Chapter 18
Thermally Degradable Photocross-Linking Polymers Masamitsu Shirai, Satoshi Morishita, Akiya Kawaue, Haruyuki Okamura, and Masahiro Tsunooka Downloaded by IOWA STATE UNIV on March 2, 2017 | http://pubs.acs.org Publication Date: February 7, 2004 | doi: 10.1021/bk-2004-0874.ch018
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
A novel monomer (MOBH) which has both epoxy moiety and thermally cleavable tertiary ester moiety in a molecule was synthesized and characterized. Homopolymer of M O B H and copolymers of MOBH with tert-butyl methacrylate, tertbutoxystyrene or styrenesulfonates were synthesized. On U V irradiation the polymer films containing photoacid generators became insoluble in organic solvents. When the crosslinked polymer films were baked at 100-220 °C, they became soluble in methanol. The effective baking temperature was strongly dependent on polymer structure. The crosslinked polymers having styrenesulfonic acid ester units became soluble in water after bake treatments.
Introduction Polymers which become insoluble in solvents on UV irradiation are used as photosensitive materials such as photoresists, printing plates, inks, coatings and photocurable adhesives (/). Since photochemically crosslinked polymers are insoluble and infusible networks, scratching or chemical treatments with strong acid or base must be applied to remove these networks from substrates. However, it is difficult or impossible to thoroughly remove crosslinked polymers without damaging underlying materials. Recently, some thermosets which are thermally or chemically degradable under a given condition have
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© 2004 American Chemical Society
Lin et al.; Polymers for Microelectronics and Nanoelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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237 been reported. Epoxy resins containing disulfide linkages were reported. They could be cleaved by treatment with triphenylphosphine to generate thiols (2, 3). Epoxy resins having acetal linkages were also studied (4) because acetal linkages can be easily decomposed by acids. Furthermore, epoxy resins with tertiary ester linkages (5, 6) or carbamate linkages (7) were reported to undergo network breakdown upon heating. Diacrylate and dimethacrylate monomers containing thermally cleavable tertiary ester linkages were synthesized and the networks obtained by photopolymerization were observed to decompose on heating to form partially dehydrated poly(acrylic acid) or poly(methacrylic acid) and volatile alkenes (6). Decomposed products were soluble in basic solutions and could be removed by a simple thermal treatment followed by washing with a basic solution. Re-usable polymers based on bicyclic ortho esters and spiro ortho esters were also reported (8, 9). These polymers can be converted to monomers by depolymerization.
soluble
Δ 1
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insoluble
soluble
xrosslinkable unit ) :cleavabte unit
Concept of thermally degradable photocrosslinking polymer.
In this paper, we report the synthesis and characterization of photocrosslinkable polymers bearing thermally degradable property (10, 11). The concept of the present system is shown in Figure 1. On irradiation network formation takes place by the photoinduced-acid catalyzed reactions of the crosslinkable moieties. A thermal treatment of the crosslinked polymers induces the cleavage of the network linkages. Based on this concept, we have
Lin et al.; Polymers for Microelectronics and Nanoelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
238
synthesized polymers bearing both an epoxy moiety and a tertiary ester linkage in the side chain. These polymers are important as a photocrosslinkable material which can be removed by baking after use.
Experimental
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Materials Benzene, i^-dimethylformamide (DMF), dichloromethane, methyl methacrylate,terf-butylmethacrylate (TBMA) and p-iert-butoxystyrene (tBOSt) were purchased and distilled before use. 2,2'-Azobisisobutyronitrile (AIBN) was purified by recrystallizationfromethanol. 9-Fluorenilideneimino ptoluenesulfonate (FITS) (72), neopentyl styrenesulfonate (NPSS) (73), cyclohexyl styrenesulfonate (CHSS) (14) and phenyl styrenesulfonate (PhSS) were prepared according to the literature. Oxone (potassium peroxymonosulfate) (Aldrich) and triphenylsulfonium triflate (TPST) (Midori Kagaku) were used as received. l-Methyl-l-(4-methyl-cyclohex-3-enyl)ethyl methacrylate (MMCEM)
MMCEM was prepared by the reaction of methacryloyl chloride with ctterpineol (Figure 2). To a cold (< 5 °C) solution of α -terpineol (34.8 g, 0.226 mol) and 4-dimethylaminopyridine (DMAP) (2.7 g, 0.0221 mol) in anhydrous pyridine (31 mL) was slowly added a solution of 24.0 g (0.230 mol) of methacryloyl chloride in 110 mL of anhydrous dichloromethane. The mixture was stirred at ambient temperature for 40 h and then thoroughly washed with 2N H S0 . The organic phase was separated and washed with saturated NaHC0 solution and then with water. The organic layer was dried over anhydrous MgS0 . The product was purified by column chromatography; yield 25.6 g (51.0 %). *H NMR (400 MHz, CDC1) £5.92 (s, 1H, C7/=C), 5.39 (s, 1H, C7/=C), 5.30 (s, 1H, -C77=C-), 1.83 (s, 3H, C77), 1.57 (s, 3H, C7/ ), 1.41 (d, 6H, 2C7/ ), 1.20-2.10 (m, 11H, CH, CH ). 2
4
3
4
3
2
2
3
3
3
2
7 -Methyl-]-(6-methyl- 7-oxabicyclo[4.7.0Jhept-3-yl) ethyl methacrylate (MOBH)
MOBH was obtained by epoxidation (75) of MMCEM. Into a threenecked round-bottom flaskfittedwith an efficient magnetic stirrer, a Claisen adapter, two addition runnels, and a pH meter electrode were placed MMCEM (25.6 g, 0.115 mol), dichloromethane (160 mL), acetone(190 mL, 2.64 mol),
Lin et al.; Polymers for Microelectronics and Nanoelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
239 phosphate buffer (pH = 7.4, 630 mL), and 18-crown-6 (1.26 g, 0.00477 mol). The flask was cooled to 0 - 5 °C using an ice-water bath. Oxone (2KHSO5 · K H S O 4 · K S 0 ) (107 g, 0.174 mol) in 390 mL of water was added dropwise over the course of 2 h. At the same time, a solution of K O H (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 an additional 4 h. The resulting mixture was filtered and extracted with three 80 mL aliquots of dichloromethane, and the combined organic layers were washed with water and dried over anhydrous MgS0 . The oily residue was subjected to column chromatography to obtain the pure product; yield 14.3 g (52.1 %). H N M R (400 MHz, CDC1 ) δ 5.92 (s, 1H, CH =C), 5.40 (s, 1H, CH =C), 2.95 (d, 1H, epoxy HCOC), 1.83 (s, 3H, CH ), 1.38 (s, 3H, CH ), 1.25 (d, 6H, 2CH ), 1.20-2.10 (m, 7H, CH, CH ). 2
4
4
!
3
2
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1
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