A new room-temperature liquid, high-performance tricyanate ester

Article abstract of DOI:10.1002/pola.24246

A new tricyanate ester monomer of a tris(4-hydroxyphenyl)benzene derivative was synthesized in 6-steps with a 63% overall yield. The geminal substitution of phenyl rings on ethane, in addition to the creation of a racemic/ diastereomeric mixture, resulted in a liquid monomer whereas compounds with similar structure and symmetry have melting points typically over 100 °C. Key properties of the polycyanurate, such as the glass transition temperature and moisture resistance, were positively influenced by the higher crosslink density provided by the monomer.

Full text of DOI:10.1002/pola.24246

A New Room-Temperature Liquid, High-Performance Tricyanate Ester
LEE R. CAMBREA,1 MATTHEW C. DAVIS,1 THOMAS J. GROSHENS,1 ANDREW J. GUENTHNER,2
KEVIN R. LAMISON,2 JOSEPH M. MABRY2
1Michelson Laboratory, Chemistry & Materials Division, Naval Air Warfare Center, China Lake, California 93555
2Air Force Research Laboratory, Propulsion Directorate, Edwards AFB, California, 93525
Received 4 June 2010; accepted 12 July 2010
DOI: 10.1002/pola.24246
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A new tricyanate ester monomer of a tris(4-hydroxy-
phenyl)benzene derivative was synthesized in 6-steps with a
63% overall yield. The geminal substitution of phenyl rings on
ethane, in addition to the creation of a racemic/diastereomeric
mixture, resulted in a liquid monomer whereas compounds with
similar structure and symmetry have melting points typically
over 100 ꢀC. Key properties of the polycyanurate, such as the
glass transition temperature and moisture resistance, were posi-
tively influenced by the higher crosslink density provided by the
monomer.
2010 Wiley Periodicals, Inc.† J Polym Sci Part A:
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Polym Chem 48: 4547–4554, 2010
KEYWORDS: cyclopolymerization; differential scanning calorime-
try; high performance polymers; thermosets
INTRODUCTION Polycyanurates (also known as ‘‘cyanate
ester resins’’) have emerged as an important class of thermo-
setting polymers in both current and developmental aero-
space, automotive, and electronics applications.1–3 Polycyanu-
rates are valued for their inherent fire resistance, low
moisture uptake, stability in cold water, and maximum use
temperatures that significantly exceed those of epoxy resins
with corresponding temperature-dependent monomer viscos-
ity characteristics.4(a) In addition, cyanate ester monomers
exhibit low toxicity, very little shrinkage or outgassing upon
cyclotrimerization, and particularly for systems that are
liquids at room temperature, favorable solubility and viscos-
ity characteristics for the addition of comonomers4(b) or
toughening agents.4(c)
temperature liquids from mixed oligomers with balanced
flexible, aromatic, and hydrophobic chemical groups by
Yameen et al.7
One of the few commercially available dicyanate ester mono-
mers that can be handled as a low-viscosity (supercooled)
R
liquid at room temperature is PrimasetV LECy.8 The mono-
mer’s low melting point (29 ꢀC)9 may be the result of the
unique thermochemical properties10 of the parent hydro-
carbon, 1,1-diphenylethane. As can be seen from Table 1, the
geminal substitution of two phenyl rings on ethane is
associated with a large depression in melting temperature
compared to various aromatic hydrocarbon analogs. This
relative trend in melting point is somewhat mirrored in the
corresponding cyanate esters.
Cyanate ester monomer systems that are liquids at room
temperature may suffer from limitations due to lowered
maximum use temperature (typically determined by the
glass transition temperature), comparatively higher moisture
uptake, brittleness, or unfavorable solubility and viscosity
characteristics for blending and/or toughening. Decreased
thermomechanical performance is often the result of incor-
porating asymmetric or highly flexible chemical functional-
ities into the monomer backbone, while the other limitations
are typically associated with highly aromatic cyanate esters
that are rendered oligomeric to prevent crystallization.
