Living anionic polymerization of 1,4-diisopropenylbenzene

Article abstract of DOI:10.1021/acs.macromol.5b00575

The anionic polymerization of diisopropenylbenzene (DIPB) derivatives was conducted in THF at -78 °C with a specially designed initiator system prepared from oligo(α-methylstyryl)lithium and an excess potassium tert-butoxide (KOBu^t) (2.7-5.0 equiv to the Li salt). Among the ortho-, meta-, and para-isomers of DIPB derivatives, it was found that the para-isomer (p-DIPB) successfully underwent the living polymerization in a selective manner through one of the two isopropenyl groups under the above stated conditions. With this living polymerization system, soluble polymers with controllable M_n values ranging from 7620 to 31 500 g/mol and near monodisperse distributions (M_w/M_n ≤ 1.03) were obtained for the first time. The obtained living polymers were stable at -78 °C even after 168 h and at -40 °C after 6 h, in which the intermolecular addition reaction of the chain-end anion to the pendant isopropenyl group could be completely suppressed. In contrast, the living polymerization of either the ortho- or meta-isomer was not successful under the same conditions. The block copolymerization of p-DIPB with either styrene (S), 2-vinylpyridine (2VP), or tert-butyl methacrylate (^tBMA) by the sequential addition of such monomers was conducted. Four new PS-block-P(p-DIPB), P2VP-block-P(p-DIPB), P(p-DIPB)-block-P2VP, and P(p-DIPB)-block-P^tBMA containing reactive P(p-DIPB) segments were synthesized. On the basis of the block copolymerization results, it is understood that p-DIPB is comparable to 2VP and located between S and ^tBMA in monomer reactivity. The reactivity increases as follows: S < 2VP ～ p-DIPB < ^tBMA, while the nucleophilicity of the living chain-end anion decreases in the following order: PS^- > P2VP^- ～ P(p-DIPB) ^- > P(^tBMA) ^-.

Full text of DOI:10.1021/acs.macromol.5b00575

Article
pubs.acs.org/Macromolecules
Living Anionic Polymerization of 1,4-Diisopropenylbenzene
Raita Goseki,† Suguru Onuki,† Shunsuke Tanaka,† Takashi Ishizone,† and Akira Hirao*,†,‡,§
†Polymeric and Organic Materials Department, Graduate School of Science and Engineering, Tokyo Institute of Technology, S1-13,
2-12-1, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan
‡Institute of Polymer Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
§College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren Ai Road, Suzhou Industrial Park,
Suzhou 215123, China
S
* Supporting Information
ABSTRACT: The anionic polymerization of diisopropenylbenzene
(DIPB) derivatives was conducted in THF at −78 °C with a specially
designed initiator system prepared from oligo(α-methylstyryl)lithium
and an excess potassium tert-butoxide (KOBut) (2.7−5.0 equiv to the
Li salt). Among the ortho-, meta-, and para-isomers of DIPB
derivatives, it was found that the para-isomer (p-DIPB) successfully
underwent the living polymerization in a selective manner through
one of the two isopropenyl groups under the above stated conditions.
With this living polymerization system, soluble polymers with
controllable Mn values ranging from 7620 to 31 500 g/mol and
near monodisperse distributions (Mw/Mn ≤ 1.03) were obtained for the first time. The obtained living polymers were stable at
−78 °C even after 168 h and at −40 °C after 6 h, in which the intermolecular addition reaction of the chain-end anion to the
pendant isopropenyl group could be completely suppressed. In contrast, the living polymerization of either the ortho- or meta-
isomer was not successful under the same conditions. The block copolymerization of p-DIPB with either styrene (S), 2-
vinylpyridine (2VP), or tert-butyl methacrylate (tBMA) by the sequential addition of such monomers was conducted. Four new
PS-block-P(p-DIPB), P2VP-block-P(p-DIPB), P(p-DIPB)-block-P2VP, and P(p-DIPB)-block-PtBMA containing reactive P(p-
DIPB) segments were synthesized. On the basis of the block copolymerization results, it is understood that p-DIPB is
t
comparable to 2VP and located between S and BMA in monomer reactivity. The reactivity increases as follows: S < 2VP ∼ p-
t
DIPB < BMA, while the nucleophilicity of the living chain-end anion decreases in the following order: PS− > P2VP− ∼ P(p-
−
−
DIPB) > P(tBMA) .
The success of the living anionic polymerization of p-DVB
may possibly be attributed by the following two issues: First,
the ion-dissociation of the chain-end anion may be significantly
suppressed by an excess KOBut (actually K+) via the common
salt effect. In addition, a steric bulkiness around the chain-end
anion may be provided by the coordination of several KOBut
molecules to the chain-end anion by the ionic interaction. Thus,
it is considered that the less dissociate, that is, less reactive and
bulkier chain-end anion is formed by the addition of KOBut.
