Photolysis and thermal active polymerization of bis (benzylidene) based benzoxazine monomers

Article abstract of DOI:10.1016/j.molstruc.2012.06.002

A new class of photopolymerizable benzoxazine monomers has been developed, using photoactive bisphenol derivatives such as bis (4-hydroxybenzylidene)-4- phenyl cyclopentanone and bis (4-hydroxybenzylidene)-4-phenyl cyclohexanone, p-toludine and formaldehyde (37%). The structure of the benzoxazine monomers has been confirmed by Fourier Transform Infra Red spectroscopy (FTIR) and ¹H and ¹³C Nuclear Magnetic Resonance spectroscopy (NMR). Upon photolysis with radiation of 365 nm, the monomers undergo photolysis via [2π + 2π] cyclo-addition reaction, which was confirmed by UV spectroscopy. After photolysis, the benzoxazine monomers were subsequently cured at 200 °C for 2 h. The photo irradiated cured products possessed enhanced glass transition temperature and thermal stability, with an increase in the photodimerization time.

Full text of DOI:10.1016/j.molstruc.2012.06.002

Journal of Molecular Structure 1027 (2012) 162–166
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Photolysis and thermal active polymerization of bis (benzylidene) based
benzoxazine monomers
M.R. Vengatesan a, S. Devaraju a, M. Selvi a, A. Chandramohan a, A. Ashok Kumar b, M. Alagar a,
⇑
a Polymer Composites Lab, Department of Chemical Engineering, Anna University, Chennai 600 025, India
b Department of Chemistry, A.P.A College of Arts and Culture, Palani 647 818, India
h i g h l i g h t s
"
"
"
New class of photoactive benzoxazine monomers has been developed.
Upon photolysis on 365 nm, the monomers undergoes photolysis via [2
The UV irradiated benzoxazine gives a highly thermally stable polybenzoxazine.
p
+ 2p] cycloaddition reaction.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of photopolymerizable benzoxazine monomers has been developed, using photoactive
bisphenol derivatives such as bis (4-hydroxybenzylidene)-4-phenyl cyclopentanone and bis (4-hydrox-
ybenzylidene)-4-phenyl cyclohexanone, p-toludine and formaldehyde (37%). The structure of the ben-
zoxazine monomers has been confirmed by Fourier Transform Infra Red spectroscopy (FTIR) and 1H
and 13C Nuclear Magnetic Resonance spectroscopy (NMR). Upon photolysis with radiation of 365 nm,
Received 2 February 2012
Received in revised form 4 June 2012
Accepted 4 June 2012
Available online xxxx
the monomers undergo photolysis via [2p + 2p] cyclo-addition reaction, which was confirmed by UV
Keywords:
spectroscopy. After photolysis, the benzoxazine monomers were subsequently cured at 200 °C for 2 h.
The photo irradiated cured products possessed enhanced glass transition temperature and thermal sta-
bility, with an increase in the photodimerization time.
Bis (benzylidene)
Polybenzoxazine
Photo cross linking
Glass transition temperature
Thermal stability
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
been the basis of numerous conventional applications in coatings,
adhesives, inks, printing plates, optical waveguides, and microelec-
In recent years, the word ‘‘polybenzoxazine’’ is more familiar in
the thermosetting world, because it offers a lot of unique proper-
ties, such as low moisture absorption, super chemical resistance,
good flame retardance, excellent electrical properties, high thermal
stability and mechanical properties, which are discussed in several
works in literature [1–5]. Polybenzoxazines are prepared from
thermal ring opening polymerization of benzoxazine monomers
or benzoxazine prepolymers. Due to its numerous advantages,
polybenzoxazine based chemistry is one of the growing fields rec-
ognized in polymer research. Polybenzoxazine research is going on
in several dimensions, among them a new molecular designing is
preferable as this one will be very useful to improve the properties
of polybenzoxazines.
