Main-Chain and Side-Chain Sequence-Regulated Vinyl Copolymers by Iterative Atom Transfer Radical Additions and 1:1 or 2:1 Alternating Radical Copolymerization

Article abstract of DOI:10.1021/jacs.5b11631

Main- and side-chain sequence-regulated vinyl copolymers were prepared by a combination of iterative atom transfer radical additions (ATRAs) of vinyl monomers for side-chain control and 1:1 or 2:1 alternating radical copolymerization of the obtained side-chain sequenced "oligomonomers" and vinyl comonomers for main-chain control. A complete set of sequence-regulated trimeric vinyl oligomers of styrene (S) and/or methyl acrylate (A) were first synthesized via iterative ATRAs of these monomers to a halide of monomeric S or A unit (X-S or X-A) under optimized conditions with appropriate ruthenium or copper catalysts, which were selected depending on the monomers and halides. The obtained halogen-capped oligomers were then converted into a series of maleimide (M)-ended oligomonomers with different monomer compositions and sequences (M-SSS, M-ASS, M-SAS, M-AAS, M-SSA, M-ASA, M-SAA, M-AAA) by a substitution reaction of the halide with furan-protected maleimide anion followed by deprotection of the furan units. These maleimide-ended oligomonomers were then radically copolymerized with styrene or limonene to enable the 1:1 or 2:1 monomer-sequence regulation in the main chain and finally result in the main- and side-chain sequence-regulated vinyl copolymers with high molecular weights in high yield. The properties of the sequence-regulated vinyl copolymers depended on not only the monomer compositions but also the monomer sequences. The solubility was highly affected by the outer monomer units in the side chains whereas the glass transition temperatures were primarily affected by the two successive monomer sequences.

Full text of DOI:10.1021/jacs.5b11631

Article
pubs.acs.org/JACS
Main-Chain and Side-Chain Sequence-Regulated Vinyl Copolymers
by Iterative Atom Transfer Radical Additions and 1:1 or 2:1
Alternating Radical Copolymerization
†
,†,‡
,†
Takamasa Soejima, Kotaro Satoh,* and Masami Kamigaito*
†
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
‡
Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
*
S Supporting Information
ABSTRACT: Main- and side-chain sequence-regulated vinyl
copolymers were prepared by a combination of iterative atom
transfer radical additions (ATRAs) of vinyl monomers for side-
chain control and 1:1 or 2:1 alternating radical copolymeriza-
tion of the obtained side-chain sequenced “oligomonomers”
and vinyl comonomers for main-chain control. A complete set
of sequence-regulated trimeric vinyl oligomers of styrene (S) and/or methyl acrylate (A) were first synthesized via iterative
ATRAs of these monomers to a halide of monomeric S or A unit (X−S or X−A) under optimized conditions with appropriate
ruthenium or copper catalysts, which were selected depending on the monomers and halides. The obtained halogen-capped
oligomers were then converted into a series of maleimide (M)-ended oligomonomers with different monomer compositions and
sequences (M−SSS, M−ASS, M−SAS, M−AAS, M−SSA, M−ASA, M−SAA, M−AAA) by a substitution reaction of the halide
with furan-protected maleimide anion followed by deprotection of the furan units. These maleimide-ended oligomonomers were
then radically copolymerized with styrene or limonene to enable the 1:1 or 2:1 monomer-sequence regulation in the main chain
and finally result in the main- and side-chain sequence-regulated vinyl copolymers with high molecular weights in high yield. The
properties of the sequence-regulated vinyl copolymers depended on not only the monomer compositions but also the monomer
sequences. The solubility was highly affected by the outer monomer units in the side chains whereas the glass transition
temperatures were primarily affected by the two successive monomer sequences.
INTRODUCTION
which can discriminate the comonomers by subtle differences
in their structures and/or reactivities during the polymer-
izations. Another is to repeat the propagation reactions one by
one iteratively by artificially adding one monomer and then
another in the desired orders.
■
Natural macromolecules such as proteins and nucleic acids
possess perfectly ordered molecular structures in terms of chain
lengths, stereostructures, and monomer sequences, which are
1
essential for their special and unique functions. Although
One of the most successful achievements based on the latter
synthetic macromolecules or polymers are widely used in our
life, their primary structures are not perfectly controlled unlike
natural ones. Most of these synthetic polymers are prepared via
statistically governed successive chemical reactions, i.e.,
polymerization, which usually do not have perfect selectivity
compared to biosynthesis. However, many efforts have been
made to control the polymer structures more precisely because
this is expected to contribute to improving polymer properties
and providing new functions to the resulting polymers. The
control of molecular weights and stereochemistry has thus
become possible at sufficient levels, although not perfectly,
through recent remarkable developments of various living
polymerizations and stereospecific polymerizations, in which
selective propagations are achieved in terms of chemoselectivity
and stereoselectivity, respectively.
approach might be solid phase synthesis using Merrifield’s
49
resins along with the automation process, which enables the
perfectly controlled monomer sequence for artificial peptide
and DNA. In view of the polymerization, this could be regarded
as a kind of chain-growth process in which the reactions occur
between the injected monomer and the attached polymer chain
on the resin. Such one-by-one iterative methods can also be
valid for a step-growth process in which the reactions occur first
between the monomers and then between the resulting
oligomers and polymers. The representative might be iterative
2
,3
5
0,51
exponential growth via stepwise reaction,
which has
recently been developed into a semiautomated flow system
and enables the synthesis of a large amount of single molecular
weight polymers consisting of sequence-regulated repeating
52,53
units.
