Sequence-controlled radical polymerization of N-substituted maleimides with 1-methylenebenzocycloalkanes and the characterization of the obtained copolymers with excellent thermal resistance and transparency

Article abstract of DOI:10.1021/ma400454u

The radical copolymerization of N-methyl, ethyl, n-butyl, and 2-ethylhexylmaleimides (RMIs) with 1-methylenebenzocycloalkanes (BCms) was carried out to fabricate thermally stable and transparent polymer materials. The copolymerization reactivity of BCms significantly depended on the carbon number of the ring structure of BCms. The analyses of the comonomer-copolymer composition curves and theoretical calculations results revealed that the sequence-controlled copolymerization of RMIs with BCms occurred in alternating and 2:1 fashions according to the BCms reactivity, which depends on the coplanarity of the exomethylene moiety and the benzene ring. The alternating and high-molecular-weight copolymers of BCms and RMIs exhibited excellent thermal and optical properties. We have demonstrated that the radical copolymerization of RMIs is useful not only as the method for high-performance transparent polymer production but also as the tool for the fundamental research of radical polymerization mechanism.

Full text of DOI:10.1021/ma400454u

Article
pubs.acs.org/Macromolecules
Sequence-Controlled Radical Polymerization of N‑Substituted
Maleimides with 1‑Methylenebenzocycloalkanes and the
Characterization of the Obtained Copolymers with Excellent Thermal
Resistance and Transparency
Miki Hisano,† Kyota Takeda,† Tsutomu Takashima,‡ Zhengzhe Jin,‡ Akira Shiibashi,‡
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
‡Central Technical Research Laboratory, Research & Development Division, JX Nippon Oil & Energy Corporation, 8, Chidoricho,
Naka-ku, Yokohama 231-0815, Japan
S
* Supporting Information
ABSTRACT: The radical copolymerization of N-methyl,
ethyl, n-butyl, and 2-ethylhexylmaleimides (RMIs) with 1-
methylenebenzocycloalkanes (BCms) was carried out to
fabricate thermally stable and transparent polymer materials.
The copolymerization reactivity of BCms significantly
depended on the carbon number of the ring structure of
BCms. The analyses of the comonomer−copolymer compo-
sition curves and theoretical calculations results revealed that
the sequence-controlled copolymerization of RMIs with BCms
occurred in alternating and 2:1 fashions according to the
BCms reactivity, which depends on the coplanarity of the
exomethylene moiety and the benzene ring. The alternating
and high-molecular-weight copolymers of BCms and RMIs
exhibited excellent thermal and optical properties. We have demonstrated that the radical copolymerization of RMIs is useful not
only as the method for high-performance transparent polymer production but also as the tool for the fundamental research of
radical polymerization mechanism.
alt-IB)] exhibited excellent properties superior to those of the
conventional transparent polymers such as poly(methyl
methacrylate) and polycarbonates.30−32 Electron-donating
monomers, such as IB and other olefins, styrenes, and vinyl
ethers, are favorably combined with the electron-accepting
RMIs for the facile production of alternating copolymers.33−40
Omayu et al. previously demonstrated that exomethylene
monomers had a copolymerization reactivity higher than those
of cycloalkenes with an internal olefin structure during the
copolymerization with the RMIs, and that the introduction of
bulky substituents, such as the alicyclic group next to the main
chain, effectively increased the Tg values of the obtained
copolymers.41 Recently, it was demonstrated that the RMI
monomers are useful for the sequence regulated radical
copolymerization in alternating and 2:1-sequence controlled
fashions.42−45 Sequence-controlled radical polymerization has
INTRODUCTION
■
N-substituted maleimides (RMIs) readily polymerize in the
presence of a radical initiator to give a high-molecular-weight
polymer with excellent thermal stabilities,1−9 although they are
one of the 1,2-disubstituted ethylenic monomers including a
high steric hindrance between the propagating radical and the
reacting monomer during polymerization.10−13 The high glass
transition temperatures (Tg) of the poly(RMI)s originate in the
rigid poly(substituted methylene) structure, which has no
methylene spacer as a flexible joint in their main chain,10,14−21
and the high onset temperatures of decomposition arise from a
robust imide-ring structure included in the repeating units of
the polymers.22−27 Poly(RMI)s are difficult to use as films and
sheets because of their brittleness, while the RMI monomers
have often been used as comonomers in order to modify the
thermal properties of conventional polymers. In 1996, Doi et al.
reported that the alternating copolymerization of the RMIs
with isobutene (IB) produced high-molecular-weight copoly-
mers with an excellent thermal stability, transparency, high
strength, and high modulus.28,29 For example, the alternating
copolymer of N-methylmaleimide (MMI) and IB [poly(MMI-
Received: March 4, 2013
Revised: March 29, 2013
Published: April 16, 2013
© 2013 American Chemical Society
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where nF, nD, and nC represent the refractive indices at 486, 589, and
656 nm, respectively.
attracted significant attention in the research fields of polymer
synthesis and radical polymerization.46−52
In this study, the radical copolymerization of the RMIs with
1-methylenebenzocycloalkanes (BCms) was carried out in
order to fabricate the maleimide copolymer with more superior
thermal properties (Scheme 1). The BCm monomers are α-
Theoretical calculations were carried out using density functional
theory (DFT) at the B3LYP/6-31G*//B3LYP/6-31G* for the
activation energy, and at the B3LYP/6-311+G**//B3LYP/6-31G*
level for the energy difference of the reactant and the product. A
Spartan’10 software package (Wave function, Inc.) was used for all the
calculations.
