PAA-g-PPO amphiphilic graft copolymer: Synthesis and diverse micellar morphologies

Article abstract of DOI:10.1021/ma901526j

A series of well-defined amphiphilic graft copolymers consisting of hydrophilic poly(acrylic acid) backbone and hydrophobic poly(propylene oxide) side chains were synthesized by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer nitroxide radical coupling (ATNRC) chemistry followed by selective hydrolysis of poly(tert-buty1 acrylate) backbone. A new Br- containing acrylate monomer, tert-butyl 2-((2-bromopropanoyloxy)methyl) acrylate, was first prepared, and it can be polymerized via RAFT in a controlled way to obtain a well-defined homopolymer with narrow molecular weight distribution {M_wM_n = 1.06). Grafting-onto strategy was employed to synthesize PiBA-gPPO well-defined graft copolymers with narrow molecular weight distributions {M_wM _n = 1.05-1.23) via ATNRC reaction between Br-containing PiBA-based backbone and poly(propylene oxide) with 2, 2, 6, 6-tetramethylpiperidine-l-oxyl (TEMPO) end group using CuBr/PMDETA or Cu/PMDETA as catalytic system. The final PAA-g-PPO amphiphilic graft copolymers were obtained by the selective acidic hydrolysis of PrBA backbone in acidic environment without affecting the side chains. The critical micelle concentrations in aqueous media were determined by a fluorescence probe technique. Diverse micellar morphologies were formed with varying the content of hydrophobic PPO segment.

Full text of DOI:10.1021/ma901526j

2
62 Macromolecules 2010, 43, 262–270
DOI: 10.1021/ma901526j
PAA-g-PPO Amphiphilic Graft Copolymer: Synthesis and Diverse
Micellar Morphologies
†
Yaogong Li, Yaqin Zhang, Dong Yang, Yongjun Li, Jianhua Hu,* Chun Feng,
†
‡
†
,‡
†
†
Sujuan Zhai, Guolin Lu, and Xiaoyu Huang*
†
,†
†
Key Laboratory of Organofluorine Chemistry and Laboratory of Polymer Materials, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China, and
‡
Key Laboratory of Molecular Engineering of Polymers (Ministry of Education), Laboratory of Advanced
Materials and Department of Macromolecular Science, Fudan University, 220 Handan Road, Shanghai 200433,
P. R. China
Received July 13, 2009; Revised Manuscript Received September 3, 2009
ABSTRACT: A series of well-defined amphiphilic graft copolymers consisting of hydrophilic poly(acrylic
acid) backbone and hydrophobic poly(propylene oxide) side chains were synthesized by sequential reversible
addition-fragmentation chain transfer (RAFT) polymerization and atom transfer nitroxide radical coupl-
ing (ATNRC) chemistry followed by selective hydrolysis of poly(tert-butyl acrylate) backbone. A new
Br- containing acrylate monomer, tert-butyl 2-((2-bromopropanoyloxy)methyl) acrylate, was first prepared,
and it can be polymerized via RAFT in a controlled way to obtain a well-defined homopolymer with narrow
molecular weight distribution (M
PPO well-defined graft copolymers with narrow molecular weight distributions (M
ATNRC reaction between Br-containing PtBA-based backbone and poly(propylene oxide) with 2,2,6,
-tetramethylpiperidine-1-oxyl (TEMPO) end group using CuBr/PMDETA or Cu/PMDETA as catalytic
w
/M
n
=1.06). Grafting-onto strategy was employed to synthesize PtBA-g-
w
/M =1.05-1.23) via
n
6
system. The final PAA-g-PPO amphiphilic graft copolymers were obtained by the selective acidic hydrolysis
of PtBA backbone in acidic environment without affecting the side chains. The critical micelle concentrations
in aqueous media were determined by a fluorescence probe technique. Diverse micellar morphologies were
formed with varying the content of hydrophobic PPO segment.
Introduction
copolymers can also be obtained via other types of PPO-based
macromonomers by the condensation polymerization or cata-
Poly(propylene oxide) (PPO) is a kind of nonionic polymer,
and it is soluble in most common organic solvents whereas hardly
soluble in water. PPO can be applied in many fields such as
2
4,25
lyzed coupling polymerization.
Moreover, nucleophilic sub-
2
6,27
stitution reactions
were also applied to synthesize the graft
1
-3
copolymers containing PPO side chains, whose backbone and
side chains were prepared independently; however, the grafting
lubricants, surfactants, hydraulic pressure liquid, and so on.
In particular, PPO segment can be used to modulate the proper-
ties of polymeric solution by its incorporation into a macromo-
2
6
densities of these graft copolymers were not clear or very low
2
7
4
(60/1230 = 4.88%), and they possessed broad molecular
lecule. Many synthetic pathways are available for the
2
6,27
weight distributions,
this indicating that these graft copoly-
preparation of well-defined block copolymers comprising PPO
5,6
mers might not be well-defined. The pendant hydroxyls of
ethylene-vinyl alcohol copolymer were also employed to initiate
anionic graft polymerization of propylene oxide to afford the
block. Each segment can be prepared independently and
connected by the postpolymerization coupling of functional
end groups. Moreover, PPO homopolymer prepared by anionic
polymerization contains one or two hydroxyl end functionalities,
and this kind of commercially available functional PPO homo-
polymer is in favor of almost unlimited chemical modification.
PPO-based block copolymers (PEO was usually selected as
2
8
denselygraftedpolymers containing PPO side chains. However,
none has reported the well-defined amphiphilic graft copolymer
bearing hydrophilic backbone and hydrophobic PPO side chains.
