Phthalonitrile resin bearing cyanate ester groups: synthesis and characterization

Abstract

Novolac resins bearing both cyanate and phthalonitrile groups in different proportions in the same backbone were synthesized and characterized. The resin was thermally cured to a network. Differential scanning calorimetric studies showed that cyanate and phthalonitrile groups underwent independent curing reactions. No evidence was obtained for co-curing of the two groups. Polymerization of cyanate groups preceded that of phthalonitrile groups and the former reaction was susceptible for catalysis. Incorporation of cyanate groups facilitated an early gelation of the resin and thus, the processability of the resin. However, this impaired marginally the moisture resistance and thermal stability of the otherwise inherently water resistant and thermally stable phthalonitrile resins.

---

A. Introduction

Phthalonitrile polymers and their composites are considered as the material of choice for high performance applications due to their excellent thermal stability and mechanical properties, elevated glass transition temperatures, high chemical resistance and non-flammability characteristics.^1–8 These attractive features are the outcome of the nitrogen containing rigid networks formed by the addition polymerization.⁹ However, the –CN groups in phthalonitrile unit are not easily polymerizable and require high temperature or an external curing agents to initiate the crosslinking reactions. Generally, phenol, aromatic amines, metals, metallic salts and complexes are used for nitrile polymerization.^10–18 However, volatility of the external curatives at high temperatures adversely affects the structural integrity of polymer network and could result in void containing composites. One of the widely accepted methods to overcome this issue is to design monomers with in-built curing groups that enable an easy curing of resin. To date, a lot of research efforts have focused on developing novel phthalonitrile monomers and polymers with desirable properties.^19–22 Some recent attempts like incorporation of aryl ethers, siloxane linkages, phthalazinone moieties and addition of inorganic fillers were successful in improving the processability, solubility and mechanical properties of the resin without compromise in thermal stability.^23–27 Introduction of certain compatible addition curable groups is an effective way to tailor the overall polymer properties.²⁸ In this direction, we have investigated the influence of propargyl groups on phthalonitrile curing.²⁹ The two groups co-react in a novolac system when both propargyl and phthalonitrile groups are located on the same polymer backbone.³⁰

Cyanate esters are addition curable resins with lots of academic and scientific interests. Their polymerization products have many attractive properties that enhanced their demand in high performance and technological applications.^31–34 The OCN groups in cyanate and CN groups in PN having similar mechanism of curing; it was of interest to investigate the cure behaviour of a resin possessing both the groups on the same back bone. The objective was also to evaluate the compatibility of cyanate ester (–OCN) and phthalonitrile (PN) groups anchored on to the same backbone and to examine the implications of these structural modifications on cure behavior and thermal properties of the resultant resins. For this, a precursor polymer with free phenolic functions that give provision for further chemical modifications was required. Novolac resin perfectly meets these criteria and can also provide a moderately thermally stable backbone for realizing a precursor bearing both cyanate ester–phthalonitrile groups. To our knowledge, this is the first report on the copolymers of phthalonitrile and cyanate ester systems.

B. Experimental

1. Raw materials

Cyanogen bromide was purchased from Alfa Aesar, USA and was sublimed prior to use. Triethylamine (Ottokemie, Mumbai) was used after purification by distillation and potassium carbonate (SRL, Mumbai) was dried at 120 °C for 5 h. Zinc octoate was purchased from Blue chem, India. Novolac cyanate ester was procured from Lonza chemicals, USA. A.R. acetone was used as received from SRL, Mumbai.

2. Characterization

FT-IR spectra were recorded using KBr pellets on a Perkin Elmer spectrum GXA spectrophotometer in the range of 400–4000 cm⁻¹. Proton NMR and ¹³C NMR were performed using Bruker Avance NMR spectrometer operating at proton frequency of 300 MHz and the corresponding carbon frequency using acetone-d₆ as solvent. Elemental analysis was done using Perkin Elmer 2400 CHN Analyzer. Differential Scanning Calorimetry (DSC) was performed using a TA instrument DSC Q-20 at a heating rate 10 °C min⁻¹ and thermogravimetric analysis (TG) was carried out by TA Instruments SDT Q-600 thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ and both analyses were done in flowing nitrogen.

