Self-assembly behavior of amphiphilic linear-block-dendritic copolymers with long subchains: Dependences on dendron generation and mixing dynamics

Article abstract of DOI:10.1002/pola.29026

Block copolymers composed of acrylic acid and methyl methacrylate with three topologies of double linear blocks, poly(acrylic acid) (PAA) linear block/poly(methyl methacrylate) (PMMA) G1-dendron and PAA linear block/PMMA G2-dendron have been prepared by the combination of atom transfer radical polymerization and azide–alkyne click reaction. Proton nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and gel permeation chromatography have been adopted thoroughly to identify the chemical structure of those block copolymers with expected topologies. The self-assembly of those block copolymers in the selective solvent has been performed through two mixing routes of gentle and abrupt variation in solvent selectivity, and the morphology of the obtained self-assemblies/aggregates was observed by transmission electron microscopy. Because the abrupt variation route altered sharply the solvent quality during the mixing, the intermolecular association of polymer chains resulted in the smaller self-assemblies but the further growth of smaller self-assemblies was not observed. On the contrary, the gentle variation route changed gradually the solvent quality during the mixing, favoring not only the intermolecular association but also the further growth of self-assemblies to result in larger aggregates. The final morphology of those assemblies/aggregates also exhibited the dependence of PMMA dendron generation.

Full text of DOI:10.1002/pola.29026

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Self-Assembly Behavior of Amphiphilic Linear-Block-Dendritic
Copolymers with Long Subchains: Dependences on Dendron
Generation and Mixing Dynamics
1
1,2
1
1
1
1
Peng-Yun Li, Chen He, Wei-Xiang Xie, Hui-Juan Li, Abdul Haleem, Chuan-Shan Hu,
1
1
Xi-Chuan Li, Wei-Dong He
1
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and
Technology of China, Hefei, Anhui 230026, China
2
Institute of Aerospace Material and Processing, Beijing 100076, China
Correspondence to: W.-D. He (E-mail: wdhe@ustc.edu.cn)
Received 4 March 2018; accepted 30 March 2018; published online 00 Month 2018
DOI: 10.1002/pola.29026
ABSTRACT: Block copolymers composed of acrylic acid and
methyl methacrylate with three topologies of double linear
blocks, poly(acrylic acid) (PAA) linear block/poly(methyl meth-
acrylate) (PMMA) G1-dendron and PAA linear block/PMMA G2-
dendron have been prepared by the combination of atom
transfer radical polymerization and azide–alkyne click reaction.
Proton nuclear magnetic resonance spectroscopy, Fourier
transform infrared spectroscopy, and gel permeation chroma-
tography have been adopted thoroughly to identify the chemi-
cal structure of those block copolymers with expected
topologies. The self-assembly of those block copolymers in the
selective solvent has been performed through two mixing
routes of gentle and abrupt variation in solvent selectivity, and
the morphology of the obtained self-assemblies/aggregates
was observed by transmission electron microscopy. Because
the abrupt variation route altered sharply the solvent quality
during the mixing, the intermolecular association of polymer
chains resulted in the smaller self-assemblies but the further
growth of smaller self-assemblies was not observed. On the
contrary, the gentle variation route changed gradually the sol-
vent quality during the mixing, favoring not only the intermo-
lecular association but also the further growth of self-
assemblies to result in larger aggregates. The final morphology
of those assemblies/aggregates also exhibited the dependence
of PMMA dendron generation. V
2018 Wiley Periodicals, Inc. J.
C
Polym. Sci., Part A: Polym. Chem. 2018, 00, 000–000
KEYWORDS: amphiphiles; block copolymers; dendron genera-
tion; linear-b-dendron copolymer; mixing dynamics; self-
assembly
1
0
10
INTRODUCTION Self-assembly of amphiphilic copolymers in
the selective solvent has prevailed in the past decades. The stud-
special surface adsorption,
regio-selective modification,
2
3,24
25
stimuli-responsive device,
photoluminescence, and elec-
1
26
ied topologies of amphiphilic copolymer can be linear diblock,
tro-chemistry. However, the dependence of linear-b-dendron
self-assembly on the dendron generation is still in the way of
investigation.
2,3
4
linear triblock, and miktoarm star-block, further extending to
5
–12
13
linear-b-dendron,
g-multi-arm,
linear-b-hyperbranched, hyperbranched-
1
4,15
16
hyperbranched-b-linear-b-hyperbranched,
1
7
The self-assembly behavior of linear block copolymers in the
selective solvent has been well recognized. Generally, the
morphology is affected by the parameters of block lengths,
block length ratio, solvent selectivity, and polymer concentra-
tion. The dynamics of mixing polymer solution with selective
solvent is also an important factor to control the self-
assembly morphology. For example, Nose and coworkers dis-
closed the strong dependence of poly(N,N-dimethylacryla-
mide)-g-poly(methyl methacryl-ate) self-assembled structure
on the rate of mixing polymer/methanol solution with
and hyperbranched-b-hyperbranched. Among them, linear-b-
dendron copolymers are more attractive due to their combina-
tion of the loose chain conformation from linear block and the
compact chain conformation from dendritic segment, leading to
more complicated phase-separation behavior both in bulk and
solution. The synthesis of linear-b-dendron copolymer can be
mainly divided to the following: the coupling strategy, the linear-
18
first strategy, and the dendron-first strategy. As for their func-
tionality, linear-b-dendron copolymers have been exploited in
1
9
20
the areas of drug release,
gene delivery,
semi-
21
22
27
interpenetrating network materials,
biomineralization,
water. Fast mixing induced the small self-assemblies with
Additional Supporting Information may be found in the online version of this article.
