Control of thermal, mechanical, and optical properties of three-component maleimide copolymers by steric bulkiness and hydrogen bonding

Article abstract of DOI:10.1002/pola.29421

N-substituted maleimides polymerize in the presence of a radical initiator to give polymers with excellent thermal stabilities and transparency. In this study, we synthesized various maleimide copolymers with styrenes and acrylic monomers to control their thermal and mechanical properties by the introduction of bulky substituents and intermolecular hydrogen bonding. Three-component copolymers of N-(2-ethylhexyl)maleimide in combination with styrene, α-methylstyrene (MSt), or 1-methylenebenzocyclopentane (BC5) as the styrene derivatives, and n-butyl acrylate, 2-hydroxyethyl acrylate, 4-hydroxybutyl acrylate, or acrylic acid as the acrylic monomers were prepared by radical copolymerization. These copolymers were revealed to exhibit excellent heat resistance by thermogravimetric analysis. Glass transition temperatures increased by the introduction of bulky MSt and BC5 repeating units. The mechanical properties of the copolymer films were improved by the introduction of intermolecular hydrogen bonding. Optical properties, such as transmittance, refractive index, Abbe number, and birefringence, were determined for the copolymers.

Full text of DOI:10.1002/pola.29421

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Control of Thermal, Mechanical, and Optical Properties of Three-
Component Maleimide Copolymers By Steric Bulkiness and Hydrogen
Bonding
Soichiro Nagase, Koki Miyama, Akikazu Matsumoto
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai,
Osaka 599-8531, Japan
Correspondence to: A. Matsumoto (E-mail: matsumoto@chem.osakafu-u.ac.jp)
Received 1 May 2019; Revised 1 June 2019; accepted 5 June 2019
DOI: 10.1002/pola.29421
ABSTRACT: N-substituted maleimides polymerize in the presence
of a radical initiator to give polymers with excellent thermal stabilities
and transparency. In this study, we synthesized various maleimide
copolymers with styrenes and acrylic monomers to control their ther-
mal and mechanical properties by the introduction of bulky substitu-
ents and intermolecular hydrogen bonding. Three-component
copolymers of N-(2-ethylhexyl)maleimide in combination with
styrene, α-methylstyrene (MSt), or 1-methylenebenzocyclopentane
(BC5) as the styrene derivatives, and n-butyl acrylate, 2-hydroxyethyl
acrylate, 4-hydroxybutyl acrylate, or acrylic acid as the acrylic
monomers were prepared by radical copolymerization. These
copolymers were revealed to exhibit excellent heat resistance by
thermogravimetric analysis. Glass transition temperatures increased
by the introduction of bulky MSt and BC5 repeating units. The
mechanical properties of the copolymer films were improved by the
introduction of intermolecular hydrogen bonding. Optical properties,
such as transmittance, refractive index, Abbe number, and birefrin-
gence, were determined for the copolymers. © 2019 Wiley Periodi-
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019
KEYWORDS: birefringence; glass transition temperature; hydrogen
bond; maleimide; mechanical property; polymer film; radical
polymerization; steric hindrance; thermal property; ther-
mogravimetric analaysis (TGA); thin films; transparent polymer
copolymerization of the N-substituted maleimides with sty-
rene (St) derivatives, such as α-methylstyrene (MSt) and
1-methylenebenzocycloalkanes, provided thermally stable poly-
mers with high Tg, but they were brittle to be provided for mechan-
ical tests.30,37,38 The brittleness of the maleimide copolymers often
becomes an obstacle for actual processing and applications. It is
also noteworthy that the maleimide copolymers including an allyl
group in the side chain were conveniently fabricated during the
copolymerization of N-allylmaleimide with olefins.39 The allyl-
containing copolymers are useful as the precursor polymers for
the synthesis of heat-resistant network polymers by a thiol–ene
reaction in the presence of multifunctional crosslinkers,40 but net-
work structure formation is disadvantageous for the fabrication of
transparent polymer films. The formation of a pseudonetwork
structure using intermolecular hydrogen bonding may be effective
for the maintenance of the desired thermal resistance and mechan-
ical properties of transparent polymer films as well as the excellent
processability.41–43
INTRODUCTION Thermoplastics are widely used because of
the merits of easy processing with the application of heat and
pressure as well as advantages for reuse and recycle, while
thermosets readily exhibit excellent heat resistance and mechanical
strength due to their network structures. It is expected to simulta-
neously have both features for thermoplastics and thermosets for
forthcoming advanced polymer materials. Actually, the additions of
certain thermal and mechanical properties are required depending
on the case for the development of thermoplastic transparent poly-
mer materials; for example, high glass transition (Tg) and decompo-
sition temperatures, high mechanical strength and toughness,
and optical properties such as high transparency and unique
birefringence.1–23 N-substituted maleimides copolymerize with var-
ious kinds of vinyl monomers in the presence of a radical initiator to
give copolymers with excellent thermal stabilities.24–26 The copoly-
merization of the maleimides with isobutene provided an alternat-
ing copolymer with an excellent thermal stability, transparency, and
mechanical properties.27–29 AAB-type sequence-controlled poly-
mers were also produced during the copolymerization of the N-
substituted maleimides with diisobutene,30–32 limonene,32,33
β-pinene,34 and other sterically hindered olefins35,36 under pen-
ultimate unit control. The AAB-type copolymers exhibited a high
Tg value due to a high content of the maleimide unit. The
In this study, we designed the maleimide copolymers by the
introduction of bulky and cyclic structures into the repeating
units and intermolecular hydrogen bonding in the polymer
side chains in order to control the thermal and mechanical
Additional supporting information may be found in the online version of this article.
