Photo-cross-linking and de-cross-linking of modified polystyrenes having degradable linkages

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.01.008

Modified polystyrenes having both epoxy moieties and tertiary ester linkages, tertiary ether linkages, or carbonate linkages were synthesized. The polymer films containing a photoacid generator were photo-cross-linkable. The cross-linked polymer films bec

Full text of DOI:10.1016/j.reactfunctpolym.2011.01.008

Reactive & Functional Polymers 71 (2011) 480–488
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Photo-cross-linking and de-cross-linking of modified polystyrenes
having degradable linkages
⇑
Haruyuki Okamura , Etsushi Yamauchi, Masamitsu Shirai
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Modified polystyrenes having both epoxy moieties and tertiary ester linkages, tertiary ether linkages, or
carbonate linkages were synthesized. The polymer films containing a photoacid generator were photo-
cross-linkable. The cross-linked polymer films became soluble in tetrahydrofuran after thermolysis.
Photo-cross-linking and thermal decomposition behaviors of the polymers were investigated and dis-
cussed in terms of the chemical structures of degradable units. The thermal stability of the polymers
was found to be in the order: tertiary ester > tertiary ether > tertiary carbonate.
Received 21 December 2010
Received in revised form 17 January 2011
Accepted 20 January 2011
Available online 24 January 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Photo-cross-linking
Thermal de-cross-linking
Epoxy
Ester
Ether
Carbonate
1. Introduction
relationship between chemical structures of degradable units of
the resins and decomposition or dissolution properties of the
Epoxy resins are widely used to increase the toughness of
adhesives, inks, coatings, and matrix materials for fiber-reinforced
composites because they form insoluble and infusible networks
through cross-linking processes. The network structure generates
the chemical toughness and mechanical strength of the cured
resins. However, the stability can be significantly disadvantageous
when the network is needed to be removed since mechanical
scratching or strong acid treatment methods must be applied.
Thus, removing the cross-linked resins is difficult or impossible
without damaging underlying materials.
Recently, much attention has been paid to develop epoxy resins
which are thermally or chemically degradable under a given condi-
tion [1]. These resins were multifunctional epoxides connected
with degradable units in the molecule. As degradable units, disul-
fide [2,3], acetal [4], primary [5–8], secondary [5,9] or tertiary
[5,9–13] ester of carboxylic acid, carbamate [14], carbonate
[15,16], and sulfonate [17–19] were studied. In the previous paper,
we have reported the polymers having both epoxy moieties and
thermally cleavable tertiary ester moieties in the side chain [10].
On UV irradiation, the polymer films containing photoacid genera-
tors became insoluble in organic solvents, and the cross-linked
films became soluble in methanol when baked at 160–180 °C.
However, no systematic work has been done to clarify the
decomposed ones. For practical use, it is important to optimize
the structures of de-cross-linking units.
In this paper, we report the synthesis of a series of modified poly-
styrene having epoxy units and various types of thermally degrad-
able linkages. As thermally degradable units, tertiary ester, tertiary
ether, and tertiary carbonate were chosen. We have reported the
synthesis of p-styrenesulfonate-based monomers 7-oxabicyclo
[4.1.0] hept-3-yl)methyl p-styrenesulfonate and 2-(6-methyl-7-
oxabicyclo[4.1.0] hept-3-yl)propyl p-styrenesulfonate [17]. More-
over, poly(p-tert-butoxystyrene) (PTBOS) [20] having tertiary ether
units and poly(p-tert-butoxycarbonyloxystyrene) (PBOCS) [21]
bearing tertiary carbonate units were reported to be precursors
of poly(4-hydroxystyrene) by acidolysis. Thus, 1-ethenyl-4-[1-
methyl-1-(6-methyl-7-oxabicyclo[4.1.0]hept-3-yl)ethoxy]benzene
(MMOES) and 1-ethenyl-4-[1-methyl-1-(6-methyl-7- oxabicy-
clo[4.1.0]hept-3-yl)ethoxycarbonyloxy]benzene (MMOCS) were
newly synthesized as monomers. MMOCS was synthesized using
4-ethenylphenyl 2-(3-methylcyclohex-3-enyl)-2-propyl carbonate
[15]as a starting material. In addition, wenewly synthesized 4-ethe-
nylbenzoic acid 1-methyl-1-(6-methyl-7-oxabicyclo[4,1,0]hept-3-
yl)ethyl ester (MMOVBA) via the reaction of 4-ethenylbenzoyl
chloride and the alkoxide of
a-terpineol. Regio-selective epoxida-
tion of the alkene of the precursor monomers proceeded to give
monomers MMOES, MMOCS, and MMOVBA.
