Effects of o -methoxy groups on the properties and thermal stability of renewable high-temperature cyanate ester resins

Article abstract of DOI:10.1021/acs.macromol.5b00496

Renewable phenols derived from biomass sources often contain methoxy groups that alter the properties of derivative polymers. To evaluate the impact of o-methoxy groups on the performance characteristics of cyanate ester resins, three bisphenols derived from the renewable phenol creosol were deoxygenated by conversion to ditriflates followed by palladium-catalyzed elimination and hydrolysis of the methoxy groups. The deoxygenated bisphenols were then converted to the following cyanate ester resins: bis(4-cyanato-2-methylphenyl)methane (16), 4,4′-(ethane-1,1′-diyl)bis(1-cyanato-3-methylbenzene) (17), and 4,4′-(propane-1,1′-diyl)bis(1-cyanato-3-methylbenzene) (18). The physical properties, cure chemistry, and thermal stability of these resins were evaluated and compared to those of cyanate esters derived from the oxygenated bisphenols. 16 and 18 had melting points 37 and >95 °C lower, respectively, than the oxygenated versions, while 17 had a melting point 14 °C higher. The T_gs of thermosets generated from the deoxygenated resins ranged from 267 to 283 °C, up to 30 °C higher than the oxygenated resins, while the onset of thermal degradation was 50-80 °C higher. The deoxygenated resins also exhibited water uptakes up to 43% lower and wet T_gs up to 37 °C higher than the oxygenated resins. TGA-FTIR of thermoset networks derived from 16-18 revealed a different decomposition mechanism compared to the oxygenated resins. Instead of a low-temperature pathway that resulted in the evolution of phenolic compounds, 16-18 had significantly higher char yields and decomposed via evolution of small molecules including isocyanic acid, CH₄, CO₂, and NH₃.

Full text of DOI:10.1021/acs.macromol.5b00496

Article
pubs.acs.org/Macromolecules
Effects of o‑Methoxy Groups on the Properties and Thermal Stability
of Renewable High-Temperature Cyanate Ester Resins
Benjamin G. Harvey,*,† Andrew J. Guenthner,‡ William W. Lai,† Heather A. Meylemans,†
Matthew C. Davis,† Lee R. Cambrea,† Josiah T. Reams,§ and Kevin R. Lamison§
†NAWCWD, Research Department, Chemistry Division, US Navy, China Lake, California 93555, United States
‡Rocket Propulsion Division, Air Force Research Laboratory, Edwards AFB, California 93524, United States
§Rocket Propulsion Division, ERC, Inc., Air Force Research Laboratory, Edwards AFB, California 93524, United States
S
* Supporting Information
ABSTRACT: Renewable phenols derived from biomass sources often
contain methoxy groups that alter the properties of derivative polymers. To
evaluate the impact of o-methoxy groups on the performance characteristics
of cyanate ester resins, three bisphenols derived from the renewable phenol
creosol were deoxygenated by conversion to ditriflates followed by
palladium-catalyzed elimination and hydrolysis of the methoxy groups.
The deoxygenated bisphenols were then converted to the following cyanate
ester resins: bis(4-cyanato-2-methylphenyl)methane (16), 4,4′-(ethane-1,1′-
diyl)bis(1-cyanato-3-methylbenzene) (17), and 4,4′-(propane-1,1′-diyl)bis-
(1-cyanato-3-methylbenzene) (18). The physical properties, cure chemistry,
and thermal stability of these resins were evaluated and compared to those
of cyanate esters derived from the oxygenated bisphenols. 16 and 18 had melting points 37 and >95 °C lower, respectively, than
the oxygenated versions, while 17 had a melting point 14 °C higher. The Tg’s of thermosets generated from the deoxygenated
resins ranged from 267 to 283 °C, up to 30 °C higher than the oxygenated resins, while the onset of thermal degradation was
50−80 °C higher. The deoxygenated resins also exhibited water uptakes up to 43% lower and wet Tgs up to 37 °C higher than
the oxygenated resins. TGA-FTIR of thermoset networks derived from 16−18 revealed a different decomposition mechanism
compared to the oxygenated resins. Instead of a low-temperature pathway that resulted in the evolution of phenolic compounds,
16−18 had significantly higher char yields and decomposed via evolution of small molecules including isocyanic acid, CH4, CO2,
and NH3.