© 2019 Wiley Periodicals, Inc.
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properties of the copolymers. We prepared various copolymers
using N-(2-ethylhexyl)maleimide (EHMI), a styrene monomer
(RS), such as St, MSt, and 1-methylenebenzocyclopentane (BC5),
and an acrylic monomer (RA), such as n-butyl acrylate (BA),
2-hydroxyethyl acrylate (HEA), 4-hydroxybutyl acrylate (HBA),
and tert-butyl acrylate (tBA), as shown in Figure 1. The tBA
repeating unit in the copolymers was transformed to an acrylic
acid (AA) unit by heating in the presence of an acid. The thermal
and mechanical properties as well as the optical properties of
the cast films of the copolymers were investigated.
(5 wt %) and dried under atmospheric conditions for 24 h, then
further dried in vacuo for 1 h with an oven at a desired temperature.
Elasticity (Young modulus) was determined from the initial slope of
the stress–strain curves (within 5% strain). The number of test
pieces (50.0 mm width × 9.0–12.0 mm length × 120–340 μm
thickness) was 2–10 for each measurement. The film thickness was
measured using a thickness gauge (Peacock, 0.01 mm dial type,
Ozaki MFG. Co., Ltd., Tokyo, Japan).
Optical Properties
The refractive indices (nF, nE, nD, and nC) were measured at the
wavelengths of 486, 546, 589, and 656 nm, respectively, using an
Abbe-type refractometer (Atago DRM2, 1-bromonaphthalene, a
halogen lamp). The Abbe number (νD) was calculated as follows.
EXPERIMENTAL
General Procedures
The nuclear magnetic resonance (NMR) spectra were recorded in
deuterated chloroform (CDCl3) using JEOL ECS-400 and ECX-400
spectrometers. The infrared (IR) and ultraviolet–visible (UV–vis)
spectra were recorded using JASCO FTIR-4600 and Shimadzu UV-
2600 spectrometers, respectively. Size exclusion chromatography
was carried out using Chromato Science CS-300C, JASCO PU-
2080PLUS, JASCO DG-2080-53, JASCO RI-2031-PLUS, TOSOH
TSK-gel columns, GMHHR-N and GMHHR-H, and tetrahydrofuran
(THF) as the eluent. Number- and weight-average molecular
weights (Mn and Mw, respectively) and polydispersity (Mw/Mn)
values were determined by calibration with standard polysty-
renes. The thermogravimetric (TG) analysis was carried out in a
nitrogen stream at the heating rate of 10 ꢀC min−1 using a Shimadzu
DTG-60. The onset temperature of decomposition (Td5) was deter-
mined as the 5% weight-loss temperature in the TG curves. The
maximum temperature of the decomposition (Tmax) was deter-
mined as the peak temperature of the differential TG curves. Differ-
ential scanning calorimetry (DSC) was carried out using a Shimadzu
DSC-60 at the heating rate of 10 ꢀC min−1. The Tg values were deter-
mined from the curves observed during a second heating process.
The tensile test was carried out using a Shimadzu Autograph AGS-X
(10 kN cell load) at the tensile rate of 0.5 mm min−1. Polymer films
were prepared by casting the chloroform solution of a polymer
νD = ðnD – 1Þ=ðnF – nCÞ
ð1Þ
The calculated refractive index values were fitted with the
experimental results using the Cauchy equation (eq 2) as the
two-parameter (n∞ and A) analysis method.44
nλ = n∞ + A=λ2
ð2Þ
where λ is the wavelength, n∞ is the refractive index at the
infinite wavelength, nλ is the refractive index at the wave-
length of λ, and A is the constant.
The birefringence property of the selected copolymer films was
evaluated according to the method reported in the literature9,45
at Material Solutions Research Institute, Kaneka Co. (Settsu,
Japan). The cast films were stretched at a rate of 2.5 mm s−1 and
a determined temperature to fabricate test pieces used for orien-
tational birefringence evaluation. Photoelastic birefringence was
ꢀ
determined at 23 C and 586.4 nm using a cast film of a copoly-
mer consisting of EHMI, St, and HEA (3a), according to eq 3.
Δn = Cσ
ð3Þ
where C and σ are the photoelastic constant and the stress,
respectively.
