Radical polymerization of 2,5-norbornadienes containing ester groups by AIBN and oxygen gas

Article abstract of DOI:10.1002/pola.27268

New 2,5-norbornadiene-type monomers bearing 1-adamantyl and cyclohexyl ester groups on their 2-position polymerized with azobisisobutyronitrile to form the polymers consisting of two types of polymer unit structures. The major part had a saturated nortricyclene framework, which was formed by 2,6-addition along with intramolecular cyclization on the norbornadiene moiety. The minor part consisted of 2-norbornene-type units constructed via 2,3-addition. A series of norbornadiene-based monomers spontaneously polymerized in the presence of oxygen. Because a radical inhibitor, namely hydroquinone, could suppress this spontaneous reaction, it was indicated that the oxygen-induced polymerization proceeds via free-radical polymerization mechanism. Changing a quantity of provided oxygen gas (O₂) to a norbornadiene monomer significantly affected on polymerization results, in specific, molecular weight of the formed polymer, which indicated that oxygen serves as one of the key reagents for the formation of free-radical initiating species. It was proven that the combination of norbornadiene ethyl ester with O₂ was applicable as a new free-radical initiator for polymerization of methyl methacrylate.

Full text of DOI:10.1002/pola.27268

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Radical Polymerization of 2,5-Norbornadienes Containing Ester Groups by
AIBN and Oxygen Gas
Masashi Shiotsuki, Hironobu Kai, Takeshi Endo
Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka, Fukuoka 820–8555, Japan
Correspondence to: T. Endo (E-mail: tendo@moleng.fuk.kindai.ac.jp)
Received 13 March 2014; accepted 30 May 2014; published online 16 June 2014
DOI: 10.1002/pola.27268
ABSTRACT: New 2,5-norbornadiene-type monomers bearing
1-adamantyl and cyclohexyl ester groups on their 2-position
polymerized with azobisisobutyronitrile to form the polymers
consisting of two types of polymer unit structures. The major
quantity of provided oxygen gas (O2) to a norbornadiene
monomer significantly affected on polymerization results, in
specific, molecular weight of the formed polymer, which indi-
cated that oxygen serves as one of the key reagents for the for-
mation of free-radical initiating species. It was proven that the
combination of norbornadiene ethyl ester with O2 was applica-
part had
a saturated nortricyclene framework, which was
formed by 2,6-addition along with intramolecular cyclization on
the norbornadiene moiety. The minor part consisted of
2-norbornene-type units constructed via 2,3-addition. A series
of norbornadiene-based monomers spontaneously polymerized
in the presence of oxygen. Because a radical inhibitor, namely
hydroquinone, could suppress this spontaneous reaction, it
was indicated that the oxygen-induced polymerization pro-
ceeds via free-radical polymerization mechanism. Changing a
ble as a new free-radical initiator for polymerization of methyl
C
V
methacrylate.
2014 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2014, 52, 2528–2536
KEYWORDS: initiators; multicyclic structure; NMR; 2,5-norborna-
diene; oxygen-induced polymerization; radical polymerization
Graham’s pioneering work displayed that free-radical poly-
merization of the NBD monomer bearing ethyl ester group
(1-Et, Scheme 1) could incorporate such an unsaturated nor-
bornene unit formed by 2,3-addition, and that it may cause
cross-linking to result in gelation with high conversion of
polymerization.1 Mizuno et al. revealed that the same 2,3-
addition could partly take place in the cationic polymeriza-
tion of simple NBD, proven by the 13C CP-MAS NMR analy-
INTRODUCTION 2,5-Norbornadiene (NBD) and its deriva-
tives show unique polymerization behavior caused by their
rigid multicyclic structure with non-conjugated double ole-
finic moieties. Radical,1–4 cationic,5–12 and anionic12 poly-
merizations of NBD monomers mainly proceed via
consecutive vinyl-addition reactions of the two CAC dou-
ble bonds along with cyclopropanation, which eventually
forms a multicyclic nortricyclene structure in the polymer
units. It is called as the 2,6-addition. This characteristic
structure of poly(NBD)s contributes their remarkable
properties such as high transparency and dry etch resist-
sis.10 Niu and Frechet showed the only case in which the
ꢀ
2,3-addition and resulting cross-linking can be suppressed if
the NBD monomers bearing bulky substituent’s such as tert-
butyl (t-Bu) or trimethylsilyl (TMS) ester groups were
applied.3 This shed light on the chemistry of poly(NBD)s
from the viewpoints of control of their polymerization by
means of arranging structures of NBD monomers or modify-
ing polymerization conditions. However, there have still been
a limited number of studies with narrow range of NBD
monomers. This led us to the thorough investigation with a
wider range of NBD ester monomers. Herein we report free-
radical polymerizations of novel NBD monomers bearing
1-adamantyl and cyclohexyl esters (1-Ad and 1-Cy, respec-
tively, Scheme 1). On the course of the current research, we
also found that the combination of an NBD ester and oxygen
gas (O2) constructs a new free-radical polymerization system,
ꢀ
ance. Frechet’s group demonstrated the applications of the
related polymers as photoresist materials.3 Polymerization
of NBDs possibly takes the other route, namely 2,3-addi-
tion, in which only one of the two olefinic bonds of NBDs
is participating in addition polymerization. Therefore, the
repeating unit given in 2,3-addition has a 2-norbornene-
type framework with an unreacted olefinic moiety.
