Synthesis and properties of new photosensitive triazene and o-nitrobenzene methacrylates

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.04.014

New photoactive polymerizable monomers were synthesized in order to photomodulate the mechanical properties of photocurable materials. These monomers are end-capped by at least two polymerizable units (using irradiation wavelength λ1) connected by a spacer including at least one photocleavable unit (irradiation wavelength λ2 ≠ λ1) i.e. aryltriazene or 2-nitrobenzyl core. The photochemical behaviour of these systems was studied in solution and in resin formulations. Their ability to promote the hardening and crosslinking of a curable resin at λ1, then the subsequent degradation or modification of crosslinked resins under an actinic light (λ2) was evaluated by DMA for formulations including these new monomers. These new monomers were shown, in resin formulation, to readily rigidify the material upon λ1 irradiation and for most of them the subsequent photolysis of their internal linkers at λ2 irradiation was followed by a decrease of the hardness at low temperature (-20 °C). For one of them (compound 6) λ2 illumination at room temperature provoked the decrease of the mechanical properties of the solid material making it of interest for dental applications.

Full text of DOI:10.1016/j.reactfunctpolym.2012.04.014

Reactive & Functional Polymers 72 (2012) 521–532
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis and properties of new photosensitive triazene and o-nitrobenzene
methacrylates
c,
Vincent Gaud a, Fabien Rougé a, Yves Gnanou b, , Jean-Pierre Desvergne
⇑
⇑
a Polyrise SAS, 16 Avenue Pey Berland, 33607 Pessac Cedex, France
b Laboratoire de Chimie des Polymères Organiques, UMR CNRS 5629, 16 Avenue Pey Berland, 33607 Pessac Cedex, France
c Institut des Sciences Moléculaires, UMR CNRS 5255, Université de Bordeaux, 351 Cours de la Libération, 33405 Talence Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2011
Received in revised form 26 April 2012
Accepted 28 April 2012
Available online 17 May 2012
New photoactive polymerizable monomers were synthesized in order to photomodulate the mechanical
properties of photocurable materials. These monomers are end-capped by at least two polymerizable
units (using irradiation wavelength k1) connected by a spacer including at least one photocleavable unit
(irradiation wavelength k2 – k1) i.e. aryltriazene or 2-nitrobenzyl core. The photochemical behaviour of
these systems was studied in solution and in resin formulations. Their ability to promote the hardening
and crosslinking of a curable resin at k1, then the subsequent degradation or modification of crosslinked
resins under an actinic light (k2) was evaluated by DMA for formulations including these new monomers.
These new monomers were shown, in resin formulation, to readily rigidify the material upon k1 irra-
diation and for most of them the subsequent photolysis of their internal linkers at k2 irradiation was fol-
lowed by a decrease of the hardness at low temperature (ꢀ20 °C). For one of them (compound 6) k2
illumination at room temperature provoked the decrease of the mechanical properties of the solid mate-
rial making it of interest for dental applications.
Keywords:
Aryltriazene diacrylates
o-Nitrobenzene
Photochemistry
Resins
Mechanical properties
Hardness
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
triazene compounds have excellent pharmacokinetic properties
[18,19], making them interesting photocleavable groups for medi-
Photosensitive materials are of great interest for their potential
applications in various fields such as optical data storage devices
[1], photoresists [2], photo- [3–5] and micro-lithography [6,7], bio-
logical and medical purposes [8].
cal applications (dentistry, photoremovable brackets/clips, etc.)
[20] and other uses requiring environment-friendly photorespon-
sive materials. Thus, triazenes appear as excellent candidates for
weakening or modifying a polymeric network upon irradiation,
and combined with other photoactive centres they can serve to ob-
tain materials with light-tunable mechanical properties.
In this regard, photoresponsive polymers are attractive materi-
als because their structure and accordingly their properties could
be finely tuned both by appropriate functionalization of the molec-
ular backbone and polymerization. Incorporation of a photolabile
linker or pendant group could endow the material with the re-
quested behaviour for specific applications, for instance in selective
solid phase chemistry [9,10] or surface modification. In this respect,
polymers incorporating triazene core are intensively investigated
for excimer laser ablation lithography [11–13]. The triazene linker
is known to be easily cleaved upon near UV irradiation generating
molecular nitrogen and two separated radical species; this photore-
action induces the breaking of the spacer [14–17] and a clear
change in the mechanical properties of the material. Additionally,
In the present paper, we describe the synthesis of an unprece-
dented series of photopolymerizable bi- and tri-(meth)-acrylic
monomers (compounds 1–5) including at least one triazene core
as internal linker (Scheme 3). The ability of these monomers to
polymerize in resin formulations, and the photosensitivity of the
resulting materials (Scheme 1) were investigated and reported
here. In particular, the conditions that are prone to the cleavage
of the triazene moiety and the subsequent weakening of the net-
works formed by crosslinking of the (meth)acrylic unsaturations
will be examined. In parallel and for comparison purpose, two
bi-(meth)-acrylic monomer compounds (6 and 7) including a o-
nitrobenzyl group (which implies a different mechanism for the
photocleavage as shown in Scheme 2) have also been investigated
(Scheme 3), this efficient photolabile group [21–23] being used in
photolithography and for other purposes [24–29] when incorpo-
rated in polymeric chains.
⇑
Corresponding authors. Tel.: +33 (0)5 40 00 84 40; fax: +33 (0)5 40 00 61 58
(J.-P. Desvergne).
E-mail addresses: gnanou@enscpb.fr (Y. Gnanou), jp.desvergne@ism.u-bor
deaux1.fr (J.-P. Desvergne).
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.04.014

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V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
Scheme 1. Initial(left) resin including two different bis-methacrylate monomers (squares: methacrylate group, circles: photocleavable subunit incorporated as linker
between the methacrylates). Centre: crosslinked resin. Right: photo-induced fragmentation of the functional linker groups.
Chromatographic separations were performed on SDS silica chro-
magel (40–63 lm). Mass spectra were obtained on an AutoSpeq
EQ spectrometer (EI and LSIMS) and a TosSpec Maldi–Tof Micro-
mass spectrometer (Manchester, UK). Elemental analyses were per-
formed by the Microanalytical Service, Institut du Pin, University
Bordeaux 1. Electronic absorption spectra and transmission spectra
were measured on an Hitachi U-3300 spectrophotometer. Dynamic
mechanical analysis (DMA) was measured with a Perkin Elmer 7
apparatus equipped with a temperature controller (Perkin Elmer
Thermal Analysis Controller TAC7/DX) and a cooling system (Perkin
Elmer CCA7). Differential Scanning Calorimetry (DSC) measure-
ments were performed on a Perkin Elmer DSC 7. Irradiations of
the resins were performed with a Hg–Xe Efos Lite E3000 source
(50W) equipped with interferential filters (320–480 nm) and an
optical fibre (5 mm diameter). Irradiance was measured with a
EXFO 250–600 nm R5000 radiometer. A 0.5 cm band-pass filter
(>426 nm) was placed between the optical fibre and the sample
subjected to irradiation.
Scheme 2. photocleavage reaction of aryltriazene and 2-nitrobenzyl chromophores
into smaller fragments.
2. Experimental
2.1. Materials
Melting points were determined in capillary tubes on a Digital
Melting Point apparatus (Electrothermal Eng., Ltd.) and are uncor-
rected. FTIR spectra were recorded on a Perkin Elmer Paragon
1000 spectrophotometer and on a Bruker Tensor 27 spectrometer.
1H NMR spectra at 250 MHz and 13C NMR spectra at 62.9 MHz were
recorded on a Bruker 250 AC and Avance 300 instruments.
Solvents were distilled and dried prior used following standard pro-
cedures. Solvents of spectrometric grade were purchased from Aldrich
and used without further purification for UV and NMR studies.
To evaluate the mechanical properties (elastic modulus) of
uncharged and charged photosensitive resins, the material was
presented as bars in order to make a 3-point bending DMA
Scheme 3. Molecular structures of photosensitive bi- and tri-methacrylates 1–7.

