I ,3-Dialkylimidazoliurn Salts as Latent Catalysts in the Curing of Epoxy Resins *

Introduction

From an industrial point of view, it is often desirable to store premixed curing agents and epoxy resins. Since typical imidazole catalysts react at room temper- ature over a period of time, attempts have been made to prepare deactivated imidazoles to prolong the shelf life of premixed resins. To this end, Dearlove ( I ) , for example, introduced electron-withdrawing groups at the 4- and 5- positions of an imidazole. Although the shelf life of these compounds did improve dra- matically, their catalytic activity was destroyed.

block the active site effectively, extend the shelf life considerably, and maintain the catalytic activity in a latent form. During the course of investigations de- signed to ascertain the mechanism of imidazole catalysis in the curing of epoxy resins (2), we observed that 1,3-dimethylimidazoIium iodide (l), when heated at temperatures above 200"C, afforded I-methylimidazole and methyl iodide. Since curing temperatures may rise above 200"C, the possibility of breaking an N-C bond and forming an active imidazole during the course of a polymerization appeared of interest. To test this possibility we investigated the decomposition of 1 in the presence of JXEBA (the diglycidyl ether of bisphenol A, a commer- cial epoxy resin). The polymerization of DGEBA by 1 supported our original belief. Therefore, we prepared several other imidazolium salts and subjected them to thermolysis under the same conditions. In most cases, imidazolium salts decomposed at around 200°C giving satisfactory heat outputs and inducing polymerization.

We thought that quaternization of an imidazole with a heat labile group could

Experimental

General Information

melting point apparatus (<200"C) or a Mel-temp capillary melting point appa- ratus (>200"C). All reported values are uncorrected and are reported in "C. The elemental microanalyses were performed by Midwest Microlabs, Ltd., Indianapolis, Indiana. Solids were recrystallized from the designated solvent and dried over potassium hydroxide in vacuo in an Abderhalden drying pistol. Li- quids were distilled in vacuo in a Kugelrohr (air bath) apparatus and were dried in vacuo at 25°C. Infrared spectra (IR) were obtained on a Perkin-Elmer 137

All melting points were determined on a Thomas-Hoover Unimelt capillary

*Experiments using a differential scanning calorimeter and Fourier transform instruments were performed at the Western Electric Engineering Research Cen- ter, Princeton, New Jersey. Taken from the Ph.D. thesis of Fiore Ricciardi, Department of Chemistry, University of Pennsylvania, 1982.

Journal of Polymer Science: Polymer Letters Edition, Vol. 21,633-638 (1983) 0 1983 John Wiley & Sons, Inc. CCC 0360-6384/83/080633-06$01.60

6 34 POLYMER LETTERS EDITION

sodium chloride spectrophotometer. Solid samples were examined as potassium bromide (KBr) disks; liquid samples were recorded as neat films between sodium chloride plates. Absorptions are reported as wavenumbers (cm-' ), and their in- tensities are designated as very strong (vs), strong (s), medium (m), weak (w), and very weak (vw). Broad bands and shoulders are designated as (br) and (sh), respectively. IR spectra were calibrated with the 1601-cm-' band of polysty- rene (Perkin-Elmer 137). Proton nuclear magnetic resonance (NMR) spectra were recorded in the designated solvents on Varian EM-360 (60 MHz) or Bruker WP-250 (250 MHz) spectrometers. Chemical shifts are reported in parts per mil- lion (6) and are relative to tetramethylsilane (TMS) used as an internal standard. Where deuterium oxide (DzO) was used as the solvent, 3-(trimethylsilyl)tetra- deuterio sodium propionate (TTP) was used as the internal standard. Multipli- city is designated as s = singlet, bs = broad singlet, d = doublet, t = triplet, q = quartet, and m = multiplet. Coupling constants are reported in hertz (Hz). Carbon nuclear magnetic resonance (I3C-NMR) spectra were taken in the desig- nated solvents on a Bruker WP-250 (operating at 62.9 MHz) spectrometer. Chemical shifts are reported in parts per million (6) relative to tetramethylsilane (TMS) as an internal standard. All differential scanning calorimetry (DSC) was performed at Western Electric Engineering Research Center (Princeton, NJ) on a DuPont model 1090 differential scanning calorimeter.

Materials

Preparation of 1,3-Dimethylimidazolium Iodide (1) and Other 1,3-Dialkyl Imidazolium Salts

mL) and the solution cooled to 10°C. Methyl iodide (8.5 g, 0.06 mol) was added dropwise over a period of 10 min. After 20 min, the mixture was refluxed for 2 h. Evaporation of the solvent gave an oil which was washed with 20 mL of anhydrous ether to induce crystallization and afford 1 I .7 g (87% yield) of the salt (1); mp 88°C [lit. (3) mp 88"CI.

