Published: May 12, 2011
r
2011 American Chemical Society
4057 dx.doi.org/10.1021/ma200514n
|
Macromolecules 2011, 44, 4057–4064
ARTICLE
pubs.acs.org/Macromolecules
Poly(2-cyclopropyl-2-oxazoline): From Rate Acceleration by
Cyclopropyl to Thermoresponsive Properties
Meta M. Bloksma,
†,‡,§
Christine Weber,
‡,§
Igor Y. Perevyazko,
§
Anette Kuse,
§
Anja Baumg€artel,
‡,§
Antje Vollrath,
§
Richard Hoogenboom,
†,^,
* and Ulrich S. Schubert
†,‡,§,
*
†
Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,
The Netherlands
‡
Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands
§
Laboratory of Organic and Macromolecular Chemistry (IOMC) and Jena Center for Soft Matter (JCSM), Friedrich-Schiller-University
Jena, Humboldtstrasse 10, 07743 Jena, Germany
^
Supramolecular Chemistry group, Department of Organic Chemistry, Ghent University, Krijgslaan 281 S4, 9000 Ghent, Belgium
b
S Supporting Information
’ INTRODUCTION
There is a high potential for thermoresponsive polymers to be
used in various applications, such as drug delivery systems
15
and separation processes,
6,7
and, therefore, these materials
received significant attention over the last few years. Polymers
that exhibit “lower critical solution temperature” (LCST) beha-
vior are soluble in water below their LCST
3,7,8
due to effective
hydration of the polymer based on hydrogen bonds between the
polymer and the solvent. With increasing temperature, the
hydrogen bonds are weakened resul ting in dehydration when
the LCST is reached. This entropically driven phase transition,
i.e., release of water molecules, leads to a collapse of the hydro-
phobic polymer chains and the formation of aggregates. There-
fore, the LCST can be tuned, e.g., by variation of the polymer side
chains or by copolymerization with other monomers, as fully
explored for the most widely studied thermo-responsive polymer,
poly(N-isopropylacrylamide) [PNIPAM].
3,6,9,10
Poly(2-oxazoline)s with methyl, ethyl, isopropyl, or n-propyl
side chains are wate r-soluble and, except for the most hydrophilic
poly(2-methyl-2-oxazoline) (pMeOx), show LCST behavior in
water.
11,12
The cloud points (CP) of these poly(2-oxazoline)s
increase with increasing hydrophilicity and depend on the
degree of polymerization (DP) and concentration.
1315
The
CP can easily be tuned by copolymerization of various 2-oxazo-
line monomers, as well as by controlling the length and end
groups.
1619
Poly(2-ethyl-2-oxazoline) (pEtOx) is known to
only reveal a CP when the DP is above 100, since smaller
polymer chains are soluble up to 100 C.
15
Poly(2-isopropyl-2-
oxazoline) (piPropOx) is an interesting thermorespons ive poly-
mer, since its CP is close to body-temperature, making it suitable
for biomedical applications.
20
However, due to its semicrystalli-
nity the thermo-responsiveness becomes irreversible after an-
nealing above the LCST.
2123
Poly(2-n-propyl-2-oxazoline)
(pnPropOx) is amorphous but has a lower LCST of ∼24 C.
15
In addition, the rather low T
g
of ∼40 C, which decreases in the
presence of water, makes it difficult to handle and to store the
polymer at ambient temperature. Therefore, an alternative
thermo-responsive poly(2-oxazoline) with a reversible critical
temperature close to body temperature is desired.
