High Anionic Polymerizability of Benzofulvene: New Exo-Methylene Hydrocarbon Monomer

Article abstract of DOI:10.1021/acs.macromol.5b00944

The anionic polymerization of benzofulvene (BF) quantitatively proceeded with various initiators, such as sec-BuLi, diphenylmethyllithium, diphenylmethylpotassium, triphenylmethyllithium, and triphenylmethylpotassium, in THF at -78 °C to give polymers having predicted molecular weights based on the molar ratios between BF and the initiators and narrow molecular weight distributions (MWD, M_w/M_n < 1.1). Although the initiation efficiencies were not quantitative, BF could also be polymerized with benzylmagnesium chloride, potassium tert-butoxide, and the anionic living poly(tert-butyl methacrylate), indicating the high anionic polymerizability. The planar conformation and the polarized electron density of BF obtained by the density functional theory (DFT) calculation supported the observed anionic polymerizability higher than that of 2-phenyl-1,3-butadiene, the acyclic analogue of BF. A series of new block copolymers, polystyrene-b-poly(BF), poly(2-vinylpyridine)-b-poly(BF), and poly(tert-butyl methacrylate)-b-poly(BF), were synthesized by the sequential anionic copolymerization of BF and the comonomers. On the other hand, no polymerization of styrene and 2-vinylpyridine took place with the living poly(BF), indicating the low nucleophilicity of the propagating indenyl anion formed from BF. BF readily underwent the free-radical polymerization with α,α′-azobis(isobutyronitrile), while the observed cationic polymerizability of BF was quite low. ¹H and ¹³C NMR analyses revealed that the repeating units of poly(BF) consisted of a 1,2-addition unit and a 1,4-addition unit without a 3,4-addition unit regardless of the polymerization conditions. The exo-methylene moiety (CH₂=) of BF always participated in the addition polymerization. The addition modes were dependent on the polymerization temperature and not on the solvent or the countercation of the anionic initiator. For instance, polymer obtained with sec-BuLi in THF at 40°C contained 72% of the 1,4-addition unit and 28% of the 1,2-addition unit. Therefore, BF acted as a polymerizable 1,3-diene possessing a fixed transoid framework.

Full text of DOI:10.1021/acs.macromol.5b00944

Article
pubs.acs.org/Macromolecules
High Anionic Polymerizability of Benzofulvene: New Exo-Methylene
Hydrocarbon Monomer
Yuki Kosaka, Susumu Kawauchi, Raita Goseki, and Takashi Ishizone*
Department of Organic and Polymeric Materials, Tokyo Institute of Technology 2-12-1-S1-13 Ohokayama, Meguro-ku, Tokyo
152-8552, Japan
S
* Supporting Information
ABSTRACT: The anionic polymerization of benzofulvene (BF) quantitatively proceeded with various initiators, such as sec-
BuLi, diphenylmethyllithium, diphenylmethylpotassium, triphenylmethyllithium, and triphenylmethylpotassium, in THF at −78
°C to give polymers having predicted molecular weights based on the molar ratios between BF and the initiators and narrow
molecular weight distributions (MWD, Mw/Mn < 1.1). Although the initiation efficiencies were not quantitative, BF could also be
polymerized with benzylmagnesium chloride, potassium tert-butoxide, and the anionic living poly(tert-butyl methacrylate),
indicating the high anionic polymerizability. The planar conformation and the polarized electron density of BF obtained by the
density functional theory (DFT) calculation supported the observed anionic polymerizability higher than that of 2-phenyl-1,3-
butadiene, the acyclic analogue of BF. A series of new block copolymers, polystyrene-b-poly(BF), poly(2-vinylpyridine)-b-
poly(BF), and poly(tert-butyl methacrylate)-b-poly(BF), were synthesized by the sequential anionic copolymerization of BF and
the comonomers. On the other hand, no polymerization of styrene and 2-vinylpyridine took place with the living poly(BF),
indicating the low nucleophilicity of the propagating indenyl anion formed from BF. BF readily underwent the free-radical
1
polymerization with α,α′-azobis(isobutyronitrile), while the observed cationic polymerizability of BF was quite low. H and 13C
NMR analyses revealed that the repeating units of poly(BF) consisted of a 1,2-addition unit and a 1,4-addition unit without a 3,4-
addition unit regardless of the polymerization conditions. The exo-methylene moiety (CH2) of BF always participated in the
addition polymerization. The addition modes were dependent on the polymerization temperature and not on the solvent or the
countercation of the anionic initiator. For instance, polymer obtained with sec-BuLi in THF at 40 °C contained 72% of the 1,4-
addition unit and 28% of the 1,2-addition unit. Therefore, BF acted as a polymerizable 1,3-diene possessing a fixed transoid
framework.
BuLi) and potassium naphthalenide (K-Naph), are necessary to
quantitatively initiate the polymerization of hydrocarbon
monomers. It is noteworthy that the range of the hydrocarbon
monomers capable of the living anionic polymerization is still
limited regardless of the recent progress of this research field.
In our recent communication,3 we proposed a novel
hydrocarbon monomer, benzofulvene (BF, α-methylenein-
dene). We have succeeded in the living anionic polymerization
of BF initiated with sec-BuLi in THF at −78 °C, as illustrated in
Scheme 1.3 The resultant poly(BF)s had predictable molecular
weights from the molar ratios between BF and initiators and
narrow MWDs (Mw/Mn = 1.1). Similarly, we recently found
INTRODUCTION
■
It has been established that conventional hydrocarbon vinyl
monomers, such as styrene, α-methylstyrene, 1,3-butadiene,
and isoprene, undergo the anionic polymerization to provide
stable living polymers with well-defined chain structures
including predicted molecular weights and narrow molecular
weight distributions (MWD, Mw/Mn < 1.1).1 These living
polymerization systems also allow the syntheses of tailored
block copolymers by the sequential copolymerization and end-
functionalized polymers by the suitable termination reactions.
