Naphthodioxinone-1,3-benzodioxole as photochemically masked one-component type II photoinitiator for free radical polymerization

Article abstract of DOI:10.1002/pola.26034

A 1,3-benzodioxole derivative of naphthodioxinone, namely 2-(benzo[d][1,3]dioxol-5-yl)-9-hydroxy-2-phenyl-4H-naphtho[2,3-d][1,3] dioxin-4-one was synthesized and characterized. Its capability to act as caged one-component Type II photoinitiator for free r

Full text of DOI:10.1002/pola.26034

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Naphthodioxinone-1,3-benzodioxole as Photochemically Masked
One-Component Type II Photoinitiator for Free Radical Polymerization
Volkan Kumbaraci,1 Binnur Aydogan,1 Naciye Talinli,1 Yusuf Yagci1,2
1Department of Chemistry, Istanbul Technical University, Maslak, TR-34469 Istanbul, Turkey
2Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
Correspondence to: N. Talinli (E-mail: talinlin@itu.edu.tr) or Y. Yagci (E-mail: yusuf@itu.edu.tr)
Received 31 December 2011; accepted 1 March 2012; published online
DOI: 10.1002/pola.26034
ABSTRACT: A 1,3-benzodioxole derivative of naphthodioxi-
none, namely 2-(benzo[d][1,3]dioxol-5-yl)-9-hydroxy-2-phenyl-
4H-naphtho[2,3-d][1,3]dioxin-4-one was synthesized and char-
acterized. Its capability to act as caged one-component Type II
photoinitiator for free radical polymerization was examined.
Upon irradiation, this photoinitiator releases 5-benzoyl-1,3-ben-
zodioxole possessing both benzophenone and 1,3-dioxole
groups in the structure as light absorbing and hydrogen donat-
ing sites, respectively. Subsequent photoexcitation of the ben-
zophenone chromophore followed by hydrogen abstraction
generates radicals capable of initiating free radical polymeriza-
C
V
tion of appropriate monomers.
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J Polym Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: irradiation; naphthodioxinone; photopolymeriza-
tion; radical polymerization; Type II photoinitiator
INTRODUCTION Photoinitiated polymerization is a powerful
industrial process widely used in various applications includ-
ing printing inks, adhesives, surface coating, optical wave-
guides, microelectronics, and printing plates.1–4 Photoiniti-
ated polymerizations have gained considerable attention
because of their superior advantages such as rapid curing
even at ambient temperatures, environmental issues, and
low energy consumption.5 Much effort has been devoted to
free radical systems mainly due to the availability of wide
range of monomers polymerizable by free radical mechanism
and photoinitators having sensitivity in both UV and visible
range.6 Among the most studied, photoinitiator systems are
those in which initiating free radicals are formed by a bimo-
lecular process consisting of a triplet excited state of an aro-
matic carbonyl compound and a hydrogen donor compound
as a co-initiator. The photopolymerization of vinyl monomers
is usually initiated by the radicals produced from the hydro-
gen donor. The ketyl radicals are usually not effective in ini-
tiating free radical polymerization due to the steric hin-
drance and the delocalization of unpaired electrons.
Benzophenone and derivatives,7–9 thioxanthones,10–14 ben-
zil,8 quinones,8 and organic dyes15–19 are typical examples of
this type of photoinitiators. Based on their hydrogen donat-
ing capability, alcohols, amines, ethers, and thiols are used as
co-initiators. Benzophenone is one of the most widely used
photoinitiator due to its good surface curing and solubility
properties together with the high quantum efficiency of the
hydrogen abstraction particularly with amines.
