A New Facile and Rapid Synthesis of Polyamides and Polyimides by Microwave-
Assisted Polycondensation
Yoshio Imai
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan
A new facile method for the rapid synthesis of aliphatic polyamides and polyimides was developed by using a domestic microwave oven to facilitate the polycondensation of both ω-amino acids and nylon salts as well as of the salt monomers composed of aliphatic diamines and pyromellitic acid or its diethyl ester in the presence of a small amount of a polar organic medium. Suitable organic media for the polyamide synthesis were tetramethylene sulfone, amide-type solvents such as N-cyclohexyl-2-pyrrolidone (CHP) and 1,3-dimethyl-2-imidazolidone (DMI), and phenolic solvents like m-cresol and o-chlorophenol, and for the polyimide synthesis amide-type solvents such as N-methyl-2-pyrrolidone, CHP, and DMI. In the case of the poly-amide synthesis, the polycondensation was almost complete within 5 min, producing a series of polyamides with inherent viscosities around 0.5 dL/g, whereas the polyimides having the viscosity values above 0.5 dL/g were obtained quite rapidly by the microwave-assisted polycondensation for only 2 min.
Recently there has been growing interest in applying microwave energy to synthetic organic chemistry and synthetic polymer chemistry as well. In the latter field, microwave energy has been utilized for the radical polymerization of vinyl monomers such as 2-hydroxyethyl methacrylate (7), methyl methacrylate (2), and styrene (3)y and for the curing of polymers such as epoxy resins (4-10) and polyurethanes (77,72), as well as for the imidization of polyamic acids (13,14). Among them, much efforts have been directed toward the curing of epoxy resins from the practical viewpoint. In most cases, the high heat efficiency gave rise to remarkable rate enhancements and dramatic reduction of reaction times. However, there is no report so far except for our studies (15,16) on the synthesis of condensation polymers by using microwave energy. We have already reported the rapid synthesis of aromatic polyamides by microwave-assisted direct polycondensation of aromatic diamines and aromatic dicarboxylic acids with condensing agents(75j, and a preliminary study on the microwave-assisted rapid synthesis of polyamides from nylon salts (16). This article reviews the first successful rapid synthesis of aliphatic polyamides and polyimides by using a microwave oven to facilitate the polycondensation of both ω-amino acids and nylon salts, as well as of the salt monomers composed of aliphatic diamines and pyromellitic acid or its diethyl ester (16-19).
In Step-Growth Polymers for High-Performance Materials; Hedrick, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
422 STEP-GROWTH POLYMERS FOR HIGH-PERFORMANCE MATERIALS
Experimental Section
Apparatus. The apparatus used for the polycondensation was a Mitsubishi RR-32 domestic microwave oven (500 W, 2.45 GHz). We made a minor modification to drill a small hole on the top of the microwave oven, and adopted an open reaction system using a 30 mL wide-mouth vial as a reaction vessel.
Monomers and Reagents. Commercial 6-aminohexanoic acid (ε-aminocaproic acid) and 11-aminoundecanoic acid were purified by recrystallization with water. Hexamethylenediamine and dodecamethylenediamine were obtained commercially and purified by distillation under reduced pressure. 12-Aminododecanoic acid, hexanedioic acid (adipic acid), octanedioic acid (suberic acid), decanedioic acid (sebacic acid), and dodecanedioic acid, as well as the solvents employed for the polymerization, were obtained commercially and used without further purification.
A series of nylon salts were prepared by a previously reported procedure (20) which involved mixing an ethanol solution of an aliphatic diamine with an ethanol solution of an aliphatic dicarboxylic acid, followed by recrystallization of the resultant precipitates with water.
