Synthesis of polymeric 1-iminopyridinium ylides as photoreactive polymers

Article abstract of DOI:10.1002/pola.23832

Two synthetic routes to polymeric 1-imino pyridinium ylides as new photoreactive polymeric architectures were investigated. In the first approach, polymerization of newly synthesized 1-imino pyridinium ylide containing monomers yielding their polymeric analogues was achieved by free radical polymerization. Alternatively, reactive precursor polymers were synthesized and converted into the respective 1-imino pyridinium ylidepolymers by polymer analogous reactions on reactive precursor polymers. Quantitative conversion of the reactive groups was achieved with pentafluorophenyl ester containing polymers and newly synthesized photoreactive amines as well as by the reaction of poly(4-vinylbenzoyl azide) with a photoreactive alcohol. The polymers obtained by both routes were examined regarding their photoreaction products and kinetics in solution as well as in thin polymer films. Contact angle measurements of water on the polymer films before and after irradiation showed dramatic changes in the hydrophilicity of the polymers.

Full text of DOI:10.1002/pola.23832

Synthesis of Polymeric 1-Iminopyridinium Ylides
as Photoreactive Polymers
DANIEL KLINGER,1* KATJA NILLES,1 PATRICK THEATO1,2
1Institute of Organic Chemistry, Johannes Gutenberg-Universitat Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
¨
2School of Chemical and Biological Engineering, WCU program of Chemical Convergence for Energy and Environment (C2E2),
College of Engineering, Seoul National University, 151-744 Seoul, Korea
Received 23 July 2009; accepted 7 November 2009
DOI: 10.1002/pola.23832
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Two synthetic routes to polymeric 1-imino pyridin-
ium ylides as new photoreactive polymeric architectures were
investigated. In the first approach, polymerization of newly syn-
thesized 1-imino pyridinium ylide containing monomers yielding
their polymeric analogues was achieved by free radical polymer-
ization. Alternatively, reactive precursor polymers were synthe-
sized and converted into the respective 1-imino pyridinium ylide
polymers by polymer analogous reactions on reactive precursor
polymers. Quantitative conversion of the reactive groups was
achieved with pentafluorophenyl ester containing polymers and
newly synthesized photoreactive amines as well as by the reac-
tion of poly(4-vinylbenzoyl azide) with a photoreactive alcohol. The
polymers obtained by both routes were examined regarding their
photoreaction products and kinetics in solution as well as in thin
polymer films. Contact angle measurements of water on the poly-
mer films before and after irradiation showed dramatic changes in
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the hydrophilicity of the polymers.
2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 48: 832–844, 2010
KEYWORDS: change of hydrophilicity; functionalization of poly-
mers; photochemistry; photoisomerization; polymer analogous
reaction; radical polymerization; reactive polymers; stimuli
responsive polymers
INTRODUCTION In recent years, much effort has been
devoted to the synthesis of stimuli-responsive polymers for a
broad variety of applications. Stimuli-responsive materials
are able to change certain properties in response to specific
external stimuli, such as temperature, pH, or light.1–8 Among
them, light represents an outstanding stimulus as it can be
applied in a very precise manner by selecting suitable wave-
lengths, polarization directions, and intensities in a noncon-
tact approach, respectively. In addition, light offers the op-
portunity to change the polymer properties in very confined
spaces and, hence, can be used to create patterns in polymer
thin films with nanometer precision, e.g., for potential use in
optical data storage.9–12 Regarding the photochemistry of
polymers, light induced changes in solubility are of special
interest.13 Different photosensitive groups attached to a poly-
mer backbone are known to alter their polarity by irradia-
tion with light and, hence, influence the overall polymer sol-
ubility. As an example for the reversible change in polarity,
spirobenzopyrane moieties undergo an isomerization process
from the colorless closed spiro isomer to the open inten-
sively colored merocyanine isomer under irradiation with
UV-light.14 This change is characterized by an increase in po-
larity and is completely reversible by thermal relaxation or
irradiation with visible light.14–16
Irreversible transformation of polarity can be achieved by
different approaches, whereby most of them include the
detachment of a molecule fragment resulting in a polar moi-
ety left on the polymer. Hereby, o-nitrobenzylic esters repre-
sent photocleavable groups which generate carboxylic acid
moieties on the polymer backbone upon irradiation.17,18 Fur-
thermore, pyrenylmethyl ester side groups on polymers19, 2-
diazo-1,2-naphtoquinone (DNQ) based amphiphiles20 and
azido-functionalized polystyrenes21,22 have been reported in
the literature. Only little attention has been paid to the irre-
versible photochemical reaction of 1-imino pyridinium ylides,
which is accompanied by a dramatic change in hydrophilicity
without fragmentation of the molecule. As shown in Scheme
1, two reaction pathways compete in this photoreaction23–27
:
(a) The ring expansion via an excited singlet state leads to
the corresponding 1,2-diazepines that exhibit a hydrophobic
character and (b) the cleavage of the NAN bond under the
detachment of pyridine via a triplet state leads to reactive
nitrenes and isocyanato groups.
The ratio of diazepine formation versus NAN bond cleavage
strongly depends on the substituent on the side of the car-
bonyl group opposite to the ylide nitrogen atoms. Heteroa-
toms, such as oxygen or nitrogen favor the diazepine
Additional Supporting Information may be found in the online version of this article. *Present address: Max Planck Institute for Polymer Research,
Ackermannweg 10, D-55128 Mainz, Germany. Correspondence to: P. Theato (E-mail: theato@uni-mainz.de)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 832–844 (2010) 2010 Wiley Periodicals, Inc.
832
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ARTICLE
guished by their good solubility in a broad variety of organic
solvents and their fast and quantitative reaction with amines
at room temperature.35–38 Additionally, for the reaction with
alcohols as nucleophiles, poly(4-vinylbenzoyl azide) as an
isocyanato groups generating polymer has recently been
reported by our group.39
The aim of this work is the synthesis of novel 1-imino pyri-
dinium ylide groups containing polymers. Two different syn-
thetic pathways will be investigated and compared: (i) syn-
thesis of new monomers containing 1-imino pyridinium ylide
groups and their free radical polymerization, and (ii) synthe-
sis of reactive precursor polymers and their conversion into
polymeric 1-imino pyridinium ylides by reactions on the
polymers.
EXPERIMENTAL
Materials
All chemicals were commercially available and used without
further purification unless otherwise stated. Anhydrous THF
and dioxane were distilled over sodium and benzophenone.
CHCl3 was dried over CaCl2 and phosphorous pentoxide and
freshly distilled before use. AIBN was recrystallized from
diethylether. 4-Vinylbenzoylazide39 as well as the activated
ester monomers pentafluorophenyl methacrylate,35 penta-
fluorophenyl acrylate,36 and pentafluorophenyl 4-vinylben-
zoate37 were synthesized according to the literature.
