Phenolic Naphthoxazines as Curing Promoters for Benzoxazines

Article abstract of DOI:10.1021/acs.macromol.8b00218

The effect of phenolic hydroxyl bearing naphthoxazines as catalysts for the ring-opening polymerization (ROP) of simple 1,3-benzoxazines was investigated. The latent catalytic role of napthoxazines in the curing process of the mixtures was demonstrated by

Full text of DOI:10.1021/acs.macromol.8b00218

Article
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Phenolic Naphthoxazines as Curing Promoters for Benzoxazines
Gizem Kaya, Baris Kiskan,* and Yusuf Yagci*
Department of Chemistry, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey
S
* Supporting Information
ABSTRACT: The effect of phenolic hydroxyl bearing
naphthoxazines as catalysts for the ring-opening polymer-
ization (ROP) of simple 1,3-benzoxazines was investigated.
The latent catalytic role of napthoxazines in the curing process
of the mixtures was demonstrated by DSC and FT-IR
investigations. It was found that phenolic naphthoxazines
cause clear reduction in ROP temperature, particularly when
electron-withdrawing groups are attached and an onset ROP
temperature as low as 169 °C is attained. Thermal properties
of the final polymers were also analyzed, and no significant
effect of naphthoxazines on the thermal stability of the cured
polybenzoxazines was observed.
flexibility for the corresponding polymers with tailored
properties.18,19 For example, cross-linking density of the PBZs
can be increased by using allyl,20,21 propargyl,22 coumarine,23 or
heteronucleobase24 functionalities. Alternatively, PBZs can be
altered by selecting long-chain alkylamines25,26 and amino
alcohols27,28 in monomer synthesis, etc. Moreover, polymeric
benzoxazines were also synthesized using several chemistries to
produce main-, side-, and end-chain polybenzoxazine pre-
cursors.29−39 Consequently, many different specific benzox-
azines for various purposes are readily accessible.
The polymerization of these monomers is a kind of thermally
driven cationic ring-opening process (ROP) that proceeds over
an oxazine ring.40−42 The required polymerization temper-
atures generally lie between 220 and 260 °C, and for some
monomers even higher temperatures are needed. As stated,
ROP is based on cationic species that can easily be stabilized by
N and O atoms on an oxazine ring.43−45 Generally, initiation is
triggered by the residual phenolic −OH groups present in
monomers. These residues provide protons that are acting as
cationic initiator and protonate either N or O atom of the
heterocycle. Depending on the protonated site, the bond
cleavage of O−CH2−N differs, producing either aryl ether or
aryl aminomethyl repeat units. Eventually, due to the thermal
effect, aryl ether units rearrange and PBZ forms (Scheme 2).
Interestingly, polymerization of benzoxazines through photo-
chemically generated acids yield mostly soluble oligomers with
complex structures.46
INTRODUCTION
■
Polybenzoxazines (PBZs) and polynaphthoxazines (PNZs)
emerged as contender alternative phenolics in the past decade
because these materials exhibit superior properties compared to
many other resins. PBZs and PNZs have high glass transition
temperatures (Tg) generally more than 160 °C up to 400 °C,
high char yields, and flame retardancy with low smoke and
toxicity.1−5 Moreover, these materials display remarkable
tensile strength and modulus, limited water uptake unlike
other traditional phenolics, dimensional stability during curing,
and chemical resistance.6−9 Hence, PBZs and PNZs are
preferred for applications where harsh conditions present
such as those using aerospace materials and high performance
circuit boards, etc.10
PBZs and PNZs can be synthesized only from their
corresponding monomers, namely 1,3-benzoxazines or naph-
thoxazines. The other isomers, however, do not yield analogous
polymers. Although benzoxazines were first reported in the
1940s and their initial applications were for medicinal purposes,
the potency of these compounds in material science was
recognized much later.11−17 One of the important aspects of
these compounds is their simple synthesis. A primary amine, a
suitable phenolic compound, and formaldehyde are the
components of the synthesis (Scheme 1). Most of the amines
and phenols are already commercially available, and therefore
these monomers can be produced in a large scale with cost
efficiency. Moreover, using various types of phenols and amines
with different functional groups attached provides a high design
Benzoxazine monomers can readily be polymerized without
any added catalyst, but generally high temperatures are required
which is considered to be a major drawback in terms of energy
consumption and necessity of thermal resistant equipment.
