Synthesis and kinetic studies on the photochemical behavior of polymeric mesoions from novel methacrylic monomers and of mesoionic copolymers with liquid crystalline properties

Article abstract of DOI:10.1021/ma034700a

A new method for synthesis of mesoionic methacrylates with various spacer lengths was developed. Copolymers with methyl methacrylate and cholesteric monomers were prepared; the latter showed liquid crystalline properties. The rate constant of the photocyc

Full text of DOI:10.1021/ma034700a

7552
Macromolecules 2003, 36, 7552-7559
Synthesis and Kinetic Studies on the Photochemical Behavior of
Polymeric Mesoions from Novel Methacrylic Monomers and of Mesoionic
Copolymers with Liquid Crystalline Properties
Alexa n d er Th eis a n d Helm u t Ritter *
Institute of Organic Chemistry and Macromolecular Chemistry, Heinrich Heine University,
Universitaetsstrasse 1, D-40225 Duesseldorf, Germany
Received May 26, 2003; Revised Manuscript Received J uly 29, 2003
ABSTRACT: A new method for synthesis of mesoionic methacrylates with various spacer lengths was
developed. Copolymers with methyl methacrylate and cholesteric monomers were prepared; the latter
showed liquid crystalline properties. The rate constant of the photocyclization to the bis(â-lactame)
structure of various spin-coated polymeric films was determined by irradiation with a HeCd laser (442
nm), whereby two different parameters controlling the rate constant could be found. It was shown that
the length of the alkyl side chain on position 5 of the pyrimidinium-olate and the embedding of the
mesoionic chromophore, which can be controlled by copolymerization with methyl methacrylate or liquid
crystalline methacrylates, causes significant changes in the rate constant. In addition, heating of a polymer
film caused a strong acceleration of the photoreaction above the glass transition temperature.
In tr od u ction
ionic systems was inspected to various parameters like
temperature, spacer length, and chemical environment.
During the past few years we have described the
synthesis and polymerization behavior of various styrene-
substituted mesoionic 6-oxo-1,6-dihydropyrimidin-3-
ium-4-olates.1-3 Recently, for the first time, the prepa-
ration of a polymerizable methacryl derivative that
contained a short dimethylene spacer was carried out
successfully as shown in Scheme 1.4
We also prepared polymers with mesoionic groups in
the main chain5,6 and mesoionic side chain polymers,
which are based on phenols as polymerizable groups.7
An interesting feature of the mesoionic moieties is their
sensitivity to light. Upon irradiation with UV light
between 320 and 490 nm, which depends on the
substitution of the pyrimidinium-olate ring, the mol-
ecule 6 undergoes an intramolecular disrotatoric pho-
tocyclization, which results in formation of a bicyclic
lactame structure 7, as shown in Scheme 2.
The course of the photoreaction of mesoionic 6-oxo-
1,6-dihydropyrimidin-3-ium-4-olates has been investi-
gated on dissolved low molecular weight compounds8
and polymers1 as well as on polymeric films.4 The
change of the molecular structure has an effect on
several properties of mesoionic polymer films such as
color, specific volume, dipole moment, and refractive
index. For this reason, mesoionic polymers are interest-
ing candidates for manufacturing photosensitive mate-
rials for lithography or optical devices. It was shown
by waveguide spectroscopy that irradiation of mesoionic
polymer films causes strong effects on refractive index
and film thickness.6,7 Hence, these materials could also
be used for optical data storage and relief printing.
Regarding technical applications, the kinetics of the
photoreaction is an important factor. For many applica-
tions, a fast switching is required. However, also stabil-
ity toward daylight, oxygen, and temperature of pho-
tosensitive compounds has to be considered.
Exp er im en ta l Section
Meth od s. Chemicals were used as received from Acros,
Fluka, or Aldrich. Technical solvents were distilled before use.
For chromatography silica gel with 30-60 µm particle size
(Baker) or 32-63 µm particle size, 60 Å (ICN) was used. 1H
and 13C NMR spectra were recorded on a Bruker AC200 or
AM400 at room temperature. The deuterated solvent was used
as lock and internal standard for the δ-scale relative to TMS.
Infrared spectra were recorded using a Nicolet FT-IR spec-
trometer 5DXC (DTGS detector) or 5SXB (MCT detector).
Mass spectra were performed on a Finnigan MAT 95 (FD).
UV-vis spectra were carried out with a Shimadzu UV-2102
PC UV-vis spectrometer. Elementary analysis was done in
the Microanalytical Lab of the Institute of Organic Chemistry,
University of Mainz. SEC measurements were measured on
a PSS installation with THF or chloroform (30 °C) as eluent.
