9 Polyimides Javier de Abajo and Jose ´ G. de la Campa Institute of Polymer Science and Technology, Madrid, Spain I. INTRODUCTION Polyimides are polymers incorporating the imide group in their repeating unit, either as an open chain or as closed rings. However, only cyclic imides are actually of interest concerning polymer chemistry. Thus, under the generic name polyimides, we will exclusively refer to cyclic polyimides in this chapter. The first reference to a polyimide was dated at the beginning of the 20th century [1], but the actual emergence of polyimides as a polymer class took place in 1955 with a patent of Edwards and Robinson on polymers from pyromel litic acid (1,2,4,5-tetracarboxy- benzene) and aliphatic diamines [2]. Since then, growing interest in polyimides has brought about a big expansion of the science and technology of this family of special polymers, which are characterised by excellent mechanical and electrical properties along with outstanding thermal stability. Among the wide list of reported heat-resistant condensati on polymers [3–5], polyimides have gained a prominent position due to their good properties– price–processability balance. And from the production figures, it can be inferred that polyimides stand virtually alone with respect to providing useful, available, technological materials. Furthermore, while at the beginning polyimides found application in a rather restricted variety of technologies, mainly on the form of films and varnis hes for the aerospace and electrical industries, the discovering of addition polyimides, and, more recently, of thermoplastic, processable aromatic polyimides has widened the range of properties and application possibilities to a great extent. Presently, they should be considered as versatile polymers with an almost unlimited spectrum of applications as specialty polymers for advanced technologies [6–12]. In a list of applications of polyimides, the following should be included:  Insulating films, coatings and laminates  Molded parts  Structural adhesives  Insulating foams  High-modulus fibers  High-temperature composites  Permselective membranes Copyright 2005 by Marcel Dekker. All Rights Reserved. From the beginning, the major proportion of research effort on polyimides was directed to the development of wholly aromatic species, seeking for high thermal stability. In this respect, wholly aromatic polyimides are materials that can retain their properties almost unchanged for long periods at 250–300  C. But it was soon realized that the application of aromatic polyimides, and in general aromatic polyheterocycles, was not possible from the melt and, furthermore, their extreme structural rigidity and high density of cohesive energy made them insoluble in any organic media. Given the excellent properties of the aromatic polyimides, structural modifications were soon outlin ed in order to overcome these limitations, and as a consequence of the many research efforts made in this direction, the chemistry of polyimides has greatly enriched thanks to the many improvements achieved in the last thirty years [9–11,13–15]. II. CONDENSATION POLYIMIDES A. Polyimides via Poly(amic acid) from Dianhydrides and Diamines. Reaction Conditions and Monomers Reactivity The polycondensation of an organic dianhydride and a diamine is the traditional method employed in the synthesis of polyimides (Scheme 1). ð1Þ This general scheme is valid for both aliphatic and aromatic polyimides. Since this is the route preferably used for aromatic, aliphatic and cycloaliphatic polyimides of technical importance, it has been the subject of numerous studies, and the main aspects of the mechanisms and kinetics are fairly well known [16]. It is a two-step reaction. In the first step the nucleophilic attack of the amine groups to the carbonyl groups of the dianhydride gives rise to the opening of the rings yielding an intermediate poly(amic acid) (Scheme 2). ð2Þ The symmetrical and unsymmetrical poly(amic acid)s are intended, since both are possible. Copyright 2005 by Marcel Dekker. All Rights Reserved. The poly(amic acid) is converted, in the second step, to the corresponding polyimide through a cyclodehydration reaction (Scheme 3). ð3Þ This simplified scheme may be envisioned in a more complete form by using monofunctional species (Scheme 4). ð4Þ The first step is crucial to attain high molecular weight, and the second has a great influence in the final nature of the polyimide since a quantitative conversion in the cyclodehydration process is needed to have a pure, fully cyclized polyimide. Highly polar solvents are suitable med ia to dissolve monomers and poly(amic acid)s. N,N-dimethyl- acetamide (DMA), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), and N-methyl-2-pyrrolidinone (NMP) are the most adequate. Purity of solvents and reactants, and strict stoichiometric balance are requirements of polycondensation reactions that fully fit polyimides synthesis, where a careful control of the reaction variables is essential to achieve high molecular weight [17–19]. For instance, rigorous exclusion of water is a key condition, as well as a moderate polymerization temperature (about 0  C or less) in poly(amic acid) formation in