Scheme 1
Materials. 2,2′-Azobis(isobutyronitrile) (AIBN) (Wako Pure
Chemical Industries, Ltd.) was recrystallized from methanol. N-
Methylmaleimide (MMI, mp 97 °C), N-n-butylmaleimide (BMI), and
N-(2-ethylhexyl)maleimide (EHMI) were synthesized according to the
methods reported in previous papers.58,59 N-Ethylmaleimide (EMI,
mp 46 °C) (Tokyo Chemical Industry Co., Ltd.) was used as received
without further purification. 1-Methylenebenzocyclobutane (BC4) was
synthesized according to the method reported in a previous paper.57 1-
Methyleneindane (BC5), 1-methylenetetralin (BC6), and 1-methyl-
enebenzocycloheptane (BC7) were synthesized by the Wittig reaction
of the respective precursor ketones, i.e., 1-indanone, 1-tetralon, and 1-
benzosuberone, which were commercially available (Tokyo Chemical
Industry Co., Ltd.).
Synthesis of BCms. A four-necked flask was charged with
methyltriphenylphosphonium bromide (32.0 g, 90 mmol) (Wako
Pure Chemical Industries, Ltd.) suspended in dry THF (200 mL)
under argon. Potassium tert-butoxide (14.6 g, 0.13 mol) (Tokyo
Chemical Industry Co., Ltd.) was then added, and the yellow mixture
was stirred at ambient temperature for 30 min. The mixture was
cooled to 0 °C, and the corresponding ketone, i.e., 1-indanone (11.9 g,
90 mmol), 1-tetralon (13.2 g, 90 mmol), or 1-benzosuberon (14.4 g,
90 mmol) was dropwise added. After being warmed to room
temperature, the mixture was stirred overnight. A small amount of
water (2−3 mL) was added to quench the excess reagent, and the
mixture was filtered through silica gel. The silica was rinsed with THF,
and the solvent was evaporated under reduced pressure. The residue
was poured into n-hexane (ca. 1 L) to precipitate the triphenylphos-
phine oxide as the byproduct. The mixture was filtered and the solvent
was evaporated under reduced pressure. The residual crude yellow oil
was purified by silica gel column chromatography with n-hexane as the
eluent. The obtained BCms were colorless oils.
substituted styrene derivatives with a reactive exomethylene
group. The m indicates the carbon number of the ring structure
of the BCm molecules. It has been reported that cyclic 1,1-
disubstituted ethylene monomers with an exomethylene moiety
have a high polymerization reactivity during the radical homo-
and copolymerizations.53−56 The homopolymerization of the
BCms and the alternating copolymerization with maleic
anhydride were previously reported,56,57 but there is no report
on the copolymerization of the BCms with the RMIs in the
literature. During the course of our studies on the
copolymerization of the RMIs with olefins, we found that the
sequence-controlled radical copolymerization occurred and the
copolymers consisting of the AB- and AAB-repeating units
(alternating and 2:1 sequence controls, respectively) were
produced depending on the m number of the BCms. We now
discuss the ring-size effects of the BCms on the copolymeriza-
tion reactivity and the sequence control based on the results of
the monomer reactivity analysis during copolymerization as
well as their theoretical calculations using model reactions. The
thermal and optical properties of the copolymers were also
investigated.
1
BC4. H NMR (300 MHz, CDCl3): δ 7.25−7.16 (m, C6H4, 4H),
5.31 (t, CCH2, 1H), 4.94 (s, CCH2, 1H), 3.63 (s, CH2, 2H). 13
C
NMR (75 MHz, CDCl3): δ 145.2 (CCH2), 145.3, 144.8, 128.8,
127.2, 122.8, and 118.5 (C6H4), 103.2 (CCH2), 38.8 (CH2).
1
BC5. Yield: 53.2%. H NMR (300 MHz, CDCl3): δ 7.54−7.46 (m,
C6H4, 1H), 7.30−7.15 (m, C6H4, 3H), 5.45 (t, CCH2, 1H), 5.03 (t,
CCH2, 1H), 3.02−2.92 (m, CH2CCH2CH2C6H4, 2H), 2.84−
2.75 (m, CH2CCH2CH2C6H4, 2H). 13C NMR (CDCl3, 75 MHz): δ
151.0 (CCH2), 147.1, 141.4, 128.6, 126.8, 125.8, and 121.0 (C6H4),
102.8 (CCH2), 31.6 (CH2CCH2CH2C6H4), 30.5 (CH2
CCH2CH2C6H4).
EXPERIMENTAL SECTION
■
1
BC6. Yield: 60.3%. H NMR (300 MHz, CDCl3): δ 7.68−7.60 (m,
General Procedures. The NMR spectra were recorded in CDCl3
using a Bruker AV300N spectrometer. The number-average molecular
weight (Mn), weight-average molecular weight (Mw), and polydisper-
sity (Mw/Mn) were determined by size exclusion chromatography
(SEC) with tetrahydrofuran (THF) as the eluent using a Tosoh
CCPD RE-8020 system and calibration with standard polystyrenes.
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 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 and cooling rates of 10 °C/min. The
UV−vis spectra were recorded using a JASCO V-550 spectropho-
tometer. The refractive index and Abbe number (νD) were determined
using an Abbe-type refractometer (Atago Co., Ltd., Model DR-M2).