The concept of the present system is shown in Fig. 1. On
irradiation, network formation occurs by the photoinduced-acid
⇑
Corresponding author. Tel.: +81 72 254 9293; fax: +81 72 254 9291.
E-mail address: okamura@chem.osakafu-u.ac.jp (H. Okamura).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.01.008

H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
481
2. Experimental
2.1. Materials
Tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and
toluene were distilled over CaH2 before use. 2,20-Azobis(isobutyro-
nitrile) (AIBN) was purified by recrystallization from ethanol.
9-Fluorenylideneimino p-toluenesulfonate (FITS) [24] and 4-ethe-
nylphenyl 2-(3-methylcyclohex-3-enyl)-2-propyl carbonate [15]
were prepared according to the method described elsewhere.
Poly(4-vinylphenol) (PVP) (Mn = 8000, Mw/Mn = 1.1), OXONE, mon-
opersulfate compound (2KHSO5ꢀKHSO4ꢀK2SO4), 4-vinylbenzoic
acid, and p-tert-butoxycarbonyloxystyrene (BOCS) were purchased
from Sigma–Aldrich and used without further purification. Other
solvents and reagents including p-tert-butoxystyrene (TBOS) were
purchased from Wako Chemicals and used as received.
Fig. 1. Concept of photo-cross-linking and de-cross-linking of the polymers used in
this work.
catalyzed reaction of the epoxy moieties of the polymer. Thermal
treatment of the cross-linked polymers induces the cleavage of
the thermally degradable linkages. The cross-linking and de-
cross-linking properties were discussed in terms of the structure
of the degradable units chosen. The reaction mechanism was stud-
ied by TGA analysis and FT-IR spectroscopy. This system is of
importance as photo-cross-linkable materials which can be re-
moved from substrates after use. Moreover, this system is essen-
tially applicable to the fabrication of functional materials such as
KrF photolithography for double exposure technologies [22,23],
since poly(4-vinylphenol) (PVP), a useful polymer for KrF resist,
is formed by thermal treatment.
2.2. Synthesis of monomers
Monomers were synthesized according to the route shown in
Scheme 1.
2.2.1. 1-Cyano-4-[1-methyl-1-(4-methylcyclohex-3-enyl)ethoxy]-
benzene
Introduction of a tertiary ether group was carried out according
to the method reported. [25]. Into a four-necked round-bottom
flask fitted with an efficient magnetic stirrer, a Claisen adapter,
and two addition funnels were placed sodium hydride (60%, dis-
Scheme 1. Synthesis of monomers.

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H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
persed in mineral oil) (7.14 g, 119 mmol),
a
-terpineol (15.3 g,
chloroform, and the combined organic layers were washed with
water and dried over anhydrous MgSO4. After removal of the
solvents on a rotary evaporator, the oily residue was subjected to
column chromatography (silica gel, hexane:ethyl acetate:diethyl
ether = 3:1:1 (v/v)); colorless liquid; yield 1.05 g (32%). 1H NMR
(270 MHz, CDCl3) d 7.28 (d, 2H, phenyl), 6.98 (d, 2H, phenyl),
6.62 (dd, 1H, CH2 = CH-), 5.68 (s, 1H, CH2 = CH-), 5.14 (s, 1H,
CH2 = CH-), 3.05 (m, 1H, epoxy HCOC), 2.36–1.11 (m, 16H, CH,
CH2, CH3). Anal. Calcd for C18H24O2: C, 79.37; H, 8.88. Found: C,
79.66; H, 9.09.
82.6 mmol), catalytic amount of 15-crown-5, and THF (200 mL)
under nitrogen atmosphere. A THF solution (200 mL) of 4-fluoro-
benzonitrile (10.0 g, 82.6 mmol) was added to the solution and
heated at 60 °C for 20 h. A 200 mL aliquot of toluene was added
to the solution and then 100 mL of water was added to quench
remaining sodium hydride. The organic phase was separated and
washed with water five times. The organic layer was dried over
anhydrous MgSO4. The product was purified by column chroma-
tography (silica gel, chloroform); yellow liquid; yield 3.65 g (17
%). 1H NMR (270 MHz, CDCl3) d 7.48 (d, 2H, phenyl), 6.94 (d, 2H,
phenyl), 5.32 (s, 1H, C = CH-), 2.14–1.20 (m, 16H, CH, CH2, CH3).
