Efficient cationic ring-opening polymerization of diverse cyclic imino ethers: Unexpected copolymerization behavior

Article abstract of DOI:10.1021/ma200426y

The recently developed fast microwave-assisted cationic ring-opening polymerization procedure for 2-oxazolines seems to be ideally suited for slower polymerizing cyclic imino ether monomers. In this study we report the effect of the cyclic imino ether structure on the polymerization rate under exactly the same microwave-assisted conditions revealing that indeed less reactive cyclic imino ethers, including 2-oxazines as well as 4- and 5-substituted 2-oxazolines, can be polymerized to at least 50% conversion for the slowest monomer, namely 5-methyl-2-butyl-2-oxazoline, within 10 h. In addition, the copolymerization behavior of 4-ethyl-2-butyl-2-oxazoline with 2-methyl-2-oxazoline and 2-phenyl-2-oxazoline unexpectedly revealed faster incorporation of the less reactive 4-ethyl-2-butyl-2-oxazoline monomer compared to 2-phenyl-2-oxazoline due to the increased bulk of the latter monomer amplifying the sterical hindrance for polymerization onto the 4-ethyl-2-butyl-2-oxazolinium propagating species.

Full text of DOI:10.1021/ma200426y

ARTICLE
pubs.acs.org/Macromolecules
Efficient Cationic Ring-Opening Polymerization of Diverse Cyclic
Imino Ethers: Unexpected Copolymerization Behavior
†
†
†
†
Hanneke M. L. Lambermont-Thijs, Martin W. M. Fijten, A. J. (Ton) van der Linden, Bart M. van Lankvelt,
†
,‡,§
,†,‡,§
,†,^
Meta M. Bloksma,
Ulrich S. Schubert,*
and Richard Hoogenboom*
†
Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, Den Dolech 2, 5612AZ 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
§
Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands
^
Supramolecular Chemistry Group, Department of Organic Chemistry, Ghent University, Krijgslaan 281 S4, B-9000, Ghent, Belgium
ABSTRACT: The recently developed fast microwave-assisted
cationic ring-opening polymerization procedure for 2-oxazo-
lines seems to be ideally suited for slower polymerizing cyclic
imino ether monomers. In this study we report the effect of the
cyclic imino ether structure on the polymerization rate under
exactly the same microwave-assisted conditions revealing that
indeed less reactive cyclic imino ethers, including 2-oxazines as
well as 4- and 5-substituted 2-oxazolines, can be polymerized to
at least 50% conversion for the slowest monomer, namely
5
2
-methyl-2-butyl-2-oxazoline, within 10 h. In addition, the copolymerization behavior of 4-ethyl-2-butyl-2-oxazoline with
-methyl-2-oxazoline and 2-phenyl-2-oxazoline unexpectedly revealed faster incorporation of the less reactive 4-ethyl-2-butyl-2-
oxazoline monomer compared to 2-phenyl-2-oxazoline due to the increased bulk of the latter monomer amplifying the sterical
hindrance for polymerization onto the 4-ethyl-2-butyl-2-oxazolinium propagating species.
16,17
’
INTRODUCTION
groups.
Importantly, a detailed comparison between micro-
wave heating and thermal heating revealed that the observed
The cationic ring-opening polymerization (CROP) of 2-ox-
acceleration is solely due to the increased reaction temperature
azoline and 2-oxazine cyclic imino ethers was first reported by
1
4,18
1
for both 2-ethyl-2-oxazoline and 2-phenyl-2-oxazoline).
Litt and co-workers in a patent application in 1965. Shortly after
Nonetheless, this accelerated CROP protocol seems to be ideally
the (living) polymerization of 2-oxazolines using various initiating
2ꢀ5
suited for slower polymerizing cyclic imino ether monomers as
systems was reported by four independent research groups.
1
9
was indeed demonstrated for 2-phenyl-2-oxazine by Ritter and
The first scientific publication on 2-oxazine polymerization also6
20
for 4-ethyl-2-butyl-2-oxazoline by our group. However, the
effect of cyclic imino ether structure on the polymerization rate
has never been evaluated for a wider range of structures; in
particular not under exactly the same conditions. Therefore, this
study aims to address the potential of the fast microwave-assisted
CROP protocol for various cyclic imino ethers (Scheme 1),
including very slow polymerizing monomers, by comparison of
their polymerization behavior. In addition, the copolymerization
behavior of 4-ethyl-2-butyl-2-oxazoline with 2-methyl-2-oxazo-
line and/or 2-phenyl-2-oxazoline will be discussed.
appeared in this early days of cyclic imino ether polymerization.
