Polycyanurate networks with enhanced segmental flexibility and outstanding thermochemical stability

Article abstract of DOI:10.1021/ma302300g

The synthesis and physical properties of cyanurate networks formed from two new tricyanate monomers, 1,3,5-tris[(4-cyanatophenylmethyl]benzene and 3,5-bis[(4-cyanatophenylmethyl)]phenylcyanate, are reported and compared to those of 1,1,1-tris[(4-cyanatoph

Full text of DOI:10.1021/ma302300g

Article
pubs.acs.org/Macromolecules
Polycyanurate Networks with Enhanced Segmental Flexibility and
Outstanding Thermochemical Stability
Andrew J. Guenthner,*,† Matthew C. Davis,‡ Michael D. Ford,‡ Josiah T. Reams,§ Thomas J. Groshens,‡
Lawrence C. Baldwin,‡ Lisa M. Lubin,∥ and Joseph M. Mabry†
†Air Force Research Laboratory, Propulsion Directorate, Edwards AFB, California 93524, United States
‡Naval Air Warfare Center, Weapons Division, China Lake, California 93555, United States
§National Research Council/Air Force Research Laboratory, Edwards AFB, California 93524, United States
∥ERC Incorporated, Edwards AFB, California 93524, United States
S
* Supporting Information
ABSTRACT: The synthesis and physical properties of cyanurate networks formed from two new tricyanate monomers, 1,3,5-
tris[(4-cyanatophenylmethyl]benzene and 3,5-bis[(4-cyanatophenylmethyl)]phenylcyanate, are reported and compared to those
of 1,1,1-tris[(4-cyanatophenyl)]ethane (also known as ESR-255). All three networks possessed somewhat different aromatic
contents and cross-link densities; however, the thermochemical stability of these networks, as determined by TGA, was
outstanding, with that of 1,3,5-tris[(4-cyanatophenylmethyl)]benzene being among the best known for organic cyanate esters
despite its comparatively high segmental flexibility. Moreover, the moisture uptake of cured 1,3,5-tris[(4-cyanatophenylmethyl)]-
benzene, at 2.2% after 96 h immersed in 85 °C water, was comparatively low for a cyanate ester network with a glass transition
temperature of 320 °C at full cure. When cured for 24 h at 210 °C, the dry glass transition temperatures of the networks ranged
from 245 to 285 °C, while the wet glass transition temperatures ranged from 225 to 240 °C. The similarity in glass transition
temperatures resulted from a lower extent of cure in the networks with more rigid segments. In essence, for networks with very
high glass transition temperatures at full cure, the process conditions, rather than the rigidity of the network, determined the
attainable glass transition temperature. Because networks with a higher extent of cure tend to exhibit slower long-term
degradation, in this case, the networks with greater segment flexibility enabled superior performance despite exhibiting a lower
glass transition temperature at full cure. These results illustrate that, in contrast to the prevailing heuristics for improving the
performance of high-temperature thermosetting polymer networks, a more flexible network with a lower glass transition tem-
perature at full cure can offer an optimal combination of thermomechanical and thermochemical performance.
ideally suited to the study of high-performance thermosetting
networks, such as the highly selective formation of a single cross-
linked network product10 (a polycyanurate network), no need to
balance stoichiometry, effective control of polymerization rate
through a wide range of available catalysts, low shrinkage on
cure that minimizes residual stresses,11,12 outstanding short-term
thermal stability of the network, and simple verification of the
chemical structure of the insoluble cured network (including
quantitative information on side product formation) through
INTRODUCTION
■
Tightly cross-linked macromolecular networks with maximum
continuous use temperatures above 175 °C, formed by thermal
cure of polyfunctional monomers, represent an extremely important
class of polymeric structural and adhesive materials, with a
broad range of applications in the electronics, energy storage,
and aerospace industries. Among these, cyanate ester resins1−3
(R−O−CN) made from trifunctional monomers have been
studied4,5 as potential materials for applications ranging from aircraft
engine turbine brush seals6−8 to nanostructured biosensors,9
due to their excellent high-temperature mechanical perform-
ance combined with ease of processing. However, cyanate ester
monomers also possess unusual characteristics that make them
Received: November 6, 2012
Revised: November 20, 2012
Published: November 30, 2012
© 2012 American Chemical Society
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FT-IR spectroscopy.13 As a result, studies of the relationships
among structure, processing, and properties involving poly-
cyanurate networks can provide significant insights into the
properties of thermosetting networks in general.
When developing new high-performance thermosetting
monomers, it has generally been assumed that the glass tran-
sition temperature (Tg) of the fully cured network (herein
referred to as a “fully-cured Tg”) should be as high as possible,
provided that an acceptable degree of toughness can be
maintained. For single-components networks with one well-
defined type of chemical cross-link, the fully cured Tg depends
on the density of cross-links and branch points, along with the
rigidity of the network segments, but not on the details of the
cure process. As a result, the “fully cured” Tg provides a useful
quantitative (though limited) indication of the relative flexibility
of network segments. For cyanate esters such as Primaset
PT-30 with fully cured Tg’s as high as 400 °C,14 however, complete
conversion of the monomer to cyanurate may require undesirably
high temperatures.15 Therefore, the network properties depend
on the “as cured” rather than the fully cured Tg. In many
thermosetting materials, vitrification (the point when network
development drives the glass transition temperature above the
cure temperature) has been assumed to result in cessation of
cure, resulting in an “as cured” Tg that is roughly the same as
the maximum cure temperature. As a result of these assumptions,
the long-established approach to developing high Tg thermosetting
resins has been to (1) make the network segments as rigid, and
cross-link densities as high, as constraints imposed by brittle-
ness permit and (2) utilize the maximum permissible cure
temperature. Even though cyanate esters have been known for
many years to violate the assumptions relating the Tg and maxi-
mum cure temperature,16 the approach to developing cyanate
esters has been the same as the one outlined above for high-
temperature thermosetting networks in general.
Figure 1. Schematic illustration of the envisioned effect of network
segment flexibility on the reactivity of cyanate esters in the glassy state.
After vitrification, portions of the network away from the chain ends
(indicated by the darker ellipses) exhibit highly restricted motion, and
the ability of the reactive groups to move into the correct position and
orientation for further cure depends on the local motions of the nearby
chain segment, as illustrated by the arrows. (a) A portion of the
network formed from the rigid monomer ESR-255, having a limited
range of motion. (b) A portion of the network formed from the more
flexible monomer 1 (see Scheme 1) that exhibits a greater range of
plausible motion due to a larger number of more freely rotating bonds.
Recently, however, it has become more widely appreciated
that in thermosetting networks slow conversion of monomer
continues to take place in the glassy state17 and thus that “as
cured” Tg’s may significantly exceed maximum cure temper-
atures. The kinetics of cure for both epoxy and cyanate ester
resins near Tg have been studied recently.18,19 However, many
aspects of cure well below Tg remain relatively unexplored.
From a performance standpoint, though, the highly desirable
combination of good thermomechanical performance (which
demands a high Tg) and affordable processing (which requires
low cure temperatures) means that a deeper understanding of
cure well below Tg will become increasingly important.
