Polymers for Microelectronics - American Chemical Society

Chapter 25

Photoinitiated Thermolysis of Poly(5-norbornene 2,3-dicarboxylates) A Way to Polyconjugated Systems and Photoresists

Downloaded by FUDAN UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch025

Ernst Zenkl, Michael Schimetta, and Franz Stelzer Christian Doppler Laboratorium für katalytische Polymerisation Institut für Chemische Technologie organischer Stoffe, Technische Universität Graz, Stremayrgasse 16, A-8020 Graz, Austria Linear and aromatic conjugated polymers have gained great interest in last decade for several reasons. The problem of inprocessibility was circumvented either by the use of substituted monomers [e.g. poly(alkylthiophene) (1), poly(alkylphenylene) (2), etc.] or by preparation via precursor polymers as shown for polyacetylene (3), poly-p-phenylene (4) and poly(arylene vinylene) (5). The latter ones are mainly prepared via sulfonium polymers in a condensation reaction. This gives bad control of the average molar mass and of the molar mass distribution. The ring opening metathesis polymerization (ROMP) of bicycloalkenes leads to polymers where a ring is embedded between two vinylene groups. If this ring is substituted with the proper number of leaving groups, double bonds can be created thus forming a conjugated system. This has already been shown for the formation of poly(cyclopentadienylene vinylene) (6, 7) from poly(2,3-di– acetoxy-5-norbornene) 1, equ.1.

There are several disadvantages in this method: a) high temperature (> 300 °C for the exo,exo-diester) is necessary for the thermal elimination; b) this temperature is even higher if an endo,endo-diester is used, because the elimination is a typical syn-elimination (8) according to equation 2, which means that the favoured structure for the elimination is that of the polymer from the

0097-6156/94/0537-0370$06.00/0 © 1994 American Chemical Society Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

25. ZENKLETAL.

Pofy(5-norbornene 2,3-dicarboxylates)

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exo,exo-monomer; c) at so high temperatures different side reactions (crosslinking and degradation) may occur; and d) the acid elimination from the ester does not come to completeness before oxidative degradation begins and therfore is not well controllable.

It is also known, that the temperature of elimination is lower for different leaving groups in the following order: -COCH3 > -COPh > -(CO)OCH3 > (CS)SR > -(CO)NHPh. Therefore we varied the acid groups of the diesters (see polymers 2 and 3 in equ.l) and the stereochemistry (ex0,eaa?-disubstituted = a, endo,endo-= b, see Table 1). Thermolysis of esters can be catalyzed through H + . In this case the stereochemistry plays a less important role. The proton may be created from several photo acid generators (PAG) through UV-irradiation (e.g. by photochemical decomposition of onium salts) (9). So we used triphenylsulfonium salts as P A G to catalyze the thermal extrusion of the acid groups. Furthermore this method offers an easy and quick way to build conducting structures in a nonconducting matrix. In this paper we present our first results of these experiments. EXPERIMENTAL Monomers were synthesized in standard esterification procedures from exo,exo2,3-dihydroxy-5-norbornene (prepared from bicyclo[2.2.1]hepta-2,5-diene by selective oxidation of one double bond with K M n 0 (10) or with Os6 /N-Methylmorpholine-N-oxide) (11) or from endo-5-norbornenylene-2,3-carbonate (prepared from cyclopentadiene and vinylene carbonate in a Diels-Alder condensation) (12). 4

4

Table 1. Characterization of Polymers T (Temp of decomposition) Heat of decomposition*" (J/g) (J/unit) (°C) d

0

Polymer lal a2 2 3

M 82000 34000 1250000 52700 82000 w

PDI 1.07 1.09 1.24 1.09 1.08

330

-140-10

390 310 270

-0.9-10 -27-10

«

3

«

-3-10

-

7

3

3

-3-10 -6-10

5 6

'minimum of the DSC-Plot **determined by DSC "^polymerized with K RuCl 2

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

5

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POLYMERS FOR MICROELECTRONICS