The first CROP of chiral 4- and 5-substituted-2-oxazolines was
7,8
reported almost a decade later.
Despite these early reports on the CROP of 2-oxazines and 4-
and 5- substituted 2-oxazolines, the major interest of polymer
9,10
scientists has been on the CROP of 2-oxazolines, resulting in
their exploration for various applications, including biomedical
materials, advanced (hierarchical) self-assembly and thermore-
11ꢀ13
sponsive properties.
The properties and applications of
poly(cyclic imino ether)s beyond the common poly(2-oxazoline)s
have hardly been explored; although there is no reason to assume
less application potential or property tuning by copolymerization
as well as side-chain variation of the other poly(cyclic imino
ether)s. The only rationale for the tremendously lower attention
for these other poly(cyclic imino ether)s might be their relatively
slow polymerization ranging from tens of hours to multiple days.
Recently we have introduced a microwave-assisted protocol to
accelerate the CROP of 2-oxazolines from multiple hours to
’
EXPERIMENTAL DETAILS
Materials. Solvents were purchased from Biosolve. Acetonitrile was
dried over molecular sieves (size 3 Å) before usage as polymerization
Received: February 23, 2011
Revised:
April 4, 2011
1
4,15
minutes,
which has been adopted by several research other
Published: May 11, 2011
r 2011 American Chemical Society
4320
dx.doi.org/10.1021/ma200426y
|Macromolecules 2011, 44, 4320–4325

Macromolecules
ARTICLE
Scheme 1. Schematic Representation of the Structures of the Investigated Cyclic Imino Ethers (Left) and the General Mechanism
of the Cationic Ring-Opening Polymerization of Cyclic Imino Ethers (Right)
1
solvent. MeOx (Aldrich) and PhOx (Aldrich) were distilled over barium
oxide and stored under argon. Methyl tosylate (Aldrich) was distilled
without drying agent and stored under argon. Valeronitrile, 2-amino-1-
ethanol, 3-amino-1-propanol, 2-amino-1-propanol, 2-amino-1-butanol,
2-Butyl-2-oxazine. H NMR (CDCl
3
): δ 4.15 (t, J = 5.2 Hz, 2H,
),
1.85 (t, J = 5.8 Hz, 2H, OCH CH ), 1.55 (quintet, J = 7.3 Hz, 2H,
2 2
OCH ), 3.36 (t, J = 5.8 Hz, 2H, NCH), 2.13 (t, J = 7.7 Hz, 2H, CCH
2
2
CCH
7.2 Hz, 3H, CH
2
CH
2
), 1.35 (sextet, J = 7.3 Hz, 2H, CCH
2
CH
2
CH
2
), 0.91 (t, J =
þ
1-amino-2-propanol, and zinc acetate were obtained from Aldrich and
3
), GCꢀMS: M = 140 (100%).
1
used without further purification.
RS-4-Methyl-2-butyl-2-oxazoline. H NMR (CDCl ): δ 4.27 (t, J =
3
General Methods and Instrumentation. The polymerizations
were performed under microwave irradiation with temperature control
in the Emrys Liberator single-mode microwave synthesizer from Per-
sonal Chemistry (now Biotage) equipped with a noninvasive IR sensor
8.6 Hz, 1H, OCH), 4.10 (sextet, J = 6.8 Hz, 1H, NCH), 3.705 (t, J = 7.8
Hz, 1H, OCH), 2.23 (t, J = 7.1 Hz, 2H, CCH
2
), 1.58 (quintet, J = 7.1 Hz,
CH CH ), 1.21
), 0.89 (t, J = 7.3 Hz, 3H, CH
), GCꢀMS:
2H, CCH CH ), 1.33 (sextet, J = 7.1 Hz, 2H, CCH
2
2
2
2
2
(d, J = 7.1 Hz, 3H, NCHCH
3
3
þ
(accuracy: (2%). Microwave vials were heated to 110 ꢀC overnight and
M
= 140 (100%).
