Low-melting phthalonitrile monomers containing maleimide group: Synthesis, dual-curing behavior, thermal and mechanical properties

Article abstract of DOI:10.1016/j.reactfunctpolym.2021.104932

Novel dual-functional phthalonitrile monomers containing a maleimide group were synthesized and characterized. Ortho, meta, and para isomers were obtained and only the meta-isomer can be considered as a low melting phthalonitrile monomer with a melting po

Full text of DOI:10.1016/j.reactfunctpolym.2021.104932

Reactive and Functional Polymers 164 (2021) 104932
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Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Perspective Article
Low-melting phthalonitrile monomers containing maleimide group:
Synthesis, dual-curing behavior, thermal and mechanical properties
S.S. Nechausov*, A.A. Aleksanova, O.S. Morozov, B.A. Bulgakov, A.V. Babkin, A.V. Kepman
M.V. Lomonosov Moscow State University, Department of Chemistry, Division of Chemical Technology and New Materials, 119991, Leninskie gory st, 1-11, Moscow,
Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Novel dual-functional phthalonitrile monomers containing a maleimide group were synthesized and character-
ized. Ortho, meta, and para isomers were obtained and only the meta-isomer can be considered as a low melting
phthalonitrile monomer with a melting point of 107.4◦С. Analysis of the dual-curing behavior of monomers
showed radical homopolymerization of maleimides followed by phthalonitrile polymerization with the formation
of isoindoline, triazine, and phthalocyanine ring structures. Two alternative copolymerization processes of
maleimide groups via ene reaction and ring-opening amidation were investigated. The approach of dual-curing
maleimide and phthalonitrile fragments in a single molecule allowed combining the excellent inherent me-
chanical properties of bismaleimide resins and the outstanding thermo-oxidative stability of phthalonitriles.
After curing of both functional groups impact strength was as high as 11.81 kJ mꢀ 2, flexural strength was 108
MPa, Young’s modulus was 4.38 GPa. On the contrary, Tg of this polymeric system was above 370◦С and T5% was
higher than 450◦С in air.
Phthalonitrile
Maleimide
Thermosets
Processability
Thermal properties
Curing behavior
1. Introduction
Phthalonitrile polymerization is usually induced by aromatic diamines
or phenols [28–32] leading to the formation of a cross-linked and void-
Thermosetting polymers with high thermal stability and superior
mechanical properties are widely used in aerospace fields. Heat-resistant
thermosets include benzoxazines [1–3] propargylated and allylated
phenolics [4–9], cyanate esters [10–12], bismaleimides [13–16], and
phthalonitriles [17–21]. Bismaleimides (BMI) are a well-known indus-
trially produced class of high-performance thermosetting resins. They
possess a range of attractive properties for aerospace applications such
as high glass transition temperatures (230–380 ◦C), good hot-wet per-
formance, and superior mechanical properties [13,22]. In particular,
bismaleimide blends with 2,2′-diallylbisphenol A (DABA) can be viewed
as a benchmark against which other commercially available thermosets
for structural composite applications [23,24] are compared. BMI were
also of interest in the fields of self-healing polymers [25], click-
chemistry and UV-induced polymerization [26], and 3D-printing [27].
However, the thermostability of polymer systems composed of mal-
eimide monomers cannot be compared with that of phthalonitriles –
some of the most heat resistant polymers. The phthalonitrile resins
attract attention due to their outstanding thermal and thermo-oxidative
stability, excellent mechanical properties, and good moisture resistance.
free polymeric network consisting of aromatic heterocycles [33–36].
These structures provide high thermal stability and low flammability of
the resulting polymers (Т > 400 ◦C, T5% > 500 ◦C, LOI > 60%)
g
[18,37–39]. Nevertheless, the high melting temperature of the common
phthalonitrile monomers (usually >180 ◦C) [17,40] which is close to
their polymerization onset temperature results in a narrow processing
window and limits their application.
Many approaches to improve processability of phthalonitriles have
been developed. One of them is the introduction of flexible linkages to
the molecular structure of phthalonitrile monomers. Aryl [15] or alkyl
[41] ether units are often used as such linkages. Wu et al. suggested to
combine fluorinated fragments and flexible ether linkages to construct
high-performance functional PN with good processability [42].
Recently, we synthesized phosphorus- [43–46] and silicon-containing
[47–50] linkers with phthalonitrile fragments to achieve glass transi-
tion temperatures of monomers as low as 0 ◦C. This allowed to obtain
CFRP via cost-effective injection techniques [51,52] and dry pre-preg
fabrication [20,53] for the first time.
Another approach was the development of dual-function
* Corresponding author.
E-mail address: nechersergey@inumit.ru (S.S. Nechausov).
https://doi.org/10.1016/j.reactfunctpolym.2021.104932
Received 13 March 2021; Received in revised form 4 May 2021; Accepted 5 May 2021
Available online 6 May 2021
1381-5148/© 2021 Elsevier B.V. All rights reserved.

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
phthalonitrile molecules. Introduction of additional flexible and func-
tional moieties can promote curing of phthalonitriles and at the same
time usually decreases the softening temperature of monomers. Auto-
catalytic phthalonitrile monomers with amino groups demonstrate this
approach [54] and are also used as synthesis precursors and as curing
agents in the present work. A molecule containing both propargyl ether
and phthalonitrile fragments was recently developed and described by
2,2′-diallylbisphenol A were purchased from HOS-technik (Austria)
under trade names Homide 121 and Homide 127A respectively and used
as received.
