Preparation of 4-Vinylbenzil and Photochemical Properties of Its Homopolymer and Copolymer with Styrene

Article abstract of DOI:10.1021/ma030213j

When irradiated at >400 nm in air, pendant benzil groups of 1-phenyl-2-(4-vinylphenyl)-1,2-ethanedione/styrene (VBz/S) copolymer films are transformed almost quantitatively into benzoyl peroxide (BP) groups. Subsequent heating at 91°C converts the pendant benzoyl peroxide groups to esters and benzoic acid moieties, and there is significantly more cross-linking than main-chain cleavage. Irradiation of the VBz/S copolymer films at 366, 313, and 254 nm also results in formation of BP groups, but they are transformed in situ upon absorption of a second photon by the matrix. The ratios of the relative rate constants for BP formation and subsequent transformation upon absorption by a second photon decrease with decreasing wavelengths of radiation. Irradiation of a film composed of a nonmiscible intimate mixture of poly(1-phenyl-2-(4-vinylphenyl)-1,2-ethanedione) (PVBz) and polystyrene (PS) at >400 nm in air does not lead to discernible BP concentrations as well. Instead, the unreacted pendant benzil groups act as photosensitizers to transform the peroxy moieties almost immediately. In addition, we demonstrate that cross-linking of the VBz/S copolymer film, induced by 254 nm radiation, can be utilized to record a negative image.

Full text of DOI:10.1021/ma030213j

1304
Macromolecules 2004, 37, 1304-1311
Preparation of 4-Vinylbenzil and Photochemical Properties of Its
Homopolymer and Copolymer with Styrene
J a r osla v Mosn a´cˇek ,† Rich a r d G. Weiss,‡ a n d Iva n Lu k a´cˇ*,†
Polymer Institute, Centre of Excellence for Degradation of Biopolymers, Slovak Academy of Sciences,
Du´bravska´ cesta 9, 842 36 Bratislava, Slovak Republic, and Department of Chemistry,
Georgetown University, Washington, D.C. 20057-1227
Received April 9, 2003; Revised Manuscript Received November 19, 2003
ABSTRACT: When irradiated at >400 nm in air, pendant benzil groups of 1-phenyl-2-(4-vinylphenyl)-
1,2-ethanedione/styrene (VBz/S) copolymer films are transformed almost quantitatively into benzoyl
peroxide (BP) groups. Subsequent heating at 91 °C converts the pendant benzoyl peroxide groups to
esters and benzoic acid moieties, and there is significantly more cross-linking than main-chain cleavage.
Irradiation of the VBz/S copolymer films at 366, 313, and 254 nm also results in formation of BP groups,
but they are transformed in situ upon absorption of a second photon by the matrix. The ratios of the
relative rate constants for BP formation and subsequent transformation upon absorption by a second
photon decrease with decreasing wavelengths of radiation. Irradiation of a film composed of a nonmiscible
intimate mixture of poly(1-phenyl-2-(4-vinylphenyl)-1,2-ethanedione) (PVBz) and polystyrene (PS) at >400
nm in air does not lead to discernible BP concentrations as well. Instead, the unreacted pendant benzil
groups act as photosensitizers to transform the peroxy moieties almost immediately. In addition, we
demonstrate that cross-linking of the VBz/S copolymer film, induced by 254 nm radiation, can be utilized
to record a negative image.
Sch em e 1
In tr od u ction
Benzil is an industrially important member of the
class of molecules with 1,2-dicarbonyl functionality. It
has been utilized in the preparation of photographic
materials and polymer resists and as a photoinitiator
in radical polymerizations.1-3 The solution-phase pho-
tochemistry of benzil has been investigated extensively,
in both the presence and absence of molecular oxygen.
When molecular oxygen is available, photooxidation of
benzil in benzene leads to phenyl benzoate, benzoic acid,
biphenyl, and a small amount of benzoyl peroxide (BP).4
In addition, we have demonstrated that benzil can be
converted almost quantitatively to BP in aerated poly-
mer films upon irradiation at >400 nm (i.e., the long
wavelength edge of the n f π* absorption band, where
BP does not absorb).2,3 Covalently attached BP pendant
groups have been formed also by irradiation of copoly-
mer films of 1-[4-(2-methacroyloxyethoxy)phenyl]-2-
phenyl-1,2-ethanedione-co-styrene5 and 1-phenyl-2-(4-
propenoylphenyl)-1,2-ethanedione-co-styrene6 (Scheme
1).
Decomposition of pendant benzoyl peroxide groups
can be an efficient method to effect cross-linking of
polymer chains. In fact, even when irradiated at >400
nm in air at room temperature, a small part of the
peroxide pendant groups is decomposed; films of both
copolymers above become partially insoluble in organic
solvents after irradiation.5,6 However, decomposition of
low molecular weight peroxides, doped into polymer
films like those of polystyrene, results in a net decrease
in polymer molecular weight (i.e., chain scission is more
important than chain cross-linking under these condi-
tions).7
eminate predominantly from easy radical abstraction
of hydrogen atoms from the oxyethoxy (-O-CH2-CH2-
O-) groups.5 Cross-linking of (unheated) films of 1-phen-
yl-2-(4-propenoylphenyl)-1,2-ethanedione-co-styrene by
>400 nm is thought to occur as a result of absorption
by photons in the red edge of the n,π* band of mono-
carbonyl groups and subsequent energy transfer to
benzoyl peroxide groups.6
We report here our investigations of the photochem-
istry of copolymer films of a novel monomer, 4-vinyl-
benzil (VBz), and styrene (VBz/S). Heretofore, benzil
pendant groups have been attached to polymer chains
via 1-methacroyloxy-2-ethoxy5 and vinyl ketone6 tethers.