Although the propagating carbanions derived from the
hydrocarbon monomers show extremely high nucleophilicities,
the anionic polymerizability of hydrocarbon monomers is
significantly lower than those of polar monomers including
vinylpyridines, (meth)acrylates, and N,N-dialkylacrylamides.1,2
Typical strong anionic initiators, such as sec-butyllithium (sec-
Received: May 4, 2015
Revised: June 13, 2015
Published: June 26, 2015
© 2015 American Chemical Society
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Scheme 1. Anionic Polymerization of BF with sec-BuLi in THF or in Benzene
that BF underwent the living anionic polymerization with sec-
BuLi in a nonpolar hydrocarbon, such as benzene, at 0−55 °C.4
Based on its chemical structure, BF is a cyclic analogue of 2-
phenyl-1,3-butadiene (2PB)5,6 and a derivative of fulvene,7 5-
methylene-1,3-cyclopentadiene, showing an extremely high
reactivity derived from the planar cross-conjugated π-electron
system including the exo-methylene group.8 BF indeed acted as
a 1,3-butadiene derivative during the anionic polymerization,
since the resulting polymer possessed 1,4- and 1,2-addition
repeating units depending on the polymerization modes
(Scheme 1).3,4 The exo-methylene carbon in BF always took
part in the addition polymerization, and no 3,4-structure was
certainly observed in the repeating unit of the resulting
poly(BF).
Scheme 2. Synthesis of BF from Indene
On the other hand, the 3-phenyl-7-methyl-9 and 2-
ethoxycarbonyl-3-phenyl-substituted BF derivatives10 were
synthesized and polymerized. Interestingly, their polymer-
izations exclusively proceeded in the 1,2-addition mode on
the exo-methylene groups. Thus, their polymerizabilities were
significantly different from that of the nonsubstituted BF, since
the 3,4-carbons in these BF derivatives were substituted with
the bulky groups.9,10 The intriguing polymerizability of the
novel hydrocarbon monomer, BF, has just been discovered
regardless of its simple and attractive fulvene framework. We
herein report the polymerization behavior of BF in more detail.
The various anionic initiators, including a series of anionic
living polymers, are newly employed in order to clarify the
relative reactivity of BF. We also compare the anionic
polymerizability of BF with 2PB in order to discuss the relative
reactivity of the exo-methylene BF monomer with the acyclic
counterpart. The stable conformations and the electron
densities of BF and 2PB are estimated by the density functional
theory (DFT) calculations11−15 to compare the relative
polymerizabilities. The free-radical and cationic polymerizations
of BF are also examined to clarify the reactivity. The
microstructure and the property of the new polymers are of
great interest.
paraformaldehyde, since this reaction was very sensitive to the
molar ratios of the starting compounds. We followed their
experimental procedure to abstract the acidic CH2 proton of
indene with n-BuLi in a mixed solvent of hexane and THF at 0
°C. HMIs were obtained in 60% yield in a high reproducibility,
and the following dehydration produced the BF monomer.
Herein, the difficulty of isolation of BF should be noted. The
resultant BF was a yellow solid and spontaneously polymerized
in the solid state. When we checked the melting point of BF
using a differential scanning calorimetry (DSC) measurement,
we observed only an exothermic peak starting from 70 °C
presumably due to the thermal polymerization of BF. In fact, an
attempt at the vacuum distillation of BF using a rotary pump
with heating (∼0.5 mmHg/80 °C) produced an insoluble
polymeric product. Therefore, we have purified BF by a flash
column silica gel chromatography and completely removed the
solvents in order to form the solid BF. We then degassed the
solidified BF using a rotary pump for 5 min and immediately
diluted it with dry THF. Although this purification was
sufficient to prepare a poly(BF) with controlled molecular
weight by the anionic polymerization, it was preferable to distill
BF from CaH2 under high-vacuum conditions using an oil
diffusion pump (∼10−6 mmHg) on a vacuum line. The distilled
BF was frozen with liquid nitrogen during the course of the
distillation. After the distillation, the isolated BF was
immediately diluted with dry THF, and the resultant yellow
solution was stored at −30 °C. The BF monomer in solution
was rather stable and could be stored in a sealed glass ampule
for several weeks.
RESULTS AND DISCUSSION
■
Monomer Synthesis. BF could be synthesized by the
dehydration of hydroxymethylindenes (HMIs) including 1-
hydroxymethylindene (1HMI) and 3-hydroxymethylindene
(3HMI) using potassium tert-butoxide (tBuOK), as shown in
Scheme 2. In our previous studies, the HMIs were obtained
from the reaction of indenylmagnesium bromide or inden-
yllithium with paraformaldehyde.3,4 In fact, the latter route was
superior to the former one from the viewpoints of yield of the
HMIs and reproducibility. Chong and co-workers reported the
synthesis of 9-hydroxymethylfluorene by the reaction of
paraformaldehyde and fluorenyllithium.16 They used an
equivalent quantity of n-BuLi to fluorene and used 1.1-fold of
Anionic Polymerization of BF and Microstructure of
Poly(BF). The anionic polymerization of BF in THF was
carried out with sec-BuLi under various conditions as shown in
Table 1. Upon the addition of the yellow THF solution of BF
to sec-BuLi in n-heptane, the color of the system immediately
changed to the characteristic orange. The orange color was
maintained during the polymerization and instantaneously
disappeared to afford a colorless solution by quenching with
methanol. This suggests the presence of propagating chain ends
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a
Table 1. Anionic Polymerization of BF in THF
microstructure
(%)
Mn (kg/mol)
b
c
d
run
BF (mmol)
initiator (mmol)
sec-BuLi, 0.0921
temp (°C)
time (h)
calcd
obsd
8.9
13
Mw/Mn
1,2-
1,4-
1
6.35
4.69
8.21
8.04
5.45
3.96
3.24
3.38
6.65
5.91
5.16
6.35
5.58
5.02
4.49
6.59
5.09
6.00
−78
−78
−78
−78
−78
−95
−40
40
15
8.9
12
1.11
1.09
1.03
1.11
1.11
1.11
1.17
2.10
1.08
1.09
1.10
1.05
1.18
1.07
1.11
1.14
3.21
1.35
2
sec-BuLi, 0.0473
sec-BuLi, 0.0645
sec-BuLi, 0.0380
10 min
3
20
16
18
28
15
12
10
41
59
4
1
27
e
5
sec-BuLi, 0.0588/TMEDA, 0.242
sec-BuLi, 0.0499
1
12
43
41
36
28
38
41
41
40
36
44
57
59
64
72
62
59
59
60
64
56
6
30 min
10
7
sec-BuLi, 0.0423
2.5
8.6
10
8
sec-BuLi, 0.0402
15 min
8.3
8.2
12
9
K-Naph, 0.195
−78
−78
−78
−78
−78
−78
−78
−78
−78
−78
1
1
8.8
9.3
15
10
11
12
13
14
15
16
17
18
Ph2CHLi, 0.0808
Ph2CHNa, 0.0452
Ph2CHK, 0.0921
1
15
13
1
11
f
Ph2CHK, 0.0616/18-crown-6, 0.282
1
12
9.5
Ph2CHCs, 0.0465
Ph3CLi, 0.0843
Ph3CK, 0.0884
BzMgCl, 0.163
tBuOK, 0.496
1
11
14
17
2
13
2
9.2
3.6
1.6
9.9
17
1
193
13
43
57
a
b
c
Conversion ∼100%. Mn(calcd) = (MW of monomer) × [M]/[I] + MW of initiator residue. Mn(obsd) was determined by SEC-RALLS equipped
with triple detectors, such as refractive index (RI), light scattering (LS), and viscometer detectors. Mw/Mn was determined by SEC calibration using
polystyrene standards in THF. N,N,N′,N′-Tetramethylethylenediamine. 1,4,7,10,13,16-Hexaoxacyclooctadecane.
