Ammonium salt catalyzed ring-opening polymerization of 1,3-benzoxazines

Article abstract of DOI:10.1016/j.polymer.2017.06.077

The effect of amine HCl salts as catalysts on the ring-opening polymerization (ROP) of simple 1,3-benzoxazines was investigated. The catalyst effects on the structure of the final polymer was examined and compared. The curing process for each salt-benzoxa

Full text of DOI:10.1016/j.polymer.2017.06.077

Accepted Manuscript
Ammonium salt catalyzed ring-opening polymerization of 1,3-benzoxazines
Azra Kocaarslan, Baris Kiskan, Yusuf Yagci
PII:
S0032-3861(17)30657-2
10.1016/j.polymer.2017.06.077
JPOL 19810
DOI:
Reference:
To appear in: Polymer
Received Date: 7 April 2017
Revised Date: 12 June 2017
Accepted Date: 29 June 2017
Please cite this article as: Kocaarslan A, Kiskan B, Yagci Y, Ammonium salt catalyzed ring-opening
polymerization of 1,3-benzoxazines, Polymer (2017), doi: 10.1016/j.polymer.2017.06.077.
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ACCEPTED MANUSCRIPT

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Ammonium Salt Catalyzed Ring-Opening Polymerization of
1,3-Benzoxazines
Azra Kocaarslan, Baris Kiskan1,*, Yusuf Yagci1,*
1Istanbul Technical University, Faculty of Science and Letters, Department of Chemistry, 34469, Maslak,
Istanbul, Turkey
ABSTRACT
The effect of amine HCl salts as catalysts on the ring-opening polymerization (ROP) of simple
1,3-benzoxazines was investigated. The catalyst effects on the structure of the final polymer was
examined and compared. The curing process for each salt-benzoxazine mixture was monitored by
using Differential Scanning Calorimetry (DSC) and it was found that amine salts cause clear
1
reduction in ROP temperature. As followed by H NMR investigations, the ring-opening of
oxazine can take place even at r.t. when suitable solvent is used. Moreover, the analysis gave
evidences that amine HCl salts act as nucleophilic catalyst and their efficiency is dependent on
ionization ability and solubility of the salts. Thermal stability of the final polymers was also
analyzed using Thermal Gravimetric Analysis (TGA).
Keywords: Benzoxazine, Amine Salt, Ring-opening polymerization, Curing, Polybenzoxazine,
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INTRODUCTION
Polybenzoxazines (PBZs) as alternative contender to classical phenolics have emerged in the last
few decades and become one of the rare polymers found industrial applications. This is mainly
due to their unique structural features such as high glass transition temperatures (Tg) (170–340
°C), char yields (generally 20–60% at 800 °C), tensile strength and modulus (100–125 MPa, 3.8–
4.5 GPa, respectively), flame retardancy with low smoke and toxicity, limited water uptake,
dimensional stability during curing and resistance against acids and bases.[1-4] Consequently,
these materials have high service temperatures and their stability under harsh conditions makes
them good candidates especially for aerospace and similar applications.[5] The structural
difference between PBZs and other phenolic resins is the presence of tertiary amine group in each
repeat unit, which generate a great number of intra- and inter-molecular hydrogen bonds that
determine many properties of these materials.[6-11] PBZs are obtained from their corresponding
1,3-benzoxazine monomers via ring-opening polymerization (ROP) of oxazine moiety (Scheme
1). The polymerization is thermally driven and temperatures between 140–220 °C, sometimes
even more, are required for the ring-opening process depending on the monomer structure.[12-
14] Although, low ROP temperatures are often observed for specially designed monomers such
as carboxylic acid or phenolic –OH containing benzoxazines,[15-20] common benzoxazine
monomers have ROP temperatures over 220 °C when dynamic heating is applied.[21-23]
Scheme 1: Ring-opening polymerization of 1,3-benzoxazine monomer yielding PBZ.
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It is generally accepted that the polymerization proceeds over a kind of cationic pathway initiated
by the reaction of residual phenolic –OH of monomers and oxazine ring. This issue was simply
tested by using pure benzoxazines and ROP temperatures increased drastically. The N and O
atoms present on the oxazine ring stabilize the cations formed and ring-opening takes place with
a subsequent electrophilic aromatic substitution to form PBZ. As expected, there is two
protonation possibility either on N or O atom and thus, initiation of polymerization proceeds
concomitantly in two routes. [24-28] Although, the initiation site has effect on the intermediates,
the final polymer for both pathways is considered to have the same structure (Scheme 2).