150 µL was injected on column arrangement of PSS SDV 5 µ,
100, 1000, and 10000 Å porosity. A TSP UV 2000 UV-vis
detector, a Shodex differential refractometer RI 71, and a WGE
Dr. Bures η1001 differential viscosimeter were used as detec-
tors. The data were evaluated with PSS-WinGPC 6.20; mo-
lecular weights were also calculated with universal calibration
using the intrinsic viscosity. Therefore, the concentration was
calculated by the RI signal. For determination of the rate
constants, the 5-10 wt % solutions of the different polymers
in chloroform were spin-coated with a thickness of about 1.5
µm on microscope object slides with a BLE Delta 10 spin-
coater, using a spin rates of 600 U/min at highest acceleration
for 1 min. For the liquid crystalline copolymers, dioxane was
used as solvent, and exhaust air separation was used during
the spin-coat process for 10 min. The kinetic measurements
were performed using a layout as shown in Figure 1.
The light beam of a HeCd laser (442 nm wavelength) was
adjusted by a lens to a definite cross section and aligned
through a chopper to the sample. Behind the sample, the
residual light intensity was detected by a laser power meter,
and the data were recorded every second by a computer. At
the beginning of the irradiation, the intensity of the transmit-
ted radiation is reduced by the absorption of the sample. With
progressing conversion to the bis(â-lactame), the absorption
is reduced and the transmission increases, until the level of
saturation is reached. Using constant laser intensity, the speed
of the photoreaction should follow the laws of pseudo-first-
order kinetics. For evaluation of the data, the intensity at the
In this study, films of novel mesoionic polymers were
prepared. The kinetics of the photoresponse of the meso-
* Corresponding author: Tel +49 211 8114760; Fax +49 211
8115840; e-mail H.Ritter@uni-duesseldorf.de.
10.1021/ma034700a CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/30/2003

Macromolecules, Vol. 36, No. 20, 2003
Photochemical Behavior of Polymeric Mesoions 7553
Sch em e 1. Or igin a l Syn th esis of Meth a cr yl-Su bstitu ted Mesoion s
2852 (aliph C-H), 1719 (CdO), 1638 (CdC); further signals
at 1453, 1404, 1322, 1297, 1252, 1165, 1018, 942, 815, 750,
650, 563.
F igu r e 1. Schematic layout to determine the rate constant
of photosensitive polymer films.
Sch em e 2. P h otoch em ica l Rea ction of Mesoion ic
6-Oxo-1,6-d ih yd r op yr im id in -3-iu m -4-ola tes
S-4-(Isop r op en ylca r bon yloxy)bu tyl-N-4-eth oxyp h en yl-
N′-p h en ylisoth iou r ea (12). 10.0 g (45.2 mmol) of 4-brom-
butyl methacrylate, 0.4 g of hydroquinone monopropyl ether,
and 12.3 g (78 mmol) of N-4-ethoxyphenyl-N′-phenylthiourea4
were added to 45 mL of ethanol p.a. and boiled for 4 h under
reflux. Then the reaction mixture was cooled to room temper-
ature, poured into a mixture of 170 mL of water and 17 mL of
concentrated ammonia, and stirred for 1 min. After 10 min,
the aqueous phase was decanted and extracted with chloro-
form. The chloroform phase and the oily phase were combined
and dried with magnesium sulfate. The crude product, which
was obtained by evaporating the chloroform, was purified by
column chromatography over silica gel (ethyl acetate/petro-
leum ether 4/1 v/v); yield 9.2 g (49%). Colorless highly viscous
saturated state Is was obtained by signal averaging at the end
of the measurement after the photoreaction was complete. The
relative amount of unconverted mesoionic functions [M]t at the
time t was calculated by eq 1 using the measured intensities
at time t.
1
oil; 200 MHz H NMR (CDCl3): δ [ppm] ) 1.38 (t, 3H, 29-H,
J ) 7.2 Hz), 1.55-1.75 (m, 4H, 6,7-H), 1.90 (s, 3H, 18-H), 2.78
((br), 2H, 8-H), 3.98 (q, 2H, 28-H, J ) 7.2 Hz), 4.10 (t, 2H,
5-H, J ) 6.6 Hz), 5.51 (s, 1H, 1-H), 6.05 (s, 1 H, 1-H), 6.81 (d,
2H, 22,26-H, J ) 8.8 Hz), 6.98-7.36 (m, 7H, 13,14,15,16,17,-
23,25-H). 50 MHz 13C NMR (CDCl3): δ [ppm] ) 14.91 (C-29),
18.34 (C-18), 26.21/27.76/30.95 (C-6, C-7, C-8), 63.69 (C-28),
64.09 (C-5), 114.87 (C-23,25), 121.72 (C-13,17), 123.39 (C-15),
124.37 (br, C-22,26), 125.47 (C-1), 129.17 (C-14,16), 135.48 (br,
C-21), 136.37 (C-2), 145.47 (br, C-12), 151.25 (br, C-10), 156.09
(C-24), 167.41 (C-3). IR (NaCl): λ-1 [cm-1] ) 3350 (N-H), 3055,
3032 (ar C-H), 2977, 2956, 2927, 2872 (aliph C-H), 1716 (Cd
O), 1622 (CdC, CdN), 1589 (ar CdC); further signals at 1510,
1497, 1437, 1408, 1393, 1298, 1240, 1221, 1167, 1130, 1117,
1090, 1048, 942, 915, 822, 757, 697. MS (FD): m/z (%) ) 413
(100) [M+].
