Photo-cross-linking of polymethacrylates with stilbene chromophores in the side chains

Article abstract of DOI:10.1002/hlca.201100252

Methacrylates (=2-methylpropenoates) 5 with (E)-stilbene (=(E)-1,2-diphenylethene) building blocks on tethers of variable length were prepared (Scheme 2) and polymerized (i.e., 5→6; Scheme 3) in the presence of AIBN (=2,2′-azobis(2-methylpropanenitrile). 4-[(E)-2-Phenylethenyl] phenyl acetate (7) as model compound established the cyclodimerization as a single irreversible photoreaction. i.e., (7→8-11; Scheme 4) in the absence of oxygen. The solution photolysis of the polymers 6 provided a similar result, whereby [2π+2π] cycloadditions of stilbene units of neighboring tethers predominated. On the contrary, the desired photo-cross-linking of chaines occurred in the irradiation of polymer films. Copyright

Full text of DOI:10.1002/hlca.201100252

Helvetica Chimica Acta – Vol. 94 (2011)
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Photo-Cross-Linking of Polymethacrylates with Stilbene Chromophores
in the Side Chains
by Angelika Jahnke, Bernhard Beile, and Herbert Meier*
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10 – 14, D-55099 Mainz
(phone: þ 49-6131-3922605; fax: þ 49-6131-3925396; e-mail: hmeier@mail.uni-mainz.de)
Methacrylates (¼2-methylpropenoates) 5 with (E)-stilbene (¼(E)-1,2-diphenylethene) building
blocks on tethers of variable length were prepared (Scheme 2) and polymerized (i.e., 5 ! 6; Scheme 3) in
the presence of AIBN (¼2,2’-azobis(2-methylpropanenitrile). 4-[(E)-2-Phenylethenyl]phenyl acetate
(7) as model compound established the cyclodimerization as a single irreversible photoreaction. i.e.,
(7 ! 8 – 11; Scheme 4) in the absence of oxygen. The solution photolysis of the polymers 6 provided a
similar result, whereby [2p þ 2p] cycloadditions of stilbene units of neighboring tethers predominated.
On the contrary, the desired photo-cross-linking of chaines occurred in the irradiation of polymer films.
1. Introduction. – Various stilbene (¼1,2-diphenylethene) derivatives have been
used as building blocks or dopants in materials for photochemical imaging and resist
techniques [1 – 8]. On the other hand, irradiation of oligomers or polymers with
stilbene units in the main chain leads often to undesired photoreactions, which make
the material worse or even useless [8]. Similar observations have been made for
stilbenoid dendrimers [9].
The major irreversible photoprocess of stilbene substructures in the absence of
oxidants is due to the formation of CꢀC bonds between the initially olefinic C-atoms.
Apart from the well-known concerted [p2s þ p2s] cycloadditions to four-membered
rings, radical reactions can occur, which lead to a cross-linking of several stilbene units
[8][9]. The latter process is particularly important when energy-rich UV light is used.
Scheme 1 illustrates the two CꢀC bond-forming processes.
Scheme 1. Formation of CꢀC Bonds by Irradiation of Stilbene Building Blocks in Oligomers,
Dendrimers, etc.
We tried now to apply these CꢀC bond formations for the development of imaging
techniques and the generation of photoresists by photo-cross-linking of polymers that
contain (E)-stilbene units in the side chains.
ꢀ 2011 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Such polymers can be prepared by applying two strategies: either by polymer-
analog reactions, in which prepolymers with suitable functionalities are reacted with
compounds, which contain stilbene units or form them in this process [1][10 – 13], or by
polymerization of monomers, which already contain stilbene building blocks [1][14 –
18]. We decided to use the second concept and prepared various polymethacrylates
(¼ poly(2-methylpropenoates).
So far, polymethacrylates with stilbene building blocks in the side chains have been
studied mainly in the context of liquid-crystalline phases and/or optoelectronic effects
[15][19 – 39].
2. Results and Discussion. – Our target molecules were polymethacrylates with side
chains which have (E)-stilbene chromophores on variable tethers. The length of the
side chains could have a considerable influence on the film-forming properties and
possibly on the photoreactions within one polymer chain, or between two or more
stilbene units which belong to different main chains. For this purpose, a variety of
methacrylates 5a – 5f were synthesized which contain terminal (E)-stilbene units on a
tether of variable length.
4-(E)-Styrylphenol (¼4-[(E)-2-phenylethenyl]phenol; 1) was alkylated by w-
bromo- or chloro-alkanols 2a – 2f in the presence of EtONa (Scheme 2). The yields
reach a maximum for 2f (n ¼ 11) and a minimum for 2c (n ¼ 5), because 5-
chloropentan-1-ol cyclizes under these conditions to tetrahydropyran. The resulting
w-(4-styrylphenoxy)alkanols 3a – 3f were then subjected to Einhorn reactions with 2-
methylacryloyl chloride (4). The obtained methacrylates 5a – 5f, as well as the alcohols
3a – 3f, showed decreasing melting points with increasing chain length (ester 5f with
eleven CH2 groups is an exception).
Scheme 2. Preparation of w-{4-[(E)-2-Phenylethenyl]phenoxy}alkyl 2-Methylprop-2-enoates 5a – 5f

