Controlled radical polymerization of 3-methylenecyclopentene with N-substituted maleimides to yield highly alternating and regiospecific copolymers

Article abstract of DOI:10.1021/ma4020092

High-molecular-weight diene copolymers with a regiospecific repeating structure were produced in a high yield during the alternating radical copolymerization of N-substituted maleimides (RMIs) and 3-methylenecyclopentene (MCP) as the cyclic 1,3-diene monomer including a reactive exomethylene moiety. The eminent copolymerization reactivity of MCP was in contrast to the predominant occurrence of the Diels-Alder reaction of isoprene with the RMIs rather than copolymerization. The highly alternating structure of the copolymers was confirmed based on the monomer reactivity ratios for the copolymerization of MCP (M₁) and N-phenylmaleimide (PhMI, M₂), r ₁ = 0.010 and r₂ = 0.0080. A mechanism for the highly controlled 1,4-regiospecific propagation, which consists of the addition of an RMI radical to the exomethylene group of MCP and subsequent 1,4-regiospecific propagation, was supported by the DFT calculations using model reactions as well as the precise structure determination of oligomers produced during telomerization in the presence of 1-butanethiol as a chain transfer agent. The resulting copolymers exhibited no weight-loss under 340 C during heating in a nitrogen atmosphere and their glass transition temperature was over the wide temperature range of 66-159 C, depending on the structure of the N-alkyl substituents. The transparent and flexible films were fabricated by a casting method. The optical properties of the films were as follows: the visible light transmittance over 95% at 380 nm, the refractive indices of 1.54-1.58, and the Abbe number of 42-45.

Full text of DOI:10.1021/ma4020092

Article
pubs.acs.org/Macromolecules
Controlled Radical Polymerization of 3‑Methylenecyclopentene with
N‑Substituted Maleimides To Yield Highly Alternating and
Regiospecific Copolymers
Daisuke Yamamoto† and Akikazu Matsumoto*,‡
†Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka 558-8585, Japan
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku,
Sakai-shi, Osaka 599-8531, Japan
S
* Supporting Information
ABSTRACT: High-molecular-weight diene copolymers with a
regiospecific repeating structure were produced in a high yield
during the alternating radical copolymerization of N-substituted
maleimides (RMIs) and 3-methylenecyclopentene (MCP) as
the cyclic 1,3-diene monomer including a reactive exomethylene
moiety. The eminent copolymerization reactivity of MCP was in
contrast to the predominant occurrence of the Diels−Alder
reaction of isoprene with the RMIs rather than copolymeriza-
tion. The highly alternating structure of the copolymers was
confirmed based on the monomer reactivity ratios for the
copolymerization of MCP (M1) and N-phenylmaleimide (PhMI, M2), r1 = 0.010 and r2 = 0.0080. A mechanism for the highly
controlled 1,4-regiospecific propagation, which consists of the addition of an RMI radical to the exomethylene group of MCP and
subsequent 1,4-regiospecific propagation, was supported by the DFT calculations using model reactions as well as the precise
structure determination of oligomers produced during telomerization in the presence of 1-butanethiol as a chain transfer agent.
The resulting copolymers exhibited no weight-loss under 340 °C during heating in a nitrogen atmosphere and their glass
transition temperature was over the wide temperature range of 66−159 °C, depending on the structure of the N-alkyl
substituents. The transparent and flexible films were fabricated by a casting method. The optical properties of the films were as
follows: the visible light transmittance over 95% at 380 nm, the refractive indices of 1.54−1.58, and the Abbe number of 42−45.
conjugated 1,3-dienes27−29 because diene compounds and the
INTRODUCTION
■
RMIs predominantly give Diels-Adler adducts,30 not copoly-
Transparent polymer materials with a thermal stability are one
of the key materials for various application fields, such as
mers. A maleimide moiety is favorably used as a reactive
functional group for the synthesis of self-healing materials,31−35
electronics, optoelectronics, and photonics.1−11 The trans-
bioconjugated functional polymers,36−38 well-controlled poly-
parent and heat-resistant polymers are industrially fabricated by
mer architectures,39−44 and controlled polymer networks45−49
condensation polymerization and metal-catalyzed polymer-
ization processes, as seen in most cases of the mass-production
of polycarbonates and cycloolefin polymers as the transparent
by polymer couplings using the Diels−Alder and thiol−ene
reactions.50−54 3-Methylenecyclopentene (MCP) is a cyclic
polymers and polyimides as the high-temperature polymers.12
diene compound including an exomethylene structure with a
fixed s-tans conformation. Hillmyer et al. reported that MCP
A radical chain polymerization process has a significant number
was synthesized from myrcene as a naturally occurring
of merits for the fabrication of polymeric materials due to the
monoterpenoid by the one-step reaction using the Grubbs
formation of high-molecular-weight polymers from commodity
catalyst and that the cationic polymerization of MCP gave a
monomers, the design of polymer sequences using controlled
polymer with a 1,4-regiocontrolled repeating structure,55,56
polymerizations, and finely tunable polymer structures and
properties by copolymerization.13−15 The radical copolymeriza-
while low-molecular-weight oligomers were produced via a
radical polymerization process. The structure of MCP is
suitable for use as the comonomer of the radical copolymeriza-
tion because the Diels−Alder reaction of MCP is entirely
tion of electron-accepting N-substituted maleimides (RMIs)
with electron-donating olefins readily produces high-molecular-
weight and alternating copolymers in a high yield.16−19
Especially, the copolymers of the RMIs with isobutene20,21
and other olefins22−26 exhibited an excellent thermal stability,
transparency, high strength, and high modulus. In contrast, very
few studies have reported the radical copolymerization with
Received: September 27, 2013
Revised: November 29, 2013
Published: December 12, 2013
© 2013 American Chemical Society
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Materials. 2,2′-Azobis(isobutyronitrile) (AIBN) (Wako Pure
Chemical Industries, Ltd., Osaka) was recrystallized from methanol.
