Polymer International
Polym Int 53:142–148 (2004)
DOI: 10.1002/pi.1318
Synthesis of well-defined comb-like
amphiphilic copolymers with protonizable
units in the pendent chains: 2. Poly(2(dimethylamino)ethyl methacrylate) grafted
poly(methyl methacrylate-co-2-hydroxyethyl
methacrylate) copolymers and their
association behavior in aqueous solution
Lan Jin, Ping Liu, Jianhua Hu and Changchun Wang∗
Department of Macromolecular Science, Fudan University and the Key Laboratory of Molecular Engineering of Polymers, Ministry of
Education, Shanghai 200 433, China
Abstract: Narrow-distribution, well-defined comb-like amphiphilic copolymers are reported in this work.
The copolymers are composed of poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) (P(MMAco-HEMA)) as the backbones and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) as the
grafted chains, with the copolymer backbones being synthesized via atom-transfer radical polymerization
(ATRP) and the grafted chains by oxyanionic polymerization. The copolymers were characterized by
gel permeation chromatography (GPC), Fourier-transform infrared (FT-IR) spectroscopy and 1 H NMR
spectroscopy. The aggregation behavior in aqueous solutions of the comb-like amphiphilic copolymers
was also investigated. 1 H NMR spectroscopic and surface tension measurements all indicated that the
copolymers could form micelles in aqueous solutions and they possessed high surface activity. The results
of dynamic light scattering (DLS) and scanning electron microscopy (SEM) investigations showed that
the hydrodynamic diameters of the comb-like amphiphilic copolymer aggregates increased with dilution.
Because of the protonizable properties of the graft chains, the surface activity properties and micellar
state can be easily modulated by variations in pH.
 2004 Society of Chemical Industry
Keywords: oxyanionic polymerization; comb-like amphiphilic copolymers; poly(2-(dimethylamino)ethyl
methacrylate); micelles; atom-transfer radical polymerization
INTRODUCTION
Comb-like amphiphilic copolymers are composed
of either a hydrophobic backbone and hydrophilic
branches, or a hydrophilic backbone and hydrophobic
branches. Their physico-chemical properties are determined by, but vary from, the component homopolymers. For their special properties and potential applications, such as impact-resistant plastics, thermoplastic elastomers, compatibilizers, polymeric surfactants
and stabilizers, amphiphilic copolymers have recently
attracted more and more attention.1 – 4
Generally, there are three routes to comb-like
amphiphilic copolymers. The first is to directly graft
branch chains onto a backbone through chemical
reaction,5,6 the second is through copolymerization of
macromomers with small molecular monomers,7,8 and
the third is to initiate polymerization of the monomer
from backbone pendent functional groups.9 – 11 In
previous research, the former two methods were
used widely, and in the preparative method, radical
polymerization is the first-choice process, because
radical polymerization is a very easy process to
carry out, although it does not give good control
over the structures of the resulting polymers. In the
last decade, there have been rapid developments in
the area of controlled ‘‘living’’ polymerization, such
as atom-transfer radical polymerization (ATRP),12
nitroxide-mediated living free-radical polymerization
(NMP),13 reversible addition–fragmentation chain
transfer (RAFT) polymerization14 and oxyanionic
polymerization.15 By using these methods, the polymer
structures can be well-controlled. A number of
∗
Correspondence to: Changchun Wang, Department of Macromolecular Science, Fudan University and the Key Laboratory of Molecular
Engineering of Polymers, Ministry of Education, Shanghai 200 433, China
(Received 26 December 2002; revised version received 15 April 2003; accepted 28 April 2003)
 2004 Society of Chemical Industry. Polym Int 0959–8103/2004/$30.00
142
PD MAEMA grafted P(MMA-co-HEMA) copolymers
papers16,17 about comb-like copolymer have be
published through the controlled living free radical
polymerization recently, the main chain and the
branched chain can be well controlled by these
method.