The νD value is represented by the following equation.
C6H4, 1H), 7.21−7.17 (m, C6H4, 3H), 5.47 (d, CCH2, 1H), 4.95
(dd, CCH2, 1H), 2.85 (t, CH2CCH2CH2CH2C6H4, 2H) 2.55 (m,
C H 2  C C H 2 C H 2 C H 2 C 6 H 4 , 2 H ) , 1 . 8 9 ( m , C H 2 
CCH2CH2CH2C6H4, 2H). 13C NMR (75 MHz, CDCl3): δ 143.4
(CCH2), 137.3, 134.7, 129.2, 127.6, 125.9, and 124.2 (C6H4), 107.9
(CCH2), 33.2, 30.4, and 23.8 (CH2).
1
BC7. Yield: 38.9%. H NMR (300 MHz, CDCl3): δ 7.22−7.07 (m,
C6H4, 4H), 5.10 (dt, CCH2, 1H), 4.98 (d, CCH2, 1H), 2.76 (t,
CH2 CCH2 CH2 CH2 CH2 C6 H4 , 2H), 2.39 (t, CH2 
CCH2 CH2 CH2 CH2 C6 H4 , 2H), 1.90−1.67 (m, CH2 
CCH2CH2CH2CH2C6H4, 4H). 13C NMR (75 MHz, CDCl3): δ
152.8 (CCH2), 144.2, 140.2, 128.9, 128.1, 127.0, and 126.1 (C6H4),
113.7 (CCH2), 36.5, 36.3, 31.5, and 27.3 (CH2).
Polymerization Procedures. RMI, BCm, AIBN, and 1,2-
dichloroethane (for solution polymerization) 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, and then the
polymerization mixture was poured into a large amount of methanol to
precipitate the copolymers with BC4 and BC5, a methanol/water
νD = (nD − 1)/(nF − nC)
(1)
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a
Table 1. Synthesis and Thermal Properties of Poly(MMI-co-BCm)s
b
BCm
yield (%)
Mn/103
Mw/103
Mw/Mn
BCm mol % in copolymer
Td5 (°C)
Tmax (°C)
Tg (°C)
BC4
BC5
BC6
BC7
89.9
36.6
1.4
24.9
10.3
2.6
67.2
18.5
3.1
2.7
1.6
1.3
1.7
48.2
51.3
47.3
25.6
346
345
c
368
385
c
143
212
200
0.5
1.0
1.6
c
c
c
a
b
1
Copolymerization conditions: [MMI] = [BCm] = 0.2 mol/L, [AIBN] = 10 mmol/L in 1,2-dichloroethane at 60 °C for 5 h. Determined by H
c
NMR spectroscopy. Not determined.
a
mixture (4/1 in volume) for the copolymers with BC6, and n-hexane
for the copolymers with BC7. The copolymers were filtered out,
washed, and then dried in vacuo. The yield of the copolymers was
gravimetrically determined. The copolymers were purified by repeated
precipitation procedures using chloroform as the solvent and the
appropriate precipitants. The composition of the obtained copolymers
Table 2. Synthesis and Thermal Properties of Poly(BCm)s
[BCm]
(mol/L)
yield Mn/ Mw/ Mw/
Td5
(°C)
Tmax
Tg
BCm
(%)
103
103
Mn
(°C) (°C)
BC4
BC5
BC6
BC7
BC7
1.0
1.0
10.3
4.0
0
4.6
9.5
6.5
1.4
2.1
305
228
397
339
133
119
19.6
1
was determined by H NMR spectroscopy.
1.0
Poly(MMI-alt-BC4). 1H NMR (300 MHz, CDCl3): δ 7.80−5.72 (m,
C6H4, 4H), 4.36−0.65 (m, CHCH, CH2, and CH3, 9H). 13C NMR
(75 MHz, CDCl3): δ 179.2 and 177.2 (CO), 146.3, 141.4, 128.8,
127.3, 123.5, 122.1, and 120.5 (C6H4), 55.2−45.5 (CC6H4, CH2, and
CH), 45.2−25.4 (CH2C6H4 and CH3). IR (KBr): νCO (cm−1) 1699.
Poly(MMI-alt-BC5). 1H NMR (300 MHz, CDCl3): δ 7.68−5.41 (m,
C6H4, 4H), 4.44−0.20 (m, CHCH, CH2, and CH3, 11H). 13C NMR
(75 MHz, CDCl3): δ 179.4 and 177.7 (CO), 152.3, 145.0, and
128.0−123.2 (C6H4), 62.5−49.1 (CH2 and CH), 40.3 (CH), 36.8
(CH2CH2C6H4), 30.9 (CH2C6H4), 24.3 (CH3). IR (KBr): νCO
(cm−1) 1698.
1.0
0
bulk
0
a
Polymerization conditions: [AIBN] = 10 mmol/L in 1,2-dichloro-
ethane or bulk, at 60 °C for 20 h.
143 °C, similar to the previously reported values for poly(MMI-
alt-IB).30 It is noted that the Tg values were over 200 °C for the
copolymers with BC5 and BC6 in spite of their decreased
molecular weights by the increase in the m number. The Td5
and Tg values for poly(BC4) and poly(BC5) as the
homopolymers were lower than those for the copolymers
with MMI.