Generally, polymer chemists employed three strategies includ-
ing grafting-through, grafting-from, and grafting-onto to synthe-
5-19
another block)
have been extensively studied due to their
29
size the graft copolymer. Graft copolymers can be obtained via
the polymerization of macromonomers using the grafting-
through strategy, the resulting graft copolymers via the conven-
tional radical polymerization possessed a broad chain-length
convenient syntheses, and only a few examples touched on PPO-
2
0-28
based graft copolymers.
were synthesized from PPO-based macromonomers containing
meth)acrylate end groups using living or traditional radical
Most of these graft copolymers
(
3
0
distribution, and living polymerization yielded well-defined
polymerization, and ester linkages were employed to connect
PPO side chains with the hydrophobic backbone in these “graft”
copolymers, implying PPO segments would be detached after
3
1
graft copolymers with low molecular weights. The grafting-
from approach utilized the pendant initiation groups on the
backbone to initiate the polymerization (generally employing
living radical polymerization including atom transfer radical
2
0-23
the hydrolysis of ester connections.
PPO-based graft
3
2-34
polymerization (ATRP),
reversible addition-fragmenta-
*To whom correspondence should be addressed: e-mail hujh@fudan.
35,36
tion chain transfer (RAFT) polymerization,
tron-transfer living radical polymerization (SET-LRP)
another monomer to form the side chains.
and single-elec-
edu.cn, Tel þ86- 21-55665280, Fax þ86-21-65640293 (J.H.); e-mail
3
7,38
) of
xyhuang@mail.sioc.ac.cn, Tel þ86-21-54925310, Fax þ86-21-
3
9,40
64166128 (X.H.).
The grafting-onto
pubs.acs.org/Macromolecules
Published on Web 11/04/2009
r 2009 American Chemical Society

Article
Macromolecules, Vol. 43, No. 1, 2010 263
method is to attach the side chains onto the backbone by a
monomer, tert-butyl 2-((2-bromopropanoyloxy)methyl)acrylate
(tBBPMA), was first prepared, and it was homopolymerized via
RAFT to form well-defined Br-containing poly(tert-butyl
acrylate)-based backbone. Next, well-defined poly(tert-butyl
acrylate)-g-poly(propylene oxide) (PtBA-g-PPO) graft copoly-
mers with narrow molecular weight distributions were synthe-
sized via ATNRC between PtBA-based backbone and TEMPO-
containing PPO side chains (PPO-TEMPO) catalyzed by CuBr/
PMDETA or Cu/PMDETA. The final amphiphilic graft copo-
lymers were obtained by the selective acidic hydrolysis of PtBA
backbone, and they can form diverse micellar morphologies in
aqueous media.
41
coupling reaction. In this situation, both the backbone and side
chains can be prepared and characterized independently, so it is
easy to synthesize the graft copolymers with certain structures.
Since propylene oxide cannot be polymerized by living radical
polymerization, it is impossible to employ the grafting-from
strategy to obtain the graft copolymers with PPO side chains.
Thus, the grafting-onto strategy is a suitable approach to
synthesize well-defined amphiphilic graft copolymers containing
hydrophobic PPO side chains due to the commercial availability
of well-defined PPO homopolymer possessing hydroxyl end
functionalities. However, this technique requires high coupling
4
1
efficiency between the backbone and the side chains.
42
As a highly efficient coupling reaction, click chemistry has
been extensively applied for the construction of well-defined
Experimental Section
4
3-46
Materials. tert-Butyl acrylate (tBA, Aldrich, 98%) was
washed with 5% aqueous NaOH solution to remove the in-
hibitor, then washed with water, dried over CaCl , and distilled
2
(
functional) polymer architectures;
however, special care
4
7
must be taken due to the instability of azide compounds.
Recently, Huang and co-workers reported a new coupling reac-
tion called atom transfer nitroxide radical coupling (ATNRC)
twice from CaH
(Aldrich, 98%) was purified by stirring overnight over
CH CO H at room temperature, followed by washing the solid
2
under reduced pressure prior to use. CuBr
47-51
chemistry,
which was realized by mixing a TEMPO-contain-
3
2
ing polymer with another Br-containing polymer in the presence
of CuBr/PMDETA catalytic system. The coupling efficiency of
with ethanol, diethyl ether, and acetone prior to drying at 40 °C
under vacuum for 1 day. Pyrene (Aldrich, 98%) was purified by
48,49
ATNRC was as high as 90%,
which is similar to that of click
0
recrystallization in ethanol for three times. 2,2 -Azobis-
chemistry. Thus, ATNRC can serve as a relatively convenient and
safe way with “click-like” efficiency to synthesize the graft copo-
lymers with certain structures using the grafting-onto strategy.
In this work, we reported the synthesis of poly(acrylic acid)-g-
poly(propylene oxide) (PAA-g-PPO) well-defined amphiphilic
graft copolymer, whose backbone and side chains were both
with certain and defined structures (Scheme 1). A new acrylic
(
isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized from
anhydrous ethanol. Triethylamine (TEA, Aldrich, 99.5%) was
dried over KOH and distilled over CaH under N prior to use.
2
2
Tetrahydrofuran (THF, Aldrich, 99%), dichloromethane
(Aldrich, 99.5%), chloroform (Aldrich, 99%), and toluene
(Aldrich, 99.8%) were dried over CaH2 and distilled from
2
sodium and benzophenone under N prior to use. Cumyl
Scheme 1. Synthesis of PAA-g-PPO Well-Defined Amphiphilic Graft Copolymer

2
64 Macromolecules, Vol. 43, No. 1, 2010
Li et al.