Another successful achievement in synthesizing
Monomer-sequence control in synthetic polymers remains
one of the most challenging topics in polymer chemistry.
4
−48
One strategy for controlling monomer sequences in synthetic
polymers is to develop highly comonomer-selective reactions,
Received: November 6, 2015
©
XXXX American Chemical Society
A
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Scheme 1. Synthesis of Sequence-Regulated Oligomonomers and Their Alternating 1:1 and 2:1 Radical Copolymerization and
Homopolymerization for Main- and Side-Chain Sequence-Regulated Vinyl Copolymers
discrete and sequence-defined macromolecules might be the
In this study, we took advantage of two strategies: artificially
iterative addition reactions for making a perfect monomer
sequence in the short side chains and chemically induced
comonomer-selective chain-growth copolymerization for mak-
ing the high-molecular-weight polymers with controlled main-
chain sequences in high yield. For this, we used two reactions:
iterative atom transfer radical additions (ATRAs) of vinyl
monomers for sequencing the comonomers in the side-chains
and AB or AAB-sequenced chain-growth radical copolymeriza-
tion for ordering the monomer sequences in the long main
chains of the high-molecular-weight vinyl copolymers.
development of dendrimers via iterative methods including
54−56
divergent and convergent approaches.
In principle, these
iterative methods enable the nearly perfect construction of the
polymers but are unsuitable for very high-molecular-weight
polymers because the yields must decrease with the successive
reactions. The synthesis of sequence-regulated vinyl copoly-
mers has especially been considered to be very difficult by the
iterative methods.
With regard to the other strategy that will be achieved by
developing highly comonomer-selective reactions, there might
be various approaches, e.g., using differences in the comonomer
reactivities, additives that can preferentially interact with one
comonomer to alter the reactivity, or templates that can arrange
the comonomers in desired orders during the reaction. In
chain-growth radical copolymerizations for some pairs of
monomers, such as styrene and cyclic maleic compounds, a
ATRA or Kharasch addition reaction is not only recognized
as a highly efficient radical addition reaction to produce well-
59,60
defined organic molecules
but is also extended to chain-
growth polymerization as atom transfer radical polymerization
61−67
(
ATRP) or metal-catalyzed living radical polymerization.
The polymerization is effective for regulating the molecular
weights via reversible activation of the dormant carbon−
halogen bonds by the metal catalysts and is widely used for the
synthesis of various well-defined polymers such as block, end-
functionalized, and star polymers. In addition, multiblock
copolymers consisting of sequence-regulated polymeric or
1:1 or AB alternating monomer sequence can be attained by
the highly ordered statistics originating from the specific
5
7,58
comonomer reactivity ratios.
Furthermore, we have
recently succeeded in a higher regulation such as 1:2 or ABB
sequence in radical copolymerization of limonene and
maleimide derivatives in some specific solvent that can alter
68−71
oligomeric blocks can be produced
similar to other
controlled/living radical polymerizations such as reversible
22−25
the reactivity of comonomers.
Although higher-ordered
addition−fragmentation chain transfer (RAFT) polymeriza-
72
monomer sequencing is basically difficult in chain-growth
copolymerizations, the chemically ordered selective reaction
can easily provide high-molecular-weight polymers in high
yield.
tion. However, the ATRP process itself cannot regulate the
monomer sequence in the copolymerization because the
comonomer reactivity ratio is in principle the same as that in
conventional radical copolymerization.
B
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Scheme 2. Synthesis of a Series of Maleimide-Ended Sequence-Regulated Oligomonomers (M−SSS, M−ASS, M−SAS, M−AAS,
M−SSA, M−ASA, M−SAA, M−ASA, M−SAA, M−AAA) by Iterative ATRAs with (i) RuCp*Cl(PPh ) or (ii) CuBr/PMDETA
3
2
a
and (iii) S 2 Reactions Followed by Retro Diels−Alder Reactions
N
a
The values in percentages below arrows and in parentheses indicate yields for each reaction and total yields of final products, respectively.
We have recently used single ATRAs iteratively between
vinyl monomers via one-by-one addition for the synthesis of
sequence-regulated telechelic oligomers with the reactive C
Cl bond at one end and the nonconjugated CC bond at the
We herein conducted similar iterative ATRAs for styrene (S)
and methyl acrylate (A) to synthesize a complete set of
maleimide (M)-ended perfectly sequence-regulated “oligomo-
nomers” consisting of trimeric vinyl monomer units (SSS, ASS,
SAA, AAS, SSA, ASA, SAA, AAA) as substituents for the
2
6
other. The obtained telechelic oligomers were then
polymerized using similar metal catalysts via a step-growth
mechanism to result in the sequence-regulated vinyl copoly-
mers in the main chain. Although the monomer sequencing in
the main chain was perfectly controlled, the molecular weight
was limited to the scale of thousands owing to the step-growth
radical addition mechanism.
maleimide derivatives by further evolving the method we
73,74
reported previously (Scheme 1).
To see the effects of
monomer sequences, the maleimide-ended oligomonomes were
then radically homopolymerized or copolymerized with styrene
75
22−25
(
S) or limonene (L)
to produce high-molecular-weight
vinyl copolymers comprising M, MS, or MML monomer
sequences in the main chain in high yield. The obtained vinyl
C
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copolymers should have a perfect trimeric sequence in the short
side chains and 1:0, 1:1, or 2:1 monomer sequences in the long
main chains. This would clarify the effects of monomer
sequences on the properties of sequence-regulated vinyl
copolymers.