Determination of Monomer Reactivity Ratios. To
clarify the monomer reactivity depending on the ring size of
the BCms, the copolymerization of BMI with the BCms (m =
5−7) was carried out over various compositions in the feed,
and the monomer reactivity ratios were determined based on
the analysis of the obtained comonomer-copolymer composi-
tion curves. The liquid BMI monomer enabled us to perform a
copolymerization in bulk, and consequently, we obtained
sufficiently high-molecular-weight copolymers to accurately
estimate their copolymer composition, while MMI with a
melting point at 97 °C was only used for the solution
polymerization. The copolymerization with the highly reactive
BCms was carried out in 1,2-dichloroethane under a high
monomer concentration ([BMI] + [BCm] = 2 mol/L). The
results are summarized in Table 3. The comonomer-copolymer
composition curves are shown in Figure 1.
All the copolymers were recovered at a low conversion to
allow the analysis of the copolymerization parameters using the
Mayo−Lewis equation (eq 2). The copolymer compositions
were determined by 1H NMR spectroscopy. For the
copolymerizations of BMI with BC5 and BC6 (Scheme 2),
the monomer reactivity ratios, r1 and r2, were successfully
determined by the terminal unit model (Scheme 3). The r1 and
r2 values, which are defined by eqs 3 and 4, respectively, were
determined by the nonlinear least-squares curve fitting
method,60 as shown in Figure 1. The monomer reactivity
ratios were also estimated by the Fineman−Ross61 and Kelen-
Tudos62 methods, but these methods gave scattered plots and
the comonomer-copolymer composition curves based on the
calculated values partially failed to fit the experimental data
(Table S1 and Figure S8−S10, Supporting Information). In
contrast, the curve fitting method resulted in good results over
the whole range of compositions.
Poly(MMI-alt-BC6). 1H NMR (300 MHz, CDCl3): δ 7.98−4.71 (m,
C6H4, 4H), 4.47−0.10 (m, CHCH, CH2, and CH3, 13H). 13C NMR
(75 MHz, CDCl3): δ 179.5−176.8 (CO), 140.3, 136.6, 129.8, and
126.3 (C6H4), 60.7−53.1 (CH2 and CH), 44.9−41.4 (CC6H4 and
CH), 31.4 and 30.2 (CH2CH2CH2C6H4 and CH2CH2CH2C6H4), 24.6
(CH3), 18.5 (CH2CH2CH2C6H4). IR (KBr): νCO (cm−1) 1697.
Poly(MMI-per-MMI-per-BC7). 1H NMR (300 MHz, CDCl3): δ
7.99−6.08 (m, C6H4, 4H), 4.86−0.33 (m, CHCH, CH2, and CH3,
15H).
RESULTS AND DISCUSSION
■
Copolymerization of MMI with BCms. The radical
copolymerization of MMI with the BCms (m = 4−7) was
carried out in the presence of AIBN as the radical initiator at 60
°C for 5 h under equimolar comonomer conditions ([MMI] =
[BCm] = 0.2 mol/L in 1,2-dichloroethane). All the copolymers
were isolated as colorless powders after reprecipitation. The
copolymer yield, the Mn, Mw, and Mw/Mn values determined by
SEC measurement, and the Td5, Tmax, and Tg values determined
by TG and DSC measurements are summarized in Table 1.
The BC4 monomer gave a copolymer with the highest
molecular weight in the highest copolymer yield. The yield
and the Mn value varied in the ranges of 0.5−90% and 1−25 ×
103, respectively, sensitive to the structure of the BCms. Both
values drastically decreased with an increase in the ring size of
the BCms. The NMR spectroscopy revealed the formation of
copolymers with an equimolar composition during the
copolymerization of MMI with all the BCms except for BC7
(Figures S1−S7 in Supporting Information). The polymer-
ization reactivity significantly depended on the BCm structure,
and similar results were observed during the homopolymeriza-
tion (Table 2). The BC4 and BC5 monomers gave the
corresponding homopolymers with an Mn of 5−10 × 103 in low
yields (4−10%), while the BCms with a higher m number
provided no polymer even during bulk polymerization.
̈
̃
The obtained copolymers exhibited excellent thermal
properties. The Td5 and Tmax values were over 345 and 365
°C, respectively. The Tg value of the copolymers with BC4 was
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Table 3. Radical Copolymerization of BMI (M2) with BCms
(M1) at 60 °C
Scheme 3
a
BCm mol %
in
comonomer
BCm mol %
in
copolymer
BCm
(M1)
time yield
Mn
Mw
Mw/
Mn
(h)
(%) /104 /104
a
BC5
8.6
17.5
26.7
36.1
46.0
56.0
66.5
77.2
88.4
9.5
0.5
0.5
0.5
0.5
0.5
1
7.7
7.4
7.4
5.6
5.0
8.0
4.9
5.3
2.0
3.4
7.3
4.6
4.8
7.9
4.8
4.0
1.0
0.8
1.8
6.4
4.6
3.6
2.4
1.9
3.5
5.8
1.5
7.1
7.1
5.3
4.4
3.4
3.0
1.9
1.2
0.7
1.0
0.9
0.7
0.8
0.6
0.6
0.5
0.4
0.1
1.0
0.8
0.8
0.7
0.7
0.7
0.6
0.5
0.5
13.5
13.5
10.9
8.9
6.6
5.6
3.4
1.9
0.9
1.6
1.3
1.0
1.0
0.9
0.7
0.6
0.5
0.2
1.3
1.0
0.9
0.9
0.8
0.8
0.7
0.6
0.6
1.9
1.9
2.1
2.0
1.8
1.9
1.7
1.6
1.3
1.5
1.5
1.3
1.4
1.2
1.2
1.2
1.2
1.4
1.3
1.3
1.2
1.2
1.2
1.2
1.2
1.2
1.2
48.2
50.5
51.9
49.5
50.5
55.0
50.7
53.1
57.3
42.5
45.4
45.5
46.5
47.0
47.6
51.0
53.0
52.8
18.0
26.1
32.3
32.9
32.5
39.4
39.3
41.6
39.9
d[M1]
d[M2]
[M1](r [M1] + [M2])
[M2](r2[M2] + [M1])
1
=
1
(2)
2
2
r = k11/k12
(3)
(4)
a
1
BC6
2
19.0
28.7
38.5
48.5
58.5
68.4
79.0
89.4
10.3
20.5
30.7
40.8
50.8
60.8
70.7
80.3
90.3
5
r2 = k22/k21
5
5
where M1 and M2 are BCm and BMI, respectively.
