Photo-induced crosslinking and thermal de-crosslinking in polynorbornenes bearing pendant anthracene groups

Accepted Manuscript Photo-Induced Crosslinking and Thermal De-Crosslinking in Polynorbornenes Bearing Pendant Anthracene Groups Simone Radl, Meinhart Roth, Martina Gassner, Archim Wolfberger, Andreas Lang, Benjamin Hirschmann, Gregor Trimmel, Wolfgang Kern, Thomas Griesser PII: DOI: Reference:

S0014-3057(13)00516-8 http://dx.doi.org/10.1016/j.eurpolymj.2013.10.024 EPJ 6280

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

5 July 2013 25 October 2013 27 October 2013

Please cite this article as: Radl, S., Roth, M., Gassner, M., Wolfberger, A., Lang, A., Hirschmann, B., Trimmel, G., Kern, W., Griesser, T., Photo-Induced Crosslinking and Thermal De-Crosslinking in Polynorbornenes Bearing Pendant Anthracene Groups, European Polymer Journal (2013), doi: http://dx.doi.org/10.1016/j.eurpolymj. 2013.10.024

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Photo-induced Crosslinking and Thermal De-crosslinking in Polynorbornenes Bearing Pendant Anthracene Groups Simone Radl1, Meinhart Roth2, Martina Gassner2, Archim Wolfberger2,4, Andreas Lang1, Benjamin Hirschmann1, Gregor Trimmel3, Wolfgang Kern1,4, Thomas Griesser2,4* 1

Polymer Competence Center Leoben GmbH, Roseggerstrasse 12, A-8700 Leoben, Austria.

2

Christian Doppler Laboratory for Functional and Polymer based Ink-Jet Inks, Otto-Glöckel-Strasse 2, A-

8700 Leoben, Austria. 3

Institute for Chemistry and Technology of Materials and Christian Doppler Laboratory for

Nanocomposite-Solar Cells, Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austria 4

Chair of Chemistry of Polymeric Materials, University of Leoben, Otto-Glöckel-Strasse 2, A-8700 Leoben,

Austria * corresponding author: Thomas Griesser, email: [email protected], Tel. +43-3842-4022358;

Keywords: photochemistry, photolithography, ring opening metathesis polymerization.

1

Abstract: Functional polynorbornenes bearing anthracene molecules in their side chain were synthesized by ring opening methathesis polymerization (ROMP). The pendant anthracene molecules undergo a [4π+4π] cycloaddition upon irradiation with UV-light, which was studied by means of UV-Vis spectroscopy in thin films of these polymers. This photodimerization reaction also leads to a crosslinking of the macromolecules due to the photodimer formation, resulting in a decrease in solubility of the UV illuminated areas. A thermally induced de-crosslinking could be obtained, revealing the reversibility of this photoreaction. The influence of the flexibility of the macromolecules, i.e. the mobility of the anthracene groups in the polymeric material, on the conversion rate, reversibility and the crosslinking behavior was studied by means of spectroscopy and sol-gel analysis. Furthermore, photo-patterned films were obtained by structured illumination and subsequent development with dichloromethane revealing a negative resist behavior.

2

Introduction Already in the 19th century, Fritzsche and co-workers reported on the reversible photochromism of anthracene crystals.1,2 They discovered that an exposure of a saturated solution of anthracene to sunlight yields a colorless crystalline precipitate, which regenerates anthracene upon melting. A great number of studies were carried out on this photoinduced dimerization reaction, revealing a mechanism based on a [4π+4π] cycloaddition of the central ring of the anthracene π-system.3,4,5,6 A reversion of this reaction can be achieved either thermally or by irradiation with deep UV-light (<300nm) as shown in Scheme 1. It has to be noted, that the photodimerization reaction takes only place in the absolute absence of oxygen. otherwise the formation of endoperoxides occurs.7,8 Scheme 1: Reversible photo-induced dimerization reaction of anthracene

In addition to a great number of investigations performed with anthracene derivatives in solution, the photodimerization reaction in the solid state has drawn great interest in material science. In particular, anthracene dissolved in polymeric materials induces considerable changes in their optical properties. 9,10 This fact has been exploited for the realization and development of photo-chromic devices11. In recent polymer related studies, anthracene units have been covalently attached to the polymer backbone. These polymeric materials have the advantage that a high anthracene concentration can be incorporated without crystallization, phase separation, or the formation of concentration gradients.