The maleimide (M) group was introduced by an SN2
reaction with furan-masked potassium maleimide anion and the
halides followed by a retro Diels−Alder reaction for the
deprotection of the furan group to yield the complete set of
maleimide-ended perfectly sequence-regulated vinyl oligomo-
nomers consisting of S and/or A trimeric units (M−SSS, M−
ASS, M−SAS, M−AAS, M−SSA, M−ASA, M−SAA, M−AAA)
with different monomer compositions or sequences, although
they were each mixtures of diasteromers and enantiomers
(Figure 1 and Figure S1). In addition, MALDI-TOF-MS
analyses showed the formation of single molecular weight
RESULTS AND DISCUSSION
■
Maleimide-Ended Sequence-Regulated Oligomono-
mers by Iterative ATRAs. Monomer Sequencing in the
Side Chain. A series of the maleimide-ended sequence-
regulated oligomonomers were prepared via the ruthenium-
or copper-catalyzed ATRAs of vinyl monomers, i.e., styrene (S)
and methyl acrylate (A), to alkyl halides possessing monomeric
S or A units iteratively (Scheme 2). We first conducted ATRA
between S or A and the corresponding monomeric halide X−S
(
(
1-bromoethylbenzene or 1-chloroethylbenzene) or X−A
methyl 2-bromopropionate or methyl 2-chrolopropionate) in
excess of the halides (2−5 mol equiv to vinyl monomer) using
appropriate ruthenium or copper complexes depending on the
monomers and halides. The metal catalysts were chosen from
those that especially give well-controlled molecular weights and
narrow molecular weight distributions (MWDs) in the ATRP
of the corresponding vinyl monomer with the halide initiators.
This is because the catalyst that enables highly controlled
molecular weights possesses a high ability to induce fast
deactivation of the resulting radical species into the halide and
is expected to provide the dimer in high yield by suppressing
further polymerization as side reactions. According to the
previous reports on the ATRPs, RuCp*Cl(PPh ) was selected
3
2
for activation of styrene-terminal halide while CuBr/PMDETA
76,77
or RuCp*Cl(PPh ) was used for acrylate terminal halide.
3
2
One of the notable features of ATRAs for the iterative radical
addition reactions is that the chlorides and bromides are
generally stable under air and can be purified by column
chromatography or distillation. However, for preparation of the
dimer with styryl terminal halide (Cl−SS or Cl−SA), the
chloride (Cl−S or Cl−A) was used because the bromide
terminal of the styrene proved to be less stable at high
temperatures for the distillation.
Although the yields depended on the monomers, halides, and
metal catalysts, all of these reactions resulted in the
corresponding 1:1 adducts, i.e., dimers (X−SS, X−AS, X−SA,
X−AA), which were purified by distillation or column
chromatography. All of these four dimers were then iteratively
reacted with S or A again in the presence of the appropriate
metal catalysts to afford a complete set of the eight halogen-
ended trimers (X−SSS, X−ASS, X−SAS, X−AAS, X−SSA, X−
ASA, X−SAA, X−AAA).
In these iterative reactions, the yields for addition reactions
between the acrylate halide terminal (X−A, X−AS, X−AA) and
styrene or acrylate monomer were generally high (=70−90%)
owing to the high reactivity of the acrylate halide terminal. In
contrast, the addition reactions between the styrene halide
terminal and styrene or acrylate resulted in lower yields. The
yields between the styrene halide (X−S, X−SS, X−SA) and
styrene were approximately 20−40% owing to the low
78
reactivities of styrene halide, which also resulted in a low
conversion of styrene. Those between styrene halide (X−S, X−
SS, X−SA) and acrylate were approximately 15−20%, where
acrylate oligomers were formed owing to the higher reactivity
of the resulting acrylate halide terminal than that of the starting
styrene halide terminal.
1
Figure 1. H NMR spectra (CDCl ; rt, room temperature) of a series
3
of maleimide-ended sequence-regulated oligomonomers (M−SSS, M−
ASS, M−SAS, M−AAS, M−SSA, M−ASA, M−SAA, M−ASA, M−
SAA, M−AAA).
D
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a
Table 1. 1:1 Alternating Free Radical Copolymerization of Maleimide-Ended Oligomonomer (M ) and Styrene (S)
1
b
b
f
f
conv (%)
incorp (%)
cloud point (°C)
cloud point (°C)
c
c
n
d
e
M1
time (h)
M /S
1
Mn
M /M
w
M /S
1
Tg (°C) Td5 (°C)
(THF/MCH)
(THF/MeOH)
M−SSS
M−ASS
M−SAS
M−AAS
M−SSA
M−ASA
M−SAA
M−AAA
5
7
7
7
7
8
4
5
92/91
94/94
93/94
96/98
97/97
95/93
95/98
89/90
102 000
181 000
125 000
176 000
150 000
172 000
258 000
183 000
3.13
3.76
3.75
3.29
3.72
3.20
2.84
2.93
49/51
50/50
49/51
50/50
49/51
49/51
49/51
50/50
138
131
112
100
130
121
102
92
380
370
370
360
377
372
365
357
21.4 (35/65)
20.6 (35/65)
soluble (50/50)
49.2 (35/65)
20.5 (50/50)
26.9 (50/50)
34.0 (50/50)
10.8 (50/50)
27.8 (45/55)
3.3 (50/50)
23.0 (45/55)
22.7 (45/55)
a
b
1
c
d
Polymerization condition: [M ] = [S] = 1.0 M, [AIBN] = 20 mM, toluene, 60 °C. Determined by H NMR. Determined by SEC. Obtained
by differential scanning calorimetry. Determined by thermogravimetric analysis. Determined by transmittanttance measurement: 10 mg/mL
solution; cooling rate = 1 °C/min; wavelength 500 nm.