N-n-Butylmaleimide (BMI) and N-cyclohexylmaleimide (CHMI) were
synthesized according to the methods reported in a previous paper.57
Commercially available N-methylmaleimide (MMI) and N-phenyl-
maleimide (PhMI) (Wako Pure Chemical Industries, Ltd., Osaka)
were used after recrystallization. Myrcene (Tokyo Chemical Industry
Co., Ltd., Tokyo) and all solvents were distilled before use.
avoided. Furthermore, it was previously reported that
exomethylenecycloalkanes readily copolymerized with the
RMIs.23,24 In this study, we investigated the radical
copolymerization of MCP and the RMIs and found that the
copolymerization occurred via a precisely controlled alternating
and 1,4-regiospecific propagation mechanism (Scheme 1). We
Scheme 1
Synthesis of 3-Methylenecyclopentene (MCP). MCP was
synthesized by the ring-closure metathesis reaction of myrcene
(Scheme 2).55 A 500-mL flask was charged with 500 mg of the
Scheme 2
discussed a mechanism for the regiospecific propagation during
the radical copolymerization based on the results of NMR
spectroscopy and DFT calculations as well as the structure
determination of the oligomers produced during the telome-
rization. The thermal and optical properties of the resulting
copolymers are also described.
second generation Grubbs catalyst (Sigma-Aldrich Co., Ltd.) under a
nitrogen stream, then decahydronaphthalene (cis/trans mixture, bp
187 °C, 375 mL) and myrcene (bp 166 °C, 46.8 g) were added. The
reaction mixture was stirred at 40 °C for 48 h and the produced MCP
and isobutene as the volatile products were transferred to a dry ice−
methanol trap under reduced pressure. The isobutene as the
byproduct was removed by bubbling argon into the liquid products
for 2 h at room temperature under atmospheric conditions. The liquid
residue was purified by distillation before use [bp 73−76 °C].55 The
structure and purity were checked by the NMR spectroscopies.
EXPERIMENTAL SECTION
■
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 tetrahy-
drofuran (THF) as the eluent using a CCPD RE-8020 system (Tosoh
Co., Ltd., Tokyo) and calibration with standard polystyrenes.
Preparative SEC was carried out using an LC-9201 (Japan Analytical
Industry Co., Ltd., Tokyo) equipped with UV/RI dual detectors and
JAIGEL-1H (exclusion limit, 1000) and JAIGEL-2H (exclusion limit,
5000) columns using chloroform as the eluent. The FT-IR spectra
were recorded by a JASCO FT/IR 430 spectrometer using the KBr
pellet method. The thermogravimetric and differential thermal
analyses (TG/DTA) were carried out using a Seiko TG/DTA 6200
(Seiko Instruments Inc., Tokyo) in a nitrogen stream at the heating
rate of 10 °C/min. The onset temperature of decomposition (Td5) was
determined 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 °C/min. The UV−vis spectra were
recorded using a V-550 spectrophotometer (JASCO Co., Tokyo). The
refractive index and Abbe number (νD) were determined using a DR-
M2 refractometer (Atago Co., Ltd., Tokyo). The νD value is
represented by the following equation.
1
MCP. Yield: 27%, colorless liquid. H NMR (300 MHz, CDCl3): δ
2.47−2.57 (m, CH2, 4H), 4.77 (s, CCHaHb, 1H), 4.86 (s, C
CHaHb, 1H), 6.13−6.20 (m, CHCH, 2H). 13C NMR (75 MHz,
CDCl3): δ 29.1 (CHCHCH2CH2), 32.4 (CHCHCH2CH2),
102.3 (=CH2), 134.6 (CH2CCHCH), 139.7 (CH2CCH
CH)), 155.2 (CH2C).
Copolymerization. The RMIs, MCP, AIBN, and 1,2-dichloro-
ethane were placed in a glass ampule. After the freeze−thaw cycles, the
ampule was sealed. The solution was heated at 60 °C for a given time,
then the polymerization mixture was poured into a large amount of
methanol to precipitate the copolymers. The copolymers were filtered
out, washed, then dried in vacuo. The yield of the copolymers was
gravimetrically determined. The copolymers were purified by a
precipitation method using chloroform and cyclohexane as the solvent
and nonsolvent, respectively. The precipitation process was repeated
three times in order to remove a small amount of the MCP
homopolymer, which was produced by spontaneous cationic polymer-
ization during the radical copolymerization procedures. The
copolymers of MCP with the RMIs were isolated by a centrifugal
separator after the precipitation procedure. The composition of the
repeating units in the copolymers was determined by 1H NMR
spectroscopy.
1
Poly(MCP-alt-MMI). H NMR (300 MHz, CDCl3): δ 0.78−3.60
nD = (nD − 1)/(nF − nC)
(1)
(m, CH, CH2, and CH3, 12H), 5.01−5.59 (m, CH, 1H). 13C NMR
(75 MHz, CDCl3): δ 24.8 (CH3), 25.9−28.1, 31.9−33.6, 34.0−35.6,
and 40.9−42.8 (CH2), 45.9−47.3 and 47.9−49.5 (CH), 126.7−129.1
(CHC), 142.9−144.5 (CHC), 178.2−178.8 (CO). Td5 = 363
°C; Tg = 120 °C. IR (KBr): 3456, 2945, 1774 (CO), 1700 (CO),
1436, 1385, 1281, 1125, 1031, and 785 cm−1.
where nF, nD, and nC represent the refractive indices at 486, 589, and
656 nm, respectively. The nE values were also determined at 546 nm.