Although nortricyclenes contain a highly strained cyclopro-
pane moiety in their frameworks, it was reported that
they are unexpectedly thermally stable. The thermal stabil-
ity of nortricyclene is sometimes beyond those of corre-
sponding norbornene, which was elucidated by Roberts
et al. and von Rauge Schleyer et al.13,14
C
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5973 inert (MS) for the volatiles given from poly(1-Et) and
poly(1-Me) at 315 ꢀC simulating their first stage of decom-
position on the TGA. High resolution mass spectra (HRMS)
were taken on a JEOL JNM-700 mass spectrometer.
Monomer Synthesis
1-Adamantyl 2,5-norbornadiene-2-carboxylate (1-Ad)
The synthesis was carried out by a modified method based
on the reports.3,22 For the synthesis of 1-adamantyl propio-
late, a mixture of propiolic acid (2.65 g, 37.7 mmol) and
1-adamantanol (6.08 g, 39.9 mmol) in toluene (60 mL) with
a few drops of sulfuric acid was refluxed in Dean–Stark
apparatus for 12 h. Removing volatiles from the reaction
mixture afforded brown oil, which was subjected to the col-
umn chromatography (silica gel) eluted by hexane and then
hexane:AcOEt (10/1, v/v) to isolate 1-adamantyl propiolate
(79% yield). 1H NMR (CDCl3): d 2.73 (s, HCBC, 1H), 2.17
(brs, CH at 3-Ad, 3H), 2.12 (s, CH2 at 2-Ad, 6H), 1.64 (virtual
triplet [AB quartet], J 5 13.2 Hz, CH2 at 4-Ad, 6H). 13C NMR
(CDCl3) d 151.3 (C@O), 84.4 (HCBC), 76.1 (HCBC), 72.2
(quaternary C at 1-Ad), 41.4 (CH2 at 2-Ad), 36.2 (CH2 at 4-
Ad), 31.0 (CH at 3-Ad). The obtained 1-adamantyl propiolate
(2.19 g, 10.7 mmol) was made to react with freshly prepared
SCHEME 1 Polymerization of NBD derivatives bearing alkyl
ester groups.
which is applicable to polymerization of methyl methacrylate
(MMA). As for the related polymerization system initiated
with O2, organoboranes are well known,11,12,15–20 whereas the
combination of the other compounds with O2 is extremely
rare.21
EXPERIMENTAL
ꢀ
cyclopentadiene (5.1 g, 77.2 mmol) in a sealed tube at 80 C
for 2 days. From the reaction mixture, all volatiles were
removed under vacuum (20 Pa) with heating at 140 ꢀC to
give a crude product. The obtained brown oil was then sub-
jected to column chromatography (silica gel) eluted by hexa-
ne:AcOEt (20/1, v/v). Evaporation of the solvents gave 1-Ad
(1.88 g, 64% yield) as an air-sensitive colorless oil. 1H NMR
(CDCl3) d 7.49 (d, J 5 3.0 Hz, olefinic HC, 1H), 6.89 (dd,
J 5 5.0 and 2.8 Hz, olefinic HC, 1H), 6.70 (dd, J 5 5.0 and 3.2
Hz, olefinic HC, 1H), 3.82 (brs, bridgehead, 1H), 3.65 (brs,
bridgehead, 1H), 2,15 (brs, CH at 3-Ad, 3H), 2.12 (s, CH2 at
2-Ad, 6H), 2.09 (AB quartet, apparent J 5 21 and 6.8 Hz,
bridging gem-CH2 of NBD, 2H), 1.65 (virtual triplet [AB quar-
tet], J 5 14.8 Hz, CH2 at 4-Ad, 6H). 13C NMR (CDCl3) d 164.3
(C@O), 154.3 (olefinic CH), 151.4 (quaternary olefinic C),
143.9 (olefinic CH), 141.9 (olefinic CH), 80.3 (quaternary C
at 1-Ad), 74.4 (bridging CH2 of NBD), 51.5 (bridgehead CH
of NBD), 50.1 (bridgehead CH of NBD), 41.5 (CH2 at 2-Ad),
36.3 (CH2 at 4-Ad), 30.9 (CH at 3-Ad). HRMS (FAB): calcu-
lated for [C18H22O2 1 H]1 : m/z 271.1698, found: m/z
271.1703.