3. Control oligomers

Novolac phthalonitrile (NPN) and novolac cyanate ester (NOCN) were taken as reference oligomers to study the copolymerization of cyanate ester–phthalonitrile functions in novolac backbone. For the study, novolac cyanate ester (P-T resin) which was commercially available was used. It has an average molecular weight of 380 g mol⁻¹. Novolac phthalonitrile (NPN) was synthesized and characterized according to the procedure in reported literature.³⁵ Cured networks prepared from these resins were compared with those of derived from the cyanate ester–phthalonitrile reactions. Cured NPN and NOCN resins are abbreviated as CNPN and CNOCN respectively.

4. Synthesis

4.1. Synthesis of one-component cyanate ester–phthalonitrile oligomers (NPN–OCN). Novolac resins bearing PN groups to varying extents were synthesized by a reported procedure.³⁵ The polymers were characterized by –OH value and nitrogen content for the degree of phthalonitrilation. The remaining OH groups were transformed to the cyanate function by reaction with CNBr systems as follows. Typically, for NPN–OCN1; at −20 °C under nitrogen atmosphere, 8 mL of triethyl amine was added to 8 g of CNBr dissolved in 8 mL CCl₄ taken in a three neck 250 mL R.B. flask. A 50% cyanogen bromide solution was prepared by dissolving the freshly sublimed CNBr in CCl₄. Novolac phthalonitrile oligomer (NPN–OH1) dissolved in THF was added drop-wise to the above solution and thoroughly stirred for 2 h. The reaction mixture was filtered to remove the triethyl amine hydrobromide and the filtrate was poured into cold isopropanol. The precipitate was filtered, washed with cold isopropanol and dried in vacuum and was stored in refrigerator. The product was characterized by spectroscopic techniques. The cure conditions of all the neat resins are described in Section C.3.

¹³C NMR (300 MHz, acetone-d₆, δ ppm): 109 (OCN), 115.8 (C≡N), 30 (Ar–CH₂–Ar), 150 and 162 (aromatic C–O–C), 122, 127, 131 and 134 (aromatic carbon). FT-IR (KBr, cm⁻¹): 3030 (aromatic C–H), 2920 (Ar–CH₂–Ar), 2231 (–CN), split band at 2200–2240 (–OCN), 1580 and 1480 (aromatic C=C), 1100 and 1200 (C–O).

C. Results and discussion

1. Characterization of NPN–OCN oligomers

Synthesis of NPN–OH systems involve nucleophilic displacement of nitro group in 4-nitro phthalonitrile by the phenolate functions of novolac in a dry dipolar aprotic solvent NMP at room temperature. The general synthesis protocol is shown in Scheme 1. By varying the concentrations of phenolic novolac resin and nitro phthalonitrile, phenolic resins with different extents of PN functionalization were synthesized and the extent of conversion was estimated from nitrogen content and hydroxyl values (Table 1). The structures of polymers were further confirmed by spectroscopic techniques and the molecular weights were verified by GPC (included in the ESI†).

Scheme 1 Synthesis of NPN–OH oligomers.

Table 1 Composition of NPN–OH oligomers

NPN oligomers	PN : phenol (mol ratio)
NPN–OH1	1 : 0.15
NPN–OH2	1 : 0.75
NPN–OH3	1 : 2.3
NPN	1 : 0

---

Free phenolic functionalities in NPN–OH oligomers offer provision for further chemical modifications. Cyanate ester groups are incorporated into the novolac phthalonitrile systems as shown in the Scheme 2. Extent of cyanation and phthalonitrilation are determined on the basis of elemental analysis (Table 2). Typical calculation for finding out the phthalonitrile and cyanate content in NPN–OCN2 is shown in ESI.†

Scheme 2 Synthesis of cyanate ester–phthalonitrile (NPN–OCN) oligomer.