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Radical polymerization of the selected methacrylates 5b and 5d – 5f afforded the
polymers 6b and 6d – 6f, respectively. Monomer concentrations of 0.2m in THF were
used; 6 mol-% of AIBN (¼2,2’-azobis(2-methylpropanenitrile)) led in 20 – 22 h to the
yields compiled in Scheme 3. The polymers were moderately soluble in benzene,
CHCl3, THF, etc. Their molecular masses were estimated by gel permeation
chromatography (GPC) in CHCl3 by using polystyrene standards. The obtained Mw
values were in the range of 30 ꢁ 103 D, only 6d was an exception with almost Mw of
200 ꢁ 103 D. The polydispersity was broad – in particular for 6d: Mw/Mn ¼ 17.96. The
melting of the polymers 6 was studied by differential scanning calorimetry (DSC;
Table). None of the polymers exhibited liquid-crystalline properties.
Scheme 3. Polymerization 5 ! 6 in the Presence of AIBN
Table. Phase Transitions of the Polymers 6b and 6d – 6f: Maxima in the Differential Scanning Calorimetry
Compound
Second heating curve
Second cooling curve
T [8]
DH [J/g]
T [8]
6b
6d
162
124
120
121
24
27
21
24
147
114
111
116
6e
6fa)
a) Compound 6f has an additional glass transition, which appears at Tg 688 in the second heating curve.
The UV absorption of the polymers 6 corresponds to the absorption of the
monomers 5; 5e and 6e, for example, have both in CH2Cl2 maxima at lmax 326 nm and e
values of 15180 and 16120 cmꢀ1 mꢀ1, respectively. In the IR spectra in KBr, the band at
1630 cmꢀ1, which is typical for the unsaturated esters 5, lacks in the polymers 6. The
1H-NMR spectra of 6 in CDCl3 contain broad signals. The region 5.5 ꢂ d ꢂ 6.1 ppm,
which contains the signals of the olefinic AB part of an ABX3 spin system of 5, is empty
in the 1H-NMR spectra of 6. 13C-NMR Solution spectra of 6 were not obtained because
of the low solubility. The solid-state cross-polarization magic-angle spirring (CP-MAS)
13C-NMR spectrum of 6e (cf. Fig. 2), contains, in the region 4 ꢂ d ꢂ 60 ppm, the signals
of Cq-atoms and CH2 group of the main chain – originally sp2-C atoms in the monomer
5e, now sp3-C-atoms.

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The photodimerization of (E)-stilbene derivatives has been studied for a variety of
compounds [8]. The regio- and stereoselectivity of the dimerization depends strongly
on the special system and on the reaction conditions. Among the investigated examples,
4-methoxystilbene [40][41] represents the only 4-alkoxy compound – comparable to
5a – 5f. However, the irradiation of 4-methoxystilbene was performed for an inclusion
complex in g-cyclodextrin, a constrained medium [40]. The obtained mixture of syn-
head-to-tail and syn-head-to-head dimers was characterized without separation by a
1H-NMR spectrum. anti-Head-to-head and anti-head-to-tail cycloadducts were not
found. Moreover, it was shortly stated that the solution photolysis was very inefficient
[40].
These results motivated us to study first a model compound. We selected 4-[(E)-2-
phenylethenyl]phenol acetate ((E)-7) [42]. Irradiation (l> 290 nm) of a 0.12m solution
of (E)-7 in oxygen-free benzene yielded (Z)-7 and the dimers 8 – 11 (Scheme 4). Fig. 1
depicts the amounts of (E)-7, (Z)-7, and 8 – 11 vs. the irradiation time. The long-
wavelength absorption bands of (E)-7 (lmax 312 nm; CH2Cl2) and (Z)-7 (lmax 282 nm;
CH2Cl2) overlapped in the irradiation range; both isomers absorbed light, but the
photodimerization (E)-7 ! 8 – 11 prevented the formation of a photostationary state
(E)-7/(Z)-7. A photo-oligomerization could not be observed under these conditions.
However, when 254-nm light was used, a small amount of polymers was generated.
The dimers 8 and 10 were obtained in a pure state by column chromatography and
fractionary crystallization; 9 and 11 were obtained as a mixture. The total yield of
Scheme 4. Photoreactions of (E)-7

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Fig. 1. Product formation from the irradiation of (E)-7 (0.12m in oxygen-free benzene) with a 450-W
medium-pressure Hg lamp equipped with a Pyrex filter (l ꢃ 290 nm)
dimers (Fig. 1) amounted to 23% after a reaction time of 44 h, the corresponding
distribution: 8/9/10/11 was ca. 40 :18 :40 :2.
The structure determination of 8 – 11 is based on NMR and MS data. The
differentiation between the head-to-head fraction 8/9 and the head-to-tail fraction 10/
11 was achieved by the orthogonal decay of the molecular ions (EI, linked scan
technique) [41].
1
The H-NMR signals of the cyclobutane H-atoms were very characteristic. The
AA’BB’ spin pattern and the degenerate singlet signal at d ca. 4.4 ppm belonged to 8
and 10, respectively. Each four-membered ring H-atom has only one neighboring
benzene ring in cis-position. The cycloadducts 9 and 11 contain four-membered ring H-
atoms ꢂbetweenꢃ two cis-oriented benzene rings; therefore, their resonances appeared
at higher field (3.6 < d < 3.8 ppm). The spin patterns allowed a further distinction [43].
Isomer 9 had four-membered ring H-atoms, which form an AA’BB’ spin system,
whereas 11 exhibited an A2B2 spin system. Due to the very low portion of 11 (2%), its
1H-NMR signals were difficult to identify. Its A2B2 spin pattern was hidden at 3.7 ppm
in the foot of the AA’BB’ pattern of 9. Nevertheless, isomer 11 provided a complete set
of 13C-NMR signals.
Whereas the regioselectivity of the cycloaddition was low: (8 þ 9)/(10 þ 11) 58 :42,
the stereoselectivity was high: 8/9 40 :18 and 10/11 40 :2. As usual in (E)-stilbene
photochemistry [8], the trans arrangement of the benzene rings in (E)-7 was
completely preserved in the products 8 – 11.
The irradiation (l ꢃ 290 nm, CH2Cl2, 48 h) of the polymers 6b and 6d – 6f yielded
yellow-to-light brown powders. The insoluble portion was smaller than 5%. Integration
of the 1H-NMR spectra (400 MHz, CDCl3) of the soluble parts revealed that 60 – 80%
of the olefinic CH groups were transformed to saturated CH groups. New signals
appeared in the range of 3.3 ꢂ d ꢂ 4.5 ppm. We assume for the soluble portion
intramolecular [2p þ 2p] cycloadditions as major reaction route. According to the