tronics [6–11]. Thermal polymerization is usually carried out at
elevated temperatures, whereas photo polymerization can be per-
formed even at temperatures below the room temperature. The
polymerization of benzoxazine, using the combination of mecha-
nism of photo and thermal polymerization has been several addi-
tional advantages of high performance applications. Photo
chemically active groups are incorporated into the benzoxazine
structure; it undergoes sequential photochemical reaction fol-
lowed by thermal activation and this leads to a highly cross-linked
polybenzoxazine network. This would obviously influence the
glass transition temperature (Tg) and consequently the mechanical
properties of the resultant polymer. Only a few works are available
on photo cross linkable benzoxazine [12–17]. A bifunctional ben-
zoxazine monomer containing a coumarin core has been reported,
which is capable of undergoing photodimerization, and the ther-
mal ring-opening polymerization processes results highly dense
cross-linked networks [12]. Recently, Jin et al. [17] reported a
methacrolyl functionalized benzoxazine monomer; it was sub-
jected to photo polymerization followed by thermal polymeriza-
Currently, photo polymerization attracts more rapid interest in
the field of synthetic polymer chemistry. Photo polymerization has
⇑
Corresponding author. Tel./fax: +91 44 22359164.
E-mail address: mkalagar@yahoo.com (M. Alagar).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.06.002

M.R. Vengatesan et al. / Journal of Molecular Structure 1027 (2012) 162–166
163
tion. The photo polymerization was successfully carried out in the
2.3. Synthesis of (2E, 6E)-2, 6-bis ((3, 4-dihydro-3-p-tolyl-2H-
benzo[e][1, 3]oxazin-6-yl) methylene)cyclohexanone (BZH)
presence and absence of a photo initiator, while the thermally acti-
vated polymerization is compromised by low temperature degra-
dation [17]. The bis (benzylidene) cycloalkanone core in organic
monomers is notable mesogenic and having photoactive character.
It was used as a photoactive core in wide range of polymer com-
pounds. Several reports are available on photo cross linkable ben-
zylidene based polymers [18–22].
BZH was synthesised by using p-toludine, bis (benzylidene)
cyclohexanone and 37% aqueous formaldehyde, the reaction fol-
lowed as per the procedure reported in Section 2.2 (Scheme 1).
Yield: 86% [23].
To the best of our knowledge, no reports are available on the
photo cross linkable benzylidene based benzoxazine monomer.
Hence, in the present work, new bis (benzylidene) based benzox-
azine monomers were synthesised and their photo polymerization
behavior as well as thermal polymerization was studied.
2.4. Photo polymerization followed by thermal polymerization of
benzoxazine
Each 0.001 M CHCl3 solution of benzoxazine monomers (BZP
and BZH) was irradiated in a UV photo reactor separately at 5,
30,60 and 120 min intervals each, and the photo chemical activity
of benzoxazine monomer was studied, using UV spectroscopy in
each time interval. After UV irradiation, the solution was slowly
evaporated, and then cured at 200 °C for 2 h.
2. Experimental
2.1. Materials
Cyclohexanone, cyclopentanone, 4-hydroxy benzaldehyde, p-
toluidine, formaldehyde (37%), sodium hydroxide, anhydrous so-
dium sulfate and all the solvents were used and received from
SRL (India) Ltd. 2,6-bis(4-hydroxybenzylidene)-cyclohexanone
(BBCH), 2,5-bis(4-hydroxybenzylidene)-cyclopentanone (BBCP)
were used and synthesised as per the literature [18].
2.5. Characterization
2.5.1. Spectroscopy
The FT-IR spectrum was recorded on a Perkin Elmer 6X FT-IR
spectrometer. About 100 mg of optical-grade KBr was ground with
a sufficient quantity of the solid sample to make the wt.% mixture
for making KBr pellets. After the sample was loaded, a minimum of
16 scans were collected for each sample at a resolution of 4 cmÀ1
.
2.2. Synthesis of (2E, 5E)-2, 5-bis ((3, 4-dihydro-3-p-tolyl-2H-
benzo[e][1,3]oxazin-6-yl)methylene)cyclopentanone (BZP)
The 1HNMR and 13CNMR spectra of benzoxazine monomers (BZP
and BZH) were recorded with a BRUCKER 300 MHz NMR spectrom-
eter. Samples were diluted using deuterated chloroform (CDCl3),
and tetramethylsilane (TMS) was used as an internal standard.