d
e
f
Figure 1. SEC curves of poly(BF)s obtained with sec-BuLi in THF at −78 °C after 15 h (A) and at 40 °C after 15 min (B). Peak (A): Mn,calcd = 8900,
Mn,RALLS = 8900, Mw/Mn = 1.11. Peak (B): Mn,calcd = 10 000, Mn,RALLS = 8300, Mw/Mn = 2.10.
and also supports the complete consumption of the yellow BF
monomer. In fact, the polymerization of BF rapidly proceeded
and was completed within 10 min at −78 °C to quantitatively
give a polymer. The poly(BF)s produced at −78 °C (runs 1−4)
possessed molecular weights predictable from the molar ratios
between BF and sec-BuLi and narrow MWDs (Mw/Mn < 1.11).
The SEC curve of poly(BF) maintained a unimodal and narrow
shape even after 15 h, indicating the absence of cross-linking
and chain degradation at −78 °C in THF. The controlled
polymerization of BF also occurred with sec-BuLi in the
presence of 4.1-fold N,N,N′,N′-tetramethylethylenediamine
(TMEDA) as a strong chelating reagent (run 5). Tailored
poly(BF)s were similarly formed with sec-BuLi at −95 and −40
°C in THF (runs 6 and 7). On the other hand, the MWD of
the polymer obtained at 40 °C significantly broadened, and the
polydispersity index, Mw/Mn, increased to 2.10 (run 8). A
temperature lower than −40 °C was preferable to control the
polymerization of BF. At −78 °C for 1 h, potassium
naphthalenide (K-Naph), a radical anion, produced a well-
defined poly(BF) in 100% yield (run 9).
We then employed a series of diphenylmethyl carbanions,
diphenylmethyllithium (Ph2CHLi), diphenylmethylsodium
(Ph2CHNa), diphenylmethylpotassium (Ph2CHK), and diphe-
nylmethylcesium (Ph2CHCs), to examine the effect of the
countercation of the initiators (runs 10−12 and 14). In each
case, the polymerization of BF quantitatively proceeded within
1 h at −78 °C, and the produced poly(BF) had a controlled Mn
and narrow MWD (Mw/Mn = 1.05−1.10). Ph2CHK could
initiate the polymerization of BF in the presence of 4.6-fold
crown ether, 18-crown-6, while the MWD of the resulting
poly(BF) slightly broadened (run 13, Mw/Mn = 1.18). Thus, BF
smoothly underwent the controlled anionic polymerization
with these diphenylmethyl carbanions regardless of the counter
cations.
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Next, the anionic polymerizations using the bulkier
triphenylmethyl carbanions, such as triphenylmethyllithium
(Ph3CLi) and triphenylmethylpotassium (Ph3CK), were
examined in THF at −78 °C for 2 h. Ph3CLi and Ph3CK
also induced the polymerization of BF to provide a polymer
having narrow MWDs and predicted molecular weights in
quantitative yields (runs 15 and 16). These results clearly
indicate that the bulky π-conjugated triphenylmethyl carban-
ions can initiate the polymerization of BF in quantitative
efficiencies similar to strong anionic initiators, such as sec-BuLi
and K-Naph. The control of the chain length of the poly(BF)s
was successfully attained using these bulky nucleophiles. This
observed polymerizability of BF is remarkably higher than those
of other hydrocarbon monomers, such as styrene and isoprene,
since the bulky π-conjugated carbanions are often intact or
show low efficiencies to initiate the conventional hydrocarbon
monomers.
Surprisingly, BF could be initiated with benzylmagnesium
chloride (BzMgCl) and tBuOK at −78 °C in THF (runs 17
and 18), although both nucleophiles cannot effectively initiate
the hydrocarbon monomers, such as styrene and 1,3-diene,
under similar polymerization conditions. In each case, the
polymer yield was quantitative, though the polymerization
behavior was far from a controlled fashion. In fact, the MWDs
were broad and the molecular weights were much higher than
the calculated values. The initiation efficiency of BzMgCl and
tBuOK was roughly estimated to be 2 and 12%, respectively.
Nevertheless, it should be emphasized again that even the low
nucleophilic Grignard reagent and alkoxide anion could initiate
the polymerization of this particular hydrocarbon monomer,
BF. This strongly supports the fact that BF is categorized as a
new class of hydrocarbon monomers from the viewpoint of
anionic polymerizability.1,2
workers reported that 2PB can be initiated by highly reactive
anionic initiators, such as sec-BuLi, n-BuLi, and cumylpotas-
sium, to afford well-defined poly(2PB)s having narrow MWDs
(Mw/Mn < 1.1) and predicted molecular weights.6 Similarly, the
tailored poly(2PB)s were obtained by the quantitative initiation
of K-Naph, Ph2CHLi, and Ph2CHK in THF at −78 °C, as
shown in Table 2 (runs 20−22). On the other hand, the
Table 2. Anionic Polymerization of 2PB in THF at −78 °C
for 24 h
Mn × 10−3
a
2PB
(mmol)
initiator
(mmol)
yield
(%)
b
c
d
run
19
calcd RALLS Mw/Mn
3.12
4.12
5.66
4.16
sec-BuLi,
92
100
100
100
10
13
12
11
18
1.04
1.08
1.05
1.03
0.0403
20
21
22
K-Naph,
0.0960
11
Ph2CHLi,
0.0881
8.5
17
Ph2CHK,
0.0323
23
24
25
26
5.96
3.24
6.49
6.94
Ph3CLi, 0.0852
Ph3CK, 0.0577
BzMgCl, 0.121
39
31
0
3.6
2.3
93
1.23
1.19
82
no polymer
no polymer
tBuOK, 0.626
0
a
b
Methanol-insoluble part. Mn(calcd) = (MW of monomer) × [M]/
c
[I] × (yield of polymer) + MW of initiator residue. Mn(obsd) was
determined by SEC-RALLS equipped with triple detectors, such as
refractive index (RI), light scattering (LS), and viscometer detectors.
