Renewable benzoxazine monomers from Lignin-like naturally occurring phenolic derivatives

Article abstract of DOI:10.1002/pola.26918

The benzoxazines of three naturally occurring phenylpropanoid phenols: ferulic, coumaric, and phloretic acids, and their esters are described. Benzoxazines with conjugated unsaturated chains exhibit unusual poor thermal stability and degrade partially at the polymerization temperature making necessary the use of a catalyst (BF₃.Et₂O) to low the polymerization temperature and prevent degradation. Polybenzoxazines are prepared thermally and characterized by DSC and TGA techniques. The resulting materials have superior Tgs when compared with those prepared from an unsubstituted monofuctional benzoxazine due to the additional crosslinking through the ester and carboxylic moieties.

Full text of DOI:10.1002/pola.26918

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Renewable Benzoxazine Monomers from “Lignin-like” Naturally
Occurring Phenolic Derivatives
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Marc Comı, Gerard Lligadas, Juan C. Ronda, Marina Galia, Virginia Cadiz
ꢀ
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ꢀ
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Departament de Quımica Analıtica i Quımica Organica, Universitat Rovira i Virgili, Campus Sescelades, Marcel.lı Domingo s/n,
Tarragona 43007, Spain
Correspondence to: J. C. Ronda (E-mail: juancarlos.ronda@urv.cat)
Received 19 June 2013; accepted 15 August 2013; published online 00 Month 2013
DOI: 10.1002/pola.26918
ABSTRACT: The benzoxazines of three naturally occurring phe-
nylpropanoid phenols: ferulic, coumaric, and phloretic acids,
and their esters are described. Benzoxazines with conjugated
unsaturated chains exhibit unusual poor thermal stability and
degrade partially at the polymerization temperature making nec-
essary the use of a catalyst (BF3.Et2O) to low the polymerization
temperature and prevent degradation. Polybenzoxazines are pre-
pared thermally and characterized by DSC and TGA techniques.
The resulting materials have superior Tgs when compared with
those prepared from an unsubstituted monofuctional benzoxa-
zine due to the additional crosslinking through the ester and car-
C
V
boxylic moieties.
2013 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2013, 00, 000–000
KEYWORDS: cationic polymerization; crosslinking; polybenzoxa-
zines; renewable resources; thermal properties; thermosets
Cardanol is a monofunctional phenol present in the cashew
nut shell oil and is obtained as a waste byproduct by the
cashew industry. Cardanol-derived polybenzoxazines have
technological potential as matrix for natural fiber-reinforced
composites6 and reactive diluents in the synthesis process of
bisphenol A (BPA) based benzoxazines.7 In a more recent
work, the synthesis of fully renewable benzoxazines from
guaiacol and furfuryl or stearylamine has been also
described.8
INTRODUCTION Polybenzoxazines are a relatively new class
of phenolic type thermosets developed to overcome most of
the short-comings associated with the traditional phenolic
resins. During the last decade, polybenzoxazines have
attracted significant attention of both industry and research
community because of their unique advantages over most of
the known polymers,1 becoming one of a rare few new poly-
mers commercialized in the past 30 years.
The chemistry of benzoxazines is responsible for a number
of inherent processing advantages, including low melt viscos-
ity, no harsh catalyst required, and no volatiles release and
minimal shrinkage during cure. The resulting materials have
also remarkable properties such as heat resistance, superior
electronic performance, low water absorption, low surface
energy, and excellent dimensional stability.2
Recently, we have described the synthesis of new benzoxa-
zine monomers derived from diphenolic acid (DPA).9 DPA is
a condensation derivative of levulinic acid which is a cheap
platform chemical produced commercially from cellulose rich
biomass, especially waste biomass.10 The additional carbox-
ylic acid functionality of these monomers makes possible
additional technological applications like the preparation of
low curing temperature impregnating resins and self-
foaming thermostable rigid foams.11
Clasically, benzoxazine monomers are synthesized either in
solution or by a melt-state reaction using a combination of
petroleum-based phenolic derivatives, formaldehyde, and pri-
mary amines. Nowadays, the scarcity of nonrenewable
resources is encouraging the scientific community to develop
and commercialize new biobased products that can alleviate
the wide-spread dependence on fossil fuels and, enhance
security, the environment, and the economy.3 Until now, only
few examples of renewable resources-based benzoxazine
materials have been reported into the literature. The most
significant example is the cardanol-derived benzoxazine.4,5
Nature offers a rich variety of readily available phenolic and
polyphenolic compounds that have not yet considered for
the synthesis of monomers for polymer synthesis.12 Natu-
rally occurring phenols are among the most widespread class
of metabolites in vegetal organisms, and their distribution is
almost ubiquitous.13 Most of these compounds arise from
phenylalanine and tyrosine which after deamination feeds
the phenylpropanoid way to give
a large variety of
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SCHEME 1 Structure of CA, FA and PA and synthesis of the benzoxazine derivatives. (1) MeOH/CH(OCH3)3/p-CH3C6H4SO3H. (2)
1,3,5-(C6H5)3C3N3/(CH2O)n, 120–130 ꢀC. (3) 1,3,5-(C6H5)3C3N3/(CH2O)n, toluene, 110 ꢀC.
