Photoinduced switching from living radical polymerization to a radical coupling reaction mediated by organotellurium compounds

Article abstract of DOI:10.1021/ja300869x

An efficient and simple method for the synthesis of symmetric macromolecules by photoinduced switching from radical polymerization to a radical coupling reaction is reported. Structurally well-defined telechelic polyisoprenes and ABA-triblock copolymers were prepared by successive organotellurium-mediated living radical polymerization (TERP) under thermal conditions, followed by a polymer-end radical coupling reaction under photoirradiation.

Full text of DOI:10.1021/ja300869x

Communication
pubs.acs.org/JACS
Photoinduced Switching from Living Radical Polymerization to a
Radical Coupling Reaction Mediated by Organotellurium
Compounds
Yasuyuki Nakamura,†,‡ Takahiro Arima,† Sora Tomita,† and Shigeru Yamago*,†,‡
†Institute for Chemical Research, Kyoto University, Gokasyo, Uji, 611-0011 Japan
‡CREST, Japan Science and Technology Agency
S
* Supporting Information
Scheme 1. Sequential LRP-RC, and Its Application to the
Synthesis of ABA-Triblock Copolymers
ABSTRACT: An efficient and simple method for the
synthesis of symmetric macromolecules by photoinduced
switching from radical polymerization to a radical coupling
reaction is reported. Structurally well-defined telechelic
polyisoprenes and ABA-triblock copolymers were prepared
by successive organotellurium-mediated living radical
polymerization (TERP) under thermal conditions, fol-
lowed by a polymer-end radical coupling reaction under
photoirradiation.
odulation of the reaction pathway using external stimuli
= Et) under thermal conditions. The monomer conversion
reached 35% after heating at 120 °C for 36 h, and polyisoprene
(PIp) 2a with a narrow molecular weight distribution (MWD,
Mw/Mn = 1.15, where Mw and Mn refer to weight and number
average molecular weights, respectively) was obtained (Table 1,
M
has been a significant challenge in organic and polymer
syntheses, because such methods would considerably expand
the structural diversity of products from a limited number of
precursors. Although a variety of chemical stimuli have been
used for this purpose, physical stimuli, such as thermal and
photochemical stimuli, like those used in the pericyclic
reactions,1 are attractive due to their “green” character.
We have already reported that organotellurium-mediated
living radical polymerization (TERP) possesses several
synthetic advantages over other living radical polymerization
(LRP) methods,2 such as high monomer versatility, high
compatibility toward functional groups and solvents,3 and ease
of the living-end transformation for the synthesis of block
copolymers4 and end-functionalized polymers.5 In addition, we
have reported that TERP can be initiated by thermal,2c,4a
chemical,6 and photochemical stimuli.3d While expanding the
synthetic applicability of TERP to dienes, we found that the
reaction pathway can be selectively switched from LRP to a
radical-coupling reaction (RC) by using photostimuli. Fur-
thermore, this method made it possible to synthesize telechelic
polymers and symmetrical ABA-triblock copolymers with
controlled macromolecular structures (Scheme 1).
a
Table 1. Organotellurium-Mediated LRP of Ip
c
Ip
(TeMe)2 time conv
Mn(GPC)
b
c
run (equiv)
(equiv)
(h)
(%)
(Mn(NMR)
)
Mw/Mn
1
2
3
4
5
6
100
0
1
1
1
1
0
24
14
12
8
35
25
12
8
4700 (2500)
3500 (1700)
6100 (2700)
11500 (5800)
19900 (12100)
9300 (3100)
1.15
1.10
1.10
1.12
1.14
1.43
100
300
1000
2000
100
8
9
d
24
44
a
A mixture of 1a (1 equiv) and isoprene was heated at 120 °C.