ROMP was carried out in absolute chlorobenzene under N2 in a glove box using Mo(CH-M)ut)(NAr)(0-f-but) (13) as the catalyst. Than the polymers were precipitated in a > 5 fold excess of methanol, washed and dried. If impurities (monomer, catalyst, etc.) were detected in the polymer (colored polymer, or by means of IR- or NMR-spectroscopy) the polymer was reprecipitated from C H a / M e O H for further purification. In one case K R u C l • H 0 and 2,3-diacetoxy-7-oxa-5-norbornene (ca. 5 mol% of the 2,3-diacetoxy-5-norbornene) in ethanol/water (1:1) was used as the initiator, but no drastic differences in the polymer properties were observed (9) (more details about the initiating system see ref.) (14). Films were spin cast on discs of BaF , KBr or glass from solutions of the polymer (10 wt%) and the photoinitiator (12 mol% relative to the number of ester functions in the dissolved polymer) in dichloromethane (filtered through a 0.5 fim Teflon filter). Then the films were dried in vacuo for 15 minutes, exposed to the light of a low pressure Hg-lamp and developed by heating in vacuo. 2

2

2

2

5

2


2

RESULTS AND DISCUSSION In Table 1 the experimental data (weight average of molar mass, polydispersity index, decomposition temperature and heat of decomposition) of polymers with varius acid groups — OR (see equ.l) and different stereochemistry are listed. It can be seen, that the decomposition temperature decreases from ca. 390 °C for poly(endo,endo-2,3-diacetoxy-5-norbornene), polymer 1 b, down to ca. 270 °C for poly(exo,eaw-5-norbornene-2,3-bis(methylcarbonate)), polymer 3 a. As expected, the polymers from the exe>,exo-isomers decomposed at lower temperatures as the endo,endo-isomers because the thermal elimination reaction is a typical synelimination: By means of mass spectroscopy we proved, that only the acids and products from the photolysis of the P A G (such as biphenylsulfide and fragments of the BrFnsted acid HAsF6) were eliminated at temperatures lower than the DSCmaximum. At temperatures above 400 °C scission of the polymer backbone was detectable. Solid state MAS-NMR spectra showed that the elimination of the acid was still incomplete even after thermolysis at this high temperature with theoretical loss of weight. Amongst the onium salts investigated triphenylsulfonium hexafluoroarsenate was the best P A G to catalyse the extrusion of the acid for polymer 1. A sample with a content of 12% P A G (referred to the number of ester groups in the polymer) showed the best results in the catalyzed decomposition. As can be seen from the DSC plots in Figure 1, the decomposition maximum shifted down to ca 150 °C with an onset already below 80 °C. In the DSC plot a small peak remained at the original decomposition temperature, even after a strong increase of the initiator concentration. In contrast to the uncatalyzed thermolysis, polymer 1 showed the best effect and no difference was found between the isomers a and b. With our equipment at least one hour of irradiation was necessary, shorter irradiation times showed lower yields of conversion, the high temperature conversion peak remained with higher intensity.


Pofy(5-norbornene 2,3-dicarbojylates)

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25. Z E N K L E T A L .

0 , 0 0

JE»

SUS

5T5 iSTn

I3T5

idn

HOOD

temperature

iSTo

35T55 3T5 no.«

(°C)

Figure 1. DSC-plots of poly(2,3-diacetoxy-5-norbornene) containing 12 mol% triphenylsulfonium hexafluoroarsenate a) not irradiated b) after U V irradiation (low pressure Hg-lamp, 60 min)


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Inspite of the drastic changes in color, the IR spectra of polymers treated at temperatures below 150 °C even for several hours show that the elimination reaction does not come to completion at this temperature, see Figure 2, plot c. The baseline drift of this graph comes from the slope of the broad absorption peak seen in the UV-VIS-IR spectrum with an onset at 2080 cm' (= 0.26 eV). Only after heating to higher temperatures (> 200 °C) the intensity of the ester modes decreased drastically, plot d. Further heating up to the original Td led to a far going destruction of the polymer structure, plot e. The color of these overheated films was reddish brown. Upon thermal treatment within the DSC range (50-400 °C) the color of the exposed film changed from colorless to green, metallic dark blue and finally to black. UV-VIS-NIR-spectroscopy can be used to follow this changes in color. Figure 3 shows the UV-VIS-NIR-spectra of a pol)

Polymers for Microelectronics - American Chemical Society

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