In this present paper, we have combined
together the advantages of ATRP and oxyanionic
polymerization to prepare for the first time a
form of comb-like amphiphilic copolymer with
protonizable units in the pendent chains. In these
systems, poly(methyl methacrylate-co-2-hydroxyethyl
methacrylate) (P(MMA-co-HEMA)) was prepared
as the hydrophobic backbone and poly(2(dimethylamino)ethyl methacrylate) (PDMAEMA) as
the hydrophilic branch, where this branched chain is
sensitive to pH, temperature and ionic strength. At
the same time, the aggregation behavior of the comblike amphiphilic copolymers in aqueous solutions was
investigated.
EXPERIMENTAL
Materials
Dimethyl sulfoxide (DMSO) was purchased from
Shanghai Feida Co Ltd, while 2-(dimethylamino)ethyl
methacrylate (DMAEMA) was purchased from Tokyo
Kasei Kogyo Co Ltd. Both were distilled before use.
KH, used to prepare DMSO− K+ , and p-tosyl chloride (p-TsCl) were obtained from Aldrich Chemical
Company. Tetrahydrofuran (THF) (Shanghai Feida
Co Ltd.) was refluxed in the presence of potassium
hydroxide (KOH) for 8 h and then refluxed in the
presence of sodium wire before use. Diphenyl ketone
(Shanghai Chemical Reagents Company) is used as
an indicator for titration of the hydroxyl group of
the copolymers.18 2,2 -bipyridyl(bpy), CuCl (purification method was same as reference 19), methyl
methacrylate (MMA), cyclohexanone, basic aluminum oxide, petroleum ether and triphenylmethane
were all purchased from Shanghai Reagents Chemical
Company. 3-(trimethylsilyl)propyl methacrylate(ProHEMA) was prepared according to Beers et al.20
Backbone copolymer synthesis
The hydrophobic backbone was prepared according
to the following process. In a 100 ml round–bottomed
flask equipped with a magnetic stirrer, a mixture
of 2,2 -bipyridyl (bpy) (0.226 g; 1.42 × 10−3 mol)
and CuCl (0.1411 g; 1.42 × 10−3 mol) was charged
(both purchased from Shanghai Chemical Reagents
Company). The mixture was degassed under vacuum
and then purged with dry argon. This procedure
was repeated three times. Following this, a mixture
of MMA (9.5 ml; 0.0892 mol), 3-(trimethylsilyl)
propyl methacrylate20 (Pro-HEMA) (8.5 ml; 0.0385 ×
10−3 mol) and cyclohexanone (42 ml), which had been
bubbled through with argon for at least 1 h, was
added to the flask. A fixed amount of p-TsCl was
added to a glass tube, degassed and purged with
argon (three times), and dissolved in cyclohexanone
Polym Int 53:142–148 (2004)
(3% w/v). Then, the reaction flask was immersed
in a thermostatic bath at 80 ◦ C immediately after a
quantitative amount of p-TsCl solution was added.
After 8 h, the solution was passed through a column
of basic aluminum oxide to remove the catalyst.
The copolymer was precipitated from solution by
adding petroleum ether (boiling range, 60–90 ◦ C),
dried under vacuum at 120 ◦ C for 48 h, and its
composition determined by 1 H NMR spectroscopy.
The MMA/Pro-HEMA copolymer was stirred in
a THF/water solution under acidic conditions at
ambient temperature for 24 h, and precipitated by
adding petroleum ether (boiling range, 60–90 ◦ C).
This procedure of dissolution and precipitation
was repeated three times in order to remove the
homopolymer. The resulting copolymer was finally
dried under vacuum at 60 ◦ C for 24 h. For a detailed
characterization of this copolymer (‘sample MP-1’),
please see Liu et al.19
Comb-like amphiphilic copolymer synthesis
All glassware used in the polymerization process was
dried overnight at 120 ◦ C, and then cooled in a desiccator before use. A typical polymerization procedure
for producing comb-like amphiphilic copolymers can
be described as follows P(MMA-co-HEMA)- and
(0.5 g) triphenylmethane(0.5 mg) were added to a
50 ml round-bottomed flask fitted with a rubber septum, degassed and purged with argon (three times).