SCHEME 1 Photoreactions of 1-imino pyridinium ylides.
formation, while compounds with aliphatic substituents react
mainly under pyridine elimination.26,28,29 As such, the reac-
tion pathway of ylides can be controlled by careful design of
the chemistry. As mesoionic functionalities, 1-imino pyridin-
ium ylides represent very hydrophilic groups, whereas the
formed 1,2-diazepines are rather hydrophobic. In comparison
the generated nitrenes or isocyanates may lead to further
functionalization. On the basis of these considerations, poly-
meric 1-imino pyridinium ylides represent a very interesting
class of photosensitive polymers, because their photoreaction
can be tuned to yield either of the two different products
resulting in different polymer properties.
Instrumentation
1H and 19F NMR spectra were measured using a Bruker AMX
300 at 300 MHz. FTIR measurements were performed using
a Bruker Vector 22. UV-vis measurements were performed
using a Shimadzu UV 2102 PC. GPC measurements were per-
formed using tetrahydrofuran (THF) as eluant (flow rate ¼ 1
mL/min) at 25 ꢀC using a light scattering and RI detector.
Polystyrene standards were used for calibration.
1-Aminopyridinium Iodide (1API)
The procedure of Go¨sl and Meuwsen was followed. Briefly,
40
Up to now, two different ways to synthesize polymeric 1-im-
ino pyridinium ylides are known. Taylor et al.30 presented
the polymerization of a monomer based on the reaction of
isocyanatoethyl methacrylate with 1-aminopyridinium ylide
and investigated the photoreaction of the polymer and its
potential use as an aqueous photoresist system. On the other
hand, Kondo et al.31 reported on the synthesis of polymeric
pyridinium ylides by the reaction of poly(4-vinylpyridine)
with a nitrene that was formed in situ by pyrolysis of ethyl
azidoformate.
three equivalents of pyridine were added to a freshly pre-
pared solution of one equivalent hydroxylamine-O-sulfonic
ꢀ
acid in cold water. After heating the mixture to 90 C for 20
min and cooling it back down to room temperature, one
equivalent of potassium carbonate was added. Water and
excess pyridine were removed under reduced pressure and
after the treatment of the residue with ethanol, the insoluble
precipitate was removed by filtration. The reaction mixture
was acidified with 57% hydroiodic acid. By storing the
ꢀ
resulting solution for 1 h at ꢁ20 C, yellowish needles sepa-
rated. Recrystallization from ethanol yielded 70% of pure
In general, the polymerization of functional monomers to
yield high molecular weight polymers can be difficult. There-
fore, alternative preparation routes utilizing reactive precur-
sor polymers have been established. After synthesis of a re-
active prepolymer, a big variety of functionalities can be
introduced by polymer analogous reactions. Activated esters
are well established in peptide chemistry32 and the concept
was successfully transferred to the preparation of reactive
polymers.33,34 Hereby, pentafluorophenyl esters are distin-
ꢀ
1API as pink crystals. mp 160–161 C.
1H NMR (D2O): d ¼ 8.73 (d, J ¼ 6.6 Hz, 2H), 8.35 (t, J ¼ 7.9
Hz, 1H), 7.99 (t, J ¼ 7.4 Hz, 2H), 4.79 (br s, 2H). 13C NMR
(D2O): d ¼ 141.74, 140.14, 128.00.
Synthesis of Monomers
The structures of the monomers M1, M2, and M3 are given
in Scheme 2.
SYNTHESIS OF POLYMERIC 1-IMINOPYRIDINIUM YLIDES, KLINGER, NILLES, AND THEATO
833

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Photoreactive monomers based on 1-imino pyridinium ylides.
{[2-(Metacryloyloxy)ethoxy]carbonyl}(pyridinium-1-yl)
azanide (M1)
10.9 Hz, 1H), 5.79 (d, J ¼ 17.6 Hz, 1H), 5.26 (d, J ¼ 10.9 Hz,
1H). 13C NMR (DMSO-d6): d ¼ 168.45, 143.51, 138.44,
138.22, 137.43, 136.57, 128.09, 126.75, 125.55, 114.97. MS
(FD) m/z 224 (Mþ). FTIR (cmꢁ1): 1580 (C¼¼O).
A solution of hydroxyethyl methacrylate (HEMA) (2.60 g, 20.0
mmol) in THF (20 mL) was added dropwise to a stirred solu-
tion of carbonyldiimidazole (CDI) (3.36 g, 20.7 mmol) in anhy-
drous THF (50 mL). The reaction mixture was stirred for 4 h
at room temperature and was added to a vigorously stirred
suspension of 1API (5.00 g, 22.5 mmol) and potassium car-
bonate (25.00 g, 0.195 mol) in 50 mL of anhydrous THF. After
stirring for additional 4 h, the inorganic salts were separated
by filtration and the solvent was removed under reduced pres-
sure. The deep blue residue was purified by column chroma-
tography over silica using CHCl3/MeOH (20:1) as eluant and
yielded 95% of M1 (4.92 g, 19.7 mmol) as a yellow oil.
Pyridinium-1-yl[(4-vinylphenyl)carbamoyl]azanide (M3)
4-Vinylbenzoyl azide (2.00 g, 11.5 mmol) was dissolved in 8
mL of anhydrous 1,4-dixoane and added dropwise under vig-
orous stirring to 20 mL of anhydrous 1,4-dioxane at 90—100
ꢀC. The solution was stirred at this temperature for 3 h. The
solvent was evaporated and the yellow oil of the isocyanate
was dissolved in 10 mL of anhydrous acetone. Meanwhile,
1API (2.80 g, 12.6 mmol) together with potassium carbonate
(14.00 g, 0.1013 mol) were grinded and suspended in 40 mL
of anhydrous acetone. The suspension turned deep purple af-
ter 2 h of stirring at room temperature. The solution of the
isocyanate was added dropwise under nitrogen atmosphere to
the so prepared solution of the deprotonated 1API. After stir-
ring for 3 h at room temperature, the inorganic solids were
removed by filtration and the obtained deep blue colored solid
was extracted three times with 250 mL of chloroform. The fil-
trates were combined and after evaporation of the solvents,
the remaining solid was purified by column chromatography
over silica using chloroform/methanol (5:1) as eluant. M3
(1.23 g, 5.1 mmol) was obtained in 44% yield after recrystalli-
1H NMR (CDCl3): d ¼ 8.81 (d, J ¼ 7.1 Hz, 2H), 7.79 (t, J ¼
7.2 Hz, 1H), 7.57 (t, J ¼ 7.2 Hz, 2H), 6.12 (s, 1H), 5.52 (s,
1H), 4.40–4.30 (m, 4H), 1.91 (s, 3H). 13C NMR (DMSO-d6): d
¼ 166.64, 143.21, 137.46, 135.91, 126.83, 126 02, 63.75,
61.47, 18.11. MS (FD) m/z 250 (Mþ). FTIR (cmꢁ1): 1717
(C¼¼O), 1635 (C¼¼O).
Pyridinium-1-yl-(4-vinylbenzoyl)azanide (M2)
A solution of 4-vinylbenzoic acid (6.00 g, 40.0 mmol) in THF
(30 mL) was added dropwise to a stirred solution of CDI (6.62
g, 41.0 mmol) in anhydrous THF (80 mL). After the addition of
5 drops of triethylamine, the reaction mixture was stirred for 5
h at room temperature and was then added to a vigorously
stirred suspension of 1API (9.83 g, 44.3 mmol) and potassium
carbonate (48.00 g, 0.347 mol) in 50 mL of anhydrous THF.
