Synthesis and properties of de-cross-linkable acrylate polymers based on hexaarylbiimidazole

Article abstract of DOI:10.1016/j.polymer.2013.05.066

The radical polymerization of 4-(4,5-diphenyl-1H-imidazol-yl)-phenyl acrylate (DPIPA) was carried out in N,N-dimethylformamide at 60 C for 24 h by using 2,2′-azobis(isobutyronitrile) (5 mol%) as an initiator to give a poly(DPIPA) (1) as a homopolymer. Poly(DPIPA-co-MMA) (2) was obtained by the radical copolymerization of DPIPA with methyl methacrylate (MMA) in a good yield. Moreover, copolymeriztion parameters of DPIPA were evaluated by the copolymerization of MMA. The cross-linking reaction of the obtained 2 proceeded with K₃[Fe(CN)₆] and KOH. The resulting cross-linked poly(DPIPA-co-MMA) (3) was de-cross-linked by pressurization or visible light irradiation. Consequently, the thiol-capped de-cross-linked poly(DPIPA-co-MMA) (4) was obtained in a good yield.

Full text of DOI:10.1016/j.polymer.2013.05.066

Polymer 54 (2013) 4161e4170
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and properties of de-cross-linkable acrylate polymers based
on hexaarylbiimidazole
,
b
Takeru Iwamura a , Saori Nakamura
*
a Department of Chemistry and Energy Engineering, Faculty of Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya-ku,
Tokyo 158-8557, Japan
b Institute for Environmental Sciences, University of Shizuoka, 52-1, Yada, Suruga-ku, Shizuoka 422-8526, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The radical polymerization of 4-(4,5-diphenyl-1H-imidazol-yl)-phenyl acrylate (DPIPA) was carried out
in N,N-dimethylformamide at 60 ꢀC for 24 h by using 2,20-azobis(isobutyronitrile) (5 mol%) as an initiator
to give a poly(DPIPA) (1) as a homopolymer. Poly(DPIPA-co-MMA) (2) was obtained by the radical
copolymerization of DPIPA with methyl methacrylate (MMA) in a good yield. Moreover, copolymeriztion
parameters of DPIPA were evaluated by the copolymerization of MMA. The cross-linking reaction of the
obtained 2 proceeded with K3[Fe(CN)6] and KOH. The resulting cross-linked poly(DPIPA-co-MMA) (3)
was de-cross-linked by pressurization or visible light irradiation. Consequently, the thiol-capped de-
cross-linked poly(DPIPA-co-MMA) (4) was obtained in a good yield.
Received 8 March 2013
Received in revised form
18 May 2013
Accepted 29 May 2013
Available online 5 June 2013
Keywords:
Cross-linking
De-cross-linking
Triphenylimidazole
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
recycled materials. We have proposed a possible approach which
consists of the introduction of a de-cross-linking system through
Since cross-linked polymers, which have a three-dimensional
network structure, cannot be melted by heating, it is difficult to
reproduce the cross-linked polymer by processing. Consequently, it
is difficult to recycle cross-linked polymers. In many cases, the used
cross-linked polymers are practically buried or incinerated. How-
ever, in recent years, due to increasing environmental concerns, the
recycling of cross-linked polymers has been studied [1e5]. Gener-
ally, it is difficult to thermoplasticize and to recycle these polymers.
However, if the cross-linked polymers that became useless can be
recycled as a thermoplastic resin, the moulded items which were
derived from the cross-linked polymers could be used by material
recycling in a wide range of fields. In addition, the thermoplastici-
zation of the cross-linked polymers having insoluble and infusible
properties means that these polymers would be converted into
solvent-soluble liner polymers. Such a thermoplasticization tech-
nology would promote recycling of the cross-linked polymers. The
thermoplasticization or solubilization of cross-linked polymers is
disturbed by the covalent bond of a cross-linked moiety. If the cross-
linked moiety, which is included in the cross-linked polymers, can
be cleaved under external stimulation, these polymers will become
the use of weak covalent bonding into cross-linked polymers.
Therefore, we paid attention to hexaarylbiimidazole (HABI) as a
molecule with a weak covalent bonding.
HABI, which is known as photo, thermo, and piezochromic
compound, was first synthesized by Hayashi and Maeda in 1960 [6].