Recent efforts to overcome these limitations include the
creation of polydisperse room temperature liquids from mix-
tures of oligomers with (monodisperse) melting points well
over 100 ꢀC by Laskoski et al.,5,6 and the synthesis of room
We therefore sought to use 1,1-diphenylethane as a template
on which to build a tricyanate ester monomer, which would
be liquid at or near room temperature. An additional feature
of the approach is the creation of a racemic/diastereomeric
mixture of compounds containing multiple chiral centers.
This approach enables further depression of the melting
point12 by mixing monomers of the same molecular weight
and identical nonenantiomeric chemical functionality. It thus
maintains virtually equal reactivity of the constituent cyanate
ester groups and the solubility and viscosity characteristics
necessary for successful toughening, while reducing the need
to incorporate flexible chemical linkages and/or asymmetry
in the chemical structure of the monomer. We report on
both the results of the successful monomer synthesis and
the properties of its cured polycyanurate.
Additional Supporting Information may be found in the online version of this article. Correspondence to: M. C. Davis (E-mail: matthew.davis@navy.
mil)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4547–4554 (2010) 2010 Wiley Periodicals, Inc. †This article is a U.S. Government
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work, and as such, is in the public domain in the United States of America.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Melting Points of Parent Aromatic Hydrocarbons and Corresponding Di and Stricyanate Esters
R¼OCN
R¼H
Mp (ꢀC)a
Name
Compound Structure
Name
Mp (ꢀC)
95–96b
104
1,1,1-Tris(4-cyanatophenyl)ethane ESR-255
1,1,1-Triphenylethane
131
4,40-Dicyanatodiphenyl
Diphenyl
69–72c
50–53c
112–114d
1,2-Bis(4-cyanatophenyl)ethane
1,2-Diphenylethane
R
79
2,20-Bis(cyanatophenyl)isopropylidene PrimasetV BADCy
2,2-Diphenylpropane
Diphenylmethane
1,1-Diphenylethane
29c
108
29
4,40 Dicyanatodiphenylmethane
22–24c
ꢁ18e
R
1,1-Bis(4-dicyanatophenyl)ethane PrimasetV LECy
a
d
e
Ref. 9.
Ref. 11.
Synthesis and characterization of this compound will be published
elsewhere.
b
c
Sigma-Aldrich Chemical catalog 2003–2004.
Material Safety Data Sheet OHS33053.
EXPERIMENTAL
The density of the cured resin was determined by placing
some discs in solutions of CaCl2 (as the dihydrate) and
deionized water and varying the CaCl2 concentration until
neutral buoyancy was observed on bubble-free samples over
a period of several minutes. The density of the neutrally
buoyant solution was determined by placing 10.00 mL in a
volumetric flask (calibrated with deionized water at 20 ꢀC)
and weighing, and checked against the predicted density of
the solution at ambient temperature based on the known
concentration of CaCl2. Prior to thermomechanical analysis,
some discs were also placed in ꢂ300 mL of deionized
Materials
The 4-methoxyacetophenone, N,N-dimethylformamide dimethyl
acetal, pyridine, methyltriphenylphosphonium bromide, potas-
sium tert-butoxide, 5% Pd/C, pyridine hydrochloride, cyano-
gen bromide, triethylamine and solvents were purchased from
Sigma-Aldrich (Milwaukee) and used as received.
Measurement and Characterization
General Methods
Melting points were collected on a Mel-Temp II from Labora-
tory Devices (Holliston, MA) and are not corrected. Infrared
analysis of monomer 7 and all cured and uncured samples
were analyzed by Attenuated Total Reflection Fourier Trans-
form Infrared (ATR-FTIR) spectroscopy using a single bounce
diamond ATR crystal. The instrument used was a Nexus 870
FTIR spectrometer with a liquid N2 cooled mercury cadmium
telluride (MCTA) detector. Each spectrum is an average of 28
scans at 4 cmꢁ1 resolution. All NMR data were collected on a
Bruker Avance II 300 MHz spectrometer. Nuclear magnetic
resonance data (free-induction decay’s) were processed using
NUTS software from Acorn NMR (Livermore, CA). All spectra
are referenced to solvent or tetramethylsilane. Elemental ana-
lyses were performed by Atlantic Microlab, (Norcross, GA).