The effect of abstractable H atoms of these tethers on
cross-linking5 or how the carbonyl groups may modify
the photochemical properties of the polymer films6 may
be different for VBz/S and related copolymers. The
VBz/S copolymer may exhibit a more regular distribu-
tion of benzil pendant groups along the polymer chains
(vide infra), and, therefore, a more regular cross-linked
network, than the related copolymers. Additionally, the
absence of carbonyl groups, other than those associated
Cross-linking of 1-[4-(2-methacroyloxyethoxy)phenyl]-
2-phenyl-1,2-ethanedione-co-styrene was presumed to
* Corresponding author: e-mail upolluka@savba.sk.
† Slovak Academy of Sciences.
‡ Georgetown University.
10.1021/ma030213j CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/24/2004

Macromolecules, Vol. 37, No. 4, 2004
Preparation of 4-Vinylbenzil 1305
1-[4-(1-Acetyloxyethyl)phenyl]-2-phenyl-1,2-ethandione (IV).
A mixture of III (28 g, 0.09 mol) and anhydrous potassium
acetate (53.5 g, 0.546 mol) was refluxed in 99% acetic acid (150
mL) for 4 h. The mixture was concentrated under reduced
pressure, and the residue was added to aqueous sodium
bicarbonate and extracted with diethyl ether. After drying
(anhydrous sodium sulfate) and evaporation of the ether, 26
g (∼100%) of crude product was obtained as a brown-red oil.
It was used without further purification.
1H NMR (CDCl3): δ (ppm) 1.45 (d, J ) 6.6 Hz, 3H, CH3);
2.00 (s, 3H, CH3COO); 5.80 (q, J ) 6.6 Hz, 1H, CH); 7.39 (m,
2H, arom); 7.42 (m, 2H, arom); 7.60 (m, 1H, arom); 7.88 (m,
4H, arom).
UV/vis (CHCl3/MeOH): λmax[nm] (ꢀ [L mol-1 cm-1]) ) 265/
262 (104600/82100) and 378/376 (118/86).
1-[4-(1-Hydroxyethyl)phenyl]-2-phenyl-1,2-ethanedione (V).
Aqueous sodium hydroxide (30 g in 30 mL of water) was added
to a solution of IV (26 g, 0.09 mol) in methanol (300 mL). The
mixture was stirred at room temperature for 15 min. After
acidification with hydrochloric acid, the solid (salt) was
removed by filtration, and the filtrate was concentrated under
reduced pressure. The soluble portion of the residue was taken
up in diethyl ether and yielded a red oil (23 g, ∼100%) after
removal of the ether under reduced pressure. It was eluted
on a silica gel column with benzene initially to remove a small
amount of impurities and with ethyl acetate to obtain the
product as yellowish oil.
1H NMR (CDCl3): δ (ppm) 1.47 (d, J ) 6.5 Hz, 3H, CH3);
3.88 (s, 1H, OH); 4.95 (q, J ) 6.4 Hz, 1H, CH); 7.48 (m, 2H,
arom); 7.50 (m, 2H, arom); 7.75 (m, 1H, arom); 7.90 (m, 4H,
arom).
UV/vis (CHCl3/MeOH): λmax[nm] (ꢀ [L mol-1 cm-1]) ) 261/
260 (65800/58500) and 375/375 (53/55).
with the benzil moiety or other groups that absorb in
the infrared region where oxidative changes are easily
diagnosed, should facilitate quantitative analyses of the
photochemical and subsequent events.
Exp er im en ta l P a r t
In str u m en ta tion . IR spectra were recorded on a Nicolet
400 (Nicolet, Germany) FT spectrophotometer. UV/vis absorp-
tion spectra were measured on a SPECORD M40 (Carl Zeiss,
J ena, Germany) spectrophotometer. Molecular weights were
estimated by gel permeation chromatography (GPC) with THF
(HPLC grade) as eluent, a PSS SDV 5 µm column (d ) 8 mm,
l ) 300 mm), a Waters 515 pump, and a Waters refractive
index detector. The instrument was calibrated with PS stan-
dards. 1H and 13C NMR spectra were measured on Bruker AM
300 (Germany) and Varian VXR-300 (USA) spectrometers with
TMS as internal standard. Differential scanning calorimetry
(DSC) measurements were carried out on a Mettler-Toledo
DSC 8211e calorimeter that was calibrated for temperature
and heat of fusion with indium.
Syn th eses. Poly[1-(4-methacroyloxyethoxyphenyl)-2-phen-
yl-1,2-ethanedione] was available from previous investiga-
tions.8
1-(4-Ethylphenyl)-2-phenyl-1-ethanone (I). Phenylacetyl chlo-
ride (74.1 g, 0.48 mol) was added to a cooled (0 °C), stirred
suspension of anhydrous aluminum chloride (69.5 g, 0.52 mol)
in carbon tetrachloride (290 mL). Then, ethylbenzene (55 g,
0.52 mol) was added dropwise at a rate that maintained the
temperature below 5 °C. Thereafter, the temperature was
increased to 20 °C, and the reaction mixture was poured onto
a mixture of concentrated hydrochloric acid and ice, extracted
with chloroform, dried with anhydrous sodium sulfate, and
concentrated under vacuum. The crude product was distillated
at 170-200 °C/15 Torr and crystallized from ligroin to yield
67 g (62%); mp 59-61 °C (lit. mp 62-64 °C9).
