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Aromatic Keto Compounds as Initiators in Photopolymerizations.

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Aromatic Keto Compounds as Initiators in Photopolymerizations
By H.-G. Heine, H.-J. Rosenkranz, and H. Rudolph[*]
Dedicated to Professor Otto Bayer on the occasion of his 70th birthday
The present article deals with the primary photochemical reactions of carbonyl compounds commonly used as photoinitiators. It is shown that radical formation results
mainly from three processes, i. e. hydrogen abstraction, P-cleavage, and a-cleavage. The
difference in the suitability of the radicals for chain initiation is discussed.
Photopolymerizations are initiated by radical formation
through the action of light on light-sensitive compounds
(photoinitiators) added to the vinyl monomers. Compounds of many classes can be used for this initiation[’];
the most important of these are carbonyl compounds
with UV absorptions in the range between 300 and
400 nm. The known compounds of this type that are
suitable for practical use and the reactions of these compounds that lead to the initiation of a polymerization
chain form the subject of the present article.
1. Chain Initiation by Hydrogen Abstraction
The photoreduction of benzophenone ( 1 ) to benzopinacol
( 3 ) is probably one of the best-known light-induced reactions. There have been numerous attempts to make use
(2) +
these esters with a solvent containing ether groups (diethyl ether, tetrahydrofuran, polyethylene glycol, or methyl
glycol acetate), polymerization begins after a very short
timeC3].Benzophenone has an equally good initiator
action even without solvents in esters of acrylic acid
with an alcohol containing ether groups, such as diethylene glycol diacrylate. Very slow polymerization is
observed, on the other hand, when benzophenone and
alkyl acrylates are exposed to light in isopropanol.
This observation is surprising at first, since kinetic
measurements indicate that hydrogen abstraction occurs
at almost equal rates on exposure of benzophenone to
light in ethers and in isopropan~l[~I,
and the quantum
yield of the photoreduction in isopropanol is almost 1.
According to investigations by E l a d 5 ] , however, the free
radicals formed from ethers have a strong tendency to
add to double bonds. Though a corresponding addition
of semipinacol radicals has occasionally also been observed[‘], ether radicals add preferentially when present
together with semipinacol radicals[71.
The difference in the initiator action of benzophenone
in the presence of isopropanol and ethers corresponds
to the variation in the ease of addition of the free radicals formed by hydrogen abstraction from the solvent.
The role of the semibenzopinacol radicals (2) invariably
formed in photoinitiation with benzophenone, which, according to Braun[*], also initiate polymerizations by
hydrogen transfer, appears to be of minor importance
according to these findings.
As mentioned previously, the initiation of polymerization
by hydrogen abstraction is effected by triplet-excited
of the ability of benzophenone to form free radicals by
hydrogen abstraction in the excited triplet state for the
initiation of vinyl polymerizations[21.However, these attempts have led to conflicting results. Thus in the common
acrylic or methacrylic esters, such as methyl, ethyl, or
n-butyl acrylate or methacrylate, benzophenone has only
a very weak initiator action, if any. On the other hand,
if benzophenone is exposed to light in a mixture of
[*] Dr. H.-G. Heine, Dr. H.-J. Rosenkranz, and Dr. H. Rudolph
Wissenschaftliches Hauptlaboratoriumder Bayer A G
41 5 Krefeld-Uerdingen (Germany)
0 0
H ~ C - ~ H - C O O+
Angew. Chem. internat. Edit. 1 Vol. 11 (1972) No. I 1
benzophenone (ET= 68.5 k c a l / m ~ l ~ ~It] )is. therefore not
surprising that benzophenone is practically inactive in
polymerizable mixtures containing styrene (ET= 61 kcal/
rnoI['Ol), 4.9. in solutions of unsaturated polyesters in
styrene" '1, even if the unsaturated polyester contains
groups suitable for hydrogen abstraction, e.g. by incorporation of oxaalkyldiols. In this case, exposure to light
results in quenching of the triplet-excited benzophenone
by styrene without any chemical reaction.
C-halogen bond. This cleavage has been described in the
literature for the formation of bidesyl (8) by photolysis
of desyl chloride ( 7)[19].
