Photoinduced depolymerization of poly(olefin sulfone)s possessing photobase generator side-chains: Effect of spacer-chain length

Article abstract of DOI:10.1002/pola.26787

Photoinduced depolymerization of poly(olefin sulfone)s possessing photobase generators in the side-chain was investigated. Irradiation with UV light generated base on the side-chains and induced depolymerization based on proton abstraction on the main-cha

Full text of DOI:10.1002/pola.26787

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Photoinduced Depolymerization of Poly(olefin sulfone)s Possessing
Photobase Generator Side-Chains: Effect of Spacer-Chain Length
Takeo Sasaki, Takumi Yoneyama, Shota Hashimoto, Sumie Takemura, Yumiko Naka
Department of Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
Correspondence to: T. Sasaki (E-mail: sasaki@rs.kagu.tus.ac.jp)
Received 21 March 2013; accepted 14 May 2013; published online 11 June 2013
DOI: 10.1002/pola.26787
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ABSTRACT: Photoinduced depolymerization of poly(olefin sul-
fone)s possessing photobase generators in the side-chain was
investigated. Irradiation with UV light generated base on the
side-chains and induced depolymerization based on proton
abstraction on the main-chain. The effect of the length of the
spacer chain, which connects the photobase-generating moiety
to the polymer main chain on the photoinduced
depolymerization, also was investigated. 2013 Wiley Periodi-
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3873–
3880
KEYWORDS: degradation; lithography; photobase generator;
photoinduced depolymerization; photoresists; poly(olefin
sulfone)s
INTRODUCTION The photoinduced depolymerization reaction
of poly(olefin sulfone)s possessing a photobase generator
(PBG) has been investigated.1–3 The poly(olefin sulfone) is a
1:1 alternating copolymer of an olefin monomer and sulfur
dioxide.4,5 The protons on the carbon adjacent to the sul-
fonyl group are easily abstracted by bases.6 When this
occurs, a poly(olefin sulfone) that possesses a photobase-
generating chromophore undergoes a photoinduced unzip-
ping reaction and the main chain of the poly(olefin sulfone)
is depolymerized back to the olefin monomer and sulfur
dioxide (Fig. 1). This type of polymer is suitable for a wide
variety of applications such as removable adhesives, stereoli-
thography, and printable micro-circuit fabrication.
EXPERIMENTAL
Samples
The molecular structures of the polymers used in this study
are shown in Figure 2. Polymers that possess an [(o-nitroben-
zyl) oxy]carbonyl group as a photobase-generating group in the
side-chain were synthesized. The [(o-nitrobenzyl)oxy]carbonyl
group degrades upon exposure to UV light to produce an amino
group.7–9 The base generation mechanism is shown in Scheme
1. The photobase-generating group was designed to produce a
secondary amino group because a secondary amino group can
induce effective depolymerization of poly(olefin sulfone)s.1 The
polymers were synthesized according to Scheme 2.
A series of compounds that generates amino groups after UV
irradiation was used as a PBG. These polymers underwent
photoinduced depolymerization. In this study, the effect of
the length of the spacer chain, which connects the PBG
chromophore to the polymer main chain, on photoinduced
depolymerization of poly(olefin sulfone)s was investigated.
The mobility of the PBG chromophore in the side chain was
dependent on the length of the spacer chain. Depolymeriza-
tion of poly(olefin sulfone) is caused by proton abstraction
from the polymer main chain by the photogenerated base.
Thus, the mobility of the PBG is expected to affect depoly-
merization. The polymer film was irradiated with 254-nm
UV light, causing decomposition of the poly(olefin sulfone)
polymer. Decomposition and depolymerization behaviors
were compared in the polymers.
Synthesis of Olefin Monomer (PNC2-8 Monomer)
2-Nitrobenzyl p-Nitrophenyl Carbonate (Step 1)
2-Nitrobenzyl alcohol (30.64 g, 0.200 mol) and pyridine
(15.93 g, 0.200 mol) were dissolved in 300 mL of dry THF
and cooled to 0 ꢀC. 4-Nitrophenyl chloroformate (51.12 g,
0.254 mol) dissolved in 150 mL of dry THF was added drop-
wiꢀse to the solution while maintaining the temperature at
0 C. Then the solution was refluxed for 8 h. After the solu-
tion was cooled to room temperature, 1000 mL of water was
added and the product was extracted with 1000 mL of chlo-
roform. The chloroform solution was washed with 1000 mL
of water and dried over sodium sulfonate. The solvent was
evaporated to yield a yellow-green powder. The product was
recrystallized from 1:1 toluene:ethanol. Yellow crystals
(56.38 g) were obtained in 88% yield.