1-Phenyl-2-(4-vinylphenyl)-1,2-ethanedione (VBz). Hydro-
quinone (0.1 g) and P2O5 (6 g, 15 mmol) were added to a
solution of V (2 g, 8 mmol) in benzene (60 mL), and the mixture
was stirred at room temperature for 3 h, filtered, and concen-
trated under vacuum. The product was isolated by column
chromatography on silica gel with ligroin:benzene (4:1 f 2:1)
as eluent to yield 0.8 g (40%) of a liquid that was stored as a
ligroin:benzene (3:1) solution in a refrigerator until being used.
One spot by silica gel thin-layer chromatography and one peak
by HPLC using a 250 × 4 mm Separon SGX C18 column
(Tessek Ltd. Prague, Czech Republic), UV detector LCD 254
(Laboratory equipments, Prague, Czech Republic), and 8:2
methanol:water as eluent.
1H NMR (CDCl3): δ (ppm) 1.25 (t, J ) 7.5 Hz, 3H, CH3);
2.69 (q, J ) 7.8 Hz, 2H, CH2); 4.26 (s, 2H, CH2); 7.30 (m, 7H,
arom); 7.94 (d, J ) 8.1 Hz, 2H, arom).
13C NMR (CDCl3): δ (ppm) 15.2 (CH3); 28.9 (CH2); 45.4
(CH2); 126.8 (CHAr); 128.2 (CHAr); 128.6 (CHAr); 128.9 (CHAr);
129.4 (CHAr); 134.3 (CAr); 134.8 (CAr); 150.2 (CAr); 197.3 (Cd
O).
1-(4-Ethylphenyl)-2-phenyl-1,2-ethanedione (II). At room
temperature, selenium dioxide (24 g, 0.21 mol) was added to
a stirred solution of I (22.4 g, 0.1 mol) in 70% acetic acid (500
mL), and the mixture was stirred at 90-100 °C for 9 h. Almost
all of the solvent was removed under reduced pressure, and
the residue was placed in an aqueous sodium bicarbonate
solution and extracted with benzene. The extract was dried
with anhydrous sodium sulfate and concentrated under vacuum
to give 22 g (90%) of an orange oil (one spot by silica gel thin-
layer chromatography) that was used without additional
purification (solid, mp 55 °C,10 or a liquid, bp 170 °C/1 Torr11).
1H NMR (CDCl3): δ (ppm) 1.25 (t, J ) 7.6 Hz, 3H, CH3); 2.70
(q, J ) 7.6 Hz, 2H, CH2); 7.35 (d, J ) 8.2 Hz, 2H, arom); 7.50
(m, 2H, arom); 7.65 (m, 1H, arom); 7.88 (m, 2H, arom); 7.97
(m, 2H, arom).
1H NMR (CDCl3): δ 5.44 (d, J cis ) 10.8 Hz, 1H, CH2); 5.91
(d, J trans ) 17.4 Hz, 1H, CH2); 6.75 (2d, J cis ) 11.1 Hz, J trans
)
17.7 Hz, 1H, CH); 7.52 (m, 4H, arom); 7.65 (m, 1H, arom); 7.93
(m, 2H, arom); 7.97 (m, 2H, arom).
13C NMR (CDCl3): δ (ppm) 118.0 (CH2); 126.7 (CH); 129.0
(CHAr); 129.9 (CHAr); 130.3 (CHAr); 132.1 (CAr); 133.0 (CAr);
134.9 (CHAr); 135.7 (CHAr); 143.9 (CAr); 194.0 (CdO); 194.5 (Cd
O).
UV (CCl4): λmax[nm] (ꢀ [L mol-1 cm-1]) ) 294 (100000) and
400 (88).
Mass m/e: 236 (M+, 10%), 131 (CH2dCH-C6H4-CO, 90%),
105 (PhCO, 36%), 103 (CH2dCH-C6H4, 30%), 77 (Ph, 100%).
Poly[1-Phenyl-2-(4-vinylphenyl)-1,2-ethanedione] (PVBz). The
homopolymer was prepared by allowing a neat sample of VBz
to remain in a refrigerator for 2 weeks. It was precipitated
three times from chloroform solution into methanol. By GPC
analysis using PS standards for calibration, Mw ) 1 221 000
and Mn ) 33 500.
UV/vis (CHCl3/MeOH): λmax [nm] (ꢀ [L mol-1 cm-1]) 269/
265 (45500/72900) and 394/375 (218/132).
1-[4-(1-Bromoethyl)phenyl]-2-phenyl-1,2-ethanedione (III).
N-Bromosuccinimide (17.8 g, 0.1 mol) and 2,2′-azobis(2-meth-
ylpropionitrile) (AIBN; 0.225 g, 1.5 mmol) were added to a
solution of II (22 g, 0.09 mol) in 80 mL of carbon tetrachloride.
The reaction mixture was warmed slowly and allowed to reflux
for 5 h. After cooling the mixture to ambient temperature, the
solid (succinimide) was removed by filtration, and the filtrate
was concentrated at reduced pressure to give 28 g (∼100%) of
crude product as an orange-red oil that was used without
further purification.
Elemental analysis calcd for C16H13O2: C 81.34%; H 5.12%.
Found: C 81.12 ( 0.14%; H 5.13 ( 0.01%.
1H NMR (CDCl3): δ 1.60 (b (broad peak), 2H, CH2); 2.00 (b,
1H, CH); 6.61 (b, 2H, arom o-CH); 7.47 (b, 2H, arom m-CdO);
7.60 (b, 3H, arom p-CdO and m-CH); 7.89 (b, 2H, arom o-Cd
O).