Besides linear poly(2-oxazoline)s, we recently also reported comb
polymers containing oligo(2-ethyl-2-oxazoline) (OEtOx) side-
chains and a methacrylate (MA) backbone as thermo-responsive
Received: March 7, 2011
Revised: April 22, 2011
ABSTRACT: The synthesis and microwave-assisted living
cationic ring-opening polymerization of 2-cyclopropyl-2-oxazo-
line is reported revealing the fastest polymerization for an
aliphatic substituted 2-oxazoline to date, which is ascribed to
the electron withdrawing effect of the cyclopropyl group. The
poly(2-cyclopropyl-2-oxazoline) (pCPropOx) represents an
alternative thermo-responsive poly(2-oxazoline) with a rever-
sible critical temperature close to body temperature. The cloud
point (CP) of the obtained pCPropOx in aqueous solution was evaluate d in detail by turbidimetry, dynamic light scattering (DLS)
and viscos ity measurements. pCPropOx is amorphous with a significan tly higher glass transition temperature (T
g
∼ 80 C)
compared to the amorphous poly(2-n-propyl -2-oxazoline) (pnPropOx) (T
g
∼ 40 C), while poly(2-isopropyl-2-oxazoline) piPropOx is
semicrystalline. In addition, a pCPropOx comb polymer was prepared by methacrylic acid end-capping of the living cationic species
followed by RAFT polymerization of the macromonomer. The polymer architecture does not influence the concentration dependence of
the CP, however, both the CP and T
g
of the comb polymer are lower due to the increased number of hydrophobic end groups.
4058 dx.doi.org/10.1021/ma200514n |Macromolecules 2011, 44, 4057–4064
Macromolecules
ARTICLE
polymers.
24
The utilized macromonomers are interesting be-
cause in combination with controlled radical polymerization
techniques it is possible to introduce further functional moieties
into the polymeric architecture, such as carboxylic acids that
enable additional pH responsiveness.
25
It is believed that the
OEtOx side-chains shield the MA backbone resulting in a similar
LCST behavior of the comb polymer compared to pEtOx, even
though the absolute CP of the comb polymer is significantly
lower than the CP of the linear polymer.
24
Graft copolymers with
a poly(N-isopropylacrylamide) backbone and grafts containing
EtOx and 2-(2-methoxycarbonylethyl)-2-oxazoline units are
reported as well. Alrea dy small amounts of carboxylic groups
stabilize the micelle formation in a broad temperature range.
26
Furthermore, it is believed that the architecture of the polymer is
important for its function as polymeric carrier.
27
In this contribution, the synthesis of a new linear poly(2-
oxazoline), namely poly(2-cyclopropyl-2-oxazoline) (pCPropOx),
and of the comb polymer, poly[(oligo-2-cyclopropyl-2-
oxazoline)methacrylate] (p[(OCPropOx)MA]), containing
OCPropOx side-chains and a MA backbone, are described
(Figure 1). It is expected that due to the rigid and bulky ring
structure of the cyclopropyl side-chains in pCPropOx an amor-
phous poly(2-oxazoline) is formed, while the T
g
of pCPropOx is
expected to be higher than pnPropOx. The LCST behavior
of pCPropOx is discussed based on turbidity measurements,
dynamic light scattering (DLS) and viscosimetry. Besides the
influence of the molar mass and concentration on the crit ical
temperature, also the influence of the polymer architecture is
reported by comparing linear and comb polymers . Differential
scanning calorimetry (DSC) measurements were used to deter-
mine the thermal properties of the polymers.
’ EXPERIMENTAL SECTION
Materials and Instrumentation. Cyclopropionitrile (Aldrich),
2-aminoethanol (Aldrich), methacrylic acid (99%, Aldrich) and the chain
transfer agent 2-cyanopropan-2-yl dithiobenzoate (Aldrich, CPDB) were
used as received. Acetonitrile (Aldrich) was dried in a solvent purifica-
tion system (Pure Solv EN, Innovative Technology) before use as a
polymerization solvent. Methyl tosylate (Aldrich) was distilled over
barium oxide and stored under argon. Triethylamine (NEt
3
) was distilled
over potassium hydroxide and stored under argon. 2,2
0
-Azobis(2-
methylpropionitrile) (98%, Acros, AIBN) was recrystallized from etha-
nol. All phase transitions were determined in demineralized water by
turbidimetry, dynamic light scattering (DLS) and viscometry.
Small-scale reactions of 1 mL were carried out in capped reaction vials
designed for the Biotage Sixty microwave system equipped with an
IR temperature sensor. The vials were dried in the oven at 105 C and
cooled under argon to room temperature before use.
