Photosensitive polymers with cinnamate units in the side position of chains : Synthesis, monomer reactivity ratios and photoreactivity 1 R. Mahy et al. / European Polymer Journal 42 (2006) 2389–2397 作者 :Rachid Mahy, Boufelja Bouammali, Abdelkader Oulmidi, Allal Challioui, Daniel Derouet, Jean Claude Brosse Reporter : Chih-Hao Li Advisor : Ching-Dong Hsieh Data :

2 Introduction Introduction Experimental Experimental Results and discussion Results and discussion Conclusions Conclusions Outline

3 IntroductionIntroduction The synthesis of different kinds of functional polymers has received much attention in recent years, such as temperature or pH sensitive polymer, electric active materials, photosensitive polymers and so on. Alignment technique Contact Contactless Photo-alignment Plasma beam alignment Ion beam alignment Rubbing alignment

4 This technique has been studied by many researchers for the practical application to the rubbing-free liquid crystal alignment layers, which are inevitable materials in the fabrication of liquid crystal display devices. In this work, we have found that the cinnamate side groups could be also reacted by thermal energy, and this reaction is presumed to attribute to the radical reaction of carbon double bond in the cinnamate groups. We studied the effect of the chain flexibility of polymer backbone on the reaction behavior of cinnamate side groups induced by UV irradiation and heating. Photo-alignment

5 photochemical and thermal reaction The photocycloaddition and thermal crosslinking reactions were supposed to be based on two different reaction mechanisms, pericyclic reaction and radical reaction.

6Experimental Cinnamoyl chloride Poly(vinyl cinnamate) PVCi Poly(3-ethylene-alt-1 vinyl cinnamate) PEVCi 4,4’-(Hexafluoroisopropylidene)diphthalic anhydride–3,3’-dihydroxy-4,4’- diaminobiphenyl 6FDA–HAB 7-(Methacryloyloxy)coumarin MOC Triethylamine TEA Isoquinoline 1,1’-azobis(cyclohexanecarbonitrile) ACCN Tetrahydrofuran THF N,N-dimethylformamide DMF PVCi PEVCi

7 Synthesis of Photoreactive Polyimide 6FDA-HAB-Ci 6FDA–HAB(1g ， mol) and TEA(0.26g ， mol) were dissolved in THF(10ml). Cinnamoyl chloride(0.43g ， mol) was dissolved in THF(10ml). Stirring was maintained for 24 h. Stirring was maintained for 24 h. Ice bath(0 ℃ ) The precipitate was filtered, the solution was poured toluene under vigorous stirring. The precipitate was filtered and dried under vacuum to give 87% of 6FDA-HAB-Ci.

8 Synthesis of Poly(7-(Methacryloyloxy)coumarin) (PMOC) Water bath (60 ℃ ) Stirring was maintained for 24 h. Stirring was maintained for 24 h. MOC(1g ， mol) and ACCN(0.052g ， mol) were dissolved in DMF(10ml). The cooled reaction solution was diluted with more DMF(10 ml) and added dropwise with vigorous stirring to methanol (500 ml). The resultant precipitate was filtered off, dissolved again in DMF(30 ml) and reprecipitated from methanol (500 ml). This procedure was repeated until no more monomer was present by PMOC and a white solid was obtained(0.67 g, yield 67%).

9 Photochemical and thermal reactions of polymers Thin film of a polymer was prepared by spin-coating a polymer solution onto a quartz substrate. Thin film of a polymer was prepared by spin-coating a polymer solution onto a quartz substrate. The thermal reaction of polymer film was conducted by placing the polymer film on the hot plate at 200 ℃. The film was irradiated by a 300W high-pressure mercury arc lamp passed through UV filter. The degree of reaction was monitored by UV spectroscopy. hνhνheat

10 Model compound In order to elucidate the mechanism of photochemical and thermal reactions of the cinnamate side groups,ethyl cinnamate was selected as a model compound and the photochemical and thermal reactions of the model compound were investigated. We checked the change of UV absorbance of ethyl cinnamate with UV irradiation or heating time. In order to elucidate the mechanism of photochemical and thermal reactions of the cinnamate side groups,ethyl cinnamate was selected as a model compound and the photochemical and thermal reactions of the model compound were investigated. We checked the change of UV absorbance of ethyl cinnamate with UV irradiation or heating time. Ethyl cinnamate

11 Results and discussion Thermal properties of polymers Ethylene backbone of PEVCi can increase the chain flexibility of polymer and thus the glass transition temperature of PEVCi is found to be lowest among the three different cinnamate polymers. Ethylene backbone of PEVCi can increase the chain flexibility of polymer and thus the glass transition temperature of PEVCi is found to be lowest among the three different cinnamate polymers. 6FDA–HAB–Ci showed the highest glass transition temperature and this would be attributed to the rigid polyimide backbone of 6FDA–HAB–Ci.