HABI has two kinds of 2,4,5-triphenylimidazole dimers which are
prepared from 2,4,5-triphenylimidazole with K3[Fe(CN)6] under
basic conditions. One of them shows photochromic and thermo-
chromic properties with irradiation and heating [7,8]. The other
shows a piezochromic property on grinding [9]. The photochromic
compound is a HABI dimer and photo, thermo, and piezochromism
of the two compounds are due to reversible dissociation of the di-
mers into a 2,4,5-triphenylimidazolyl radical which forms by a
reversible reaction. The photochromic and piezochromic dimers are
isomeric dimers which differ in their positions of combination of
the 2,4,5-triphenylimidazolyl radicals and are at equilibrium with
the radical in solution [10]. HABI derivatives are readily cleaved
under mild conditions such as visible light irradiation and pressure.
We have reported a novel de-cross-linking system utilizing
pressure or visible light irradiation [11]. The cross-linking reaction
of a polymer having a 2,4,5-triphenylimidazole moiety which was
incorporated into the side chains of the polymers proceeded with
K3[Fe(CN)6] and KOH, under pressure or visible light. The signal of
the 2,4,5-triphenylimidazolyl radicals which indicated that the de-
* Corresponding author. Fax: þ81 3 5707 1089.
E-mail address: iwamurat@tcu.ac.jp (T. Iwamura).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.05.066

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T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
cross-linking reaction had proceeded, was detected by ESR spec-
troscopy. However, because the reaction conditions of de-cross-
linking could not be sufficiently examined, the yield of the de-
cross-linking polymer could not be clarified. Therefore, we aimed
to further investigate the de-cross-linking reaction of the cross-
linked polymers. The properties of HABI derived from triphenyli-
midazole may be applied in not only recycling materials, but also
adhesion field, and printing field. In this article, a new acrylate
monomer having a 2,4,5-triphenylimidazole moiety was synthe-
sized. To examine the radical polymerization behaviour of this
monomer in detail, it was polymerized. In addition, de-cross-
linking properties of the obtained de-cross-linkable polymers are
also described.
2.3. Synthesis of 4-(4,5-diphenyl-1H-imidazol-yl)-phenyl acrylate
(DPIPA)
Acryloyl chloride (0.145 g, 1.60 mmol) was added dropwise to a
stirred solution of 4-(4,5-diphenyl-1H-imidazol-2-yl)-phenol
(0.441 g, 1.41 mmol) and triethylamine (0.171 g, 1.69 mmol) in anhy-
drousTHF (8 mL). After being stirred at room temperature for15 h, the
reaction mixture was diluted with diethyl ether and then was washed
with water. The organic phase was dried over MgSO4, and evaporated
under reduced pressure to leave a solvent. The residuewas purified by
chromatography on silica gel with chloroform as an eluent to isolate
DPIPA. Yield: 87%. Mp. 203e205 ꢀC; IR (KBr): 3087, 3060, 3034, 1739,
1633,1605, 1492, 1450, 1248, 1205, 1169, 1149, 847, 767, 697 cmꢁ1; 1H
NMR (CDCl3, 500 MHz)
d
: 6.03 (dd, J ¼ 10.30 and 1.15 Hz, 1H, CH2]
CHe cis), 6.30 (dd, J ¼ 17.20 and 10.30 Hz, 1H, CH2]CHe), 6.60 (dd,
J ¼ 17.20 and 1.15 Hz, 1H, CH2]CHe trans), 7.11 (d, J ¼ 8.88 Hz, 2H, e
COOeC6H4e), 7.20e7.40 (m, 6H, PheH), 7.40e7.60 (m, 4H, PheH),
7.80 (d, J ¼ 8.88 Hz, 2H, eC6H4eC3N2He) ppm; 13C NMR (CDCl3/
2. Experimental procedure
2.1. Materials
DMSO-d6, 125 MHz) d: 121.46, 126.18, 127.03, 127.40, 128.10, 132.75,
1-Butylimidazolium tetrafluoroborate ([Hbim]BF4) was pre-
pared by a reported procedure [12]. 4-(4,5-Diphenyl-1H-imidazol-
2-yl)-phenol was prepared by Siddiqui’s reported procedure [13].