Dry THF (30 ml) was added via a syringe to the flask,
and the solution stirred at 0 ◦ C and then titrated
by DMSO–K+ , which was prepared according to
the literature.21 After the color of the solution had
changed to pink (stable for 30 min), an appropriate
amount of DMAEMA was added to the flask through
a syringe. The flask was then transfered to a 30 ◦ C oil
bath for 3–4 h, before quenching with methanol. The
final polymer product was then precipitated by adding
hexane and dried under vacuum.
Characterization
IR spectra were recorded on a Nicolet Magna 550
FT-IR spectrometer, with the samples being prepared
by casting the polymer solutions onto pieces of
aluminum foil. All of the 1 H NMR spectra were
recorded with a DM500 NMR instrument, using
D2 O or CDCl3 as the solvent. The degree of branch
grafting was determined via 1 H NMR spectroscopic
analysis. Molecular weights and their distributions
were determined by gel permeation chromatography
(GPC), using an HP-1100 instrument, which was
equipped with a Zorbax HV1618 column connected
to a refractive index detector (G 1362A). Calibration
was achieved by using polystyrene standards. THF
was used as the eluent, at a flow rate of 1 ml min−1 .
Surface tension measurements were carried out with
a JYW-200A automatic surface tensiometer, equipped
with an electrical torsion balance and platinum
ring (Chengde Experimental Instrument Company,
China). All measurements were performed at room
143
L Jin et al
CH3
(a) CH2
CH3
C
+
CH2
monomer:solvent (v/v) = 3:7; T = 80 °C
[M]0:[I]0:[CuCl]0:[bpy]0 = 90:1:1:1
C
C O
C O
O
O
CH3
CH2
CH2
O
CH3
Si CH3
CH3
CH3
CH2
C
x
C
CH3
CH2 C
O
Hydrolysis
y
CH3
CH2
C
CH3
CH2 C
x
y
C O
C O
C O
O
O
O
O
CH3
CH2
CH3
CH2
CH3
CH2
CH2
O
OH
Si CH3
CH3
CH3
(b)
CH2 C
x
CH3
CH2 C
y
DMSO−K+/ THF
DMAEMA
CH3
CH3
CH2 C
CH2 C
x
y
C O
C O
C O
C O
O
O
O
O
CH3
CH2
CH3
CH2
CH2
CH2
CH3
OH
C CH2 O
m
C O
O
CH2CH2
N
CH3 CH3
Scheme 1. Procedure used for the synthesis of comb-like amphiphilic copolymers.
temperature (20 ◦ C), and the values obtained were
checked periodically by measuring the surface tension
of deionized water (72–73 mN m−1 at 20 ◦ C).The
average comb-like copolymer micelle size was obtained
by using a DLS apparatus (Malvern Autosizer 4700)
which was equipped with a 100 mW argon laser and
operated under a wavelength of 514.5 nm at 20 ◦ C.
The intensity of the scattered light was detected at 90◦
to the incident beam unless otherwise stated. The data
were fitted using ‘CONTIN’ analysis. Images of the
amphiphilic comb-like copolymers were recorded with
a Philips XL30 scanning electron microscope. From
latex dispersions dried at room temperature on glass
plates.
RESULTS AND DISCUSSION
Preparation of comb-like amphiphilic
copolymers
Motivated by the great range of potential applications,
research on well-defined amphiphilic copolymers has
witnessed great progress in the last decade.22,23,24 The
main characteristic of amphiphiles is their tendency
144
to undergo intramolecular microphase separation
and spontaneous selforganization into well-defined
supermolecular assemblies. Up until new, studies
of the self-assembly of amphiphilic copolymers have
been mainly focused on the AB diblock copolymers
and ABA triblock copolymers, because the physical
profiles of the micelles are much simpler and clear or
in such systems. However, for comb-like amphiphilic
copolymers, both the chemical structures of the
copolymers and the physical structures of the micelles
are more complex, and hence studies of formation of
the micelles also becomes more complicated. In this
present work, in order to properly characterize the
experimental process, we have prepared a series of
well-defined comb-like amphiphilic copolymers. The
copolymer backbones were prepared by atom-transfer
radical polymerization (ATRP), while the grafted
chain were prepared by oxyanionic polymerization.
