Refractive index changes in polymers bearing pendant active ester groups by thermal rearrangement reaction

Article abstract of DOI:10.1246/cl.2011.1363

Linear and branched polymers bearing S-(2-benzoxazolyl) thioester groups were selectively synthesized by free radical and photoinitiated radical polymerization, respectively, of S-(2-benzoxazolyl)-4-vinylthiobenzoate. Upon heating, the pendant S-(2-benzox

Full text of DOI:10.1246/cl.2011.1363

doi:10.1246/cl.2011.1363
Published on the web November 19, 2011
1363
Refractive Index Changes in Polymers Bearing Pendant Active Ester Groups
by Thermal Rearrangement Reaction
Makoto Miyasaka, Ayano Higurashi, and Atsushi Kameyama*
Department of Chemistry, Faculty of Engineering, Kanagawa University,
Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa 221-8686
(Received September 12, 2011; CL-110754; E-mail: kameya01@kanagawa-u.ac.jp)
Linear and branched polymers bearing S-(2-benzoxazolyl)
O
C
O
C
thioester groups were selectively synthesized by free radical and
photoinitiated radical polymerization, respectively, of S-(2-
benzoxazolyl)-4-vinylthiobenzoate. Upon heating, the pendant
S-(2-benzoxazolyl) thioester groups were converted to 3-acyl-
benzoxazoline-2-thione groups, accompanied with a change of
refractive index (¦n) of about +0.01 in both cases. The
structural conversion was confirmed by UV-vis and IR
spectroscopy, as well as comparison with authentic polymers
bearing 3-acylbenzoxazoline-2-thione groups (similarly synthe-
sized from 3-(4-vinylbenzoyl)benzoxazoline-2-thione). Applica-
tion of such thermally induced rearrangement reactions may
provide a new approach for obtaining materials whose refractive
index can be changed by heat (e.g., generated by a laser beam).
N
O
Δ
H3CO
N
O
H3CO
S
S
2
5
Scheme 1. Thermal rearrangement of 2 to 5.
changes. Here, we focused on the thermal rearrangement
reaction of S-(2-benzoxazolyl) thioester (S-acyl) to 3-acyl-
benzoxazoline-2-thione (N-acyl).12,13 The rearrangement gener-
ates a polar functional C=S bond, which is expected to cause an
increase of the molar refraction R. Therefore, we speculated that
thin films of polymer containing S-benzoxazolyl thioester
moieties might show an increase of refractive index on exposure
to heat. In this study, we designed and synthesized a styrene
monomer bearing S-(2-benzoxazolyl) thioester moiety, and
synthesized linear and branched polymers bearing S-(2-benzox-
azolyl) thioester moieties by means of simple radical and
photoinitiated radical polymerizations, respectively. We then
examined the refractive index change of thin films of these
polymers upon heating. We also examined whether the structural
difference between the linear and branched polymers influenced
the change of refractive index.
The thermal rearrangement reaction from S-(2-benzox-
azolyl) thioester to 3-acylbenzoxazoline-2-thione was confirmed
using model compound 2 (Scheme 1). 2 exhibited a sharp
exothermic at 101 °C and an endotherm at 112 °C, which reflect
melting point and thermal rearrangement of 2, respectively. The
melting point and spectroscopic data of compound obtained
from thermal rearrangement of 2 was identical with the authentic
N-acyl derivative 5.
The styrene monomer, S-(2-benzoxazolyl)-4-vinylthioben-
zoate (1), was synthesized by the reaction of 4-vinylbenzoyl
chloride and 2-sulfanylbenzoxazole with triethylamine (TEA) in
THF at below 10 °C. The polymerization of 1 was carried out as
a free radical polymerization with 2,2¤-azobis(4-methoxy-2,4-
dimethylvaleronitrile) (V-70) as the initiator in 1,4-dioxane at
30 °C for 18 h, to afford a homopolymer P-1 with Mn = 1.7 ©
104, Mw/Mn = 2.29, in 58% yield (Scheme 2). 1 has an
interesting structure because it contains a polymerizable styrene
group and an initiating/propagating moiety consisting of a
benzoxazolylthio (BT) group. Photolysis of 1 leads to the
initiating benzoyl radical with inactive BT radical. This benzoyl
radical can react with the vinyl group to form dimer-like
structure (ABB¤ monomer).14 Branched polymer (P-2, Mn =
5.2 © 103, Mw/Mn = 2.83) with terminal S-benzoxazolyl thio-
benzoate (SBT) was synthesized by photoinitiated radical
polymerization of 1 with S-(2-benzoxazolyl)-4-methoxythioben-
zoate as a capping agent15 (2, 2 mol %) at room temperature
in 26% yield (Scheme 2). The structures were confirmed by
Materials whose refractive index (n) can be tuned or
modified are required for fabrication of optical waveguides,
switches, and data storage devices,1 and various UV-responsive
functional polymers have been developed for such applications.2
Kern et al. reported that exposure of thiocyanate-bearing
polymer films to UV light resulted in isomerization of
thiocyanate (-S-C¸N) to isothiocyanate (-N=C=S), accompa-
nied with a refractive index change (¦n about +0.03).3
Recently, photo-Fries rearrangement of aromatic ester4 or N-
phenylamide5 moieties was also reported to result in a significant
increase of polymer refractive index (up to ¦n = +0.1).