2018 Wiley Periodicals, Inc.
V
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SCHEME 1 Schematic self-assembly morphologies of L-PAA-b-L-PMMA, L-PAA-b-G1-PMMA, and L-PAA-b-G2-PMMA via two mix-
ing routes. [Color figure can be viewed at wileyonlinelibrary.com]
unimodal size distribution, while slower mixing rate yielded
the larger self-assemblies with bimodal size distribution. Han
and coworkers prepared the self-assemblies of poly(acrylic
acid)-g-poly(ethylene oxide)-g-dodecyl (PAA-g-PEO-g-dodecyl)
through two mixing dynamics of gentle and abrupt variation in
solvent selectivity. It was found that at low pH value and
high dodecyl content, abrupt variation promoted intrachain
association. Especially, Gillies and coworkers synthesized lin-
ear-b-dendron copolymers composed of linear poly(c-benzyl-
L-glutamate) and hydroxyl-capped polyester dendron (PBLGx-
EXPERIMENTAL
Materials
Methyl methacrylate (MMA) and tert-butyl acrylate (tBA) (Sino-
pharm Chemical) were distilled under vacuum to remove the
inhibitor. Tetrahydrofuran (THF) was refluxed over CaH2 for
2
8
6
h, over sodium/benzophenone until it turned black-purple,
then distilled for use. N,N-Dimethylformamide (DMF) was dried
with anhydrous MgSO and distilled under reduced pressure.
Copper bromide (Sinopharm Chemical) was reduced by 0.01 M
4
Na SO3 aqueous solution, filtered, and subsequently washed
2
b-G -OH, x and y indicates the chain length and the generation
y
with 1 wt % HBr aqueous solution, acetic acid, and alcohol for
twice, sequentially. Ethyl 2-bromoisobutyrate (EBiB) and
propargyl 2,2-bis((2 -bromo-2 -methyl-propanoyloxy)methyl)-
2
9
number, respectively). They found that fast injection of
31
PBLG -b-G -OH/DMF or PBLG -b-G -OH/DMF solution into
0
0
2
8
3
54
3
water mainly induced the formation of solid aggregates,
whereas slow injection of water into the polymer solution pro-
moted the formation of assemblies like micelles and larger
spherical objects.
propionate (PBMP, NMR spectrum in Supporting Information
32
Fig. S2)
were prepared according to the literature. 2-
0
00 00
Bromoisobutyryl bromide and N,N,N ,N ,N -pentamethyldiethy-
lenetriamine (PMDETA) (Sigma-Aldrich) and other chemical
reagents (Aladdin) were used without further purification.
Deionized water was used in the preparation of self-assemblies.
Although the self-assembly of amphiphilic copolymers com-
posed of linear block and short-subchain dendron has been
5
–9,29,30
studied,
those concerning with linear block/long-sub-
Synthesis of Propargyl 2-Bromoisobutyrate
chain dendron architecture are still rarely reported. Fan et al.
A 250-mL round-bottomed flask was charged with propiolic
alcohol (5.61 g, 100 mmol) and CH Cl (100 mL). Then, trie-
synthesized linear poly(D- or L-lactide)-block-long-subchain
2
2
1
2
dendritic PAA and studied the self-assembly properties.
thylamine (11.14 g, 110.1 mmol) was added. After the mix-
ture was kept in an ice bath for 30 min, 2-bromoisobutyryl
bromide (25.29 g, 110 mmol) in CH Cl (50 mL) was added
However, as for any kind of linear-b-dendron copolymers, the
influence of dendron generation was still not concerned.
2
2
dropwise and the reaction was left at ambient temperature
overnight. The reaction mixture was washed with water (4
In this work, we obtained amphiphilic PAA/poly(methyl
methacrylate) (PMMA) block copolymers with three
topologies such as double linear blocks, PAA linear block/
long-subchain PMMA G1-dendron, and PAA linear block/
long-subchain PMMA G2-dendron, namely, L-PAA-b-L-PMMA,
L-PAA-b-G1-PMMA, and L-PAA-b-G2-PMMA. Their self-
assemblies were prepared by two mixing dynamics: gentle
and abrupt variation in solvent selectivity. As illustrated in
Scheme 1, the effects of mixing dynamics and dendron gen-
eration on the self-assembly morphology have been studied
in detail.
3
100 mL) and the combined organic phase was dried with
anhydrous Na SO . Finally, CH Cl was removed by rotary
2
4
2
2
evaporation, and the product (yield: 14.36 g, 70%) was
obtained by vacuum distillation. Its NMR spectrum was
shown in Supporting Information Figure S1.
Synthesis of Block Copolymers with Different Topologies
Synthesis of Linear Poly(tert-butyl acrylate) Through ATRP
A 100-mL round-bottomed flask was charged with EBiB
(441.1 mg, 2.3 mmol), tBA (31.5 g, 245.8 mmol), PMDETA
2
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(
395.5 mg, 2.3 mmol), and acetone (24 mL). After three
Yield:
m(AC(CH
2.72
g,
M
5 18,000,
PDI 5 1.20.
FTIR:
n,GPC
1
2
21
freeze–pump–thaw cycles, CuBr (333.1 mg, 2.3 mmol) was
added. The flask was kept in an oil bath controlled at 60 8C
and the reaction was allowed to last for 10 h. After the reac-
tion was quenched in liquid nitrogen, the reaction mixture
was diluted with THF and passed through the neutral alu-
mina column to remove the copper salt. After concentrated,
the mixture was precipitated in methanol/water (50/50; v/
v) for three times. After decantation, the polymer was dried
under vacuum at ambient temperature for 24 h and obtained
as a solid.
