Radical copolymerization of N-phenylmaleimide and diene monomers in competition with diels–alder reaction

Article abstract of DOI:10.1002/pola.28248

Radical copolymerization of N-phenylmaleimide (PhMI) is carried out with various diene monomers including naturally occurring compounds and the copolymers are efficiently produced by the suppression of Diels–Alder reaction as the competitive side reaction. Diene monomers with an exomethylene moiety and a fixed s-trans diene structure, such as 3-methylenecyclopentene and 4-isopropyl-1-methyl-3-methylenecyclohexene, exhibit high copolymerization reactivity to produce a high-molecular-weight copolymer in a high yield. The copolymerization of sterically hindered noncyclic diene monomers, such as 2,4-dimethyl-1,3-pentadiene and 2,4-hexadiene, also results in the formation of a high-molecular-weight copolymer in a moderate yield. The NMR spectroscopy reveals that the obtained copolymers consist of predominant 1,4-repeating structures for the corresponding diene unit. The copolymers have excellent thermal stability, that is, an onset temperature of decomposition over 330 °C and a glass transition temperature over 130 °C. The copolymerization reactivity of these diene monomers is discussed based on the results of the DFT calculations. The efficient copolymer formation in competition with Diels–Alder addition is investigated under various conditions of the temperature, solvents, and initiators used for the copolymerization.

Full text of DOI:10.1002/pola.28248

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Radical Copolymerization of N-Phenylmaleimide and Diene
Monomers in Competition with Diels–Alder Reaction
Akikazu Matsumoto,1 Daisuke Yamamoto2
1Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-Cho, Naka-Ku, Sakai-Shi, Osaka, 599-8531, Japan
2Department of Applied Chemistry and Bioengineering, Graduate School of Engineering,
Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-Ku, Osaka, 558-8585, Japan
Correspondence to: A. Matsumoto (E-mail: matsumoto@chem.osakafu-u.ac.jp)
Received 29 June 2016; accepted 26 July 2016; published online 00 Month 2016
DOI: 10.1002/pola.28248
ABSTRACT: Radical copolymerization of N-phenylmaleimide
(PhMI) is carried out with various diene monomers including
naturally occurring compounds and the copolymers are effi-
ciently produced by the suppression of Diels–Alder reaction as
the competitive side reaction. Diene monomers with an exo-
methylene moiety and a fixed s-trans diene structure, such
as 3-methylenecyclopentene and 4-isopropyl-1-methyl-3-
methylenecyclohexene, exhibit high copolymerization reactivity
to produce a high-molecular-weight copolymer in a high yield.
The copolymerization of sterically hindered noncyclic diene
monomers, such as 2,4-dimethyl-1,3-pentadiene and 2,4-hexa-
diene, also results in the formation of a high-molecular-weight
copolymer in a moderate yield. The NMR spectroscopy reveals
that the obtained copolymers consist of predominant 1,4-
repeating structures for the corresponding diene unit. The
copolymers have excellent thermal stability, that is, an onset
temperature of decomposition over 330 8C and a glass transi-
tion temperature over 130 8C. The copolymerization reactivity
of these diene monomers is discussed based on the results of
the DFT calculations. The efficient copolymer formation in
competition with Diels–Alder addition is investigated under
various conditions of the temperature, solvents, and initiators
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used for the copolymerization. 2016 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2016, 00, 000–000
KEYWORDS: alternating copolymer; DFT calculations; glass tran-
sition temperature; naturally occurring materials; radical copo-
lymerization; thermal stability
conformation of this cyclic monomer (Scheme 1). More
recently, we succeeded in the synthesis of alternating copoly-
mers of maleic anhydride (MAn) and noncyclic 1,3-diene
monomers by simple molecular design based on the DFT cal-
culations for the conformational analysis of various kinds of
diene compounds.33 Actually, we demonstrated that the copo-
lymerization of 2,4-dimethyl-1,3-pentadiene (DMPD) with
MAn produced an alternating copolymer in the highest copol-
ymer yield among various diene monomers33 (Scheme 2). The
anhydride moiety and carbon-to-carbon double bond in the
repeating unit of the copolymer were available for postpoly-
merization reactions; for example, crosslinking using epoxy
compounds, dialcohols, and diamines as the polyfunctional
crosslinkers, as well as the oxidative ozonolysis of the carbon-
to-carbon double bonds in the main chain, leading to the
polymer chain scission, decrosslinking of cured resins, and
polymer surface modification.33–35
INTRODUCTION Alternating radical copolymerization has
many advantages for polymer synthesis, such as the produc-
tion of a high-molecular-weight copolymer in a high yield,
well-defined alternating sequence structures independent of
the comonomer ratio in the feed, and the use of various
monomers.1 N-Substituted maleimides (RMI) as the electron-
accepting monomers readily provide alternating copolymers
with excellent thermal, optical, and mechanical properties
during radical copolymerization with electron-donating mono-
mers, such as styrene, vinyl ethers, and isobutene.2–28 In con-
trast to the facile copolymerization of the RMIs with olefins,
they react with 1,3-diene compounds, such as butadiene,
cyclopentadiene, and furan to predominantly yield Diels–Alder
adducts rather than alternating copolymers.29–31 We previous-
ly reported the production of an alternating copolymer of
3-methylenecyclopentene (MCP) as the cyclic 1,3-diene mono-
mer containing a reactive exomethylene moiety during radical
copolymerization with the RMIs,32 because Diels–Alder reac-
tion of MCP was totally suppressed due to the fixed s-trans
In this study, we synthesized the copolymers of N-phenylma-
leimide (PhMI) with various diene monomers including
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Radical alternating copolymerization of MCP and
RMI.