and decomposition pathways of derivative thermoplastics and
thermosetting resins.27−33
INTRODUCTION
■
In recent years the development of thermoplastics and
thermoset resins from renewable sources has undergone a
renaissance.1−9 In large part this remarkable degree of activity
has been driven by the need to establish sustainable feedstocks
that are not dependent on finite petroleum resources. One key
area of study is the generation of renewable phenols that can
potentially replace or supplement the ubiquitous bisphenol
A.10−20 A number of research groups are exploring this field,
not only with the goal of generating full-performance materials,
but also with an eye toward developing less toxic alternatives to
bisphenol A.21−23 The most logical source of renewable
phenolic precursors for thermoplastic and thermoset resins is
lignin. This abundant biofeedstock is currently generated by the
commercial paper industry and is a significant component of
forestry residue and agricultural waste. Crude forms of lignin
can be acquired for ∼$0.05/lb,24 and it is estimated that as
many as 250 million tons could be sustainably produced
annually in the US by 2030.25,26 Despite the promise of lignin-
derived and other natural phenols, these molecules typically
contain additional functional groups (primarily methoxy
groups) that can affect the material properties, thermal stability,
Cyanate esters are an important class of thermosetting resins
that can be readily prepared from polyphenols. The high glass
transition temperatures, low dielectric constants, and low water
uptakes afforded by these resins make them suitable for use in a
variety of demanding environments.34−37 The physical proper-
ties of the thermosets can be readily modified by altering the
position, number, and nature of substituents on the aromatic
rings.38−44 Alternatively, similar effects can be obtained by
altering the bridging groups between rings.45−56 In a recent
paper we described the synthesis and characterization of a series
of bis(cyanate) esters derived from the renewable phenol
creosol (2-methoxy-4-methylphenol).57 Unlike the majority of
modified cyanate ester resins that have aliphatic or aromatic
substituents, the resins derived from creosol have methoxy
groups ortho to the cyanate ester groups. Although these
“oxygenated” cyanate esters exhibit thermal stabilities and water
Received: March 8, 2015
Revised: April 30, 2015
Published: May 14, 2015
This article not subject to U.S. Copyright.
Published 2015 by the American Chemical
Society
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Article
temperature and was then filtered through a coarse frit, poured into
water (100 mL), and extracted with ether (3 × 75 mL). The organic
fractions were combined, washed with water (3 × 50 mL) and brine (3
× 50 mL), and dried over MgSO4. The organic solution was passed
through a pad of silica gel, and the solvent was removed under reduced
pressure to give 10, 11, or 12 as light tan to off-white solids. These
crude solids were suitable for further manipulation, but analytically
pure samples were prepared by recrystallization from methanol or
chromatography on silica gel eluting with Et2O/hexanes.
uptake levels that render them useful for high performance
applications, the methoxy groups result in lower Tg’s, lower
thermal stability, and increased water uptake compared to
conventional resins.
In perhaps the first study of its kind, the current work seeks
to directly evaluate the impact of o-methoxy groups on the
properties and decomposition pathways of renewable cyanate
esters by comparing close structural analogues. It is expected
that the information generated in this work will provide insight
into how o-methoxy groups affect other polymers derived from
renewable sources. Further, this study will allow the benefits of
deoxygenation to be evaluated by considering thermoset
performance in the context of synthetic atom economy and
cost.
Bis(4-methoxy-2-methylphenyl)methane (10). Yield: 76.9%. 1H
NMR (DMSO-d6) δ: 6.77 (d, J = 2.6 Hz, 2H), 6.73 (d, J = 8.3 Hz,
2H), 6.65 (dd, J = 8.3, 2.6 Hz, 2H), 3.70 (s, 6H), 3.33 (s, 2H), 2.17 (s,
6H). 13C NMR (DMSO-d6) δ: 157.9, 137.8, 131.0, 130.1, 116.1, 111.4,
55.3, 34.8, 19.8. Anal. Calcd for C17H20O2: C, 79.65; H, 7.86. Found:
C, 79.45; H, 7.84.
4,4′-(Ethane-1,1′-diyl)bis(1-methoxy-3-methylbenzene) (11).
