N-Activated 1,3-Benzoxazine Monomer as a Key Agent in Polybenzoxazine Synthesis

Article abstract of DOI:10.1021/acs.macromol.0c02036

A novel and successful application of ring-closing reactions of aminophenols has been proposed for the formation of a new type of 1,3-benzoxazine ionic derivatives. The optimization of the reaction and detailed computational studies have been reported for the estimation of heterocyclic ring stability and its further transformation, which is crucial in the polymerization process. The molecular structure of the obtained compounds has been fully characterized by applying X-ray analysis and spectroscopic methods. The novel benzoxazines undergo an intriguing thermal reaction leading to classical benzoxazines and chloroalkanes, which is the first step of transformation before polymerization. To gain more insights into the transformation behavior of ionic benzoxazine derivatives, the Fourier transform infrared (FT-IR) spectra of gaseous products were recorded in experiments with near simultaneous FT-IR/TGA measurements. The combination of thermogravimetry with FT-IR spectroscopy enables the quantitative and qualitative characterization of thermal transformation products and clarification of the reaction mechanism. The experimental data have been verified by applying DFT(B3LYP) and DFT(M062x) theoretical studies.

Full text of DOI:10.1021/acs.macromol.0c02036

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Article
N‑Activated 1,3-Benzoxazine Monomer as a Key Agent in
Polybenzoxazine Synthesis
́
Danuta Trybuła, Aleksandra Marszałek-Harych, Małgorzata Gazinska, Sławomir Berski,
Dawid Jędrzkiewicz, and Jolanta Ejfler*
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ABSTRACT: A novel and successful application of ring-closing
reactions of aminophenols has been proposed for the formation of
a new type of 1,3-benzoxazine ionic derivatives. The optimization
of the reaction and detailed computational studies have been
reported for the estimation of heterocyclic ring stability and its
further transformation, which is crucial in the polymerization
process. The molecular structure of the obtained compounds has
been fully characterized by applying X-ray analysis and
spectroscopic methods. The novel benzoxazines undergo an
intriguing thermal reaction leading to classical benzoxazines and
chloroalkanes, which is the first step of transformation before
polymerization. To gain more insights into the transformation
behavior of ionic benzoxazine derivatives, the Fourier transform
infrared (FT-IR) spectra of gaseous products were recorded in experiments with near simultaneous FT-IR/TGA measurements. The
combination of thermogravimetry with FT-IR spectroscopy enables the quantitative and qualitative characterization of thermal
transformation products and clarification of the reaction mechanism. The experimental data have been verified by applying
DFT(B3LYP) and DFT(M062x) theoretical studies.
polybenzoxazines with predictable fixed properties. On the
other hand, they also have some disadvantages, such as low
solubility, a high curing temperature, low fracture toughness,
and a sometimes expensive fabricating process, which would
require some improvements to be introduced into the
monomer design on the synthesis level.
The basic strategy usually applied involves synthesizing
curable polymeric benzoxazine precursors, blending the
benzoxazine monomer with toughening agents, and other
resins prior to the polymerization process. However, within
scientific research and in industrial use, there is a growing
amount of attention being paid to conducting detailed studies
aimed at designing new benzoxazine monomers for the
reduction of some deficiencies in order to improve their
properties and add new functions. One such study presented in
this paper involves the synthesis of new benzoxazine
monomers and the evaluation of their heterocyclic ring
stability crucial in the polymerization process. To the best of
1. INTRODUCTION
Polybenzoxazines are perceived as both well-defined and still
novel thermoset polymers. The appropriate tailor-made
structural modification of benzoxazine monomers provides an
opportunity to introduce new valuable properties into final
polybenzoxazines in comparison to traditional phenolic resins.1
The existing application of benzoxazines and polybenzoxazines
in various fields is incontestable and still requires further
attention. They are already widely applied in the areas of
coatings, adhesives, microelectronics, and aerospace technol-
ogy.2 The advantages and features worth underlining include
low water absorption,3,4 excellent dimensional stability, nearly
zero volumetric shrinkage,5,6 thermal stability, chemical
resistance,7,8 stable low dielectric properties, self-extinguish-
ing/flame retardance, and good electronic properties.9−12
Additionally, it is important to note from a green chemistry
point of view that the polymerization process proceeds without
catalysts, byproducts, and harmful volatile formation,13−15 and
the monomers can be obtained from the renewable feedstock.
In this context, biobased polybenzoxazines are a vibrant area
for research.16,17
Received: September 7, 2020
Although benzoxazine monomers possess rich molecular
design flexibility, which enables the synthesis of polybenzox-
azines with tailor-made properties,18−23 some features are still
being analyzed and require detailed studies in order to obtain
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Scheme 1. Synthetic Strategy for the Ring-Closure Reaction for 1,3-Benzoxazine Derivative Formation
our knowledge, no examples of ionic benzoxazines have been
synthesized until now. While it is true that no ionic
benzoxazine monomers have yet been reported, ionic
polybenzoxazines have been obtained but in the post-
polymerization process.24,25 The advantages of the obtained
compounds result from both the novel type of functional
benzoxazine monomers and their unexpected but surprising
reactivity, that is, their thermal-induced transformation to a
neutral entity and chloroalkanes. The experimental results have
been verified through detailed theoretical studies, which clearly
demonstrate that an unexpected synthetic pathway and further
transformation are possible and selective. Additionally, the new
benzoxazine monomers give new insights into gaining a better
understanding of the initiation stage of the ring-opening
reaction of benzoxazine monomers and the new next stage of
this reaction which has never been discussed in the available
literature on the topic. The title compounds present a
structural motif of an N-activated benzoxazine species
postulated in the polymerization mechanism.
yields. Other synthetic routes include the ring closure of ortho-
hydroxybenzylimine with an aldehyde,27 the rhodium-catalyzed
rearrangement reaction of functionalized benzylamines with a
mixture of H2/CO,28 the direct reaction of ortho-lithiated
phenols with N,N-bis[(benzotriazol-1-yl)methyl]amines,29 the
redox reaction of aromatic hydroxyl aldehydes and cyclic
amines.30 Both benzoxazine ring formation and modifications
of the heterocyclic ring of 1,3-benzoxazines are achieved by
using simple formaldehyde or more complex aldehydes. It
seems that the potential of the Mannich reaction has been
exhausted; however, the formation of the heterocyclic ring may
proceed in a different way without using aldehyde substrates.
Therefore, we studied the possibility of the ring-closing
reaction of aminophenols by replacing the formaldehyde
with dichloromethane as a source of the methylene bridge
necessary for the formation of the 1,3-benzoxazine motif
(Scheme 1).
Recently, we reported a unique method of bisphenol/
bisnaphthol synthesis found as an invisible stage of an
aminophenol methylation reaction. In this unexpected case,
sodium complexes generated during the activation of the
aminophenols played the role of crucial intermediates in the
synthetic scenario.31 These studies became the inspiration to
undertake investigations of the new benzoxazine motif. Along
this line, a slight change in the reaction medium and the careful
analysis of synthetic nuances has provided the opportunity to
2. RESULTS AND DISCUSSION
The most widely adopted method for the synthesis of
benzoxazine monomers is the classical multi-component
Mannich condensation of phenols, primary amines and
formaldehyde in a 1/1/2 molar ratio.26 Until now, this was
the most explored and basic method and is expanded on many
different modifications to optimize the results and improve the
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discover the ring-closing reaction of activated aminophenols,
constituting a totally new approach in this area.
and comparable to reported neutral ones (for details of the X-
ray data, see Tables S1 and S2),14,21,32 but it should be
emphasized that this is the first example of ionic 1,3-
benzoxazine in the literature. The heterocyclic ring presents
a preferential distorted semichair conformation, making it
possible for the oxazine ring to undergo ring-opening
polymerization (ROP). This structural feature is a paradigm
for classical benzoxazines, but here it also indicates the ability
to achieve ring opening.
