Imidazoles-Intercalated α-Zirconium Phosphate as Latent Thermal Initiators in the Reaction of Glycidyl Phenyl Ether ( GPE ) and Hexahydro-4-Methylphthalic Anhydride ( MHHPA )

The capabilities of imidazoles-intercalated α-zirconium phosphate (α-ZrP·imidazole): imidazol (α-ZrP·Im), 2-methylimidazole (α-ZrP·2MIm), and 2-ethyl-4-methylimidazole (α-ZrP·2E4MIm) as latent thermal initiators were examined by the copolymerization of glycidyl phenyl ether (GPE) and hexahydro-4-methylphthalic anhydride (MHHPA) with the imidazoles-intercalated α-zirconium phosphate at varying temperatures for one-hour periods. Polymerization was not observed until the reactants were heated to 100 ◦C or above. Increasing the temperature, polymerization in the presence of α-ZrP·Im, α-ZrP·2MIm, or α-ZrP·2E4MIm proceeded at 140 ◦C for 1 h with over 90% conversion. The thermal stabilities of α-ZrP·Im, α-ZrP·2MIm, and α-ZrP·2E4MIm in the reaction at 40 ◦C for 264 h were tested. With α-ZrP·2MIm, the conversion was less than 15% up to 96 h. In the cases of α-ZrP·Im and α-ZrP·2E4MIm, the conversion reached less than 15% at 264 h. The thermal stabilities of α-ZrP·Im, α-ZrP·2MIm, and α-ZrP·2E4MIm at 40 ◦C were superior to those of the commercially available thermal latent initiators: HX-3088 and HX-3722.

Aside from DBU and DABCO, imidazoles have been widely used for curing epoxy resins as cured epoxy resins give good thermal resistance and physical properties because of the reactivity in chain polymerization with epoxide [12,13].Latent thermal initiators of imidazoles have been developed.Romanchick prepared 1,3-dialkylimidazolium salts in the curing of epoxy resin [14].In this case, the activation of catalyst needs a high temperature around 200 • C. Shin reported using microencapsulated imidazoles with polycaprolactone [15].Arimitsu et al. proposed using the Michel addition products of imidazoles and fumarate ester to improve the low miscibility in epoxy resin [16].However, these catalysts remain the blocking residues following the reaction to epoxy groups.These residues might be attributed to the deterioration of the resin.Amine-intercalated α-ZrP can act as latent thermal initiators [17,18] and the blocking residues of α-ZrP can act as additives for reinforcement of resins [19][20][21].
Figure 3 shows the conversions of GPE in the reaction with α-ZrP•Im, α-ZrP•2MIm, or α-ZrP•2E4MIm at these temperatures over a 1 h period.In each case, GPE did not substantially change below 80 °C.Particularly in the case of α-ZrP•Im, the reaction did not proceed at 100 °C.With an increase in temperature, the reactions with any α-ZrP intercalation compound gradually proceeded and reached over 94% of conversions at 140 °C.These α-ZrP intercalation compounds were thermally stable and required 140 °C of curing temperature as initiators.Commercially available latent thermal curing agents such as the microencapsulated imidazoles of HX-3088 and HX-3722 also did not effectively initiate the reaction up to 80 °C.When the temperature was increased, the reaction fairly proceeded at 100 °C with 66% conversion for HX-3088 and 75% conversion for HX-3722.However, these initiators required 140 °C of curing temperature as well as the imidazoles-intercalated α-ZrP.Arimitsu et al. reported that the imidazole derivatives prepared from fumarate ester as latent thermal initiators of butyl glycidyl ether.The reaction completely proceeded at 150 °C for 30 min [16].The capabilities of the α-ZrP•Im, α-ZrP•2MIm, and α-ZrP•2E4MIm as latent initiators were examined in the reactions of GPE with MHHPA containing 3 mol % of each intercalation compound for GPE at varying temperatures for 1 h periods by 1 H-NMR technique as shown in Figure 2. The conversions of GPEs were measured from the area ratio of the aromatic protons at δ 6.9 and the methylene protons of δ 2.92 (indicated by arrows), which relatively decreased with an increase in the reaction temperature from 80 to 140 °C (Figure 2).
