Synthesis, characterization, and non-isothermal curing kinetics of two silicon-containing arylacetylenic monomers

Article abstract of DOI:10.1007/s11164-011-0291-1

Vinyltri(phenylethynyl)silane ((ph-C≡C)₃-Si-C=CH ₂; VTPES) and phenyltri(phenylethynyl)silane ((ph-C≡C) ₃-Si-ph; PTPES) were synthesized by Grignard reaction. Their molecular structures were characterized by means of

Full text of DOI:10.1007/s11164-011-0291-1

Res Chem Intermed (2011) 37:831–845
DOI 10.1007/s11164-011-0291-1
Synthesis, characterization, and non-isothermal curing
kinetics of two silicon-containing arylacetylenic
monomers
•
Dexin Tan Tiejun Shi Zhong Li
•
Received: 15 October 2010 / Accepted: 20 January 2011 / Published online: 6 February 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Vinyltri(phenylethynyl)silane ((ph–C:C)3–Si–C=CH2; VTPES) and
phenyltri(phenylethynyl)silane ((ph–C:C)3–Si–ph; PTPES) were synthesized by
1
Grignard reaction. Their molecular structures were characterized by means of H
NMR, 13C NMR, 29Si NMR, and FT-IR spectroscopy. Their nonisothermal thermal
curing processes were characterized by DSC, and the corresponding kinetic data, for
example activation energy (E), pre-exponential factor (A), and the order of the
reaction (n), were obtained by the Kissinger method. The results showed that the
melting points of VTPES and PTPES were 84 and 116 °C, respectively. Their
curing reaction rates were consistent with first-order kinetic equations. VTPES
monomer had a lower activation energy and curing temperature as a result of
coordination between reactive groups.
Keywords Vinyltri(phenylethynyl)silane ꢀ Phenyltri(phenylethynyl)silane ꢀ
Grignard reaction ꢀ Structural characterization ꢀ Curing kinetics
Introduction
In 1968, Luneva first reported the synthesis of diethynyldiphenylsilane in a nitrogen
atmosphere [1]. This kind of silicon-containing arylacetylenic monomer can be
cured thermally without formation of volatiles. The polymers’ high-temperature
properties and high char yield resulted in their rapid development, because of their
D. Tan ꢀ T. Shi (&) ꢀ Z. Li
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, Anhui,
People’s Republic of China
e-mail: stjhfut@163.com
D. Tan ꢀ Z. Li
School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001,
Anhui, People’s Republic of China
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D. Tan et al.
potential applications in biomaterials, optoelectronics, and ceramics precursors, etc.
[2–8].
Synthesis of different alkynylsilane monomers has been reported in recent years
[9, 10]. The main synthetic routes were dehydrogenation coupling reactions
between hydrosilanes and alkynic compounds using metal compounds as catalysts.
H2PtCl6/LiI, I2, IrH2(SiEt3)(COD)(AsPh3), [IrH(H2O](bp)L2)]SbF6, and RhClL3
(L = PPh, bp = 7,8-benzoquinolinato) have been used as catalysts [11]. In these
cases, dehydrogenating cross-coupling reactions were found to accompany the
significant hydrosilylation reactions to produce the corresponding alkenylsilanes,
and dehydrocoupling in low yield was also found to accompany hydrosilation and
hydrogen-transfer reactions [12, 13]. Calas and Bourgeois obtained alkynylsilane
(n-Bu–C:C–SiEt3) from Et3SiH and 1-hexyne using Na or NaH as a catalyst [11].
Liu and Harrod reported that the CuCl/amine catalyst led to highly selective
dehydrogenative cross-coupling reactions of hydrosilanes with alkynes [14]. Itoh
et al. also found that solid bases (MgO) and metal hydrides (LiAlH4, NaAlH4,
LiBH4, and LiAlH(Ot–Bu)3) catalyze the same selective dehydrogenating cross-
coupling reactions [15]. However, these synthetic methods were carried out under
moisture-free and oxygen-free conditions, the catalysts and raw materials (e.g.
LiAlH4 and phenylsilane) used were dangerous, and the products were rather
expensive. Further, the hydrosilanes were mainly synthesized from chlorosilanes
and lithium aluminum hydride in anhydrous diethyl ether or dioxane [16–18]. As a
result, those methods were mainly limited to laboratory preparations, and the
synthesized monomers were only characterized in the structures. However, the
Grignard reaction is a common reaction method and the reaction conditions are mild
[19]. Methyldi(phenylethynyl)silane and [–SiH(R)–C:C–C6H4–C:C–] (R = Ph,
CH3, H) were synthesized by use of the Grignard reaction and yields were better
than 75% [20, 21], but the synthesis of vinyltri(phenylethynyl)silane and
phenyltri(phenylethynyl)silane by use of Grignard reactions has not been reported.
The main objectives in this work were to prepare vinyltri(phenylethynyl)silane
and phenyltri(phenylethynyl)silane monomers by the Grignard reaction, using the
relatively inexpensive vinyltrichlorosilane or phenyltrichlorosilane as raw materials,
and to characterize the monomers’ structures by use of FT-IR and NMR, and the
non-isothermal curing process by use of DSC.
Experimental
Materials and methods
Phenylacetylene was purchased from Hanwang Reagent Company, China. Vinyl-
trichlorosilane and phenyltrichlorosilane were purchased from Huarong Chemical
Materials Company, China. Solvents were bought from Shanghai Reagent
Company, and all chemicals and solvents were distilled or dried if necessary
according to standard procedures.
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Kinetics of two silicon-containing arylacetylenic monomers
833