[M]t Is - It
(1)
The first value of [M]t is per definition [M]0. Using the law of
first-order kinetics (eq 2), ln([M]0/[M]t) was spread against t.
[M]0
ln
) kt
(2)
(
)
[M]t
The slope calculated by linear regression is the rate constant
k.
Syn th esis. 4-Br om obu tyl Meth a cr yla te (10). 400 g (1.85
mol) of 1,4-dibromobutane, 46 g (0.37 mol) of sodium meth-
acrylate, 1.85 g of tetrabutylammonium iodide as phase
transfer catalyst, and 0.74 g of hydroquinone monopropyl ether
were dissolved in 520 mL of ethyl acetate and heated to 80 °C
for a period of 20 h. After cooling to room temperature, the
precipitated NaBr was filtered off and the ethyl acetate was
evaporated in a vacuum. The product was isolated by fractional
distillation in high vacuum (bp 47-49 °C, 2 × 10-3 mbar); yield
59.5 g (73%). Colorless oil; 200 MHz 1H NMR (CDCl3): δ [ppm]
) 1.73-2.08 (m, 7H, 6,7,10-H), 3.42 (t, 2H, 8-H, J ) 6.3 Hz),
4.15 (t, 2H, 5-H, J ) 6.1 Hz), 5.54 (s, 1H, 1-H), 6.07 (s, 1 H,
1-H). 50 MHz 13C NMR (CDCl3): δ [ppm] ) 18.23 (C-10), 27.26
(C-7), 29.35 (C-6), 33.02 (C-8), 64.08 (C-5), 125.32 (C-1), 136.26
(C-2), 167.15 (C-3). IR (NaCl): λ-1 [cm-1] ) 2960, 2929, 2899,

7554 Theis and Ritter
Macromolecules, Vol. 36, No. 20, 2003
Mesoion ic 5-Bu tyl-1-(4-eth oxyp h en yl)-2-[4-(isop r op e-
n ylca r bon yloxy)b u t ylt h io]-6-oxo-3-p h en yl-1,6-d ih yd r o-
p yr im id in -3-iu m -4-ola te (14). 1.0 g (2.42 mmol) of S-4-
(is op r op en ylca r bon yloxy)bu t yl-N -4-et h oxyp h en yl-N ′-
phenylisothiourea and 1.0 g (4.84 mmol) of DCC were dissolved
under stirring in 5 mL of dry dichloromethane. Then 0.39 g
(2.42 mmol) of butylmalonic acid was added batchwise for a
period of 10 min under exclusion of moisture. After 50 min of
additional stirring, the precipitated urea was filtered off and
washed with a small amount of dichloromethane. The com-
bined dichloromethane fractions were concentrated in a vacuum,
and 250 mL of petroleum ether was added under stirring. After
15 min, the petroleum ether was decanted, and the yellow
residue was dried in a vacuum. For further purification, the
crude product was chromatographed over silica gel (ethyl
acetate/petroleum ether 7/3 v/v); yield 0.6 g (47%). Yellow
added to 55 mL of ethyl acetate and boiled under reflux for 24
h. The reaction mixture was cooled to room temperature, the
precipitate was filtered off, and the ethyl acetate was removed
by distillation in a vacuum. For purification, the crude product
was dissolved in 6 mL of dichloromethane and precipitated in
70 mL of methanol. The product was filtered off, washed with
additional methanol, and dried in a vacuum; yield 3.2 g (68%).