Benzodioxinones were recently reported as new photosensi-
tive compounds, which form functionalized salicylate esters
upon irradiation in the presence of alcohol and phenols.20 In
our previous studies, we reported several photoinduced reac-
tions of benzodioxinones in polymer synthesis and UV curing
applications.21 Typical examples include oligoester synthe-
sis,22 initiation of free radical and cationic polymeriza-
tions,23,24 UV curing of monomers or preformed polymers,25
and graft copolymer synthesis.26 In these processes, depend-
ing on the particular application, two different reactive spe-
cies are involved. Upon irradiations, benzodioxinone forms
benzophenone and ketene. Although benzophenone can par-
ticipate in radical forming process by a second photon excita-
tion under variety of conditions, the ketene compound readily
reacts with nucleophiles such as hydroxyl and amine groups
leading to the formation of esters and amides. The overall use
of benzodixoxinones in synthetic polymer chemistry and as
photoinitiators for free radical polymerization is summarized
in Scheme 1. Reportedly, ketene type reactive intermediates
can also be produced thermally from dialkyl Meldrum’s acid
and successfully used for the modification of macromolecular
structures and crosslinking of polyolefines.27,28
Cyclic acetals were shown to be alternative hydrogen donors
for bimolecular photoinitiating systems. Typical example
includes
a natural component, 1,3-benzodioxole, which
replaced conventional hazardous amine co-initiators for dental
applications.29 More recently, it was shown30 that 6-benzoyl-
1,3-benzodioxolane (BBDO) can initiate the polymerization of
C
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SCHEME 1 Photoinduced reactions of benzodioxinone. Ester formation (a) and initiation of free radical polymerization (b).
acrylates and multifunctional monomers without an additional
hydrogen donor in the mixture. This benzophenone derivative
contains both light absorbing and hydrogen donating sites in
the structure.
benzophenone (Fig. 2). Notably, BHND has a spectrum which
is typical for napthodioxinone chromophore absorbing
strongly in far UV and possessing an absorption maximum of
the p ! p* transition of the long conjugated p-ring system
around 376 nm typically with high extinction coefficient.23
On the other hand, BBDO has an additional absorption at
316 nm which belongs to benzodioxole structure.
As part of our continuing interest in design and development
of photoinitiating systems, and expand their use in macro-
molecular syntheses, we report herein the use of naphtho-
dioxinone-1,3-benzodioxole as one-component Type II
photoinitiator.
UV spectral change of BHND upon irradiation at k > 300 nm
was also investigated. As shown in Figure 3(a), upon irradia-
tion, the absorption at 238 nm which belongs to benzophe-
none diminished and prolonged irradiation resulted in the
development of a new absorption band at 315 nm corre-
sponding to the absorption of BBDO. Considering BBDO has
both benzophenone and hydrogen donors in the structure,
further irradiation induces radical generation. Thus, the
absorption of BBDO correspondingly decreases [Fig. 3(b)].
RESULTS AND DISCUSSION
As stated in the ‘‘Introduction’’ section, the aim of this work
is to exploit a new masked Type II photoinitiator which does
not require additional hydrogen donor. The approach is to
use specially designed benzodioxinone containing hydrogen
donating sites that upon irradiation with UV light undergoes
photodecomposition to release benzophenone for the subse-
quent photoinitiation process. For this purpose, first, the pre-
cursor 5-benzoyl-1,3-benzodioxole (BBDO), known as Type II
photoinitiator possessing hydrogen donating groups, was
synthesized by a Friedel-Crafts reaction. The ultimate com-
pound, 2-(benzo[d][1,3]dioxol-5-yl)-9-hydroxy-2-phenyl-4H-
naphtho[2,3-d][1,3]dioxin-4-one (BHND), was then synthe-
sized by reacting 3,5-dihydroxynaphthoic acid with BBDO
according to literature procedures (Scheme 2).31
To gain more insight into the reaction mechanism, the forma-
tion of descendant product, BBDO during the photolysis of
BHND at k > 300 nm in the absence of monomer was inves-
tigated by GC analysis. It is apparent from Figure 4 that the
concentration of BBDO as determined with respect to the
The structures of both BBDO and BHND were confirmed by
1H NMR and UV–vis analysis. Figure 1 shows the 1H NMR
spectra of BBDO and the corresponding masked photoinitia-
tor BHND. As can be seen, both compounds exhibit charac-
teristic protons and signals emerging from the dioxolane and
aromatic groups. UV–vis spectral analysis also revealed that
both compounds exhibit absorption characteristics similar to
SCHEME 2 Synthesis of 2-(benzo[d][1,3]dioxol-5-yl)-9-hydroxy-
2-phenyl-4H-naphtho[2,3-d][1,3]dioxin-4-one (BHND).