In some cases, however, an energy transfer of this type
is the basis of the initiation of a polymerization chain,
as in the benzophenone-sensitized system maleic anhydride/cyclohexene[' ']. This initiation mechanism is of
no practical importance, since it leads only to oligomeric
Many other, mainly aromatic carbonyl compounds and
a-diketones behave photochemically in the same way as
benzophenone[' 31,and are fundamentally capable of initiating polymerizations by hydrogen abstraction" 41. However, these compounds are Iess important, either because
of a less favorable UV absorption or because of a lower
quantum yield in the primary step. Only a few quinones,
e.g. anthraquinone derivatives, exhibit good initiator
activity[' These compounds have a long-wave absorp
tion, which allows excitation even with daylight. They are
used as initiators in copyhg systems involving crosslinking by photopolymerization (dry-resist foils for the production of printed circuits).
We have investigated photoinitiation by p-halomethylbenzophenones. After exposure of p-chloromethylbenzophenone (9) to light in benzene, we were able to isolate
the dimer (20) as a secondary product of the phenylogous p-cleavage' 'O].
2. Chain Initiation by P-Cleavage
Another type of initiation is exhibited by aromatic a-haloketones ( 4 ) which are frequently mentioned in the Iiteraturel I 61 as photoinitiators. In addition to phenacyl
chloride and bromide, which are possibly the best-known
representatives, this group of compounds includes desyl
chloride and practically all phenyl ketones with one or
more halogen atoms in the a-position.
X = C1, B r
R' = RZ = C1, Br, H, Alkyl, Phenyl
R2 = C1, Br, H, Alkyl, Phenyl
The photolysis of this group of initiators evidently yields
two radicals of differing reactivity, i. e. halogen and phenacyl radicals or their vinylogs and phenylogs. The pronounced tendency of halogen radicals to add to double
bonds and thus to initiate polymerization processes is well
known[''], and there can be little doubt that chlorine or
bromine radicals liberated when these initiators are exposed to light are of primary importance in chain initiation.
The resonance stabilization of the phenacyl radical ( I O U )
and to an even greater extent of the p-benzoylbenzyl
radical (20b) probably explains a reduced tendency for
these radicals to add to unsaturated systems.
It is not surprising that o-trichlorocrotonophenone ( 5 ) [ ' '1
and halogenated p-alkylbenzophenones (6)l' are also
good photoinitiators. These vinylogous and phenylogous
a-haloketones react in the same way as the compounds
of type ( 4 ) on exposure to light, with homdysis of a
X = C1, Br
R' = R2 = C1, Br, H, Alkyl
R' f R2 = C1, Br, H, Alkyl
Angew. Chem. internat. Edit. i Vol. I 1 (1972) i No. 11
The chloromethylated benzophenones, as very effective
initiators, are used in the photopolymerization of acrylic
acid derivatives. Moreover, bromomethylated benzophenones are suitable for the light-induced curing of unsaturated polyester resin-styrene mixtures. The cleavage
into radicals is evidently faster in this case than the
quenching of the excited benzophenone chromophore by
styrene'' 1' .
As well as the haloketones, the ketosulfides (11) and
(12), which are obtainable by reaction of these compounds with thiophenols, can also be used as photoinitiators[221(Table I). They react in the same way as the
A further ten years passed before the light-induced copolymerization of unsaturated polyesters with styrene in
the presence of benzoin ethers as initiators was developed
into an industrially used vanishing and coating process[26!
As the importance of this process grew, a more thorough
knowledge of the classes of substances that could be used
became desirable. We have therefore synthesized a large
number of benzoin derivatives (131, a selection of which
is shown in Table 2["1.
Table I. Reactivity of the keto sulfides ( / / I and ( 1 2 ) in the lightinduced curing of unsaturated polyesters [*].
Table 2. Reactwity of some benzoin derivatives ( 1 3 ) in the lightinduced curing of unsaturated polyester resins.
Bleeding time of
paralfin (min)
[*] The polymerization experiments were carried out with a solution of
an unsaturated polyester (Roskydal W 13, Farbenfabriken Bayer, solids
content 54 wt. YO in styrene) containing 0.1 wt. YO of parafin (m.p.
52-53'C) in the form of a 10% (w/w) solution in toluene. 2 wt. Yo of
photoinitiator was added t o this mixture. Coated films 500 pm thick
were exposed t o a fluorescent tube (Osram L 40 W/70-1) at a distance
of 5 cm, and the time taken for tile paraffin to bleed out was determined. The exposure time required for complete hardening was taken
as the time when the coating could no longer be marked with a
rounded pencil lead of hardness 6 H.