d
Mw/Mn was determined by SEC calibration using polystyrene
standards in THF.
poly(2PB)s obtained with Ph3CLi and Ph3CK had rather broad
MWDs with long tailings and molecular weights higher than
the calculated values (runs 23 and 24). In fact, the initiation
efficiencies of Ph3CLi and Ph3CK for 2PB were estimated to be
9.6% and 8.7%, respectively. Furthermore, low nucleophilic
initiators, such as a Grignard reagent (BzMgCl) and alkoxide
(tBuOK), could not initiate 2PB at all in THF at −78 °C,
resulting in the quantitative recovery of the monomer (runs 25
and 26). It is again noteworthy that BF can be readily
polymerized with BzMgCl or tBuOK under similar conditions,
while the initiation efficiency was far from quantitative (Table
1, runs 17 and 18). These results indicate that the anionic
polymerizability of 2PB, the acyclic counterpart of BF, is
certainly lower than that of BF. Nevertheless, 2PB showed an
anionic polymerizability higher than 1,3-butadiene or isoprene
due to the phenyl substituent conjugated with the 1,3-diene
framework. For example, no polymeric product was obtained
by the reaction of isoprene with Ph3CK under similar
conditions, while 2PB underwent the polymerization.17
To theoretically understand such a distinct anionic polymer-
izability between 2PB and BF, we optimized the equilibrium
structures of BF and 2PB in the ground state in vacuo by the
DFT calculation and focused on four factors including the
dihedral angle, bond angle of the sp2 carbon of the exo-
methylene (vinyl) group, atomic charge on the β-carbon of the
vinyl group, and LUMO value, as shown in Table 3. As can be
seen in Figure 2, both monomers showed quite different
geometries. We evaluated two kinds of dihedral angles: one was
a twist angle between the vinyl and phenyl groups (∠C1−C2−
C3−C4), and the other was a twist angle in the 1,3-diene
framework (∠C1−C2−C3′−C4′). In fact, both angle values
Finally, we discuss the microstructures of the obtained
polymers as also shown in Table 1. The microstructures can be
determined by the integral ratio in the 13C NMR spectra of the
methylene signal of poly(BF), as previously reported (Figure
S1).3,4 The exo-methylene (CH2) group in BF was always
consumed during the propagation, and no 3,4-addition mode
was observed in the resulting polymers (Scheme 1). In other
words, the repeating unit of poly(BF) contained only the 1,2-
and 1,4-addition modes regardless of the polymerization
conditions. As can also be seen in Table 1, the 1,4-addition
mode was always a major unit and varied from 56 to 72%. The
effects of the countercation of the initiator and the additive
effect of the chelating reagents, such as TMEDA and crown
ether, were not significant but negligible. In fact, the 1,4-
contents of poly(BF) obtained with sec-BuLi, sec-BuLi/
TMEDA, K-Naph, Ph2CHLi, Ph2CHNa, Ph2CHK, Ph2CHCs,
Ph2CHK/18-crown-6, and BzMgCl at −78 °C ranged from 56
to 64%. On the other hand, the 1,4-addition mode tended to
slightly increase from 59 to 72% by increasing the polymer-
ization temperature from −95 to 40 °C. Thus, BF acted as a
1,3-diene during the addition polymerization, since not only the
exo-methylene group but also the CC linkage in the
cyclopentadiene part of BF participated in the polymerization
(Scheme 1). It is noted that the polymerization temperature
certainly affects the addition mode of the repeating units of
poly(BF).
Anionic Polymerization of 2PB. Next, the anionic
polymerization of 2PB, an acyclic analogue of BF, was
investigated with various initiators employed in Table 1 to
compare their polymerization behaviors. Fujimoto and co-
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a
Table 3. Structural Data, Atomic Charge, and LUMO Value of BF and 2PB
dihedral angle (deg)
bond angle (deg)
b
monomer
∠C1−C2−C3−C4
∠C1−C2−C3′−C4′
∠C3−C2−C3′
atomic charge on C1
LUMO (eV)
BF
0.0
0.0
105.4
119.5
−0.301
−0.346
−0.162
+0.706
2PB
57.3
8.7
a
b
All structures were optimized by ωB97X-D/6-311G(d,p). Natural charges were obtained by NBO analysis.
due to the lower atomic charge. The same tendency was
observed in the calculated LUMO values. The LUMO values of
BF and 2PB were −0.162 and +0.706 eV, respectively. BF is
considered to be more reactive to the nucleophiles because the
energy level of the unoccupied molecular orbital of BF is much
lower than that of 2PB. All these calculated results supported
the experimental observations that the exo-methylene-type BF
had a higher anionic polymerizability than the corresponding
vinyl-type 2PB.
Block Copolymerization of BF. In this section, we
performed the sequential copolymerization of BF with
comonomers possessing different anionic polymerizabilities,
such as styrene, isoprene, 2-vinylpyridine (2VP), 4-cyanostyr-
ene (4CNSt),18 MMA, and tert-butyl methacrylate (tBMA) for
the synthesis of various block copolymers (Table 4). In
addition, the results of the sequential copolymerization
(crossover reaction) will provide the relative reactivity of the
monomers including BF. All the employed comonomers
undergo the living anionic polymerizations to provide polymers
with well-defined chain structures.19 At first, a series of anionic
living polymers of styrene, isoprene, 2VP, 4CNSt, MMA, and
tBMA were synthesized for the macroinitiators for BF in THF.
All the living polymers could initiate the polymerization of BF,
and the copolymers were obtained in quantitative yields (runs
27−32). The Mn values of the block copolymers possessing
poly(BF) segments were estimated from either the RALLS-
Figure 2. Molecular frameworks and most stable structures of BF and
2PB determined by the DFT calculation.