3

Over the past few years, reactions of macromolecular anthracene derivatives have undergone a great revival due to their orthogonal “click” like behavior. Besides the photo-induced dimerization also the Diels-Alder reaction with maleimide groups has been used for the creation of highly defined polymer architectures.12,13 Moreover, the photodimerization reaction has already been applied for a selective crosslinking of macromolecules, leading to changes in solubility14,15 as well as in refractive index16. Very recent endeavors focus on the reversible photoreaction between polymer based anthracenederivatives and singlet oxygen to give endoperoxides, which significantly alters the surface potential.17 This interesting material property can be used for the realization of photochromic storage devices that can be read out by Kelvin probe force microscopy. Apart from polymeric materials, this photoreaction has also been exploited for the fabrication and crosslinking of nanoparticles, respectively.18,19 A very important aspect of this photodimerisation reaction in the solid state is the required steric configuration of the anthracene moieties. While the 9- and 9,10substituted anthracene derivatives generate these configuration very easily in solution and yield tail-tail and/or tail-head photoproducts20, in polymeric materials the mobility and diffusion are restricted, which can prevent the required configuration and thus photodimerization. Many studies pointed out that the anthracene mobility in the polymer chain is more important for the successful photodimerization than the density of anthracene moieties.21,22 Several investigations are related to this specific feature in different classes of polymeric materials. Tazuke et. al. explored the influence of the glass transition temperature of polyesters and polyurethanes bearing anthracene units in the side chain on the conversion, rate and reversibility of the photodimerization reaction.23 It turned out that the mobility of the anthracene units in the polymeric material mainly determines the efficiency of the photoreaction, which is in good accordance with theoretical considerations of an appropriate alignment. Moreover, also an efficient cleavage of the dimers needs a certain degree of flexibility.7

4

In the present contribution, we focus on the preparation of anthracene containing polymers via ringopening metathesis polymerization (ROMP). ROMP is a versatile technique for the preparation of highly defined functional polymers. In particular, polynorbornenes have been intensively used as polymeric model compounds for the investigation of several photo-induced reactions due to their good film forming- and optical- properties.24,25,26,27 In order to provide a better mobility of the anthracene moieties, also copolymers with oligoethyleneglycol groups were prepared in order to lower the glass transition temperature. The photochromism will be utilized for a selective crosslinking of the macromolecules, leading to changes in the solubility, which is exploited for a photolithographic patterning of thin films of the obtained functional (co)polymers. A particular focus of this study is the influence of the glass transition temperature, i.e. the mobility of the pendant anthracene groups, on the conversion rate and reversibility of the photoreaction as well as on the crosslinking behaviour. The thermally induced reversion of the photodimerization is evaluated as a way to de-crosslink polymeric materials.

5

Experimental Materials preparation All chemicals were purchased from commercial sources and were used without further purification. All experiments were carried out under inert atmosphere using Schlenk technique or a glove box. Endo,exobicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, bis[2-[2-(2-ethoxyethoxy)ethoxy]ethyl] ester (mono-2) was synthesized as previously reported.28 For the ROM polymerization [1,3-bis(2,4,6-trimethylphenyl)-2imidazolidinylidene]dichloro-(3-phenyl-1H-inden-1-ylidene)(pyridyl)ruthenium(II)

(Umicore

M31)