V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
523
measurement. A Teflon mould of dimensions 45 ꢁ 4 ꢁ 2 (l ꢁ w ꢁ h,
mm) was stuck on a glass microscope slide using double-sided
tape, the portion of the adhesive in the bottom of the mould being
removed. The formulation was photopolymerized by irradiating
during 100 s both upper and lower surfaces, a bandpass filter
k > 426 nm (thickness 0.5 cm) being interposed between the opti-
cal fibre (light ‘‘Efos Lite’’ standard filter) and the surface of the
mould (or the glass plate for irradiation on the underside of the
sample). The cured bar was then removed from the mould and
then irradiated again (50 s) on each side, employing the same filter.
The bar was tested in DMA at ꢀ25 °C and 75 °C with a gradient of
5 °C/min and a frequency of 1 Hz. The bar was ‘photodegraded’ by
irradiating each side 300 s using the optical fibre located at 1 mm
of the surface of the sample (320 nm < k < 480 nm, 4.65 W cmꢀ2).
The mechanical properties were evaluated again in DMA between
ꢀ25 °C and 250 °C, other parameters being unchanged.
Intermediates a1 and b1 were prepared by slow and dropwise
addition of the diazonium salt to a cold solution of N-methyletha-
nol and Na2CO3 in THF/H2O (90/10). The compounds spontane-
ously crystallized in the medium after completion of the reaction
(73–90% yield). c1 was synthesised as already described in the lit-
erature [32], using acetonitrile or THF as solvent which allows a
better dispersion of the tetrafluoroborate salt than DMF (mostly
because such solvents allow for an easier isolation of the coupling
products). c1 was directly obtained as a pure compound after pre-
cipitating in the solvent (74% yield).
The esters functions of compounds a1–c1 were efficiently re-
duced by LiAlH4 in THF with two equivalents of hydride by ester
function instead of 0.5 equivalent as reported by Nuyken et al.
[5,33] for parent derivatives (this ratio generating lower yields).
Due to the acidic sensitivity of the triazene group, the crude prod-
uct was treated with a THF/ethyl acetate mixture and pure a2–c2
was obtained after washing the resulting solid with methanol
(and evaporating resulting filtrated solutions).
2.2. Synthesis of compounds 1–7
For compounds 1–3, introduction of the methacrylate groups
was performed using methacrylic anhydride and 4-dimethylami-
nopyridine (DMAP) instead of methacryloyl chloride or activated
acids with triethylamine as described by Nuyken for polyesters
incorporating triazene subunits [5,33], which in our conditions
afforded impure products with poor yields (that probably due to
the high acidic sensitivity of triazenes). In our experimental condi-
tions DMAP was used in excess and methacrylic anhydride was
slowly added for trapping protonic acid as soon as produced. The
moderate yields did not exceed 65% mainly due to the difficulties
2.2.1. triazene derivatives (1–5)
Aryltriazene units were prepared, as shown in Scheme 4, by
coupling a diazonium salt of an aromatic amine with a primary
or secondary amine in the presence of a base. Although Nuyken
et al. [30] prepared triazene derivatives with the diazonium chlo-
ride salts of the corresponding aromatic amines, we preferred, after
several attempts, to use tetrafluoroborate salts [31] as they were
obtained with excellent yields (96%) and displayed a higher ther-
mal stability.
Scheme 4. Reagents and conditions: (a) tBuNO2 (1.2 eq), BF3/Et2O (1.5 eq), CH2Cl2/THF (-25 °C); (b) N-methylaminoethanol (1.4 eq) Na2CO3, (4.4 eq), THF/H2O (9/1), 0 °C; (c)
diethanolamine (1.2 eq), Na2CO3 (4.4 eq), THF/H2O (9/1), 0 °C; (d) N,N0-dimethylethylenediamine (0.5 eq), Na2CO3 (4.4 eq), CH3CN, -10 °C; (e) LiAlH4 (4 eq), THF, 50 °C; (f)
LiAlH4 (2 eq), THF, 50 °C, (g) LiAlH4 (2.3 eq), THF, 50 °C, (h) DMAP (3.6 eq), HQME (0.055 g), THF, pyridine (1.7 eq), methacrylic anhydride (3.6 eq), 50 °C ; (i) DMAP (5.9 eq),
HQME (0.050 g), THF, methacrylic anhydride (4.5 eq), 50 °C ; (j) DMAP (5.1 eq), HQME (0.055 g), anhydrous THF, methacrylic anhydride (14.8 eq), 50 °C ; (k) DABCO (0.08 eq),
HQME (0.065 g), dibutyltin dilaurate (four drops), 2-(methacryloyloxy)ethyl isocyanate (2.2 eq), THF reflux ; (l) DABCO (0.12 eq), HQME (0.050 g), dibutyltin dilaurate (four
drops), 2-(methacryloyloxy)ethyl isocyanate (3.5 eq), THF reflux.