The polymer films for the UV−vis measurements were prepared by
casting the polymer solution (8 wt % in chloroform) on a quartz or
Teflon plate (20 × 20 mm2) and slow drying at ambient pressure and
temperature. The thickness of the obtained polymer films was 90−100
μm.
Theoretical calculations were carried out using the Spartan’10
software package (Wave function, Inc., Irvine, CA, USA). The
production of the initial molecular structure and the preliminary
optimization of the structure were carried out using 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/6-311+G**//B3LYP/6-31G* level.
Poly(MCP-alt-BMI). 1H NMR (300 MHz, CDCl3): δ 0.65−4.01 (m,
CH, CH2, and CH3, 18H), 5.02−5.59 (s, CH, 1H). 13C NMR (75
MHz, CDCl3): δ 13.7 (CH3), 25.8−28.1, 27.0, 29.8, 32.4−35.4, 38.6,
and 41.1−42.6 (CH2), 46.3−49.4 (CH), 127.0−129.2 (CHC),
143.2−144.4 (CHC), 178.2 −178.8 (CO). Td5 = 346 °C; Tg = 66
°C. IR (KBr): 3454, 2961, 1771 (CO), 1695 (CO), 1440, 1401,
1191, 1131, 923, 808, 750, and 620 cm−1.
1
Poly(MCP-alt-CHMI). H NMR (300 MHz, CDCl3): δ 0.72−4.32
(m, CH and CH2, 20H), 3.75−4.24 (m, NCH, 1H), 4.94−5.94 (m, 
CH, 1H). 13C NMR (75 MHz, CDCl3): δ 25.1, 25.9, and 27.3−27.9
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a
Table 1. Properties of Copolymers Produced during Radical Copolymerization of MCP with RMIs
b
c
d
RMI
Mn/105
Mw/Mn
RMI mol % in copolymer
Td5 (°C)
Tg (°C)
τ
nD
νD
MMI
BMI
1.04
1.65
3.45
1.97
2.03
3.11
2.22
2.32
54.3
50.5
51.9
50.1
363
346
378
364
120
66
95
96
97
96
1.541
1.535
1.542
1.577
45.3
43.6
43.9
41.6
CHMI
PhMI
142
159
a
b
Copolymerization conditions: [MCP] = [RMI] = 0.50 mol/L and [AIBN] = 1.0 mmol/L in 1,2-dichloroethane at 60 °C for 2 h. Transmittance at
c
d
380 nm (%). Refractive index at 589 nm. Abbe number.
(CH2), 28.8 (C6H11), 28.4−29.3, 32.3−33.8, and 34.3−35.2 (CH2),
40.9−42.3, 46.4−48.9, and 51.6 (CH), 127.3−129.0 (CHC),
143.2−144.4 (CHC), 178.3−179.0 (CO). Td5 = 378 °C; Tg =
142 °C. IR (KBr): 3452, 2933, 2855, 1770 (CO), 1700(CO),
1454, 1397, 1376, 1345, 1259, 1190, 1144, and 895 cm−1.
126.5, 128.8, 129.3, and 132.0 (C6H5), 146.0 (CC), 175.8 and 178.3
(CO).
BT/MCP/PhMI-1,4-Adduct (Isomer II). Rf = 0.92 (hexane/ethyl
1
acetate =1/1). H NMR (300 MHz, CDCl3): δ 0.91 (t, CH3, 3H),
1.40 (ddd, CH2, 2H), 1.53 (m, CH2, 2H), 1.60 (m, CH2, 1H), 2.19
(m, CH2, 1H), 2.43 (t, CH2, 2H), 2.48 (m, CH2, 2H), 2.57 (dd, CH2,
1H), 2.88 (dd, CH2, 1H), 3.12 (ddd, CH, 1H), 3.22 (d, CH2, 2H),
3.35−3.50 (m, CH, 1H), 5.53−5.58 (m, CH, 1H), 7.10−7.76 (m,
C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 13.9 (CH3), 22.2, 25.9,
31.4, 31.5, 32.0, 32.3, and 33.6 (CH2), 43.9 and 47.0 (CH), 126.6 (
CH), 127.8, 128.8, 129.3, and 132.0 (C6H5), 144.1 (CC), 176.0 and
178.2 (CO).
1
Poly(MCP-alt-PhMI). H NMR (300 MHz, CDCl3): δ 0.45−4.15
(m, CH and CH2, 12H), 5.08−5.95 (s, CH, 1H), 6.45−8.85 (m,
C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 26.2−28.3, 32.6−33.7, and
34.5−35.5 (CH2), 46.8−48.2 (CH), 48.3−49.8 (CH), 125.8−126.8
(C6H5), 127.4−128.2 (CHC) 128.2−128.8, 128.8−129.6, and 132.2
(C6H5), 143.2−144.5 (CHC), 176.7−178.0 (CO). Td5 = 364 °C;
Tg = 159 °C. IR (KBr): 3473, 2936, 1777 (CO), 1713 (CO),
1598 (CC), 1500 (CC), 1384, 1179, 753, 692, and 619 cm−1.
Diels−Alder Reactions. The reaction of IP and PhMI was carried
out using CDCl3 as the solvent at 60 °C for a given time, then an
aliquot was analyzed by NMR spectroscopy in order to determine the
conversion of PhMI and the yield of the resulting Diels−Alder adduct.
IP/PhMI Diels−Alder Adduct: 1H NMR (300 MHz, CDCl3): δ 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): δ 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).