1
cooled to room temperature under argon before usage. Gas chromatogra-
phy (GC) was measured on an Interscience Trace gas chromatograph with
a Trace Column RTX-5 connected to a PAL autosampler. Size exclusion
chromatography (SEC) measurements were performed on a Shimadzu
system equipped with a SCL-10A system controller, a LC-10AD pump, a
RID-10A refractive index detector, a SPD-10A UV-detector at 254 nm and a
PLgel 5 μm Mixed-D column at 50 ꢀC utilizing a chloroform:triethyl-
amine:2-propanol (94:4:2) mixture as eluent at a flow rate of 1 mL/min.
RS-4-Ethyl-2-butyl-2-oxazoline. H NMR (CDCl
3
): δ 4.26 (t, J = 8.0
Hz, 1H, OCH), 4.01 (quintet, J = 7.1 Hz, 1H, NCH), 3.84 (t, J = 7.6
Hz,1H, OCH), 2.27 (t, J = 7.8 Hz, 2H, CCH
), 1.67ꢀ1.45 (m, J = 7.1
Hz, 4H, CCH CH and OCH CH ), 1.37 (sextet, J = 7.4 Hz, 2H,
), 0.96ꢀ0.90 (m, J = 7.6 Hz, 6H, 2 CH ), GCꢀMS:
2
2
2
2
CCH
2
CH
2
CH
2
3
þ
M
= 154 (100%).
1
RS-5-Methyl-2-butyl-2-oxazoline. H NMR (CDCl
1H, OCH), 4.05 (m, 2H, NCH
(d, J = 7.1 Hz, 3H, OCHCH
3
): δ 4.57 (m,
), 2.25 (t, J = 7.1 Hz, 2H, CCH ), 1.79
), 1.56 (quintet, J = 7.1 Hz, 2H, CCH CH ),
), 0.90 (t, J = 7.1 Hz, 3H,
2
2
1
The molar masses were calculated against polystyrene standards. H NMR
3
2
2
spectra of the products were recorded on a Varian AM-400 spectrometer at
1.35 (sextet, J = 7.1 Hz, 2H, CCH
CH CH
2 2
2
þ
room temperature in CDCl as solvent. The chemical shifts are given in
CH
3
), GCꢀMS: M = 140 (100%).
3
ppm relative to residual nondeuterated solvent signals. Thermal transitions
were determined on a DSC 204 F1 Phoenix by Netzsch under a nitrogen
atmosphere from ꢀ100 to þ220 ꢀC with a heating rate of 20 K/min and a
cooling rate of 40 K/min (for the calculations, only the third heating curve
of three runs was considered).
Kinetic Investigations on the Microwave-Assisted Poly-
merizations. The polymerization kinetics were studied by preparing
first a stock solution of the monomer with 4 M concentration in
acetonitrile together with methyl tosylate resulting in a monomer to
initiator ratio of 60. This stock solution was divided over several
microwave vials (1 mL each) that were heated for different predefined
times to 140 ꢀC. After microwave heating, the polymerization mixture
was cooled to 38 ꢀC and quenched by the addition of water. GC and SEC
samples were prepared from the polymerization mixtures to determine
the monomer conversion and the molar mass (distribution) of the
resulting polymers. The monomer conversion was calculated from
integration of the GC signals, whereby the solvent signal was used as
internal standard.
General Monomer Synthesis Procedure. The oxazoline
monomers are all synthesized in a one-step reaction between valeroni-
trile and the corresponding aminoalcohol using zinc acetate as
2
1,22
catalyst.
The aminoalcohol (1 mol equiv) was added dropwise to
(0.02 mol equiv).
a mixture of the nitrile (1.1 mol equiv) and Zn(OAc)
2
After complete addition of the aminoalcohol the temperature was
increased to 130 ꢀC. The reaction was kept under argon at 130 ꢀC for
15 to 20 h followed by addition of dichloromethane. The resulting
The copolymerization kinetics were determined in a similar manner
yellow suspension was washed with water (2 times) and brine (1 time).
The separated organic layer was dried with magnesium sulfate, filtered
and the solvent was evaporated under reduced pressure. The pure
products where obtained as colorless liquids in 50ꢀ70% yield after
using a total monomer to initiator ratio of 100 with [M
1 2
] = [M ] for two
monomers or [M ] = [M ] = [M ] for three monomers.
1
2
3
vacuum distillation with barium oxide.