Materials
Propiolic acid (TCI), methyl propiolate (TCI), 1-adamantanol
(TCI), cyclohexanol (Wako), hydroquinone (HQ) (Wako), and
bromotrichloromethane (TCI) were purchased and used
without further purification. Azobisisobutyronitrile (AIBN)
was purified by recrystallization from methanol. Cyclopenta-
diene was prepared by cracking of dicyclopentadiene at 180
ꢀC. MMA was purified by distillation before use. Monomer
1-Et was prepared by the reported method.1 O2 was used as
received in the gas cylinder. Solvents such as toluene,
anisole, tetrahydrofuran (THF), and N,N-dimethylformamide
(DMF) were distilled by the standard methods.
Measurements
1H (400 MHz) and 13C (100 MHz) NMR were recorded on
JEOL JNM ECS 400 with CDCl3 and tetramethylsilane as a
solvent and an internal standard, respectively. Number-
average and weight-average molecular weights (Mn and Mw,
respectively) and polydispersity indices (Mw/Mn) of the
formed polymers were estimated by size exclusion chroma-
tography (SEC) on Tosoh chromatograph model HLC-
8320GPC equipped with Tosoh TSKgel SuperHM-H styrogel
columns (3.0 mm / 3 15 cm, 3 and 5 mm bead sizes), UV–
vis detector (254 nm) and refractive index (RI) detector,
under the following conditions: THF as the eluent, 40 ꢀC, a
flow rate of 0.6 mL/min, calibrated with polystyrene stand-
ards. Thermal gravimetric analysis (TGA) was performed on
Seiko Instrument TG-DTA 6200 using aluminum pan under a
Cyclohexyl 2,5-norbornadiene-2-carboxylate (1-Cy)
Synthesis of this compound was carried out in the same
manner for monomer 1-Ad by using cyclohexylalcohol. Cyclo-
hexyl propiolate (a precursor of 1-Cy, 98% yield): 1H NMR
(CDCl3) d 4.85 (tt, J 5 9.1 and 3.7 Hz, CH of cHex, 1H), 2.84
(s, HCBC, 1H), 1.88 (m, cHex, 2H), 1.74 (m, cHex, 2H), 1.56–
1.18 (m, cHex, 6H). 13C NMR (CDCl3) d 152.4 (C@O), 75.4
(HCBC), 75.2 (HCBC), 74.1 (CH of cHex), 31.4 (cHex), 25.3
(cHex), 23.7 (cHex). Monomer 1-Cy (85% yield): 1H NMR
(CDCl3) d 7.58 (d, J 5 3.2 Hz, olefinic CH, 1H), 6.89 (dd,
J 5 5.1 and 3.2 Hz, olefinic CH, 1H), 6.69 (dd, J 5 5.0 and 3.2
Hz, olefinic CH, 1H), 4.78 (tt, J 5 12.4 and 3.7 Hz, CH of cHex,
1H), 3.87 (brm, bridgehead CH, 1H), 3.67 (brm, bridgehead
ꢀ
50 mL/min N2 flow at a heating rate of 10 C/min. Differen-
tial scanning calorimetry (DSC) was carried out with Seiko
Instrument DSC-6200 using aluminumꢀpan under a 20 mL/
min N2 flow at the heating rate of 10 C/min. Pyrolysis GC–
MS was performed on Japan Analytical Industry model JPS-
330 and Agilent Technologies Inc. model GC6890A (GC) and
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TABLE 1 Results of Polymerization of NBD Monomersa
Polymer
Initiatorb
Yield (%)
Mn
PDIc
c
Run
Monomer
Atmosphere
Solvent
Time (h)
1
1-Ad
N2
N2
N2
N2
N2
N2
air
N2
N2
air
N2
N2
Toluene
DMF
24
24
48
24
48
24
24
24
24
24
24
24
AIBN
AIBN
AIBN
AIBN
AIBN
AIBN
–
72
70
80
32
50
87
50
66
61
50
55
60
5,000
5,000
4,500
5,400
5,600
ndd
2.0
2.0
2.2
1.8
2.0
ndd
4.5
2.4
2.8
4.5
7.8
5.4
2
3
DMF
4
Anisole
Anisole
(Bulk)
5
6
7
(Bulk)
17,300
8,200
13,300
17,300
16,284
21,300
8
1-Cy
Toluene
Anisole
(Bulk)
AIBN
AIBN
–
9
10
11
12
1-Me
1-Et
Anisole
Anisole
AIBN
AIBN
a
c
Conditions: 50 wt % monomer solution [except for the bulk polymer-
ization (runs 6, 7 and 10)], 60ꢀ C, 24 h, and under N2.
Estimated by GPC (THF, PSt standards).
Not determined because of insolubility of the polymer in organic
d
b
2 mol % based on monomer quantity.
solvents.
