co-AA) (Fig. 6). The glass transition
temperature of the precursor gel (a) is 56.2 ꢀC which is
obviously higher than that in the literature,39 43 ꢀC, and is
attributed to the reduced mobility of P(t-BA) segments ꢀin
the network. The Tg of the polymer conetwork (b) is 84.5 C,
FIGURE 5 GPC traces for cleaved precursor gel (a,b), cleaved
polymer conetworks (c–g) and cleaved amphiphilic conetwork
(h). Precursor gel samples were taken for different feed ratios,
N (N ¼ [t-BA]0/[di-AET]0), polymer conetwork samples were
taken at different polymerization times, tp. The overall yields of
the gels are defined as percentage of styrenic units incorpo-
rated into the gel over styrene feed amount (measured after
extraction). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
an intermediate value between the literature Tgs of PS
39
ꢀ
ꢀ
(100 C) and P(t-BA) (43 C). The measured value is close
to theoretical value, 84.8 ꢀC, calculated from Fox equation
(eq. 1). However, the explanation is tentative because the co-
network is segmented copolymer of styrene and t-BA. Micro-
phase separation of the two components may be prevented by
the cross-linking. For amphiphilic conetworks (c), the Tgs of
39
ꢀ
ꢀ
PAA (105 C) and PS segments overlap around 110.3 C.
WPðt-BAÞ
Tg Tg;PS Tg:Pðt-BAÞ
1
WPS
¼
þ
(1)
diffusibility to cap propagating radicals, leading to partially
uncontrolled polymerization. In addition, in the measure-
ment, the cleaved samples for GPC analysis contain both PS
and P(t-BA), the latter is the skeleton of the precursor gel
and does not grow in network extension reaction. This small
part of P(t-BA), which is located at Mn ꢃ 104 g/mol as
determined using cleaved precursor gel (Curves a and b),
also contributes to the broad molecular-weight distribution.
Figure 7 shows the FT-IR spectra of precursor gel P(t-BA)
(a), polymer conetworks poly(St-co-(t-BA)) (b) and hydro-
lyzed product P(St-co-AA) (c). The characteristic bands of
tert-butyl ester are clearly observed in Curves a and b,
centered at 1369, 1394 cmꢂ1 [d(CH3) of tert-butyl] and
1730 cmꢂ1 [m(C¼¼O) of carbonyl]. After NMRP of styrene, PS
After the network extension, the precursor gel becomes a
conetwork of PS and P(t-BA). The mesh size increases
because the cross-linking point becomes longer from the
initial small molecular alkoxyamine to polystyrene chains.
This can be proved by the change of swelling ratio of pre-
cursor gels and polymer conetworks. As shown in Figure 4,
the swelling ratios of the latter are several folds of those of
the former. In addition, larger swelling ratio is observed
when more styrene has been incorporated into the gel
(extending the polymerization time), which can be seen by
comparing Sample d and e in Figure 4. Besides, the exchange
between thermally generated PS which is monofunctional37
and PS radical cleaved from the network leads to the forma-
tion of dangling PS chains. The coupling termination between
PS radicals forms longer cross-linkages and reduces number
of cross-linkages. These factors also result in decreasing
cross-linking density and increased swelling ratio.
FIGURE 6 DSC curves of networks. Sample: (a): precursor gel,
P(t-BA), [t-BA]0/[di-AET]0
¼
50; (b): polymer conetworks,
Synthesis of Amphiphilic Conetworks by
poly(St-co-(t-BA)), polymerized with styrene for 60 h; (c):
amphiphilic conetwork, P(St-co-AA). Performed over the tem-
perature range ꢂ20 to 150 ꢀC under nitrogen with a scan rate
of 10 ꢀC/min, glass transition temperatures (Tgs) were obtained
after a heating/cooling cycle using the midpoint method.
Hydrolysis of P(t-BA) Segments
Matyjaszewski and coworkers34 have reported that both
homopolymer and copolymer of tert-butyl acrylate can
hydrolyze efficiently by refluxing with HCl in dioxane. In this
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72 h for the measurement of swelling ratio. Two kinds of
mixed solvents are used, DMF/CH2Cl2 and DMF/aqueous
NaOH solution (pH ¼ 12.9).