Determined from H NMR spectroscopy. Determined from GPC
b
c
1
calibrated against PSt standards. Mn determined from 1H NMR
spectroscopy is shown in parentheses. Polymerization was carried out
d
in the presence of 1-[(1-cyano-1-methylethyl)azo]formamide (1
equiv).
run 1). The Mn(GPC) obtained from gel permeation chromatog-
raphy (GPC) by using polystyrene (PSt) standards was
considerably different from the value calculated by H NMR
It has been reported that the reaction course can be changed
from LRP to RC by adding an excess amount of Cu(0) to
polystyrene prepared via Cu(I) catalyzed atom transfer radical
polymerization7 and by adding dienes to polyacrylonitrile,
poly(vinyl acetate), and poly(N-vinyl pyrrolidone) prepared via
cobalt-mediated radical polymerization.8 However, to the best
of our knowledge, there is no report on the selective switching
of the reaction course without chemical stimuli.
1
[Mn(NMR)] due to the lack of appropriate standards, but the
M
n(GPC) is used for the following discussions. 1H NMR spectral
analysis showed that the polymer main chain mainly contained
(1,4) isomeric units (90%) together with (1,2) and (3,4) units
(5% each) (Chart 1).9 The isomeric ratio is identical to that for
We first examined TERP of isoprene (Ip, 100 equiv) in the
presence of organotellurium chain transfer agent (CTA) 1a (R
Received: February 6, 2012
Published: March 13, 2012
© 2012 American Chemical Society
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Communication
Chart 1. Structure of PIp 2 and Dimer 3
PIp prepared by using free radical polymerization and other
LRP methods,10 and the results are consistent with the
involvement of free radicals in the carbon−carbon bond
formation step. Although the polymerization exhibited high
MWD control at low monomer conversions, MWD became
broad as the monomer conversion increased due to the
formation of high molecular weight polymers probably from
RC of polymer-end radicals (see Figure S7 in Supporting
Information, SI). Therefore, the polymerization was stopped at
low monomer conversions in the following experiments.10a
The addition of dimethyl ditelluride (1.0 equiv) considerably
enhanced the rate of monomer conversion and MWD control
(run 2, Figure S8 in SI). The results are consistent with our
previous reports on the role of the ditelluride in the
polymerizations of styrene and methacrylate.4a,11,12 Structurally
well-controlled PIps with high molecular weights and narrow
MWDs (Mn ≈ 19900, Mw/Mn < 1.14) were obtained by
increasing the Ip/1a ratio (runs 3−5). Although addition of an
azo-initiator, 1-[(1-cyano-1-methylethyl)azo]formamide, in-
creased the rate of polymerization,6 MWD control decreased
(run 6). This is because azo-initiator-derived radicals generated
significant amounts of PIps which lead to the formation of large
amounts of dead polymers.13
Figure 1. MALDI-TOF MS spectrum of 3a (Table 2, run 1). The
molecular ions were observed as silver ion adducts [m/z = (M + Ag)+].
peak resolution method.7a,15 On the other hand, RC under
thermal conditions at high temperatures was sluggish and did
not go to completion even after heating at 140 °C for 24 h
(Table 2, run 2, and Figure S12 in SI). Since the dimer forms
via a coupling reaction involving polymer-end radicals, the low
xc under thermal conditions is due to the low efficiency of the
generation of polymer-end radicals from 2a and also the
potential side reaction at high temperature.
Selective RC proceeded for higher molecular weight PIps.
The corresponding dimers were selectively obtained with a high
xc (0.87−0.95) (Table 2, runs 3 and 4). Since the propagation
rate of Ip polymerization was slow at room temperature,16
dimerization selectively occurs without chain elongation. The
results clearly show that selective switching from LRP to RC
occurred upon photoirradiation.
Hydroxyl-telechelic PIp 3c was prepared from either CTA 1b
or 1c having trimethylsiloxy or hydroxyl groups, respectively.
Polymerization of Ip (200 equiv) in the presence of 1b gave α-
trimethylsiloxy PIp 2b (Mn = 2300, Mw/Mn = 1.11 after 8%
monomer conv.), which was converted to 3b (Mn = 4400, Mw/
Mn = 1.12) with high coupling efficiency (xc = 0.93) by
photoirradiation. The silyl group was removed by tetrabutyl
ammonium fluoride to give 3c (Mn = 4200, Mw/Mn = 1.12).