dithiobenzoate (CDB) was synthesized according to previous
2-((2-bromopropanoyloxy)methyl)acrylate) (PtBBPMA) 2, was
obtained after drying in vacuo. GPC: M
52
literature. 2-Bromopropionic acid (Aldrich, 99%), copper
powder (Aldrich, 99%), 4-hydroxy-2,2,6,6-tetramethylpiperi-
dine-1-oxyl (HTEMPO, Aldrich, 97%), formalin (Aldrich, 38
wt %), poly(propylene oxide) monobutyl ether (PPO-OH,
n
= 6200, M
w n
/M =
-1
1.06. FT-IR: ν (cm ): 2978, 2932, 1736, 1448, 1394, 1368, 1253,
1
1143, 998, 970, 842, 753. H NMR: δ (ppm): 1.45 (9H, C(CH
1.86 (2H, CH CCO and 3H, CH CH), 4.53 (1H, CH CH and
2H, CO CH ). C NMR: δ (ppm): 21.9 (CH CH), 28.2
3 3
) ),
2
2
3
3
1
3
M
n
= 1000 and 2500, Aldrich), 1,4-diazabicyclo[2.2.2]octane
0
2
2
3
(DABCO, Aldrich, 98%), N,N -dicyclohexylcarbodiimide
(DCC, Aldrich, 99%), 4-di(methylamino)pyridine (DMAP,
(C(CH ) ), 40.5 (CH CCO and CH CH), 49.4 (CH CCO ),
C-
3
3
2
2
3
2
2
65.6 (CO
(CH ) ). Element analysis: Br% = 27.12%.
2
CH
2
), 83.2 (C(CH
3
)
3
), 169.5 (CO
2
CH
2
), 171.5 (CO
2
0
0
00
Aldrich, 99%), N,N,N ,N ,N -pentamethyldiethylenetriamine
PMDETA, Aldrich, 99%), thionyl chloride (Aldrich, 99%),
succinic anhydride (Aldrich, 99%), and trifluoroacetic acid
3 3
(
Functionalization of PPO. In a typical procedure, PPO-OH
(10.00 g, M =1000, 10 mmol), succinic anhydride (2.5018 g,
n
(
TFA, Aldrich, 99%) were used as received.
Measurements. FT-IR spectra were recorded on a Nicolet
AVATAR-360 FT-IR spectrophotometer with a resolution of
25 mmol), DMAP (0.1222 g, 1 mmol), and dry chloroform
(250 mL) were added to a 500 mL three-neck flask, and the
mixture was stirred at 80 °C for 18 h. Chloroform was removed
by rotary evaporation, and unreacted succinic anhydride was
eliminated by filtration. The filtrate was diluted with chloroform
and washed with brine. A yellowish liquid, carboxyl-functiona-
lized PPO (PPO-COOH, 7.76 g, 70.5%) was obtained by silica
column chromatography. FT-IR: ν (cm ): 3489 (COOH),
2971, 2870, 1736 (CdO), 1459, 1374, 1345, 1297, 1112, 1014,
928, 866.
PPO-COOH (3.8315 g, 3.48 mmol) and thionyl chloride
(20 mL, 275.4 mmol) were added to a 50 mL flask, and the
mixture was stirred at room temperature for 2.5 h. Unreacted
thionyl chloride was removed in vacuo at 50 °C to obtain acyl-
chloride-functionalized PPO (PPO-COCl). Next, HTEMPO
(2.1750 g, 13.92 mmol), DMAP (0.0510 g, 0.4176 mmol), TEA
(0.5 mL, 3.48 mmol), and dry THF (60 mL) were added to a
250 mL three-neck flask, and the solution was cooled to 0 °C for
adding PPO-COCl (dissolved in 60 mL of dry THF). The system
was stirred at 0 °C for 1 h and was raised to room temperature
with further stirring for 1 day. Brine was added to terminate the
reaction. The organic phase was washed with water until the
aqueous layer turned colorless. After drying over anhydrous
-
1
1
cm . All H NMR (500 MHz) and C NMR (125 MHz)
13
4
analyses were performed on a Bruker Avance 500 spectrometer
in CDCl
measured in CD OD in the presence of stoichiometric
1
3
, except that H NMR spectrum of PPO-TEMPO was
3
4
and Pd/C. TMS ( H NMR) and CDCl
8
1
13
( C
-1
HCOONH
4
3
NMR) were used as internal standards. Elemental analysis
was carried out on a Carlo-Erba1106 system. Bromine content
was determined by the titrations with Hg(NO ) . ESI-MS was
measured by an Agilent LC/MSD SL system, and MALDI-
TOF-MS was measured by an Applied Biosystems Voyager DE-
STR spectrometer. Relative molecular weights and molecular
weight distributions were measured by conventional gel permea-
tion chromatography (GPC) system equipped with a Waters
3
2
1
detector, and a set of Waters Styragel columns (HR3, HR4, and
515 isocratic HPLC pump, a Waters 2414 refractive index
HR5, 7.8 ꢀ 300 mm). GPC measurements were carried out at 35
°
C using THF as eluent with a flow rate of 1.0 mL/min. The
system was calibrated with linear polystyrene standards. Steady-
state fluorescent spectra were recorded on a Hitachi F-4500
spectrofluorometer with the bandwidths of 10 nm for excitation
and 2.5 nm for emission, where λex = 339 nm. Transmission
electron microscope (TEM) images were obtained by a JEOL
JEM-1230 instrument operated at 80 kV.
4
MgSO , an orange liquid, TEMPO-functionalized PPO (PPO-
TEMPO) 3a (2.846 g, 65.1%), was obtained by silica column
chromatography. MAIDI-TOF-MS (m/z): found M : 1317.
þ
-
1
Synthesis of tert-Butyl 2-((2-Bromopropanoyloxy)methyl)-
acrylate. Acrylic monomer 1 was synthesized in two steps
using commercially available tert-butyl acrylate, formalin, and
FT-IR: ν (cm ): 2971, 2870, 1737 (CdO), 1458, 1374, 1345,
1
1297, 1259, 1111, 1014, 929, 868. H NMR (CD
3
OD in the
presence of Pd/C and HCOONH ): δ (ppm): 0.92 (3H,
4
2
-bromopropionic acid as starting materials. First, tert-butyl
OCH
of TEMPO), 1.46 (4H, OCH
TEMPO), 2.57 (4H, CO CH
CHCH ), 3.60 (2H, OCH
CH CH ).