Materials
Maleic anhydride (MAn), 2-ethylhexylamine, acetic acid, sodium
acetate, 1-indanone, methyltriphenylphosphonium bromide, and
potassium tert-butoxide were purchased from Tokyo Chemical
Industry, Co., Ltd. (Tokyo, Japan) and used as received. 2,20-
Azobisisobutyronitrile (AIBN) was purchased from Fujifilm
Wako Pure Chemical Co. (Tokyo, Japan) and used after recrystal-
lization from methanol. St and vinyl acetate (VAc) were pur-
chased from Fujifilm Wako Pure Chemical Co., and MS, BA, HEA,
HBA, and tBA were from Tokyo Chemical Industry, Co., Ltd. They
were used after distillation. The solvents were purchased from
Nacalai Tesque (Kyoto, Japan) and used after distillation. EHMI
and BC5 were synthesized according to the methods reported in
the literature.30,46
FIGURE 1 Monomers used for the synthesis of copolymers.
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N-(2-Ethylhexyl)maleimide
(11.2 g, 100 mmol) was added, then the yellow mixture was
stirred at 0 ꢀC for 30 min. 1-Indanone (13.0 g, 100 mmol) was
added dropwise, then the mixture was stirred for 16 h after being
warmed to room temperature. A total volume of 150 mL of water
was added to quench the excess reagent, and the product was
extracted with diethyl ether (50 mL, six times). After dried on anhy-
drous sodium sulfate, the silica gel column chromatography with n-
hexane/ethyl acetate (9/1 in volume) as the eluent provided BC5. It
should be careful to the undesirable isomerization of BC5 to
3-methylindene during the preparation and purification processes
of BC5. The isomerization of BC5 and other exomethylene com-
pounds, such as itaconic derivatives, in the presence of a basic com-
pound was reported in the literature.47,48 In this study, acetone-d6
was used for monitoring the reaction. The chemical structure of
BC5 was checked again by 1H NMR spectroscopy using CDCl3 after
purification and isolation.
To 14.8 g (0.15 mol) of MAn dissolved in 150 mL of diethyl
ether, 19.4 g (0.15 mol) of 2-ethylhexylamine in 50 mL of diethyl
ether was slowly added at 0 ꢀC, then the mixture was further
stirred at room temperature for 1.5 h. After the reaction mixture
was kept at −40 ꢀC for 1 h, the precipitated N-(2-ethylhexyl)mal-
eamic acid was isolated by filtration. The yield was 31.1 g. A mix-
ture of the maleamic acid (31.1 g, 0.14 mol), acetic anhydride
ꢀ
(140 mL), and sodium acetate (7.03 g) was stirred at 100 C for
1 h. After the mixture was cooled to 0 ꢀC, 180 mL of distilled
water was added, then further stirred at room temperature for
2 h. The product was extracted with 120 mL of diethyl ether,
washed with saturated aqueous NaHCO3 and saturated aqueous
NaCl, and then dried over anhydrous sodium sulfate. Silica gel
column chromatography with n-hexane and ethyl acetate (8/2 in
volume) provided 8.70 g of EHMI.
Colorless oil, yield 17%,1H NMR(400 MHz, CDCl3): δ = 7.48–7.51
(m, C6H4, 1H), 7.18–7.28 (m, C6H4, 3H), 5.45 (t, J = 2.5 Hz, CH2,
1H), 5.03 (t, J = 2.5 Hz, CH2, 1H), 2.97–3.00 (m, CH2C6H4, 2H),
and 2.78–2.82 (m, CCH2, 2H).
N-(2-Ethylhexyl)maleamic Acid
Colorless solid, yield 90%, 1H NMR (400 MHz, CDCl3):
δ = 6.29 (d, J = 12.9 Hz, CHCOOH, 1H), 6.20 (d, J = 12.9 Hz,
CHCONH, 1H), 3.24–3.28 (m, NHCH2, 2H), 1.45–1.49 (m, CH,
1H), 1.22–1.32 (m, CH2, 8H), and 0.81–0.86 (m, CH3, 6H).
Polymerization
The mixture of EHMI, RS, RA, or other monomers, AIBN, and chlo-
roform or methyl ethyl ketone (MEK) were cooled with liquid
nitrogen, degassed for 10 min at the same temperature, then the
mixture was kept in a water bath for 20 min. These freeze–thaw
cycles were repeated three times. The mixture was heated at
60 ꢀC, and then the reaction mixture was poured into a large
amount of methanol. The precipitated copolymer was filtered out,
washed, and then dried in vacuo at room temperature. The yield
of copolymers was gravimetrically determined. The copolymers
N-(2-Ethylhexyl)maleimide
Colorless oil, yield 30%, H NMR (400 MHz, CDCl3): δ = 6.68 (s,
CH=, 2H), 3.41 (d, J = 7.7 Hz, NCH2, 2H), 1.70–1.73 (m, CH, 1H),
1.22–1.28 (m, CH2, 8H), and 0.8–1.0 (t, J = 7.1 Hz, CH3, 6H).