compounds which share the C6-C3 phenyl propanoid skele-
ton and contain one, two or more hydroxylic groups: benzoic
acids, cinnamic acids, flavonoids, proanthocyanidins, coumar-
ins, stilbenes, lignans, and lignins. Even they are present in
many different plants,14 most of these compounds are pro-
duced at commercial scale in their pure form processing
lignocellulosic bagasse. Lignin is a fundamental component
of plants accounting for about 25–35% of the organic matrix
of wood and comprises roughly 20% of the total mass of the
planet biosphere. Lignin is a phenolic high molecular mass
CA and FA are present in relatively large quantities in vegeta-
bles like peanuts, tomatoes, fennel, coffee, or artichokes but, as
main components of lignocellulose, they are commercially pro-
duced mainly by chemical or enzymatic transformation of corn,
sugarcane, and other agricultural bagasses.16–19 PA can be
obtained by hydrogenation of p-coumaric acid but it is com-
monly produced by chemical or enzymatic treatment of phlore-
tin, a dihydrochalcone present in the apple tree leaves.
biopolymer composed of
a
highly branched phenyl-
EXPERIMENTAL
propanoid framework based on p-coumaryl, coniferyl, and
sinapyl moieties, which has long been recognized as a poten-
tial feedstock for producing chemicals.12,15
Materials
The following chemicals were obtained from the sources indi-
cated and used as received: p-ferulic acid (Aldrich), p-couma-
ric acid (Aldrich), phloretic acid (Aldrich) paraformaldehyde
(Mallinkrodt), trimethyl orthoformate (Fluka), p-toluenesul-
fonic acid monohydrate (Aldrich). 1,3,5-Triphenylhexahydro-
1,3,5 triazine and 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine
were synthesized according to a reported procedure.20 All sol-
vents were purified by standard procedures.
In this work, we explore trans-3-(4-hydroxyphenyl)propenoic
acid (p-coumaric acid, CA), trans-3-(3-methoxy-4-hydroxy-
phenyl)propenoic acid (ferulic acid, FA), and 3-(4-hydroxy-
phenyl)propanoic acid (phloretic acid, PA) and their methyl
ester derivatives as platform chemicals for producing valua-
ble benzoxazine monomers and polymers (Scheme 1).
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Synthesis of 3-(3-Phenyl-3,4-dihydro-2H-1,3-benzoxazin-
6-yl)propenoic acid (CA-Bz), 3-[8-Methoxy-(3-phenyl-3,4-
dihydro-2H-1,3-benzoxazin-6-yl)] propenoic acid (FA-Bz),
and 3-(3-Phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-
yl)propanoic acid (PA-Bz; Scheme 1)
1,3,5-Triphenylhexahydro-1,3,5-triazine (0.04 mol), parafor-
maldehyde (0.12 mol), p-coumaric, ferulic, or phloretic acid
(0.12 mol) and 25 mL of toluene were placed into 50 mL
two-necked round-bottom flask equipped with a condenser
and a teflon coated stirring bar. The reaction mixture was
gently stirred and heated at 110 ꢀC for 16 h. The resulting
light yellow to orange solution was filtered and concentrated
in the rotary evaporator obtaining syrup that was dried
under high vacuum for 48 h. The obtained solid mass was
finely powdered, washed several times with diethyl ether,
and finally dried again under vacuum.
179.0 (s, C@O), 153.0 (s), 148.5 (s), 132.7 (d), 129.5 (d),
127.9 (d), 126.6 (s), 121.6 (s), 121.0 (d), 118.4 (d), 117.2
(d), 79.6 (t, OACH2AN), 50.6 (t, ArACH2AN), 36.0 (t,
CH2AAr), and 30.0 (t, CH2ACO).
Synthesis of Methyl 3-(4-hydroxyphenyl)propenoate
(CAM), Methyl 3-(3-methoxy-4-hydroxyphenyl)
propenoate (FAM), and Methyl 3-(4-hydroxyphenyl)
propanoate (PAM; Scheme 1)
A 250 mL round-bottom flask equipped with a condenser
was charged with coumaric, phloretic, or ferulic acid (0.42
mol), p-toluenesulfonic acid monohydrate (4 3 1024 mol),
trimethyl orthoformate (0.21 mol) and 100 mL of methanol.
The mixture was heated under reflux 24 h and the methanol
was removed under vacuum. The resulting oil was dissolved
in ethyl ether and washed several times with a saturated
NaHCO3 solution and with water. The organic layer was
dried with anhydrous MgSO4, the solvent was removed in
the rotary evaporator and the product was dried under vac-
uum at room temperature for 12 h. The resulting colorless
oils, which crystallize after several days, were used without
further purification in the next step. Yields of 98% were
obtained in all cases.