Stirring for additional 4 h was followed by removing the sol-
vent under reduced pressure and the addition of 250 mL of
CHCl3 to the deep blue residue. The inorganic salts were sepa-
rated and the remaining solution was reduced on a rotary
evaporator. Purification of the residue was first done by col-
umn chromatography over silica using CHCl3/MeOH (12:1) as
an eluant and was followed by recrystallization of the obtained
solid out of acetone. Seventy-four percent of M2ꢀ(6.67 g, 29.7
mmol) were obtained as yellow needles. mp 186 C.
ꢀ
zation out of acetone. mp 188 C.
1H NMR (CDCl3): d ¼ 8.80 (d, J ¼ 7.0 Hz, 2H), 7.74 (t, J ¼
7.7 Hz, 1H), 7.55 (t, J ¼ 7.2 Hz, 2H), 7.41 (d, J ¼ 8.7 Hz, 2H),
7.29 (d, J ¼ 8.7 Hz, 2H), 6.63 (dd, Jtrans ¼ 17.6 Hz, Jcis
¼
10.9 Hz, 1H), 6.55 (br s, 1H), 5.59 (d, J ¼ 17.6 Hz, 1H), 5.07
(d, J ¼ 10.9 Hz, 1H). 13C NMR (DMSO-d6): d ¼ 161.64,
142.76, 142.68, 136.77, 135.59, 128.43 126.45, 126.41,
117.23, 110.44. MS (FD) m/z 239 (Mþ). FTIR (cmꢁ1): 1621
(C¼¼O), 1575 (NAH).
Synthesis of Functionalized 1-Imino Pyridinium Ylides
Four different ylides have been synthesized. The structures
are presented in Scheme 3.
Y1
1H NMR (CDCl3): d ¼ 8.79 (d, J ¼ 7.0 Hz, 2H), 8.09 (d, J ¼
8.3 Hz, 2H), 7.87 (t, J ¼ 7.7 Hz, 1H), 7.63 (t, J ¼ 7.2 Hz, 2H),
The free amine of 1API was generated in situ by stirring one
equivalent of 1API with two equivalents of triethylamine in
the respective solvent for 1 h at room temperature.
7.43 (d, J ¼ 8.2 Hz, 2H), 6.74 (dd, Jtrans ¼ 17.6 Hz, Jcis
¼
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ARTICLE
SCHEME 3 Amino- and hydroxy-functionalized pyridinium ylides for reactions on reactive precursor polymers.
[(2-Hydroxyethoxy)carbonyl](pyridinium-1-yl)azanide (Y4)
1API (10.00 g, 45.0 mmol) and potassium carbonate (50.00
g, 0.362 mol) were grinded and suspended in 90 mL of anhy-
drous THF. After stirring for 2 h at room temperature under
nitrogen, the color turned deep blue. A solution of ethylene
carbonate (4.05 g, 46.0 mmol) in THF (20 mL) was added
dropwise to the suspension. The reaction mixture was stirred
for 2 days at room temperature and for an additional day at
50 ꢀC. The solvent was then evaporated and the deep blue
residue was treated with 250 mL of chloroform. After filtra-
tion of the suspension, the solvent was removed under
reduced pressure and the remaining blue solid was purified
by column chromatography using methanol as eluant. Recrys-
tallization of the so obtained red solid out of acetone yielded
was removed under reduced pressure and the obtained beige
solid was suspended for 1 h in anhydrous ethanol. The insolu-
ble salts were removed and the solvent was evaporated. Y2
(0.652 g, 3.13 mmol) was obtained as an orange colored solid
ꢀ
in 74% yield. mp 186 C.
1H NMR (MeOD-d4): d ¼ 8.62 (d, J ¼ 8.2 Hz, 2H), 8.14 (t, J
¼ 7.8 Hz, 1H), 7.84 (t, J ¼ 8.6 Hz, 2H), 3.23 (t, J ¼ 6.7 Hz,
2H), 3.14 (t, J ¼ 6.7 Hz, 1H), 2.70 (t, J ¼ 6.9 Hz, 1H), 1.59 (s,
4H). 13C NMR (MeOD-d4): d ¼ 145.67, 139.18, 127.92, 42.38,
41.56, 31.37, 29.00. FT-IR (cmꢁ1): 1585 (C¼¼O), 1470 (NAH).
[(2-{[(2-Aminoethyl)caramoyl]oxy}ethoxy)carbonyl]
(pyridinium-1-yl)azanide (Y3)
CDI (0.921 g, 5.57 mmol) was dissolved in 20 mL of anhy-
drous chloroform and a solution of Y4 (1.028 g, 5.64 mmol) in
chloroform was added slowly. After 5 h of stirring at room
temperature, the reaction mixture was added over a period of
1 h to a solution of 1,2-diaminobutane (0.518 g, 8.63 mmol) in
the same solvent. The reaction mixture was stirred over night,
the solvent was evaporated and the remaining solid was puri-
fied by column chromatography over silica using methanol as
eluant. The product was obtained in 27% yield (0.403 g, 1.50
mmol) as a yellow-colored oil.
ꢀ
66% of Y4 (5.38 g, 29.5 mmol) as yellow needles. mp 118 C.
1H NMR (CDCl3): d ¼ 8.78 (d, J ¼ 7.1 Hz, 2H), 7.82 (t, J ¼
7.7 Hz, 1H), 7.60 (t, J ¼ 7.2 Hz, 2H), 4.25–4.20 (m, 2H), 3.82
(m, 2H), 3.42 (br s, 1H). 13C NMR (CDCl3): d ¼ 164.19,
142.73, 136.03, 126.13, 66.82, 62.37. MS (FD) m/z 182
(Mþ). FTIR (cmꢁ1): 1635 (C¼¼O), 1602 (NAH).
[(4-Aminobutyl)carbamoyl](pyridinium-1-yl)azanide (Y2)
A solution of N-BOC-1,4-diaminobutane (1.367 g, 7.26 mmol)
in 5 mL of anhydrous THF was added dropwise to a stirred
solution of CDI (1.193 g, 7.36 mmol) in 15 mL of anhydrous
THF. The reaction mixture was stirred for 4 h at room tem-
perature and was then added to a suspension of 1API (1.750
g, 7.88 mmol) and potassium carbonate (8.750 g, 63.31
mmol) in THF. After stirring the mixture for 4 h at room tem-
perature and removing the solvent, the remaining deep blue
solid was suspended for 1 h in chloroform. The insoluble
inorganic salts were removed by filtration and the solvent
was evaporated. For purification, column chromatography
using methanol as eluant was followed by recrystallization
from acetone. The obtained crystals were dissolved in deion-
ized water and concentrated hydrochloric acid (9.58 g, 97.26
mmol) was added under cooling the solution in an ice bath.