Table 2 Composition of cyanate ester–phthalonitrile oligomers (NPN–OCN)

Composition	Mole fraction of –OCN
NPN–OCN1	0.13
NPN–OCN2	0.43
NPN–OCN3	0.69
NPN	0
NOCN	1

---

The molecular weight distribution and polydispersity index of oligomers were compared from their GPC profiles (Fig. 1). Number average molecular mass of NPN oligomer was 1324 g mol⁻¹ and polydispersity index 1.5. NPN–OCN oligomers exhibited M_n in the range of 2600–2000 with polydispersity index around 1.7. However, these values are with reference to polystyrene standards and are hence not absolute.

Fig. 1 GPC curves of NPN and NPN–OCN oligomers.

FT-IR spectra of NPN–OCN3 and their NPN–OH3 precursor are displayed in Fig. 2. In the case of NPN–OCN, the prominent –OH stretching bands at around 3430 cm⁻¹ which was found in NPN–OH disappeared, suggesting a complete conversion of free hydroxyl functions to –OCN linkages. Characteristic peak at 2231 cm⁻¹ due to phthalonitrile unit overlapped with the split bands at around 2200–2300 cm⁻¹ due to –OCN groups.³⁶ The characteristic absorption of the C–O–C group was seen at 1200 cm⁻¹ which got intensified on cyanation of NPN–OH precursors.

Fig. 2 FT-IR spectra of NPN–OH3 and NPN–OCN3 oligomers.

The ¹³C NMR spectra of NPN–OCN oligomers conformed well to the expected structures. A typical ¹³C NMR spectrum is shown in Fig. 3 which shows signals due to aromatic carbons at around 116–155 ppm and those due to –OCN groups at 109 ppm. The C≡N group in phthalonitrile was observed at 115 ppm.

Fig. 3 Typical ¹³C NMR spectrum of NPN–OCN oligomer.

2. Cure characteristics

The FT-IR spectra of NPN–OCN1 and its cured form, CNPN–OCN1 are shown in Fig. 4. On heating, the characteristic absorption band of –OCN (in NPN–OCN) at 2220 cm⁻¹ disappears indicating their complete conversion in the cure process. However, the –CN peak at 2231 cm⁻¹ remained as one with reduced intensity, suggesting the curing of phthalonitrile functions is incomplete under the cure schedule adopted. Difficulty to attain complete phthalonitrile curing due to the steric factors prevailing in many systems has been reported.^37–39 Expected peaks characteristic of triazine rings (1325 and 1560 cm⁻¹) formed by cyclotrimerization of –OCN and –CN groups are unresolvable in the spectrum due to the overlapping of the respective peaks. However, the formation of structures such as isoindoline is indicated by appearance of a broad peak around 3440 cm⁻¹ (NH stretch).^40,41 These changes are observed in the IR spectra of all cured NPN–OCN compositions (Fig. 5).

Fig. 4 FT-IR spectra of NPN–OCN1 and its cured form CNPN–OCN1.

Fig. 5 FT-IR spectra of cured NPN–OCN oligomers.

Thermally activated polymerization of oligomers was investigated by DSC studies (Fig. 6). NOCN and NPN were independently analyzed by DSC and were found to exhibit a single cure exotherm for each case. In the case of NOCN, the trimerization exotherm initiates at around 115 °C with peak maximum at 210 °C whereas, the cure exotherm in NPN occurs at a higher temperature regime of around 320 °C. However, in cyanate ester–phthalonitrile system NPN–OCN1, two well resolved exotherms are observed at 120 °C and 335 °C. Obviously, the first exotherm corresponds to cyclization of cyanate groups and the second one, to phthalonitrile crosslinking. These results imply that the two functional groups undergo independent curing and do not influence the crosslinking of the other group. A comparison of DSC curves of NPN–OCN3 and their precursor NPN–OH3 given in Fig. 6 clearly support this observation.

Fig. 6 DSC scans of NPN, NOCN, NPN–OH3, NPN–OCN1 and NPN–OCN3 oligomers.