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IR (KBr) 1244 (s, asymm CAO str), 1341 (s, symm NAO
str), 1522 (s, asymm NAO str), 1764 (s, C@O str).
4-Hydroxy-N-[[(o-nitrobenzyl)oxy]carbonyl]pyridine
(Step 2)
2-Nitrobenzyl p-nitrophenyl carbonate (69.95 g, 0.220 mol),
4-hydroxypyridine (22.96 g, 0.227 mol), and 1-
hydroxybenzotriazole (HOBt, 14.94 g, 0.111 mol) were dis-
solved in 450 mL dry THF. The solution was refluxed for
6 h. The solvent was removed by evaporation and the
residue (yellow solid) dissolved in chloroform (1000 mL)
and washed with saturated aq. sodium hydrocarbonate
(1000 mL) and water (1000 mL). The solution was dried
over sodium sulfonate. After the solvent was removed, the
product was recrystallized from 1:1 mixture ethyl acetate:-
hexane. A slightly yellow solid (50.69 g) was obtained in
82% yield.
FIGURE 1 Photoinduced depolymerization of poly(olefin sul-
fone)s possessing a photobase generator in the side-chain.
Anal. Calcd. for C13H16N2O5: C, 55.71%; H, 5.75%; N, 9.99%.
Found: C, 56.34%; H, 5.80%; N, 10.13%.
1H NMR (CDCl3, 500 MHz) d 1.48–1.57 (m, 2H, CyAH), 1.87–
1.92 (m, 2H, CyAH), 3.17–3.22 (s, 2H, CyAH), 3.87–3.94
(m, 3H, CyAH), 5.52 (s, 2H, ArACH2AOA), 7.47 (t, 1H,
ArAH), 7.55 (d, 1H, ArAH), 7.63 (t, 1H, ArAH), 8.05 (d, 1H,
ArAH).
FIGURE 2 Structure of poly(olefin sulfone)s PNC2-x (x 5 2–8).
IR (KBr) 1341 (s, symm NAO str), 1524 (s, asymm NAO
str), 1764 (s, C@O str), 3484 (s, OAH str).
Anal. Calcd. for C14H10N2O7: C, 52.84%; H, 3.17%; N, 8.80%.
Found: C, 52.08%; H, 3.12%; N, 8.67%.
4-[9-Decenylcarbonyl]oxy-N-[[(o-nitrobenzyl)oxy]
carbonyl]piperidine (PNC2-8 Monomer, Step 3)
10-Undecenoic acid (2.00 g, 0.010 mol), 4-hydroxy-N-
[[(o-nitrobenzyl)oxy]carbonyl]pyridine (3.16 g, 0.011 mol),
1H NMR (CDCl3, 500 MHz) d 5.75 (s, 2H, ArACH2AOA), 7.42
(d, 2H, ArAH), 7.58 (m, 1H, ArAH), 7.74 (t, 2H, ArAH), 8.20
(d, 1H, ArAH), 8.30 (d, 2H, ArAH).
SCHEME 1 Degradation mechanism of the photobase generator.
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SCHEME 2 Synthetic route for PNC2-x.
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC, 3.02 g,
0.016 mol), and 4-dimethylaminopyridine (DMAP, 3.04 g,
0.025 mol) were dissolved in dry chloroform (120 mL). The
solution was stirred for 40 h at room temperature. A 3 N
solution of hydrochloride (250 mL) was added and the
resulting solution stirred for 30 min at room temperature.
The chloroform solution was washed with 3 N hydrochloride
(1000 mL) and saturated aq. of sodium chloride (1000 mL).