Well below Tg, large-scale molecular motion in the devel-
oping network is generally absent. As illustrated in Figure 1, the
ability for cure to proceed in a system containing reactive
groups only at chain ends is likely to depend strongly on
the degree of localized motion available to these chain ends.
The flexibility of the individual network segments thus plays a
key role in either facilitating or hindering the formation of networks
with very high levels of conversion. The incorporation of
flexible chemical linkages that enable such motions, however,
tends to result in a lower fully cured Tg. Because of the historical
considerations outlined earlier, the concept of incorporating more
flexible network segments to facilitate cure well below Tg has thus
far received little consideration in the development of high-
temperature thermosetting polymers. The trade-offs, however,
between achieving full cure and maintaining a high Tg are
technologically quite important because the long-term hydro-
lytic stability of cyanate esters, for instance, depends critically
on achieving close to 100% conversion,20 while the toughness
of cyanate esters has been shown to more than double when
conversion is increased from 95% to near 100%.21 Despite their
importance, these trade-offs have not been explored systemati-
cally in many types of thermosetting resins.
We recently compared the performance of the commercial
polycyanurate Primaset PT-30, which forms a highly rigid
network, with a more flexible analogue, and found that many,
though not all, of the expected disadvantages associated with
more flexible network segments were mitigated by the relative
ease of achieving close to complete conversion in the more
flexible network.22 However, we also found that the thermo-
chemical stability of the more flexible network was significantly
reduced. In addition to degrading application-related perform-
ance, the reduction in thermal stability prevented a clear deter-
mination of the fully cured Tg of the more flexible network. A
thermochemically more stable network with enhanced segment
flexibility was thus needed to systematically understand the
trade-offs between segment flexibility, “as cured” and fully cured
Tg, and performance.
As reported in this paper, a newly synthesized cyanate ester
monomer (1) affords enhanced network segment flexibility with
no loss in thermochemical stability compared to more rigid
analogues (see Chart 1 for chemical structures). The synthesis
of this monomer, along with the synthesis of more rigid single-
component analogues, thus enabled a clear comparison of
the effect of network segment flexibility on both extent of cure
and Tg. The comparisons indicated that the “as cured” Tg
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the enaminoketone generated from acetophenones and ex-
pensive dialkyl acetals of dimethylformamide.23,24 In this case,
p-methoxyacetophenone was condensed with methyl formate
using potassium tert-butoxide as base to give the potassium salt
4 in good yield.25−27 This salt was then cyclotrimerized in hot
glacial acetic acid to give 5 in good yield.28 Reduction of the
carbonyls by sodium borohydride gave the diastereomeric mixture of
triol 6. Hydrogenolysis of 6 by catalytic transfer hydrogenation29−32
gave the unreported symmetrical tribenzylbenzene 7. Deme-
thylation of 7 by molten pyridine hydrochloride gave the
triphenol 8.33 Single-crystal X-ray analysis proved the structure
of 8 (see Supporting Information Section S1). Finally, 8 was
cyanated with cyanogen bromide and triethylamine base to
give tricyanate 1 at 30% overall yield in the six steps from p-
methoxyacetophenone. NMR spectra for monomers 1, 2, and
ESR-255 are shown in Supporting Information Section S1.
The synthesis of tricyanate 2 is shown in Scheme 2. 5-
Hydroxyisophthalic acid was heated with dimethyl sulfate and
potassium carbonate in acetone to give the trimethyl derivative
9.34 Hydrolysis by refluxing in aqueous potassium hydroxide
gave the diacid 10.35 Reaction with thionyl chloride followed by
Friedel−Crafts acylation of anisole gave the unreported diketone
11 in 52% yield over the two steps. Reduction with lithium
tetrahydroaluminate in tetrahydrofuran gave the diastereomeric
mixture of diol 12. Hydrogenolysis of the benzylic alcohols in
refluxing acetic acid with palladium catalyst gave the dibenzyl
compound 13.36 Deprotection followed by cyanation in similar
fashion to that described above gave tricyanate 2 in 8% yield
over the seven steps.
Chart 1. Chemical Structures of New Tricyanate Esters 1 and
2 and More Rigid Analogues ESR-255 and PT-30
(using identical cure conditions) was, though far above the cure
temperature itself, still limited to a maximum value determined
primarily by the cure conditions rather than by the rigidity
of the network segments. In contrast, the fully cured Tg of the
networks clearly decreased with increasing segment flexibility.
Therefore, under a given set of cure conditions, near-complete
conversion was only possible in networks having sufficient seg-
ment flexibility. Because near-complete conversion is often a
requirement for acceptable long-term performance, in this case
the more flexible network, contrary to expectations based on
the prevailing heuristics described previously, may provide the
best performance. Therefore, in addition to developing a new
cyanate ester monomer with superior performance, new insights
have been gained into the role of network segment flexibility
and its usefulness in optimizing performance. These insights
will greatly aid both process and product development for a
wide range of thermosetting polymers used in many high-
performance electronics, aerospace, and energy applications.
In addition to the new monomers 1 and 2, the rigid cyanate
ester known as ESR-255, originally reported by Shimp et al.,37
was also prepared. These three compounds form a convenient
series for comparison purposes: all are composed of three
cyanated phenyl rings; ESR-255 has no flexible methylene
linkages between rings, 2 has two flexible methylene linkages
and is a small-molecule analogue of the oligomeric commercial
Primaset PT-30 resin, while 1 has three methylene linkages and
thus features the greatest degree of network segment flexibility.
RESULTS AND DISCUSSION
■
Synthesis of the New Monomers 1 and 2. The synthesis
of tricyanate 1 is shown in Scheme 1. The preparation of 1,3,5-
triaroylbenzenes such as 5 has been typically accomplished via
Scheme 1. Chemical Synthesis Route to Tricyanate 1
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Scheme 2. Chemical Synthesis Route Tricyanate 2
Thermomechanical Analyses. By avoiding aliphatic
network junctions, themochemical stability comparable to
Primaset PT-30 was maintained in the more flexible network
formed from cured 1, as illustrated by the comparative TGA
data in Figure 2. Note that 2 shows similar thermochemical
Table 1. Chemical Characteristics of Cyanurate Networks
Relevant to TGA Results
wt fraction of pure cyanurate
network classified as
av mol wt
between
network
junctions
(g/mol)
char yield
in air at
600 °C
(wt %)
d
a
b
c
monomer phenyl aliphatic cyanurate
1
0.65
0.60
0.60
0.09
0.08
0.07
0.27
0.33
0.33
157
127
122
74
73
59
2
ESR-255
a
Includes phenyl rings (not cyanurate rings) and their associated
b
hydrogens. Includes aliphatic carbons and their associated hydrogens.
c
d
Includes cyanurate rings and their neighboring oxygen bridges. Note
that branch points in the monomer count as network junctions, and
short side chains (e.g., terminal methyl groups not along a path
between junctions) are not included.
the network formed by curing 1 was the most chemically stable
at elevated temperatures. Comparing the data for cured 1 and
cured 2 reveals that the increased phenyl content relative to
cyanurate did not improve the char yield.
