Using Dicyclopentadiene-Derived Polyarylates as Epoxy Curing Agents To Achieve High T g and Low Dielectric Epoxy Thermosets.

To achieve high-T g and low-dielectric epoxy thermosets, four dicyclopentadiene-derived polyarylates (26-P, 26-M, 236-P, and 236-M) were prepared from 2,6-dimethyl (or 2,3,6-trimethyl) phenol-dicyclopentadiene adduct with terephthaloyl (or isophthaloyl) chloride by high-temperature solution polymerization. The resulting polyarylates, exhibiting active ester linkages (Ph-O-(C=O)-) are found to be reactive toward a commercial dicyclopentadiene phenol epoxy (HP7200) in the presence of some lone-pair electron-containing compounds. Five compounds including 4-dimethylaminopyridine (DMAP), imidazole, 2-methylimidazole, triphenylphosphine, and triphenylimidazole have been evaluated as a catalyst for the curing reactions. We found that DMAP, with the smallest pK b among them, is the best catalyst according to differential scanning calorimetry, infrared, and thermal analyses. The thermal and dielectric properties of the polyarylate/HP7200 thermosets are evaluated. We found that they exhibit a high T g characteristic (e.g., T g is 238 °C for DMAP-catalyzed, 236-P/HP7200 thermoset). Furthermore, because of the hydrophobic methyl and cycloaliphatic moieties, and the secondary hydroxyl-free structure, polyarylate/HP7200 thermosets show a relative low-dielectric constant of around 2.75 U. The detailed structure-properties relationship is discussed in this work.

Introduction

Because of the good chemical resistance, adhesion, dimensional stability, and insulation after thermal curing, epoxy resins have been widely applied in the electronic field such as encapsulation and printed circuit boards. Generally, epoxy resins are cured by reacting the epoxy groups with active hydrogen-containing curing agents, such as phenol novolac, diamines, and thiols.[1−11] The curing reaction leads to a highly polar secondary alcohol linkages, which will increase the dielectric constant of epoxy thermosets. Therefore, compared with other low-dielectric thermosets,[12] epoxy thermosets show a moderate-to-high dielectric constant.

Using active ester-type epoxy curing agents (curing agents with Ph–O–(C=O)– structure) is a solution to prepare secondary alcohol-free epoxy thermosets.[13−15] (Note that the active esters mentioned in this work are different from those reviewed by Das and Theato for postpolymerization modification).[16] Nakamura and Arima prepared a triphenol and its active ester as epoxy curing agents,[15] and they found that epoxy thermosets cured by the active ester exhibit better dielectric properties than those cured by the triphenol. They also (1) cured copolymers of phenyl methacrylate (contains active ester moiety after polymerization of the C=C bond) by polyfunctional epoxy;[17,18] (2) prepared polyethers with pendant ester by the condensation of p-phenylene dibenzoate with diepoxy in the presence of quaternary salts or the mixture of crown ether with organic and inorganic salts;[19,20] and (3) studied thermocrosslinking reactions of polymers containing pendant epoxy with various active esters.[21−23]

Nelson and Jackson,[24] Hiroaki,[25] Wang et al.,[26,27] have synthesized low-dielectric hydrocarbon-containing epoxy thermosets. Among these hydrocarbons, dicyclopentadiene (DCPD) and higher oligomers of cyclopentadiene are the most commonly used starting materials because of their availability (DCPD is a byproduct of C5 streams in oil refineries) and low cost. Taking advantage of DCPD, Dainippon Ink and Chemicals Corporation has commercialized EPICLON HPC-8000, a phenol-DCPD adduct-based active ester for preparing low-dielectric epoxy thermosets.[28] In addition to HPC-8000, DC808, a phenol novolac-based active ester, has been commercialized by Japan Epoxy Resins for preparing low-dielectric epoxy thermosets.[29]