DMF is a good solvent for both PS and PAA segments,
therefore the swelling study starts from pure DMF. As shown
in Figure 8, along with the increase of CH2Cl2 percentage, a
good solvent for PS and nonsolvent for PAA, the gel shrinks
because of the shrinkage of PAA segments. Then the gel
undergoes expansion to the largest swelling ratio when
CH2Cl2 volume fraction increases from 20 to 60%. This is
because CH2Cl2 has larger affinity to PS segments, whereas
DMF fraction still swells PAA segments. However, further
increase in CH2Cl2 leads to gel shrinkage again. It seems that
PAA shrinkage dominates in this region because of
the incompatibility of CH2Cl2 and PAA segments. For DMF/
aqueous NaOH solution, the swelling ratio decreases along
with the increase of aqueous NaOH solution fraction because
of the shrinkage of PS segments. The expansion of the gel is
observed when the percentage of aqueous NaOH solution
increases to 60%, which causes ionization of ACOOH group.
The swelling curves of the gel are characteristic of amphi-
philic conetworks.40
FIGURE 7 FT-IR spectra of networks. Sample: (a): precursor
gel, P(t-BA), [t-BA]0/[di-AET]0 ¼ 50; (b): polymer conetworks,
poly(St-co-(t-BA)), polymerized with styrene for 60 h; (c):
amphiphilic conetwork, P(St-co-AA).
We do not go in depth on the effect of solvent–solvent inter-
action on swelling behavior, because it is very complex keep-
ing in mind we have two kinds of segments and two
solvents. In a simpler way, we can consider only one seg-
ment, i.e., PAA, because both DMF and CH2Cl2 are good
solvents for PS segment. Therefore, PAA/DMF/CH2Cl2
becomes a typical three-component system. With increasing
volume fraction of CH2Cl2, PAA segment tends to precipitate
from the system, causing shrinkage of the gel. However, this
tendency seems to be interrupted by the interaction of PS
segments with the solvent, as indicated by Figure 8. The
other system with DMF/aqueous NaOH is similar but occurs
segments are incorporated into the polymer conetworks and
their characteristic bands at 3025 and 3060 cmꢂ1 [m(ring
CAH)], 1601, 1493, and 1450 cmꢂ1 [m(ring CAC)] and 2923,
2852 cmꢂ1 [d(CH2)] significantly appear in Curve b. Efficient
hydrolysis is indicated by the appearance of m(OAH) broad
band at 3400 cmꢂ1 and disappearance of tert-butyl bands at
1369 and 1394 cmꢂ1, and by the band shift of C¼¼O from
1730 to 1720 cmꢂ1 in Curve c.
The amphiphilicity of the poly(St-co-AA) gel is investigated
by measuring the swelling ratio in mixed solvents. The
amphiphilic conetwork is immersed in mixed solvents for
FIGURE 9 The swelling ratio of the PAA gel (*) and poly(St-
co-AA) gel (D) at different aqueous NaOH solutions and aque-
ous HCl solutions. The swelling ratio was measured at 20 ꢀC
for three samples at each point. The PAA gel was hydrolyzed
from the precursor gel P(t-BA) (Sample 3 in Table 1) in the
same condition with the hydrolysis of P(St-co-(t-BA)) gel.
FIGURE 8 The relationships between swelling ratio of the
amphiphilic conetwork and the volume percentage of DMF.
The solvent was mixed with DMF, and the swelling ratio was
measured at 20 ꢀC for three samples at each point. Sample:
(l): aqueous NaOH solution (pH ¼ 12.9) mixed with DMF. (~):
CH2Cl2 mixed with DMF.
POLYMER CONETWORKS RADICAL POLYMERIZATION, ZHAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
8 (a) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.;
Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.;
Moad, G.; Rizzardo, E.; Thang, S. H.; Macromolecules 1998, 31,
5559–5562; (b) Moad, G.; Rizzardo, E.; Thang, S. H. Aust J
Chem 2009, 62, 1402–1472.
within different range of volume fraction. It is difficult to
discuss the effect of the solvent–solvent interaction on
swelling behavior with the inference of solvent–polymer and
polymer–polymer interactions.