Scheme 2: Proposed polymerization mechanisms for 1,3-benzoxazines initiated by
protonation of either N or O atoms. [24-28]
Though the ROP of benzoxazines does not require additional catalyst or curative, high curing
temperature is an obvious restriction both in terms of energy consumption and limitation on the
usage of thermally susceptible functional groups on PBZs. Hence, lower ROP temperatures
would be a significant advantage. To address this issue, several catalysts such as organic or Lewis
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acids and compounds bearing nucleophilic character have been employed to promote the ROP.
For example, PCl5, POCl3, TiCl4, AlCl3, FeCl3 with solvents have been shown to have good
catalytic activities.[25, 28-30] Another important example concerns the use of the mixture of a
Lewis acid and a nucleophilic catalyst to effectively promote the polymerization. Typically,
acetylacetonato complexes of transition metals of the 4th period was found to act as highly
efficient catalysts.[31] However, the admixing of such active catalysts to benzoxazines often
initiates ROP at r.t. and increases the viscosity significantly during the storage, thus, reducing the
shelf life in practical use. Latent catalysts, dormant at r.t. or certain temperatures, but generate
active initiators by heating were shown to overcome such storage problems. For example, toluene
sulfonates, diamines, thiols and elemental sulfur were successfully used to reduce ROP
temperatures for classical mono- and di-functional benzoxazines.[32-41] Apart from admixed
catalysts, specially designed benzoxazine monomers can also be used as latent catalysts.
Generally, free phenolic –OH group bearing benzoxazines or naphthoxazines are able to reduce
the ROP temperature to certain values.[19, 42] Despite these considerable efforts, a further
reduction of polymerization temperature with cheap catalysts is highly desirable and major
concern for benzoxazine based high performance thermosets. Herein, we are interested in
developing a latent catalyst based on amine HCl salts to reduce the ROP of benzoxazines to
practical temperatures.
EXPERIMENTAL
Materials
Paraformaldehyde (Aldrich, 96%), aniline (Merck, 99.5%), 4,4′- isopropylidenediphenol (Alfa
Aesar, 97%), 1,4-dioxane (Merck, 99%), diethyl ether (Merck, 99.7%), sodium hydroxide
(Merck, 99%), aAmmonium chloride (Merck, 99.8%), phenylhydrazine hydrochloride (Fluka
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AG, 99%), anilinium hydrochloride (Merck, 97%), benzyl triethyl ammonium chloride, hydroxy
ammonium chloride (Merck, 99%), ethylamine hydrochloride (Sigma-Aldrich, 98%)
Characterization
1
All H 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 Perkin-Elmer FTIR Spectrum One spectrometer. Differential Scanning
Calorimetry (DSC) was performed on Perkin-Elmer Diamond DSC from 30 °C 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 Perkin-Elmer Diamond
TA/TGA with a heating rate of 10 °C.min-1. under nitrogen flow.
Synthesis of Monomers
Bisbenzoxazine monomer (BA-a) synthesis was performed according to the literature [43]. In a
250 mL round bottomed flask, 3 g (100 mmol) of paraformaldehyde, 2.33 g (25 mmol) aniline,
5.7 g (25 mmol) bisphenol A and were dissolved in 180 mL 1,4-dioxane and the reaction mixture
was refluxed for 12h. After completing reaction, 1,4-dioxane was completely evaporated using a
rotary evaporator. The remaining sticky crude product was dissolved in 150 mL diethylether and
washed with 200 mL, 0.2 M NaOH solution in three portions, then, washed with 100 mL distilled
water. After washing process, the solution was dried using anhydrous Na2SO4. After filtering the
drying agent, diethyl ether was removed under reduced pressure. The remaining semi-solid was
dried under vacuum at 50 °C for 24h, resulted a solid (yield≈ 52 %). Note: In the case of mono-
functional monomers phenol, paraformaldehyde, p-Toluidine and benzylamine were used. Mono-
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functional monomers are abbreviated as P-tol (p-Toluidine derived) and P-bz (benzyl amine
derived).
Curing Procedure
Typical curing procedure is as follows: BA-a or P-tol monomer was mixed with a selected amine
HCl salt (5 mol%) by grinding in a mortar with a pestle. Acetone (2-3 mL) was added to this
mixture and all the mass was stirred with a spatula for 1-2 min. Then, acetone was removed by
blowing air or N2 gas. The remaining mixture placed in a Teflon mold and then heated up to 170-
220 °C according to the DSC data for each admixed amine HCl salt.