Due to partial phthalonitrilation in NPN–OH systems, the free hydroxyl groups trigger the curing of –CN functionalities and thereby exhibit an inherent cure promoting behavior. This is clearly visible in DSC thermogram of NPN–OH3 oligomer which shows a strong exotherm with significant reduction in cure temperature to 170 °C against 320 °C observed for pure phthalonitrile polymers. Studies have shown that phenol mediated phthalonitrile crosslinking result in heteroaromatic structures such as isoindoline, triazine and phthalocyanines.^35,42–44 On converting OH groups to cyanate esters, the trend in cure behavior was altered. Two independent cure exotherms due to the homopolymerization of –OCN groups and PN groups were evolved. In NPN–OH, there are free phenolic OH groups that are known to catalyze the polymerization of C≡N groups. This causes a shift in cure temperature of C≡N groups in phthalonitrile system to the lower temperature regime (∼170 °C). Upon transforming the OH groups to –OCN, NPN–OH loses this advantage and the resultant resin shows the curing of PN groups at 280 °C. Hence, the co-curing of cyanate groups with nitriles (–CN) is ruled out. The high propensity of cyanate esters to undergo cyclotrimerization to form phenolic triazine networks in contrast to the phthalonitrile groups that demand higher activation energy is the reason for this (Schemes 3 and 4). Thus, these two groups, though possess similar chemical nature and follow similar cure mechanisms (independently), do not co-cure at all. This was further proved by a deliberate catalysis of the NPN–OCN precursor resin. For this, NPN–OCN2 oligomer was doped with 0.3% and 0.8% of zinc octoate (which is a known catalyst for OCN trimerization) and the mixtures are abbreviated as NPN–OCN2–ZnO1 and NPN–OCN2–ZnO2 respectively. On examination of the cure pattern by DSC (Fig. 7), it was found that the low temperature exotherm due to –OCN groups showed a further shift to still lower temperature regime with the increase in quantity of metal catalyst, while no change was observed in the case of phthalonitrile cure exotherm. Presence of metal entities induced the cyclotrimerization of –OCN groups. This undoubtedly proved the independent curing of the two functional groups located on novolac backbone.

Scheme 3 Phenol – mediated phthalonitrile crosslinking.

Scheme 4 Phenolic–triazine network formation.

Fig. 7 DSC scans of neat and catalyzed NPN–OCN2.

The overall co-cured cyanato–phthalonitrile network structure can be depicted as in Scheme 5 when the network consists of crosslinks caused by polymerization of both the groups independently.

Scheme 5 Postulated network structure of cyanato–phthalonitrile resins.

3. Cure schedule of resins

All the polymer precursors were cured in vacuum according to the temperature profile given.

NPN: 150 °C, 200 °C and 250 °C for 1 h each, 300 °C for 2 h and 350 °C for 3 h;

NOCN: 150 °C for ½ h, 200 °C for 1 h and 230 °C for 2 h;

NPN–OCN: 150 °C, 200 °C and 250 °C for 1 h each, 300 °C for 2 h and 330 °C for 3 h.

Cured form of NPN, NOCN and NPN–OCN oligomers are denoted as CNPN, CNOCN and CNPN–OCN respectively.

4. Water uptake level of cured polymers

Cured phthalonitrile is known to be resistant to water, so is the case with cyanate ester resins too. However, how their copolymer behaviour was of interest. Relative rate of water absorption of cured polymers was determined according to ASTM standard D 570–98.⁴⁵ Cured samples were initially dried in an oven at 110 °C for 1 h, cooled in a dessicator and weighed. These conditioned specimens are then immersed in a container of distilled water maintained at a temperature of 23 ± 1 °C. After 24 h, the specimens are removed from the water and the surface is carefully wiped with a dry cloth and weighed. Percentage of water absorption is found from the weight differences before and after immersion as shown in the eqn (1)where W_i and W_f denote weight of initial dry sample and weight after exposure respectively. Water uptake (%) of cured polymers is plotted against their composition and is shown in Fig. 8. It has been found that the water repellent nature of phthalonitrile systems are not significantly impaired by introducing cyanate ester groups into the polymer backbone. Though the triazine polymer generated out of curing of the cyanate ester are inherently water repellent, they do absorb moisture which are accommodated in the free volume generated during curing. Generally 1 to 1.5% water is absorbed by these systems. The copolymer also behaved as expected. The water uptake, though negligible, increased proportional to the cyanate content in the back bone (Fig. 8) probably due to the free volume contributed by the triazine groups formed on curing.