Designed for the needs of this study, the aminophenol
R ,Me
L ′ −H was activated by an NaH in a CH2Cl2/THF
R,tBu
solution. The applied solvent CH2Cl2 generated a methylene
bridge in a ring-closure reaction leading to 1,3-benzoxazine
derivative formation [R,tBuBxR′,Me]Cl (Scheme 1). The
synthetic strategy has been verified by using two types of
aminophenols with a free or bulky ortho-position of the aryl
core and two types of amine arms containing an alkyl chain
(C12) or cyclohexyl ring (Cy). The structure of the obtained
compounds [R,tBuBxR′Me]Cl in the solution and in the solid
state were determined by using NMR spectroscopy and the X-
ray crystallography method (for details of the X-ray data and
NMR spectra, see Supporting Information, Figures S1−S4,
Therefore, joint computational and experimental studies
have been undertaken, as they may provide some mechanistic
information for the better understanding of both the stability
and ability to transform the benzoxazine heterocyclic ring and
for the comparison of neutral and ionic species.
Both types of benzoxazines, the neutral R,tBuBxCy and the
[
dtBuBxCy,Me]Cl, have been studied, and the effect of the solvent
1
Tables S1 and S2). The H NMR spectrum of [R,tBuBxR′Me]Cl
has been taken into account by means of micro solvation with
four CH2Cl2 molecules. The calculations have been carried out
using the DFT method with four electron density functionals
(B3LYP,33−36 M062x,37 PBE0,38 B3PW9139 and the 6-
31+G(d,p) basis set40−43). Exemplary structures optimized at
the DFT(M062x)/6-31+G(d,p) computational level42−44 for
both dtBuBxCy and [dtBuBxCy,Me]Cl with four dichloromethane
molecules models have been presented in Figure S11 (see
Supporting Information).45,46
compounds revealed signals revealing the presence of tert-butyl
groups, aryl protons, and a Cy or dodecyl chain. However, the
most important were the signals in the range 3.38−3.59 ppm
which was assigned to the presence of the methyl substituent
on the nitrogen atom and the signals in the range 5.39−5.76
ppm attributed to a newly formed methylene bridge that
appeared during the ring-closing reaction. The formation of
the heterocyclic ring has been observed in the presence of
CD2Cl2 in the reaction media (Figure 1, example for
[
dtBuBxCy,Me]Cl).
The calculated energies of the highest occupied (HOMO)
and lowest unoccupied molecular orbitals (LUMO) have been
presented in Figure 3 and Table S2. The HOMO−LUMO gap
Figure 1. Fragment of 1H NMR spectra for [dtBuBxCy,Me]Cl formed in
the presence of CH2Cl2 (A) or CD2Cl2 (B).
Figure 3. HOMO and LUMO. Ionic [dtBuBxCy,Me]Clleft, neutral
dtBuBxCyright.
The absolute assignment of the molecular structures of two
isomers for [dtBuBxCy,Me]Cl was achieved by applying X-ray
crystallography (Figure 2). The isomers represent a different
arrangement of the methyl and cyclohexyl substituents
occurring on the nitrogen atom. The crystallographic data
reveal that all bond lengths and angles are in the normal ranges
for the R,tBuBxCy model oscillates between 3.32 (PBE0) and
7.37 eV (M062x), while for the [R,tBuBxCy,Me]Cl (ionic)
modelbetween 2.80 (PBE0) and 7.40 eV (M062x). With
the exception of the M062x calculations, carried out for the
molecules with substituents in the ortho-positions and solvent
molecules, which indicate a slightly (0.105 eV) larger LUMO−
HOMO gap for the neutral form, in all other cases the
HOMO−LUMO gap for the [R,tBuBxCy,Me]Cl (ionic) model is
smaller than for the R,tBuBxCy model. Thus, one may expect
smaller thermodynamic stability for the ionic system. It is
worth emphasizing that the difference in the energy gaps
between both molecules is only 0.52 eV in the PBE0
calculations, while for the B3LYP, B3PW91, and M062x
methods similar values are obtained in the range: 1.27−1.41
Figure 2. Molecular structures of two benzoxazine [dtBuBxCy,Me]Cl
isomers (isomer 1right, isomer 2left).
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Scheme 2. Mechanism of Benzoxazine Polymerisation Leading to Phenoxy-Type Polybenzoxazine by O-Activation (Left) or N-
Activation (Right)
Scheme 3. Mechanism of Benzoxazine Polymerisation Leading to Phenolic-Type Polybenzoxazine by O-Activation (Left) or
N-Activation (Right)
eV. The results indicated that neutral benzoxazine is more
stable than the N-activated benzoxazine species R,tBuBxCy.
In the commonly proposed mechanism of the ROP of
benzoxazine, the oxygen over nitrogen as the initiation site is
preferred, and then with a cyclic tertiary oxonium ion formed
by the O-activated oxazine ring. Polymerization by the O-
activated species leads to a phenoxy-type polybenzoxazine
structure (Scheme 2, left).
However, an alternative polymerization pathway is also
possible through the N-activated oxazine ring, giving the same
type of polybenzoxazine (Scheme 2, right). Although this
alternative mechanism has been proposed, it has never been
proven experimentally. Additionally, the substituents located
on the aryl core of the benzoxazine monomer influence the
type of polymer formation created, which was described in
detail in our earlier studies.14 The obstructed ortho position in
the aryl core of the benzoxazine monomer selectively forms
phenoxy-type polybenzoxazine. On the other hand, the
benzoxazine monomers with a free ortho position possess
high reactivity toward polymerization with or without catalysts
and commonly form a phenolic-type polymer. Similarly, for
this type of polymers, the O-activated species is favored. The
obtained here benzoxazine monomers are the well-defined N-
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Scheme 4. General Scheme of Three Possible Decomposition Reactions of the [dtBuBxCy,Me]Cl Benzoxazine
Figure 4. Schematic energy profiles of three decomposition reactions of the [dtBuBxCy,Me]Cl benzoxazine. (A) Blue line, (B) green line, and (C)
orange line. The numerical values correspond to the activation energies (Ea) calculated from the side of the reagents and the reaction energies. In
cursive the values corrected by ΔZPVE.
activated form synthesized in two versions of the aryl core
(Scheme 3).
The first stage of the decomposition reaction giving
chloromethane (Scheme 4A, marked as light blue in Figure
4) has been carried out through the single transition state
(TS), located at 45.08 kcal/mol (activation energy, Ea), which
is above the energy level of the Cl−···dtBuBxCy,Me. The lower
value of Ea equaling 41.96 kcal/mol is obtained when the value
of the zero-point vibrational energy difference (ΔZPVE) is
taken into account. The analysis of the geometrical structure of
TS shows that the N−CH3 bond (2.295 Å) is already broken
and the H3C−Cl bond (2.637 Å) has not yet been formed.
Thus, the TS contains the [CH3]0.32+ moiety interacting with
the [dtBuBxCy]0.28+ fragment and the Cl0.60− anion. Such an
interpretation is supported by the topological analysis of the
electron localization function (ELF),50−52 which shows the
lack of the bonding basin V(C,N) and the lack of any bonding
basin formed by the Cl atom (separated Cl atom). On the
other hand, one may observe a lone pair in the vicinity of the N
atom, V(N), with 1.74 e, which is the remainder of the
heteropolar N−C(CH3) bond. The N−C bond, formed by the
C atom from the cyclohexyl group, is described by the bonding
basin V(C,N) with 1.84 e, thus, it is a single bond depleted
with electron density.
The evaluation of the potential application of benzoxazine
monomers required an analysis of their thermal behavior, since
the most important factor is the stability of the monomer’s
temperature just before polymerization. In order to gain more
insight into the process of the transformation of the obtained
ionic benzoxazines [dtBuBxCy,Me]Cl, three potential reactions
have been investigated, one giving chloromethane (Scheme
4A) as a by-product, two otherchlorocyclohexane (Scheme
4B,C). The studies have been performed at the DFT
computational level,42−44 using the B3LYP electron density
functional33−36 and 6-31+G(d,p) basis set.41−43 The reaction
path has been simulated in vacuum at 0 K using the intrinsic
reaction coordinate (IRC) procedure47,48 (see computational
details, Supporting Information).49
The reaction starts with the formation of the
Cl−···dtBuBxCy,Me molecular complex, and the interaction energy
(Eint) equals −87.49 kcal/mol. Removing the basis set
superposition error, using the counterpoise technique (CP),
CP
int
yields the value of −87.19 kcal/mol (E ).