ꢀ
water at 85 C for 96 h with sample dimensions and weight
measured before and after exposure.
Differential scanning calorimetry (DSC) was performed on
ꢂ10 mg of monomer reserved after degassing using a TA
Instruments Q2000 calorimeter under 50 mL/min of flowing
nitrogen. Samples were heated to 350 ꢀC, then cooled to
100 ꢀC and reheated to 350 ꢀC, all at 10 ꢀC/min. Thermo-
gravimetric analysis (under both nitrogen and air) was per-
formed on ꢂ10 mg samples of the cured discs that were
removed immediately after cure, using a TA Instruments
Q5000 analyzer, at a gas flow rate of 10 mL/minꢀ(balance)
and 25 mL/min (purge), heating the samples at 5 C/min to
600 ꢀC followed by a 2-h isothermal hold. The discs were
then tested via dynamic thermomechanical analysis (dynamic
TMA) with a TA Instruments Q400 series analyzer under
50 mL/min of nitrogen flow. The discs were initially held in
place via a 0.2 N compressive force with the standard
ꢂ5 mm diameter flat cylindrical probe. The force was then
modulated at 0.05 Hz over an amplitude of 0.1 N (with a
mean force of 0.1 N) and the temperature wꢀas ramped twice
(heating and cooling) between 100 and 200 C (to determine
Thermal Analysis
Prior to cyclotrimerization, monomer
7 was partially
ꢀ
degassed along with a silicone mold at 95 C for 30 minutes
under reduced pressure (300 mm Hg). The monomer was
then poured into the mold and cured under flowing nitrogen
for 1 h at 150 ꢀC followed by 24 h at 210 ꢀC to produce
void-free discs measuring ꢂ13.7 mm in diameter and 1.6 mm
thick and weighing 200–300 mg. The temperature ramp rate
ꢀ
ꢀ
thermal lag) with a final heating to 350 C, all at 10 C/min.
For samples previously exposed to hot water, the heating
ꢀ
during cure was 5 C/min.
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ARTICLE
TABLE 2 Crystal Structure Data and Refinement Details
for Compound 4
(160.22 g) as yellow microcrystals after washing with Et2O.
The filtrate was rotary evaporated leaving a second crop of
product (181.3 g). The title compound was obtained as light
yellow plates from toluene. Yield: 341.5 g (83%). Mp: 82–
85 C (lit. 90 C). H NMR (300 MHz, CDCl3, d, ppm): 7.91
(d, J ¼ 8.8 Hz, 2H), 7.76 (d, J ¼ 12.5 Hz, 1H), 6.90 (d, J ¼
8.8 Hz, 2H), 5.69 (d, J ¼ 12.4 Hz, 1H), 3.81 (s, 3H), 2.96 (bs,
6H). 13C NMR (75 MHz, CDCl3, d, ppm): 187.15, 161.86,
153.66, 133.07, 129.31, 113.21, 91.59, 55.21. Anal. calcd for
Compound
4
13
1
ꢀ
ꢀ
Empirical formula
Formula weight
Temperature (K)
Crystallization Solvent
Crystal system
C
33H30O3
474.57
296(2)
2-propanol
Monoclinic
P 1 21 1
C12H15NO2: C, 70.22; H, 7.37; N, 6.82. Found: C, 70.38; H,
Space group
7.33; N, 6.79.
ꢀ
˚
Unit cell dimensions (A,
)
1,3,5-Tris(4-methoxybenzoyl)benzene (3)
a
b
c
a
b
10.0023(2)
A 2-L round-bottomed flask equipped with magnetic stirbar
and reflux condenser was charged with glacial HOAc (683
mL, 11.4 mol) followed by pyridine (171 mL, 2.16 mol)
and then 3-(dimethylamino)-40-methoxyacrylophenone (2)
(341.5 g, 1.66 mol) was added. The mixture was refluxed for
12 h. The mixture was allowed to cool to RT whereby copi-
ous solids precipitated. An addition funnel was equipped and
H2O (300 mL) was added dropwise with vigorous stirring.