1
Yield: 74.6%. H NMR (CDCl3) δ: 6.92 (d, J = 8.4 Hz, 2H), 6.71−
EXPERIMENTAL SECTION
6.66 (m, 4H), 4.21 (q, J = 7.2 Hz, 1H), 3.69 (s, 6H), 2.15 (s, 6H), 1.39
(d, J = 7.2 Hz, 3H). 13C NMR (CDCl3) δ: 157.1, 136.5, 136.3, 127.1,
115.7, 110.8, 54.8, 21.1, 18.9. Anal. Calcd for C18H22O2: C, 79.96; H,
8.20. Found: C, 80.17; H, 8.17.
■
General. 5,5′-Methylenebis(2-methoxy-4-methylphenol) (1), 5,5′-
(ethane-1,1-diyl)bis(2-methoxy-4-methylphenol) (2), and 5,5′-(pro-
pane-1,1-diyl)bis(2-methoxy-4-methylphenol) (3) were prepared as
previously described.12 All solvents and chemicals were purchased
from Sigma-Aldrich and used as received except for triethylamine
which was distilled from sodium/benzophenone under nitrogen. NMR
spectra were collected with a Bruker Avance II 300 MHz NMR
spectrometer (spectra for compounds 7−18 are included in the
Supporting Information). 1H and 13C NMR chemical shifts are
reported versus the deuterated solvent peak [CDCl3: δ 7.27 ppm (1H),
77.23 ppm (13C). DMSO-d6: δ 2.50 (1H), 39.51 (13C)]. Elemental
analysis was performed by Atlantic Microlabs Inc., Norcross, GA.
General Procedure for Triflation. A round-bottomed flask
equipped with a magnetic stirring bar was charged with bisphenol 1,
2, or 3 (30 mmol) and CH2Cl2 (100 mL). The mixture was cooled in
an ice bath under a slow stream of nitrogen, and in one portion
pyridine (3 equiv) was added. Then triflic anhydride (2.5 equiv) was
slowly added over 20 min with a pressure equalizing addition funnel,
and the reaction was stirred at 0 °C for 6 h. The mixture was then
washed with H2O several times and finally with brine. The organic
phase was dried over MgSO4, and the solvent was removed under
reduced pressure, leaving a pale yellow solid. The product was then
crystallized from hot MeOH, and a white solid (7, 8, or 9) was
obtained by filtration.
4,4′-(Propane-1,1′-diyl)bis(1-methoxy-3-methylbenzene) (12).
1
Yield: 85.4%. H NMR (DMSO-d6) δ: 6.98 (d, J = 8.7 Hz, 2H),
6.69 (m, 4H), 3.96 (t, J = 7.4 Hz, 1H), 3.69 (s, 6H), 2.19 (s, 6H), 1.83
(dt, J = 7.4 Hz, 2H), 0.86 (t, J = 7.4 Hz, 3H). 13C NMR (DMSO-d6) δ:
156.9, 137.0, 134.8, 127.6, 115.6, 110.9, 54.8, 42.4, 28.5, 19.2, 12.7.
Anal. Calcd for C19H24O2: C, 80.24; H, 8.51. Found: C, 80.52; H, 8.46.
General Hydrolysis Process. To a −78 °C solution of
bis(methoxy) 10, 11, or 12 (15 mmol) in methylene chloride (30
mL) was added BBr3 (2.1 mL, 21.8 mmol) dropwise. The reaction was
stirred at −78 °C for an hour and then allowed to warm to room
temperature and stirred for an additional hour. Anhydrous methanol
was then added, and the solution was stirred for another hour before
being placed under reduced pressure to give crude bisphenol. The
bisphenol was recrystallized from ethanol/water to give 13, 14, or 15
as white solids.
1
4,4′-Methylenebis(3-methylphenol) (13). Yield: 70.5%. H NMR
(DMSO-d6) δ: 9.03 (s, 2H), 6.59 (d, J = 8.2 Hz, 2H), 6.57 (d, J = 2.6
Hz, 2H), 6.46 (dd, J = 8.2, 2.6 Hz, 2H), 3.62 (s, 2H), 2.09 (s, 6H). 13C
NMR (DMSO-d6) δ: 155.3, 137.0, 129.6, 129.0, 116.8, 112.5, 34.3,
19.3. Anal. Calcd for C15H16O2: C, 78.92; H, 7.06. Found: C, 78.79; H,
6.96.
1
Methylenebis(2-methoxy-4-methyl-5,1-phenylene) Bis(trifluoro-
4,4′-(Ethane-1,1′-diyl)bis(3-methylphenol) (14). Yield: 74.5%. H
1
methanesulfonate) (7). Yield: 85.6%. H NMR (CDCl3) δ: 6.89 (s,
NMR (DMSO-d6) δ: 9.02 (s, 2H), 6.80 (d, J = 7.9 Hz, 2H), 6.50 (m,
4H), 4.11 (q, J = 6.9 Hz, 1H), 2.08 (s, 6H), 1.35 (d, J = 6.9 Hz, 3H).
13C NMR (DMSO-d6) δ: 155.0, 136.2, 134.8, 127.0, 116.9, 112.4, 35.2,
21.4, 18.9. Anal. Calcd for C16H18O2: C, 79.31; H, 7.49. Found: C,
79.23; H, 7.44.
2H), 6.69 (s, 2H), 3.91 (s, 6H), 3.77 (s, 2H), 2.26 (s, 6H). 13C NMR
(CDCl3) δ: 149.2, 137.5, 137.0, 134.8, 120.7, 115.3, 56.1, 28.5, 19.5.