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Scheme 5. Major Substructures Assumed for the Irradiated Polymers 6b and 6d – 6f (according to the
model compound)
photodimerization of the model compound 7, the formation of head-to-head and head-
to-tail cycloadducts (Scheme 5) could be detected.
The photodimerization of stilbenes is characterized by the intermediate generation
of excimers and pericyclic minima [8]. This effect can facilitate the formation of large
rings, especially when the stilbene units of two neighboring pendants react (Scheme 5).
Moreover, (Z)-configured C¼C bonds were detectable by UV spectroscopy (lmax
1
284 nm) and H-NMR spectroscopy in the region 6.4 ꢂ d ꢂ 6.8 ppm. The photo-cross-
linking between different chains led to the small insoluble portion of the photoproduct.
The assumption of prevailing intrachain photocycloadditions is further sustained by
GPC measurements. The GPC diagrams of irradiated and unirradiated 6e are very
similar. The Mw value changed by irradiation from 38900 to 37600, and Mn from 17300
to 17800.
Fig. 2 shows a comparison of the solid-state 13C-NMR spectra of the unirradiated
1
and the irradiated polymer 6e. In contrast to the H-NMR solution recordings, the
insoluble portions of irradiated 6e are included. Related to the signal at 15 ꢂ d ꢂ
35 ppm (CH3, pendant CH2), the intensity of the aromatic/olefinic region 105 ꢂ d ꢂ
140 ppm was decreased, and the intensity of the CH, Cq, CH2 (main chain) region
40 ꢂ d ꢂ 60 ppm was strongly increased in the irradiated probe. (arrows in Fig. 2,b).
When films of 6e or 6f were irradiated, a completely insoluble cross-linked material
was generated fast. Fig. 3 shows the UV film spectra for a monochromatic irradiation of
6e at 366 nm – in the long-wavelength tail of the absorption. It is known that four-
membered rings of the type as in 8 – 11 can be cleaved by energy-rich UV light [8]. This
effect can be excluded for 366 nm irradiations.

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Fig. 2. Solid-state CP-MAS 13C-NMR spectra of a) unirradiated 6e (cross polarization, 50 ms) and b)
irradiated 6e (cross-polarization, 10 ms). The recordings were performed at 100 MHz and a rotation
frequency of 5000 Hz. Inset: The quaternary C-atoms are more visible and distinguishable from side-
bands (for example d 195) for the cross polarization of 2000 ms. For the detailed assignment of the
signals, see Exper. Part.

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Fig. 3. Reaction spectra of the irradiation (lirr. 366 nm) of films of 6e (casted of a solution in benzene)
The film spectrum of 6e exhibited a maximum (lmax 292 nm) which is hypsochromi-
cally shifted in comparison to the solvent spectrum (lmax 326 nm). This indicates the
generation of H aggregates, which are favorable for both types of CꢀC bond formations
in the cross-linking (Scheme 1).
The photo-cross-linking was very fast in the beginning and slowed down after ca.
30 min. The final state of the films was reached after ca. 6 h. Then, an absorption
maximum at 282 nm was deteceted for 6e as well as for the film irradiation of 6b, 6d,
and 6f. The tailing of the curves in Fig. 3 from 375 to 400 nm is due to some light
scattering.
The film-forming properties of 6b and 6d were less favorable. Films of 6b and 6d
become soon turbid.
3. Conclusions. – Polymethacrylates, which contain (E)-stilbene building blocks
fixed on tethers of variable length, were prepared from the corresponding monomers
by radical polymerization.
Irradiation of 4-[(E)-2-phenylethenyl]phenyl acetate (7) as model compound
revealed the formation of head-to-head and head-to-tail cycloadducts as only
irreversible photoreaction in the absence of oxidants.
The polymethacrylates, dissolved in CH2Cl2, behaved on irradiation in a similar way
whereby intrachain [2p þ 2p] cycloadditions are favored. Irradiated and unirradiated
probes gave nearly identical GPC diagrams (Mw and Mn values). On the contrary, the
irradiation (lirr. 366 nm) in neat films afforded very fast completely insoluble materials
by an interchain photo-cross-linking. Increasing length of the tethers improved the
film-forming properties. The photochemistry of the films represents the basis for
imaging techniques and negative photoresists.