1
0
0
0
e
f
Figure 2. MALDI-TOF-MS spectrum (linear mode) of poly(M−SAA-alt-S) obtained in the RAFT copolymerizations ([M ] /[S] /[AIBN] /
1
0
0
0
[
PEDB] = 1000/1000/5.0/20 mM, 20% for M−SAA and 21% for styrene, M = 3900, M /M = 1.33).
0
n
w
n
compounds (Figure S2). These results indicate that the
synthesis of perfectly sequence-regulated vinyl oligomers is
possible in 5−10 g scales using appropriate metal catalysts and
halides under optimized conditions in iterative ATRAs,
although this has been considered to be almost impossible
owing to the concurrently occurring ATRPs. However, there
might be a practical limitation for the synthesis of higher
oligomers based on this approach.
maleimide-ended oligomonomers and styrene were thus
measured by the peak intensity ratios in the H NMR spectra,
and all proved to be nearly 50/50 irrespective of differences in
the trimeric monomer units in the side chains.
1
In addition, the reversible addition−fragmentation chain
transfer (RAFT) copolymerization of the maleimide-ended
oligomonomers and styrene was investigated using 1-
pheynlethyldithiobenzoate (PEDB: PhEtSC(S)Ph) as a RAFT
agent in the presence of AIBN at 60 °C to investigate the
controllability of the molecular weight and chain-end groups of
the resulting copolymers and clarify the monomer sequences in
the main chain. The RAFT copolymerization proceeded at the
same consumption rate of the maleimide-ended oligomonomer
(M−SAA) and styrene (Figure S5) as in the free radical
copolymerization. The resulting copolymers exhibited narrow
MWDs (M /M ∼ 1.2) and controlled molecular weights that
1:1 Alternating Radical Copolymerization of Malei-
mide-Ended Oligomonomers and Styrene. AB Monomer
Sequencing in the Main Chain. The series of maleimide-ended
sequence-regulated oligomonomers was then radically copoly-
merized with styrene at the 1:1 feed ratio with AIBN as the
initiator in toluene at 60 °C. In all cases, both of the
oligomonomers and styrene were consumed at the same rate
almost quantitatively for 10 h to yield the copolymers with
w
n
considerably high molecular weights (M > 100 000) (Table 1,
increased in direct proportion to monomer conversion. The
slightly lower molecular weights than the calculated values can
be attributed to the lower estimation of the hydrodynamic
volume of the resulting copolymers with pendent trimeric
monomer units than that of standard polystyrene. The matrix-
assisted laser desorption−ionization time-of-flight mass spec-
troscopy (MALDI-TOF-MS) revealed that the resulting
copolymers possess the well-defined RAFT chain end-groups
n
Figure S3).
1
The obtained copolymers were then analyzed by H NMR.
Similar but slightly different spectra were obtained for the
polymers because of the presence of styrene and maleimide
and/or acrylate units as their components and slight differences
in the monomer compositions and sequences in the obtained
vinyl copolymers (Figure S4). The incorporated ratios of the
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Figure 3. Transmittance of a series of main- and side-chain sequence-regulated vinyl copolymers [poly(M -alt-S)] obtained in 1:1 alternating free
1
radical copolymerization of maleimide-ended oligomonomers (M ) and styrene at varying temperatures in (A) methylcyclohexane/THF (65/35)
1
(
M : M−SSA, M−SAS, M−ASS), (B) THF/EtOH (50/50) (M : M−SSA, M−SAS, M−ASS), (C) MCH/THF (50/50) (M : M−SAA, M−ASA,
1
1
1
M−AAS), and (D) THF/EtOH (45/55) (M : M−SAA, M−ASA, M−AAS): 10 mg/mL; cooling rate = 1 °C/min; wavelength = 500 nm.
1
and alternating sequences comprising nearly the same numbers
of each unit [(n, m) = (k, k − 1), (k, k), (k, k + 1)], where n
and m represent the unit number of M−SAA and styrene in the
main chain, respectively, with small exceptions [(n, m) = (k, k −
structural isomers of the copolymers. When a series of
copolymers containing two styrene and one acrylate units,
i.e., poly(M−ASS-alt-S), poly(M−SAS-alt-S), and poly(M−
SSA-alt-S), was dissolved in MCH/THF (65/35) with a lower
polarity, poly(M−SSA-alt-S) with acrylate in the outer unit was
less soluble than the others: it became turbid at approximately
50 °C (T = 49.2 °C, T : cloud point), whereas the others did
so at a lower temperature, approximately 20 °C (Figure 3A). In
contrast, in THF/EtOH (50/50) with a higher polarity, the
solubility of the copolymers increased as the acrylate unit came
out from the inner to the outer of the monomer sequences in
2
)] (Figure 2). The most major series are those of (n, m) = (k,
k − 1), and the second most major are (n, m) = (k, k),
indicating that the RAFT copolymerization was most
preferentially started from the addition of 1-phenylethyl
fragment radical, i.e., unimer styryl radical, to M−SAA, and
was predominantly terminated by the capping of dithioben-
zoate to the M−SAA terminal.