The commercial active ester-type curing agents such as EPICLON HPC-8000 and DC808 are oligomer types. They are generally prepared from the esterification of (a) phenol-DCPD adduct, or (b) naphthol-DCPD adduct, or (c) phenol-formaldehyde novolac, or (d) biphenol A-formaldehyde novolac (Figure ). It is worth noticing that main-chain-type epoxy curing agents show a series of advantages,[30−32] such as improvement in the thermal and mechanical properties of the epoxy thermosets. For example, Francis et al. prepared a hydroxyl-terminated poly(ether ether ketone) with different molecular weights and used them as epoxy curing agents.[33−35] They found that the mechanical properties of thermosets increase with the increase in the molecular weight of the curing agents. In our previous work,[36] we have successfully prepared a phenol-functionalized phosphinated polyether (P1). P1, with phenol linkages, can react with cresol novolac epoxy to get a flexible and transparent of an epoxy thermosetting film with a Tg value as high as 250 °C. We have also demonstrated that high-MW polybenzoxazine precursors can lead benzoxazine thermosets to a much higher Tg than those cured by monomer-type benzoxazines.[37,38] The secondary alcohol resulting from the curing reaction of active hydrogen and epoxy can provide intermolecular interactions, so epoxy thermosets cured by active hydrogens show higher Tg than those cured by active ester-type curing agents. This is the penalty for epoxy thermosets cured by active ester-type curing agents. However, our previous work[36−38] indicates that using polymeric-type curing agent can result in epoxy thermosets with a high Tg characteristic.

Figure 1

Structure of phenolic oligomers (the precursor of active esters) (a) phenol-DCPD adduct, (b) naphthol-DCPD adduct, (c) phenol-formaldehyde novolac, and (d) biphenol A-formaldehyde novolac.

In this work, combining the concept of (1) active ester, (2) polymeric-type curing agent, and (3) hydrophobic DCPD and methyl groups, we prepare four DCPD-derived polyarylates (26-P, 26-M, 236-P, and 236-M) for preparing high-Tg, low-dielectric epoxy thermosets. The resulting polyarylates, all with active ester Ph–O–(C=O)–linkages, are reactive toward epoxy resin in the presence of some lone-pair electron-containing compounds. To the best of our knowledge, no DCPD-derived polyarylates have been applied as curing agents. Five compounds including 4-dimethylaminopyridine (DMAP), imidazole (Imi), 2-methylimidazole (2-MI), triphenylphosphine (TPP), and triphenylimidazole (TPI) have been evaluated as a catalyst for the curing reactions. We found that the basicity of the compound has a strong influence on curing reactions and thermal and dielectric properties of the resulting epoxy thermosets. The synthesis of polyarylates and the influence of the catalyst on curing reaction and properties of thermosets will be discussed in this work.

Results and Discussion

Synthesis of 2,6-Phenol-DCPD and 2,3,6-Phenol-DCPD

2,6-Phenol-DCPD and 2,3,6-phenol-DCPD were prepared from the alkylation of DCPD with 2,6-dimethyl phenol or 2,3,6-dimethyl phenol, respectively, in the presence of AlCl3 (Scheme ). The reaction involves two steps. In the first step, DCPD reacted with AlCl3 to form carbocations, which then reacted with 2,6-dimethyl phenol or 2,3,6-dimethyl phenol in the second step. Figure S1 shows the 1H NMR spectra of 2,6-phenol-DCPD and 2,3,6-phenol-DCPD. There are no unsaturated −HC=CH– signals of DCPD at 5.4 and 5.9 ppm in both spectra, indicating a complete reaction. The signals of aliphatic hydrogens and methyl in the range of 1.2–3.6 ppm, the characteristic peaks of Ar-H at 6.6–7.0 ppm, and the OH peaks at 4.43 and 4.46 ppm support the structure of 2,6-phenol-DCPD and 2,3,6-phenol-DCPD. There are multiple constitutional isomers in both 2,6-phenol-DCPD[39] and 2,3,6-phenol-DCPD, so the peak patterns are not as simple as a pure compound. Figure S2 shows the infrared (IR) spectra of DCPD, 2,6-phenol-DCPD, and 2,3,6-phenol-DCPD. The characteristic peak of unsaturated HC=CH signals at 3053 and 750 cm–1 disappeared, and characteristic absorption of the aromatic ring at 1490–1450 cm–1 and the OH absorption at 3400–3230 cm–1 support the structure of 2,6-phenol-DCPD and 2,3,6-phenol-DCPD.