2.2. Methods
NMR spectra were recorded on a Bruker Avance II 600 at 600 MHz
for 1H and 151 MHz for 13C. Chemical shifts (ppm) are given relative to
solvent: references for DMSO were 2.50 ppm (1H NMR) and 39.50 ppm
our research group [38,55]. Tm of the obtained monomers was 117 ◦
C
and their superior processability was demonstrated for vacuum infusion
resin molding process (VIM) [38]. Recently, Yue Han et al. synthesized
low-melting (Tm = 68 ◦C) phthalonitrile monomers with allyl moieties
and showed increased toughness of the cured resin (impact strength was
(
13C NMR). Fourier Transform Infrared (FT-IR) spectra were recorded on
a Bruker Tensor-27 spectrophotometer in the range of 4000–400 cmꢀ 1
using KBr pellets. HPLC and GPC analysis was performed on an Agilent
1260 chromatography system equipped with reversed-phase column
ZORBAX Eclipse Plus C18 (Т column = 30◦С; eluent – 50% aqueous
acetonitrile (v/v); flow rate = 1.0 ml/min) for HPLC and two
PL1113–6300 ResiPore (300 × 7.5 mm) columns (Т column = 30◦С;
eluent – tetrahydrofuran; flow rate = 1.0 ml/min) for GPC. Differential
scanning calorimetry (DSC) was performed on Netzsch DSC214 Polyma
at heating rates of 5 ◦C/min, 10 ◦C/min, and 15 ◦C/min. Rheological
behavior was measured with MCR 302 rheometer using cone 7 at 200
rpm in the temperature range from 50 to 400 ◦C at a heating rate of
13.63 kJ mꢀ 2) compared to bi-functional phthalonitriles (8.74 kJ mꢀ 2
)
[56,57]. In general, the presence of several functional groups capable of
participating in different polymerization mechanisms allows to control
the polymerization pathways, which, in turn, alters the properties of the
resulting polymer [3].
Prior to this study, several blends containing maleimides and
phthalonitriles were prepared to improve thermal stability of the bis-
maleimide thermosets [3,58,59]. However, only Kaliavaradhan et al.
examined the effect of the presence of both maleimide and phthaloni-
trile in the same molecular structures obtained by Michael addition of 4-
(4-aminophenoxy)phthalonitrile to different bismaleimides [59]. Un-
fortunately, these monomers were characterized by high melting tem-
peratures (>170 ◦C). Additionally, the introduction of phthalonitrile
fragments was carried out through the Michael addition to maleimide
double bond, and as a result, the functionality of maleimide group was
absent in the resulting monomers.
2
◦C/ min. Thermal stability was evaluated via thermogravimetric
analysis (TGA) on Netzsch TG 209 P3 Tarsus, at a heating rate of 10 ◦C/
min in range of 40–980 ◦C in Ar or air purge rate of 50 ml/min.
Elemental analysis was performed at the Laboratory of Microanalysis of
INEOS RAS, Moscow.
2.3. Synthesis
The general drawback of both phthalonitriles and maleimides is the
high brittleness of the cured polymeric material, which is an obstacle for
using them as matrices in high-performance composites [13,44].
Fortunately, it is possible to adjust the mechanical properties of mal-
eimide polymers by introducing different comonomers which change
the mechanism of polymerization. Due to the presence of the electron-
deficient double bond, the maleimide group could be easily involved
in such polymerization processes as: anionic or radical homopolymeri-
zation, copolymerization by ene reaction, Michael addition with nu-
cleophiles (amines or thiols [60]), as dienophiles in Diels-Alder
reactions [61]. Such methods to reduce the brittleness of maleimide
polymers were studied in many published studies, but usually the
improvement of toughness occurred at the cost of decreased Tg and
thermooxidative stability [62–64].
1,3-Bis(3,4-dicyanophenoxy)benzene was synthesized according to a
well-known procedure with quantitative yield (98%) [65]. Synthetic
procedure and NMR spectra for 3O, 3M, 3P, 4O, 4M, 4P are presented
in the supplementary materials.
2.3.1. 4-[3-(2,5 dione-1H-pyrrole-1-yl)phenoxy]benzene-1,2-
dicarbonitrile (PNB-M)
4-((3-(3,4-Dicyanophenoxy)phenyl)amino)-4-oxobut-2-enoic acid
4M (20.0 g, 0.069 mol) was dissolved in 25 ml of propionic anhydride
(0.2 mol) in a 250 ml round-bottom flask equipped with a reflux
condenser and magnetic stirrer. Sodium acetate (4.00 g, 0.049 mol) was
added as a catalyst to promote the dehydration reaction. The reaction
mixture was stirred at room temperature and monitored by thin layer
chromatography (TLC). After 7 h, the solution was slowly poured into
400 ml of aqueous solution of sodium bicarbonate. The precipitated
product was filtered and dissolved in acetone. The acetone solution was
poured into distilled water. This procedure was repeated several times
until neutral pH of the product aqueous solution was achieved. The
product was filtered and dried under vacuum at 70 ◦C. Yield: 18.1 g
(96%) of white powder.
The present article deals with the synthesis and characterization of
novel para, meta, and ortho isomers of phthalonitrile monomers con-
taining maleimide moieties N-[4-(4-phthalonitrile)phenoxy]maleimide
(PNB-P), N-[3-(4-phthalonitrile)phenoxy]maleimide (PNB-M), and N-
[2-(4-phthalonitrile)phenoxy]maleimide (PNB-O). Blends of PNB-M
with DABA, 1,3-bis(4-aminophenoxy)benzene (APB), or 4-(4-amino-
phenoxy)phthalonitrile (3P) were prepared and studied in order to
counter brittleness and improve processability, curing, thermal and
mechanical properties. Processes of maleimide and phthalonitrile
polymerization were studied by liquid-state NMR, FT-IR, and DSC.
1H NMR (DMSO‑d6, 600 MHz) δ: 8.14 (d, 1H7, J6,7 = 8.70 Hz), 7.87
(d, 1H8, J6,8 = 2.51 Hz), 7.60 (dd, 1H4, J3,4 = 8.37, J3,5 = 7.70 Hz), 7.45
(dd, 1H6, J6,7 = 8.70 Hz, J6,8 = 2.51 Hz), 7.32 (d, 1H3, J3,4 = 8.37 Hz),
7.24 (d, 1H5, J3,5 = 7.70 Hz); 7.23 (s, 1H2), 7.19 (s, 2H1) (for 1H-spec-
trum with peak assignment see Fig. S15). 13C NMR (DMSO‑d6, 151 MHz)
δ: 169.58, 160.43, 153.93, 136.42, 134.79, 133.36, 130.79, 123.65,
123.15, 122.44, 119.19, 118.24, 116.80, 115.86, 115.37, 108.71 (16/
16) (Fig. S16). Anal. Calc. for C18H9N3O3 (Mr = 315.28): C, 68.57; H,
2.88; N, 13.33%. Found C, 68.42; H, 2.91; N, 13.41%.