Preparation of vinyltriphenylethynylsilane (VTPES) (monomer a)
An organic magnesium Grignard reagent was prepared as follows: Magnesium
turnings (6.72 g, 0.28 mol) were introduced into a 500-mL four-necked flask and
the atmosphere of the flask was replaced with dry nitrogen gas. THF (80 mL) which
had been dried over sodium was subjected to simple distillation and introduced into
the flask. A small piece of iodine was then added and the mixture was stirred to
activate the magnesium. To the activated magnesium was added dropwise a solution
of 20 mL (0.268 mol) ethyl bromide in THF (100 mL) at room temperature over
approximately 1 h. The resulting mixture was reacted while being heated under
reflux for 3 h to produce ethylmagnesium bromide. The reaction system was then
added dropwise to a solution of 29 mL (0.264 mol) phenylacetylene in THF
(100 mL) in a ice-bath over 2 h, with stirring, and the reaction was continued for an
additional 3 h while being heated under reflux to produce the intended phenyle-
thynylene Grignard reagent (0.264 mol).
Monomer a was then prepared by the following procedure. The reaction was
performed subsequent to the foregoing preparation of the phenylethynylene reagent.
A solution of 11 mL (0.086 mol) vinyltrichlorosilane in THF (100 mL) was added
dropwise to the flask in a ice-bath over 1 h, with stirring, and the reaction system
was further reacted for 3 h while being heated under reflux, following by the color
variation from green to yellow. The reaction system was then post-treated as
follows. An aqueous solution of hydrochloric acid (1 mol/L, 300 mL) was placed in
another 500-mL flask and the reaction solution prepared in the 500-mL flask was
transferred to a dropping funnel. The aqueous hydrochloric acid solution was gently
stirred while the reaction solution was slowly added dropwise by use of the
dropping funnel (over 30 min) in a ice-bath. Benzene (50 mL) was added to the
reaction solution and the resulting oil phase was separated by use of a separatory
funnel, washed with distilled water until no acid was present, then dried by adding
anhydrous calcium chloride while standing overnight. The solution was filtered
through a ceramic filter to remove the dehydrating agent. The solvent was removed
from the solution by distillation under vacuum, eventually giving a viscous, oily,
crude product. The crude product was repeatedly recrystallized in methanol to cause
precipitation. The resulting precipitates were isolated by filtration and dried to
give pin-like crystals (approx. 23 g, 74.7% yield) of the intended monomer
a (Scheme 1).
Mg
+
Br-CH2-CH3
MgBr
EtMgBr
Cl
Si
Cl
Cl
R
EtMgBr
a R=vinyl
b R=ph
Si
R
Scheme 1 Synthetic routes to the two monomers
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D. Tan et al.
Preparation of phenyltriphenylethynylsilane (PTPES) (monomer b)
The same procedures used for preparation of monomer a were repeated, except that
phenyltrichlorosilane was substituted for vinyltrichlorosilane to give columnar
crystals of monomer b (70% yield).
Characterization
FT-IR spectroscopic characterization was carried out with a Nicolet 8700 FT-IR
spectrometer and use of potassium bromide (KBr) pellets. 1H NMR, 13C NMR, and
29Si NMR spectra were recorded on an Avance AV-400 superconducting Fourier
1
digital NMR spectrometer (400 MHz for H NMR, 100.61 MHz for 13C NMR and
79.49 MHz for 29Si NMR). The chemical shifts were recorded relative to
tetramethylsilane (d, 0.0 ppm) for 1H NMR and 29Si NMR, and CDCl3 (d,
77.7 ppm) for 13C NMR. The differential scanning calorimetry (DSC) study was
performed with a Mettler-Toledo-DSC 821e/400 under nitrogen atmosphere.
Results and discussion
Structural characterization of the monomers
Typical proton NMR spectra of the two monomers are shown in Fig. 1. The
aromatic hydrogen resonated at 7.35–7.59 ppm and silicon vinyl protons resonated
at 6.33 ppm. In a precise study of NMR peak integration, vinyl protons (m, 3H,
CH=CH2), aromatic hydrogen bonded to ethynyl (m, 6H, PhH–C:C), and aromatic
hydrogen (m, 9H, PhH) were in the ratio 3:6:9 for VTPES (Fig. 1a). For PTPES,
aromatic hydrogen bonded to silicon (m, 2H, PhH–Si), aromatic hydrogen (m, 3H,
PhH–Si), aromatic hydrogen bonded to ethynyl (m, 6H, PhH–C:C), and aromatic
hydrogen (m, 9H, PhH–C:C) were in the ratio 2:3:6:9 (Fig. 1b). These results were
in agreement with the structures of the silicon-containing arylacetylenic monomers
[20–23].
The carbon spectra of the two monomers, with band assignments are shown in
Fig. 2. The acetylenic carbons appeared as a pair of resonances at 86 and 107 ppm
as they were bonded to the silicon and phenyl units of the two monomers. The
phenyl carbon bonded to C:C and the side-chain phenyl carbon bonded to Si were
observed at 128–134 ppm. For VTPES (Fig. 2a), 137 ppm corresponded to carbon
attached to C=C. The aromatic carbons bonded to C:C and vinyl carbons bonded
to Si were similar and difficult to separate between 128 and 134 ppm, because of
similar chemical environments. Accurate chemical shift information will be
investigated in the future.
29Si NMR spectra of two monomers are shown in Fig. 3. The two signal peaks
assigned to the silicon were observed at -71.34 ppm (VTPES) (Fig. 3a) and
-69.15 ppm (PTPES) (Fig. 3b) because of the different chemical environments
[18]. On the basis of the 1H NMR, 13C NMR and 29Si NMR spectra we can see that
123