All of these results indicate that the radical copolymerization
of maleimide-ended oligomonomers and styrene proceeded in a
highly alternating fashion to result in the AB monomer
sequences in the main chain of the resulting vinyl copolymers,
which also have perfectly sequence-regulated trimeric vinyl
monomer units in the side chains.
c
c
the side chains (T = 34.0, 10.8, and 3.3 °C for M−ASS, M−
c
SAS, and M−SSA sequences, respectively) (Figure 3B). This
indicates that monomer sequences in the side chains indeed
affect the solubility and that the outer segments have more
important roles for determining the solubility of the vinyl
copolymers.
Effects of Side-Chain Monomer Sequences on
Polymer Properties. The effects of side-chain monomer
sequences on the polymer properties were first investigated in
terms of solubility of the polymers. The solubility was evaluated
in two solvent mixtures with different polarity, i.e., methyl-
cyclohexane (MCH)/THF as less polar solvents and THF/
EtOH as more polar solvents, at varying temperatures by
measuring the light transmittance of the polymer solutions at
Another series of the isomeric copolymers that contain one
styrene and two acrylate units, i.e., poly(M−AAS-alt-S),
poly(M−ASA-alt-S), and poly(M−SAA-alt-S), was generally
more soluble in polar solvents than the former series because it
contains a higher amount of acrylate units. Regarding the effects
of monomer sequences in this series on the solubility, similar
tendencies were observed. As the hydrophobic styrene unit
went in from the outside to the inside of the monomer
sequences, the solubility in a less polar solvent, MCH/THF
(50/50), decreased (Figure 3C). In contrast, in THF/EtOH
(45/55) with a higher polarity, poly(M−AAS-alt-S) with
styrene in the outer unit became turbid at higher temperature
(T = 27.8 °C) than the others (T = 23.0 and 22.7 °C for M−
500 nm (Figure 3).
To see the main effects of monomer sequences, the solubility
was compared for a series of the copolymers with the same
monomer composition but different monomer sequences.
These series of copolymers can be regarded as a kind of
c
c
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ASA and M−SAA, respectively) (Figure 3D). These results
again indicate that not only the monomer compositions but
also the monomer sequences are crucial for determining the
solubility of the vinyl copolymers and that the solubility can be
gradually changed by the monomer sequences in the side
chains.
The effects of monomer compositions and sequences on the
thermal properties of the sequence-regulated vinyl copolymers
were then evaluated by differential scanning calorimetry (DSC).
All copolymers are amorphous and exhibit glass transition
acrylate units, the T values for the copolymers possessing two
g
sequential styrene units, i.e., poly(M−SSA-alt-S) and poly(M−
ASS-alt-S), were almost the same at approximately 130 °C,
whereas that without sequential styrene units, i.e., poly(M−
SAS-alt-S), exhibited a lower T at 112 °C. Similarly, in the
g
second series of isomeric copolymers with one styrene and two
acrylate units, the copolymers possessing two consecutive
acrylate units, i.e., poly(M−SAA-alt-S) and poly(M−AAS-alt-
S), exhibited nearly the same T values of approximately 100
g
°
C, whereas the T of the copolymers without successive
g
temperatures (T ’s) between 90 and 140 °C, which were
g
acrylate units, i.e., poly(M−ASA-alt-S), was higher (121 °C)
than those of the two isomers. As we previous reported, for the
copolymers consisting of the sequence-regulated dimeric units,
dependent on both the monomer compositions and sequences
(
Figure 4). The highest T was observed for the copolymers of
g
no such effects of the monomer sequences on T values were
g
74
observed. These results indicate that the presence of two
successive styrene (SS) units in the side chains is crucial for
yielding relatively high T values of approximately 130 °C. A
g
similar conclusion concerning two sequential acrylate (AA)
units for relatively low T values of approximately 100 °C can
g
also be derived. In addition, it is interesting to note that
poly(M−ASA-alt-S) has a higher T than poly(M−SAS-alt-S)
g
(
121 °C vs 112 °C) irrespective of the lower styrene contents
in the side chains. Thus, the monomer sequences even in the
side chains can affect the thermal properties of vinyl
copolymers.
The degradation temperatures (T ; temperature at 5%
d5
weight loss) of these copolymers were also measured (Figure
S6). All copolymers possessed similar Td5 values between 360
and 380 °C and showed relatively high thermal stability mainly
originating from the main-chain alternating maleimide and
styrene sequences. There were very few specific dependences of
thermal decompositions on the monomer sequences.
Figure 4. DSC curves and T values of a series of main- and side-chain
sequence-regulated vinyl copolymers obtained in 1:1 alternating free
g
2
:1-Sequenced Radical Copolymerization of Malei-
radical copolymerization of maleimide-ended oligomonomers (M1)
mide-Ended Oligomonomers and Limonene. AAB Mono-
mer Sequencing in the Main Chain. To control the monomer
sequences in the main chain in a higher order, a series of the
maleimide-ended sequence-regulated oligomonomers were
copolymerized with limonene (L) at a feed ratio of 2:1
and styrene [poly(M -alt-S)] (A).