1
1-Methylenebenzocyclopentane
A 1000 mL four-necked flask was charged with methyltriphenyl-
phosphonium bromide (35.7 g, 100 mmol) suspended in dry
THF (200 mL) under nitrogen. Potassium tert-butoxide
TABLE 1 Radical Copolymerization of RS, EHMI, and RA at 60 ꢀC
RS/EHMI/RA
AIBNa
RS/EHMI/RA in
Copolymer RS
St
RA
in Feed (mmol) (mol %) CHCl3 (g) Time (h) Yield (%) Copolymer (mol %)
M
w × 10−4 Mw/Mn
b
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4b0
4b00
4c
5a
5b
5c
None 5.0/5.0/0
2.5
5
5
69
65
47
72
54
35
51
46
19
45
48
41
19
35
57
46
64
50/50/0
7.2
2.0
2.1
1.7
2.5
1.9
2.4
1.8
1.9
1.8
1.7
1.9
1.8
1.6
2.0
2.0
1.8
1.8
MSt None 10.0/10.0/0
BC5 None 10.0/10.0/0
1.0
0
50/50/0
14.4
21.7
8.3
1.0
0
c
5
52/48/0
St
BA
8.0/8.0/8.0
10.0/10.0/10.0
4.0/4.0/5.3
7.5/7.6/7.6
10.0/10.0/10.0
8.0/8.0/8.0
7.5/7.0/9.0
8.0/8.0/8.0
8.0/8.0/16.0
8.0/4.0/16.0
8.5/9.1/10.0
8.0/8.0/8.0
8.0/8.0/8.0
7.3/7.3/7.3
1.7
10
10
10
2
48/36/16
48/37/15
45/40/15
44/38/18
51/41/8
MSt BA
BC5 BA
0.67
1.0
0
10.0
11.7
32.3
16.2
10.2
26.3
14.9
16.9
9.7
0
St
HEA
1.0
7.6
8.3
0
MSt HEA
BC5 HEA
0.67
0.67
1.9
10
10
2
49/40/11
43/39/18
46/38/16
46/36/18
46/35/19
43/46/11
45/41/14
44/48/8
St
HBA
10.7
1.6
2.1
1.8
0
MSt HBA
MSt HBA
MSt HBA
BC5 HBA
0.75
0.75
1.0
4
4
4
0.72
0.83
0.75
1.0
7.5
12.7
28.0
12.4
15.7
St
tBA
8.3
1.6
0
2
8
MSt tBA
BC5 tBA
10
46/48/6
aBased on the total amount of monomers.
b[EHMI] = [St] = 0.40 mol L−1
c[EHMI] = [St] = [BA] = 0.40 mol L−1
.
.
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FIGURE 2 Repeating structure of copolymers synthesized in this study.
were purified by repeated precipitation procedures using chloro-
form and methanol, then the copolymer composition was deter-
mined by 1H NMR spectroscopy. For the determination of the tBA
content, the weight loss observed during TG measurement was
also available (see typical TG curves in Fig. S4 of Supporting
Information).
results of copolymerization are summarized in Table 1.
Figure 2 shows the repeating unit structures of the copoly-
mers obtained in this study.
The alternating copolymers of EHMI and the RSs (1a–1c) with
a high molecular weight (Mw = 7.2–21.7 × 104) were readily
produced in a high yield (47–69%), being in good agreement
with the results reported in the literature.24–26,37 The further
addition of BA to the copolymerization systems of EHMI and
the RSs provided the corresponding three-component copoly-
mers (2a–2c). The contents of BA in the copolymers were
always lower than those in the feed. This was because the
cross-propagation between EHMI and the RSs was preferred
rather than the propagation of BA. It was reported that acry-
late and maleimide monomers exhibited similar monomer
RESULTS AND DISCUSSION
Synthesis of Copolymers
The maleimide copolymers consisting of EHMI and the RSs
(St, MSt, and BC5) were prepared in the absence of the RAs
(1a–1c) and in the presence the RAs (2a–2c for BA, 3a–3c for
HEA, 4a–4c for HBA, and 5a–5c for tBA) by radical copoly-
merization with AIBN in chloroform or in bulk at 60 ꢀC. The
SCHEME 1 Transformation of the tBA-containing copolymers (5a–5c) to the AA-containing copolymers (6a–6c).
4
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TABLE 2 Radical Copolymerization of RS (M1) and Various Monomers (M2) at 60 ꢀC
RS/M2 in
Feed (mmol)
AIBNa
(mol %)
RS/M2 in
Copolymer (mol %)
M1 (RS)
M2
Solvent (mL)
Time (h)
Yield (%)
M
w × 10−4
Mw/Mn
BC5
BC5
BC5
BC5
BC5
MSt
MSt
St
BA
5.0/5.0
5.0/5.1
1.0
1.0
1.0
1.0
0.56
1.0
1.1
1.0
1.0
None
10
10
10
5
3
63/37
1.0
1.5
St
None
1
76/24
b
0.5
b
1.8
b
VAc
EHMI
MAn
BA
5.0/5.0
None
Trace
47
30
35
50
82
83
10.0/10.0
1.8/1.8
None
MEKc (0.5)
52/48
47/53
44/56
50/50
22
1.7
2.2
1.7
1.7
2.0
4.4
10
17
4
3.4
3.6
10.6
25
21.0/9.0
14.0/14.0
21.0/9.0
14.8/14.8
None
MAn
BA
MEKc (4.0)
None
11
10
32/68
b
St
MAn
MEKc (15.0)
24.4
aBased on the total amount of monomers.
bNot determined.
cMEK, methyl ethyl ketone.
reactivity ratios during the mutual copolymerization systems;
for example, monomer reactivity ratios were r1 = 0.71 and
r2 = 0.77 for the copolymerization of methyl acrylate (M1) and
N-cyclohexylmaleimide (M2).25 It is speculated that the copoly-
mers produced in the present three-component copolymerization
mainly consist of an alternating structure of EHMI and RS units,
of which the EHMI repeating unit was partly substituted by BA.