E
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Atomic rearrangement after TS leads to a molecular complex
being formed between the chloromethane and the benzoxazine
form a system stabilized by noncovalent interactions. The
hydrogen−chlorine bond is broken and the H atom is partially
transferred to one of the sp2 hybridized C atoms in the C6H10
molecule. Such an interpretation is supported by the
population of the V(H,C) basin of 1.71 e, which is much
smaller than 2.00−2.11 e computed for the unperturbed H−C
bonds. The Cl atom is localized at 2.892 Å from the second sp2
hybridized C atom and chemical bonding is observed between
those atoms. The topological analysis of ELF does not show
any bonding basin associated with the Cl atom. The
topological charge of Cl is −0.72 e. In the region of the C
C bond (cyclohexene ring), only one V(C,C) basin is observed
instead of two Vi=1,2(C,C) basins, which have been localized for
the C6H10 molecule in the HCl···dtBuBxMe···C6H10 complex.
Such a topological change is associated with the elongation of
the CC bond, which is partially transformed from a double
to a single bond. The population of V(C,C) of 2.25 e is close
to a formal value of 2 e expected for the C−C bond.
molecules (H3CCl···dtBuBxCy). Both molecules interact through
CP
noncovalent interactions with Eint of −1.16 kcal/mol and E
int
of −0.85 kcal/mol. The total reaction is exothermic with
reaction energies of −11.63 and −14.50 kcal/mol, when the
ΔZPVE value is taken into account.
A second reaction that leads to the formation of the dtBuBxMe
and chlorocyclohexane molecules has been carried out through
two TSs, TS1 and TS2 (Scheme 4B, marked as green in Figure
4). In the first stage of the studied reaction scheme, the HCl
and cyclohexene molecules are formed and the next “portion”
of HCl reacts with the cyclohexene ring leading to
chlorocyclohexane molecule formation (Scheme 4B).
The first TS, TS1, is located at 17.65 kcal/mol, which is
above the energy level of the Cl−···dtBuBxCy,Me system. When
the ΔZPVE is added, the value of Ea is decreased to 13.35
kcal/mol. It is worth noting that this activation energy is much
smaller that the energy needed for the breaking of the N−CH3
bond (45.08 and 41.96 kcal/mol).
After TS2, the reacting atoms rearrange to form the post-
reaction system, which consists of chlorocyclohexane and
dtBuBxMe molecules. The molecules interact mainly through the
In TS1, the N−C bond (2.351 Å) is already broken and the
H−C bond (1.164 Å) is essentially elongated when this bond
length is compared with that obtained for the H−C bonds
(1.09 Å) not involved in any interactions. Thus, a partial
transfer of the H atom toward the Cl atom is observed. The
topological analysis of ELF shows that the basin population for
the elongated H−C bond is 1.84 e. This population is
noticeably smaller than the populations in the range 2.00−2.10
e computed for the unperturbed H−C bonds. In the region of
the N−C bond, only the lone electron pair V(N) is observed
with a basin population of 1.99 e. Thus, the N−C bond in TS1
has been broken and the V(N) basin is the remainder of the
N−C bond. No bonding basin has been observed that is
associated with the Cl atom, which stays unbound in TS1. The
topological charge of Clδ− is −0.72 e. The charge of the H
atom approaching the Cl atom is +0.23 e.
C−H···N hydrogen bond with an interaction energy of −2.27
CP
int
kcal/mol (E = −1.65 kcal/mol).
The formation of the chlorocyclohexane is an exothermic
process with respect to the energy of the HCl···dtBuBxMe···
C6H10 molecular complex (see Figure 4) with a reaction
energy of −14.11 kcal/mol (−9.77 kcal/mol, including
ΔZPVE). It seems more probable that such a process,
investigated in the chosen physical conditions, will occur
than the decomposition, which leads to the formation of the
CH3Cl molecule.
The third possibility (Scheme 4C, marked as orange in
Figure 4) assumes the breaking of the C−C bond between the
cyclohexyl group and the dtBuBxMe fragment. Next, the chlorine
atom forms the covalent C−Cl bond with the cyclohexyl group
yielding the chlorocyclohexane molecule. The observed
mechanism is similar to the reaction, which yields the
H3CCl···dtBuBxCy complex. The product of the reaction is the
molecular complex formed between the chlorocyclohexane and
the benzoxazine molecules (C6H11Cl···dtBuBxMe). The reaction
has been carried out through the single TS, located at 34.24
(Ea) and 29.97 kcal/mol (including ΔZPVE) above the energy
level of the Cl−···dtBuBxCy,Me complex. The total reaction is
exothermic with the reaction energies of −11.63 and −14.50
kcal/mol (including ΔZPVE).
It is worth noting that the activation energy needed to break
the C−C bond with the cyclohexyl group (29.97 kcal/mol) is
smaller than the energy needed to detach the methyl group
(41.96 kcal/mol). Thus, the route leading to the formation of
the benzoxazine and C6H11Cl molecules seems to be more
probable than the formation of the H3CCl molecule. On the
other hand, the activation energy for the detachement of the
cyclohexyl group in a one stage process is higher than the
values of Ea calculated for a two stage process proceeding
through the addition of HCl to the CC bond in the
cyclohexene ring.
After TS1, the rearrangement of the atoms leads to a post-
reaction compound, which consists of the HCl molecule
interacting with the benzoxazine, dtBuBxMe, and cyclohexene
molecules. The optimized length of the CC bond in
cyclohexene is 1.345 Å. The topological analysis of ELF shows
two bonding basins Vi=1,2(C,C) in the region of the CC
bond, and such a topology of ELF supports the double type of
carbon−carbon bonding. The total value of the Vi=1,2(C,C)
basin population amounts to 3.46 e, which is much larger than
2 e expected for the single C−C bond (2 e). The
HCl···dtBuBxMe···cyclohexene complex is weakly stabilized by
noncovalent forces with an interaction energy of −5.87 kcal/
CP
int
mol (E = −4.46 kcal/mol). The main stabilizing interaction
comes from the C−H···N hydrogen bond. The reaction is
exothermic with a reaction energy of −12.16 or −18.98 kcal/
mol (+ΔZPVE).
The second TS, TS2, determines the energetics of the
second reaction stage, which leads to the formation of the
chlorocyclohexane and benzoxazine dtBuBxMe molecules. The
reaction proceeds by the addition of the HCl to the double
CC bond in the cyclohexene ring (see Scheme 4B). The
activation energy from the side of the reagents (the
HCl···dtBuBxMe···C6H10 molecular complex) is 28.90 or 29.06
kcal/mol when the correction for ΔZPVE is included.
The analysis of the chemical bonds in TS2, performed by
means of the topological analysis of ELF, shows that the
[C6H11]δ+ molecule, the Clδ− anion, and the dtBuBxMe molecule
In order to investigate the effect of the solvent (dichloro-
methane) on the energetics of studied reactions the micro-
solvation with single CH2Cl2 molecule has been considered.
For the reaction yielding the dtBuBxCy and H3CCl molecules,
the value of the activation energy has been increased by 0.45
kcal/mol and the reaction energy has been increased by 0.93
kcal/mol. Thus, the influence of the solvent on the energetics
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of the reaction is negligible. Similarly, a small effect has been
found for the reaction yielding the dtBuBxMe and C6H11Cl
molecules. The value of Ea has been increased by 1.18 kcal/
mol and the reaction energy increased by 2.16 kcal/mol.
In the next step of our investigations, the verification of the
theoretical results has been achieved through experimental
studies. The thermal transformation behavior of 1,3-benzox-
azine ionic derivatives has been studied by means of a
combination of thermogravimetric analysis (TGA) and gas-
phase Fourier transform infrared (FT-IR) spectroscopy.