1H NMR (CDCl3): d ¼ 8.67 (d, J ¼ 7.0 Hz, 2H), 7.73 (t, J ¼
7.7 Hz, 1H), 7.54 (t, J ¼ 7.1 Hz, 2H), 6.31 (br t, J ¼ 5.7 Hz,
1H), 4.13 (s, 4H), 3.04 (q, J ¼ 5.9 Hz, 2H), 2.63 (t, J ¼ 5.9
Hz, 2H), 1.90 (br s, 2H). 13C NMR (CDCl3): d ¼ 163.24,
156.81, 142.53, 135.66, 126.08, 63.59, 62.67, 43.82, 41.72.
FTIR (cmꢁ1): 1706 (C¼¼O), 1628 (C¼¼O), 1538 (NAH).
Polymerizations
For all polymerizations, the monomers and 0.5 mol % of the
respective initiator were solubilized in approximately 0.5 mL
of the particular solvent for every 100 mg of monomer. The
mixture was degassed by three freeze–thaw cycles and
stirred under argon atmosphere for a predetermined poly-
merization time at a constant temperature. The polymeriza-
tions were terminated by rapid cooling and freezing. The
homopolymers were purified by precipitation in a respective
solvent. The crude polymers were dissolved in a good sol-
vent, precipitated again, and finally dried in vacuum.
ꢀ
The mixture was heated slowly up to 50 C and stirred for 20
h at this temperature. After the successful deprotection of the
amino group, the mixture was cooled down to 2 ꢀC and
treated with potassium carbonate until pH ꢂ 11. The water
SYNTHESIS OF POLYMERIC 1-IMINOPYRIDINIUM YLIDES, KLINGER, NILLES, AND THEATO
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Conditions of the Polymerizations of the Ylide Monomers M1, M2, and M3
Polymer
Monomer
Solvent
Initiator
T (ꢀC)
t (h)
Precipitated In
Yield (%)
PM1
PM2
PM3
M1
M2
M3
NMP
NMP
NMP
AIBN
AIBN
AIBN
70
70
70
20
42
42
Acetone
Acetone
THF
87
72
14
Free Radical Polymerizations of the Photoreactive
Monomers
Table 1 summarizes the synthetic details of the different
reactions.
Free Radical Polymerization of 4-Vinylbenzoyl Azide
(4VBA)
1H NMR (CDCl3): d ¼ 7.61 (br s, 2H); 6.40 (br s, 2H); 1.75–
1-14 (br m, 3H). FTIR (cmꢁ1): 2138 (N3), 1688 (C¼¼O). GPC
(THF): Mn ¼ 31,500 g/mol. Mw/Mn ¼ 2.4.
Free Radical Polymerization of M1
1H NMR (DMSO-d6): d ¼ 8.76 (br s, 2H); 8.01 (br s, 1H);
7.79 (br s, 2H); 4.10 (br s, 4H); 2.24–0.39 (br m, 5H). FTIR
(cmꢁ1): 1725 (C¼¼O), 1635 (C¼¼O).
Polymer Analogous Reactions
Polymer analogous reactions were carried out under nitro-
gen atmosphere in reaction tubes sealed with a rubber sep-
tum. The respective polymer was dissolved in a small
amount of the appropriate solvent (Solvent 1) and added to
the reaction tube. A solution of the functionalized pyridinium
ylide in an appropriate solvent (Solvent 2) miscible with the
solvent for the polymer (Solvent 1) was then added to the
polymer solution. If necessary, the reaction phase was kept
homogeneous by adjusting the solvent polarity by adding
supplemental Solvent 2 in small portions. Conversion of the
reactive groups was monitored by FTIR spectroscopy. After
completion of the reaction, the polymer was precipitated,
dissolved and precipitated again. For further purification size
exclusion chromatography with Sephadex LH-20 as station-
ary phase was used. Scheme 4 shows the different reactions
and Table 3 summarizes the synthetic details.
Free Radical Polymerization of M2
1H NMR (DMSO-d6): d ¼ 8.78 (br s, 2H); 8.03 (br s, 1H);
7.75 (br s, 4H); 6.66 (br s, 2H); 1.83 (br s, 3H). FTIR
(cmꢁ1): 1672 (C¼¼O).
Free Radical Polymerization of M3
1H NMR (DMSO-d6): d ¼ 8.79 (br s, 2H); 7.88 (br s, 1H);
7.70 (br s, 2H); 7.35 (br s, 2H); 6.60 (br s, 2H); 1.52 (br s,
3H). FTIR (cmꢁ1): 1632 (C¼¼O), 1514 (NAH).
Free Radical Polymerizations of Activated Ester
Monomers
Table 2 summarizes the synthetic details of the different
reactions.
Free Radical Polymerization of Pentafluorophenyl
Methacrylate (PFPMA)
Polymer Analogous Reaction of PPFPMA with Y1 (PY1a)
1H NMR (MeOD-d4/dioxane-d8 (3:4)): d ¼ 9.30–7.67 (br m,
5H); 3.26–0.86 (br m, 13.5 H).
1H NMR (CDCl3): d ¼ 2.71–1.89 (br m, 2H); 1.61–1.17 (br m,
3H). FTIR (cmꢁ1): 1776 (C¼¼O). GPC (THF): Mn ¼ 10,700 g/
mol. Mw/Mn ¼ 1.9.
Polymer Analogous Reaction of PPFPMA with Y2 (PY2a)
1H NMR (MeOD-d4/dioxane-d8 (3:1)): d ¼ 8.66 (br s, 2H);
8.08 (br s, 1H); 7.82 (br s, 2H); 7.82 (br s, 2H); 3.23 (br s,
4H); 2.73–0.72 (br m, 18 H).
Free Radical Polymerization of Pentafluorophenyl
Acrylate (PFPA)
1H NMR (CDCl3): d ¼ 3.07 (br, 1H); 2.09 (br, 2H). FTIR
(cmꢁ1): 1781 (C¼¼O). GPC (THF): Mn ¼ 3200 g/mol. Mw/Mn
¼ 1.7.
Polymer Analogous Reaction of PPFPMA with Y3 (PY3a)
1H NMR (DMSO-d6/MeOD-d4 (7:1)): d ¼ 8.87 (br s, 2H);
8.04 (br s, 1H); 7.79 (br s, 2H); 7.25 (br s, 1H); 4.07 (br s,
5H); 3.04 (br s, 4H); 2.19–0.28 (br m, 6.1H).
Free Radical Polymerization of Pentafluorophenyl
Vinylbenzoate (PFPVB)
1H NMR (CDCl3): d ¼ 8.03–7.57 (br, 2H); 6.96–6.35 (br, 2H);
2.26–1.12 (br, 3H). FTIR (cmꢁ1): 1760 (C¼¼O). GPC (THF):
Mn ¼ 16,340 g/mol. Mw/Mn ¼ 2.1.
Polymer Analogous Reaction of PPFPA with Y1 (PY1b)
1H NMR (MeOD-d4): d ¼ 8.82 (br s, 2H); 8.14 (br s, 1H);
7.78 (br s, 2H); 2.89 (br s, 1H); 2.38–1.60 (br m, 2H).