2
CH
2
CH
2
CH
3
), 1.12 (3H, OCH
2
CHCH
3
and 12H, CH
3
acrylate reacted with formalin to give an intermediate of tert-
butyl (2-hydroxymethyl)acrylate with a yield of 77.0%. Next,
the desired acrylic monomer 1 was obtained via the reaction
between the intermediate and 2-bromopropionic acid at room
2
CH
2
CH
CH ), 1.84 (4H, CH
2 3
2
of
2
2
CH
CHCH
2
CO
and 2H, OCH
2
2
), 3.49 (1H, OCH
2
-
-
CH
3
2
3
2
2
3
temperature with a yield of 94.4%. ESI-MS (m/z): found (M -
ATNRC between PtBBPMA and PPO-TEMPO Catalyzed by
CuBr/PMDETA. In a typical procedure, CuBr (0.0286 g, 0.1991
mmol) and PtBBPMA 2 (0.0587 g, M =6200, M /M =1.06, Br
þ
5
Br, 27.30%. Found: C, 45.37%; H, 5.58%; Br, 27.20%. FT-IR:
6) : 237. Anal. Calcd for C11
H
17
O
4
Br: C, 45.05%; H, 5.80%;
n
w
n
-
1
%
= 27.12%) were added to a 50 mL Schlenk flask (flame-dried
under vacuum prior to use) sealed with a rubber septum under
. After three cycles of evacuating and purging with N , PPO-
ν (cm ): 2979, 2934, 1748, 1720, 1643, 1448, 1394, 1369, 1146,
1
1
1
5
2
1
063, 848, 816, 761, 515. H NMR: δ (ppm): 1.38 (9H, C(CH
3 3
) ),
N
2
2
.71 (3H, CH CH), 4.31 (1H, CH CH), 4.72 (2H, CO CH ),
2
.71 and 6.17 (1H, CH dC). C NMR: δ (ppm): 21.5 (CH CH),
3
3
2
1
3
TEMPO 3a (0.3000 g, 0.2389 mmol) in 10 mL of dry toluene and
PMDETA (0.042 mL, 0.1991 mmol) were charged via a gastight
syringe. The flask was degassed by three cycles of free-
zing-pumping-thawing followed by stirring at room tempera-
ture for 10 min. Next, the flask was immersed into an oil bath set
at 90 °C. The coupling reaction lasted 1 day, and it was
2
3
7.9 (C(CH
26.5 (CH
CH ).
3
)
3
), 39.6 (CH
dC), 136.1 (CH
3
CH), 63.6 (CO
2
CH
2
3 3
), 81.3 (C(CH ) ),
2
2
2 3 3
=C), 163.9 (CO C(CH ) ), 169.4
(
CO
2
2
RAFT Homopolymerization of Acrylic Monomer. AIBN
0.082 g, 0.50 mmol) and CDB (0.408 g, 1.50 mmol) were first
(
terminated by putting the flask into liquid N . The reaction
added to a 25 mL Schlenk flask (flame-dried under vacuum
prior to use) sealed with a rubber septum for degassing and kept
under N . Next, tBBPMA 1 (11.00 g, 37.52 mmol) and dry
2
mixture was diluted with THF and passed through a neutral
alumina column to remove the residual copper complex. The
purification of raw product was performed by dissolving in
2
toluene (3.0 mL) were charged via a gastight syringe. The flask
was degassed by three cycles of freezing-pumping-thawing
followed by immersing the flask into an oil bath set at 70 °C. The
polymerization was terminated by immersing the flask into
CH
separator. The remaining CH
was concentrated and dried in vacuo overnight to obtain
PtBA-g-PPO 4c graft copolymer. GPC: M = 25 800, M
= 1.05. FT-IR: ν (cm ): 2974, 2933, 1736, 1458, 1370,
3
OH and filtering through an ultrafiltration membrane
3
OH solution in the separator
liquid N
2
after 12 h. THF was added to dilute the solution,
n
w
/
-
1
and the solution was precipitated into hexane. After repeated
purification by dissolving in THF and precipitating in hexane,
M
n
1
1343, 1253, 1149, 925, 842, 749. H NMR: δ (ppm): 0.91 (3H,
OCH CH CH CH ), 1.14 (3H, OCH CHCH and 12H, CH of
TEMPO), 1.47 (9H, O(CH and 4H, OCH CH CH CH ),
1.81 (3H, CH CHBr, 2H, CH CCO and 4H, CH of TEMPO),
4.75 g of pink powder was obtained after drying in vacuo.
AIBN was used to remove the dithiobenzoate moiety according
2
2
2
3
2
3
3
)
3
3
2
2
2
3
5
to previous report. Finally, a white powder, poly(tert-butyl
3
3
2
2
2

Article
Macromolecules, Vol. 43, No. 1, 2010 265
2
(
CH
.60 (4H, CO
2H, OCH CHCH
CHBr and 2H, CO
2
CH
2
CH
and 2H, OCH
CH ), 5.03 (1H, CH of TEMPO).
CH CH CH ), 17.5 (OCH
CH ), 21.3 (NCCH ), 22.0
), 29.4 (CO CH CH CO ), 31.9
), 33.8 (CO CH(CH ), 40.6 (CH CCO
CHBr), 44.6 (NCCH ), 49.4 (CH CCO ), 60.4
), 70.5 (CO CH(CH ), 71.4 (OCH CH CH
CHON), 73.5 (OCH CHCH ), 75.5 (OCH CHCH
3.3 (C(CH ), 169.7 (CO CH ), 172.0 (CO C(CH
CH CO ).