Scheme 1

Synthesis of 2,6-Phenol-DCPD and 2,3,6-Phenol-DCPD and Four Polyarylates

Synthesis of Polyarylates

Polyarylates were prepared from 2,6-phenol-DCPD or 2,3,6-phenol-DCPD with terephthaloyl chloride (TPC) or isophthaloyl chloride (IPC) by high-temperature solution polymerization in o-dichlorobenzene (Scheme ). Figure shows the 1H NMR of the four polyarylates. The Ar-H from the terephthalic structure was clearly observed at 8.4 ppm for 26-P and 236-P. The Ar-H from the isophthalic structure was clearly observed at 7.8, 8.5, and 9.2 ppm for 26-M and 236-M. Figure S3 shows the IR spectra of the four polyarylates. The broad and strong OH absorption at around 3500 cm–1 disappeared, and the ester absorption at 1735 cm–1 was clearly observed. Figure S4 shows the differential scanning calorimetry (DSC) thermograms of synthesized polyarylates. The Tg values are 224, 199, 187, and 177 °C for 26-P, 26-M, 236-P, and 236-M, respectively. The Tg values depend on the structure of polyarylates. Polyarylates 26-P and 236-P, with the para-substituted structure, are more rigid than those (26-P and 236-P) with the meta-substituted structure, leading to higher Tg values. Furthermore, the 236-series polyarylates (236-P and 236-M) have lower Tg values than 26-series polyarylates (26-P and 26-M), probably because of the fact that the three methyl substitutions hinder the molecular packing.

Figure 2

1H NMR spectra of polyarylates (a) 26-P, (b) 26-M, (c) 236-P, and (d) 236-M.

Influence of Catalysts on the Exothermic Peak of DSC Thermograms

Figure shows DSC heating scans of polyarylate/HP7200 blends using different types of catalysts. Note that the word blend means polyarylate/HP7200 has not been thermally cured before DSC scans. The order of the exothermic peak temperature is DMAP-catalyzed < 2MI-catalyzed < Imi-catalyzed systems, suggesting that DMAP is the best catalyst among them. No exothermic peak was found for TPP- and TPI-catalyzed systems, indicating that no reaction occurs in both systems. Figure lists the pKb and structure of catalysts used in this work. According to Figures and 4, we found that a catalyst with smaller pKb has the lower exothermic peak temperature. These results indicate that a catalyst with a stronger basicity displays a better catalytic effect, and DMAP is found to be the best catalyst among them.

Figure 3

DSC heating scans of polyarylate/HP7200 blends with different types of catalysts. (a) 26-P/E-X blend, (b) 26-M/E-X blend, (c) 236-P/E-X blend, and (d) 236-M/E-X blend. X is the catalyst: DMAP, 2MI, Imi, TPP, and TPI, respectively. The catalyst used is marked in the figure.

Figure 4

pKb value and the structure of catalysts used in this work.

Influence of Catalysts on Carbonyl Absorption of IR Spectra

In our previous work, we have designed a model reaction of glycidyl phenyl ether and phenyl acetate at 120 °C in a molten state in the presence of 2.0 wt % (based on the weight of glycidyl phenyl ether) DMAP.[40] After careful evaluation of the product’s peaks, we found the reaction product is 1,3-diphenoxy-2-acetoxypropane (Figure ). The carbonyl absorption changes from Ar–O–C=O– to R–O–C=O–. According to the IR absorption principle, aromatic ester with Ar–O–(C=O) has higher absorption than that of aliphatic ester with R–O–(C=O).[41] Therefore, we can use the shift of C=O absorption to monitor the reaction of polyarylate and HP7200 in the presence of a catalyst.

Figure 5

Model reaction of glycidyl phenyl ether and phenyl acetate in the presence of DMAP.

Figure shows enlarged Fourier transform infrared (FTIR) spectra of polyarylates and polyarylate/HP7200 thermosets catalyzed by different catalysts. Note that the word thermoset means polyarylate/HP7200 has been thermally cured. The C=O absorption value of epoxy thermosets is listed in Figure . For DMAP-, Imi-, and 2MI-catalyzed system, the shift is obvious from around 1737 cm–1 to around 1725 cm–1. The order of IR shift is DMAP-catalyzed > 2MI-catalyzed > Imi-catalyzed systems, suggesting that DMAP is the best catalyst among them. No shift in carbonyl absorption was found for TPP- and TPI-catalyzed systems, indicating that no reaction occurs in both systems.

Figure 6

Enlarged FTIR spectra of polyarylate and polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P and 26-P/E-X, (b) 26-M and 26-M/E-X, (c) 236-P and 236-P/E-X, and (d) 236-M and 236-M/E-X. X is the catalyst: DMAP, 2MI, Imi, TPP, and TPI, respectively. The catalyst used is marked in the figure.

Figure 7

C=O absorption values of polyarylate and polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P and 26-P/E-X, (b) 26-M and 26-M/E-X, (c) 236-P and 236-P/E-X, (d) 236-M and 236-M/E-X. X is the catalyst: DMAP, 2MI, Imi, TPP, and TPI, respectively.

The results indicate that a catalyst with a stronger basicity has a better catalytic effect, and DMAP is found to be the best catalyst among them. This result is consistent with the DSC thermogram in Figure .