order to limit the competition of side reactions and a premature release of imidation water. A comparative study of the influence of side reactions has been made by Kolegov et al. [20], who have considered the following sequence of possible reactions (Scheme 5). The concurrence of these reactions can obviously alter the progress of the main reactions 1 and 2 and may prevent a high molecular weight. Experimental data of polycondensations of diamines and dianhydrides can generally be treated as second order reversible reactions, but the comparatively great magnitude of K 1 allows the calculation of rate constants according to an irrversible reaction. In fact K 1 is greater than K 2 , K 4 and Copyright 2005 by Marcel Dekker. All Rights Reserved. K 5 and K 5 by approximately seven orders of magnitude and over fifteen times greater than K 3 [21]. ð5Þ The reactants concentration also plays a determinant role. It has been stated that on plotting the inherent viscosity of poly(amic acid) against the initial concentration of monomers, a curve with a maximum can be attained. This maxi mum is presumably different for each monomers combination and solvent, but from the available data it is accepted that for high molecular weight to be obtained 0.4 to 0.8 mol/L monomer concentration is to be used [22–24]. The figures correlate well with data reported for the synthesis of aromatic polyamides from aromatic diamines and aromatic diacid chlorides [25]. In order to carry out a successful polymerization, a fixed mode of monomers addition has been suggested. Traditionally, the addition of the dianhydride (preferably as a solid) on the diamine solution has been considered as the right mode of addition, and that because the anhydride is sensitive to solvent impurities (water, amines), and even to solvent reaction, in much greater degree than the diamine, so that with the diamine in large excess the main reaction will be favoured [22,26,27]. Furthermore, unlike aromatic diamines, aromatic dianhydrides are not easily dissolved at low temperature. Nevertheless, the same results can be obtained regardless the order of monomers addition in the synthesis of poly(amic acid)s from pyromellitic dianhydride and oxydianiline if the reaction conditions are stretched in terms of dryness, stoichiometry, and solvent and monomers purity [28]. This indicates that the classical order of mono mers Copyright 2005 by Marcel Dekker. All Rights Reserved. addition has been imposed by the sensitivity of dianhydrides to water and solvent impurities more than by reactivity or solubility concerns. The progress of the polycondensation reaction largely depends also on the nature of the monomers, and particularly on the monomers reactivity. As a rule, electron deficient diamines will react more slow ly than electron rich diamines. At this respect, some studies have been made on the reactivity of diamines by conventional methods. A reliable approach to quantify the reactivity of diamines and dianhydrides, is the calculation of molecular parameters by means of the modern methods of Computational Chemistry. The reactivity of diamines against acylating monomers like acid chlorides have been reported [29,30]. Likewise, theoretical calculations can be made to estimate the relative reactivity of diamine and dianhydride monomers. Quantum semiempirical methods are reliable tools for the determination of parameters involved in the reactivity of organic reactants [31]. In fact, some partial studies were performed by Russian researchers more than twenty years ago to relate electronic parameters with reactivity of polyimide monomers [16] . However, the methods they used to calculate these parameters have been nowadays ov ercome, and consequently, it seems interesting to obtain new theoretical data that could be correlated with experimental results. Thus, the method AM1 [32] included in the MOPAC package, version 6.0 [33] has been used for the calculations that follow. In spite of the commercial importance of polyimides and of the huge number of new monomers synthesized in the last twenty years, the amount of kinetic data for the acylation reaction of diamines and dianhydrides is very scarce, and we have only been able to find data for a few diami nes and an even shorter number of dianhydrides [34]. As commented before, the acylation reaction between a diamine and a dianhydride takes place by the attack of the lone pair of the nitrogen of the amine to the centre of low electronic density located in the carbonylic carbon of the anhydride. Therefore, the reaction will be controlled by the interaction between the occupied orbitals of the diamine and the unoccupied orbitals of the dianhydride. The reactivity of the amines will be affected by both the electronic density on the nitrogen and by the energy of the Highest Occupied Molecular Orbital (HOMO) [29,30]. In dianhydrides, the reactivity will be determined by the electronic deficiency on the carbonylic carbon and by the energy of the Lowest Unoccupied Molecular Orbital (LUMO). As the reactivity will be higher when the difference between both orbitals will be lower, higher values of E HOMO and lower values of E LUMO will indicate the more reactive diamines and dianhydrides