2
CO
2
), 3.43 (1H, OCH
2
CHCH
3
), 3.56
2
3
2
CH CH CH
2
2
3
), 4.51 (1H,
1
3
3
2
2
C
NMR: δ (ppm): 14.1 (OCH
CHCH ), 19.5 (OCH CH CH
CH), 28.3 (C(CH
OCH CH CH CH
2
2
2
3
2
-
3
2
2
2
3
3
(
(
CH
3
3
)
3
2
2
2
2
2
2
2
3
2
2
)
2
2
2
and CH
CO CH
1.9 (CH
3
3
2
2
(
2
2
2
2
)
2
2
2
2
CH
3
),
),
7
8
3
2
3
2
3
3
)
3
2
2
2
3 3 2
) and CO -
CH
2
2
2
ATNRC between PtBBPMA and PPO-TEMPO Catalyzed by
Cu/PMDETA. Cu powder (0.0126 g, 0.1991 mmol) and
PtBBPMA 2 (0.0587 g, M
2
n w n
= 6200, M /M = 1.06, Br% =
7.12%) were added to a 50 mL Schlenk flask (flame-dried
under vacuum prior to use) sealed with a rubber septum under
N . After three cycles of evacuating and purging with N , PPO-
2
2
TEMPO 3a (0.3000 g, 0.2389 mmol) in 10 mL of dry toluene and
PMDETA (0.042 mL, 0.1991 mmol) were charged via a gastight
syringe. The flask was degassed by three cycles of free-
zing-pumping- thawing followed by stirring at 20 °C for 2
1
13
Figure 1. H NMR (A) and C NMR (B) spectra of tBBPMA 1 in
CDCl .
3
2
days, and it was terminated by putting the flask into liquid N .
The reaction mixture was diluted with THF and passed through
a neutral alumina column to remove the residual copper com-
plex. The purification of raw product was performed by dis-
5
4,55
enolate or the activation of aldehyde.
Next, 2-bromo-
propionic acid reacted with tert-butyl (2-hydroxymethyl)-
acrylate to provide the desired Br-containing tBBPMA 1
monomer via the common esterification using DMAP as
catalyst.
FT-IR, H NMR, and C NMR were used to characterize
the chemical structure of tBBPMA 1. Typical signals of
carbonyl and double bond appeared at 1748, 1720, and
1643 cm in the FT-IR spectrum. The resonance signals
of double bond were found to locate at 5.71 and 6.17 ppm in
the H NMR spectrum (Figure 1A). The peaks at 4.31 and
.72 ppm belonged to 1 proton of CHBr and 2 protons of
CO CH , respectively. The typical peaks at 126.5, 136.1,
63.9, and 169.4 ppm in the C NMR spectrum (Figure 1B)
solving in CH
membrane separator. The remaining CH
separator was concentrated and dried in vacuo overnight to
obtain PtBA-g-PPO 4d graft copolymer. GPC: M = 25 300,
/M =1.05.
Selective Acidic Hydrolysis of PtBA-g-PPO. PtBA-g-PPO 4
was dissolved in CH Cl and treated with excess trifluoroacetic
3
OH and filtering through an ultrafiltration
3
OH solution in the
1
13
n
M
w
n
-
1
2
2
acid (10-fold) at 40 °C for 32 h. The solution was concentrated
and precipitated into cold hexane. After filtration, PAA-g-PPO
1
5
amphiphilic graft copolymer was obtained after drying in
4
-
1
vacuo overnight. FT-IR: ν (cm ): 3445 (COOH), 2973, 2933,
2
1
584, 1736, 1449, 1376, 1236, 1158, 1104, 1007, 832, 758. H
2
2
13
1
NMR: δ (ppm): 0.91 (3H, OCH CH CH CH ), 1.14 (3H,
2
2
2
3
also witnessed the presence of double bond and carbonyl. All
these evidence demonstrated the successful synthesis of Br-
containing acrylic monomer tBBPMA 1.
OCH CHCH and 12H, CH of TEMPO), 1.54 (4H, OCH -
2
3
3
2
CH
CH
2
CH CH ), 1.82 (3H, CH CHBr, 2H, CH
of TEMPO), 2.59 (4H, CO
2
3
3
2
CCO
2
and 4H,
CH CH CO ), 3.43 (1H,
2
2
2
2
2
Preparation of Well-Defined PtBA-Based Backbone.
tBBPMA 1 was homopolymerized via RAFT in toluene
using AIBN as initiator and CDB as chain transfer agent.
AIBN was employed to remove the moiety of dithiobenzoate
end group according to previous literature. The pink
powder turned into white powder after the reaction, indicat-
ing the absence of dithiobenzoate residue.
OCH CHCH ), 3.56 (2H, OCH CHCH and 2H, OCH2-
2
3
2
3
CH CH CH ), 4.50 (1H, CH CHBr and 2H, CO CH ), 5.03
2
2
2
3
3
2
1
3
(
CH CH CH ), 17.5 (OCH CHCH ), 19.5 (OCH CH CH
1H, CH of TEMPO). C NMR: δ (ppm): 14.1 (OCH
2
-
-
2
2
3
2
3
2
2
2
5
3
CH ), 21.2 (NCCH ), 22.9 (CH CH), 29.9 (CO CH CH CO ),
3
3
3
2
2
2
2
32.0 (OCH
CCO
2
CH
2
CH CH
2
3
), 33.7 (CO
2
CH(CH
), 47.8 (CH
), 71.4 (OCH CH
2
)
2
), 40.5 (CH
CCO ), 60.6
CH CH ),
2
-
2
and CH
3
CHBr), 44.3 (NCCH
CH(CH
3
2
2
(
CO CH ), 70.5 (CO
2
2
2
2
)
2
2
2
2
3
The FT-IR spectrum showed the disappearance of typical
-
1
7
1.9 (CH CHON), 73.5 (OCH CHCH ), 75.5 (OCH CHCH ),
3
signal of double bond at 1643 cm after RAFT homopo-
lymerization. The peak of polyacrylate backbone appeared
at 1.86 ppm in the H NMR spectrum after the homopo-
2
3
2
3
1
69.7 (CO CH
2
2
), 172.1 (CO CH
2
2
CH CO
2
2
), 179.7 (COOH).