1H NMR (CDCl3): δ (ppm) 2.02 (d, J ) 6.9 Hz, 3H, CH3);
5.18 (q, J ) 6.9 Hz, 1H, CH); 7.48 (m, 2H, arom); 7.52 (m, 2H,
arom); 7.65 (m, 1H, arom); 7.93 (m, 4H, arom).
13C NMR (CDCl3): δ (ppm) 41.2 (CH); 41.3 (CH2); 127.4
(CHAr); 128.0 (CHAr); 129.1 (CHAr); 130.0 (CHAr); 131.3 (CAr);
132.8 (CAr); 135.0 (CHAr); 151.5 (CAr); 193.4 (CdO); 194.1 (Cd
O).
UV/vis (CHCl3/MeOH): λmax[nm] (ꢀ [L mol-1 cm-1]) 272/270
(88800/125200) and 380/377 (93/109).

1306 Mosna´cˇek et al.
Macromolecules, Vol. 37, No. 4, 2004
Sch em e 2
Poly[1-phenyl-2-(4-vinylphenyl)-1,2-etha nedione-co-sty-
rene] (VBz/ S). An ampule containing 0.5 g of VBz, 8.8 g of
styrene, and 20 mg of AIBN was sealed under argon and
polymerized at 59 °C for 12 h. The copolymer was precipitated
three times from chloroform solution into methanol to yield
4.8 g (51.6 wt %) of a slightly yellowish polymer. From the
ratio of integrated peak areas of the aromatic hydrogens in
the 1H NMR spectrum (chloroform-d), the VBz content was
10.4 wt %; estimates from elemental analysis (see below) are
10.1 ( 1.8 wt % (carbon) and 9.4 ( 1.0 wt % (hydrogen). By
The postirradiated image was developed by placing the film
in isobutyl methyl ketone.
Resu lts a n d Discu ssion
Syn th esis of 4-Vin ylben zil. 4-Vinylbenzil was pre-
pared in six steps (Scheme 2), the first five of which,
yielding 1-[4-(1-hydroxyethyl)phenyl]-2-phenyl-1,2-ethane-
dione (V), have been described previously for the similar
compounds.12 Compounds II-IV are high boiling liquids
that were prepared in nearly quantitative yields. Be-
cause they are difficult to purify by chromatographic
methods, they were used in crude form, and only
intermediate V and VBz were purified by chromatog-
raphy. The final step, dehydration of V, was attempted
with various reagents including triphenylphosphine in
CCl4,13 POCl3 in pyridine,14 BF3OEt2 in dichlormethane,15
FeCl3 on silica gel,16 and p-toluenesulfonic acid in
benzene.17 Unfortunately, low yields of the desired
product, contaminated with byproducts, were achieved
in most cases. In addition, significant amounts of
polymer, perhaps from the desired VBz, were also
detected. VBz was finally prepared in 40% yields by
dehydration with P2O5 in benzene18 at room tempera-
ture (Scheme 2).
GPC analysis (PS calibration), Mw ) 216 000 and Mn
113 000. By DSC, Tg ) 107.7 °C.
)
Elemental analysis: C 91.15 ( 0.19%; H 7.49 ( 0.04%.
1H NMR (CDCl3): δ 1.45 (b, CH2 of S), 1.86 (b, CH of S and
CH2 of VBz), 2.07 (b, CH of VBz), 6.50 (b, 2H-arom of S and
2H-arom of VBz), 7.08 (b, 3H-arom of S), 7.54 (b, 3H-arom of
VBz), 7.66 (b, 2H-arom of VBz), 8.00 (b, 2H-arom of VBz).
13C NMR (CDCl3): δ (ppm) 40.5 (CH), 43.6 (CH2), 125.6
(CHAr of S), 127.6 (CHAr of S and CHAr of VBz), 127.9 (CHAr of
S and CHAr of VBz); 129.0 (CHAr of VBz); 129.7 (CHAr of VBz);
130.7 (CAr of VBz), 133.1 (CAr of VBz); 134.8 (CHAr of VBz),
145.3 (CAr of S), 194.3 (CdO), 195.2 (CdO). Note that no
quaternary CAr signal was observed although it was detected
in the spectrum of PVBz at 151.5 ppm.
Ir r a d ia tion s a n d Mea su r em en ts. Films (10 cm2 area)
consisting of 40 mg of copolymer (VBz/S), 4 mg of homopolymer
PVBz in 16 mg of PS, and 4 mg of poly[1-(4-methacroyloxy-
ethoxyphenyl)-2-phenyl-1,2-ethanedione] in 16 mg of PS were
prepared from solution as described2,3 and irradiated at
ambient temperature through a UV CL SR HPR plastic film
filter (LLumar) at >400 nm in a Spectramat (Ivoclar A.G.,
Schaan, Liechtenstein) apparatus (350-530 nm). A homemade
“merry-go-round” apparatus was employed in some experi-
ments. It consists of three concentric quartz walls in which a
125 W medium-pressure mercury arc is in the inner cylinder
and is surrounded by a layer of cooling water and a 1 cm thick
layer of a liquid filter (optional). The distance of each sample
(placed in rotating holder) from arc is about 9 cm. The
radiation was filtered with a 4 mm Corning 5860 glass filter
(for 366 nm) or a 1 cm thick layer of aqueous potassium
Syn th eses of 4-Vin ylben zil Hom op olym er a n d
VBz/S Cop olym er a n d Th eir Ch a r a cter iza tion .