After the chloroform solution was dried over sodium sulfo-
nate, the solvent was removed by evaporation. The yellow
oil obtained was purified by silica gel column chromatogra-
phy (eluent: 1:1 ethyl acetate:chloroform). The product was
obtained as white crystals (4.30 g, yield: 89%).
ACH2ACH2ANA of Cy), 33.1 (CH2@CHACH2A), 33.7
(ACH2ACOAOA),
(AOACH2AAr),
40.8
68.7
(ACH2ANA
(AOACHA
of
Cy),
63.2
114.5
(CH2)2A),
(CH2@CHA), 124.6 (ACHA (Ar)), 129.1 (ACHA of Ar), 132.0
(ACA (Ar)), 133.9 (ACHA of Ar), 138.7 (CH2@CHA), 147.4
(ACHA (Ar)), 153.8 (NACOAOA), 172.2 (ACH2ACOAOA)
IR (KBr) 1237 (s, asymm COAO str), 1333 (s, symm NAO
str), 1528 (s, asymm NAO str), 1697 (s, C@O str), 2922
(s, CAH str).
Polymerization (Step 4)
The olefin monomers were dissolved in liquefied SO2 and
polymerized with tert-butylperoxide (tBuOOH) as an initia-
tor. tBuOOH acted as a redox initiator with SO2, producing a
tert-butyloxy radical. The initiator (1.56 3 1022 mol), olefin
monomer (1.0 g, 7.1 3 1023 mol–1.8 3 1022 mol), and SO2
(10 g, 1.6 3 1021 mol) were added to a pressurized glass
tube at 2196 ꢀC.ꢀWhen the temperature of the tube was
raised above 270 C, the frozen solution fused and polymer-
ization occurred. The tube was maintained at 213 ꢀC for
1 h. After polymerization, the polymer was purified by pre-
cipitation from methanol, then washed several times with
methanol, and dried under vacuum at room temperature.
The presence of SO2 in the polymer was confirmed by FT-IR
Anal. Calcd. for C13H16N2O5: C, 64.55%; H, 7.67%; N, 6.27%.
Found: C, 64.66%; H, 7.65%; N, 6.23%.
1H NMR (CDCl3, 500 MHz) d 1.44 (m, 6H, ACH2A), 1.62–
1.68 (m, 6H, ACH2A, CyAH), 1.85–1.90 (m, 2H, CyAH),
2.040 (q, 4H, ACH2A), 2.33 (t, 2H, ACH2A), 3.37 (s, 2H,
CyAH), 3.75 (m, 2H, CyAH), 4.88 (m, 1H, CyAH), 4.98 (d,
2H, CH2@CHA), 5.05 (d, 2H, CH2@CHA), 5.52 (s, 2H,
ArACH2AOA), 5.80 (m, 1H, CH2@CHA), 7.48 (t, 1H, ArAH),
7.55 (d, 1H, ArAH), 7.63 (t, 1H, ArAH), 8.06 (d, 1H, ArAH).
13C NMR (DMSO, 500MHz) d 24.4 (ACH2ACH2ACOAOA),
28.2–28.6 (CH2@CHACH2ACH2ACH2ACH2ACH2ACH2ACH2A,
1
(1311 and 1130 cm21) and H NMR. Molecular weights and
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TABLE 1 Physical Properties of the Polymers
a
b
Polysulfone
PNC2-2
Mw
Mw/Mn
Xw
Tg (ꢀC)c
Td (ꢀC)d
39,000
41,000
46,000
87,000
95,000
137,000
69,000
18,000
44,000
56,000
98,000
90,000
96,000
102,000
105,000
119,000
80,000
19,000
25,000
41,000
80,000
95,000
2.96
2.98
2.62
2.07
1.85
2.48
2.06
2.33
2.79
3.56
2.27
2.58
3.01
3.98
2.46
2.80
2.35
2.34
2.43
2.85
2.67
2.79
91
96
74.8
79.7
76.4
77.0
76.7
79.2
63.6
49.7
53.2
52.4
42.4
45.5
35.5
39.0
40.2
32.2
34.2
24.7
29.2
22.3
24.1
27.6
186.7
194.3
210.9
219.5
221.0
222.3
205.7
181.3
210.4
191.3
179.5
108
204
223
321
157
40
PNC2-3
PNC2-4
97
123
216
192
199
211
218
247
161
37
FIGURE 4 Dependence of the photochemical reaction of the
photobase generator in PNC2-x film on irradiation energy. UV
Light (254 nm) from a super-high-pressure mercury lamp was
used to irradiate the film. Molecular weights (Mw) of the poly-
mers were 87,000 for PNC2-2, 56,000 for PNC2-4, 96,000 for
PNC2-6, and 80,000 for PNC2-8.