2.2.5. 1-Ethenyl-4-[1-methyl-1-(6-methyl-7-oxabicyclo[4.1.0]hept-3-
yl)ethoxycarbonyloxy]benzene(MMOCS)
2.2.2. 1-Formyl-4-[1-methyl-1-(4-methylcyclohex-3-enyl)ethoxy]-
benzene
MMOCS was synthesized by the epoxidation of 4-ethenylphenyl
2-(3-methylcyclohex-3-enyl)-2-propyl carbonate [15] (1.28 g, 4.26
mmol) using the same method employed for MMOES. After purifi-
cation by flash chromatography (silica gel, chloroform), MMOCS
was obtained as colorless liquid; yield 0.70 g (55%). 1H NMR
(270 MHz, CDCl3) d 7.37 (d, 2H, phenyl), 7.08 (d, 2H, phenyl),
6.66 (dd, 1H, CH2 = CH-), 5.68 (s, 1H, CH2 = CH-), 5.23 (s, 1H,
CH2 = CH-), 3.02 (m, 1H, epoxy HCOC), 2.30–1.02 (m, 16H, CH,
CH2, CH3). Anal. Calcd for C18H24O2: C, 72.13; H, 7.65. Found: C,
72.01; H, 7.90.
Into a four-necked round-bottom flask fitted with an efficient
magnetic stirrer and two addition funnels were placed 1-cyano-
4-[1-methyl-1-(4-methylcyclohex-3-enyl)ethoxy]benzene (7.87 g,
30.8 mmol and toluene (32 mL) under nitrogen atmosphere and
cooled to –15 °C. Diisobutylaluminum hydride solution (1.0 M in
toluene, 31 mL, 31 mmol) was added dropwise and the solution
was kept at 0 °C for 3.5 h. A 100 mL aliquot of 2N sulfuric acid
was added to the solution and the organic phase was extracted
with diethyl ether twice, and washed with sodium bicarbonate
and then with water. The organic layer was dried over anhydrous
MgSO4. The product was purified by column chromatography (sil-
ica gel, hexane:ethyl acetate = 7:2, v/v); yellow liquid; yield 4.57 g
(57%). 1H NMR (270 MHz, CDCl3) d 9.88 (s, 1H, HC = O), 7.77 (d, 2H,
phenyl), 7.07 (d, 2H, phenyl), 5.40 (s, 1H, C = CH-), 2.14–1.20 (m,
16H, CH, CH2, CH3).
2.2.6. 4-Ethenylbenzoic acid 1-methyl-1-(3-methylcyclohex-3-enyl)-
ethyl ester
Into a three-necked round-bottom flask fitted with an efficient
magnetic stirrer and two addition funnels were placed 4-vinylben-
zoic acid (3.00 g, 20.2 mmol), oxalyl chloride (2.4 mL, 17.4 mmol),
one drop of DMF, and benzene (30 mL) and stirred for 2 h. The
solution was added dropwise into a chilled solution of the mixture
2.2.3. 1-Ethenyl-4-[1-methyl-1-(4-methylcyclohex-3-enyl)ethoxy]-
benzene
of a-terpineol (3.73 g, 20.2 mmol) and butyllithium solution (1.6 N
in hexanes, 12.6 mL, 20.2 mmol) in 30 mL of THF at 0 °C. The mix-
ture was refluxed for 1 h. A 40 mL aliquot of water was added to
the solution and the organic phase was extracted with diethyl
ether twice, and washed with sodium bicarbonate and then with
water. The organic layer was dried over anhydrous MgSO4. The
product was purified by column chromatography (silica gel, hex-
ane:ethyl acetate = 5:1 (v/v)); colorless liquid; yield 4.40 g (76 %).
1H NMR (270 MHz, CDCl3) d 7.96 (d, 2H, phenyl), 7.43 (d, 2H, phe-
nyl), 6.72 (dd, 1H, CH2 = CH-), 5.83 (s, 1H, CH2 = CH-), 5.34 (s, 2H,
C = CH- and CH2 = CH-), 2.21–0.85 (m, 16H, CH, CH2, CH3).
Into a four-necked round-bottom flask fitted with an efficient
magnetic stirrer and two addition funnels were placed methyltri-
phenylphosphonium bromide (6.20, 17.4 mmol), potassium tert-
butoxide (1.95 g, 17.4 mmol), and THF (60 mL) under nitrogen
atmosphere and kept at the room temperature for 1 h. A THF solu-
tion (10 mL) of 1-formyl-4-[1-methyl-1-(4-methylcyclohex-3-
enyl)ethoxy]benzene (3.00 g, 11.6 mmol) was added dropwise to
the solution and kept at the room temperature for 48 h. A
200 mL aliquot of water was added to the solution and the organic
phase was extracted with ethyl acetate twice, and washed with so-
dium bicarbonate and then with water. The organic layer was dried
over anhydrous MgSO4. The product was purified by column chro-
matography (silica gel, chloroform); yellow liquid; yield 0.650 g
(22%). 1H NMR (270 MHz, CDCl3) d 7.24 (d, 2H, phenyl), 6.85 (d,
2H, phenyl), 6.59 (dd, 1H, CH2 = CH-), 5.55 (s, 1H, CH2 = CH-),
5.32 (s, 1H, C = CH-), 5.05 (s, 1H, CH2 = CH-), 2.11–1.21 (m, 16H,
CH, CH2, CH3).