Colorless crystals; 400 MHz 1H NMR (CDCl3): δ [ppm] ) 0.65
(s, 3 H, 30-H), 0.77-2.02 (m, 49 H, 3,8,9,10,16,17,20,21,22,-
23,24,26,27,29,31,32,33,34,35,36,37,38,39,40,41-H), 2.27 (t, 2
H, 11-H, J ) 7.3 Hz), 4.11 (t, 2 H, 7-H, J ) 6.6 Hz), 4.59 (m,
1 H, 15-H), 5.34 (m, 1 H, 25-H), 5.52 (s, 1 H, 1-H), 6.06 (s, 1
H, 1-H). 100 MHz 13C NMR (CDCl3): δ [ppm] ) 11.77 (C-30),
18.17 (C-3), 18.66 (C-35), 19.21 (C-21), 20.99 (C-26), 22.46/22.71
(C-40,41), 23.79 (C-37), 24.21 (C-33), 24.60 (C-9), 25.50 (C-10),
27.76 (C-16), 27.91 (C-39), 28.14 (C-32), 28.27 (C-8), 31.84 (C-
23), 31.89 (C-24), 34.41 (C-11), 35.71 (C-34), 36.13 (C-36), 36.51
(C-18), 36.95 (C-17), 38.10 (C-20), 39.46 (C-27), 39.71 (C-38),
42.26 (C-28), 50.02 (C-22), 56.14 (C-31), 56.65 (C-29), 64.35
(C-7), 73.71 (C-15), 122.50 (C-25), 124.98 (C-1), 136.42 (C-2),
1
highly viscous oil; 400 MHz H NMR (DMSO-d6): δ [ppm] )
0.85 (t, 3 H, 28-H, J ) 6.8 Hz), 1.16-1.44 (m, 11 H, 6,7,26,-
27,38-H), 1.83 (s, 3 H, 16-H), 2.17 (t, 2 H, 8-H, J ) 6.8 Hz),
2.25 (t, 2 H, 25-H, J ) 7.6 Hz), 3.89 (t, 2 H, 5-H, J ) 6.1 Hz),
4.02 (q, 2 H, 37-H, J ) 7.2 Hz), 5.65 (s, 1 H, 1-H), 5.94 (s, 1 H,
1-H), 7.00 (d, 2 H, 32,34-H, J ) 8.8 Hz), 7.38 (d, 2 H, 31,35-H,
J ) 8.8 Hz), 7.42-7.53 (m, 5 H, 19,20,21,22,23-H). 50 MHz
13C NMR (DMSO-d6): δ [ppm] ) 13.93 (C-28), 14.49 (C-38),
17.87 (C-16), 22.33/24.27/25.39/26.62/30.00/34.79 (C-6, C-7,
C-8, C-25, C-26, C-27), 63.39 (C-5), 64.21 (C-37), 94.60 (C-13),
114.22 (C-32,34), 125.51 (C-1), 128.69 (C-19,20,22,23), 128.88
(C-21), 129.79 (C-31,35), 130.49 (C-30), 135.81 (C-2), 138.22
(C-18), 158.61/159.09/159.25/160.06 (C-10, C-12, C-14, C-33),
166.32 (C-3). IR (KBr): λ-1 [cm-1] ) 3061 (ar C-H), 2954,
2929, 2869 (aliph C-H), 1716 (CdO), 1641 (mesoion CdO),
1594 (ar CdC); further signals at 1510, 1478, 1455, 1394, 1366,
1319, 1299, 1249, 1171, 1116, 1068, 1045, 758, 699. UV
(dichloromethane): λmax [nm] (log ꢀ) ) 266 (4.03), 375 (3.26).
MS (FD): m/z (%) ) 537 (100) [M+].
139.58 (C-19), 167.24 (C-4), 172.66 (C-12). IR (KBr): λ-1 [cm-1
]
) 2945, 2867 (C-H), 1733, 1723 (CdO); further signals at
1638, 1467, 1377, 1366, 1321, 1297, 1255, 1236, 1169, 1029,
1012, 940. DSC: ∆Cp (negative) ) 4 °C. Peak ) 35 °C. Peak
) 64 °C. MS (FD): m/z (%) ) 569 (2) [M+], 387 (10)
[cholesterol+], 369 (100) [cholesteryl+].
EA: C30H36N2O5S (536.70). Calcd: C, 67.14; H, 6.76; N, 5.22;
S, 5.97. Found: C, 66.35; H, 7.09; N, 5.33; S, 5.80.
EA: C37H60O4 (568.89). Calcd: C, 78.12; H, 10.63. Found:
C, 77.72; H, 10.34.