2. Experimental
2.1. Materials
All manipulations with the oxidation and moisture sensitive com-
pounds were carried out under Ar atmosphere. Acetone, N,N-dimethy-
lacetamide, potassium carbonate, sodium bicarbonate were purchased
from Chimmed (Russia), anhydrous sodium acetate was purchased from
PanReac AppliChem (Spain), propionic anhydride, 2-aminophenol, 3-
aminophenol, 4-aminophenol, maleic anhydride, tret-butyl perox-
ybenzoate were purchased from Acros Organics (USA), 4-nitrophthalo-
nitrile and 1,3-bis(4-aminophenoxy)benzene were purchased from
Central Drug House (India), 4,4′-diphenylmethanebismaleimide and
4-[4-(2,5 dione-1H-pyrrole-1-yl)phenoxy]benzene-1,2-dicarbonitrile
PNB-P and 4-[2-(2,5 dione-1H-pyrrole-1-yl)phenoxy]benzene-1,2-
dicarbonitrile PNB-O were synthesized by the same procedure from 4P
and 4O respectively, with minor modifications described in the results
section.
2.3.1.1. PNB-P. Yield 92%. 1H NMR (DMSO‑d6, 600 MHz) δ: 8.13 (d,
2

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
1H5, J4,5 = 8.70 Hz), 7.89 (d, 1H6, J4,6 = 2.51 Hz), 7.47 (d, 2H3, J2,3
9.01), 7.45 (dd, 1H4, J4,5 = 8.70 Hz, J4,6 = 2.51 Hz), 7.33 (d, 2H2, J2,3
=
=
3. Results and discussion
9.01 Hz), 7.21 (s, 2H1) (Fig. S13). 13C NMR (DMSO‑d6, 151 MHz) δ:
169.84, 160.62, 153.04, 136.40, 134.74, 128.9, 128.79, 123.21, 122.41,
120.53, 116.78, 115.88, 115.37, 108.60б (14/14), (Fig. S14). Anal.
Calc. for C18H9N3O3 (Mr = 315.28): C, 68.57; H, 2.88; N, 13.33%. Found
C, 68.36; H, 2.97; N, 13.43%.
3.1. Synthesis of dual-function monomers
Dual functional monomers PNB-P, PNB-M, PNB-O were successfully
synthesized in three steps from the corresponding aminophenols
(Fig. 1). In the first step phthalonitrile functionalized anilines 2P, 2M,
2O were prepared via nucleophilic aromatic substitution reaction of
aminophenols and 4-nitrophthalonitrile according to the literature
procedure [54].
2.3.1.2. PNB-O. Yield 86%. 1H NMR (DMSO‑d6, 600 MHz) δ: 8.09 (d,
1H7, J6,7 = 8.70 Hz), 7.69 (d, 1H8, J6,8 = 2.18 Hz), 7.60 (dd, 1H4, J3,4
8.37, J4,5 = 7.70 Hz), 7.52 (d, 1H5, J4,5 = 7.70 Hz), 7.46 (dd, 1H3, J3,4
=
=
The most challenging step was the preparation of the maleimide ring.
Despite the fact that it is possible to obtain maleimide from amine by a
one-pot reaction in acetone [66], N-maleamic acids 3P, 3M and 3O were
isolated separately to decrease side-product formation. It has been
previously reported in [14] that high temperature (around 80 ◦C) and an
excess of anhydride are necessary to avoid the formation of iso-
maleimide instead of the desired maleimide (Fig. S19). The reaction
condition optimization was carried out using amide 3M. The reaction
was first carried out in propionic anhydride without any other solvent
using sodium acetate as the base. A significant amount (around 10%) of
the product of Michael addition to the maleimide carbon‑carbon double
bond was observed in the NMR spectra of the product mixture (Fig. S20).
The strong electron-withdrawing effect of the phthalonitrile group re-
sults in high susceptibility of the maleimide double bond to Michael
addition. Therefore, trimethylamine was used to promote condensation
between the amide and the carboxylic acid under milder conditions.
However, the reaction proceeded slowly, and undesirable Michael
addition side-product was also formed (Fig. S21). Finally, sodium ace-
tate was used as the Brønsted base without the addition of trimethyl-
amine. The reaction was carried out at room temperature and no
evidence of the isomaleimide formation was found by NMR-
spectroscopy. The other two isomers PNB-O and PNB-P were also ob-
tained under these optimized conditions. The reactivity and yield of the
cyclization reaction increased in the following order: 3O < 3M ≈ 3P.
The lower reactivity of 3O might be primarily explained by steric
hindrances.
8.37 Hz, J2,3 = 7.03 Hz), 7.36 (d, 1H2, J2,3 = 7.03 Hz), 7.23 (s, 2H1)
(Fig. S17). 13C NMR (DMSO‑d6, 151 MHz) δ: 169.28, 160.14, 150.17,
136.21, 135.07, 131.27, 131,16,126.45, 123.37, 122.63, 122.09,
121.58, 116.55, 115.76, 115.24, 108.8 (16/16) (Fig. S18). Anal. Calc.
for C18H9N3O3 (Mr = 315.28): C, 68.57; H, 2.88; N, 13.33%. Found C,
68.32; H, 2.95; N, 13.51%.
2.4. Monomer mixing and curing
Blends were prepared by mixing PNB-M with comonomers in a 100
ml round-bottom flask with a magnetic stirrer at 120 ◦C under vacuum
(1 mmHg) until a clear homogeneous mixture was obtained. After stir-
ring 15 g of the blend (PNBM-DABA or PNBM-3P) was poured into a
steel mold (100 × 100 × 2 mm or 100 × 100 × 4 mm) to obtain molded
specimens for analytical studies or mechanical tests. The mold was
placed into an oven and cured at 180 ◦C for 8 h. The molded plate of
polymer was released and post-cured in an oven by heating to 240 ◦C,
280 ◦C, 330 ◦C or 375 ◦C (10 ◦C/h) and held at the final temperature for
8 h. A similar procedure was used to prepare the samples of control
blends (PN-3P, BMI-DABA, BMI-PN, BMI-PN-DABA).