2.2.7. 4-Ethenylbenzoic acid 1-methyl-1-(6-methyl-7-oxabicyclo-
[4,1,0]hept-3-yl)ethyl ester(MMOVBA)
MMOVBA was synthesized by the epoxidation of 4-ethenylben-
zoic acid 1-methyl-1-(3-methylcyclohex-3-enyl)ethyl ester (2.00 g,
7.00 mmol) using the same method employed for MMOES. After
purification by flash chromatography (silica gel, chloroform),
MMOVBA was obtained as colorless liquid; yield 1.36 g (64 %). 1H
NMR (270 MHz, CDCl3) d 7.86 (d, 2H, phenyl), 7.36 (d, 2H, phenyl),
6.67 (dd, 1H, CH2 = CH-), 5.78 (s, 1H, CH2 = CH-), 5.29 (s, 1H,
CH2 = CH-), 2.93 (m, 1H, epoxy HCOC), 2.24–1.07 (m, 16H, CH,
CH2, CH3). Anal. Calcd for C18H24O2: C, 75.97; H, 8.05. Found: C,
75.56; H, 8.27.
2.2.4. 1-Ethenyl-4-[1-methyl-1-(6-methyl-7-oxabicyclo[4.1.0]hept-3-
yl)ethoxy]benzene (MMOES)
Into a three-necked round-bottom flask fitted with an efficient
magnetic stirrer, a Claisen adapter, two addition funnels, and a
pH meter electrode were placed 1-ethenyl-4-[1-methyl-1-(4-
methylcyclohex-3-enyl)ethoxy]benzene (2.34 g, 9.14 mmol), chlo-
roform (23 mL), acetone (21 mL), phosphate buffer (pH = 7.4,
100 mL), and 18-crown-6 (0.11 g, 0.42 mmol). The flask was cooled
to 0–5 °C using an ice-water bath. Oxone (2KHSO5ꢀKHSO4ꢀK2SO4)
(6.43 g, 10.5 mmol) in 26 mL of water was added dropwise over
the course of 30 min. At the same time, a solution of KOH (40 g,
0.713 mol) in 190 mL of water was also added dropwise to keep
the reaction mixture at pH 7.1–7.5. After the addition of Oxone,
the reaction mixture was stirred at 5 °C for 2.5 h. The resulting
mixture was filtered and extracted with three 50 mL aliquots of
2.3. Synthesis of polymers
Poly(MMOES) (PMMOES), poly(MMOES-co-TBOS) (MMOES(20)-
TBOS),
poly(MMOCS)
(PMMOCS),
poly(MMOCS-co-BOCS)
(MMOCS(43)-BOCS), and poly(MMOVBA) (PMMOVBA) were
prepared by photo-radical polymerization in degassed DMF solu-
tion at 30 °C using AIBN as an initiator with irradiation using a
medium-pressure mercury lamp (Toshiba SHL-100UV) with a cut-
off filter (Toshiba UV-35). Poly(TBOS) (PTBOS) and poly(BOCS)
(PBOCS) were prepared by thermal radical polymerization of the

H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
483
Scheme 2. Structures of polymers and a photoacid generator.
corresponding monomers in degassed toluene solution at 60 °C
using AIBN as an initiator. The resulting polymers were purified
by reprecipitation from chloroform / hexane. The fraction of the
MMOES or MMOCS incorporated into the polymer was determined
from 1H NMR spectra. The structures of the polymers and the
photoacid generator FITS are shown in Scheme 2. Polymerization
conditions and characteristics of the polymers are summarized in
Table 1.
sample films (1.9 lm) for FT-IR measurements. Irradiation was
performed at 254 nm in air using a low-pressure mercury lamp
(USHIO ULO-6DQ) without a filter. The intensity of the light was
measured with an Orc Light Measure UV-M02. Baking treatment
was carried out using a conventional hot plate. Insoluble fraction
was determined by comparing the film thickness before and after
dipping the samples into THF. Thickness of films was measured
by interferometry using a Nanometrics Nanospec M3000.