The TGA and the first derivative TGA (dTGA) curves of
1,3-benzoxazine ionic derivatives are presented in Figure 5.
Based on traces of each presented TGA curve, the thermal
degradation process can be divided into a few partially
overlapping stages.
Table 1. Thermal Stability of Ionic Benzoxazines
benzoxazine
mass loss at the first degradation stage [%] T−5% [°C]
[
[
[
[
H,tBuBxCy,Me]Cl
dtBuBxCy,Me]Cl
H,tBuBxC12,Me]Cl
dtBuBxC12,Me]Cl
43.1
65.5
29.4
25.3
115
174
207
161
The course of the dTGA curve of the benzoxazine derivative
with an alkyl chain as the amine arm and a tert-butyl
substituent in the ortho-position of phenol [dtBuBxC12,Me]Cl is
different: the first stage of degradation occurs with the highest
rate at the lowest temperature (177 °C). The thermal stability
of this benzoxazine derivative set as the temperature
corresponding to 5 wt % mass loss (T−5%) is lower with
respect to the benzoxazine with a free ortho-position. Other
differences refer to only two distinct degradation steps and the
24 wt % of residue at 800 °C.
The exemplary 3D FT-IR spectra of the gaseous
decomposition products of [H,tBuBxCy,Me]Cl and the Gram−
Schmidt curves of the benzoxazines measured near simulta-
neously with the TGA signal are presented in Figure 6. The
Gram−Schmidt curve represents the absorbance intensity of
the total gases emitted at a particular temperature; thus, the
mass losses observed in the TGA are accompanied by an
increase in the Gram−Schmidt curve. On the Gram−Schmidt
curve of the benzoxazines, a few maxima can be distinguished.
Compared with the first derivative of the TGA, there is a delay
in the Gram−Schmidt maxima positions because of the
transport of degradation gases through a transfer line. The
combination of thermogravimetry with FT-IR spectroscopy
enables the quantitative and qualitative characterization of the
thermal degradation/transformation products and clarification
of the reaction mechanism.
Figure 7 presents the FT-IR spectra of the gaseous
degradation products of benzoxazine derivatives recorded at
two temperatures selected from the temperature range
corresponding to the first maximum of the Gram−Schmidt
curves. The common features of FT-IR spectra presented in
Figure 7 are the absorption bands characteristic for gaseous
and liquid water visible at 3500−3950 and 3400−3500 cm−1,
respectively, and absorption bands in the range of 2250−2400,
670, and 2050−2230 cm−1 from carbon dioxide and carbon
monoxide, respectively. The analysis of the spectra of
[
H,tBuBxC12,Me]Cl acquired at a lower temperature indicates a
Figure 5. TGA and the first derivative TGA curves of [H,tBuBxCy,Me]Cl,
[
mixture of methyl chloride and chlorododecane. The most
important absorption band of stretching vibrations of the C−
Cl bond, specific for alkyl chlorides, is present at 747 cm−1.53
The presence of chlorododecane is identified by character-
istic absorption bands at 2967, 2934, 2865, and 1459 cm−1.
The bands at 2967 and 2934 cm−1 correspond to the
asymmetric stretching vibrations (νas) of C−H bonds of the
methyl and methylene groups, respectively, whereas the band
at 2865 cm−1 corresponds to the symmetric stretching
vibrations (νs) of C−H bonds of methylene groups over-
lapping with νs of methyl groups.53
The absorption bands of the asymmetric scissoring
vibrations (δas) of the C−H bonds of −CH3 and the
symmetric scissoring vibrations (δs) of the −CH2− groups
overlap at the range of 1476−1430 cm−1. A characteristic
absorption band of the symmetric scissoring vibration of the
−CH3 group is visible at 1373 cm−1. Other bands at 2980 cm−1
dtBuBxCy,Me]Cl (A) and [H,tBuBxC12,Me]Cl, and [dtBuBxC12,Me]Cl (B).
The common future of the dTGA curves is a distinct sharp
peak with a maximum at 215 and 217 °C for the benzoxazine
derivatives [H,tBuBxCy,Me]Cl and [dtBuBxCy,Me]Cl, respectively,
and at 214 °C for [H,tBuBxC12,Me]Cl. The high temperature limit
of the first degradation/transformation stage is presented in
Table S3 (see Supporting Information) and corresponding
mass loss occurring at this stage of degradation is in the range
of 25 wt % for [dtBuBxC12,Me]Cl and 65 wt % for [dtBuBxCy,Me]Cl
(Table 1).
The degradation or more precisely transformation process of
these three derivatives at higher temperatures is similar, three
additional degradation stages are visible as weak overlapping
peaks. The positions of the respective peaks differ depending
on the structure of the benzoxazine derivatives.
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Figure 6. Time-dependent 3D FT-IR spectra of the thermal degradation of [H,tBuBxCy,Me]Cl (left) and Gram−Schmidt curves of ionic benzoxazines
(right).
chloride from the infrared database of National Institute of
Standards and Technology54 are included in Supporting
Information (Figure S12A).
The spectra of [H,tBuBxC12,Me]Cl measured at a higher
temperature lack absorption bands characteristic of methyl
chloride indicating that the main degradation product is
chlorododecane (Figure 8). At the range of the unique group
Figure 8. FT-IR profiles of wavenumbers 2939 and 747 cm−1
recorded during the degradation of benzoxazine [dtBuBxCy,Me]Cl.
frequency of the C−Cl bond stretching at 800−700 cm−1
remains the band with maximum at 725 cm−1 and weak
intensity as typical for chlorododecane. The changes are also in
Figure 7. FT-IR gas phase spectra of characteristic degradation
the range 3020−2800 cm−1, there is no band from methyl
products of ionic benzoxazines [H,tBuBxC12,Me]Cl, [dtBuBxC12,Me]Cl (A),
chloride at 2980 cm−1.
and [H,tBuBxCy,Me]Cl, [dtBuBxCy,Me]Cl (B) recorded at two temperatures
from the range of the first degradation stage.
The infrared spectra recorded for [H,tBuBxCy,Me]Cl and
[
dtBuBxCy,Me]Cl correspond to chlorocyclohexane as evidenced
by the characteristic absorption bands at 2935, 2868, 1458,
(C−H stretching), 1373, and 1338 cm−1 attribute to methyl
chloride.
1374, 1339, and 747 cm−1.54
The thermal transformation of benzoxazine [H,tBuBxCy,Me]Cl
and [dtBuBxCy,Me]Cl proceeds in an analogous manner, with the
release of methyl chloride and chlorocyclohexane. The FT-IR
spectra recorded at a lower temperature allow identification of
methyl chloride and chlorocyclohexane, whereas the spectrum
acquired at higher temperatures does not have the band
characteristic of methyl chloride; the presence of chlorocyclo-
The absorption band of rocking vibrations, visible typically
for more than five connected −CH2− groups, is present at 723
cm−1. Moreover, the absorption bands of C−C skeletal
vibrations at the range of 1300−800 cm−1 are unique for
chlorododecane.53 The recorded FT-IR spectra compared with
FT-IR vapor phase spectra of chlorododecane and methyl
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Table 2. Thermal Properties of the Benzoxazine Derivatives Estimated from the First Heating DSC Curves
peak
exo
[°C]
endset
exo
[°C]
onset
exo
Tonset
exo a
T
T
ΔHexo
mass loss up to T
mass loss at the range of exothermic
width of exothermic effect
[°C]
benzoxazine
[°C]
[J/g]
[%]
effect [%]
[
[
[
H,tBuBxCy,Me]Cl
dtBuBxCy,Me]Cl
219
215
212
221
220
226
227
244
252
−94.3
−52.8
−62.1
35.4
29.8
9.3
6.0
41.5
25.7
8
29
40
H,tBuBxC12,Me
Cl
]
a
onset
exo
Lower limit of the temperature range of exothermic peak (T ) is estimated as the onset because of overlapping of endo- and exothermic effects.
hexane is evidenced by characteristic absorption bands at 2939,
2868, 1460, 820, and 738 cm−1 (see Supporting Information,
Figure S12B).54
Detailed analysis of the polymerization mechanism of
cationic benzoxazine derivatives [H,tBuBxC12,Me]Cl,
[
H,tBuBxCy,Me]Cl, and [dtBuBxCy,Me]Cl is planned to be described
The rapid disappearance of methyl chloride and the
evolution of chlorododecane and chlorocyclohexane as a
main degradation product of the first transformation stage in
the case of benzoxazine [H,tBuBxC12,Me]Cl, [H,tBuBxCy,Me]Cl, and
separately, and an investigation along this line is currently
underway. The present study is concentrated on the
explanation of the initial structural changes in benzoxazine
structures occurring during heating and preceding polymer-
ization.