Proposed Mechanism

According to the conclusion of the model reaction of glycidyl phenyl ether and phenyl acetate in the presence of DMAP[40] and IR spectra in Figure , we propose a curing mechanism of polyarylate (236-P) and HP7200 in the presence of DMAP in Scheme . The first step is the attack of DMAP on the epoxy group of HP7200, forming intermediate (I) with an alkoxy anion. The second step is the nucleophilic substitution of the alkoxy anion of (I) on the ester group of polyarylate, forming intermediate (II) and polyarylate with a phenoxy anion. The third step is the nucleophilic substitution of the phenoxy anion of polyarylate on the carbon of the C–N bond of (II), forming intermediate (III) and releasing DMAP that can repeat step 1. Another way to obtain (III) is through steps 4 and 5. The fourth step is the nucleophilic addition of the phenoxy anion of polyarylate on the epoxy group of HP7200, forming intermediate (IV) with an alkoxy anion. The fifth step is the nucleophilic substitution of the alkoxy anion of (IV) on the ester group of polyarylate, forming (III) and polyarylate with a phenoxy anion that can repeat step 4.

Scheme 2

Proposed Reaction Mechanism of Polyarylates (236-P) and HP7200 in the Presence of DMAP

Solubility of Polyarylate/HP7200 Thermosets

Table lists the solubility of the polyarylates and polyarylate/HP7200 thermosets. With 10 wt % concentration, the four polyarylates are soluble in tetrahydrofuran (THF), NMP, toluene, N,N-dimethyl acetamide (DMAc), and CHCl3 at room temperature, and are soluble in methyl ether ketone (MEK) on heating. The multiple methyl groups and multiple constitutional isomers of 2,6-dimethyl (or 2,3,6-trimethyl) phenol-DCPD adduct make them soluble. The good solubility of polyarylates makes them easy for solution processing for future application in copper clad laminate. In contrast to the good solubility of polyarylates, the thermosets are insoluble, suggesting that network structures are formed.

Table 1

Solubility of Polyarylate and Polyarylate/HP7200 Thermosetsa,b

sample	THF	MEK	NMP	toluene	DMAc	CHCl3
26-P	+	+h	+	+	+	+
26-P/E-Xc	–	–	–	–	–	–
26-M	+	+h	+	+	+	+
26-M/E-Xc	–	–	–	–	–	–
236-P	+	+h	+	+	+	+
236-P/E-Xc	–	–	–	–	–	–
236-M	+	+h	+	+	+	+
236-M/E-Xc	–	–	–	–	–	–

Solubility was tested with a 5 mg in 0.5 mL of solvent.

+, soluble at room temperature; +h, soluble on heating, −, insoluble.

All of the thermosets are insoluble regardless of X (the catalyst: DMAP, 2MI, or Imi).

Thermomechanical Property

To discuss the influence of the catalyst on Tg of a thermoset, we evaluate polyarylate/HP7200 thermosets catalyzed by different catalysts by dynamic mechanical analysis (DMA, Figure ). The data are listed in Table . With the same polyarylate, the order in the Tg value is DMAP-catalyzed > 2MI-catalyzed > Imi-catalyzed systems. For example, for 26-P/E-X, Tg values are 239, 222, and 217 when X is DMAP, 2MI, and Imi, respectively. The results indicate that a catalyst with a stronger basicity has a better catalytic effect, and DMAP is found to be the best catalyst among them.

Figure 8

DMA thermograms of polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P/E-X, (b) 26-M/E-X, (c) 236-P/E-X, and (d) 236-M/E-X. X is the catalyst: DMAP, 2MI, and Imi. The catalyst used is marked in the figure.

Table 2

Thermal Properties of Polyarylates/HP7200 Thermosets

sample	Tg (°C)a	Tg (°C)b	CTE (ppm/°C)c	Td,onset (°C)d	Td5% (°C)e	CY (%)f
26-P/E-DMAP	239	197	57	402	416	14
26-M/E-DMAP	225	188	52	401	415	10
236-P/E-DMAP	238	195	59	401	407	12
236-M/E-DMAP	209	179	80	400	406	11
26-P/E-2MI	222	187	68	404	414	10
26-M/E-2MI	213	176	74	399	412	10
236-P/E-2MI	220	181	78	402	407	11
236-M/E-2MI	213	175	70	399	411	10
26-P/E-Imi	217	186	75	404	412	12
26-M/E-Imi	212	173	49	401	412	15
236-P/E-Imi	212	176	68	401	406	12
236-M/E-Imi	209	173	54	400	407	13

Measured by DMA at a heating rate of 5 °C/min.