RESULTS AND DISCUSSIONS
As stated, previous studies on the polymerization mechanism of benzoxazines disclosed that ROP
begins by the cleavage of methylene bridge on the oxazine ring. Formation of different cationic
intermediates by thermal activation followed by N-, O- and aryl attack and rearrangement of
labile bonds finally produce polybenzoxazines. In a such mechanism, latent activation of catalyst
during benzoxazine curing would be beneficial both to lower the ROP temperature and solve the
storage problems. Addition to this classical pathway, initiation by a nucleophile by using primary
amines was shown to be an alternative route.[32, 35] It was revealed that nucleophilic catalysts
have the capacity to reduce ROP temperature and could act as latent catalyst in certain
formulations. In the light of these developments, it seemed appropriate to consider amine HCl
salts as a latent catalyst system since most of the amine HCl salts are very stable compounds at
r.t. and do not release HCl or any other species. However, at certain temperatures an equilibrium
may be established between amine and its HCl salt form. For example, ammonium chloride
decomposes into ammonia and hydrogen chloride gas upon heating.[44] And after such
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decomposition, the released HCl would eventually act as initiator for ROP of benzoxazines by
protonation of N and O atom. However, this possibility is so small since decomposition of many
amine HCl salts have higher temperatures than required for ROP of benzoxazines. Apart from
this excluded pathway, amine HCl salts can ionize at certain temperatures in melt benzoxazine
and generate chlorine anion that could act as nucleophilic catalyst. It should be noted that the
ionization of many salts is limited in organic compounds and thus, in a benzoxazine medium,
solubility and ionization constant of the salt, polarity of benzoxazine monomer would affect the
catalyst performance. In order to gain more insight on the influence of amine HCl salts on ROP
of benzoxazines, first, classical benzoxazine monomers derived from bisphenol A and phenol
was synthesized using aniline, p-toluidine and benzylamine as amine sources by the conventional
method (Scheme 3). Their thermally activated curing behavior in the presence and absence of
several amine salts was investigated and plausible mechanism for the ring-opening process was
proposed (Scheme 4).
Aniline,
O
O
CH2O
N
N
HO
OH
OH
RNH2,
CH2O
(BA-a)
O
N
R
(P-tol) or (P-bz)
RNH2: p-toluidine
or benzylamine
Scheme 3: Synthesis of benzoxazine monomers.
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Scheme 4: Plausible mechanism for ROP of 1,3-benzoxazines with ammonium salts.
1
Spectral characterization of these monomers was performed using H NMR and FTIR
spectroscopies (Figure S1, S2, S3).
As stated, depending on the functionalities present, benzoxazines have the ROP temperature
(Curing maximum, Tmax) generally over 220 °C and this event can be monitored as an exotherm.
Thus, ROP of BA-a, P-tol and P-bz with amine salts were analyzed under N2 environment with a
heating rate of 10 °C. min-1 using a DSC device. Accordingly, samples were prepared as follows:
P-tol and BA-a monomers were mixed with several amine salts by grinding with a mortar and
pestle. Acetone was also added to these mixtures to obtain a better homogeneity, and
immediately evaporated by using air or N2 blow. The overlaid DSC thermograms for P-tol and its
amine salt mixtures presented in Figure 1 and tabulated DSC data in Table 1 disclose the ROP
reduction trend according to the used amine HCl salt. The results clearly indicate that amine HCl
salts have a positive effect on the ROP temperature of P-tol monomer. The most ROP
temperature depression is provided by hydroxylamine HCl salt. Besides, aromatic amine salts are
also highly effective on ROP temperature. The impact of catalyst amount on ROP was also
analyzed by increasing the anilinium chloride content from 5 to 10 mol% and the on-set of curing
temperature dropped to 158 °C. Actually, similar effect was observed for each salt, but we
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deliberately kept the catalyst amount as 5 mol% because high amounts of the salt content in
polybenzoxazines may generate voids and phase separation that eventually affect the physical
properties in a deleterious manner.