DMPD, as shown in Figure 1. The diene monomers used in
this study were commercially available as the chemicals or
conveniently obtained by the one-step transformation of
chemical sources including naturally occurring materi-
als.36–40 For example, DMPD, 2,4-hexadiene (HD), and iso-
prene (IP) as the hydrocarbon diene monomers were
commercially available as well as methyl sorbate (MS). Sev-
eral terpenes and the related chemicals, that is, myrcene
(Myr), abietic acid (AA), a-phellandrene (T3), and a-
terpinene (T4), were available as the naturally occurring
materials. The other terpenoids, 4-isopropyl-1-methyl-3-
methylenecyclohexene (T1) and (-)-2-methylene-4,6,6-trime-
thylbicyclo[3.1.1]-3-heptene (T2) were transformed from the
corresponding ketones as the naturally occurring com-
pounds, that is, piperitone and verbenone, respectively. MCP
was obtained by the metathesis reaction of Myr using the
Grubbs catalyst.41 The reaction behavior of these diene
monomers during radical copolymerization with PhMI and
the structure and properties of the obtained copolymers
were investigated.
FIGURE 1 Chemical structures of diene monomers used in this
study.
molecular mechanics (MMFF) and the semiemprical method
(PM3), then the density functional theory (DFT) was used
for the final structure determination and the calculations of
the energy and electron densities at the B3LYP/6-31G* level.
Materials
2,20-Azobis(isobutyronitrile) (AIBN; Wako Pure Chemical
Industries, Ltd., Osaka) and dimethyl 2,20-azobis(4-methoxy-
2,4-dimethylvaleronitrile) (AMVN; Wako Pure Chemical
Industries, Ltd., Osaka) were recrystallized from methanol.
2,20-Azobis(2,4,4-trimethylpentane) (ATMP) was supplied
from Wako Pure Chemical Industries, Ltd., Osaka and used
without further purification. Commercially available N-phe-
nylmaleimide (PhMI; Wako Pure Chemical Industries, Ltd.,
Osaka) was used after recrystallization. All solvents were dis-
tilled before use. Myrcene (Myr; Tokyo Chemical Industry
Co., Ltd., Tokyo) as the mixture of two isomers was distilled
before use. The composition of the isomers was determined
to be a-Myr/b-Myr 5 16/84 by 1H NMR spectroscopy (Fig.
S1 in Supporting Information). MCP was synthesized by the
ring-closure metathesis reaction of Myr according to the
method described in the literature.41 (2)-a-Phellandrene
(T3) and a-terpinene (T4) were purchased from Tokyo
Chemical Industry Co., Ltd., Tokyo, and used without further
purification. Abietic acid (AA; Tokyo Chemical Industry Co.,
Ltd., Tokyo) was used after purification by the following pro-
cedure: 3 g of AA in 13 mL of ethanol was heated to 60 8C,
then 4 mL of water was added and cooled to room tempera-
ture. The precipitated crystals were filtered, dried, then used
for copolymerization. The isolated AA was the mixture of
several compounds and the purity of AA was 95%. The pre-
dominant contaminant was dehydroabietic acid (Fig. S1 in
Supporting Information), which was inert for the copolymeri-
zation and Diels–Alder reaction.