CH, 1H), 2.12 (dt, J 5 6.4 and 1.8 Hz, bridging gem-CHH of
NBD, 1H), 2.08 (virtual d, apparent J 5 6.4 Hz, bridging gem-
CHH of NBD, 1H), 1.83 (brm, cHex, 2H), 1.71 (brm, cHex,
2H), 1.54-1.20 (m, cHex, 6H). 13C NMR (CDCl3) d 164.5
(C@O), 155.1 (olefinic CH), 150.3 (quaternary olefinic C),
143.9 (olefinic CH), 141.8 (olefinic CH), 74.4 (bridging CH2
of NBD), 72.3 (CH of cHex), 51.6 (bridgehead CH of NBD),
50.1 (bridgehead CH of NBD), 31.8 (cHex), 25.6 (cHex), 23.8
(cHex). HRMS (FAB): calculated for [C14H18O2 1 H1]: m/z
219.1385, found: m/z 219.1382.
Polymerization of 1-Et with O2 Gas
A solution of monomer 1-Et was placed in a Schlenk tube
filled with N2 gas. After closing a stop cock, O2 gas was
injected by a gas-tight syringe into the closed system
through an equipped rubber septum. The polymerization
was performed under the condition as shown in Table 2. The
reaction was quenched by addition of large quantity of
MeOH into the reactor to produce the formed polymer as
white precipitate. Following filtration and vacuum drying the
corresponding polymer was obtained.
Methyl 2,5-norbornadiene-2-carboxylate (1-Me)
Reaction of 1-Et and BrCCl3 in the Presence of Oxygen
Compound 1-Et (328 mg, 2.0 mmol) and BrCCl3 (1.59 g, 8.0
mmol) were mixed in a Schlenk tube under N2. With a gas-
tight syringe, O2 gas (0.96 mL in standard condition at
The synthesis was performed in the same manner as for
monomer 1-Ad using methyl propiolate (16% yield). 1H
NMR (CDCl3) d 7.60 (d, J 5 3.3 Hz, olefinic CH, 1H), 6.86 (dd,
J 5 5.0 and 3.2 Hz, olefinic CH, 1H), 6.83 (dd, J 5 5.0 and 3.2
Hz, olefinic CH, 1H), 3.85 (brs, bridgehead CH, 1H), 3.68 (s,
Me, 3H), 3.67 (brs, bridgehead CH, 1H), 2.08 (AB quartet,
J 5 11.9 and 6.4 Hz, bridging CH2, 2H). 13C NMR (CDCl3) d
165.4 (C@O), 155.9 (olefinic CH), 149.5 (quaternary olefinic
C), 143.8 (olefinic CH), 141.8, (olefinic CH), 74.4 (bridging
CH2 of NBD), 51.6 (bridgehead CH of NBD), 51.4 (Me), 50.1
(bridgehead CH of NBD). HRMS (EI): calculated for C9H10O2:
m/z 150.0681, found: m/z 150.0682.
ꢀ
21 C, 0.04 mmol, 2 mmol%) was ꢀinjected in to the reaction
system. Heating the mixture at 70 C for 17 h resulted in the
formation of several compounds. The conversion of starting
1-Et and the yields of the formed 2a and 2b were deter-
1
mined by H NMR spectra based on the assignments carried
out to the practically isolated 2a and 2b (each included two
diastereomers) by column chromatography eluted by hexa-
ne:AcOEt (20/1, v/v). Compound 2a (composed of two
stereo isomers): 1H NMR (stereo isomer #1, CDCl3): d 4.19–
4.06 (m, OCH2, 2H), 3.99 (brs, BrCH, 1H), 3.12 (d, J 5 1.3 Hz,
HCCCl3, 1H), 2.72 (virtual quintet, J 5 1.5 Hz, HCCl3, 1H),
2.44 (dq, J 5 5.0 and 0.9 Hz, BrCH-CH on cyclopropane moi-
ety, 1H), 2.31 (dt, J 5 5.0 and 1.4 Hz, CH on cyclopropane
moiety, 1H), 2.57 (d, J 5 12.4 Hz, CHH, 1H), 2.11 (d, J 5 12.4
Hz, CHH, 1H), 1.22 (t, J 5 6.9 Hz, CH3, 3H). 13C NMR (stereo
isomer #1, CDCl3): d 169.3 (C@O), 98.8 (CCl3), 63.4 (HCCCl3),
61.2 (OCH2), 51.8 (CHBr), 46.3 (HBrCACHACHCCl3), 35.6
(quaternary CAC@O), 34.1 (CH of cyclopropane moiety),
28.3 (bridging CH2), 23.3 (CH of cyclopropane moiety), 14.4
Polymerization of NBD Esters with AIBN
A stock solution of monomer 1-Ad, 1-Cy, 1-Me, or 1-Et was
mixed with a solution of AIBN, followed by degassing by
three consecutive freeze-pump-thaw cycles. After filling N2,
the polymerization was performed under the reaction condi-
tion as stated in Table 1. The formed polymer was isolated
by pouring the reaction solution into large amount of ace-
tone for poly(1-Ad) or methanol for the other polymers. The
obtained white precipitate was collected by filtration and
then dried under vacuum.