1
the highest styrene composition (T = 138 °C, poly(M−SSS-
g
alt-S)), whereas the lowest T was found for the highest acrylate
g
(
[oligomonomer] /[L] = 2/1) using V-70 as the initiator in
composition (Tg = 92 °C, poly(M−AAA-alt-S)). This
demonstrates that the difference in monomer compositions in
the side chains of the same main-chain alternating maleimide
0 0
22−25
PhC(CF
)
3
OH at 20 °C.
In all cases, oligomonomers and
2
limonene were consumed at the same rate irrespective of the
differences in the side-chain monomer compositions and
sequences of the oligomonomers (Figure S7) to yield products
of their molecular weights of approximately 10 000−20 000
based on the polystyrene standard calibration by SEC (Table
and styrene units can alter the T values in relatively wide
g
ranges (∼50 °C), in which the order is the same as that for the
homopolymers of styrene (T = 100 °C) and methyl acrylate
g
(
Tg = 10 °C).
1
The effects of side-chain monomer sequences on T values
2). H NMR analysis of the products shows that the products
g
were indeed observed for both series of isomeric copolymers. In
the first series of isomeric copolymers with two styrene and one
are vinyl copolymers between the maleimide-ended oligomo-
nomers and limonene and that the incorporated oligomo-
a
Table 2. 2:1 Alternating Free Radical Copolymerization of Maleimide-Ended Oligomonomer (M ) and Limonene (L)
1
b
c
c
b
d
e
M1
time (h)
conv (%) M /L
Mn
M /Mn
w
incorp (%) M /L
Tg (°C)
Td5 (°C)
1
1
M−SSS
M−ASS
M−SAS
M−AAS
M−SSA
M−ASA
M−SAA
M−AAA
360
480
240
480
480
480
216
240
58/57
55/52
76/74
61/61
61/61
56/55
78/78
63/62
8300
1.69
1.53
1.76
1.83
1.65
1.81
1.88
1.64
66/34
65/35
66/34
65/35
67/33
65/35
67/33
64/36
138
126
112
101
129
113
104
97
336
327
315
326
325
325
322
325
8000
19 000
20 900
11 500
19 000
22 700
24 900
a
b
1
c
Polymerization condition: [M ] = 800 mM, [L] = 400 mM, [V-70] = 8.0 mM, PhC(CF ) OH, 20 °C. Determined by H NMR. Determined
1
0
0
0
3 2
d
e
by SEC. Obtained by differential scanning calorimetry. Determined by thermogravimetric analysis.
G
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Journal of the American Chemical Society
Article
nomers ratios are invariably 64−67%, independent of the side-
chain compositions and sequences and are nearly the same as
the value 2/3, assuming that the ideal 2:1 copolymerization
proceeds (Figure S8).
To clarify the monomer sequences in the main chain of the
copolymers, the RAFT copolymerization of one of the
maleimide-ended oligomonomers (M−SAA) and limonene
was conducted at the 2:1 feed ratio using S-cumyl S′-butyl
trithiocarbonate (CBTC: CumSC(S)Bu) as the RAFT agent
and AIBN as the initiator in PhC(CF ) OH at 60 °C. Both
3
2
monomers were consumed at the same rate as in the free
radical copolymerization without RAFT agents (Figure S9).
The molecular weights of the copolymers increased with
monomer conversion but were lower than the calculated values.
One of the main reasons for the lower molecular weights is the
difference in the hydrodynamic volumes of standard poly-
styrene, as suggested by the fact that the absolute molecular
weight measured by multiangle light scattering (MALLS)
equipped with the SEC was close to the calculated value
(
M (MALLS) = 10 800, M (calcd) = 9400). The controlled
n
n
Figure 6. DSC curves and T values of a series of main- and side-chain
sequence-regulated vinyl copolymers obtained in 2:1 alternating
radical copolymerization of maleimide-ended oligomonomers (M1)
g
terminal structures with RAFT end-groups and the AAB
sequence-regulated main-chain structures were confirmed by
the MALDI-TOF-MS spectrum (Figure 5). The molar masses
and limonene [poly(M -co-Lim)].
1
Interestingly, these T values (138 and 97 °C) were nearly the
g
same as those of the above-mentioned main-chain AB-
sequenced alternating vinyl copolymers (138 and 92 °C)
irrespective of the differences in the main-chain components
and sequences, i.e., MML and MS.
With regard to the effects of side-chain sequences, the
copolymer with alternating SAS sequences in the side chain has
a lower T (112 °C) than that with SSA (129 °C) or ASS (126
g
°
C), in which both the latter copolymers with a homo-SS side-
chain sequence exhibited similarly higher Tg values of
approximately 125−130 °C. These values of the main-chain
AAB-sequenced copolymers with two styrene and one acrylate
units in the side chains are almost the same as those of the
main-chain AB-sequenced copolymers with the same monomer
sequences as mentioned above. For another series of isomeric
main-chain AAB-sequenced vinyl copolymers, the copolymer
with alternating ASA sequence in the side chain has a higher Tg
Figure 5. MALDI-TOF-MS spectrum (linear mode) of poly(M−SAA-
co-Lim) obtained in the RAFT copolymerizations ([M ] /[Lim] /
(
113 °C) than that with SAA (104 °C) and AAS (101 °C), in
1
0
0
which both copolymers with homo-AA side-chain sequence
exhibited similarly lower T ’s of approximately 100−105 °C.