2.4. Photo-cross-linking and thermal de-cross-linking
2.5. Measurements
All sample films were prepared on silicon wafers by spin-cast-
ing from solutions of cyclohexanone containing sample polymer
and FITS. The sample films were dried on a hot plate at 120 °C
1H NMR spectra were observed at 270 MHz using a JEOL GX-270
spectrometer. Elemental analysis was carried out using a Yanako
CHN coder MT-3. UV–vis spectra were taken on a Shimadzu UV-
2400 PC. FT-IR measurements were carried out using a JASCO IR-
for 2 min. The thickness of films was about 0.5 lm except for the

484
H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
Table 1
Polymerization conditions and characteristics.
Monomer in feed (mmol)
Polymer
MMOES TBOS MMOCS BOCS MMOVBA Solvent
AIBN
(mmol)
Polym time. Yield
Compositione
(mol%)
Mn ꢁ 10ꢂ3 Mw
/
Td
(°C)g
f
(mL)
(h)
(%)
Mn
PMMOESa
MMOES(20)-
TBOSa
3.7
0.38
1.0c
0.061
0.014
48
49
52
59
100
20
6.0
11.0
1.84
2.07
250
252
0.59
11.5
0.20c
PTBOSb
2.0d
0.50c
1.2c
0.140
0.073
0.037
4
45.5
29
25
52
59
0
100
43
37.7
9.1
21.0
1.82
1.80
1.70
314
182
173
PMMOCSa
MMOCS(43)-
BOCSa
1.55
1.7
2.48
11.5
PBOCSb
2.6d
0.120
0.11
17.5
7
50
42
0
100
85.0
24.7
1.80
2.00
185
223
PMMOVBAa
2.00
0.60c
a
b
c
d
e
f
Polymerization was carried out at 30 °C with UV irradiation.
Polymerization was carried out at 60 °C.
DMF.
Toluene.
Composition of MMOES, MMOCS, or MMOVBA.
Determined by SEC.
Determined by TGA.
g
410. In situ FT-IR measurements were carried out using a Lithotech
Japan PAGA-50. Thermal decomposition behavior of films was
investigated with a Shimadzu TGA 50 thermogravimetric analyzer
(TGA) under a nitrogen flow. Size exclusion chromatography (SEC)
was carried out in THF on a JASCO PU-980 chromatograph
equipped with polystyrene gel columns (Shodex GMNHR-H
+
GMNHR-N; 8.0 mm i.d. ꢁ 30 cm each) and a differential refractom-
eter JASCO RI1530. Number-average molecular weight (Mn) and
molecular weight distribution (Mw/Mn) were estimated on the ba-
sis of a polystyrene calibration.
3. Results and discussion
3.1. Synthesis of polymers
Fig. 2. 1H NMR spectra of (a) MMOES, (b) PMMOES, and (c) MMOES(20)-TBOS in
CDCl3 at 270 MHz.
A series of modified polystyrene having epoxy units and various
types of thermally degradable linkages was prepared. In this work,
polystyrene was chosen as a backbone due to easy incorporation of
various functional groups. 1-Ethenyl-4-[1-methyl-1-(6-methyl-7-
oxabicyclo[4.1.0]hept-3-yl)ethoxy]benzene (MMOES) and 1-eth-
enyl-4-[1-methyl-1-(6-methyl-7-oxabicyclo[4. 1. 0]hept-3-yl)
ethoxycarbonyloxy]benzene (MMOCS) were chosen as monomers.
There are few reports on the incorporation of a tertiary ether link-
age in a phenyl moiety. The aromatic nucleophilic substitution
reaction of p-cyanofluorobenzene using a bulky alkoxide was em-
ployed according to the method of DeCosta et al. [25]. The cyano
group of the resulting compound was transformed into the vinyl
group via reduction into the formyl group and followed by Wittig
reaction without the loss of the tertiary ether linkage as shown
in Scheme 1. MMOCS was synthesized using 4-ethenylphenyl
2-(3-methylcyclohex-3-enyl)-2-propyl carbonate [15] as a starting
material. We have attempted the synthesis of 4-ethenylbenzoic
acid 1-methyl-1-(6-methyl-7-oxabicyclo[4,1,0]hept-3-yl)ethyl es-
ter (MMOVBA) via the reaction of 4-ethenylbenzoyl chloride
shown in Fig. 2. Although the peaks ascribed to vinyl groups
(5.39–6.70 ppm) of the monomers disappeared, the peak due to
aliphatic epoxides (ca. 3 ppm) remained. A characteristic peak
around 1.3 ppm ascribed to tert-butoxy groups of TBOS units was
observed in the spectrum of MMOES(20)-TBOS (Fig. 2c). Monomer
composition of MMOES(20)-TBOS was calculated using the aro-
matic peaks (6–7 ppm) and the epoxy peaks (ca. 3 ppm). Poly
(MMOCS) (PMMOCS) and poly(MMOCS-co-BOCS) (MMOCS(43)-
BOCS) were also prepared to investigate the effect of epoxy
contents of the polymers. Abbreviations of the polymers and poly-
merization conditions and characteristics of the polymers are sum-
marized in Table 1. The molecular weights of PMMOES and
PMMOCS were smaller than that of PMMOVBS. The bulkiness of
the terpene moiety may affect the polymerization rates and chain
length of the polymers. Insolubilization properties of the cross-
linked polymers strongly were affected by the molecular weights
as mentioned below.