Alternatively, 1c prepared from 1b could be directly used for
the polymerization of Ip to give α-hydroxyl PIp 2c (Mn = 3200,
Mw/Mn = 1.09, 14% monomer conv.), which afforded 3c (Mn =
6400, Mw/Mn = 1.10) in an excellent coupling efficiency (xc =
0.96) by photoirradiation.
The RC of 2a was examined next. After thermal polymer-
ization of Ip (100 equiv), as shown above, crude 2a (Mn =
1800, Mw/Mn = 1.15, 9% monomer conversion) was irradiated
with a 500 W Hg lamp through a >390 nm cutoff filter at room
temperature for 1 h.14 2a completely disappeared, and coupling
product 3a, which had an Mn (3500) nearly double that of 2a
and a narrow MWD (1.13), was obtained (Table 2, run 1). The
a
Table 2. Radical Coupling Reaction of PIps
PIp-TeMe
Product
Symmetric ABA triblock copolymers were synthesized from
organotellurium macroCTAs. For example, poly(methyl
methacrylate) (PMMA)-TeMe (Mn = 4300, Mw/Mn = 1.16),
prepared from 1a and methyl methacrylate (MMA), was
treated with Ip (300 equiv) in the presence of dimethyl
ditelluride at 120 °C for 10 h, affording the structurally well-
controlled block copolymer PMMA-b-PIp-TeMe (Mn = 7000,
Mw/Mn = 1.11, 10% monomer conv.). Irradiation of the
copolymer afforded symmetric triblock copolymer PMMA-b-
PIp-b-PMMA with a nearly double Mn (12800) and a narrow
MWD (1.11) with xc = 0.92 (Table 3, run 1, Figure 2). The
same triblock copolymers with high molecular weight (Mn =
22100 and 28900) were successfully synthesized in a controlled
manner (Table 3, runs 2 and 3). Structurally well-defined PSt-
b-PIp-b-PSt (Mn = 17000, Mw/Mn = 1.18) was also synthesized
from PSt-TeMe (Mn = 5900, Mw/Mn = 1.15) by using
sequential TERP of Ip and RC (Table 3, run 4). Since various
di- and triblock copolymers have already been prepared by
using TERP,3e,4a,c,d such copolymers would be transformed to
time
(h)
b
b
b
b
c
run
1
Mn(GPC)
Mw/Mn
Mn(GPC)
Mw/Mn
xc
1800
3000
3000
9700
1.15
1.09
1.09
1.09
1
3500
4100
5900
17200
1.13
1.26
1.10
1.11
0.97
0.51
0.95
0.87
d
2
24
2
3
4
1
a
A 500 W high-pressure Hg lamp was irradiated through a >390 nm
cutoff filter. Determined from GPC calibrated against PSt standards.
b
c
Coupling efficiency (xc) calculated by a GPC peak resolution
method.7a,15 The reaction was carried out in the dark at 140 °C in
d
benzene.
methyltellanyl group was quantitatively recovered as dimethyl
ditelluride. The dimeric structure was further confirmed by
using matrix-assisted laser desorption/ionization time-of-flight
mass spectrometry (MALDI-TOF MS), which showed a series
of molecular ion peaks corresponding to 3a (Figure 1). The
efficiency of RC (xc) was determined to be 0.97 using a GPC
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Journal of the American Chemical Society
Communication
Table 3. Synthesis of ABA Triblock Copolymers from Organotellurium MacroCTAs
a
b
MacroCTA
Ip polymerization
AB diblock copolymer
RC
ABA triblock copolymer
c
c
d
c
c
e
c
c
run
structure
Mn(GPC)
Mw/Mn
equiv
time (h)
conv (%)
Mn(GPC)
Mw/Mn
xc
Mn(GPC)
Mw/Mn
f
1
PMMA-TeMe
PMMA-TeMe
PMMA-TeMe
PSt-TeMe
4300
9100
13700
5900
1.16
1.18
1.16
1.15
300
400
400
500
10
8
10
7
7000
1.11
1.15
1.18
1.12
0.92
0.90
0.88
0.86
12800
22100
28900
17000
1.11
1.13
1.15
1.18
fg
2 ,
12300
16600
9700
fg
3 ,
8
6
4
10
8
a
b
A mixture of macroCTA (1 equiv), (TeMe)2 (1 equiv), and Ip was heated at 120 °C. The polymerization mixture was irradiated with a 500 W
c
d
high-pressure Hg lamp through a >390 nm cutoff filter at 20 °C. Determined from GPC calibrated against PMMA or PSt standards. Determined
from 1H NMR spectroscopy. Determined from GPC using a peak resolution method.7a 1,4-Dioxane was used as a solvent. The polymerization of
e
f
g
isoprene was carried out without (TeMe)2.