Figure 3 shows the conversions of GPE in the reaction with α-ZrP•Im, α-ZrP•2MIm, or α-ZrP•2E4MIm at these temperatures over a 1 h period.In each case, GPE did not substantially change below 80 °C.Particularly in the case of α-ZrP•Im, the reaction did not proceed at 100 °C.With an increase in temperature, the reactions with any α-ZrP intercalation compound gradually proceeded and reached over 94% of conversions at 140 °C.These α-ZrP intercalation compounds were thermally stable and required 140 °C of curing temperature as initiators.Commercially available latent thermal curing agents such as the microencapsulated imidazoles of HX-3088 and HX-3722 also did not effectively initiate the reaction up to 80 °C.When the temperature was increased, the reaction fairly proceeded at 100 °C with 66% conversion for HX-3088 and 75% conversion for HX-3722.However, these initiators required 140 °C of curing temperature as well as the imidazoles-intercalated α-ZrP.Arimitsu et al. reported that the imidazole derivatives prepared from fumarate ester as latent thermal initiators of butyl glycidyl ether.The reaction completely proceeded at 150 °C for 30 min [16].

Scheme 1. Synthesis of poly(GPE-co-MHHPA).
The capabilities of the α-ZrP•Im, α-ZrP•2MIm, and α-ZrP•2E4MIm as latent initiators were examined in the reactions of GPE with MHHPA containing 3 mol % of each intercalation compound for GPE at varying temperatures for 1 h periods by 1 H-NMR technique as shown in Figure 2. The conversions of GPEs were measured from the area ratio of the aromatic protons at δ 6.9 and the methylene protons of δ 2.92 (indicated by arrows), which relatively decreased with an increase in the reaction temperature from 80 to 140 • C (Figure 2).
Figure 3 shows the conversions of GPE in the reaction with α-ZrP•Im, α-ZrP•2MIm, or α-ZrP•2E4MIm at these temperatures over a 1 h period.In each case, GPE did not substantially change below 80 • C. Particularly in the case of α-ZrP•Im, the reaction did not proceed at 100 • C. With an increase in temperature, the reactions with any α-ZrP intercalation compound gradually proceeded and reached over 94% of conversions at 140 • C.These α-ZrP intercalation compounds were thermally stable and required 140 • C of curing temperature as initiators.Commercially available latent thermal curing agents such as the microencapsulated imidazoles of HX-3088 and HX-3722 also did not effectively initiate the reaction up to 80 • C. When the temperature was increased, the reaction fairly proceeded at 100 • C with 66% conversion for HX-3088 and 75% conversion for HX-3722.However, these initiators required 140 • C of curing temperature as well as the imidazoles-intercalated α-ZrP.Arimitsu et al. reported that the imidazole derivatives prepared from fumarate ester as latent thermal initiators of butyl glycidyl ether.The reaction completely proceeded at 150 • C for 30 min [16].[11], α-ZrP•DBU (Δ) [11], HX-3088 (◆), and HX-3722 (•) at 40 °C.
This reaction system of GPE-MHHPA has an advantage for characterization of the resulting imidazoles-intercalated α-ZrPs after the reaction.Thus, the resulting ones can be easily isolated by simply washing the products with an organic solvent.After the reaction of GPE with MHHPA in the presence of imidazoles-intercalated α-ZrPs at 140 °C for 2 h or 1 h, the resulting imidazolesintercalated α-ZrPs (hereafter abbreviated as α-ZrP•Im-RXN, α-ZrP•2MIm-RXN, and α-ZrP•2E4MIm-RXN) were isolated and characterized by XRD, NMR, and elemental analyses.Basal distances, chemical shifts of the main signals, and elemental analyses are listed in Table 1.This reaction system of GPE-MHHPA has an advantage for characterization of the resulting imidazoles-intercalated α-ZrPs after the reaction.Thus, the resulting ones can be easily isolated by simply washing the products with an organic solvent.After the reaction of GPE with MHHPA in the presence of imidazoles-intercalated α-ZrPs at 140 °C for 2 h or 1 h, the resulting imidazolesintercalated α-ZrPs (hereafter abbreviated as α-ZrP•Im-RXN, α-ZrP•2MIm-RXN, and α-ZrP•2E4MIm-RXN) were isolated and characterized by XRD, NMR, and elemental analyses.Basal distances, chemical shifts of the main signals, and elemental analyses are listed in Table 1.This reaction system GPE-MHHPA has an advantage for characterization of the resulting imidazoles-intercalated α-ZrPs after the reaction.Thus, the resulting ones can be easily isolated by simply washing the products with an organic solvent.After the reaction of