1
Poly(IP-alt-PhMI). H NMR (300 MHz, CDCl3): δ 0.60−3.81 (m,
CH3, CH2, and CH, 9H), 4.89−5.95 (m, CH, 1H), 6.81−7.91 (m,
C6H5, 5H). 13C NMR (75 MHz, CDCl3): δ 15.7−16.9 and 23.7
(CH3), 27.3−29.4, 32.4−34.1, and 40.1−42.4 (CH2), 42.4−45.9
(CH), 122.7−123.7 (CHC), 126.4, 128.7, 129.2, and 131.9 (C6H5),
134.9−136.3 (CHC), 177.0−178.3 (CO). IR (KBr): 3472, 2916,
1778 (CO), 1712 (CO), 1597 (CC), 1498 (CC), 1382,
1184, 756, 691, and 620 cm−1.
Hydrogenation.58 The poly(MCP-alt-PhMI) (95 mg, Mn = 1.08
× 105, Mw/Mn = 2.89) and p-toluenesulfonyl hydrazide (1.23 g, 13
equiv), tri-n-propylamine (287 mg), and dibutylhydroxytoluene (5
mg) were stirred in 1,2-dichlorobenzene (5 mL) at 120 °C for 10 h
under a nitrogen atmosphere, and then the reaction mixture was
poured into a large amount of methanol. The precipitated polymer was
filtered out, washed with methanol, then dried in vacuo at room
temperature. The recovered polymer was purified by precipitation
using chloroform and methanol. The conversion was determined by
1H NMR spectroscopy. The Mn and Mw/Mn values of the
hydrogenated copolymer were 1.04 × 105 and 2.93, respectively.
Monomer Reactivity Ratios. All the copolymers were recovered
at a low conversion to allow the analysis of the copolymerization
parameters using the Mayo−Lewis equation (eq 2)
d[M1]
d[M2]
[M1](r [M1] + [M2])
[M2](r2[M2] + [M1])
1
=
(2)
(3)
(4)
r = k11/k12
1
r2 = k22/k21
1
Hydrogenated Poly(MCP-alt-PhMI). Conversion: 61%. H NMR
where M1 and M2 are MCP or IP and PhMI, respectively. The
monomer reactivity ratios, r1 and r2, which are defined by eqs 3 and 4,
respectively, were successfully determined using a terminal unit model
and the nonlinear least-squares curve fitting,59 Fineman−Ross,60 and
Kelen−Tudos61 methods.
(300 MHz, CDCl3): δ 0.40−3.80 (m, CH2 and CH), 5.15−5.78 (m,
CH, 1H), 6.91−7.96 (m, C6H5, 5H). 13C NMR (75 MHz, CDCl3):
δ 27.5−50.5 (CH, CH2, and CH3), 127.7−129.5 (CCH), 126.1−
126.8, 128.6, 129.2, and 131.8 (C6H5), 143.4−144.3 (CCH),
177.2−178.5 (CO). Td5 = 375 °C; Tg = 149 °C.
̈
̃
Telomerization. PhMI (0.5 mol/L), MCP (0.5 mol/L), AIBN
(0.01 mol/L), and 1-butanethiol (BT, 2.0 mol/L) in 10 mL of 1,2-
dichloroethane were placed in a glass ampule. After the freeze−thaw
cycles, the ampule was sealed. The solution was heated at 60 °C for 1
h, then the reaction mixture was poured into a large amount of
methanol to precipitate the copolymers as the byproduct. The filtrate
was concentrated under reduced pressure and several oligomers with a
BT fragment were isolated by preparative SEC and subsequent silica
gel chromatography (hexane/ethyl acetate =7/3 in volume).
RESULTS AND DISCUSSION
Copolymerization of MCP with RMIs. The radical
■
copolymerization of MCP with the RMIs was carried out in
1,2-dichloroethane at 60 °C in the presence of AIBN. The high-
molecular-weight copolymers (Mn > 105) were produced in a
high yield, independent of the N-alkyl substituent structures of
the RMIs, as shown in Table 1 and Figure 1. The
copolymerization of MCP with PhMI gave a copolymer with
higher yields than those with the other N-alkyl-substituted
RMIs. The eminent reactivity of MCP during the copoly-
merization with the RMIs was quite different from the poor
homopolymerization ability55 leading to the formation of
1
BT/PhMI-Adduct. H NMR (300 MHz, CDCl3): δ 0.94 (t, CH3,
3H), 1.37−1.74 (m, CH2, 4H), 2.69 (dd, CH2, 1H), 2.90 (m, CH2,
2H), 3.31 (dd, CH2, 1H), 3.87 (dd, CH, 1H), 7.27−7.50 (m, C6H5,
5H).
BT/MCP/PhMI-1,4-Adduct (Isomer I): Rf = 0.82 (hexane/ethyl
acetate =1/1). 1H NMR (300 MHz, CDCl3) δ 0.90 (t, CH3, 3H), 1.38
(ddd, CH2, 2H), 1.53 (m, CH2, 2H), 1.69 (m, CH2, 1H), 2.31 (m,
CH2, 1H), 2.41 (t, CH2, 2H), 2.48 (t, CH2, 2H), 2.59 (dd, CH2, 1H),
2.87 (dd, CH2, 1H), 3.08 (ddd, CH, 1H), 3.21 (d, CH2, 2H), 3.46−
3.58 (m, CH, 1H), 5.27−5.33 (d, =CH, 1H), 7.10−7.76 (m, C6H5,
5H). 13C NMR (75 MHz, CDCl3): δ 13.9 (CH3), 22.1, 28.3, 31.3,
31.4, 31.5, 32.4, and 33.8 (CH2), 44.3 and 46.7 (CH), 125.2 (=CH),
1
oligomers with an Mn value less than 103. The H NMR
spectroscopy revealed the formation of the 1:1 composition
copolymers over the entire range of the comonomer
composition, i.e., the poly(MCP-alt-RMI)s, as will be shown
later. In general, an electron-deficient ene compound tends to
react with an electron-donating 1,3-diene to produce a Diels−
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Figure 1. Time-copolymer yield relationships during the radical
copolymerization of MCP with the RMIs in 1,2-dichloroethane at 60
○
°C; [MCP] = [RMI] = 0.50 mol/L; [AIBN] = 1.0 mmol/L. Key: ( )
MMI, (Δ) BMI, ( ) CHMI, and ( ) PhMI.