Figure 3 shows the TGA data for cured ESR-255 compared
to 1 and 2. Despite having as low or lower aliphatic content
than either 1 or 2, the weight loss at high temperatures in ESR-
255 was significantly higher than that of both 1 and 2 under
both nitrogen and air. As with previously studied flexible tri-
cyanate esters,22 ESR-255 features an aliphatic network junction
comprised of an aliphatic carbon next to a methyl group. Solely on
the basis of empirical observation of both these and previously
studied cyanate esters without considering mechanistic details, we
surmise that while some types of junctions formed by tertiary or
quaternary aliphatic carbons may adversely affect thermochemical
stability, the presence of methylene bridges between phenyl
groups does not significantly detract from the thermochemical
stability of highly aromatic cyanate esters. In the specific case of
ESR-255, the presence of unreacted cyanate esters groups at
high temperatures (as detailed in later sections) may also negatively
affect thermochemical stability. The aforementioned conclusions
are in general agreement with the results of previous compara-
tive studies of thermochemical stability in cyanate esters.1,42
Figure 2. (a) TGA of cured cyanate ester networks featuring only phenyl
rings and methylene bridges, under nitrogen. Note that data for PT-30
has previously been published (ref 22). (b) TGA of cured cyanate ester
networks featuring only phenyl rings and methylene bridges, in air.
degradation behavior to that previously reported for PT-30,22,38
as expected. The char yields of over 70% at 600 °C in both
nitrogen and air observed in cured 1 are among the best known
for cyanate esters, though many other recent approaches39−41
may ultimately provide better heat and flame resistance overall,
and more detailed long-term testing will be needed to deter-
mine the applicability of these results to, for example, the
thermo-oxidative stability of composite structures. Despite having
the highest aliphatic content and largest molecular weight between
network junctions (see Table 1) of all of the materials studied,
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from continued slow cure of the sample. Although the apparent
enthalpy of cure of ESR-255 based only on the nonisothermal
DSC curve was a lower than expected 93 kJ/equiv, combined
FT-IR and isothermal DSC analysis (see Supporting Information
Section S3) determined that the actual enthalpy of cure for
ESR-255 was, as expected, 110 7 kJ/equiv. Hence, ESR-255
attained a conversion of only around 85%, even when heated to
350 °C.
Insight into the differing nonisothermal DSC behavior of
these three monomers can be gained by considering the evolution
of the sample Tg during the DSC scan, as illustrated in Figure 5.
Figure 3. (a) Comparative TGA of all samples synthesized for this
work, under nitrogen. (b) Comparative TGA of all samples
synthesized for this work, in air.
The outstanding thermochemical stability of cured 1 also means
that any significant loss in mechanical stiffness below 400 °C in
the fully cured network may be attributed definitively to a com-
paratively low value of the glass transition temperature brought
about by more flexible network segments, rather than, as was the
case with previously synthesized flexible cyanurate networks,22
some combination of mechanical and chemical effects.
Cure Kinetics and Tg. The impact of selectively incor-
porating methylene groups on flexibility and the subsequent
impact on ease of cure in cyanate esters can be seen by exami-
nation of nonisothermal DSC data. Figure 4 compares the DSC
Figure 4. Comparative nonisothermal DSC at 10 °C/min of tricyanate
monomers, showing exothermic region.
exotherm of monomers 1, 2, and ESR-255 calculated using a
rescanned baseline (that is, by subtracting the heat flow seen on
the second heating from that seen on the first heating).
Whereas 1 exhibited a symmetrical curve with an enthalpy of
cyclotrimerization of around 120 kJ/cyanate equiv (slightly
higher than expected), 2 exhibited a more asymmetric curve
with a close to expected enthalpy of 106 kJ/equiv. In contrast,
ESR-255 showed a highly asymmetrical curve with an “open”
tail. Previous work on cyanate ester resins16 as well as our own
verification using modulated DSC (see Supporting Information
Section S2) indicated that the open-ended nature of the exo-
thermic peak was not due to a baseline shift, but rather resulted
Figure 5. (a) Comparison of sample temperature and estimate of Tg
(with heat flow also plotted for reference) during the DSC scan of 1.
(b) Comparison of sample temperature and estimate of Tg (with heat
flow also plotted for reference) during the DSC scan of 2. Note that
the error in the estimated Tg is comparatively large for 2 due to greater
uncertainty in the diBenedetto parameters. (c) Comparison of sample
temperature and estimate of Tg (with heat flow also plotted for
reference) during the DSC scan of ESR-255.
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The Tg of the sample, which changes due to cure during the DSC
scan, is estimated with the aid of the diBenedetto equation from
conversion data computed via a running integration of the heat
flow. Figures 5a−c show the sample Tg superimposed on the
scan temperature and the heat flow, all as a function of time
elapsed from the start of heating at 50 °C. The diBenedetto
equation previously has been used quite successfully to predict
the Tg of partly cured cyanate esters,43,44 most likely due to
their well-defined cure chemistry. The same well-defined cure
chemistry of cyanate esters enables reliable computation of the
conversion from the heat flow data.1 Details of the determi-
nation of the diBenedetto equation parameters for monomers
1, 2, and ESR-255 are provided in Supporting Information
Section S4.
Table 2. Results of Thermomechanical Analysis on Cured
Cyanate Esters
a
b
c
monomer “as cured” Tg (°C) “fully cured” Tg (°C) “as cured” conv
1
280 15
286 15
245 15
338 10
(>340)
0.96 0.03
0.91 0.03
0.72 0.03
2
d
ESR-255
(>419)
a
After 1 h at 150 °C followed by 24 h at 210 °C, under nitrogen. Tg
indicated by peak in loss component of stiffness, corrected for thermal
lag and heated at 50 °C/min to avoid in situ cure. Determined on
b
heating at 10 °C/min, subsequent to initial temperature ramp to
c
350 °C. Based on the diBenedetto equation (see Supporting
d
Information Section S4). Heated to decomposition, which takes
place either before or simultaneous with mechanical softening.
In Figure 5a, it can be seen that the Tg rapidly approaches,
but never actually reaches, the scan temperature for monomer
1. For monomer 2, as seen in Figure 5b, the Tg surpasses the
scan temperature (that is, the sample vitrifies) but then the Tg
stabilizes as conversion nears completion, remaining only
modestly higher than the scan temperature. For ESR-255, as
seen in Figure 5c, even after vitrification, the cure is still far
from complete. Additional cure drives the Tg upward quickly
enough that the sample is pushed deeper into the glassy state.