EXPERIMENTAL
General Procedures
The NMR spectra were recorded in CDCl3 using a Bruker
AV300N spectrometer (Bruker-Biospin, Ltd., Yokohama). The
number-average molecular weight (Mn), weight-average
molecular weight (Mw), and polydispersity (Mw/Mn) were
determined by size exclusion chromatography (SEC) with tet-
rahydrofuran (THF) as the eluent using a CCPD RE-8020 sys-
tem (Tosoh Co., Ltd., Tokyo) and calibration with standard
polystyrenes. The thermogravimetric (TG) analysis was car-
ried out using a Seiko TG 6200 (Seiko Instruments Co.,
Tokyo) in a nitrogen stream at the heating rate of 10 8C/min.
The onset temperature of decomposition (Td5) was deter-
mined as the 5% weight-loss temperature in the TG curves.
The differential scanning calorimetry (DSC) was carried out
using a Seiko DSC-6200 at the heating rate of 10 8C/min.
Theoretical calculations were carried out using the Spar-
tan’10 software package (Wavefunction, Inc., Irvine, CA). The
production of the initial molecular structure and the prelimi-
nary optimization of the structure were carried out using
Synthesis of T1 and T2
A four-necked flask was charged with methyltriphenylphos-
phonium bromide (34.3 g, 0.1 mol) (Wako Pure Chemical
Industries, Ltd., Osaka) suspended in dry THF (240 mL)
SCHEME 2 Radical alternating copolymerization of DMPD and
MAn.
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under nitrogen. Potassium tert-butoxide (15.6 g, 0.14 mol;
Tokyo Chemical Industry Co., Ltd., Tokyo) was then added,
and the yellow mixture was stirred at ambient temperature
for 30 min. The mixture was heated to 50 8C, and piperitone
for the synthesis of T1 or verbenone for the synthesis of T2
(9.2 g, 0.1 mol) in 60 mL of anhydrous THF was dropwise
added. After the mixture was stirred for 24 h at 50 8C, a
small amount of water (2 mL) was added to quench the
excess reagent. A large amount of n-hexane was added, then
washed with water and saturated NaCl aqueous solution
three times. The organic layer was dried on Na2SO4 and
purified by silica gel column chromatography (eluent: n-hex-
ane). The solvent was removed at a reduced pressure (20
mmHg) and room temperature over 5 h, then T1 or T2 was
distilled under the identical reduced pressure with an oil
bath maintained at 90 8C. The isolated yield of T1 was 13%
and the purity was 94%. The yield and purity of T2 were
14% and 98%, respectively.
Diels–Alder Adduct of PhMI and DMPD: 1H-NMR (300 MHz,
CDCl3) d 0.98 (s, CH3, 3H), 1.33 (s, CH3, 3H), 1.74 (s, CH3,
3H), 2.45 (dd, CH2, 1H), 2.50 (m, CH2, 1H), 2.74 (d, CCH,
1H), 3.27 (ddd, CH2CH, 1H), 5.38 (s, @CH, 1H), 7.22–7.45
(m, C6H5, 5H); 13C-NMR (75 MHz, CDCl3) d 23.3, 23.6, and
25.7 (CH3), 29.5 (CH2), 34.6 and 39.2 (CH), 49.3 (C), 126.3,
128.4, and 129.0 (C6H5), 131.8, 131.9, and 132.0 (C6H5,
@CH, @C), 177.3 and 179.3 (C@O).
Diels–Alder Adduct of PhMI and HD: 1H-NMR (300 MHz,
CDCl3) d 1.50 (d, CH3, 3H), 2.52 (q, CHCH3, 2H), 3.17 (dd, CH,
2H), 5.77 (s, @CH, 2H), 7.14–7.52 (m, C6H5, 5H); 13C-NMR
(75 MHz, CDCl3) d 17.0 (CH3), 31.4 and 45.8 (CH), 126.8,
128.6, 129.2, and 132.1 (C6H5), 134.3 (@CH), 176.8 (C@O).
Diels–Alder Adduct of PhMI and IP: 1H-NMR (300 MHz,
CDCl3) d 1.75 (s, CH3, 3H), 2.15–2.37 (m, @CHCH2CH,
2H), 2.53–2.66 (m, @CCH2CH, 2H), 3.13–3.25 (m, CH,
2H), 5.55–5.64 (m, @CH, 1H), 7.14–7.52 (m, C6H5, 5H);
13C NMR (75 MHz, CDCl3) d 23.5 (CH3), 24.5 and 28.8
(CH2), 39.2 and 39.6 (CH), 120.1 (CH@C), 126.4, 128.5,
129.0, and 132.1 (C6H5), 136.5 (CH2@C), 179.1 and
179.4 (C@O).