Thermal properties of the polymers were investigated by TGA
measurements. Fig. 3 shows the TGA curves of PMMOES, PMMOCS,
and PMMOVBS together with those of PTBOS and PBOCS. The TGA
curve of PMMOES was quite different form that of PTBOS. PMMOES
started the first weight loss from ca. 250 °C. The observed weight
loss (50 %) was correspondent to the complete elimination of the
terpene units in PMMOES to form poly(4-vinylphenol) (PVP) (cal-
culated weight loss: 56%) by the cleavage of the tertiary ether link-
ages. PTBOS started to decompose at ca. 300 °C. No characteristic
information indicating the structural changes of PTBOS such as
the fragmentation of isobutene was obtained from the TGA curves.
and
1-(3-methylcyclohex-3-enyl)ethyl ester was not obtained. The
use of the alkoxide of -terpineol gave a good result. Regio-selec-
a-terpineol. The precursor 4-ethenylbenzoic acid 1-methyl-
a
tive epoxidation of the alkene of the precursor monomers pro-
ceeded to give the monomers MMOES, MMOCS, and MMOVBA.
MMOES, an analogue of TBOS having an epoxy moiety was
homopolymerized and copolymerized with TBOS by the photopo-
lymerization using AIBN at 30 °C with UV irradiation to prevent
gel formation. The 1H NMR spectra of MMOES, poly(MMOES)
(PMMOES), and poly(MMOES-co-TBOS) (MMOES(20)-TBOS) are

H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
485
degree of the PMMOES and MMOES(20)-TBOS films increased,
reached to a maximum value, decreased and increased again with
a rise of baking temperature. The maximum insoluble fraction was
observed for the films when baked at 120 °C for PMMOES and at
100 °C for MMOES(20)-TBOS. Insolubilization efficiency of the
films increased with the fraction of a MMOES moiety when baked
at between 80 and 140 °C for 10 min after irradiation. The insolu-
bilization was due to the photoinduced-acid catalyzed polymeriza-
tion of epoxy moieties of the polymers. As mentioned above,
insolubilization of the polymers was strongly affected by the
molecular weights. Polymers having large molecular weights easily
became insoluble in solvents by cross-linking reactions. The lower
insoluble fraction of MMOES(20)-TBOS than PMMOES was mainly
due to the lower molar fraction of the epoxide.
When the irradiated PMMOES and MMOES(20)-TBOS films
were baked at 140 or 160 °C for 10 min, insolubilization was not
observed. Cleavage of tertiary ether linkages in the polymers was
observed by in situ FT-IR measurements as discussed below. The
re-insolubilization for PMMOES and MMOES(20)-TBOS films on
baking at above 180 °C for 10 min was due to undesired cross-link-
ing of PVP derivatives which were formed by the thermolysis of
PMMOES and MMOES(20)-TBOS polymers. The reaction mecha-
nism was supported from the insolubilization of the irradiated
PTBOS films on baking at 200 °C. Crivello reported the deblocking
studies of polymers bearing tert-butoxy groups [20]. The model
study of acidolysis of tert-butoxytoluene revealed that PTBOS
decomposed to form PVP as a major product with compounds
substituted by a tert-butyl group as minor products as shown in
Scheme 3. Thus, the re-insolubilization may be due to electrophilic
aromatic substitution reaction of phenyl moieties in PVP may oc-
cur to form phenyl ether linkages.
Fig. 3. TGA curves of (a) PMMOES, (b) PMMOCS, (c) PMMOVBA, (d) PTBOS, and (e)
PBOCS in N2. Heating rate: 10 °C/min.
On the other hand, TGA curves of PMMOCS and PBOCS resembled
with each other. The first weight loss of PMMOCS and PBOCS
started at 182 and 185 °C, respectively. The results strongly indi-
cated that decomposition of the polymers started from the scission
of tertiary carbonate linkages. The first weight loss was 60% for
PMMOCS and 48% for PBOCS, and those values strongly suggested
the formation of PVP (calculated weight loss: 65% and 52%, respec-
tively). The first weight loss of PMMOVBA started at 223 °C, which
was similar to that observed for poly(1-methyl-1-(6-methyl-7-
oxabicyclo[4.1.0]hept-3-yl)ethyl methacrylate)) (216 °C) [10]. The
weight loss at 250 °C was 42%, which was ca 10% lower than that
of the complete elimination of the terpene units in PMMOVBA to
form poly(4-vinylbenzoic acid) (weight loss: 51%).