Both PVBz and VBz/S are high molecular weight
polymers. PVBz, owing to its polymerization without
added initiator, has a broad molecular weight distribu-
tion. The polydispersity of the VBz/S, prepared by
polymerization initiated with AIBN, is near the theo-
retical value. In the monomer mixture, VBz reacts faster
than styrene. Therefore, the average content of VBz
units in the copolymer (4.9 mol % at 51.6 wt %
conversion) is higher than in the monomer mixture (2.44
mol %). From the contents of the monomer units in the
monomer mixture and in the polymers, as well as from
additional information in the literature8,19 and those
presented here, the distribution of benzil groups at-
tached to main chains by different tethers in copolymers
with styrene cannot be discerned precisely. It is only
possible to conclude that the VBz polymerizes with
styrene equally fast or faster than monomers with very
reactive vinyl ketone and methacroyl tethers! Therefore,
the distribution of VBz units in the styrene copolymer
is similar to or somewhat less regular than the previ-
ously prepared copolymers. On the basis of comparisons
chromate (0.3884 g dm-3) and sodium carbonate (50 g dm-3
)
in combination with a 0.5 cm Corning glass filter N 7-59 (9863)
(for 313 nm). For irradiations at mainly 254 nm, films were
mounted in the “merry-go-round” apparatus, and the lamp was
a low-pressure mercury arc (TNK 6/20, Hanau GmbH, Ger-
many). Thermal decompositions of benzoyl peroxide groups in
irradiated films were conducted in air at 91 ( 1 °C in the
column oven of a Shimadzu gas chromatograph.
The photolithography experiments employed the low-pres-
sure mercury arc. A lithographic mask on a quartz glass was
placed over a silicon wafer with a spin-surface coated VBz/S
film (ca. 0.5 µm thickness), made from a chloroform solution.

Macromolecules, Vol. 37, No. 4, 2004
Preparation of 4-Vinylbenzil 1307
F igu r e 2. UV-vis absorption spectra of a VBz/S copolymer
film irradiated at >400 nm in a Spectramat apparatus at RT
in air for various periods.
F igu r e 1. FT-IR spectra of a VBz/S copolymer film irradiated
at >400 nm in a Spectramat apparatus at RT in air for various
periods.
with published data,20 it follows that the copolymerizing
properties of VBz are similar to those described for
copolymerization of unsubstituted styrene and styrene
with strongly deactivating substituents at the para
position. Owing to the copolymerization properties, the
content of VBz structures in the VBz/S copolymer
formed at the beginning of polymerization is higher than
at the end of the reaction. Despite the chemical inho-
mogenities of the VBz/S copolymer, the prepared films
are perfectly transparent, and most probably VBz
groups are distributed regularly throughout the sample.
P h otoch em ica l Gen er a tion of Ben zoyl P er oxid e
Gr ou p s in VBz/S (Mola r R a t io 5:95) Cop olym er
F ilm s. Conversions of pendant benzil groups during
irradiations of VBz/S at >400 nm and ambient temper-
ature were followed by FT IR spectroscopy. As reaction
progressed, the intensity of the characteristic IR stretch-
ing band of the 1,2-dicarbonyl groups at 1660-1690
cm-1 decreased, and their nearly quantitative transfor-
mation to benzoyl peroxide (BP) groups was complete
after 3 h (Figure 1, Scheme 1). Formation of the BP
groups is accompanied in IR spectra by the growth of
new intense bands in the 1750-1800 cm-1 region.2 The
nearly quantitative transformation was also indicated
by the very similar pseudo-first-order rate constants for
loss of benzil groups (k ) 0.98 h-1, obtained from plots
of the log of the decrease of absorbance at 1675 cm-1 vs
time) and formation of BP groups (k ) 1.04 h-1, from
plots of the log of the increase of absorbance at 1766
cm-1 vs time).
F igu r e 3. FT-IR spectra of a VBz/S copolymer film irradiated
at 366 nm at RT in air for various periods in a merry-go-round
apparatus.
photochemical properties. Although formation of the BP
groups was observed also during irradiation of VBz/S
copolymer films at 366 nm (Figure 3), FT IR spectra
indicate that the peroxides were converted slowly to
esters (growth of peaks in the 1700-1750 cm-1 region)
and benzoic acid (growth of peaks in the 1670-1700
cm-1 region). It is possible to fit the temporal intensity
changes of the 1766 cm-1 band in Figure 3 (dotted
spectrum) to an exponential function and calculate a
pseudo-first-order rate constant for decomposition of BP
groups (8.5 × 10-3 h-1) that is considerably smaller than
the one for their formation (5.9 × 10-2 h-1), based on
the decreases of absorbance of 1,2-diketo groups at 1675
cm-1. For this reason, the intensities of absorbance
bands from the peroxide increase up to 90% benzil
conversion and decrease only when concentration of
benzil groups becomes very small. After 40 h of irradia-
tion, only conversion of peroxides can be detected
(Figure 3, dotted spectra). This experiment also shows
that the decomposition of the peroxides is due princi-
pally to their small absorption at 366 nm and not by
sensitization (energy transfer) from the remaining ben-
zil groups. Although a sensitization mechanism was
operative in the PVBz and poly[1-(4-methacroyloxy-
ethoxyphenyl)-2-phenyl-1,2-ethanedione] homopolymers
(see below), it is not here because the concentration of
1,2-dione groups is much smaller, their average separa-
UV-vis absorption spectra were also recorded in the
region of the diketone n f π* absorption band (∼390
nm) during irradiation (λ > 400 nm) of VBz/S films
(Figure 2). There is a continuous decrease in absorbance
at 390 nm, and only the tail of a stronger π f π* band
is detectable after 3 h of irradiation.