The following salts were prepared by the same method: I-ethyl-3-methyl- imidazolium iodide (2), 69.3% yield, mp 80-8 1°C [lit. (4) mp 79-80"Cl ; 1 -methyl-3-(2-propenyI)imidazolium chloride (3), 56.1% yield, liquid, dec. on heating; I-benzyl-3-methylimidazolium chloride (4), 78.6% yield, dec. on heat- ing; I-butyl-3-methylimidazolium chloride ( S ) , 5 1.4% yield, liquid, dec. on heat- ing; 1,3-diethylimidazolium iodide (6), 64.3% yield, mp 127°C [lit. (4) mp 128"CI. 1 (4) 'H-NMR (DMSOd6): 6 3.90 (6H, s), 7.51 (2H, s). I3C-NMR

1 .SO (3H, t, J = 7.0 Hz), 3.88 (3H, s), 4.20 (2H, q, J = 1 1 .O Hz), 7.40 ( lH, s), 7.41 (IH, s); I3C-NMR (DMSOd6):6 15.2 (q), 36.6 (q), 45.4 (t), 122.3 (d), 123.9(d), 135.9.3 'H-NMR (DMSO-d6): 6 3.90 (3H,s), 4.70-4.90 (2H,m),5.18- 6.45(3H,m),7.49(1H,m),7.51 (lH,m),8.88(1H,s). 4'H-NMR(DMSo-d6): 6 3.94 (3H, s), 5.36 (2H, s), 7.32 (2H, s), 7.46 (5H, m), 8.66 (IH, s). 5 , Anal. Calcd. forCaH15NzCI: C, 55.00%; H, 8.66%; N, 16.04%. Found: C, 55.28%;

1-Methylimidazole (5.0 g, 0.06 mol) was dissolved in methylene chloride (50

(DMSOd6)6 38.2 (q), 125.4 (d), 135.1 (d). 2 (4) 'H-NMR (DMSO-d6): 6

POLYMER LETTbXS EDITION 635

usc

Yo ..

" i a t

.o a a tm (10 fa ia la zm rn 20

TLMPFRATURE (dcp C)

Fig. 1. Differential scanning calorimeter (DSC) plot of reaction between 1,3- dimethylimidazolium iodide and EPON 828.

H, 8.78%; N , 15.86%. 'H-NMR (DMSO-d6): 6 1.10 (3H, t, J = 6.0 Hz), 1.2- 2.1 (4H,m),3.97(3H,s),7.54(1H,s),7.56(1H,s),8.82(1H,s). 6'H-NMR (DMSOd6): 6 1.57 (6H, t, J = 8.0 Hz), 4.29 (4H, 9 , J = 12.0 Hz), 7.54 (2H, s) .

Preparation of 1 -Methyl-3-(2-Hydroxy-2-Phenoxypropyl)imidaolium Iodide (7)

lmidazole and phenylglycidyl ether were treated as described above. After reflux, the solution was cooled to 10°C and treated with methyl iodide ( 1 equiv) t o afford a 53.2% yield of the product, mp 238°C. Anal. Calcd for CI3Hl7N2- 021: C, 43.35%; H, 4.76%; N,7.78%. Found: C, 43.64%; H, 4.82%; N, 7.66%. 'H-NMK (DMSO-d,): 6 3.93 (3H, s), 4.05 (2H, m), 4.18 ( IH, m). 4.39 (ZH, m), 4.72 ( IH, br. s), 6.7-6.9 (5H, m), 7.08 ( l H , m), 7.29 ( IH, m), 7.80 ( l H , s); "C-NMK (DMSO-d6): 6 35.96 (q), 52.14 (t), 67.31 (d), 69.43 (t), 114.75 (d), 120.42 (d), 120.89 (d), 123.10(d), 123.22 (d), 137.13 (d), 158.27 (s).

Differential Scanning Calorimetry

The decomposition of the 1,3-dialkylimidazolium salts was followed by dif- ferential scanning calorimetry (DSC) using two identical miniature reaction vessels. One was left empty and the other was charged with approximately 10

636 POLYMER LETTERS EDITION

TABLE I

Differential Scanning Calorimetry Data

X- R2

Exotherm Exotherm Exotherm T o t a l

R 1 R2 R3 o n s e t ' peak' enda h e a t a P o l y .

(1) CH3 H CH3 1 7 6 . 6 2 2 2 . 1 2 5 0 . 0 426 +

(2) CH3 H CH2CH3 1 9 5 . 1 2 4 0 . 9 2 8 0 . 0 596 + (2) CH3 H CH2CH=CH2 2 1 1 . 5 2 5 6 . 3 3 0 4 . 0 527 +

(4) CH3 H CH2Ph 2 1 8 . 4 2 5 0 . 3 2 8 2 . 6 558 + (5) CH3 H (CH2l3CH3 2 1 3 . 2 2 5 2 . 1 2 9 1 . 7 5 8 5 + (5) m2M3 H CH2CH3 2 0 3 . 5 2 3 9 . 3 2 8 0 . 0 240 +

(1) CH3 H CH2CHOHCH20Ph 2 0 1 . 5 2 4 8 . 8 2 7 7 . 5 315 + H CH3 80.1 1 1 5 . 2 1 9 7 . 1 2 5 3 + H CH3 4 - n i t r o 1 3 9 . 4 1 8 1 . 4 1 9 3 . 0 4 . 4 2 -

2 2 7 . 8 2 5 7 . 0 2 8 7 . 0 6 . 7 3

'Temperatures are "C. bHeat units are J/g.

mg of epoxy resin to which had been added 5% by weight of the 1,3-dialkyI- imidazolium iodide. Both vessels were heated at a constant rate of 10°C/min and the heat flow required to keep them both at an equal temperature was meas- ured. Figure 1 shows a plot of the heat flow versus the ambient temperature.