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V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
encountered for the purification of the compounds which formed
gels with the solvents and required several chromatographies on
silica for getting pure products.
fluoroborate 20 g (77.22 mmol), gave b1 as a yellow solid (15.99 g,
73%), mp: 60 °C. 1H NMR (DMSO-d6) dppm: 1.31 (t, 3H), 3.67 (m,
4H), 3.89 (t, 4H), 4.30 (q, 2H), 4.89 (q, 2H), 7.42(d, 2H), 7.91(d, 2H).
13C NMR (DMSO-d6) dppm: 14.16, 50.25, 56.54, 57.82, 59.59, 60.37,
119.95 (2C), 125.79, 130.06 (2C), 154.25, 166.52. IR, KBr (cmꢀ1):
3447, 3330, 3203, 2991, 2973, 2955, 2941, 2914, 2878, 2855, 1706,
1638, 1602, 1505, 1478, 1462, 1431, 1420, 1400, 1358, 1278, 1158,
1194, 1167, 1102, 1051, 869, 855, 772.
The synthesis of monomers 4 and 5 has also been considered as
they include urethane links which are known to improve the prop-
erties of the resins (ageing, surface adhesion, etc.), the kinetics of
polymerization [34–37] and their biodegradability [38]. They were
synthesised from intermediates a2 and b2 in refluxed THF in the
presence of 2-isocyanatoethylmethacrylate (1.1 eq.) and dibutylst-
annylene dilaurate [39]/DABCO [40] as catalyst. 4 and 5 were iso-
lated in 80% and 67% yields, respectively, and found very sensitive
to polymerization even at low temperature (refrigerator).
The yellow coloured compounds 1–5 were fully characterized by
the usual techniques (as well as all the intermediates, the data for
known compounds are in agreement with those given in the
literature).
2.2.1.4. 1,2-Di-[1-(4-ethyloxycarbonyl-phenyl)-3-methyl)-triaz(1)en-
3-yl]-ethane (c1). At ꢀ10 °C and under stirring, N,N0-dimethyleth-
ylenediamine (1.65 g, 18.78 mmol) in solution in CH3CN (0.1 L)
was slowly added (6 h) to a CH3CN solution (0.5 L) of 4-(ethyloxy-
carbonyl)benzene
diazonium
tetrafluoroborate
(9.78 g,
37.42 mmol) and Na2CO3 (17.38 g, 164 mmol). After addition, stir-
ring was maintained for 15 h at RT. After usual workup, the prod-
uct was purified following several precipitations at 0 °C from
concentrated diethylether solutions giving c1 as a yellow solid
(6.10 g, 74%), mp: 104 °C. 1H NMR (DMSO-d6) dppm: 3.18 (s broad,
6H), 4.04 (s, 4H), 4.31 (q, 4H), 7.32 (d, 4H), 7.93 (d, 4H). 13C NMR
(DMSO-d6) dppm: 14.36 (2C), 35.29 (2C), 54.40 (2C), 60.76 (2C),
120.33 (4C), 127.28 (2C), 130.51 (4C), 154.00 (2C), 166.60 (2C).
IR, KBr (cmꢀ1): 3009, 2978, 2932, 2915, 1706, 1638, 1540, 1461,
1421, 1361–1346, 1282, 1211, 1164, 1102, 858, 772.
Their IR spectrum showed a characteristic band at ca 1350 cmꢀ1
ascribed to the triazene subunit (ANAN'NA) [19,41]. For 4 and 5,
the urethane functions were revealed through the NH ‘stretching’
of (RANHACOAOR0) (3331 cmꢀ1 for 4), the intermolecular hydro-
gen bonds between 3556 and 3285 cmꢀ1 and the NH ‘bending’ at
ca 1540 cmꢀ1 for 4. Additionally, 1H NMR and 13C NMR analyses
fully supported the presence of these subunits and confirmed the
assignment of the molecular structure.
2.2.1.1. 4-(Ethyloxycarbonyl)benzene diazonium tetrafluoroborate. To
32.2 g of boron trifluoride etherate (227 mmol) contained in a
three-neck round-bottom flask was added 25 g (151 mmol) of
ethyl 4-aminobenzoate. A mixture of anhydrous CH2Cl2 (0.5 L)
and THF (0.2 L) was rapidly added to the reactants and cooled at
ꢀ25 °C. tert-butyl nitrite (20.98 g, 183 mmol) in 100 mL of anhy-
drous THF was added dropwise to the rapidly stirred reaction solu-
tion over a 3-h period. After 1.5 h addition, 150 mL THF were
added. Following complete addition the temperature of the reac-
tion solution was maintained at ꢀ20 °C for 4 h and then allowed
to warm to 0 °C and pentane (400 mL) was added. The solid precip-
itate was filtered, washed (three times) with diethylether and des-
olvated at RT under reduced pressure during 10 h. The diazonium
salt was obtained as a pink solid (37.5 g, 96%), mp: 48 °C. 1H
NMR (DMSO-d6), dppm: 1.38 (t, 3H), 4.44 (q, 2H), 8.44 (d, 2H),
8.80 (d, 2H). 13C (DMSO-d6), dppm: 14.19, 62.34, 120.12, 131.14
(2C), 133.10 (2C), 139.37, 165.49. IR, KBr (cmꢀ1): 3560, 3487,
3414, 2230, 1716, 1638, 1617, 1285, 1166, 1109, 1083, 861, 762.
2.2.1.5. 1-(4-Hydroxymethyl-phenyl)-3-methyl-3-(2-hydroxyethyl) tria-
z(1)ene (a2). LiAlH4 (3.2 0 g, 84.32 mmol) was portionwise added
(1 h) to a THF solution (250 mL) of a1 (5.30 g, 21.13 mmol) at
50 °C, under stirring and nitrogen atmosphere. After complete addi-
tion, stirring was maintained at 50 °C during 5 h. The reaction mix-
ture was then poured with caution to a 300 mL THF/ethyl acetate
solution (1/1) cooled at 0 °C. After filtration, the solid residue was ex-
tracted (four times) with a hot THF/MeOH (1/1) solution. After
removing the solvents under reduced pressure, a2 was obtained as
a yellow solid (2.66 g, 60%), mp: 57–60 °C. 1H NMR (DMSO-d6) dppm:
3.18 (s broad, 3H), 3.44 (s broad, 2H), 3.66 (t, 2H), 3.82 (t, 2H), 4.47 (s,
2H), 5.03 (s broad, 1H), 7.28 (m, 4H). 13C NMR (DMSO-d6) dppm:
35.02, 57.68, 59.53, 65.31, 119.75 (2C), 127.03 (2C), 139.17, 149.36.
IR/KBr (cmꢀ1): 3417, 2950, 2926, 2864, 1602, 1540, 1508, 1464,
1446, 1431, 1397, 1343, 1179, 1073, 1044, 863, 849, 816, 778. UV/
THF e
max/L molꢀ1 cmꢀ1: 9113 (287 nm), 8423 (310 nm).
2.2.1.6. 1-(4-Hydroxymethyl-phenyl)-3,3-bis-(2-hydroxyethyl) triaz
(1)ene (b2). Following the same procedure used for preparing a2,
from b1 (14.99 g, 53.33 mmol) and LiAlH4 (4.05 g, 106.70 mmol)
in 0.45 L THF, b2 was obtained as a yellow solid (12.94 g, 96%),
mp: 65–66 °C (dec.). 1H NMR (DMSO-d6) dppm: 3.66 (t broad,
4H), 3.83 (t, 4H), 4.46 (s, 2H), 4.68 (s broad, 2H), 5.03 (s broad,
1H), 7.27 (m, 4H). 13C NMR (DMSO-d6) dppm: 50.01, 56.99 (2C),
59.85, 62.74, 119.83 (2C), 127.08 (2C), 139.28, 149.39. IR/KBr
(cmꢀ1): 3385, 2925, 2855, 1638, 1620, 1559, 1507, 1453, 1429,
1402, 1357–1337, 1190, 1167, 1071, 1044, 891, 850, 810. UV/THF
2.2.1.2. 1-(4-Ethyloxycarbonyl-phenyl)-3-methyl-3-(2-hydroxyethyl)
triaz(1)ene (a1). In a three-neck round-bottom flask, N-methyleth-
anol (4.8 g, 63.9 mmol) was solubilized under stirring in a THF/H2O
solution (9/1). After addition of Na2CO3 (21.6 g, 204 mmol), the
solution was cooled at 0 °C. Under stirring, 4-(ethyloxycarbon-
yl)benzene diazonium tetrafluoroborate (12 g, 46.33 mmol) was
added portionwise over a 3-h period at 0 °C. After addition, stirring
was maintained for 12 h at RT. After filtering the reaction mixture
extraction with CH2Cl2, drying of the organic phase and solvent
removing under reduced pressure, a1 was isolated as a yellow solid
(10.61 g, 91%), mp: 64 °C. 1H NMR (CDCl3), dppm: 1.39 (t, 3H), 2.95
(s broad, 1H), 3.35 (s broad, 3H), 3.95 (s broad, 4H), 4.35 (q, 2H),
7.46 (d, 2H), 8.03 (d, 2H). 13C (CDCl3), dppm: 14.34, 35.88, 58.15,
59.25, 60.83, 120.23 (2C), 126.98, 130.57 (2C), 154.22, 166.66. IR,
KBr (cmꢀ1): 3550, 3479, 3415, 3339, 3291, 2992, 2977, 2943,
2875, 1704, 1638, 1616, 1604, 1478, 1464, 1425, 1412, 1344,
1315, 1276, 1162, 1150, 1187, 1178, 1105, 1052, 861, 773.
e
max/L molꢀ1 cmꢀ1: 13618 (288 nm), 13604 (314 nm).
2.2.1.7. 1,2-Bis-[1-(4-hydroxmethyl-phenyl)-3-methyl-triaz(1)en-3-
yl]-ethane (c2). Following the same procedure used for preparing
a2 and b2, from c1 (6.85 g, 15.57 mmol) and LiAlH4 (1.38 g,
36.36 mmol) in 0.4 L THF, c2 was obtained as a yellow solid
(6.85 g, 73%), mp: 131 °C. 1H NMR (DMSO-d6) dppm: 3.06 (s, 6H),
3.96 (s, 4H), 4.38 (s, 4H), 5.14 (s broad, 2H), 7.12 (m, 8H). 13C NMR
(DMSO-d6) dppm: 34.41 (2C), 53.38 (2C), 62.67 (2C), 119.86 (4C),
126.99 (4C), 139.42 (2C), 149.25 (2C). IR/KBr (cmꢀ1): 3426, 2960,
2925, 2875, 1654, 1638, 1569, 1502, 1470, 1454, 1438, 1394,
2.2.1.3. 1-(4-Ethyloxycarbonyl-phenyl)-3,3-bis-(2-hydroxyethyl) triaz
(1)ene (b1). Following the same procedure used for preparing a1,
diethanolamine 9,74 g (92.63 mmol), THF/H2O (9/1) 1.5 L, Na2CO3
(36 g, 340 mmol), 4-(ethyloxycarbonyl)benzene diazonium tetra-
1351, 1207, 1161, 1055, 859, 842, 798. UV/THF
12735 (288 nm), 13110 (310 nm).
e :
max/L molꢀ1 cmꢀ1