Scheme 1. Synthesis of Benzoxazine and Naphthoxazine
Received: January 30, 2018
© XXXX American Chemical Society
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and effectively reduced ROP temperatures.59 Despite these
considerable efforts, further improvements in the reduction of
polymerization temperature, particularly with cheap catalysts,
still remains highly desirable and major concern for
benzoxazine-based high performance thermosets. Herein, we
describe a latent catalyst based on naphthoxazines to reduce the
ROP of benzoxazines to practical temperatures, and the system
would contain only benzoxazines and naphthoxazines. Techni-
cal aspects regarding applications of naphthoxazines as curing
promoters of benzoxazine-based systems are reported else-
where.63
Scheme 2. Acid-Catalyzed ROP of a 1,3-Benzoxazine
EXPERIMENTAL SECTION
■
Materials. 1-Naphtol (Fluka AG, ≥99%), 4-amino-2,6-dichlor-
ophenol (Alfa Aesar, 97%), paraformaldehyde (Aldrich, 95.0−
100.5%), 4-aminophenol (Merck, ≥99%), p-anisidine (Aldrich,
≥99%), phenol (Aldrich, ≥99%), benzylamine (Aldrich, 99%), sodium
hydroxide (Merck, ≥99%), potassium hydroxide (reagent grade,
Aldrich, 90%), magnesium sulfate (Acros, 97%), methanol (MeOH,
Merck, ≥99.9%), acetonitrile (ACN, Merck, 99.9%), ethyl acetate
(EtOAc, Merck, ≥99.5%), tetrahydrofuran (THF, Sigma-Aldrich,
≥99.9%), dimethyl sulfoxide (DMSO, Sigma-Aldrich, ≥99%), N,N-
dimethylformamide (DMF, Sigma-Aldrich, ≥99.8%), and ethanol
(EtOH, Carlo Erba, ≥99.9%) were used as received. B-a monomer
(bisphenol A and aniline based) was synthesized according to the
literature.2
Obviously, lower curing temperatures would be practical for
many applications. Therefore, several catalysts to promote ROP
of benzoxazines and naphthoxazines were reported tried by
various research groups. For example, Lewis acids were found
to reduce the curing temperatures down to 120−160 °C.41
Typically, PCl5, POCl3, TiCl4, AlCl3, FeCl3, etc., acted as
efficient catalysts particularly when with solvents and ring-
opening of oxazine took place even at room temperature.
Moreover, acetylacetonato complexes of fourth period
transition metals with nucleophiles promoted the polymer-
ization efficiently and 120 °C as onset of ROP temperatures
were obtained.47 On the other hand, due to highly active
catalytic nature of Lewis acid catalysts, they often initiate ROP
during storage and lead to viscosity increase in formulations.
Thus, latent catalytic systems which are dormant at room
temperature or certain temperature intervals are highly
desirable. Structurally different molecules other than benzox-
azines or special benzoxazines such as diamines,48,49 thiols,50−54
toluene sulfonates,55 and colloidal sulfur56,57 can act as ROP
latent catalysts. It should be noted that thiols are generally
highly active at low temperatures but prone to storage
problems. However, in some specific cases, disulfides play a
better latent catalytic role than thiols.58 As stated, designed
benzoxazines with certain groups in the structure can also
catalyze the ROP process. This approach provides additional
for some applications since the system would only contain
benzoxazines based structures and final product would not be
contaminated by residual catalysts. Functional groups having
acidic character on benzoxazines can obviously reduce the ROP
temperatures. Thus, several benzoxazines were synthesized
bearing −COOH and −OH functionalities.59−61 In this
connection, it should be pointed out that decarboxylation
observed with COOH groups during the polymerization
process generate voids in the final polymer.62 Thus, −OH
groups appear to be more beneficial as catalyst preventing
problems associated with gas formations. For example,
hydroxyethyl terminated ether chain-functional benzoxazines
were found to reduce ROP temperature compared to the
nonfunctional benzoxazines.27 The mechanistic role of
hydroxylethyl group in the polymerization was studied, and it
was found that hydroxyethyl groups accelerate polymerization
via interaction between −OH group and cationic intermediates
formed during ROP. Similar effects were also observed for
methylol containing monomers via hydrogen bonding between
heteroatoms of oxazine and methylol group.28 Moreover,
phenolic monomers were also synthesized as built-in catalysts
Characterization. All 1H NMR spectra were recorded on an
Agilent NMR System VNMRS 500 spectrometer at room temperature
in CDCl3 or DMSO-d6 with Si(CH3)4 as an internal standard. FTIR
spectra were recorded on a PerkinElmer FTIR Spectrum One
spectrometer. Differential scanning calorimetry (DSC) was performed
on PerkinElmer Diamond DSC from 30 to 320 °C with a heating rate
of 10 °C min−1 under nitrogen flow. A typical DSC sample was 2−5
mg in a 30 μL aluminum pan. Thermal gravimetric analysis (TGA)
was performed on PerkinElmer Diamond TA/TGA with a heating rate
of 10 °C min−1 under nitrogen flow.