Anhydrous N,N-dimethylformamide (DMF) and anhydrous ethanol
were purchased from Wako Pure Chemical Industries, Ltd. 2,20-
Azobis(isobutyronitrile) (AIBN) was recrystallized from ethanol.
Methyl methacrylate (MMA) was distilled and stored under nitro-
gen. Triethylamine was dried over CaH2, distilled, and stored under
nitrogen. Other solvents and reagents were used as supplied.
144.81, 149.96, 163.69 ppm; Anal. calcd for C24H18N2O2: C, 78.67%; H,
4.95%; N, 7.65%. Found: C, 78.69%; H, 4.85%; N, 7.65%.
2.4. Radical homopolymerization of DPIPA (typical procedure)
DPIPA (0.304 g, 0.83 mmol) and AIBN (7.1 mg, 5 mol%) were
dissolved into DMF (0.83 mL) in a test tube, which was then
degassed and sealed in vacuo. After being stirred at 60 ꢀC for 24 h,
the reaction mixture was evaporated under reduced pressure to
remove DMF and the residue was dissolved in THF. The resulting
THF solution was poured into distilled water, and the precipitated
polymer was separated by using a TOMY LC-131 (3000 rpm,
20 min). The obtained poly(DPIPA) (1) was dried in vacuo. Yield:
97%. GPC (eluent: THF): Peak 1: Mn ¼ 32,600 (Mw/Mn ¼ 3.89),
94.4%; Peak 2: Mn ¼ 2600 (Mw/Mn ¼ 1.05), 5.6%; IR (KBr): 3059,
2957, 2933, 2809, 1753, 1659, 1605, 1539, 1491, 1449, 1387, 1203,
2.2. Measurements
1H NMR and 13C NMR spectra were recorded on a JEOL ECA-500
spectrometer. IR spectra were measured on a JASCO FT/IR-4200
spectrometer. Gel permeation chromatographic analyses were
carried out on a Tosoh Co. HLC-8020 GPC system (TSK-GEL
G3000XL, DMF containing LiBr (5.8 mM) as an eluent, and an ul-
traviolet (UV) detector) using polystyrene as a standard. Melting
points were determined on a MEL-270 Shibata melting point
apparatus and are uncorrected.
1168, 1140, 969, 915, 842, 768, 697 cmꢁ1
500 MHz) : 1.54e2.61 (m, eCH2eCH(COOR)e, 2H), 2.83e3.38 (m,
eCH2eCH(COOR)e, 1H), 6.80e8.28 (m, PheH, 14H) ppm; 13C NMR
;
1H NMR (DMSO-d6,
d
Scheme 1. Synthesis of DPIPA.

T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
4163
60 ꢀC, the reaction mixture was evaporated under reduced pressure
to remove DMF and the residue was dissolved in THF. The resulting
THF solution was poured into distilled water, and the precipitated
polymer was separated by centrifuging at 3000 rpm for 20 min. The
obtained poly(DPIPA-co-MMA) (2) was dried in vacuo. Yield: 92%.
Peak 1: Mn ¼ 34,000 (Mw/Mn ¼ 3.28), 94.4%; Peak 2: Mn ¼ 2700
(Mw/Mn ¼ 1.08), 6.5%; IR (KBr): 3059, 2987, 2951, 2853, 1751, 1731,
1657, 1605, 1541, 1491, 1450, 1387, 1203, 1167, 1136, 970, 915, 843,
768, 697 cmꢁ1 1H NMR (DMSO-d6, 500 MHz)
; d: 0.62e3.05 (m, e
CH2eC(CH3)(COOR), 5H ꢂ 0.52, eCH2eCH(COOR)e, 3H ꢂ 0.48),
3.49 (s, eOCH3, 3H ꢂ 0.52), 6.80e7.70 (m, PheH, 12H ꢂ 0.48), 8.10
(s, PheH, 2H ꢂ 0.48) ppm; 13C NMR (DMSO-d6, 125 MHz)
d: 16.62,
18.10, 20.07, 21.66, 24.2, 26.3, 35.79, 36.88, 37.34, 41.03, 43.90,
44.43, 44.93, 46.15, 46.2, 47.9, 51.75, 53.5, 54.7, 121.61, 126.41,
127.00, 127.69, 128.07, 128.31, 128.54, 130.93, 135.00, 137.12, 144.72,
150.04, 173.04, 173.61, 175.54, 176.20, 177.27 ppm.