Kinetics of two silicon-containing arylacetylenic monomers
835
Fig. 1 1H NMR spectra of VTPES (a) and PTPES (b)
the chemical shift of phenylethynyl bonded to Si has not been affected dramatically
by vinyl and phenyl, probably because of small steric and inductive effects in the
silicon-containing monomers [22].
123

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D. Tan et al.
Fig. 2 13C NMR spectra of VTPES (a) and PTPES (b)
Non-isothermal curing kinetics of the two monomers
Cure reactivity of the two monomers
The heat curing behavior of VTPES and PTPES was examined by DSC (Fig. 4).
The monomers gave distinct endothermic peaks at 84 and 116 °C corresponding to
123

Kinetics of two silicon-containing arylacetylenic monomers
837
Fig. 3 29Si NMR spectra of VTPES (a) and PTPES (b)
their melting points. The curing temperatures of the two monomers were also shown
in Table 1. It was noticed that with increasing heating rate, the curing time
decreased, the peak became sharp, and the thermal curing temperature moved to the
high-temperature region (Fig. 5). This is because with increasing heating rate, the
123

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D. Tan et al.
Fig. 4 DSC scans of VTPES and PTPES at a heating rate of 5 °/min
thermal effect per unit time increased, and a larger temperature difference was
generated. As a result, the endothermic peak was shifted to higher temperature
region. At the same time, we could see that the peak temperature of polymerization
(Tp) of VTPES was 316 °C, which was 47 °C lower than that of PTPES (Table 1),
and the reactive activation energy of VTPES (114.25 kJ/mol) was also 40 kJ/mol
lower than that of PTPES. It is well known that thermal polymerization of
phenylethylene monomers is a radical mechanism [24, 25]. Thus, it was
theoretically possible to deduce that radicals formed by VTPES would be stabilized
to a greater extent and more active than those formed by PTPES, and the
coordination role reduced the reactive energy barrier at the unitary level between
double bonds and acetylenic bonds.
Non-isothermal curing kinetics analysis of the two monomers by DSC
The generation of accurate time–temperature-degree of conversion curves is of
great practical importance for establishing optimum cure schedules. Mechanisti-
cally, evaluation of the activation energy of the cure reaction could give valuable
information about barriers to reaction and about mechanisms. The dynamic DSC
method was used in this study to obtain kinetic data on the basis of the variation in
the peak exotherm temperature (Tp) with heating rate (u). A simple relationship
between activation energy, heating rate, and peak exotherm temperature was based
on the work of Kissinger, which had been shown to be effective in calculation of
cure kinetics data [26]. The general rate equation is:
123