) ): 1369 cm ; m(AC@O): 1728 cm ; m(CACAO):
3 3
2
1
1
1191 cm
.
H NMR (CDCl
AOAC@O (5.24); CH
ACHAtriazole (4.01); ACH ACHAC@O (2.15); BrACA
(1.86); ACH ACHAC@O, OAC(CH (1.85–1.22);
ACH AOAC@O (1.19); O@CACA (CH (1.06).
3
) d : proton in tirazole ring
(7.73); triazoleACH
ACH
(CH
CH
ACH AOAC@O,
2
3
2
2
2
)
3
)
3 3
)
3 2
2
2
3
2
L-PtBA-b-L-PMMA (1.102 g, 0.06 mmol) and CH Cl (20 mL)
were added into a 100-mL round-bottomed flask. After com-
plete dissolution, trifluoroacetic acid (TFA, 3 mL) was added.
2
2
After standing at ambient temperature for 2 days, CH Cl2
2
and TFA were removed under low vacuum. The crude prod-
uct was dissolved in THF and precipitated in n-hexane for
three times. After dryness under vacuum at ambient temper-
ature for 24 h, L-PAA-b-L-PMMA was obtained.
Yield: 8.26 g, M
5 4600, PDI 5 1.12. FTIR: m(AC(CH ) ):
3 3
n,GPC
2
1
21
1
1369 cm ; m(AC@O): 1728 cm
.
H NMR (CDCl ) d :
3
ACH AOAC@O, ACH ACHABr (4.01); ACH ACHAC@O
2
2
2
(2.15);
ACH ACHAC@O,
AOAC(CH3)3
(1.95–1.22);
2
CH ACH AOAC@O (1.19); O@CACA (CH ) (1.06).
3
2
3 2
21
Yield: 0.82 g. FTIR: m(AC@O): 1728 cm ; m(CACAO):
2
1
1
1191 cm
.
H
NMR (DMF-d7)
d
:ACOOH (12.69);
ACH ACHAC@O (2.48); ACH ACHAC@O, ACH ACAC@O
Synthesis of Linear PAA-Block-PMMA (L-PAA-b-L-PMMA)
A 100-mL round-bottomed flask was charged with linear
poly(tert-butyl acrylate) (L-PtBA-Br) (5.26 g, 1.1 mmol),
2
2
2
(
1.89); CH
3
ACAC@O (0.60–1.20).
NaN (936.7 mg, 14.4 mmol), and DMF (50 mL), and then it
was kept in an oil bath at 40 8C. The reaction proceeded for
3
Synthesis of L-PAA-b-G1-PMMA and L-PAA-b-G2-PMMA
A dry Schlenk flask was charged with L-PtBA-N (3.2 g, 0.7
3
36 h. The reaction mixture was diluted with THF and passed
mmol), PBMP (0.4927 g, 1.05 mmol), PMDETA (0.1820 g,
through a neutral alumina column to remove NaN . After-
3
1.05 mmol), and DMF (20 mL). After three freeze–pump–
ward, the solution was concentrated by rotary evaporation
and subsequent vacuum distillation. The product L-PtBA-N3
was obtained by thrice precipitation in methanol/water (50/
thaw cycles, CuBr (0.1507 g, 1.05 mmol) was added. Then
the flask was kept in an oil bath preheated at 40 8C and the
reaction lasted for 12 h. Subsequently, the reaction mixture
was diluted with THF, passed through a neutral alumina col-
umn. Afterward, the mixture was concentrated by rotary
evaporation and precipitated in methanol/water (50/50; v/
v) for three times and dried under vacuum to obtain L-PtBA-
PBMP with two ATRP initiation sites. Yield: 2.62 g.
50; v/v) and dried under vacuum at room temperature.
2
1
Yield: 4.74 g. FTIR: m(AC(CH ) ): 1369 cm ; m(AC@O):
3
21
3
2
1
1728 cm ; m(AN
3
): 2114 cm
.
To a 25-mL Schlenk flask, L-PtBA-N3 (1.43 g, 0.3 mmol),
propargyl 2-bromoisobutyrate (PBiB) (0.1235 g, 0.6 mmol),
PMDETA (52.0 mg, 0.3 mmol), and DMF (15 mL) were
added, followed by the addition of CuBr (43.2 mg, 0.3 mmol)
after three freeze–pump–thaw cycles. After sealed under vac-
uum, the Schlenk flask was kept in an oil bath preheated at
The synthesis of L-PtBA-b-G1-PMMA was carried out as fol-
lows. A 50-mL Schlenk flask charged with PtBA-PBMP (1.6 g,
0
0
.32 mmol), MMA (22.9 g, 228.7 mmol), PMDETA (111.0 mg,
.64 mmol), and THF (4 mL) was degassed by three freeze–
pump–thaw cycles. Then, CuBr (91.8 mg, 0.64 mmol) was
added and the flask was kept in an oil bath at 40 8C for 15
min. Subsequently, the reaction was stopped by dipping the
flask in liquid nitrogen. The reaction mixture was diluted
with THF, passed through a neutral alumina column. After
rotary evaporation, thrice precipitation in n-hexane, and dry-
ness under vacuum, the product was obtained as powder.
40 8C. The reaction proceeded for 12 h and quenched in liq-
uid nitrogen. After dilution with THF, the reaction mixture
was passed through neutral alumina column. Subsequently,
the mixture was concentrated, precipitated in methanol/
water (50/50; v/v) for three times, and dried under vacuum
at ambient temperature to obtain L-PtBA-PBiB as white pow-
der (yield: 1.12 g).