This procedure is shown in Scheme 1.
IR spectra of the MMA/HEMA copolymer, comblike copolymers and PDMAEMA are shown in Fig 1.
The characteristic absorption at around 3500 cm−1
for the hydroxyl group of poly(MMA/HEMA) has
Polym Int 53:142–148 (2004)
PD MAEMA grafted P(MMA-co-HEMA) copolymers
120
a
Transmittance (%)
100
80
b
60
c
d
40
20
e
0
4000
3500
3000
2500
Wavenumbers
2000
1500
1000
(cm−1)
Figure 1. FT-IR spectra of the copolymers of: (a) P(MMA-co-HEMA); (b) MHD-1; (c) MHD-2; (d) MHD-3; (e) PDMAEMA.
almost disappeared after oxyanionic polymerization,
indicating that the hydroxyl groups on the polymer
backbone have successfully initiated the oxyanionic
polymerization of DMAEMA. Compared to the IR
spectrum of MMA/HEMA, PDMAEMA have two
characteristic absorptions at 2700 and 2800 cm−1
for the –CH2 –groups close to the nitrogen atom.
In the spectra of the comb-like copolymers, these
characteristic adsorptions can be clearly seen, hence
indicating that comb-like amphiphilic copolymers have
been successfully produced.
Molecular weight and polydispersity data obtained
for the comb-like amphiphilic copolymers are summarized in Table 1. From this Table, we can see
that the Mn of the copolymers decreases with increasing branch length of the DMAEMA (from GPC).
This strange behavior may be caused by absorption of
DMAEMA segments on to the packing material in the
analytical columns—this same result has previously
been reported by Creutz25 and Baines26 —the larger
the DMAEMA content, then the greater the deviation
obtained. From the GPC studies, we also find that
Dh /6
FDMAEMA
=
FMMA
Dc /3
FPMMA
Dc /3
=
FPHEMA
(Dd –Dg )/2
Table 1. Molecular weight data for the comb-like amphiphilic
copolymers, obtained by 1 H NMR spectroscopy and GPCa
Sample
Average
grafted
chain
lengthb
Mn
(×104 g
mol−1 )c
Mn
(×103 g
mol−1 )d
Mw /Mn d
MHD-1
MHD-2
MHD-3
18
33
48
3.08
5.74
7.86
9.6
6.9
5.6
1.34
1.65
1.41
a
Note: the main chain of the comb-like copolymer is MMA-coHEMA (Mn = 1.08 × 104 g mol−1 ; Mw /Mn = 1.05; FMMA /FHEMA = 9.1
(measured by GPC)); the graft chain is DMAEMA.
b Number of units.
c Measured by 1 H NMR spectroscopy.
d Measured by GPC.
Polym Int 53:142–148 (2004)
only one peak can be detected, which means that the
precursor polymer is absent from the product. Because
the comb-like copolymers can absorb strongly on the
packing material in the analytical columns, the retention times of the precursor and comb-like copolymers
will exhibit large differences, and two peaks will be
seen in the GPC curves if the MMA/HEMA copolymer was present in the final copolymer products. In
order to measure the molecular weights of the comblike copolymers precisely, 1 H NMR spectroscopy, in
a non-selective solvent (CDCl3 ), was carried out to
determine the branch lengths. Figures 2 and 3 shown
the 1 H NMR spectra of the comb-like copolymers
and their precursor (polymer A). In these spectra,
signals at 2.2–2.4 ppm and 2.5–2.7 ppm correspond
to methyl and methane protons (h and g), close to
the nitrogen atom of the PDMAEMA grafted on the
backbones, while the signals at 3.7 ppm correspond
to the methyl ester protons (c) of MMA. Then, the
grafted chain length can be calculated according to the
following equations:
(1)
(2)
Thus, we can obtain the branch length (Lbranch ) as
follows:
Lbranch =
FDMAEMA
Dh
=
FPHEMA
3(Dd –Dg )
(3)
where Dc , Dd , Dg and Dh are the peaks areas of the
protons indicated in Fig 2. The results obtained are
shown in Table 1. As expected, with an increase in feed
amount, the branched chain length of the comb-like
copolymers gradually increases.