The photo cross linking behavior was monitored by the absorp-
tions between 320 nm and 420 nm on a Shimadzu UV-160A UV
visible recording spectrophotometer in a chloroform solution
(0.001 M).
The reaction was conducted in the same manner as the proce-
dure reported in our earlier work [23]. 37% aqueous formaldehyde
solution 11 g (0.3666 mol) and p-toludine 6.95 g (0.0648 mol)
were mixed with 1, 4-dioxane (50 mL) in a 250 ml three necked
flask at 0–5 °C for 30 min. Then 10 g (0.0326 mol) of bis (benzyli-
dene) cyclopentanone was added. The temperature of the reaction
mixture was gradually raised to reflux and maintained for 6 h.
After the completion of the reaction, the solvent and water were
removed under vacuum and the residue was dissolved in chloro-
form (CHCl3) (100 mL). The product solution was washed with
water and 1 N sodium hydroxide (NaOH) several times, to remove
any impurities and unreacted monomers. The product solution was
then dried by using anhydrous sodium sulfate and the solvent
evaporated at room temperature, to yield a wine red semi solid
product (Scheme 1). Yield: 82%.
2.5.2. Thermal properties
The Glass transition temperature (Tg) of the samples was deter-
mined, using DSC 200 PC differential scanning calorimeter (Net-
zsch Gerateban GmbH) in the temperature range between 25 °C
and 300 °C at a heating rate of 10 °C per min in a nitrogen atmo-
sphere. The thermogravimetric analysis (TGA) was performed on
a Netzsch STA 409 thermogravimetric analyzer. The instrument
was calibrated with calcium oxalate and aluminum supplied by
O
NH2
O
1,4-dioxane
O
O
CH2O
5h/Reflux
HO
OH
N
N
BBCP
paraformaldehyde
BZP
O
p-Toluidine
O
1,4-dioxane
5h/Reflux
NH2
O
O
CH2O
HO
OH
N
N
paraformaldehyde
BBCH
p-Toluidine
BZH
Scheme 1. Synthesis of photo active benzoxazine monomers (BZP and BZH).

164
M.R. Vengatesan et al. / Journal of Molecular Structure 1027 (2012) 162–166
Netzsch. The samples (about 50 mg) were heated from ambient
temperature to 800 °C under
(60 mL/min), at 10 °C/min.
a continuous flow of nitrogen
3. Results and discussion
3.1. Spectral analysis of bis (benzylidene)based benzoxazine monomers
Photoactive benzoxazine monomers (BZP and BZH) were syn-
thesised from appropriate photoactive bisphenols, p-toluidine
and paraformaldehyde; it has been presented in Scheme 1. The
FT-IR spectrum of (BZP and BZH) is presented in Fig. S1. A strong
intensive band appeared at 2942 and 2852 cmÀ1 and it represent
the symmetric and asymmetric stretching frequency of the cyclo-
pentanone moiety. The formation of a strong intensive band at
1500 cmÀ1 indicates the presence of a tri substituted benzene ring
of benzoxazine monomer. The band appeared at 1247 cmÀ1 indi-
cating the carbonyl stretching frequency of the benzylidene unit.
The appearance of a strong intensive band at 992 cmÀ1 confirms
the presence of bis benzylidene group in the benzoxazine mono-
mers. The appearance of a band at 930 cmÀ1 indicates N–C–O
stretching, which in turn, confirms the formation of benzoxazine
monomer. The 1H NMR spectra of BZP and BZH are presented in
Figs S2 and S4. The benzylidene protons of the benzoxazine mono-
mer appear as a singlet at 7.72 ppm. The methylene protons of –N–
CH2–O– and –N–CH2–Ar appear as a singlet at 5.3–5.4 ppm and
4.3–4.6 ppm respectively which confirmed the formation of ben-
zoxazine monomer. The cyclic protons are appearing as multiplet
in the range between 1.2 and 1.7 ppm. The 13C Spectrum of BZP
is presented in Fig. S3. The two types of methylene bridge carbon
(–N–CH2–O– and –N–CH2–Ar) appears in range between 50 and
80 ppm respectively and the carbonyl carbon appear at 181 ppm.