Scheme 2. Radical homopolymerization of DPIPA.
2.6. Determination of monomer reactivity ratios
Table 1
Radical homopolymerization of DPIPA.
The copolymerization of DPIPA and MMA was performed under
various feed ratios. This copolymerization was carried out in the
same manner described in Section 2.5 (polymerization
time ¼ 60 min). The copolymer compositions were determined by
their 1H NMR spectra.
Run Initiator Temp Yield Peak 1
Peak 2
(ꢀC)
(%)
Mn
Mw/Mn Peak
area (%)
Mn
Mw/Mn Peak
area
(%)
1
2
3
AIBN
BPO
DTBPO 120
60
80
97
11
21
32,600 3.89
94.4
41.4
68.4
2600 1.05
1100 1.08
970 1.04
5.6
58.6
31.6
8300
9300
4.90
2.84
2.7. Cross-linking reaction of 2
Acold 1% aqueous solutionof potassium ferricyanide(50 mL) was
added slowly to a solution of 2 (0.280 g) dissolved in a mixed solvent
of ethanol (11 mL) and THF (14 mL) containing potassium hydroxide
(137.6 mg). During addition, the reaction mixture was maintained at
5e10 ꢀC and vigorously stirred. After 6 h at 5e10 ꢀC, the reaction
mixture was neutralized by HCl. The precipitated cross-linked
polymer was separated by centrifuging at 3000 rpm for 20 min.
The obtained precipitated polymer was dried in vacuo. Inorganic
reagents were removed by centrifugation/washing (3 times) with
distilled water. Then, the obtained suspension, including cross-
(DMSO-d6, 125 MHz) d: 35.09, 41.50, 121.71, 121.98, 126.44, 126.96,
127.11, 128.35, 130.99, 135.07, 137.20, 144.77, 150.09, 173.15 ppm.
2.5. Radical copolymerization of DPIPA and MMA (typical procedure)
DPIPA (0.916 g, 2.50 mmol), MMA (0.259 g, 2.59 mmol) and
AIBN (42.6 mg, 5 mol%) were dissolved in DMF (5.0 mL) in a test
tube, which was then degassed and sealed in vacuo. After 24 h at
Fig. 1. 1H NMR (DMSO-d6) spectrum of 1.

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T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
Fig. 2. 13C NMR (DMSO-d6) spectrum of 1.
linked poly(DPIPA-co-MMA) (3), was freeze-dried in vacuo. Yield:
Mn ¼ 5000 (Mw/Mn ¼ 3.38); IR (KBr): 3093, 3070, 3028, 2925, 2855,
61%. IR (KBr): 3133, 3095, 3067, 3020, 2989, 2953, 2907, 2841, 1731,
1720, 1678, 1600, 1557, 1494, 1408, 1363, 1293, 1238, 1179, 1136, 846,
1606, 1502, 1448, 1394, 1235, 1174, 1140, 848, 754, 696 cmꢁ1
.
756, 696 cmꢁ1 1H NMR (DMSO-d6, 500 MHz)
; d: 0.60e1.40 (m, e
CH2eC(CH3)(COOR), 3H ꢂ 0.52, CH3eCH2eSe, 1.01H [¼3H ꢂ 0.34]),
1.40e2.47 (m, eCH2eC(CH3)(COOR), 2H ꢂ 0.52, eCH2eCH(COOR)e,
3H ꢂ 0.48), 3.48e3.70 (m, eOCH3, 3H ꢂ 0.52), 3.68e4.35 (m, CH3e
CH2eSe, 0.68H [¼2H ꢂ 0.34]), 6.82e8.22 (m, PheH, 14H ꢂ 0.48)
2.8. De-cross-linking of 3 under visible light irradiation
3 (20.0 mg) was dispersed with DMF (1.0 mL) in a Pyrex test
tube. The suspension of 3 was irradiated under visible light irra-
diation using a desk lamp (HITACHI RS3205, 34W). After visible
ppm; 13C NMR (DMSO-d6, 125 MHz)
d: 13.72, 13.90, 16.55, 17.79,
17.96, 19.88, 21.76, 22.18, 23.10, 23.40, 24.79, 25.48, 27.44, 29.06,
30.47, 31.42, 33.26, 36.40, 42.58, 43.13, 43.88, 44.24, 46.44, 46.74,
47.73, 50.63, 51.70, 53.66, 115.61, 115.79, 118.00, 119.06, 126.90,
127.42, 127.84, 128.34, 128.50, 129.08, 129.29, 129.64, 130.81, 131.34,
132.88, 139.88, 141.17, 174.69, 175.10, 176.13, 176.31, 176.46, 177.24,
177.73 ppm.