The polymerization ability of the benzoxazine derivatives
was verified through differential scanning calorimetry. The
polymerization of benzoxazine can be confirmed by the
presence of the exothermic effect at a temperature range of
200−250 °C.1,58 The DSC curves of ionic benzoxazine
derivatives are presented in Figure 9 and the estimated
thermal parameters are collected in Table 2. On the DSC
thermograms of [H,tBuBxCy,Me]Cl, [dtBuBxCy,Me]Cl, and
[
dtBuBxCy,Me]Cl, respectively, is confirmed by the intensity
profiles of wavenumbers 747, 2934, and 2939 cm−1 (Figures 8
and S13 in Supporting Information). The profile of wave-
number 747 cm−1 represents emission of methyl chloride,
whereas wavenumbers 2934 and 2939 cm−1 were chosen as a
characteristic for chlorododecane and chlorocyclohexane,
respectively.
In the case of benzoxazine [dtBuBxC12,Me]Cl degradation, the
mechanism is different than the one presented above. The first
stage of the weight loss of [dtBuBxC12,Me]Cl occurs at a lower
temperature as compared to other presented benzoxazines.
Mass loss occurs with the highest rate at 177 °C (Figure 5).
On the FT-IR spectrum of the evolved gases acquired at a
temperature corresponding to the first maximum on the
Gram−Schmidt curve, no characteristic absorption bands of
the predicted product can be precisely distinguished (Figure
7A). The presence of other bands indicates that the evolved
gases are a mixture of volatile products and the decomposition
is more complex. We can hypothesize that thermal
decomposition proceeds through the formation of a mixture
of chloroalkanes and subsequent or sudden fragmentation of
the benzoxazine ring. Such a degradation mechanism has not
yet been described and requires further research on a broader
group of compounds.
In the spectra of evolved gases, as shown in Figure 7,
characteristic absorption bands of oxazine moieties observed at
1235 and 1228 cm−1 (C−O−C asymmetric stretching mode)
and 943 cm−1 (oxazine−related bands)55 are present and
indicate on possible benzoxazine derivatives [H,tBuBxC12,Me]Cl,
[
H,tBuBxCy,Me]Cl, and [dtBuBxCy,Me]Cl evaporation at the
beginning of mass loss. For comparison, ATR-FTIR spectra
of benzoxazine derivatives are presented, as shown in Figure
S14 in Supporting Information. This findings agree with
differential scanning calorimetry (DSC) results. The presence
of the large endothermic effect prior to the exothermic peak of
ROP and high mass loss taking place during heating up to the
onset of the ROP exotherm (Table 2) confirms some
benzoxazine derivative evaporation. Partial evaporation of
monomers does not disturb the elimination of chlorocyclohex-
ane and chlorododecane during the first stage of mass loss.
Evaporation of oxazine derivatives prior to polymerization was
observed and described in the case of many naphthoxazine
derivatives.56,57 The absence of the characteristic absorption
oxazine bands in the case of [dtBuBxC12,Me]Cl suggests
degradation of this benzoxazine with ring opening and explains
the lack of an exotermic effect on the DSC curve.
Figure 9. First heating DSC curves of benzoxazine derivatives
[
[
H,tBuBxCy,Me]Cl, [dtBuBxCy,Me]Cl (top), and [H,tBuBxC12,Me]Cl,
dtBuBxC12,Me]Cl (bottom).
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[
H,tBuBxC12,Me]Cl, the exothermic effect is visible with the peak
been fully characterized in solution by NMR spectroscopy and
in the solid state by X-ray. The resulting 1,3-benzoxazine
derivatives are well soluble in aqua media, which is crucial inter
alia in medical applications. Furthermore, these new
compounds can be identified as useful synthons in polymer-
ization reactions, organic synthesis, and in the construction of
diverse heterocycles. The most interesting results have been
obtained by combined experimental and theoretical studies
proving the unique transformation reactions of ionic 1,3-
benzoxazine monomers leading to a classical neutral form and
chloroalkane, which could be the key stages modifying the
commonly proposed mechanism of the ROP process.
Generally, both heteroatoms (N, O) on the oxazine ring can
be potential cationic polymerization initiation sites, but the
oxygen atom is preferred over nitrogen. The alternative route
of polymerization is only conceptually possible, also with the
nitrogen atom as the initiation site. The current mechanism
enforced in the appropriate literature is classical and described
extensively, but the intermediate derivatives have not been
isolated up to now. The above-presented benzoxazines are
examples of such intermediates which illustrate the first stage
of monomer initiation occurring on the nitrogen atom.
Additionally, the proven alternative route by the chloroalkane
formation indicates a new perspective in the discussion on the
initiation of monomers.
peak
maximum (T ) located between 210 and 226 °C. An
exo
endothermic effect overlaps at the beginning of the exothermic
peak. The temperature range of this endothermic event
indicates the emission of cyclohexyl chloride occurring during
the first transformation stage. The endothermic peak temper-
atures amount to 215 °C for [H,tBuBxCy,Me]Cl and 211 °C for
[
dtBuBxCy,Me]Cl and agree with the peak temperature on the
dTGA curves of the respective benzoxazine derivatives. The
exothermic effect corresponds to the ROP of the benzox-
azine.58,59 In the case of typical benzoxazine precursors, ROP is
accompanied by excessive monomer volatilization which limits
its applications.59 Thus, it is favorable to reduce the ROP
temperature to avoid the partial degradation of the
benzoxazine precursors. In the case of the investigated
benzoxazine derivatives, the degradation mechanism is differ-
ent. The degradation of [H,tBuBxC12,Me]Cl, [dtBuBxC12,Me]Cl, and
[
H,tBuBxCy,Me]Cl starts with the elimination of chlorododecane
or chlorocyclohexyl as a main product, respectively. Such an
elimination seems to initiate ROP and causes the favorable
reduction of the temperature of the ROP. Within the presented
benzoxazine derivatives, the lowest onset of the exothermic
peak of ROP (T ) is exhibited by [H,tBuBxC12,Me]Cl. This
onset
exo
benzoxazine has the highest thermal stability, estimated as a
temperature of 5 wt % of mass loss (T−5%) (Table 1) and
which is directly related to thermal stability, and exhibits the
lowest mass loss during heating up to the onset of ROP.
The measured values of the enthalpy of the exothermic effect
(ΔHexo) are in the range of the ROP of common benzoxazine
monomers, that is, 50−200 J/g.59 The highest enthalpy ΔHexo
is achieved by the monomer [H,tBuBxCy,Me]Cl (94.3 J/g), while
the exothermic peak of this monomer is narrower. The lower
value of ΔHexo of [dtBuBxCy,Me]Cl could stem from the sterical
hindrance of the tert-butyl in the ortho-position of the phenol.