1
H NMR and
13
C NMR spectra were recorded in CDCl
3
on a Bruker
Avance 250 or 300 MHz spectrometer. Chemical shifts are given in
ppm relative to TMS. GC measurements were performed on a Shimadzu
GC-2010 equipped with a Restek Rtx-5 column, a FID detector and a
PAL autosampler. ESI-Q-TOFMS measurements were performed
with a microTOF Q-II (Bruker Daltonics) mass spectrometer equipped
with an automatic syringe pump from KD Scientific for sample injection.
GCMS measurements were performed on a Shimadzu GC-17A
(Column: DB-SMS, 5% phenyl-/95% dimethylpolysiloxane, length =
30 m, inner diameter = 0.25 mm, film thickness = 0.1 μm) connected to
an AOC-20i autoinjector and a GCMS-QP5050A mass spectrometer.
Ionization was managed by EI (electron impact). MALDITOF spectra
were recorded on an Ultraflex III TOF/TOF of Bruker Daltonics,
Bremen, Germany equipped with a Nd:YAG laser and a collision cell.
Size exclusion chromatography (SEC) was measured on a Shimadzu
system equipped with a LC-10AD pump, a RID-10A refractive index
detector, a SCL-10A VP system controller, and a PSS SDV pre/lin S
column utilizing chloroform:triethylamine:2-propanol (94:4:2) as elu-
ent at a flow rate of 1 mL min
1
and a column temperature of 40 C. A
polystyrene (PS) or poly(methyl methacrylate) (PMMA) calibration
was used to calculate the molar mass values. Turbidity measurements
were performed on a Crystal 16 from Avantium Technologies connected
to a chiller (Julabo FP 40) at a wavelength of 500 nm. The concentration
was systematically varied between 1 and 30 mg mL
1
in water. The
transmittance was measured at a temperature range from 0 to 100 C
with heating and cooling rates of 1 C min
1
. The cloud point (CP) was
defined as the temperature where the transmittance goes through 50%.
Dynamic light scattering (DLS) was performed on a Zetasizer Nano ZS
of Malvern instruments, Malvern, U.K., equipped with a 10 mW HeNe
laser with a wavelength of 633 nm, operating at an angle of 173. The
aqueous polymer solution with a concentration of 1 mg mL
1
was
measured three times at different temperatures for 60 s. The hydro-
dynamic radius (R
h
) and polydispersity index (PDI
DLS
) was determined
by the Cumulants method, assuming a spherical shape. Furthermore, the
particle size distribution was obtained by the multi narrow mode. Viscosity
measurementswereconductedonanAMVn(AntonPaar,Graz,Austria)
rolling ball viscometer with a manually filled capillary with an internal
diameter of 0.8 mm. The viscosity of the solution, η, and of the solvent, η
0
,
were obtained from the rolling times of the steel ball, measured at three
inclination angles (from 30 to 70) of the capillary. The temperature
dependence of the dynamic viscosity is described by the following equation:
ηðTÞ¼η
0
exp
E
RT
ð1Þ
The intrinsic viscosities of the polymer solutions at 20 C were
estimated according to the Kraemer extrapolation procedure:
ln η
r
c
¼½ηþk
00
½η
2
c þ :::
ð2Þ
where η
r
is the relative viscosity, c is the concentration, and k
00
is
Kraemer’s dimensionless parameter. For studying the temperature
dependence, the dynamic viscosity was used.
Monomer Synthesis. The monomer has been synthesized by a
standard procedure in which cyclopropionitrile (1 equiv) and zincace-
tate dehydrate (catalyst, 0.02 equiv) were heated to 130 C under reflux
conditions, and 2-aminoethanol (1.1 equiv) was added dropwise. After
a reaction time of ∼20 h, the reaction mixture was allowed to cool to
room temperature, and dichloromethane was added. The organic phase
was washed 3 times with water and once with brine. After removing
the dichloromethane under reduced pressure, the monomer was further
purified by distillation over barium oxide. The monomer was analyzed
with
1
H NMR and
13
C NMR spectroscopy as well as GCMS and
HRESIMS to demonstrate the correct molecular structure.