The characteristic open-ended “L”-shaped DSC trace thus
appears to signify a sample that has vitrified well short of
complete conversion. Figures 5b,c also show that, as has been
well-documented for cyanate esters, cure below Tg can take
place at a slow but significant rate. The particular shape of the
Tg curve in Figure 5c after vitrification suggests that the “as
cured” Tg should be approximately a linear function of the cure
temperature (with a slope greater than one) for samples that
have vitrified but are well short of full conversion, as we
recently observed for dicyanate esters.13
The behavior of ESR-255 seen in Figure 5c has significant
implications for the study of high-temperature thermosetting
polymers. For one, it suggests that, if following the standard
practice of using a “residual heating” to determine the extent of
cure by DSC, precautions must be taken to ensure that full cure
of the sample is in fact achieved. If not, then a technique such as
the combined DSC/FT-IR approach we describe in Supporting
Information Section S3 must be utilized if true conversions are to
be measured. As the ESR-255 data illustrates, the errors in
conversion caused by ignoring incomplete cure may be
substantial. The significant amount of cure below Tg implies
that in situ cure of vitrified cyanate esters can occur during
standard characterization procedures; such in situ cure can lead
to incorrect Tg measurements and may even cause the spurious
appearance of two separate Tg’s when in fact only one is
present.45 Also, such behavior can cause an “as cured” Tg values
to move outside the range of measurement during a standard
DSC scan, leading to the incorrect conclusion that because no
glass transition was observed, the “as cured” Tg must lie outside
the scan range. Such phenomena are important to consider
when investigating high-Tg thermosetting polymers such as
cyanate esters.
contrast with the “fully cured” Tg (determined by reheating
samples previously cured to 350 °C), which, as also seen in
Table 2, varied as expected with the rigidity of the network
segments and cross-link density. Using the diBenedetto equation
to convert the “as cured” Tg values to “as cured” conversions
confirms the trend seen in the nonisothermal DSC data; namely,
monomer 1 cured the most readily of the three monomers
examined, while ESR-255 cured least readily. These results have
important technological implications. Specifically, when cyanu-
rate networks are not fully cured, the residual cyanate ester
groups are prone to reactions with water at elevated temperature
that eventually lead to formation of volatiles, which in turn can
lead to blistering and loss of structural integrity.20,46 Hence, the
more rigid ESR-255, although exhibiting a similar “as cured” Tg,
provides in some aspects a lower level of performance compared
to monomer 1 due to a lower level of conversion (∼72% for the
former versus ∼96% for the latter).
In order to better understand the reasons for the lack of
complete cure in ESR-255, a more detailed analysis of the cure
kinetics based on the Kamal model47 analyzed via a modified
form of Kenny’s graphical method48 was undertaken using iso-
thermal DSC data for monomers 1, 2, and ESR-255, with
baselines determined in a manner similar to that reported by
Sheng et al.44 Full details and complete results are provided in
Supporting Information Section S6. The activation energy, the
graphical determination of which is depicted in Figure 6, was
Figure 6. Arrhenius plot of autocatalytic rate constant k2 (prior to
gelation) for the cyanate ester monomers studied. Note that, because
the exponents for each monomer are different, a higher k2 value does
not necessarily translate into a faster reaction rate.
Because of the possibility of in situ cure, dynamic TMA
measurements must be carried out at a very rapid heating rate
to determine the “as cured” Tg for dry samples (shown in Table 2,
with plots and more details provided in Supporting Information
Section S5). The Tg values were similar after 24 h of cure at
210 °C under a nitrogen atmosphere with no added catalyst,
with the value for the more rigid ESR-255 being slightly lower,
perhaps due to its markedly slower cure kinetics. These results
only slightly lower for monomer 1 than for monomer 2 or ESR-
255 for the autocatalytic portion of the cure, suggesting that the
rigidity of the ESR-255 monomer was not a major impediment
to cure prior to gelation. Interestingly, the initial rates of conversion,
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as seen in Figure 7 at 290 °C, for example, were always significantly
higher in monomers 1 and 2 than for ESR-255. The final
sufficient time, Tg values well in excess of the cure temperature
are possible. As with the short-term cure at higher temper-
atures, however, it would appear that the Tg values tend to
converge over time. The faster cure and somewhat higher “as
cured” (for 210 °C for 24 h under nitrogen) Tg values for
monomers 1 and 2 than for ESR-255 could be due to differing
levels of impurities in the samples. van’t Hoff purity determi-
nations (see Supporting Information Section S7) showed a
substantially higher apparent purity for ESR-255. However, a
small sample of monomer 1, after purification by flash chro-
matography through silica gel in an ethyl acetate/hexane mixture,
showed no differences in either cure kinetics or apparent “as
cured” Tg despite a significant increase in the van’t Hoff purity
estimate (details provided in Supporting Information Section S8).
A more interesting explanation for the data in Table 2 is that,
paradoxically, more flexible network segments in cyanate esters
allow for higher “as cured” Tg due to their ability to facilitate
continued cure after vitrification, as described in the Introduction.
In fact, somewhat higher “as cured” Tg values for more flexible
networks have been reported in both dicyanates13 and other
tricyanates15 under similar cure conditions. Because the
differences involved are fairly small, more intensive measure-
ments, perhaps in combination with recent developments in the
molecular modeling of cyanurate networks,49,50 will be needed
to determine with certainty whether, and under what conditions, a
more flexible network can routinely provide a higher Tg. Despite
being much higher than the cure temperature, the “as cured” Tg
values for these networks are much more similar than the
corresponding set of fully cured Tg values. These results imply
that the parameters of cure process, rather than the rigidity of
network segments, are the primary determinants of “as cured”
Tg in these networks.
Figure 7. Conversion as a function of time for cyanate esters cured at
290 °C under nitrogen.
conversions achieved were also always much lower in ESR-255,
despite ample time to reach complete conversion based on the
observed rate of cure at the higher temperatures.
To help explain the observed conversion behavior, an estimate
of the Tg of the cyanurate networks during the isothermal DSC
cure was created (with the same procedure based on the
diBenedetto equation used for Figure 5) and is shown in Figure 8.
Other Physical Properties. The lower cyanurate ring
density of monomer 1 would be expected to lead to a lower
moisture uptake compared to monomer 2 and ESR-255, based
on previous correlations developed for cyanurate networks at
high conversion.51 Indeed, as Table 3 shows, such was the case.
Table 3. Other Key Physical Properties of Cured Cyanate
Esters
Figure 8. Estimated Tg as a function of time for cyanate esters cured at
290 °C under nitrogen.
theor cyanurate
moisture
uptake
(wt %)
“as cured”
ring density at full
“wet” Tg
(°C)
monomer density (g/cm3) cure (mmol/cm3)
1
1.171 0.003
1.297
2.48 0.01
3.40
2.2 0.2
3.3
238 12
232
Although the slower cure kinetics of ESR-255 lead to an initially
slower increase in Tg, in all cases, the rise in Tg slows
dramatically as the Tg reaches the cure temperature (290 °C),
causing the sample to vitrify. Since cyanate esters possess the
capability to cure slowly in the glassy state, the Tg continues to
rise slowly, even after vitrification. In monomer 1, vitrification
does not take place until conversion is nearly complete; thus,
further conversion is limited by the low concentration of remaining
unreacted groups, and further increases in Tg are small. By contrast,
in ESR-255, vitrification takes place well short of full con-
version, and thus the rate of conversion in the glassy state
appears to be somewhat higher. This result, when combined
with a higher sensitivity of the Tg to small changes in con-
version (as seen in the diBenedetto parameters for ESR-255),
leads to a comparatively rapid rise in Tg, at least initially, after
vitrification in ESR-255.
2
ESR-255
1.270
3.33
3.5
224
In fact, the moisture uptake of cured 1, at just 2.2 wt %, is
among the lowest observed for a cyanurate network with a
correspondingly high fully cured Tg, though similar tricyanates
have come fairly close.52 Although the lower moisture uptake in
combination with the similar dry Tg of cured 1 compared to
cured 2 and ESR-255 might be expected to lead to a sig-
nificantly higher “wet” Tg for cured 1 compared to both cured 2
and ESR-255, previous work on cyanate esters has shown that
the drop in Tg on exposure to hot water becomes significantly
less pronounced as conversion decreases.13 Therefore, the lower
conversion of cured ESR-255 is expected to compensate some-
what for the higher moisture uptake, resulting in more modest
differences in “wet” Tg, as observed (see Table 3).