Figure 1: DSC Thermograms of P-tol and amine HCl salt mixtures (5 mol%), P-tol (a), P-
tol/EtNH3Cl (b), P-tol/NH4Cl (c), P-tol/PhNH3Cl (d), P-tol/PhNHNH3Cl (e), P-tol/NH3OHCl (f),
P-tol/PhNH3Cl (10 mol%)
Table 1: DSC Thermogram data of P-tol/amine salts (5 mol%) mixtures
On-Set of
Curing (°C) Curing (°C)
End-Set of
Maximum Curing
Temp. (Co)
Amount of
Exotherm (J/g)
Sample
190
167
173
158
166
189
237
232
225
236
240
222
224
185
205
189
200
210
-124
-156
-74
P-tol
P-tol/NH3OHCl
P-tol/PhNH3Cl
P-tol/PhNH3Cl (10 mol
%)
-161
-110
-84
P-tol/PhNHNH3Cl
P-tol/NH4Cl
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181
232
210
-160
P-tol/EtNH3Cl
Similar trend was observed for the BA-a monomer and salt mixtures (Figure 2 and Table 2).
Aromatic amine salts are the most effective salts in this case and NH2OH.HCl has a drastic ROP
temperature lowering effect, too. For both monomers, aromatic amine HCl salts are effective that
signifying the importance of solubility of the salts in melt benzoxazine.
Figure 2: DSC Thermograms of BA-a and amine HCl salt mixtures (5 mol %), BA-a (a), BA-
a/ NH4Cl (b), BA-a/EtNH3Cl (c), BA-a/NH3OHCl (d), BA-a/PhNHNH3Cl (e), BA-a/PhNH3Cl
(f)
Table 2: DSC Thermogram data of BA-a/amine salts (5 mol %) mixtures
Sample
On-Set of
Curing (°C) Curing (°C)
End-Set of
Maximum Curing
Temp. (°C)
Amount of
Exotherm (J/g)
211
170
174
170
260
255
242
255
237
214
213
213
-125
-106
-35
BA-a
BA-a/NH3OHCl
BA-a/PhNH3Cl
BA-a/PhNHNH3Cl
-105
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181
171
254
255
208
216
-124
-108
BA-a/NH4Cl
BA-a/EtNH3Cl
In contrast, the solubility of NH3OHCl can be expected to be relatively lower compared to
aromatic amine HCl and EtNH3Cl in benzoxazines. However, this salt is more effective than
NH4Cl, EtNH3Cl for both monomers and as effective as aromatic amine salts. To further clarify
this issue, DSC analysis was performed to check whether a decomposition takes place or any
exotherm overlaps is present during curing for NH3OHCl catalyst. In Figure 3, DSC traces of P-
bz, NH3OHCl and P-bz/NH3OHCl (2:1, w/w) are presented. Notably, high amount of catalyst is
used to augment any effect for clarity. As seen, this salt has a melting at 158 °C and a rigorous
exotherm after Tm, probably due to decomposition, and an evaporation endotherm starting at 220
°C. Thus, the catalyzing mechanism for this salt can be different than proposed for the other
amine HCl salts. Moreover, when this salt is mixed with P-bz the on-set of ROP temperature
decreases drastically down to 91 °C and Tmax lowered from 260 to 155 °C. However, this
pronounced effect was observed in the case of high amount of NH3OHCl in benzoxazine.
Figure 3: DSC thermograms of P-bz (a), P-bz/NH3OHCl (2:1, w/w) (b), NH3OHCl (c)
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As stated, the solubility of amine HCl salts in the reaction medium is crucial factor for ring-
opening of benzoxazines. Hence, it can be expected that in suitable solvents, amine HCl salts can
catalyze the ROP more easily. It is even likely that ring-opening can be achieved at r.t. in certain
solvents. However, it is important to point out that it does not mean that the polymerization can
1
take place at r.t. To test this probability, we have conducted a reaction followed by H NMR
spectroscopy. P-tol and anilinium chloride was dissolved in CDCl3 and NMR spectrum was
recorded immediately. Then, after 1h, 3h, 6h and 24h 1H NMR spectra of the same sample were
taken to track N-CH2-O protons of oxazine ring (Figure 4). It is obvious that in solution medium
the catalytic effect of amine HCl salt is operative and ring-opening takes places at r.t. These
results can also be considered as an evidence for the role of Cl- ions on the mechanism. Since
amine HCl salts cannot decompose at r.t. to form HCl, the possible formation and catalyzing
effect of HCl can be excluded from the ROP mechanism.
Figure 4: 1H NMR spectra of BA-a/PhNH3Cl mixture in CDCl3 at r.t. for given
interaction period.