Synthesis of Naphthoxazines with Phenolic −OH Groups. A
typical procedure is as follows: In a 100 mL round-bottomed flask, 1-
naphthol (3 g, 0.021 mol) and 4-amino-2,6-dichlorophenol (3.7 g,
0.0208 mol) were dissolved in acetonitrile (30 mL), and the solution
was mixed at ambient temperature. Formaldehyde solution was as
follows; paraformaldeyhde (1.25 g, 0.042 mol), ethanol (5 mL), and
≈50 mg of NaOH were mixed and heated until the solution became
clear. This solution was added to acetonitrile solution immediately.
The whole mixture was magnetically stirred at ambient temperature
for 24 h. The reaction solution was concentrated using a rotary
evaporator and then precipitated in warm water. The resulting solid
was filtered and washed with warm water. After that, the solid
dissolved in hot ethanol and reprecipitated in water, filtered, and
washed with copious amounts of water. The product was dried in a
vacuum oven at 50 °C for 24 h, and an orange solid was obtained
(yield: ≈69%) Note: 4-aminophenol were used instead of 4-amino-
2,6-dichlorophenol for aminophenol-based monomer. These naph-
thoxazines were abbreviated as Npz-PhOHCl2 for 4-amino-2,6-
dichlorophenol and Npz-PhOH for 4-aminophenol derivatives.
Synthesis of Anisidine-Based Naphthoxazine. Acetonitrile (30
mL) was added in a 100 mL round-bottom flask containing 1-naphthol
(3 g, 0.021 mol) and p-anisidine (2.56 g, 0.021 mol). Formaldehyde
solution (1.25 g, 0.042 mol) was added portionwise to this solution.
The whole mixture was magnetically stirred at ambient temperature
for 24 h. The solvent was evaporated using a rotary evaporator. The
resulting product was dissolved in ethyl acetate (100−120 mL) and
extracted with 0.4 N sodium hydroxide for various times. Then, ethyl
acetate solution was washed with distilled water (100 mL) for two
times to neutralize the solution. The solution was dried with
anhydrous MgSO4 and filtered. Ethyl acetate was evaporated with a
rotary evaporator, and the product was dried in a vacuum oven at 50
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°C for 24 h. Finally, a brown solid was obtained (yield: ≈63%). The
monomer is abbreviated as Npz-PhOCH3.
Synthesis of Benzylbenzoxazine. Benzylamine (10.7 g, 0.1
mol), phenol (9.4 g, 0.1 mol), and paraformaldehyde (6 g, 0.2 mol)
were mixed in 200 mL of 1,4-dioxane. The solution was heated to 105
°C for 16 h. After cooling the solution, 1,4-dioxane was removed using
a rotary evaporator. Oily crude product was dissolved in diethyl ether,
washed with 0.5 M NaOH solution 3−4 times, and finally washed with
deionized water 2 times. The ether solution was dried with anhydrous
Na2SO4 and then filtered to remove the salt. Then, diethyl ether was
completely removed with a rotary evaporator, and viscous raw product
remained. The raw product was recrystallized from an ethanol−water
mixture and dried under vacuum at room temperature (rt). The
product was obtained as white crystals (yield: 59%). The monomer is
abbreviated as P-Bn (see Supporting Information for 1H NMR
spectrum, Figure S1).
1
Figure 1. H NMR spectra of Npz-PhOCH3 (a), Npz-PhOHCl2 (b),
and Npz-PhOH (c).