(0°) of BF clearly indicated its complete planar structure and
the effective conjugation between the exo-methylene group and
phenyl ring. On the other hand, the resonance effect between
the vinyl moiety and the phenyl group in 2PB might be
negligible due to the highly twisted conformation (57°), while
the transoid 1,3-diene framework in 2PB maintained an
effective conjugation (8.7°). The bond angles (∠C3−C2−
C3′), which might reflect the distortion at the sp2-conjugated
C2 carbon of BF and 2PB, were 105.4° and 119.5°, respectively.
The former value of BF was apparently smaller than the ideal
value of 120° for the typical sp2-conjugated carbon. It is thus
suggested that the C2 carbon of BF is more strained and
reactive compared to that of 2PB. The strained sp2-conjugated
C2 carbon changes to the less strained sp3 carbon by releasing
the stress, if the polymerization of BF proceeds in the 1,2-
addition mode. The calculated atomic charges on the β carbons
(C1) of the exo-methylene (vinyl) groups in BF and 2PB were
−0.301 and −0.346, respectively. This means that the β carbon
of BF can be more easily attacked by the nucleophiles than 2PB
1
SEC or H NMR measurement. In fact, the resulting novel
block copolymers possessed relatively narrow MWDs (Mw/Mn
= 1.09−1.22) along with predicted molecular weights. Since the
SEC curves of the block copolymers were almost unimodal, the
efficiencies of the crossover reactions from the various living
polymers to BF were estimated to be quantitative.
As a typical example, the polymerization of BF was
performed with a living poly(tBMA), prepared with Ph2CHK
in THF at −78 °C for 2 h (run 32). When BF was added to the
living poly(tBMA) to reinitiate the copolymerization, the color
a
Table 4. Block Copolymerization of BF with Styrene, Isoprene, 2VP, 4CNSt, MMA, and tBMA in THF at −78 °C
b
block copolymer (homopolymer )
Mn (kg/mol)
c
d
run
initiator
sec-BuLi
1st monomer
2nd monomer
calcd
obsd
Mw/Mn
27
28
29
30
styrene
isoprene
2VP
BF
BF
BF
BF
BF
BF
19 (5.0)
23 (11)
19 (5.5)
12 (5.2)
18 (4.7)
30 (6.7)
14 (4.5)
26 (11)
19 (3.2)
11 (3.3)
22 (4.4)
29 (5.5)
1.12 (1.10)
1.14 (1.19)
1.22 (1.10)
1.11 (1.11)
1.09 (1.05)
1.11 (1.10)
K-Naph
Ph2CHLi
Ph2CHK
4CNSt
MMA
tBMA
e
31
sec-BuLi/DPE/LiCl
Ph2CHK
32
e
33
sec-BuLi/LiCl
BF
BF
MMA
tBMA
15 (6.0)
14 (9.6)
18 (6.7)
10 (8.0)
1.18 (1.16)
f
34
Ph2CHLi
1.50 (1.22)
a
b
c
Yield of copolymer was quantitative in each case. Homopolymer was obtained at the first-stage polymerization. Mn(obsd) was determined by
d
SEC-RALLS equipped with triple detectors, such as refractive index (RI), light scattering (LS), and viscometer detectors. Mw/Mn was determined
by SEC calibration using polystyrene standards in THF. Reference 3. Bimodal SEC curve.
e
f
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of the polymerization system rapidly changed from colorless to
orange upon the addition of BF, indicating the occurrence of a
rapid crossover reaction (Scheme 3). A copolymer was
comonomers, such as styrene, isoprene, 2VP, and 4CNSt,
when these monomers were added to the living poly(BF). The
homopolymers of BF, formed during the first stage polymer-
ization, were recovered from the reaction systems along with
the comonomers. This is due to the low nucleophilicity of the
propagating indenyl anion formed from BF (Scheme 3). Since
the BF monomer shows an extremely high anionic polymer-
izability, the resulting propagating carbanion has a very low
nucleophilicity. In contrast, the living poly(BF) could initiate
the polymerization of MMA, an α,β-unsaturated carbonyl
compound showing a high electron affinity, with a quantitative
efficiency to give a well-defined poly(BF)-b-poly(MMA) as
previously reported (run 33).3 On the other hand, the initiation
efficiency of the copolymerization of tBMA was not
quantitative and less than 50% to give a mixture of a block
copolymer and a homopoly(BF), as shown in Figures 3C and
3D (run 34). The relative polymerizability of tBMA might be
lower than that of MMA probably due to the steric effect of the
bulky tert-butyl group.
Scheme 3. Crossover Reaction of BF with Living
Polymethacrylate
Thus, these results strongly indicate the comparable anionic
polymerizability of BF and methacrylates, since the reversible
sequential copolymerization of BF and methacrylates is possible
to produce the block copolymers. In particular, the quantitative
crossover reaction of polymethacrylates to BF is of great
interest from a synthetic viewpoint. This high anionic
polymerizability can be explained by the relative acidity of the
conjugated acids for indene (pKa = 20.1) and ethyl acetate (pKa
= 24.4).20 The less basic π-stabilized indenyl anion of BF forms
via the initiation with the more basic propagating ester enolate
of the polymethacrylates, as shown in Scheme 3. In summary,
the relative anionic polymerizability of BF is estimated from the
results of the homopolymerization and block copolymerization
as follows.19
Figure 3. SEC curves of poly(tBMA) (A, dotted line) and
poly(tBMA)-b-poly(BF) (B, solid line). Peak (A): Mn,calcd = 6500,
Mn,RALLS = 5500, Mw/Mn = 1.10. Peak (B): Mn,calcd = 28 000, Mn,RALLS
= 29 000, Mw/Mn = 1.11 (run 32). SEC curves of poly(BF) (C, dotted
line) and poly(BF)-b-poly(tBMA) (D, solid line). Peak (C): Mn,calcd
=
9500, Mn,RALLS = 8000, Mw/Mn = 1.22. Peak (D): Mn,calcd = 14 000,
Mn,RALLS = 10 000, Mw/Mn = 1.50 (run 34).
relative anionic polymerizability:
MMA ∼ BF > 4CNSt > 2VP > styrene ∼ isoprene
quantitatively obtained after 1 h. Figures 3A and 3B show the
SEC curves of the poly(tBMA) and a poly(tBMA)-b-poly(BF).