One more important issue is that the reactivity of the pendant
CH2CH bond reduced to some extent by the electron-
donating character of the −CH2−CH− backbone chain formed
by the polymerization in comparison to the reactivity of each
INTRODUCTION
■
It is known that the intermolecular addition reaction of the
chain-end anion to the pendant CH2CH bond significantly
occurs during the polymerization of 1,4-divinylbenzene (p-
DVB). Such the addition reaction results in the formation of
highly cross-linked polymers insoluble in solvents. Therefore, p-
DVB is generally used as a cross-linking agent to prepare micro-
and macro-gels, but not considered as a typical monomer.1−9 In
2011, we successfully found that the oligo(α-methylstyryl)-
lithium (αMSLi)-initiated anionic polymerization of p-DVB
proceeds in a living manner by adding an excess potassium tert-
butoxide (KOBut) (10 equiv or more).10 In this polymer-
ization, one of the two CH2CH bonds selectively
participated the polymerization to quantitatively give the
soluble poly(p-DVB)s with predictable Mn values up to 60
500 g/mol and near monodisperse distributions (Mw/Mn ≤
1.05).11 The living nature of this system was also guaranteed by
the quantitative formation of poly(p-DVB)-block-poly(2-vinyl-
pyridine) and poly(p-DVB)-block-poly(tert-butyl methacrylate)
by the sequential addition of p-DVB and then 2-vinylpyridine
or tert-butyl methacrylate to the above initiator system.12
CH2CH bond of p-DVB. This is clearly evident from the 13
C
NMR observation that the resonance of 113.9 ppm assigned to
the vinyl β-carbon of p-DVB is shifted to 113.1 ppm after the
polymerization.
Received: March 18, 2015
Revised: April 26, 2015
Published: May 12, 2015
© 2015 American Chemical Society
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polymerize at −78 °C for 3 h. The same work-up as that used in the
homopolymerization gave the expected P(p-DIPB)-block-PtBMA. The
other block copolymers were synthesized by the similar manner as that
described above except for the reaction time for each polymerization
steps.
On the basis of the above presumption and observation, the
living polymerization of p-DVB is considered to proceed in
such a way that the less reactive and bulkier chain-end anion
preferentially reacts with the more reactive CH2CH bond
(113.9 ppm) of p-DVB to initiate the polymerization of p-DVB.
On the other hand, the less reactive pendant CH2CH bond
(113.1 ppm) incorporated into the backbone chain remains
intact during the polymerization.
RESULTS AND DISCUSSION
■
In 1978, Okamoto and Mita first reported that p-DIPB was
anionically polymerized with potassium naphthalenide (K-
Naph) in THF at −30 °C to afford a soluble polymer during
the early stage of the polymerization.14 As expected, the
gelation by the intermolecular addition reaction of the chain-
end anion to the pendant isopropenyl group occurred at the
end of the polymerization to afford an insoluble material.
Although they indicated that the polymerization was an
equilibrium polymerization, no detailed characterization results
were available in their paper. Soon after, Lutz, Rempp, and their
co-workers also reported the anionic polymerization of p-DIPB
in THF at −30 °C with 1-phenylethylpotassium.15 A soluble
polymer was obtained in 65% yield after 2 h. The resulting
polymer was observed to possess an observed Mn value of 20
900 g/mol by GPC close to that calculated (Mn = 19 500 g/
mol) and a somewhat narrow molecular weight distribution
(Mw/Mn = 1.25), indicating that the polymerization proceeded
in a living manner. However, the broadening of the molecular
weight distribution was progressively pronounced as the
polymerization proceeded and the insoluble material by
cross-linking was formed at the high conversion. The anionic
polymerization of the meta-isomer (m-DIPB) also similarly
behaved. Thus, the anionic polymerization of p-DIPB or m-
DIPB could be controlled to a certain extent during the early
stage of the polymerization, because of the slow addition
reaction, possibly due to the steric bulkier chain-end anion and
pendant isopropenyl group.
This success encouraged us to apply the above-designed
initiator system, effectively working in the living anionic
polymerization of p-DVB, to the polymerization of other
divinylbenzene derivatives, which were difficult to polymerize in
a controlled manner. We now report the possibility of the living
anionic polymerization of diisopropenylbenzene (DIPB)
derivatives using such an effective initiator system. The DIPB
derivatives herein employed involve not only the para- but also
the meta- and ortho-isomers to compare the positional effect on
the polymerization behavior.
EXPERIMENTAL SECTION
■
Materials and Instruments. Since all the materials including their
syntheses and purification procedures and instruments are the same as
those reported in the previous papers,10−12 they are described in the
Supporting Information.
Synthesis of 1,4-Diisopropenylbenzene (p-DIPB). After
dissolving in 1,4-bis(2-hydroxyisopropyl)benzene in THF, the
insoluble portion was removed by filtration. It was further purified
by recrystallization from toluene. The reaction mixture of the purified
1,4-bis(2-hydroxyisopropyl)benzene (9.06 g, 46.6 mmol) and p-
toluenesulfonic acid (10 mg) in toluene (100 mL) was heated at
110 °C for a few hours, while the formed water was removed by a
Dean−Stark condenser. After quenching with NaHCO3 aqueous
solution, the usual work-up (extraction with diethyl ether, drying over
MgSO4, concentration, and column chromatography with hexane) to
afford p-DIPB as a colorless solid (6.40 g, 40.5 mmol) in 87% yield
1
(mp 62−63 °C). H NMR (CDCl3, 300 MHz; ppm): δ 7.44 (s, 4H,
In 2006, Hirao and his co-workers reported that a structurally
analogous 4-isopropenylstyrene substituted with vinyl and
isopropenyl groups successfully underwent a selective anionic
polymerization through only the vinyl group in a living manner
in THF at −78 °C with an initiator bearing K+ such as K-Naph
and cumylpotassium.16,17 The selective living anionic polymer-
ization was also achieved with αMSLi by the addition of 5 equiv
of KOBut, in which oligo(α-methylstyryl)potassium might be in
situ formed by exchanging the counteraction from Li+ to K+ and
C6H4), 5.39 (s, 2H, (Z)−CH2C(CH3)-C6H4), 5.08 (s, 2H, (E)−
CH2C(CH3)-C6H4), 2.16 (s, 6H, CH2C(CH3)−). 13C NMR
(CDCl3, 75 MHz; ppm): δ 142.9(CH2C(CH3)−), 140.3, 125.5,
112.4 (CH2C(CH3)−), and 21.9 (CH2C(CH3)−).