V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
525
2.2.1.8.1-(4-Methacryloyloxymethyl-phenyl)-3-(2-methacryloyloxyeth-
yl)-3-methyl-triaz(1)ene (1). In a three-neck round-bottom flask with
stirring, a2 (3.41 g, 16.31 mmol), 4-dimethylaminopyridine (DMAP)
(7.22 g, 59.09 mmol) and hydroquinone monomethylether (HQME)
(0.055 g) were solubilized in anhydrous THF (0.25 L) under inert
vents: petroleum ether 90–85 mL/ethyl acetate 10–15 mL/ethanol
2–10 mL). The oily product was rinsed several times with diethyl
ether and desiccated over P2O5 under reduced pressure. 4 was ob-
tained as a viscous dark yellow oil (8.86 g, 79%). (The product was
stabilized after purification with addition of HQME (0.050 mg). 1H
NMR (DCCl3) dppm: 1.92 (s, 6H), 3.32 (s broad, 3H), 3.47 (m, 4H),
3.96 (t broad, 2H), 4.24 (m, 4H), 4.33 (t broad, 2H), 5.05 (s, 2H),
5.25 (t broad, 2H), 5.57 (m, 2H), 6.10 (m, 2H), 7.31–7.38 (m, 4H).
13C NMR (DCCl3) dppm: 18.18 (2C), 35.58, 39.27, 40.08, 53.87,
62.47, 63.45, 63.62, 66.49, 120.61 (2C), 126.02 (2C), 128.89 (2C),
133.45, 135.88 (2C), 150.55, 156.10, 158.13, 167.38 (2C). IR/NaCl
film (cmꢀ1): 3331, 3041, 2958, 2916, 2885, 1903, 1717, 1634,
1623, 1610, 1605, 1535, 1452, 1535, 1394, 1353, 1322, 1296,
atmosphere before adding with
a syringe pyridine (2.25 g,
28.43 mmol).Thesolutionwasthenwarmedto50 °Cbeforedropwise
addition, over a 5-h period, of a 50 mL THF solution of methacrylic
anhydride(9.68 g, 59.02 mmol). Afteraddition,thestirringwasmain-
tained at RT during 15 h. The solvent was partially removed (1/2 total
volume) under reduce pressure before addition of 0.1 L of diethyl-
ether which produced a beige precipitate then removed by filtration
on celite R545 (eluent THF). The filtrate was concentrated upon re-
duced pressure and dissolved in diethylether (0.25 L). The organic
phase was treated (three times) with an aqueous HCl solution (pH
5–6) evaporated and submitted to a flash chromatography on silica
(gradient petroleum ether/ethylacetate: 100–5/95–5). After three
successive chromatographies, 1 was obtained as a yellow amorphous
solid (3.65 g, 64%), mp: 37–38 °C. 1H NMR (DCCl3) dppm: 1.90 (s, 3H),
1.96 (s, 3H), 3.30 (s broad, 3H), 4.04 (t, 2H), 4.40 (t, 2H), 5.17 (s, 2H),
5.54–5.57 (m, 2H), 6.09 (s, 1H), 6.14 (s, 1H), 7.34 (m, 4H). 13C NMR
(DCCl3) dppm: 18,64–18.73 (2C), 36.11, 54.55, 62.89, 66.71, 121.08
(2C), 126.11, 126.51, 129.24 (2C), 133.62, 136.26, 136.67, 150.92,
167.46, 167.67. IR/KBr (cmꢀ1): 2987, 2955, 2925, 1714, 1638, 1608,
1558, 1540, 1505, 1455, 1399, 1346, 1317, 1294, 1161, 1123, 1017,
1162, 1088, 1041, 814, 860, 777. UV/THF
e :
max/L molꢀ1 cmꢀ1
11637 (287 nm), 11135 (310 nm). (LSIMS) m/z: 542.22 [M + Na]+.
2.2.1.12.
1-[4-(2-Methacryloyloxyethyl)-aminocarbonyloxymethyl-
phenyl]-3,3-di-[(2-methacryloyloxyethyl)amino-(2-carbonyloxyeth-
yl)]triaz(1)ene (5). Following the same procedure for preparing 4,
b2 (2.5 g, 10.64 mmol), DABCO (0.140 mg, 1.25 mmol), HQME
(0.050 g), dibutyltin dilaurate (four drops), 2-(methacryloyl-
oxy)ethyl isocyanate (5.96 g, 37.65 mmol), anhydrous THF (0.6 L)
gave 5 as a dark yellow viscous oil (4.97 g, 67%). 1H NMR (DCCl3)
dppm: 1.92 (s, 9H), 3.47 (m, 6H), 3.97 (t, 4H), 4.21 (m, 6H), 4.33
(t, 4H), 4.86 (t broad, 2H), 5.07 (s, 2H), 5.44 (t broad, 1H), 5.57
(m, 3H), 6.10 (s broad, 3H), 7.29–7.36 (m, 4H). 13C NMR (DCCl3)
dppm: 18.73 (3C), 39.89 (2C), 40.60, 54.03 (2C), 62.66 (2C), 63.99
(2C), 64.15, 67.16, 121.25 (2C), 126.59 (3C), 129.41 (2C), 134.32,
136.35–136.47 (3C), 150.54, 156.93 (2C), 158.55, 167.76–167.95
(3C). IR/NaCl film (cmꢀ1): 3556, 34.10, 3285, 29.57, 2929, 2891,
2844, 1717, 1637, 1617, 1539, 1454, 1539, 1405, 1364, 1322,
814, 863, 831, 781. UV/THF
e
max/L molꢀ1 cmꢀ1: 14483 (287 nm),
13830 (310 nm). (EI) m/z: 345.17 [M]+.
2.2.1.9.
1-(4-Methacryloyloxymethyl-phenyl)-3,3-bis-(2-methacry-
loyloxyethyl) triaz(1)ene (2). According to the same procedure for
preparing 1, b2 (2.5 g, 10.12 mmol), DMAP (7.42 g, 60.13 mmol),
HQME (0.050 g), anhydrous THF (0.55 L), methacrylic anhydride
(7 g, 45.43 mmol) gave 2 as a pale viscous yellow oil (2.43 g,
52%). 1H NMR (DCCl3) dppm: 1.92 (s, 6H), 1.96 (s, 3H), 4.07 (t,
4H), 4.42 (t, 4H), 5.17 (s, 2H), 5.56–5.57 (m, 3H), 6.09 (s, 2H),
6.14 (s, 1H), 7.39 (m, 4H). 13C NMR (DCCl3) dppm: 18.77 (3C),
46.95, 54.45, 61.75, 62.55, 66.75, 121.32 (2C), 126.22, 126.57
(2C), 129.32 (2C), 134.20, 136.41 (2C), 136.78, 150.49, 167.60
(2C), 167.77. IR/NaCl film (cmꢀ1): 2987, 2957, 2926, 1898, 1721,
1638, 1608, 1558, 1540, 1505, 1456, 1404, 1316, 1295, 1155,
1298, 1166, 1090, 1045, 814, 874, 856, 775. UV/THF emax
/
L molꢀ1 cmꢀ1: 13363 (287 nm), 13040 (310). (LSIMS) m/z: 727.29
[M + Na]+.
2.2.2. o-Nitrobenzyl derivatives (6–7)
As the presence of a substituent on the aromatic ring is known
to improve the photoreactivity of the o-nitrobenzyl group (as well
as on the a-carbon of the benzyl group) [22], the structures 6 and 7
bearing 1 or 2 nitro groups on the ortho positions to the benzylic
1088, 1031, 1010, 814, 891, 834, 781. UV/THF
e :
max/L molꢀ1 cmꢀ1
group were also designed.
14475 (287 nm), 13605 (310 nm). (EI) m/z: 443.21 [M]+.
Compound 6 was prepared in four steps as shown in Scheme 5.
Alkylation of acetovanillon (d1) with t-butylbromoacetate fol-
lowed by the selective nitration of the protected phenol (d2) gen-
erated compound d3 as already reported by Holmes [42] (overall
yield 63%). d3 was transformed into the diol d4 with BH3/THF at
45 °C during 48 h, the oily product was obtained pure (70% yield)
after chromatography on silica. Finally, d4 treated in THF (RT,
36 h) with an excess (3 eq) of methacrylic anhydride, pyridine
(5 eq) and one equivalent of 4-dimethylaminopyridine (DMAP)
afforded, after flash chromatography on silica, 6 as a yellow oil
(79%) which appeared to be very sensitive to polymerization even
in the presence of hydroquinone. IR spectroscopy revealed the
characteristic vibration modes of the nitro group ((1522 and
1334 cmꢀ1), together with the bands characteristic of the methac-
rylates (1720 cmꢀ1 C@O stretching, 1635 cmꢀ1 C@C stretching
(conjugated) and 1164 cmꢀ1 CA(C@O)AO stretching)).
2.2.1.10. 1,2-Di-[1-(4-methacryloyloxymethyl-phenyl)-3-methyl-tria-
z(1)en-3-yl]-ethane (3). According to the same procedure for pre-
paring 1 and 2, c2 (5 g, 14.04 mmol), DMAP (8.85 g, 72.44 mmol),
HQME (0.055 g), anhydrous THF (0.6 L), methacrylic anhydride
(10.41 g, 67.52 mmol) gave 3 as a yellow solid (3.85 g, 55%), mp:
74–76 °C. 1H NMR (DCCl3) dppm: 1.95 (s, 6H), 3.25 (s broad, 6H),
4.05 (s, 4H), 5.17 (s, 4H), 5.57 (m, 2H), 6.14 (s, 2H), 7.33 (m, 8H).
13C NMR (DCCl3) dppm: 18.69 (2C), 34.98 (2C), 54.02 (2C), 66.66
(2C), 121.04 (4C), 126.05 (2C), 129.41 (4C), 133.57 (2C), 136.65
(2C), 150.90 (2C), 167.63 (2C). IR/KBr (cmꢀ1): 2991, 2980, 2962,
2944, 2915, 1714, 1638, 1605, 1540, 1505, 1467, 1452, 1394,
1323, 1299, 1181, 814, 896, 834, 757. UV/THF
e :
max/L molꢀ1 cmꢀ1
24036 (287 nm), 24657 (313 nm). (EI) m/z: 492.25 [M]+.
2.2.1.11.
1-[4-(2-Methacryloyloxyethyl)-aminocarbonyloxymethyl-
Compound 7 was prepared from 1,4-dichloromethylbenzene
(e1) as the starting material (Scheme 6). Compound e1 was trans-
formed into the dinitro derivative e2 with a solution of H2SO4/
HNO3 as described by McMurdie et al. [25] (yellow solid, 70%
yield). Diol e3 was obtained according to the same author [43]
by submitting e2 to potassium formate in refluxed DMF and puri-
fied by crystallization in water (yellow solid, 58% yield). Compound
7 was finally synthesized as a yellow solid, by treating diol e3 as
done for 6 but with a catalytic amount of DMPA. The yield in pure
phenyl]-3-methyl-3-[(2-methacryloyloxyethyl)amino-(2-carbonyl-
oxyethyl)]triaz(1)ene (4). Under nitrogen atmosphere, to a refluxed
anhydrous THF (0.5 L) solution of a2 (4.5 g, 21.53 mmol), 1,4-
diazabicyclo [2.2.2]octane (DABCO) (0.195 mg, 1.73 mmol), HQME
(0.065 g) and dibutyltin dilaurate (four drops) was added 2-(meth-
acryloyloxy)ethyl isocyanate (7.34 g, 47.35 mmol) and stirred for
24 h. The viscous residue obtained after removing the solvent,
was submitted to flash chromatography (silica gel, gradient of sol-