Georgiou and Patrickios reported that the swelling ratio of
amphiphilic conetworks is affected by conetwork composition
and architecture.24 We have investigated the swelling nature of
hydrolyzed conetwork in different pH values using a sample
containing 11.8% molar fraction of PAA (Sample h in Fig. 5
before cleavage). As shown in Figure 9, the swelling ratios are
quite stable at around 2.0 below pH ¼ 10, and increase quickly
above pH ¼ 10. This phenomenon is very similar to that of the
hydrolyzed precursor gel (pure PAA network). The abrupt
increase in swelling ratio is because of the electrostatic repul-
sion in fully charged state at high pH. It is interesting to note
that the increase of swelling ratio of poly(St-co-AA) occurs at
larger pH than PAA network, possibly because of the presence
of PS segments. Nevertheless, the similarity in behavior of the
two samples indicates that the pH dependence of the swelling
ratio is dominated by the nature of PAA segments.
9 Hawker, C. J. J Am Chem Soc 1994, 116, 11185–11186.
˚
10 Hawker, C. J.; Hedrick, J. L.; Malmstrolm, E. E.; Trollsas, M.;
Mecerreyes, D.; Moineau, G.; Dubois, P.; Je´roˆ me, R. Macro-
molecules 1998, 31, 213–219.
11 Hawker, C. J.; Fre´chet, J. M. J.; Grubbs, R. B.; Dao, J. J Am
Chem Soc 1995, 117, 10763–10764.
12 (a) Matyjaszewski, K.; Gaynor, S. G.; Edelman, S.; Macromo-
lecules 1996, 29, 1079–1081; (b) Matyjaszewski, K; Gaynor, S.
G.; Kulfan, A.; Podwika, M. Macromolecules 1997, 30,
5192–5194; (c) Matyjaszewski, K.; Gaynor, S. G.; Muller, A. H. E.
¨
Macromolecules 1997, 30, 7034–7041; (d) Matyjaszewski, K.;
Gaynor, S. G. Macromolecules 1997, 30, 7042–7049.
13 Li, C. M.; He, J. P.; Li, L.; Cao, J. G.; Yang, Y. L. Macromole-
cules 1999, 32, 7012–7014.
14 Tao, Y. F.; He, J. P.; Wang, Z. M.; Pan, J. Y.; Jiang, H. J.;
Chen, S. M.; Yang, Y. L. Macromolecules 2001, 34, 4742–4748.
15 He, X. H.; Yan, D. Y. Macromol Rapid Commun 2004, 25,
CONCLUSIONS
949–953.
Well-defined polymer conetworks have been synthesized
through two steps of controlled/‘‘living’’ radical polymeriza-
tions, ATRP of t-BA and di-AET, and NMRP of styrene initiated
by di-AET moieties. The sequential method allows the prepara-
tion of P(t-BA) and PS segments in separate steps, thus facili-
tates the tuning of segment length and cross-linking density.
Amphiphilic conetwork is obtained by hydrolyzing the P(t-BA)
segments, whereas alkoxyamine moieties remaining untouched.
The product exhibits amphiphilic swelling feature in mixed
solvents. Therefore, this work provides an alternative method
in the synthesis of amphiphilic conetworks.
16 Wang, Z. M.; He, J. P.; Tao, Y. F.; Liu, Y.; Jiang, H. J.; Yang,
Y. L. Macromolecules 2003, 36, 7446–7452.
17 (a) Ide, N.; Fukuda, T. Macromolecules 1997, 30, 4268–4271;
(b) Ide, N.; Fukuda, T. Macromolecules 1999, 32, 95–99.
18 (a) Tsarevsky, N. V.; Matyjaszewski, K. Macromolecules
2005, 38, 3087–3092; (b) Gao, H. F.; Min, K.; Matyjaszewski, K.
Macromolecules 2007, 40, 7763–7770.
19 (a) Li, Y. T.; Armes, S. P. Macromolecules 2005, 38, 8155–8162;
(b) Bannister, I.; Billingham, N. C.; Armes, S. P.; Rannard, S. P.;
Findlay, P. Macromolecules 2006, 39, 7483–7492.
This work is subsidized by the National Science Foundation of
China (20574009) and the National Basic Research Program of
China (2005CB623800).
20 (a) Wang, A. R.; Zhu, S. P.; Polym Eng Sci 2005, 45,
720–727; (b) Wang, A. R.; Zhu, S. P. J Polym Sci Part A: Polym
Chem 2005, 43, 5710–5714.
21 Erdodi, G.; Kennedy, J. P. Prog Polym Sci 2006, 31, 1–18.
22 Khan, A.; Malkoch, M.; Montague, M. F.; Hawker, C. J.
J Polym Sci Part A: Polym Chem 2008, 46, 6238–6254.