C H ~ C H ~ C D O C ~ H ,c2n5
CH 2-0-Tos
Paraffin bleeding
time [a] (rnin)
I .5
I .2
Hardness [a]
[a] See footnote [*] t o Table 1
haloketones on exposure to light, with p-cleavage, which
is not suppressed by triplet quenchers such as 1,3-cyclohexadiene or piperylene. The thiyl radicals formed are
probably mainly responsible for the initiation of polymerization ; addition reactions of thiyl radicals to double
bonds are generally known, and are used in preparative
chemistry1231.The equally well-known reversibility of these
reactions, which partly forms the basis of the action of
thiols as regulators in thermal polymerization processes,
is probably the reason for the unsatisfactory quality of
some polymers prepared with photoinitiators listed in
Table 1.
3. Initiation of Polymerization by a-Cleavage
Benzoin and its derivatives are extremely important as
photoinitiators. They are currently used in unsaturated
polyester resin varnishes, in compositions for the production of acrylate-based printing plates, and in printing inks
that are dried by UV light. Benzoin was first claimed
for light-induced polymerization in 19451241,but it was
more than ten years before a polymerization process
initiated by benzoin ethers was used in practice[251.This
was the process introduced by DuPont under the name
Dycryl for the production of plastic printing plates for
relief printing and dry offset.
It can be seen that benzoin alkyl ethers and x-alkylated
benzoins are particularly reactive. Benzoin itself has only
a relatively low reactivity in the system investigated.
Among the initiators listed, however, only those that
exhibit high reactivity without impairing the storage life
of the resin are suitable for practical use. This condition
is satisfactorily fulfilled only by benzoin ethers of secondary alcohols and by the %-substituted benzoin and their
ethers. Some of these compounds were used for investigations on the photochemical behavior of this class of
substances as well as on the initiation process.
The photochemical decomposition of benzoin has been
known for a long
The cleavage into benzoyl and
hydroxybenzyl radicals[301as postulated by De M u ~ o [ ~ ~ ~
seems plausible also in the light of studies on deoxybenzoin~[~~].
It seemed obvious that %-cleavage must also be the
primary photochemical process in photopolymerizations
initiated by benzoin alkyl ethers. When benzoin ethers
( 1 4 ) are exposed to light, benzaldehyde and benzil can
in fact be isolated as reaction products of the benzoyl
radical, and the dimerization product ( 1 5 ) as a reaction
product of the alkoxybenzyl radical~"zl.
In the presence of oxygen, the initially formed radicals
are completely captured with formation of benzoic acid
Angew. Chem. infernal. Edit. 1 Vol. 11 (1972) / N o . I 1
and esters of benzoic acid, respectively. As is shown by
quenching experiments with naphthalene o r piperylene,
r-cleavage of the benzoin ethers cannot be quenched in
yet very highly developed, and radical-stabilizing substituents could thus have little effect on the rate of a-cleavage.
An explanation that seems much more plausible is one
involving polar effects, such as have recently been discussed by Riichardt for thermal free-radical reactions1371.
Whereas it was possible to establish the facts presented
above by kinetic and analytical studies, it is relatively
difficult to determine the subsequent steps of the photoinitiation of free-radical polymerizations by benzoin derivatives.
benzene. Even with piperylene as the solvent, no inhibition is observed. %-Cleavage is evidently faster than the
transfer of energy to the triplet quenchers piperylene
(ET=59 kcal/m01)[~~1
and naphthalene (ET=61 kcal/
mol)[’], the rate of which is practically diffusion-controlled
(5 x lo9 liter/mol.s in benzene). a-Cleavage thus competes
with intersystem crossing, whose rate for acetophenone
has been estimated to be 10’’sand therefore
probably takes place from an excited singlet state. The
solvent dependence of the long-wave UV maximum
(h,,,=330 nm in ethanol; 345 nm in cyclohexane) indicates that this is an (n-m*) state.
This could be one of the reasons for the difference in the
reactivities of the benzoin derivatives listed in Table 2,
since, as was mentioned earlier, styrene like piperylene
has a strong quenching action on (n-m*) triplet excited
phenyl ketones191. The photoreactions of some of the
benzoin derivatives that are not very effective as photoinitiators in this system take place preferentially from an
excited triplet state. For example, the photolysis of benzoin
acetate in benzene is suppressed by piperylene ( 1 M ) [ ~ ~ ] .