1-Br om h exa n oic Acid Ch olester yl Ester (17). 15.46 g
(40 mmol) of cholesterol, 8.25 g (40 mmol) of DCC, and 0.49 g
(4 mmol) of 4-(dimethylamino)pyridine were dissolved in 70
mL of dichloromethane. Then 7.8 g (40 mmol) of 1-bromohex-
anoic acid was added under stirring over a period of 20 min,
whereby the temperature was kept at room temperature by
use of a water bath. The solution was stirred for additional 2
h, and then the precipitated urea was filtered off and washed
Cop olym er of Meth yl Meth a cr yla te a n d Mesoion ic
5-Bu tyl-1-(4-eth oxyph en yl)-2-[4-(isopr open ylcar bon yloxy)-
bu t ylt h io]-6-oxo-3-p h en yl-1,6-d ih yd r op yr im id in -3-iu m -
4-ola te (21). 0.50 g (0.93 mmol) of mesoionic 5-butyl-1-(4-
ethoxyphenyl)-2-[4-(isopropenylcarbonyloxy)butylthio]-6-oxo-
3-phenyl-1,6-dihydropyrimidin-3-ium-4-olate, 0.37 g (3.72 mmol)
of methyl methacrylate, and 23.0 mg (0.14 mmol) of AIBN were
dissolved in 2 mL of absolute DMF. The mixture was degassed
by exposure to nitrogen bubbles for 30 min. Then the solution
was polymerized for 20 h at 60 °C, and the polymer was
precipitated in a mixture of 100 mL of methanol and 200 mL
of water. After drying in a vacuum, the polymer was purified
by dissolving in 5 mL of dichloromethane and reprecipitating
in 150 mL of diethyl ether. The product was dried in a vacuum
again; yield 0.6 g (69%). Yellow powder; 400 MHz 1H NMR
(CDCl3): δ [ppm] ) 0.46-2.10 (m, 1,6,7,16,25,26,27,28,38,39,-
44-H), 2.42 (m, 8-H), 3.55 (s, 43-H), 3.65-4.12 (m, 5,37-H),
6.32-7.59 (m, ar-H). IR (KBr): λ-1 [cm-1] ) 2987, 2950, 2871
(aliph C-H), 1730 (CdO), 1651 (mesoion. CdO); further
signals at 1510, 1479, 1453, 1365, 1248, 1192, 1151, 1064, 995,
973, 924, 852, 757, 701. UV (dichloromethane): λmax [nm] )
263, 380. DSC: Tg ) 104 °C. SEC (chloroform, universal cali-
bration by viscosity): Mn ) 27 000, Mw ) 280 000, PD ) 11.
EA for n:m ) 4:1. Calcd: C, 64.08; H, 7.31; N, 2.99; S, 3.42.
Found: C, 63.35; H, 7.29; N, 3.07; S, 3.89.
with
a small amount of dichloromethane. The combined
dichloromethane fractions were evaporated in a vacuum. The
dried crude product could be used without further purification;
yield 22.5 g (100%). Colorless crystals; mp 110-113 °C; 400
1
MHz H NMR (CDCl3): δ [ppm] ) 0.64 (s, 3 H, 25-H), 0.77-
2.02 (m, 46 H, 3,4,5,11,12,15,16,17,18,19,21,22,24,26,27,28,-
29,30,31,32,33,34,35,36-H), 2.27 (t, 2 H, 6-H, J ) 7.5 Hz), 3.38
(t, 2 H, 2-H, J ) 6.8 Hz), 4.58 (m, 1 H, 10-H), 5.34 (m, 1 H,
20-H).
IR (KBr): λ-1 [cm-1] ) 2946, 2886, 2867 (C-H), 1733 (Cd
O); further signals at 1652, 1456, 1436, 1384, 1373, 1365, 1323,
1255, 1229, 1187, 1172, 999.
5-(Ch olester olca r bon yl)p en tyl Meth a cr yla te (19). 4.7
g (8.33 mmol) of 1-bromohexanoic acid cholesteryl ester, 1.03
g (8.33 mmol) of potassium methacrylate, 20 mg of hydro-
quinone monopropyl ether, and 0.044 g (0.12 mmol) of tet-
rabutylammonium iodide as phase transfer catalyst were

Macromolecules, Vol. 36, No. 20, 2003
Photochemical Behavior of Polymeric Mesoions 7555
1H NMR (CDCl3): δ [ppm] ) 0.65 (s, 63-H), 0.72-2.15 (m, 1,-
14,23,33,34,36,41,42,43,49,50,53,54,55,56,57,59,60,62,64,65,-
66,67,68,69,70,71,72,73,74-H), 2.19-2.33 (m, 44-H), 2.40 (m,
6-H), 3.60 (m, 5-H), 3.90 (m, 40-H), 4.03 (m, 32-H), 4.57 (m,
48-H), 5.34 (m, 58-H), 6.94 (m, 27,29-H), 7.10 (m, 26,30-H),
7.24 (m, 19-H), 7.30-7.54 (m, 17,18,20,21-H). IR (KBr): λ-1
[cm-1] ) 2945, 2867 (aliph C-H), 1732 (CdO), 1653 (mesoion
CdO); further signals at 1510, 1467, 1376, 1251, 1170, 1058,
1012, 960, 843, 746, 702. UV (dichloromethane): λmax [nm] )
264, 370. DSC: ∆Cp ) 59 °C. Peak ) 129 °C. SEC (THF, PS
standard): Mn ) 21 000, Mw ) 36 000, PD ) 1.8.