1
’ RESULTS AND DISCUSSION
2
-Butyl-2-oxazoline. H NMR (CDCl ): δ 4.21 (t, J = 9.5 Hz, 2H,
3
OCH
CCH
2
), 3.82 (t, J = 9.5 Hz, 2H, NCH
),1,62 (quintet, J = 7.8 Hz, 2H, CCH
2
), 2.72 (t, J = 7.7 Hz, 2H,
CH ), 1.37 (sextet, J =
The investigated cyclic imino ethers were prepared by a
condensation reaction between valeronitrile and the correspond-
2
2
2
20,21
7
.8 Hz, 2H, CCH CH CH ), 0.92 (t, J = 7.2 Hz, 3H, CH ), GCꢀMS:
ing amino-alcohols using zinc acetate as catalyst.
All mono-
2
2
2
3
þ
M
= 126 (100%).
mers were prepared bearing a butyl-group on the two position to
4
321
dx.doi.org/10.1021/ma200426y |Macromolecules 2011, 44, 4320–4325

Macromolecules
ARTICLE
23
exclude side-group effects on the polymerization behavior. The
synthesized cyclic imino ether monomers are 2-butyl-2-oxazo-
line, 2-butyl-2-oxazine, 4-methyl-2-butyl-2-oxazoline, 4-ethyl-2-
butyl-2-oxazoline, and 5-methyl-2-butyl-2-oxazoline, whereby
24
the latter three are racemic mixtures.
The cationic ring-opening polymerization of these cyclic
imino ethers was investigated using our previously determined
optimal reaction conditions for 2-oxazolines, namely polymeri-
zation with methyl tosylate as initiator in acetonitrile at 140 ꢀC
using a monomer concentration of 4 M and a monomer to
14,15
initiator ratio of 60.
Upon heating the polymerization
mixture, nucleophilic attack of the cyclic imino ether onto the
methyl tosylate results in the formation of a cationic propagating
species having a weakened CꢀO bond. Subsequent attack of the
next monomer onto this carbon atom yields the ring-opened
poly(cyclic imino ether) still bearing the reactive cationic group.
At the end of the polymerization, the living chain ends were
quenched by the addition of water resulting in a mixture of
hydroxy and ester end-groups due to water addition to the 2- and
25,26
5
-positions of the propagating species.
This general poly-
merization mechanism is shown in Scheme 1.
The polymerization kinetics were determined by polymerizing
separate samples of the polymerization mixture for different
times under microwave irradiation. The resulting mixtures were
analyzed by gas chromatography (GC) to determine the mono-
mer conversion as well as by size exclusion chromatography
(
SEC) to determine the molar mass and molar mass distribution
of the polymers. The living CROP of 2-oxazolines is known to
follow first order kinetics based on the constant concentration of
cationic propagating species. When plotting the conversion data
in a first order kinetic plot (ln([M] /[M] ) versus time) a linear
Figure 1. Top: First order kinetic plot for the cationic ring-opening
polymerization of the cyclic imino ethers. Bottom: Evolution of number-
0
t
average molar mass (M ) and polydispersity index (PDI) with conver-
n
correlation was found for all investigated monomers confirming
that the polymerizations followed first order kinetics (Figure 1).
In addition, the linear increase of the number-average molar mass
sion. Microwave polymerizations performed at 140 ꢀC in acetonitrile
using methyl tosylate as initiator, [M]/[I] = 60 and [M] = 4 M. SEC
3
analysis was performed with CHCl : 2-propanol:triethylamine (94:2:4)
mixture as eluent and a polystyrene calibration.
(
M ) with conversion together with the relatively low polydis-
n
persity indices (PDI < 1.35) indicate that the polymerizations
were controlled; although the PDI values indicate the occurrence
of some chain transfer side reactions. For most monomers, the
intercept of the molar mass versus conversion plots is rather high,
which can be ascribed to the relative molar mass determination
against polystyrene standards. The only exception is the polym-
erization of 4-methyl-2-butyl-2-oxazoline that resulted in poly-
mers with broader molar mass distributions (PDI > 1.4) and
sterical hindrance for nucleophilic monomer addition resulting
from the gauche conformation of the nonplanar oxazonium ring
compared to the flat oxazolinium species. Nonetheless, com-
plete monomer conversion is still reached within 1 h for the
CROP of 2-butyl-2-oxazine. Substitution of the oxazoline ring
has a much more drastic effect on the polymerization rate leading
27
to an almost 30-fold decrease in k for 4-methyl-2-butyl-2-
p
rather low M values most likely resulting from a larger extent of
oxazoline compared to 2-butyl-2-oxazoline. This slower polym-
erization can be ascribed to increased sterical hindrance for the
attack of the next monomer as is clearly evidenced by the
n
chain transfer reactions presumably caused by impurities in the
monomer or trace amounts of water. The lower purity of this
monomer is caused by the smaller scale synthesis (<5 mL)
complicating the work-up and purification procedures.