1
HNMR
(250 MHz, δ in ppm, CDCl
3
): 4.15 (t, CH
2
O, 2H), 3.75 (t, CH
2
N, 2H),
1.6 (q, NCCH, 1H), 0.8 (br, CCHCH
2
CH
2
, 4H).
13
C NMR (250 MHz,
δ in ppm, CDCl
3
): 169.0 (NC=O, 1C), 66.9 (CH
2
O = C, 1C), 54.1
Figure 1. Schematic representation of the linear pCPropOx and the
comb polymer p[(OCPropOx)
5
MA]
42
.
4059 dx.doi.org/10.1021/ma200514n |Macromolecules 2011, 44, 4057–4064
Macromolecules
ARTICLE
(NCCH
2
, 1C), 8.2 (CCH, 1C), 6.4 (CH(CH
2
)
2
,2C).HRMS
(ESITOF): calculated for C
6
H
9
NO þ H
þ
= 112.0757; found =
112.0759. GCMS: 110 [M
þ
], 96 [M
þ
CH
2
], 80 [M
þ
C
2
H
4
],
68 [M
þ
C
3
H
5
], 54 [M
þ
C
4
H
7
], 41 [M
þ
C
5
H
9
].
Kinetic Investigation of the Microwave-Assisted Polym-
erization.
Kinetic investigations of the CPropOx polymerization
were performed under microwave irradiation at 140 C with an initial
monomer concentration of 4 M in acetonitrile and three monomer
to initiator ([M]/[I]) ratios of 52, 107, and 155 using methyl tosylate
as the initiator. These conditions are similar to the previously
reported conditions used for the polymerization of EtOx, nPropOx,
and iPropOx.
15,28
1
H NMR (300 MHz, CDCl
3
): δ 3.83.1 (NCH
2
; backbone);
3.23.0 (NCH
3
; small initiator signals), 2.01.6 (CCHC; c-propyl),
1.10.7 (CH
2
CH
2
, c-propyl) ppm.
Polymer Synthesis. All polymers with varying [M]/[I] values
were synthesized under similar conditions. The initial monomer con-
centration (M
0
) was 4 M for all polymerizations. The [M]/[I] (I =
methyl tosylate) ratio was changed between 25 and 150 to obtain six
polymers with varying degree of polymerization (DP). These polymer-
ization mixtures were heated to 140 C under microwave irradiation for
1 to 5 min, depending on the [M]/[I] value, to reach ∼80% conversion.
After cooling to <40 C, the polymerization mixtures were quenched by
the addition of water. After drying under reduced pressure, the polymers
were redissolved in chloroform and precipitated in cold n-hexane. The
polymer was isolated by filtration followed by drying under reduced
pressure, before further characterization. Turbidity measurements, dy-
namic light scattering (DLS), and viscosity measurements were carried
out to determine the lower critical solution temperature (LCST). The
thermal properties were determined by differential scanning calorimetry
(DSC) measurements.
Comb Polymer Synthesis. The comb polymer was synthesized in
a similar way as p[(OEtOx)MA], described in a recently published
paper.
24
In a representative example, MeOTs, CPropOx and acetonitrile
with M
0
of 4 mol L
1
and [M]/[I] of 5 were heated to 140 C under
microwave irradiation for 20 s. After cooling to <40 C, OCPropOx was
end functionalized by the addition of 1.5-fold excess of MAA and 2-fold
excess of NEt
3
via syringe through the septum of the microwave vial.
After completion of the reaction at 50 C for 15 h the reaction mixture
was dissolved in chloroform and washed with saturated aqueous sodium
hydrogen carbonate and saturated brine, dried over sodium sulfate, and
filtered. The solvent was evaporated under reduced pressure, and the
resulting white sticky macromonomer was dried under reduced pres-
sure.