Fig. 2. Changes in UV spectral characteristics during photolysis of BZH in CHCl3 at
various time intervals.
the formation of the dimerization of olefinic double bond of bis
(benzylidene) cycloalkanone chromophores, which involves the
2p + 2p cycloaddition reactions (Figs. 1 and 2). The absorption at
360 nm disappeared completely after 120 min of irradiation. The
relative rate of photo cross linking of benzoxazine monomers are
shown in Fig. 3, where the relative reactivity Ao–At/Ao is plotted
against the irradiation time, in which, ‘‘Ao’’ is the absorption before
irradiation and ‘‘At’’ is the absorption after irradiation at time ‘‘t’’. It
is observed that, among the two types of benzoxazine monomers,
the cyclopentanone containing monomer undergoes photolysis
faster than the cyclohexanone containing monomer. This may be
attributed to an increase in the unfavorable geometry for 2p + 2p
cycloaddition reactions in cyclohexanone containing monomer
[18].
3.2. Photo cross linking studies of benzoxazine monomers
Two independent reactions, such as photo induced dimerization
and ring-opening polymerization of photo active monomers, were
investigated. The benzoxazine monomer in a chloroform solution
was irradiated in a UV photo reactor and its structural changes
were monitored by UV spectrophotometer. The representative
spectral changes in the photolysis of benzoxazine monomers are
shown in Figs. 1 and 2 respectively. The UV spectra of benzoxazine
monomer in a chloroform solution showed that there is a decrease
in the intensity of absorption at 360 nm. This may be ascribed to
3.3. Effect of photo irradiation on the curing behavior of benzoxazine
monomers
The DSC thermograms taken before and after the UV irradiation
of benzoxazine monomers (BZP and BZH) and the data are pre-
sented in Table 1. All the monomers show a single exothermic
peak. Before UV irradiation, the DSC thermogram of benzoxazine
(BZP) exhibits an onset, and the maximum curing temperature at
Fig. 1. Changes in UV spectral characteristics during photolysis of BZP in CHCl3 at
various time intervals.
Fig. 3. Rate of photo cross linking of BZP and BZH under UV irradiation in different
time intervals (A0 and At are the absorbance at time 0 min and absorbance at time t).

M.R. Vengatesan et al. / Journal of Molecular Structure 1027 (2012) 162–166
165
Table 1
Curing studies of benzoxazine monomers before and after UV irradiation.
250
Polybenzoxazines from BZP and UV irradiated BZP
Polybenzoxazines from BZH and UV irradiated BZH
Sample code
Ti (°C)
Tmax (°C)
240
230
220
210
200
190
BZP
163
166
177
169
169
181
229
232
251
235
238
255
BZPa
BZPb
BZH
BZHa
BZHb
Ti – initial polymerization temperature; Tmax – maximum curing temperature.
a
BZP (BZP subjected to UV irradiation for 5 min).
BZP (BZP subjected to UV irradiation for 120 min).
b
163 °C and 229 °C respectively (Table 1), whereas the monomer
BZH exhibits then at 169 °C and 235 °C respectively (Table 1).
The photolysis was performed in a UV reactor using irradiation
of CHCl3 solution (0.001 M) of benzoxazine monomers (BZP and
BZH) at 365 nm for 5 min and 120 min. After solvent evaporation,
the curing behaviors of both the irradiated solid samples were ana-
lyzed using the DSC instrument. The onset and peak at the maxi-
mum curing exotherm of BZPa (BZP subjected to UV irradiation
for 5 min) are 166 °C and 242 °C respectively. The values are al-
most similar to the onset and peak at the maximum curing exo-
therm of BZHa (BZH subjected to UV irradiation for 5 min) viz,
168 °C and 237 °C respectively. The curing behavior of the 5 min
UV irradiated benzoxazine monomers (BZPa and BZHa) is not much
varied, while the monomers irradiated for 120 min (BZPb and
BZHb), exhibited a significant variation in the onset and maximum
curing temperatures (Table 1). Fig. 4 shows the variation of the
maximum curing temperature with respect to UV irradiation time
of benzoxazine monomers. The changes occurring in the curing
exotherm are due to the dimerization and higher cross-linking for-
mation of the UV irradiated samples. The cross-linking of benzox-
azine could be due to the hydrogen abstraction from the carbon
atom adjacent to the nitrogen atom or from the vinylene proton
of bisbenzylidene. Due to this hydrogen abstraction, a new free
radical species has been generated that result in two reacting sites
in one molecule [17].