Yield:
6.68
g,
M
5 24,000,
PDI 5 1.20.
FTIR:
n,GPC
21
L-PtBA-PBiB (1.003 g, 0.21 mmol), MMA (7.511 g, 75.0
mmol), PMDETA (42.5 mg, 0.25 mmol), and THF (1.25 mL)
were added into a 25-mL Schlenk flask. After three freeze–
pump–thaw cycles, CuBr (30.5 mg, 0.21 mmol) was added.
Then the Schlenk flask was sealed under vacuum and kept
in an oil bath at 40 8C. The reaction lasted for 15 min and
quenched in liquid nitrogen. Subsequently, the reaction mix-
ture was diluted with THF, passed through a neutral alumina
column. L-PtBA-b-L-PMMA as white solid was obtained after
thrice precipitation in n-hexane and dryness under vacuum
at room temperature for 24 h.
21
m(AC(CH ) ): 1369 cm ; m(AC@O): 1728 cm ; m(CACAO):
3
3
21
1
1191 cm
.
H
NMR (CDCl
3
)
d
:
AOCH
3
(3.53);
ACH ACHAC@O (2.15); ACH ACHAC@O (1.98–1.64);
2
2
OAC(CH ) (1.37); CH ACAC@O (0.38–1.08).
3
3
3
The synthesis of L-PtBA-b-G2-PMMA follows the same proce-
dure as the preparation of L-PtBA-b-G1-PMMA, such as the
azidation of bromo end-groups of L-PtBA-b-G1-PMMA-Br,
click reaction of L-PtBA-b-G1-PMMA-N with PBMP and final
ATRP of MMA with L-PtBA-b-G1-PMMA-PBMP as the
3
macroinitiator.
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SCHEME 2 Synthesis of linear diblock copolymer of L-PAA-b-L-PMMA. [Color figure can be viewed at wileyonlinelibrary.com]
2
1
Mn,GPC 5 28,400, PDI 5 1.21. FTIR: m(AC(CH ) ): 1369 cm
;
Characterization
Fourier transform infrared (FTIR) spectra were recorded on
a Bruker VECTOR-22 IR spectrometer using KBr pellets. Pro-
3 3
2
1
21
1
m(AC@O): 1728 cm ; m(CACAO): 1191 cm
.
H NMR
(
CDCl3) AOCH3 (3.53); ACH ACHAC@O (2.15);
ACH ACHAC@O (1.98–1.64); AC(CH ) (1.37); CH ACAC@O
d
:
2
1
ton nuclear magnetic resonance ( H-NMR) analysis was per-
2
3 3
3
(
0.38–1.08).
formed on a Bruker DRX-300 NMR (300 MHz) instrument in
CDCl3 or DMF-d7 at room temperature with tetramethylsi-
lane as the internal standard. The morphology of self-
assemblies was observed under transmission electron
microscopy (TEM, Hitachi H-7650, 100 kV). In a typical pro-
tocol of preparing TEM samples, 5 lL thinned aqueous dis-
persion of the self-assemblies was deposited to copper grid.
Then, water was evaporated naturally. The molecular weight
and its distribution were determined by gel permeation
chromatography (GPC) using Waters 1515 apparatus
equipped with a refractive index detector (RI, Wyatt WREX-
The hydrolysis of PtBA block in L-PtBA-b-G1-PMMA and L-
PtBA-b-G2-PMMA was carried out by the same procedure as
that for L-PAA-b-L-PMMA.
Self-assembly of Block Copolymers Through Different
Mixing Dynamics
DMF was selected as the common solvent to dissolve the
block copolymers and water as the selective solvent to
induce the self-assembly.
02), a multiangle LLS detector (MALLS, Wyatt DAWN EOS),
The typical procedure for gentle variation in solvent selectiv-
2
1
ity was as follows. Copolymer solution (5.0 mL, 2 mgꢀmL
)
in DMF was added into a glass vial equipped with a stirring
bar after passing through a 0.45-lm nylon filter. Under vig-
2
1
orous stirring, water (2.0 mL) was injected at 10 mL h by
syringe pump. As for L-PtBA-b-G2-PMMA, mixed solvent
(Vwater:VDMF 5 1) was slowly injected to copolymer solution
in DMF to avoid the precipitation. All of the self-assemblies
were dialyzed against water for 3 days with dialysis mem-
21
brane (cut-off molecular weight: 3500 g mol ) to remove
DMF. Finally, polymer concentration of self-assembly disper-
21
sion was adjusted to 1 mg mL
.
Compared with the gentle variation in solvent selectivity, the
abrupt variation in mixing dynamic was achieved by inject-
ing copolymer/DMF solution into water at the rate of 10
2
1
mL h . The concentration and volume of initial copolymer
solution as well as water volume were the same as above.
Finally, those self-assembly dispersions were adjusted to the
1
FIGURE 1 H NMR spectra of L-PtBA-Br (A) and L-PtBA-PBiB (B)
21
concentration of 1 mg mL
.