light irradiation, ethanethiol (25 mL, 4.0 eq.) was added to a stirred
solution of the reaction mixture. After stirring, the reaction mixture
was filtered using a membrane filter (Millipore, MillexÒ-LH,
0.45 mL). The filtrate was poured into distilled water (10 mL), and
the precipitated polymer was separated by centrifuging at
3000 rpm for 15 min. The obtained thiol-capped de-cross-linked
polymer (4) was dried in vacuo. Yield: 99%. GPC (eluent: THF):
2.9. De-cross-linking of 3 under pressure
3 (13.1 mg) was pressurized by an agate mortar and pestle. After
being pressurized for 5 min, the pressurized solid was dispersed in
DMF (1.0 mL), and then ethanethiol (25 mL, 4.0 eq.) was added to a
stirred solution of the pressurized solid in DMF. After stirring, the
reaction mixture was filtered using a membrane filter (Millipore,
MillexÒ-LH, 0.45
mL). The filtrate was poured into distilled water
(10 mL), and the precipitated polymer was separated by centri-
fuging at 3000 rpm for 15 min. The obtained thiol-capped de-cross-
linked polymer (4) was dried in vacuo. Yield: 80%. GPC (eluent:
THF): Mn ¼ 7000 (Mw/Mn ¼ 3.27); IR (KBr): 2981, 2949, 2925, 1728,
1655, 1608, 1554, 1519, 1492, 1447, 1385, 1094, 1034, 987, 918, 847,
749, 698 cmꢁ1 1H NMR (DMSO-d6, 500 MHz)
; d: 0.60e1.40 (m, e
CH2eC(CH3)(COOR), 3H ꢂ 0.52, CH3eCH2eSe, 0.83H [¼3H ꢂ 0.28]),
1.40e2.47 (m, eCH2eC(CH3)(COOR), 2H ꢂ 0.52, eCH2eCH(COOR)e,
3H ꢂ 0.48), 3.48e3.70 (m, eOCH3, 3H ꢂ 0.52), 3.68e4.35 (m, CH3e
CH2eSe, 0.56H [¼2H ꢂ 0.28]), 6.82e8.22 (m, PheH, 14H ꢂ 0.48)
ppm; 13C NMR (DMSO-d6, 125 MHz)
d: 13.71, 13.89, 17.72, 17.99,
Fig. 3. FT-IR spectrum (KBr) of 1.
18.54, 19.69, 19.90, 21.69, 22.11, 23.11, 25.37, 25.66, 28.71, 29.01,

T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
4165
Scheme 3. Radical copolymerization of DPIPA and MMA.
Fig. 4. 1H NMR (DMSO-d6) spectrum of 2.
30.87, 31.21, 32.16, 32.91, 36.33, 36.76, 36.95, 42.30, 42.61, 43.00,
DPIPA (Scheme 1, eq. 2). It was well characterized by 1H NMR, 13
C
NMR, and FT-IR spectroscopy. DPIPA was also characterized by
elemental analysis.
43.86, 44.26, 47.40, 50.57, 51.66, 52.35, 115.59, 115.77, 117.98, 119.04,
126.87, 127.40, 127.81, 128.32, 128.48, 129.06, 129.25, 129.55, 129.61,
131.31, 131.77, 139.87, 140.34, 141.16, 174.29, 174.77, 175.02, 175.66,
176.07, 176.45, 176.89, 177.23, 177.39, 177.82, 178.70 ppm.
3.2. Radical homopolymerization of DPIPA
3. Results and discussion
The radical homopolymerization of DPIPA was carried out at
60 ꢀC for 24 h using a radical polymerization initiator (5 mol%) such
as AIBN (Scheme 2). The results of the polymerization are summa-
rized in Table 1. 1 was obtained as a water-insoluble part. In the case
of the AIBN initiation system, 1 was obtained in a good yield.