[
AIBN] /[CBTC] = 800/400/5.0/12 mM, 8% for M−SAA, and 8%
0
0
g
for limonone, M = 1800, M /M = 1.41).
n
w
n
These values of the main-chain AAB-sequenced copolymers
with one styrene and two acrylate units in the side chains are
similar to those of the main-chain AB-sequenced copolymers
with the same monomer sequences, although that with a side-
chain ASA sequence was slightly lower for the main-chain AAB
than AB (113 °C vs 121 °C). Such small dependences on the
main-chain monomer sequences are most probably due to small
differences in the structure of styrene and limonene, both of
which possess pendent six-membered rings.
The degradation temperatures of the main-chain AAB-
sequenced copolymers were between 315 and 336 °C (Figure
S10) and were invariably lower than those of the main-chain
AB-sequenced copolymers (360−380 °C). The lower thermal
stability of the main-chain AAB-sequenced copolymer can be
attributed to several factors such as the presence of a remaining
CC double bond in the limonene unit, a main-chain
quaternary carbon originating from the vinylidene structure of
limonene, and a bulky maleimide-maleimide sequence in the
main chain.
of the main series of peaks were separated by the same intervals
(
883.05) as those for AAB (A, M−SAA; B, limonene)
monomer sequences and were very close to those calculated
for the copolymers consisting of (Cum-(AAB) -A-SC(S)-
n
2
2−25
Bu).
These results indicate that the 2:1 sequenced radical
copolymerization proceeded similarly, even for the maleimide-
ended oligomonomers and limonene in the fluorinated alcohol
to give the AAB monomer sequences in the main chain.
DSC was also conducted for the series of main-chain AAB-
sequenced vinyl copolymers of maleimide-ended sequence-
regulated olimonomers and limonene (Figure 6). The effects of
side-chain monomer components and sequences on T values
g
of the copolymers were quite similar to those observed for the
series of main-chain AB-sequenced vinyl copolymers of
oligomonomers and styrene. The Tg values were varied
between 138 °C for the copolymers of the highest styrene
composition and 97 °C for the highest acrylate composition.
H
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Journal of the American Chemical Society
Article
a
Table 3. Free Radical Homopolymerization of Maleimide-Ended Oligomonomer (M1)
b
c
c
d
e
Tg∞ (°C)
f
g
M1
time (h)
conv (%) M1
Mn
M /Mn
w
Mn
Tg (°C)
Td5 (°C)
M−SSS
M−ASS
M−SAS
M−AAS
M−SSA
M−ASA
M−SAA
M−AAA
180
24
36
20
120
25
30
7
92
95
96
95
94
95
92
95
3000
1.31
2.19
1.66
1.72
1.36
1.69
1.40
2.47
6300
122
131
103
77
354
354
342
342
348
372
340
335
8300
15 600
17 700
36 300
13 400
29 400
31 700
24 900
138
106
82
9500
16 900
7900
129
130
122
142
132
140
17 200
16 100
11 500
53
a
b
1
c
d
Polymerization condition: [M ] = 1.0 M, [AIBN] = 10 mM, toluene, 60 °C. Determined by H NMR. Determined by SEC. Determined by
1
0
0
e
f
g
MALLS. Determined by DSC. Determined by Flory−Fox equation. Determined by thermogravimetric analysis.
Radical Homopolymerization of Maleimide-Ended
Oligomonomers. Homosequencing in the Main Chain. In
general, maleimides can also be radically homopolymerized,
although the rate for homopropagation is smaller than that for
the alternating copolymerization with styrene. To see the
effects of the side-chain monomer sequences on the
homopolymers and differences among the main-chain A-
homo, AB-alternating, and AAB-monomer sequences, a series
of maleimide-ended oligomonomers were homopolymerized in
toluene at 60 °C using AIBN as the initiator.
Both of these values were lower than those of the main-chain
AB-alternating or AAB-monomer-sequenced polymers. This
might be ascribed to lower molecular weights of the obtained
homopolymers but are based on SEC.
To exclude the effects of molecular weights and compare
only the effects of side-chain monomer compositions and
sequences on T values, the resulting polymers were then
g
fractionated by preparative SEC and subjected to the DSC
measurement because there were relatively large differences in
molecular weights of the resulting homopolymers depending
In all cases, the homopolymerizations occurred quantitatively
on the side chains. The T
fraction and plotted against the reciprocal M
to estimate the T for an infinite molecular weight, i.e., T ,
g
g
values were measured for each
(Figure S11) although the rates were smaller than those of the
n
of each fraction
∞
alternating copolymerization with styrene, which were com-
pleted in 10 h under similar conditions (Figure S3). It is
interesting to note that the homopolymerization was fastest for
the maleimide-ended oligomonomer with the side-chain AAA
sequence and slowest for the SSS sequence. In addition, the
rate decreased as the side-chain styrene unit came into the
inner sequence or the number of the styrene unit increased.