2.5. Kinetics studies
Dynamic DSC studies of curing kinetics were carried out using
NETZSCH Thermokinetics 3.1 Software. Reaction parameters such as
activation energy and pre-exponential factor were obtained by the
Ozawa-Flynn-Wall method. Then the obtained parameters were
assumed as initial values for the non-linear regression analysis of DSC
curves. Experimental and predicted curves of the dual-stage curing re-
actions of monomers and their blends were shown in Figs. S12–15.The
proposed kinetic models were in good agreement with the experimental
DSC curves.
It is worth noting that the purification of PNBs monomers from re-
sidual propionic acid and anhydride following synthesis is critical for an
accurate polymerization study. Even minor impurities of acids could
significantly inhibit both the radical homopolymerization of maleimides
and the Michael addition reaction due to protonation of the imide
carbonyl oxygens, enhancing the electrophilicity of the maleimide cycle
(see Fig. S22) [67]. Therefore, all monomers were precipitated by
pouring their acetone solutions into distilled water. This procedure was
repeated several times until all acidic impurities were removed.
High purities of all monomers were confirmed by HPLC, elemental
analysis, 1H and 13C NMR spectra. The NMR spectra of all synthesized
compounds with peak assignments can be found in the supplementary
materials in Figs. S1–18. HPLC data can be found in Fig. S23.
2.6. Mechanical testing
Dynamic mechanical analysis (DMA) was performed on DMA Q800
by scanning the specimens (55 mm × 5 mm × 2 mm) over a temperature
range of 50–500 ◦C with frequency of 1 Hz and under inert atmosphere.
The dependences of storage modulus (E′), loss modulus (E′′), and loss
factor (tan δ) of the polymer specimens on temperature were obtained.
Mechanical tests were performed using Hounsfield H100KS, Tinius
Olsen H5KS, and Instron 5985 testing machines.
3.2. Melting and curing behavior of dual-curing monomers and their
blends
Non-isothermal DSC curves of the PNB-monomers are presented in
Fig. 2. Parameters calculated from DSC measurements of the investi-
gated monomers and blends are shown in Table S1. All three monomers
showed an endothermic transition due to melting followed by two
exothermic peaks corresponding to the polymerization reactions of
maleimide and phthalonitrile moieties. 4,4′-diphenylmethanebismalei-
mide (BMI) and 1,3-bis(3,4-dicyanophenoxy)benzene (PN) were shown
in Fig. 2 for comparison to demonstrate the absence of phthalonitrile
polymerization without catalysts (PN) and the presence of an
exothermic peak of maleimide homopolymerization (BMI). The melting
point of PNB-M was the lowest (107.4◦С) compared to PNB-P (210.4◦С)
and PNB-O (200. 8◦С). Subsequent cooling revealed that crystallization
of PNB-M from the melt-state is significantly hindered and the DSC
The impact strength was determined using Charpy impact tester
equipped with a 2.5 J impact energy pendulum according to ISO 179-2
standard. Fracture toughness in terms of critical-stress-intensity factor
(KIc) and critical strain energy release rate (GIc) were determined ac-
cording to ASTM D5045 on single-edge-notch bending specimens. KIc
and GIc parameters were not monitored for specimens cured at a series of
different temperatures because of the complex procedure for testing. At
least five samples were utilized to obtain the results of each mechanical
test. Average values and their standard deviations were calculated.
3

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
Fig. 1. (A) Synthetic routes towards PNB-P, PNB-M, PNB-O (PNB-s).
standards). The maximum conversion of maleimide polymerization at
180◦С was about 55% when determined by integration of the NMR
peaks. A further increase in temperature above 180◦С leads to the
polymerization of phthalonitrile fragments and therefore, to the for-
mation of insoluble cross-linked polymer.
The radical mechanism of maleimide polymerization was observed
on the DSC curve of blend PNB-M with 1% wt. tret-butyl perox-
ybenzoate as free-radical initiator (Fig. S28). After addition of the
initiator the maleimide polymerization DSC peak shifted to a lower
temperature, the original maleimide homopolymerization peak of PNB-
M remained but significantly decreased. The double bonds are likely
sterically hindered at the high degree of crosslinking and cannot be fully
polymerized below the decomposition temperature of the initiator. The
addition of hydroquinone, on the contrary, shifts the polymerization
peak to a higher temperature due to the inhibition of free-radical
polymerization. There was no evidence that the phthalonitrile frag-
ments underwent free-radical polymerization as was suggested by Z Liu
et al. [70]. The peak maximum of phthalonitrile polymerization on the
DSC curve of the PNB-M blend with tret-butyl peroxybenzoate
decreased only by 10◦С probably due to the earlier maleimide poly-
merization the product of which in turn promoted phthalonitrile poly-
merization. It was reported that succinimide structures obtained from
the homopolymerization of maleimides can insignificantly degrade into
amines at 300◦С [71,72]. It is well known that even minor amounts of
amine could initiate phthalonitrile polymerization. Therefore, we pro-
posed that the mechanism of self-catalyzed polymerization of the
phthalonitrile fragments may be related to the minor decomposition of
the succinimide structures into amines at 300◦С, which in turn can
catalyze the polymerization of phthalonitrile fragments. The equimolar
blend of BMI and PN was also studied by DSC, and its curve showed an
exothermic peak of phthalonitrile polymerization (Fig. S29) similar to
those observed for the PNB monomers, which serves as additional proof
of the catalytic effect of the polymerized maleimide moieties on the
curing of phthalonitrile fragments.
Fig. 2. DSC curves of dual-function PNB-monomers compared to 4,4′-diphe-
nylmethanebismaleimide
(BMI)
and
1,3-bis(3,4-dicyanophenoxy)ben-
zene (PN).
curve collected during the second heating showed only an amorphous
solid-state with glass transition around 33.7◦С (see Fig. S24). Polymer-
ization of PNB-P and PNB-O started immediately after melting, which
does not allow performing the heating-cooling-heating cycle DSC ex-
periments with them and leads to short processing windows for these
monomers.