2
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FIGURE 1 1H NMR spectra of (a) BBDO and (b) BHND in CDCl3.
standard 1,4-dimethoxybenzene increased rapidly up to the
certain irradiation time from which point a leveling off is
observed. Presumably, at this point, the precursor BHND is
completely consumed or the competing absorption of BBDO
prevents further decomposition. It should also be pointed
out that concomitantly formed naphthalene ketene transient
may also absorb light. However, its absorption could not be
detected by stationary photolysis as it is short lived and rap-
idly reacts with the hydroxyl group present in the structure.
The absorption of the resulting ester does not interfere with
that of BBDO since naphthalene derivatives have absorption
below 300 nm.23
conversion was obtained in the case of dimethylformamide
as it is known that radical polymerizations are not sensitive
to the polarity of the solvent.
BHND was used as a photoinitiator for the polymerization of
methyl methacrylate (MMA) under nitrogen atmosphere. A
comparison of the results obtained with different solvents
and initiator concentrations is listed in Table 1. As can be
seen, the conversion increased with increasing photoinitiator
concentration. Among the several solvents tested, tetrahydro-
furan (THF) seemed to be a quite suitable solvent for the po-
lymerization that is known to be a good hydrogen-donating
solvent for excited aromatic carbonyl compounds. The lowest
FIGURE 2 UV spectra of BBDO (1.2 ꢀ 10ꢁ5 mol Lꢁ1) and BHND
(1.2 ꢀ 10ꢁ5 mol Lꢁ1).
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FIGURE 3 Typical UV spectral change of BHND (a) at first 4 min and (b) after 4 min on irradiation at k > 300 nm under nitrogen in
CH2Cl2.
The overall polymerization process can be represented by
Scheme 3. The excited triplet of BBDO, which is populated
by the successive absorption of the ground state BBDO
released by the photolysis of BHND, abstracts hydrogen from
another ground state BBDO to generate the initiating radi-
cals. The ketene intermediate also formed (see Scheme 3) in
the first stage rapidly reacts with hydroxyl groups to form
an ester.
The efficiency of BHND in photocuring of formulations con-
taining multifunctional monomer was also studied. The heat
released during the photocuring of the formulations contain-
ing BHND, BBDO, and BP/TEA was followed by photo-DSC.
Kinetic profiles referring to the polymerization of trimethy-
lolpropane triacrylate (TMPTA) under polychromatic light
emitting at k ¼ 315–450 nm are shown in Figure 6.
Although the photoinitiation occurred only after a sufficient
amount of BBDO is released by the photolysis of BHND, the
final conversion was nearly the same with bare BBDO used
formulation. Apparently, BHND can initiate the photopolyme-
rization of TMPTA more efficiently than BP/TEA. The system
has the advantage of overcoming the presence of migratables
in a coating due to the incorporation of both initiating and
hydrogen donating sites in the same structure. This is partic-
ularly important for curing applications because formula-
The results of the polymerization allow no definitive conclu-
sion whether or not the hydrogen abstraction occurs from
the BHND under these conditions as well. UV–vis absorption
spectrum of poly(methyl methacrylate) obtained by the
described photoinitiating system (Table 1, Run 1) exhibits a
weak absortion band at longer wavelengths indicating incor-
poration of BHND units into polymer chain (Fig. 5). The
observed behavior gives strong hint that the precursor
BHND also takes part in the hydrogen abstraction process to
generate initiating radicals.
tions containing amine at high concentrations cause
a
decrease in the pendulum hardness of the cured films as a
result of the plasticizing effect of amines.32 For comparison,
BHND containing formulation was also cured in air and a
relatively long induction period was observed which can be
attributed to the quenching of the excited precursor
TABLE 1 Photoinitiated Free Radical Polymerization of Methyl
Methacrylate at Room Temperature in CH2Cl2 for 4 h at
k > 300 nm
b
[BHND] ꢀ
Conversiona Mn
Run 10ꢁ3 (mol Lꢁ1
)
Solvent (%)
(g molꢁ1
) Mw/Mn
b
1
2
3
4
4.85
9.70
4.85
4.85
CH2Cl2 10
CH2Cl2 16
72,000
60,000
54,000
45,000
2.9
2.3
2.0
1.9
DMF
THF
8
12
CH2Cl2, dichloromethane; DMF, dimethylformamide; THF, tetrahydrofuran.
a
FIGURE 4 Plot of the released BBDO concentration during the
Calculated conversion in weight.
b
photolysis of BHND versus irradiation time in CH2Cl2.