However, in the case of the benzoyl peroxide (BPO) and di-tert-butyl
peroxide (DTBPO) initiation system, the corresponding polymers
were obtained in low yields. Otsu et al. reported the relative
3.1. Synthesis of DPIPA
DPIPA was prepared by a two-step reaction. First, 4-(4,5-
diphenyl-1H-imidazol-2-yl) phenol was synthesized by means of
Siddiqui’s protocol (Scheme 1, eq. 1) [13]. Then, 4-(4,5-diphenyl-
1H-imidazol-2-yl) phenol reacted with acryloyl chloride to yield

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T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
Fig. 5. 13C NMR (DMSO-d6) spectra of 2.
reactivity of MMA toward primary radicals [14]. The relative reac-
tivity of MMA toward 2-cyano-2-propyl radical ((CH3)2(CN)C$)
derived from AIBN was reported as being 0.56. They also reported
that the relative reactivity values of MMA derived from BPO or
DTBPO were low (0.12 and 0.06, respectively). DPIPA is a kind of
acrylic monomer similar to MMA. Therefore, these relative re-
activities might be similar. Since the initiator efficiencies of BPO and
DTBPO were low, the yields of the corresponding polymer might be
low. In the gel permeation chromatography (GPC) analysis, the ob-
tained homopolymers showed bimodal peaks. The reason for these
peaks is probably the chain-transfer reaction to initiator which may
be taking place in the growing polymer chain.
The 1H and 13C NMR spectra of 1 are shown in Figs. 1 and 2,
respectively. In the 1H NMR spectrum, the signals resulting from
the aromatic protons (a) in the triphenylimidazole moieties were
observed at
d
6.80e8.28 ppm. Additionally, the peaks due to the
2.83e
methine and methylene protons (b and c) were observed at
d
3.38 and 1.54e2.61 ppm, respectively (Fig. 1). This result supported
the fact that the polymerization of DPIPA involved a radical
homopolymerization process. The 13C NMR spectrum of 1 also
supported radical homopolymerization (Fig. 2). The peaks due to
methine and methylene carbons (f and g) were observed at
and 35.1 ppm, respectively.
d 41.5
In Fig. 3, the FT-IR spectrum of 1 showed an absorption band at
1753 cmꢁ1 based on the ester carbonyl group (
C]O). Moreover, the
absorption bands of the imidazole ring observed at 1659 cmꢁ1 and
n
Fig. 6. FT-IR spectrum of 2.

T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
4167
Table 2
Radical copolymerization of DPIPA and MMA.
Run
1/MMA (mol ratio)
Yield (%)
x/y
Peak 1
Peak 2
Mn
Mw/Mn
Peak area (%)
Mn
Mw/Mn
Peak area (%)
1
2
3
4
5
6
7
100.0/0.0
80.4/19.6
60.6/39.4
49.2/50.8
40.3/59.7
20.1/79.9
0.0/100.0
97
98
100
92
90
97
1.00/0.00
0.79/0.21
0.59/0.41
0.48/0.52
0.40/0.60
0.19/0.81
0.00/1.00
32,600
34,000
33,200
34,000
26,500
20,900
15,700
3.89
3.76
3.75
3.28
3.59
2.71
1.86
94.4
92.9
93.1
93.5
89.4
83.6
40.8
2600
2600
2600
2700
2400
3000
1800
1.05
1.06
1.07
1.08
1.10
1.10
1.44
5.6
7.1
6.9
6.5
10.6
16.4
59.2
91
GPC analysis of the copolymer from DPIPA and MMA was carried
out in THF using polystyrene as the standard. In the GPC profiles,
the obtained polymers showed bimodal peaks (Table 2). The peak
area decreased on the high-molecular-weight side of the peak and
increased on the low-molecular-weight side of the peak as the
amount of MMA increased. Moreover, Mn, which was calculated
from the high-molecular-weight side of the peak, decreased as the
amount of MMA increased.
Monomer reactivity ratios were evaluated by using the Mayoe
Lewis equation [15]. The calculated values, r1, r2 and r1r2, and
AlfreyePrice Qee values are shown in Table 3 [16]. Q and e values
indicate that DPIPA has a large resonance effect and electron-
withdrawing character due to the ester and triphenylimidazole
groups, playing an important role in alternating copolymerizability
with MMA.