3
in CDCl . [Color figure can be viewed at wileyonlinelibrary.com]
4
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FIGURE 3 GPC curves of L-PtBA-Br, L-PtBA-b-L-PMMA, L-PtBA-
b-G1-PMMA, and L-PtBA-b-G2-PMMA. [Color figure can be
viewed at wileyonlinelibrary.com]
originates from the overlapping of tertiary butyl group and
methylene group of PtBA backbone. The signals at 1.19 ppm
(
a) and 1.06 ppm (c) come from the methyl group of EBiB
residue and that at 4.01 ppm (b, g) is attributed to the over-
lapping of methylene protons from EBiB and the methine
proton of tBA unit beside bromo end-group. The polymeriza-
tion degree (DP) of tBA calculated from the integral height
ratio of signal “e” to that of signal “b 1 g” (H /Hb1g) is 38.
e
As illustrated in Figure 1(B), after the click reaction of PtBA-
N with PBiB, the characteristic signals of triazole proton at
3
7.73 ppm (h), methylene protons next to the electron-
withdrawing triazole ring at 5.24 ppm (i) and methyl pto-
tons from PBiB at 1.86 ppm (j) appear. The integral height
ratio of “H /Hb1g” is about 2:3. The above results suggest the
successful synthesis of L-PtBA-PBiB.
i
1
FIGURE 2 H NMR spectra of L-PtBA-b-L-PMMA in CDCl
3
(A)
and L-PAA-b-L-PMMA in DMF-d
7
(B). [Color figure can be
Subsequently, L-PtBA-b-L-PMMA was prepared by ATRP of
viewed at wileyonlinelibrary.com]
MMA using L-PtBA-PBiB macroinitiator. As shown in Figure
0
2
(A), after the polymerization, the signal at 3.6 ppm (a ) and
and three Waters Styragel columns (HR2, HR4, HR6). Meas-
urements were performed in THF at the flow rate of 1
mL min at room temperature and the calibration was built
on polystyrene standards.
0
the signal near 0.98 ppm (e ) corresponding to the protons
of methoxy group and a-methyl group of MMA unit appear.
2
1
The DP of PMMA block determined from the integral height
ratio of “Ha /H ” is 86. Furthermore, as revealed in Figure 3,
e
0
GPC traces of L-PtBA-Br and L-PtBA-b-L-PMMA are narrow
and unimodal. The elution time of L-PtBA-b-L-PMMA shifts
to lower elution time compared with that of L-PtBA-Br, indi-
cating the successful polymerization of MMA monomer. Mn,
RESULTS AND DISCUSSION
Preparation of L-PAA-b-L-PMMA
The synthesis procedure of L-PAA-b-L-PMMA is illustrated in
Scheme 2. L-PtBA-Br was first obtained by ATRP of tBA with
M , and PDI for both polymers are summarized in Table 1.
w
3
EBiB as initiator, then converted to PtBA-N through the azi-
dation of bromo end-group. Afterward, click reaction
between PtBA-N and PBiB produced the macroinitiator of L-
PtBA-PBiB for ATRP of MMA.
3
TABLE 1 Molecular Characteristics of L-PtBA-Br and L-PtBA-b-
L-PMMA
1
H NMR analysis was used to confirm the click reaction
Sample
M
n,GPC
M
w,GPC
PDI
M
n,NMR
between L-PtBA-N and PBiB. Figure 1(A) reveals NMR spec-
3
trum of L-PtBA-Br. The signal at 2.15 ppm (e) is ascribed to
the methine group of PtBA and that at 1.22–1.95 ppm (f, d)
L-PtBA-Br
4600
5100
1.12
1.20
5100
L-PtBA-b-L-PMMA
18,000
21,600
13,800
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21
ring. As shown in Figure 4(D), the absorbance at 1277 cm
2
1
and 1190 cm can be assigned to the symmetrical deforma-
tion vibration of a-methyl group and asymmetrical stretching
vibration of CACAO group of PMMA, respectively, which also
reveals the introduction of PMMA block. After the selective
hydrolysis to remove tert-butyl group from L-PtBA block, L-
PAA-b-L-PMMA was finally obtained. Comparing Figure 4(D)
with Figure 4(E), it is easy to find that the characteristic sig-
nal of symmetric stretching vibration of tertiary butyl at
2
1
1
369 cm disappears after the selective hydrolysis. As well,
2
1
21
a broad peak from 3500 cm to 2300 cm attributed to
carboxylic acid groups appears.
1
The selective hydrolysis was also confirmed by H NMR anal-
ysis as shown in Figure 2(B). The appearance of new signal
at 12.69 ppm (f) can be attributed to the proton of newly
formed carboxylic group. Meanwhile, the signal at ꢁ1.60
3
FIGURE 4 FTIR spectra of L-PtBA-Br (A), L-PtBA-N (B), L-PtBA-
0
ppm (d 1 d ), resulting from the methylene protons of poly-
PBiB (C), L-PtBA-b-L-PMMA (D), and L-PAA-b-L-PMMA (E).
Color figure can be viewed at wileyonlinelibrary.com]
mer backbone, decreased dramatically due to the removal of
[
tert-butyl group. The relative integral heights of the signal at
0
1
2.69 ppm and that at 0.60–1.20 ppm (H and H ) is consis-
f
e
Figure 4 reveals FTIR spectra of L-PtBA-Br, L-PtBA-N , L-
3
tent with quantitative hydrolysis.
PtBA-PBiB, L-PtBA-b-L-PMMA, and L-PAA-b-L-PMMA. The
21
peak at the wavenumber of 1369 cm
is assigned to the
Preparation of L-PAA-b-G1-PMMA and L-PAA-b-G2-PMMA
Scheme 3 reveals the whole synthesis procedure of L-PtBA-
b-G1-PMMA and L-PAA-b-G2-PMMA. To ensure the well con-
trol over ATRP of MMA, the bromo end-groups of L-PtBA-Br
and L-PtBA-b-G1-PMMA-Br were converted into azido
groups, followed with the introduction of tertiary bromo at
the corresponding macro-initiators such as L-PtBA-PBMP for
L-PtBA-b-G1-PMMA and L-PtBA-b-G1-PMMA-PBMP for L-
characteristic symmetric stretching vibration of tertiary butyl
from PtBA. Compare Figure 4(A) with Figure 4(B), it can be
found that, after the azidation of PtBA-Br, characteristic
2
1
absorbance at 2114 cm attributed to azido group appears,
demonstrating the successful substitution of bromine group.