The mixture was filtered on a medium porosity glass frit to
obtain a light tan, granular solid that was recrystallized from
toluene. Yield: 223.3 g (84%). Mp: 179–183 ꢀC (lit.14 175–
9.2100(2)
14.5901(5)
90ꢀ
100.7870(10)ꢀ
c
90ꢀ
3
˚
Volume (A )
1320.31(7)
Z
2
Density (Mg/cm3)
Crystal size (mm)
Reflections collected
Independent reflections
Completeness (%)
1.194
0.17 ꢃ 0.20 ꢃ 0.47
25409
1
ꢀ
176 C). H NMR (300 MHz, DMSO-d6, d, ppm): 8.14 (s, 3H),
7.83 (d, J ¼ 8.8 Hz, 6H), 7.08 (d, J ¼ 8.8 Hz, 6H), 3.84 (s,
9H). 13C NMR (75 MHz, DMSO-d6, d, ppm): 192.97, 163.43,
138.03, 132.89, 132.44, 128.73, 114.10, 55.63. Anal. calcd for
4675
100.0
Data/restraints/parameters
Goodness-of-fit on F2
Final R [I > 2sigma(I)]
4675/1/329
C30H24O6: C, 74.99; H, 5.03. Found: C, 75.03; H, 4.80.
1.024
R1 ¼ 0.0373, wR2 ¼ 0.1012
1,3,5-Tris(1-(4-methoxyphenyl)vinyl)benzene (4)
A 250 mL round-bottomed flask equipped with magnetic
stirring bar and reflux condenser was charged with methyl-
triphenylphosphonium bromide (43.48 g, 122 mmol, 3.5
equiv), KOtBu (13.7 g, 122 mmol, 3.5 equiv) and THF
(300 mL). After 1 h, 1,3,5-tris(4-methoxybenzoyl)benzene
(3) (16.73 g, 34.8 mmol) was added portionwise over
30 min. After the addition, the mixture was refluxed for 1 h.
Upon cooling to RT, H2O (50 mL) was added and the mixture
was neutralized by adding HOAc (1 g, 17 mmol, 0.5 equiv).
The organic layer was separated and washed with brine
(50 mL). The organic layer was dried over anhydrous MgSO4
and rotary evaporated to a thick oil, which slowly crystal-
lized. The crude was dissolved in hot iPrOH (200 mL) and
allowed to cool, whereby the title compound crystallized.
Filtration on a medium porosity glass frit gave the title
compound as white needles. Yield: 13.29 g (97%). Mp: 116–
118 ꢀC. 1H NMR (300 MHz, CDCl3, d, ppm): 7.30 (d, J ¼
8.8 Hz, 6H), 7.28 (s, 3H), 6.85 (d, J ¼ 9 Hz, 6H), 5.36 (d, J ¼
1.0 Hz, 3H), 5.33 (d, J ¼ 1.0 Hz, 3H), 3.82 (s, 9H). 13C NMR
(75 MHz, CDCl3, d, ppm): 159.56, 149.43, 141.77, 133.88,
129.48, 128.08, 113.78, 113.33, 55.49. Anal. Calcd for
ꢀ
rate was increased to 20 C/min and the segments were run
in reverse order to minimize drying before determination of
the glass transition temperature. All reported dynamic TMA
temperatures are corrected for thermal lag (typically 3–10
ꢀC) determined separately for each sample.
X-Ray Structure Determination of 4
Some selected data and refinement details are collected in
Table 2. CCDC 776623 (4) contains the supplementary crys-
tallographic data for this article. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif, by e-mailing
data_request@ccdc.cam.ac.uk, or by contacting CCDC 12
Union Road, Cambridge CB2 1EZ, UK; fax: þ44 1223
336033.