12
12
24
48
48
1
Both the r1 and r2 values determined for the copolymeriza-
tions with BC5 and BC6 were close to zero; r1 = 0.046 and r2 =
0.0043 for the BC5(M1)−BMI(M2) system and r1 = 0.019 and
r2 = 0.047 for the BC6(M1)−BMI(M2) system, as summarized
in Table 4. This indicates the production of the copolymers
with highly controlled alternating repeating structures. The
value of 1/r2 represents the relative reactivity of the BMI radical
to the BCm monomers. The 1/r2 value for the BC5
copolymerization (1/r2 = 233) was ten times higher than that
for BC6 (1/r2 = 21). The lower reactivity of BC6 was reflected
in the yield and molecular weight of the resulting copolymers
shown in Table 3. For the copolymerization of BMI with BC7,
the copolymerization behavior was entirely different from the
alternating one observed for the cases with BC5 and BC6. The
monomer reactivity ratios were determined using the
penultimate unit model42−45,63,64 for the BMI−BC7 copoly-
merization, while appropriate values were not obtained using
the terminal model. Similar penultimate group effects were
reported for the copolymerization of RMIs with various olefins
containing bulky substitutents, leading to the formation of the
2:1 sequence controlled copolymers.45
b
BC7
5
5
5
5
5
12
48
48
a
[BMI] + [BCm] = 2.0 mol/L, [AIBN] = 20 mmol/L in 1,2-
b
dichloroethane. ([BMI] + [BC7])/[AIBN] = 100/1 in bulk.
In general, the penultimate unit model involves the use of
eight propagation reactions (Scheme 4) with four monomer
reactivity ratios (eqs 5−8)
r = k111/k112
(5)
(6)
(7)
(8)
11
r = k122/k121
12
r21 = k211/k212
r22 = k222/k221
Figure 1. Comonomer-copolymer composition curves for the radical
⧫
□
●
copolymerization of BMI (M2) with BC5 ( ), BC6 ( ), and BC7 ( )
(M1) in 1,2-dichloroethane at 60 °C.
With these four monomer reactivity ratios, the copolymer
composition is represented as eq 9.
Scheme 2
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Table 4. Monomer Reactivity Ratios for Radical Copolymerization of RMIs (M2) with BCms and Other Olefins (M1) at 60 °C
olefin (M1)
RMI (M2)
solvent
r1
r2
r11
r12
r21
r22
ref
BC5
BMI
1,2-dichloroethane
1,2-dichloroethane
bulk
0.046
0.019
0.0043
0.047
this work
this work
this work
42
BC6
BMI
BC7
BMI
0
0
0
0
2.92
0
0
0
0
0.252
0.0042
0.20
limonene
β-pinene
β-pinene
PhMI
MMI
PhMI
PhC(CF3)2OH
1,2-dichloroethane
1,2-dichloroethane
18.7
7.09
10.3
43
0.236
44
Scheme 4
Scheme 5
r F + 1
r21F + 1
11
1 + r21F
(
)
f =
r
r22 + F
r12 + F
12
1 +
(
)
F
(9)
where F = [M1]/[M2] and f = d[M1]/d[M2]. In this case, the
11 and r21 values can be regarded as zero due to the lack of the
energy differences between the reactant and the product for the
homopropagations and the ΔE12 and ΔE21 values correspond
to those for the cross-propagations. The results are summarized
in Table 5.
r
homopolymerization ability of BC7 (Table 2). Therefore, eq 9
is reformulated as eq 10.
1 − 2f
F(1 − f )
f
1
r
12
=
r22 −
F2(1 − f )
Table 5. Activation Energies (Eact) and Energy Differences in
Reactant and Product (ΔE) for Propagations during
(10)
a
The monomer reactivity ratios, r12 and r22, were determined
as r12 = 2.9 and r22 = 0.25 by the curve fitting method. The
values of r12 > 1 and r22 < 1 agreed well with the fact that the
AAB-type periodic copolymer with a 2:1 composition of BMI
and BC7, i.e., poly(BMI-per-BMI-per-BC7), was predominantly
produced. The penultimate group effect was indispensable for
the production of the 2:1 sequence-controlled copolymers, but
the origin of the effect has not been revealed yet.63,64 Both the
polar and steric effects are important to determine the
monomer reactivity, and a significant solvent effect was also
pointed out for the RMI−olefin copolymerization sys-
tems.28,44,65,66 In this study, we found that the coplanarity of
the exomethylene moiety and the benzene ring of BCms
significantly determined the reactivity of the monomer and the
corresponding propagating radicals as follows.