purchased from Umicore AG & Co. KG was used. Caution: For preparative work, hazardous chemicals and solvents were used. Reactions must be carried out in a fume hood and protective clothes and goggles must be used! Endo, exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, bis(anthracen-9-ylmethyl) ester (mono-1): (9anthracenyl)methanol (1.05 g; 5 mmol) was added to a stirred solution of bicyclo[2.2.1]hept-5-ene-2,3dicarbonyl dichloride (0.372 mL; 2.3 mmol) and pyridine (0.405 mL; 5 mmol) in dichloromethane. The solution was stirred for 48 h at ambient temperature. The reaction mixture was then filtered to remove the pyridinium salt and extracted with dichloromethane. The organic layer was extracted with 3 × 20 mL of 5% hydrochloric acid solution to remove excess pyridine, and with 20 mL of saturated sodium bicarbonate solution, and then dried over sodium sulphate. The solvent was removed in vacuo and subsequently a column separation using cyclohexane/ethyl acetate (20:1) for product purification was performed. Yield: 1.12 g of a white solid (87% of theoretical yield). 1H-NMR: (δ, 400 MHz, 20°C, CDCl3): 8.51 (d, 2H an10), 8.26 (vt, 4H an1,8), 8.03 (d, 2H an5), 7.50 (m, 8H an2,3,6,7), 6.20-6.00 (m, 5H, O-CH2, nb6), 5.83 (m, 1H, nb5), 3.47 (t, 1H nb3), 3.11 (s, 1H, nb4), 3.01 (s, 1H, nb1), 2.77 (dd, 1H, nb2), 1.54, 1.32 (d, 2H, nb7); 13C-NMR: (δ, 125 MHz, 20°C, CDCl3): 174.5, 173.3 (2C, C=O), 137.63, 134.94 (2C, nb5,6), 131.35 (1C, an9), 131.03, 130.98 (2C, an8a,9a), 129.18 (2C, an4a,4b), 129.07 (2C, an4,5), 126.62 (1C, an10), 126.54 (2C, an2,7), 125.09 (2C, an3,6), 123.92, 123.87 (2C, an1,8), 59.44, 59.13 (2C, O-CH2), 47.91

6

(1C, nb1,3), 47.28 (1C, nb7), 47.23 (1C, nb2), 45.86 (1C, nb4); IR-Data (CaF2, cm-1): 2854, 1722, 1448, 1305, 1262, 1161, 1111, 985, 889, 739. Polymerization of mono-1 to give poly(bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, bis(anthracen-9ylmethyl) ester) (poly-1): a solution of mono-1 (200 mg, 0.355 mmol) in 2.5 mL of dichloromethane was added to a solution of Umicore M31 (1.78 mg, 2.38 µmol) in 1 mL of CH2Cl2 and was stirred at ambient temperature for 12 h. Afterwards, the reaction was stopped by adding 0.10 mL of ethylvinylether. The polymer was precipitated by dropping the solution into a tenfold excess of cold methanol. The precipitate was dried in vacuo. Yield: 127 mg of a white solid (64%). 1H-NMR: (δ, 400 MHz, 20°C, CDCl3): 8.4 -6.8 (18H, ph); 6.1-4.1 (2H, (CH=CH) + 4H, (O-CH2-ph)); 3.4-2.3 (4H, nb1,2,3,5); 1.9-1.3 (2H, nb4) ppm. FTIR (film on CaF2, cm-1): 3110-2928, 1724, 1635, 1528, 1450, 1384, 1259, 1159, 1059, 975. SEC: (CHCl3): Mn=37800 g/mol; Mw= 41600 g/mol; PDI= 1.1; Tg: 140°C

Copolymerization of mono-1 and mono-2 to give poly[(bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, bis(anthracen-9-ylmethyl)

ester)-co-

(bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic

acid,

bis[2-[2-(2-

ethoxyethoxy)ethoxy]ethyl] ester] (poly-(1-co-2): a solution of Umicore M31 (1.1 mg, 1.47 µmol) in 2 mL of CH2Cl2 was added to a mixture of mono-1 (64.9 mg, 0.11 mmol) and mono-2 (58.3 mg, 0.11 mmol) in 3 mL of dichloromethane and was stirred at RT for 12 h. Afterwards the reaction was stopped by adding 0.10 mL of ethylvinylether. The polymer was precipitated by dropping the solution into a tenfold excess of cold methanol. The precipitate was dried in vacuo. Yield: 90 mg (73%) of a white solid. SEC (CHCl3): Mn= 61800 g/mol; Mw= 76400 g/mol; PDI= 1.2; Tg= 23 °C. (δ, 400 MHz, 20°C, CDCl3): 8.6-6.6 (18H, ph); 6.3-4.8 (4H, CH=CH) (4H, O-CH2-ph); 4.4-3.3(24H, CH2); 3.33.5 (4H, nb1,2,3,5); 2.2-1.5 (4H, nb4); 1.4-1 (3H, CH3) ppm. Composition: 55 mol% mono-1 and 45 mol% mono-2 as estimated from NMR spectroscopy; FTIR (film on CaF2, cm-1): 2871, 1728, 1510, 1453, 1389, 1258, 1112, 965.