respectively. Tables 1 and 2 show the main parameters calculated for several diamines and dianhydrides, from which kinetic data could be found in the literature. The calculated values correspond, in all cases, to the more stable conformation. In both cases, diamines and dianhydrides, the differences of charge, either on the amino nitrogen or on the carbonylic carbon, are very scarce and, furthermore, in the case of diamines, because of the fact that the C Ar –N bond is out of the plane of the aromatic ring, the charge transfer from the amine to the ring is difficult. Therefore, the presence of electronwithdrawing groups does not cause a decreas e of the charge on the nitrogen but an increase on the polarizability of the N–H bonds. The values of E HOMO in the diamines are controlled by the character of the groups present in the structure, being higher (higher reactivity) in the case of electron donating groups. In that way, the higher reactivity should correspond to p-phenylene diamine, where the second amino group acts as activating of the first one. The lowest reactivity corresponds to the sulfon yldianiline (DDS O), because of the strong electron withdrawing character of the sulfone group. These values of E HOMO can be related with the Copyright 2005 by Marcel Dekker. All Rights Reserved. experimental values of acylation constants shown in Table 1 as it can be seen in Figure 1. A very good linear relationship can be observed, thus confirming the influence of the electronic parameters of the diamines in the determination of reactivity. The reaction of the first amino group, that is converted to amide, causes a decrease of the reactivity of the second amino group, as it could be expected, which is reflected by a decrease of E HOMO (Table 1). However, contrarily to the expected, a smal l increase of the electronic density in the amine nitrogen is observed. This effect is probably related with the out of plane situation of the C Ar –N bond, that has been commented above. The decrease in E HOMO is very small in all cases, even for p-phenylene diamine and practically no influence of the structure of the diamine can be observed. In Table 2 are shown the electronic characteristics of the dianhydrides (E LUMO and charge on the carbonylic carbon) and their acylation constants. In this case, the presence of electronwithdrawing groups causes a decrease of E LUMO . Thus, the most reactive compound is the pyromellitic dianhydride, because of the strong activation produced by the presence of the second anhydride group. Next in reactivity is the dianhydride with the sulfonyl group, and the lower reactivity corresponds to monomers with a long separation between both anhydrides, and with electron donating ether groups. However, in this case, the correlation between theoretical and experimental data is not as good as in the case of diamines, mainly because of the strong deviation of the linear behaviour observed in the case of the pyromellitic dianhydride. Table 1 Electronic parameters and kinetic data for several diamines and their corresponding monobenzamides. Diamine Q N a E HOMO Q N b amide E HOMO amide log K acylation À0.314 À7.92 À0.319 À8.06 2.48 À0.329 À8.26 À0.330 À8.40 0.00 À0.327 À7.94 À0.328 À8.06 0.37 À0.323 À8.11 À0.323 À8.25 0.79 À0.337 À8.65 À0.338 À8.73 À2.17 À0.326 À8.29 À0.326 À8.39 0.56 À0.354 À8.89 À0.356 À8.99 À2.66 À0.330 À8.32 À0.330 À8.36 0.15 a Charge on the nitrogen of any of the amino groups in the diamine. b Charge on the remaining amino group after the formation of the benzamide on the other side. Copyright 2005 by Marcel Dekker. All Rights Reserved. This must be attributed to the effect produced on the reactivity of the second anhydride group by the form ation of the amide in the first one. Also in this case, the occurrence of the first reaction causes a decrease in the reactivity of the second anhydride (an increase of E LUMO ), but a very small change of the charge on the carbonylic carbon. However, in this case, the change in the orbitalic energy is significantly higher than for diamines and it depends very much on the structure of the dianhydride (as most of the Table 2 Electronic parameters and kinetic data for several dianhydrides and their corresponding monoamides. Dianhydrides Q CðC¼OÞ a E LUMO Q CðC¼OÞ b amide E LUMO amide log K acylation 0.344 À2.86 0.349 À2.18 0.79 0.349(m) c À2.20 0.350(m) À1.85 0.13 0.348( p) 0.349( p) 0.349(m) À2.03 0.349(m) À1.68 À0.006 0.351( p) 0.353( p) 0.350(m) À2.30 0.350(m) À2.02 À0.66 0.346( p) 0.346( p) 0.351(m) À2.45 0.352(m) À2.15 1.04 0.341( p) 0.342(p) 0.349(m) À1.67 0.349(m) À1.59 À0.32 0.354( p) 0.354( p) 0.349(m) À1.53 0.349(m) À1.46 À0.80 0.355( p) 0.354( p) 0.351(m) À2.09 0.349(m) À2.01 0.326 0.345( p) 0.355( p) a Charge on any of the carbonyl groups in the dianhydride. b Charge on the remaining carbonyl groups after the formation of amide on the other side. c m and p refer to the carbonyl in meta or para position to the substituent. Copyright 2005 by Marcel Dekker. All Rights Reserved. dianhydrides are not symmetrical there are two possibilities of ring opening, one with the amide group in meta to the substituent and one with the amide in para). Although there are small differences between