EA for n:m ) 1:4. Calcd: C, 75.78; H, 9.78; N, 1.02; S, 1.17.
Found: C, 74.90; H, 10.07; N, 1.40; S, 1.92.
Cop olym er of Mesoion ic 5-Bu tyl-1-(4-eth oxyp h en yl)-
2-[4-(isop r op en ylca r bon yloxy)bu tylth io]-6-oxo-3-p h en yl-
1,6-d ih yd r op yr im id in -3-iu m -4-ola te a n d 5-(Ch olester ol-
ca r bon yl)p en tyl m eth a cr yla te (23). 0.25 g (0.466 mmol) of
mesoionic 5-butyl-1-(4-ethoxyphenyl)-2-[4-(isopropenylcarbo-
nyloxy)butylthio]-6-oxo-3-phenyl-1,6-dihydropyrimidin-3-ium-
4-olate, 0.75 g of (1.32 mmol) 5-(cholesterolcarbonyl)pentyl
methacrylate, and 10.0 mg (0.0609 mmol) of AIBN were
dissolved in 5 mL of absolute THF. The mixture was degassed
by exposure to nitrogen bubbles for 10 min. Then the solution
was polymerized for 15 h at 60 °C, and the polymer was
precipitated in 100 mL of methanol. The product was dried in
a vacuum; yield 0.96 g (96%). Pale yellow powder; 400 MHz
1H NMR (CDCl3): δ [ppm] ) 0.65 (s, 68-H), 0.71-2.05 (m,
1,6,7,16,26,27,28,38,39,41,46,47,48,54,55,58,59,60,61,62,64,65,-
67,69,70,71,72,73,74,75,76,77,78,79-H), 2.06-2.40 (m, 25,49-
H), 2.54 (m, 8-H), 3.79 (t, 5-H), 3.90 (m, 45-H), 4.02 (q, 37-H),
4.57 (m, 53-H), 5.34 (m, 63-H), 6.94 (d, 32,34-H), 7.10 (d, 31,-
35-H), 7.21 (t, 21-H), 7.40-7.54 (m, 19,20,22,23-H). IR (KBr):
λ-1 [cm-1] ) 2947, 2867 (aliph C-H), 1731 (CdO), 1654
(mesoion CdO); further signals at 1510, 1467, 1375, 1248,
1169, 1064, 1012, 960, 842, 754, 699. UV (dichloromethane):
Resu lts a n d Discu ssion
1,ω-Dibromoalkanes (9) were used for the synthesis
of mesoionic monomers with various spacer lengths. The
methacryl based ω-bromoalkane (10) was prepared
under phase-transfer conditions using potassium meth-
acrylate (8) and a 5-fold excess of 1,4-dibromobutane
(9). The distilled product (10) was boiled in alcoholic
solution with N-(4-ethoxyphenyl)-N′-phenylthiourea (11)
and worked up under basic conditions to form the
isothiourea structure (12). The mesoionic monomer (14)
was prepared using butylmalonic acids (13) and DCC
as condensation reagent (Scheme 3).
λ
max [nm] ) 264, 380. DSC: ∆Cp ) 54 °C. Peak ) 128 °C. SEC
(THF, PS standard): Mn ) 32 000, Mw ) 110 000, PD ) 3.5.
EA for n:m ) 1:4. Calcd: C, 76.02; H, 9.89; N, 1.00; S, 1.14.
Found: C, 75.16; H, 10.23; N, 1.09; S, 2.03.
For embedding the mesoionic functions into a liquid
crystalline environment, copolymers containing liquid
crystalline components were prepared. For this purpose,
the cholesteric methacrylate 19 was synthesized. This
monomer could be copolymerized statistically with the
mesoionic methacrylates and does not show any self-
absorption in the interesting wavelength region, which
could be used for irradiation. The cholesteric monomer
was obtained by a new synthetic route in high yield
(Scheme 4).