Theoretical Calculations. In order to discuss the
copolymerization reactivity of the BCms, we estimated the
activation energies (Eact) and energy differences between the
reactant and the product (ΔE) by the DFT calculations using
model reactions for the homo- and cross-propagations of MMI
and the BCms (Scheme 5). The Eact‑22 value represents the
activation energy for the homopropagation of MMI and the
Eact‑12 and Eact‑21 values are activation energies for the cross-
propagations. Similarly, the ΔE11 and ΔE22 values indicate
Copolymerization of MMI with BCms (Unit is kJ/mol)
cross-propagations
homopropagation
BCm
Eact‑12
ΔE12
Eact‑21
ΔE21
ΔE11
BC4
BC5
BC6
BC7
18.3
29.4
38.8
74.5
−61.3
−27.7
−14.4
−2.5
9.2
8.2
−85.4
−97.9
−97.3
−71.6
−60.9
−35.5
24.4
7.1
33.4
19.7
a
Calculated by the DFT method at the B3LYP/6-31G*//B3LYP/6-
31G* and B3LYP/6-311+G**//B3LYP/6-31G* levels of theory for
the Eact and ΔE values, respectively, using the model reactions in
Scheme 5. For the MMI homopropagation, the Eact‑22 and ΔE22 values
were calculated to be 20.9 and −79.1 kcal/mol, respectively.
The calculated Eact and ΔE values were closely related to
each other; i.e., the lower the Eact values, the negatively larger
the ΔE values. This tendency satisfies a common rule accepted
for many reactions. The Eact‑21 values (7.1−9.2 kJ/mol) for the
cross-propagation of the MMI radical to BCms (m = 4−6) were
lower than the Eact‑22 value (20.9 kJ/mol) for the MMI
homopropagation, while the Eact‑21 value (33.4 kJ/mol) for BC7
was far greater than the Eact‑22 one. The ΔE21 and ΔE22 values
showed similar results; the ΔE21 values were in the range of
−85.4 to −97.9 kJ/mol for BC4 to BC6 and ΔE21 = −71.6 kJ/
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Macromolecules
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mol for BC7, while the ΔE22 was −79.1 kJ/mol for the MMI
homopropagation. Typical energy diagrams are shown in
Figure 2. This result indicates that the cross-propagation of
is quite consistent with the fact that the copolymerization
reactivity (deduced from the yield and the molecular weight of
the produced copolymers) decreased with an increase in the
ring size of the BCms, as already shown in Table 1. The ΔE11
values calculated for the homopropagation of the BCms also
well agreed with the homopolymerization results in Table 2. No
polymer was produced during the polymerization systems
exhibiting a positive ΔE value, i.e., for the cases of the
endothermic homopropagation of the BCms.
We further investigated the molecular conformations in the
transition state during the cross-propagations in order to
discuss the reactivities of the BCm monomers and their radicals
(m = 5−7). The molecular structures in the transition state are
shown in Figure 3. The carbon-to-carbon distances between the
carbon atoms making a new covalent bond during the reaction
were almost the same (RCC = 2.22−2.37 Å) in the transition
states. No steric interaction was observed during the reaction of
the MMI radical to the BCms (Figure 3a−c). This is consistent
with the low Eact‑21 values and the fast addition of the MMI
radical to the BCms. During the reactions of the BCm radicals
to the MMI monomer (Figure 3d−f), the distances between
the closest hydrogen atoms on the maleimide carbon-to-carbon
double bond and the cyclic methylene moieties of the BCm
radicals decreased according to the increased ring size. This
suggests the enhanced steric repulsion during the transition
state leading to an increase in the Eact values with an increase in
the m number.
It was revealed that the conformation of the exomethylene
moieties varied from a planar structure for BC5 and BC6 to a
nonplanar structure for BC7, as noted by the dihedral angles
(∠C1−C2−C3−C4) in Table 6. In this table, the values
estimated based on the 13C coupling constants in the NMR
spectra are also shown.67 The BC7 monomer had large dihedral
angles in the ground and transition states; ∠C1−C2−C3−C4 =
55.3 and 44.3°, respectively. The electron density on the C1
carbon of the exomethylene group of BC7 was slightly lower
than those for BC5 and BC6 with a coplanar molecular
conformation. The deviation from the coplanar structure of the
BCm monomers was reduced by the radical addition due to a
Figure 2. Energy profiles for the homopropagation of the MMI radical
and the cross-propagation to the BCms (m = 5 and 7). BC4 and BC6
provide diagrams similar to that for BC5. M and M• indicate the MMI
monomer and radical, respectively.
the MMI radical predominantly occurs rather than the
homopropagation of MMI for the cases of BCms (m = 4−6).
In contrast, the MMI homopropagation is preferred to the
cross-propagation for the BC7 case. The formation of the
copolymers with the 2:1 sequence is based on the reduced
reactivity of the BC7 radical during the cross-propagation to
BMI.