Similarly, HEA, HBA, and tBA were incorporated into the copoly-
mers of EHMI and the RSs. Here, the tBA was used for the prep-
aration of the precursor copolymers for those containing an AA
repeating unit. The AA-containing copolymers (6a–6c) were pre-
pared by heating of the corresponding tBA-containing copoly-
mers (5a–5c) in the presence of p-toluenesulfonic acid in
toluene at 110 ꢀC for 5 h (Scheme 1).13
TABLE 3 Thermal and Mechanical Properties of the Copolymers Synthesized in this Study
Thermal Properties Mechanical Properties
Number of
Samples
Young Modulus
(MPa)
Maximum
Strength (MPa)
Copolymer
T
g (ꢀC)
T
d5 (ꢀC)
T
max (ꢀC)
Strain at Break (%)
a
a
a
a
a
a
a
a
a
a
a
a
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4b’
4b”
4c
5a
5b
5c
6a
6b
6c
7b
8b
90
116
109
53
351
317
330
298
314
320
328
294
311
358
322
342
339
319
258b
303b
265b
366
339
297
308
296
431
378
377
424
3
a
224 ꢁ 38
1.86 ꢁ 1.13
1.49 ꢁ 0.72
a
a
a
77
380
a
a
a
a
92
384
78
434
10
5
707 ꢁ 65
674 ꢁ 73
531 ꢁ 109
620 ꢁ 92
662 ꢁ 47
–
13.7 ꢁ 1.54
3.97 ꢁ 2.42
0.95 ꢁ 0.32
10.02 ꢁ 1.71
3.68 ꢁ 0.72
–
3.38 ꢁ 0.95
0.75 ꢁ 0.48
0.24 ꢁ 0.07
2.61 ꢁ 0.70
0.66 ꢁ 0.15
–
103
104
73
387
388
5
426
3
93
388
5
91
392
–
87
388
–
–
–
–
97
389
10
–
766 ꢁ 92
–
3.47 ꢁ 1.30
–
0.53 ꢁ 0.20
–
86
257b, 435
269b, 388
245b, 375
427
106
97
–
–
–
–
–
–
–
–
102
126
120
233
70
3
641 ꢁ 30
694 ꢁ 18
793 ꢁ 23
7.72 ꢁ 2.13
2.71 ꢁ 0.15
2.58 ꢁ 0.39
1.93 ꢁ 0.70
0.47 ꢁ 0.05
0.37 ꢁ 0.05
390
2
381
2
342
4
a
1385 ꢁ 171
2.18 ꢁ 0.13
0.21 ꢁ 0.04
a
a
a
341
aToo brittle to be measured.
bFor the elimination of isobutene from the tert-butyl ester in the side
chain.
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FIGURE 4 Effect of the structure of the RSs and the RAs on the Tg
values of the copolymers with EHMI. See Table 1 for the copolymer
compositions. [Color figure can be viewed at wileyonlinelibrary.com]
presence of EHMI, the copolymers with a low molecular
weight (Mw < 104) were produced in a low yield (less than 3%)
during copolymerization of BC5 with BA, St, and Vac in the
absence of EHMI, as shown in Table 2. The two-component copo-
lymerization of BC5 combined with EHMI provided a high molec-
ular weight and alternating copolymer in a high yield, as was
expected.30 The copolymerization of MSt or St with BA produced
random copolymers. The BA content in the copolymers increased
in the following order: St (68 mol % for BA unit) > MSt (56 mol
% for BA unit) > BC5 (37 mol % for BA unit). These random
copolymers with BA were of high molecular weight, but it was
difficult to provide tough films.
FIGURE 3 TG curves of (a) 4a (dash), 4b (dot), and 4c (solid) and
(b) 5c (solid) and 6c (dash) in nitrogen stream at the heating rate
of 10 ꢀC min−1
.
The RS contents were almost constant in a range of 43–52 mol
% for the all copolymerization systems. The RA contents varied
in 6–19 mol % gently depending on the RSs, that is, the RA con-
tent slightly decreased according to the substituent in the RS unit
as follows; St > MSt > BC5. The apparent copolymerization rate
and the molecular weight of the copolymers showed similar ten-
dency, but more detailed discussion for the relative reactivity in
this system was difficult because we have no data under unified
copolymerization conditions. Recently, the acceleration of the
propagation rate of vinyl monomers containing a hydroxy group
was reported for several polymerization systems, in which inter-
molecular and intramolecular hydrogen bonding is important to
extraordinary enhancement of monomer reactivity.49–51 Being
different from the results reported in the literature, no outstand-
ing incorporation of HEA and BHA was confirmed in this study
because of fast cross-propagations between EHMI and RS. When
the copolymerization was carried out using various comonomer
ratios in the feed, that is, [RS]/[EHMI]/[HBA] = 1/1/1, 1/1/2,
and 1/0.5/2 in molar ratio for the preparation of the copolymers
4b, 4b0, and 4b00, respectively, the copolymer composition slightly
varied (see Table 1). The control of the RA content was difficult
by a change in the comonomer feed under the copolymerization
conditions used in the present study. Consequently, we used the
three-component copolymers containing the RA units in a limited
range of 6–19 mol %, as shown in Table 1.