GPE with MHHPA in the presence of imidazoles-intercalated α-ZrPs at 140 • C for 2 h or 1 h, the resulting imidazoles-intercalated α-ZrPs (hereafter abbreviated as α-ZrP•Im-RXN, α-ZrP•2MIm-RXN, and α-ZrP•2E4MIm-RXN) were isolated and characterized by XRD, NMR, and elemental analyses.Basal distances, chemical shifts of the main signals, and elemental analyses are listed in Table 1.The XRD patterns of pristine α-ZrP is shown in Figure 5a.The peak corresponding to the basal distance of 7.6 Å was expanded with the intercalation of imidazoles.The XRD patterns of α-ZrP•Im and α-ZrP•Im-RXN are shown in Figure 5b,c.In addition to the peak corresponding to 10.7 Å of α-ZrP•Im, a small broad peak was observed at 24.4 Å (2θ = 3.6) in that of α-ZrP•Im-RXN.In the 31 P MAS NMR spectra of α-ZrP•Im and α-ZrP•Im-RXN, the main signals at −15.9, −16.8, and −23.7 ppm shifted to −16.1 and −22.0 ppm as shown in Figure 6a,b.In the 31 P MAS NMR spectra of intercalation compounds of α-ZrP•Im and α-ZrP•2E4MIm, the peak of deprotonated phosphate groups were observed at δ value of −15.9 and −16.8 for α-ZrP•Im and −16.1 for α-ZrP•2E4MIm.The single peak of pristine α-ZrP is shown at a δ value of −20.1 [10].The peak δ value of higher than that of −20.1 for α-ZrP•2MIm doesn't observed.That might show the separation of 2MIm from phosphate group.In the 13 C CPMAS NMR spectra presented in Figure 6d,e, the aromatic carbons associated with the GPE at δ = 158.8,132.4,129.8, and 116.7 ppm and the structural main chain methylene groups at δ = 67.9 were observed.However, the carbonyl carbon of the resulting poly(GPE-co-MHHPA) at δ = 172.9 in Figure 6e was not observed in that of α-ZrP•Im-RXN.The resulting products were derived from GPE.A possible phosphate ester formed by PO 4 of α-ZrP with GPE may be denied by the chemical shifts in the 13 C CP MAS NMR spectra.Phosphate esters produced by the reaction of α-ZrP and 1,2-epoxydodecane, the carbons generated from epoxide were observed at δ = 61.6 and 71.2 for β-cleavage and δ = 79.6 and 71.2 for α-cleavage in 13C CP MAS NMR spectra [25].Epoxy-ring opening products, such as the homo-oligomer of GPE, exist in the interlayer.In the following cases, the epoxy-ring opening products are formed in the interlayer.
In the XRD patterns of α-ZrP•2MIm and α-ZrP•2MIm-RXN (shown in Figure 5d,e), the basal distances were shortened from 12.1 to 11.3 Å and a broad peak was observed at 24.4 Å (2θ = 3.6).In the 31 P MAS NMR spectra, the main signals at −21.0, −22.2, and −23.4 ppm shifted to −21.5 ppm.Similarly, the aromatic carbons and methylene groups derived from GPE were observed in the 13 C CPMAS NMR spectra.
The XRD patterns of pristine α-ZrP is shown in Figure 5a.The peak corresponding to the basal distance of 7.6 Å was expanded with the intercalation of imidazoles.The XRD patterns of α-ZrP•Im and α-ZrP•Im-RXN are shown in Figure 5b,c.In addition to the peak corresponding to 10.7 Å of α-ZrP•Im, a small broad peak was observed at 24.4 Å (2θ = 3.6) in that of α-ZrP•Im-RXN.In the 31 P MAS NMR spectra of α-ZrP•Im and α-ZrP•Im-RXN, the main signals at −15.9, −16.8, and −23.7 ppm shifted to −16.1 and −22.0 ppm as shown in Figure 6a,b.In the 31 P MAS NMR spectra of intercalation compounds of α-ZrP•Im and α-ZrP•2E4MIm, the peak of deprotonated phosphate groups were observed at δ value of −15.9 and −16.8 for α-ZrP•Im and −16.1 for α-ZrP•2E4MIm.The single peak of pristine α-ZrP is shown at a δ value of −20.1 [10].The peak δ value of higher than that of −20.1 for α-ZrP•2MIm doesn't observed.That might show the separation of 2MIm from phosphate group.In the 13 C CPMAS NMR spectra presented in Figure 6d,e, the aromatic carbons associated with the GPE at δ = 158.8,132.4,129.8, and 116.7 ppm and the structural main chain methylene groups at δ = 67.9 were observed.However, the carbonyl carbon of the resulting poly(GPE-co-MHHPA) at δ = 172.9 in Figure 6e was not observed in that of α-ZrP•Im-RXN.The resulting products were derived from GPE.A possible phosphate ester formed by PO4 of α-ZrP with GPE may be denied by the chemical shifts in the 13 C CP MAS NMR spectra.Phosphate esters produced by the reaction of α-ZrP and 1,2epoxydodecane, the carbons generated from epoxide were observed at δ = 61.6 and 71.2 for -cleavage and δ = 79.6 and 71.2 for α-cleavage in 13C CP MAS NMR spectra [25].Epoxy-ring opening products, such as the homo-oligomer of GPE, exist in the interlayer.In the following cases, the epoxy-ring opening products are formed in the interlayer.