Anal. Calcd for C19H18F6O8S2: C, 41.31; H, 3.28. Found: C, 41.62; H,
3.27.
Ethane-1,1-diylbis(2-methoxy-4-methyl-5,1-phenylene) Bis-
(trifluoromethanesulfonate) (8). Yield: 78.2%. 1H NMR (CDCl3)
δ: 6.85 (s, 2H), 6.83 (s, 2H), 4.23 (q, J = 7 Hz, 1H), 3.89 (s, 6H), 2.23
(s, 6H), 1.47 (d, J = 7 Hz, 3H). 13C NMR (CDCl3) δ: 149.3, 137.0,
136.9, 136.2, 120.3, 115.3, 56.2, 36.4, 20.7, 19.2. Anal. Calcd for
C20H20F6O8S2: C, 42.40; H, 3.56. Found: C, 42.46; H, 3.60.
Propane-1,1-diylbis(2-methoxy-4-methyl-5,1-phenylene) Bis-
(trifluoromethanesulfonate) (9). Yield: 86.3%. 1H NMR (CDCl3)
δ: 6.87 (s, 2H), 6.82 (s, 2H), 3.98 (t, J = 7 Hz, 1H), 3.88 (s, 6H), 2.26
(s, 6H), 1.87 (q, J = 7 Hz, 2H), 0.96 (t, J = 7 Hz, 3H). 13C NMR
(CDCl3) δ: 149.2, 137.5, 137.0, 134.8, 120.8, 115.3, 56.1, 43.4, 28.5,
19.5, 12.5. Anal. Calcd for C21H22F6O8S2: C, 43.45; H, 3.82. Found: C,
43.53; H, 3.75.
General Procedure for Detriflation. To a solution of triethyl-
amine (22 mL, 160 mmol) in anhydrous DMF (45 mL) was added
98% formic acid (6 mL, 26 mmol). The mixture was stirred for 5 min,
and then ditriflate 7, 8, or 9 (19 mmol), Pd(OAc)2 (0.17 g, 0.78
mmol), and diphenylphosphinoferrocene (DPPF, 0.84 g, 1.5 mmol)
were added to the reaction flask. The resulting solution was stirred at
80 °C for 1 h under a blanket of N2. The solution transitioned from a
yellow color to deep red, and after several minutes a yellow precipitate
was observed. The reaction mixture was allowed to cool to room
4,4′-(Propane-1,1′-diyl)bis(3-methylphenol) (15). Yield: 70.0%. 1H
NMR (DMSO-d6) δ: 9.01 (s, 2H), 6.85 (d, J = 9.2 Hz, 2H), 6.51 (m,
4H), 3.87 (t, J = 7.4 Hz, 1H), 2.12 (s, 6H), 1.79 (dt, J = 7.2 Hz, 2H),
0.84 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6) δ: 154.8, 136.7, 133.3,
127.6, 116.9, 112.5, 42.3, 28.7, 19.2, 12.8. Anal. Calcd for C17H20O2: C,
79.65; H, 7.86. Found: C, 78.94; H, 7.77.
General Procedure for Cyanate Ester Synthesis. To a −78 °C
solution of bisphenol 13, 14, or 15 (4 mmol) and cyanogen bromide
(10 mmol) in anhydrous THF (30 mL), triethylamine (8 mmol) was
added dropwise. The reaction was stirred for 1 h and then allowed to
warm to 0 °C. A substantial amount of white precipitate was then
removed by filtration and washed with ether (3 × 50 mL). The organic
fractions were combined, washed with water (3 × 25 mL) and brine (3
× 25 mL), and dried over MgSO4. The solvent was removed under
reduced pressure to give 16, 17, or 18.
Bis(4-cyanato-2-methylphenyl)methane (16). Yield: 81%. 1H
NMR (CDCl3) δ: 7.17 (d, J = 2.8 Hz, 2H), 7.05 (dd, J = 8.5, 2.9
Hz, 2H), 6.90 (d, J = 8.5 Hz, 2H), 3.89 (s, 2H), 2.29 (s, 6H). 13C
NMR (CDCl3) δ: 151.6, 139.8, 136.6, 130.9, 117.0, 113.0, 109.1, 35.6,
19.9; mp = 88 °C (DSC). Anal. Calcd for C17H14N2O2: C, 73.37; H,
5.07; N, 10.07. Found: C, 73.36; H, 5.03; N, 10.07.
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4,4′-(Ethane-1,1′-diyl)bis(1-cyanato-3-methylbenzene) (17).