The SEC curve of the copolymer shifts toward the higher
molecular weight region from the SEC curve of poly(tBMA)
maintaining a unimodal and narrow MWD. This clearly
indicates the formation of the tailored AB diblock copolymer
of tBMA and BF, poly(tBMA)-b-poly(BF). In other words, a
hydrocarbon monomer, BF, can be polymerized with a low
nucleophilic enolate anion of poly(tBMA) similar to the
previous results using the living poly(MMA).3 This high
electrophilicity of BF is in good accordance with the
polymerization results observed in the former section.
Radical and Cationic Polymerization of BF. We then
attempted to polymerize BF with radical and cationic initiators
as summarized in Table 5. At first, stirring of BF and α,α′-
azobis(isobutyronitrile) (AIBN) in benzene at 70 °C for 15 h
led to the quantitative formation of an insoluble gel-like
product (run 35). We then shortened the polymerization time
to 6 h (runs 36 and 37). The conversion of BF was monitored
by 1H NMR measurements. In each case, the radical
polymerization of BF with AIBN was completed within 6 h
to afford a soluble polymer as a powder in a good to
quantitative yield. The SEC curves of the obtained polymers
were unimodal and the observed molecular weights were
14 000−33 000 by the SEC-RALLS analysis. As shown in
Next, we used BF as the first monomer to prepare block
copolymers in the reversed sequences. However, a living
poly(BF) could not initiate at all the polymerization of
1
Figure 4, the H NMR spectrum of the radically obtained
polymer, poly(BF-r), is significantly different from that of the
Table 5. Polymerization of BF with AIBN and BF3OEt2
a
b
b
run
initiator, mol %
solvent
time (h)
temp (°C)
yield (%)
Mn (kg/mol) obsd
Mw/Mn
35
36
37
38
39
40
AIBN, 1.1
benzene
benzene
benzene
CH2Cl2
CH2Cl2
CH2Cl2
15
6
70
70
70
0
100
80
96
0
insoluble
AIBN, 1.1
14
33
2.46
4.63
AIBN, 0.87
BF3OEt2, 8.0
BF3OEt2, 37
BF3OEt2, 43
6
48
4
0
46
98
3.9
15
24.3
c
24
0
3.18
a
b
c
Methanol-insoluble part. Mn(obsd) and Mw/Mn were determined by SEC calibration using polystyrene standards in THF. Multimodal.
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Solubility and Thermal Property of Poly(BF). Poly(BF-
a) and poly(BF-r) were off-white powders and showed similar
solubilities in various solvents. In fact, they were insoluble in n-
hexane, cyclohexane, dimethyl sulfoxide, methanol, and water
but soluble in benzene, toluene, chloroform, and THF at room
temperature. It is noted that the microstructure of poly(BF)
does not considerably affect their solubilities.
The thermal stabilities of poly(BF-a) and poly(BF-r) were
measured by a thermogravimetric analysis (TG/DTA) under
nitrogen (Figure 5). When we heated the samples from room
Figure 4. 1H NMR spectra of poly(BF)s measured in CDCl3 at room
temperature: (A) poly(BF-a) obtained with sec-BuLi in THF at −78
°C; (B) poly(BF-r) obtained with AIBN in benzene at 70 °C; (C)
poly(BF-c) obtained with BF3OEt2 in CH2Cl2 at 0 °C.
anionically obtained polymer, poly(BF-a), produced with sec-
BuLi in THF. We could assign several signals of poly(BF-r)
with using 2D NMR (HMQC), since its spectrum was much
simpler than that of poly(BF-a). However, it is also difficult
Figure 5. TGA curves of poly(BF)s: (A) poly(BF-a), 10% weight loss
temperature (T10) = 326 °C; (B) poly(BF-r), T10 = 311 °C.
1
from the H NMR spectrum to estimate the contents of the
addition modes in the radical polymerization. The 13C NMR
measurements then revealed that the contents of the 1,2- and
1,4-units of poly(BF-r) were 9 and 91%, respectively. Similar to
the anionic polymerization, BF acted as a polymerizable 1,3-
diene derivative during the course of the radical polymerization.
We next attempted to polymerize BF with boron trifluoride
etherate (BF3OEt2) (8.0 mol %) at 0 °C in CH2Cl2 for 48 h
(Table 5, run 38). The reaction mixture showed a black color
during the reaction and was quenched with degassed methanol.
However, no polymeric product was obtained and only BF was
recovered from the reaction system. We then increased the
amount of BF3OEt2 to 37 mol % and reacted BF at 0 °C in
CH2Cl2 for 4 h (run 39). In this case, the polymerization of BF
managed to proceed, and the conversion of BF reached 46%.
After 24 h, BF was completely consumed, and a quantitative
yield of a polymeric product was obtained (run 40). Thus, the
cationic polymerization of BF with BF3OEt2 at 0 °C slowly
proceeded, while the anionic polymerization was completed
within 10 min even at −78 °C in THF. This cationically
obtained polymer, poly(BF-c), possessed a molecular weight
around 15 000 and a broad MWD (Mw/Mn = 3.18). More
temperature to a higher temperature, the thermal degradations
of the poly(BF)s occurred around 270 °C with decreasing the
weight of the polymer samples. At 600 °C, the residual weights
of the ashes derived from poly(BF-a) and poly(BF-r) were
observed to be 32 and 23 wt %. The 10% weight loss
temperatures, T10, of poly(BF-a) and poly(BF-r) were observed
at 326 and 311 °C, respectively. These observations suggest
that no significant relationship is present between the thermal
stability and the microstructure of the poly(BF) samples.
The glass transition temperature (Tg) of a series of poly(BF)
was analyzed by DSC measurements as shown in Figure 6. In
addition to poly(BF-a) and poly(BF-r), the poly(BF-a-Bz)
samples, obtained with sec-BuLi in benzene at 0 °C,4 were
added in Figure 6. The microstructure of poly(BF-a-Bz) (1,4-
content ∼76%, 1,2-content ∼24%) rather resembled that of
poly(BF-r), while the samples were produced with the anionic
1
interestingly, the poly(BF-c) showed a H NMR spectrum
significantly different from those of poly(BF-a) and poly(BF-r),
as can be seen in Figure 4C. In fact, it was very difficult to
estimate the microstructure of the poly(BF-c) based on the 13
C
NMR spectrum due to the broadness of the spectra, as shown
in Figure S1. However, it was obvious that the microstructure
of poly(BF-c) differed from those of poly(BF-a) and poly(BF-
r). Under the cationic conditions, some possible side reactions
related to the aromatic ring might occur along with the normal
propagation of the exo-methylene group of BF. Thus, the
observed polymerizability of BF toward the cationic initiator
was remarkably low, although the propagation barely occurred
to form a polymer. This observation is reasonable, since BF
presents a very high anionic polymerizability as discussed
above.