Determination of Critical Micelle Concentration. Pyrene was
1
used as fluorescence probe to measure the critical micelle
concentration (cmc) of PAA-g-PPO 5 amphiphilic graft copo-
lymer. Acetone solution of pyrene (0.1286 mM) was added to a
large amount of water until the concentration of pyrene reached
0.0006 mM. Different amounts of THF solutions of PAA-g-
PPO 5 (10, 1, 0.1, 0.01, 0.001, or 0.0001 mg/mL) were added to
pyrene-containing water ([pyrene]=0.0006 mM).
lymerization (Figure 2A) whereas the resonance signals of
the double bond disappeared. In particular, the resonance
signal of 1 proton of CHBr was found to locate at 4.53 ppm,
and the bromine content after the homopolymerization
(
(
27.12%) was almost the same as that of the monomer
27.20%), which showed that the CHBr group was not
affected during the homopolymerization. The disappearance
of the resonance signals of the double bond was also verified
by the C NMR spectrum (Figure 2B), and the peak of
Results and Discussion
13
Synthesis of Br-Containing Acrylic Monomer. The key
intermediate, tert-butyl (2-hydroxymethyl)acrylate, was first
prepared via Baylis-Hillman reaction using tert-butyl acry-
late as starting material under base-catalyzed and water-
promoted conditions. DABCO and TEA were used as basic
carbonyl of CO C(CH ) shifted from 163.9 to 171.5 ppm
2
after RAFT homopolymerization.
3 3
In addition, only a unimodal and symmetrical elution
peak with narrow molecular weight distribution (M /M
= 1.06) was found in the GPC curve (Figure 3) after RAFT
w
n
cocatalysts, and a protonic solvent H O was employed to
2
accelerate the reaction through either the stabilization of
homopolymerization of tBBPMA 1. From the data of
molecular weight (M = 6200), we can estimate that every
n

2
66 Macromolecules, Vol. 43, No. 1, 2010
Li et al.
Table 1. Preparation of TEMPO-Functionalized PPO
c
n
d
n
e
n
f
sample
M
(g/mol)
M
(g/mol)
M /M
w
N
PPO
a
3
3
a
b
1254
2754
1317
1.07
1.11
16.0
41.9
b
a
b
n
Synthesized from PPO-OH, M =1000. Synthesized from PPO-
d
c
n
OH, M =2500. Theoretical molecular weight. Measured by MAIDI-
e f
TOF-MS. Measured by GPC in THF. The number of PO repeating
unit per chain.
1
13
Figure 2. H NMR (A) and C NMR (B) spectra of PtBBPMA 2 in
CDCl
3
.
Figure 4. GPC traces of PtBA-g-PPO 4c and 4d before and after the
ultrafiltration.
Table 2. Synthesis of PtBA-g-PPO Graft Copolymer
d
n
d
n
e
f
sample [3]/[Br group]
M
(g/mol)
M
w
/M
n
PPO
D
graft (%)
a
4
4
4
4
4
4
4
a
b
c
d
e
f
g
0.4/1
0.7/1
1.2/1
1.2/1
0.4/1
0.7/1
1.2/1
14 700
19 500
25 800
25 300
35 900
47 500
52 500
1.10
1.07
1.05
1.05
1.17
1.23
1.13
4.0
6.9
11.6
12.0
4.7
19.5
33.7
56.6
58.5
22.9
42.0
65.4
a
a
b
c
c
8.6
13.4
c
a
Synthesized from PPO-TEMPO 3a catalyzed by CuBr/PMDETA at
b
9
0 °C for 24 h. Synthesized from PPO-TEMPO 3a catalyzed by Cu/
c
PMDETA at 20 °C for 24 h. Synthesized from PPO-TEMPO 3b
d
catalyzed by CuBr/PMDETA at 90 °C for 24 h. Measured by GPC
e
1
in THF at 35 °C. The number of grafted PPO side chain obtained by H
NMR. The grafting density of PPO side chain.
Figure 3. GPC traces of PtBBPMA 2 and PtBA-g-PPO 4a, 4b, 4c, and
d in THF.
f
4
(1254 g/mol). All these points demonstrated the successful
PtBBPMA 2 chain possesses 20.5 repeating units. All the
above results confirmed the formation of well-defined PtBA-
based backbone.
preparation of functionalized PPO with TEMPO end group.
The approximate number of PO repeating unit per chain
(NPPO) can be calculated from the molecular weight of PPO-
OH, and the values are 16.0 and 41.9 for PPO-TEMPO 3a
and 3b, respectively.
Preparation of TEMPO-Functionalized PPO. PPO-OH
was first esterified by succinic anhydride to provide PPO-
COOH. The successful introduction of carboxyl group was
Synthesis of PtBA-g-PPO Graft Copolymer via ATNRC.
ATNRC reaction between Br-containing PtBBPMA 2
homopolymer and PPO-TEMPO 3 was performed in toluene
-
1
evidenced by two new peaks at 3489 and 1736 cm in the
FT-IR spectrum after the reaction. Next, PPO-COOH was
almost quantitatively converted to PPO-COCl by reacting
47-51
at 90 °C using CuBr/PMDETA as the catalytic system.
with SOCl followed by adding HTEMPO in situ to obtain
2
The crude product was a mixture of PtBA-g-PPO graft
copolymer and unreacted PPO-TEMPO as evidenced by
GPC (Figure 4A). The mixture was separated by an ultra-
filtration membrane separator to provide pure PtBA-g-PPO
4 graft copolymer as shown in Figure 4B. It is clear that all
the molecular weights of the obtained graft copolymers are
much higher than that of PtBBPMA 2 homopolymer as
listed in Table 2, indicating the occurring of the coupling
reaction. All graft copolymers’ GPC curves (Figure 3)
showed a unimodal and symmetrical elution peak with
narrow molecular weight distribution (M /M e 1.23),
the orange PPO-TEMPO compared to the colorless PPO-
OH. A typical signal of CH of TEMPO located at 1.84 ppm
2
1
in the H NMR spectrum after the reaction, confirming the
existence of TEMPO radical. The peaks of CH and CH of
3
TEMPO were overlapped with those of PPO and CD OD,
3
respectively. In addition, the peak at 2.57 ppm is attributed
to 4 protons of CO CH CH CO connection between TEM-
2
2
2
2
PO end group and PPO segment, which also illustrated the
successful incorporation of TEMPO radical. The molecular
weight of the product derived from PPO-OH (M = 1000 g/
n
w
n
mol) is 1317 g/mol (Table 1) measured by MAIDI-TOF-MS,
which is in good accordance with the theoretical value
and the molecular weights increased with the raising of the
composition of PPO-TEMPO 3 in the feed ratios.