Measured by TMA at a heating rate of 5 °C/min.

CTE is recorded from 50 to 150 °C.

Onset decomposition temperature, recorded by thermogravimetry at a heating rate of 20 °C/min.

5 wt % decomposition temperature, recorded by thermogravimetry at a heating rate of 20 °C/min.

Residual wt % at 800 °C in N2 atmosphere.

According to the IR spectra (Figure ), the shift of the C=O signal strongly depends on the basicity of the catalyst. Therefore, the difference in Tg is attributed to the different curing degrees. To discuss the influence of polyarylate on Tg of the thermoset, DMA thermograms of polyarylate/HP7200 thermosets cured by different polyarylates are shown in Figure S5. With the same catalyst, the order in Tg values is generally 26-P/E-X > 236-P/E-X > 26-M/E-X > 236-M/E-X. For example, the Tg values are 239, 238, 225, and 209 °C for 26-P/E-DMAP, 236-P/E-DMAP, 26-M/E-DMAP, and 236-M/E-DMAP, respectively. Thermosets based on the para-substituted polyarylate (26-P and 236-P) are more rigid than those based on the meta-substituted polyarylates (26-M and 236-M). The Tg values in this work are in the range of 209–239 °C, which are much higher than that (165 °C, DMA data) of the DMAP-catalyzed EPICLON HPC-8000/HP-7200 thermoset.[42] This work demonstrates that the epoxy thermoset cured by polymeric-type curing agents with active ester in the main chain, such as polyarylates in this work, displays higher Tg than those cured by oligomeric-type curing agents with active ester in the side chain, such as HPC-8000.

To further discuss the influence of the catalyst on Tg of a thermoset, we evaluate polyarylate/HP7200 thermosets catalyzed by different catalysts by thermomechanical analysis (TMA) (Figure ). The data are listed in Table . With the same polyarylate, the order in the Tg value is DMAP-catalyzed > 2MI-catalyzed > Imi-catalyzed systems. For example, for 26-P/E-X, Tg values are 197, 187, and 186 when X is DMAP, 2MI, and Imi, respectively. The relation between the type of the catalyst and Tg is the same as those observed in DMA data. However, no obvious trend between structure and the coefficient of thermal expansion (CTE) was observed. To discuss the influence of polyarylate on Tg of a thermoset, TMA thermograms of polyarylate/HP7200 thermosets cured by different polyarylates are shown in Figure S6. With the same catalyst, the order in Tg values is 26-P/E-X > 236-P/E-X > 26-M/E/X > 236-M/E-X. For example, Tg values are 197, 195, 188, and 179 °C for 26-P/E-DMAP, 236-P/E-DMAP, 26-M/E-DMAP, and 236-M/E-DMAP, respectively. The relation between the structure of polyarylates and Tg is the same as those observed in DMA data.

Figure 9

TMA thermograms of polyarylate/HP7200 thermosets catalyzed by different catalysts. (a) 26-P/E-X, (b) 26-M/E-X, (c) 236-P/E-X, and (d) 236-M/E-X. X is the catalyst: DMAP, 2MI, and Imi. The catalyst used is marked in the figure.

Thermal Stability

Figure S7 shows the thermal gravimetric analysis (TGA) thermograms of the thermosets in a nitrogen atmosphere. Table lists the thermal stability data of polyarylate/HP7200 thermosets. The onset degradation temperature in a nitrogen atmosphere is around 400 °C, and the 5 wt % degradation temperatures (Td5) in nitrogen atmosphere range from 406 to 416 °C, and char yields are in the range of 10–15%. These values are slightly higher than typical epoxy thermosets, suggesting the benefit in thermal stability using polyarylates as the curing agent. However, no obvious structure–thermal stability relationship was found.

Dielectric Property

Table lists the dielectric constant and dissipation factor of the polyarylate/HP7200 thermosets. The dielectric constant ranges from 2.74 to 2.88 U, and the dissipation factor ranges from 10.1 to 15.4 mU. With the same catalyst, meta-substituted polyarylate/HP7200 thermosets show better dielectric properties than para-substituted polyarylate/HP7200 thermosets. For example, the dielectric constant (dissipation factor) is 2.88 U (15.1 mU) and 2.83 U (14.3 mU) for 26-P/E-2MI and 26-M/E-2MI, respectively; and it is 2.83 U (14.0 mU) and 2.76 U (12.5 mU) for 236-P/E-2MI and 236-M/E-2MI, respectively. In addition, epoxy thermosets cured by trimethyl-substituted polyarylates (236-P and 236-M) show better dielectric properties than those cured by dimethyl-substituted polyarylates (26-P and 26-M). For example, the dielectric constant (dissipation factor) is 2.85 U (14.8 mU) and 2.75 U (13.5 mU) for 26-P/E-DMAP and 236-P/E-DMAP, respectively; and it is 2.83 U (13.6 mU) and 2.74 U (10.1 mU) for 26-M/E-DMAP and 236-M/E-DMAP, respectively.