After the click reaction with PBiB, this absorbance signal dis-
appears due to the conversion of azido group into triazole
SCHEME 3 Synthetic route to linear block dendritic copolymers of L-PtBA-b-G1-PMMA and L-PAA-b-G2-PMMA. [Color figure can
be viewed at wileyonlinelibrary.com]
6
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FIGURE 6 H NMR spectra of L-PtBA-b-G1-PMMA (A) and L-
3
PtBA-b-G2-PMMA (B) in CDCl . [Color figure can be viewed at
wileyonlinelibrary.com]
for 15 min. As shown in Figure 6(A), after ATRP of MMA
0
monomer, the signals of AOCH at 3.53 ppm (a ) and ACH
3
3
0
at 0.38–1.08 ppm (e ) originated from PMMA appear. In the
case of L-PtBA-b-G2-PMMA [Fig. 6(B)], the integral height
0
e a
ratio of signal e to signal a (H /H ) decreases due to the
0
formation of the second generation of PMMA dendron.
The subchain length of G1-PMMA (DPG1) was determined as
follows. The integral height of signal f (H ) was set as 342,
f
0
and the integral height of signal a (H ) was automatically
a
0
given as 532. Thus, DPG1 was calculated to be about 89
based on H /6. When it came to DPG2, H was set as 342,
and H was automatically given as 1864. Thus, DPG2 was
obtained to be 110 based on (Ha - 6DPG1)/12. The success-
0
a
f
1
FIGURE 5 H NMR and FTIR spectra of L-PtBA-PBMP. [Color
0
a
figure can be viewed at wileyonlinelibrary.com]
0
ful preparation of L-PtBA-b-G1-PMMA and L-PtBA-b-G2-
PMMA was further confirmed by GPC technique as shown in
Figure 3. Their GPC traces are symmetrically narrow and
keep moving to lower elution time with the increase in den-
dron generation, indicating the well-defined and expected
chain structure. Molecular weight information obtained by
GPC and NMR analysis are summarized in Table 2. As can be
seen, Mn,GPC and Mn,NMR of L-PtBA-b-G1-PMMA are similar,
while those of L-PtBA-b-G2-PMMA deviate from each other.
This phenomenon can be explained by the following reasons.
GPC characterization obtains the results based on the elution
volume and the calibration was built using linear polystyrene
standards in this work.
PtBA-b-G2-PMMA through azide–alkyne click reaction
between the AB2 type initiator of PBMP and the related
precursor.
1
Figure 5 exhibits H NMR and FTIR spectra of L-PtBA-PBMP.
As seen in Figure 5(A), after the click reaction, the character-
istic signal of triazole ring at 7.72 ppm (h) appears. The new
signals of methylene protons adjacent to the electron-
withdrawing triazole ring at 5.20 ppm (i), methylene protons
from PBMP residue [AC(CH
2 2
OCO) A] at 4.27 ppm (j) and
methyl protons from PBMP residue [ACBr(CH ) ] at 1.82
3
2
ppm (k) are also verified. The integral height ratio of “Hb1g
/
H ” is close to 1.5 and is consistent with chemical structure.
i
As for FTIR spectrum, the disappearance of stretching absor-
21
bance of azido group at 2114 cm
clarifies the complete
TABLE 2 Molecular Characteristics of L-PtBA-b-G1-PMMA and
L-PtBA-b-G2-PMMA
reaction of L-PtBA-N . All the above results confirm the suc-
3
cessful preparation of L-PtBA-PBMP.
Samples
M
n,GPC
M
w,GPC
PDI
M
n,NMR
Then, ATRP of MMA monomer was carried out using L-PtBA-
PBMP as the macroinitiator. To avoid chain coupling, the
temperature was controlled at 40 8C and the reaction lasted
L-PtBA-b-G1-PMMA
L-PtBA-b-G2-PMMA
24,000
28,400
28,900
34,500
1.20
1.21
23,300
68,200
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1
FIGURE 7 H NMR spectra of L-PAA-b-G1-PMMA (A) and L-PAA-b-G2-PMMA (B) in DMF-d
7
. [Color figure can be viewed at
wileyonlinelibrary.com]
As dendritic polymer has more compact conformation com-
pared with its linear counterpart, so its hydrodynamic radius
is relatively smaller and its elution volume from GPC is
simultaneously shorter, making its Mn,GPC relatively smaller,
especially when the dendron generation is higher. Therefore,
Mn,NMR is calculated from the integration height, which is
strictly proportional to the proton number.
PMMA increases in its intensity compared with Figure 8(A–
C), while the absorbance of tertiary butyl of PtBA at
2
1
1369 cm decreases in its intensity, resulted from the for-
mation of the second generation PMMA dendron. The broad
absorbance of hydroxyl groups of PAA is also found in FTIR
spectrum of L-PAA-b-G2-PMMA [Fig. 8(F)]. These aforemen-
tioned analyses indicate the successful preparation of L-PAA-
b-G1-PMMA and L-PAA-b-G2-PMMA.