REFERENCES
■
(1) Fleming, I. Frontier Orbitals and Organic Chemical Reaction;
Wiley: London, 1976.
(2) (a) Yamago, S. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1−
12. (b) Yamago, S. Chem. Rev. 2009, 109, 5051−5068. (c) Yamago, S.;
Iida, K.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 2874−2875.
(3) (a) Yamago, S.; Ray, B.; Iida, K.; Yoshida, J.; Tada, T.; Yoshizawa,
K.; Kwak, Y.; Goto, A.; Fukuda, T. J. Am. Chem. Soc. 2004, 126,
13908−13909. (b) Sugihara, Y.; Kagawa, Y.; Yamago, S.; Okubo, M.
Macromolecules 2007, 40, 9208−9211. (c) Yamago, S.; Kayahara, E.;
Kotani, M.; Ray, B.; Kwak, Y.; Goto, A.; Fukuda, T. Angew. Chem., Int.
Ed. 2007, 46, 1304−1306. (d) Yamago, S.; Ukai, U.; Matsumoto, A.;
Nakamura, Y. J. Am. Chem. Soc. 2009, 131, 2100−2101. (e) Mishima,
Figure 2. GPC profiles of block copolymerization and RC for the
E.; Yamago, S. Macromol. Rapid Commun. 2011, 32, 893−898.
preparation of PMMA-PIp-PMMA (Table 3, run 1). The polymers
were analyzed without purification.
(4) (a) Yamago, S.; Iida, K.; Yoshida, J. J. Am. Chem. Soc. 2002, 124,
13666−13667. (b) Ray, B.; Kotani, M.; Yamago, S. Macromolecules
2006, 39, 5259−5265. (c) Yusa, S.; Yamago, S.; Sugahara, M.;
the corresponding symmetrical multiblock copolymers under
the current conditions.
Morikawa, S.; Yamamoto, T.; Morishima, Y. Macromolecules 2007, 40,
5907−5915. (d) Kumar, S.; Changez, M.; Murthy, C. N.; Yamago, S.;
Lee, J.-S. Macromol. Rapid Commun. 2011, 32, 1576−1582.
(e) Mishima, E.; Yamada, T.; Watanabe, H.; Yamago, S. Chem.
Asian J. 2011, 6, 445−451.
(5) (a) Yamada, T.; Mishima, E.; Ueki, K.; Yamago, S. Chem. Lett.
2008, 650−651. (b) Yamago, S.; Kayahara, E.; Yamada, H. React.
Funct. Polym. 2009, 69, 416−423. (c) Yamago, S.; Yamada, T.; Togai,
M.; Ukai, Y.; Kayahara, E.; Pan, N. Chem.Eur. J. 2009, 15, 1018−
1029. (d) Kayahara, E.; Yamada, H.; Yamago, S. Chem.Eur. J. 2011,
17, 5272−5280.
In summary, Ip was polymerized in a controlled manner by
TERP under thermal conditions. Since conjugated dienes are
one of the challenging monomers in LRP,10,17,18 TERP
provides a new synthetic route to polydienes with controlled
structures. Furthermore, selective switching from LRP to RC
upon photoirradiation occurred with high coupling efficiency,
and this method provides structurally well controlled telechelic
PIps and symmetrical ABA-triblock copolymers. Considering
the high versatility of TERP for the synthesis of homo and
block copolymers, we believe that the current method would be
useful for providing varieties of structurally well controlled new
polymers for advanced macromolecular engineering.
(6) Goto, A.; Kwak, Y.; Fukuda, T.; Yamago, S.; Iida, K.; Nakajima,
M.; Yoshida, J. J. Am. Chem. Soc. 2003, 125, 8720−8721.