Table 3

Dielectric Properties of DCPD Series Cured Epoxy Thermosets

sample	thickness (μm)	Dk (U, 1 GHz)	Df (U, 1 GHz)
26-P/E-DMAP	403	2.85 ± 0.002	0.0148 ± 0.0002
26-M/E-DMAP	384	2.83 ± 0.002	0.0136 ± 0.0003
236-P/E-DMAP	377	2.75 ± 0.001	0.0135 ± 0.0002
236-M/E-DMAP	371	2.74 ± 0.003	0.0101 ± 0.0003
26-P/E-2MI	238	2.88 ± 0.002	0.0151 ± 0.0001
26-M/E-2MI	211	2.83 ± 0.001	0.0143 ± 0.0002
236-P/E-2MI	203	2.83 ± 0.002	0.0140 ± 0.0001
236-M/E-2MI	198	2.76 ± 0.003	0.0125 ± 0.0003
26-P/E-Imi	202	2.87 ± 0.01	0.0154 ± 0.0001
26-M/E-Imi	217	2.84 ± 0.03	0.0146 ± 0.0002
236-P/E-Imi	233	2.81 ± 0.01	0.0143 ± 0.0001
236-M/E-Imi	200	2.79 ± 0.02	0.0129 ± 0.0003

Hougham et al. reported that the dielectric constant can be reduced by increasing the molecule’s hydrophobicity and free volume and by decreasing polarization.[43] The fact that the meta-substituted polyarylates provide higher free volume than the para-substituted polyarylates might be responsible for the better dielectric property of meta-substituted polyarylate/HP7200 thermosets. The hydrophobic methyl group might be the reason for the better dielectric property of trimethyl-substituted polyarylate/HP7200 thermosets. As listed in Table , with the same polyarylate, the order of the dissipation factor is generally DMAP-catalyzed < 2MI-catalyzed < Imi-catalyzed systems. For example, the dissipation factor is 14.8, 15.1, and 15.4 mU for 26-P/E-DMAP26-P/E-2MI and 26-P/E-Imi; and it is 10.1, 12.5, and 12.9 mU for 236-M/E-DMAP, 236-M/E-2MI, and 236-M/E-Imi, respectively.

Previous data (Figure , 6–9) show that the order of the catalytic effect is DMAP > 2MI > Imi, so the higher conversion (with less residual high-polar epoxy) might be the reason for the order of the dissipation factor. The dielectric constant is in the range of 2.74–2.88 U, which is much smaller than that (around 3.5)[44−48] for common epoxy thermosets. We think that the hydrophobic methyl and cycloaliphtic moiety and the secondary alcohol-free structure of the thermosets are responsible for the good dielectric properties. Moreover, the Tg corresponding to the dielectric constant of 2.74–2.88 is as high as 209–239 °C (Tables and 3), demonstrating that high-Tg, low-dielectric epoxy thermosets can be achieved through this facile approach.

Conclusions

We have successfully prepared four DCPD-derived polyarylates. The resulting polyarylates, exhibiting active ester linkages (Ph–O–(C=O)−), are found to be reactive toward epoxy (HP7200) in the presence of some lone-pair electron-containing compounds. Among the compounds, DMAP, with the lowest pKb, has the lowest exothermic peak for the polyarylate/HP7200 blend in the DSC thermogram (Figure ), the highest C=O shift of the thermoset in the IR spectrum (Figure ), and the highest Tg of thermosets (Figures and 9). Tg values of thermosets cured by the four polyarylates are in the range of 209–239 °C, which are much higher than that (165 °C) of the DMAP-catalyzed commercialized EPICLON HPC-8000/HP-7200 thermoset. This result demonstrates the advantage of polyarylates over commercialized oligomeric-type curing agents in thermal properties. With the same catalyst, thermosets based on the para-substituted polyarylate (26-P and 236-P) display higher Tg than those based on the meta-substituted polyarylates (26-M and 236-M) (Table ). Thermosets based on meta-substituted and trimethyl-substituted polyarylates (236-M) show the best dielectric properties. With the same polyarylate, the value of the dissipation factor is generally DMAP-catalyzed < 2MI-catalyzed < Imi-catalyzed systems (Table ), which is the inverse order of reactivity of the catalyst. In other words, a catalyst with a better reactivity will lead to a better dissipation factor, probably because of the better conversion that will transform highly polar epoxy groups to less polar ester groups. The dielectric constant of epoxy thermosets ranges from 2.74 to 2.88 U which is much smaller than that (around 3.5) for common epoxy thermosets. The Tg values corresponding to the dielectric constant of 2.74–2.88 are as high as 209–239 °C, demonstrating that high-Tg, low-dielectric epoxy thermosets can be achieved through this facile approach. To the best of our knowledge, this is the first example using DCPD-derived polyarylates as epoxy curing agents to achieve high-Tg and low-dielectric epoxy thermosets.