Article
Macromolecules, Vol. 43, No. 1, 2010 267
0
Scheme 2. Mechanism of Cu -Catalyzed ATNRC Reaction
71.4 (OCH CH CH CH ), 73.5 (OCH CHCH ), and 75.5
ppm (OCH CHCH ) are attributed to PPO side chains com-
2
2
2
3
2
3
2
3
13
pared to C NMR spectrum of PPO-OH (Figure 6C). Typical
signals of tert-butyl of PtBA backbone are found to locate
at 28.3 (C(CH ) ) and 83.3 ppm (C(CH ) ). The TEMPO
3
3
3 3
1
connection between PtBA backbone and PPO side chains is
also evidenced by the peaks at 21.3 (NCCH ), 33.8 (CO CH-
Figure 5. H NMR spectra of PtBA-g-PPO 4 (A) and PAA-g-PPO 5
B) in CDCl
(
3
.
3
2
(
CH ) ), 44.6 (NCCH ), and 70.5 ppm (CO CH(CH ) ).
2 2 3 2 2 2
Generally, ATNRC reaction was catalyzed by a relatively
expensive catalyst, CuBr (Aldrich 212865, 98%, US $70.90/
2
50 g), under a relatively high temperature (90 °C), which
may cause some side reactions including the chain transfer
and the cross-linking. Is it possible to optimize ATNRC so
that it can be run under the relatively mild conditions? Percec
3
7,38
0
and co-workers have developed SET-LRP, in which Cu
powder (Aldrich 207780, 99%, US $41.60/500 g) was used as
I
an economical catalyst instead of Cu at ambient
temperature.
5
6-67
0
Thus, it is feasible to adopt Cu powder
instead of CuBr to generate the macroradicals according to
the mechanism of SET-LRP to synthesize PtBA-g-PPO graft
copolymer under the relatively mild conditions. This experi-
0
ment was carried out in toluene at 20 °C using the Cu /
PMDETA catalytic system, and a well-defined PtBA-g-PPO
graft copolymer 4d (M = 25 300, M /M = 1.05) was
n
w
n
successfully synthesized with a similar grafting density
Table 2) and GPC curve (Figure 4D) in comparison with
the entry of 4c catalyzed by CuBr/PMDETA at 90 °C
Table 2). This strategy is similar to SET-NRC recently
(
(
6
8
reported by Huang and co-workers, in which THF and
ethanol were as solvents. In the current system, equimolar
Cu transferred an electron to Br-containing repeating unit
0
I
(P-Br) to afford the macroradical and Cu by the SET
mechanism,
3
7,38,56-67
as shown in Scheme 2. The formed
macroradicals were captured by TEMPO-functionalized
PPO so that PPO side chains were linked to the backbone.
Since the molecular weight of graft copolymer measured
1
3
Figure 6. C NMR spectra of PtBA-g-PPO 4 (A), PAA-g-PPO 5 (B),
and PPO-OH (C) in CDCl
3
.
6
9
The resonance signals of the corresponding protons of
both PtBA backbone and PPO side chains are found in the
H NMR spectrum as shown in Figure 5A. The peaks at 1.14
OCH CHCH ), 3.43 (OCH CHCH ), and 3.56 ppm
2 3 2 3
OCH CHCH ) are attributed to PPO side chains. A typical
3
signal of 9 protons of tert-butyl of PtBA backbone locates at
.47 ppm, which is overlapped with the signal of 4 protons of
OCH CH CH CH end group of PPO side chains. The
minor peak at 5.03 ppm (peak “m”) belongs to 1 proton of
CH of piperidine, which demonstrated the existence of TEM-
PO linkage between PtBA backbone and PPO side chains.
Figure 6A shows the C NMR spectrum of PtBA-g-PPO 4.
The peaks at 14.1 (OCH CH CH CH ), 17.5 (OCH CH-
CH ), 19.5 (OCH CH CH CH ), 31.9 (OCH CH CH CH ),
by GPC is very different from the “real” value, the number
of grafted PPO side chains (nPPO) was determined by H
1
1
NMR instead of GPC. This value was calculated according
to eq 1 (S is the peak area, and NPPO (16.0 or 41.9) is the
number of PO repeating unit per side chain) as listed in
Table 2. Furthermore, the grafting density of PPO side chain
(Dgraft) was calculated via eq 2 (20.5 is the number of tBA
repeating unit of the backbone), and the results are summar-
ized in Table 2. Thus, the grafting efficiency can be estimated
to be in the range from 48% to 65%, which is similar to that
of a recent report about synthesizing the graft copolymers via
(
(
2
1
2
2
2
3
13
48
ATNRC strategy. Another recent article also reported the
I
moderate coupling efficiency (36-75%) of the Cu -catalyzed
click “grafting-onto” reaction. Relatively low efficiency is
2
2
2
3
2
7
0
3
2
2
2
3
2
2
2
3

2
68 Macromolecules, Vol. 43, No. 1, 2010
Li et al.
typical in synthesizing the graft copolymer via the grafting-
onto strategy due to the steric hindrance of the dense
4
8,70
structure of the graft copolymer.