TABLE 2 Conditions of the Polymerizations of the Activated Ester Monomers
Polymer
Monomer
Solvent
Initiator
T (ꢀC)
t (h)
Precipitated In
Yield (%)
PPFPMA
PPFPA
PPFPVB
P4VBA
a
Pentafluorophenyl methacrylate
Pentafluorophenyl acrylate
Pentafluorophenyl vinylbenzoate
4-Vinylbenzoyl azide
THF
THF
THF
THF
AIBN
AIBN
AIBN
V-70a
60
60
60
32
20
20
8
Methanol
Methanol
Methanol
Methanol
43
80
76
48
16
V-70: 2,20-Azobis(4-methoxy-2,4-dimethyl valeronitrile).
836
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ARTICLE
SCHEME 4 Reactions on reactive
precursor polymers to yield the re-
spective photoreactive polymers.
1
Polymer Analogous Reaction of PPFPVB with Y3 (PY3B)
1H NMR (DMSO-d6): d ¼ 8.71 (br s, 2H); 8.38 (br s, 2H);
8.01 (br s, 1H); 7.74 (br s, 2H); 7.64–7.22 (br m, 3H); 6.49
(br s, 1H); 4.08 (br s, 4H); 3.15 (br s, 4H); 2.27–0.87 (br s,
3H).
Photoisomerization Studies via H NMR Spectroscopy
Twenty to thirty milligram of each sample were dissolved in
1 mL of deuterated solvent. The solution was placed in a
quartz cuvette and then irradiated for 24 h with 500 W at
the wavelengths of 315–390 nm. The reaction mixture was
added to an NMR tube, sealed and directly placed in a 300
MHz NMR spectrometer.
Polymer Analogous Reaction of P4VBA with Y4 (PY4)
1H NMR (DMSO-d6): d ¼ 9.50 (br s, 1H); 8.72 (br s,
2H); 7.98 (br s, 1H); 7.71 (br s, 2H); 7.18 (br s,
2H); 6.46 (br s, 2H); 4.16 (br s, 4H); 2.23–0.50 (br s,
3H).
Kinetic UV-Vis Measurements
Kinetic measurements were performed by irradiating a solu-
tion of each sample for 1 min at 315–390 nm before every
measurement.
SYNTHESIS OF POLYMERIC 1-IMINOPYRIDINIUM YLIDES, KLINGER, NILLES, AND THEATO
837

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 Conditions of the Postpolymerization Functionalization of the Polymeric-Activated Esters Yielding Polymeric Ylides
Precursor
Polymer
Functionalized
Ylide
Solvent 2
(Total)
Yield
(%)
Product
PY1a
Solvent 1
T (ꢀC)
t (h)
Precipitated In
THF
PPFPMA
Dioxane
2.4 mL
Dioxane
2.4 mL
CHCl3
Y1 (1API þ NEt3)
0.95 mmol
Y2
Methanol
1.2 mL
Methanol
3.6
25a
50a
25a
50a
50c
25c
25
24
72
24
72
48
120
12
78
77
75
78
74
34
0.79 mmol PFPMA
PPFPMA
PY2a
PY3a
PY1b
PY3b
PY4
a
Acetone
THF
0.64 mmol PFPMA
PPFPMA
1.00 mmol
Y3
Methanolb
2.98 mmol PFPMA
PPFPA
3.0 mL
DMSO
1.0 mL
CHCl3
3.7 mmol
Y1 (1API þ NEt3)
0.91 mmol
Y3
1 mL
DMSO
1.0 mL
CHCl3
Acetone
Acetone
Acetone
0.84 mmol PFPA
PPFPVB
25
70
12
4d
0.32 mmol
P4VBA
1.5 mL
DMF
0.37 mmol
Y4
1.5 mL
–
1.15 mmol
4 mL
1.39 mmol
d
Addition was performed at room temperature (25 ꢀC), after 24 h the
reaction temperature was increased to 50 ꢀC.
After 4 h unreacted isocyanato groups were quenched by adding 0.5
mL of anhydrous methanol.
b
Triethylamine was added (0.37 mmol).
c
Addition was performed at 50 ꢀC, after 24 h the reaction temperature
was decreased to room temperature (25 ꢀC).
RESULTS AND DISCUSSION
tive of M2 as a carbon based substituent should influence
the photoreaction to yield a significant amount of pyridine
elimination.24,26,28,29 M141 was synthesized by carbonyldiimi-
dazole (CDI) activated coupling of hydroxyethyl methacrylate
(HEMA), with 1-aminopyridinium iodide (1API) under basic
conditions. The ethyl spacer group was introduced to pre-
vent deactivation of the double bond due to conjugation
effects with the pyridinium ylide ring. For the synthesis of
M2, 4-vinyl benzoic acid was also activated with CDI and
allowed to react with the free amine of 1API. The synthesis
of M3 started with 4-vinylbenzoyl azide39 (4VBA), which is
known to undergo a Curtius rearrangement at elevated tem-
peratures and thereby rearranges into the corresponding iso-
cyanate. The in situ formed isocyanate was then reacted with
the free amine of 1API yielding the urethane M3.
Two synthetic routes to obtain polymeric 1-imino pyridinium
ylides were investigated. First, three different monomers
containing photoreactive 1-imino pyridinium ylide groups
were synthesized and successfully polymerized by free radi-
cal polymerization. Second, functional molecules containing
pyridinium ylide groups were attached to reactive precursor
polymers via polymer analogous reactions. For this purpose,
three different activated ester polymers, namely poly(penta-
fluorophenyl methacrylate), poly(pentafluorophenyl acrylate),
poly(pentafluorophenyl 4-vinylbenzoate) as well as poly(4-
vinylbenzoyl azide) as an isocyanato group generating poly-
mer were synthesized by free radical polymerization. Two
different photoreactive amines and one alcohol were synthe-
sized to attach the 1-imino pyridinium ylide groups to the
reactive polymers mentioned earlier. The reactions on the
polymers were studied with the focus on the conversion of
reactive groups. All polymers obtained by both synthetic
routes were examined in respect to their photoreaction.
Especially, the kinetics of the photoreactions in the polymers
were examined by UV-vis spectroscopy and compared to
those of their low molecular weight analogues. Thin films of
the polymers were fabricated and illuminated. Contact angle
measurements were used to reveal differences in wettability.
All monomers showed an explicit hydrophilic character,
which was verified by their solubility in polar solvents, such
as methanol, ethanol, DMSO, DMF, NMP and even water for
monomer M1. UV-vis spectroscopy showed in each case two
characteristic p-p* absorption peaks corresponding to the
pyridinium ylide rings.42 The absorption maxima of the long
wave transition depends on the substituents on the pyridin-
ium ylide carbonyl group due to their electronic influences
on the p-p* transition, which is accompanied by the electron
donating character of the ylide nitrogen to the pyridinium
ring. Hence, the energy gap can be decreased by substituents
with a þM effect and the resulting red shift could be
detected in the UV-vis spectra. The absorption maxima were
determined to be 330 nm for M1, 353 nm for M2 and 373
nm for M3 in chloroform (see Supporting Information). In
addition, a strong negative solvatochromic effect could be
observed for all monomers. The bathochromic shifts of the
absorption maxima in chloroform relative to methanol con-
firmed the highly hydrophilic character of the monomers
(see Supporting Information).