Polymerization of the maleimide double bond proceeds through
radical homopolymerization with peak maxima at 287.9◦С (PNB-P),
231.3◦С (PNB-M), 278.9◦С (PNB-O) [13]. We assumed that the peak
maximum of PNB-M was at the lowest temperature because the with-
drawing effect of the phthalonitrile fragment on the maleimide double
bond increased from meta position in PNB-M to para and ortho positions
in PNB-P and PNB-O. It has been reported that the presence of a with-
drawing group in the backbone of maleimide increased the homo-
polymerization temperature and reduced the reactivity of the double
bond [68]. 1H NMR spectrum of PNB-M bulk-polymerized at 180◦С for
24 h showed a linear homopolymer pPNB-M soluble in methylene
chloride or acetone (Fig. S25) [69]. DSC curve of this polymerized
product established three glass transitions that could result from the
wide and complex molecular weight distribution (MWD) of pPNB-M
(Fig. S26). Indeed, the GPC experiment showed a linear polymer with a
wide MWD (Fig. S27) and a low degree of polymerization (polystyrene
To investigate the dual-curing behavior of PNB-monomers, different
blends with comonomers (DABA, APB, 3P) were obtained (Table 1).
DSC curves for these blends are shown in Fig. 3, and relevant parameters
are listed in Table S1. The reaction mechanisms were examined by NMR
of the adducts obtained from PNB-M heated with DABA or 3P
(Figs. S30–31). Proposed mechanisms for the polymerization of PNB-M,
PNBM-DABA and PNBM-3P were summarized in Fig. 4.
Since DABA is the most common comonomer used for improving the
mechanical properties of bismaleimides, an investigation of the PNB-M
blend with DABA was a task of the highest priority. There were two
exothermic peaks on the DSC curves of blends containing PNB-M and
DABA (Fig. 3). The first peak was an assigned to the copolymerization of
DABA with PNB-M. The second peak was related to the phthalonitrile
curing. The temperature of phthalonitrile curing of all DABA-containing
4

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
the case of diallyl bisphenol A, Diels-Alder reaction did not occur due to
steric hindrance [73,74].
Table 1
Abbreviations of the blends and their components mass ratios.
On the DSC curves of the PNB-M blend with the amine-containing
compounds 3P and APB, two peaks were observed. The first one
might be related to the amidation of maleimide or Michael addition of
amine to the maleimide double bond [75] and the second one may be
associated with the polymerization of phthalonitrile. However, an NMR
study of the reaction between 3P and PNB-M showed only the imide ring
opening with the formation of amide (amidation) and no adducts of the
reaction between 3P and PNB-M by Michael addition were observed
(Fig. S31). Moreover, the homopolymer can be seen in the NMR spectra
of the PNBM-3P blend heated at 180◦С due to the anionic polymeriza-
tion of the maleimide groups promoted by amines [14]. Maleic diamide
derivative 5 pm (Fig. S31) was isolated by flash column chromatog-
raphy from the products obtained after heating of blend PNBM-3P. We
assumed that the amidation reaction of PNB-M might be enabled by the
withdrawing effect of the phthalonitrile affecting the maleimide double
bond [76,77]. The amidation reaction was additionally confirmed by an
IR-study described further.
Blend
Dual-
Amine
comonomer
DABA
Phthalonitrile
with amino group
(3P)
functional
monomer
PNB-M
comonomer
(APB)
4PNBM-
DABA
2PNBM-
DABA
PNBM-
DABA
PNBM-
3P
89.1
80.4
67.2
57.3
90.1
–
10.9
19.6
32.8
–
–
–
–
–
–
–
42.7
PNBM-
APB
9.9
–
–
Control
blends
Bisphthalonitrile
monomer PN
Bismaleimide
BMI
comonomer
DABA
Phthalonitrile
with amino
group (3P)
PN-3P
BMI-
93.9
–
90.7
–
9.3
6.1
–
–
DABA
BMI-PN
BMI-PN-
DABA
3.3. IR-study of monomers and blends
50.2
35.2
49.8
34.8
–
30
–
–
FT-IR measurements were performed on PNB-M, PNB-O, PNB-P
monomers and PNBM-DABA, PNBM-3P, PNBM-APB blends polymer-
ized at different temperatures to additionally investigate their curing
behaviors, and the IR-spectra are presented in Fig. 5 and in Figs. S32–34
for PNB-O, PNB-P, and PNBM-APB.
In the FT-IR spectra of PNB-M and PNBM-DABA, PNBM-3P, PNBM-
APB blends polymerized above 180◦С and PNB-P and PNB-O poly-
◦
–
merized at 280 С the characteristic absorption bands of C C maleimide
–
linkage around 830 cmꢀ 1 and 698 cmꢀ 1 disappeared [78]. The signal
located at 3105 cmꢀ 1 specific to the maleimide double bonds also dis-
appeared. An absorption band corresponding to the C-N-C maleimide (at
1147 cmꢀ 1) decreased for all samples while a new absorption band
around 1180 cmꢀ 1 (C-N-C succinimide) [79] increased. A decrease of
the absorption peak at 1627 cmꢀ 1 indicated that the allyl group reacted
at 180◦С in the case of PNBM-DABA blend curing.
–
In the case of PNBM-3P and PNBM-APB, stretching bands of C
O
–
(amide I mode) at 1641 cmꢀ 1 and 1668 cmꢀ 1 can be seen after poly-
merization in addition to the band of the succinimide ring. These bands
overlapped with the isoindoline characteristic absorption band at 1652
cmꢀ 1 when further curing was carried out at the temperature higher
than 280◦С. After polymerization of PNBM-3P and PNBM-APB, the
amino bands at 3472 and 3379 cmꢀ 1 disappeared (NH2 asymmetric and
symmetric stretching vibrations). Presence of the amide absorption
bands on the IR-spectra together with the NMR analysis show the ring-
opening amidation of imide in the polymerization processes of PNBM-
3P at 180◦С.
Fig. 3. DSC curves of the PNB-M blends with comonomers DABA, 3P and APB.