PNC2-5
PNC2-6
243.8
188.9
222.6
176.4
203.6
213.9
187.8
188.3
189.2
190.5
223.5
PNC2-7
PNC2-8
Measurements
Infrared spectra were obtained with a Jeol JIR-5500 FT-IR
spectrophotometer. The weight-average molecular weight
(Mw) of the polymers was determined by gel permeation
47
80
157
186
a
Molecular weights of polymers were determined by GPC with polysty-
rene standards.
b
Degree of polymerization calculated from weight-average molecular
weight.
c
Glass transition temperature Tg was obtained by DSC using a heating
rate of 10 ꢀC/min under N2.
d
Degradation temperature measured by thermogravimetric analysis
(TGA); temperature at 10 wt % loss.
thermal properties of the polymers obtained are listed in
Table 1. The polymers were soluble in chloroform, tetrahy-
drofuran, N,N-dimethylformamide, dimethyl acetamide, and
dimethyl sulfoxide.
FIGURE 5 1H NMR spectra of DMSO-d6 solutions of (a) PNC2-2
and (b) PNC2-2 after irradiation at 254 nm (1.5 J/cm2) followed
by heating at 140 ꢀC for 15 min; and (c) PNC2-2 monomer.
FIGURE 3 IR absorption spectra of the PNC2-4 (Mw 5 56,000)
film spin-coated on a KBr plate. Film thickness was 2 lm. Sam-
ple film was irradiated at 254 nm.
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FIGURE 6 Decomposition ratio and depolymerization ratio of
PNC2-2 plotted as a function of degree of polymerization (Xw).
(a) Heated as a film. Polymer films were irradiated at 254 nm
(1.5 J/cm2), followed by heating at 120 ꢀC for 30 min. (b)
Heated in DMSO-d6 solution. Polymer films were irradiated at
254 nm (1.5 J/cm2) and dissolved in DMSO-d6, followed by
heating at 120 ꢀC for 30 min and NMR analysis.
FIGURE 7 Decomposition ratio and depolymerization ratio of
PNC2-4 plotted as a function of degree of polymerization (Xw).
(a) Heated as a film. Polymer films were irradiated at 254 nm
(1.5 J/cm2), followed by heating at 120 ꢀC for 30 min.
(b) Heated in DMSO-d6 solution. Polymer films were irradiated
at 254 nm (1.5 J/cm2) and dissolved in DMSO-d6, followed by
heating at 120 ꢀC for 30 min and NMR analysis.
irradiation to yield a secondary amine. When the polymer
film was exposed to UV light, the nitro group of the PBG was
converted to a nitroso group and carbon dioxide was
released. The photochemical reaction was followed by moni-
chromatography (GPC; Tosoh HLC-8220 with Super Multi-
pore HZ-M column, tetrahydrofuran eluent), and glass transi-
tion temperatures (Tg) were determined by differential
scanning calorimetry (DSC; Mettler, DSC822e). The decompo-
sition temperature was confirmed by thermogravimetric
analysis (TGA, TA instruments, Hi-Res TGA2950). 1H NMR
spectra were measured using a Jeol EX-400 spectrometer
(500 MHz). The polymer film was irradiated using a 250 W
super high-pressure mercury lamp (Ushio, SX-UI250HQ).
The intensity of the UV light was maintained at 23 mW/cm2
(at 254 nm). A color filter (Toshiba, UVD-36C) and an inter-
ference filter were used to obtain monochromatic light.