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V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
Scheme 5. Reagents and conditions: (a) K2CO3 (4 eq), t-butylbromoacetate (1.08 eq), CH3CN reflux; (b) NO3H (12.5 eq), acetic anhydride, 0 °C; (c) BH3-THF (10.8 eq), 45 °C ;
(d) DMPA (2 eq), hydroquinone (0.050 g), methacrylic anhydride (6 eq), pyridine (10 eq), THF reflux.
Scheme 6. Reagents and conditions: (a) H2SO4 98%/HNO3 100%, 0 °C; formic acid 90%, DMF, aq KOH, reflux; (c) e3 (7.43 g, 32.01 mmol) and DMAP (0.1 eq), pyridine (10 eq),
methacrylic anhydride (5.1 eq), THF reflux.
product (34%) is moderate mostly due to the difficulty to remove
impurities formed along the reaction. Compound 7 displayed in
IR the expected characteristic vibration modes of NO2 and those
of methacrylate groups (see experimental part).
eral times) in water for giving d3 as a pale yellow solid (9.95 g,
69%), mp: 168 °C. 1H NMR (DMSO-d6) dppm: 2.52 (s, 3H), 3.94 (s,
3H), 4.90 (s, 2H), 6.70 (s, 1H), 7.58 (s, 1H), 13.20 (s broad, 1H).
13C (DMSO-d6), dppm: 29.99, 56.65, 65.15, 108.43, 110.06,
131.71, 137.94, 147.61, 153.34, 169.40, 199.26. IR/KBr (cmꢀ1):
3556–3420, 3254, 3066, 2973, 2858, 1635, 1614, 1585, 1525,
1505, 1446, 1340, 1287, 1246, 1082, 1028, 881, 787, 757.
2.2.2.1. t-Butyl-(2-methoxy-4-acetylphenoxy)acetate (d2). Under
nitrogen atmosphere, to a stirred freshly distilled acetonitrile solu-
tion (0.5 L) containing acetovanillon d1 (18.97 g, 114 mmol), oven
dried K2CO3 (63 g, 456 mmol) was added t-butylbromoacetate
(24 g, 123 mmol). The mixture was then refluxed for 24 h under
stirring. After filtration and evaporation of a large part of the sol-
vent, the residual organic phase was washed (upon ultrasonic acti-
vation) with an aqueous saturated NaHCO3 solution and extracted
with diethylether. After removing the solvent, the crude residue
was dissolved in CH2Cl2 and purified by several precipitations in
cooled hexane. d2 was obtained as a white solid (28.57 g, 89%),
mp: 94 °C. 1H NMR (DCCl3) dppm: 1.46 (s, 9H), 2.55 (s, 3H), 3.96
(s, 3H), 4.65 (s, 2H), 6.75 (d, 1H), 7.51 (m, 2H). 13C NMR (DCCl3)
dppm: 26.18, 27.97 (3C), 56.01, 66.09, 82.60, 110.86, 111.75,
122.73, 131.35, 149.28, 151.52, 167.17, 196.63. IR/KBr (cmꢀ1):
2994, 2942, 2900, 1744, 1699, 1638, 1618, 1591, 1508, 1448,
1280, 1249, 1164, 1146, 1082, 1030, 873, 822.
2.2.2.3. 2-[4-(1-Hydroxyethyl)-5-methoxy-2-nitrophenyloxy]ethanol
(d4). Under nitrogen atmosphere and stirring d3 (5 g, 18.5 mmol)
was added to a 1 M BH3–THF solution (200 mL, 200 mmol) and
the mixture was warmed at 45 °C for 48 h. The reaction mixture
was then poured in a ice-cooled ethanol/water solution (150 mL/
100 mL). After removing the solvents, the solid residue was sub-
mitted to flash chromatography (silica gel, petroleum ether/ethyl
acetate/ethanol, gradient: 85/15/1–5). d4 was obtained as a dark
yellow oil (3.24 g, 70%). 1H NMR (DCCl3) dppm: 1.55 (d, 3H), 2.6
(broad peak, 1H), 3.98 (s, 3H), 4.00 (t, 2H), 4.17 (t, 2H), 4.98 (broad
peak, 1H), 5.56 (q, 1H), 7.32 (s, 1H), 7.60 (s, 1H). 13C NMR (DCCl3)
dppm: 24.44, 56.37, 60.89, 65.64, 71.11, 108.87, 109.54, 137.62,
139.53, 146.71, 153.95. IR/NaCl film (cmꢀ1): 3483, 3108, 2973,
2931, 2858, 1614, 1577, 1515,1502, 1464, 1427, 1334, 1269,
1215, 1075, 1058, 1044, 1017, 884, 894, 802, 757. UV/THF emax
/
L molꢀ1 cmꢀ1: 3609 (300), 3959 (344). (EI) m/z: 257.08 [M]+.
2.2.2.2. (2-Methoxy-4-acetyl-5-nitrophenoxy)acetic acid (d3). To a
stirred and ice-cooled solution of acetic anhydride (31.86 g,
312 mmol) and nitric acid 70% (44.3 mL, NO3H 43.41 g, 689 mmol)
was slowly added d2 (14.85 g, 55.20 mmol) dissolved in acetic
anhydride (44.3 mL). After addition, the mixture was maintained
at 0 °C for 2 h before stirred at room temperature for a supplemen-
tary 4-h period. The mixture in then poured in ice-cooled water
(650 mL) and stirred at this temperature for 15 h before adding
1,4-dioxane (150 mL). The yellow precipitate was crystallized (sev-
2.2.2.4. 1-(2-Methacryloyloxyethyloxy)- 2-methoxy-4-(1-methacry-
loyloxy-1-methyl-ethyl)-5-nitro -benzene (6). Under strirring and
nitrogen atmosphere, d4 (2.31 g, 9 mmol), DMPA (2.19 g,
17.9 mmol), hydroquinone (0.050 g) and methacrylic anhydride
(8.85 g, 53.9 mmol) were dissolved in anhydrous THF (0.45 L).
The solution was warmed to reflux (12 h) after adding pyridine
(7.11 g, 89.9 mmol). The stirring was continued at room tempera-