Kinetics of two silicon-containing arylacetylenic monomers
839
Table 1 Characteristics of non-isothermal DSC scans for the two monomers (N2, 100 mL/min)
Characteristic information
Monomers
VTPES
PTPES
5 °C (min)
Tia
289
336
Tp
316.4
341.9
10.6
362.9
381.9
9.2
Te
Dt (min)b
DH (kJ/mol)c
346.5
285.6
15 °C (min)
Ti
311
340.8
364
3.5
350
Tp
376.7
397.1
3.1
Te
Dt (min)
DH (kJ/mol)
338
288
20 °C (min)
Ti
315
362.6
392
Tp
347.8
369.4
2.7
Te
412
Dt (min)
2.5
DH (kJ/mol)
337.2
297.4
25 °C (min)
Ti
322.9
366.6
397.5
417.6
2
Tp
354.7
Te
374.9
Dt (min)
2.1
DH (J/g)
393.8
313.3
Linear-fitting equation of ln(b/T2p)*1/Tp
Correlation coefficient (R)
Linear-fitting equation of lnb*1/Tp
Correlation coefficient (R)
Activation energy, E (kJ/mol)
Frequency factor, A (1/s)
Reaction order (n)
Y = -13.75x ? 12.23
0.9978
Y = -18.51x ? 17.75
0.9977
Y = -14.9x ? 26.96
0.9984
Y = -19.83x ? 32.73
0.9975
114.25
2.82 9 109
153.89
9.42 9 1011
0.92
0.93
a
Ti initial reaction temperature; Tp peak temperature; Te eliminated reaction temperature
b
c
Dt curing time
DH enthalpy of reaction
da
n
¼ kð1 ꢁ aÞ
ð1Þ
ð2Þ
dt
k ¼ A expðꢁE=RTÞ
Where a is the degree of cure, t the reaction time, k the rate constant defined by the
Arrhenius relationship, n the order of the cure reaction, A the frequency factor, E the
123

840
D. Tan et al.
Fig. 5 DSC scans of VTPES (a) and PTPES (b) at a heating rates of 5, 15, 20, and 25 °/min
activation energy, R the universal gas constant, and T the temperature in Kelvin.
Combining Eqs. (1) and (2):
ꢀ
ꢁ
da
dt
E
n
¼ A exp ꢁ
ð1 ꢁ aÞ
ð3Þ
RT
123

Kinetics of two silicon-containing arylacetylenic monomers
841
d lnðu=Tp2Þ
dð1=TpÞ
E
ꢁ
¼
ð4Þ
R
A was found from the relationship:
A ¼
uE expðE=ðRTpÞÞ
ð5Þ
ð6Þ
RTp2
and n was determined by use of the Crane equation:
d ln u
¼ ꢁðE=nR þ 2TpÞ
dð1=TpÞ
A plot of Ln u versus 1/Tp from several DSC curves would give a straight line with
slope (-E/nR), when E/nR ꢂ 2Tp.
In our study, the cure reactions of the above-mentioned monomers were
investigated by use of the non-isothermal DSC technique at different heating rates
of 5, 15, 20, and 25 °/min under a nitrogen atmosphere with a flow rate of 100 mL/
min (Fig. 5). Table 1 shows the relationship between ln(b/T2p) and (1/Tp) or lnb and
(1/Tp), obtained by use of the Kissinger method. The numerical values were fitted to
a linear equation and the correlation coefficients were almost unity. The
corresponding activation energy, frequency factor, and reaction order were also
obtained. The results of calculation showed that the curing reactions of VTPES and
PTPES approached first-order kinetics, with reaction orders of 0.92 and 0.93,
respectively. Similar studies have been conducted on methyldi(phenylethynyl)silane
and multisubstituted acetylenic monomers; a first-order cure reaction was reported
for both, suggesting that the order n might not be a function of the substituted
groups on the resin and was probably only correlated with acetylenic bonds [27, 28].
However, the exact cure mechanism to account for the observed value of n = 1 in
these thermal polymerizations was unclear. The theoretically predicted value for
cyclotrimerization was 189 10 kJ/mol based on bond energy calculations for the
thermal polymerization of the acetylenic group [29], and the experimentally
determined heats of polymerization for the phenyl and vinyl-substituted acetylenic
monomers were 338 and 288 kJ/mol, respectively, which were higher than the
predicted enthalpy of cyclotrimerization, suggesting that trimerization of the
acetylenic groups might be the preferred reaction pathway, although trimerization of
these secondary acetylenic groups to form hexasubstituted benzene rings seemed to
be a sterically unfavorable process. Surprisingly, the frequency factor for PTPES
was two orders of magnitude larger than that for VTPES. The differences between
the activation energies and frequency factors were probably because of the existence
of relatively stable radicals and lower steric factors [30], which caused the VTPES
to react more easily. The analyzed results were in accordance with cure reactivity of
the two monomers.
At the same time, a precursor monomer suitable for preparing heat-resistant
polymer must have an adequately large processing window, i.e. a temperature range
above the melting point of the monomer and below the onset of appreciable cure
where the monomer can be held in a state of liquid flow. The ideal processing
123