The increase of curing temperature resulted in the decrease of peak
height of the characteristic absorption band of the cyano groups at 2233
cmꢀ 1. However, the significant decrease of the cyano groups absorption
was not observed for the cured PNB monomers until 280◦С due to the
higher temperature of the phthalonitrile polymerization without any
comonomers. Furthermore, the nitrile absorption band shifted to lower
wavenumbers, which could indicate the formation of triazine de-
rivatives [80]. The characteristic absorption bands of triazine rings
could also be found at the range from 1520 to 1545 cmꢀ 1 but only after
curing at 375◦С. The characteristic peak of triazine ring at 1325 cmꢀ 1
was unidentifiable due to the overlapping of peaks. At the same time,
there was obvious formation of phthalocyanine rings observed as ab-
sorption bands in the range from 1010 to 1020 cmꢀ 1 appeared starting
at the low-temperature curing stages (180–240◦С) [45]. The charac-
teristic band of isoindoline structures at 1652–1660 cmꢀ 1 appeared only
after polymerization at the temperatures above 240◦С. Such behavior of
polymerization might be related to the steric factor hindering the for-
mation of phthalocyanines only at the high degrees of crosslinking. The
systems was lower than for the pure PNB-M monomer due to the fact
that DABA can additionally facilitate the polymerization of phthaloni-
trile fragments by OH-groups [3]. Increasing DABA content in the blend
from 4PNBM-DABA to PNBM-DABA lead to a shift of the peak
maximum to a lower temperature from 326.5◦С to 299.4◦С. The DSC
curves of the phthalonitrile-bismaleimide control systems with and
without DABA (BMI-PN-DABA and BMI-PN) showed a similar differ-
ence in the peak maxima of the phthalonitrile polymerization due to the
catalytic effect of the OH-groups on the phthalonitrile fragments curing
(Fig. S29).
The reaction of DABA and PNB-M proceeded by a well-known
pathway through an ene reaction that was proven by IR and NMR
spectroscopy. Only products of the mono- 6 m and di-addition 7 m via
ene reactions were observed for the PNBM-DABA blend (Fig. S30). No
evidence of products of Diels-Alder reactions of PNB-M with the ene
adducts were observed in NMR spectra. It was earlier reported that in
5

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
Fig. 4. Proposed polymerization pathways of PNB-M, PNBM-DABA and PNBM-3P.
Fig. 5. IR spectra of neat and cured at different temperatures PNB-M (A), PNBM-DABA (B) and PNBM-3P (C).
results of the IR study indicate a high degree of conversion of the
phthalonitrile and maleimide moieties during polymerization reactions
of all investigated blends and monomers.
(Figs. S35–37). These values are similar to the activation energies of the
maleimide homopolymerization reported in the range 120–160 kJ/mol.
The highest activation energy of PNB-O can be related to steric hin-
drance of the maleimide double bond. Activation energy of the PNBM-
DABA blend was 85.9 kJ/mol, which is lower than the activation energy
of pure PNB-M, while pre-exponential factor of PNB-M was similar to
PNBM-DABA. It can explain the absence of PNB-M’s homopolymeri-
zation adducts in the cured PNBM-DABA blend. The calculated activa-
tion energies of phthalonitrile polymerization were similar to those
reported earlier [54,81]. Phthalonitrile polymerization was described as
autocatalytic first-order reactions probably due to the formation of
catalytic moieties during phthalonitrile polymerization. It was reported
that isoindoline structures were able to accelerate further phthalonitrile
polymerization [81].
3.4. Non-isothermal polymerization kinetic analysis
Kinetic parameters such as activation energy and pre-exponential
factor were calculated as described in the experimental section
(Table S2). Maleimide polymerization was described as an n-th order
reaction (Eq. (1)) and phthalonitrile polymerization was described with
the Prout–Tompkins equation, which is
a
short form of the
ˇ
´
Sestak–Berggren equation (Eq. (2)).
n
f(α
) = (1 ꢀ
α
)
(1)
(2)
f(α
) = (1 ꢀ
α
)
nαa
3.5. Rheological behavior and processability
The DSC curves of all investigated monomers and the PNMB-DABA
blend were successfully fitted as two-step sequential reactions
(Fig. S38). The calculated Ea values for PNB-P, PNB-M, and PNB-O were
91.5 kJ/mol, 106.9 kJ/mol and 134.1 kJ/mol, respectively
The dynamic viscosity changes in the course of curing reactions were
monitored via viscosity studies. Viscosity-temperature and isothermal
curves for the PNB-M monomer, PNBM-3P, and PNBM-DABA are
6

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
shown at Fig. 6. The temperature of viscosity decrease to 1000 mPa s
(T1000mPas), curing onset temperature (Tonset), processing window, and
gel time at 180 ◦C are the parameters determining the possibility of resin
application in composite manufacturing using such methods as resin
transfer molding (RTM) or vacuum infusion (VIM) [52]. These param-
eters were obtained from the viscosity data and summarized in Table 2.
Processing window was calculated as the difference between Tonset and
Table 2
Parameters calculated from rheological study of PNB-monomers and blends.
Monomer/
Blend
T1000mPas
,
Tonset
,
Processing
Gel time at
◦C
◦C
window (◦C)
180 ◦C, min
PNB-P
214.7
107.4
202.5
76.2
241.2
232.6
224.0
258.1
253.4
26.5
–
PNB-M
125.2
21.5
143
–
PNB-O
PNBM-DABA
2PNBM-
DABA
181.9
157.9
92
127
T
1000mPas [57]. Viscosity-temperature profiles for all PNB monomers can
95.5
be found in Fig. S39. The presence of a peak on the viscosity-
temperature curves is evidence of the formation of a linear polymer
during PNB monomer homopolymerization. After the peak viscosity was
reached, a further increase of temperature resulted in the viscosity
decrease due to polymer melting. Increasing temperature to 280 ◦C leads
to a rapid increase of viscosity due to the phthalonitrile fragment
polymerization in all monomers and blends.
PNBM-3P
BMI-PN-
DABA
107.3
97.5
248.2
212.1
143.9
114.6
223
–
- – parameter was not investigated.
The temperature of 180 ◦C is commonly used as a curing temperature
for phthalonitriles. Therefore, the resin gel time at this temperature is an
important parameter for estimating the processability character and
curing time. At 180 ◦C the viscosity increase of the system containing
DABA occurs more rapidly than for the pure PNB-M monomer likely due
to the lower activation energy of the ene reaction compared to the
homopolymerization reaction [13]. The viscosity of PNBM-3P increased
to 2000 mPa s during the first 30 min of isothermal heating at 180 ◦
C
likely due to the amidation reaction between PNB-M and 3P with the
formation of maleic diamide, followed by a further rapid increase after
220 min due to phthalonitrile fragments curing. The T1000mPas of PNBM-
DABA was 76.2 ◦C, which is lower than 87 ◦C observed for the recently
reported low-melting phthalonitrile system [57] and could be accept-
able for RTM and VIM fabrication. At the same time, this value was
lower than 97.5 for the BMI-PN-DABA blend due to the lower melting
point of PNB-M compared to PN and BMI.