3-(3-Phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-
yl)propenoic acid (CA-Bz)
Yield. 90%, clear yellow powder M.p.: 119–120 ꢀC. FTIR
(cm21): 3463–2176 (r COOH), 1679 (r C@O), 1628 (r
C@C), 1236 (r CAN), 1210, 1029 (rs CAOAC),
941(NACH2AO Bz ring).
1H NMR (CDCl3, tetramethylsilane (TMS) d ppm): 7.98 (1H,
d, J 15.6 Hz, ACH@CHACOOA), 7.63–7.12 (8H, ArAH), 6.58
(1H, d, J 15.6 Hz, ACH@CHAAr), 5,71 (2H, s, OACH2AN),
4.96 (2H, s, ArACH2AN).
Methyl Coumarate (CAM)
Yield. 98%. White crystalls M.p.: 130–132 ꢀC. ESI-TOF
(M 1 Na)1: 201.0532. FTIR: (cm21): 3510–3058 (r OAH),
1682 (r C@O), 1631 (r C@C), 1431 (ra CAOAC), 1195–
1103 (rs CAOAC).
13C NMR (CDCl3, d ppm): 172.5 (s, C@O), 157.1 (s), 148.4
(s), 147.0 (s), 129,7 (d), 128.7 (d), 127.7 (d), 127.2 (d),
122.3 (s), 121.6 (s), 118.9 (d), 118.0 (d), 115.1 (d), 80.4 (t,
OACH2AN), and 50.8 (t, ArACH2AN).
1H NMR (DMSO d6, TMS, d ppm): 10.00 (1H, s, ArAOH), 7.56
(1H, d, J 13.9 Hz, CH@CHACOO), 7.54–6.77 (7H, ArAH), 6.38
(1H, d, J 13.9 Hz, CH@CHAAr), 3.68 (3H, s, COOCH3). 13C
NMR (DMSO d6, d ppm): 167.1 (s, C@O), 159.0 (s),144.8 (s),
130.3 (d), 125.1 (d), 115.8 (d), 113.9 (d), and 51.2 (q,
COOCH3).
3-[8-Methoxy-(3-phenyl-3,4-dihydro-2H-1,3-benzoxazin-
6-yl)]propenoic acid (FA-Bz)
Yield. 92%. White powder. M.p.: 120–121 ꢀC. FTIR (cm21):
3578–2185 (r COOH), 1680 (r C@O), 1625 (r C@C), 1253
(r CAN), 1200, 1082 (rs CAOAC), 929 (NACH2AO Bz ring).
Methyl Ferulate (FAM)
Yield. 98%. White crystalls M.p.: 62–64 ꢀC. ESI-TOF
(M 1 Na)1: 231.0631. FTIR: (cm21): 3572–3103 (r OAH),
1697 (r C@O), 1671 (r C@C), 1512, 1431 (ra CAOAC),
1250–1159 (rs CAOAC).
1H RMN (CDCl3, TMS, d ppm): 7.65 (1H, d, J 16.0 Hz,
CH@CHACOO), 7.27–6.84 (7H, ArAH), 6.28 (1H, d, J 16.0 Hz,
CH@CHAAr), 5,47 (2H, s, OACH2AN), 4.64 (2H, s,
ArACH2AN), and 3.88 (3H, s, COOCH3).
1H NMR (CDCl3, TMS, d ppm): 7.61 (1H, d, J 16.0 Hz,
CH@CHACOOA), 7.26–6.90 (7H, m, ArAH), 6.28 (1H, d, J
16.0 Hz, ACH@CHAAr), 6.09 (1H, s, ArAOH), 3.92 (3H, s,
ACOOCH3), and 3.79 (3H, s, ArOCH3).
13C RMN (CDCl3, d ppm): 172.2 (s, C@O), 148.5 (s), 147.9
(s), 146.9 (s), 146.4 (s), 129.3 (d), 126.4 (d), 121.9 (d),
121.5 (s), 120.4 (s), 118.5 (d), 115.1 (d), 108.5 (d), 80.4 (t,
OACH2AN), 55.9 (q, ArAOCH3), and 50.2 (t, ArACH2AN).
13C NMR (CDCl3, d ppm): 167.9 (s, C@O), 148.2 (s), 146.9
(s), 145.2 (d), 127.0 (d), 123.2 (d), 115.2 (d), 114.9 (d),
109.6 (d), 56.1 (q, COOCH3), 51.2 (q, ArAOCH3).
3-(3-Phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-
yl)propanoic acid (PA-Bz)
Yield. 95%. Yellow powder M.p.: 121–122 ꢀC. FTIR (cm21):
3574–2158 (r COOH), 1692 (r C@O), 1597 (r C@C), 1498
(ra CAOAC), 1252 (r CAN), 1039 (rs CAOAC), and 921
(NACH2AO Bz ring).
Methyl Phloretate (PAM)
Yield 90% colourless liquid. ESI-TOF (M 1 Na)1: 203.0680.