Thicknesses of the films were measured by atomic force
microscopy (AFM; Keyence, VN-8000).
toring the IR absorption of the carbonyl group (1700 cm21
)
and the nitro group (1530 cm21). Films were irradiated with
UV light (254 nm) and the photochemical reaction of the
PBG was examined. Figure 3 shows the IR absorption spec-
tra of a PNC2-4 film coated on a KBr plate. The absorptions
at 1700 and 1530 cm21 decreased as irradiation time
increased. Decomposed fractions of PBG in the polymer films
PNC2-2, PNC2-4, PNC2-6, and PNC2-8 are plotted as a func-
tion of irradiation energy in Figure 4. The decomposition
fraction increased with irradiation energy. Decomposition
of the PBG moiety in the side-chain was not affected by
spacer length.
RESULTS AND DISCUSSION
Photochemical Reaction of PBG in the Polymer Film
Base generation of the photobase generator (PBG) moieties
in the poly(olefin sulfone)s was investigated. As shown in
Scheme 1, the PBG underwent decarboxylation upon UV light
Effect of Degree of Polymerization on Photoinduced
Depolymerization
The films of PNC2-x (2-lm thick) were transparent and
colorless. When
a
PNC2-x film was heated without
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FIGURE 8 Decomposition ratio and depolymerization ratio of
PNC2-6 plotted as a function of degree of polymerization (Xw).
(a) Heated as a film. Polymer films were irradiated at 254 nm
(1.5 J/cm2), followed by heating at 120 ꢀC for 30 min. (b)
Heated in DMSO-d6 solution. Polymer films were irradiated at
254 nm (1.5 J/cm2) and dissolved in DMSO-d6, followed by
heating at 120 ꢀC for 30 min and NMR analysis.
FIGURE 9 Decomposition ratio and depolymerization ratio of
PNC2-8 plotted as a function of degree of polymerization (Xw).
(a) Heated as a film. Polymer films were irradiated at 254 nm
(1.5 J/cm2), followed by heating at 120 ꢀC for 30 min. (b)
Heated in DMSO-d6 solution. Polymer films were irradiated at
254 nm (1.5 J/cm2) and dissolved in DMSO-d6, followed by
heating at 120 ꢀC for 30 min and NMR analysis.
photoirradiation at 140 ꢀC for 15 min, no change was
observed. In contrast, when a PNC2-x film was irradiated
with 254-nm UV light, the film became slightly yellow due to
absorption of 2-nitrosobenzaldehyde produced by photoreac-
tion of the PBG. At this stage, no change was observed on
the surface of the film except for color. After the PNC2-x film
was irradiated and then heated at 140 ꢀC for 15 min, the
film became soluble in methanol.
depolymerization ratios in DMSO solution as well as those in
polymer films were measured. Polymer films were irradiated
with 254 nm UV light (1.5 J/cm2) and dissolved in DMSO-d6.
The solution in the NMR tube then was immersed in an oil
ꢀ
bath at 120 C for 30 min.
The effect of degree of polymerization on the photoinduced
depolymerization of PNC2-x was investigated. The degree of
polymerization (Xw) is defined as:
Figure 5 shows the NMR spectra of PNC2-2 without light
irradiation and after 254 nm light irradiation of 1.5 J/cm2
and heating at 140 ꢀC for 15 min, and PNC2-2 monomer.
After photoirradiation and heating, proton signals of the ole-
fin group appeared. The decomposition ratio was estimated
from the decrease in signal of the methylene protons on the
polymer main chain (4 ppm), and the depolymerization ratio
was obtained from the increase in the signal of the olefin
protons (5.8 ppm). After 254 nm irradiation of 1.5 J/cm2
and post-exposure heating (120 ꢀC, 30 min), the film was
dissolved in deuterated dimethylsulfoxide (DMSO-d6) and
1H-NMR spectra were obtained. The decomposition and
Weight2average molecular weightðMwÞmeasured by GPC
Xw5
Molecular weight of the olefin monomer and sulfur dioxide
Degree of polymerization of the polymers used in this study
is listed in Table 1. The decomposition and depolymerization
ratios in PNC2-x as a function of the degree of polymeriza-
tion are shown in Figures 6–9. Both the decomposition ratio
and depolymerization ratio were higher in solution than in
films and were nearly independent of the degree of polymer-
ization regardless of the condition of the polymer (in film or
in solution).