The copolymerization of the mesoionic monomer 14
with methyl methacrylate (20) was carried out in N,N-
dimethylformamide (DMF) as solvent using 2,2′-azobis-
(isobutyronitrile) (AIBN) as initiator. Probably due to
the long time of polymerization (24 h), the polydispersity
of polymer 21 was relatively high. Surprisingly, the
copolymerization of monomer 14 with the cholesteric
monomer 19 in DMF resulted in a mixture of the
homopolymers. Obviously, the cholesteric monomer has
a tendency to form aggregates in the polar DMF. In
contrast, by using tetrahydrofuran (THF) as solvent, a
nearly homogeneous statistical copolymer was obtained
(Scheme 5).
Cop olym er of Mesoion ic 1-(4-Eth oxyp h en yl)-2-[4-(iso-
p r op en ylca r bon yloxy)eth ylth io]-5-m eth yl-6-oxo-3-p h en -
yl-1,6-d ih yd r op yr im id in -3-iu m -4-ola te a n d 5-(Ch olester -
olca r bon yl)p en tyl Meth a cr yla te (29). 0.25 g (0.536 mmol)
of mesoionic 1-(4-ethoxyphenyl)-2-[2-(isopropenylcarbonyloxy)-
ethylthio]-5-methyl-6-oxo-3-phenyl-1,6-dihydropyrimidin-3-
ium-4-olate,4 0.75 g (1.32 mmol) of 5-(cholesterolcarbonyl)-
pentyl methacrylate, and 10.0 mg (0.0609 mmol) of AIBN were
dissolved in 5 mL of absolute THF. The mixture was degassed
by exposure to nitrogen bubbles for 10 min. Then the solution
was polymerized for 15 h at 60 °C, and the polymer was
precipitated in 100 mL of methanol. The product was dried in
a vacuum; yield 0.91 g (91%). Pale yellow powder; 400 MHz
For comparison, the homopolymer 254 of the me-
soionic monomer 1-(4-ethoxyphenyl)-2-(2-methacryleth-

7556 Theis and Ritter
Macromolecules, Vol. 36, No. 20, 2003
Sch em e 3. P a th w a y for P r ep a r a tion of th e Mesoion ic Meth a cr yla te
Sch em e 4. Syn th esis of th e Liqu id Cr ysta llin e Mon om er s
Sch em e 5. P r ep a r a tion of Mesoion ic Cop olym er s

Macromolecules, Vol. 36, No. 20, 2003
Photochemical Behavior of Polymeric Mesoions 7557
Sch em e 6. Ad d ition a l (Co)p olym er s
Ta ble 1. P r op er ties of th e P r ep a r ed Cop olym er s
chain length in position 5 of the mesoionic function led
to an increase of the cyclization rate. This can be
explained by the improved mobility, if fewer atoms have
to be moved. Thus, the methyl substituent in the
5-position allowed the highest conversion rate during
UV irradiation. Depending on the copolymer, the rate
constant could be enlarged by a factor of 2-3.
polymer
21
23
29
yield [%]
69
96
91
n:m (polymer)
Mn [g/mol]
Mw [g/mol]
PD
4:1
4:1
4:1
27000
400000
11
32000
110000
3.5
21000
36000
1.8
∆Cp (Tg) [°C]
peak (Tcl) [°C]
104
54
128
59
129
As expected, also a strong influence of the liquid
crystalline state was observed. The liquid crystalline
copolymers 23 and 29 permit a switching rate to the
bis(â-lactame) structure about a factor of 3.5-4.5 higher
than the analogous homopolymer 25 or copolymers 21
and 27 with methyl methacrylate, which do not show
any remarkable deviation to the homopolymer with
respect to the glass transition temperature and rate
constant. In this connection, less hindrance for the
photochemical rearrangement in the liquid crystalline
polymers is caused by the low glass transition temper-
ature next to the temperature, where the measurements
were achieved. Therefore, the quantum yield and reac-
tion rate are enlarged.
ylthio)-5-methyl-6-oxo-3-phenyl-1,6-dihydropyrimidinium-
4-olate4 and the copolymer 274 with methyl methacrylate
were used. Additionally, the copolymer 29 of 1-(4-
ethoxyphenyl)-2-(2-methacrylethylthio)-5-methyl-6-oxo-
3-phenyl-1,6-dihydropyrimidinium-4-olate4 and the cho-
lesteric monomer 19 was prepared (Scheme 6).
The data obtained from all new polymers are sum-
marized in Table 1.
The liquid crystalline phase of the cholesteric copoly-
mers was confirmed by polarizing microscopy. The
kinetic constant of the photoreaction of the different
polymers was determined in the solid state using spin-
coated films of a thickness of about 1.5 µm and a HeCd
laser (442 nm). The calculated first-order rate constants
k are summarized in Table 2.