Each Eact‑12 value (18.3−74.5 kJ/mol) was higher than the
corresponding Eact‑21 one (7.1−33.4 kJ/mol) for all the BCms,
indicating that the cross-propagation of the BCm radical to
RMI is the rate-determining step. Therefore, the copolymeriza-
tion rate is significantly affected by the cross-propagation to the
RMI monomers during the copolymerization of the BCms and
RMIs. This suggests the order of the copolymerization
reactivity as follows: BC4 > BC5 > BC6 ≫ BC7. Actually, it
Figure 3. Transition-state structures determined by the DFT calculations for (a) the MMI radical···BC5 (RCC = 2.37 Å), (b) the MMI radical···BC6
(RCC = 2.37 Å), (c) the MMI radical···BC7 (RCC = 2.26 Å), the MMI radical, (d) the BC5 radical···MMI (RCC = 2.26 Å), (e) the BC6 radical···MMI
(RCC = 2.22 Å), and (f) the BC7 radical···MMI (RCC = 2.29 Å). RCC is the distance between the carbons which make a new covalent bond.
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a
Table 6. Dihedral Angles Exhibiting Coplanarity of Exomethylene Group and Benzene Rings of BCms and BCm Radicals
dihedral angles (∠C1−C2−C3−C4, deg)
in the ground state
in the transition state
b
Mulliken atomic charge
BCm
BCm•
MMI−BCm•
BCm•···MMI
MMI•···BCm
c
c
BC5
BC6
BC7
−0.433
−0.433
−0.385
9.3 (1.4)
1.6
6.7
1.4
5.4
26.7
31.8
5.9
6.3
9.2
19.8 (1.8)
c
55.3 (59.0)
34.0
23.1
44.3
a
Dihedral angles (∠C1−C2−C3−C4) are defined as in Figure 4. Calculated by DFT method at the B3LYP/6-31G*//B3LYP/6-31G* levels of
b
c
theory. C1 carbon of the exomethylene moiety of the BCms (see Table S2, Supporting Information). Reference 67.
change in the sp2 carbon to the benzyl-type conjugated radicals,
of which the dihedral angle values decreased to 34.0 and 23.1°
when the adduct radicals, BCm• and MMI−BCm• were
formed, respectively. A smaller deviation from the planar
structure was observed for both the monomers (9.3−19.8°)
and the radicals (1.4−6.7°) of BC5 and BC6. Ring strain can be
released during the radical addition reactions. The
MMI•···BCm in the transition state exhibited intermediate
molecular structures (∠C1−C2−C3−C4 = 6.3−44.3°), as
shown in Table 6. The BCm radicals in the transition state
(BCm···MMI) were revealed to have a molecular structure
entirely different from the BCm radicals in the ground state due
to their late transition state during the slow cross-propagation
reactions as the rate-determining steps.
were high at 55−67% and 9−22 × 104, respectively. The lower
values were obtained during the BC6 copolymerization due to
the reduced monomer reactivity, i.e., 16−18% yields and 0.4−2
× 104 Mw values. All the poly(RMI-alt-BCm)s were confirmed
to be thermally stable and the Td5 and Tmax values were high
enough. The decomposition of the copolymers proceeded via a
one-step reaction and a trace amount of residue was observed
after heating to 420 °C, independent of the N-alkyl substituents
and the m number (Figure 5, see also Figures S11 and S12,
Figure 4. Numbering for the carbons for dihedral angle determination
of the BCms and the related radicals.
Figure 5. TG curves of (―) poly(MMI-alt-BC5), (- - - -)
poly(EHMI-alt-BC5), and (― · ―) poly(MMI-alt-BC6) in a
nitrogen stream at the heating rate of 10 °C/min.
Synthesis and Properties of High-Molecular-Weight
Copolymers. The bulk and solution polymerizations at a high
monomer concentration were carried out to obtain high-
molecular-weight copolymers using BC5 and BC6 as the BCms
and MMI, EMI, BMI, and EHMI as the RMIs. The results are
summarized in Table 7. The yield and Mw values of the
copolymers produced during the copolymerization with BC5
Supporting Information, for details). The Tg values of
poly(MMI-alt-BC5) and poly(MMI-alt-BC6) were 212 and
213 °C, respectively, being higher than the Tg values previously
reported for poly(MMI-alt-IB) (152 °C),30 poly(MMI-alt-
a
Table 7. Thermal and Optical Properties of Poly(RMI-alt-BC5)s and Poly(RMI-alt-BC6)s
([olefin] + [RMI])/
yield
(%)
Mw/
Mw/
Mn
olefin mol % in
copolymer
Td5
(°C)
Tmax
(°C)
Tg
(°C)
b
olefin
RMI (mol/L)
[AIBN]
104
nD
νD
BC5
BC6
MMI(0.2)
EMI(2.0)
BMI(bulk)
EHMI(bulk)
MMI(0.4)
EMI(2.0)
BMI(bullk)
EHMI(bulk)
MMI
40
100
100
100
40
36.6
54.6
66.7
60.6
17.6
16.0
16.3
17.9
1.9
8.5
1.8
2.1
2.1
1.7
1.4
1.5
1.7
1.7
51.3
50.6
50.3
49.3
47.7
51.8
49.3
50.3
50.0
49.5
48.8
345
344
341
330
330
320
317
319
396
334
307
303
454
385
381
381
377
362
359
350
348
212
203
158
109
213
221
194
139
157
178
198
100
140
1.559
1.554
1.543
38
40
43
16.6
21.7
0.4
100
100
100
1.0
2.0
1.9
c
IB
22.0
0.4
1.53
51
d
d
CA5
CA6
MMI
416
395
MMI
0.3
c
poly(MMA)
1.49
1.58
53
29
c
polycarbonate
a
b
Copolymerization conditions: [BCm]/[MMI] = 1/1 in 1,2-dichloroethane or in bulk at 60 °C for 5 h (BC5) or 20 h (BC6). Abbe number: νD =
c
d
(nD − 1)/(nF − nC). The nF, nD, and nC values were determined at 486, 589, and 656 nm, respectively. Reference 30. Reference 41.