RESULTS AND DISCUSSION
■
Generally, naphthols exhibit higher reactivity in several
reactions than many phenols due to their extra electron-rich
aromatic rings.64 Based on Mannich reactions with these
compounds conducted at low temperatures, aminophenols can
produce −OH bearing oxazine monomers in one step.59 In the
case of ordinary phenols, similar monomers could only be
synthesized by a three-step synthetic procedure containing
imine formation, reduction of imine to amine, and finally ring-
closing with formaldehyde.65 By taking the advantage of
reactivity of naphthols, free −OH-containing naphthoxazines
were synthesized in one step at rt (Scheme 3). Two different
attached to −OH groups. In Figure S2, the oxazine carbons are
visible at 80.0 ppm (O−CH2−N) and 51.4 ppm (Ar−CH2−
N). Moreover, the aromatic carbon attached to oxazine ring
from the oxygen side is detectable at 149.5 ppm, and the
aromatic carbon that has linkage with −OH appears at 159.9
ppm. FTIR spectra of the Npz-PhOH and Npz-PhOHCl2 give
additional evidence for the formation of oxazine ring in both
structures (Figure 2). A vibration band of the substituted
Scheme 3. Synthesis of Naphthoxazines Npz-PhOH (R: −H,
R1: −H), Npz-PhOHCl2 (R: −Cl, R1: −H), and Npz-
PhOCH3 (R: −H, R1: −CH3)
aminophenols, namely 4-amino-2,6-dichlorophenol and 4-
aminophenol, were used in the synthesis to produce phenolic
naphthoxazines (Npz-PhOHCl2 and Npz-PhOH, respectively).
Anisidine-based naphthoxazine was also synthesized as an
−OH-capped sample with a methyl group (Npz-PhOCH3). It
should be noted that Npz-PhOH and Npz-PhOCH3 were
synthesized previously for organic or medicinal chemistry
purposes by other or similar chemical methods.66,67 Also,
although Npz-PhOH was previously reported,68 Npz-PhOHCl2
is synthesized for the first time as a novel promoter, and the
other two naphthoxazines were used only for comparison.
The spectral characterization of monomers was performed
via H NMR and FTIR spectroscopies. H NMR spectra of
naphthoxazines are shown in Figure 1. The characteristic
oxazine N−CH2−O peaks emerge at 5.47, 5.52, and 5.61 ppm;
also, Ar−CH2−O peaks are observable as wide bands at 4.57,
4.52, and 4.55 ppm for Npz-PhOCH3, Npz-PhOH, and Npz-
PhOHCl2, respectively. Moreover, the NMR spectrum of Npz-
PhOCH3 exhibits a −OCH3 proton signal at 3.76 ppm. The
phenolic nature of Npz-PhOH and Npz-PhOHCl2 can be
proved by −OH peaks at 8.88 and 9.01 ppm, respectively. 13C
NMR spectroscopy was used to further verify the chemical
structure of Npz-PhOHCl2 since this technique is useful to
identify the characteristic signal of oxazine and the carbon
Figure 2. FTIR spectra of Npz-PhOH (a) and Npz-PhOHCl2 (b).
aromatic ring of naphthoxazines also appears at 1490 and 1510
cm−1, respectively. The C−O vibration band of monomers
further verifies the presence of oxazine structure. The phenolic
−OH groups are clearly visible as wide bands in the range of
3620−3176 cm−1. Moreover, the stretching vibrations of
aromatic C−H (3006−3096 cm−1) and aromatic CC
(1453−1660 cm−1) and the out-of-plane bending vibrations
of aromatic C−H at 928 and 914 cm−1 are detected for both
structures.
Figure 3 shows the DSC profile of the three compounds, and
onset, end-set, and maximum curing temperatures are tabulated
in Table 1.
As seen both from Table 1 and Figure 3, the onset of curing
and the maximum curing temperature are as low as 152 and
180 °C for Npz-PhOHCl2. Moreover, the endothermic at 117
°C corresponding to the melting of the Npz-PhOHCl2 crystal is
1
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temperature enough for the complete ring-opening (Figure S3).
Accordingly, a complete ROP was observed after heating at 150
°C for 10 min.
In Figure 3, there is another broad exotherm after ROP of
Npz-PhOHCl2 between 237 and 279 °C. The end-set region of
this exotherm is distorted and can be attributed to the
degradation. However, onset and maximum temperature zones
are uniform and correspond to an exothermic reaction. This
reaction can possibly be an etherification reaction between
−OH and −Cl groups. Although, such reactions are usually
performed in the presence of a catalyst at mild conditions,
etherification may still take place at such high temperatures
with the effect of tertiary amine on polynaphthoxazines
(Scheme 4). The basicity of tertiary amine can abstract proton
from −OH generating a negatively charged nucleophilic
oxygen. At such high temperature, this nucleophile can attack
aromatic carbons where Cl atoms are attached. Subsequently,
an amine salt can be formed through the reaction of amine with
evolved HCl. Accordingly, the possibility of the formation of
the aromatic C−O bond was tracked by FTIR spectroscopy.