Although the data shown in Figure 8 are for relatively short
cure times and involve Tg values only modestly higher than the
cure temperature, the data in Table 2 confirm that, if given
Taken together, these results have important implications for
the manner in which thermosetting polymers are selected for
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high-temperature applications. Resins are typically evaluated
solely on the basis of a “cured” Tg, with catalyst packages then
selected in order to attain sufficient cure for a given set of
processing conditions. In such a scenario, a higher fully cured
Tg is most often deemed a superior choice. This prevalent
heuristic, however, ignores both the steep dependence of the Tg
on the extent of cure and the effect of different catalyst
packages on long-term performance degradation. For cyanurate
networks in particular, these two effects are especially pro-
nounced in comparison to other types of thermosetting polymer
networks, and thus the prevalent heuristic is less appropriate.
When there is a steep dependence of Tg on the extent of
cure, it is most likely, as seen in the data presented herein, that
the Tg will reach and eventually exceed the cure temperature,
causing the cure process to slow dramatically. As a result, the
“as cured” Tg of the network will be limited largely by the cure
conditions themselves rather than the fully cured Tg, provided
that the fully cured Tg is high enough. As a result, systems with
a higher fully cured Tg will exhibit a lower extent of cure, which
in many cases is detrimental to long-term performance.20 The
extent of cure (and hence, the attainable Tg) may be increased
by substitution of a more active catalyst package, with the
trade-off that more active catalyst packages may also accelerate
degradation. Under such circumstances, the best performing
network may not be the one with the highest fully cured Tg.
The optimal network may well be one in which the fully cured
Tg equals the maximum attainable Tg for a given set of cure
conditions and a catalyst package with acceptable degradation
penalties.
85 °C water. The dry Tg of the fully cured network produced
from this monomer was around 320 °C. Because fully cured
networks prepared from similar monomers with fewer methylene
spacers exhibited significantly higher Tg’s, the networks did show
the expected effects of higher segmental flexibility. However,
the Tg of all networks studied was similar after a 24 h cure at
210 °C, as was the “wet” Tg. Therefore, in practical curing and
use situations, the Tg’s of these networks are likely to be
primarily limited by parameters of the cure process itself,
specifically, the dramatic slowing of the cure rate that occurs
upon vitrification and the degree of catalysis, regardless of the
degree of flexibility. In addition, when Tg’s of the fully cured
networks are sufficiently high, the more flexible networks will
exhibit the highest conversions for a given cure process. Because
near-complete conversion of cyanate esters typically provides
networks with superior physical properties, in this case the most
flexible network, with its high thermochemical stability and low
water uptake, may be the most desirable. These results illustrate
that, in contrast to the prevailing heuristics for improving the
performance of high-temperature thermosetting polymer net-
works, a more flexible network with a lower fully cured Tg can
offer an optimal combination of thermomechanical and thermo-
chemical performance. Both the newly synthesized materials and
the new insights into structure−process−property relationships
that they illustrate are expected to help advance the state of the
art in high-performance polymeric materials for the electronics,
aerospace, and energy storage industries.
As an example, consider the networks formed from the three
monomers synthesized in this study. If the cure process were
constrained to a temperature of 210 °C and an available time of
24 h (such constraints may be imposed, for instance, by the
thermal stability of tooling or resin transfer equipment), then,
for systems with no added catalyst, the maximum Tg attainable
appears to be, on average, roughly 250−280 °C. In such cases, a
network with a fully cured Tg of just over 280 °C would be ideal
because such a system would exhibit both the highest attainable
Tg and full cure. Networks with a higher fully cured Tg, such as
ESR-255, will, after cure, attain the same “as cured” Tg (250−
280 °C) but will, as a result, exhibit significantly less than full
conversion, leading to potential long-term degradation. There-
fore, for many practical situations, particularly with cyanurate
networks, there will exist monomers that form networks that
are “too rigid”; that is, Tg of the fully cured network is pro-
hibitively high. The discovery of monomer 1, which exhibits a
lower Tg in the fully cured state with no loss in thermo-
chemical stability, and with a low water uptake, thus represents
a significant step forward because it will allow for the formation
of optimal networks under a wider variety of processing
conditions (particularly those involving lower temperatures or
milder catalysts) than has previously been possible.
EXPERIMENTAL SECTION
■
Synthesis of the Monomers 1, 2, and ESR-255. General
Consideration and Instrumentation. The melting points were
collected on a Mel-Temp II from Laboratory Devices (Holliston, MA)
and are not corrected. All NMR data were collected on a Bruker
Avance II 300 MHz spectrometer (1H at 300 MHz, 13C at 75 MHz).
Nuclear magnetic resonance data (free-induction decays) were
processed using NUTS software from Acorn NMR (Livermore,
CA). All spectra are referenced to solvent or tetramethylsilane. The
1,1,1-tris(4-hydroxphenyl)ethane was purchased from TCI America
(Portland, OR), and all other reagents were purchased from Sigma-
Aldrich (Milwaukee, WI). Elemental analyses were performed by
Atlantic Microlab, Inc. (Norcross, GA).
Synthesis of Monomer 1. 4-Methoxybenzoylacetaldehyde
Potassium Salt Hydrate (4).
A round-bottomed flask (500 mL) equipped with magnetic stirring
bar, addition funnel, and N2 bubbler was charged with KOtBu (9.3 g,
83 mmol, 1 equiv) and anhydrous THF (100 mL). After complete
dissolution, 4-methoxyacetophenone (12.45 g, 83 mmol) was added,
and after 20 min, methyl formate (4.98 g, 5.1 mL, 83 mmol, 1 equiv)
was added dropwise. The mixture was stirred at rt for 2 h and then
diluted with hexanes (200 mL). The mixture was filtered through a
medium porosity glass frit. The product was dried to a constant weight
in a vacuum oven (10 Torr, 50 °C) overnight. The yield was 15.3 g
(85%); mp > 240 °C. 1H NMR (300 MHz, DMSO; δ, ppm): 9.34 (d,
J = 9.6 Hz, 1H), 7.68 (d, J = 8.5 Hz, 2H), 6.85 (d, J = 8.7 Hz, 2H),
5.37 (d, J = 9.5 Hz, 1H), 3.75 (s, 3H). Elemental analysis calcd for
C10H9KO3·0.4H2O: C, 53.74; H, 4.42; found: C, 53.36; H, 4.07%.
CONCLUSIONS
■
The selective use of methylene spacers in the network segments
of highly aromatic thermosetting cyanate ester resins has
proven to be an especially valuable technique for providing
controlled segmental flexibility while maintaining thermochem-
ical stability and low moisture uptake. In particular, a newly
synthesized monomer, 1,3,5-tris[(4-cyanatophenylmethyl)]-
benzene, exhibits char yields near 75% at 600 °C in air,
equivalent to the best known common cyanate ester resins,
with a moisture uptake of only 2.2% after 96 h immersed in
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1,3,5-Tri(4-methoxybenzoyl)benzene (5).