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CA5) (178 °C),41 and poly(MMI-alt-CA6) (198 °C),41 of
which the repeating structures are shown in Figure 6. The CA5
Figure 6. Repeating structures and the Tg values of poly(MMI-alt-
BCm)s and poly(MMI-alt-CAm)s.
and CA6 are exomethylene cyclic olefin monomers containing
no benzene ring. These results indicate that the introduction of
the benzene moiety into the position closest to the polymer
main chain effectively increases the Tg values of the polymers.
Furthermore, the copolymers containing the BC6 and CA6 unit
with bulkier structures resulted in the higher Tg values than the
corresponding copolymer containing the BC5 and CA5 units.
The poly(RMI-alt-BCm)s were soluble in many organic
solvents, such as chloroform, THF, toluene, and acetone, and
transparent thin films were obtained by casting the solution and
drying, as shown Figure 7. The film strength, i.e., brittleness,
was dependent on the N-alkyl groups. The flexibility of the
poly(RMI-alt-BCm)s films increased in the order of MMI <
EMI < BMI < EHMI due to the increased free volume. On the
basis of the transmittance experiments of visible light using the
films (ca. 60-μm thickness), the excellent transparency of
poly(BMI-alt-BC5) and poly(EHMI-alt-BC5) was confirmed.
The nD values of the poly(RMI-alt-BCm)s (1.54−1.56) were
higher than that of poly(MMA) and the νD values of the
poly(RMI-alt-BCm)s (38−43) were lower (Table 7). In Figure
7c, the dispersion was fitted by the simplified Cauchy’s formula,
Figure 7. (a) Photographs of poly(EHMI-alt-BC5) film. (b)
Transmittance of poly(RMI-alt-BC5) films (thickness ca. 60 μm) in
the UV and visible regions (―) poly(BMI-alt-BC5) and (- -)
poly(EHMI-alt-BC5). (c) Wavelength dispersion of the poly(EMI-alt-
■
●
▲
BC5) ( ), poly(BMI-alt-BC5) ( ), and poly(EHMI-alt-BC5) ( ).
copolymerization of BMI with the BCms. As a result, highly
alternating copolymers were produced during the copolymer-
izations of BMI with BC5 and BC6, while the copolymerization
of BMI with BC7 produced copolymers with a 2:1 composition.
The monomer reactivity ratios were determined by the terminal
and penultimate group models based on the obtained
comonomer-copolymer composition curves. The DFT calcu-
lations for the propagation reactions were carried out in order
to clarify the difference in the copolymerization reactivity and
fashion of the copolymerization, i.e., alternating or 2:1 sequence
controlled, depending on the ring size of the BCms. It was
revealed that the coplanarity of the exomethylene group and
the benzene ring of the BCms was important for the
determination of the reactivity of the BCm monomers and
the corresponding radicals. We also investigated the polymer
properties, such as the thermal stability and transparency. The
introduction of the benzocycloalkane units neighboring the
main chain improved the Tg values of the copolymers, as was
expected. We concluded that the high-molecular-weight and
high-Tg poly(RMI-alt-BCm)s were produced during the
copolymerization of BC5 and BC6 combined with the RMIs
with an appropriate N-alkyl substituent and the transparent and
nλ = n∞ + D/λ2
(11)
where n∞ and D are the estimated refractive index at the infinite
wavelength and the dispersion coefficient. The n∞ values were
1.537, 1.533, and 1.524, and the D values were 7781, 7335, and
6752 nm2, for poly(EMI-alt-BC5), poly(BMI-alt-BC5), and
poly(EHMI-alt-BC5), respectively. The poly(MMI-alt-BCm)s
with the highest Tg values provided transparent films, but they
were too brittle to determine the nD and νD values.
CONCLUSIONS
■
The copolymers of RMI with BCms (m = 4−7) were
synthesized by the radical copolymerization process. The
yield and molecular weight of the resulting copolymers
decreased with an increase in the ring size of the BCm. In
order to investigate the copolymerization reactivity of the
BCms, we determined the monomer reactivity ratios for the
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ASSOCIATED CONTENT
* Supporting Information
NMR spectra, monomer reactivity ratios data, DFT calculation
■
(27) Takeda, K.; Matsumoto, A. Macromol. Chem. Phys. 2010, 211,
782−790.
S
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is available free of charge via the Internet at http://pubs.acs.
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AUTHOR INFORMATION
Corresponding Author
■
(32) Doi, T.; Yukioka, S.; Inoue, H.; Akimoto, A. J. Appl. Polym. Sci.
1997, 63, 925−929.
*(A.M.) Present address: Department of Applied Chemistry,
Graduate School of Engineering, Osaka Prefecture University,
1-1, Gakuen-cho, Naka-ku, Sakai 599-8531, Japan. Phone and
Fax: +81-72-254-9292; E-mail: matsumoto@chem.eng.osakafu-
u.ac.jp.
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The authors declare no competing financial interest.
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