Therefore, two Npz-PhOHCl2 samples were treated with
different temperatures to distinguish the aromatic ether C−O
bands from the C−O stretching vibrations of ring-opened
oxazine. One of the samples was heated up to 170 °C to ring-
open the oxazine and to prevent further possible etherification
and the other sample exposed to heat up to 300 °C for the
claimed etherification. The overlaid spectra of both samples are
presented in Figure 4 which clearly exhibits different aromatic
Figure 3. DSC thermograms of Npz-PhOCH3 (a), Npz-PhOH (b),
and Npz-PhOHCl2 (c).
a
Table 1. DSC Characteristics of the Monomers
ΔH of
onset temp of
curing (°C)
max curing end-set temp of
curing
(J/g)
monomer
temp (°C)
curing (°C)
Npz-
PhOCH3
180
201
226
−23
Npz-PhOH
166
152
196
180
219
206
−37
−25
Npz-
PhOHCl2
a
DSC measurements were performed under nitrogen flow (20 mL
min−1) with a scan rate of 10 °C min−1.
lower than that of many other naphthoxazines reported.
Typically, the related temperatures for naphthoxazine function-
alized with a cyanate ester group and aromatic amine-based
naphthoxazine are 206 and 214 °C, respectively. In fact, these
values are lower than the monomers Npz-PhOH and Npz-
PhOCH3 synthesized for comparison. As stated, acidic
conditions would obviously decrease ROP temperature of the
1,3-oxazine ring. Therefore, a low ROP temperature for Npz-
PhOHCl2 is expected due to the contribution of phenolic −OH
on the structure which has a more acidic character arising from
the electron-withdrawing nature of the chlorine atoms. The
effect of −OH group on ROP of benzoxazines was also
demonstrated by using Npz-PhOCH3 monomer as blank.
Expectedly, the curing exotherm is higher than the other two
monomers since this monomer contains no −OH groups.
From these results, it can be deduced that the main effect on
ROP temperature is the acidity of −OH group, and the
strength of the acidity can be arranged by using additional
electron-withdrawing groups on the aromatic ring of the amine
in the synthesis. Moreover, Npz-PhOHCl2 exposed to heat at
different temperatures for 10 min to confirm the suitable
Figure 4. FTIR spectra of thermal treated Npz-PhOHCl2 at 170 °C
(a) and Npz-PhOHCl2 at 300 °C (b).
C−O bonds for different temperatures. Accordingly, FTIR
spectra comparison is giving clues about the possible
Scheme 4. Possible Etherification Reaction between PNZ Chains
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Scheme 5. Acid−Base Reaction between P-Bn and Npz-PhOHCl2 Generating Two Different Types of Intermediates (Electron-
Withdrawing Atoms on the Catalyst Are Highlighted)
etherification at high temperatures that is already observed as
an exotherm in DSC analysis.
As previously stated, the initial stage of the polymerization of
benzoxazines is the cleavage of methylene bridge on the oxazine
by the residual protons (vide ante, Scheme 2). After bond
cleavage, the formed cationic intermediates are stabilized by N
and O atoms followed by the attack on aryl and rearrangement
of labile bonds, finally producing PBZs. In a such polymer-
ization, latent activation of catalyst during curing would be
beneficial to solve the storage problems as reflected by the
viscosity increment at the shelf. The DSC profile of Npz-
PhOHCl2 did not change after two months, indicating its
stability for long time and latent catalytic nature.
Although Npz-PhOHCl2 is curable at low temperatures,
cured Npz-PhOHCl2 is slightly soluble in common organic
solvents such as DMSO, DMF, and THF, indicating that the
polymerization of this monomer is limited and some oligomers
mol %) (b), and P-Bn/Npz-PhOHCl2 (5 mol %) (c).
remain in the final product. Therefore, postcuring was applied
Figure 5. DSC Thermograms of P-Bn (a), P-Bn/Npz-PhOHCl2 (3
to have a complete network, but soluble parts still remained. As
Table 2. DSC Characteristics of P-Bn and Its Mixtures with
a
measured by GPC, the soluble part is mainly dimer having a
Npz-PhOHCl2
molecular weight of Mn ≈ 700 Da. These results revealed that
Npz-PhOHCl2 has limitation to be used in high performance
ΔH of
onset temp of max curing end-set temp
curing (°C) temp (°C) of curing (°C)
curing
(J/g)
applications where a stiff network is required. In light of these
experiments, it seemed appropriate to use Npz-PhOHCl2 in a
different manner such as latent catalyst in certain formulations.