1
H NMR (300 MHz, CDCl
3
): δ 6.1 (dCH
2
), 5.6 (dCH
2
), 4.3
(CH
2
COO), 3.83.1 (NCH
2
); 3.23.0 (NCH
3
), 2.01.6
(CCHC), 1.10.7 (CH
2
CH
2,
c-propyl) ppm. For the RAFT
polymerization of the macromonomer, 1 g of OCPropOxMA was
dissolved in ethanol and subsequently the appropriate amounts of
CPDB (5.68 mg) and AIBN (1.05 mg) were added from stock solutions
in ethanol. The monomer concentration was kept at 0.5 mol L
1
and the
molar ratio of [monomer]:[CPDB]:[AIBN] at 60:1:0.25. In order to
remove oxygen, an argon flow was bubbled through the solution for
30 min before the reaction was carried out in an oil bath at 70 C for 22 h
(conversion =70%). The obtained polymer was precipitated into cold
diethyl ether and, subsequently, dissolved in THF in order to separate
unreacted OCPropOxMA monomer by column chromatography on a
BioBeads S-X1 column (solvent THF, exclusion limit 14 000 g/mol).
Finally, the purified comb polymer was precipitated into cold diethyl
ether and dried in a vacuum oven at 40 C. Turbidity measurements of
aqueous polymer solutions were carried out to determine the cloud
point (CP), to determine the thermal properties of the comb polymer,
DSC measurements were performed.
’ RESULTS AND DISCUSSION
The monomer 2-cyclopropyl-2-oxazoline (CPropOx) could
be synthesized by the reaction of cyclopropionitrile with 2-ami-
no-1-ethanol using zinc acetate as the catalyst following standard
synthetic procedures.
29
The polymerization kinetics of CPropOx
was determined at 140 C, using an initial monomer concentra-
tion (M
0
) of 4 M and a monomer to initiator ratio ([M]/[I]) of
52, 107, and 155 using methyl tosylate (MeOTs) as initiator and
acetonitrile as solvent. The first order kinetics (Figure 2) do not
pass the origin of the plots, indicating that the first data points in
Figure 2a correspond to the conversions after 1s reaction time
at 140 C; i.e., the reaction already started while heating the
reaction mixture to the reaction temperature of 140 C.
All three polymerizations of CPropOx revealed linear first-
order kinetics up to a conversion of approximately 80% as well as
a linear increase of the molar mass with conversion, demonstrat-
ing a constant concentration of propagating species and that the
polymerizations proceeded in a controlled manner. The polym-
erization rate constants (k
p
) calculated from the linear fitupto
80% are 0.257 ((0.006) L mol
1
s
1
, 0.255 ((0.00 8) L mol
1
s
1
and 0.214 ((0.008) L mol
1
s
1
for [M]/[I] = 52, [M]/[I]
= 107 and [M]/[I] = 155, respectively. These similar k
p
values
confirm that the polymerization proceeded in a controlled
fashion. Com pared to other 2-oxazolines, CPropOx polymerizes
approximately two times faster under similar conditions; the k
p
of
Figure 2. (a) Kinetic plots for CPropOx polymerizations initiated with methyl tosylate ([M]/[I] = 52, 107, and 155) in acetonitrile (M
0
= 4 M) at 140 C.
(b) Corresponding molar mass (M
n
) against conversion plot, including polydispersity indices (PDI).
4060 dx.doi.org/10.1021/ma200514n |Macromolecules 2011, 44, 4057–4064
Macromolecules
ARTICLE
nPropOx is 0.117 ((0.004) L m ol
1
s
1
,
30
while in contrast
iPropOx polymerizes slower than nPropOx.
18
In fact, the k
p
of
CPropOx is the fastest reported so far for a 2-oxazoline with an
aliphatic side chain; only 2-phenyl-2-oxazolines with electron-
withdrawing o-fluoro groups were slightly faster.
31
The higher
reactivity of CPropOx is not yet completely understood, but
might be related to the electron withdrawing effect of the
cyclopropyl substituent caused by the small angle of 60 between
the C atoms resulting in partial π-character of the bonds making
the remaining orbitals s-rich and, thus, electron withdrawing.
32
Even though this electron withdrawing effect lowers the nucleo-
philicity of the monomer, it enhances the reactivity of the cationic
propagating species apparently dominating the overall polymer-
ization rate. The lower steric bulk of cyclopropyl compared to
n-propyl or isopropyl most likely does not influence the polym-
erization rate since it has been demonstrated that 2-oxazolines
with ethyl to n-nonyl substituents all have similar polymerization
rates.