The syntheses of 1,3-diisopropenylbenzene (m-DIPB), 1,2-
diisopropenylbenzene (o-DIPB), and 1-isopropenyl-3-(α-
isopropylethenyl)benzene are described in the Supporting Informa-
tion.
Polymerization Procedure. All of the polymerizations were
conducted according to the usual high vacuum technique with break-
seals.13 Typically, p-DIPB was mixed with the initiator prepared from
αMSLi and an excess KOBut (2.7−5.0 equiv) at −78 °C with vigorous
stirring. After allowing to polymerize for a prescribed polymerization
time, the usual work-up (quenching with degassed methanol,
precipitation in methanol, reprecipitation twice (THF/methanol),
and freeze-drying) gave P(p-DIPB)s. Since the resulting P(p-DIPB)s
were not stable in the air for a long time, the freeze-drying was
conducted in the dark for overnight. The resulting polymers were
stored in ice box (−18 °C) under a N2 atmosphere to avoid the cross-
linking. 1H NMR (CDCl3, 300 MHz; ppm): δ 7.6−6.2 (br, 4H,
−C6H4−), 5.6−5.2 (br, 1H, (Z)−CH2C(CH3)-C6H4-), 5.2−4.8 (br,
work as
a real initiator. The soluble poly(4-
isopropenylstyrene)s with controllable molecular weights (Mn
= 5000−61 000 g/mol) and narrow distributions (Mw/Mn ≤
1.08) were quantitatively obtained. The pendant isopropenyl
group completely remained intact for 5 min under the stated
conditions. It should be noted that the chain-branching always
occurred in the polymerization when used the initiators bearing
Li+ like sec-BuLi and αMSLi. Thus, the influence of the
countercation on the polymerization behavior is important and
critical.
Anionic Polymerization of p-DIPB. The anionic polymer-
ization of p-DIPB was conducted with αMSLi in the presence
of an excess KOBut, which effectively worked in the living
anionic polymerization of both p-DVB and 4-isopropenylstyr-
ene, as already mentioned. Although several other potassium
alkoxides were found to be also effective in the polymerization
of p-DVB, KOBut was always used throughout this study
because it was one of the best additives.11 The polymerization
of p-DIPB readily proceeded in THF at −78 °C to afford a
soluble polymer in 95% yield after 48 h. The dark red color,
1H, (E)−CH2C(CH3)-C6H4-), 2.6 (br, 8H, −CH2−C(CH3)−). 13
C
NMR (CDCl3, 75 MHz; ppm): δ 149.7−147.8, 143.2, 137.4, 128.0−
123.0, 111.3 (CH2C(CH3)−), 63.2−58.8, 43.4, 25.6, and 21.8.
The block copolymers were synthesized by the sequential addition
of the corresponding monomers to the above-designed initiator system
at −78 °C for the times shown in Table 6. The amount of each
monomer was also listed in the same table. Typically, the synthesis of
P(p-DIPB)-block-PtBMA was described. Similar to the homopolyme-
rization of p-DIPB, p-DIPB was polymerized with the above initiator
system in THF at −78 °C for 48 h. Then, tBMA in THF was added to
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characteristic of the living styryl anion derived from p-DIPB,
remained unchanged during the polymerization. The resulting
polymer shows a sharp SEC peak (Mw/Mn = 1.03) without any
higher-molecular-weight portion (Figure 1). The observed Mn
value (Mn = 7620 g/mol) agreed with that calculated (Mn = 6
750 g/mol) based on the monomer to initiator ratio (Table 1).
polymerization, indicating the lower anionic reactivity of the
pendant vinylene group incorporated into the backbone chain
as compared to the vinylene group of p-DIPB. This is
undoubtedly attributed to the electron-donating effect of the
−CH2−C(CH3)- backbone chain.
For the preparation of the high-molecular-weight polymers, a
series of polymerizations were conducted by increasing the
ratio of monomer to initiator. As shown in Table 1, the Mn
value of the polymer could be controlled in the range from 11
200 to 31 500 g/mol. All these polymers possessed near
monodisperse distributions (Mw/Mn ≤ 1.03). The polymer-
ization slowly proceeded and was not complete after 24 h. At
the monomer concentration of ca. 0.2 M, the polymer yield was
around 90% even after 48 h and nearly quantitative after 168 h.
Doubling the monomer concentration (ca. 0.4 M) did not
affect the polymerization rate. It was observed under such
conditions that the polymerization heterogeneously proceeded
because p-DIPB was not completely soluble in THF at −78 °C.
The higher-molecular-weight portion was not formed at all in
each of the polymers obtained after 24−48 h. A very small
amount (∼2%) of a dimeric product produced by the same
intermolecular 1:1 addition reaction was present in the polymer
obtained after 168 h, as shown in Figure 3. We presently
consider that this is almost a negligible amount because only
1% of all the polymer chain-end anions may participate in the
addition reaction. In contrast to the polymerization of p-DVB,
in which 10 equiv or more of KOBut were required, the use of
2.7 equiv of KOBut was sufficient enough to achieve the living
polymerization of p-DIPB.18
Next, the polymerization was conducted by raising the
polymerization temperature to −40 °C in order to increase the
polymerization rate. In fact, the polymerization rate became
much faster at −40 °C and the polymers were obtained in 89
and 92% yields after 2 and 6 h, respectively. The unwanted
higher-molecular-weight portion is not formed in each case, as
shown in Figure 4, parts 4a and 4b.