7

Physico-chemical measurements 1

H-NMR and 13C-NMR spectra were recorded with a Varian 400-NMR spectrometer operating at 399.66

MHz and 100.5 MHz, respectively, and were referenced to Si(CH3)4. A relaxation delay of 10 s and 45° pulse were used for acquisition of the 1H-NMR spectra. Solvent residual peaks were used for referencing the NMR spectra to the corresponding values given in literature.29 For UV-Vis and FTIR measurements a 10 mg/mL solution of the corresponding polymer dissolved in tetrahydrofuran (THF) was spincast on CaF2 discs. The film thickness was in the range of 20-25 nm for ellipsometry. For UV-Vis and FTIR-spectroscopy measurements 100 nm thick polymer films were applied. UV-Vis spectra were recorded with a Varian Cary 50 UV-Vis spectrophotometer. All UV-VIS spectra were taken in absorbance mode. Differential scanning calorimetry (DSC) measurements were carried out on a Perkin Elmer “Pyris Diamond” under a nitrogen flow of 20 mL/min and a heating rate of 10 °C/min. Glass transition temperatures (Tg) from the second heating run were read as the midpoint of change in heat capacity. FTIR spectra were recorded with a Perkin Elmer “Spectrum One” instrument (spectral range between 4000 and 450 cm-1). All FTIR spectra were recorded in transmission mode. Size exclusion chromatography: The weight and number average molecular weights (MW and Mn) as well as the polydispersity index (PDI) were determined by gel permeation chromatography with tetrahydrofuran (THF) as solvent using the following arrangement: Merck Hitachi L6000 pump, separation columns of Polymer Standards Service, 8 x 300 mm STV 5 μm grade size (106 Å, 104 Å, and 103 Å), combined refractive index – viscosity detector from Viscotec, Viscotec 200. Polystyrene standards purchased from Polymer Standard Service (PSS) were used for calibration.

8

UV irradiation experiments UV irradiation experiments were carried out with a medium pressure Hg lamp (Omnicure S1000) equipped with a filter which excludes UV-light < 300 nm, and an ozone free low pressure Hg lamp (Hereaus Noblelight; 254 nm). In these experiments, the light intensity (power density P; in µW cm -2) at the sample surface was measured with a spectroradiometer (Solatell, Sola Scope 2000TM, spectral range from 230 to 470 nm). The power density was 64 µW cm-2 for 300-450 nm and 178 µW cm-2 for 254 nm, respectively. All UV illuminations were carried out under inert gas atmosphere (N2). Caution: UV irradiation causes severe eye and skin burns. Precautions (UV protective goggles, gloves) must be taken! Photolithographic patterning was carried out with a mask aligner (model MJB4 from SUSS, Germany) using a 500 W HgXe lamp equipped with a filter transmissive for the wavelength range from 350 nm to 380 nm (P= 37 mW cm−2).

Results and Discussion Synthesis of the polymers poly-1 and poly-(1-co-2) The photoreactive monomer mono-1 is easily accessible by an esterification reaction of bicyclo[2.2.1]hept-5-ene-2,3-dicarbonyl dichloride with (9-anthracenyl)methanol (see Scheme 2), analogous to the synthesis of endo, exo-dinaphtyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate as reported previously.27 Mono-1 was purified by column chromatography and was obtained in high yield (87%). 1H NMR, 13C NMR and FTIR-spectroscopy are in accordance with the proposed structure. Details are given in the experimental section.