◊
■
Alder adduct. However, no Diels−Alder adduct was produced
during the reaction of MCP with the RMIs. This is due to the
structure of the diene moiety of MCP as the fixed s-trans
conformation. In contrast to the exclusive formation of the
copolymers of MCP with the RMIs, cyclic diene compounds
with fixed s-cis conformations, such as cyclopentadiene and
furan, readily undergo the Diels−Alder reaction with the RMIs.
Noncyclic dienes with a flexible and nonfixed conformation
provide both a Diels−Alder adduct and a copolymer. The
corresponding Diels−Alder adduct was actually produced as the
major product during the reaction of isoprene (IP) with PhMI.
Figure 2a shows the time−conversion curves for the reaction of
IP and PhMI in CDCl3 at 60 °C in the presence of AIBN. The
rate of Diels−Alder adduct formation was much greater than
that of the copolymer formation.
Figure 2. (a) Competition of alternating copolymerization and Diels−
Alder addition during the reaction of IP with PhMI in CDCl3 at 60 °C.
[IP] = [PhMI] = 0.50 mol/L, [AIBN] = 10 mmol/L. The PhMI
conversion and Diels−Alder adduct yield were determined by 1H
NMR spectroscopy. The copolymer yield was determined based on
The thermal properties of the resulting poly(MCP-alt-RMI)s
were investigated by TG and DSC measurements in a nitrogen
stream at the heating rate of 10 °C. The results are summarized
in Table 1. The Td5 values were over 345 °C for all the
copolymers. The Tg values depended on the N-substituents,
and increased by the introduction of bulky cyclohexyl and
phenyl groups. All the poly(MCP-alt-RMI)s were soluble in
chloroform, dichloromethane, 1,2-dichloroethane, and tetrahy-
drofuran, and insoluble in methanol, ethanol, acetone,
acetonitrile, and n-hexane. The solubility in the other solvents
was dependent on the N-substituent structure of the
copolymers (see Table S3). The copolymer containing a
large cyclohexyl substituent was soluble in toluene, while the
methyl- and phenyl-substituted derivatives were soluble in polar
solvents, such as N,N-dimethylformamide and dimethyl
sulfoxide. The casting of the chloroform solution of the
copolymers provided transparent and flexible films, of which
the optical properties were investigated. The visible light
transmittance was greater than 95% and the nD and νD values
were 1.54−1.58 and 42−45, respectively. The poly(MCP-alt-
PhMI) exhibited a slightly higher nD value and a lower νD value
than those of the other N-alkyl-substituted poly(MCP-alt-
RMI)s. A double bond in the MCP repeating unit was
hydrogenated by a chemical reduction using p-toluenesulfonyl
hydrazide at 120 °C for 1 h.58 No change in the molecular
weight was observed during the reaction. The hydrogenation
efficiency reached only a 61% conversion for the reaction of
poly(MCP-alt-PhMI) under the conditions used in this study.
Nevertheless, the polymer after transformation exhibited the
1
the weight of the isolated copolymer. (b) H NMR spectrum of a
mixture of PhMI and the Diels−Alder adduct.
Td5 of 375 °C, which was 10 °C higher than the original one
(364 °C). The Tg value decreased from 159 to 149 °C due to
an increase in the chain flexibility.
Alternating Copolymerization Mechanism. In order to
evaluate the copolymerization reactivity of MCP and IP with
PhMI, the copolymerizations were carried out under the
conditions of various comonomer compositions. The monomer
reactivity ratios, r1 and r2, were determined based on the
comonomer−copolymer composition curves (Figure 3), which
were drawn using the results obtained by the curve fitting
method. It is noted that the MCP copolymerization precisely
produced alternating copolymers for all the comonomer
compositions. The r1 and r2 values were 0.010 and 0.0080,
respectively, for the MCP (M1)−PhMI (M2) system (Table 2).
The both reactivity ratios were much lower than those for the
IP (M1)−PhMI (M2) copolymerization, i.e., r1 = 0.12 and r2 =
0.036. The Fineman−Ross and Kelen−Tudos methods gave
̈
̃
similar r1 and r2 values, as summarized in Table 2. The 1/r2
values indicated that the relative reactivity of MCP toward the
PhMI radical was 5−20 times higher than that of IP due to the
enhanced monomer reactivity of MCP with an exomethylene
group.
1
The H and 13C NMR spectra of the copolymers of MCP
with the RMIs (Figures 4 and 5) suggested the exclusive
formation of the 1,4-repeating structure of the MCP units and
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Figure 3. Comonomer-copolymer composition curves for the radical
copolymerization of (a) MCP and (b) IP (M1) with PhMI (M2).
Copolymerization conditions: ([diene]+[PhMI]) = 1.0 mol/L,
[AIBN] = 10 mmol/L in 1,2-dichloroethane at 60 °C. The curves
were drawn using the monomer reactivity ratios determined by the
nonlinear least-squares curve fitting method.
1
Figure 4. H NMR spectrum of the copolymer produced during the
copolymerization of MCP with MMI in 1,2-dichloroethane at 60 °C.
See Table 1 for the copolymerization conditions.
no formation of any other structures, i.e., 1,2- and 3,4-forms.