RESULTS AND DISCUSSION
■
1
Yield: 79.6%. H NMR (DMSO-d6) δ: 7.21 (m, 6H), 4.40 (q, J =
Cyanate esters derived from the renewable phenol creosol, with
the cyanate ester group meta to the bridging group between
aromatic rings, have recently been studied57 (compounds 4−6
in Figure 1). Direct analogues of these structures without the o-
7.0 Hz, 1H), 2.25 (s, 6H), 1.46 (d, J = 7.0 Hz, 3H). 13C NMR
(DMSO-d6) δ: 150.7, 142.5, 139.0, 128.5, 116.9, 112.9, 108.8, 36.0,
20.7, 18.7; mp = 105 °C (DSC). Anal. Calcd for C18H16N2O2: C,
73.95; H, 5.52; N, 9.58. Found: C, 73.82; H, 5.36; N, 9.42.
4,4′-(Propane-1,1′-diyl)bis(1-cyanato-3-methylbenzene) (18).
1
Yield: 95.6% H NMR (DMSO-d6) δ: 7.24 (m, 6H), 4.16 (t, J = 7.5
Hz, 1H), 2.29 (s, 6H), 1.89 (dt, J = 7.2 Hz, 2H), 0.88 (t, J = 7.2 Hz,
3H). 13C NMR (DMSO-d6) δ: 150.5, 141.0, 139.5, 129.0, 116.9, 112.8,
108.8, 42.7, 28.1, 19.0, 12.4. Anal. Calcd for C19H18N2O2: C, 74.49; H,
5.92; N, 9.14. Found: C, 73.95; H, 5.88; N, 9.10.
Preparation of Networks. Monomers were further purified by
dissolution in dichloromethane, followed by passage through a W-
Prep2XY Yamazen flash chromatography column, and removal of the
solvent under reduced pressure. All monomers were first melted at 90
°C or 15 °C above the melting point. The liquid monomer was then
either placed directly into a hermetically sealed differential scanning
calorimetry (DSC) pan and/or poured into one or more silicone
rubber molds having a disc-shaped cavity and cured under flowing
nitrogen at 150 °C for 1 h and 210 °C for 24 h using a ramp rate of 5
°C/min, then cooled, and demolded. The resultant orange to red discs
measured approximately 12 mm in diameter by 4 mm thick. The
densities of the discs prepared from compounds 17 and 18 were 1.19
and 1.16 g/mL, respectively. The disc prepared from 16 was porous,
and a density was not determined.
Water Immersion Testing. Cured discs were dried to a 0.0001 g
constant weight in a vacuum desiccator, then weighed, and immersed
in approximately 250 mL of deionized water maintained at 85 °C for
96 h. After removal from the water, samples were patted dry and
weighed to determine the moisture uptake (on a dry weight basis).
These samples were used for wet Tg determinations
Characterization Techniques for Networks. Differential
Scanning Calorimetry (DSC). 5−10 mg pieces of cured resins were
removed from the molded discs and hermetically sealed in aluminum
DSC pans. Samples were then ramped under 50 mL/min of flowing
nitrogen at 10 °C/min, first heating to 350 °C, cooling to 100 °C, and
then reheating to 350 °C, using a TA Instruments Q200 differential
scanning calorimeter. The melting points for compounds 16 and 17
were obtained from these experiments and are simply the temperatures
corresponding to the maximum endothermic heat flows.
Oscillatory Thermomechanical Analysis (OTMA). Cured discs
were tested via oscillatory thermomechanical analysis (OTMA) with a
TA Instruments Q400 series analyzer under 50 mL/min of nitrogen
flow. The discs were initially held in place with a compressive force of
0.2 N using the standard ∼5 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 was ramped twice
(heating and cooling) between −50 and 200 °C (to aid in
determination of thermal lag) followed by heating to 350 °C, cooling
to 100 °C, and reheating to 350 °C, all at 50 °C/min. For samples
previously exposed to hot water, the heating rate was decreased to 20
°C/min, and the order of segments was: heating to 350 °C, cooling to
100 °C, two cycles between 100 and 200 °C for thermal lag
determination, and finally heating to 350 °C.
Thermogravimetric Analysis Fourier Transform Infrared Spec-
troscopy (TGA-FTIR). Samples were analyzed using a Thermo Nicolet
Nexus 870 FTIR interfaced via a heated gas cell and transfer line (held
at 150 °C) to a TA Instruments Q50 TGA. The TGA was set to ramp
from room temperature to 500 °C at a rate of 10 °C/min. FTIR
spectra are an average of 32 scans at 4 cm−1 resolution. A liquid
nitrogen cooled MCTA detector was used. Spectra were background
corrected with the gas cell heated and under a nitrogen purge.