9

Scheme 2: Synthesis of an anthracene functionalized polymer (poly-1) by ROMP

The polymers were prepared by ring opening metathesis polymerization (ROMP) using [1,3-bis(2,4,6trimethylphenyl)-2-imidazolidinylidene]dichloro-(3-phenyl-1H-inden-1-ylidene)(pyridyl)ruthenium(II)

as

initiator for ROMP. The polymerizations were performed in dichloromethane under inert atmosphere at room temperature in a glove box. A homopolymer, poly-1, with a theoretical polymerization degree Xn= 150 as well as a statistical copolymer poly-(1-co-2) (Scheme 3, Xn= 150, mono-1 : mono-2= 50:50) were prepared by this method. Scheme 3: Structure of poly-(1-co-2)

The integrals of the proton NMR signals were used to determine the content of mono-1 and mono-2 in

10

poly-(1-co-2). The value found for the molar fraction of the incorporated monomers (55% mono-1; 45% mono-2) is - within experimental error - in good agreement with the theoretical fraction of 50% mono-1 and 50% mono-2. However, a full conversion of the monomers could not be achieved in both polymerization reactions, even after 12 h of reaction time, which is usually more than sufficient for a complete polymerization using this type of catalyst. This fact explains also the rather low yields of 64% (poly-1) and 73% (poly-2), respectively, and also the strong deviation of the molar mass from the theoretically expected value as shown in Table 1. The observed deviations are more pronounced for the homopolymer and are not in accordance with the living type of ROMP. One possible explanation is that the polymerization is hampered at a certain degree by steric hindrance, imposed by the polymer chain attached to the initiator.28 This assumption is also corroborated by the low yields. Table 1: Physico-chemical data of poly-1 and poly-(1-co-2)

Polymer

Molar ratio

a

Yield

Mn (theor.)

Mn (exp.)

PDI

Tg

(%)

(g mol-1)

(g mol-1)

(Mw Mn-1)

(°C)

a

poly-1

1:150

64

84400

37800

1.1

140

poly-(1-co-2)

1:150b

73

80000

61800

1.2

23

molar ratio initiator to monomer, b molar ratio of monomers mono-1:mono-2= 1:1

For poly-(1-co-2) the glass transition temperature (Tg= 23 °C) is far lower than the Tg of poly-1 (140 °C), which results from the flexible oligoethylenglycol sidechains of mono-2. Both polymers show excellent film forming properties when spin-cast from CH2Cl2 solutions, and fully transparent films with optical quality were obtained.

11

Investigation of the photoreaction For the investigation of the photo-induced dimerization reaction, homogeneous and transparent polymer films were prepared by spin-casting from solutions (10 mg/mL) of poly-1 and poly-(1-co-2). The UV-irradiation of those polymers causes a [4 π +4 π] cycloaddition reaction in 9,10 position of the pendant anthracene units accompanied by a destruction of the conjugated π- system. This photoinduced rearrangement of the π-electrons can be observed by UV-Vis spectroscopy, as shown in Figure 1 (left) for poly-(1-co-2). Both anthracene absorption bands, between 350-400 nm, and at 250 nm decrease with prolonged illumination due to the depletion of the extended π-system. At the same time an increase of the band at about 200 nm is observed, which can be explained by the formation of benzene type photoproducts. In order to prevent endoperoxide formation, the illumination of the polymer films was performed under inert gas atmosphere. Therefore, no oxidation product could be detected during UV illumination by FTIR spectroscopy.30

Absorbance / a.u.

Remaining anthracene groups / %

100

90

80

70

60

50

200

250

300

350

wavelength / nm

400

0

100

200

300

400

500

600

time / sec

Figure 1: Left: Time dependent UV-Vis spectra of poly-(1-co-2) under UV irradiation (nitrogen atmosphere). Right: Conversion of the anthracene groups during illumination (>300nm) in poly-1 (diamonds) and poly-(1-co-2) (open squares). The power density was 64 µW cm-2.