Pyromellitic acid (PMA) was prepared by a usual hydrolysis of pyromellitic dianhydride, purified by sublimation, with water. 1,4-Diethyl ester of pyromelitic acid, i.e. 2,5-di(ethoxycarbonyl)terephthalic acid (PME), was synthsized according to the reported procedure (21) by the reaction of PMDA with absolute ethanol. A series of nylon-salt-type monomers XPMA and XPME, where X stands for the number of methylene unit in the aliphatic diamine, were prepared by a usual procedure by mixing an ethanol solution of an aliphatic diamine with an ethanol solution of PMA and PME, respectively, followed by recrystallization of the resultant precipitates with water (22,23).
Polymerization Procedure. The reaction vessel containing a mixture of reactants and a solvent was placed on the center of the turn table in the microwave oven, then nitrogen gas was introduced from the top of the reaction vessel through a thin Teflon tube to minimize danger of fire in the reaction system. The microwave was irradiated for a prescribed time. The temperature of the reaction mixture was determined just immediately after removal of the reaction vessel from the microwave oven by using a thermocouple and a temperature recorder. The polymer formed was isolated by washing the reaction mixture with methanol, followed by drying under vacuum.
Results and Discussion
Microwave Heating of Solvents. It is well known that the heat generation by microwave irradiation is proportional to the product of dielectric constant ε and dielectric loss tangent tan δ of the material (4). It is also known that a material of larger dielcetric constant ε generally has a larger dielectric loss tangent tan δ. Hence, a polar material generates much heat quickly by microwave irradiation.
In fact, highly polar solvents such as water, 13-dimethyl-2-imidazolidone (DMI), and tetramethylene sulfone (sulfolane) generated heat quickly by the microwave irradiation. For example, water (ε = 78) was heated within 1 min to reach the boiling point near 100°C. Highly polar and high-boiling-point solvents such as DMI (ε = 37 and bp 225°C) and sulfolane (ε = 43 and bp 287°C) were heated to 180°C after 1 min of the microwave irradiation. The heat generation of less polar m-cresol (ε = 12 and bp 202°C) was intermediate between that of water and DMI or sulfolane.
Polyamide Synthesis from ω-Amino Acids. The polycondensation of ω-amino acids was carried out with a domestic microwave oven under nitrogen atmosphere (Eq 1).
H 2 N ( C H 2 ) x C O O H »> [-NH(CH2)xCO-]n + H 2 O (1)
x = 5, 10, 11
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In Step-Growth Polymers for High-Performance Materials; Hedrick, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
27. IMAI Polyamide and Polyimide Synthesis Using Microwaves 423
In order to induce effective homogeneous heating of the monomers and hence cause efficient polycondensation, we employed a small amount of an organic medium that acts as solvent for both the starting monomers and the resultant polyamides. Figure 1 exhibits the solvent effect on the polymerization temperature and the inherent viscosity of the polyamide formed by the polycondensation of 12-aminododecanoic acid as a function of microwave irradiation time. Here we used m-cresol, DMI, and sulfolane as solvents. The temperature of the reaction mixture rose quickly and reached tenperatures in excess of 270°C after 4 min of microwave irradiation. At that time, most of the solvent evaporated leaving a melt (melting temperature of the polymer is 179°C). During this initial period, the polycondensation proceeded rapidly, producing the polyamide having inherent viscosities of around 0.4 dL/g. Between 4 min and 10 min, the reaction temperature remained almost constant, while the inherent viscosity of the polymer gradually increased, finally reaching a value of about 0.7 dL/g.