Isothermal TGA. Samples were broken into granular chunks and
analyzed with a TA Instruments Q5000 TGA. The temperature was
ramped from ambient up to 350 °C at 10 °C/min in air. The samples
were then held at 350 °C for an additional 6 h.
Figure 1. Synthesis of deoxygenated cyanate esters from renewable
bisphenols.
methoxy group would be difficult to prepare from the parent
bisphenol given that the methoxy group would have to be
eliminated while protecting the phenol for later conversion to a
cyanate ester. A more direct approach to deoxygenated
derivatives would involve elimination of the phenol followed
by hydrolysis of the methoxy group to a hydroxyl group.
Although the resulting compounds would not be direct
analogues (i.e., the hydroxyl group would be para to the
bridging group between aromatic rings), these structures would
still allow for a close comparison between methoxy-function-
alized (oxygenated) and methoxy-free (deoxygenated) cyanate
ester resins and would also represent the most practical
deoxygenated compounds that could be generated from
creosol. In regard to the latter, the comparison between the
two sets of resins would then allow for a quantitative analysis of
the benefits derived from chemically upgrading the renewable
resins.
Synthesis of Deoxygenated Cyanate Ester Resins. In
order to evaluate the deoxygenated cyanate esters, an efficient
synthesis of the precursor bisphenols was conceived (Figure 1).
Deoxygenation was achieved by conversion of the bisphenols to
triflates followed by a palladium catalyzed reductive elimination
using HEt3N(CHO2) as the hydride source.58,59 The triflates
were readily isolated in recrystallized yields of up to 86%. The
ease of recrystallization from methanol solutions made it
convenient to utilize crude bisphenols as starting materials
which both improved the overall conversion of creosol to the
(bis)triflates and simplified the process. The deoxygenation
protocol was similarly efficient and resulted in yields of 75−
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85%. Use of a dppf-functionalized palladium catalyst con-
veniently led to precipitation of the catalyst as an oligomeric
species60 after the reaction was complete. This allowed for
removal of the catalyst by filtration which eliminated the need
for extensive chromatography. Although a relatively high
catalyst loading (4 mol %) was used for the detriflation
reaction, the amount of catalyst used was selected based on
convenience, and significantly lower loadings are likely feasible,
particularly because TLC and precipitation of the catalyst
showed that the reactions were complete within several
minutes. After developing an efficient detriflation protocol,
two methods were employed to convert the bis(methoxy)
compounds to bisphenols. Reaction with either pyridinium
hydrochloride or BBr3 generated the products in 70−75%
yields. In the case of BBr3, treatment with methanol yielded a
volatile methanol adduct of the residual boron compound that
could be conveniently removed under reduced pressure.
With the deoxygenated bisphenols in hand, synthesis of the
bis(cyanate)esters was straightforward with crude yields of 80−
96%. These products were obtained as analytically pure
materials without further purification via crystallization or
column chromatography. However, to make certain that trace
impurities did not affect the network properties, all of the
cyanate ester resins were purified by flash chromatography
immediately prior to curing.
provide insight into the molecular design of liquid resins with
optimal Tgs.
After establishing the effect of methoxy groups on the
melting points of the resins, the cure behavior of the
deoxygenated resins was evaluated by DSC and IR spectros-
copy. The resins had cure exotherms between 109 and 117 kJ/
(mol −OCN). These numbers are in line with the generally
accepted value of ∼100 kJ/(mol −OCN).61 To determine the
degree of cure, the DSC was cycled up to 350 °C, cooled to 50
°C, and then reheated. In all cases the cure was complete as
evidenced by the lack of an exotherm on the second heating
cycle (Figures 2 and 3 and Figure S25). As expected, the DSC
Comparison of Oxygenated and Deoxygenated
Cyanate Ester Resins. The first structure−property relation-
ship studied was the difference in melting points between the
two classes of cyanate esters. Low melting or liquid resins are
much easier to process than high melting resins and can be
easily integrated into conventional composite fabrication
methods. As an initial hypothesis it seemed likely that removal
of the o-methoxy groups would decrease the melting points of
the cyanate esters due to both the decrease in molecular weight
and the decreased polarity of the molecules. In general, this was
reflected in the results with 16 having a melting point 37 °C
lower than 4 and 18 having a melting point at least 95 °C lower
than 6 (Table 1). In contrast, 17 actually had a melting point
Figure 2. DSC data for compound 17.
Table 1. Effect of Deoxygenation on the Melting Points of
the Resins
a
b
compound
Tm (°C)
ΔTm (°C)
c
4
125
16
5
88
91
−37
+14
c
Figure 3. DSC data for compound 18.