Synthesis of the Monomer
3-(Dimethylamino)-40-methoxyacrylophenone (2)
A 2-L round-bottomed flask equipped with magnetic stirbar
and reflux condenser was charged with 4-methoxyacetophe-
none (300.4 g, 2 mol) and N,N-dimethylformamide dimethyl
acetal (358 g, 3 mol, 1.5 equiv) and the mixture was
refluxed. The reaction progress was followed by 1H NMR
analysis and was complete after 48 h reflux. Shortly after
cooling to RT the reaction mixture solidified. Filtration on a
medium porosity glass frit gave a first crop of product
C33H30O3: C, 83.51; H, 6.37. Found: C, 83.31; H, 6.35.
Racemic 1,3,5-Tris(1-(4-methoxyphenyl)ethyl)benzene (5)
mixture of 1,3,5-tris(1-(4-methoxyphenyl)vinyl)benzene
A
(4) (19.61 g, 41 mmol), 5% Pd/C (500 mg), THF (50 mL),
R
and EtOH (50 mL) was hydrogenated (50 torr) on a ParrV
apparatus for 5 h. After this time the uptake of hydrogen
HIGH-PERFORMANCE TRICYANATE ESTER, CAMBREA ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ceased. The mixture was filtered through diatomaceous earth
to remove the catalyst and rotary evaporated leaving the title
compound as a colorless, viscous oil. Yield: 19.6 g (100%).
1H NMR (300 MHz, CDCl3, d, ppm): 7.06 (d, J ¼ 8.7 Hz, 6H),
6.88–6.84 (m, 3H), 6.79 (d, J ¼ 8.9 Hz, 6H), 4.0 (q, J ¼
7.5 Hz, 3H), 3.78 (s, 9H), 1.52 (d, J ¼ 7.3 Hz, 9H). 13C NMR
(75 MHz, CDCl3, d, ppm): 157.16, 145.83, 145.78, 145.74,
138.03, 137.99, 137.97, 127.55, 123.54, 123.46, 123.41,
113.36, 54.61, 42.94, 42.92, 21.29. Anal. calcd for C33H36O3:
C, 82.46; H, 7.55. Found: C, 82.23; H, 7.40.
Racemic 1,3,5-Tris(1-(4-hydroxyphenyl)ethyl)benzene (6)
A 250 mL round-bottomed flask equipped with magnetic
stirring bar was charged with racemic 1,3,5-tris(1-(4-meth-
oxyphenyl)ethyl)benzene (5) (19.6 g, 41 mmol) and pyridine
hydrochloride (56.8 g, 0.49 mol, 12 equiv). The mixture was
protected with an N2 bubbler and heated in a 180 ꢀC oil
bath for 5 h. The miꢀxture was allowed to cool briefly to
ꢀ
ꢂ120 C and then 55 C H2O (100 mL) was added slowly to
the mixture with vigorous stirring. The mixture was
extracted three times with EtOAc (100 mL portions). The
collected organic phase was washed with brine (200 mL)
and dried over anhydrous MgSO4. Rotary evaporation gave a
thick, colorless oil that foamed under vacuum. Yield: 17.17 g
SCHEME 1 Synthetic route to monomer 7.
1
(95%). H NMR (300 MHz, DMSO-d6, d, ppm): 9.12 (s, 3 OH),
RESULTS AND DISCUSSION
6.98 (d, J ¼ 8.6 Hz, 6H), 6.90 (s, 3H), 6.63 (d, J ¼ 8.6 Hz,
6H), 3.92 (q, J ¼ 7.2 Hz, 3H), 1.44 (d, J ¼ 7.5 Hz, 9H). 13C
NMR (75 MHz, DMSO-d6, d, ppm): 155.35, 146.59, 146.53,
146.48, 136.69, 136.67, 136.65, 128.04, 123.88, 123.65,
114.96, 43.49, 43.46, 22.09, 22.04. Anal Calcd for C30H30O3 ꢄ
0.65 H2O: C, 80.02; H, 7.01. Found: C, 80.38; H, 7.41.
Synthesis
The synthesis of tricyanate ester 7 is shown in Scheme 1.