Table 1 summarizes the solvent effect on the inherent viscosity of the polyamide formed by the polycondensation of 12-aminododecanoic acid for the microwave irradiation time of 4 or 5 min. From this Table, large temperature differences were observed clearly between the final temperature of the solvent alone and the final temperature of the polymerization mixture. This is a clear indication of the participation of the polar amino acid monomer dissolved in the solvent to generate heat during the microwave irradiation polymerization. When water, which has the highest dielectric constant, was used as the solvent, the polyamide having an inherent viscosity of 0.35 dL/g was obtained. In general, the use of the solvents having both high dielectric constant and high boiling point, reaching high final polymerization temperature, led to the formation of the polyamides having inherent visocisties higher than 0.5 dL/g. Such suitable solvents were sulfolane, and amide-type solvents such as A/-cyclohexyl-2-pyrrolidone and DMI. Hydroxyl-containing solvents having high dielectric constant like ethanediol, and those with less polarity such as benzyl alcohol, m-cresol, and o-chlorophenol, were also effective for producing the polyamide with the viscosity values around 0.5 dL/g or higher. The fact that m-cresol and 0-chlorophenol were very good solvents for the aliphatic polyamides is probably related to the ready formation of the polyamide with high viscosity values. The solvent with a high boiling point but low dielectric constant, like diphenyl ether, was not heated too much, probably due to limited solubility of the monomer, thereby giving no polymer.
It is apparent again from Table 1 that the viscosity values were greatly affected by the final reaction temperature, and higher final temperature afforded the polyamide having higher viscosity value. In general, the final reaction temperatures in excess of 280°C were required to obtain this type of polyamide with inherent viscosities exceeding 0.5 dL/g. Thus, the solvents played a very important role for the microwave-assisted polycondensation.
Figure 2 shows a comparison of the time dependence curve of the inherent viscosity of the polyamide formed by the microwave-assisted polycondensation of 12-aminododecanoic acid with the same polymer as obtained by conventional melt polycondensation. It is evident that the microwave-assisted polycondensation, curve A, proceeded much faster than the melt polycondensation, curve B. Thus, it is concluded that the internal heat generation of both solvent and the monomer under the microwave irradiation was much more effective for the progress of the polycondensation, producing the polyamide having a high viscosity value in a shorter polymerization time, compared with conventional external heating.
The microwave-assisted polycondensation was extended to the polycondensation of ω-amino acids such as 6-hexanoic acid and 11-aminoundecanoic acid in addition to 12-aminododecanoic acid. Table 2 summarizes the results of the microwave-assisted synthesis of various polyamides. All three amino acids readily afforded the polyamides having reasonable inherent viscosities around 0.5 dL/g in such solvents as m-cresol, DMI, and sulfolane for only 4 min of the microwave irradiation.
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424 STEP-GROWTH POLYMERS FOR HIGH-PERFORMANCE MATERIALS
400
ο I · i • · 1
0 2 4 6 8 10 12
Reaction time (min)
Figure 1. Time dependence of final temperature and inherent viscosity of the polyamide formed by the microwave-assisted polycondensation of 12-aminododecanoic acid (2 g) in different solvents (2 mL): (·) m-cresol, (•) DMI, and (•) sulfolane.
Table 1. Solvent Effect on Microwave-Assisted Polycondensation of 12-Aminododecanoic Acid a
a The polymerization was carried out with 2 g of the monomer and 2 mL of the solvent under microwave irradiation, b Final temperature of the solvent alone after 2 min of microwave irradiation. c Final temperature of the reaction mixture, d Measured at a concentration of 0.5 g/dL in m-cresol at 30°C. e Bp = 154°C/ 7 Torr.
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IMAI Polyamide and Polyimide Synthesis Using Microwaves 425
1.0
o « - · · • ' — ι
0 10 20 30 40 50 60
Reaction time (min)
Figure 2. Time dependence of inherent viscosity of the polyamide formed by the polycondensation of 12-aminododecanoic acid (2 g): (A) the microwave-assisted polycondensation in m-cresol, and (B) the melt polycondensation at 210°C.
Table 2. Microwave-Assisted Synthesis of Various Polyamides from Both ω-Amino Acids and Nylon Salts a
a The polymerization was carried out with 2 g of the ω-amino acid or nylon salt and 1 mL of the solvent under microwave irradiation for 4 min. b Measured at a concentration of 0.5 g/dL in m-cresol at 30°C. c DMI: 13-dimethylimidazolidone, and the polymerization was conducted for 5 min. d Sulfolane: tetramethylene sulfone.