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TABLE 2 Effect of O2 Quantity on the O2-Induced Polymerization of 1-Eta
Polymer
Yieldb (%)
Mn
PDIc
c
Run
Solvent
O2 (mol %)
1
2
3
4
5
6
7
8
9
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
CHCl3
0
0
–
–
1
40
44
48
53
55
5
65,400
37,000
21,000
15,400
11,400
7,000
4.8
3.9
2.8
2.6
2.3
1.5
1.7
1.8
2
4
6
10
d
–
2
2
39
44
4,200
THF
7,400
a
c
Conditions: 50 wt % monomer solution, 60ꢀ C, 24 h, and under N2.
Methanol-insoluble part.
Estimated by GPC (THF, PSt standards).
Under O2 atmosphere (with an O2 balloon).
b
d
(CH3). 1H NMR (stereo isomer #2, CDCl3): d 4.27–4.11
(m, OCH2, 2H), 4.05 (d, J 5 1.4 Hz, HCCCl3, 1H), 3.97
(t, J 5 1.4 Hz, BrCH, 1H), 2.70 (virtual quintet, J 5 1.3 Hz,
BrHCCHCCl3, 1H), 2.33 (m, CH on cyclopropane moiety, 1H),
2.30 (dt, J 5 11.5 and 1.4 Hz, bridging CHH, 1H), 2.27 (m, CH
on cyclopropane moiety, 1H), 1.59 (brd, J 5 11.9 Hz, bridging
CHH, 1H), 1.27 (t, J 5 6.9 Hz, CH3). 13C NMR (stereo isomer
#2, CDCl3): d 169.9 (C@O), 99.6 (CCl3), 63.9 (HCCCl3), 61.1
(OCH2), 53.0 (BrCH), 46.3 (HBrCACHACHCCl3), 34.9 (CH of
cyclopropane moiety), 33.4 (quaternary CAC@O), 29.6
(bridging CH2), 25.5 (CH of cyclopropane moiety), 14.2
(CH3). HRMS (EI): calculated for C11H12BrCl3O2: m/z
361.9062, found: m/z 361.9055.
calculated for C11H12BrCl3O2 : m/z 361.9062, found: m/z
361.9070.
RESULTS AND DISCUSSION
Polymerization of the New NBD Ester Monomers, 1-Ad
and 1-Cy, with AIBN
First, we attempted to polymerize a novel NBD mono-
mer 1-Ad (Scheme 1), having a bulky substituent, 1-
adamantyl (Ad) ester group. The polymerization results
of 1-Ad are summarized in Table 1. Monomer 1-Ad was
successfully polymerized in solution with AIBN to afford
poly(1-Ad) with 4500–5600 of Mn in 32%–80% yields
(runs 1–5). The difference of polymerization solvents did
not show significant effect on the molecular weight of
the polymer. In bulk polymerization (run 6), the highest
polymer yield (87%) was achieved among runs 1–6, but
the formed polymer became insoluble in contrast to the
case of the solution polymerizations. This result is iden-
tical to the report by Grahams treating polymerization of
the NBD monomer having an ethyl ester group in terms
of the insolubility of the polymer in the case of high
conversion of a NBD monomer, possibly indicating cross-
linking.1 Several reports mentioned that 2-substituted
NBD derivatives having alkyl ester groups also spontane-
ously polymerize under air, in specific, in the presence
of O2, especially in the case of NBD esters.1,3,4 Monomer
1-Ad also spontaneously polymerized under air without
AIBN to afford the polymer with Mn of 17,300 in 50%
yield (run 7). The PDI of the obtained polymer under
air was relatively large, possibly suggesting cross-linking.
Compound 2b (composed of two stereo isomers): 1H NMR
(stereo isomer #1 of 2b, CDCl3): d 4.38–4.19 (m, OCH2, 2H),
2.82 (brm, BrCACHACHCCl3, 1H), 2.80 (brs, CHCCl3, 1H),
2.52(dt, J 5 12.4 and 3.5 Hz, bridging CHH, 1H), 2.24 (dt,
J 5 11.9 and 1.4 Hz, bridging CHH, 1H), 2.13 (dd, J 5 10.5
and 0.9 Hz, CH on cyclopropane moiety, 1H), 1.78 (m, 2CHs
on cyclopropane moiety, 2H), 1.31 (t, J 5 7.3 Hz, CH3, 3H).