Fig. 8 Percentage of water absorption versus composition of polymer.

5. Thermal stability of the cured networks

As the cyanate groups, appropriately catalysed, help in early gelation of the otherwise difficult-to-cure PN resins, these resins can be preferred to the conventional PN resins from processing point of view. However the impact of structural modification on thermal stability is to be seen. For this, the cured samples were evaluated by TGA. TGA and DTG curves are shown in Fig. 9 and 10 respectively and their thermal data are compiled in Table 3. It is evident that cyanation results in a marginal penalty in the intial decomposition temperature for the phthalonitrile resins. Cyanate ester polymers are more susceptible to thermal dissociation compared to phthalonitrile modified networks. Char formation is also marginally affected by cyanation. However, there is a disproportionate increase in char residue at high temperature (Table 3), though the phenolic triazines are comparatively fragile than phthalonitrile components.

Fig. 9 TGA curves of cured polymers in nitrogen.

Fig. 10 DTG curves of cured polymers.

Table 3 Thermal properties of homopolymers and copolymers^a

Sample	T i (°C)	T p (°C)	T f (°C)	% Char at 900 °C
a T i – initial decomposition temperature; Tp – peak decomposition temperature; Tf – final decomposition temperature.
CNPN	407	551	837	78
CNOCN	310	405	796	48
CNPN–OCN1	352	552	833	72
CNPN–OCN2	347	542	830	73
CNPN–OCN3	343	540	830	70

---

D. Conclusions

The –OCN and –CN groups present in a novolac resin undergo independent polymerization to form a network comprised of polycyanurate and polyphthalonitrile. Introduction of –OCN groups did not confer any special characteristics to the resin except that their presence helped achieve early gelation of the resin at lower temperature as these groups are prone to be catalyzed by a host of materials. Cyanation of phthalonitrile polymer precursors marginally impaired the water repellent nature of phthalonitrile systems and their overall thermal stability.

Acknowledgements

The authors thank Director, VSSC for granting permission to publish this work. One of the authors (DA) thanks ISRO for the research fellowship. They also thank Analytical and Spectroscopy Division, VSSC for the support in characterization of the materials.