1,3,5-Tris[(4-hydroxyphenyl)methyl]benzene (8).
A round-bottomed flask (500 mL) equipped with magnetic stirring bar
and reflux condenser was charged with 7 (9.87 g, 22 mmol) and
pyridine hydrochloride (50 g, 434 mmol, 19 equiv). The mixture was
refluxed for 4 h. The mixture was carefully diluted with H2O (300 mL)
whereby a white precipitate formed. The precipitate was collected on a
medium porosity glass frit. The precipitate was dissolved in EtOAc
(100 mL) and washed with H2O (100 mL) followed by brine
(100 mL). After drying over anhydrous MgSO4, the solvent was rotary
evaporated, leaving a white solid. Recrystallization from MeOH/H2O
gave the title compound colorless plates. The yield was 8 g (90%); mp
A round-bottomed flask (250 mL) equipped with magnetic stirring bar
and reflux condenser was charged with 4 (15.1 g, 68 mmol) and glacial
HOAc (50 mL). The mixture was refluxed for 30 min. After cooling
to rt, the precipitate was filtered on a coarse porosity glass frit.
Recrystallization from MeCN gave the title compound as clear, pale
yellow rhombs. The yield was 9.2 g (82%); mp 179−181 °C [lit.23 177
°C]. 1H NMR (300 MHz, CDCl3; δ, ppm): 8.29 (s, 3H), 7.86 (d, J =
8.9 Hz, 6H), 6.98 (d, J = 8.9 Hz, 6H), 3.89 (s, 9H). 13C NMR (75
MHz, CDCl3; δ, ppm): 193.98, 163.93, 138.85, 133.34, 132.79, 129.33,
114.08, 55.73. Elemental analysis calcd for C30H24O6: C, 74.99; H,
5.03; found: C, 74.87; H, 5.01%.
1
205−207 °C. H NMR (300 MHz, DSMO; δ, ppm): 9.32 (s, 3OH),
6.93 (d, J = 8.6 Hz, 6H), 6.81 (s, 3H), 6.62 (d, J = 8.4 Hz, 6H), 3.69
(s, 6H). 13C NMR (75 MHz, DMSO; δ, ppm): 155.61, 142.24, 131.86,
129.82, 126.78, 115.46, 40.58. Elemental analysis calcd for C27H24O3:
C, 81.79; H, 6.10; found: C, 81.48; H, 6.16%.
Racemic 1,3,5-Tris[(4-methoxyphenyl)hydroxymethyl]benzene (6).
1,3,5-Tris[(4-cyanatophenyl)methyl]benzene (1).
A round-bottomed flask (2 L) equipped with magnetic stirring bar,
reflux condenser, and N2 bubbler was charged with 5 (20 g, 41 mmol)
and EtOH (1 L). The mixture was refluxed, and pulverized NaBH4
(4.7 g, 125 mmol, 3 equiv) was added in portions over 2 h. Reflux was
continued 8 h, resulting in a deep red solution. The mixture was
cooled to rt and quenched with HOAc. The mixture was poured into
H2O (2 L), and the white precipitate was collected on a medium
porosity glass frit. Recrystallization from MeCN gave the title
compound as a white, microcrystalline powder. The yield was 18 g
A round-bottomed flask (250 mL) equipped with magnetic stirring
bar, N2 bubbler, and addition funnel was charged with 8 (3 g,
7 mmol), BrCN (2.81 g, 26 mmol, 3.5 equiv), and acetone (100 mL).
The mixture was cooled in a −20 °C bath before TEA (2.1 g,
21 mmol, 3 equiv) was added dropwise in 20 min. Copious solids
(TEA·HBr) precipitated during the addition. After 2 h, the mixture
was poured into cold H2O (250 mL) and extracted with Et2O (2 ×
100 mL). The extracts were collected and washed with H2O (200 mL)
followed by saturated aqueous NaHCO3 (100 mL) and finally brine
(100 mL). After drying over anhydrous MgSO4, rotary evaporation left
an off-white solid. Recrystallization from EtOH gave the title com-
pound as white needles. The yield was 1.97 g (60%); mp 97−99 °C.
1H NMR (300 MHz, CDCl3; δ, ppm): 7.21 (s, 12H), 6.81 (s, 3H),
3.91 (s, 6H). 13C NMR (75 MHz, CDCl3; δ, ppm): 151.53, 141.19,
139.99, 130.85, 127.75, 115.54, 109.05, 40.99. Elemental analysis calcd
for C30H21N3O3: C, 76.42; H, 4.49; N, 8.91; found: C, 76.19; H, 4.37;
N, 8.89%.
1
(90%); mp 181−183 °C. H NMR (300 MHz, DMSO; δ, ppm):
7.27−7.13 (m, 9H), 6.83 (d, J = 9.4 Hz, 6H), 5.67 (d, J = 3.7 Hz, 3H),
5.55 (d, J = 3.7 Hz, 3H), 3.70 (s, 9H). 13C NMR (75 MHz, DMSO; δ,
ppm): 158.00, 145.32, 137.82, 122.39, 113.32, 74.01, 54.98. Elemental
analysis calcd for C30H30O6: C, 74.06; H, 6.21; found: C, 74.04; H,
6.26%.
1,3,5-Tris[(4-methoxyphenyl)methyl]benzene (7).
Synthesis of Monomer 2. Dimethyl 5-Methoxyisophthalate (9).
A round-bottomed flask (500 mL) equipped with magnetic stirring
bar, reflux condenser, and N2 bubbler was charged with 6 (12 g,
25 mmol), 5% Pd/C catalyst (2 g), ammonium formate (8.8 g,
140 mmol, 6 equiv), and glacial HOAc (250 mL). The mixture was
refluxed for 5 h. After cooling to rt, the mixture was filtered through
diatomaceous earth to remove the catalyst. The filtrate was partitioned
between H2O (500 mL) and Et2O (500 mL). The organic layer was
separated and washed with H2O (500 mL) and then brine (500 mL).
Rotary evaporation of the volatiles gave a colorless oil that gradually
solidified into a white, waxy solid. The yield was 9.87 g (91%); mp
55−57 °C. 1H NMR (300 MHz, DMSO; δ, ppm): 7.07 (d, J = 8.6 Hz,
6H), 6.85 (s, 3H), 6.81 (d, J = 8.6 Hz, 6H), 3.76 (s, 6H), 3.69 (s, 9H).
13C NMR (75 MHz, DMSO; δ, ppm): 157.48, 141.71, 133.09, 129.49,
126.58, 113.72, 54.90, 54.81, 40.16. Elemental analysis calcd for
C30H30O3: C, 82.16; H, 6.89; found: C, 82.08; H, 6.88%.
A round-bottomed flask (1 L) equipped with magnetic stirring bar and
reflux condenser was charged with 5-hydroxyisophthalic acid (18.2 g,
100 mmol), acetone (300 mL), freshly pulverized anhydrous K2CO3
(40 g, 289 mmol, 2.8 equiv), and dimethyl sulfate (50 mL, 66.5 g, 520
mmol, 5.2 equiv). After refluxing overnight, the mixture was cooled
and poured into H2O (1 L). The resulting precipitate was collected on
a medium porosity glass frit and air-dried several hours. Recrystalliza-
tion from cyclohexane gave the title compound as a white
microcrystalline powder. The yield was 21.1 g (94%); mp 110−112 °C
[lit.53 110−111 °C]. Elemental analysis calcd for C11H12O5: C, 58.93; H,
5.39; found: C, 58.66; H, 5.31%.