The polymer with a Mn value of 15 000 g/mol could also be
obtained in 92% yield at −40 °C after 6 h, although a negligible
amount (∼2%) of the dimeric byproduct was also formed in
this case. The amount of the dimer increased up to 15% at −40
°C after 24 h. A very small amount of the trimer was also
formed. The deviation of the observed Mn value (Mn = 93 100
g/mol) from that calculated (Mn = 52 100 g/mol) may possibly
Figure 1. SEC profile of P(p-DIPB).
In the 1H NMR spectra of p-DIPB and the resulting polymer
(see Figure 2, parts a and b), the characteristic resonances at 5.4
and 5.0 ppm to the vinylene (CH2C) protons remained at a
50% intensity, while new peaks (2.0−0.1 ppm) corresponding
to the −CH2−C− protons of the backbone chain appeared,
along with two sharp peaks at 2.2−2.0 ppm for the α-CH3
protons. The 13C NMR spectra also show the similar signal
changes before and after the polymerization (see Figure 2, parts
c and d). These data clearly demonstrated that p-DIPB
undergoes the living anionic polymerization in a selective
manner through one of the two isopropenyl groups and the
thus obtained living polymer is stable at −78 °C for 48 h
without occurrence of the intermolecular addition reaction of
the chain-end anion to the pendant isopropenyl group.
Moreover, the 13C NMR resonance at 112.4 ppm for the
vinylene β-carbon of p-DIPB was shifted to 111.3 ppm after the
Table 1. Anionic Polymerization of p-DIPB with Initiators Bearing K+ in THF at −78 or −40 °C
Mn (kg/mol)
a
b
c
d
c
c
initiator system
temp (°C)
time (h)
conversion (%)
calcd
SEC
RALLS
Mw/Mn
coupling product (%)
e
sBuLi/αMS/KOBut (4.2 equiv)
sBuLi/αMS/KOBut (4.0 equiv)
sBuLi/αMS/KOBut (5.0 equiv)
sBuLi/αMS/KOBut (4.7 equiv)
sBuLi/αMS/KOBut (4.7 equiv)
sBuLi/αMS/KOBut (4.5 equiv)
sBuLi/αMS/KOBut (4.4 equiv)
sBuLi/αMS/KOBut (4.6 equiv)
sBuLi/αMS/KOBut (4.1 equiv)
K-Naphf/KOBut (6.2 equiv)
K-Naph
−78
−78
−78
−78
−78
−40
−40
−40
−40
−78
−78
48
24
48
168
24
2
95
91
86
98
69
89
92
92
94
91
91
6.75
8.82
6.53
9.00
7.62
11.2
19.2
31.5
7.22
7.83
8.96
15.0
93.1
15.0
16.5
1.03
1.03
1.02
1.02
1.05
1.04
1.07
1.03
1.10
1.08
1.06
0
0
16.7
33.7
7.66
16.6
22.9
5.23
0
∼ 2
0
6.76
7.79
6.00
7.20
0
6
0
6
17.3
19.1
∼2
15
0
24
24
24
52.1
19.0
18.6
70.9
13.9
15.3
9.3
a
b
c
Determined by 1H NMR. Mn(calcd) = (MW of monomer) × (convn)/100 × [monomer]/[initiator] + (MW of initiator fragment). Evaluated by
d
e
SEC calibrated with polystyrene standards. Evaluated by SEC equipped with triple detectors. Oligo(α-methylstyryl)lithium was prepared from sec-
BuLi (sBuLi) and a 3-fold excess of α-methylstyrene (αMS).
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1
Figure 2. H NMR spectra of p-DIPB (a) and P(p-DIPB) (b) and 13C NMR spectra of p-DIPB (c) and P(p-DIPB) (d).
The polymerization of p-DIPB also proceeded with K-Naph
in the presence of 6.2 equiv of KOBut. As also listed in Table 1,
a soluble polymer with a narrow molecular weight distribution
and a predictable molecular weight was obtained in 91% yield.
No higher-molecular-weight portion was present in the
resulting polymer. Under the same conditions, a polymer was
obtained in 91% yield by the polymerization with K-Naph
without KOBut. In this case, the dimeric product was formed in
9.3% yield. Once again, the use of excess KOBut was found to
be essential to suppress the unwanted addition reaction.
The polymerization of p-DIPB with sec-BuLi at −78 °C was
observed to proceed faster than that with the above-prepared
initiator. A soluble polymer with a Mn value of 21 500 g/mol
was obtained in 98% yield after 15 h as listed in Table 2.
However, the higher-molecular-weight portion including not
only dimer, but also oligomers, was observed in the resulting
polymer (see Figure 5). The higher-molecular-weight portion
of 8.5% was already formed in the polymer yield of 50% for 3 h.
This clearly indicated that the addition reaction competitively
occurs during the polymerization under such conditions. As
also seen in Table 2, the addition of either LiOBut or LiCl was
not effective to suppress the formation of the higher-molecular-
weight portion even at −95 °C. Thus, the resulting polymers
always included the higher-molecular-weight portions in the
polymerization with the initiators bearing Li+, but were
completely soluble.
Figure 3. SEC profile of the P(p-DIPB) obtained after 168 h.