Condensation of 4-methoxyacetophenone (1) with N,N-dime-
thylformamide dimethyl acetal by the reported method15
gave the enaminoketone 216 in good yield. Cyclotrimerization
of 2 using conditions of Elghamry17 gave the 1,3,5-triben-
zoylbenzene 318–20 in good yield. Triple Wittig reaction21,22
of 3 with methyltriphenylphosponium bromide using potas-
sium tert-butoxide as base gave the new triene 4 but only
after partially selective recrystallization from heptane
followed by silica gel chromatography to remove the byprod-
uct triphenylphospine oxide (TPPO). It was discovered that
the three equivalents of TPPO could be completely removed
from 4 simply by recrystallizing the crude product from iso-
Racemic 1,3,5-Tris(1-(4-cyanatophenyl)ethyl)benzene (7)
A 250 mL round-bottomed flask equipped with magnetic
stirring bar was charged with racemic 1,3,5-tris(1-(4-
hydroxyphenyl)ethyl)benzene (6) (4.36 g, 99 mmol), cyano-
gen bromide (3.68, 34.7 mmol, 3.5 equiv) and anhydrous
acetone (50 mL). The mixture was cooled in a ꢁ20 ꢀC bath
before dropwise addition of TEA (2.99 g, 297 mmol, 3 equiv).
Near the end of the addition, copious solids (TEAꢄHBr) pre-
cipitated. After 1 h, ice cold H2O (100 mL) and CH2Cl2
(100 mL) were added and the phases were separated. The
organic layer was washed with cold saturated NaHCO3
(50 mL) and then brine (50 mL). After drying over anhy-
drous MgSO4, the solvent was evaporated leaving a viscous,
pale yellow oil. The crude product was dissolved in CH2Cl2
and filtered through a pad of silica gel and rotary evaporated
to give the title compound in analytically pure form as a
thick, colorless oil. Yield: 5.0 g (98%). 1H NMR (300 MHz,
CDCl3, d, ppm): 7.21 (s, 12 H), 6.84–6.81 (m, 3H), 4.09 (q,
J ¼ 7.2 Hz, 3H), 1.57 (d, J ¼ 7.2 Hz, 9H). 13C NMR (75 MHz,
CDCl3, d, ppm): 151.44, 146.23, 146.19, 146.15, 145.45,
145.42, 145.39, 129.66, 125.13, 125.08, 115.47, 109.14,
44.29, 22.16. FTIR (KBr, cmꢁ1): 2263 (OCN), 2233 (OCN).
Anal Calcd for C33H27N3O3: C, 77.17; H, 5.30; N, 8.18. Found:
C, 77.17; H, 5.32; N, 8.33.
propanol as demonstrated by Carey and coworkers.23
A
crystal structure for triene 4 was obtained, Figure 1, along
with some selected crystallographic data in Table 2. In the
following step, catalytic hydrogenation of 4 reduced the
methylene bridges and gave liquid product 5 presumably as
a statistical mixture (1:3) of a pair of racemic diastereomers
(R,R,R/S,S,S:S,S,R/R,R,S).24 Demethylation of 5 to give the
glass-like25 tris(4-hydroxyphenyl) derivative 626 by molten
pyridine hydrochloride27 was a clean and convenient proce-
dure when compared to the typical boron tribromide method.19
Finally, cyanation of 6 with cyanogen bromide in the presence
of triethylamine gave the tricyanate ester 7 in excellent yield.
Traces of the impurity diethylcyanamide, from von Braun deg-
radation, could be eliminated by maintaining the reaction tem-
perature below ꢁ10 C. The H and 13C NMR of 7 is shown in
1
ꢀ
Figure 2. Although, 1H NMR did not indicate a mixture, 13C
NMR clearly shows three multiplets in the aromatic region
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ARTICLE
Characterization
Based on a comparison of qualitative flow characteristics
with other epoxy and cyanate ester resins of known viscos-
ity, the room temperature viscosity of monomer 7 was
estimated to be on the order of 10,000 cP. While about
two orders of magnitude thicker than the analogous dicya-
R
nate ester PrimasetV LECy, the viscosity is similar to low mo-
lecular weight ‘‘novolac’’ type tricyanate esters (such as
R
PrimasetV PT-15).28 Although storage for several days at
ꢁ20 ꢀC induced a hazy appearance, no other signs of phase
separation or crystallinity could be detected. Based on these
physical characteristics and the structural similarity to the
R
readily toughened PrimasetV LECy, monomer 7 should be
relatively easy to toughen via the addition of thermoplastic
modifiers. Taken together, these results indicate that mono-
mer 7 exhibits very favorable processing characteristics for a
tricyanate ester.
