Practical protocols for organotellurium-mediated living radical polymerization by in situ generated initiators from AIBN and ditellurides

Article abstract of DOI:10.1021/ma034211a

A new initiating system for TERP was developed using shelf stable diazo compounds and ditellurides. The obtained polymer possesses an organotellanyl end group, which would be useful for polymer-end manipulations and for block copolymer synthesis.

Full text of DOI:10.1021/ma034211a

Macromolecules 2003, 36, 3793-3796
3793
P r a ctica l P r otocols for
Sch em e 1
Or ga n otellu r iu m -Med ia ted Livin g Ra d ica l
P olym er iza tion by in Situ Gen er a ted
In itia tor s fr om AIBN a n d Ditellu r id es
Sh iger u Ya m a go,*,†,‡,§ Ka zu n or i Iid a ,†
Mitsu r u Na k a jim a , a n d J u n -ich i Yosh id a *
†
,†
Department of Synthetic Chemistry and Biological
Chemistry, Graduate School of Engineering, Kyoto
University, Sakyo-ku, Kyoto 606-8501, J apan, and
PRESTO, J apan Science and Technology Corporation
Ta ble 1. P olym er iza tion of Vin yl Mon om er s w ith AIBN
a n d Ditellu r id es
Received February 18, 2003
condns yield
Revised Manuscript Received April 17, 2003
run methoda monomer
R
(°C/h)
(%)
Mn b
PDIb
Living radical polymerization (LRP) has become an
increasingly important tool for the synthesis of nano-
structural macromolecules with defined structures pos-
sessing a variety of functional groups since radical
species are compatible with many polar functional
groups that do not tolerate ionic and metal-catalyzed
1
2
3
4
5
6
7
8
9
1
1
12
13
1
1
1
A
A
A
A
B
B
B
B
B
B
C
C
C
C
C
C
St
BA
MMA
MMA
St
St
BA
BA
MMA
MMA
St
St
BA
BA
BA
MMA
Me 100/15
Me 100/24
Me 80/13
Me 80/13
Me 90/36
Ph 80/56
Me 100/48
Ph 100/24
Me 80/13
Ph 80/4
Me 90/32
Ph 90/45
Me 100/24
Me 100/24
Ph 100/24
Me 80/0.5
98
69
74
81
9400 1.15
8300 1.12
8600 1.37
8300 1.12
c
54 11700 1.20
55 19000 1.20
>3
25
95 28800 1.14
96 50100 1.22
88
93
38
89
88
1
2900 1.06
4600 1.07
polymerization conditions. LRP relies on the controlled
and reversible generation of active polymer-end radicals
from dormant species, and a variety of new methods
have recently been developed for the precise control of
0
1
9200 1.38
6900 1.48
3400 1.10
7800 1.24
7200 1.18
2
this process. Among these methods, organotellurium-
mediated living radical polymerization (TERP) has
unique versatility, molecular weight control, functional
group compatibility, and ease of polymer-end transfor-
d
e
4
5
6
100 13100 1.55
3
mations. However, a drawback of TERP may be the
a
sensitivity of initiators toward oxygen, and thus the
initiators must be stored under an inert atmosphere and
manipulated using standard syringe techniques. There-
fore, the development of a new initiating system using
shelf stable reagents is highly desirable. Because ditel-
A: Purified 1a was used as an initiator (1:monomer ) 1:100).
B: The initiator was prepared in situ by heating a solution of AIBN
and ditelluride in R,R,R-trifluoromethylbenzene at 80 °C for 3 h
(AIBN:ditelluride:monomer ) 1:1:100). C: AIBN, ditelluride and
b
monomer (1:1:100) was heated as indicated. Calibrated by gel
permeation chromatography using polystyrene standards for
polystyrene samples or polyMMA standards for polyMMA and
polyBA samples. c Dimethyl ditelluride (1 equiv) was added. 0.7
4
lurides are excellent radical capturing reagents, we
d
anticipated that radicals generated from diazo com-
pounds would react with ditellurides to form organo-
tellurium compounds, which could be used directly as
initiators for TERP. We report here one such binary
system for TERP starting from shelf stable reagents,
namely azoisobutyronitrile (AIBN) and ditellurides
e
equiv of dimethyl ditelluride was used. 10 equiv of dimethyl
ditelluride were used.
Sch em e 2
(
Scheme 1). While the effect of diphenyl ditelluride on
the molecular weight control in the AIBN-initiated
polymerization of styrene has been reported, there is
5
a
AIBN (0.1 equiv), Bu
3
SnH(D) (3 equiv), C
6 5 3
H CF , 80 °C, 1
no direct experimental evidence for the involvement of
organotellurium compounds during the polymerization
process. This work, however, clearly reveals the crucial
role of the carbon-tellurium bond both in the initiators
and in the dormant polymer species for the precise
control of the polymerization process.6
h; 89% and 96% yields for 4 and 4-d
1
, respectively.
for the precise control of the polymerization of MMA,3b
a afforded polyMMA with an acceptable level of
polydispersity without ditelluride (PDI ) 1.37, entry 2).
The addition of 1 equiv of dimethyl ditelluride further
controlled the polymerization of MMA (PDI ) 1.12,
entry 3). The existence of the methyltellanyl end group
in polySt 3 was confirmed by the characteristic benzylic
proton signal at 3.9 ppm in the H NMR spectrum of a
sample prepared from 1a and 30 equiv of styrene
Scheme 2). In addition, reduction of 3 by either tribu-
tyltin hydride or deuteride afforded 4 or 4-d1, respec-
1
Before attempting the binary initiating system, we
first examined the polymerization of styrene (St), butyl
acrylate (BA), and methyl methacrylate (MMA) using
purified 1a (R ) Me), which was prepared from 2-bromo-
1
2
1
-methylpropionitrile and methyltellanyllithium (Table
, entries 1-4, method A). The desired polymers formed
(
with the predicted molecular weight and low polydis-
persity in high yields in all cases. While the previous
initiators required the addition of dimethyl ditelluride
7
tively. Selective incorporation of the deuterium atom
in the polymer was ascertained by MALDI-TOF mass
spectroscopy by an increase of one mass number in the
ions seen for 4-d1 compared to those for 4 (Figure 1).
The mass spectroscopy analyses also revealed the
formation of the well-defined polySt (PDI ) 1.04). The
†
Kyoto University.
J apan Science and Technology Corporation.
Current address: Division of Molecular Material Science,
‡
§
Graduate School of Science, Osaka City University, Osaka 558-
2
8
585, J apan.
H NMR spectrum of 4-d1 further supported the selec-
1
0.1021/ma034211a CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/03/2003