Fig.
5 shows the insolubilization profiles of PMMOCS,
MMOCS(43)-BOCS, and PBOCS. The insolubilization profiles of the
polymers were similar to those for PMMOES shown in Fig. 4. The
insolubilization degree of the PMMOCS and MMOCS(43)-BOCS
films increased, reached to a maximum value, decreased and in-
creased again with a rise of baking temperature. The decrease of
the insolubilization degree was due to the cleavage of the carbon-
ate linkages in MMOCS units. The reaction was the analogue of that
for PBOCS as shown in Scheme 4. Complete re-dissolution for
PMMOCS was observed on baking at 130 °C, whereas the incom-
plete re-dissolution for MMOCS(43)-BOCS films was observed.
We consider the reactions of de-cross-linking by the cleavage of
the carbonate linkages in MMOCS units and re-cross-linking reac-
tions of PVP derivatives which were formed by the thermolysis of
MMOCS(43)-BOCS occurred simultaneously. The re-insolubiliza-
tion of irradiated PMMOCS, MMOCS(43)-BOCS, and PBOCS films
was observed at lower temperatures than those of PMMOES,
MMOES(20)-TBOS, and PTBOS films. Investigation of the mecha-
nism of the re-insolubilization is now in progress. A plausible reac-
tion mechanism is discussed below.
3.2. Photo-cross-linking, thermal cross-linking, and thermal de-cross-
linking
Polymer films containing a PAG were irradiated at 254 nm and
insoluble fraction was studied. FITS (see Scheme 2) as a PAG was
photolyzed to generate p-toluenesulfonic acid [22]. The photoin-
duced acid initiated cationic polymerization of epoxy units in the
side chain to generate networks. Fig. 4 shows the insolubilization
properties of PMMOES, MMOES(20)-TBOS, and PTBOS. The insolu-
ble fraction was determined by comparing the thickness of the
films before and after dipping into THF. When the films were
irradiated with 60 mJ/cm2 and baked below 60 °C for 10 min, insol-
ubilization was not observed for all films. The insolubilization
The effect of the ether, ester, and carbonate linkages on the
insolubilization profiles of the polymers was investigated. Fig. 6
shows the insolubilization profiles of PMMOES, PMMOCS,
PMMOVBA, and PVP. For irradiated samples (open symbols), insol-
ubilization efficiency on baking at lower temperatures (<150 °C)
decreased in the order PMMOCS > PMMOES > PMMOVBA. Gener-
ally, insolubilization efficiency of polymers increases with their
molecular weights. The low efficiency for PMMOVBA contradicts
the effect of the molecular weights of the polymers which
decreased in the order PMMOVBA > PMMOCS > PMMOES.
The decreased insoluble fraction of the films on baking at
120–150 °C was due to the cleavage of the ether, ester, and carbon-
ate linkages in the polymers. The stability of the linkages in the
presence of p-toluenesulfonic acid was estimated to be in the or-
der: tertiary carbonate (PMMOCS) > tertiary ether (PMMOES) > ter-
Fig. 4. Photoinduced insolubilization of ( ) PMMOES, ( ) MMOES(20)-TBOS, and
(
Dissolution: THF for 10 min.
)
PTBOS films containing 5 wt.% FITS. Exposed at 254 nm with 60 mJ/cm2.

486
H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
Scheme 3. Reaction mechanism of photoinduced acid-catalyzed decomposition of PTBOS.
decomposed films. The irradiated films of PVP became insoluble
on baking at above 220 °C. The insolubilization may be due to
dehydration of phenolic OH groups in PVP. The re-insolubilization
of irradiated PMMOES and PMMOCS occurred by baking at above
180 °C which was 40 °C lower than that of PVP. PTBOS formed
compounds substituted by a tert-butyl group as shown in Scheme
3. The reaction concerned with the decomposed fragments such as
limonene, a difunctional alkene, generated from the dehydration of
a-terpineol, may occur. Moreover, the re-insolubilization profiles
of the irradiated PMMOES and PMMOCS coincided with the insol-
ubilization curves of unirradiated (solid symbols) PMMOES and
PMMOCS. The insolubilization of PMMOES and PMMOCS may be
due to the complicated reactions of cross-linking of epoxides,
decomposition of tertiary ether or carbonate linkages, and the re-
cross-linking of PVP generated by the decomposition of PMMOES
and PMMOCS, simultaneously. The re-increase of the insoluble
fraction of the irradiated PMMOVBA films occurred when baked
at above 200 °C due to the formation of carboxylic anhydride link-
ages. The plausible reaction mechanism of the cross-linking and
de-cross-linking of PMMOES, PMMOCS, and PMMOVBA is shown
in Scheme 5.