17
6
105
120
c
trace for 18 showed no melting point (Figure 3), providing
further evidence that the resin exists as a liquid at room
temperature. IR spectra of cured resins (Figures S34−S36)
showed the complete disappearance of the CN stretching band,
confirming that a high degree of cure had been achieved.
To evaluate the glass transition temperatures of resin
networks, cured disks of each resin were prepared and
subjected to dynamic TMA analysis. Both as-cured Tgs as
well as fully cured Tgs were determined for compounds 16−18.
To obtain fully cured Tgs, disks were postcured to 350 °C in
the TMA. In the case of the oxygenated resins 4−6, heating
above 300 °C leads to network degradation and subsequent
decreases in the Tg.57 Therefore, the maximum achievable Tgs
of both sets of resins were compared (as cured for 4−6 and
fully cured for 16−18). Compounds 16, 17, and 18 showed
increases in Tg of 12, 30, and 18 °C compared to compounds 4,
18
liquid (RT)
>−95
a
b
Determined by DSC. Change in Tm due to deoxygenation of the
resin. Taken from ref 57.
c
14 °C higher than 5. Clearly, there is not always a direct
correlation between the presence of o-methoxy groups and an
increase in melting point. Instead, other factors including the
molecular symmetry, polarity, and lattice energy have a
significant impact on the melting points of the resins. In
contrast to the other resins, compound 18 is a liquid at room
temperature. It is not clear why 18, with a bridging propylidene
group, has a melting point 80 °C lower than 17, with a bridging
ethylidene group, but the study of how bridging structural
groups impact the melting points of cyanate ester resins may
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5, and 6, respectively (Table 2). The Tg of 16 was lower than
expected, likely as a result of atypical cure behavior. The sample
Table 3. Effect of Deoxygenation on the Water Uptake and
Wet Tgs of the Resins
water uptake
(%)
Δ(water uptake)
wet Tg
b
Δ(wet Tg,
a
a
Table 2. Effect of Deoxygenation on the Glass Transition
Temperatures of the Resins
compound
(%)
(°C)
°C)
c
c
4
2.05
184
a
a
b
16
5
NM
unknown
NM
unknown
+29
compound
Tg (as-cured, °C)
Tg (fully cured, °C)
ΔTg (°C)
c
c
c
2.61
178
4
255
248
17
6
2.11
−19
207
16
5
267
+12
+30
+18
c
c
c
3.21
161
253
271
254
256
219
18
1.84
−43
b
198
+37
17
6
283
a
c
Change due to deoxygenation. Measured via TMA (loss profile).
Taken from ref 57.
231
c
18
272
a
b
Measured by TMA (loss profile). Difference in Tg due to
This provides support for the theory advanced by Shimp38,39
and more recently by Guenthner44 that the decreased water
uptake in the ortho-substituted resins is due to inhibited access
of water to hydrophilic sites and not due to differences in
network cross-link density and void space.
deoxygenation. The maximum Tg values (as cured for 4, 5, and 6
and fully cured for 16, 17, and 18 are compared). Taken from ref 57.
c
prepared from 16 was porous and anisotropic and a high
quality test article could not be generated using the standard
cure protocol. Interestingly, no extraneous exothermic reactions
were observed in the DSC data (Figure S25), and cyanate
esters are known for curing without the generation of volatiles,
so it is not clear what led to the voids in the sample. All of the
cyanate esters were prepurified by flash chromatography on
silica gel directly before cure reactions, and no significant
impurities were observed in either the NMR spectrum of 16 or
the elemental analysis results. On this basis it does not seem
likely that impurities led to the anisotropic cure. One other
possibility is that the high purity of the cyanate ester increased
the cure temperature into a regime which resulted in
volatilization of the monomer as cure proceeded. It is likely
that an altered cure protocol or addition of a catalyst would
result in a fully cured and well-formed thermoset material.
Despite the difficulties with 16, the surface of the resin disk was
well formed and rendered a fragment that was suitable for TMA
analysis. Even though 4 has the highest as-cured Tg of the
oxygenated cyanate esters (255 °C), the deoxygenated version
still has a Tg 12 °C higher. It is also interesting to note that the
as-cured Tgs of 6 and 18 are very similar. This suggests that the
o-methoxy group does not intrinsically reduce the Tg of the
resulting thermoset. Instead, the methoxy group is responsible
for the decomposition of the oxygenated resins at the higher
temperatures required for complete cure of the resins as
represented by reduced fully cured Tgs.