FTIR (cm21) 3572–3080 (r OAH), 1708 (r C@O), 1438 (ra
CAOAC), and 1261–1148 (rs CAOAC).
1H NMR (CDCl3, TMS, d ppm): 7.28–6.73 (8H, ArAH), 5.33
(2H, s, OACH2AN), 4.60 (2H, s, ArACH2AN, 2.85 (2H, t,
CH2ACOO), 2.62 (2H, t, CH2AAr). 13C NMR (CDCl3, d ppm):
1H NMR (CDCl3, TMS, d ppm): 7.02–6.74 (4H, ArAH), 3.66
(3H, s, COOCH3), 2.86 (2H, t, CH2ACOO), and 2.60 (2H, t,
ACH2AAr).
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13C NMR (CDCl3, d ppm): 174.3 (s, C@O), 154.3 (s), 131.9
(s), 129.2 (d), 115.3 (d), 51.77 (q, COOCH3), 35.9 (t,
ACH2AAr), and 29.9 (t, ACH2ACO).
1H NMR (CDCl3, TMS, d ppm): 7.19–6.65 (8H, ArAH), 5.26
(2H, s, OACH2AN), 4.53 (2H, s, ArACH2AN), 3.59 (3H, s,
COOCH3), 2.70 (2H, t, ACH2ACOO), 2.50 (2H, t, CH2AAr).
Synthesis of Methyl 3-(3-phenyl-3,4-dihydro-2H-1,3-
benzoxazin-6-yl) propenoate (CAM-Bz), Methyl-3-[8-
methoxy-(3-phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-yl)]
propenoate (FAM-Bz), and 3-(3-Phenyl-3,4-dihydro-2H-
1,3-benzoxazin-6-yl) propanoate (PAM-Bz; Scheme 1)
A 100 mL round-bottom flask, was charged with 1,3,5-tri-
phenylhexahydro-1,3,5-triazine (0.04 mol), paraformaldehyde
(0.12 mol), and (0.04 mol) of methyl coumarate, methyl
phloretate, or methyl ꢀ ferulate. The reaction mixture was
heated foꢀr 2 h at 130 C in the case of methyl coumarate, 4
h at 120 C in the case of methyl ferulate and methyl phlore-
tate. The resulting light yellow solid was dissolved in ethyl
ether, filtered, and washed several times with 20% NaOH.
The organic layer was dried with anhydrous MgSO4 and con-
centrated under vacuum to obtain syrup that was subse-
quently dried under high vacuum. The resulting solid was
powdered and recrystallized using a heptanes/1,2-dichloro-
ethane 7:3 mixture.
13C NMR (CDCl3, d ppm): 173.7 (s, C@O), 154.8 (s), 148.5
(s), 132.9 (d), 129.4 (d), 127.8 (d), 126.6 (s), 121.5 (s),
121.3 (d), 118.3 (d), 117.0 (d), 79.5 (t, OACH2AN), 51.8 (t,
ArACH2AN), 50.5 (a, OCH3), 36.2 (t, CH2AAr), and 30.2 (t,
CH2ACO).
Curing
Curing was performed in teflon coated molds under pressure
following a preset temperature cycle. Samples of 50 3 8 3
3 mm3 were prepared starting from finely powdered mono-
ꢀ
mer degassed at 60 C under vacuum for 1 h. Catalyzed sam-
ples were prepared adding 3% (w/w) of BF3.Et2O to a
dichloromethane solution of the monomer and evaporated to
dryness prior to the degassing step.
Instrumentation
1H NMR (400 MHz) and 13C NMR (100.6 MHz) spectra were
recorded using a Varian Gemini 400 spectrometer. Chemical
shifts were reported in ppm relative to TMS or CHCl3 as
internal standards.
3-(3-Phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-yl)
propenoate (CAM-Bz)
FTIR spectra were recorded on a Bomem Michelson MB 100
FTIR spectrophotometer with a resolution of 4 cm21 in the
absorbance mode. An attenuated total reflection (ATR)
device with thermal control and a diamond crystal (Golden
Gate heated single-reflection diamond ATR, Specac-Tknok-
roma) was used.
Yield. 91%. White powder M.p.: 110–111 ꢀC. FTIR (cm21):
1687 (r C@O), 1641 (r C@C), 1232 (r CAN), 1035 (rs
CAOAC), and 968 (NACH2AO Bz ring).
1H NMR (CDCl3, TMS, d ppm): 7.41 (1H, d, J 13.9 Hz,
ACH@CHACOO), 7.14–6.77 (7H, ArAH), 6.62 (1H, d, J 13.9
Hz ACH@CHAAr), 5,22 (2H, s, OACH2AN), 4.46 (2H, s,
ArACH2AN), and 3.61 (3H, s, COOCH3).