The hydrolysis of tert-butyl group was performed by treating
L-PtBA-b-G1-PMMA and L-PtBA-b-G2-PMMA with trifluoro-
1
acetic acid. Figure 7(A) exhibits H NMR spectrum of L-PAA-
Self-assembly of L-PAA-b-L-PMMA, L-PAA-b-G1-PMMA,
and L-PAA-b-G2-PMMA by Gentle Variation in Solvent
Selectivity
Gentle variation in solvent selectivity to induce self-assembly
of three block copolymer with different topologies was con-
ducted by injecting water slowly into polymer solution in
the common solvent. The self-assembly morphologies of
b-G1-PMMA.
The appearance of the signal at 12.72 ppm (f) attributed to
the carboxylic group of PAA as well as the disappearance of
the signal ascribed to the tertiary butyl group of PtBA estab-
lishes the successful preparation of L-PAA-b-G1-PMMA. Fig-
1
ure 7(B) reveals H NMR spectrum of L-PAA-b-G2-PMMA,
the signal at 12.72 ppm (f) also shows up. By comparison,
0
the integral height ratio of “H /H ” decreases owing to the
f
e
increase in PMMA dendron generation. Figure 8 illustrates
FTIR spectra of different linear-block-dendritic copolymers.
As seen in Figure 8(B), a characteristic stretching peak of
21
azido group appears at 2119 cm after the azidation. When
L-PtBA-b-G1-PMMA-PBMP was prepared, this signal disap-
pears, verifying that the AB
2
initiator of PBMP has been
linked to the chain ends of L-PtBA-b-G1-PMMA.
In addition, the disappearance of the characteristic symmetric
21
stretching absorbance of tertiary butyl at 1369 cm and the
21
appearance of broad absorbance from 3500 to 3200 cm
attributed to hydroxyl groups of PAA, as well as the broaden-
2
1
ing of the stretching absorbance around 1730 cm owing to
the overlapping of the absorbance signals of ester carbonyl
groups of PMMA and carboxylic acid groups of PAA can con-
firm the successful hydrolysis of PtBA block.
FIGURE 8 FTIR spectra of L-PtBA-b-G1-PMMA (A), L-PtBA-b-
G1-PMMA-N3 (B), L-PtBA-b-G1-PMMA-PBMP (C), L-PAA-b-G1-
PMMA (D), L-PtBA-b-G2-PMMA (E), and L-PAA-b-G2-PMMA (F).
[Color figure can be viewed at wileyonlinelibrary.com]
In case of L-PtBA-b-G2-PMMA and L-PAA-b-G2-PMMA, it is
21
observed that the absorbance at 1191 cm assigned to the
asymmetrical stretching vibration of CACAO groups of
8
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FIGURE 9 TEM images of the self-assemblies of L-PAA-b-L-PMMA (A), L-PAA-b-G1-PMMA (B), and L-PAA-b-G2-PMMA (C) pre-
pared by gentle variation in solvent selectivity. [Color figure can be viewed at wileyonlinelibrary.com]
those block copolymers of L-PAA-b-L-PMMA, L-PAA-b-G1-
PMMA, and L-PAA-b-G2-PMMA were observed under TEM.
L-PAA-b-G2-PMMA, the spherical micelles are relatively
larger than those of linear block copolymer, caused by the
topology change in PMMA component as well as its volume
fraction. More crowded chain conformation and compact
structure of G2-PMMA dendrons should force the chains
repel to each other and increase the curvature simulta-
neously, resulting in the twist morphology of the self-
assemblies of L-PAA-b-G2-PMMA.
As exemplified in Figure 9(A) (Supporting Information Fig.
S3), L-PAA-b-L-PMMA self-assemblies are mainly spherical
micelles with much small diameter (<10 nm), which further
aggregated into caterpillar-like micelles with a diameter of
about 30 nm. As for L-PAA-b-G1-PMMA shown in Figure
9
(B) (Supporting Information Fig. S4), spherical micelles are
also present but their diameter is larger (approximately
0 nm) and their number is much low. Meanwhile, much
It is worthy to notice that the image contrast of spherical
micelles is deeper than that of caterpillar-like micelles in Fig-
ure 9(B,C), indicating the variation in chain package density in
2
more and much longer caterpillar-like micelles are mostly
observed. Besides the block length ratio of amphiphilic block
3
8
the micelle cores. While the image contrast of spherical
micelles and caterpillar-like micelles is almost the same in Fig-
ure 9(A), PMMA linear blocks might be normally packed in
both cases. A possible explanation for this observation might
be as follows. From L-PAA-b-L-PMMA, L-PAA-b-G1-PMMA to L-
PAA-b-G2-PMMA, the hydrophobic component of PMMA
changes its topology from linear, Y-type to dendron. Moreover,
the equilibrium morphology of the obtained micelle should
3
3
copolymers, the topology difference of Y-type block copoly-
mer from linear diblock copolymer also contributes to the
3
4
variation in their self-assembly morphologies. In the case
of L-PAA-b-G2-PMMA, spherical micelles and caterpillar-like
micelles with different length were formed, as shown in Fig-
ure 9(C) (Supporting Information Fig. S5). Compared with
Figure 9(B), the caterpillar-like micelles of L-PAA-b-G2-
PMMA are rather twisted in shape and large in width, proba-
bly due to the increase in dendron generation.
3
7
keep the minimum value of interfacial free energy. As the
interfacial tension between PMMA core and PAA shell is con-
stant, the interfacial area should be dominative. Therefore, in
the spherical micelle formation of L-PAA-b-G1-PMMA and L-
PAA-b-G2-PMMA with PMMA dendron, more PMMA chains
tend to densely pack to form the core, lessening the interfacial
area and further interfacial free energy.