4-Isopropyl-1-methyl-3-methylenecyclohexene (T1): Colorless
liquid, yield 13%, purity 94%, 1H NMR (300 MHz, CDCl3)
d 0.89 (d, CH3, 3H), 0.92 (d, CH3, 3H), 1.74 (s, CH3, 3H), 1.50–
2.20 (m, CH2, CH, 6H), 4.63 (s, @CH2, 1H), 4.74 (s, @CH2, 1H),
5.86 (s, @CH, 1H); 13C NMR (75 MHz, CDCl3) d 20.0, 21.9, and
23.7 (CH3), 24.7 and 26.9 (CH2), 27.7 and 46.1 (CH), 109.3
(@CH2), 124.9 (@CH), 138.1 and 146.6 (@C).
Diels–Alder Adduct of PhMI and MS: 1H-NMR (300 MHz,
CDCl3) d 1.45 (d, CH3, 3H), 2.40–2.56 (m, CH, 1H), 3.14–
3.24 (m, CHC@O, 1H), 3.25 (dd, CH, 1H), 3.82 (s, OCH3, 3H),
3.86 (dd, CH, 1H), 5.84 (ddd, CH, 1H), 6.43 (ddd, CH, 1H),
7.14–7.46 (m, C6H5, 5H); 13C-NMR (75 MHz, CDCl3) d 16.7
(CH3), 31.6, 40.5, 43.8, and 52.4 (CH), 126.4, 128.6, 129.1,
and 131.6 (C6H5), 134.7 (@CH), 171.2, 175.8, and 176.3
(C@O).
(2)-2-Methylene-4,6,6-trimethylbicyclo[3.1.1]-3-heptene (T2):
Colorless liquid, yield 14%, purity 98%, 1H NMR (300 MHz,
CDCl3) d 0.82 (s, CH3, 3H), 1.35 (s, CH3, 3H), 1.45 (d, CH2,
1H), 1.78 (d, CH3, 3H), 2.10 (dt, CH2, 1H), 2.51 (dt, CH, 1H),
2.59 (dt, CH, 1H), 4.57 (s, CH2, 2H), 5.78 (s, CH2, 1H); 13C
NMR (75 MHz, CDCl3) d 21.9, 23.0, and 26.3 (CH3.), 35.7
(CH2), 43.7 (C), 48.3 and 51.4 (CH), 104.5 (@CH2), 120.7
(@CH), 148.6 and 150.3 (@C).
RESULTS AND DISCUSSION
We carried out the radical copolymerization of PhMI with
various diene monomers shown in Figure 1 under the condi-
tions of [PhMI] 5 [Diene] 5 0.5 mol/L (50/50 in a molar
ratio) and [AIBN] 5 0.01 mol/L in 1,2-dichloroethane at 60
8C for various polymerization times (1–24 h). The results of
the copolymerization are summarized in Table 1. The copoly-
mers with a high molecular weight were produced in a high
yield during the copolymerizations of DMPD, MCP, HD, and
T1 with PhMI. Among these diene monomers, MCP and T1
with a fixed s-trans conformation due to their cyclic mono-
mer structure showed the highest copolymerization reactivi-
ty (Mn 5 6.5–15 3 104, 82%–97% yield), as was expected.
The copolymerization of DMPD and HD as the noncyclic
diene monomers also resulted in the formation of a high-
molecular-weight copolymer in a moderate yield (Mn 5 1.6–
3.6 3 104, 47%–51% yield), being consistent with the previ-
ously reported results for the copolymerization with MAn.33
The copolymerization of IP with PhMI produced a copolymer
in a lower yield (Mn 5 1.8–2.4 3 104, 13%–20% yield).