Calorimetric studies were carried out on a Mettler DSC821
differential scanning calorimeter using N2 as a purge gas (20
mL/min). Thermal stability studies were carried out on a
Mettler TGA/SDTA851e/LF/1100 with N2 as purge gas. The
13C NMR (CDCl3, d ppm): 168.1 (s, C@O), 156.4.2 (s), 148.2
(s), 144.6 (s), 129.6 (d), 128.1 (d), 127.7 (d), 127.2 (s),
122.1 (s), 121.5 (s), 118.7 (d), 117.8 (d), 115.7 (d), 80.2 (t,
OACH2AN), 51.8 (q, AOCH3), 50.7 (t, ArACH2AN).
ꢀ
studies were performed in the 30–800 C temperature range
at a scan rate of 10 ꢀC/min. Degradation studies were car-
ried out in a Carbolite TZF 12/38/400 oven connected to a
condenser cooled by liquid nitrogen. GC-MS measurements
were carried out using an HP 6890 gas chromatograph with
an Ultra 2 capillary column (crosslinked 5% PH ME siloxane)
and an HP 5973 mass detector.
Methyl 3-[8-methoxy-(3-phenyl-3,4-dihydro-2H-1,3-
benzoxazin-6-yl)] propenoate (FAM-Bz)
Yield. 93%. Yellow powder M.p.: 166–167 ꢀC. IR (cm21):
1692 (r C@O), 1632 (r C@C), 1233 (r CAN), 1033 (rs
CAOAC), and 941 (NACH2AOA Bz ring).
RESULTS AND DISCUSSION
1H NMR (CDCl3, TMS, d ppm): 7.56 (1H, d, J 16.0 Hz,
ACH@CHACOO), 7.26–6.82 (7H, ArAH), 6.28 (1H, d, J 16.0 Hz,
ACH@CHAAr), 5,47 (2H, s, OACH2AN), 4.64 (2H, s, ArACH2AN),
3.88 (3H, s, ACOOCH3), and 3.79 (3H, s, ArAOCH3).
As commented in the introduction, the main aim of this
work is to explore the synthesis of new benzoxazine mono-
mers and polymers derived from some 4-hydroxycynamic
acid derivatives of renewable origin: CA, FA, and PA. These
building blocks share a 3-phenylpropanoic acid skeleton
with a phenolic group in the para position and a double
bond in trans configuration in the case of CA and FA.
13C NMR (CDCl3, d ppm): 167.8 (s, C@O), 148.6 (s), 148.2 (s),
146.2 (s), 144.9 (s), 129.5 (d), 126.9 (d), 122.1 (d), 121.6 (s),
120.1 (s), 118.7 (d), 115.8 (d), 108.5 (d), 80.5 (t, OACH2AN),
56.1 (q, AOCH3), 51.8 (q, ArAOCH3), and 50.2 (t, ArACH2AN).
One of the main drawbacks in the synthesis of benzoxazine
monomers is the extensive formation of oligomers, which
reduces the yield of benzoxazine and makes the purification
difficult. This undesired side-reaction has been reported to
be favoured mostly by water, polar solvents and high tem-
peratures.21–25 One of the synthetic approaches described to
3-(3-Phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-yl)
propanoate (PAM-Bz)
Yield. 97% White solid Mp: 156–157 ꢀC. FTIR (cm21): 1728
(r C@O), 1226 (r CAN), 1023 (rs CAOAC), and 943
(NACH2AOA Bz ring).
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ylether the final product. By this procedure, benzoxazine
monomers CA-Bz, FA-Bz, and PA-Bz were obtained in high
yield. (Scheme 1).
In the case of methyl ester derivatives, the lower melting
point of the CAM, FAM, and PAM allowed to carry out the
reaction in bulk. The reaction was complete in 3 h working a
110 ꢀC in the case of FAM-Bz and PAM-Bz but required
higher temperatures and longer times in the case of CAM-Bz
due to the higher melting point of CAM. Oligomeric traces
were removed washing with 10% NaOH and the crude
monomers were purified by recrystallization to yield essen-
tially pure CAM-Bz and FAM-BZ. The formation of the ben-
zoxazine ring is detected by the FTIR characteristic band at
about 945 cm21 due to the out-of-plane CAH vibration of
the benzene ring to which the oxazine ring is attached and
by the 1H NMR methylene singlet signals at c.a. 5.2 ppm
(OACH2AN) and at about 4.5 ppm (ArACH2AN). The struc-
ture of all benzoxazine monomers was confirmed by FTIR-
1
ATR and H and 13C NMR spectroscopy. As example, Figure 1
1
shows the H NMR spectra of some benzoxazines.
Thermal behavior and stability were characterized by DSC
and TGA (Table 1). By DSC all benzoxazine monomers show
a melting endotherm (with low associated enthalpy) in most
cases followed by a relatively narrow exotherm at higher
temperatures. Usually, this exothermic peak is attributed to
the thermal benzoxazine ring opening polymerization. How-
ever, for all unsaturated monomers TGA data clearly indi-
cates that this process is overlapped with thermal
decomposition of the monomer, which involves weight losses
around 20–25% of the initial mass. A different behaviour
can be inferred for PA-Bz and PAM-Bz, which have a satu-
rated C3 substituent. For these monomers, the weight loss is
negligible and the exothermic process can consequently be
attributed to the benzoxazine polymerization. This behavior
is represented in Figure 2 in the case of CA and PA
benzoxazines.