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426 STEP-GROWTH POLYMERS FOR HIGH-PERFORMANCE MATERIALS
Polyamide Synthesis from Nylon Salts. The polycondensation of nylon salts was also accelerated by the microwave irradiation (Eq 2).
H2N(CH2)XNH2 + HOOC(CH2)yCOOH
> +H 3N(CH2) xNH3- |-OOC(CH2)yCOO-
> [-NH(CH2)xNHCO(CH2)yCO-]n + 2 H 2 0 (2)
x = 6,12 y = 4 ,6 ,8,10
Table 2 also lists the results of the microwave-assisted syntheses of a series of polyamides starting from the nylon salts. The polycondensation of the nylon salts was again accelerated by the microwave irradiation in the presence of polar organic solvents, yielding the polyamides having viscosity values between 0.2 and 0.5 dL/g. In more detail, the hexamethylenediamine-based nylon salts such as 66, 68, 610, and 612 tended to discolor partly during the microwave heating, thereby giving the polyamides with rather lower viscosity values around 0.2 dL/g. On the contrary, such a discoloration was not observed for the polycondensations of the dodecamethylenediamine-bearing nylon salts such as 126, 128, 1210, and 1212, resulting in the polyamides with higher inherent viscosities in the range of 0.3-0.6 dL/g.
Polyimide Synthesis from Nylon-Salt-Type Monomers. The microwave-assisted polycondensation was further applicable to the effective, rapid synthesis of aliphatic polypyromellitimides from the salt monomers XPMA and XPME composed of aliphatic diamines and both pyromellitic acid PMA and its diethyl ester PME (Eqs 3 and 4).
For the polycondensation of the nylon-salt-type monomers leading to aliphatic polypyromelitimides, we selected here high-boiling-point and polar organic media such as N-methyl-2-pyrrolidone (NMP, ε = 32 and bp 202°C), N.-cyclohexyl-2-pyrrolidone (CHP, bp 154°C/7 Torr), l,3-dimethyl-2-imidazolidone (DMI), and tetramethylene sulfone (sulfolane). Figure 3 shows the time dependence of the inherent viscosity of the polyimide formed by the microwave-assisted polycondensation of the dodecamethylenediamine-based salt monomers, 12 PMA and 12 PME, in the presence of DMI. The polycondensation proceeded very rapidly, and only 2 min of the microwave irradiation afforded readily the polyimide having an inherent viscosity of 0.7 dL/g. After that time, the increase in the inhernt viscosity was rather gradual with prolonged
XPMA
+ 4 H 2 O (3)
XPME
+ 2 H 2 O (4) + 2 EtOH
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27. IMAI Polyamide and Polyimide Synthesis Using Microwaves 427
microwave irradiation. No appreciable differences in the reactivity were observed between these two types of the salt monomers, 12 PMA and 12 PME, with respect to the inherent viscosities of the resultant polyimides.
The solvent effect on the microwave-assisted polycondensation of two salt monomers 12PMA and 12PME is summarized in Table 3. The best polymerization medium for this polycondensation was DMI, which gave the polyimides with inherent viscosities about 0.7 dL/g or higher. Other amide-type solvents such as NMP and CHP were also effective for producing the polyimides with high viscosity values. Sulfolane was the next suitable solvent. However, m-cresol, which was found to be one of the best solvent for the microwave-assisted polyamide synthesis, was inadequite for the polyimide synthesis because of lower solubility of these pyromellitic acid-based salt monomers.
1.2
Reaction time (min)
Figure 3. Time dependence of inherent viscosity of the polyimide formed by the microwave-assisted polycondensation of 12PMA (-·-) and 12PME (-•-) in DMI solvent (1 mL), where the monomer was 1 g.
Table 3. Solvent Effect on Microwave-Assisted Polycondensation of Salt Monomers of 12PMA and 12PME a
a The polymerization was carried out with 1 g of the monomer and 2 mL of the solvent under microwave irradiation for 2 min. b Measured at a concentration of 0.5 g/dL in concentrated sulfuric acid at 30°C. c A partial gelation occurred and the viscosity measurement was conducted after filtration of the solution. d Bp= 154°C/7Torr.