Thermal Property
Table 3 summarizes the thermal properties of the copolymers
synthesized in this study. The Tg values were determined by DSC
and the Td5 and Tmax values were determined by TG analysis at
the heating rate of 10 ꢀC min−1 in a nitrogen stream.
Typical TG curves are shown in Figure 3. The copolymers con-
taining a tert-butyl ester (5a–5c) showed the Td5 and Tmax values
lower than those for the other copolymers because the elimina-
tion of isobutene from the ester alkyl group quantitatively
occurred upon heating.52–54 In fact, the weight loss started
ꢀ
ꢀ
at approximately 250 C and the Tmax values were 245–269 C.
The copolymers isolated after heating were confirmed to have
the repeating unit structure, of which the tBA repeating units
were transformed to the AA repeating units (6a–6c) by NMR
and IR spectroscopies (Scheme 1). For the copolymers other than
5a–5c, the Td5 values were 294–358 ꢀC and the Tmax values
were 377–434 ꢀC, being much higher than decomposition tem-
peratures for conventional vinyl polymers. The copolymers with
MSt and BC5 exhibited lower Td5 and Tmax values compared with
those for the copolymers with St. This was due to the occurrence
of depolymerization during the thermal decomposition of copoly-
mers containing 1,1-disubstituted ethylene monomer units.
In contrast to the production of the high molecular weight
copolymers from BC5 during the copolymerization in the
In the DSC traces for the copolymers, the glass transition was
clearly observed, as shown in Figure S6. No peak due to a
6
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FIGURE 5 Stress–strain curves for the tensile test of various three-component maleimide copolymer films. (a) 2a (solid), 3a (dash), 4a
(dot), (b) 3a (solid), 3b (dash), 3c (dot), (c) 4a (solid), 4b (dash), 4c (dot), and (d) 3c (solid), 4c (dash), and 6c (dot).
crystalline polymer indicates that the copolymers synthesized
substituent gave only brittle films. The film of 2a was provided
for the tensile test, but a maximum strength, a strain at break,
and Young modulus were much low as 1.86 MPa, 1.5%, and
224 MPa, respectively. Being different from the result of 2a,
the copolymers of St and EHMI with various RAs including
polar substituents exhibited maximum strength values in a range
of 7.7–13.7 MPa and strain at break of 1.9–3.4% [Fig. 5(a)]. The
Young moduli also increased to 531–766 MPa by the combination
with HEA, HBA, and AA. Unfortunately, the introduction of bulky
MSt and BC5 units decreased in the toughness of the films, that is,
both the maximum strength and strain at break decreased as
shown in Figure 5(b,c). The main chain mobility might be reduced
by the introduction of 1,1-disubstituted ethylene units, such as MSt
and BC5, leading to an increase in the Tg of the copolymers and
simultaneously a decrease in the mechanical strength. The tough-
ness of the copolymers depended on the ester alkyl groups of the
RA unit. The maximum strength and strain values decreased in the
following order: 4c > 6c > 3c, as shown in Figure 5(d).
in this study are amorphous. The Tg values for the two-component
ꢀ
copolymers were 90–116 C (1a–1c). The copolymers with MSt
and BC5 including the bulky substituents exhibited the higher Tg
values. While the introduction of BA into the copolymers decreased
the Tg values (Tg = 53–92 ꢀC for 2a–2c), the presence of a hydroxy
group kept the Tg values in the range of 73–97 ꢀC for 4a–4c and the
shorter alkyl spacer further increased (Tg = 78–104 ꢀC for 3a–3c),
as shown in Figure 4. The copolymers containing AA (6a–6c)
showed the highest Tg values. This indicates the significant contribu-
tion of intermolecular interaction via strong hydrogen bonding
between the carboxylic acids.28,43 For the all cases, it was revealed
that the introduction of the bulky MSt and BC5 units and the hydro-
gen bonding were valid for an increase in Tg of the copolymers.
Mechanical Property
Table 3 also summarized the results of the mechanical properties
evaluated by the tensile test of the copolymer films. The films with a
thickness of 120–340 μm were prepared by a casting method using
chloroform as the solvent. Transparent and tough films were
obtained from the copolymers including the RA units with hydroxy
and carboxylic acid groups. In contrast, two-component copoly-
mers and the three-component copolymers including no polar
As already described, the two-component copolymers of EHMI
with the RSs including no hydroxy and carboxylic acid groups
are too brittle to be used as the polymer films for the mea-
surement of mechanical properties. One of the reasons for the
poor mechanical property is the presence of ring structure in
SCHEME 2 Synthesis of the alternating copolymer of MAn and MSt (7b) and transformation to the diester copolymer (8b) by the
reaction of 7b with n-butanol in MEK by reflux for 3.5 h in the presence of an acid catalyst. R00 = (CH2)3CH3.