The peaks due to the protons and carbons of an exomethylene
moiety in the 3,4-repeating unit, which should be observed in
the region of 4.7−4.9 ppm,55 were not found in the spectra of
the poly(MCP-alt-RMI)s. The results of the 13C NMR analysis
of the copolymers also determined the regiospecific formation
of the 1,4-repeating MCP unit, although the 1,4- and 4,1-
propagations could not be differentiated based on the NMR
information because of the symmetrical RMI repeating unit.
In contrast, the weak signals due to the 1,2- and 3,4-
structures were observed in the 1H NMR spectrum of poly(IP-
alt-PhMI) in the addition to the major peaks due to the 1,4-
repeating unit (Figure 6), when the IP content in the
copolymer was as high as 68 mol %. The high regulation of
the regioselectivity of MCP was due to the high alternating
propagation during the copolymerization, while the addition of
the IP propagating radical to the IP monomer occurred in a
less-controlled fashion. The comonomer-copolymer composi-
tion curve for the copolymerization of IP and PhMI in Figure
3b also indicated the presence of the successive IP units in the
copolymer chains. Thus, the NMR spectroscopy indicated the
predominant formation of 1,4-repeating structure of MCP.
DFT Calculations for Regiospecific Propagation
Mechanism Analysis. The regiospecific propagation of
MCP was further investigated by the DFT calculations using
the model reactions shown in Figure 7. The energy difference
(ΔE) between the reactant and the product and the difference
in the competitive reactions for each step [Δ(ΔE)] are
summarized in Table 3. The DFT calculation results indicated
the plausible addition of the RMI radical to the 1- and 4-
positions of the MCP monomer with the ΔE values of −27.4
and −23.9 kcal/mol, respectively. They were thermodynami-
cally favored compared to the 2- and 3-additions (ΔE = −3.0
and −8.9 kcal/mol, respectively). The reaction selectivity
should strictly be discussed based on the activation energies for
each propagation, but not the ΔE values. In a previous paper,
Figure 5. 13C NMR spectra of the copolymers produced during the
copolymerization of MCP with (a) MMI, (b) CHMI, and (c) PhMI in
1,2-dichloroethane at 60 °C. See Table 1 for the copolymerization
conditions. Asterisk denotes the solvent.
however, we already confirmed that the ΔE values were closely
related to the absolute activation energies for the RMI
copolymerization with styrene derivatives;24 i.e., the ΔE values
negatively higher, and the activation energies lower. It was
expected that the 1-addition favorably occurred due to less
steric repulsion between the radical and the diene monomer
during the propagation. The Mulliken atomic charges on the
carbon atoms of the diene moiety of MCP were also calculated
by the DFT method. The results are summarized in Table 4, in
which the observed 13C NMR chemical shift values are also
shown. It was confirmed that an electron density on the C1
carbon was higher than that on C4. This result agreed well with
the NMR chemical shift values. The highest density and the
Table 2. Monomer Reactivity Ratios Determined for Copolymerizations of MCP and IP (M1) with PhMI (M2) in 1,2-
Dichloroethane at 60 °C
curve fitting
Fineman−Ross
Kelen−Tudos
̈ ̃
comonomers
r1
r2
r1
r2
∼0
0.046
r1
r2
MCP (M1)−PhMI (M2)
IP (M1)−PhMI (M2)
0.010
0.12
0.0080
0.036
0.0099
0.12
0.012
0.18
0.0026
0.059
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1
the structures of the products were investigated by H NMR
spectroscopy (Figure 9). Fraction I consisted of almost a single
component and assigned as the 1:1 adduct of BT and PhMI
(BT/PhMI-adduct). The NMR spectrum of fraction II included
a large number of sharp and well-split peaks (Figure 9b),
suggesting the formation of the other oligomers, such as the
BT/MCP-adducts, BT/MCP/PhMI-adducts, BT/PhMI/MCP-
adducts, and others. The entire spectral pattern of fraction III
was partially similar to that of fraction II, but much broader.
This suggested that Fraction III may include higher oligomers,
such as the BT/MCP/PhMI/MCP-adducts, BT/MCP/PhMI/
MCP/PhMI-adducts, or larger oligomers. In the NMR
spectrum of Fraction II, no peak was observed due to the
methine proton of the maleimide ring adjacent to the butylthiyl
substituent, while the corresponding doublet peaks were
observed at 3.9 ppm in the spectrum of the BT/PhMI-adduct
(Figure 9a). This indicated no formation of BT/PhMI/MCP-
adduct. Using silica gel column chromatography with a hexane/
ethyl acetate mixture (7/3 in volume) as the eluent, two major
products were successfully isolated from fraction II. They were
assigned as the BT/MCP/PhMI-1,4-adducts (Scheme 3)
according to an analytical process for the precise structure
determination of the products as follows.
Figure 6. 1H NMR spectra of the copolymers produced during
copolymerization of IP with PhMI in 1,2-dichloroethane at 60 °C. The
IP contents in the copolymers: (a) 50.4 mol %; (b) 67.8 mol %.
lowest chemical shift value on the C1 atom supported the
occurrence of the radical addition at the C1 position of MCP
rather than that at the C4 position. This result was further
confirmed by the isolation of the 1:1 adduct of the MCP and
RMI and the structure analysis of the product formed during
telomerization (see next section). The second step of the
regiospecific propagation is the RMI addition to the allyl
radical. The 1,4-propagation was estimated to be energetically
favored. The calculations for the larger oligomers provided
similar results, as shown in Table 3.