both, they are not significant and consequently the values of E LUMO shown in Table 2 are the mean of both possibilities). The reaction of one group in pyromellitic dianhydride increases E LUMO in 0.68 eV (maximum change for diamines was 0.14 eV), but in the case of the dianhydride with the aliphatic chain and the ether groups between both rings, only an increase of 0.07 eV is observed. This means that the reactivity for the global acylation does not depend on the reactivity of the dianhydride but on the reactivity of the less reactive molecule, that is, the monoreacted anhydride. Consequently, it can be confirmed that the reactivity of these species is controlled by the energy of the LUMO. A representation of E LUMO (monoamide) versus log K is shown in Figure 2. The correlation in this case is very good, thus confirming the useful ness of the electronic parameters to predict the reactivity, even in a semiquantitative way. Thus, the value of E LUMO can be used to predict the reactivity of dianhydrides, when no kinetic data are available. In Tabl e 3 are shown the E LUMO values of several important dianhydrides, for which kinetic data are not available. All these dianhydrides should have a very high reactivity, because of the lower values of E LUMO for both the dianhydride and the monoamide. In fact, hexafluoroisopropyliden 4,4 0 -diphthalic anhydride should be only slightly less reactive than benzophenone tetracarboxylic dianhydride, and 2,3,6,7-naphthalene tetracarboxylic dianhydride should be very similar to biphenyl dianhydride. But if the reaction is controlled by the monoamide, as we have postulated, the most reactive dianhydride should be 1,4,5,8- naphthalene tetracarboxylic dianhydride, because E LUMO is almost the same than for pyromellitic dianhydride, but E LUMO monoamide is lower than E LUMO monoamide of the pyromellitic (À 2.33 versus À 2.18 eV). To conclude, it can be said that the reactivity of diamines and dianhydrides to give polyamic acids, and consequently polyimides, is co ntrolled by the energy of the frontier orbitals of both types of molecules. Although the charges could also play a role in Figure 1 Correlation between E HOMO of the diamines and log K. Copyright 2005 by Marcel Dekker. All Rights Reserved. the control of reactivity, the differences between them are very small and, in addition, in the case of diamines it is very difficult to determine the real value of charge on the nitrogen because the amino group is not in the same plane that the aromatic ring. For the theoretical study of reactivities, selected diamines and dianhydrides have been chosen along those more frequently used in the preparation of aromatic polyimides. Most of them are commercially available, but some of them have been produced only at laboratory scale. Table 3 Electronic parameters for dianhydrides from which there are no kinetic data available. Dianhydride E LUMO E LUMO monoamide À2.237 À2.03 À2.15 À1.92 À2.85 À2.33 À2.20 À1.905 Figure 2 Correlation between E LUMO of the monoreacted dianhydrides and log K. Copyright 2005 by Marcel Dekker. All Rights Reserved. For some specific applications, particularly for microelectronics, the purification of these monomers is sometimes so critical that the isolation of suitable reactants requires sophisticated purification methods. For instance, miniaturization and tougher processing requirements for advanced microelectronics have forced researchers to attain ultrapure poly(amic acid)s from monomers purified by zone refining, and dianhydrides isolated in solid ingot form [35]. As to the molecular weights of poly(amic acid)s and polyimides, they had been only very seldom measured and reported. Thus, the usual criterion for molecular size in poly(amic acid)s and soluble polyimides had traditionally been the inherent viscosity ( inh ) until the size exclusion chromatography techniques (GPC) were refined and implemented in last years. The development of many new soluble thermoplastic polyimides has moved also for a growing interest in knowing the molecular weights, and for an improvement of the analytical technique for the determination of M n ’s and M w ’s. GPC columns, that can work with aggressive solvents like DMF, DMA or m-cresol at temperatures up to 70–80  C, are available nowadays and can be used for the analysis of many soluble polyimides [36,37]. Of greatest importance is the cyclodehydration reaction leading from poly(amic acid)s to polyimides. The general approach in the application of insoluble, wholly aromatic polyimides as materials involves the elimination of solvent and water at high temperature. When a poly(amic acid) solution is heated over 200  C, or at a lower temperature in the presence of a dehydrating agent, such as acetic anhydride/base, the polyimide is attained in few hours. Logically, the first approach received much more attention in the early years, because the research effort was mainly focussed to insoluble polyim ides based on pyromellitic dianhydride, although the chemical imidization of poly(amic acid) films in the solid state has been the subject of several studies [38–40]. Thermal imidization associated to classical aromatic polyimides actual ly needs temperatures of about 300  C