12 Photochemical reaction of polymers Fig. 1. The change of UV absorbance of polymer films with UV irradiation, (a)PEVCi, (b) PVCi, (c) 6FDA–HAB–Ci. There was no clear relationship between the chain flexibility of polymer and the photocycloaddition reaction of cinnamate side groups. The degree of reaction was monitored by the decrease in the peak intensity of –C=C– bond of the cinnamoyl group at 284 nm by UV spectroscopy.

13 Thermal reaction of polymers Fig. 2. The change of UV absorbance of polymer films with heating at 200 ℃, (a) PEVCi, (b) PVCi, (c) 6FDA–HAB–Ci. Glass transition temperature : 6FDA–HAB–Ci > PVCi > PEVCi The radical reaction of the cinnamate side groups can be affected by thechain flexibility of polymers.

14 Model reaction of cinnamate group Fig. 3. The change of UV absorbance of ethyl cinnamate solutions (a) with UV irradiation, (b) with heating at 200 ℃. This result provides an additional support on the effect of the chain flexibility on the thermal reaction of the cinnamate groups.

15 Model reaction of cinnamate group Fig H NMR spectra of ethyl cinnamate before and after UV irradiation for 20 h. After Before δ = 7.4 、 7.5ppm δ = 7.7ppm δ = 6.5ppm δ = 4.2ppm δ = 6 、 7ppm δ = 3.3 、 3.8ppm

16 Model reaction of cinnamate group Fig H NMR spectra of ethyl cinnamate before and after heating at 200 ℃ for 20 h. After Before There was only one group of new peaks at 3.5 and 4 ppm and these correspond to the new protons generated by breaking the carbon double bond of ethyl cinnamate.

17 Model reaction of cinnamate group Fig C NMR spectra of ethyl cinnamate after UV (a)irradiation for 20h, (b)heating at 200 ℃ for 20 h. (a)(b) δ = 45 、 47ppm Linear carbon δ = 35 、 40ppm

18 Photo-dimerizable groups and radical reaction Fig. 7. The change of UV absorbance of PMOC film with heating at 200 ℃. The glass transition temperature of the coumarin polymer was about 109 ℃ and this is much lower than the heating temperature 200 ℃. Due to the strong electron delocalization of coumarin groups, the radical reaction of carbon double bond becomes more difficult and thus the thermal crosslinking of coumarin polymer was not observed.

19Conclusions The photocycloaddition is based on the [2+2] cycloaddition reaction mechanism and the thermal crosslinking is supposed to be based on the radical reaction mechanism. Due to the radical mechanism of thermal crosslinking reaction of the cinnamate side groups, the flexibility of polymer has the considerable effect on the thermal crosslinking of cinnamate polymers. The difference between the photocycloaddition and thermal crosslinking of the cinnamate side groups was confirmed by the 1 H NMR and 13 C NMR analysis of the photochemical and thermal reaction of model compound. The possibility of the radical reaction of photodimerizable molecule was also closely related to the electron delocalization state of carbon double bond. The photocycloaddition is based on the [2+2] cycloaddition reaction mechanism and the thermal crosslinking is supposed to be based on the radical reaction mechanism. Due to the radical mechanism of thermal crosslinking reaction of the cinnamate side groups, the flexibility of polymer has the considerable effect on the thermal crosslinking of cinnamate polymers. The difference between the photocycloaddition and thermal crosslinking of the cinnamate side groups was confirmed by the 1 H NMR and 13 C NMR analysis of the photochemical and thermal reaction of model compound. The possibility of the radical reaction of photodimerizable molecule was also closely related to the electron delocalization state of carbon double bond.

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