V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
527
ture during 36 h before removing a large part of the solvent under
reduce pressure. The crude reaction product was treated with 1 N
HCl (0.2 L) and diethyl ether (0.2 L). The organic phase was washed
with 1 N HCl (4 ꢂ 150 mL) and dried over Na2SO4. After removing
the solvent, the residue was submitted to flash chromatography
(silica gel, petroleum ether/ethyl acetate 99–85/1–5). 6 was ob-
tained as a pale yellow oil (2.79 g, 79%). 1H NMR (DCCl3) dppm:
1.66 (d, 3H), 1.94 (s, 3H), 1.96 (s, 3H), 3.92 (s, 3H), 4.34 (t, 2H),
4.52 (t, 2H), 5.59 (m, 1H), 5.62 (m, 1H), 6.12 (s, 1H), 6.18 (s, 1H),
6.52 (q, 1H), 7.04 (s, 1H), 7.66 (s, 1H). 13C NMR (DCCl3) dppm:
18.24, 18.28, 22.01, 56.25, 62.76, 67.62, 68.69, 108.45, 110.17,
125.72, 126.24, 134.16, 135.82, 136.31, 139.67, 146.95, 154.25,
166.03, 167.16. IR/NaCl film (cmꢀ1): 3087, 2973, 2931, 2858,
2004, 1887, 1720, 1635, 1614, 1580, 1522, 1502, 1454, 1334,
1276, 1218, 1164, 1123, 1082, 1048, 1017, 873, 815, 802, 757,
8.03 (s, 2H). 13C NMR (DCCl3) dppm: 18.63, 18.85, 58.60, 64.15,
125.98, 127.22 (2C), 127.86, 127.90, 135.49, 135.87, 140.41,
151.58 (2C), 166.89, 167.12. IR/KBr (cmꢀ1): 3082, 3056, 2981,
2957, 2924, 2905, 2853, 1726, 1711, 1638, 1617, 1577, 1550,
1537, 1507, 1487, 1449, 1354, 1321, 1294, 1166, 1150, 1015, 937,
902, 875, 821, 814, 794, 749, 729. UV/THF e
max/L molꢀ1 cmꢀ1: 643
(310 nm), 778 (323 nm). (EI) m/z: 364.09 [M]+.
3. Results and discussion
3.1. Photochemical behaviour of compounds 1–6 in solution
UV/Vis spectra exhibited the characteristic absorption bands of
triazene [5,33] and o-nitrobenzyl [42] groups for compounds 1–5
and 6, respectively. They all absorb in the near UV (triazenes) with
a tail in the visible range for the nitro derivatives. In order to test
their photosensitivity, the compounds were dissolved in THF (conc.
ca 0.2 mM) and irradiated in a 1 cm quartz cell (kirr. 320–480 nm,
irradiance output of the optical fibre 4.65 W/cm2), the experimen-
tal set-up being maintained on the sample holder of a UV spec-
trometer. The spectra of the solutions were strongly affected by
the irradiation as shown in Figs. 1–3. For compounds 1–5 the dis-
appearance of the broad non-structured band corresponds to the
photolysis of the triazene moiety as already demonstrated [5,33].
This band completely vanished after only 10 s of irradiation
(t½ ꢃ 4 s for all the triazene derivatives). These data confirmed
the great photosensitivity of the triazene group towards light
irradiation.
The spectrum evolution of the o-nitro derivative 6 is given in
Fig. 2a. During irradiation, the absorption at 300 nm disappears
in favour of a very intense band at 373 nm characteristic of the ni-
troso group resulting from the reduction of NO2 [21–23]. The latter
band reached its maximum after 7.5 s of irradiation. The photolysis
yields methacrylic acid which is the expected photoproduct [21–
23]. If the irradiation is continued after the absorbance reaches
its maximum at 373 nm, the spectrum continues to change
(Fig. 2b). Secondary photochemical reactions on nitroso groups
could explain this phenomenon. Indeed, these groups could,
among other products, form nitroso dimers and generate nitrox-
ides [44–46], in agreement with the decrease of absorption ob-
served in the visible and near UV. The molar extinction
coefficient of the primary photoproduct at 373 nm is ca
5700 L molꢀ1 cmꢀ1 which means that during photolysis, the photo-
product competes with the o-nitrobenzyl moiety for light
absorption.
716. UV/THF e
max/L molꢀ1 cmꢀ1: 4440 (296 nm), 4681 (344 nm).
(EI) m/z: 393.14 [M]+.
2.2.2.5. 2,6-Dinitro-1,4-di-chloromethylbenzene (e2). H2SO4 98%
(14.85 mL), H2SO4 93–94% (108.5 mL) and HNO3 100% (171 mL)
were mixed under stirring with caution at 0 °C before adding por-
tionwise 1,4-dichloromethylbenzene e1 (40 g, 228 mmol). Stirring
was maintained during 30 min. At room temperature a solution of
H2SO4 93% (5.8 mL), H2SO4 98% (5.8 mL) and HNO3 100% (5.8 mL)
was added over a 20-min period to the mixture. After stirring for
2 h, a deep yellow precipitate was formed. The reaction mixture
was poured with caution in a beaker containing ice (1 kg) and
water (0.5 L). The yellow precipitate was filtered and washed with
aqueous saturated Na2CO3 solution (3 ꢂ 0.3 L) and with water
(2 ꢂ 300 mL). The crude solid was crystallized in ethanol (95%)
and washed three times with CH2Cl2/pentane (15 mL/185 mL)
solution. After desiccation over P2O5 under reduced pressure, e2
was obtained as a bright yellow solid (42.66 g, 70%), mp: 78%. 1H
NMR (DCCl3) dppm: 4.68 (s, 2H), 5.03 (s, 2H), 8.13 (s, 2H). 13C
NMR (DCCl3) dppm: 35.92, 42.91, 124.74, 128.89 (2C), 141.69,
149.68 (2C). IR/KBr (cmꢀ1): 3080, 3066, 3023, 2967, 2877, 1637,
1617, 1573, 1547, 1526, 1443, 1351, 896, 878, 853, 812, 713,
681, 728.
2.2.2.6. 2,6-Dinitro-1,4-di-hydroxymethylbenzene (e3). Under nitro-
gen atmosphere, to a refluxed solution of e2 (30 g, 115 mmol) in
a mixture of formic acid 90% (24 g) and DMF (8 mL) was added
an aqueous solution of KOH (17.5 g in 0.050 L of distilled water),
after addition the mixture was maintained under reflux for 12 h.
After neutralization (pH 7–8) with an aqueous NaHCO3 solution
(0.15 L), a solid precipitated at 0 °C. After crystallization (several
times) in water, e3 was isolated as yellow crystals (15.15 g, 58%),
mp: 123 °C. 1H NMR (DMSO-d6) dppm: 4.63 (s, 2H), 4.74 (s, 2H),
5.69 (m, 2H), 8.07 (s, 2H). 13C (DMSO-d6), dppm: 57.33, 62.57,
125.59, 129.41 (2C), 146.51, 151.63 (2C). IR/KBr (cmꢀ1): 3549,
3476, 3414, 3227, 3082, 2947, 2884, 1638, 1617, 1550, 1524,
1482, 1352, 1031, 906, 891, 805, 792.
As the molar extinction coefficient
e of 6 is in the 300–400 nm
range and is significantly lower than that of aryltriazenes, it should
be an advantage to ensure a greater transparency for formulations
including these monomers which have no large overlap with the
2.2.2.7. 2,6-Dinitro-1,4-di-(methacryloyloxymethyl)benzene (7). Under
nitrogen atmosphere, to a solution of e3 (7.43 g, 32.01 mmol) and
DMAP (0.41 g, 3.32 mmol) in anhydrous THF (0.5 L) were added pyr-
idine (25.43 g, 321.46 mmol) and methacrylic anhydride (26.91 g,
164.07 mmol). The mixture was refluxed for 2 h and then main-
tained under stirring for 15 h at room temperature. After solvent
evaporation, the crude product was dissolved in diethyl ether
(0.3 L) and washed with aqueous HCl 3.5% (4 ꢂ 0.2 L). After the
usual workup, the crude product was submitted to flash chromatog-
raphy (petroleum ether/ethyl acetate: 100–95 mL/0–5 mL). 7 was
obtained as a amorphous yellow solid (4.11 g, 34%), mp: 57–59 °C.
1H NMR (DCCl3) dppm: 1.85 (s, 3H), 1.98 (s, 3H), 5.53 (s, 2H), 5.57
(m, 1H), 5.59 (s, 2H), 5.69 (m, 1H), 6.01 (m, 1H), 6.21 (m, 1H),
Fig. 1. Evolution of the electronic absorption spectrum of compound
1 upon
irradiation (320–480 nm) in THF (2.02 ꢂ 10ꢀ4 M) with increasing time from 0 to 6 s.