Fig. 7. TGA curves for cured polymers under nitrogen atmosphere.
3.6. Thermal properties of cured polymers
succinimide moiety and as a consequence – an increase of the thermal
stability of the polymer [43]. The trend observed for oxidation resis-
tance was similar to the one for thermal stability.
Thermal stability of all obtained polymers was studied using thermal
analysis under nitrogen and air atmospheres. TGA curves are shown in
Figs. 7 and 8. Parameters characterizing degradation behavior including
weight loss temperatures (T5% and T10%) and char yields at 900 ◦C (Yc)
are presented in Table 3.
Polymerized blends PNBM-DABA and PNBM-3P showed high ther-
mal stability. T5% was above 450 ◦C, and char yield in nitrogen was as
high as 75.9% for PNBM-DABA. As expected, these values exceed the
T
5% (less than 400 ◦C) and char yield (less than 50%) of the polymerized
As can be seen from Table 3, thermal stability increases in the
following order: PNB-O - PNB-P - PNB-M. The electron-withdrawing
phthalonitrile ring in the para- and ortho- positions of the phenyl ring
in monomers PNB-O and PNB-P could reduce the electron density at the
imide nitrogen compared with the meta- position of PNB-M. Delocal-
ization of electron density from the imide nitrogen to the carbonyl group
blend of bismaleimide BMI with DABA [13,83] because phthalonitriles
possess better thermal properties. The higher char yields also lead to the
high limiting oxygen indeces (LOI) of the cured PNBM-DABA and
PNBM-3P systems according to Van Krevelen equation [84]. However,
thermo-oxidative stability of these cured blends was still lower than in
the case of the system containing only the phthalonitrile monomer (PN-
–
would result in an increase of the strength of the C N bond in the
Fig. 6. Time sweep curves of viscosity collected at 180 ◦C (A) and viscosity-temperature profiles (B) for monomer PNB-M and its blends.
7

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
of BMI-DABA remained almost the same. It is obvious from the DMA and
TGA experiments that thermal performance of the cured PNBM-DABA
blend significantly exceeded the commonly used BMI-DABA system.
Storage modulus for the PNBM-3P blend cured at 180 ◦C was lower
than 3 GPa due to the incomplete crosslinking. Two glass transition
temperatures were also detected if the samples were cured at tempera-
tures below 240 ◦C. It is likely that one of them was related to the
transition of the homopolymerized double bond of maleimide and
another to the phthalocyanine and isoindoline moieties of the poly-
merized phthalonitrile. The PNBM-DABA and PNBM-3P blends cured at
375 ◦C showed a decrease of modulus simultaneously with an increase of
glass transition temperature. We assumed that a high degree of cross-
linking density might lead to an increase in the internal stress that
caused damage to the structure of the polymer. Moreover, minor
destruction of the maleimide polymer structure might also have taken
place.
Fig. 8. TGA curves for cured polymers under air atmosphere.
3.7. Mechanical properties of cured polymers
Table 3
Results obtained from TGA measurements of cured polymers.
Flexural, fracture, and impact tests of the blends (PNBM-DABA and
PNBM-3P) which were cured at different temperatures were performed
to evaluate the influence of the maleimide and phthalonitrile polymer-
ization on the mechanical properties of the polymers. Flexural modulus,
flexural strength, and impact strength obtained are shown in Fig. 11.
Flexural strength and modulus of PNBM-DABA cured at 180 ◦C were
123 MPa and 5.07 GPa, respectively, which was much higher than those
observed for the conventional phthalonitrile systems [44] and at the
same level as the strength and modulus of the bismaleimide-DABA resin
systems [13,85,86]. These mechanical properties of PNBM-3P were
lower and they can be considered typical for phthalonitrile resins. This
can be explained by the absence of flexible molecular structures pro-
duced by the ene reaction of the maleimide group with DABA.
The flexural modulus, impact, and flexural strength of the PNBM-
DABA cured blend decreased as the curing temperature increased.
Flexural modulus (4.69 MPa) of the PNBM monomer cured with DABA
at 240 ◦C was much higher than that (4.02 GPa) of the cured BMI-DABA
resin. At the same time, flexural strengths of these polymers were almost
the same.
monomer/blend
Nitrogen
Air
T5%,◦C
T
5%,◦C
T10%,◦C
Yc, %, 900 ◦
C
T10%,◦C
PNB-P
403
414
385
456
488
407
503
452
467
423
505
530
423
543
65.1
68.0
61.3
75.9
77.5
413.8
419.0
392.9
458.8
470.9
–
469.6
475.8
439.8
507.9
510.5
–
PNB-M
PNB-O
PNBM-DABA
PNBM-3P
BMI-DABA [82]
PN-3P [57]
37 (at 800 ◦C)
73.2
504
534
3P).
Curves of storage modulus (G’) and tanδ were obtained by DMA
measurements for the PNBM-DABA (Fig. 9) and PNBM-3P (Fig. 10)
blends cured at different temperatures. Parameters including G’ at RT
and 300 ◦C, Tg are all presented in Table 4. Once more, the measurement
could not be performed for cured PNB monomers due to the brittleness
of samples caused by excessive cross-linking.
For the PNBM-DABA blend cured at 240 ◦C two peaks were observed
on the tanδ curves. This can be associated with a relatively high content
of the more flexible products of copolymerization of DABA with mal-
eimides compared to the cured phthalonitrile fragments. The increase of
curing temperature results in the decrease of storage modulus at 50 ◦C
possibly due to a greater impact of cured phthalonitrile fragments on the
storage modulus. The first glass transition temperature of PNBM-DABA
was similar to BMI-DABA when both were cured at 240 ◦C, but after the
curing temperature was increased to 280 ◦C, Tg of PNBM-DABA
increased to 357.6 ◦C due to phthalonitrile polymerization while the Tg
The flexural modulus and strength of PNBM-3P cured at 180 ◦C were
very low (2.82 GPa and 58 MPa, respectively). According to the IR study,
the phthalonitrile polymerization at this temperature was characterized
by low conversion and as a result, the polymerization proceeds with the
formation of a material with a cross-linking density which is too low.