to ensure total rings closing, and that is far from being an optimal approach in many instances because elimination of solvent and water at high temperatures can approach about surface irregularities, microvoids and even polymer degradation. Furthermore, high temperatures help for cross-linking side reactions, for example (Scheme 6): 1. ÀNH 2 end groups with the imide rings of the chains: 2. Thermal imidization by means of non-cyclized ortho-carboxyamides: ð6Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... Properties of selected polyimides from monomers containing flexible bridging groups Solubility Polymer Tg ( C) NMP 2 59 267 Æ þþ 299 305 223 m-cresol þþ þ þþ þþ þþ diamines and the so-called ‘cardo’ monomers, can be considered in this section, and they can be seen also as valuable alternatives for the preparation of processable polyimides [92 94 ] For the preparation of this new generation of aromatic... copolyimides In fact, many of the countless processable, aromatic polyimides described in last years, have ether linkages in their backbone [74,81, 197 – 199 ] The method of synthesis is the general method via poly(amic acid) in most instances, using ether-containing dianhydrides or ether-containing diamines One of the most representative structure is shown in Scheme ( 39) ð 39 Some representative monomers... the synthesis of many poly(ester imide)s of varied chemical structure [154–158], including a great deal of novel aliphatic–aromatic and aromatic thermotropic poly(ester imide)s [1 59] 3 Poly(ester imide) Resins Most of these copolymers have been described as linear because they are formulated to be soluble in amidic solvents and cresols, however, they become thermosets upon crosslinking at the moment of. .. react in solution of appropriate organic solvents to yield poly(amic acid ester) in a first step, and poly(ester imide) by cyclodehydration in the second step A number of dianhydrides containing ester groups have been used for the synthesis of poly(ester imide)s Most of them are bistrimellitates which are synthesized by condensation of diacetylbisphenols (R ¼ arylene) with two moles of trimellitic anhydride... catalysts [67,68] 9 Imidization is achieved by thermal treatment of the poly(amic ester) precursor in the usual way, with elimination of alcohol This method, because of its relative complexity, has not got practical significance for conventional polyimides However it has been of great importance in the development of photocurable condensation polyimides [8], and to study the behaviour of different isomers... force water separation For the splitting off of water, azeotropic solvents are frequently used too From a mechanistic point of view, it is to presume that bases help for the nucleophilic attack of the diamine to the anhydride to form amic acid, and acids catalyse the closing of the ring with evolution of water Nevertheless, the role of acids in the formation of six-membered ring imides, like naphthalimides,... universally adopted:     Introduction of flexible linkages, which reduces chain stiffness Introduction of side substituents, which helps for separation of polymer chains and hinder molecular packing and crystallization Use of 1,3-substituted instead of 1,4-substituted monomers, and/or asymmetric monomers, which lower regularity and molecular ordering Preparation of co-polyimides from two or more dianhydrides... used [74,75] The presence of flexible linkages has a dramatic effect on the properties of the final polymers First, ‘kink’ linkages between aromatic rings or between phthalic anhydride functions cause a breakdown of the planarity and an increase of the torsional mobility Furthermore, the additional bonds mean an enlargement of the repeating unit and, consequently, a separation of the imide rings, whose... and also the combination of some of them with conventional rigid monomers like benzenediamines, benzidine, pyromellitic dianhydride or biphenyldianhydride, offer a major possibility of different structures with a wide spectrum of properties, particularly concerning solubility and meltability [76–80] However, very few of the polymers that can be synthesized combining monomers of Tables 3 and 4 have been... readily give rise to complexes [41], and that solvent rests can remain joined to the polymer even through covalent bonds [42,43] A novel preparative method of poly(amic acid)s from aromatic diamines and dianhydrides consists of carrying out the polycondensation reaction in a precise mixture of tetrahydrofurane/methanol (9/ 1 to 6/4 by weight), at room temperature [44] Average molecular weights (Mw) exceeding . À7 .92 À0.3 19 À8.06 2.48 À0.3 29 À8.26 À0.330 À8.40 0.00 À0.327 À7 .94 À0.328 À8.06 0.37 À0.323 À8.11 À0.323 À8.25 0. 79 À0.337 À8.65 À0.338 À8.73 À2.17 À0.326 À8. 29 À0.326 À8. 39 0.56 À0.354 À8. 89. 0.342(p) 0.3 49( m) À1.67 0.3 49( m) À1. 59 À0.32 0.354( p) 0.354( p) 0.3 49( m) À1.53 0.3 49( m) À1.46 À0.80 0.355( p) 0.354( p) 0.351(m) À2. 09 0.3 49( m) À2.01 0.326 0.345( p) 0.355( p) a Charge on any of the. of the electronic parameters of the diamines in the determination of reactivity. The reaction of the first amino group, that is converted to amide, causes a decrease of the reactivity of the second