Thus, the order of the rate is as follows: M−AAA > M−AAS >
M−ASA > M−SAA ∼ M−ASS > M−SAS > M−SSA > M−
SSS. This could be related to the bulkiness of the side-chain
groups near the maleimide groups because the styrene unit is
bulkier than methyl acrylate. In homopolymerization of the
maleimide-ended oligomonomers, the steric hindrance around
the propagating maleimide radical species is more crucial than
the copolymerization, in which the steric hindrance between
the propagating chain and the incoming monomer can be eased
by the relatively small comonomer such as styrene.
g
using the Flory−Fox equation (Figure S13). Among a series of
isomeric homopolymers of the maleimide-ended oligomono-
mers possessing two styrene and one acrylate units, those with
∞
SS sequences show higher T ’s (142 °C for poly(M−SSA)
g
and 138 °C for poly(M−ASS)) than those of the alternating
∞
SAS sequence (Tg = 106 °C) in a similar way to the same
series of isomeric main-chain AB-alternating or AAB-sequenced
copolymers. Regardless of homopolymers of maleimide
derivatives, which are expected to have rigid main chains
originating from the cyclic repeating units, the T values of the
g
homopolymers are similar to those of the main-chain AB- and
AAB-copolymers. With regard to another series of isomeric
homopolymers with one styrene and two acrylate units in the
∞
side chains, Tg with the alternating ASA sequence was higher
∞
∞
(
Tg = 140 °C) than those with AA sequences (Tg = 132 °C
for poly(M−SAA) and 82 °C for poly(M−AAS)) similar to
those of the same series of main-chain AB- and ABB-sequenced
As shown in Table 3, such differences in the polymerization
rate originating from the side-chain sequences affect the
molecular weights of the resulting homopolymers. The
∞
copolymers. However, the Tg of poly(M−SAA) with side-
chain AA sequences was closer to that of poly(M−ASA) than
poly(M−AAS), suggesting that the homopolymer of M−SAA
molecular weights were lowest (M (SEC) = 3000 based on
n
has a relatively high T , which is difficult to expect from other
polystyrene calibration, M (MALLS) = 6300) for poly(M−
g
n
results. The decomposition temperatures of the homopolymers
were varied from 335 to 372 °C and were roughly between
those of the main-chain AAB- and AB-sequenced copolymers.
SSS) obtained in the slowest polymerization and were still
lower (M (SEC) = 8000−10000, M (MALLS) = 13 000−
n
n
18 000) for the polymers containing two styrene units in the
side chains. The polymers that contain two or three acrylate
units in the side chains had invariably higher molecular weights
CONCLUSIONS
■
(
M (SEC) = 10 000−20 000, M (MALLS) = 25 000−36 000).
n
n
We succeeded in the synthesis of a complete set of maleimide-
ended sequence-regulated oligomonomers consisting of
trimeric vinyl monomer units of styrene and/or methyl acrylate
via iterative ATRAs by choosing appropriate metal catalysts
depending on the monomers and halide terminals in each step.
The obtained maleimide-ended oligomonomers were radically
copolymerized with styrene via alternating AB-fashion or with
limonene via 2:1 AAB-fashion successfully to result in the high-
molecular-weight vinyl copolymers with sequence-regulated
structures in both main and side chains. The main- and side-
1
H NMR analysis also showed that the obtained products were
homopolymers of the maleimide-ended oligomonomers via
polymerization of the maleimide moiety (Figure S12).
Finally, the thermal properties of the homopolymers with
different side-chain monomer compositions and sequences
were evaluated by DSC and TGA. The T values were highest
g
(
122 °C) again for the homopolymers consisting of SSS side-
chain sequences and lowest (53 °C) for AAA sequences
irrespective of the lower molecular weights for poly(M−SSS).
I
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Journal of the American Chemical Society
Article
chain monomer compositions and sequences affected not only
the solubility but also the thermal properties of the sequence-
regulated vinyl copolymers. The outer monomer unit in the
side chains is crucial for the solubility, whereas the consecutive
monomer unit, i.e., homo- or alternating sequences, in the side
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(
2
(
chains is important for determining T . This study first
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This synthetic strategy can be applied to various vinyl
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ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b11631.
Soc. 2010, 132, 10003−10005.
(23) Matsuda, M.; Satoh, K.; Kamigaito, M. J. Polym. Sci., Part A:
Polym. Chem. 2013, 51, 1774−1785.
Experimental procedures and supplementary data (PDF)
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5
(
(
473−5482.
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AUTHOR INFORMATION
■
26) Satoh, K.; Ozawa, S.; Mizutani, M.; Nagai, K.; Kamigaito, M.
Corresponding Authors
satoh@apchem.nagoya-u.ac.jp
kamigait@apchem.nagoya-u.ac.jp
Nat. Commun. 2010, 1, 6.
(27) Tong, X. M.; Guo, B. H.; Huang, Y. B. Chem. Commun. 2011,
*
*
47, 1455−1457.
(28) Lv, A.; Deng, X.-X.; Li, L.; Wang, Y.-Z.; Du, F.-S.; Li, Z.-C.
Notes
Polym. Chem. 2013, 4, 3659−3662.
29) Wang, C.-H.; Song, Z.-Y.; Deng, X.-X.; Zhang, L.-J.; Du, F.-S.;
Li, Z.-C. Macromol. Rapid Commun. 2014, 35, 474−478.
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33) Han, J.; Zheng, Y. Z.; Zhao, B.; Li, S.; Zhang, Y.; Gao, C. Sci.
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
(
7
(
2
(
2
(
■
This work was supported in part by a Grant-in-Aid for Scientific
Research (A) (No. 15H02181) for M.K. by the Japan Society
for the Promotion of Science and Program for Leading
Graduate Schools “Integrative Graduate Education and
Research Program in Green Natural Sciences”.
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