(7) (a) Yoshikawa, C.; Goto, A.; Fukuda, T. e-Polym. 2002, 13, 1−12.
(b) Yurteri, S.; Cianga, I.; Yagci, Y. Macromol. Chem. Phys. 2003, 204,
1771−1783. (c) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101,
2921−2990. (d) Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev.
2009, 109, 4963−5050.
ASSOCIATED CONTENT
■
S
* Supporting Information
(8) (a) Debuigne, A.; Jer
Ed. 2009, 48, 1422−1424. (b) Debuigne, A.; Poli, R.; Winter, J. D.;
Laurent, P.; Gerbaux, P.; Debois, P.; Wathelet, J.-P.; Jer me, C.;
́ ̂
ome, C.; Detrembleur, C. Angew. Chem., Int.
Synthesis of 1b and 1c, polymerization and RC reaction
procedures, GPC profiles, and NMR spectra of PIp and new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
́ ̂
o
Detrembleur, C. Chem.Eur. J. 2010, 16, 1799−1811. (c) Debuigne,
A.; Poli, R.; Winter, J. D.; Laurent, P.; Gerbaux, P.; Wathelet, J.-P.;
Jer
(d) Debuigne, A.; Poli, R.; Jer
Prog. Polym. Sci. 2009, 34, 211−239.
(9) Sato, H.; Ono, A.; Tanaka, Y. Polymer 1977, 18, 580−586.
(10) (a) Benoit, D.; Harth, E.; Fox, P.; Waymouth, R. M.; Hawker, C.
J. Macromolecules 2000, 33, 363−370. (b) Wegrzyn, J. K.; Stephan, T.;
Lau, R.; Grubbs, R. B. J. Polym. Sci., Part A: Polym. Chem. 2005, 43,
2977−2984. (c) Jitchum, V.; Perrier, S. Macromolecules 2007, 40,
1408−1412. (d) Germack, D. S.; Wooley, K. L. J. Polym. Sci., Part A:
Polym. Chem. 2007, 45, 4100−4108.
́
o
̂
me, C.; Detrembleur, C. Macromolecules 2010, 43, 2801−2813.
AUTHOR INFORMATION
́
o
̂
me, C.; Jer me, R.; Detrembleur, C.
́ ̂
o
■
Corresponding Author
yamago@scl.kyoto-u.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by the Core Research for
Evolution Science and Technology (CREST) of the Japan
Science and Technology Agency and Toray Science and
Technology Grant.
(11) Kwak, Y.; Tezuka, M.; Goto, A.; Fukuda, T.; Yamago, S.
Macromolecules 2007, 40, 1881−1885.
(12) Since dimethyl ditelluride possesses a similar bond dissociation
energy to 1, dimethyl ditelluride would thermally generate a
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Journal of the American Chemical Society
Communication
methyltellanyl radical under the reaction conditions. By reacting the
tellanyl radical with 1 or the dormant species generating the initiating
radicals, the rate of polymerization was increased.
(13) Nakamura, Y.; Kitada, Y.; Kobayashi, Y.; Ray, B.; Yamago, S.
Macromolecules 2011, 44, 8388−8397.
(14) Direct irradiation without a cutoff filter led to the formation of a
black precipitate, probably the tellurium element, which decreased the
irradiation efficiency (see Figure S11 in SI).
(15) For the determination of the coupling efficiency of RC, the
equation xc = (1 − Mn(precursor)/Mn(Product))/2 is often used. However
due to the difference of the structure of polyisoprene and polystyrene
standards, the GPC peak resolution method should be more reliable.
For details about the equation, see ref 7a.
(16) Kamachi, M.; Kajiwara, A. Macromolecules 1996, 29, 2378−2382.
(17) Germack, D. S.; Harrisson, S.; Brown, G. O.; Wooley, K. L. J.
Polym. Sci., Part A: Polym. Chem. 2006, 44, 5128−5228.
(18) Asandei, A. D.; Simpson, C. P.; Yu, H. S.; Adebolu, O. I.; Saha,
G.; Chen, Y. ACS Symp. Ser. 2009, 1024, 149−166.
5539
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