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can be derived from the copolymers of MAn with the corres-
ponding comonomers by the polymer reactions consisting of
ring opening with an amine and the subsequent maleimide-ring
formation by dehydration upon heating.56 According to a simi-
lar procedure, the diester copolymers would be readily pro-
duced.57,58 In the present study, we synthesized the alternating
copolymer (8b) of MSt and di-n-butyl maleate by the reaction
of the alternating copolymer of MSt and MAn (7b) with n-
butanol in MEK by reflux (Scheme 2).
As shown in Table 2, the copolymerization of MAn with St and MSt
produced high molecular weight polymers (Mw = 10.6–24.4 × 104)
in high yield (50–83%). Being different from them, the copolymer
with BC5 with a lower Mw was obtained in a lower yield. The ring-
opening reaction of the alternating copolymer of MAn with MSt
(7b) was monitored by IR spectroscopy. From comparison of the
IR spectra of 7b before and after the reaction [Fig. 6(a)], an inten-
sity of peaks characteristic to an anhydride group (the symmetric
and asymmetric stretching vibrations of a C O bond at 1858 and
1780 cm−1, respectively) decreased and the peaks due to the ester
group newly appeared at 1735 cm−1 (C O stretching vibration)
and 1170 cm−1 (C( O)─O stretching vibration). In 1H NMR spec-
trum of the product [Fig. 6(b)], peaks assigned as the n-butyl ester
groups were detected. It was concluded based on the intensity
ratio of the characteristic peaks that the transformation quantita-
tively proceeded. When the mechanical properties of 7b were eval-
uated, it was revealed to a high Young modulus (1385 MPa) and
low strength and strain values at break (2.18 MPa and 0.21%,
FIGURE 6 (a) IR and (b) 1H NMR spectra of 7b and 8b (after
transformation of 7b to the diester copolymer). Asterisk indicates
peaks due to water and solvents. [Color figure can be viewed at
wileyonlinelibrary.com]
the polymer main chain. It has been reported that the homopol-
ymers of N-substituted maleimides have outstanding heat resis-
tance, but too brittle and fragile to be used for various
applications.55 In addition, the homopolymerization reactivity
of the N-substituted maleimides is not so high due to their ste-
rically hindered 1,2-disubstituted ethylene monomer structure.
In the most cases, therefore, they were used as the copolymers
with conventional vinyl monomers. The maleimide copolymers
TABLE 4 Optical Properties of the Maleimide Copolymer Films
Transmittance at
380 nm (%)
a
Copolymer
nD (589 nm)
νD
1a
2a
3a
4a
6a
1cb
2c
3c
4c
6c
96.7
97.9
97.3
96.7
94.5
94.8
96.3
97.0
93.9
91.9
1.533
1.533
1.523
1.524
1.530
1.543
1.548
1.550
1.538
1.543
41.8
38.6
41.5
42.2
43.1
42.9
45.3
40.7
42.3
37.0
FIGURE 7 (a) Transmittance of the copolymer films in UV–vis
region of 4c (solid) and 1c (dash). (b) Wavelength dependence
of refractive indices for 4a, 4c, 6a, and 6c.
aAbbe number: νD = (nD–1)/(nF–nC).
bCited from Ref. 30.
8
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TABLE 5 Birefringence Properties of 3a and Other Polymers
Intrinsic Birefringence
Photoelastic Coefficient
Polymer
n0 (×10−3
)
C (×10−12, Pa−1
)
Ref.
3a
–
0.7
This study
59,60
60,61
10
Polystyrene
−110
−91
8.8
Poly(MSt)
−3.7
11.9
Poly(N-methylmaleimide)
Poly(methyl methacrylate)
Poly(benzyl methacrylate)
Poly(cyclohexyl methacrylate)
53.8
−5.6, −6.1
19.5
−5.5, −4.3
48.4
4,45
4
−2.3
6.21
45
respectively). The Tg of 7b was much high as 233 ꢀC and the
decomposition temperature were also high (Td5 = 308 ꢀC and
In Table 5, the results for the evaluation of the birefrin-
gence properties of the copolymers are shown. The orienta-
tional birefringence was evaluated for the copolymer films
stretched at different drawing ratios (Fig. 8). In this study,
intrinsic birefringence values were not determined because
the degree of orientation was unknown. As a result, 3a and
6a had small and minus birefringence values. It has been
reported that polystyrene and poly(MSt) show a large
intrinsic birefringence with a minus sign59–61 while poly(N-
methylmaleimide) shows a plus intrinsic birefringence.10
Poly(alkyl methacrylate)s including poly(methyl methacry-
late) have a small intrinsic birefringence.4,45 The three-
component copolymers produced in this study consist of
the styrene, maleimide, and acrylate units as the repeating
structures. The EHMI and the RSs units are expected to
exhibit birefringence properties similar to those for poly(N-
methylmaleimide) and poly(alkyl methacrylate)s, respec-
tively. Based on the copolymer compositions of 3a and 5a,
a small and minus birefringence was considered to be of a
result for the offset of polarization along the main chain by
the maleimide ring included in the EHMI repeating unit and
polarization orthogonal to the main chain by the benzene
ring included in the St repeating unit.