30
At higher conversion (>80%), the solution became very
viscous explaining the observed decrease in polymerization rate
due to limited diffusion. This can also explain the slightly slower
polymerization rate at higher [M]/[I] ratios due to the higher
viscosity resulting from longer polymer chains. In addition, SEC
characterization revealed monomodal molar mass distributions
for the polymers with polydispersity index (PDI) values below
1.3 when the [M]/[I] ratio is 52 or 107 demon strating that
the polymers were prepared in a controlled manner. At a higher
[M]/[I] ratio, the PDI values start to increase indicating the
occurrence of more chain transfer reactions. It is known that
the control over the polymerization is lost for most poly( 2-
oxazoline)s when the [M]/[I] value is above 200.
33
On the basis of the kinetic investigations, six CPropOx
polymers with different degrees of polymerization (DP) were
synthesized under microwave-assisted conditions using MeOTs
as initiator and acetonitrile as solvent. The DP values were
calculated from the conversion measured by GC, assuming
the absence of chain transfer and termination reactions. After
the polymerization, the polymers were precipitated in n-hexane
resulting in white solid powders. Mos t polymers show some
tailing in the SEC traces (Figure 3), indicating the occurrence of
some chain transfer and/or termination reactions during the
polymerization. When M
n
(determined with SEC) is plotted
against DP (calculated from the GC results), it is observed that
the values progressively deviate from the theoretical values with
increasing DP. This increasing deviatio n is a direct conseque nce
of the broadening of the molar mass distributions, ascribed to
chain transfer reactions, causing a decrease in M
n
. It should be
noted that this effect might be overestimated by the used SEC
calibration standards.
The MALDI TOF MS (Figure 4a) spectrum of pCPropOx
with a DP of 20 revealed two main distributions corresponding to
the methyl initiated pCPropOx (CH
3
[C
6
H
9
NO]
n
OH) and the
proton initiated pCPropOx (H[C
6
H
9
NO]
n
OH), confirming the
expected polymer structure as well as indicating the occurrence
of minor chain transfer reactions leading to the proton initiated
chains (ESI).
Besides linear pCPropOx, also a comb CPropOx polymer
was synthesized to demonstrate the versatility of the CPropOx
monomer as well as to investigate the influence of the polymeric
architecture on the polymer properties. During the macromono-
mer synthesis, a well-defined oligo(2-cyclopropyl-2-oxazoline)-
methacrylate (OCPropOxMA) with a DP of 5 was obtained by
end-capping the living polymer chains with triethylammonium
methacrylate. The macromonomer has a molar mass of 750 g/
mol and a PDI value of 1.17 (SEC using PS standards, ESI). The
MALDITOF MS spectrum revealed that next to the desired
compound (CH
3
[C
6
H
9
NO]
n
C
4
H
5
O
2
), also two minor mass
distributions are present that can be assigned to proton initiated
(H[C
6
H
9
NO]
n
C
4
H
5
O
2
) and water end-capped polymer chains
(CH
3
[C
6
H
9
NO]
n
OH) (Figure 4b, ESI). Comparison of the peak
integrals derived from the vinylic protons of the MA end group
and aromatic protons of the tosylate counterion in the
1
H NMR
spectrum of the reaction mixture revealed a degree of functiona-
lization of 91% with the desired MA end group (ESI). In the
second step, the macromonomer was polymerized via reversible
additionfragmentation chain transfer (RAFT) polymerization
up to a conversion of 70%, as determined from the integrals of the
macomonomer and the polymer in the SEC curve (ESI), resulting
in a comb polymer with a theoretical DP of 42 and a theoretical
molar mass of 27,500 g mol
1
(estimated from [M]:[CTA] and
conversion). As frequently observed for polymacromonomers,
the molar mass determined by SEC (9400 g mol
1
), is signifi-
cantly underestimated as a result of the large difference in
hydrodynamic volume of the dense “bottle brush” like polymer
structure when compared to the linear PMMA standards. Never-
theless, SEC reveals a narrow and symmetric molar mass dis-
tribution with a PDI value of 1.19 (ESI) as well as a complete
removal of residual macromonomer after purification by prepara-
tive SEC using Biobeads.