Determined by GPC according to polystyrene standards.
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SCHEME 3 Photoinitiated free radical polymerization by using 2-(benzo[d][1,3]dioxol-5-yl)-9-hydroxy-2-phenyl-4H-naphtho[2,3-
d][1,3]dioxin-4-one (BHND).
compound, BHND. The later stage of the polymerization is
expected to involve similar mechanism that is described for
BBDO.30
1485, 1445, 1352; UV (CH2Cl2): kmax., nm (e, L molꢁ1 cmꢁ1);
316 (9050), 279 (6492), 238 (20941).
Synthesis of 2-(Benzo[d][1,3]dioxol-5-yl)-9-hydroxy-2-
phenyl-4H-naphtho[2,3-d][1,3]dioxin-4-one
BHND was prepared according to the modified literature
procedure.31 Thus, 3,5-dihydroxy-2-naphthoic acid (1.5 g, 7.4
mmol) and above prepared BBDO (2.5 g, 11 mmol) were put
into the flask under nitrogen atmosphere and a mixture of
trifluoroacetic acid (10 mL) and trifluoroacetic anhydride
(11 mL) were added at 0 ꢃC. Then, the solution was stirred
for 24 h at room temperature. The crude product was dis-
solved in ethylacetate and washed with NaHCO3 solution
three times. Organic phase was dried over MgSO4 and sol-
vent was evaporated. The obtained product was purified
with column chromatograpy using ethylacetate/hexane 10/
90 mixture as eluent.
EXPERIMENTAL
Materials
Unless otherwise noted, commercially available materials
were used without further purification. Thus, 1,3-benzodiox-
ole (Acros, %97), benzoyl chloride (Across, %99), benzophe-
none (ꢂ99%, Aldrich), 3,5-dihydroxy-2-naphthoic acid
(ꢂ95.0%, Fluka), trifluoroacetic anhydride (99þ%, Acros),
trifluoroacetic acid (99.5%, Acros), sodium sulfate anhydrous
(ꢂ99.0%, Aldrich), and silicagel 60 (Merck, 0.040–0.063
mm) were used as received. MMA (99%, Acros) and TMPTA
(techn., Aldrich) were passed through a column of basic alu-
mina before use. All solvents were of HPLC or ACS grade.
Solvents used for moisture sensitive operations were dis-
tilled from drying reagents under a nitrogen atmosphere:
Et2O and THF from sodium benzophenone ketyl; toluene
from sodium; CH2Cl2 from CaH2 and acetone over 4 Å molec-
ular sieves.
Yield: 0.16 g, (%5), mp 245–247 ꢃC; 1H NMR (CDCl3): d ¼
8.44 (s, 1H), 7.86 (s, 1H), 7.62 (d, J ¼ 6.5, 1H), 7.42–7.18 (m,
6H), 7.08 (m, 2H), 6.89 (d, J ¼ 7.5, 1H), 6.71 (d, J ¼ 7.5, 1H),
Synthesis of 5-Benzoyl-1,3-benzodioxole
Anhydrous FeCl3 (9 g, 55 mmol) was added at 0–5 ꢃC to a
mixture of 50 mL of anhydrous CH2Cl2 (6 g, 49 mmol) of
1,3-benzodioxole and benzoyl chloride (7.5 g, 53 mmol). The
mixture was stirred for 2 h at room temperature. The mix-
ture was poured into ice, acidified with HCl, and extracted
with CH2Cl2. The extract was washed with water until to
become neutral, dried with Na2SO4. Crude product was
obtained as brown oil after the evaporation of CH2Cl2 with
rotary evaporator. It was purified by column chromatography
(over silicagel 10% EtOAc-Hexane).