145
L Jin et al
a
CH3
b
CH2
C
O
a
CH3
co
m
C
O
b
CH2
C
O
C
c
CH3
O
n
O
C
e
d
CH2 CH2 O
CH2
b
C
g
d
CH2 CH2
O
CH3
N
h
CH3
x
CH3
a
h
a, b
g
d
c
MHD-3
MHD-2
MHD-1
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
δ (ppm)
Figure 2. 1 H NMR spectra of the copolymers MHD-1, MHD-2 and MHD-3 in CDCl3 .
Polymer A
b
H2
C
O
a
CH3
C
m
b
H2
C
co
C
O
O
c
CH3
MHD-3
O
a
CH3
C
C
O
g
d
O CH2 CH2
C
n
h
d
H2
C
e
H2
C
O
f
OH
CH2
b
C
x
CH3
N
h
CH3
CH3
a
a, b
d
g
c
(MHD-3 in CDCl3)
(MHD-3 in D2O)
(Polymer A in CDCl3)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
δ (ppm)
Figure 3. 1 H NMR spectra of P(MMA-co-HEMA) and MHD-3.
From the FT-IR spectroscopic, GPC and 1 H NMR
spectroscopic results, we can conclude that wellcontrolled comb-like amphiphilic copolymers can be
prepared by a combination of ATRP and oxyanionic
polymerization.
The aggregation behavior of amphiphilic
copolymers in aqueous solutions
Figure 3 shows the results obtained from 1 H NMR
spectroscopic measurements on polymer A and
the comb-like amphiphilic copolymer (MHD-3) in
146
CDCl3 and/or D2 O. From this figure, we can
see that the characteristic signal of the methoxy
protons of PMMA in the amphiphilic copolymer
(δ = 3.6 ppm) disappeared when D2 O is used as the
solvent. This may be due to the P(MMA-co-HEMA)
backbone which is less soluble in water and forms
a packed core of micelles–thus, the signal cannot
be detected. When CDCl3 is used as the solvent,
the copolymer can be dissolved completely and no
micelles are formed, and therefore all proton signals
can be clearly detected. This result tells us that
Polym Int 53:142–148 (2004)
PD MAEMA grafted P(MMA-co-HEMA) copolymers
the hydrophobic P(MMA-co-HEMA) backbone could
form the micellar core in aqueous media, while the
hydrophilic DMAEMA chain branch comprises the
shell and stabilizes the micelles in aqueous solutions.
SEM studies further proved the formation of micelles.
Figure 4 shows a scanning electron micrograph of the
MHD-1 micelles in water (0.0625 wt%). We can see
from the figure that the micelles are almost sperical in
shape, and have a size of ca. 150 nm.
The results obtained from, surface tension measurements of the aqueous micellar solution are shown in
Fig 5. Under neutral conditions, the curve shows the
typical trend of change in surface tension of surfactant
with concentration. With an increase in concentration,
however, the surface tension decreased rapidly–this
means that a proportion of the amphiphilic copolymers can be easily attracted at the air–water interface
to lower the surface tension, with the final surface
tension of the micellar solution remaining constant
at around 43 mN m−1 . This indicates that the comblike amphiphilic copolymers possess very good surface
activity. At the same time, we also find that pH strongly
influences the surface activity of the amphiphilic
copolymers, since DMAEMA can be protonized under
acid conditions, and the solubilities of the amphiphilic
160
Hydrodynamic diameter (nm)
Figure 4. Scanning electron micrograph of the copolymer MHD-1 in
aqueous solution at a concentration of 0.0625 wt%.
copolymers in water can be improved dramatically.
Then, the copolymer molecules tend to move into the
water phase, instead of staying at the air–water interface, and the surface activity quickly decreases. From
the above results, we can see that the surface tension plots of the protonizable amphiphilic copolymers
show a strongly dependence on the pH. Indeed, by
using pH, we can modulate the state of the comb-like
amphiphilic copolymers in aqueous solution.