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D. Tan et al.
window should be as broad as possible. Therefore practically, a processing window
of at least 5–10 °C is necessary to ensure adequate flow behavior within processing
temperature fluctuations. From Table 1, we can see that both monomers had a wide
processing window above 200 °C, and that for the PTPES monomer was
approximately 20 °C higher than that for VTPES. These broad processing windows
mean the monomers have adequate time to impregnate other materials for preparing
high-temperature polymers. Thus, the two monomers should be easily processed and
are excellent candidates for further study as high-temperature-resistant polymers.
Non-isothermal curing analysis of the two monomers by FT-IR
In order to better understand the curing process of two monomers, FT-IR spectra
were used to monitor the precise extent of the cure process and the chemistry of
various reactions taking place. Spectra were taken every 5 min by means of four
scans with a resolution of 4 cm-1. Figure 6 shows the FT-IR spectra obtained as the
two monomers were cured at different temperatures. The strong peaks at
approximately 2160 cm-1 indicate the presence of acetylenic functionality; the
absence of primary acetylenes is also apparent from the lack of absorption at
3295 cm-1 (C:C–H). Other functionality was as follows: 1486–1629 cm-1
(aromatic, C=C), 1009–1220 cm-1 (CH2–H), 756 cm-1 and 838 cm-1 (Si–C) from
curve 1. The peak at 2160 cm-1 broadened with increasing curing temperature and
vanished when the curing temperature reached 450 °C. This decrease in peak
intensity was caused by conformational distribution of the products by a
crosslinking reaction and was accompanied by an increase in intensity at
3056 cm-1 assigned to absorption of C–H bonds in the benzene ring with curing.
No new absorption peaks were observed in spectra except for the increased intensity
of the absorption peak of the benzene ring, which indicated the occurrence of a
trimerization reaction [31]. The results were in agreement with the DSC analysis,
and a small red shift in the position of the triple bonds peak occurred from 2160 to
2139 cm-1, which further confirmed occurrence of the trimerization reaction. It is
worth mentioning that the peak of VTPES at 1392 cm-1 attributed to absorption of
vinyl C–H bonds bonded to silicon showed changes and vanished eventually,
accompanied by an increase in the intensity of the 2928 cm-1 peak assigned to
absorption of methylene C–H bonds when the curing temperature was beyond
360 °C. These results suggest that, in the presence of oxygen, benzene rings are
formed from two C:C bonds, and the C=C bonds changed into C–C bonds with
curing [25]. Thus, the introduction of unsaturated groups contributed to the lower
thermal curing temperature of the phenylacetylsilanes and improved process
conditions.
Conclusions
Vinyltriphenylethynylsilane and phenyltriphenylethynylsilane were synthesized as
functional monomers. Spectral analysis confirmed the structures of the products.
123

Kinetics of two silicon-containing arylacetylenic monomers
843
Fig. 6 FT-IR spectra of VTPES (a) and PTPES (b) at different curing temperatures: (1) 25 °C, (2)
150 °C, (3) 270 °C, (4) 300 °C, (5) 330 °C, (6) 360 °C, (7) 390 °C, (8) 420 °C, (9) 450 °C
The curing reactions were consistent with a first-order reaction according to
the Kissinger method. The two monomers had a broad processing window for
preparation of highly resistant materials by cyclotrimerization. The introduction
123

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D. Tan et al.
of unsaturated groups contributed to improving the process conditions for the
phenylethynylsilane monomers.
Acknowledgments The authors gratefully acknowledge the support of the National Natural Science
Foundation of China (NSFC) (Grant No. 50773017, 50973024).
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