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428 STEP-GROWTH POLYMERS FOR HIGH-PERFORMANCE MATERIALS
The microwave-assisted polycondensation was extended to the polycondensation of a series of PMA- and PME-based salt monomers having 6 to 12 methylene units. The results of the polycondensation are tabulated in Table 4. All the rx>lycondensations were carried out in the presence of DMI, and proceeded very rapidly under microwave irradiation for only 2 min, yielding quantitatively the polyimides with high inherent viscosity values. When the pyromellitic acid based XPMA salt monomers were used, a series of polyimides having inherent viscosities in the range of 0.5 and 0.8 dL/g were readily obtained. In the case of the XPME salt monomers composed of aliphatic diamines and pyromellitic acid diethyl ester, these salt monomers dissolved more readily in DMI compared with the parent mPMA salt monomers, and hence the microwave-assisted polycondensation rapidly produced a series of polyimides with higher viscosity values between 0.8 and 1.6 dL/g. In more detail, when the salt monomers having 11 and 12 methylene units were used, the polymerization mixture remained in a melt state even after the solvent was evaporated, because the resultant polyimides had melting temperatures around 300°C. In the case of the polycondensation of the salt monomer having 6 methylene unit, the powdery polymer was obtained due to having high polymer melt temperature about 450°C, while all the other salt monomers having 7 to 10 methylene units afforded the polyimides with partly melted appearance. Hence the polymerization temperature must have reached to 300°C but not exceeded 400°C.
Table 4. Microwave-Assisted Synthesis of A Series of Polyimides from Nylon-Salt-Type Monomers a
a The polymerization was carried out with 1 g of the monomer and 2 mL of 13-dimethylimidazolidone under microwave irradiation for 2 min. b An endothemic peak temperature determined by DTA at a heating rate of 10°C/min in nitrogen. c Measured at a concentration of 0.5 g/dL in concentrated sulfuric acid at 30°C. d A partial gelation occurred and the viscosity measurement was conducted after filtration of the solution.
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27. IMAI Polyamide and Polyimide Synthesis Using Microwaves 429
Figure 4 shows a comparison of the time dependence curve of the inherent viscosity of the polyimide formed by the microwave-assisted polycondensation of 12PMA with that by a conventional solid-state polycondensation at 250°C. It is obvious that the microwave-assisted polycondensation for curve A proceeded much faster than the solid-state polycondensation for curve B. Thus it is concluded that the internal heating by the microwave irradiation was highly effective compared with a conventional external heating, yielding the polyimide with a high inherent visocisty in a very short polymerization time.
0 5 10 15 20
Reaction time (min)
Figure 4. Time dependence of inherent viscosity of the polyimide formed by the polycondensation of 12PMA (1 g): (A) the microwave-assisted polycondensation in DMI (1 mL), and (B) the thermal polycondensation at 250°C.
Conclusions
We have developed a new facile method for the rapid synthesis of aliphatic polyamides and aliphatic polypyromellitimides by using a domestic microwave oven by the polycondensation of both ε-amino acids and nylon salts as well as nylon-salt-type monomers in the presence of an appropriate polar organic medium. The use of such an organic medium was necessary to induce effective homogeneous heating of the monomers and hence cause efficient polycondensation leading to the formation of the polyamides and polyimides having high inherent viscosities around 0.5 dLg or above. The microwave-assisted polycondensation proceeded rapidly and was almost complete within 5 min for the polyamide synthesis and within 2 min for the polyimide. This method should become more and more important from the viewpoint of high efficiency utilization of heat energy. Further studies on this project for the rapid synthesis of other types of condensation polymers are now in progress.
References and Notes
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430 STEP-GROWTH POLYMERS FOR HIGH-PERFORMANCE MATERIALS
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