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V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
Fig. 2a. Evolution of the electronic absorption spectrum of compound 6 upon
irradiation (320–480 nm) in THF (2.05 ꢂ 10ꢀ4 M) with increasing time from 0 to
7.5 s.
Scheme 7. Photocleavage mechanism of BAPO (Irg 819) and molecular structure of
DUMA.
selected a formulation allowing the excitation of the initiator in the
visible range, 426 nm 6 k1 6 480 nm (different to the absorption
region of triazene or nitro benzyl core). Additionally, non-toxic
materials suitable for environment-friendly purposes have been
preferentially employed. For these reasons, the traditional system
camphorquinone/tertiary amine (DABE) [47] has been retained
(DABE: N,N-dimethyl-p-aminobenzoic acid ethylester). However,
this system did not function efficiently with the nitro derivative
6, as only a partial polymerization occurred accompanied with a
deep red colouration of the medium (which is certainly due to a
charge transfer complex between the nitro benzyl (acceptor) and
the tertiary amine). While bis(2,4,6-trimethylbenzoyl)phenylphos-
phine oxide (BAPO) absorbing light up to >420 nm [48] was found
a suitable photoinitiator for formulations involving compound 6, it
did not work for 7 in the same experimental conditions. That was
probably due to the strong acceptor nature of dinitrobenzene
derivatives which makes them radical traps and thus efficient
polymerization inhibitors [49].
Regarding the co-monomers, we selected compounds used in
dental composites as the physical behaviours of their formulations
are relatively well-known [50–52] and their toxicity evaluated
[53–55]. Thus in addition to a di or tri (meth)acrylate photosensi-
tive monomer (1–6), the photoinitiator (and co-initiator), the resin
blends also contained commercial linear mono and bismethacry-
lates as shown in Table 1: TEGDMA (triethylenglycol dimethacry-
late), PEGDMA (polyethylene dimethacrylate), DUMA (diurethane
dimethacrylate, Scheme 7), BisGMAE (bisphenol A ethoxylate
dimethacrylate), BMA (butylmethacrylate). As mentioned in Table
1, different resin blends were investigated. The mechanical proper-
ties of the materials were evaluated, successively after k1 and k2
irradiation, through dynamic mechanical analysis (DMA). They
were obtained from a thin bar-shaped sample (after 300 s photoir-
radiation at k1 > 426 nm on each face, 25 °C), the storage modulus
being determined at ꢀ15 °C and 75 °C. The latter temperature limit
was not crossed for the first scanning to avoid any thermal modi-
fication of the material (thermal post-curing or thermal degrada-
Fig. 2b. Evolution of the electronic absorption spectrum of compound 6 upon
irradiation (320–480 nm) in THF (2.05 ꢂ 10ꢀ4 M) with increasing time from 7.5 to
150 s (after reaching the maximum of the absorbance at 373 nm).
excitation zone of the initiator (BAPO, bis-(2,4,6-trim-
ethylbenzoyl)-phenylphosphine (Irg 819)) for curing phase (see
next section and Scheme 7).
The photosensitivity of 7 was not investigated, because in our
experimental conditions compound 7 could not be copolymerized
with methacrylates or divinylbenzenes using a radical photoiniti-
ator or by thermal initiation (AIBN).
3.2. Photocuring and photodegradation of formulations incorporating
monomers 1–6
The aforementioned monomers were tested in formulations
including co-monomers and initiators as described hereafter. The
formulations have to be compatible with two different excitation
wavelengths, k1 (or a range of wavelengths) is supposed to excite
selectively the photoinitiator (or combination of photoinitiator,
coinitiator, photosensitizer) without affecting the photolabile sub-
unit and k2 the photocleavable chromophore. In this work, we have
Fig. 3. Evolution versus temperature of G0k curves after k1 and k2 exposure for formulations FNC 3/3.0 (left) and FC 3/3.0 (right) including compound 3.

Table 1
Evolutions of hardness (G0) of various resin formulations involving compounds 1–6.
Compound
Weight %
Formulation
TEGDMA
PEGDMA
DUMAb
BisGMAE
BMA
Cem-Bridgea
CQ
AIBN 140 °C
15 min/air
DABE
Irg 819b
(BAPO)
G0k1 (1010 Pa)
25 °C (ꢀ20 °C)
G0k2 (1010 Pa)
25 °C (ꢀ20 °C)
Breaking
temp. °C (k2)
1
35
FNC 1/1.0
FC 1/1.0
FC 1/4.0
53
53
45
12
12
20
0.25
0.25
0.25
0.25
0.25
0.25
0.8
1.25
1.5
1.6
2.65
3.25
180
185
165
60
60
2
2
35
FNC 2/1.0
FC 2/1.0
53
53
12
12
0.25
0.25
0.25
0.25
0.9
0.56
(0.80)
0.48
(0.55)
2.25
1.9
115
>250
60
0.69
(0.74)
0.48
(0.39)
7.9
FNC 2/4.0
45
0.25
0.25
0.25
0.25
130
FC 2/4.0
45
20
60
200
180
40
55
FNC 2/5.1
40
40
30
1
22
(26.6)
0.30
(0.39)
2.75
(4.46)
0.61
23
(21.4)
0.30
(0.29)
2.4
(2.80)
0.69
FNC 2/5.2
FNC 2/6.0
FC 2/8.0
5
1
180
145
45
25
1
100
60
60
1.5
>250
(0.92)
(0.76)
3
35
FC 3/1.0
FNC 3/2.0
FC 3/2.0
FNC 3/3.0
53
13
13
35
12
13
13
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.41
0.45
0.265
0.50
(1.05)
1.4
(2.86)
1.75
1.1
0.80
0.60
0.36
0.50
(0.65)
2.0
(2.86)
3.1
1.2
180
175
–
39
39
30
30
210
FC 3/3.0
35
60
60
0.25
0.25
145
FNC 3/4.0
FC 3/4.0
45
45
20
20
0.25
0.25
0.25
0.25
110
150
4
35
FNC 4/1.0
FNC 4/2.0
53
13
12
13
0.25
0.25
0.25
0.25
0.30
0.40
0.48
0.35
160
160
39
(1.12)
0.14
(0.20)
(0.59)
0.27
(0.337)
60
50
35
FNC 4/9.0
FNC 5/2.2
FNC 6/1.0
20
25
20
25
1.5
165
>250
225
5
6
0.25
0.25
0.20
(0.50)
0.28
(0.40)
53
53
12
12
1
3.0
3.0
(1.18)
1.7
0.75
(1.49)
0.74
(1.05)
0.75
(0.92)
(4.82)
1.35
2.3
(6.0)
0.87
(1.29)
0.75
(1.17)
FC 6/1.0
FNC 6/2.1
60
60
1
1
210
150
35
30
30
30
45
FNC 6/6.0
FC 6/6.0
25
35
1
1
115
125
a
Auto-photopolymerizable composite luting cement (Bis GMA resin (methacrylates), silica oxide, ionomeric glass, excipients (inorganic filler content: 26% vol., ACTEON Group). w/w% given in function of the total mass of the
material (resin + loadings).
b
See molecular structure in Scheme 7.

530
V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
tion of labile sites). The sample was then subjected to the k2 irra-
diation (320–480 nm, 600 s on each side, 25 °C) before being ana-
lyzed and the corresponding modulus determined under the same
conditions, for temperatures between ꢀ20 °C and 250 °C. The irra-
diation time required for a full photocuring was found shorter than
300 s. However, in order to minimize the amount of residual
photoinitiator, the period of k1 irradiation was set longer.
formulation 3.0 (see Table 1) can be considered similar to a dental
formulation in which the triazene monomer is the crosslinking
agent partially replacing BisGMAE usually found in the traditional
dental formulations. Other formulations have been prepared in or-
der to confer ‘flexibility’ to the material by introducing PEGDMA a
poly(ethylene glycol) dimethacrylate (Mn ꢄ 875 g/mol) whose
long chains reduce the crosslinking density or DUMA, a urethane
dimethacrylate able to bring stronger cohesion to the material
while preserving a high degree of elasticity. Butyl methacrylate
(BMA) was also used to create longer segments between the
cross-links which helped increasing the overall conversion due to
the low Tg of the formulation (20 °C).
Soon after subjecting the samples to the k2 irradiation, we man-
aged to determine their storage modulus first at ꢀ20 °C and then at
increasing temperature up to 250 °C (Figs. 4–6). In most cases
when recorded, the storage modulus G0k2 measured at ꢀ20 °C
was lower than G0k1 obtained at the same temperature. In one
case, however, that involving compound 4 and the formulation
FC4/9.0 and corresponding to the least rigid material synthesized,
G0k2 > G0k1 at ꢀ20 °C, which might reflect an initially loose net-
work undergoing further crosslinking after k2 irradiation. Indeed,
at this wavelength the triazene moieties underwent photolysis,
loosening the initial network and produced active radicals (see
G0k1 and G0k2 denote the curves of storage modulus after photo-
crosslinking (k1) and photodegradation (k2), respectively.
3.2.1. Triazenes 1–5
The photocuring of the majority of liquid resins based on triaz-
enes 1–5 could be achieved by using the pair camphorquinone/
DABE as the photoinitiator. Only certain formulations of monomer
3 generated materials with a crosslinking density smaller than the
one measured for other triazene monomers (G0 ꢄ 5109 Pa). This
lower hardness may be explained by a greater opacity of the resin
compared to other formulations. The solubility of 3 in the resin is
assumed to be not optimal and strongly dependent on the
formulation.
A series of formulations with a proportion of 35% in triazene
(Fw35) was fully investigated because lower percentages (ꢄ15%)
did not lead to significant changes of G0 after k2 irradiation. The
Fig. 4. Evolution versus temperature of G0k curves after k1 and k2 exposure for formulations FNC 4/2.0 (left) and FC 4/9.0 (right) including compound 4.
Fig. 5. Evolution versus temperature of G0k curves after k1 and k2 exposure for formulations FNC 2/5.1 (left) and FC 2/6.0 (right) including compound 2.
Fig. 6. Evolution of G0 versus T after k1 and k2 irradiation for formulations including compound 6. Left: FNC 6/2.1 (cured with AIBN), centre: FNC 6/6.0, right FC 6/6.0.