3
5,36
Eisenberg and coworkers
studied the self-assembly of
linear polystyrene-b-PAA copolymers, revealing that micelle-
like aggregates were the primary morphology. Compared
with polystyrene, PMMA is more hydrophilic and micible
with PAA. Therefore, the spherical micelle size of L-PAA-b-L-
PMMA in this case is much small. What is more, the spheri-
cal micelles should have the higher probability to grow into
caterpillar-like micelles, whose diameter is several times
larger than that of spherical micelles. This observation sug-
gests that the caterpillar-like micelles of L-PAA-b-L-PMMA
might be formed by aggregation of spherical micelles. Espe-
cially, we have studied the morphologies of L-PAA-b-L-PSt
However, high compactness of chain packing is accompanied
3
9
with the increase of curvature, leading to the increase in
interfacial free energy. Therefore, in the formation of
caterpillar-like micelles, less PMMA chains would be packed
to the core, leading to the low packing density. The nonlinear
topology of PMMA chains in G1-PMMA and G2-PMMA is also
responsible for the loose packing because those PMMA
chains are little difficulty to fold and interpenetrate with
3
7
assemblies throughout RAFT dispersion polymerization.
With the increase in the reaction time from 1 to 2 h and 9 h,
the sequence of the morphologies follows the order of spher-
ical micelles, spherical/cylindrical micelles, and vesicle, con-
vincing us to believe that the cylindrical micelle should be
derived from spherical micelles and the vesicle should be
derived from cylindrical micelles. In this work, the
caterpillar-like micelles should be derived from spherical
micelles. As for the self-assemblies of L-PAA-b-G1-PMMA and
40
each other. Thus, the image contrast of caterpillar-like
micelles is low in Figure 9(B,C).
Self-assembly of L-PAA-b-L-PMMA, L-PAA-b-G1-PMMA,
and L-PAA-b-G2-PMMA by Abrupt Variation in Solvent
Selectivity
Along with the influence of dendron generation on the mor-
phology of self-assemblies, the mixing dynamics could also
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FIGURE 10 TEM images of the self-assemblies of L-PAA-b-L-PMMA (A), L-PAA-b-G1-PMMA (B), and L-PAA-b-G2-PMMA (C) pre-
pared by abrupt variation in solvent selectivity. [Color figure can be viewed at wileyonlinelibrary.com]
take their effect. The self-assemblies have been prepared by
abrupt variation in solvent selectivity, where the amphiphilic
copolymer solution in DMF was injected slowly to the non-
solvent of water. The concentration and volume of polymer
solution as well as the volume of water are the same as the
gentle variation in solvent selectivity.
investigated in detail. In the self-assembly induced by gentle
variation in solvent selectivity, with the increasing dendron
generation from L-PAA-b-L-PMMA to L-PAA-b-G2-PMMA, the
main morphology of the self-assemblies changes from the
spherical micelles, straight caterpillar-like micelles to twisted
caterpillar-like micelles, consistent with the stiffness of
PMMA component. The self-assembly via abrupt variation in
solvent selectivity produced mainly the spherical micelles
and lessoned the formation of caterpillar-like micelles with
the increase in PMMA dendron generation. The dependence
of self-assembly morphology on the PMMA dendron genera-
tion and variation in solvent selectivity should be attributed
to the chain compactness of PMMA component and the
dynamics to mix polymer solution with non-solvent. Gentle
variation in solvent selectivity enabled the intermolecular
association and allowed the associates to grow into longer
caterpillar-like micelles, while abrupt variation in solvent
selectivity favored the intramolecular association and inhib-
ited the associates to grow further. As well, with the increase
in dendron generation to G2, PMMA component in different
block copolymers became more compact, leading to the
twisting of caterpillar-like micelles in the case of gentle vari-
ation in solvent selectivity and no formation of caterpillar-
like micelles in the case of abrupt variation in solvent
selectivity.
As can be observed in Figure 10, in comparison with the
gentle variation in solvent selectivity, the self-assemblies pre-
pared by abrupt variation in solvent selectivity are mostly
spherical micelles along with the presence of short and little
caterpillar-like micelles. Slow addition of polymer solution
into water caused the abrupt variation in solvent quality and
endowed the polymer chains the higher possibility of the
2
7
intramolecular association.
Therefore, amphiphilic PAA/
PMMA block copolymers would first undergo the intramolec-
ular collapse of PMMA segments, then several collapsed
block copolymers aggregated into the micelle-like self-assem-
blies, whose diameter is much larger than that of spherical
micelles in Figure 9(A). Meanwhile, the abrupt variation in
solvent quality made it very difficult for polymer chains to
achieve the thermodynamically favored state of self-
assemblies and left much lower possibility for the collapsed
block copolymers to grow into long and straight caterpillar-
like micelles. Thus, in this case of abrupt variation in solvent
quality, only short and twisted caterpillar-like micelles were
observed for L-PAA-b-L-PMMA and L-PAA-b-G1-PMMA (Fig.
10; Supporting Information Figs. S6 and S7). As for the self-
assemblies of L-PAA-b-G2-PMMA prepared by abrupt varia-
tion in solvent selectivity, dendritic topology leads to rela-
tively compacted chain conformation of G2-PMMA dendrons,
making them much more difficult to form caterpillar-like
micelles. Thus, spherical micelles with about 20 nm diameter
but no caterpillar-like micelles are found as shown in Figure
ACKNOWLEDGMENTS
The authors thank the assistance of Prof. Chi Wu, the University
of Science and Technology of China, in getting GPC measure-
ment. This work was supported by the National Natural Science
Foundation of China (20934005 and 21274136).
10(C) (Supporting Information Fig. S8).
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