In conclusion, p-DIPB underwent living anionic polymer-
ization in a selective manner through one of the two
isopropenyl group with the designed initiator system prepared
from αMSLi and an excess KOBut in THF at −78 °C. The
polymerization was slow and complete after 168 h. Under such
conditions, the living anionic polymer was stable at −78 °C. It
was observed that raising the polymerization temperature to
−40 °C increased the polymerization rate. The resulting living
polymer was stable for 6 h at −40 °C, but the addition reaction
gradually occurred after 6 h. The presence of KOBut is of
course essential to suppress the addition reaction and 2.7 equiv
of KOBut are sufficient enough in the living polymerization of
p-DIPB. This is in sharp contrast to the polymerization of p-
DVB, in which 10 equiv or more of KOBut are required to
suppress the similar addition reaction even within a few
minutes.
Figure 4. SEC profiles of P(p-DIPB)s obtained after 2 h (a) and after
6 h (b).
be due to the impurities in p-DIPB. Accordingly, the living
polymer was almost stable at −40 °C for 6 h, but the unwanted
addition reaction gradually occurred for the longer time of 24 h.
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Table 2. Anionic Polymerization of p-DIPB with Initiators Bearing Li+ in THF at −78 or −95 °C
Mn (kg/mol)
a
b
c
d
c
c
initiator system
temp (°C)
time (h)
conversion (%)
calcd
9.39
16.4
SEC
8.74
18.9
RALLS
Mw/Mn
coupling product (%)
e
e
e
e
e
sBuLi
sBuLi
−78
−78
−78
−78
−95
3
15
24
24
24
50
98
98
99
27
9.91
21.5
19.3
21.9
5.00
1.18 (1.03)
1.11 (1.02)
1.16 (1.05)
1.26 (1.02)
1.16 (1.04)
8.5
11
sBuLi/LiOBut (4.1 equiv)
sBuLi/LiCl (4.4 equiv)
sBuLi
16.5
14.8
14.8
15.7
11
11
5.51
4.71
7.5
a
b
c
Determined by 1H NMR. Mn(calcd) = (MW of monomer) × (convn)/100 × [monomer]/[initiator] + (MW of initiator fragment). Evaluated by
SEC calibrated with polystyrene standards. Evaluated by SEC equipped with triple detectors. Mw/Mn of main peak.
d
e
Figure 6. SEC profile of P(m-DIPB).
Figure 5. SEC profile of P(p-DIPB) obtained with sec-BuLi.
Thus, the unwanted addition reaction cannot be suppressed
in the polymerization of m-DIPB. As illustrated in Scheme 1,
the chain-end anion of p-DIPB takes five resonance structures,
including the pendant isopropenyl group (10π system), while
four resonance structures are taken in the case of m-DIPB (8π
system). Thus, the less stabilized and more reactive chain-end
anion may facilitate the addition reaction leading to the chain-
branching, like m-DVB previously reported.11
It should be noted that higher-molecular-weight portions
gradually increased in the obtained P(p-DIPB)s when the
polymers were allowed to stock in the air at room temperature.
In contrast, they were completely stable even after one year at
−18 °C under a nitrogen atmosphere.
Anionic Polymerizations of 1,3- and 1,2-Diisoprope-
nylbenzenes. To study the influence of the substituent
position on the polymerization, the polymerizations of 1,3- and
1,2-diisopropenylbenzenes, that is, the meta-isomer (m-DIPB)
and ortho-isomer (o-DIPB), were conducted under the above-
mentioned conditions (Table 3). In the polymerization of m-
DIPB, the polymer having a controllable Mn value (Mn = 3310
g/mol) and a narrow distribution (Mw/Mn = 1.07) was
obtained in 43% yield after 2.5 h. After 24 h, the polymer (Mn =
21 300 g/mol) was obtained in 80% yield. Although the main
peak was very narrow (Mw/Mn = 1.04), a dimeric product was
formed in 19% yield, as shown in Figure 6. Increasing the
amount of KOBut to an 11-fold excess slightly decreased the
dimer formation to 11%.
The polymerization of o-DIPB was also carried out under the
identical conditions. In this case, only a small amount of
oligomer (average molecular weight = ca. 600 g/mol) was
obtained in 5.9% yield after 24 h. The polymerization was
performed by changing the conditions using K-Naph or sec-
BuLi as the initiator at −78, −40, or even 0 °C. Similar
molecular weight oligomers were always obtained in 4−14%
yields. The intramolecular cyclization was suggested to partly
1
occur by a H NMR analysis of the resulting oligomers, as
illustrated in Scheme 2. We initially thought that o-DIPB
behaved similar to p-DIPB during the polymerization, since the
intermediate chain-end anion might take five resonance
Table 3. Anionic Polymerization of m-DIPB in THF at −78 °C
Mn (kg/mol)
a
b
c
d
c
c
initiator system
time (h)
conversion (%)
calcd
3.41
19.0
16.4
SEC
2.74
15.4
13.7
RALLS
Mw/Mn
1.07
coupling product (%)
sBuLi/αMS/KOBut (3.7 equiv)
sBuLi/αMS/KOBut (5.1 equiv)
sBuLi/αMS/KOBut (11 equiv)
2.5
24
43
80
81
3.31
21.3
16.2
0
19
11
e
e
1.10 (1.04)
1.13 (1.06)
24
a
b
c
Determined by 1H NMR. Mn(calcd) = (MW of monomer) × (convn)/100 × [monomer]/[initiator] + (MW of initiator fragment). Evaluated by
d
e
SEC calibrated with polystyrene standards. Evaluated by SEC equipped with triple detectors. Mw/Mn of main peak.
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Scheme 1. Resonance Structures of Chain-End Anions Derived from p-, m-, and o-DIPB
Scheme 2. Intramolecular Cyclization during the Polymerization of o-DIPB
structures (see Scheme 1) like the case derived from p-DIPB.