3
794 Communications to the Editor
Macromolecules, Vol. 36, No. 11, 2003
F igu r e 1. MALDI-TOF MS spectra of 4 (top) and 4-d
1
(bottom) obtained by method A. The molecular ions were observed as
+
silver ion adducts [m/z ) (M + Ag) ]. Molecular weight (M
n
) and polydispersity (PDI) were directly obtained by MS spectra.
3
tive incorporation of deuterium at the benzylic position
δ ) 2.30 ppm, broad singlet). The same labeling
previously reported initiators in terms of versatility and
(
molecular weight controllability.
experiments with polyBA and polyMMA also revealed
the existence of the methyltellanyl group at the polymer
end (see Supporting Information). The desired polymers
formed with very narrow molecular weight distributions
We next examined the binary system using in situ
generated initiators (method B). Thus, initiator 1a was
prepared by heating an equimolar amount of AIBN (0.20
mmol) and dimethyl ditelluride (0.20 mmol) in C D (0.8
mL) at 80 °C for 3 h. The H NMR of the crude mixture
indicated the formation of 1a (0.07 mmol, 17% yield
based on the tellurium atom) and 2,2,3,3-tetramethyl-
6
6
1
(
PDI ) 1.06 for polyBA and PDI ) 1.11 for polyMMA).
These results clearly indicate that 1a also possesses
excellent or even superior versatility compared with the