12

Figure 1 (right) shows the depletion of the anthracene units during UV illumination (>300 nm) in poly-1 (diamonds) and poly-(1-co-2) (squares). The conversion of the anthracene units during UV-illumination was determined using the area under the characteristic anthracene absorption peaks between 324-411 nm. A comparison of both polymers reveals that in the copolymer the conversion rate is higher than in the homopolymer in the first 600 s of illumination. Prolonged irradiation (1200 s) leads to a conversion of 45% (poly-1) and 55% (poly-(1-co-2)) of the anthracene units, respectively. This behaviour can be explained by the lower glass transition temperature of poly-(1-co-2) offering a higher mobility and thus enabling the required alignment of the anthracene moieties in the polymeric material.7,23 These results confirm the finding of Hargreaves and Weber, who have found that for an efficient cycloaddition not the anthracene densitiy, but the anthracene mobility is crucial.21 Besides the anthracene conversion rate also the reversibility of this photoreaction has been determined by UV-Vis spectroscopy. For that purpose illuminated (1200 s) films of poly-1 and poly-(1-co-2) were either treated with UV-light of 254 nm (inert gas atmosphere) or annealed at 150°C for 90 min in vacuo. While the irradiation with deep UV-light causes no significant regeneration of the consumed anthracene moieties, the thermal annealing results in a considerable recovery of the UV-Vis peak at 324-411 nm. The superiority of the thermal dissociation of the dimers over the deep UV illumination has also been observed by S. Tazuke et. al..23 Although the illumination of poly-(1-co-2) leads to a more efficient conversion, a subsequent thermal treatment recovers anthracene to a value of 62% in both polymer films. This indicates that also the yield of thermal dissociation is mainly determined by the flexibility of anthracene units, i.e. the glass transition temperature of the polymers. In order to evaluate the stability of this photoreaction several cycles of illumination and annealing were performed a shown in Figure 2.

13

90

1 -h .

80

8 - T

7 -h .



 5-h.

4 - T

6 - T

50

3-h.

60



70

2 - T

Remaining antracene groups / %



100

40

Figure 2: Remaining anthracene units in poly-1 (diamonds) and poly-(1-co-2) (open squares) after repeated cycles of UV irradiation (1200s) and annealing (90 min at 150°C). Besides the conversion rate of the photoreaction, it turned out that poly-(1-co-2) is also superior in terms of stability. Although both polymers show significant decrease in reversibility during the four illumination-annealing cycles, the overall recovery is more efficient in poly-(1-co-2). Going a step beyond the investigation of UV-Vis absorption changes, also the crosslinking behaviour of both polymers were investigated by means of sol-gel analysis. The photo-induced dimerization of anthracene units mainly leads to intermolecular crosslinking as only a head to tail orientation causes efficient photodimer formation in macromolecules.23 This intermolecular crosslinking leads to changes in the solubility of anthracene containing polymers as reported by Chae et. al. 14 and Paul et. al.15. For the investigation of dimer formation and its influence on crosslinking, thin films of poly-1 and poly(1-co-2) were illuminated for different periods of time. After subsequent development with CH2Cl2 the insoluble fraction (gel fraction) was determined by quantitative FTIR spectroscopy.

14

Figure 3 shows the gel fraction of poly-1 and poly-(1-co-2) with increasing illumination time. An exposure for 5 s already causes an insoluble fraction of 86% in poly-1, which corresponds to a conversion of only 1% of anthracene moieties (Figure 1 left) in the polymer. In contrast to this, poly-(1-co-2) is far less sensitive, which can be explained by the lower concentration of anthracene moieties per volume. In this case an illumination for 10 seconds leads to a comparable insoluble fraction of 84%. It has to be noted that 10 seconds of UV illumination, as shown in Figure 1 (left), corresponds to a conversion of 11% converted anthracene units.

90

90

80

80

Gelfraction / %

Gelfraction / %

70 60 50

70

60

50

40 40 30 30 20 0

10

20

30

40

50

60

0

time / sec

5

10

15

20

25

30

time / sec

Figure 3: Gel fraction (squares) as a function of the illumination time for poly-1 (left) and poly-(1-co-2) (right) after development with CH2Cl2 (20 min), and gel fraction (diamonds) as a function of the illumination time of poly-1 (left) and poly-(1-co-2) (right) after development (CH2Cl2, 20 min), annealing (90 min, 150°C) and repeated development (CH2Cl2, 20 min). Moreover, also the reversibility of the crosslinking reaction and its influence on the solubility has been investigated by means of sol-gel analysis. For that purpose the already developed samples were annealed for 90 min at 150°C and subsequently immersed in dichloromethane again. In both polymers an increase in solubility could be observed for 5 s (poly-1) and 10 s (poly-(1-co-2) illumination time as shown in Figure 3 (diamonds). As expected, also the thermally induced increase in solubility is more