Nishikubo et al. prepared polymers with pendant bicyclo ortho
ester groups and observed ¦n values in the range between
+0.011 and +0.023 after photoinitiated cationic curing of the
polymers.6 The refractive index change ¦n is considered to be
primarily due to the difference of molar refraction (R) of the
functional groups before and after reaction, according to the
Lorentz-Lorenz equation:7
n2 ꢀ 1
n2 þ 2
M
n2 ꢀ 1
n2 þ 2
R ¼
ꢁ
¼
ꢁ V
ð1Þ
μ
where M is the molecular weight (M g mol¹1), n is the refractive
index, μ is the density, and V is the molecular volume.
To our knowledge, there are only a few reported examples
of the use of heating to generate an increase of refractive index,
i.e., it was reported that the heat-induced conformational change
of poly(di-n-hexylsilane) from trans-planar to random helix was
accompanied with a refractive index increase.8 Also, it was
reported that the thermal isomerization of quadricyclane (QC) to
norbornadiene (NB) groups resulted in an increase of refractive
index (this is the reverse reaction of the photoisomerization from
NB to QC).9 Since many heat-induced structural changes of
organic molecules are known,10,11 we considered that other
such reactions might be available to generate refractive index
Chem. Lett. 2011, 40, 1363-1365
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1364
n
n
n
C
S
O
C
S
O
C
S
O
Δ
C
N
O
V-70
N
O
S
1,4-Dioxane
O
N
SBT =
S
O
N
O
N
O
O
P-3
SBT
SBT
SBT
SBT
SBT
1
P-1
P-1
SBT
SBT
H3CO
SBT
Scheme 4. Thermal rearrangement of P-1 to P-3.
SBT
N
O
SBT
h
ν
SBT
S
SBT
SBT
2, THF
SBT
O
SBT
SBT
P-2
SBT
SBT
Scheme 2. Selective synthesis from 1 of linear and branched
polymers bearing S-(2-benzoxazolyl) thioester moieties.
n
V-70
C
N
O
O
C
N
O
O
O
1,4-Dioxane
N
NBT =
S
S
S
O
3
P-3
NBT
NBT
NBT
SBT
BT
N
NBT
NBT
NBT
NBT
NBT
Figure 1. FT-IR spectra of P-1; (a) prior to heating, and
(b) after heating at 135 °C for 30 min.
NBT
O
hν
NBT
N
NBT
S
THF
O
NBT
SBT
NBT
P-4
NBT
NBT
Scheme 3. Selective synthesis from 3 of linear and branched
polymers bearing 3-acylbenzoxazoline-2-thione moieties.
1H NMR and FT-IR spectroscopy.16 The FT-IR spectrum showed
a characteristic absorption peak at 1702 cm¹1 due to the carbonyl
groups.
In order to confirm the structures of the products obtained
by thermal reaction of the above polymers, we also synthesized
authentic samples of the N-acyl-type polymers from 3-(4-
vinylbenzoyl)benzoxazoline-2-thione (3) as a monomer, using
the same methods. Linear polymer P-3 (Mn = 2.3 © 104) and
branched polymer P-4 (Mn = 1.7 © 104) were obtained in 79
and 20% yield, respectively (Scheme 3). The FT-IR spectra
showed a clear shift of the carbonyl vibration (¯C=O) toward
higher wavenumber (around 1717 cm¹1) for P-3 and P-4
compared to P-1 and P-2.
Figure 2. UV-vis spectra of a film of P-1 on a quartz cell.
Broken line: prior to heating (P-1); solid line: after heating at
135 °C for 10 min.
which can be attributed to the formation of 3-acylbenzoxazoline-
2-thione groups.18,19 It is noteworthy that the previously used
photo-Fries rearrangement3-5 is accompanied with photodecar-
boxylation, whereas the present thermal rearrangement proceed-
ed selectively without any side reaction.
A chloroform solution of the polymer was spin-coated onto
a silicon wafer and dried in vacuo at room temperature to
prepare a thin film with a thickness of about 0.1 ¯m. The
refractive indices of polymer films were investigated by means
of ellipsometry before and after heating at 135 °C for 10 min.
The values of nb (refractive index before heating), na (refractive
index after heating), and ¦n (the change of refractive index) are
summarized in Table 1.