FIGURE 1 1H NMR spectra of (a) PA-Bz, (b) CAM-Bz, and (c)
CA-Bz.
minimize the formation of oligomers consist of the use of
1,3,5-triphenylhexahydro-1,3,5-triazine and paraformalde-
hyde as reagents in bulk conditions or with the help of sol-
vents like toluene or xylenes.20,26 The high melting point of
CA, FA, and FA, prevent the synthesis of the corresponding
benzoxazines in bulk conditions and toluene had to be added
as solvent. The reactions required 16h to be complete.
Attempts to shorten the reaction time by increasing the tem-
perature using xylenes, gave products with high oligomeric
content. Usually, phenolic oligomers are eliminated by wash-
ing with 2M sodium hydroxide but in this case the presence
of acid functionalities prevents this treatment. Purification
was done by filtering solid impurities and rinsing with dieth-
Most benzoxazine monomers reported in the literature are
ꢀ
stable at the polymerization temperature 150–250 C allow-
ing preparing well defined thermosetting resins. At higher
TABLE 1 DSC and TGA Characterization of the Benzoxazine Monomers
Mon.
Tm (ꢀC)a
Texoo (ꢀC)b
Texom (ꢀC)c
Tdo (ꢀC)d
Tdm (ꢀC)e
Wexp (%)f
W
theo.(%)g
CA-Bz
117
121
122
109
166
156
131
132
131
213
224
180
137
130
212
216
203
197
137
130
212
216
203
197
169
168
263
252
246
278
14,5
19.2
2.8
44.8
40.5
44.5
42.6
38.7
42.3
FA-Bz
PA-Bz
CAM-Bz
FAM-Bz
PAM-Bz
26.5
23.5
3.8
a
f
Maximum temperature of the melting endotherm by DSC.
Onset temperature of the exotherm by DSC.
Percentage of weight loss after the first decomposition step by TGA.
b
c
gTheoretical weight percent loss, supposing the elimination of one mol-
ecule of aniline and one molecule of formaldehyde.
Maximum temperature of the exotherm by DSC.
d
e
Onset temperature of the decomposition by TGA.
Temperature of the maximum decomposition rate for the first decom-
position step by TGA.
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ester, and methoxy groups were observed in all cases, indi-
cating that these moieties remain unaffected at these tem-
peratures. The spectra of the polymeric residues show the
characteristic signals of the aniline ring, indicating that a
part of the aniline groups remain in the polymer and are not
volatilized. This can be confirmed by the TGA results col-
lected in Table 1. As can be seen, the percentage of weight
loss after the first degradation step is always lower than the
theoretical expected for the loss of one molecule of aniline
and one molecule of formaldehyde.
Aniline has been described as one of the initial decomposi-
tion products in aniline-derived benzoxazine resins, which
are detected at temperatures above 270 ꢀC.31 However, this
ꢀ
compound is detected at temperatures as low as 130–150 C
in the case of the carboxylic acid benzoxazines CA-Bz and
FA-Bz indicating a lower thermal stability of these mono-
mers. These results suggest that the presence of a double
bond conjugated to the benzoxazine phenolic ring causes
unstabilization to benzoxazine ring.
FIGURE 2 DSC and TGA plots for CA-Bz and PA-Bz monomers.
(Melting of CA-Bz is not detected in this DSC plot). [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Taking into account, the above evidences and the generally
accepted benzoxazine polymerization mechanisms,33,34 we
propose a possible degradation mechanism which is consist-
ent with the formation of the detected products from the
free and polymerized monomer (Scheme 2).
temperatures polybenzoxazine resins undergo well known
thermal degradation processes, which have been the matter
of several studies.27–31 The unsaturated monomers studied
in this work show an unusual low thermal stability, which
must be related with some structural characteristics. Prece-
dent studies show an important influence of the Mannich
base and the phenolic structure in the thermal degradation
of polybenzoxazines, especially in the initial degradation
steps.27 To gain more insight about these degradation proc-
esses, an analysis of the volatile components at the maxi-
mum temperature of the degradation exotherm was carried
out by pyrolysis-GC-MS.