It was possible to monitor the formation of peroxides
in a VBz/S copolymer film in deuteriochloroform by 13
C
NMR spectroscopy. A VBz/S film was irradiated at >400
nm and ambient temperature to about 70% conversion
of the 1,2-dicarbonyl groups (to yield peroxides). The
spectrum included a strong new peak at 163.1 ppm
attributed to the peroxide and residual peaks at 194.3
and 194.7 ppm from the unreacted 1,2-dicarbonyl
groups.
Copolymer films were irradiated also at 366, 313, and
254 nm to investigate influence of wavelength on their

1308 Mosna´cˇek et al.
Macromolecules, Vol. 37, No. 4, 2004
F igu r e 4. FT-IR spectra of a VBz/S copolymer film irradiated
at 313 nm at RT in air for various periods in merry-go-round
apparatus.
F igu r e 6. FT-IR spectra of a VBz/S copolymer film irradiated
at 254 nm at RT in air for various periods.
F igu r e 5. IR absorbance changes at 1766 cm-1 (from Figure
4) in a VBz/S copolymer film vs time of irradiation (λ ) 313
nm) at RT in air.
F igu r e 7. Dependence of diketone consumption on time of
irradiation (λ ) 254 nm) in a VBz/S copolymer film at RT in
air. See text for details.
tion is much larger, and they are more randomly
distributed in VBz/S than in the PVBz and poly[1-(4-
methacroyloxyethoxyphenyl)-2-phenyl-1,2-ethanedione]-
homopolymers. As a result, the distance between VBz
donors and peroxide acceptors formed in the glassy
matrix provided by VBz/S is too large for efficient energy
transfer.
Irradiation of VBz/S at ambient temperature with 313
nm radiation leads also to the in situ transformation of
benzil to BP groups (Figure 4). As expected from the
molar extinction coefficients of the species involved
(Figure 2), the ratio (R) of the relative pseudo-first-order
rate constants for formation and subsequent transfor-
mation of BP groups, as calculated from the temporal
changes in the relevant IR bands (vide ante), is smaller
with 313 nm radiation (R ) 0.11 h-1/0.045 h-1 or <3)
than with 366 nm radiation (R ≈ 7). For this reason,
the peroxide absorbance increases up to only ca. 70%
conversion of benzil groups at 313 nm (Figures 4 and
5) vs ca. 90% conversion at 366 nm. After ca. 20 h of
irradiation at 313 nm, only loss of peroxides can be
observed.
formed. The 254 nm wavelength region corresponds to
the strong aromatic π f π* absorptions. As a result of
the high optical densities in this region, radiation
penetrates primarily the parts of the film near the
surface, and the remainder of the film is not exposed to
large photon fluxes. As a result, only about 12% (6% on
each exposed side) of the benzil groups are converted
to peroxides after protracted irradiation periods (Figure
7). If it is assumed reasonably that only those benzil
groups near the film surfaces react, the depth of
penetration of the radiation (and the space of reaction)
is limited to only about 2.4 µm (i.e., 6% of the total 40
µm thickness). Consistent with this assumption, the
pseudo-first-order rates for benzil consumption on each
side are the same within experimental error.
Th er m al Decom position of P h otoch em ically Gen -
er a ted Ben zoyl P er oxid e Gr ou p s in VBz/S Cop oly-
m er F ilm s. Thermal decomposition (at 91 °C; Scheme
3) of BP groups, generated by prior irradiation of VBz/S
copolymer films at >400 nm (Figure 1), was followed
by IR spectral changes in the regions of the peroxide
(decreases at 1750-1800 cm-1), ester (increases at
1700-1750 cm-1), and benzoic acid (increases at 1670-
1700 cm-1) absorbances (Figure 8).
No absorptions attributable to BP groups could be
discerned in FT IR spectra of a VBz/S copolymer film
irradiated principally at 254 nm (Figure 6). We believe
that transformation of benzil to BP groups is still the
major reaction pathway, but photoinduced decomposi-
tion of BP groups proceeds more rapidly than they are
The first-order rate constant for thermal decomposi-
tion of BP groups, kt ) 0.08 h-1, was calculated from
IR peak intensity changes at 1750-1780 cm-1. Recently,
we noted6 that the rate constants for thermal decom-

Macromolecules, Vol. 37, No. 4, 2004
Preparation of 4-Vinylbenzil 1309
Sch em e 3
position of low molecular weight BP derivatives in PS
as well as two different BP pendant groups of copoly-
mers are larger when there is an electron-donating
substituent at the 4-position of one of the aromatic rings
than when the substituent is electron-withdrawing.
Although the BP groups generated photochemically in
the VBz/S copolymer contain an electron-donating sub-
stituent, their kt value is comparable to that of the BP
group with an electron-withdrawing substituent (i.e.,
from the irradiated copolymer of 1-phenyl-2-(4-prope-
noylphenyl)-1,2-ethandione and styrene; Table 1).
Other factors besides the electronic nature of the
substituents must influence the rates of BP thermolyses
in styrene copolymers. In this regard, it is important to
note that kt (and its associated rate) is not a simple
process. What is measured is the net loss of BP groups
with timesthe difference between the total number of
peroxy bonds lysed and the number of radical pairs that
combine to re-form BP groups. This difference can be
mediated by the ability of the “walls” of the cages21 to
act as hydrogen atom donors to trap one (or both) of
the radicals in an intimate pair. Thus, the relatively
high rate of BP decomposition in irradiated 1-{4-(2-
methacroyloxyethoxy)phenyl}-2-phenyl-1,2-ethanedione/
styrene copolymer5 may be related to the presence of
relatively easily abstractable hydrogen atoms from the
-O-CH2-CH2-O- tether segments.