Copolymerization Procedures
PhMI, a diene monomer, a radical initiator, and a solvent
were placed in a glass ampoule. After the freeze–thaw cycles,
the ampoule was sealed. The solution was heated at a deter-
mined temperature for a given time, then the polymerization
mixture was poured into a large amount of methanol to pre-
cipitate the copolymers, which were filtered out, washed,
then dried in vacuo. The yield of the copolymers was gravi-
metrically determined. The copolymers were purified by a
precipitation method using chloroform and methanol as the
solvent and non-solvent, respectively. The composition of the
repeating units in the copolymers was determined by 1H
NMR spectroscopy. Photopolymerization was carried out in
the presence of an azo initiator under UV irradiation at a
distance of 10 cm from a high-pressure Hg lamp (Toshiba
SHL-100UVQ-2) at 0 8C.
Diels–Alder Reactions
The reaction of PhMI with diene monomers was carried out
in various solvents at a determined temperature, then an ali-
quot was analyzed by NMR spectroscopy in order to deter-
mine the conversion of PhMI and the yield of the
corresponding Diels–Alder adduct.
In Table 1, the obtained polymer yields were almost con-
stant, independent of the time for several copolymerizations.
This was because the copolymer yield already reached a cer-
tain value after polymerization for several hours and the
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TABLE 1 Results of the Copolymerization of PhMI with Various Diene Monomers and Thermal Properties of the Obtained
Copolymersa
PhMI Content
in
Time
(h)
Copolymer
Yieldb (%)
Copolymer
(mol %)
Mn
3
Td5
Tg
Diene
1023
Mw/Mn
(8C)
(8C)
DMPD
2.5
5
16.5 (34.9)
26.3 (51.7)
37.9 (68.1)
43.3 (76.1)
50.7 (80.2)
97.3
35.8
31.7
27.1
24.8
22.5
147
1.57
1.57
10
15
20
6
1.62
1.64
49.5
50.1
1.67
331
364
130
159
MCP
HD
2.97
2
28.2 (71.0)
33.6 (84.7)
41.7 (91.8)
44.8 (92.9)
47.3 (93.7)
13.1 (87.6)
19.6 (99.9)
2.5 (28.5)
5.3 (40.5)
6.1 (90.8)
9.3
25.8
22.5
18.1
17.2
15.8
23.9
18.0
10.0
8.3
1.67
6
1.82
12
15
20
2
1.94
1.96
51.1
49.4
49.4
2.06
355
144
IP
1.67
19
1
2.26
MS
1.26
3
1.24
20
4.5
4.5
7
49.0
52.3
51.4
49.0
53.8
62.4
61.8
7.0
1.24
339
340
348
340
339
349
380
142
142
210
Myrc
AAd
T1
9.8
3.90
2.0
6.1
1.24
82.1
64.6
2.21
189
e
T2
7
8.9
6.6
1.29
T3
24
24 [10]f
8.0 [16.0]f
1.0 [3.8]f
2.4 [3.6]f
1.5 [3.9]f
1.63 [1.99]f
1.39 [2.54]f
208
151
T4
a
d
e
f
[PhMI] 5 [Diene] 5 0.5 mol/L, [AIBN] 5 0.01 mol/L, in 1,2-dichloroethane
at 60 8C.
[PhMI] 5 [AA] 5 0.7 mol/L.
Not detected in a temperature range below the Td5 by DSC.
Values in brackets indicate the values for photopolymerization:
b
Values in parentheses indicate PhMI conversion in % determined in
CDCl3.
[ATMP] 5 0.01 mol/L in 1,2-dichloroethane at 0 8C under UV irradiation
with a high-pressure Hg lamp.
c
[PhMI] 5 [Myr] 5 1.0 mol/L.
copolymerization occurred in competition with Diels–Alder
reaction. In Figure 2, the yields of the copolymers and the
Diels–Alder adducts were plotted as the function of a reac-
tion time during the reaction of various diene monomers at
60 8C. The conversion of PhMI was simultaneously moni-
suppressed copolymer formation, as was seen in the reaction
of MS. It was concluded that MS was not suitable to the
copolymerization with PhMI, although it readily reacted with
molecular oxygen to produce an alternating copolymer.42–44
1
The PhMI contents in the copolymers were revealed to be
close to 50 mol %, which was determined from the peak
intensity ratios of the aromatic hydrogens and the others in
the 1H-NMR spectra of the copolymers (Fig. 3). It was also
clarified that the diene units in the copolymers consisted of
mainly 1,4-repeating units, based on the chemical shifts and
the intensity of the peaks due to the unsaturated groups
observed at 4–6 ppm in the NMR spectra. It was previously
revealed that the peaks due to the 1,2- and 3,4-repeating
units of the copolymers of the RMIs or MAn with diene
monomers were observed as those with chemical shifts
clearly different from the 1,4-repeating one.32,33 In this study,
the peaks due to the 1,4-repeating units with a relatively
large intensity were observed at 5–5.5 ppm for the all
copolymers and minor several peaks were additionally
tored by H-NMR spectroscopy.