References

1. M. Laskoski, D. D. Dominguez and T. M. Keller, Synthesis and properties of a bisphenol A based phthalonitrile resin, J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 4136–4143.
2. S. B. Sastri, J. P. Armistead and T. M. Keller, Carbon-based composites derived from phthalonitrile resins, US Patent 5965268, 1999.
3. M. L. Warzel and T. M. Keller, Tensile and fracture properties of a phthalonitrile polymer, Polymer, 1993, 34, 663.
4. M. D. Smith, B. E. Spencer and P. V. Dine, Processing fiberglass and carbon fiber reinforced phthalonitrile composites, 30th ISAMPE technical conference, 1998, vol. 20–24, pp. 138–145.
5. U. Sorathia and I. Perez, Navy R&D Programs for Improving the Fire Safety of Composite Materials, ACS Symp. Ser., 2009, 922, 185–198.
6. F. Zhao, R. Liu, C. Kang, X. Yu, K. Naito, X. Qu and Q. Zhang, A Novel High-Temperature Naphthyl-based Phthalonitrile Polymer: Synthesis and Properties, RSC Adv., 2014, 4, 8383–8390.
7. P. J. Burchill, On the formation and properties of a high-temperature resin from a bisphthalonitrile, J. Polym. Sci., Part A: Polym. Chem., 1994, 32, 1–8.
8. H. N. Jones and T. M. Keller, NRL review, 2002, http://www.nrl.navy.mil/research/nrl-review/2002/featured-research.
9. L. Zong, C. Liu, S. Zhang, J. Wang and X. Jian, Enhanced thermal properties of phthalonitrile networks by cooperating phenyl-s-triazine moieties in backbones, Polymer, 2015, 77, 177–188.
10. P. J. Burchill and T. M. Keller, Curing phthalonitrile resins with acid and amine, US Patent 5237045, 1992.
11. T. M. Keller and T. R. Price, Amine-Cured Bisphenol-Linked Phthalonitrile Resins, J. Macromol. Sci., Chem., 1982, 18, 931–937.
12. T. M. Keller, Phenolic-cured phthalonitrile resins, US Patent 4410676, 1983.
13. W. T. Li, F. Zuo, K. Jia and X. B. Liu, Study of catalytic effect of ammonium molybdate on the bisphthalonitrile resins curing reaction with aromatic amine, Chin. Chem. Lett., 2009, 20, 348–351.
14. H. Guo, Z. Chen, J. Zhang, X. Yang, R. Zhao and X. Liu, Self-promoted curing phthalonitrile with high glass transition temperature for advanced composites, J. Polym. Res., 2012, 19, 9918.
15. K. Zeng, K. Zhou, S. Zhou, H. Hong, H. Zhou, Y. Wang, P. Miao and G. Yang, Preparation process and properties of exfoliated graphite nanoplatelets filled Bisphthalonitrile nanocomposites, Eur. Polym. J., 2009, 45, 1328–1335.
16. H. Guo, Y. Zou, Z. Chen, J. Zhang, Y. Zhan, J. Yang and X. Liu, Effects of self-promoted curing behaviors on properties of phthalonitrile/epoxy copolymer, High Perform. Polym., 2012, 24, 571–579.
17. D. Augustine, K. P. Vijayalakshmi, R. Sadhana, D. Mathew and C. P. Reghunadhan Nair, Hydroxyl terminated PEEK-toughened epoxy–amino novolacphthalonitrile blends – Synthesis, cure studies and adhesive properties, Polymer, 2014, 55, 6006–6016.
18. X. L. Yang and X. B. Liu, Study on curing reaction of 4-amino phenoxy phthalonitrile/bisphthalonitrile, Chin. Chem. Lett., 2010, 21, 743–747.
19. F. Zhao, R. Liu, X. Yu, K. Naito, C. C. Tang, X. Qua and Q. Zhang, Synthesis of a novel naphthyl-based self-catalyzed phthalonitrile polymer, Chin. Chem. Lett., 2015, 26, 727–729.
20. B. Amir, H. Zhou, F. Liu and H. Aurangzeb, Synthesis and characterization of self-catalyzed imide-containing pthalonitrile resins, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 5916–5920.
21. B. Amir, R. K. Michael, H. Zhou, H. Z. Javid, H. Shahid and H. Aurangzeb, An efficient approach to prepare ether and amide-based self-catalyzed phthalonitrile resins, Polym. Chem., 2013, 4, 3617–3622.
22. H. Sheng, X. Peng, H. Guo, X. Yu, K. Naito, X. Qu and Q. Zhang, Synthesis of high performance bisphthalonitrile resins cured with self-catalyzed 4-aminophenoxy phthalonitrile, Thermochim. Acta, 2014, 577, 17–24.