additional 3-fold decrease in k when changing the 4-substituent
p
from methyl to ethyl giving 4-ethyl-2-butyl-2-oxazoline. Placing a
methyl substituent on the 5-position, i.e., where the next mono-
mer will attack, resulting in 5-methyl-2-butyl-2-oxazoline results
in the slowest polymerizing cyclic imino ether among the studied
variants, which again can be ascribed to increased sterical
The polymerization rate constants (k ) for the different cyclic
p
imino ethers were calculated from the slope of the first order
kinetic plots assuming complete initiation and absence of termi-
nation (chain transfer will not affect the polymerization rate) and
the values are summarized in Table 1. The assumption of
hindrance. Even though the k is drastically lowered by incorpo-
p
1
complete initiation is confirmed by the H NMR spectra of the
rating 4- and 5-substituents, the microwave-assisted polymeriza-
tion procedure still enables reasonable monomer conversion of
at least 50% for the slowest 5-methyl-2-butyl-2-oxazoline cyclic
imino ether within 10 h polymerization time.
polymerization mixtures revealing a complete shift, already for
the first kinetic samples, of the aromatic signals from 7.35 and
7
.80 ppm for the methyl tosylate to 7.16 and 7.74 ppm for the
anionic tosylate counterion (data not shown). The polymeriza-
The effect of the cyclic imino ether structure on the thermal
properties of the resulting polymers was evaluated based on
differential scanning calorimetry (DSC) measurements. The
tion of 2-butyl-2-oxazoline revealed the highest k , which is
p
25
similar to the values reported for other 2-n-alkyl-2-oxazolines.
Expanding the ring size to the 2-butyl-2-oxazine lowers the
polymerization rate, which can be ascribed to the increased
glass transition temperatures (T ) noticeably demonstrate that
increasing the ring size leads to enhanced chain flexibility and,
g
4
322
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Macromolecules
Table 1. Polymerization Rate Constants (k ) of the Cyclic Imino Ethers and Glass Transitions Temperatures (T ) of the Resulting
ARTICLE
p
g
Polymers
a
b
a
b
monomer
k
p
T
g
, ꢀC
23.8
ꢀ 5.1
monomer
k
p
g
T , ꢀC
2
-butyl-2-oxazoline
-butyl-2-oxazine
139 ( 2
4-methyl-2-butyl-2-oxazoline
4-ethyl-2-butyl-2-oxazoline
5.0 ( 0.4
1.6 ( 0.1
51.3
58.8
34.0
2
22.2 ( 1.5
5
-methyl-2-butyl-2-oxazoline
0.32 ( 0.03
a
ꢀ3
b
In 10 L/(mol ꢁ s); Calculated from the slope of the first order kinetic plots. Determined by differential scanning calorimetry with a heating rate of
2
0 K/min.
thus, a lower T (Table 1). In contrast, adding substituents to the
propagating species accelerate the incorporation of 4-ethyl-2-
butyl-2-oxazoline compared to its homopolymerization while the
polymerization of 2-methyl-2-oxazoline is slowed down compared
to its homopolymerization due to the higher sterical hindrance of
the 4-ethyl-2-butyl-2-oxazolinium propagating species.
g
4
- and 5-position of the 2-oxazoline ring enhances the rigidity of
the resulting polymers bearing main-chain substituents leading to
an increase in T . The unexpectedly low T for poly(5-methyl-2-
g
g
butyl-2-oxazoline) might be related to the low 50% monomer
conversion of the final kinetic sample that was used for analysis,
i.e., the fraction of flexible chain ends is higher resulting in a lower
Tg and/or a minor fraction of residual monomer might act as
plasticizer.