Synthesis of Monomers M1, M2, and M3
To examine the influence of the substituent on the side of
the carbonyl group opposite to the ylide nitrogen atoms on
the ratio of diazepine formation versus NAN bond cleavage,
three different monomers with a systematically varied mo-
lecular structure were synthesized (Scheme 2). Hereby, the
heteroatoms in the carbamate derivative of M1 and the urea
derivative of M3 should favor the diazepine formation upon
irradiation, while the aryl substituent in the benzoyl deriva-
838
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
Polymers PM1, PM2, and PM3
Supporting Information). As a result of their similar molecu-
lar attachment of the spacers to the carboxylic group of the
pyridinium ylide, the carbamate derivatives Y4 and Y3
showed the same absorption maxima at 330 nm. In contrast,
the urea derivative of Y2 showed a shifted absorption maxi-
mum at 358 nm due to the same electronic effects as already
discussed for the UV-vis spectra of the monomers M1 and
M3 (see Supporting Information). The solubility of the func-
tionalized pyridinium ylides was restricted to polar solvents
such as methanol, DMSO, DMF, and NMP. Y3 showed also
good solubility in CHCl3 and Y2 was soluble in water.
Monomers M1, M2, and M3 were polymerized under free
radical polymerization conditions yielding the respective
polymers PM1, PM2, and PM3 (see Table 1). Investigations
of the purified polymers by 1H NMR spectroscopy showed
the characteristic broad peaks in the aromatic region, which
could be assigned to the pyridinium ylide ring, confirming
that the photoreactive group remained intact during the po-
lymerization. UV-vis measurements confirmed this observa-
tion by detecting the same absorption maxima that was
found for the respective monomers. Molecular weight char-
acterization was extremely difficult for these mesoionic poly-
mers due to their solubility restriction to polar solvents such
as methanol, water, DMSO, NMP, and DMF. GPC measure-
ments of PM1 in water were not successful in determining
the molecular weight. Similarly, GPC measurements of PM1,
PM2, and PM3 in DMF failed in determining the molecular
weight. Also the investigation of the molecular weight by
MALDI-TOF was not successful.
Reactions on the Precursor Polymers
Polymeric activated esters are well known to react fast and
quantitatively with primary and secondary amines in a broad
variety of organic solvents.36 The synthesis and characteriza-
tion of pentafluorophenyl ester polymers is well established
and their reactions with 1-imino pyridinium ylides therefore
represent an alternative way to obtain photoreactive poly-
mers with a known molecular weight and polydispersity.
However, the restricted solubility of the amino functionalized
pyridinium ylides to polar solvents limits the conditions for
the reactions on the precursor polymers. In a first attempt
poly(pentafluorophenyl methacrylate) (PPFPMA) was reacted
with the different amines Y1, Y2 and Y3. The reactions were
complicated by differences in solubility of the amines, the
precursor polymer and the formed photoreactive polymer.
Hence, different solvent mixtures were used to ensure best
possible solubility. During the reactions the polymers were
kept in solution by changing the composition of the solvent
mixture, thereby adjusting its polarity. However, solubility
problems still prevented a complete conversion of the active
ester groups in PPFPMA. The percentages of attached photo-
reactive groups could be calculated by integration of the re-
spective 1H NMR spectra and were determined for the reac-
tion of PPFPMA with Y1 to yield PY1a to be in the order of
36%, for the reaction of PPFPMA with Y2 yielding PY2a to
be in the order of 82% and in the order of 37% for the reac-
tion of PPFPMA with Y3 yielding PY3a. In other words, in all
reactions only copolymer structures were obtained. Never-
theless, UV-vis measurements showed in every case the char-
acteristic long wave p-p* transition band attributed to the
pyridinium ylide groups and, therefore, proved the successful
coupling to the polymer backbone. The absorption maxima
of 300 nm for PY1a, 342 nm for PY2a and 328 nm for PY3a
also indicated that the photoreactive moieties were not
affected by the coupling reactions.
The problems of molecular weight determination of PM1,
PM2, and PM3 motivated us to break new ground and follow
an alternative synthetic route via reactive precursor poly-
mers to yield polymeric pyridinium ylides. In this case the
precursor polymer molecular weights could easily be deter-
mined by GPC measurements in THF. Reactions on these
polymers required, however, specifically functionalized low
molecular weight pyridinium ylides (see Scheme 3).
Synthesis of Functionalized 1-Imino Pyridinium Ylides
To investigate the possible tuning of the photoreaction of pyri-
dinium ylides to yield different products, four distinct function-
alized ylides have been synthesized. Since the substituent on
the side of the carbonyl group opposite to the ylide nitrogen
atoms influences the photoreaction pathway, the connection of
the spacer group to the ylide moiety was, in analogy to the
monomers M1–M3, systematically varied. Y1 as a free amine
yields by reaction with a reactive precursor polymer an ali-
phatic connection of the pyridinium ylide to the polymer back-
bone and thus should favor the pyridine elimination. In con-
trast, Y2, Y3, and Y4 as carbamate or urea derivatives should
enhance the diazepine formation due to their connection via a
hetereoatom.25,26,28,29 Y1 was formed by deprotonation of
1API with potassium carbonate. The photoreactive alcohol Y4
was synthesized in one step by a ring opening reaction of eth-
ylene carbonate with the free amine of 1API. Y3 as an amino
terminated ylide molecule was synthesized by attaching ethyl-
ene diamine onto the carboxylated alcohol of Y4. For the prep-
aration of the photoreactive amine Y2, the CDI activated com-
mercially available N-BOC-1,4-diaminobutane was reacted with
the deprotonated 1API to yield the BOC-protected photoreac-
tive ylide Y2. Deprotection with hydrochloric acid in aqueous
media yielded Y2.
To overcome the restrained solubility of the PPFPMA poly-
mers during the conversion of activated ester groups, poly
(pentafluorophenyl acrylate) (PPFPA) was reacted with Y1 to
yield PY1b. In comparison to PPFPMA, which had to be
reacted in solvent mixtures, the acrylate-based polymer was
soluble in DMSO, which is also a good solvent for the photo-
reactive amines. Consequently, the reaction with Y1 pro-
ceeded quantitatively within 12 h at room temperature in
homogeneous solution. Conversion of all activated ester
groups was determined by 1H NMR spectroscopy and the
even more sensitive method of 19F NMR spectroscopy, which
1H NMR spectroscopy of the functionalized pyridinium ylides
showed in all cases the characteristic multiplets attributed to
the pyridinium ring. The UV-vis spectra of Y4, Y2, and Y3 in
DMSO clearly showed the long wave absorption band due to
the p-p* transition of the aromatic pyridinium cycle (see
SYNTHESIS OF POLYMERIC 1-IMINOPYRIDINIUM YLIDES, KLINGER, NILLES, AND THEATO
839

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Photoreactions of 1-Imino Pyridinium Ylides in Solution
Sample
Solvent
Conversion (%)
Diazepine-Formation (%)
:
Pyridine-Elimination (%)
M1
DMSO
100
100
78
100
100
100
75
:
:
:
:
:
:
:
:
:
:
:
:
0
PM1
M2
DMSO
0
DMSO
0
PM2
M3
DMSO
100
0
25
0
DMSO
0
PM3
PY1a
PY2a
PY3a
PY1b
PY3b
PY4
DMSO
0
0
0
MeOH/dioxane (3:4)
MeOH/dioxane (1:1)
MeOH/DMSO (1:7)
DMSO
83
17
83
0
100
100
100
100
100
100
100
7
0
93
0
DMSO
100
100
DMSO/dioxane (6:1)
0
1
showed no signals of any remaining pentafluorophenyl
groups. In addition, the characteristic bands of the penta-
fluorophenylester groups at 1781 cmꢁ1 and 1515 cmꢁ1 in
the FTIR spectrum vanished completely. The absorption max-
imum at 320 nm in the UV-vis spectrum proved the success-
ful attachment of the chromophore to the polymer.