15

pronounced for poly-(1-co-2) (24% for 5 s) than for poly-1 (14% for 10 s), which is in accordance with the results of the UV-Vis study. Interestingly, at higher irradiation doses the reversibility decreases significantly in both polymers. Presumably, the mobility and flexibility of the anthracene units is restricted by a denser crosslinking preventing a successfully dissociation of the photo-generated dimers.15

Patterning of thin films of poly-(1-co-2) To demonstrate the applicability of this photodimerization reaction for the realization of negative-toned photoresists, thin films of poly-(1-co-2) were photo patterned using a mask aligner system equipped with a suitable quartz-chromium mask (MJB4 from SUSS, 500 W HgXe lamp, 2 s). The features of the inscribed structures were visualized by optical microscopy after a subsequent development in CH2Cl2, which leads to the dissolution of the uncured areas of the poly-1 as shown in Figure 4. Without any optimisation, resolutions of 4 µm were achieved in this experiment.

Figure 4: Optical micrograph of a photo-patterned film of poly-(1-co-2) after development with dichloromethane (20 min). Conclusions In this contribution, we report on the synthesis of polynorbornene based polymers bearing anthracene molecules in their side chain. Besides the homopolymer poly-1 also a statistical copolymer (poly-(1-co-2)) bearing oligoethylenglycole groups, which provides a higher flexibility (Tg=23°C) of the macromolecules

16

at room temperature, was synthesized using this technique. The aromatic anthracene molecules undergo a [4π+4π] cycloaddition upon irradiation with UV-light, leading to significant changes in the UVVis spectra in thin films of poly-1 and poly-(1-co-2). This photodimerization reaction causes also a crosslinking of the macromolecules, resulting in a decrease in solubility of the UV illuminated areas. A thermally induced de-crosslinking could be obtained revealing the reversibility of this photoreaction. In general, it has been found that the conversion rate of the anthracene molecules, the reversibility and also the crosslinking behavior of poly-(1-co-2) is superior over poly-1, which can be explained by the higher mobility and flexibility of the anthracene groups in the copolymer poly-(1-co-2). Moreover, photo-patterned films of poly-1 were obtained after structured illumination and subsequent development revealing the applicability of these photoreaction for the realization of negative toned photoresists. Acknowledgements M. Roth, M. Gassner, A. Wolfberger, T. Griesser and G. Trimmel thank the Christian Doppler research association and the Austrian Ministry of Economics, Family and Youth (BMWFJ) for financial support. Part of this work also was performed at the Polymer Competence Center Leoben GmbH (PCCL) within a strategic research project (IV-4S1). PCCL is funded by the Austrian Government and the State Governments of Styria and Upper Austria within the COMET program.

1

Fritsche J, (1866) Bull. Acad. Imper. Sci. St.-Petersbourg 9:406. Fritsche J, (1887) Bull. Acad. Imper. Sci. St.-Petersboug 11:385. 3 Coulson C A, Orgel, L E, Taylor W, Weiss J, (1955) J. Chem. Soc., 2961. 4 Greene F, Ocampo S R, Kaminski L A, (1957) J. Am. Chem. Soc., 79:5957. 5 Applequist D E, Litle R L, Friedrich E C, Wall R E, (1959) J. Am. Chem. Soc. 81:452. 2

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Photo-induced Crosslinking and Thermal De-crosslinking in Polynorbornenes Bearing Pendant Anthracene Groups Simone Radl, Meinhart Roth, Martina Gassner, Archim Wolfberger, Andreas Lang, Benjamin Hirschmann, Gregor Trimmel, Wolfgang Kern, Thomas Griesser

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Polynorbornenes bearing anthracene groups were synthesized by ROMP. The anthracene groups undergo a reversible cycloaddition upon UV- irradiation. The photoreaction has been studied by UV-Vis spectroscopy. This photodimerization reaction also leads to a crosslinking of the macromolecules. The reversibility depends on the mobility of the anthracene groups in the polymer.