Yield: 4.0 g (36%); 1H NMR (CDCl3): d ¼ 7.74–7.71 (d, J ¼
7.5, 2H), 7.55 (t, J ¼ 7.5, 1H), 7.52–7.42 (m, 2H), 7.37–7.35
(m, 2H), 6.85 (d, J ¼ 7.5, 1H), 6.05 (s, CH2, 2H); FTIR
(cmꢁ1): 3065, 3025, 2903, 1746, 1651, 1601, 1578, 1503,
FIGURE 5 UV spectrum of PMMA (8.1 ꢀ 10ꢁ6 mol Lꢁ1) initiated
by BHND.
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FIGURE 6 Rate of photopolymerization (a) and conversion-time plot (b) for the photoinitiated free radical polymerization of
TMPTA at 30 ꢃC. ([BHND] ¼ [BBDO] ¼ [BP] ¼ 0.1 mol %, [TEA] ¼ 1 mol %, light intensity ¼ 76 mW ꢀ cmꢁ2).
5.91 (s, CH2, 2H); FTIR (cmꢁ1): 3447, 3359, 1702, 1628,
tion (Rp) is directly related to the heat flow (dH/dt) by eq 2:
1579, 1504, 1484, 1282, 1240, 1103, 1070, 1036, 830; UV
(CH2Cl2): kmax, nm (e, L molꢁ1 cmꢁ1); 376 (3458), 291
Rp ¼ dC=dt ¼ ðdH=dtÞ=DHtheory
(2)
0
(12933), 263 (52391).
CONCLUSIONS
Photopolymerization
Monomer MMA (1.0 mL, 14 mmol) in CH2Cl2 (1.5 mL) solu-
tion, BHND (5 mg, 0.01 mmol) were put into a Pyrex tube,
degassed with nitrogen and irradiated at room temperature
in a photoreactor (Rayonet) equipped with 16 lamps and
emitting light nominally at k > 300 nm. At the end of irradi-
ation, the reaction mixture was precipitated in excess metha-
nol. After being collected by filtration, the polymer was kept
in a vacuum oven for drying. Conversions were determined
gravimetrically.
In conclusion, a naphthodioxinone derivative was synthe-
sized and used as a new one-component Type II photoinitia-
tor. The actual photoinitiator, BBDO, is formed after photode-
composition of naphthodioxinone. The subsequent step
includes the radical formation by hydrogen abstraction of
photoexcited BBDO from a ground state BBDO and/or BHND.
This is an attractive photoinitiator because it does not
require an additional hydrogen donor and initiates polymer-
ization of acrylates in the presence and absence of air. The
other obvious advantage of the photoinitiating system is
related to the different absorption characteristics of the pre-
cursor and the successor compounds, that is, kmax (BHND) ¼
376 nm; kmax (BBDO) ¼ 316 nm. This behavior make the
photoinitiating system important alternative for two-photon
induced polymerizations34,35 acting at two different wave-
lengths for applications involving three-dimensional (3D)
microfabrication, optical data storage, imaging, and the con-
trolled release of biological systems. Further studies in this
line together with its use in free radical promoted cationic
polymerization are now in progress.
Photocalorimetry (Photo-DSC)
The photo-differential scanning calorimetry (Photo-DSC)
measurements were carried out by means of a modified Per-
kin-Elmer Diamond DSC equipped with a high pressure mer-
cury arc lamp. A uniform UV light intensity is delivered
across the DSC cell to the sample and reference pans. The in-
tensity of the light was measured as 76 mW cmꢁ2 by a UV
radiometer covering broad UV range. The mass of the sam-
ples was 3 mg and the measurements were carried out in an
isothermal mode at 30 ꢃC under a nitrogen flow of 20 mL
minꢁ1. The reaction heat liberated in the polymerization was
directly proportional to the number of acrylate double bonds
reacted in the system. By integrating the area under the exo-
thermic peak, the conversion of the acrylate or methacrylate
groups (C) or the extent of the reaction was determined
according to eq 1:
The authors thank Istanbul Technical University Research
Funds for the financial support.
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