Photoreaction Studies via H NMR Spectroscopy
To examine the performance of the new photoreactive poly-
mers, detailed studies of the photoreaction and especially
the ratio of 1,2-diazepine formation versus NAN bond cleav-
age were conducted. This ratio was expected to depend
strongly on the molecular structure of the chromophores
and should therefore be controllable by the systematic varia-
tion of the attachment of the pyridinium ylide group to the
spacer groups or the polymer backbone. In addition, the
photoreaction of the polymeric pyridinium ylides was com-
pared to their low molecular weight analogues. The isomeri-
zation studies were carried out in deuterated solvents in a
quartz-cuvette at concentrations of 25 mg/mL. All solutions
of polymers and low molecular weight analogues were char-
acterized by 1H NMR spectroscopy, and then irradiated for
24 h at the wavelengths of 315–390 nm. The crude reaction
mixture was then characterized again by 1H NMR spectros-
copy without any further purification. Overall conversion of
the pyridinium ylide chromophores as well as the ratio of
formed 1,2-diazepine to the elimination of pyridine by NAN
Furthermore, the activated ester polymer poly(pentafluoro-
phenyl 4-vinylbenzoate) (PPFPVB) is known for its highly re-
active character toward amines and was, therefore, chosen to
examine the influence of higher reactivity combined with
better solubility on the conversion of the reactive groups.
PPFPVB was reacted with Y3 in chloroform, which is a good
solvent for the activated ester prepolymer, the photoreactive
amine and the resulting photoreactive polymer PY3b. In this
case, the reaction proceeded also quantitatively within 12 h
at room temperature. 100% conversion of all activated ester
1
groups was again confirmed by FTIR, 19F NMR, and H NMR
spectroscopy. Of course, UV-vis spectroscopy showed in anal-
ogy to PY3a the same absorption maximum at 328 nm for
the carbamate derivative of the pyridinium ylide group of Y3
(see Supporting Information).
1
bond cleavage were calculated by H NMR and are summar-
ized in Table 4.
An alternative way to the reaction of activated ester prepoly-
mers with amines is the reaction of different alcohols with
P4VBA, an isocyanato group generating polymer.39 For the syn-
thesis of the photoreactive polymer PY4, P4VBA was reacted
Even though determination of molecular weight failed for
the polymers PM1-PM3, their photoreaction was compared
to those of the polymers formed by polymer analogous reac-
tions. Monomer M1 and the respective polymer PM1 showed
the quantitative conversion of all pyridinium ylide groups to
the 1,2-diazepines upon irradiation in DMSO-d6, as detected
by 1H NMR spectroscopy. In comparison, M2 showed only a
78% conversion of the photoreactive groups to the 1,2-dia-
zepine as only product, whereas PM2 showed a quantitative
conversion of the chromophores yielding 25% of pyridine
elimination and 75% of the photoisomer. Hereby, the differ-
ence to the monomer reaction can be explained by the
higher viscosity of the polymer solution in comparison to
the viscosity of the monomer solution in the same solvent.
The decreased molecular motion leads to fewer intermolecu-
lar collisions inhibiting a deactivation of the triplet state and
thus favoring the NAN bond cleavage.
ꢀ
with Y4 in DMF under anhydrous conditions. At 70 C isocya-
nato groups were formed in situ by a Curtius rearrangement
and reacted directly with the photoreactive alcohol Y4. Once
more, FTIR and 1H NMR spectroscopy showed a quantitative
conversion of all azide groups. PY4 was obtained as a pure
polymer in 35% yield after precipitation in acetone. The low
yield can be attributed to the formation of a small percentage
of crosslinked material due to traces of water in the reaction
mixture, similar to experiments published recently.39 As
expected, because of the same molecular structure as carba-
mate derivative of the attached pyridinium ylide Y4, PY4
showed the same absorption band as PY3a and PY3b in UV-vis
spectroscopy at 328 nm (see Supporting Information).
840
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
M3 and PM3 both showed no conversion of the chromo-
phores during irradiation at all. It was assumed that this
effect occurs due to the high optical density of the monomer
and polymer. Regarding solutions of the same chromophore
concentration of M1, M2, and M3 it could be clearly
observed that M2 in comparison to M1 already showed an
increase of the absorption by the factor 1.5, whereas M3
showed an absorption intensity of even 2.5 times compared
to M1. The photochemically formed 1,2-diazepines exhibit an
intense p-p* transition of the nonplanar ring in the range of
short wavelengths between 220–250 nm and a prohibited n-
p* transition at 370 nm for M1, 390 nm for M2, and 430 nm
for M3. Both transitions overlap with the long wave pyridin-
ium ylide band and therefore reduce the quantum yield of
the photoreaction resulting in a deceleration. This effect
increases with the optical density of the monomers from M1
over M2 to M3 and could also be observed for the respective
polymers PM1, PM2 and PM3.
The photoreactive polymers PY2a, PY3a, PY3b, and PY4, all
carbamate or urea derivates, showed quantitative conversion
to the respective 1,2-diazepines. No sign of NAN bond cleav-
age could be detected. In comparison, PY1a and PY1b con-
tained pyridinium ylides that were attached by aliphatic
groups to the polymer backbone. This different electronic
structure strongly influences the photoreaction of these poly-
mers in favor of the excited triplet state and therefore NAN
bond cleavage.26,28,29 Irradiation of PY1a for 24 h yielded in
a conversion of 83% of all pyridinium ylide groups, whereas
in the case of PY1b 100% of the chromophores reacted. The
amount of pyridine elimination was significantly higher com-
pared to all other photoreactive polymers. PY1a showed
83% of pyridine formation, while for PY1b even 93% of pyr-
idine formation was observed. In summary, it should be
noted that by systematically varying the molecular structure
of the pyridinium ylide group on the polymers the ratio of
1,2-diazepine formation to NAN bond cleavage can be influ-
enced and diversified from 100% down to 7% of diazepin
formation. As a comparison, Figure 1 shows the 1H NMR
spectra of PY4 and PY1b before and after 24 h of irradiation.
FIGURE 1 1H NMR spectra in DMSO-d6 of the photoreactive
polymers and their crude photoproducts: (a) PY4 before and
after irradiation, (b) PY1b before and after irradiation.