13C NMR (stereo isomer #1 of 2b, CDCl3): d 168.7 (C@O),
99.2 (CCl3), 68.6 (BrCC@O), 63.1 (CHCCl3), 62.2 (OCH2), 45.1
(CBrACHACHCCl3), 28.8 (bridging CH2), 23.3 (CH on cyclo-
propane moiety), 19.1 (CH on cyclopropane moiety), 15.4
(CH on cyclopropane moiety), 14.0 (CH3). 1H NMR (stereo
isomer #2 of 2b, CDCl3): d 4.24 (q, J 5 6.8 Hz, OCH2, 2H),
3.76 (brs, CHCCl3, 1H), 2.83 (brs, BrCACHACHCCl3, 1H),
2.37 (dt, J 5 12.4 and 2.0 Hz, bridging CHH, 1H), 2.11 (td,
J 5 5.5 and 0.9 Hz, CH on cyclopropane moiety, 1H), 1.88 (tt,
J 5 5.0 and 0.9 Hz, CH on cyclopropane moiety, 1H), 1.68 (t,
J 5 5.5 Hz, CH on cyclopropane moiety, 1H), 1.43 (d, J 5 12.4
and 1.5 Hz, bridging CHH, 1H), 1.30 (t, J 5 7.3 Hz, CH3). 13C
NMR (stereo isomer #2 of 2b, CDCl3): d 168.5 (CAO), 99.9
(CCl3), 69.1 (BrCC@O), 65.2 (CHCCl3), 62.2 (OCH2), 44.6
(CBrACHACHCCl3), 27.6 (bridging CH2), 22.9 (CH on cyclo-
propane moiety), 17.9 (CH on cyclopropane moiety), 15.8
(CH on cyclopropane moiety), 13.9 (CH3). HRMS (EI):
Another new NBD monomer having a cyclohexyl ester group
(1-Cy, Scheme 2) was polymerized with AIBN under the
same reaction condition as for 1-Ad to give the correspond-
ing polymer [poly(1-Cy)] with higher molecular weight than
poly(1-Ad) (runs 8 and 9). In the same case as the other
NBD esters, monomer 1-Cy was spontaneously polymerized
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and alternatively broad peaks newly emerged at aliphatic
region on the spectrum of poly(1-Ad) [Fig. 1 (bottom)].
Small multiplets at 6.5–5.4 ppm were also observed, indicat-
ing that poly(1-Ad) also incorporated 2-norbornene-type
repeating unit as a minor moiety. This result is consistent
with the report of polymerization of analogous norborna-
diene monomers.1–3 As the reported results by Niu et al.,3
NBD monomer containing bulky t-Bu or TMS ester groups
also polymerized with AIBN to give the polymer composed
solely of nortricyclene-type unit. They insisted that the steri-
cally bulky substituents at the 2-position of NBD suppress
intermolecular coupling (propagating via 2,3-addition) and
facilitate intramolecular cyclization followed by 2,6-addition
polymerization. Although the Ad group of 1-Ad is considered
as bulky as t-Bu group, the 1H NMR spectrum result of
poly(1-Ad) rather indicated the presence of 2-norbornylene-
type units formed via 2,3-addition, whose occupancy is about
7% (thus, m:n 5 93:7, Fig. 1) calculated by integration of the
corresponding peaks. As shown in Figure 2, all of the other
obtained polymers also showed similar small multiplets in
the olefinic region with almost identical occupancy of the
minor units [poly(1-Cy): 7%, poly(1-Me): 6%, and poly(1-
Et): 8%].
SCHEME 2 Proposed mechanism of thermal decomposition of
the minor repeating unit.
under air without addition of AIBN (run 10). Under the
condition at 60 ꢀC for 24 h, the corresponding polymer,
poly(1-Cy) was given with large PDI of 4.5. For comparison,
NBD monomers bearing a relatively small Me or Et groups
(1-Me and 1-Et) were polymerized in the identical condition
with AIBN in anisole, but the PDIs of the formed polymers
resulted to be apparently larger than those of poly(1-Ad)
and poly(1-Cy) (runs 11 and 12). Assumingly this is consid-
ered that it is caused by cross-linking reaction, as Niu et al.
discussed in their article about polymerization of analogous
monomers bearing bulky substituents.3
Thermal Properties
The results of thermal gravimetric analysis (TGA) of poly(1-
Ad), poly(1-Cy), poly(1-Me), and poly(1-Et) are shown in
Figure 3. All the current polymers showed the multistage
ꢀ
ꢀ
decomposition starting with the first stage at 200 C–220 C
by losing a few percent of weight. Further heating induced
the second stage with considerable weight loss. Their 5%
weight loss temperature (Td5) were about 420 ꢀC, 317 ꢀC,
364 ꢀC, and 349 ꢀC for poly(1-Ad), poly(1-Cy), poly(1-Me),
and poly(1-Et), respectively. Below these temperatures, all
The 1H NMR result of poly(1-Ad) revealed that the polymer
consisted mainly of nortricyclene-type repeating unit formed
via 2,6-addition cyclopolymerization (Fig. 1). The three ole-
finic and two bridgehead protons of monomer 1-Ad [peaks
a–e, respectively, Fig. 1 (top)] disappeared by polymerization,
FIGURE 1 1H NMR spectra of monomer 1-Ad (top) and poly(1-Ad) (bottom) [sample: run 3 in Table 1, measured in CDCl3,
s: CHCl3, *: H2O, r: residual hexane].