According to the results collected in Table 1, only monomers
PA-Bz and PAM-Bz can be thermally polymerized without
significant decomposition. For the rest of the monomers the
polymerization proceeds with partial decomposition prevent-
ing the preparation of useful materials. It must be pointed
out that, as expected, the temperature of the polymerization
and degradation exotherm shows a clear influence of the
benzoxazine structure being almost 100 ꢀC lower for the
monomers with a free carboxylic acid group due to its cata-
lytic effect on the benzoxazine ring opening.35–37
It is well known that, apart from protic acids several other
substances catalyze the benzoxazine ring-opening lowering
considerably the polymerization temperature.33,38,39 As
boron trifluoride complexes have been shown as effective
catalysts for the benzoxazine curing,36,40 we used BF3.Et2O
as catalyst in an attempt to polymerize the unsaturated ben-
zoxazine monomers at lower temperatures than the decom-
position temperature. In this way, all monomers were heated
in presence of 1, 2, and 3 mol % of BF3.Et2O and the poly-
merization behaviour was followed by DSC. The results, col-
lected in Table 2, indicate that catalyst activates the
polymerization reducing the reaction temperature for all
monomers. According to TGA data, in the case of CA-Bz,
CAM-Bz, FA-Bz, and FAM-Bz, the polymerization proceeds
without significant degradation when using 2 or 3% of
BF3.Et2O. The most relevant case is FAM-Bz, which is shown
in Figure 3.
Unsaturated benzoxazine monomers produced few volatile
products, collected as a white solid and yellowish oil, that
were identified, respectively, as paraformaldehyde and ani-
line. Also traces of phenol and methylphenols were detected.
This seems to indicate that thermal decomposition of these
monomers proceeds mainly through the ring-opening and
subsequent fragmentation of the benzoxazine ring. The
orange-red pyrolysis residue was partially soluble in DMSO-
d6 and CDCl3 especially in the case of the ferulic acid-
derived benzoxazines. The 1H NMR spectra of this soluble
fractions present broad signals at 4.4–4.0, 3.8–3.4, and 2.2–
2.0 ppm. Polymerization of benzoxazines have been reported
to proceed through the formation of N,O-ketalic bridges
(methylene signals at 4.25 ppm) that rearrange to Mannich
bridges (methylene signals at 3.80 ppm) at high tempera-
ture.32 Thus, the structure of the pyrolisis residues seems to
correspond to oligomeric fractions with both Mannich and
N,O-ketalic bridges. In addition the peak 2.1 ppm can be
attributed to the formation of methylphenolic groups.20
Moreover, signals corresponding to double bond, methyl
Without catalyst, this monomer starts to polymerize at about
210 ꢀC with extensive degradation but in presence of 1%
catalyst a double exotherm starting just after the monomer
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SCHEME 2 Proposed mechanism for the thermal degradation of the unsaturated benzoxazine monomers and polymers.
melting at about 160 ꢀC can be observed. As the amount of
catalyst increase, the enthalpy of the high temperature exo-
therm (associated to the degradation processes) progres-
sively decreases and the peak completely disappears when
using 3% BF3.Et2O. The exotherm at low temperature only
can be observed in presence of catalyst and increases its
enthalpy with the amount of catalyst and is the only peak
observed when using 3% BF3.Et2O. TGA analysis indicates
that in presence of catalyst the polymerization is favoured
and the decomposition phenomena are delayed or com-
pletely suppressed.
According to the above results, PA-Bz and PAM-Bz were
polymerized without catalyst and all the unsaturated mono-
mers CA-Bz, CAM-Bz, FA-Bz, and FAM-Bz were polymerized
in presence of 3% of BF3.Et2O. 3-phenyl-3,4-dihydro-2H-1,3-
benzoxazine Ph-Bz was also polymerized in absence of cata-
lyst as reference material. In Table 2 are collected the curing
TABLE 2 Onset (Texoo) and Maximum (Texom) Temperatures of the Exothermic Peaks in the Polymerization of Benzoxazine Mono-
mers Without Catalyst and Catalyzed with 1, 2, and 3% of BF3.Et2O
Mon.
Neat Texoo/Texom
1% Texoo/Texom
2% Texoo/Texom
3% Texoo/Texom
Curing conditions
CA-Bz
136
140
132
152
151
160
220
246
224
244
210
244
259
262
130
139
100
143
–
129
137
96
126
136
88
125 (3 h)
135 (1 h)
100 (3 h)
135 (1 h)
150 (3 h)
160 (1 h)
155 (3 h)
200 (1 h)
175 (3 h)
200 (1 h)
215 (3 h)
245 (1 h)
150 (3 h)
260 (1 h)
FA-Bz
142
–
138
–
PA-Bz
CAM-Bz
FAM-Bz
PAM-Bz
Ph-Bz
170
215
169
204/239
–
159
206
167
207/237
–
147
202
165
205
–
–
–
–
Curing and postcuring conditions selected.
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of the double bond protons are absent indicating probably
some polymerization of the unsaturated chains. Noteworthy,
no signals over 4.0 ppm can be observed confirming that
after the used curing cycles, no N,O-ketalic links remain in
the structure of these resins, which are composed basically
by Mannich type bridges.
Polybenzoxazine resins were characterized by DSC and TGA.
The Tg values and the thermal stability under air and nitro-
gen is collected in Table 3. All monofunctional benzoxazine-
based materials are characterized by low crosslinking
degrees. In our monomers, the presence of substituents in
some of the active aromatic positions, were expected to
reduce even more the crosslinking points, especially in the
case of ferulic derivatives.