The data recorded included (1) the onset of exotherm and, therefore, the temperature at which polymerization began, (2) the peak exotherm indicating the point where the most energy was given off, and (3) the end of exotherm signifying the end of reaction. Additionally, an integration of the plot afforded the total heat output for the reaction, in the case of 1,426 J/g. A high heat output indicates that an exothermic reaction, namely, polymerization, has taken place. Typical polymerizations afford heat outputs of 200-600 J/g. After the system had cooled down, the physical state of the product was checked to ver- ify that polymerization had occurred.

It was important to look at the total heat output and whether or not poly- merization occurred. The total heat output is important since too little indi- cates that no polymerization occurred and too high heat output signifies that the reaction will not be scaled up easily to industrial capacity with a satisfactory safety level.

POLYMER LETTERS EDITION 6 3 1

Results and Discussion

In most cases, the imidazolium salts subjected to thermolysis in the differen- tial scanning calorimeter decomposed at a temperature of about 200°C giving satisfactory heat outputs and polymerization. Included for comparison were 2-methylimidazole, a typical imidazole catalyst, and 4-nitroimidazole, a deac- tivated imidazole in the trend of Dearlove. We noted that a nitro substituent on the ring caused a severe drop in the heat output of the reaction, therefore limiting the degree of polymerization. The data demonstrated that for imidazo- lium salts N-dealkylation is possible under polymerization conditions. The DSC data are shown in Table I.

When the imidazolium salt is not symmetrically substituted, it is not clear which alkyl group is removed preferentially. For instance, Chan (3) studied the thermolysis of similar salts, analyzed the volatile products by gas-liquid chroma- tography (GLC) and NMR, and collected the I-substituted imidazoles in a cold trap. For the pyrolysis of 1 -ethyl-3-methylimidazolium iodide, he obtained a 1 :68 ratio for removal of methyl over ethyl while Auwers (5) reported that only removal of the ethyl group occurred. Other discrepancies are even more dramatic, for example, 1 : 1.5 for deethylation over depropylation by Chan versus total depropylation by Auwers and Mauss (5). Auwers and Maws and Sarasin and Wagmann ( 6 ) believed that a methyl group hardly ever leaves preferentially. Kost and Grandberg (7) stated that “on cleavage of a group from a quaternary salt, those radicals will be cleaved most easily which most readily form cations.” This author then listed methyl well ahead of ethyl in ability t o cleave. Whatever the case, the alkyl imidazolium salts serve the purpose of being latent catalysts and the DSC results show their activation t o occur a t a reasonable temperature.

Conclusion

It was demonstrated that the thermolysis of 1,3-dialkylimidazolium salts around 200°C produces active imidazoles capable of inducing the polymeriza- tion of DGEBA. The fact that N-dealkylation occurs makes it possible t o use 1,3-dialkylimidazolium salts as latent catalysts in the curing o f epoxy resins. The effectiveness of various substituted imidazolium salts can be measured accu- rately and conveniently by differential scanning calorimetry.

References

(1) T. J. Dearlove, “Epoxy Curing Agents, 11. Deactivated Imidazoles and Flexible Systems,” National Technical Information Service, Springfield, VA, 1971, HDL-TR-1552.

(2) F. Ricciardi, W. A. Romanchick, and M. M . Joullid, J. Polym. Sci. Polym.

(3) B. K. M. Chan, N. H. Chang, and M. R. Grimmett, Aust. J. Chem., 30, Chem. Ed.. 21. 1475 (1983).

2005 (1977).

638 POLYMER LE'ITERS EDITION

(4) M. Shimbo, M. Iwakoshi, and M. Ochi, Nippon Setchaku Kyokai Shi, 10,

(5) K. von Auwers and W. Mauss, Ber. Dtsch. Chem. Ges. B, 61,241 1 (1928). (6) J . Sarasin and E. Wagmann, Helv. Chim. Acta, 7,720 (1924). (7) A. W. Kost and I . I . Grandberg, Adv. Heterocycl. Chem., 6,417 (1966).

161 (1974).

Fiore Ricciardi William A. Romanchick* Madeleine M. Joullid

Department of Chemistry University of Pennsylvania Philadelphia, Pennsylvania 19 104

Received February 1 1, 1983 Revised March 14, 1983 Accepted March 22, 1983

*Present address: Betz Laboratories, Trevose, Pennsylvania 19047. +To whom all correspondence should be addressed.