Quenching experiments also indicate that benzoin, which
is relatively inactive in unsaturated polyester/styrene
mixtures, undergoes the r-cleavage mainly from a triplet
state. r-Alkylated benzoins and in particular their ethers,
which have a very good initiator action (Table 2), again
react preferentially from the excited singlet state[321,which
suggests that the rate of formation of free radicals is
increased for these compounds by the higher degree of
substitution on the r - C atom. They also have a more
favorable UV spectrum than benzoin for excitation with
conventional UV lamps (Fig. I).
The difference in the behavior of the benzoin derivatives
listed in Table 2, and particularly the pronounced differences between benzoin alkyl ethers and benzoin esters,
could be explained by the stabilizing effect of the sc-hetero
atom on the radicals being formed. However, kinetic
studies on the photochemical r-cleavage of some phenyl
ketones show that the transition state of the free-radical
formation lies on the reaction coordinate at a very early
stage[3h1;at this stage, the free-radical character is not
Angew. Chem. internat. Edit. 1 Vol. I I (1972) 1 No. I 1
Fig. I . U V spectra of some benzoin derivatlves in benzene. 1, Benzom
2, r-methylbenzoin: 3, r-hydroxymethylbenzoin; 4, benzoin isopropyl
ether: 5, benzoin phenyl ether: 6. r-methylbenzoin methyl ether.
The addition of acyl radicals generated thermally or by
irradiation to unsaturated compounds has been described
in numerous
that of benzoyl radicals to maleic
esters was also observed by Patrick[391.
The corresponding
reaction of acyl radicals with styrene is described as exot h e r m i ~ 1 ~Chain
~ 1 . initiation by the benzoyl radical formed
on photolysis of the benzoin derivatives i s therefore quite
O n the other hand, the behavior of the second fragment,
i.e. the substituted benzyl radical, is less uniform in the
presence of unsaturated compounds. Our experiments
have shown that benzyl ether radicals from the photolysis of benzoin alkyl ethers dimerize even in the presence
of piperylene and styrene, while the formation of benzaldehyde and benzil from the benzoyl radicals is suppressed
in the presence of small quantities of these unsaturated
compounds by addition to the double bonds[”’. These
observations indicate that r-alkoxybenzyl radicals have
less tendency to add to unsaturated compounds.
Another possible mode of reaction of the substituted
benzyl radicals is the fragmentation postulated by B m m
for semibenzopinacol radicals (2) [*I. The secondary radical
similarly formed from semibenzopinacol ether radicals IS
thought to be responsible for the actual chain initiation
in various thermally initiated free-radical polymerizat i o n ~ [ ~A’ mode
of reaction that agrees with this mechanism has been observed in photopolymerizations initiated
by benzoin derivatives only for benzoin and its r-substituted derivatives. Thus r-methylbenzoin ( 16) gives the
expected carbonyl compound acetophenone (17) on
exposure to light in styrene.
The most effective benzoin-based photoinitiators, i. e. the
benzoin alkyl ethers and the a-alkylbenzoin ethers, evidently d o not undergo fragmentations of this type in the
presence of unsaturated compounds. Thus the expected
carbonyl compounds were not detected when benzoin isopropyl ether or cr-methylbenzoin methyl ether was exposed
to lightr4*l.
However, since only very low concentrations are used in
photoinitiation with these compounds (1 to 2% based on
the unsaturated polyester) and the mobility of the free
radicals formed is very quickly restricted by the rapid
gelling of the system, it may be assumed that the dimerization observed on exposure to light in solution does not
take place during the photoinitiation of unsaturated polyesters. It is more likely chat the benzyl ether radicals
ultimately also add to double bonds in the same way as
thermally produced alkoxybenzyl radicals add to maleic
anhydride[431,and thus initiate polymerization chains, or
that they react with growing free-radical chains to cause
chain termination. This is probably the explanation of
Mochel's rep0rt~*~1
of the incorporation of radioactivity
into the polymethyl methacrylate obtained by photoinitiation with [methyl-'4C]-benzoin methyl ether.
In our view, the difference in the reactivities of the
benzoyl and substituted benzyl radicals is essential to the
suitability of many benzoin derivatives for photoinitiation.
Only in this way, apparently, is it certain (and this also
applies to the other types of initiators mentioned here)
that a chain growth initiated by an initiator radical is not
prematurely terminated by combination with another
chain initiated at the same time in its immediate vicinity
or with a reactive second radical.
Received: July 3, 1972 [A 915 IE]
German version: Angew. Chem. 84,1032 (1972)
Translated by Express Translation Service, London
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Angew. Chem. internut. Edit.
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photopolymerization, initiator, compounds, keto, aromatic
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