As a result, the rates of the both reactions of copolymeriza-
tion and Diels–Alder addition depended on the diene mono-
mer structure. The total rate of the both reactions, which is
also deduced from the consumption of PhMI, decreased
according to the steric bulkiness of the substituents of the
diene monomers in the order of IP > HD > DMPD. The yield
of the copolymers increased with a decrease in the total
reactivity of the diene monomers. The main product was a
copolymer during the reaction of DMPD, while Diels–Alder
reaction of HD rapidly proceeded in competition with the
copolymer formation. The Diels–Alder adduct was the main
product throughout the reaction of IP. The introduction of an
electron-withdrawing substituent also resulted in the
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FIGURE 2 Relationship between polymerization time and the copolymer yield (᭺), the yield of Diels–Alder adduct (᭝), and the con-
version of PhMI (w) during the radical copolymerization of PhMI with (a) DMPD, (b) HD, (c) IP, and (d) MS in the presence of AIBN
(0.01 mol/L) in 1,2-dichloroethane or CDCl3 at 60 8C.
observed in some cases, as is shown in Figure 3. It was con-
cluded that the copolymers obtained in this study consisted
of mainly 1,4-repeating structures, that is, approximately
more than 90%.
confirmed the formation of a homopolymer with a molecular
weight of 3000 in 1.2% yield (see Fig. S9 in Supporting
Information). The contribution of the both diene and vinyl
moieties should also be taken in consideration during the
propagation of Myr. It may produce a branched and partly
On the other hand, the copolymerization with other diene
monomers resulted in the formation of copolymers with Mn
less than 104 in a copolymer yield lower than 10% (Table
crosslinked polymer. In fact,
a
large Mw/Mn value
was observed for the copolymer with Myr (Mw/Mn 5 3.90),
as was shown in Table 1. In addition, the intramolecular
cross-propagation of the diene and vinyl moieties might pro-
duce a cyclic repeating structure in the main chain during
the propagation of Myr (Scheme 3). On the other hand, AA
produced no homopolymer under similar polymerization
conditions, and no occurrence of Diels–Alder reaction
because of its fixed s-trans conformation. Several less-
reactive diene monomers with considerable steric hindrance,
such as T3 and T4, led to the formation of the PhMI-rich
1
1). The H-NMR spectra of the copolymers with MS and Myr
in Figure 3 indicated the formation of the copolymer with
almost equimolar comonomer contents, but the other copoly-
mer compositions varied in a range of 54–62 mol % as the
PhMI content, depending on the reactivity of the
diene monomers. As a result of the homopolymerization of
Myr under the conditions of [Myr] 5 0.5 mol/L, [AIBN] 5
0.01 mol/L in 1,2-dichloroethane at 60 8C for 18 h, we
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FIGURE 3 1H-NMR spectra of the copolymers of PhMI with (a) DMPD, (b) MCP, (c) HD, (d) T1, (e) MS, and (f) Myr. Measured in
CDCl3. The chemical structures indicate 1,4-repeating units as the predominant structures for each copolymer.
copolymers (61.8–62.4 mol % as the PhMI content in the
copolymers). The control of copolymer sequences has been
reported for the copolymerizations of RMI with bulky olefin
monomers including b-pinene (BP), limonene, diisobutene by
several research groups.45–48 It was pointed out that the
penultimate unit effect played an important role for the for-
mation of the sequence-controlled copolymers including an
AAB-type repeating unit and that the suppressed copolymer-
ization rate was observed during the copolymerization under
the penultimate unit control. The copolymer production with
the high PhMI content may be due to the penultimate unit
effect of the sterically hindered diene monomers. We carried
out the photopolymerization of T3 and T4 as the less reac-
tive monomers with sterically ernal diene structures at 0 8C
in order to check the effect of a ceiling temperature. As a
result, the low-molecular-weight copolymers (Mn less than 4
3 103) were produced during the copolymerization with
SCHEME 3 Possible intramolecular 1,4-propagation of Myr.
6
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films were obtained by casting the solutions of the copoly-
mers, but they were brittle due to the steric bulkiness of the
substituents in the RMI and diene repeating units.