For this purpose, Npz-PhOHCl2 was used as catalyst by mixing
with P-Bn monomer (Scheme 5) in two different mole ratios (3
and 5 mol %). As stated, the acidity of Npz-PhOHCl2 is
relatively high due to electron-withdrawing Cl atoms, and
phenolic −OH group can protonate either O or N atom on the
P-Bn monomer, resulting in two different types of inter-
mediates (Scheme 5). Consequently, polymerization proceeds
in two separate pathways. The resulting polymer would be a
combination PBZ and PNZ; therefore, the final network would
only be composed of phenolics, and any other component such
as metal or noncurable organic catalysts residues are excluded
from the system. These additives may act as potential void
forming compounds or may alter the end properties of the PBZ
network. Moreover, Npz-PhOHCl2 is not corrosive compared
to classical Lewis or organic acid catalyst, thus offering
additional superiority.
The catalytic performance of Npz-PhOHCl2 was studied by
independent DSC investigations. In Figure 5 and Table 2, the
comparative DSC profiles of P-Bn and the P-Bn/Npz-
PhOHCl2 system are presented. As expected, Npz-PhOHCl2
acted as an effective catalyst for reducing the curing
temperature of P-Bn. Increasing the Npz-PhOHCl2 content
in the mixture had more impact on the ROP temperature. Only
3 mol % of catalyst decreased the onset of curing of P-Bn from
monomer
P-Bn
240
188
254
212
263
232
−40.4
−98.9
P-Bn/Npz-
PhOHCl2
(3 mol %)
P-Bn/Npz-
PhOHCl2
(5 mol %)
169
201
221
−86.3
a
DSC measurements were performed under nitrogen flow (20 mL
min−1) with a scan rate of 10 °C min−1.
240 to 188 °C, and 5 mol % catalyst dropped this temperature
as low as to 169 °C. It is clear that Npz-PhOHCl2 showed an
ability to reduce the onset of ROP of benzoxazine below 177
°C, which is considered as standard curing temperature for
aerostructures. Moreover, a second exotherm observed in the
DSC thermogram of Npz-PhOHCl2 (see Figure 3) is not
detectable because it is diluted in the system, and the possible
etherification reaction is prevented. It should be emphasized
that the Npz-PhOHCl2 content can be increased more than 5
mol % since the catalyst is also curable and takes part of the
network. Thus, increased amount of Npz-PhOHCl2 would
obviously reduce the ROP temperature even more, but in a
such case a careful consideration is needed since some
drawbacks may occur due to the composition of end-structure.
Typically, high amounts of Npz-PhOHCl2 may decrease the
network stiffness (vide ante).
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Apart from DSC analysis, a simple FTIR study was
performed for the latent catalyst property of Npz-PhOHCl2
in P-Bn. For the purpose, a mixture of P-Bn/Npz-PhOHCl2 (5
mol %) was prepared and subjected to FTIR analysis as
prepared and 1 week later. Figure 6 shows the overlaid FTIR
Table 3. Thermal Properties of the Cured Npz-PhOHCl2,
Npz-PhOH, Npz-PhOCH3, P-Bn, and P-Bn/Npz-PhOHCl2
a
sample
T5% (°C)
T10% (°C)
Tc (%)
P-Bn
257
250
272
301
263
265
258
292
327
293
35
38
41
56
59
b
P-Bn/Npz-PhOHCl2
Npz-PhOCH3
Npz-PhOH
Npz-PhOHCl2
a
T5%: the temperature for which the weight loss is 5%. T10%: the
temperature for which the weight loss is 10%.Tc: the char yield at 800
b
°C. The amount of Npz-PhOHCl2 is 5 mol %.
evaporation of these small molecules can lead having lower T5%
and T10% temperatures. On the contrary, possible self-
etherification reactions under heat exposure may contribute
to the thermal stability of cured Npz-PhOHCl2 at high
temperatures, resulting in a 59% of char yield. The char yields
of these three naphthoxazines are higher compared to P-Bn due
to ease of polyaromatization of naphthyl groups. The effect of
Npz-PhOHCl2 on char yield is also seen in TGA trace of P-Bn/
Npz-PhOHCl2 mixture since only 5 mol % Npz-PhOHCl2 has a
positive effect on char as 3% at 800 °C. On the other hand, the
effect of the Npz-PhOHCl2 on thermal properties of classical
bisbenzoxazine (abbreviated as B-a, benzoxazine derived from
bisphenol A and aniline) was also investigated by TGA (Figure
S4), and the effect of catalyst on the thermal properties was
observed as insignificant.