Table 3
Monomer reactivity ration of the copolymerization of DPIPA with MMA.
M1
M2
r1
r2
r1r2
Q1
e1
1.175
1
MMA
0.582
0.942
0.548
1.072
1539 cmꢁ1 were assigned to the C]N stretching frequency and Ne
H in-plane deformation vibration, respectively.
3.3. Radical copolymerization of DPIPA and MMA
The radical copolymerization of DPIPA and MMA was carried out
at 60 ꢀC for 24 h using AIBN as an initiator under various feed ratios
(Scheme 3). The corresponding copolymers were obtained in good
yields (90e100%). The copolymer composition was determined by
its 1H NMR spectra (Fig. 4). The unit ratio was determined from the
integral ratio between the aromatic protons (a) and methyl protons
(f). The 13C NMR spectrum of 2 is shown in Fig. 5. The signals as-
cribable to the aryl imidazole moiety (x unit) were observed at
3.4. Cross-linking reaction of 2
The cross-linking reactions of 2 were carried out by a previ-
ously reported procedure [11]. To the solution of 2 in a solution of
KOH in a mixed solvent of ethanol and THF, a cold ethanol 1%
K3[Fe(CN)6] solution was added with vigorous stirring at 5e10 ꢀC
for 6 h (Scheme 4). The precipitated cross-linked polymer was
repeatedly washed with distilled water and then freeze-dried in
water. Consequently, 3 was obtained in 61% yield. The IR spectra
of 3 are shown in Fig. 7. In the FT-IR spectrum of 3, the signals
ascribable to the ester group were 1731 cmꢁ1. In the region of
1800e1400 cmꢁ1, the FT-IR spectra of 2 and 3 were compared. In
d
120.7e151.0 (c) ppm. The signals ascribable to the MMA moiety (y
unit) were observed at 174.8e178.8 (a), 51.8 (d), and 15.3e25.2 (i)
ppm. The 13C NMR spectrum of 2 also supported the fact that DPIPA
undertook radical copolymerization.
The FT-IR spectrum of 2 showed an absorption band at
1731 cmꢁ1 based on the ester carbonyl group (
nC]O) in Fig. 6.
Moreover, the absorption bands of the imidazole ring observed at
1657 cmꢁ1 and 1541 cmꢁ1 were assigned to the C]N stretching
frequency and NeH in-plane deformation vibration, respectively.
Scheme 4. Cross-linking reaction of 2.

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T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
Table 4
De-cross-linking reaction of 3.
Run
External
stimulation
Solvent
Time
(min)
Yield
(%)
Modification
ratio (%)a
1
2
h
n
DMF
None
60
5
99
80
70
58
Pressure
a
Determined by 1H NMR spectrum.
Fig. 7. FT-IR spectra of 2 and 3.
the case of 2, the C]N stretching vibrations ascribable to the
imidazole moiety were observed at 1605 and 1583 cmꢁ1. More-
over, the C]C stretching vibration and C]NeC]C symmetric
stretching vibration were observed at 1491 and 1450 cmꢁ1
,
Fig. 8. FT-IR spectra of 2, 4 prepared under visible light (Table 4, Run 1), and 4 pre-
respectively. On the other hand, the HABI moiety as a cross-
linking point was included in 3. White et al. and Tanino et al.
reported the IR spectra of HABI isomers. Therefore, 3 was iden-
tified by these reported data. In these reported data, the 1,20-
isomer of HABI absorbed at 1604 (m), 1554 (m), and 1480 (m)
cmꢁ1. 3 absorbed at in the same region at 1606 (m), 1557 (m), and
1481 (m) cmꢁ1. White et al. reported that the 4-40-isomer of HABI
has an IR band at 1605 (m), 1562 (s), and 1489 (s) cmꢁ1 [10]. On
the other hand, Abe et al. pointing out that the 2,20-isomer was
more appropriate than the 4,40-isomers that White et al. proposed
[17]. In the FT-IR spectrum of 3, these peaks, which might be
buried in other peaks, could not be clearly confirmed. Addition-
ally, the strong band at 1610 cmꢁ1 due to the 1,40-isomer and the
very strong band at 1613 cmꢁ1 due to the 2,40-isomer were not
observed in this spectrum. Therefore, the 1,20-isomer might be
main constituent of the cross-linking point (Scheme 5).
pared under pressure (Table 4, Run 2).