However, no polymerization occurred after 24 h. The steric
hindrance of the o-isopropenyl group may be responsible for
this, although the reason is not understood at the present time.
Thus, the polymerizations of both m-DIPB and o-DIPB have
not yet been successfully controlled.
A new meta-substituted monomer with isopropenyl and α-
isopropylethenyl groups, as shown in Figure 7, was synthesized
Figure 7. 1-Isopropenyl-3-(α-isopropylethenyl)benzene.
Figure 8. SEC profile of poly(1-isopropenyl-3-(α-isopropylethenyl)-
benzene).
and polymerized under the similar conditions for 48 h. This
monomer underwent the selective polymerization via the
polymerizable isopropenyl group in a living manner (see Figure
8 and Table 4). Thus, the introduction of a bulkier isopropyl
substituent at the α-position of the m-vinyl group makes it
possible to achieve the living anionic polymerization.
Block Copolymerization. Block copolymers are usually
synthesized by the sequential addition of two or more different
monomers to an anionic initiator.19 Among the monomers
having similar reactivities, the resulting living chain-end anions
also show similar reactivities and, therefore, the crossover block
copolymerization is possible. In practice, almost all block
copolymers can be synthesized by sequentially adding the
corresponding monomers according to the block sequences.
On the other hand, the synthesis of block copolymers is
significantly limited in block type when monomers with
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Table 4. Anionic Polymerization of 1-Isopropenyl-3-(α-isopropylethenyl)benzene in THF at −78 °C
Mn (kg/mol)
a
b
c
d
c
initiator system
sBuLi/αMS/KOBut (4.2 equiv)
time (h)
48
conversion (%)
calcd
SEC
RALLS
14.4
Mw/Mn
97
11.3
13.1
1.04
a
b
c
Determined by 1H NMR. Mn(calcd) = (MW of monomer) × (convn)/100 × [monomer]/[initiator] + (MW of initiator fragment). Evaluated by
d
SEC calibrated with polystyrene standards. Evaluated by SEC equipped with triple detectors.
different reactivities are employed. The success of the block
copolymer synthesis strongly depends on the addition order of
the monomers, since a more reactive monomer generally
produces a less reactive living chain-end anion and vice versa. A
serious problem often arises such that the less reactive
monomer cannot be polymerized with the less reactive living
chain-end anion. The synthesis of the target block copolymer is
not possible by this combination. Accordingly, the monomer
must be sequentially added in such a way that the less reactive
monomer is the first to polymerize, followed by the
polymerization of the more reactive monomer.
Table 5. Summary of Block Copolymerization among S,
2VP, p-DIPB, and BMA
a
t
monomer
p-DIPB
polymer anion
S
2VP
++
tBMA
b
PS−
P2VP−
P(p-DIPB)−
P(tBMA)−
++
+
++
++
++
++
++
b
++
b
+
++
++
b
−
−
−
++
a
Key: ++, living polymerization; +, insufficient initiation efficiency; −,
b
no polymerization. Homopolymerization.
In order to synthesize new block copolymers containing
reactive P(p-DIPB) segments, the sequential polymerization of
p-DIPB with either of S, 2VP, or BMA was performed under
monomodal peak, while the peak of the starting PS almost
disappeared (Figure 9). The characterization results listed in
t
the similar conditions to those used in the homopolymerization
of p-DIPB. As already known, the above three monomers in
addition to p-DIPB undergo the living anionic polymerization
and these three monomers show different monomer reactivities.
Both the reactivities of the three monomers and their living
chain-end anions are examined based on the previously
reported crossover block copolymerization results. With regard
to the monomer reactivity, S is the least reactive monomer
t
among them, while BMA is the most reactive one. The
reactivity of 2VP is positioned between these two monomers.
Thus, the monomer reactivity increases as follows: S < 2VP <
tBMA. In contrast, the living chain-end anion reactivity
increases in the opposite direction, that is, living poly(tBMA)
(PtBMA−) < living poly(2VP) (P2VP−) < living polystyrene
(PS−).19−22 Since PS− is the most reactive living chain-end
anion, both 2VP and tBMA are readily polymerized with PS− to
afford the well-defined PS-block-P2VP and PS-block-PtBMA.23
On the other hand, the least reactive PtBMA− is not capable of
polymerizing the least reactive S and even the less reactive 2VP.
Figure 9. SEC profile of PS-block-P(p-DIPB).
Table 6 and the SEC observation clearly show the successful
synthesis of the expected PS-block-P(p-DIPB). Similarly, P2VP−
was prepared using the same initiator system, followed by
addition of p-DIPB. The polymer yield was quantitative. The
SEC peak of the resulting polymer completely moved to the
higher-molecular-weight side and the peak of the starting P2VP
almost disappeared. Agreements of the Mn values and
compositions are satisfactory between the values measured
and the calculated ones. Thus, P2VP− quantitatively initiated
the polymerization of p-DIPB to give the expected P2VP-block-
P(p-DIPB). On the other hand, the polymerization of PtBMA−
with p-DIPB did not occur at all at −78 °C for 48 h and both
the p-DIPB monomer and deactivated P(tBMA) were
quantitatively recovered. The SEC of the polymer obtained
after the second-stage polymerization exhibited the same shape
and elution count as that of the starting PtBMA.