The thermal rearrangement of the S-acyl moieties in thin
films of P-1 and P-2 (Scheme 4) was followed by means of FT-
IR spectroscopy. Figures 1a and 1b show the FT-IR spectra of
P-1 before and after heating at 135 °C for 30 min.17 The non-
heated film shows characteristic signals of the carbonyl groups
of the S-acyl groups at 1702 cm¹1 (Figure 1a). After heating, the
carbonyl vibrational band shifted to 1717 cm¹1. In addition, the
¹1
vibrational band at 1353 cm assigned to thiocarbonyl (¯C=S
)
groups became more intense. Similar results were obtained with
P-2. FT-IR spectra of P-1 and P-2 after heating coincided well
with those of P-3 and P-4, synthesized separately as authentic
standards. Further, as shown in Figure 2, heating of a film of P-1
caused a significant increase in the absorption at around 340 nm,
After thermal rearrangement of P-1, the refractive index at
- = 632.8 nm changed from nb = 1.675 to na = 1.687 (¦n =
Chem. Lett. 2011, 40, 1363-1365
© 2011 The Chemical Society of Japan
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1365
Table 1. Refractive index properties of thin films of polymersa
result in further modification of ¦n. Other polymers containing
S-acyl derivatives are currently under investigation.
b
c
Polymers
nb
na
¦nd
P-1
P-2
1.675
1.674
1.687
1.684
+0.012
+0.010
References and Notes
1
2
3
4
5
6
7
8
9
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aThe thin films (about 0.1 ¯m in thickness) were prepared from
the CHCl3 solution, followed by spin-coating onto a silicon
wafer. bRefractive index before heating. cRefractive index after
d
heating at 135 °C for 10 min. ¦n = na ¹ nb.
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+0.012). In the case of P-2, ¦n was +0.010 after heating.
Notably, n values obtained after heating were almost identical
with those of authentic P-3 (n = 1.684) and P-4 (n = 1.683),
suggesting that the rearrangement from the S-acyl to N-acyl
groups proceeds efficiently upon heating. The nb value of the
branched polymer was lower than that of the linear polymer,
possibly because the three-dimensional globular structure of the
branched polymer results in a lower density (μ), leading to a
lower value of n according to the Lorentz-Lorenz equation
(see above). We also measured the refractive index change of
monofunctional compound 2 and difunctional S,S¤-(2-benzox-
azolyl) dithioisophthalate (4, see Supportin Information22)
dispersed in a poly(methyl methacrylate) (PMMA) matrix.
Films prepared with a ratio of 2/PMMA and 4/PMMA of 2
(wt/wt) showed ¦n values of +0.015 and +0.017 after heating,
with retention of good film quality.20 Film prepared with a
4/PMMA ratio of 1 (wt/wt) showed ¦n of +0.005. These
results support the idea that thermal rearrangement of S-thioester
groups is associated with refractive index changes. The change
of refractive index for model compounds 2 and 5 (Scheme 1) is
also consistent with computational prediction.21
In summary, linear and branched polymers bearing active
S-thioester moieties were selectively synthesized by means of
free radical and photoinitiated radical polymerization, respec-
tively, starting with S-(2-benzoxazolyl)-4-vinylthiobenzoate.
The refractive index n of thin films of both these polymers
increased (¦n µ +0.01) upon heating, as a result of thermal
rearrangement of the S-(2-benzoxazolyl) thioester moieties to 3-
acylbenzoxazoline-2-thione moieties. Thermal rearrangement
reaction of functional groups in polymers is a new strategy for
creating materials whose refractive index can be changed. It
should be possible to stimulate such reaction by applying a laser
beam as the heat source. The products, i.e., polymers having N-
acyl moieties, can react with amine derivatives, and this might
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Anada, Bull. Chem. Soc. Jpn. 2004, 77, 1415.
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14 A. Kameyama, T. Yagishita, unpublished results.
15 A. Kameyama, T. Ono, M. Miyasaka, 12th International
Conference on Radiation Curing in Asia, Yokohama, Japan,
June, 2011, p. 260.
16 The degree of branching (DB) of P-2 (and P-4) were not able
1
to determine by H NMR and 13C NMR measurements.
17 Exothermic peak top at 120 °C (Tend = 135 °C) appeared in
the DSC profile for 1.
18 H. P. Koch, J. Chem. Soc. 1949, 401.
19 J. J. D’Amico, S. T. Webster, R. H. Campbell, C. E. Twine,
J. Org. Chem. 1965, 30, 3618.
20 No change (¦n = 0.000) was observed prior to and after
heating in the case of PMMA only film.
21 Computationally predicted n values for 2 and 5 by using
ACD/ChemSketch are 1.674 and 1.702, respectively.
22 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Chem. Lett. 2011, 40, 1363-1365
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/