From the above results,
it can be concluded that the synthesized graft copolymers,
PtBA-g-PPO 4, possess well-defined structure: a poly(tert-
butyl acrylate) backbone (20.5 repeating units) and 4.0-13.4
grafted PPO side chains with certain length (16.0 or 41.9
repeating units per side chain).
Sd,e,h=Si,j,l ¼ ð3NPPO þ 2ÞnPPO=ð20:5 ꢀ 9 þ4nPPOÞ ð1Þ
Dgraft ¼ nPPO=20:5
ð2Þ
Selective Hydrolysis of PtBA Backbone. Trifluoroacetic
acid was used to hydrolyze the hydrophobic PtBA backbone
to the hydrophilic PAA backbone in CH Cl according to
2
2
7
1-73
previous literatures. The chemical structure of the
hydrolyzed product was characterized by H NMR,
1
13
C
NMR, and FT-IR. The integration area of the peak at 1.47
1
ppm in the H NMR spectrum after the hydrolysis
Figure 7. FT-IR spectra of PAA-g-PPO 5 (A) and PtBA-g-PPO 4 (B).
(Figure 5B) reduced remarkably due to the disappearance
of the signal of 9 protons of tert-butyl group. Moreover, the
characteristic signals of the carbons of C(CH ) group at 28.3
3
3
(
peak “n” in Figure 6A) and 83.3 (peak “o” in Figure 6A)
1
3
ppm in the C NMR spectrum completely disappeared after
the hydrolysis as shown in Figure 6B, which demonstrated
the successful hydrolysis of PtBA backbone. A new broad
-
1
peak of -COOH appeared at 3445 cm in the FT-IR
spectrum after the hydrolysis (Figure 7A) in comparison
with that before the hydrolysis (Figure 7B), indicating the
presence of carboxyl groups and the formation of PAA
backbone. The minor peak at 5.03 ppm (peak “m”) still
appeared in Figure 5B after the hydrolysis, which verified the
remaining of TEMPO linkage between PAA backbone and
PPO side chains. Furthermore, typical signals of PPO side
1
13
chains remained in both H NMR and C NMR spectra
after the hydrolysis; this confirmed that PPO side chains
were not affected during the hydrolysis. Therefore, we can
conclude that PtBA-g-PPO was selectively hydrolyzed to
PAA-g-PPO.
Figure 8. Dependence of fluorescence intensity ratios of pyrene emis-
sion bands on the concentrations of PAA-g-PPO 5b.
Self-Assembly of PAA-g-PPO Amphiphilic Graft Copoly-
mer. The cmc values of PAA-g-PPO 5 amphiphilic graft
copolymers in aqueous media were examined by the fluor-
7
4,75
a
escence technique using pyrene as probe.
The fluores-
cence spectrum of pyrene is sensitively influenced by the
Table 3. cmc of PAA-g-PPO Amphiphilic Graft Copolymer
6
9,71
PtBA-g-PPO
PAA-g-PPO
cmc (g/mL)
environment and the polarity of its surrounding.
presence of micelles, pyrene is solubilized within the interior
In the
-
-
-
6
6
7
4a
4b
5a
5b
5c
5e
5f
2.08 ꢀ 10
1.51 ꢀ 10
9.57 ꢀ 10
1.47 ꢀ 10
of the hydrophobic part. As a result, the values of I /I of
1
3
4
4
4
4
c
e
f
the emission spectrum changed sharply. The intensity ratios
I /I ) against the logarithm of polymer concentrations were
plotted to determine cmc as the onset of micellization
-
6
(
1
3
-6
-7
1.24 ꢀ 10
9.84 ꢀ 10
g
5g
Determined by fluorescence spectroscopy using pyrene as probe.
(Figure 8).
a
The cmc values of PAA-g-PPO 5 are summarized in
Table 3. These values were comparable with those of poly-
copolymer may make sense. Eisenberg and co-workers
found that the micellar morphologies formed by PS-b-PAA
amphiphilic diblock copolymers could be tuned by the
compositions of the copolymers. Thus, it may be interest-
ing to investigate the micellar morphology of PAA-g-PPO 5
amphiphilic graft copolymer with the changing of the con-
tent of hydrophobic PPO segment.
In the current work, micellar structures were investigated
by TEM. Diverse morphologies, including spheres,
branched short rods, nanofibers, and vesicles, were formed
by PAA-g-PPO 5 graft copolymers with different molecular
weights in pure water (Figure 9). Figure 9A shows the
spherical micelles (ca. 340 nm) formed by PAA-g-PPO 5a
synthesized from PPO-TEMPO 3a. These spherical micelles
7
6-79
meric amphiphiles.
The cmc values of PAA-g-PPO
decreased when the molecular weights increased due to the
increasing of the content of hydrophobic PPO side chains.
As for PPO-based amphiphilic copolymers, only a few
examples touched on their self-assembly properties in the
selective solvents. Liu and co-workers reported the reversible
temperature-induced self-assembly process of PPO-based
8
0
5
,6
block copolymers. Furthermore, the sizes and morpholo-
gies of PEO-b-PPO amphiphilic diblock copolymer changed
as the variation of the concentration of the aqueous solu-
1
9
tion. However, none has reported the micellar morphology
of PPO-based amphiphilic graft copolymer. The studies on
the micellar morphology of PAA-g-PPO amphiphilic graft

Article
Macromolecules, Vol. 43, No. 1, 2010 269
Acknowledgment. The authors thank the financial support
from National Natural Science Foundation of China (20974117,
2
(
0904065, and 50873029), Shanghai Rising Star Program
07QA14066), Shanghai Nano-Technology Program (0952
nm05800), Shanghai Scientific and Technological Innovation
Project (08431902300), and Knowledge Innovation Program of
the Chinese Academy of Sciences.
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Conclusions
8
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