Handbook of Polymer Synthesis Second Edition Episode 9 ppsx

The polyamic acid is converted, in the second step, to the corresponding polyimidethrough a cyclodehydration reaction Scheme 3.to achieve high molecular weight [17–19].. Nevertheless, th

Polyimides

Javier de Abajo and Jose´ G de la Campa

Institute of Polymer Science and Technology, Madrid, Spain

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 interestconcerning polymer chemistry Thus, under the generic name polyimides, we willexclusively 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 pyromellitic acid benzene) and aliphatic diamines [2] Since then, growing interest in polyimides has broughtabout a big expansion of the science and technology of this family of special polymers,which are characterised by excellent mechanical and electrical properties along withoutstanding thermal stability Among the wide list of reported heat-resistant condensationpolymers [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 thatpolyimides stand virtually alone with respect to providing useful, available, technologicalmaterials

(1,2,4,5-tetracarboxy-Furthermore, while at the beginning polyimides found application in a ratherrestricted variety of technologies, mainly on the form of films and varnishes for theaerospace and electrical industries, the discovering of addition polyimides, and, morerecently, of thermoplastic, processable aromatic polyimides has widened the range ofproperties and application possibilities to a great extent Presently, they should beconsidered as versatile polymers with an almost unlimited spectrum of applications asspecialty polymers for advanced technologies [6–12]

In a list of applications of polyimides, the following should be included:

Insulating films, coatings and laminates

From the beginning, the major proportion of research effort on polyimides wasdirected to the development of wholly aromatic species, seeking for high thermal stability.

In this respect, wholly aromatic polyimides are materials that can retain their propertiesalmost unchanged for long periods at 250–300
C But it was soon realized that theapplication of aromatic polyimides, and in general aromatic polyheterocycles, was notpossible from the melt and, furthermore, their extreme structural rigidity and high density

of cohesive energy made them insoluble in any organic media Given the excellentproperties of the aromatic polyimides, structural modifications were soon outlined inorder to overcome these limitations, and as a consequence of the many research effortsmade in this direction, the chemistry of polyimides has greatly enriched thanks to themany improvements achieved in the last thirty years [9–11,13–15]

A Polyimides via Poly(amic acid) from Dianhydrides and Diamines

Reaction Conditions and Monomers ReactivityThe polycondensation of an organic dianhydride and a diamine is the traditional methodemployed in the synthesis of polyimides (Scheme 1)

ð1Þ

This general scheme is valid for both aliphatic and aromatic polyimides Since this isthe route preferably used for aromatic, aliphatic and cycloaliphatic polyimides of technicalimportance, it has been the subject of numerous studies, and the main aspects ofthe mechanisms and kinetics are fairly well known [16] It is a two-step reaction In thefirst step the nucleophilic attack of the amine groups to the carbonyl groups of thedianhydride gives rise to the opening of the rings yielding an intermediate poly(amic acid)(Scheme 2)

ð2ÞThe symmetrical and unsymmetrical poly(amic acid)s are intended, since both arepossible

The poly(amic acid) is converted, in the second step, to the corresponding polyimidethrough a cyclodehydration reaction (Scheme 3).

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 apremature 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 mainreactions 1 and 2 and may prevent a high molecular weight Experimental data ofpolycondensations of diamines and dianhydrides can generally be treated as second orderreversible reactions, but the comparatively great magnitude of K1allows the calculation

of rate constants according to an irrversible reaction In fact K1is greater than K2, K4and

K5 and K5 by approximately seven orders of magnitude and over fifteen times greaterthan K3[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 ofmonomers, a curve with a maximum can be attained This maximum is presumablydifferent 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/Lmonomer concentration is to be used [22–24] The figures correlate well with datareported for the synthesis of aromatic polyamides from aromatic diamines and aromaticdiacid chlorides [25]

In order to carry out a successful polymerization, a fixed mode of monomersaddition 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, andthat 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 inlarge excess the main reaction will be favoured [22,26,27] Furthermore, unlike aromaticdiamines, aromatic dianhydrides are not easily dissolved at low temperature

Nevertheless, the same results can be obtained regardless the order of monomersaddition in the synthesis of poly(amic acid)s from pyromellitic dianhydride andoxydianiline if the reaction conditions are stretched in terms of dryness, stoichiometry,and solvent and monomers purity [28] This indicates that the classical order of monomers

addition has been imposed by the sensitivity of dianhydrides to water and solventimpurities more than by reactivity or solubility concerns.