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We are grateful to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for
financial support.
Experimental Part
General. M.p.: Bꢀchi melting point apparatus. The phase transitions of 5 and 6 were determined with
a differential scanning calorimeter DSC-7 from Perkin-Elmer. UV/VIS Spectra: MCS-224/MCS-234
1
diode array spectrometer from Zeiss. IR Specra: Beckman Acculab 4. H- and 13C-NMR soln. spectra:
AM-400 spectrometer from Bruker; CDCl3 as solvent and Me4Si as internal standard; d in ppm and J in
Hz. Solid-state 13C-NMR spectra: DPX-400 from Bruker. FD-MS: Finnigan-MAT-95 spectrometer, 5 kV
ionization energy. EI-MS: Finnigan-MAT-95 ionization energy 70 eV. GPC: Program Millenium-2010,
polystyrene standard from PSS, CHCl3 as eluent.
Preparation of the Alkanols 3a – 3f. 4-[(E)-2-Phenylethenyl]phenol (1; 1.0 g, 5.1 mmol) was added to
a soln. of Na (160 mg, 6.9 mmol) in 70 ml of dry EtOH. The soln. turned yellow; 6.2 – 7.0 mmol of w-
bromoalkanol, 2a and 2f, or w-chloroalkanol, 2b – 2e, were added portionwise under Ar. The mixture was
stirred and refluxed, until TLC (SiO2; Et2O) indicated the complete consumption of 1. The products 3a –
3f started to precipitate after 12 h. The total reaction time amounted to 1 – 4 d. The precipitate was
washed with cold EtOH and recrystallized from EtOH.
2-{4-[(E)-2-Phenylethenyl]phenoxy}ethanol (3a). Yield 552 mg (45%). Colorless crystals. M.p. 1628
1
([44]: 148 – 1498). H-NMR (CDCl3): 2.04 (t, J ¼ 6.2, OH); 3.96 (q, J ¼ 6.2, CH2(1)); 4.11 (t, J ¼ 6.2,
CH2(2)); 6.91/7.45 (AA’BB’1), 4 arom. H); 6.97/7.07 (AB, J ¼ 15.8, 2 olef. H); 7.18 – 7.52 (m, 5 arom. H).
13C-NMR (CDCl3): 61.5 (C(1)); 69.4 (C(2)); 114.9, 126.4, 127.1, 127.4, 127.9, 128.3, 128.7 (arom. and olef.
CH); 130.8, 137.7, 158.5 (arom. Cq). EI-MS: 240 (100, Mþ), 196 (71), 195 (25). Anal. calc. for C16H16O2
(240.1): C 79.97, H 6.71; found: C 79.91, H 6.67.
3-{4-[(E)-2-Phenylethenyl]phenoxy}propan-1-ol (3b). Yield 480 mg (37%). Colorless crystals. M.p.
1638. 1H-NMR (CDCl3): 1.73 (t, J ¼ 6.2, OH); 2.00 – 2.15 (m, CH2(2)); 3.86 (q, J ¼ 5.7, CH2(1)); 4.14 (t,
J ¼ 5.9, CH2(3)); 6.89, 7.46 (AA’BB’, 4 arom. H); 6.94/7.06 (AB, J ¼ 15.8, 2 olef. H); 7.18 – 7.50 (m, 5 arom.
H). 13C-NMR (CDCl3): 32.1 (C(2)); 60.5 (C(1)); 65.8 (C(3)); 114.9, 126.3, 126.8, 127.2, 127.7, 128.2, 128.6
(arom. and olef. CH); 130.5, 137.7, 158.5 (arom. Cq). EI-MS: 254 (99, Mþ), 196 (100), 195 (67), 165 (67),
152 (41). Anal. calc. for C17H18O2 (254.3): C 80.28, H 7.13; found: C 80.41, H 7.13.
5-{4-[(E)-2-Phenylethenyl]phenoxy}pentan-1-ol (3c). Yield 290 mg (20%). Colorless crystals. M.p.
1
1488. H-NMR (CDCl3): 1.28 (s, OH); 1.51 – 1.67 (m, CH2(2), CH2(3)); 1.79 – 1.90 (m, CH2(4)); 3.63 –
3.72 (m, CH2(1)); 3.97 (t, J ¼ 6.4, CH2(5)); 6.86, 7.42 (AA’BB’, 4 arom. H); 6.94, 7.05 (AB, J ¼ 15.8, 2
olef. H); 7.21 – 7.50 (m, 5 arom. H). 13C-NMR (CDCl3): 22.4, 29.0, 32.4 (C(2), C(3), C(4)); 62.8 (C(1));
67.9 (C(5)); 114.8, 126.2, 126.6, 127.1, 127.7, 128.3, 128.6 (arom. and olef. CH); 130.5, 137.7, 158.5 (arom.
Cq). EI-MS: 282 (56, Mþ), 196 (100). Anal. calc. for C19H22O2 (282.4): C 80.82, H 7.85; found: C 80.75, H
7.83.
6-{4-[(E)-2-Phenylethenyl]phenoxy}hexan-1-ol (3d). Yield 620 mg (41%). Colorless crystals. M.p.
1448. 1H-NMR (CDCl3): 1.26 (s, OH); 1.41 – 1.66 (m, CH2(2), CH2(3), CH2(4)); 1.72 – 1.87 (m, CH2(5));
3.61 – 3.71 (m, CH2(1)); 3.96 (t, J ¼ 6.4, CH2(6)); 6.86, 7.42 (AA’BB’, 4 arom. H); 6.94, 7.05 (AB, J ¼ 15.9,
2 olef. H); 7.17 – 7.49 (m, 5 arom. H). 13C-NMR (CDCl3): 25.6, 25.9, 29.3, 32.7 (C(2), C(3), C(4), C(5));
62.9 (C(1)); 67.9 (C(6)); 114.7, 126.2, 126.5, 127.2, 127.7, 128.3, 128.6 (arom. and olef. CH); 130.0, 137.7,
158.8 (arom. Cq). EI-MS: 296 (48, Mþ), 196 (100). Anal. calc. for C20H24O2 (296.4): C 81.04, H 8.16;
found: C 80.74, H 7.76.
8-{4-[(E)-2-Phenylethenyl]phenoxy}octan-1-ol (3e). Yield 1190 mg (72%). Colorless crystals. M.p.
1418. 1H-NMR (CDCl3): 1.24 (t, OH); 1.36 – 1.58 (m, CH2(2), CH2(3), CH2(4), CH2(5), CH2(6)); 1.72 –
1.86 (m, CH2(7)); 3.59 – 3.70 (m, CH2(1)); 3.96 (t, J ¼ 6.4, CH2(5)); 6.87, 7.44 (AA’BB’, 4 arom. H); 6.94,
7.05 (AB, J ¼ 15.8, 2 olef. H); 7.21 – 7.50 (m, 5 arom. H). 13C-NMR (CDCl3): 25.7, 25.9, 29.2, 29.3, 29.3, 32.8
(C(2), C(3), C(4), C(5), C(6), C(7)); 63.0 (C(1)); 68.1 (C(8)); 114.8, 126.3, 126.6, 127.2, 127.7, 128.4,
1
)
Coupling constants of AA’BB’ spin patterns were not determined by simulation.