In order to clarify the reason for a difference in the reactivi-
ty of the diene monomers, we estimated the electron density
of a diene moiety by the DFT calculations at the B3LYP level
of theory with the 6-311G* basis set. The results are shown
in Table 2. It was revealed that the highest electron density
was observed on the C1 carbon of T1, well accounting for its
high copolymerization reactivity. The calculations also
revealed low electron density on the C1 carbon of AA and
the C1 and C4 carbons of T4, due to the presence of methyl
and isopropyl substituents. The addition of the PhMI radical
to the C4 carbon of AA was confirmed to be more favored
(DH4 5 234.9 kcal/mol) compared with that to the C1 car-
bon (DH1 5 222.0 kcal/mol). It was also considered that the
4,1-propagation predominantly proceeded rather than the
4,3-propagation during the reaction of the AA radical and
the PhMI monomer. For the other monomers, Myr, T2, and
T3, the moderate electron density values were obtained.
Judging from the molecular structure of T2, it was expected
to exhibit copolymerization reactivity similar to that of T1
with an exomethylene structure, but it produced a low-
molecular-weight polymer in a low yield. The bicyclic struc-
ture of T2 may suppress the reactivity. The propagation pro-
ceeded via a ring-opening process in competition with the
intermolecular 1,4-addition and led to the copolymer forma-
tion with complex repeating copolymer structures during the
copolymerization of T2 (Scheme 4). The structure of the
propagating radicals formed by the ring-opening reactions in
Scheme 4 were similar to those for the radicals produced
from the copolymerizations of b-pinene.45 T4 showed the
lowest copolymerization reactivity because of the disadvan-
tageous addition of the PhMI radical to the C1 and C4 car-
bons of T4 due to the low electron densities and the steric
hindrance of the alkyl substituents. The Tg value of the
copolymer of T4 with PhMI (Tg 5158 8C) lower than those
for the other copolymers of terpenes with PhMI might be
due to the lowest molecular weight.
FIGURE 4 TG curves for the copolymers obtained by the radi-
cal copolymerization of PhMI with various diene monomers at
the heating rate of 10 8C/min in a nitrogen stream.
PhMI (Table 1), suggesting that the less reactivity of these
monomers was not caused by a low ceiling temperature.
All the copolymers of PhMI with diene monomers obtained
in this study were thermally stable and the Td5 values were
higher than 330 8C, as is shown in Figure 4. The copolymer
of T4 with PhMI showed the highest Td5 value of 380 8C
despite the lowest molecular weight (Mn 5 1.5 3 103). The
Tg values varied depending on the structure of the diene
monomer units, as shown in Table 1. The Tg values of the
copolymers including noncyclic diene units were 130 8C–144
8C and the copolymers including cyclic repeating units
showed Tg values higher than 150 8C. Especially, the copoly-
mers obtained from the terpenes and the related monomers,
T1–T4 had high Tg values, being due to the rigid cyclic
repeating structure with bulky alkyl substituents. The Tg val-
ue of the copolymer of T4 with PhMI (Tg 5 158 8C) lower
than those for the copolymers of other terpenes with PhMI
might be due to the lowest molecular weight. The copoly-
mers were soluble in organic solvents, such as toluene, chlo-
roform, 1,2-dichloroethane, and THF. Highly transparent
TABLE 2 Mulliken Atomic Charge of Conjugated Diene Monomers at the B3LYP Level of Theory with the 6-311G* Basis Seta
Mulliken Atomic Charge (Electron Density)
Monomer
C1
C2
C3
C4
C5
C6
MS
Myr
AA
T1
20.327
20.300
0.023
20.130
0.111
20.178
20.062
0.000
20.155
20.298
20.209
0.083
2
2
20.169
0.079
20.212
0.156
–
–
–
–
–
–
–
–
–
–
20.489
20.455
20.386
0.093
20.207
20.246
20.211
20.215
T2
0.173
20.115
20.361
0.120
T3
20.364
20.210
T4
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temperature, for example, at 160 8C in 1,2-dichlorobenzene,
also provided the Diels–Alder adduct as the major product.
Next, the copolymerization at
a low temperature was
attempted to suppress the Diels–Alder reaction. The copoly-
merization was carried out in CDCl3 at 0 8C in the presence
of AIBN under UV irradiation with a Hg lamp. However, it
was failed as shown in Figure 5(a) and Table 3; almost no
copolymer was produced. An increase in the AIBN concentra-
tion resulted in an increase in the copolymer yield, but a
maximum copolymer yield was still less than 10% even after
polymerization for 184 h.