Kinetic UV-Vis Measurements
To confirm and quantify the observed deceleration effect of
the photoreaction due to different optical densities of the
polymers, kinetic UV-vis measurements in diluted solutions
of c ¼ 1.5 ꢃ 10ꢁ4 mol/L were conducted. The measure-
ments were restricted to the homopolymers PM1, PM2,
PY1b, PY3b, and PY4, which consisted of 100% pyridinium
ylide repeating units, and their low molecular weight ana-
logues M1, M2, M3, Y3, and Y4. All substances were dis-
solved in DMSO. Series of UV-vis spectra were recorded,
measuring a spectrum each time after every sample was
irradiated for 1 min. To prove the stability against further
reactions in the dark, a comparative sample of every sub-
stance was irradiated for 5 min; a UV-vis spectrum was
measured and was then kept in the dark for 1 h before it
was measured again. The resulting spectra of all investigated
ylides showed no change of the absorption intensity. Time
dependent UV-vis spectra were analyzed by the intensity
change of the absorption maximum. The absorption maxi-
mum at t ¼ 0 was set as [M]0 and a plot of ꢁln[M]0/[M]t
versus t resulted in a linear distribution, indicating a first
order kinetic in respect to the chromophore concentration as
expected. In this case, a linear regression directly yielded the
rate constants of the photoreactions. The half-life time for a
first-order reaction is defined as s[1/2] ¼ (ln 2)/k and was
calculated for every sample. The determined half-life times
are listed together with the calculated rate constants in Table
5. Time dependent UV-vis spectra of the photoreaction of
PY4 and the -ln[M]0/[M]t versus t plots for the calculation of
the rate constants for all investigated polymers are shown in
Figure 2(a,b), respectively.
All monomers showed a quantitative conversion of the pyri-
dinium ylide groups. In comparison to the results obtained
in concentrated solution, this confirms the suggestion that a
SYNTHESIS OF POLYMERIC 1-IMINOPYRIDINIUM YLIDES, KLINGER, NILLES, AND THEATO
841

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 5 Rate Constants and Half-Life Times for the Photoreactions of Synthesized Photoreactive Polymers and Their Low
Molecular Weight Analogues
Polymeric Ylide
k (s-1)
s1 (s)
Monomeric Ylide
k (s-1)
s1 (s)
=
=
2
2
PM1
PM2
PY1b
–
3.68 ꢃ 10ꢁ3
2.56 ꢃ 10ꢁ3
4.43 ꢃ 10ꢁ3
–
188
271
156
–
M1
M2
–
3.23 ꢃ 10ꢁ3
1.42 ꢃ 10ꢁ3
–
0.15 ꢃ 10ꢁ3
5.10 ꢃ 10ꢁ3
3.53 ꢃ 10ꢁ3
215
489
–
M3
Y3
Y4
4,620
136
196
PY3b
PY4
3.80 ꢃ 10ꢁ3
182
233
2.97 ꢃ 10ꢁ3
higher optical density results in a deceleration of the photo-
chemical rearrangement. The inherent difference of the opti-
cal density and its influence on the rate constants of the
isomerization process is strongly pronounced for M3. The
rate constant is decreased by a factor of 20 compared to M1,
while M2 only showed a deceleration factor of 2 compared
to M1. Further measurements of the reaction kinetics in
more concentrated solutions (c ¼ 3.0 ꢃ 10ꢁ4 mol/L) con-
firmed the observation that a higher chromophore concentra-
tion increased the optical density and therefore decelerated
the photoreaction as well. Comparison of the half-life times
for the photoreactions of the carbamate derivatives Y3 and
Y4 with those of the monomers M1, M2, and M3, showed
that the values were in the same order than those of M1. In
case of the polymers PM1, PY3b, and PY4, the rate constant
did not vary significantly from those of their low molecular
weight analogues, indicating that the reactions in the poly-
mers proceed in the same time scale as those in the mono-
mers. However, for polymer PM2 the photoreaction half-life
time decreased compared to M2. The observed effect contra-
dicts the expected deceleration of the isomerization process
due to the inhibited molecular motion during the ring expan-
sion in the more viscous polymer solution. It seems that in
this case the divergence could be explained by the increased
ratio of NAN bond cleavage. Although the formed 1,2-diaze-
pines absorbed in the same range of wavelengths, the pyri-
dine elimination led to no deceleration effects as they do not
absorb in that region and hence, did not lead to a reduction
of the photon density available for the photoreaction. There-
fore, PM2 with a ratio of 25% of NAN bond cleavage reacted
faster than M2 where only the diazepine was detected as
photoproduct. In addition the acceleration of the photoreac-
tion in the polymer was also observed by the conversion of
the samples for 1H NMR studies mentioned above. After
24 h of irradiation, M2 showed only a conversion of 77%
whereas PM2 already reacted quantitatively.
Photoreactions in Thin Polymer Films
The photoreaction of polymeric pyridinium ylides in a poly-
mer film has already been described in the literature and
can be exploited as a negative photoresist.27 With regard to
a possible application of the synthesized polymers for optical
data storage or surface modifications, it should be assured
that the photoreaction also proceeds quantitatively in thin
polymer films. Thin polymer films were prepared by spin
coating polymer solutions of PM1, PM2, PY1b, PY3b, and
PY4 on glass substrates to investigate the photoreaction,
which was monitored by UV-vis spectroscopy in analogy to
the experiments of the polymer solutions. The chromophores
in all polymer films were transformed to their respective
reaction products in quantitative yields after irradiation with
UV-light in less than one hour. Figure 3 exemplary shows the
UV-vis spectra of the irradiation of a film of PM2. Each
FIGURE 2 (a) Time dependent UV-vis spectra of the photoreac-
tion of PY4 in DMSO, (b) Kinetic plots for the calculation of the
rate constants of the photoreaction of PM1 (*), PM2 (h) und
PY1b (~), PY3b (l), PY4 (n).
842
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
fully be polymerized under free radical polymerization
conditions, however, resulting in problems with the determi-
nation of the molecular weight. Thus, three different 1-imino
pyridinium ylides were efficiently and quantitatively attached
to different reactive precursor polymers, such as poly(penta-
fluorophenyl acrylate), poly(pentafluorophenyl 4-vinylben-
zoate) and poly(4-vinylbenzoyl azide). Depending on the
substituents of the 1-imino pyridinium ylides, the photoreac-
tion could be controlled to be either a quantitative photoiso-
merization to the 1,2-diazepine containing polymers or a
photoelimination of 93% of pyridine. The half-life times for
the photoreactions of the synthesized photoreactive poly-
mers and their low molecular weight analogues were found
to be very similar for all cases. Investigations of the photore-
actions of thin polymer films of the synthesized ylide con-
taining polymers showed a dramatic increase of contact
angles of water on those films after irradiation.
FIGURE 3 UV-vis spectra of the irradiation of a polymer film of
PM2.
Financial support from the University of Mainz is gratefully
acknowledged. D. Klinger acknowledges the Dr. Georg Scheu-
ing-Stiftung of the University of Mainz for a scholarship.
spectrum was measured after illumination of the sample for
1 min.
Contact Angle Measurements of the Polymer Films
To investigate the change of hydrophilicity of the synthesized
polymers, contact angles were measured of the polymer
films before and after irradiation. Measurements were per-
formed using the sessile drop method with 40 individual
measurements being average. The contact angles measured
before and after irradiation are listed in Table 6.
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