REFERENCES AND NOTES
23 Themistou, E.; Patrickios, C. S. J Polym Sci Part A: Polym
1 Hawker, C. J.; Bosman, A. W.; Harth, E. Chem Rev 2001, 101,
Chem 2009, 47, 5853–5870.
3661–3688.
24 Kali, G.; Georgiou, T. K.; Iva´n, B.; Patrickios, C. S. J Polym
2 Hawker, C. J.; Barclay, G. G.; Dao, J. J Am Chem Soc 1996,
Sci Part A: Polym Chem 2009, 47, 4289–4301.
118, 11467–11471.
25 Erdodi, G.; Kennedy, J. P. J Polym Sci Part A: Polym Chem
3 Rodlert, M.; Harth, E.; Hawker, C. J. J Polym Sci Part A:
2007, 45, 295–307.
Polym Chem 2000, 38, 4749–4763.
26 (a) Patrickios, C. S.; Georgiou, T. K. Curr Opin Colloid Inter-
face Sci 2003, 8, 76–85; (b) Krasia, T. C.; Patrickios, C. S. Macro-
molecules 2006, 39, 2467–2473; (c) Achilleos, M.; Krasia, T. C.;
Patrickios, C. S. Macromolecules 2007, 40, 5575–5581; (d) Achil-
leos, M.; Legge, T. M.; Perrier, S.; Patrickios, C. S. J Polym Sci
Part A: Polym Chem 2008, 46, 7556–7565.
4 (a) Matyjaszewski, K; Xia, J. H. Chem Rev 2001, 101,
2921–2990; (b) Matyjaszewski, K. Prog Polym Sci 2005, 30,
858–875; (c) Gao, H. F.; Matyjaszewski, K. Prog Polym Sci 2009,
34, 317–350.
5 Percec, V.; Barboiu, B. Macromolecules 1995, 28, 7970–7972.
6 Kato, M.; Kamigaito, M.; Sawamoto, M; Higashimura, T.
27 (a) Higaki, Y.; Otsuka, H.; Takahara, A. Macromolecules 2006,
39, 2121–2125; (b) Amamoto, Y.; Kikuchi, M.; Masunaga, H.;
Sasaki, S.; Otsuka, H.; Takahara, A. Macromolecules 2009, 42,
8733–8738.
Macromolecules 1995, 28, 1721–1723.
7 Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.; Imam,
M. R.; Percec, V. Chem Rev 2009, 109, 6275–6540.
4148
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
28 Wong, E. H. H.; Stenzel, M. H.; Junkers, T.; Barner-Kowollik,
34 Davis, K. A.; Matyjaszewski, K. Macromolecules 2000, 33,
C. J Polym Sci Part A: Polym Chem 2009, 47, 1098–1107.
4039–4047.
35 Matyjaszewski, K.; Gaynor, S.; Greszta, D.; Mardare, D.; Shi-
29 Sciannamea, V.; Catala, J.; Jerome, R.; Je´roˆ me, C.;
Detrembleur, C. J Polym Sci Part A: Polym Chem 2009, 47,
1085–1197.
gemoto, T. Macromol Symp 1995, 98, 73–89.
36 Flory, P. J. Principles of Polymer Chemistry; Cornell Univer-
sity Press: Ithaca, NY, 1953.
30 Murthy, K. S.; Ma, Q.; Clark, C. G., Jr.; Remsen, E. E.; Woo-
ley, K. L. Chem Commun 2001, 773–774.
37 Pryor, W. A.; Coco, J. H. Macromolecules 1970, 3, 500–508.
31 Fu, Q.; Lin, W. C.; Huang, J. L. Macromolecules 2008, 41,
38 Ma, Q. G.; Wooley, K. L. J Polym Sci Part A: Polym Chem
2381–2387.
2000, 38, 4805–4820.
32 Appelt, M.; Schmidt-Naake, G. Macromol Mater Eng 2004,
39 Brandup, J.; Immergut, E. H.; Grulke, E. A., Eds. Polymer
289, 245–253.
Handbook, 4th ed.; Wiley: New York, 1999.
33 Popielarz, R.; Arnold, D. R. J Am Chem Soc 1990, 112,
40 Doura, M.; Naka, Y.; Aota, H.; Matsumoto, A. Macromole-
3068–3082.
cules 2003, 36, 8477–8482.
POLYMER CONETWORKS RADICAL POLYMERIZATION, ZHAO ET AL.
4149