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The effect of the catalyst on the structure of the final polymer was also investigated by FTIR
analysis. The overlaid FTIR spectra of the resulting polymers after catalytic and non-catalytic
curing of P-tol are presented in Figure S4. Only minor or no change in the spectra is noted
indicating that varying the amine HCl salts have limited or no impact on the end-structure.
Thermal stability of the polybenzoxazines for each catalyst was measured by using thermo-
gravimetric analysis (TGA). TGA traces and related thermal properties are presented in Figure 5
and Table 3, respectively. According to TGA data, amine salt catalyzed polybenzoxazines have
slight difference for T5%, T10% and Tc. In terms of initial degradations, except NH4Cl catalyzed
sample, the other samples have slight lower values and among them PhNHNH3Cl catalyzed
sample has the lowest value. However, the same sample has a char yield value as that of the
cured P-tol (P-P-tol). The degradation patterns of the samples, except aromatic salt catalyzed,
exhibit similarities according to Tmax values with three maxima. These analogous patterns can
also be seen in derivative TGA graph in Figure S5. Moreover, char yields at 800 °C are close and
lies between 20–28%. But, it should be noted that all the samples have lower char yields than
pristine P-P-tol. There are two possible explanations for the observed char yield depression. One
is that amine salts lead to lower char formation effecting the degradation pathways of
polybenzoxazines, and the other is the decomposition and evaporation of the amine salt at the
elevated temperature since those molecules are not part of the polybenzoxazine networks. When
the char yields are compared, the mass difference is close to the added amount of salt for most
samples which may be considered as a support for the second possibility. On the contrary, Tmax
values of the catalyzed samples are better than values for P-P-tol, possibly amine salts generate a
stiffer network.
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Figure 5: TGA traces of cured P-tol and amine salts (5 mol%) mixtures, P-tol (a), P-
tol/PhNHNH3Cl (b), P-tol/NH4Cl (c), P-tol/NH3OHCl (d), P-tol/PhNH3Cl (e), P-tol/EtNH3Cl
(f)
Table 3: Thermal properties of the cured P-tol and amine salts (5 mol%) mixtures
Sample
T
5% (°C) T10% (°C) Tmax (°C)a Tc (%)
P-tol
275
242
292
261
258
261
305
284
311
285
294
291
296, 378, 466 28
390 28
P-tol/PhNHNH3Cl
P-tol/NH4Cl
P-tol/NH3OHCl
P-tol/PhNH3Cl
P-tol/EtNH3Cl
314, 399, 458 23
300, 404, 460 22
393
21
316, 407, 460 20
a These values extracted from derivative of TGA (Fig. S5 in supporting information).
T5%: The temperature for which the weight loss is 5%
T10%: The temperature for which the weight loss is 10%
Tmax: The temperature for maximum weight loss.
Tc: The char yield at 800 °C
CONCLUSION
We have established the catalytic potential of various amine HCl salts in the polymerization of
benzoxazines. The results revealed that solubility and ionization of the salts in melt benzoxazine
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1
play important roles in the catalytic ability. The stability of the salts and H NMR analysis give
evidences that the nucleophilic initiation is operative in which amine HCl salts catalyze the ROP
via Cl– attack on the oxazine ring. However, after initial stages of ring-opening, the formed free
phenolics can alter the pathway and cationic species predominate the mechanism. Thus, both the
anionic and cationic species may involve in the overall polymerization process. It should also be
noted that among the amine HCl salts, NH3OHCl appears to have different thermal behavior and
exhibits an exotherm after its melting point overlapping ROP exotherm of benzoxazines.
Moreover, FTIR analysis were conducted to detect possible differences in the end-structures and
it was found that there is no or limited catalytic impact on the PBZ. It is clear that amine HCl
salts can be used as an effective and cheap catalyst for ROP of benzoxazines. The ability of the
salts to reduce the maximum ROP temperatures, e.g. down to 184 from 224 °C, may lead to
further extend industrial applications these high performance thermosets.
Acknowledgements
Istanbul Technical University Research Fund is acknowledged for financial support.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Corresponding Authors
Correspondence and requests for materials should be addressed to Baris Kiskan, e-mail:
kiskanb@itu.edu.tr and Yusuf Yagci, e-mail: yusuf@itu.edu.tr
Additional Information
Supplementary information accompanies this paper at http://www.nature.com/srep
Competing financial interests
The authors declare no competing financial interests.
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Highlights
•
Ammonium salts reduced the ring-opening polymerization temperature of 1,3-
benzoxazines.
•
Availability and cost of ammonium salts are additional advantages.