Figure 6. Relationship between Tg and Mn: (A) poly(BF-a), (B)
poly(BF-a-Bz), and (C) poly(BF-r).
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Initiators. Commercially available sec-BuLi (1.0 M in cyclohexane,
Kanto Chemical Co., Inc.) was used without purification and diluted
with dry n-heptane. K-Naph was prepared by the reaction of a small
excess molar quantity of naphthalene with potassium in THF.
Ph2CHLi, Ph2CHNa, Ph2CHK, and Ph2CHCs were synthesized by
the reaction of corresponding metal naphthalenides and 1.5-fold
diphenylmethane in dry THF under argon at room temperature for 48
h. Ph3CLi and Ph3CK were synthesized by the reaction of
corresponding alkali metal and triphenylmethane in dry THF under
argon at room temperature for 48 h. These initiators were sealed off
under high-vacuum conditions in ampules equipped with break-seals
and stored at −30 °C. The concentration of initiator was determined
by colorimetric titration using standardized 1-octanol in THF in a
sealed reactor under vacuum, as previously reported.21 BzMgCl was
prepared by the reaction of benzyl chloride and magnesium in THF.
AIBN for radical polymerization was purified by recrystallization from
ethanol. BF3OEt2 was distilled in the presence of CaH2 on a vacuum
line and then diluted with CH2Cl2.
Synthesis of BF. A solution of n-BuLi (2.60 M, 66.0 mL, 172
mmol) was added dropwise to a solution of indene (19.9 g, 171
mmol) in dry THF (400 mL) at 0 °C under nitrogen. The reaction
mixture was stirred at 0 °C for 5 min. Then, paraformaldehyde (5.61 g,
187 mmol) was added to the mixture in one portion at 0 °C. The
reaction mixture was stirred at 0 °C for 3 min and at rt for 5 min. The
reaction mixture was quenched with aqueous solution of NaHCO3.
The layers were separated, and the aqueous layer was extracted with
diethyl ether three times. Then, the combined organic layer was
washed with aqueous solution of NaCl twice and water once and then
dried over anhydrous MgSO4. MgSO4 was removed by filtration, and
the filtrate was concentrated in vacuo. The residual liquid was purified
by flash column chromatography (silica gel, hexane/ethyl acetate) to
give a mixture of hydroxymethylindenes (16.0 g, 109 mmol, 64%),
including 15% of 1-hydroxymethylindene (1HMI) and 85% of
isomeric 3-hydroxymethylindene (3HMI). The resulting mixture of
1HMI and 3HMI was used for the following dehydration without
further purification.
The mixture of 1HMI and 3HMI (16.0 g, 109 mmol) in dry
methanol (150 mL) was added to a solution of tBuOK (13.4 g, 120
mmol) in dry methanol (200 mL) at 0 °C. The reaction mixture was
stirred at 40 °C for 30 min. The mixture was then cooled to room
temperature and was added to ice water to quench the reaction. Then,
the mixture was extracted with hexane three times. The combined
organic phase was washed with water twice and dried over anhydrous
MgSO4. MgSO4 was removed by filtration, and the filtrate was
concentrated in vacuo. The residual liquid was purified by flash column
chromatography (silica gel, hexane) to give BF as yellow solid (5.53 g,
43.2 mmol, 40%, mp ∼80 °C (polymerized)). 1H NMR (CDCl3): δ =
5.73 and 6.07 (2s, 2H, CH2), 6.50 (d, J = 5.46 Hz, 1H, Hb), 6.87 (d,
J = 5.46 Hz, 1H, Hc), 7.16−7.28 (m, 3H, He, Hf, Hg), 7.58 (d, 1H, Hh,
J = 7.16 Hz). 13C NMR (CDCl3): δ = 114.1 (CH2), 119.8 (Ch),
120.9, 125.3, 128.1 (Ce, Cf, Cg), 129.8 (Cb), 133.4 (Cc), 135.9 (Ci),
143.5 (Cd), 147.6 (Ca).
initiator, sec-BuLi. The Tg value of poly(BF-a) increased with
the increasing molecular weight and reached 164 °C (Mn =
31 000). The Tg value of poly(BF-a-Bz) similarly increased to
150 °C in the sample with the highest molecular weight. In the
case of the poly(BF-r)s, the Tg values were observed at around
145 °C, slightly lower than those of poly(BF-a-Bz). The Tg
values of poly(BF-a-Bz) and poly(BF-r) were always lower than
that of poly(BF-a), even if the former ones possessed molecular
weights higher than poly(BF-a). Thus, the order of the Tg
values of poly(BF) was estimated as follows: poly(BF-a) >
poly(BF-a-Bz) > poly(BF-r). Therefore, the Tg values tend to
increase with the contents of the 1,2-structure, indicating the
effect of the rigid indene ring perpendicular to the polymer
main chain. This observation suggests that the movement of the
polymer main chain in the 1,2-addition mode is more restricted
compared to the repeating units consisting of the 1,4-addition
mode.
CONCLUSIONS
■
We have succeeded in the anionic, free-radical, and cationic
polymerizations of a hydrocarbon monomer, BF, possessing a
fixed transoid 1,3-diene framework in the molecule. In
particular, BF readily undergoes the living anionic polymer-
izations in the 1,2- and 1,4-addition modes to afford polymers
with tailored chain lengths. Based on the results of the
homopolymerization and sequential block copolymerization,
the high anionic polymerizability of BF, comparable to those of
the methacrylates, is clearly demonstrated. BF can be
polymerized with various anionic initiators showing a low
nucleophilicity, including the bulky π-conjugated diphenyl-
methyl and triphenylmethyl anions, Grignard reagents,
alkoxides, and ester enolates derived from methacrylates. On
the other hand, a propagating indenyl anion showing a low
nucleophilicity was formed from BF during the course of the
polymerization. The observed high anionic polymerizability of
the exo-methylene-type BF can be explained by the
conformation, the bond angle, and the electron density
determined by the DFT calculation by comparing these values
of 2PB and clearly breaks the conventional image of
hydrocarbon monomers showing low anionic polymerizabil-
ities. The polarized planar fulvene framework in BF might
induce the remarkable anionic polymerizability as a new class of
hydrocarbon monomers.