The determined values of the heat of polymerization is lower
than the actual ones because of the overlapping endothermic
effect. There is no exothermic effect on the DSC curve of
4. EXPERIMENTAL SECTION
4.1. General Materials, Methods, and Procedures. Solvents
for synthesis were purified by standard methods: THF [high-
performance liquid chromatography (HPLC), VWR] was distilled
from Na/benzophenone, MeOH (HPLC, VWR) was distilled, and
DCM (99.8% VWR) was distilled from P2O5, n-hexane (HPLC,
VWR). Solvents for a standard workup and chromatography were
used as received. All chemicals were obtained from commercial
sources and used without further purification: 2,4-tert-buthylphenol
(99%, Sigma-Aldrich), 4-tert-buthylphenol (99%, Sigma-Aldrich), N-
methylcyclohexylamine (99%, Sigma-Aldrich) N-methyldodecylamine
(98%, Alfa Aesar), formaldehyde (37% solution in H2O, Sigma-
Aldrich), and sodium hydride (95%, Sigma-Aldrich).
[
dtBuBxC12,Me]Cl.
The 1H, 13C NMR spectra were obtained using a Bruker AVANCE
500 MHz spectrometer. The chemical shifts are given in ppm relative
The common feature of benzoxazine derivatives involves the
endset
exo
high temperature limit of the endothermic effect (T
correlating with the end of the first degradation stage (see
Table S3 in Supporting Information).
)
1
to the residual signals of the solvent (CDCl3, H: 7.26 ppm, 13C:
77.16 ppm). HRMS spectra were recorded using Bruker MicOTOF-Q
spectrometers with Supporting Information ion source and time-of-
flight mass analyzer. Microanalyses were conducted with an Elementar
CHNS Vario EL III analyzer. All the reactions and operations which
required an inert atmosphere of N2 were performed by using a
glovebox (MBraun) or standard Schlenk apparatus and vacuum line
techniques.
The experimental results and theoretical calculations provide
similar conclusions. The ionic form of benzoxazine presents an
N-activated monomer which during curing undergoes trans-
formation into benzoxazine and chloroalkane, after which the
mixture forms polymers. Therefore, the mechanism of
benzoxazine polymerization by N-activated monomers should
be taken into account in theoretical calculations but in a
modified fashion that would include the transformation stage
presented in this paper.
A detailed description of the polymerization reaction of the
in situ formed mixture (benzoxazine and chloroalkane)
requires in-depth studies on a large set of benzoxazine
monomers in order to establish a well-defined structural
motif of the obtained polybenzoxazines and the optimization
of the polymerization reaction now in progress and will be
published with detailed theoretical studies.
4.2. Syntheses. Aminophenols were synthesized according to a
literature procedure:
2,4-Di-tert-butyl-6-((cyclohexyl(methyl)amino)methyl)phenol,
60
Cy,Me
[
[
[
L
dtBu
−H].
2,4-Di-tert-butyl-6-((dodecyl(methyl)amino)methyl)phenol,
61
C12,Me
L
dtBu
−H].
4-tert-Butyl-6-((cyclohexyl(methyl)amino)methyl)phenol,
62
Cy,Me
L
H,tBu
−H].
4.2.1. 4-tert-Butyl-6-((dodecyl(methyl)amino)methyl)phenol,
C12,Me
L
−H. To a solution of 0.60 g (4.00 mmol) of 4-tert-
H,tBu
butylphenol and 1.00 mL (4.00 mmol) of N-methyldodecylamine in
MeOH (20 mL), 0.45 mL (6.00 mmol) of formaldehyde (37%
solution in H2O) was added. The solution was stirred and heated
under reflux for 24 h. Next, the mixture was evaporated to give yellow
oil, washed with 20 mL of cold methanol placed in a freezer for
recrystallization. The crude product precipitated as a white solid after
1 week. It was collected by filtration, washed with cold methanol (3 ×
20 mL), and dried in vacuo to give H,tBuLC12,Me−H. Yield 72% (1.04 g,
3. CONCLUSIONS
A new type of 1,3-benzoxazines have been synthesized by ring-
closing reactions of aminophenols activated by NaH. The
molecular structure of the newly obtained compounds have
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2.88 mmol). 1H NMR (500 MHz, CDCl3): δ 9.54 (s, 1H, OH), 7.17
(dd, JHH = 8.4, 2.4 Hz, 1H, ArCH), 6.94 (d, JHH = 2.4 Hz, 1H,
ArCH), 6.75 (d, JHH = 8.4, Hz, 1H, ArCH), 3.67 (s, 2H, ArCH2N),
2.46 (t, JHH = 7.4 Hz, 2H, CH2), 2.27 (s, 3H, NCH3), 1.59−1.48 (m,
2H, CH2), 1.27 (m, 27H, C(CH3)3, CH2), 0.88 (t, JHH = 7.0 Hz, 3H,
CH3). 13C NMR (126 MHz, CDCl3): δ 155.72 (s, 1C, ArCO), 141.66
(s, 1C, ArC), 125.37 (s, 1C, ArCH), 125.22 (s, 1C, ArCH), 121.34 (s,
1C, ArC), 115.45 (s, 1C, ArCH), 62.09 (s, 1C, ArCH2N), 57.38 (s,
1C, CH2), 41.34 (s, 1C, NCH3), 34.07 (s, 1C, C(CH3)3), 32.06 (s,
1C, CH2), 31.72 (s, 3C, C(CH3)3), 30.09−29.57 (m, 5C, CH2),
29.49 (s, 1C, CH2), 27.34 (s, 1C, CH2), 27.16 (s, 1C, CH2), 22.83 (s,
1C, CH2), 14.25 (s, 1C, CH3). Anal. calcd (found) for C24H43NO: C,
79.72 (79.14); H, 11.99 (11.57); N, 3.87 (3.51)%. ESI/MS calcd
(found): 361.33 (362.3) [M + 1]+.
NCH2), 3.57 (s, 3H, NCH3), 1.89−1.82 (m, 2H, CH2), 1.73−1.69
(m, 2H, CH2), 1.36 (s, 9H, C(CH3)3), 1.29−1.22 (m, 25H, CH2,
C(CH3)3), 0.88 (t, JHH = 6.9 Hz, 3H, CH3). 13C NMR (126 MHz,
CDCl3): δ 146.90 (s, 1C, ArC-O), 146.18 (s, 1C, ArC), 138.22 (s,
1C, ArC), 124.82 (s, 1C, ArCH), 122.14 (s, 1C, ArCH), 113.40 (s,
1C, ArC), 84.05 (s, 1C, OCH2N), 62.49 (s, 1C NCH2Ar), 60.91 (s,
1C, NCH2), 45.77 (s, 1C, NCH3), 35.01 (s, 1C, C(CH3)3), 34.24 (s,
1C, C(CH3)3), 31.83 (s, 1C, CH2), 31.47 (s, 3C, C(CH3)3), 29.83 (s,
3C, C(CH3)3), 29.78−29.20 (m, 6C, CH2), 26.45 (s, 1C, CH2),
22.80 (s, 1C, CH2), 22.42 (s, 1C, CH2), 14.24 (s, 1C, CH3). Anal.
calcd (found) for C29H52ClNO: C, 74.36 (74.72); H, 11.00 (11.24);
Cl, 7.43 (7.60); N, 2.89 (3.00)%. ESI/MS calcd (found): 430.40
(430.37) [M + 1]+. IR: 1230, 962 cm−1.