The statistical copolymerization of 4-ethyl-2-butyl-2-oxazo-
line with 2-phenyl-2-oxazoline, surprisingly, revealed a faster
incorporation of 4-ethyl-2-butyl-2-oxazoline (k of (3.8 ( 0.3) ꢁ
p
ꢀ
ꢀ
3
10 L/(mol ꢁ s)) than 2-phenyl-2-oxazoline (k of (1.5 ( 0.1) ꢁ
p
3
Recently, we demonstrated the one-pot quasi-diblock copo-
10
L/(mol ꢁ s)) resulting in a monomer gradient from
lymer synthesis based on the large difference in reactivity
4-ethyl-2-butyl-2-oxazoline to 2-phenyl-2-oxazoline, despite the
20-fold faster homopolymerization of 2-phenyl-2-oxazoline com-
pared to 4-ethyl-2-butyl-2-oxazoline (Figure 2, middle). This
large discrepancy can be ascribed to the large aromatic side-chain
of 2-phenyl-2-oxazoline, which amplifies the sterical hindrance
for attack onto 4-ethyl-2-butyl-2-oxazoline oxazolinium propa-
gating species. In contrast, the attack of 4-ethyl-2-butyl-2-oxazo-
line onto 2-phenyl-2-oxazolinium is more efficient compared to
the attack on the more sterically hindered C-5 of 4-ethyl-2-butyl-
2-oxazolinium species. When analyzing the terpolymerizations
kinetics in more detail, keeping the copolymerization results in
mind, it can be calculated that the polymerization of 2-methyl-2-
ꢀ
3
of 2-methyl-2-oxazoline (k = 145 ꢁ 10
L/(mol ꢁ s) at
p
25
ꢀ3
1
40 ꢀC) and 2-phenyl-2-oxazoline (k = 32 ꢁ 10 L/(mol ꢁ s)
p
1
5,28
at 140 ꢀC).
On the basis of the even slower, but still
controlled, polymerization found for 4-ethyl-2-butyl-2-oxazoline
it was anticipated that the one-pot copolymerization of 2-methyl-
2
-oxazoline, 2-phenyl-2-oxazoline and 4-ethyl-2-butyl-2-oxazo-
line at 140 ꢀC would lead to the formation of a quasi-triblock
copolymer structure. However, kinetic analysis of this copoly-
merization surprisingly revealed the formation of a gradient
copolymer structure going from a 2-methyl-2-oxazoline rich
domain, via a 4-ethyl-2-butyl-2-oxazoline rich domain to a 2-phen-
yl-2-oxazoline rich domain at the final stages of the polymerization
ꢀ
3
oxazoline is slower (k of (86 ( 4) ꢁ 10
L/(mol ꢁ s))
p
(
Figure 2, bottom).
Unfortunately, H NMR spectroscopy could not be used to
compared to its homopolymerization, but faster compared to the
copolymerization with 4-ethyl-2-butyl-2-oxazoline. In addition,
the kinetics of 4-ethyl-2-butyl-2-oxazoline also revealed two
1
analyze the copolymer composition due to the very broad signals
of the poly(4-ethyl-2-butyl-2-oxazoline) (2.0ꢀ4.5 ppm) result-
ing in significant overlap. Nonetheless, the almost complete
monomer conversions (complete conversion of 2-methyl-2-
oxazoline and 4-ethyl-2-butyl-2-oxazoline; 75% conversion of
ꢀ
3
slopes in the terpolymerizations; a k of (13.7 ( 0.3) ꢁ 10
p
L/(mol ꢁ s) in the presence of 2-methyl-2-oxazoline, which
ꢀ
3
decreases to (6.5 ( 0.3) ꢁ 10
L/(mol ꢁ s) after full
p
conversion of 2-methyl-2-oxazoline. Finally, the k for 2-phen-
ꢀ3
2
-phenyl-2-oxazoline) in combination with a polydispersity in-
yl-2-oxazoline was found to be (3.1 ( 0.2) ꢁ 10 L/(mol ꢁ s)
in the terpolymerizations. These observed trends for the terpo-
lymerization are similar to the copolymerizations. Nonetheless,
dex below 1.30 indicate that the copolymer has the expected
composition.