Figure 3. (a) SEC traces of all six synthesized polymers dissolved in chloroform. (b) Molar mass and polydispersity indices (PS calibration) plotted
against the calculated degree of polymerization (DP). The dashed line represents the theoretical molar mass.
4061 dx.doi.org/10.1021/ma200514n |Macromolecules 2011, 44, 4057–4064
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ARTICLE
Turbidity of the aqueous pCPropOx polymer solutions was
measured as a function of temperature with a concentration of
5mgmL
1
with heating and cooling rates of 1 C min
1
in a
temperature range between 0 and 100 C. Upon heating, the
polymer precipitates resulting in a drop in transmittance, and,
upon cooling, the polymer dissolves again demonstrating the
reversibility of the solubility transition. Most polymers show
similar transmittance curves during heating and cooling indicat-
ing that no or only a slight hysteresis took place (ESI), except for
pCPropOx with a DP of 20. The reason for this larger hysteresis
is unsure, but it might be related to the formation of smaller
aggregates that cannot be detected by turbidimetry. The cloud
points (CP) taken from the first heating run decrease in a
nonlinear way from 54 C for pCPropOx with a DP of 20 to
30 C for pCPropOx with a DP of 112 (Figure 5a). The steepest
decrease occurs when the DP is increased from DP = 20 to
DP = 51, which is due to the end group effect which is more
pronounced in short polymers. Since the OH end group is
more hydrophilic than the polymer chain, the end group
will increase the CP. With the increase in concentration,
the CP decreases in a nonlinear way and only a small decrease
is observed when the concentration is increased from 10 to
30 mg mL
1
indicating that the CP at 30 mg mL
1
approaches
the LCST value (Figure 5b). Both the DP and concentration
dependence of the CP follow the same trend as for pnPropOx,
15
which is a consequence of the Type I FloryHuggins miscibility
behavior that is characterized by a shift of the LCST toward
lower polymer concentration when the polymer chain length is
increased.
34
The CP of pCPropOx (30 C) lies in between the
CP of pnPropOx (44 C) and piPropOx (27 C) (all 5 mg mL
1
and a DP of 100, ESI). This can be explained by the difference in
hydrophilicity;
35
the polymer that is most hydrophilic (pEtOx)
has the highest CP followed by piPropOx, pCPropOx, and
pnPropOx.
Like pCPropOx, the comb polymer does not show large
hysteresis; the CP during heating is comparable to the CP during
cooling (ESI). The concentration depe ndence of the CP of the
comb polymer follows the same trend as for linear pCPropOx
(Figure 5b), which is in contrast to the concentration depen-
dence of poly(2-oxazoline)s with different cloud points due to
different side chains, such as pEtOx and pnPropOx.
15
This
confirms that the side-chains shield the MA backbone and that
only the OCPropOx side-chains interact with the aqueous
environment, resulting in similar LCST behavior for the linear
Figure 5. (a) Cloud points (CPs) determined by turbidimetry as a function of the degree of polymerization (DP) of pCPropOx at a concentration of
5mgmL
1
in water. (b) CPs as a function of concentration for pCPropOx with various DP and p[(OCPropOx)
5
MA]
42
.
Figure 4. MALDI TOF MS spectrum of (a) pCPropOx with DP = 20 and (b) the macromonomer, OCPropOxMA, using DCTB as the matrix.
![Figure 5. (a) Cloud points (CPs) determined by turbidimetry as a function of the degree of polymerization (DP) of pCPropOx at a concentration of 5 mg mL 1 in water. (b) CPs as a function of concentration for pCPropOx with various DP and p[(OCPropOx)5MA]42.](/figures/figure-5-a-cloud-points-cps-determined-by-turbidimetry-as-a-2ohqfu3a.webp)
![Figure 1. Schematic representation of the linear pCPropOx and the comb polymer p[(OCPropOx)5MA]42.](/figures/figure-1-schematic-representation-of-the-linear-pcpropox-and-351z1c6y.webp)