4.2.5. 6-(tert-Butyl)-3-dodecyl-3-methyl-3,4-dihydro-2H-benzo-
[e][1,3]oxazin-3-yl Chloride, H,tBuBxC12,Me. Synthesis was carried out
analogically as for the compound dtBuBxCy,Me by using appropriate
substrates as follows: compound H,tBuLC12,Me−H (0.50 g, 1.38 mmol),
NaH (0.041 g, 1.73 mmol, 1.25 equiv). Compound H,tBuBxC12,Me was
4.2.2. 6,8-Di-tert-butyl-3-cyclohexyl-3-methyl-3,4-dihydro-2H-
benzo[e][1,3]oxazin-3-yl Chloride, [dtBuBxCy,Me]Cl. To a solution of
Cy,Me
L
−H (0.50 g, 1.50 mmol) in 10 mL of the mixture of THF/
dtBu
CH2Cl2 (1/1), NaH (0.057 g, 1.89 mmol, 1.25 equiv) was added
under a inert atmosphere of N2. The slightly yellow solution was
stirred for 1 h until gas evolution disappeared. Then, the mixture was
evaporated to dryness and the 15 mL of CH2Cl2 was added for the
precipitation of NaCl. After filtration of NaCl, the filtrate was dried
1
obtained as a white powder, yield: (0.49 g, 1.19 mmol, 85%). H
NMR (500 MHz, CDCl3): δ 7.28 (dd, JHH = 8.6, 2.3 Hz, 1H, ArCH),
7.07 (d, JHH = 2.3 Hz, 1H, ArCH), 6.91 (d, JHH = 8.6 Hz, 1H, ArCH),
5.70 (d, JHH = 8.2 Hz, 1H, OCH2N), 5.46 (d, JHH = 8.2 Hz, 1H,
OCH2N), 5.16 (d, JHH = 15.3 Hz, 1H, NCH2Ar), 4.97 (d, JHH = 15.3
Hz, 1H, NCH2Ar), 3.84−3.64 (m, 2H, NCH2), 3.59 (s, 3H, NCH3),
1.94−1.73 (m, 2H, CH2), 1.40−1.30 (m, 2H, CH2), 1.26−1.16 (m,
25H, CH2, C(CH3)3), 0.84 (t, JHH = 6.9 Hz, 3H, CH3). 13C NMR
(126 MHz, CDCl3): δ 147.41 (s, 1C, ArC−O), 147.13 (s, 1C, ArC),
127.37 (s, 1C, ArCH), 124.32 (s, 1C, ArCH), 116.78 (s, 1C, ArCH),
113.11 (s, 1C, ArC), 84.37 (s, 1C, OCH2N), 60.42 (s, 1C, NCH2Ar),
59.49 (s, 1C, NCH2), 45.56 (s, 1C, NCH3), 34.52 (s, 1C, C(CH3)3),
31.96 (s, 1C, CH2), 31.39 (s, 3C, C(CH3)3), 30.08−28.49 (m, 6C,
CH2), 26.39 (s, 1C, CH2), 22.74 (s, 1C, CH2), 22.34 (s, 1C, CH2),
14.18 (s, 1C, CH3). Anal. calcd (found) for C25H44ClNO: C, 72.98
(73.22); H, 10.75 (10.81); Cl, 8.46 (8.65); N, 3.12 (3.42)%. ESI/MS
calcd (found): 374.34 (374.23) [M + 1]+. IR: 1232, 930 cm−1.
4.3. Thermal Measurement. TGA was performed using TGA/
DSC1 Mettler Toledo thermobalance coupled by heated at 230 °C
transfer line to a Nicolet iZ10 Thermo Scientific spectrometer.
Samples of 10−12 mg were heated with a rate of 10 °C/min from 25
to 1000 °C under 30 mL/min of nitrogen flow. FT-IR gas-phase
spectra were collected with a spectral resolution of 4 cm−1 with 32 co-
added scans.
DSC measurements were performed using the Mettler Toledo
DSC1 system, coupled with a Huber TC 100 intracooler. The
instrument was calibrated using indium (Tm = 156.6 °C, ΔHm = 28.45
J/g) and zinc (Tm = 419.7 °C, ΔHm = 107.00 J/g) standards. Samples
(∼3.5 mg) were measured in 40 μL aluminum pans under a constant
nitrogen purge (60 mL/min) from 0 to 300 °C with heating rate 10°
C/min. The TGA and DSC experimental data were processed using
the generic STARe computer program. For the purpose of data
presentation, the DSC, TGA, and FT-IR profiles were exported to
OriginPro 64 (v. 9.0) as ASCII files.
4.4. Details of X-Ray Data Analysis. X-ray diffraction data for a
suitable crystal of each sample were collected using a Xcalibur CCD
Ruby or KUMA KM4 CCD (see Supporting Information, Tables S1
and S2) with a ω scan technique. The data collection and processing
utilized CrysAlis suite of programs.63 The space groups were
determined based on systematic absences and intensity statistics.
Lorentz polarization corrections were applied. The structures were
solved using intrinsic phasing SHELXT-2014/5 and refined by full-
matrix least-squares on F2. All calculations were performed using the
SHELX suite of programs.64 All non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen atom positions
were calculated with geometry and not allowed to vary. Thermal
ellipsoid plots were prepared with 50% of probability displacements
for non-hydrogen atoms by using Mercury 3.9 program.65 All data
have been deposited with the Cambridge Crystallographic Data
Centre CCDC-2002871 for isomer 1 of [dtBuBxCy,Me]Cl and -2002871
for isomer 2 of [dtBuBxCy,Me]Cl. Copies of the data can be obtained
over vacuum to give dtBuBxCy,Me as a white powder (0.51 g, 1.34
1
mmol) in 89% yield. H NMR (500 MHz, CDCl3): δ 7.27 (d, JHH
2.4 Hz, 1H, ArCH), 6.92 (d, JHH = 2.4 Hz, 1H, ArCH), 5.76 (d, JHH
=
=
8.6 Hz, 1H, NCH2O), 5.56 (d, JHH = 8.6 Hz, 1H, NCH2O), 5.24 (d,
HH = 15.3 Hz, 1H, NCH2Ar), 4.89 (d, JHH = 15.3 Hz, 1H, NCH2Ar),
J
3.89 (m, 1H, NCH), 3.42 (s, 3H, NCH3), 2.42−2.30 (m, 2H, CH2),
2.08−1.90 (m, 2H, CH2), 1.73−1.62 (m, 1H, CH2), 1.35−1.31 (m,
2H, CH2), 1.34 (s, 9H, C(CH3)3), 1.27 (s, 9H, C(CH3)3), 1.25−1.07
(m, 3H, CH2). 13C NMR (126 MHz, CDCl3): δ 154.92 (s, 1C, ArC-
O), 146.64 (s, 1C, ArC), 137.95 (s, 1C, ArC), 124.72 (s, 1C, ArCH),
123.59 (s, 1C, ArCH), 113.64 (s, 1C, ArC), 83.50 (s, 1C, OCH2N),
67.34 (s, 1C, NCH), 58.59 (s, 1C, NCH2Ar), 41.68 (s, 1C, NCH3),
34.99 (s, 1C, C(CH3)3), 34.72 (s, 1C, C(CH3)3), 31.47 (s, 3C,
C(CH3)3), 29.75 (s, 3C, C(CH3)3), 26.60−24.95 (m, 5C, CH2).
Anal. calcd (found) for C23H38ClNO: C, 72.64 (72.70); H, 9.89
(10.08); N, 3.51 (3.69)%. IR: 1230, 961 cm−1.
4.2.3. 6-(tert-Butyl)-3-cyclohexyl-3-methyl-3,4-dihydro-2H-
benzo[e][1,3]oxazin-3-yl Chloride, H,tBuBxCy,Me. Synthesis was carried
out analogically as for the compound dtCByu,BMxeCy,Me by using appropriate
substrates as follows: compound H,tBuL
−H (0.50 g, 1.80 mmol),
NaH (0.054 g, 2.27 mmol, 1.25 equiv). Compound H,tBuBxCy,Me was
1
obtained as a white powder. Yield: (0.54 g, 1.59 mmol, 88%). H
NMR (500 MHz, CDCl3): δ 7.28 (dd, JHH = 8.6, 2.3 Hz, 1H, ArCH),
7.07 (d, JHH = 2.3 Hz, 1H, ArCH), 6.91 (d, JHH = 8.6 Hz, 1H, ArCH),
5.70 (d, JHH = 8.6 Hz, 1H, OCH2N), 5.46 (d, JHH = 8.6 Hz, 1H,
OCH2N), 5.16 (d, JHH = 15.3 Hz, 1H, NCH2Ar), 4.97 (d, JHH = 15.3
Hz, 1H, NCH2Ar), 3.81 (m, 1H, NCH), 3.38 (s, 3H, NCH3), 2.35−
2.23 (m, 2H, CH2), 2.01−1.95 (m, 2H, CH2), 1.72−1.64 (m, 1H,
CH2), 1.62−1.54 (m, 2H, CH2), 1.45−1.34 (m, 2H, CH2), 1.26 (s,
9H, C(CH3)3), 1.20−1.04 (m, 1H, CH2). 13C NMR (126 MHz,
CDCl3): δ 148.65 (s, 1C, ArC−O), 147.72 (s, 1C, ArC), 127.39 (s,
1C, ArCH), 124.38 (s, 1C, ArCH), 116.77 (s, 1C, ArCH), 114.34 (s,
1C, ArC), 83.84 (s, 1C, OCH2N), 67.38 (s, 1C, NCH), 58.47 (s, 1C,
NCH2Ar), 41.69 (s, 1C, NCH3), 34.62 (s, 1C, C(CH3)3), 31.42 (s,
3C, C(CH3)3), 26.48 (s, 1C, CH2), 25.94 (s, 1C, CH2), 25.28 (s, 2C,
CH2), 24.88 (s, 1C, CH2). ESI/MS calcd (found): 303.26 (303.22)
[M + 1]+. Anal. calcd (found) for C20H33ClNO: C, 72.98 (73.22); H,
10.75 (10.81); Cl, 8.46 (8.65); N, 3.12 (3.42)%. IR: 1238, 954 cm−1.