However, the lowest photosensitivity was detected in
the case of the mesoionic polymer 31 obtained by
2-phenylpolyamidine.6 In this type of main chain poly-
mer, the photoreaction requires a certain rearrangement
of the backbone, which is strongly restricted at temper-
atures much below the glass transition at 160 °C.
By comparison of the rate constants, two different
main factors can be attributed to the speed of cycliza-
tion. Similar to the photoreaction of mesoionic pyrimi-
dinium-olates in solution,1 the reduction of the alkyl
F igu r e 2. Temperature-dependent cyclization kinetics of the copolymer (27) prepared from mesoionic 1-(4-ethoxyphenyl)-2-[2-
(isopropenylcarbonyloxy)ethylthio]-6-oxo-3-phenyl-5-propyl-1,6-dihydropyrimidin-3-ium-4-olate and methyl methacrylate at different
temperatures.

7558 Theis and Ritter
Macromolecules, Vol. 36, No. 20, 2003
Ta ble 2. Ra te Con sta n t k for th e P h otor ea ction of Differ en t P olym er s
According to the above-described results, the reaction
rate of mesoionic polymers should generally be increas-
able simply by heating the polymer films. This feature
was checked for example by measuring the reaction rate
of copolymer 27 at different temperatures between 30
and 160 °C (Figure 2).
Up to a temperature of about 100 °C, only a slight
increase of the rate constant could be observed. Above

Macromolecules, Vol. 36, No. 20, 2003
Photochemical Behavior of Polymeric Mesoions 7559
this temperature, the increase is accelerated up to about
120 °C, where acceleration stops and the increase
remains nearly constant at a high level. The region, in
which the course increases, is lying within the interval
of the glass transition temperatures of the mesoionic
polymer 27 (117 °C) and the corresponding photocon-
verted bis(â-lactame) polymer 28 (101 °C).
This temperature-dependent behavior could be ex-
plained by the fact that the mobility of mesoionic
functions is restricted by the rigid environment below
the glass transition temperature. Reaching the glass
transition temperature by heating, the free volume of
the polymer is typically increased, so the photoreaction
rate is significantly enlarged. Above 160 °C, the polymer
decomposes. It was therefore not possible to measure
the rate constants at higher temperatures. Generally,
the reaction rate could be enlarged by factor 8 simply
by heating the polymer.
polymers caused a strong increase of the rate constant.
In addition, heating of a polymer film with a Tg of 104
°C caused an acceleration of the photoreaction above the
glass transition temperature up to a factor of 8. These
results have clearly shown that the photochemical
induced photocyclization of mesoionic polymers is strongly
controlled by the mobility of the polymer segments,
especially the polymers with liquid crystalline behavior
showing relatively high rate constants.
Ack n ow led gm en t. We thank M. Deptolla (Institute
of Organic Chemistry, University of Mainz) for her great
assistance with the synthesis and S. Mittler and B.
Menges (Max Planck Institute for Polymer Research,
Mainz) for their support with the HeCd laser.
Refer en ces a n d Notes
(1) Ritter, H.; Sperber, R.; Weisshuhn, C. M. Macromol. Chem.
Phys. 1994, 195, 3823.
(2) Ritter, H.; Sperber, R. Macromol. Rapid Commun. 1995, 16,
Con clu sion s
Novel mesoionic methacrylates with various spacer
lengths were synthesized by a new route and character-
ized. Copolymers with methyl methacrylate and cho-
lesteric monomers were prepared by radical polymeri-
zation in organic solvents; the cholesteric copolymer
showed liquid crystalline properties above the Tg of 54
°C. The polymers reached molecular weights up to Mn
) 21 000-32 000 and a polydispersity range of 1.8-11.
By determination of the rate constant of the photocy-
clization to the bis(â-lactame) structure, it was shown
that a decrease of the length of the alkyl side chain on
position 5 of the pyrimidinium-olate and the embedding
of the mesoionic chromophore into liquid crystalline
407.
(3) Deutschmann, T.; Ritter, H. Macromol. Chem. Phys. 2000,
201, 1200.
(4) Theis, A.; Ritter, H. Des. Monomers Polym. 2001, 4, 177.
(5) Deutschmann, T.; Ritter, H. Macromol. Rapid Commun.
1996, 17, 723.
(6) Theis, A.; Ritter, H.; Bo¨hme, F.; Klinger, C.; Menges, B.;
Mittler, S. Chem. Mater. 2002, 14, 2109.
(7) Tonami, H.; Uyama, H.; Kobayashi, S.; Menges, B.; Mittler,
S.; Theis, A.; Ritter, H. Macromol. Chem. Phys. 2003, 204,
1079.
(8) Gotthardt, H.; Schenk, K. H. J . Chem. Soc., Chem. Commun.
1986, 687.
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