Fig. 5. Photoinduced insolubilization of ( ) PMMOCS, ( ) MMOCS(43)-BOCS, and
(
Dissolution: THF for 10 min.
)
PBOCS films containing 5 wt.% FITS. Exposed at 254 nm with 60 mJ/cm2.
tiary ester (PMMOVBA). It is noteworthy that the thermal stability
determined by TGA measurements increased in the order
PMMOCS < PMMOVBA < PMMOES. The photogenerated-acid pro-
moted the efficient cleavage of the tertiary ether linkages of
PMMOES.
3.3. In situ observation of degradation of tertiary ether and tertiary
carbonate
The photoinduced acid-catalyzed decomposition of PTBOS,
PBOCS, and poly(tert-butyl methacrylate) (TBMA) was discussed
using Ea values which were reported to correspond to the reaction
controlled by activation energy [26]. The Ea values of PTBOS,
PBOCS, and TBMA were reported to be 142, 136, and 130 kJ/mol,
respectively [26]. Though the Ea values were strongly affected by
the structure and concentration of acid, we expected that the high-
er stability of PMMOES compared to PMMOCS was due to the high
activation energy of PMMOES.
In situ FT-IR measurements revealed the precise decomposition
behaviors of tertiary ether linkages of PMMOES and tertiary car-
bonate linkages and PMMOCS. Fig. 7 shows the FT-IR spectral
changes of the irradiated (a) PMMOES and (b) PMMOCS film con-
taining 5 wt.% FITS on baking at 130 °C. The peak at 1130 cmꢂ1
due to C–O–C stretching of tertiary ether linkages in PMMOES
and the peak at 1760 cmꢂ1 due to carbonate carbonyl of PMMOCS
decreased with baking time. Fig. 8 shows the disappearance of ter-
tiary ether units of cross-linked PMMOES and carbonate units of
cross-linked PMMOCS, and appearance of hydroxy units in the
cross-linked PMMOES. Remaining ether group of PMMOES was
The re-increase of the insoluble fraction of the films on baking
at above 160 °C may be due to the network formation of the
Scheme 4. Reaction mechanism of photoinduced acid-catalyzed decomposition of PBOCS.

H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
487
Scheme 5. Plausible reaction mechanism of photo-cross-linking and thermal de-cross-linking.
Fig. 6. Effect of baking temperature on insoluble fraction of PMMOES (
,
),
PMMOCS (
,
), PMMOVBA (
,
), and PVP ( ) films. Additive: 5 wt.% FITS. Open
symbol: irradiated with a dose of 60 mJ/cm2. Solid symbol: no irradiation. Baking
time: 10 min. Dissolution: THF for 10 min.
monitored using the peak at 1130 cmꢂ1. Remaining carbonate
group of PMMOCS was monitored using the peak at 1760 cmꢂ1. In-
creased hydroxyl group of decomposed PMMOES was monitored
using the peak at 3480 cmꢂ1. Tertiary ether units gradually decom-
posed to generate aromatic OH groups on heating at 130 °C. The
decomposition of carbonate units suddenly occurred after heating
at 130 °C for 6 min. The thermal decomposition was catalyzed by
phenolic OH groups generated, which caused the self-acceleration
properties. Thus, it was concluded that the decomposition of ter-
tiary carbonate linkages was faster than that of tertiary ether link-
ages. The conclusion was supported by the discussion of Ea as
mentioned above.
4. Conclusion
Fig. 7. FT-IR spectral changes of the photo-cross-linked (a) PMMOES and (b)
PMMOCS films containing 5 wt.% FITS on baking at 130 °C. Film thickness: 1.0 lm.
Irradiation dose to cross-link the polymer: 60 mJ/cm2.
We have designed and synthesized novel photo-cross-linkable
polymers having thermally degradable linkages. Homopolymers
and copolymers which have an epoxy moiety and tertiary ether
(MMOES), tertiary ester (MMOVBA), or tertiary carbonate
(MMOCS) linkages in a molecule, were photo-cross-linkable on
if baked at a given temperature. Thermal decomposition of the
degradable linkage of the network generated linear polymers.
The stability of the linkages of the polymers was found to be in
the order: tertiary ester > tertiary ether > tertiary carbonate. As
for tertiary ester and tertiary ether, the order was accountable to
irradiation in the presence of
a photoacid generator. The
photochemically cross-linked polymers became soluble in solvents

488
H. Okamura et al. / Reactive & Functional Polymers 71 (2011) 480–488
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