Telomerization. In order to discuss the preferred 1,4-
propagation of MCP, the telomerization of MCP and PhMI
was carried out in the presence of 1-butanethiol (BT) as the
chain transfer agent and the resulting oligomers were isolated
and characterized (Scheme 3). The telomerization was carried
out under the conditions at a low monomer concentration
([MCP] = [PhMI] = 0.1 mol/L) and a high BT concentration
([BT] = 2.0 mol/L) with AIBN as the radical initiator in 1,2-
dichloroethane at 60 °C for 1 h. After the reaction, a methanol-
soluble fraction was further divided into some fractions using
preparative SEC. Fractions I to III in Figure 8 were isolated and
The 1H and 13C NMR spectra of one of the isolated adducts
(isomer I with Rf = 0.82) are shown in Figure 10. The 2D NMR
spectra (HSQC and HHCOSY) used for the structure
determination are shown in Figure 11. It was revealed that
the compound included two kinds of CO, single CC,
single phenyl, two kinds of methyls, seven kinds of methylenes,
and single methyl groups, based on the number, chemical shifts,
and the other correlation information on the peaks detected in
the NMR spectra. We considered the BT/MCP/PhMI-adducts
as the candidates for the isolated oligomers.
In the HSQC spectrum, the methyl carbon at 13.9 ppm, the
five methylene carbons at 22.1, 31.4, 31.5, 32.4, and 33.8 ppm,
and two methine carbons at 44.3 and 46.7 ppm were related to
the corresponding protons, as summarized in Table 5. Two sets
of nonequivalent methylene protons were also observed at 1.69
and 2.31 ppm as the multiplets, and at 2.59 and 2.87 ppm as the
double doublet, being correlated to the methylene carbons at
28.3 and 31.3 ppm, respectively. The HHCOSY spectrum
indicated that one set of nonequivalent methylene protons (HA
and HB, dd at 2.59 and 2.87 ppm) are correlated to one of the
Figure 7. DFT calculation results for the model reactions related to the preferred regiospecific propagation during the radical copolymerization of
MCP with the RMIs. See also Table 3 and Figure S11 for the results of the larger oligomers.
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a
Table 3. DFT Calculation Results for Possible Propagation Reactions During Alternating Copolymerization of MCP with MMI
radical
monomer
MCP
reaction
ΔE (kcal/mol)
Δ(ΔE) (kcal/mol)
CH3−MMI•
addition to 1-position of MCP
addition to 2-position of MCP
addition to 3-position of MCP
addition to 4-position of MCP
1,4-propagation of MCP unit
1,2-propagation of MCP unit
addition to 1-position of MCP
1,4-propagation of MCP unit
1,2-propagation of MCP unit
−27.4
−3.0
−8.9
−23.9
−11.5
−3.3
−26.6
−12.6
−2.2
0
24.4
18.5
3.5
0
CH3−MMI−MCP•
MMI
8.2
−
0
CH3−MMI−MCP−MMI•
MCP
MMI
CH3−MMI−MCP−MMI−MCP•
10.4
a
See Figure 7 and Figure S12 for the detailed repeating structures of the radicals.
Table 4. Mulliken Atomic Charges on the Diene Moiety of
MCP Determined by DFT Calculations and Observed 13C
NMR Chemical Shifts
a
b
carbon
Mulliken atomic charge 13C NMR chemical shift (δ in ppm)
C1
C2
C3
C4
−0.467
0.080
102.2
155.1
134.6
139.5
−0.180
−0.179
a
See Figure 7 for the numbering of the carbon atoms C1 to C4 of
b
MCP. Observed in CDCl3 at room temperature.
Scheme 3
Figure 8. Elution curve for the preparative SEC of the telomerization
mixture.
Figure 9. 1H NMR spectra of fractions I−III isolated from the
telomerization mixture by preparative SEC. Measurement solvent,
CDCl3.
methine protons (HC, ddd at 3.08 ppm). The observed
coupling constants were as follows: J(HA-HB) = 18.2 Hz, J(HA-
HC) = 9.3 Hz, and J(HB-HC) = 4.8 Hz (Figure 12, see also
Figure S16). The nonequivalent HA and HB exhibited no
further correlation to any other protons in the HHCOSY
spectrum. The methine proton HC was weakly correlated to
another methine proton, HD. Consequently, a partial structure
for the skeleton of the isolated oligomer was proposed as
shown in Figure 12. The BT/MCP/PhMI-1,4-adduct uniquely
satisfied these conditions and the other correlations between all
the carbons and protons were rationally explained. The precise
NMR analysis of the oligomeric products formed during
telomerization provided evidence for the 1,4-propagation of the
MCP repeating unit.
Another oligomer isolated from fraction II (isomer II with Rf
= 0.92) was also characterized and revealed to have a chemical
structure similar to that of isomer I. The NMR data for both
isomers are summarized in Table 5 (see also Figures S15 and
S16). As a result, isomer II was also assigned the BT/MCP/
PhMI-1,4-adduct. The BT/MCP/PhMI-1,4-adduct includes
two asymmetric carbons and exists as a set of diastereomers,
i.e., the (R*,R*)- and (R*,S*)-form isomers. The product ratio
of isomers I and II was determined to be 64/36, based on the
intensity ratio of the peaks due to the CCH bond. In this
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were calculated to be 6.5 × 107 and 2.1 × 105 mol/(L s),
respectively,63 suggesting the predominant formation of the
BT/MCP-adducts, but not the BT/PhMI-adduct. However, the
BT/PhMI-adduct was actually isolated as fraction I among the
possible oligomers produced during the telomerization
reactions (Figure 9a). The reason why the BT/MCP-adduct
could not be isolated in this study was possibly due to the
preferred addition of PhMI to the MCP radical rather than the
chain transfer to BT (Scheme 4). On the other hand, the
formation of the PhMI-terminated oligomers is quite rationally
explained by the larger chain transfer constants of the
maleimide radical to thiols; the kct values to a thiol were
reported to be 1.4 mol/(L s) and 8.2 × 103 mol/(L s) for the
polymerizations of butadiene and maleimide, respectively.63
This well accounts for the PhMI-terminated oligomer
formation as the experimental results in this study. We assumed
that the additions of the thiyl radical to MCP and PhMI
competitively occurred during the initial step during the
reactions for the oligomer formation. The produced BT/PhMI
radical was rapidly terminated by hydrogen abstraction from
BT, while another BT/MCP radical further propagated by the
reaction with PhMI and the resulting BT/MCP/PhMI radical
readily underwent chain transfer to BT to produce the BT/
MCP/PhMI-adduct, which was the main component of fraction
II. These kinetic analyses well explained the isolation of three
kinds of oligomers, i.e., the BT/PhMI-adduct and BT/MCP/
PhMI-adducts as two diastereomers, as the major products
during the telomerization.