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128.6 (arom. and olef. CH); 130.1, 137.8, 159.0 (arom. Cq). EI-MS: 324 (91, Mþ), 196 (100). Anal. calc. for
C22H28O2 (324.5): C 81.44, H 8.70; found: C 81.30, H 8.62.
11-{4-[(E)-2-Phenylethenyl]phenoxy}undecan-1-ol (3f). Yield 1550 mg (83%). Colorless crystals.
M.p. 1408. 1H-NMR (CDCl3): 1.17 – 1.60 (m, CH2(2), CH2(3), CH2(4), CH2(5), CH2(6), CH2(7),
CH2(8), CH2(9), OH); 1.70 – 1.84 (m, CH2(10)); 3.56 – 3.68 (m, CH2(1)); 3.95 (t, J ¼ 6.5, CH2(11)); 6.86,
7.41 (AA’BB’, 4 arom. H); 6.94, 7.05 (AB, J ¼ 15.9, 2 olef. H); 7.17 – 7.49 (m, 5 arom. H). 13C-NMR
(CDCl3): 25.7, 26.0, 29.2, 29.3, 29.3, 29.4, 29.4, 29.5, 32.8 (C(2), C(3), C(4), C(5), C(6), C(7), C(8), C(9),
C(10)); 63.0 (C(1)); 68.1 (C(11)); 114.7, 126.2, 126.5, 127.1, 127.6, 128.3, 128.5 (arom. and olef. CH); 130.0,
137.7, 158.9 (arom. Cq). EI-MS: 367 (59, Mþ), 196 (100). Anal. calc. for C25H34O2 (366.6): C 81.92, H 9.35;
found: C 81.98, H 9.28.
Preparation of the Methacrylates 5a – 5f. The alkanol 3a – 5f (3.0 mmol) and 2-methylprop-2-enoyl
chloride (3.5 mmol) were reacted under Ar in 100 ml of CCl4 in the presence of 800 mg molecular sieves
(3 ꢄ). TLC Control (SiO2; Et2O) provided a reaction time of 2 – 5 d. The mixture was filtered, and the
solvent was evaporated. CC (SiO2; 4 ꢁ 40 cm, CH2Cl2) to yield anal. pure products 5a – 5f the melting
points of which were determined by DSC (108/min).
2-{4-[(E)-2-Phenylethenyl]phenoxy}ethyl 2-Methylprop-2-enoate (5a). Yield 407 mg (44%). Color-
1
less solid. M.p. 1008. H-NMR (CDCl3): 1.95 – 1.96 (m, Me); 4.23 (t, J ¼ 6.5, CH2O); 4.50 (t, J ¼ 6.6,
CO2CH2); 5.58 – 5.61 (m, HEꢀC(3)); 6.14 – 6.15 (m, HZꢀC(3)); 6.91, 7.45 (AA’BB’, 4 arom. H); 6.98, 7.07
(AB, J ¼ 16.4, 2 olef. H); 7.19 – 7.51 (m, 5 arom. H). 13C-NMR (CDCl3): 18.2 (Me); 63.0 (CO2CH2); 66.1
(CH2O); 114.9, 126.3, 126.9, 127.3, 127.7, 128.1, 128.6 (arom. and olef. CH); 125.9 (C(3)); 130.7, 137.6,
158.3 (arom. Cq); 136.0 (C(2)); 167.3 (C(1)). EI-MS: 308 (5, Mþ), 113 (100), 69 (63). Anal. calc. for
C20H20O3 (308.4): C 77.90, H 6.54; found: 77.51, H 6.69.
3-{4-[(E)-2-Phenylethenyl]phenoxy}propyl 2-Methylprop-2-enoate (5b). Yield 520 mg (54%).
1
Colorless solid. M.p. 948. H-NMR (CDCl3): 1.94 – 1.95 (m, Me); 2.11 – 2.24 (m, CH2); 4.06 (t, J ¼ 6.5,
CH2O); 4.33 (t, J ¼ 6.6, CO2CH2); 5.55 – 5.58 (m, HEꢀC(3)); 6.10 – 6.11 (m, HZꢀC(3)); 6.89, 7.44
(AA’BB’, 4 arom. H); 6.94, 7.07 (AB, J ¼ 16.3, 2 olef. H); 7.15 – 7.51 (m, 5 arom. H). 13C-NMR (CDCl3):