F igu r e 8. Changes in the FT-IR spectra of the region of BP
group absorbances at 91 °C in a VBz/S copolymer film that
had been preirradiated at >400 nm at RT in air (Figure 1).
Ta ble 1. F ir st-Or d er Ra te Con sta n ts a t 91 °C for Th er m a l
Decom p osition (kt) of Va r iou s Typ es of P en d a n t Ben zoyl
P er oxid e Str u ctu r es Gen er a ted by Ir r a d ia tion of Ben zil
Con ta in in g Cop olym er F ilm s
benzoyl peroxide source and carrier
kt (h-1
)
VBz/S
0.08
1-{4-(2-methacroyloxyethoxy)phenyl}-2-phenyl-
1,2-ethanedione-co-styrene
1-phenyl-2-(4-propenoylphenyl)-1,2-ethanedione-co-
styrene
0.2565
0.0776
Sch em e 4
Also, the local flexibility of groups constituting the
“cage” environment of the BP groups may be important
to the overall rates. Factors such as “hole” free volume,
the length and nature of the tether between the BP
group and its the main chain, and, to a lesser extent,
the number of degrees under the glass transition the
thermolysis is conducted must contribute to the overall
flexibility. In fact, flexibility affects the rate of cleavage
of the peroxy linkages of the BP groups as well as the
rates at which the intimate radical pairs recombine.21
Of these factors, only the length of the tethers differ
significantly among the copolymers with styrene.
Cr oss-Lin k in g of a VBz/S Cop olym er F ilm . In
principle, benzoyloxy radicals from oxidized, low mo-
lecular weight benzil dopants or from BP groups ap-
pended to polymer chains can abstract a hydrogen atom
from the polymer, add to a phenyl ring (of a polystrene
polymer or copolymer), recombine, or disproportionate.
The latter two reaction pathways have no influence on
the average molecular weight or degree of cross-linking
of the polymer. The former two pathways can influence
both the average molecular weight and the degree of
cross-linking, depending on the nature of the benzoyloxy
radical.
yl-2-(4-propenoylphenyl)-1,2-ethanedione/styrene6 pro-
duces one “free” and one polymer-bound acyloxy radical.
Cross-linking initiated by the polymer-bound radical
(Scheme 4) was evidenced by the insolubility of these
polymers in common solvents after being heated. How-
ever, thermolysis of a low-molecular-weight BP-contain-
ing molecule doped in PS resulted in a reduction in
polymer chain lengths and no insoluble fraction.7 In this
case, neither of the benzoyloxy radicals is covalently
attached to the polymer chains. Their abstraction of a
hydrogen atom from the polymer backbone favors chain
scission, and addition to a pendant phenyl ring does not
result in cross-linking.
It is known that the reverse of the addition reaction
of benzoyloxy radicals to a variety of aromatic rings
(n.b., Scheme 4), including to styrene monomer, is
extremely fast.22-24 Despite that, decomposition of ben-
zoyl peroxide molecules in PS films leads the addition
of benzoyloxy,7 as it does when our peroxidized copoly-
mer is heated or irradiated. The reason for the additions
to the aromatic rings is related to the high concentration
of molecular oxygen in the films, presumed to be higher
than in solutions of benzene or toluene (where similar
phenomena occur23,24). Molecular oxygen can remove a
hydrogen atom at the ipso carbon of addition at a rate
For instance, thermolysis of each pendant diacyl per-
oxide group of irradiated 1-[4-(2-methacroyloxyethoxy)-
phenyl]-2-phenyl-1,2-ethanedione/styrene5 and 1-phen-

1310 Mosna´cˇek et al.
Macromolecules, Vol. 37, No. 4, 2004
that is at least comparable to the rate of the reverse
addition of benzoyloxy from the intermediate.
The structures of the reaction sites (junction points)
were investigated by 13C NMR spectroscopy. A VBz/S
film that had been irradiated at >400 nm and ambient
temperature to about 70% conversion of the 1,2-dicar-
bonyl groups was heated to decompose ca. 50% of the
so-formed peroxides. This procedure corresponds to
about 1.5 mol % (based on monomer structure unites)
of reaction sites. Because of reactions not leading to
cross-linking, especially the formation of carboxylic
acids, the concentration of cross-linking sites is <1 mol
%. The film was swollen in deuteriochloroform in a 1
cm tube for 2 days, and FIDs from it were accumulated
for 70 h to obtain a spectrum with good signal-to-noise
ratios. It contained a small peak ascribed to peroxide
carbonyls at 163.12 ppm and larger peaks from 1,2-
dicarbonyl groups at 194.29 and 194.7 ppm. However,
no peaks due to the ester groups were detected, pre-
sumably because of the long relaxation times of the
carbon atoms near cross-linking sites. Moreover, the
formed ester structures are probably a mixture from o-,
m-, and p-substitution on polystyrene phenyl rings. For
these reasons, we are unable to estimate the fraction of
peroxide groups that result in cross-links when they
undergo thermal decomposition.
Prior to irradiation, VBz/S can be dissolved completely
in chloroform and several other solvents at room tem-
perature. However, even without heating, only ca. 25%
remained soluble in chloroform after a film was irradi-
ated at >400 nm until all of the benzil groups had been
oxidized to BP moieties. After thermal decomposition
of BP groups, the same film became completely insoluble
in chloroform. Complete insolubility in chloroform was
also achieved after irradiation of VBz/S films at 366 nm
for 228 h or at 313 nm for 88 h without heating. In the
IR spectra of Figures 3, 4, and 8, the absorbances
associated with benzoic acids are about half as intense
as those of the esters, the groups probably responsible
for the cross-links. In these cases, as well as those in
which the benzil chromophore is bound to the main
chain by different tethers,5,6 aromatic ester formation
should be an important process for cross-linking with
styrene.