3.5. De-cross-linking of cross-linked polymer 3
We attempted the de-cross-linking reaction of 3 under pres-
sure or visible light irradiation. In the case of the de-cross-linking
reaction of 3 under pressure, 3 was ground manually in an agate
mortar with a pestle at room temperature for 5 min. On the other
hand, in the case of the de-cross-linking reaction of 3 under
visible light irradiation, 3 was compulsorily dispersed in DMF. The
suspension of 3 was irradiated under visible light irradiation us-
ing a desk lamp for 60 min. In our previous work, the ESR peak of
cleaved imidazolyl radicals in the side chain of the cross-linked
polymer exhibited a single peak with a width of 7.5 G and a g
Scheme 5. De-cross-linking reaction of cross-linked polymer 4.

T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
4169
Fig. 9. 1H NMR (DMSO-d6) spectrum of 4 prepared under visible light (Table 4, Run 1).
value of 2.003 after irradiation for 23 min. The intensity of the
ESR peak was 1.56 ꢂ 1017 spins/mg under visible light irradiation
for 23 min. Under visible light irradiation for 40 min, the intensity
of the ESR peak increased with time (2.41 ꢂ 1017 spins/mg). After
stopping the irradiation with visible light, the peak of the cleaved
imidazolyl radicals gradually decreased with time in the dark [11].
Therefore, to isolate the de-cross-linked polymer, ethanethiol was
added to reaction mixture after the de-cross-linking reaction. The
corresponding 4 was obtained as a water-insoluble part in a good
yield. The yield of the de-cross-linked polymer, which was un-
clear in our previous research, was clarified in this study. The
results of the de-cross-linking reaction of 3 are summarized in
Table 4.
The IR spectra of 2 and 4 are shown in Fig. 8. Especially, the FT-IR
spectra of 2 and 4 were compared in the region of 1800e1000 cmꢁ1
.
The peaks aei of 2 are peaks which can almost all be found in 4
although one of these peaks has been buried in other peaks. This
result shows that the structure of 4 is mostly maintained in 2.
Fig. 11. GPC profiles of before cross-linking polymer (2), after de-cross-linking polymer
(4) prepared under visible light (Table 4, Run 1), and after de-cross-linking polymer (4)
prepared under pressure (Table 4, Run 2).
Fig. 10. 13C NMR (DMSO-d6) spectrum of 4 prepared under visible light (Table 4, Run 1).

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T. Iwamura, S. Nakamura / Polymer 54 (2013) 4161e4170
However, in the case of 4, the peaks not observed in 2 were
observed at 1440 (region of A) and 1167 (region of B) cmꢁ1. Harrison
et al. and Krishnakumar et al. reported the IR spectra of (alkylthio)
benzimidazole. From their reported data, these peaks were
assigned to the S-alkyl group of the imidazole ring [18]. The 1H
NMR spectrum of 4 is shown in Fig. 9. A peak due to the methyl
polymer can be expected in many fields such as adhesion field,
and printing field.
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific
Research (C) (No. 23510112) from Japan Society for the Promotion
of Science.
group of the S-alkyl group was observed at around
d 1.23 ppm (g).
Additionally, in the 13C NMR spectrum of 4, a peak due to the
methyl group of the S-alkyl group was observed at around
d
13.8 ppm in Fig. 10. The GPC analysis of 4 was carried out in DMF
Appendix A. Supplementary data
containing LiBr as an eluent. The GPC profiles before cross-linking
and after de-cross-linking of 3 were accomplished by pressure or
visible light (Fig. 11).
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.05.066.
4. Conclusions
References
The radical polymerization of DPIPA prepared by the reaction
of 4-(4,5-diphenyl-1H-imidazole-2-yl)-phenol with acryloyl chlo-
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initiator. The corresponding 1 and 2 were obtained in good yields.
Copolymerization parameters of DPIPA were evaluated by the
copolymerization with MMA. The monomer reactivity ratios were
found to be r1 ¼ 0.582 and r2 ¼ 0.942 from which the Qee values
of DPIPA were estimated as Q ¼ 1.072 and e ¼ 1.175. The cross-
linking reaction of 2 proceeded with K3[Fe(CN)6] under basic
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3 proceeded by
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