EXPERIMENTAL SECTION
■
Materials. All reagents were purchased from Tokyo Chemical
Industry (TCI) Co., Ltd., unless otherwise stated. Indene, tBuOK, and
n-BuLi (2.6 M in hexane, Kanto Chemical Co., Inc.) were used
without purification. Paraformaldehyde was dried under vacuum
condition over P2O5 overnight. THF was refluxed over sodium for 3 h
and distilled from LiAlH4 under nitrogen. THF used as a
polymerization solvent was further purified by the distillation from
sodium naphthalenide solution on a vacuum line. LiCl was dried in
vacuo for 2 days and used as a THF solution. 2PB5a and 4CNSt18 were
synthesized and purified according to the previous report. DPE was
distilled from CaH2 under reduced pressure and then distilled in the
presence of 1,1-diphenylhexyllithium on a vacuum line. Styrene was
distilled over CaH2 under reduced pressure and finally distilled over
dibutylmagnesium on the vacuum line. 2VP was distilled over CaH2
under reduced pressure and finally distilled over CaH2 on the vacuum
line. MMA and tBMA were distilled over CaH2 under reduced
pressure and finally distilled over (C8H17)3Al on the vacuum line.
TMEDA and 18-crown-6 were distilled over CaH2 under reduced
pressure and diluted with THF.
Purification of BF. After flash column chromatography (silica gel,
hexane), solidified BF was degassed and sealed off in an all-glass
apparatus equipped with a break-seal in the presence of CaH2. The
sealed BF was immediately dissolved in dry THF. The monomer
solution in THF was distilled from CaH2 on the vacuum line into an
ampule fitted with a break-seal and immediately diluted with dry THF.
The resulting yellow monomer solution (0.3−0.7 M) was stored at
−30 °C until ready to use for the anionic polymerization.
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Anionic Polymerization. All anionic polymerizations were carried
out in dry THF at −78 °C in an all-glass apparatus equipped with
break-seals under high-vacuum conditions (10−6 mmHg).21 The
polymerization was terminated with degassed methanol. The polymer
was precipitated by pouring the reaction mixture in methanol, and the
precipitated polymer was recovered by filtration. The resulting
polymer was purified by freeze-drying from benzene and characterized
ASSOCIATED CONTENT
* Supporting Information
Figure S1; Tables S1 and S2. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.macromol.5b00944.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: tishizon@polymer.titech.ac.jp (T.I.).
Notes
1
by H and 13C NMR spectroscopies.
■
Block Copolymerization of BF and MMA. The anionic
copolymerization of MMA and BF was carried out in an all glass-
apparatus equipped with break-seals under high-vacuum conditions
similar to the homopolymerization. Typical procedure of sequential
copolymerization was as follows: MMA (0.544 g, 5.44 mmol) in THF
(5.20 mL) was added to a mixture of sec-BuLi (0.115 mmol) in n-
heptane (3.05 mL) and DPE (0.210 mmol) in THF (4.75 mL) at −78
°C and reacted for 10 min. The color of initiator system changed
immediately from red to colorless, indicating the rapid initiation of
MMA. Then, LiCl (0.0800 M, 0.400 mmol) in THF (5.00 mL) was
successively added to the resulting living poly(MMA) at −78 °C. After
10 min, a THF solution (12.7 mL) of BF (1.08 g, 8.45 mmol) was
finally added to the living poly(MMA) at −78 °C. The color of
polymerization system instantaneously changed from colorless to
orange, indicating the formation of indenyl anion derived from BF.
The polymerization of BF was continued for 1 h at −78 °C and
quenched with degassed methanol. The reaction mixture was poured
into methanol to precipitate a polymer. The yield of poly(MMA)-b-
poly(BF) was almost quantitative (1.60 g, 100%, Mn = 22 000, Mw/Mn
= 1.09, Table 4, run 31).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by Grant-in Aid (No.
14550833 and 18550105) from the Ministry of Education,
Science, Sports, and Culture, Japan. T.I. appreciates the
financial support from the Yazaki foundation and JX Nippon
Oil & Energy Corporation. Y.K. appreciates the support by
Grant-in Aid for JSPS fellows (No. 26·11883) from Japan
Society for the Promotion of Science (JSPS). The numerical
calculations were carried out on the TSUBAME2.5 super-
computer at the Tokyo Institute of Technology, Tokyo, Japan,
and on the supercomputer at the Research Center for
Computational Science, Okazaki, Japan.
REFERENCES
■
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1
Measurements. H and 13C NMR spectra were measured on a
Bruker DPX300 in CDCl3. Chemical shifts were recorded in ppm
1
downfield relative to CHCl3 (δ 7.26) and CDCl3 (δ 77.1) for H and
13C NMR as standard, respectively. Size exclusion chromatogram
(SEC) curves for determination of Mw/Mn were obtained in THF at
40 °C at flow rate of 1.0 mL min−1 with a Viscotek TDA302 equipped
with three polystyrene gel columns (TSKgelG5000HHR + G4000HHR
+ G3000HHR). The combination of viscometer, right angle laser light
scattering detection (RALLS), and refractive index (RI) detection was
applied for the online SEC system in order to determine the absolute
molecular weights of homopolymer and block copolymer. The Tgs of
the polymers were measured by DSC using a Seiko Instruments
DSC6220 apparatus under nitrogen flow. The polymer sample was
first heated to 120 °C, cooled to 30 °C, and then scanned at a rate of
10 °C min−1. The ground state geometries were fully optimized by
using density functional theory (DFT) calculations, long-range and
dispersion corrected ωB97X-D11 functional with the polarized split-
valence triple zeta 6-311G(d,p) basis set,12,13 denoted as ωB97X-D/6-
311G(d,p). Then, the vibrational frequencies were computed at the
same level to check each optimized structure as an equilibrium
structure. All calculations were carried out using the Gaussian 09
program.14 Natural charges were obtained by using the NBO 6.0
program.15
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