T
max = 342 ꢀC). Thus, it was confirmed that the copolymer con-
taining an anhydride ring structure in the main chain was ther-
mally stable but too hard and brittle for the use as tough film
materials. The esterification of the copolymer resulted in a
drastic decrease in the Tg value while the decomposition tem-
peratures were the same. The produced diester copolymer 8b
was too brittle to be evaluated by a tensile test, as shown in
Table 3.
Optical Property
In Table 4, transmittance at 380 nm, reflective index
(D line, 589 nm), and Abbe number [νD = (nD–1)/(nF–nC)]
for each copolymer are shown. All the copolymer films
showed excellent transmittance [Fig. 7(a)] except for 6c
with transmittance of 91.9%. The copolymers containing
the carboxylic acid group, that is, 6a and 6c, exhibited
transmittance values lower than those for the copolymers
with St (1a–4a) and BC5 (1c–4c), respectively. The reflec-
tive indices were 1.524–1.533 and 1.543–1.550 for the
copolymers with St and BC5, respectively. The latter
exhibited higher νD values because of a decrease in the RA
unit in the copolymers, resulting in an increase in the rela-
tive contents of EHMI [Fig. 7(b)]. The observed Abbe num-
bers (37.0–45.3) were closed to the values reported for the
other maleimide copolymers in the literature.14,26
In this study, the photoelastic coefficient of 3a was success-
fully determined to be 0.7 × 10−12 Pa−1. This value was one
order smaller than those for other conventional and related
polymers, for example, 8.8 × 10−12 Pa−1 for polystyrene
and −4.3 to −5.5 × 10−12 Pa−1 for poly(methyl methacry-
late). A photoelastic coefficient is the parameter which is
closely related to the fashion of the segment mobility of
polymer chains. For example, the photoelastic property
of poly(methyl methacrylate) comes from the reorientation of
the side chain. The sign of the coefficients also significantly
depends on the polymer structures. The unique birefringence
properties of the maleimide copolymers will attract attention in
the research fields of optical polymer materials and their applica-
tions. Further investigation on the birefringence of the maleimide
copolymers is now continued.
CONCLUSIONS
In this study, we synthesized various three-component copolymers
consisting of EHMI, styrenes, and acrylates containing a hydroxy or
carboxylic acid group in the side chain and investigated their
FIGURE 8 Orientational birefringence of 3a (solid) and 6a (dash).
The films of 3a and 6a were drawn at 100 and 113 ꢀC, respectively.
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18 E. K. MacDonald, M. P. Shaver, Polym. Int. 2015, 64, 6.
19 T. Higashihara, M. Ueda, Macromolecules 2015, 48, 1915.
thermal, mechanical, and optical properties. The Tg values
increased by using MSt and BC5 instead St because of the
restricted rotation of the main chain. All the copolymers
exhibited high onset temperatures for decomposition. The
mechanical properties were improved by the introduction of
intermolecular hydrogen bonding although the introduction
of a bulky styrene unit decreased the toughness of the films.
For the simultaneous achievement of a high Tg and a high
strength and strain at break, the presence of maleimide ring
structure in the main chain and intermolecular interaction
between polymer chains in addition to the balance of the
steric bulkiness along the polymer main chain. The prelimi-
nary birefringence test in this study opened a new possibil-
ity of the maleimide copolymers for the application as the
thermally stable and highly transparent polymer films with
unique optical features.
20 C.-L. Tsai, H.-J. Yen, G.-S. Liou, React. Funct. Polym. 2016,
108, 2.
21 R. Hihumi, I. Tomita, Macromolecules 2018, 51, 5594.
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24 A. Matsumoto, T. Kubota, T. Otsu, Macromolecules 1990, 23,
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25 T. Otsu, A. Matsumoto, T. Kubota, Polym. Int. 1991, 25, 179.
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H. Okamura, Kobunshi Ronbunshu 2015, 72, 243.
27 T. Doi, Y. Sugiura, S. Yukioka, A. Akimoto, J. Appl. Polym.
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28 A. Omayu, T. Ueno, A. Matsumoto, Macromol. Chem. Phys.
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29 A. Matsumoto, D. Yamamoto, J. Polym. Sci. Part A: Polym.
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30 M. Hisano, K. Takeda, T. Takashima, Z. Jin, A. Shiibashi,
A. Matsumoto, Macromolecules 2013, 46, 3314.
The authors acknowledge the experimental support from
Material Solutions Research Institute of Kaneka Corpora-
tion for the determination of the optical properties of the
maleimide copolymers.
31 M. Hisano, K. Takeda, T. Takashima, Z. Jin, A. Shiibashi,
A. Matsumoto, Macromolecules 2013, 46, 7733.
32 S. Terada, A. Matsumoto, Polym. J. in press.
33 K. Satoh, M. Matsuda, K. Nagai, M. Kamigaito, J. Am. Chem.
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CONFLICT OF INTEREST
34 D. Yamamoto, A. Matsumoto, Macromol. Chem. Phys. 2012,
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The authors declare no conflict of interest.
35 M. Matsuda, K. Satoh, M. Kamigaito, J. Polym. Sci. Part A:
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