In the XRD patterns of α-ZrP•2MIm and α-ZrP•2MIm-RXN (shown in Figure 5d,e), the basal distances were shortened from 12.1 to 11.3 Å and a broad peak was observed at 24.4 Å (2= 3.6).In the 31 P MAS NMR spectra, the main signals at −21.0, −22.2, and −23.4 ppm shifted to −21.5 ppm.Similarly, the aromatic carbons and methylene groups derived from GPE were observed in the 13 C CPMAS NMR spectra.In the case of α-ZrP•2E4MIm, similar to α-ZrP•2MIm, the basal distances were shortened from 12.9 to 11.5 Å and a broad peak was observed at 24.1 Å (2θ = 3.7).In the 31 P MAS NMR spectrum, the main signals at −16.1, and −22.2 ppm shifted to −14.8 and −20.6 ppm.In the 13 C CPMAS NMR spectrum, the aromatic carbons and methylene groups derived from GPE were similarly observed.In the case of α-ZrP•2E4MIm, similar to α-ZrP•2MIm, the basal distances were shortened from 12.9 to 11.5 Å and a broad peak was observed at 24.1 Å (2θ = 3.7).In the 31 P MAS NMR spectrum, the main signals at −16.1, and −22.2 ppm shifted to −14.8 and −20.6 ppm.In the 13 C CPMAS NMR spectrum, the aromatic carbons and methylene groups derived from GPE were similarly observed.
In any case, with the expansion of the basal distances, an increase in C content was recognized.Moreover, the 13 C CPMAS NMR suggested the presence of substances derived from GPE in the interlayer.Based on the elemental analyses, the compositions were calculated to give GPE/Im = 0.7, Zr(HPO 4 ) 2 (C 3 H 4 N 2 ) 0.51 •(GPE) 0.34 for α-ZrP•Im-RXN, GPE/2MIm = 0.8, Zr(HPO 4 ) 2 (C 4 H 6 N 2 ) 0.50 •(GPE) 0.42 In these cases, while the imidazoles were deintercalated, the intercalation of GPE and successive reactions occurred.In the preceding paper, the interlayer polymerization of GPE-MHHPA with α- In these cases, while the imidazoles were deintercalated, the intercalation of GPE and successive reactions occurred.In the preceding paper, the interlayer polymerization of GPE-MHHPA with α-ZrP•DBU; in contrast, the lack of interlayer polymerization with α-ZrP•DABCO were observed [11].
In our system, control of the deintercalation of amines was an important factor in the design of the latent thermal initiator.In addition to the pK a of the amines, the basal distances related to the molecular shape and size of the amines, the shape and size of α-zirconium phosphate, the structural design of the intercalation compounds, and the equilibrium of the reactants should be thermodynamically considered.

Preparation of Imidazole-Intercalated α-ZrP (α-ZrP•Im)
The intercalation of imidazole into the layers of Zr(HPO 4 ) 2 •H 2 O (α-ZrP) was carried out using a previously reported method [16].α-ZrP (10 g) was added to 142 mL of a 0.7 mol dm −3 imidazole aqueous solution.The reaction mixture was then allowed to stand at 40 • C for 24 h, before the product was collected by centrifugation and washed with water several times.The resulting residue was dried under vacuum.The intercalation of 2-methylimidazole (2MIm) and 2-ethyl-4-methylimidazole (2E4MIm) were carried out by the same procedure for the preparation of α-ZrP•Im.