On the contrary, the measured Tg’s of the obtained materials
were in the order of 120–150 ꢀC and higher than the Tg of
the substituent free Ph-BZ. Moreover, in all cases the
methylester-derived benzoxazine materials have higher Tg
values than their carboxylic acid counterparts. These high Tg
values can be attributed to the occurrence of additional
crosslinking reactions through the propanoic or propenoic
units. In the case of DPA derived benzoxazines, we have
recently demonstrated the existence of esterification and
specially transesterification reactions between the phenolic
hydroxylic groups resulting from the benzoxazine ring open-
ing and the free carboxylic or the methyl ester groups.9 Anal-
ysis by FTIR-ATR of the obtained materials show new peaks
at 1740–1760 cm21 for both the carboxylic acid and the
methylester-derived polybenzoxazines. In fact this higher fre-
quency peak constitutes the most intense carbonyl peak in
all the thermosets indicating the extensive formation of phe-
nolic ester linkages as extra crosslinking links.
FIGURE 3 DSC plots of FAM-Bz (————-) (a) and its mixtures
with 1% (- - - -) (b), 2% (– – –) (c), and 3% (– - – -) (d), of
BF3.Et2O. Upper plot is the TGA trace of FAM-Bz with 3% of
BF3.Et2O. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
conditions for each monomer, which were established
according to the DSC behavior.
In this way, curing was performed for 3 h at about 10 ꢀC
over the onset of the polymerization exothem, and postcur-
ing heating for 1 h at the maximum temperature of the poly-
merization exothem. The resulting thermosets were hard and
fragile orange to red solids which were completely insoluble
in solvents such as DMSO, DMF, or NMP indicating their
crosslinked structure. The only exception was the material
obtained from FAM-Bz, which was partially soluble in DMSO.
1H NMR analysis of this soluble fraction shows two main
broad peaks: one at 7.5–6.5 ppm, corresponding to the aro-
matic protons, and a second at 4.0–3.6 ppm corresponding
to the Mannich bridges, which appear overlapped with the
signals of the COOCH3 and OCH3 groups. Also small signals
at 2.9 and 2.0 ppm are observed, the first one attributable to
the formation of diphenyl-methane bridges. Moreover, signals
Referring to the thermal stability, all materials resulted less
stable than the unsubstituted polibenzoxazine Ph-BZ with
the exception of PAM-Bz. In addition CA and FA based ben-
zoxazines are less stable than PA-based benzoxazines. More-
over, materials with free carboxylic acid groups are less
stable than the corresponding methyl esters. These behaviors
can be related with the low stability of unsaturated chains
and the occurrence of decarboxylation processes. The pres-
ence of free carboxylic groups and unsaturated chains also
TABLE 3 Thermal Properties of the Benzoxazine Thermosets
DSC
TGA Nitrogen
TGA Air
Monom.
1/2DCp (ꢀC)
T5% (ꢀC)
Tmax (ꢀC)
R800 (%)
T5% (ꢀC)
Tmax (ꢀC)
R800 (%)
CA-Bz
119
120
130
154
135
143
115
220
200
293
280
259
340
320
235, 414
200, 341
281, 455
300, 432
260, 330, 440
328, 423
395, 469
30
34
43
43
45
43
46
221
190
295
255
259
352
326
235, 392, 451, 622
191, 265, 388, 552
277, 449, 650
0
0
0
0
0
0
0
FA-Bz
PA-Bz
CAM-Bz
FAM-Bz
PAM-Bz
Ph-Bz
268, 468, 640
260, 344, 400, 554, 649
330, 431, 652
335, 470, 650
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FIGURE 4 First derivative TGA plots of (a) PolyPh-Bz (. . . .) and the acid containing polybenzoxazines (——-): 1 PolyCA-Bz, 2
PolyFA-Bz, and 3 PolyPA-Bz and (b) PolyPh-Bz (. . . .) and ester containing polybenzoxazines (——-): 4 PolyCAM-Bz, 5 PolyFAM-Bz,
and 6 PolyPAM-Bz. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
seems to influence the char residue, which is clearly lower
(30–34%) when compared with the ester containing and PA
derived benzoxazines (over 40%).
presence of catalyst to produce materials with comparable
or even improved properties when compared with the corre-
sponding petroleum-based analogues.
Figure 4 shows the TGA curves and the fist derivative
recorded under nitrogen. As can be observed in Figure 4,
degradation behavior changes with the structure of the start-
ing benzoxazines. CA, CAM, PA, and PAM thermosets present
two main degradation stages, one with maximum degrada-
tion rate at 230–330 ꢀC and another, more important with
ACKNOWLEDGMENTS
Financial support by the MICINN (Ministerio de Ciencia e Inno-
vacion; MAT2011-24823) is gratefully acknowledged.
ꢀ
maximum degradation rate at 410–460 C. Both stages occur
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