From dynamic light scattering (DLS) studies, we
found that the hydrodynamic diameters of the micelles
show a strongly inverse concentration-dependence
(see Fig 6) i.e. the higher the concentration, then the
smaller the hydrodynamic diameter of the micelles.
Similar results have also been previously reported by
Winnik and co-workers.26 From this Figure, we find
that the hydrodynamic diameters are similar at higher
concentrations (for MHD-1, MHD-2 and MHD-3),
at around 10 nm. However, in dilute solutions (lower
than 0.01 wt%), differences in the hydrodynamic
diameters are obvious–the lower the DMAEMA
content, then the larger the hydrodynamic diameters
140
120
100
80
MHD-1
MHD-2
MHD-3
60
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
Concentration (wt%)
Figure 6. The influence of concentration on the aggregation sizes of
the comb-like amphiphilic copolymers in aqueous solution at pH 7.0.
160
Hydrodynamic diameter (nm)
Surface tension (mN m−1)
75
70
65
60
pH 3.05
pH 6.83
55
50
45
140
120
100
Dilution
Direct preparation
80
60
40
20
0
40
0.0
0.5
1.0
1.5
Concentration (g
e−1)
2.0
2.5
Figure 5. The influence of concentration on the surface tension of an
MHD-2 solution at pH 3.05 and 6.83.
Polym Int 53:142–148 (2004)
0.0
0.2
0.4
0.6
0.8
1.0
Concentration (wt%)
Figure 7. The influence of method of preparation on the aggregation
size of the MHD-1 micelles in aqueous solution at pH 7.0.
147
L Jin et al
of the micelles. This result is reasonable, because as the
DMAEMA chain is hydrophilic, the micellar particles
in the aqueous solution are stabilized by these chains,
and so more contained DMAEMA will result in a
lower size aggregation. Theories of micelle formation
are usually based on the equilibrium thermodynamic
approach. However, kinetic processes may sometimes
control the final properties of the micelles. In the
case of the abnormal behavior of micelle formation
for our prepared copolymers, is it possible that
their formation is controlled by kinetic processes?
Therefore, we have prepared our micelles following
different procedures, as if the final states are controlled
by kinetic processes, than these different preparative
routes will result in different micellar states. In our
experiments, one way is to directly prepare solutions
of different concentrations by the addition of MHD1/DMF solutions to water (Fig 7, direct preparation
curve), while another method is to prepare 1 wt%
solutions first, and after dialysis, then to add more
water to the solutions to produce solutions of different
concentrations (corresponding to those prepared via
the direct preparation method) (Fig 7, dilution curve).
From this figure, we find that the hydrodynamic
diameters and the change tendencies of the micelles
are both almost identical. These results tell us that
the micelles exist in a thermodynamically stable state,
and that the behavior of the hydrodynamic diameter
as a function of concentration will be controlled by the
properties of the amphiphilic copolymers themselves.
The abnormal behavior of the micelles reported in this
present work is currently under further investigations
in our laboratory.
CONCLUSIONS
In this work, well-defined P(MMA-co-HEMA) materials were prepared19 by atom-transfer radical polymerization (ATRP) as the comb-like copolymer backbones, while PDMAEMA was prepared by oxyanionic polymerization as the graft chains. The results
obtained from FT-IR spectroscopic, DSC and 1 H
NMR spectroscopic studies indicated that comblike amphiphilic copolymers were prepared. Surface
tension measurements gave evidence that these comblike amphiphilic polymers with protonizable units in
the pendent chains possessed high surface activity,
and that the latter can be modulated by changes in
pH. 1 H NMR spectroscopic, SEM and DLS measurements revealed that the amphiphilic copolymers
148
could aggregate to form micelles in aqueous solutions,
the micellar hydrodynamic diameters had a strongly
inverse concentration dependence, and this process is
controlled by thermodynamic factors.
ACKNOWLEDGEMENTS
This work was supported by the National Science
Foundation of China (Grant Number 50 173 005) and
the Association of Shanghai Science and Technology.
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Polym Int 53:142–148 (2004)