V. Gaud et al. / Reactive & Functional Polymers 72 (2012) 521–532
531
Scheme 2) which may or may not have reacted with unreacted
(meth)acrylic double bonds of their immediate surrounding
depending upon the rigidity of the initial sample (IR spectra
showed a decrease of the C@C ‘stretching’ at 1637 cmꢀ1 and the
CH ‘bending’ out of the plane of at 816 cmꢀ1 suggesting a con-
sumption of residual unsaturations. No changes of the CH bending
bands of the aromatics were detected, supporting the preservation
of the substitution of the aromatic rings after formation of amino
and phenyl radicals.).
ried out thermally). There were the largest decreases of G0 values
registered in the series (Fig. 6). In contrast, the FNC 6/1.0 formula-
tion did not provide a similar tendency while its composition in-
cluded softening PEGDMA, whereas formulation FNC 6/2.1
incorporating co-monomer BisGMAE and DUMA was expected to
impart more ‘resistance’ to the material. However, G0k2 > G0k1 for
the loaded formulation FC 6/1.0.
For large concentrations in compound 6 (>35%) the tendencies
were similar to the lowest one. The k2 irradiated unloaded formu-
lations (6 > 35%) exhibited internal and surface naked-eye visible
fractures underlying the degradation of the material.
Samples exhibiting a rather high G0k1 and thus mirroring a high
degree of crosslinking, are materials rigid enough not to favour the
mobility of dangling subchains, in particular those cleaved after
the k2 irradiation. When measuring the G0k2 values of such rigid
samples at ꢀ20 °C, one observes a clear decrease of G0k2 (ꢀ20 °C)
as compared to G0k1 (ꢀ20 °C) whatever the concentration of the
crosslinker. In the case of the FC4/9.0 formulation, the sample
being the least rigid of the materials tested, the radicals produced
upon k2 irradiation could move more easily even at ꢀ20 °C and re-
act with the unreacted double bonds left or recombine, provoking a
hardening of the entire material: hence a higher G0k2 compared to
G0k1 at all temperatures.
In all other samples, this phenomenon occurred to a greater or
lesser extent – reaction of the produced radicals with their sur-
rounding – upon increasing T, which necessarily induced a soften-
ing of the samples. As the latter become less rigid than at ꢀ20 °C,
classical radical reactions occurred (addition and recombination)
which in turn is reflected in the fact that G0k2 is systematically
higher than G0k1 at elevated temperatures.
For zero loading formulations, the results showed a larger de-
crease of G0 for nitrobenzyl derivative.
However, the G0 variations registered are too weak to promote
an efficient degradation of a material and to anticipate a direct
application. However, the resulting ‘k2’ effect is greatly improved
when the polymerization is thermally initiated (AIBN). For BAPO
(Irg 819) based formulations, a competitive post-crosslinking, ow-
ing to the photorearrangement of the nitrobenzyl a-methyl entity,
could be contemplated simultaneously with the k2 photodegrada-
tion of the material. In contrast, AIBN does not respond to k2 irra-
diation and photodegradation is the main photochemical event.
Additionally, for the loaded formulations, a light scattering effect
by the particles could decrease the photodegradation efficiency.
4. Conclusions
Regarding the effect of the components in the curing formula-
tions, it appeared that the presence of urethane groups (DUMA)
looked to be favourable for efficient photocleavages (see for in-
stance FNC2/6.0 and FNC4/2.0 formulations), see Fig. 5. It is known
that the urethane groups induce strong hydrogen interactions
endowing the elastic/rubbery character of the material. The mate-
rial retained its glassy properties after k2 irradiation, the existing
H-bonds limiting the recombination of radicals formed after the
photolysis of triazene subunit. For all other comonomers, the
changes were found to be very weak. Even butyl methacrylate
did not produce the desired effects. Its plasticizing properties could
in fact be favourable to the recombination of phenyl and amino
radicals.
It thus appeared that the nature of the glassy matrix was the
main criterion which determined the fate of the photogenerated
amino and phenyl radicals. For a matrix displaying a G0 around
1010 Pa, the diffusion of radicals is greatly reduced, particularly
for species bonded to the network (as already shown for other
crosslinked systems [56,57]). Formulations carried out with 100%
of monomers aryltriazenes monomers led to the same results.
The introduction of Cem-Bridge loading did not significantly
modify the situation. The presence of unsaturated groups on the
surface of the Cem-Bridge loading is probable (but not confirmed,
as the composition of the commercial product is not well known).
In matrices with 60 wt.% Cem-Bridge, these unsaturated groups
might be also involved in a post-crosslinking.
In a rigid matrix, the aryltriazene derivatives 1–5 behaved dif-
ferently from 2-nitrobenzyl monomers 6. This comparative study
shows the importance of the photodegradation mechanism of the
chromophores on the mechanical changes of the material. For aryl-
triazenes, whose driving force for cleavage is the release of nitro-
gen, the homolytic mechanism leads to the formation of highly
reactive radicals that can react or not with their immediate sur-
rounding (addition and recombination) depending upon the initial
rigidity of the material. When determined at low temperature
(ꢀ20 °C), the mechanical properties of the samples including aryl-
triazene derivatives and irradiated at k2 do exhibit a decrease as
compared to those measured just after crosslinking at k1, indicat-
ing the triazene photolysis. As temperature was increased, the
glassy rigid matrices observed at low temperature tend to soften,
allowing the radicals produced upon triazene photolysis to move
and react with their surroundings: this results in the hardening
of the material at high temperature. In contrast, 2-nitrobenzyl
photoclevage involves an intramolecular rearrangement releasing
two molecular species. The UV absorption spectra of solutions con-
firmed that the primary photoproducts are not stable and generate
secondary chemical reactions, the latter being, however, reduced in
a rigid matrix. Unlike the monomers 1–5, monomer 6 appears not
to react in a solid matrix with other species evolving through a
cage reaction towards nitroso-ketone.
In summary, although most samples synthesized did undergo
photolysis of their internal linker, only those including 6 could ex-
hibit a decrease of their mechanical properties at the service tem-
peratures of 25 °C for the reasons discussed above. The latter are
thus more appropriate for the dental application contemplated.
News systems with different molecular architectures and based
on the precedent chromophores are currently investigated, their
photobehaviour will be rapidly reported.
3.2.2. 2-Nitrobenzyl derivative 6
For monomer 6, formulations have been thermally cured at
140 °C in the presence of AIBN as radical initiator, then BAPO (Irg
819) was systematically used in place of camphorquinone (vide
supra).
Most of these formulations stand out from their triazene coun-
terparts as none of the zero loading formulations underwent
photo-hardening upon k2 irradiation. The general trend was re-
versed, even highly reversed in the case of the formulation FNC
6/2.1 because G0k2 ꢄ G0k1/3.5 and G0k2 ꢄ G0k1/5 at 25 °C and
ꢀ15 °C, respectively (polymerization of this formulation was car-
Acknowledgements
The authors are indebted to dental laboratories Pierre Rolland
(Acteon Group) for supporting this work and Dr A. Roubière and

532
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