4.2.4. 6,8-Di-tert-butyl-3-cyclohexyl-3-methyl-3,4-dihydro-2H-
benzo[e][1,3]oxazin-3-yl Chloride, dtBuBxC12,Me. Synthesis was carried
out analogically as for the compound dtBuBxCy,Me by using appropriate
C12,Me
substrates as follows: compound
L
−H (0.50 g, 1.20 mmol),
dtBu
NaH (0.036 g, 1.50 mmol, 1.25 equiv). Compound dtBuBxC12,Me was
obtained as a white powder. Yield: (0.52 g, 1.11 mmol, 92%). H
1
NMR (500 MHz, CDCl3): δ 7.31 (d, JHH = 2.0 Hz, 1H, ArCH), 6.92
(d, JHH = 2.0 Hz, 1H, ArCH), 5.62 (d, JHH = 8.2 Hz, 1H, OCH2N),
5.39 (d, JHH = 8.2 Hz, 1H, OCH2N), 5.11 (d, JHH = 15.3 Hz, 1H,
NCH2Ar), 4.88 (d, JHH = 15.3 Hz, 1H, NCH2Ar), 3.80−3.67 (m, 2H,
K
https://dx.doi.org/10.1021/acs.macromol.0c02036
Macromolecules XXXX, XXX, XXX−XXX

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Article
free of charge by application to CCDC, 12 Union Road, Cambridge
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have been performed in the gas phase, 0 K using Gaussian16 (C.01),
G16, programme.66 The 6-31+G(d,p) basis and electron density
functionals have been used as included in G16. Each structure has
been verified by harmonic frequency analysis with no imaginary
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PES associated with a TS have been verified by the calculation of the
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energy minima. The Ea values have been corrected for the vibrational
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.0c02036.
Polymerization of N-Phenyl Mono-1,3-benzoxazines in Aqueous
Media. Macromolecules 2018, 51, 3672−3679.
■
sı
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of N-Alkyl Groups on Thermally Induced Polymerization Behavior of
1,3-Benzoxazines. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2777−
2782.
Spectroscopic data (1H, 13C NMR), FT-IR spectra, X-
ray data, computational details, and temperature range
of the first degradation stage (PDF)
(9) Zhang, K.; Han, L.; Froimowicz, P.; Ishida, H. A Smart Latent
Catalyst-Containing Ortho-Trifluoroacetamide Functional Benzoxa-
zine: Precursor for Low Temperature Formation of Very High
Performance Polybenzoxazole with Low Dielectric Constant and
HighThermal Stability. Macromolecules 2017, 50, 6552−6560.
(10) Zhang, K.; Yu, X.; Kuo, S.-W. Outstanding Dielectric and
Thermal Properties of Main Chain-Type Poly(benzoxazine-co-imide-
cosiloxane)-Based Cross-Linked Networks. Polym. Chem. 2019, 10,
2387−2396.
(11) Liu, J.; Safronava, N.; Lyon, R. E.; Maia, J.; Ishida, H. Enhanced
Thermal Property and Flame Retardancy via Intramolecular 5-
Membered Ring Hydrogen Bond-Forming Amide Functional
Benzoxazine Resins. Macromolecules 2018, 51, 9982−9991.
(12) Shan, F.; Ohashi, S.; Erlichman, A.; Ishida, H. Non-Flammable
Thiazole-Functional Monobenzoxazines: Synthesis, Polymerization,
Thermal and Thermomechanical Properties, and Flammability
Studies. Polymer 2018, 157, 38−49.
Crystallographic data for isomers 1 and 2 (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Jolanta Ejfler − Faculty of Chemistry, University of Wrocław,
Wrocław 50-383, Poland; orcid.org/0000-0002-7467-
1312; Email: jolanta.ejfler@chem.uni.wroc.pl
Authors
Danuta Trybuła − Faculty of Chemistry, University of Wrocław,
Wrocław 50-383, Poland
Aleksandra Marszałek-Harych − Faculty of Chemistry,
University of Wrocław, Wrocław 50-383, Poland
́
̈
(13) Liu, C.; Shen, D.; Sebastian, R. M.; Marquet, J.; Schonfeld, R.
Mechanistic Studies on Ring-Opening Polymerization of Benzox-
azines: A Mechanistically Based Catalyst Design. Macromolecules
2011, 44, 4616−4622.
́
Małgorzata Gazinska − Department of Engineering and
Technology of Polymers, Faculty of Chemistry, Wrocław
University of Science and Technology, Wrocław 50-370, Poland
Sławomir Berski − Faculty of Chemistry, University of Wrocław,
Wrocław 50-383, Poland
(14) Ejfler, J.; Krauzy-Dziedzic, K.; Szafert, S.; Lis, T.; Sobota, P.
Novel Chiral and achiral Benzoxazine Monomers and Their Thermal
polymerization. Macromolecules 2009, 42, 4008−4015.
(15) Chutayothin, P.; Ishida, H. Cationic Ring-Opening Polymer-
ization of 1,3-Benzoxazines: Mechanistic Study Using Model
Compounds. Macromolecules 2010, 43, 4562−4572.
Dawid Jędrzkiewicz − Faculty of Chemistry, University of
Wrocław, Wrocław 50-383, Poland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.0c02036
(16) Salum, M. L.; Iguchi, D.; Arza, C. R.; Han, L.; Ishida, H.;
Froimowicz, P. Making Benzoxazines Greener: Design, Synthesis, and
Polymerization of a Biobased Benzoxazine Fulfilling Two Principles of
Green Chemistry. ACS Sustainable Chem. Eng. 2018, 6, 13096−13106.
(17) Dogan, Y. E.; Satilmis, B.; Uyar, T. Synthesis and Character-
ization of Bio-Based Benzoxazines Derived from Thymol. J. Appl.
Polym. Sci. 2019, 136, 47371.
(18) Arslan, M.; Kiskan, B.; Yagci, Y. Benzoxazine-Based Thermoset
with Autonomous Self-Healing and Shape Recovery. Macromolecules
2018, 51, 10095−10103.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to express their gratitude to the
National Science Centre in Poland (grant 2017/25/B/ST5/
00597) and the Wrocław Centre for Networking and
Supercomputing (http://www.wcss.wroc.pl). The authors
would like to thank Agnieszka Bondyra for performing TGA
and DSC measurements.
(19) Khan, M.; Halder, K.; Shishatskiy, S.; Filiz, V. Synthesis and
Crosslinking of Polyether-Based Main Chain Benzoxazine Polymers
and their Gas Separation Performance. Polymers 2018, 10, 221.
(20) Zhang, S.; Ran, Q.; Fu, Q.; Gu, Y. Preparation of Transparent
and Flexible Shape Memory Polybenzoxazine Film Through Chemical
Structure Manipulation and Hydrogen Bonding Control. Macro-
molecules 2018, 51, 6561−6570.
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