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5-Methoxyisophthalic Acid Monohydrate (10).
evaporated leaving a crude solid. Recrystallization from toluene gave
the title compound as a white powder. The yield was 10.1 g (60%). 1H
NMR (300 MHz, CDCl3/DMSO; δ, ppm): 7.27−7.20 (m, 4H), 7.02−
6.93 (m, 1H), 6.82−6.74 (m, 6H), 5.63 (s, 2H), 4.22 (bs, 2 OH), 3.73
(s, 6H), 3.69 (s, 3H). 13C NMR (75 MHz, CDCl3/DMSO; δ, ppm):
159.43, 159.35, 158.41, 158.40, 146.43, 146.37, 136.87, 136.85, 127.63,
127.59, 117.09, 116.99, 113.31, 110.43, 110.31, 74.83, 74.81, 54.97,
54.93. Elemental analysis calcd for C23H24O5: C, 72.61; H, 6.36;
found: C, 72.88; H, 6.42%.
A round-bottomed flask (500 mL) equipped with magnetic stirring bar
was charged with 9 (21.1 g, 94 mmol), MeOH (200 mL), and a
solution of KOH (16 g, 285 mmol, 3 equiv) in H2O (50 mL). The
mixture was refluxed for 6 h. After cooling to rt, the mixture was
poured into H2O (500 mL) and acidified to pH 3 by dropwise
addition of concentrated HCl. The precipitate was collected on a
medium porosity glass frit. Recrystallization from glacial HOAc gave
the title compound as colorless needles. The yield was 19.1 g (95%);
mp 270−272 °C [lit.54 267−268 °C]. 1H NMR (300 MHz, DMSO; δ,
ppm): 13.20 (bs, 2 CO2H), 8.09 (s, 1H), 7.61 (s, 2H), 3.83 (s, 3H).
13C NMR (75 MHz, DMSO; δ, ppm): 167.11, 159.37, 133.74, 122.56,
118.28, 55.65. Elemental analysis calcd for C9H8O5·H2O: C, 50.47; H,
4.71; found: C, 50.92; H, 4.63%.
3,5-Bis[(4-methoxyphenyl)methyl)]anisole (13).
A round-bottomed flask (500 mL) equipped with magnetic stirring bar
and reflux condenser was charged with a mixture of 12 (10.1 g, 26
mmol), glacial HOAc (300 mL), and 5% Pd/C (2 g). The mixture was
vigorously stirred and heated near boiling under an H2 atmosphere
(1 Torr). After 18 h, the mixture was cooled to rt and the catalyst was
filtered. Rotary evaporation left an oil which was chromatographed on
silica gel eluting with 20% EtOAc in hexanes. Rotary evaporation of
the fractions gave a colorless oil. Reduced pressure (0.1 Torr)
distillation gave the title compound in analytically pure form. The yield
was 3.87 g (41%). 1H NMR (300 MHz, CDCl3; δ, ppm): 7.08 (d, J =
8.6 Hz, 4H), 6.81 (d, J = 8.8 Hz, 4H), 6.62 (t, J = 1.4 Hz, 1H), 6.53 (d,
J = 1.4 Hz, 2H), 3.84 (s, 4H), 3.77 (s, 6H), 3.69 (s, 3H). 13C NMR
(75 MHz, CDCl3; δ, ppm): 160.10, 158.21, 143.28, 133.33, 130.04,
122.21, 114.09, 112.35, 55.46, 55.31, 41.26. Elemental analysis calcd
for C23H24O3: C, 79.28; H, 6.94; found: C, 79.31; H, 6.94%.
3,5-Bis(4-methoxybenzoyl)anisole (11).
A round-bottomed flask (200 mL) equipped with magnetic stirring bar
and reflux condenser was charged with 10 (18.2 g, 85 mmol) and
SOCl2 (25 mL). An N2 bubbler piped to a caustic wash bottle was
attached, and the mixture was gently refluxed for 5 h. The excess
SOCl2 was distilled under reduced pressure (10 Torr), leaving an off-
white solid of 5-methoxyisophthaloyl dichloride. The latter was
transferred to a 500 mL round-bottomed flask equipped with magnetic
stirring bar and addition funnel with carbon disulfide (50 mL). The
mixture was cooled in an ice bath before AlCl3 (17.69 g, 134 mmol, 2
equiv) was added in one portion. The addition funnel was charged
with anisole (29 g, 270 mmol, 4 equiv), which was added dropwise
over 30 min. The cooling bath was removed, and the mixture was
stirred at rt for 3 h. The mixture was carefully poured onto cracked ice
(500 g). The mixture was extracted with EtOAc (3 × 100 mL). The
extracts were collected and washed with H2O (500 mL) followed by
brine (500 mL). After drying over anhydrous MgSO4, the solvent was
rotary evaporated, leaving a pale yellow oil that eventually solidified
under vacuum. Recrystallization of the crude product from EtOH gave
the title compound as small colorless needles. The yield was 16.61 g
3,5-Bis[(4-hydroxyphenyl)methyl)]phenol (14).
A round-bottomed flask (250 mL) equipped with magnetic stirring bar
and reflux condenser was charged with 13 (3.87 g, 11 mmol) and
pyridine hydrochloride (26 g, 222 mmol, 20 equiv). The mixture was
heated by heating the mantle to 180 °C for 5 h. After cooling to rt, the
mixture was diluted with H2O (250 mL), whereupon a tan precipitate
eventually formed. Filtration followed by recrystallization of the solid
from toluene/MeOH gave the title compound as a white, micro-
1
crystalline powder. The yield was 2.5 g (75%); mp 183−186 °C. H
NMR (300 MHz, CDCl3; δ, ppm): 9.17 (2, 2OH), 9.11 (s, 1OH),
6.97 (d, J = 8.3 Hz, 4H), 6.67 (d, J = 8.0 Hz, 4H), 6.50 (s, 1H), 6.35
(s, 2H), 3.67 (s, 4H). 13C NMR (75 MHz, CDCl3; δ, ppm): 157.33,
155.48, 143.13, 131.35, 129.59, 119.72, 115.11, 113.06, 40.32.
Elemental analysis calcd for C20H18O3: C, 78.41; H, 5.92; found: C,
78.31; H, 5.78%.
1
(52%); mp 115−117 °C. H NMR (300 MHz, CDCl3; δ, ppm): 7.84
(d, J = 8.6 Hz, 4H), 7.62 (t, J = 1.4 Hz, 1H), 7.49 (d, J = 1.2 Hz, 2H),
6.96 (d, J = 9.0 Hz, 4H), 3.91 (s, 3H), 3.88 (s, 6H). 13C NMR (75
MHz, CDCl3; δ, ppm): 194.57, 163.69, 159.71, 139.76, 132.72, 129.78,
123.36, 118.27, 113.88, 55.94, 55.66. Elemental analysis: calcd for
C23H20O5: C, 73.39; H, 5.36; found: C, 73.65; H, 5.23%.