0
20
40
60
80
100
120
UV irradiadtion time of benzoxazine
monomers (min)
Fig. 5. A plot of Tg values of polybenzoxazines vs. irradiation time of benzoxazine
monomers.
obtained from non irradiated benzoxazine monomer. The Tg values
of P-PBZs increased linearly with an increase in the UV irradiation
time of benzoxazine monomers (Fig. 5). The increase in the Tg value
may be ascribed to the higher cross linking density of P-PBZs. The
thermal behavior of polybenzoxazines was evaluated by the TGA in
a nitrogen atmosphere at the heating rate of 10 °C/min and the
data are presented in Table 2. Twenty percent weight loss temper-
atures of the photo-polybenzoxazine (P-PBZP1 and P-PBZH1) were
observed at 442 °C and 428 °C, which are in the higher region than
that of neat polybenzoxazine (Table 2). The results indicate that,
the photo-polybenzoxazines were stable up to 247 °C and start
degrading thereafter in a nitrogen atmosphere. When the photo
irradiation time of benzoxazine monomer increases, its cross link-
ing density may also increase. Therefore the highly cross linkable
benzoxazine undergoes thermal polymerization resulting in ther-
mally stable polybenzoxazine. The thermal stability of polybenzox-
azines increased linearly with an increase in the irradiation time of
the benzoxazine monomers. From the data, it was observed that
the 20% wt loss temperature of the PBZP (435–473 °C) is more sta-
ble than that of PBZH (414–451 °C) (Table 2). This may be ex-
plained on the basis of the Baeyer’s strain theory, that the heat of
combustion per methylene unit of cyclopentanone was higher than
that of cyclohexanone [18].
3.4. Effect of photo cross linking on thermal properties of
polybenzoxazines
The photo-polybenzoxazines (P-PBZs) obtained from benzox-
azine involved to UV irradiation exposed by thermal curing, pos-
sess higher values of Tg than those of polybenzoxazine (PBZ)
Table 2
Thermal properties of polybenzoxazines (PBZP and PBZH).
BZP and UV irradiated BZP
Sample code
Tg (°C)
T5
T20
Char yield (wt.%) at 800 °C
260
BZH and UV irradiated BZH
PBZP
202
209
221
230
235
189
194
206
217
223
259
273
280
284
291
247
258
267
275
280
435
442
459
466
473
414
428
434
446
451
29
33
38
42
45
24
27
32
36
39
P-PBZP1
P-PBZP2
P-PBZP3
P-PBZP4
PBZH
P-PBZH1
P-PBZH2
P-PBZH3
P-PBZH4
250
240
230
PBZP: Thermal curing of BZP monomer without UV irradiation; P-PBZP1: BZP
monomer UV irradiated for 5 min followed by thermal curing; P-PBZP2: BZP
monomer UV irradiated for 30 min followed by thermal curing; P-PBZP3: BZP
monomer UV irradiated for 60 min followed by thermal curing; P-PBZP4: BZP
monomer UV irradiated for 120 min followed by thermal curing; PBZH: Thermal
curing of BZH monomer without UV irradiation.; P-PBZH1: BZH monomer UV
irradiated for 5 min followed by thermal curing; P-PBZH2: BZH monomer UV irra-
diated for 30 min followed by thermal curing; P-PBZH3: BZH monomer UV irradi-
ated for 60 min followed by thermal curing; P-PBZH4: BZH monomer UV irradiated
for 120 min followed by thermal curing.
0
20
40
60
80
100
120
UV irradiation time of benzoxazine
monomers (min)
Fig. 4. A plot of Tmax vs. UV irradiation time of benzoxazine monomers.

166
M.R. Vengatesan et al. / Journal of Molecular Structure 1027 (2012) 162–166
4. Conclusion
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.06.
002.