1
Figure 10. H and 13C NMR spectra of BT/MCP/PhMI-1,4-adduct
(isomer I) separated from the telomerization mixture by preparative
SEC, followed by silica gel column chromatography.
CONCLUSION
■
The N-substituted maleimide derivatives, the RMIs, have been
used as the comonomers during the radical alternating
copolymerization with electron-donating monomers, such as
olefins and styrenes, to produce thermally stable alternating
copolymers. The RMIs were also used for the random
copolymerization with (meth)acrylates and vinyl chloride in
order to modify the thermal stability of the commodity
polymers. Otherwise, the RMI groups were used as a highly
reactive moiety for the Diels−Alder and thiol−ene reactions,
which are valid for the structural and functional designs of
polymer materials. At the same time, however, the nature of the
study, we did not assign which of these two isomers is the
diastereomer. The other minor products were also included in
fraction II, but they could not be isolated as the pure
compounds because of their too small amount contained in the
fraction.
It is well-known that the rate of the addition of an
electrophilic thiyl radical to electron-donating alkene and
alkynes is much higher than that to electron-deficient alkenes
and alkynes.14,62 The rate constants (kct) for the addition of the
methylthiyl radical (CH3S•) to 1,3-butadiene and maleimide
Figure 11. HSQC and HHCOSY spectra of BT/MCP/PhMI-1,4-adduct (isomer I) separated from the telomerization mixture by preparative SEC,
followed by silica gel column chromatography. For isomer II, see Figure S16.
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1
Table 5. H and 13C NMR Spectral Data for the Two Isomers of BT/MCP/PhMI-1,4-Adducts (Measured in CDCl3 at Room
Temperature, Unit Is δ in ppm)
isomer I
isomer II
position
structure
1H NMR
0.90 (t)
13C NMR
1H NMR
0.91 (t)
13C NMR
a
CH3
CH2
CH2
13.9
13.9
b
1.38 (ddd)
22.1
31.5
28.3
31.4
33.8
31.3
32.4
44.3
46.7
1.40 (ddd)
22.2
32.0
25.9
31.5
33.6
31.4
32.3
43.9
47.0
c
1.53 (m)
1.53 (m)
a
g, g′
d
CH2
1.69 (m), 2.31 (m)
2.41 (t)
1.60 (m), 2.19 (m)
2.43 (t)
CH2
CH2
h
2.48 (t)
2.48 (m)
a
l, l′
e
CH2
2.59 (dd), 2.87 (dd)
3.21 (d)
2.57 (dd), 2.88 (dd)
3.22 (d)
CH2
k
CH
3.08 (ddd)
3.12 (ddd)
f
CH
3.46−3.58 (m)
5.27−5.33 (m)
7.10−7.76 (m)
3.35−3.50 (m)
5.53−5.58(m)
7.10−7.76 (m)
i, j
CCH
C6H5
CO
125.2, 146.0
126.6, 144.1
o, p, q, r
m, n
126.5, 128.8, 129.3, 132.0
175.8, 178.3
127.8, 128.8, 129.3, 132.0
176.0, 178.2
a
Nonequivalent methylene protons.
with the 1,3-diene monomers. The MCP monomer is a 1,3-
diene monomer derived from one of the monoterpenoids as the
renewable resources and its cyclic and exomethylene structure
totally prevents the Diels−Alder reaction with the RMIs. In this
study, we demonstrated the synthesis of the high-molecular-
weight and alternating copolymers of MCP and the RMIs and
their excellent thermal stability and optical properties, similar to
those for alternating copolymers of olefins and styrenes with
the RMIs. A mechanism for the regiospecific 1,4-propagation of
the MCP monomers via a radical reaction process was
discussed based on the results of DFT calculations as well as
the precise structure determination of the model oligomers.
Figure 12. Possible partial structure and coupling constants between
the methylene and methine protons for isomer I.
RMIs as strong dienophiles for Diels−Alder reactions has been
refused for use as the comonomer for the copolymerization
Scheme 4
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ASSOCIATED CONTENT
* Supporting Information
■
́
́
S
Radical copolymerization results, Diels−Alder reaction results,
solubility of polymers, monomer reactivity ratio determination,
NMR spectra, DSC curves, refractive index data, TG curves,
(30) Fringuelli, F.; Taticchi, A. The Diels-Alder Reaction; John Wiley
& Sons: West Sussex, U.K., 2002.
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DFT calculation results, Fineman−Ross and Kelen−Tudos
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charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
■
(34) Yoshie, N.; Saito, S.; Oya, N. Polymer 2011, 52, 6074−6079.
(35) Zeng, C.; Seino, H.; Ren, J.; Hatanaka, K.; Yoshie, N.
Macromolecules 2013, 46, 1794−1802.
*(A.M.) Fax: +81-72-254-9292. E-mail: matsumoto@chem.
osakafu-u.ac.jp.
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Notes
The authors declare no competing financial interest.
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