3,5-Bis[(4-cyanatophenyl)methyl)]phenylcyanate (2).
Racemic 3,5-Bis[(4-methoxyphenyl)hydroxymethyl)]anisole (12).
A round-bottomed flask (250 mL) equipped with magnetic stirring bar
was charged with 14 (1 g, 3.3 mmol), BrCN (1.38 g, 13 mmol, 3.9
equiv), and acetone (50 mL). The mixture was stirred in a −20 °C
cooling bath while TEA (1.07 g, 10.6 mmol, 3.2 equiv) was added
dropwise. Copious solids (TEA·HBr) precipitated during the addition.
After stirring 2 h, the mixture was poured into H2O (100 mL). The
mixture was extracted with EtOAc (2 × 50 mL). The extracts were
combined and washed with H2O (100 mL) followed by brine (100 mL).
After drying over anhydrous MgSO4, the solvent was rotary evaporated,
leaving the crude product. The title compound was obtained as color-
less needles by recrystallization from i-PrOH. The yield was 1.2 g
A round-bottomed flask (1 L) equipped with magnetic stirring bar was
charged with LAH (3.3 g, 87 mmol, 1.9 equiv) and anhydrous THF
(500 mL). An N2 bubbler, addition funnel, and reflux condenser were
equipped. The mixture was stirred in an ice bath, and a solution of 11
(16.61 g, 44 mmol) in THF (50 mL) was charged to the funnel. The
addition was made as a gentle stream over 1 h. After stirring at rt for 3
h the reduction was complete. The mixture was carefully quenched
by addition of H2O (3.3 mL) followed by 15% NaOH (3.3 mL)
and finally H2O (9.9 mL). After stirring overnight, the mixture was
filtered through a medium porosity glass frit. The filtrate was rotary
1
(95%); mp 119−121 °C. H NMR (300 MHz, CDCl3; δ, ppm): 7.24
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(m, 8H), 6.94 (s, 2H), 6.92 (s, 1H), 3.99 (2, 4H). 13C NMR (75 MHz,
CDCl3; δ, ppm): 153.47, 151.86, 143.97, 138.55, 130.97, 127.92,
115.89, 113.95, 108.89, 108.71, 40.82. Elemental analysis calcd for
C23H15N3O3: C, 72.43; H, 3.96; N, 11.02; found: C, 72.43; H, 3.91; N,
10.98%.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystal structure and NMR data (Section S1); modulated
DSC data (Section S2); determination of enthalpy of cure by
the DSC/IR technique (Section S3); diBenedetto equation
parameters (Section S4); TMA plots and detailed description
(Section S5); detailed cure kinetics analysis (Section S6); van’t
Hoff analysis of monomers (Section S7); properties of highly
purified 1 (Section S8). This material is available free of charge
via the Internet at http://pubs.acs.org.
Synthesis of ESR-255. 1,1,1-Tris(4-cyanatophenyl)ethane (ESR-
255).
AUTHOR INFORMATION
Corresponding Author
*E-mail: andrew.guenthner@edwards.af.mil.
Notes
■
A round-bottomed flask (250 mL) equipped with magnetic stirring bar
and addition funnel was charged with 1,1,1-tris(4-hydroxyphenyl)-
ethane (10.0 g, 32 mmol), BrCN (13.78 g, 130 mmol, 4 equiv), and
anhydrous acetone (100 mL). The mixture was cooled in a −20 °C
bath before TEA (11.55 g, 114 mmol, 3.5 equiv) was added dropwise.
During the addition, copious solids (TEA·HBr) precipitated. After-
ward, the mixture was stirred in an ice bath for 30 min before pouring
into crushed ice and H2O (500 mL). The mixture was extracted with
Et2O (3 × 100 mL). The extracts were collected and washed with H2O
(100 mL) followed by brine (100 mL). After drying over anhydrous
MgSO4, the solvent was rotary evaporated, leaving an off-white solid
(12.59 g). The crude product was recrystallized from EtOH to give the
title compound as a white powder. The yield was 6.5 g (52%); mp
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The support of the Office of Naval Research, the Air Force Office
of Scientific Research, the Air Force Research Laboratory, and the
National Research Council Research Associateship Program (J.T.R.)
is gratefully acknowledged. The authors thank Dr. Suresh Suri of
AFRL for helpful advice on the flash chromatography of monomer 1.
113−115 °C [lit.55 104 °C]. H NMR (300 MHz, CDCl3; δ, ppm):
1
REFERENCES
■
7.28 (d, J = 9.1 Hz, 6H), 7.17 (d, J = 9.0 Hz, 6H), 2.22 (s, 3H). 13C
NMR (75 MHz, CDCl3; δ, ppm): 151.48, 146.58, 130.57, 115.34,
108.65, 51.69, 30.92. Elemental analysis calcd for C23H15N3O3: C,
72.43; H, 3.96; N, 11.02; found: C, 72.35; H, 3.88; N, 11.06%.
Cured Network Preparation and Characterization. Cured
resin samples were prepared by melting the monomer powder at 115−
130 °C at a reduced pressure of 300 mmHg for 30 min and then
pouring into either a silicone casting mold (13 mm diameter × 3 mm
discs) for multistep cure or into a 6 mm diameter × 1 mm high
aluminum pan for single-step cure. Multistep cure involved heating in
an oven under nitrogen to 150 °C for 1 h, followed by 210 °C for 24 h,
with subsequent cooling to below 150 °C prior to demolding. Heating
ramps for multistep cures were 5 °C/min. Additional details such as
the mold fabrication procedures have been published elsewhere.56
Thermomechanical analysis of cured samples was performed using a
TA Instruments Q400 thermomechanical analyzer (TMA) in dynamic
(oscillatory compression) TMA mode. The oscillatory compression
mode provides qualitative information on the storage and loss
components of the sample stiffness, in addition to sample displace-
ment as a function of temperature. Heating and cooling rates of
50 °C/min were used for “as cured” dry samples, with 10 °C used for
“fully cured” samples, and 20 °C/min used for “wet” samples. These
rates are considered optimal for each given situation and represent a
balance between minimizing in situ cure, thermal lag, and (for wet
samples) in situ drying. A standard thermal cycling procedure, using
limits of 0 and 200 °C, was used to determine and correct for the
thermal lag. The procedure was employed prior to heating to 350 °C
for dry samples and after heating to 350 °C for wet samples. The mean
compressive load on the samples was 0.1 N, with an oscillatory force
applied at an amplitude of 0.1 N and a frequency of 0.05 Hz. Complete
details, examples, and the rationale for the cycling procedures have
been explained at length elsewhere.56 Thermogravimetric analysis (TGA)
was carried out using a TA Instruments Q5000 under 60 mL/min of
either nitrogen or air. For TGA analysis, ∼2 mg of small chips of multi-
step cured samples was heated at 10 °C/min to 600 °C. The density of
cured samples was determined via neutral buoyancy of 13 mm × 3 mm
discs in CaCl2/deionized water mixtures, as described elsewhere.57
Some multistep cured and postcured samples were placed in 250 mL
of deionized water at 85 °C for 96 h as a means of testing the effects of
exposure to hot water.
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