Macromolecules, Vol. 36, No. 11, 2003
Communications to the Editor 3795
succinonitrile (0.14 mmol, 69% yield), a dimer of the
AIBN-derived radical, together with the recovered dim-
ethyl ditelluride (0.18 mmol, 90%). The rather low
coupling efficiency of the AIBN-derived radical with
dimethyl ditelluride is not surprising because ca. 40%
of the radical generated from AIBN dimerizes in solvent
The existence of an organotellanyl end group in the
polySt and polyBA prepared by method C was also
1
confirmed directly by H NMR spectroscopy and by
2
MALDI TOF MS and H NMR spectra after treatment
of the polymer with tributyltin hydride or deuteride (see
1
Supporting Information). While the H NMR spectra
8
cages before diffusing to bulk solutions. The phenyl-
were very similar to those prepared by method A,
MALDI TOF MS analysis revealed the existence of a
series of new peaks. The newly observed peaks also
increased one mass number upon the deuterium-label-
ing experiment. Therefore, these peaks were also de-
rived from dormant species possessing organotellanyl
end groups; however, the origin of the new peaks is
unclear at the present time.
tellanyl-substituted 1b could also be prepared in 8%
yield by using commercially available diphenyl ditellu-
ride instead of dimethyl ditelluride. The 1/ditelluride
ratio varied upon changing the starting AIBN/ditellu-
ride ratio, but no conditions were found where all the
ditelluride was converted to 1 (see Supporting Informa-
tion).
In summary, we have developed a new initiating
system for TERP using shelf stable diazo compounds
and ditellurides. The obtained polymer possesses an
organotellanyl end group, which would be useful for
polymer-end manipulations and for block copolymer
The in situ generated 1 also initiated polymerization
of St, BA, and MMA, but the efficiency and controllabil-
ity varied depending on the vinyl monomers (Table 1,
entries 5-10). Phenyltellanyl-substituted initiator 1b
generally promoted the polymerization much faster than
3
synthesis. Since methods B and C do not require
1
a , while the controllability of the molecular weight
distribution was slightly less efficient with 1b than with
a . Because 1b has a lower bond dissociation energy
purification and handling of air-sensitive organotellu-
rium compounds, these new methods are especially
1
1
3
suitable for the practical use of TERP. Method C is
the most convenient and is useful for the polymerization
of styrene and acrylate derivatives because of the
simplicity of the reaction procedure with a high level of
controllability of the resulting polymers, while method
B should be useful for the controlled polymerization of
methacrylate derivatives. Method A using purified
initiators, however, is certainly the choice to obtain the
living polymers with the highest level of controllability
for both molecular weight and polydispersity.
than 1a (105 and 120 kJ /mol by density functional
9
theory calculations, respectively ), the observed trend
is due to the faster radical generation from the phenyl-
tellanyl-substituted initiator or dormant species than
from the methyltellanyl-substituted ones. PolyMMA
with the anticipated molecular weight and low polydis-
persity (PDI < 1.26) was successfully obtained, indicat-
ing that polymerization is initiated exclusively by 1
entries 9 and 10).10 Because the reaction mixture
(
contained the unreacted ditelluride, further addition of
the ditelluride was unnecessary. As the remaining
ditelluride also shifts the equilibrium constant to the
dormant organotellurium species from the polymer-end
radicals, the rate of the polymerization using method
B was considerably slower compared to that using
method A, especially in the polySt and polyBA synthe-
ses.11 Styrene was slowly polymerized upon prolonged
heating at 80-90 °C with low polydispersity (PDI )
Ack n ow led gm en t. This work was partly supported
by a Grant-in-Aid for Scientific Research from J apan
Society for the Promotion of Science.
Su p p or tin g In for m a tion Ava ila ble: Experimental sec-
tion. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
.20) (entries 5 and 6), while polymerization of BA was
Refer en ces a n d Notes
not complete within a period of time for practical use
(
1) Handbook of Radical Polymerization; Matyjaszewski, K.,
Davis, T. P., Eds.; Wiley-Interscience: New York, 2002.
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(entries 7 and 8).
Polymerization by heating a mixture of AIBN, ditel-
luride, and vinyl monomer (method C) was especially
effective for the controlled polymerization of BA (entries
(
12).
2) (a) Hawker, C. J .; Bosman, A. W.; Harth, E. Chem. Rev.
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1
3-15). The results could be attributed to the decrease
(
of ditelluride in the reaction mixture due to the decrease
of homocoupling of the AIBN-derived radical, which now
reacted preferentially with BA followed by the ditellu-
ride to form dormant species. The polymerization was
almost completed within 24 h at 100 °C by changing
the AIBN/ditelluride ratio (entry 14) or by using diphe-
nyl ditelluride (entry 15), and the desired polyBA formed
with low polydispersity (PDI < 1.24). PolySt with low
polydispersity was also formed by method C when the
polymerization was carried out below 90 °C with the
combination of dimethyl ditelluride (entry 11),12 whereas
polymerization using diphenyl ditelluride or a higher
temperature resulted in the formation of polySt of high
polydispersity (PDI > 1.4). Polymerization of MMA
could not be controlled even in the presence of an excess
of ditelluride (entry 16). As the polymerization pro-
ceeded very rapidly with the formation of high-molec-
ular-weight polyMMA, the result must be attributed to
the preferential occurrence of the conventional AIBN-
initiated free radical polymerization.
2
2001, 101, 2921. Kamigaito, M.; Ando, T.; Sawamoto, M.
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(
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(
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796 Communications to the Editor
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n
8% formation of 1a and 1b, respectively.
3
9, 3669. Yamago, S.; Miyazoe, H.; Yoshida, J . Tetrahedron
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(
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(
(
(
(PDI < 1.4) was obtained below 90 °C.
1
911.
(
13) The toxicity of organotellurium compounds is not clear. They
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9) B3LYP density functionals with LANL2DZ basis set for the
tellurium atom and 6-31G(d) basis set for the rest were used.
The obtained energies resulted from the sum of Hartree-
Fock and zero-point vibrational energies of 1 and the
MA034211A