With the first series block copolymerization, two new PS-
block-P(p-DIPB) and P2VP-block-P(p-DIPB) were successfully
synthesized, while the synthesis of PtBMA-block-P(p-DIPB)
was not possible because PtBMA− could not initiate the
polymerization of p-DIPB. On the basis of these results, it can
be concluded that p-DIPB polymerizable with P2VP− is more
reactive than styrene and appears to be comparable to 2VP in
t
As expected, P2VP− polymerizes the most reactive BMA to
give a P2VP-block-PtBMA. The polymerization of S with P2VP−
occurs only with an insufficient initiation efficiency, followed by
the rapid propagation of S. As a result, most of the starting
P2VP is recovered and a high-molecular-weight and high PS
content P2VP-block-PS is obtained. In addition, the generated
PS− was added to some extent to the pyridine CN moiety.
Thus, both the reactivities of the monomers and their chain-
end anions are evaluated by the block copolymerization results
summarized in Table 5.
The first series block copolymerization, in which S, 2VP, or
tBMA was polymerized, followed by addition of p-DIPB, was
conducted. With this series block copolymerization, not only
the synthetic possibility of block copolymers, but also the
monomer reactivity of p-DIPB will be understood. For the
block copolymerization of S with p-DIPB, S was first
polymerized for 30 min and then p-DIPB was polymerized
with the resulting PS− for 48 h. The polymer yield was
quantitative based on the total weights of S and p-DIPB,
indicating that the second-stage polymerization of p-DIPB
quantitatively proceeded. The SEC profile exhibited a sharp
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t
Table 6. Anionic Block Copolymerization of p-DIPB with S, 2VP, or BMA with Oligo(α-methylstyryl)lithium and a 3.5-Fold
Excess of KOBut in THF at −78 °C
composition first/second
monomers
Mn (kg/mol)
(mol %)
a
b
c
d
first
second
time (h)
conversion (%)
calcd
RALLS
Mw/Mn
calcd
1H NMR
S
0.5
48
1
100
91
6.65
13.9
5.74
14.6
5.89
15.3
6.53
13.8
9.88
10.3
6.67
15.4
6.38
16.8
7.47
15.5
1.04
1.03
1.18
1.08
1.09
1.12
1.04
1.22
1.07
1.07
1.04
1.10
p-DIPB
p-DIPB
p-DIPB
S
56/44
47/53
41/59
36/64
24/76
45/55
59/41
47/53
100/0
31/69
24/76
44/56
2VP
100
91
48
3
tBMA
100
0
11.8
29.7
48
48
0.5
48
1
p-DIPB
p-DIPB
p-DIPB
96
6.32
14.2
5.76
16.9
6.16
12.9
100
90
2VP
100
95
48
tBMA
3
100
a
b
c
Determined by 1H NMR. Mn(calcd) = (MW of monomer) × (convn)/100 × [monomer]/[initiator] + (MW of initiator fragment). Evaluated by
d
SEC equipped with triple detectors. Evaluated by SEC.
monomer reactivity. The reactivity of p-DIPB is definitely less
reactive than that of tBMA because p-DIPB cannot be
polymerized with PtBMA−.
of P(p-DIPB)-block-P2VP and P(p-DIPB)-block-P(tBMA) were
synthesized without any difficulty by the sequential addition of
p-DIPB and then 2VP or tBMA to the initiator under the same
conditions.
Next, the second series block copolymerization, in which p-
DIPB was first polymerized to prepare the P(p-DIPB)−,
The two block copolymers of P(p-DIPB)-block-P2VP and
P(p-DIPB)-block-P(tBMA) were newly synthesized by the
second series sequential polymerization. Their successful
syntheses also support the living nature of the polymerization
of p-DIPB. On the basis of the results, the anionic reactivity of
P(p-DIPB)− is slightly lower than that of PS− and comparable
to that of P2VP− because P(p-DIPB)− can initiate the
polymerization of S with an insufficient efficiency and
quantitatively polymerizes both 2VP and tBMA. In our previous
study, the living anionic polymer of p-DVB could not
polymerize S at all under very similar conditions.12 Accordingly,
P(p-DIPB)− appeared to be appreciably higher than P(p-
DVB)− in anionic reactivity, possibly to due to the electron-
donating character of the α-methyl substituent of the
isopropenyl group to increase the electron-density on the
chain-end anion.
t
followed by addition of S, 2VP, or BMA, was conducted
under the same conditions as those employed in the first series
block polymerization. In the second-stage polymerization of S
with P(p-DIPB)−, S was quantitatively polymerized at −78 °C
for 30 min. The peak of the resulting polymer moved to the
higher-molecular-weight side, but rather broad and asymmetric
in shape and tailed to the lower-molecular weight side (Figure
10). In addition, the same peak as that of the starting P(p-
In conclusion, four block copolymers, that is, PS-block-P(p-
DIPB), P2VP-block-P(p-DIPB), P(p-DIPB)-block-P2VP, and
P(p-DIPB)-block-P(tBMA), were successfully synthesized by
developing the sequential polymerization procedure. All of
them are new block copolymers containing reactive P(p-DIPB)
segments and well-defined in structure. Throughout the block
copolymerization study, as listed in Table 5, p-DIPB was
understood to be comparable to 2VP and located between S
and tBMA in monomer reactivity, namely, S < 2VP ∼ p-DIPB <
tBMA. On the other hand, the anionic reactivity of the chain-
end anion is shown as follows: PS− > P2VP− ∼ P(p-DIPB)− >
PtBMA−. The newly obtained data on p-DIPB by the block
copolymerization were added to Table 5.
Figure 10. SEC profile of P(p-DIPB)-block-PS.