18.2 (Me); 28.7 (CH2); 61.5 (CO2CH2); 64.6 (CH2O); 114.8, 126.3, 126.8, 127.2, 127.7, 128.3, 128.6 (arom.
and olef. CH); 125.4 (C(3)); 130.4, 137.7, 158.6 (arom. Cq); 136.4 (C(2)); 167.4 (C(1)). EI-MS: 322 (9,
Mþ), 254 (43), 196 (61), 195 (49), 165 (43), 127 (100). Anal. calc. for C21H22O3 (322.4): C 78.23, H 6.88;
found: C 77.99, H 6.93.
5-{4-[(E)-2-Phenylethenyl]phenoxy}pentyl 2-Methylprop-2-enoate (5c). Yield 535 mg (51%). Color-
less solid. M.p. 898. 1H-NMR (CDCl3): 1.93 – 1.94 (m, Me); 1.50 – 1.86 (m, 3 CH2); 3.98 (t, J ¼ 6.5, CH2O);
4.17 (t, J ¼ 6.6, CO2CH2); 5.54 – 5.57 (m, HEꢀC(3)); 6.08 – 6.09 (m, HZꢀC(3)); 6.86, 7.43 (AA’BB’, 4
arom. H); 6.94, 7.07 (AB, J ¼ 16.2, 2 olef. H); 7.18 – 7.50 (m, 5 arom. H). 13C-NMR (CDCl3): 18.2 (Me);
22.6, 28.4, 28.9 (CH2); 64.5 (CO2CH2); 67.7 (CH2O); 114.8, 126.2, 126.6, 127.2, 127.7, 128.3, 128.6 (arom.
and olef. CH); 125.2 (C(3)); 130.1, 137.3, 158.8 (arom. Cq); 136.5 (C(2)); 167.5 (C(1)). EI-MS: 350 (93,
Mþ), 196 (100), 69 (99), 41 (81). Anal. calc. for C23H26O3 (350.5): C 78.83, H 7.48; found: C 78.45, H 7.39.
6-{4-[(E)-2-Phenylethenyl]phenoxy}hexyl 2-Methylprop-2-enoate (5d). Yield 950 mg (87%). Color-
1
less powder. M.p. 828. H-NMR (CDCl3): 1.92 – 1.93 (m, Me); 1.41 – 1.86 (m, 4 CH2); 3.96 (t, J ¼ 6.5,
CH2O); 4.14 (t, J ¼ 6.6, CO2CH2); 5.52 – 5.55 (m, HEꢀC(3)); 6.08 – 6.09 (m, HZꢀC(3)); 6.85, 7.43
(AA’BB’, 4 arom. H); 6.94, 7.06 (AB, J ¼ 16.3, 2 olef. H); 7.17 – 7.49 (m, 5 arom. H). 13C-NMR (CDCl3):
18.3 (Me); 25.7, 25.8, 28.6, 29.1 (CH2); 64.6 (CO2CH2); 68.0 (CH2O); 114.7, 126.2, 126.6, 127.1, 127.7,
128.3, 128.6 (arom. and olef. CH); 125.0 (C(3)); 130.1, 137.7, 158.9 (arom. Cq); 136.6 (C(2)); 167.5 (C(1)).
EI-MS: 364 (78, Mþ), 196 (100). Anal. calc. for C24H28O3 (364.5): C 79.09, H 7.74; found: C 79.12, H 7.40.
8-{4-[(E)-2-Phenylethenyl]phenoxy}octyl 2-Methylprop-2-enoate (5e). Colorless powder. Yield
905 mg (77%). M.p. 758. 1H-NMR (CDCl3): 1.93 – 1.94 (m, Me); 1.37 – 1.82 (m, 6 CH2); 3.96 (t, J ¼
6.5, CH2O); 4.14 (t, J ¼ 6.6, CO2CH2); 5.52 – 5.55 (m, HEꢀC(3)); 6.08 – 6.09 (m, HZꢀC(3)); 6.88, 7.44
(AA’BB’, 4 arom. H); 6.94, 7.07 (AB, J ¼ 16.3, 2 olef. H); 7.18 – 7.50 (m, 5 arom. H). 13C-NMR (CDCl3):
18.4 (Me); 25.9, 26.0, 28.6, 29.2, 29.3, 29.3 (CH2); 64.8 (CO2CH2); 68.0 (CH2O); 114.7, 126.2, 126.5, 127.2,
127.7, 128.3, 128.7 (arom. and olef. CH); 125.2 (C(3)); 130.0, 137.7, 158.9 (arom. Cq); 136.6 (C(2)); 167.5
(C(1)). EI-MS: 392 (21, Mþ), 196 (100). Anal. calc. for C26H32O3 (392.5): C 79.56, H 8.22; found: C 79.50,
H 8.27.