Figure 6. FTIR spectra of P-Bn/Npz-PhOHCl2 after 1 week storage
(a), fresh P-Bn/Npz-PhOHCl2 mixture (b), and P-Bn (c).
spectra of P-Bn and P-Bn/Npz-PhOHCl2 mixture. As can be
seen, there is no detectable change in spectra; even the
phenolic −OH band at 3508 cm−1 remained the same in both
the bandwidth and peak. Accordingly, Npz-PhOHCl2 is stable
at rt for at least 1 week in P-Bn and thus can be used as a latent
catalyst with classical benzoxazines without a storage problem.
Thermal stability of the PNZs derived from Npz-PhOCH3,
Npz-PhOH, Npz-PhOHCl2, and PBZs from P-Bn and P-Bn/
Npz-PhOHCl2 (5 mol %) was measured by using thermogravi-
metric analysis (TGA). TGA traces and related thermal
properties are presented in Figure 7 and Table 3, respectively.
CONCLUSION
■
Npz-PhOHCl2 undergoes thermally activated curing at temper-
atures much lower than that of conventional benzoxazines. The
self-polymerization of Npz-PhOHCl2 was found to be
insufficient to produce a stiff polynaphthoxazine network;
thus, this compound cannot be used solely as a material.
However, the results revealed that Npz-PhOHCl2 has high
catalytic potential for the polymerization of benzoxazines.
Accordingly, it was shown that the electron-withdrawing groups
on naphthoxazines bearing phenolic −OHs are more acidic
than their analogues and thus reduce the onset temperature of
ROP of benzoxazines down to 169 °C. Furthermore, Npz-
PhOHCl2 was used with P-Bn and played a latent catalytic role
since the mixture of P-Bn/NpzPhOHCl2 had revealed no
spectral change after 1 week and catalyst can be activated only
by heating. Besides, Npz-PhOHCl2 was also stable and did not
undergo self-polymerization even after two months of storage.
Apart from catalytic benefits, cured Npz-PhOHCl2 exhibited
high char yield as much as 59% at 800 °C. Similarly, cured P-
Bn/Npz-PhOHCl2 (5 mol %) mixture showed better char yield
compared to pristine cured P-Bn. Consequently, naphthox-
azines bearing phenolic −OH with neighboring electron-
withdrawing functional groups have high acidity to catalyze
ROP of benzoxazines acting as dormant species, and the system
would only contain benzoxazines or naphthoxazines with
improved char yields.
Figure 7. TGA thermograms of cured Npz-PhOHCl2 (a), Npz-PhOH
(b), Npz-PhOCH3 (c), P-Bn/Npz-PhOHCl2 (5 mol %) (d), and P-Bn
(e).
According to T5%, T10%, and Tc values and TGA, all the three
PNZs are more thermally stable than PBZs. The initial
degradations of cured P-Bn and Npz-PhOHCl2 are closer to
each other, but the char yields differ up to 24%. The initial
degradation temperature of Npz-PhOHCl2 is lower than other
naphthoxazines since curing of this monomer is complicated,
and as stated before oligomers remain in the cured form; thus,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b00218.
■
S
Figures S1−S4 (PDF)
F
DOI: 10.1021/acs.macromol.8b00218
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Macromolecules
Article
fluorene-based benzoxazine monomer and its high performance
thermoset. RSC Adv. 2014, 4 (110), 64798−64801.
AUTHOR INFORMATION
Corresponding Authors
*(B.K.) E-mail: kiskanb@itu.edu.tr.
*(Y.Y.) E-mail: yusuf@itu.edu.tr.
ORCID
Baris Kiskan: 0000-0001-9476-2054
Yusuf Yagci: 0000-0001-6244-6786
Notes
■
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Istanbul Technical University Research Fund is acknowledged
for financial support.
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