Protracted irradiation of a VBz/S copolymer film at
254 nm resulted in much more film insolubility (about
22%) than conversion of benzil moieties (about 12%).
The very long chains of the polymer need few cross-links
to become insoluble, even when long chain segments
(lying deeper than the regions of radiation penetration
and benzil reaction in the film) are completely free of
cross-links.
P h otooxid a tion of P VBz Hom op olym er . As a
comparison, the photooxidation of pendant benzil groups
of the PVBz homopolymer was examined also as an
admixture with polystyrene. Since the two polymers are
immiscible, the vast majority of benzil groups are in an
environment that approximates that of neat PVBz films.
Polystyrene was added because the neat PVBz films
could not be made sufficiently thin for the progress of
the reactions to be followed by IR spectroscopy.
F igu r e 9. FT-IR spectra of a polymer film containing an
admixture of 4 mg of homopolymer PVBz and 16 mg of PS
irradiated at >400 nm in a Spectramat apparatus at RT in
air for various periods.
groups in the regions 1710-1750 and 1650-1710 cm-1
increased a great deal. These results are in stark con-
trast to those found upon irradiation at >400 nm of
VBz/S copolymer films (vide ante). Instead of the BP
groups being stable, as they were to >400 nm radiation
in the VBz/S copolymer, they react rapidly in the PVBz
homopolymer. This comportment appears to be general
for homopolymers containing pendant benzil groups be-
cause irradiations at >400 nm of an admixture of poly-
[1-(4-methacroyloxyethoxyphenyl)-2-phenyl-1,2-ethanedi-
one] in PS film produced changes in the IR spectra like
those in the PVBz-polystyrene mixture.
The conversion of the as-formed BP groups in the
homopolymers appears to be due to energy transfer from
excited triplet states of the remaining benzil groups (ET
) 53.4 kcal mol-1).25 They possess more than sufficient
energy to cleave -O-O- bonds of the BP groups (ED
∼
33 kcal mol-1).26 At the very high local concentrations
of donors and acceptors, the energy transfer must be
very efficient. The results demonstrate also that the
polystyrene is segregated from the aggregated chains
of the PVBz and PBzMA homopolymers and that stable
BP groups can be made photochemically from benzil
groups only in copolymers in which the second monomer
is in excess.
F or m a tion of a Nega tive Ton e Im a ge in th e
VBz/S Cop olym er . As shown in Figure 6, irradiation
of VBz/S copolymer at 254 nm leads to formation of a
ca. 2 µm film layer that is highly cross-linked and,
consequently, insoluble in all solvents examined. To
determine the feasibility of employing VBz/S as a
photoresist (i.e., to form nonerasable images), it was
spin-coated as ca. 0.5 µm thick films onto silicon wafers
from a chloroform solution. Then, 254 nm radiation was
passed through a mask that was placed atop a quartz
plate over the samples for 1, 2, and 10 min periods.
Finally, the unirradiated parts were dissolved in isobu-
tyl methyl ketone to develop the images. Although the
experiments were not optimized for sensitivity or reso-
lution, the pattern after 10 min irradiation contained
features as narrow as 12 µm (Figure 10), comparable
in size to the smallest apertures in the mask. Although
the photoresists commonly employed industrially are
based on photoacid- or photobase-amplified reactions
that result in image formation in aqueous bases,27,28 the
Irradiation of the mixed polymer film at >400 nm led
to a decrease in absorbances in the 1650-1690 cm-1
region where benzil bands are found (Figure 9). How-
ever, the loss of benzil was accompanied by the appear-
ance of a weak peroxide absorbance at 1750-1800 cm-1
.
Instead, the absorbances associated with ester and acid

Macromolecules, Vol. 37, No. 4, 2004
Preparation of 4-Vinylbenzil 1311
cial support. The authors thank Drs. T. Liptaj and N.
Pronayova´ for useful suggestions and for recording some
of the NMR spectra.
Refer en ces a n d Notes
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F igu r e 10. Developed images in a VBz/S copolymer film
supported on a silicon wafer achieved after 10 min of irradia-
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VBz/S copolymer may fill a niche for specialized ap-
plications.
Con clu sion s
We have demonstrated that poly(4-vinylbenzil) and
a copolymer of 4-vinylbenzil with styrene undergo
efficient peroxidation of the 1,2-dicarbonyl functionality
when their films are irradiated in air. Depending upon
the wavelength of irradiation, the conversion dicarbonyl
to peroxy moieties can be nearly quantitative (at >400
nm), or they can undergo subsequent in situ reactions
involving the cleavage of the peroxy bonds and net cross-
linking of polymer chains (at shorter wavelengths). The
reactions can be mediated by controlling the density of
1,2-dicarbonyl groups; greater spatial dispersion at-
tenuates photosensitized decompositions of peroxy groups.
Finally, the ability of these polymers to form images
when irradiated at 254 nm through a mask has been
demonstrated. In addition, we expect that these poly-
mers or variants of them will find applications in areas
where controlled cross-linking or decomposition is de-
sired.
(27) Shirai, M.; Suyama, K.; Tsunooka, M. Trends Photochem.
Photobiol. 1999, 5, 169.
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Fouasier, J . P., Rabek, J . F., Eds.; Elsevier: London, 1993;
Vol. 4, Chapter 11.
Ack n ow led gm en t. J .M. and I.L. thank the Grant
Agency VEGA through Grant 2/3002/23, and R.G.W.
thanks the U.S. National Science Foundation for finan-
MA030213J