Helvetica Chimica Acta – Vol. 94 (2011)
2121
11-{4-[(E)-2-Phenylethenyl]phenoxy}undecyl 2-Methylprop-2-enoate (5f). Yield 1040 mg (80%).
Colorless powder. M.p. 858. IR (KBr): 2920, 2850, 1705, 1630, 1600, 1510, 1470, 1325, 1290, 1270, 1250,
1
1175, 1040, 1020, 970, 960, 830, 815, 755, 695. H-NMR (CDCl3): 1.93 – 1.94 (m, Me); 1.29 – 1.81 (m, 9
CH2); 3.95 (t, J ¼ 6.5, CH2O); 4.12 (t, J ¼ 6.6, CO2CH2); 5.52 – 5.54 (m, HEꢀC(3)); 6.08 – 6.09 (m,
HZꢀC(3)); 6.87, 7.44 (AA’BB’, 4 arom. H); 6.94, 7.06 (AB, J ¼ 16.2, 2 olef. H); 7.17 – 7.50 (m, 5 arom. H).
13C-NMR (CDCl3): 18.2 (Me); 25.9, 26.0, 28.6, 29.2, 29.3, 29.3, 29.4, 29.4, 29.5 (CH2); 64.8 (CO2CH2); 68.1
(CH2O); 114.8, 126.2, 126.5, 127.1, 127.7, 128.3, 128.6 (arom. and olef. CH); 125.1 (C(3)); 130.0, 137.7,
158.9 (arom. Cq); 136.6 (C(2)); 167.5 (C(1)). EI-MS: 434 (84, Mþ), 196 (100). Anal. calc. for C29H38O3
(434.6): C 80.14, H 8.81; found: C 79.92, H 8.93.
Polymerization of 5b and 5d – 5f. Ar was purged through dry THF (10 – 20 ml) for 20 min in order to
get an oxygen-free solvent. The selected monomer 5 (2.0 mmol) and AIBN (20 mg, 0.12 mmol) were
added, and the mixture was heated to 608 for 20 – 24 h under Ar. The mixture was poured into 150 ml of
H2O. The obtained precipitate was twice dissolved in hot THF and precipitated with cold MeOH to
remove unreacted monomer. The colorless polymer was dried at 1.3 ꢁ 102 Pa and 208 for 8 h.
Polymer 6b. Yield 518 mg (80%). M.p. 1628. IR (KBr): 1705, 1590, 1500, 1240, 1170, 960. 1H-NMR
(CDCl3): 0.5 – 2.3 (m, Me, CH2, main chain and side chains2)); 3.6 – 4.2 (m, CH2O); 6.6 – 7.5 (m, arom.
and olef. H).
Polymer 6d. Yield 525 mg (72%). M.p. 1248. IR (KBr): 1705, 1590, 1500, 1240, 1170, 960. 1H-NMR
(CDCl3): 0.5 – 2.2 (m, Me, CH2, main chain and side chains); 3.4 – 4.2 (m, CH2O); 6.6 – 7.5 (m, arom. and
olef. H).
Polymer 6e. Yield 670 mg (85%). M.p. 1208. IR (KBr): 1710, 1600, 1505, 1250, 1175, 960. 1H-NMR
(CDCl3): 0.5 – 2.2 (m, Me, CH2, main chain and side chains); 3.6 – 4.2 (m, CH2O); 6.7 – 7.5 (m, arom. and
olef. H). 13C-NMR (solid state, CPMAS): 10 – 40 (Me, CH2 side chains); 40 – 50 (Cq, main chain); 50 – 60
(CH2, main chain); 60 – 70 (CH2O); 110 – 120 (arom. CHꢀCqO); 120 – 135 (arom. CH and Cq para to
CqO); 135 – 140 (arom. Cq, Ph); 155 – 165 (arom. CqO); 175 – 180 (C¼O).
Polymer 6f. Yield 715 mg (82%). M.p. 1218. Tg 668. IR (KBr): 1710, 1600, 1510, 1250, 1175, 960.
1H-NMR (CDCl3): 0.7 – 2.0 (m, Me, CH2, main chain and side chains); 3.6 – 4.1 (m, CH2O); 6.6 – 7.5 (m,
arom. and olef. H).
Due to incomplete combustion, no elemental analyses of 6b and 6d – 6f could be obtained.
Irradiation of 4-[(E)-2-Phenylethenyl]phenyl Acetate ((E)-7) as Model Compound. Ester (E)-7 [42]
(5.67 g, 23.8 mmol) was dissolved in 200 ml of oxygen- and thiophene-free benzene. Irradiation under Ar
with a Hanovia-450-W medium pressure Hg lamp equipped with a Pyrex filter (l ꢃ 290 nm)3) led to a
yellow soln. After stirring at r.t. for 44 h, the mixture was separated by CC (SiO2; 5 ꢁ 60 cm, CH2Cl2).
The first fractions consisted of pure (Z)-7 (550 mg, 10%) and (E)/(Z)-7 mixtures. SiO2 provokes a slow
catalytic isomerization (Z)-7 ! (E)-7. The following fractions consisted of the dimers 8/10, followed by
the dimers 9/11. The total yield of dimers was 1.30 g (23%), and the ratio 8/9/10/11 obtained by 1H-NMR
spectroscopy was ca. 40 :18 :40 :2. The mixture of dimers 9 – 11 was a waxy solid. Anal. calc. for C32H28O4
(476.6): C 80.65, H 5.92; found: C 80.77, H. 5.69.
The fraction 8/10 was then separated by fractionary crystallization from EtOH or EtOH/Et2O 1:1. A
separation of the stereoisomers 9 and 11 was not achieved.
The course of the photoreaction (E)-7 ! (Z)-7 þ 8 – 11, shown in Fig. 1, was determined under the
same conditions. Probes of the soln. were evaporated, and the residue was dissolved in CDCl3 to obtain
1
the amount of the components by H-NMR spectroscopy.
4-[(Z)-2-Phenylethenyl]phenyl Acetate ((Z)-7). Oil. UV (CH2Cl2): 283 (19970). 1H-NMR (CDCl3):
2.26 (s, Me); 6.53, 6.59 (AB, J ¼ 12.3, 2 olef. H); 6.93, 7.23 (AA’BB’, 4 arom. H); 7.18 – 7.25 (m, 5 arom. H).
13C-NMR (CDCl3): 21.1 (Me); 121.3, 127.2, 128.3, 128.8, 129.2, 129.9, 130.5 (arom. and olef. CH); 134.8,
137.1 (arom. Cq); 149.6 (arom. CqO); 169.3 (C¼O). EI-MS: 238 (21, Mþ), 196 (100). The compound was
indentical with an authentic sample [45].
2
)
The H-atom chemical shifts (d values) of the irradiated and unirradiated polymers are given here
with a single digit, because the second digit is not reproducible.
Without filter, the energy-rich radiation (l ¼ 254 nm) caused a polymer cover on the irradiation
vessel.
3
)