Despite the fact that amines were considered as comonomers which
could improved toughness of polymers obtained via maleimide poly-
merization through Michael addition reactions [60], as explained
earlier, significant amidation of maleimide with observable production
Fig. 9. DMA curves of the PNBM-DABA blend post-cured at different temperatures (4 h). (A) storage modulus; (B) loss factor (tan δ).
8

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
Fig. 10. DMA curves of the PNBM-3P blend post-cured at different temperatures (4 h). (A) storage modulus; (B) loss factor (tan δ).
rigid units of phthalonitrile polymerization products leading to
shrinkage and micro-cracking of the polymer samples.
Table 4
Parameters obtained from DMA measurements of the cured polymer blends.
Impact strength is one of the most important toughness parameters
which needs to be evaluated for the application of resins in composite
manufacturing. The variation caused by curing temperature was similar
to the variation in flexural and strength modulus. It is important that the
value of impact strength was as high as 11.81 kJ*mꢀ 2 for PNBM-DABA
cured at 330 ◦C which is within the range required for resin application
in structural composite matrices [13]. This value was higher than the
impact strength of the BMI-DABA system cured at 240 ◦C despite the
significant difference in glass transition temperatures of these polymer
systems. This likely originated from the balance in the flexible and rigid
moieties formed after both the phthalonitrile and the maleimide poly-
merizations. Impact strength of the PNBM-3P system cured at 330 ◦C
was only 8.49 kJ*mꢀ 2 and that can be associated with the higher stiff-
ness of the polymer structure compared to PNBM-DABA.
Prepolymer
blend
Curing
Tg1(◦C)
Tg2(◦C)
G′ RT
G′ 300 ◦
C
temperature
(tan
(tan
(MPa)
(MPa)
(◦C)
delta)
delta)
(retention of
G′ 50 ◦C)
PNBM-
DABA
180
240
280
262.5
291.2
–
366.2
369.4
357.6
4895
4612
4367
546 (11.1%)
672 (14.6%)
2125
(48.7%)
2521
330
375
180
240
280
330
375
–
368.4
436.2
349.8
352.1
364.3
377.1
446.3
4243
3421
2811
4003
4007
3965
3250
(59.4%)
1742
–
(50.9%)
1059
PNBM-3P
278.6
(37.7%)
1562
288.1
(39.0%)
2011
The critical stress intensity factor (KIc) and critical strain energy
release rate (GIc) are indicators of the material’s ability to resist fracture.
Their values were 1.08 MPa*m0.5 and 308 J*mꢀ 2 respectively, for
PNBM-DABA cured at 330 ◦C which is on the same, level as the BMI-
–
–
–
(50.2%)
2563
(64.6%)
2368
DABA systems (KIc = 0.8–1.4 MPa*m0.5; GIc = 250–500 J*mꢀ 2
)
(72.9%)
425 (9.9%)
512 (12.9%)
3122
[13,85,87]. Fracture toughness parameters of the cured PNBM-3P at
330 ◦C were significantly lower (KIc = 0.67 MPa*m0.5 and GIc = 124
J*mꢀ 2). As it was expected, KIc values decreased with the increasing
phthalonitrile content from PNBM-DABA to PNBM-3P, as reflected by
the higher glass transition temperatures.
BMI-DABA
PN-3P
240
280
375
287.3
290.1
–
–
–
4287
3954
4069
476.2
(76.7%)
of maleic diamide was observed for these blends. As the result, such
polymers consist of highly crosslinked phthalonitrile fragments, which
leads to high brittleness. The rapid decrease of mechanical properties
that was observed after curing at 375 ◦C (a typical post-curing end
temperature for phthalonitriles) is likely due to the presence of the more
4. Conclusions
Novel self-catalyzed phthalonitrile monomers containing an N-mal-
eimide group in different positions of the benzene ring were synthesized.
Chemical structures were confirmed by 1H and 13C NMR, FTIR, and
Fig. 11. Flexural strength (A), flexural modulus (B), and impact strength (C) of the PNBM-DABA and PNBM-3P blends cured at a series of temperatures (180 ◦C,
240 ◦C, 280 ◦C, 330 ◦C, 375 ◦C) with comparison to the phthalonitrile PN-3P and bismaleimide BMI-DABA systems.
9

S.S. Nechausov et al.
Reactive and Functional Polymers 164 (2021) 104932
bisphenol a-based benzoxazines and properties of corresponding polybenzoxazines,
Polymers (Basel). 12 (2020) 1225, https://doi.org/10.3390/polym12061225.
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elemental analysis. The investigation of their curing behavior revealed
homopolymerization of the maleimide double bond followed by self-
catalyzed formation of the phthalocyanine, isoindoline, and triazine
structures from the phthalonitrile moieties. Only the meta- isomer PNB-
M exhibited a low melting point of 107.4 ◦C while Tm of the para- and
ortho- isomers were 210 ◦C and 200 ◦C. Therefore, PNB-M was chosen to
obtain polymer materials and blended with comonomers for maleimide
groups such as diallylbisphenol A (DABA) and 4-(4-aminophenoxy)
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in RTM and VIM composite manufacturing technologies. The mechani-
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allowed to easily obtain a material with an enhanced thermal perfor-
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flexural modulus, flexural strength, and fracture toughness of the cured
PNB-M with DABA was on the same level as the bismaleimides: KIc was
1.08 MPa*m0.5, GIc was 308 J*mꢀ 2, and impact strength was 11.81
kJ*mꢀ 2 We showed that the mechanical properties and brittleness of
phthalo.nitriles could be significantly improved with only a minor loss of
thermal performance by introducing maleimide groups into the phtha-
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functional monomers demonstrated will allow others to utilize the
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Author statement
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All persons who meet authorship criteria are listed as authors, and all
authors certiff that they have participated sufficiently in the work to take
public responsibility for the content, including participation in the
concept, design, analysis, writing, or revision of the manuscript. Fur-
thefinore, each auth6r certifies that this material or similar material has
not been and will not be submitted to or published in any other publi-
cation before its appearance in the Hong Kong Journal of Occupational
Therapy.
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Acknowledgments
This work was financially supported by the Russian Science Foun-
dation (grant number 20-79-00146). Authors are grateful to the Moscow
State University (Russia) for the opportunity to use the NMR facilities of
the Center for Magnetic Tomography and Spectroscopy.
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