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FIGURE 2 1H NMR spectra of (a) poly(1-Cy), (b) poly(1-Me), and (c) poly(1-Et) [samples: runs 9, 11, and 12 in Table 1, measured in
CDCl3, *: H2O (omitted)].
the obtained polymers did not show any glass transition
temperatures according to the results of their DSC analyses.
As representative samples, poly(1-Me) and poly(1-Et) were
subjected on pyrolysis GC–MS. The formation of cyclopenta-
diene was confirmed when poly(1-Me) and poly(1-Et) were
inhibitor for free-radical polymerization, the phenomenon
strongly indicates that the spontaneous polymerization of
these NBD ester monomers would proceed in free-radical
polymerization mechanism. Unfortunately, it has not been
successful to detect any significant species formed by the
ꢀ
pyrolyzed at 315 C, simulating the first stage decomposition
on TGA. It strongly indicates that the first stage decomposi-
tion of the current polymers should be caused by the retro-
Diels–Alder reaction of the minor 2-norbornene-type repeat-
ing unit. Because the weight losses at the first stage on TGA
are corresponding to the calculated values based on the
occupancy of the minor repeating unit given by 1H NMR, it
is suggested that the minor 2-norbornene-type repeating
units are formed through 2,3-addition (not 5,6-addition)
with high regioselectivity.
Study of Spontaneous Polymerization of Monomer 1-Et
in the Presence of O2
It has already been known that NBD monomers bearing Et,
t-Bu, and TMS ester groups spontaneously polymerizes
under aerobic condition without addition of any free-radical
initiator.1,3 With 1-Et, spontaneous polymerization behavior
was studied in detail. Firstly, it was found that the spontane-
ous polymerization of 1-Et in the presence of O2 was sup-
pressed when a slight quantity (a few mol % to 1-Et) of HQ
was mixed with 1-Et. Because HQ is generally utilized as an
FIGURE 3 TGA curves of (a) poly(1-Ad), (b) poly(1-Cy), (c)
poly(1-Me), and (d) poly(1-Et) [samples: runs 3, 9, 11, and 12 in
Table 1, respectively].
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SCHEME 3 The reported reactions of the NBD derivatives bearing a trimethylsiloxy group and molecular O2.
reaction of the NBD esters and O2 as long as we experi-
mented with the present monomers. There is a report treat-
ing with the reaction of NBDs bearing a trimethylsiloxy
group with molecular O2, which gives several kinds of perox-
ides as shown in Scheme 3.23 It is guessed that analogous
peroxides may be related to the formation of initiating spe-
cies in the present spontaneous polymerization of the NBD
ester monomers.
1,3
ꢀ
According to Graham’s and Frechet’s respective reports,
bromotrichloromethane reacted with NBDs in the presence
of a free-radical source such as BPO or AIBN to form nortri-
cyclene products. With addition of catalytic quantity of O2
gas instead of such radical source compounds, the combina-
tion of NBD ester, 1-Et, and bromotrichloromethane also
gave nortricyclenes as shown in Scheme 4. This result
strongly indicates that NBD esters in the presence of O2 also
induce the formation free-radical species. Judging from the
1H NMR result of the products, the 3,5-adduct (2a) was the
most major product (59% NMR yield) along with the 2,6-
adduct (2b) as minor one (8%), suggesting that free-radical
addition polymerization of NBDs induced by O2 principally
proceeds through internal ring-closing reaction by consump-
tion of both double bonds of the NBD monomers.
SCHEME 5 O2-induced free-radical polymerization of 1-Et.
gas-tight syringe. Polymerization solvent gave significant
effects on the polymerization and that anisole resulted in the
highest molecular weight of the polymer (runs 3, 8, and 9).
Although the polymer yields were limited in middle for all
the cases in anisole solution, the molecular weight and the
PDI of the obtained polymer apparently tended to decrease
when more O2 was applied. Relationship between the
inverse of the O2 quantity (mol %) and Mn of the formed
poly(1-Et) were described in Figure 4. The Mn of poly(1-Et)
increased in direct inverse proportion to O2 quantity, sug-
gesting that O2 should be related to the formation reaction
of radical initiating species in the current polymerization
system. This presumption is supported by the
Effect of quantity of O2 gas on polymerization behavior was
studied using monomer 1-Et (Scheme 5). Table 2 shows the
results of polymerization of 1-Et initiated by injecting a cata-
lytic amount of O2-gas in the range of 1–10 mol % based on
monomer quantity. The experiments were carried out using
a rubber-sealed Schlenk tube holding the monomer solution,
into which accurate quantity of O2 gas was injected with a
SCHEME 4 Addition of bromotrichloromethane to 1-Et in the
FIGURE 4 Relationship between 1/[O2 quantity] (mol %21) and
Mn of the formed poly(1-Et).
presence of O2.