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FIGURE 11 (a) Decomposition ratio of PNC2-x heated after UV
irradiation (254 nm and 1.5 J/cm2) as a film as a function of
post-exposure heating temperature. (b) Depolymerization ratio
of PNC2-x film [after irradiation at 254 nm (1.5 J/cm2), dissolu-
tion of the film in DMSO-d6, and heating] as a function of post-
exposure heating temperature. Molecular weights (Mw) of the
polymers were 87,000 for PNC2-2, 56,000 for PNC2-4, 96,000 in
PNC2-6, and 80,000 in PNC2-8.
FIGURE 10 (a) Decomposition ratio of PNC2-x as a function of
spacer length x (x 5 2–8). Polymer films were irradiated at 254
nm (1.5 J/cm2), followed by heating at 120 ꢀC for 30 min (᭿) in
DMSO-d6 solution or (•) as a film. (b) Depolymerization ratio of
PNC2-x as a function of spacer length x (x 5 2–8). Polymer
films were irradiated at 254 nm (1.5 J/cm2), followed by
ꢀ
w
heating at 120 C for 30 min in ( ) DMSO-d6 solution or (᭺) as
a film.
The decomposition and depolymerization ratios of the poly-
mers in solution and in the film state also were measured.
Both decomposition and depolymerization ratios were
greater in polymers heated in solution compared to those
heated as films. The decomposition and depolymerization
ratios of the polymers heated in solution decreased with
increasing spacer chain length. Dependences of decomposi-
tion and depolymerization ratios on spacer length were
similar in solution and as film. Although mobility of the pho-
togenerated amine moieties was higher in solution, the
decomposition reaction did not depend on the spacer length.
The effect of steric hindrance of the spacer chain on proton
abstraction on the polymer main chain may be large enough
to cancel out the increase in mobility of the amine moieties.
Effect of Spacer-Chain Length on Photoinduced
Depolymerization of Poly(olefin sulfone) films
The decomposition ratios and depolymerization ratios of
PNC2-x films after UV irradiation and heating are plotted as
a function of spacer length (the number of the methylene
units in the spacer chain) in Figure 10(a). Approximately
30–50% of the polymer decomposed during the post-
exposure heating. With an increase in spacer chain length
from x 5 2 to x54, the decomposition ratio decreased and
reached to a constant value of 30%. Although flexible spacer
chain length increased, decomposition was depressed. There
seemed no evidence of the affection of the mobility of the
base moiety. The decrease in the decomposition ratio was
considered to be attributed to an increase in steric hindrance
around the polymer main chain. As the length of the spacer
chain increased, proton abstraction was inhibited. The depo-
lymerization ratio decreased slightly with spacer chain
length [Fig. 10(b)]. Approximately 32% of the decomposed
polymer was converted to monomers in film.
The decomposition ratios of PNC2-x are plotted as a function
of post-exposure heating temperature in Figure 11. The
decomposition ratio increased with baking temperature. No
effect of chain length on the temperature dependence of the
decomposition ratio was observed either in films or in
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those of PNC-x with longer side-chains, almost no difference
was observed in PNC2-x polymers.
CONCLUSIONS
Photoinduced depolymerization of poly(olefin sulfone)s that
possess photobase generators in a side-chain was investi-
gated. The effect of the length of the alkyl side chain con-
necting PBG and the polymer main chain on photoinduced
depolymerization was examined. Polymers with different
spacer-lengths and different degrees of polymerization were
prepared subjected to photoinduced degradation. The
length of the spacer of the photodegradable side-chain of the
poly(olefin sulfone)s did not significantly affect degradation
behavior. This may be due to cancellation of the increase in
the freedom of motion of the reactive chromophore photo-
base generator in the side-chain by the increase in steric
hindrance of the connecting spacer unit between the
photobase-generating moiety and the polymer main chain.
ACKNOWLEDGMENT
This work was supported by the Suzuki Foundation.
REFERENCES AND NOTES
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FIGURE 12 (a) Decomposition ratio of PNC2-x heated after UV
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[after irradiation at 254 nm (1.5 J/cm2), dissolution of the film
in DMSO-d6, and heating] as a function of post-exposure heat-
ing time. The temperature was maintained at 120 ꢀC. Molecular
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PNC2-4, 96,000 in PNC2-6, and 80,000 in PNC2-8.
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