FIGURE 1 Crystal structure of 4.
The cure characteristics of monomer 7 also compared favor-
ably to that of typical cyanate esters. Figure 3 presents DSC
data for the uncatalyzed cyclotrimerization of monomer 7
caused by heating the sample at 10 ꢀC/min. The peak exo-
therm temperature of 323 ꢀC provided corroboration that a
attibuted to the diasteromeric nature of monomer 7. No
attempts were made to separate and characterize the diastereo-
mers. The product was found to be a viscous oil at room tempera-
ture or even several months at ꢁ20 ꢀC.
FIGURE 2 1H and 13C NMR for monomer 7 with expanded regions for clarity.
HIGH-PERFORMANCE TRICYANATE ESTER, CAMBREA ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Dynamic TMA of monomer 7 after cure at 210 ꢀC
FIGURE 3 DSC traces of monomer 7 illustrating that nearly
complete cure was achieved without the addition of catalyst at
210 ꢀC. The curves have been offset on the vertical axis for
convenience.
and storage under vacuum.
that nearly complete cure was obtained while avoiding expo-
sure to temperatures above 210 ꢀC. In contrast, a similarly
R
complete cure of PrimasetV LECy could not be achieved
near quantitative conversion of the hydroxyl groups to
cyanate ester during the preparation of compound 7 was
achieved. The enthalpy of polymerization (given by the area
under the large peak) was approximately ꢁ660 J/g, or
ꢁ110 kJ/cyanate equiv, which equaled the high end of the
under identical conditions (see Supplemental Information),
R
and resins of the ‘‘novolac’’ type (PrimasetV PT-30) often
require significantly higher post-cure temperatures to
achieve full cure.29
R
range of values typically reported for PrimasetV BADCy8
Thermomechanical analysis (results shown in Figs. 5 and
6) revealed that cured 7 exhibited good resistance (for a
polycyanurate) to degradation by hot water. Figure 5 com-
pares the storage and loss components of the stiffness (i.e.,
of the oscillatory force amplitude divided by oscillatory dis-
placement amplitude) obtained for cured 7 (uncatalyzed,
with a final cure temperature of 210 ꢀC as described in the
Experimental section) after storage under vacuum. Based
on the tan delta peak shown in the figure, the ‘‘dry’’ Tg of
cured 7 was near 305 ꢀC, in agreement with the value
from determined by DSC, and also similar to the value
reported for the polycyanurate from the experimental com-
pound 1,1,1-tris(4-cyanatophenyl)ethane (also known as
ESR-255, a tricyanate ester developed by Shimp).30 Note
that because the contact area between the probe and the
sample cannot be determined precisely in this type of TMA
measurement, quantitative modulus values were not calcu-
lated, and only qualitative interpretation of the stiffness
data is warranted.
(90–110 kJ/cyanate equiv), and was somewhat higher on a
molar equivalent basis than the value measured for
R
PrimasetV LECy using the same method (ꢁ730 J/g or
ꢁ93 kJ/cyanate equiv).9 The discrepancy may have been due
to a small amount of exothermic degradationꢀin monomer
7 as it cured at temperatures approaching 350 C.