Experimental Section

Materials

DCPD (from Sigma), aluminum(III) chloride (AlCl3, from Alfa), TPC (from TCI), IPC (from TCI), 2,6-dimethylphenol (from Acros), 2,3,6-dimethylphenol (from Acros), and phenol DCPD novolac epoxy with an EEW of 250 g/equiv were kindly supplied by Dainippon Ink and Chemicals Corporation under the commercial name of HP-7200. 4-Dimethylaminopyrdine (DMAP, from Alfa), Imi (from Acros), 2-MI (from Aldrich), TPP (from Alfa), and 2,4,5-TPI (from Alfa) were used as received without purification. o-Dichlorobenzene (from Acros), N-methylpyrrolidone (NMP from Tedia), DMAc (from Tedia), MEK (from Acros), and other solvents used for the solubility test are HPLC or ACS grade and were used as received without purification.

Synthesis of 2,6-Dimethyl Phenol-DCPD Adduct (2,6-Phenol-DCPD)

2,6-Dimethyl phenol (0.7143 mol) and AlCl3 (0.015 mol) were added to a four-necked round-bottomed flask equipped with a nitrogen inlet, magnetic stirrer, thermocouple, and temperature controller. The reaction mixture was gradually heated to 120 °C. DCPD 0.1 mol was added gradually over a period of 2 h. The solution was kept at 120 °C for another 4 h. After the reaction was completed, 0.06 mol of 5 wt % NaOH(aq) was added, and the mixture was stirred for 1 h. The reaction mixture was filtered, and the filtrate was washed three times with water. Next, the organic phase was separated and distilled in a rotary evaporator to remove excess 2,6-dimethyl phenol. The crude product was dissolved in toluene and extracted with water for several times. The organic phase was distilled to remove toluene, and a deep-brown 2,6-phenol-DCPD was obtained in quantitative yield. 1H NMR (ppm, CDCl3): δ 1.05–2.80 (26H, aliphatic-H), 6.60–7.10 (4H, aromatic-H), 4.43–4.76 (2H, OH). FT-IR (KBr, cm–1): 3468 (OH), 2929, 2487 (CH3), 1200 (C–O). HR-MS (FAB+) m/z: calcd for C26H32O2, 376.24; anal., 375.2326.

Synthesis of 2,3,6-Trimethyl Phenol-DCPD Adduct (2,3,6-Phenol-DCPD)

2,3,6-Phenol-DCPD was prepared by the reaction of 2,3,6-trimethyl phenol and DCPD in the presence of AlCl3. The preparation procedure is the same as that of 2,6-phenol-DCPD. 1H NMR (ppm, CDCl3): δ 1.21–2.40 (32H, aliphatic-H), 6.81–7.00 (2H, aromatic-H), 4.45–4.80 (2H, OH). FT-IR (KBr, cm–1): 3468 (OH), 2929, 2487 (CH3), 1200 (C–O). HR-MS (FAB+) m/z: calcd for C28H36O2, 404.27; anal., 403.2368.

Synthesis of Polyarylate (26-P)

2,6-Phenol-DCPD (8 mmol), TPC (8 mmol), and 30 mL of o-dichlorobenzene were added to a four-necked round-bottomed flask equipped with a nitrogen inlet, magnetic stirrer, thermocouple, and temperature controller. The solution was heated to reflux temperature for 30 h. Then, the solution was poured into methanol to precipitate the polyarylate. After filtration, the filter cake was washed two times with methanol/water (1/1, v/v) and dried at 60 °C in a vacuum oven to obtain a brown polyarylate (26-P). The yield is quantitative. 1H NMR (ppm, CDCl3): δ 1.21–2.40 (26H, aliphatic-H), 6.81–7.20 (4H, aromatic-H), 8.20–8.62 (4H, aromatic-H). FT-IR (KBr, cm–1): 2929, 2487 (CH3), 1735 (C=O), 1200 (C–O). Gel permeation chromatography (GPC): number-average molecular weight 12 600 g/mol, weight-average molecular weight 34 500 g/mol.