Given the lower water uptake of the deoxygenated resins, it
was also expected that they would have higher wet Tgs. Again, it
was not possible to determine a wet Tg for 16, but compounds
17 and 18 had wet Tgs 29 and 37 °C higher than 5 and 6,
respectively. Overall, the wet Tg knockdowns of 64 and 58 °C62
for 17 and 18, respectively, are similar to LECy (52 °C).18
Another key property of cyanate ester resins, their thermal
stability, has been shown to be significantly affected by the
presence of o-methoxy groups.57 This is likely due to electron
donation from the methoxy groups into the aromatic rings of
the polymer. Defining the onset of thermal degradation (OTD)
as the temperature at which 5% weight loss is observed,
compounds 16−18 began to degrade at almost identical
temperatures between 409 and 414 °C (Figure 4). In contrast,
After evaluating the impact of o-methoxy groups on the Tgs
of the resins, it was of interest to examine how the methoxy
groups affected water uptake and the wet Tgs of the materials.
These considerations are critical for materials used in maritime
environments. Removal of the methoxy group resulted in a
decrease in the water uptake of 19% for 17 compared to 5 and
43% for 18 compared to 6 (Table 3). No comparison could be
made between 16 and 4 due to the poor quality test article
formed from 16. These results were expected based on the
ability of the methoxy groups to hydrogen bond with water. In
regard to the properties of the deoxygenated resins as
compared to conventional cyanate ester resins, a recent
paper44 has shown that the presence of methyl groups ortho
to the cyanate ester groups results in an up to 50% reduction in
water uptake compared to BADCy and LECy, commercial
cyanate esters derived from bisphenol A and bisphenol E,
respectively. In contrast, the current work shows that the
presence of methyl groups in the position meta to the cyanate
ester groups does not decrease the water uptake of the resins.
Figure 4. TGA data for compounds 16−18.
we recently reported that compounds 4−6 degrade between
329 and 360 °C.57 The 50−80 °C difference in the OTD
reflects the presence of two fundamentally different decom-
position pathways. In the case of the o-methoxylated cyanate
esters, previous work showed that the primary decomposition
products were phenolic fragments and isocyanic acid.57 To
explore the thermal decomposition of the deoxygenated
cyanate esters, TGA-FTIR was employed. The primary
decomposition products at 430 °C were isocyanic acid and
CO2. At 490 °C the primary decomposition products were
CO2, methane, ammonia, and some isocyanic acid (Figure 5).
The lack of electron donation into the aromatic rings appears to
increase the stability of the aliphatic bridging groups between
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resins that can be generated from creosol. Thus, this work has
shown the degree to which the properties of compounds 4−6
can be altered by deoxygenation of the resins. This in turn
allows for an assessment of the amount of manipulation
required to generate full-performance resins from renewable
phenols. From a renewable and green chemistry perspective, it
makes sense to minimize the number of steps required to
isolate the final resin. Depending on the application, the
renewable materials may exhibit suitable performance. How-
ever, when higher performance materials are required, removal
of the methoxy groups offers a pathway to improve the
properties and increase the thermo-oxidative stability of these
materials. In effect, renewable phenols can be upgraded to full-
performance resins. One of the key challenges of this approach
is the development of efficient and atom-economic methods for
selectively deoxygenating renewable phenols.
Figure 5. Gas phase IR data for the thermal decomposition products
of compound 16.
the rings. This stability then allows the polymer to survive to
significantly higher temperatures, which in turn shifts the
decomposition pathway to degradation of the triazine ring
systems. This reaction pathway is known to generate isocyanic
acid, methane, CO2, and NH3, with the latter two molecules
dominating at higher temperature.63 On the basis of the evident
decomposition pathway, one would expect that the deoxy-
genated cyanate esters would have higher char yields, not only
due to a decrease in thermally labile groups but also due to the
greater stability of the resins which allows for char formation to
be competitive with decomposition to volatile components. A
comparison of char yields at 500 °C (Table 4) shows that the
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra of new compounds, DSC, TMA, and
TGA data for thermoset networks, IR spectroscopy of resins
and thermoset networks, and gas phase IR spectra of
decomposition products. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.macromol.5b00496.
AUTHOR INFORMATION
Corresponding Author
*E-mail benjamin.g.harvey@navy.mil (B.G.H.).
■
Table 4. Char Yield of Resins at 500 °C
Notes
compound
char yield (% in N2)
Δ char yield
% mass (OMe)
18.3
The authors declare no competing financial interest.
4
38.6
59.0
30.6
56.8
30.0
51.7
16
5
+20.4
+26.2
+21.7
ACKNOWLEDGMENTS
■
17.6
16.9
The authors thank the Strategic Environmental Research and
Development Program (SERDP, WP-2214), the Office of
Naval Research (ONR), and the Air Force Office of Scientific
Research (AFOSR) for financial support of this work.
17
6
18
deoxygenated resins have char yields 20−26% higher than the
oxygenated versions and that these values are higher than can
be explained by loss of the methoxy groups.
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