To better understand the unexpected copolymer behavior, the
copolymerization of the separate monomer pairs were also
studied. Statistical copolymerization of 2-methyl-2-oxazoline
with 4-ethyl-2-butyl-2-oxazoline revealed the expected fast in-
corporation of 2-methyl-2-oxazoline resulting in a quasi-diblock
all k values calculated for the terpolymerizations are higher than
p
for the copolymerizations, which can be ascribed to the lower
amount of the more sterically hindered 4-ethyl-2-butyl-2-oxazo-
line monomer in the terpolymerizations (33% in the terpoly-
merizations versus 50% for the copolymerizations).
copolymer structure. However, the calculated k of 60 ( 3 ꢁ
The contribution of preferential microwave heating of the
4-ethyl-4-butyl-2-oxazoline as driving force for the unexpected
copolymerization kinetics can be ruled out based on the follow-
ing discussion. In our previous work we have shown for the
homopolymerization of 2-ethyl-2-oxazoline and 2-phenyl-2-ox-
azoline, i.e., two very different 2-oxazoline monomers, that the
observed acceleration under similar microwave-assisted polym-
erization conditions was solely due to the increased temperatures
p
ꢀ
3
1
0
L/(mol ꢁ s) for MeOx indicates a deceleration in the
presence of 4-ethyl-2-butyl-2-oxazoline (Figure 2, top). The first
order kinetic plot for 4-ethyl-2-butyl-2-oxazoline revealed two
ꢀ
3
regimes with an initial k of (7.7 ( 0.3) ꢁ 10 L/(mol ꢁ s) and
p
ꢀ
3
a k of (2.2 ( 0.2) ꢁ 10 L/(mol ꢁ s) during the later stages of
p
the polymerization. This later k is observed when all 2-methyl-2-
p
oxazoline has been consumed and resembles the k obtained for
p
14,18
the homopolymerization of 4-ethyl-2-butyl-2-oxazoline. The
facilitated by the closed vial conditions.
Therefore, it is
enhanced k for 4-ethyl-2-butyl-2-oxazoline in combination with
expected that this absence of a nonthermal microwave effect
can be extrapolated to 4-ethyl-2-butyl-2-oxazoline, which struc-
ture is rather similar to 2-ethyl-2-oxazoline. This assumption is
further justified by the fact that the solvent, acetonitrile, also
p
the decreased k for 2-methyl-2-oxazoline when both monomers
p
are present indicate a nonideal copolymerization behavior,
whereby the higher accessibility of the 2-methyl-2-oxazolinium
4
323
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ARTICLE
5
-methyl-2-butyl-2-oxazoline. All microwave-assisted polymeri-
zations could be performed in a controlled manner, although
significant chain transfer side reactions were observed for
4
-methyl-2-butyl-2-oxazoline presumably due to minor impuri-
ties, such as traces of water. Statistical copolymerization of
-methyl-2-oxazoline, 2-phenyl-2-oxazoline and 4-ethyl-2-
2
butyl-2-oxazoline revealed that the polymerization of 4-ethyl-2-
butyl-2-oxazoline is accelerated due to the better accessibility of
the oxazolinium species without 4-substituents. In contrast, the
polymerization of 2-methyl-2-oxazoline and 2-phenyl-2-oxazo-
line are slowed down by the presence of the more sterically
hindered 4-ethyl-2-butyl-2-oxazolinium species. This effect is
amplified by the large size of the aromatic side group of
2
-phenyl-2-oxazoline resulting in a faster incorporation of 4-eth-
yl-2-butyl-2-oxazoline compared to 2-phenyl-2-oxazoline during
the statistical copolymerization while, in contrast, 2-phenyl-2-
oxazoline exhibits a faster homopolymerization.
These results provide important new insights into the
polymerization behavior of cyclic imino ethers opening up
new possibilities for their more widespread use as comonomers
during CROP of 2-oxazolines. Moreover, the fast microwave-
assisted polymerization procedure enables efficient polymeri-
zation of 4- and 5-substituted 2-oxazolines.
’
AUTHOR INFORMATION
Corresponding Author
E-mail: ulrich.schubert@uni-jena.de (U.S.S.) or richard.
*
hoogenboom@ugent.be (R.H.).
’
ACKNOWLEDGMENT
The authors thank the Dutch Polymer Institute (DPI, tech-
nology area HTE) and NWO (VICI award for U.S.S. and VENI
award for R.H.) for financial support.
’
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(
(
7
(
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(
(
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