Synthesis of Polyarylate (26-M)

Polyarylate (26-M) was prepared by 2,6-phenol-DCPD and IPC. The preparation procedure is the same as that of 26-P. 1H NMR (ppm, CDCl3): δ 1.05–2.80 (26H, aliphatic-H), 6.70–7.20 (4H, aromatic-H), 7.58–7.8 (1H, aromatic-H), 8.28–8.64 (2H, aromatic-H), 8.86–9.16 (1H, aromatic-H). FT-IR (KBr, cm–1): 2929, 2487 (CH3), 1736 (C=O), 1200 (C–O). GPC: number-average molecular weight 13 900 g/mol, weight-average molecular weight 35 900 g/mol.

Synthesis of Polyarylate (236-P)

Polyarylate (236-P) was prepared by 2,3,6-phenol-DCPD and TPC. The preparation procedure is the same as that of 26-P. 1H NMR (ppm, CDCl3): δ 1.21–2.40 (32H, aliphatic-H), 6.61–7.20 (4H, aromatic-H), 8.17–8.57 (4H, aromatic-H). FT-IR (KBr, cm–1): 2929, 2487 (CH3), 1733 (C=O), 1200 (C–O). GPC: number-average molecular weight 21 000 g/mol, weight-average molecular weight 49 100 g/mol.

Synthesis of Polyarylate (236-M)

Polyarylate (236-M) was prepared by 2,3,6-phenol-DCPD and IPC. The preparation procedure is the same as that of 26-P. 1H NMR (ppm, CDCl3): δ 1.21–2.40 (32H, aliphatic-H), 6.7–7.2 (4H, aromatic-H), 7.58–7.8 (1H, aromatic-H), 8.28–8.60 (2H, aromatic-H), 8.86–9.16 (1H, aromatic-H). FT-IR (KBr, cm–1): 2929, 2487 (CH3), 1737 (C=O), 1200 (C–O). GPC: number-average molecular weight 11 400 g/mol, weight-average molecular weight 28 900 g/mol.

Curing Procedure for the Polyarylate/Epoxy Thermoset

The DCPD-phenol epoxy, HP7200, was thermally cured by the four polyarylates. The molar ratio of the ester moiety to oxirane (epoxy group) was controlled at one. Polyarylate, HP7200, and a catalyst (DMAP, Imi, 2MI, TPP, or TPI) 0.5 wt % (based on the weight of epoxy) were dissolved in NMP to make a solution with 30 wt % solid content. The viscous solution was cast on a glass by an automatic film applicator. The resulting thin films were dried at 80 °C for 12 h. Then, under nitrogen atmosphere, the curing process was done for 140 °C (2 h), 160 °C (2 h), 180 °C (2 h), and 200 °C (2 h). The thermosets are named in the format of “polyarylate/E-X”. X is the catalyst. For example, the thermoset cured from 26-P, HP7200, and DMAP is named 26-P/E-DMAP and that cured from 236-M, HP7200 and Imi is named 236-M/E-Imi and so on.

Characterization

DMA was performed with a PerkinElmer Pyris Diamond DMA with a sample size of 5.0 cm × 1.0 cm × 0.2 cm, and the test was performed using a tension mode with an amplitude of 25 μm. The storage modulus and tan(δ) were determined as the sample subjected to the temperature scan mode at a programmed heating rate of 5 °C/min with a frequency of 1 Hz. NMR measurements were performed using a Varian Inova 600 NMR in DMSO-d6 or CDCl3. The chemical shift of 1H NMR was calibrated by setting the chemical shift of a trace amount of DMSO-d5 in DMSO-d6 as 2.49 ppm. IR spectra were obtained in the standard wavenumber range of 400–4000 cm–1 by a PerkinElmer RX1 IR spectrophotometer. TGA was performed with PerkinElmer Pyris 1 at a heating rate of 20 °C/min in a nitrogen atmosphere. Dielectric properties were determined with Agilent E4991A at 1 GHz with a sample size with 2.5 cm in diameter and thickness at least 200 μm. TMA was performed by a SII TMA/SS6100 at a heating rate of 5 °C/min. The CTE was recorded at the temperature range of 50–150 °C. GPC was performed by Hitachi L2400 in N-methyl-2-pyrrolidone (NMP) (Hitachi, Tarrytown, NY, Tarrytown, USA), using polystyrene as the standard.