CONCLUSIONS
DIPB) remained to a certain extent. This strongly indicated the
slow initiation of S with P(p-DIPB)− with an insufficient
efficiency in the second-stage polymerization. Thus, a well-
defined P(p-DIPB)-block-PS could not be obtained by this
sequential order of the two monomers. This result also
indicated that the anionic reactivity of P(p-DIPB)− is somewhat
lower than that of PS−, which rapidly and quantitatively
initiated the polymerization of S. Two other block copolymers
■
We have successfully demonstrated that p-DIPB undergoes the
living anionic polymerization in a selective manner through one
of the two isopropenyl groups by the initiator system prepared
from αMSLi and excess KOBut. The unwanted intermolecular
addition reaction of the chain-end anion to the pendant
isopropenyl group was almost suppressed at −78 °C for 168 h
and at −40 °C for 6 h. Soluble polymers were obtained under
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(7) Schmidt, M.; Burchard, W. Macromolecules 1981, 14, 370.
such conditions. The molecular weight was precisely controlled
in the range from 7 620 to 31 500 g/mol and narrowly
distributed, the Mw/Mn values being ≤1.03.
(8) Widmaier, J. M. Makromol. Chem. 1985, 186, 2079.
(9) Okay, O.; Funke, W. Makromol. Chem. 1990, 191, 1565.
(10) Hirao, A.; Tanaka, S.; Goseki, R.; Ishizone, T. Macromolecules
2011, 44, 4579.
(11) Tanaka, S.; Matsumoto, M.; Goseki, R.; Ishizone, T.; Hirao, A.
Macromolecules 2013, 46, 146.
(12) Tanaka, S.; Goseki, R.; Ishizone, T.; Hirao, A. Macromolecules
2014, 47, 2333.
(13) Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. J. Polym.
Sci., Part A: Polym. Chem. 2000, 38, 3211.
(14) Okamoto, A.; Mita, I. J. Polym. Sci., Polym. Chem. Ed. 1978, 16,
1187.
(15) Lutz, P.; Beinert, G.; Rempp, P. Makromol. Chem. 1982, 183,
In contrast, the anionic polymerization of the corresponding
meta-isomer proceeded to afford soluble polymers with about
90% yields after 24 h under the same conditions as already
stated. However, in all of the polymerizations, the formation of
higher-molecular-weight-portions was not avoided. The poly-
merization of the ortho-isomer was more problematic, possibly
due to the steric hindrance of the o-isopropenyl group. Only
small amounts of oligomers (Mn = 300−600 g/mol) were
obtained after 24 h.
The block copolymerization was conducted by the sequential
t
2787.
addition of p-DIPB with either S, 2VP, or BMA. With the
(16) Hirao, A.; Imai, T.; Watanabe, K.; Hayashi, M.; Sugiyama, K.
Chem. Mon. 2006, 137, 855.
(17) Sugiyama, K.; Watanabe, K.; Hirao, A.; Hayashi, M. Macro-
molecules 2008, 41, 4235.
(18) The P(p-DIPB) (Mn = 12 200 g/mol and Mw/Mn = 1.03) was
obtained in THF at −78 °C using the initiator prepared from αMS
and a 2.7-fold excess of KOBut.
(19) Hsieh, H. L.; Quirk, R. P. In Anionic Polymerization; Principles
and Applications; Marcel Dekker: New York, 1996; pp 335−347.
(20) Giebeler, E.; Stadler, R. Macromol. Chem. Phys. 1997, 198, 3815.
(21) Goldacker, T.; Abetz, V.; Stadler, R.; Erukhimovich, I.; Leibler,
L. Nature 1999, 398, 137.
block copolymerization, in which p-DIPB was added as either
the first or second monomer, four new well-defined PS-block-
P(p-DIPB), P2VP-block-P(p-DIPB), P(p-DIPB)-block-P2VP,
and P(p-DIPB)-block-PtBMA containing reactive P(p-DIPB)
segments were successfully synthesized. On the basis of the
block copolymerization results, it is understood that p-DIPB is
t
comparable to 2VP and located between S and BMA in
monomer reactivity, namely, S < 2VP ∼ p-DIPB < tBMA, while
the anionic reactivity of the chain-end anion increases as
follows: PS− > P2VP− ∼ P(p-DIPB)− > P(tBMA)−.
Finally, it should be mentioned that the anionic polymer-
ization behavior of p-DIPB was different from that of p-DVB
with respect to the stability of the chain-end anion. Under very
similar polymerization conditions at −78 °C, P(p-DIPB)− was
stable even after 168 h and, on the other hand, P(p-DVB)−
retained its stability within a few minutes, although it was stable
at least for 30 min at the extremely low temperature of −95 °C.
The anionic reactivity of P(p-DIPB)− was appreciably higher
than that of P(p-DVB)− from the viewpoint of the possible
polymerization of styrene with P(p-DIPB)−.
(22) Ludwig, S.; Boker, A.; Abetz, V.; Muller, A. H. E. Polymer 2003,
̈
̈
44, 6815.
(23) For the synthesis of PS-block-P2VP and PS-block-PtBMA, the
first formed PS− is usually end-capped with 1,1-diphenylethylene prior
to the second monomer addition to avoid the undesired attack of the
PS− on the −CN− bond of the pyridine ring or the ester carbonyl.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Experimental details and H NMR spectra (Figure S1−S12).
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b00575.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ahirao@email.plala.or.jp (A.H.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.T. greatly appreciates the support by Grant-in-Aid for JSPS
fellows from Japan Society for the Promotion of Science
(JSPS). A.H. is also thankful for financial support from Denki
Chemical Co. Ltd., in Japan.
REFERENCES
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