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Helvetica Chimica Acta – Vol. 94 (2011)
r-1,c-2-Bis(4-acetoxyphenyl)-t-3,t-4-diphenylcyclobutane (¼ [(1R*,2S*,3R*,4S*)-3,4-Diphenylcyclo-
butane-1,2-diyl]dibenzene-4,1-diyl Diacetate; 8). Colorless crystals. M.p. 2008. 1H-NMR (CDCl3): 2.22 (s,
2 Me); 4.40, 4.43 (AA’BB’, 4 cyclobutane H), 6.86, 7.09 (AA’BB’, 8 arom. H); 7.02 – 7.14 (m, 10 arom. H).
13C-NMR (CDCl3): 21.1 (Me); 46.9, 47.8 (CH, cyclobutane), 121.1, 126.0, 127.9, 128.1, 129.0 (arom. CH);
138.2, 140.3 (arom. Cq); 148.3 (arom. CqO); 169.3 (CO). FD-MS: 476 (10, Mþ), 238 (100). EI-MS: 296
(5), 254 (10), 238 (55), 212 (10), 196 (100), 180 (5). The linked scan technique established, that the ion
peak at m/z 212 had the ꢂparentsꢃ 254 and 296.
r-1-t-3-Bis(4-acetoxyphenyl)-c-2,t-4-diphenylcyclobutane (¼(trans-2,4-Diphenylcyclobutane-1,3-
1
diyl)dibenzene-4,1-diyl Diacetate; 10). Colorless crystals. M.p. 2008. H-NMR (CDCl3): 2.22 (s, 2 Me);
4.43 (ꢂsꢃ, 4 cyclobutane H), 6.85, 7.08 (AA’BB’, 8 arom. H); 7.06 – 7.16 (m, 10 arom. H). 13C-NMR
(CDCl3): 21.0 (Me); 46.9, 47.7 (CH, cyclobutane); 120.9, 126.1, 128.0, 128.1, 128.9 (arom. CH); 138.2,
140.3 (arom. Cq); 148.8 (arom. CqO); 169.3 (CO). FD-MS: 476 (10, Mþ), 238 (100). EI-MS: 238 (100),
196 (80). The linked scan technique established, that the ion peak at m/z 196 had the ꢂparentꢃ 238.
r-1-t-2-Bis(4-acetoxyphenyl)-c-3,t-4-diphenylcyclobutane (¼ [(1R*,2R*,3S*,4S*)-3,4-Diphenylcyclo-
butane-1,2-diyl]dibenzene-4,1-diyl Diacetate; 9) and r-1,c-3-Bis(4-acetoxyphenyl-t-2,t-4-diphenylcyclobu-
1
tane (¼(cis-2,4-Diphenylcyclobutane-1,3-diyl)dibenzene-4,1-diyl Diacetate; 11). Oil. H-NMR (CDCl3)
of 9: 2.28 (s, 2 Me); 3.65, 3.68 (AA’BB’, 4 cyclobutane H); 7.02, 7.28 (AA’BB’, 8 arom. H); 7.25 – 7.38 (m,
10 arom. H). 1H-NMR (CDCl3) of 11: 2.31 (s, 2 Me); 3.60 – 3.67, 3.67 – 3.73 (2m, 2 H each of
cyclobutane); 7.00 – 7.03 (m, 4 arom. H); 7.25 – 7.38 (m, 10 arom. H). 13C-NMR (CDCl3) of 9: 21.1 (Me);
51.1, 51.7 (CH, cyclobutane); 121.5, 126.7, 127.0, 127.8, 128.5 (arom. CH); 139.9, 142.0 (arom. Cq); 149.3
(arom. CqO); 169.4 (CO). 13C-NMR (CDCl3) of 11: 21.0 (Me); 51.0, 51.8, (CH, cyclobutane); 121.7,
126.4, 126.6, 127.5, 128.0 (arom. CH); 139.8, 142.1 (arom. Cq); 150.0 (arom. CqO); 169.3 (CO). FD-MS:
476 (10, Mþ), 196 (100).
When an oxygen-containing soln. of 7 was irradiated, the formation of 4-[(2R*,3R*)-3-phenyl-
oxiran-2-yl]phenyl acetate and traces of phenanthren-3-yl acetate could be observed.
Irradiation of the Polymers 6b and 6d – 6f in Soln. The selected polymer 6 (1.2 mmol related to the
monomer) was dissolved in oxygen-free CH2Cl2 (30 ml) and irradiated under Ar with a Hanovia-450-W
medium-pressure Hg lamp equipped with a Pyrex filter (l ꢃ 290 nm). After 48 h, the solvent was
evaporated, and the residue was dried at r.t. and 1.3 ꢁ 102 Pa for 2 – 3 d. The yellow-to-light brown solid is
moderately soluble in CDCl3. Small amounts of insoluble products (ꢂ 5%) were obtained as coating on
the irradiation vessel.
Irradiated Polymer 6b. IR (KBr): 1715, 1605, 1510, 1250, 1180, 1050. 1H-NMR (CDCl3): 0.7 – 2.1 (m,
Me, CH2, main chain and side chains); 3.6 – 4.2 (m, CH2O, CH); 4.2 – 4.4 (m, CH); 6.4 – 6.5 (m, olef. H
Z); 6.5 – 7.5 (m, arom. and olef. H). According to the integration, 80% of the olefinic C¼C bonds reacted.
Irradiated Polymer 6d. IR (KBr): 1710, 1600, 1500, 1235, 1175. 1H-NMR (CDCl3): 0.6 – 2.2 (m, Me,
CH2, main chain and side chains); 3.6 – 4.2 (m, CH2O, CH); 4.2 – 4.4 (m, CH); 6.4 – 6.5 (m, olef. H (Z));
6.5 – 7.5 (m, arom. and olef. H). According to the integration, 60% of the olefinic C¼C bonds reacted.
Irradiated Polymer 6e. IR (KBr): 1705, 1600, 1500, 1245, 1170. 1H-NMR: 0.6 – 2.2 (m, Me, CH2, main
chain and side chains); 3.6 – 4.2 (m, CH2O, CH); 4.2 – 4.4 (m, CH); 6.4 – 6.5 (m, olef. H (Z)); 6.5 – 7.5 (m,
arom. and olef. H). According to the integration, 65% of the olefinic C¼C bonds reacted. Solid-state CP-
MAS 13C-NMR: 10 – 40 (Me, CH2, side chains); 40 – 60 (CH, Cq, main chain, CH2, main chain); 60 – 70
(CH2O); 110 – 120 (arom. CHꢀCqO); 120 – 135 (arom. and olef. CH and Cq); 135 – 140 (arom. Cq); 155 –
165 (arom. CqO); 175 – 180 (CO).
Irradiated Polymer 6f. IR (KBr): 1710, 1600, 1500, 1245, 1175. 1H-NMR: 0.5 – 2.2 (m, Me, CH2, main
chain and side chains); 3.6 – 4.2 (m, CH2O, CH); 4.2 – 4.4 (m, CH); 6.4 – 6.5 (m, olef. H (Z)); 6.5 – 7.5 (m,
arom. and olef. H). According to the integration, 65% of the olefinic C¼C bonds reacted.
Irradiation of Casted Films of 6e and 6f. Sat. solns. of 6e and 6f (4 – 5 mg/ml) in CHCl3 or thiophene-
free benzene were poured on to quartz plates. After complete evaporation of the solvent at 408/80 Torr,
transparent films were obtained, which were monochromatically irradiated (366 nm filter) for 6 h.
Transparent, completely insoluble films were obtained, which showed a panchromatic absorption
between 250 and 360 nm with a maximum at l 282 nm in both cases.
In contrast to all unirradiated polymers 6, the irradiated materials do not melt.

Helvetica Chimica Acta – Vol. 94 (2011)
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Received June 15, 2011