To avoid any chance of thermal degradation, the uncatalyzed
ꢀ
sample was successfully cured at 150 C for 1 h followed by
ꢀ
210 C for 24 h. The lower DSC traces of Figure 3 show that
the Tg of the system cured at 210 ꢀC (and then heated to
350 ꢀC in the DSC) was ꢂ305 ꢀC. The residual cure exo-
therm (ꢂꢁ20 J/g) amounted to less than 3% of the total
monomer cure exotherm, while the ATR-FTIR spectrum of
the cured sample, shown in Figure 4 revealed essentially no
residual cyanate ester groups (which appeared clearly in
the uncured monomer as the characteristic doublet near
2250 cmꢁ1) but showed strong bands for the cyanurate ring
(near 1360 and 1560 cmꢁ1). These combined results prove
FIGURE 4 ATR-IR spectra of monomer 7 before and after cure
at 210 ꢀC, normalized to the aliphatic CAH stretch region.
FIGURE 6 Dynamic TMA of monomer 7 after cure at 210 ꢀC
and immersion in water for 96 h at 85 ꢀC.
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ARTICLE
be 2.76 ꢃ 1021/cc, considerably higher than the ꢂ1.8 ꢃ
R
1021/cc for fully cured PrimasetV LECy based on its cured
density. A higher density of physical crosslinks can lead to
brittleness in polycyanurates, but our admittedly qualitative
experience during cutting and handling of cured samples of
R
both PrimasetV LECy and monomer 7 did not reveal any
noticeable differences in brittleness. Though a complete
investigation of the subject is beyond the scope of this work,
the probable ease of creating toughened polycyanurates from
monomer 7 could likely be used to compensate for any
increased brittleness.
FIGURE 7 TGA of monomer 7 after cure at 210 ꢀC.
CONCLUSIONS
A successful six step synthesis was carried out to produce
the racemic tricyanate ester monomer 1,3,5-tris(1-(4-cya-
natophenyl)ethyl)benzene (7) in 63% overall yield. The mono-
mer exhibited favorable processing characteristics, including
moderate viscosity as a stable liquid at room temperature,
and the ability to be cured to at least 97% conversion using
a final cure temperature of 210 ꢀC, without added catalyst.
Both the dry and ‘‘wet’’ glass transitioꢀn temperature of the
resultant polycyanurate (305 and 254 C, respectively) were
comparable with those of polycyanurates made from previ-
ous tricyanate esters, such as 1,1,1-tris(4-cyanatophenyl)-
ethane (ESR-255), which melts at 104 ꢀC. Thus, both the
favorable processing characteristics of cyanate esters based
on 1,1-diphenylethane (Bisphenol E) repeat units and the
enhanced thermomechanical performance in wet environ-
ments associated with tricyanate esters were realized in a
single monomer.
Figure 6 compares the storage and loss components of the
stiffness for a separate sample of cured 7 immersed in water
at 85 ꢀC for 96 h. For the ‘‘wet’’ sample, the Tg was around
255 ꢀC similar to the ‘‘wet’’ HDT reported for ESR-255.30 It
should be noted that after heating to temperatures signifi-
cantly higher than its boiling point, most of the water evapo-
rated from the samples prior to measurement of Tg, as con-
firmed by weighing experiments done during interrupted
TMA tests performed on separate samples. For cyanate
esters, however, the reversible decrease in Tg that accompa-
nies plasticization by water is known to be smaller and less
important than the irreversible decrease associated with
polycyanurate-network degradation. The increase in the
storage component of the stiffness during heating seen for
the ‘‘wet’’ sample was likely related to the gradual loss of
water from the sample. However, since such losses create
dimensional instabilities in the sample, only a rough corre-
spondence between the observed stiffness and the sample
storage modulus as a function of temperature is likely under
these conditions.
Financial support through an In-house Laboratory Independ-
ent Research award from the Office of Naval Research, and
funding from the Air Force Office of Scientific Research, is
gratefully acknowledged. Thanks to Dan Bliss (NAWCWD) for
assistance with thermogravimetric analyses.
For comparison purposes, both ‘‘dry’’ and ‘‘wet’’ TMA tests
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were run on PrimasetV LECy, cured with and without added
catalyst, (data shown in the Supplemental Information), and
resulted in apparent Tg values of 280–285 ꢀC (dry) and
approximately 190–230 ꢀC (wet). The ‘‘wet’’ Tg values were
approximate due to outgassing that took place during rapid
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