23. E. Hamciuc, C. Hamciuc, M. Cazacu, M. Ignat and G. Zarnescu, Polyimide–polydimethylsiloxane copolymers containing nitrile groups, Eur. Polym. J., 2009, 45, 182–190.
24. A. V. Babkin, E. B. Zodbinov, B. A. Bulgakov, A. V. Kepman and V. V. Avdeev, Low-melting siloxane-bridged phthalonitriles for heat-resistant matrices, Eur. Polym. J., 2015, 66, 452–457.
25. M. Laskoski, M. B. Schear, A. Neal, D. D. Dominguez, H. L. Ricks-Laskoski, J. Hervey and T. M. Keller, Improved Synthesis and Properties of Aryl Ether-Based Oligomeric Phthalonitrile Resins and Polymers, Polymer, 2015, 67, 185–191.
26. X. Fu, G. Yu, C. Liu, J. Wang, C. Pan and X. Jian, Novel High-temperature-resistant single-polymer composites based on self-reinforced phthalonitrile end-capped polyarylene ether nitrile, High Perform. Polym., 2014, 26(5), 540–549.
27. G. Yu, C. Liu, X. Li, J. Wang, X. Jian and C. Pan, Highly thermostable rigid-rod networks constructed from an unsymmetrical bisphthalonitrile bearing phthalazinone moieties, Polym. Chem., 2012, 3, 1024–1032.
28. M. Laskoski, D. D. Dominguez and T. M. Keller, Alkyne-containing phthalonitrile resins: controlling mechanical properties by selective curing, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 4774–4778.
29. D. Augustine, D. Mathew and C. P. Reghunadhan Nair, Mechanistic and kinetic aspects of the curing of phthalonitrile monomers in the presence of propargyl groups, Polymer, 2015, 60, 308–317.
30. D. Augustine, D. Mathew and C. P. Reghunadhan Nair, One Component Propargyl Phthalonitrile Novolac: Synthesis and Characterization, Eur. Polym. J., 2015, 71, 389–400.
31. C. P. Reghunadhan Nair, D. Mathew and K. N. Ninan, Cyanate ester resins: Recent developments, Adv. Polym. Sci., 2001, 155, 1–99.
32. I. Hamerton, Chemistry and Technology of Cyanate Ester Resins, Blackie Academic & Professional, Glasgow, 1994, p. 207.
33. M. R. Kessler, Cyanate ester resins, Wiley Encyclopedia of Composites, Wiley online library, 2012.
34. T. Fang and D. A. Shimp, Polycyanate esters: Science and applications, Prog. Polym. Sci., 1995, 20, 61–118.
35. D. Augustine, D. Mathew and C. P. Reghunadhan Nair, Phenol-containing Phthalonitrile Polymers – Synthesis, Cure Characteristics and Laminate Properties, Polym. Int., 2013, 62, 1068–1076.
36. L. J. Kasehagen and C. W. Macosko, Structure development in cyanate ester polymerization, Polym. Int., 1997, 44, 237–247.
37. V. N. Nemykin, S. V. Dudkin, F. Dumoulin, C. Hirel, A. G. Gürek and V. Ahsen, Synthetic approaches to asymmetric phthalocyanines and their analogues, ARKIVOC, 2014, 142–204.
38. D. I. Packham, J. D. Davies and H. M. Paisley, Polymers from Aromatic Nitriles and Amines: Polybenzimidazoles and Related Polymers, Polymer, 1969, 10, 923–931.
39. B. N. Achar, G. M. Fohlen and J. A. Parker, Phthalocyanine polymers, US 4499260 A, 1985.
40. T. M. Keller and D. D. Dominguez, High temperature resorcinol-based phthalonitrile polymer, Polymer, 2005, 46, 4614–4618.
41. K. Zeng, K. Zhou, S. Zhou, H. Hong, H. Zhou, Y. Wang, P. Miao and G. Yang, Studies on self-promoted cure behaviors of hydroxy-containing phthalonitrile model compounds, Eur. Polym. J., 2009, 45, 1328–1335.
42. B. Zhang, Z. Luo, H. Zhou, F. Liu, R. Yu, Y. Pan, Y. Wang and T. Zhao, Addition-curable phthalonitrile-functionalized novolac resin, High Perform. Polym., 2012, 24, 398–404.
43. M. J. Sumner, M. Sankarapandian, J. E. McGrath, J. S. Riffle and U. Sorathia, Flame retardant novolac–bisphthalonitrile structural thermosets, Polymer, 2002, 43, 5069–5076.
44. Y. Yang, Z. Min and L. Yi, A Novel Addition Curable Novolac Bearing Phthalonitrile Groups: Synthesis, Characterization and Thermal Properties, Polym. Bull., 2007, 59, 185–194.
45. ASTM International Designation: D 570 – 98, Standard test method for water absorption of plastics.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18045g

---