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SCHEME 6 Polymerization of MMA initiated by the mixture of 1-Et and O2.
a
TABLE 3 Polymerization of MMA Initiated by the Mixture of 1-Et and O2
Feed Ratio (mol %)
Polymer
PDIb
b
Run
1-Et
MMA
Solvent
Yield (wt %)
Mn
1
2
3
4
5
6
7
8
0
100
98
95
90
98
95
90
80
(Bulk)
(Bulk)
0
–
–
2
63
68
65
0
252,800
197,600
177,500
–
2.8
3.4
3.8
–
d
5
–
d
10
2
–
Anisole
Anisole
Anisole
Anisole
5
22
33
36
102,500
92,500
77,800
2.5
2.3
2.5
10
20
a
b
Conditions: bulk or 50 wt % monomer solution (for solution polymer-
ization in anisole), 60ꢀ C, 24 h, under N2, with catalytic amount of O2
gas (1 mol % based on the total monomer quantity [1-Et 1 MMA]).
Estimated by GPC (THF, PSt standards).
polymerization result examined under the pure O2 atmos-
phere (run 7). Under this condition, the yield of poly(1-Et)
was only 5% and its Mn considerably decreased, which indi-
cates that the reaction of 1-Et and O2 is dominant com-
pared with polymer propagation reaction with the in situ
formed radical initiating species. According to the results
shown above, it is assumed that reaction of 1-Et with O2
molecules should provide a certain radical initiating spe-
cies, which may be derived from some peroxides as investi-
gated by Jefford et al. (Scheme 3).23
(Scheme 6). With a catalytic amount of 1-Et (2–10 mol %)
and O2 gas (1 mol % based on a total monomer quantity of
1-Et and MMA), polymerization of MMA in bulk condition
successfully gave polymers (runs 2–4, Table 3). Reducing the
quantity of 1-Et ratio toward MMA did not give a substantial
effect on polymer yield, but molecular weight of the obtained
polymer tended to increase along with increasing the MMA
mol ratio. Even in the solution polymerization using anisole,
the identical polymers formed by a catalytic amount of 1-Et
and O2, although the polymer yields were lower than those
in bulk polymerization. The solution polymerization with
2 mol % of 1-Et and 1 mol % of O2 did not afford any poly-
mer as methanol-insoluble part (run 5). GPC curve of the
obtained polymers displayed a unimodal peak with rather
large PDIs (>2.3). As shown in Figure 5, the 1H NMR result
Polymerization of MMA Initiated by the In Situ Formed
Radical Initiating Species Derived From 1-Et and O2
The present results shown above led us to an idea to apply
this O2-triggered polymerization system as a free-radical ini-
tiator to polymerization of the other monomer such MMA
FIGURE 5 The 1H NMR spectra of the obtained polymer (in CDCl3, *: H2O, sample: run 3 in Table 3).
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4 V. Alupei, S. W. Choi, I. C. Alupei, H. Ritter, Polymer 2004, 45,
2111–2117.
of the obtained polymer displayed a typical spectra of poly(-
MMA), indicating that the main part of the polymer is consist
of poly(MMA). The presence of the incorporated poly(1-Et)
moieties was proven by the appearance of small peaks at
about 4.0–4.2 ppm, which is assigned to OCH2 of poly(1-Et)
as shown above in Figure 2(c). The 1H NMR result also
revealed that the content of poly(1-Et) moiety in the
obtained polymer is less than 1% even in the case of 5 mol
% feed of 1-Et (run 3, Table 3), implying that the monomer
reactivity ratio of 1-Et is much less than that of MMA.
5 J. P. Kennedy, J. A. Hinlicky, Polymer 1965, 6, 133–140.
6 M. B. Roller, J. K. Gillham, J. P. Kennedy, J. Appl. Polym.
Sci. 1973, 17, 2223–2233.
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CONCLUSIONS
11 D. E. Bergbreiter, G. F. Xu, Macromolecules 1995, 28, 4756–
4758.
In this article, the polymerization behaviors of NBD esters
with a free-radical initiator or O2 were studied. The NMR
and thermal analyses confirmed that the formed polymer
derived from various NBD esters are composed of two types
of repeating units such as nortricyclene and norbornene
frameworks. Based on the study of spontaneous polymeriza-
tion of NBD esters in the presence of O2, a novel O2-trig-
gered free-radical initiating system were displayed and
actually applicable to polymerization of MMA.
12 T. C. Chung, W. Janvikul, H. L. Lu, J. Am. Chem. Soc. 1996,
118, 705–706.
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16 A. V. Fedorov, A. A. Ermoshkin, A. Mejiritski, D. C. Neckers,
Macromolecules 2007, 40, 3554–3560.
17 M. F. Sonnenschein, S. P. Webb, P. E. Kastl, D. J. Arriola,
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ACKNOWLEDGMENTS
This work was financially supported by JSR Corporation Inc.
The authors are grateful to Mr. Tsuji Shoei and Mr. Takuma
Ebata (JSR) for the measurement of pyrolysis GC-MS analysis of
the polymers.
18 F. J. Welch, J. Polym. Sci. 1962, 61, 243–252.
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