SCHEME 4 Possible propagation mechanism of T2 accompa-
It was reported that the copolymerization rate of PhMI
increased in a solvent with a large acceptor number.45,50 In
this study, therefore, we examined the copolymerization of
PhMI with IP in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP),
which is one of the highly electron-pair accepting solvents.51
As a result, the copolymer was obtained in only 7% yield.
Thus, the solvent effect was not valid for the control of the
reaction pathways in this study, while the copolymerization
behavior of the RMIs significantly depended on the Lewis
acidity of the solvents, as reported in the literatures.45,52–54
nying radical isomerization via ring opening reactions.
In order to increase a copolymer yield during the copolymer-
ization of the less-reactive diene monomers, we examined
the optimized copolymerization conditions including poly-
merization temperature, solvent, and an initiator using IP as
the diene monomer. The copolymer yield, the Diels–Alder
adduct yield, and the PhMI conversion determined for the
copolymerization under various reaction conditions are
shown in Table 3. Unfortunately, the formation of the Diels–
Alder adducts could not be suppressed when the copolymer-
ization was carried out in DMSO-d6 at 100 8C using ATMP as
the high temperature azo initiator (kd 5 1.3 3 1025 sec21
and half-life time 5 15 h at 100 8C).49 The Diels–Alder reac-
tion rapidly proceeded at the initial stage of polymerization
and no effect of the reverse reaction, that is, retro-Diels–
Alder addition was observed under the conditions in Table 3.
The unsaturated groups in the copolymer produced in 3%–
4% yield further reacted and resulted in increasing Mw/Mn
values under high temperature conditions. The tailing of the
peaks to a high molecular weight region was observed in the
SEC elution curves of the copolymers. The trials of the copo-
Finally, the use of AMVN as the azo initiator baring a high
decomposition rate (kd 5 7.7 3 1026 sec21 and half-life
time 5 25 h at 30 8C)55 was confirmed to be valid for the
increase in the copolymer yield, as shown in Figure 5(b).
The copolymer yield reached 33% after the 21-h polymeriza-
tion, being better than the yield for the copolymerization at
60 8C using AIBN (kd 5 9.6 3 1026 sec21 and half-life
time 5 20 h at 60 8C)56 in Table 1. However, the main prod-
uct was still the Diels–Alder adduct, but not the copolymer.
We concluded that the reaction pathway was predominantly
determined by the diene monomer structure themselves
rather than the reaction conditions such as the temperature,
solvents, and initiators.
lymerization under the other conditions at
a higher
TABLE 3 Yields of Copolymer and Diels–Alder Product During the Copolymerization of PhMI With IP Under Various Conditions
Initiator
(mol/L)
Temp
Time
(h)
Copolymer
Yield (%)
Diels–Alder
PhMI
Solvent
(8C)
Mn 3 1023
Mw/Mn
Adduct Yield (%)
Conversion (%)
DMSO-d6
ATMP (0.01)
AIBN (0.01)
100
0.5
1
3.4
7.0
4.47
5.69
1.17
1.18
1.26
1.54
1.37
1.58
3.61
2.93
2.98
1.59
1.60
1.74
84.0
87.7
66.7
81.3
87.8
45.0
74.4
78.1
86.4
85.5
85.4
38.9
52.6
60.7
97.9
99.1
71.8
85.7
96.7
51.2
81.8
92.8
92.5
93.0
93.3
56.4
77.4
94.1
3.9
6.0
CDCl3
CDCl3
HFIP
0 (UV)
15
31
93
6
0.17
0.22
1.2
5.3
4.8
4.0
AIBN (0.5)
AIBN (0.01)
MAIB (0.01)
0 (UV)
60
0.17
0.21
7.5
5.8
22
78
3
6.9
6.0
7.0
23.7
23.7
21.0
23.1
18.6
13.0
10
15
1
7.4
7.2
CDCl3
30
12.5
21.3
33.2
3
21
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conditions, such as temperature and solvents. The copoly-
merization at a relatively low temperature in the presence of
an azo initiator with a large decomposition rate constant
was valid for the synthesis of the copolymers in a higher
yield. The obtained copolymers showed high onset tempera-
tures of decomposition and high Tg values. The copolymers
of the RMIs with diene monomers would be useful for vari-
ous application fields related to optics and optoelectronics
materials, similar to the copolymers of the RMIs and olefins.
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