The progress of the polycondensation reaction largely depends also on the nature ofthe monomers, and particularly on the monomers reactivity As a rule, electron deficientdiamines will react more slowly than electron rich diamines At this respect, some studieshave been made on the reactivity of diamines by conventional methods A reliableapproach to quantify the reactivity of diamines and dianhydrides, is the calculation ofmolecular parameters by means of the modern methods of Computational Chemistry Thereactivity of diamines against acylating monomers like acid chlorides have been reported[29,30] Likewise, theoretical calculations can be made to estimate the relative reactivity ofdiamine and dianhydride monomers

Quantum semiempirical methods are reliable tools for the determination ofparameters involved in the reactivity of organic reactants [31] In fact, some partialstudies were performed by Russian researchers more than twenty years ago to relateelectronic parameters with reactivity of polyimide monomers [16] However, the methodsthey used to calculate these parameters have been nowadays overcome, and consequently,

it seems interesting to obtain new theoretical data that could be correlated withexperimental 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 ofnew monomers synthesized in the last twenty years, the amount of kinetic data for theacylation reaction of diamines and dianhydrides is very scarce, and we have only been able

to find data for a few diamines and an even shorter number of dianhydrides [34]

As commented before, the acylation reaction between a diamine and a dianhydride takesplace by the attack of the lone pair of the nitrogen of the amine to the centre of lowelectronic density located in the carbonylic carbon of the anhydride Therefore, thereaction will be controlled by the interaction between the occupied orbitals of the diamineand the unoccupied orbitals of the dianhydride The reactivity of the amines will beaffected by both the electronic density on the nitrogen and by the energy of the HighestOccupied Molecular Orbital (HOMO) [29,30] In dianhydrides, the reactivity will bedetermined by the electronic deficiency on the carbonylic carbon and by the energy of theLowest Unoccupied Molecular Orbital (LUMO)

As the reactivity will be higher when the difference between both orbitals will belower, higher values of EHOMOand lower values of ELUMOwill indicate the more reactivediamines and dianhydrides respectively Tables 1 and 2 show the main parameterscalculated 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 stableconformation 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, inthe case of diamines, because of the fact that the CAr–N bond is out of the plane ofthe aromatic ring, the charge transfer from the amine to the ring is difficult Therefore, thepresence of electronwithdrawing groups does not cause a decrease of the charge on thenitrogen but an increase on the polarizability of the N–H bonds

The values of EHOMOin the diamines are controlled by the character of the groupspresent in the structure, being higher (higher reactivity) in the case of electron donatinggroups 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 reactivitycorresponds to the sulfonyldianiline (DDSO), because of the strong electron withdrawingcharacter of the sulfone group These values of EHOMO can be related with the

experimental values of acylation constants shown inTable 1as it can be seen inFigure 1.

A very good linear relationship can be observed, thus confirming the influence of theelectronic 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 adecrease of EHOMO(Table 1) However, contrarily to the expected, a small increase of theelectronic density in the amine nitrogen is observed This effect is probably related with theout of plane situation of the CAr–N bond, that has been commented above The decrease

in EHOMO is very small in all cases, even for p-phenylene diamine and practically noinfluence of the structure of the diamine can be observed

InTable 2are shown the electronic characteristics of the dianhydrides (ELUMOandcharge on the carbonylic carbon) and their acylation constants In this case, the presence

of electronwithdrawing groups causes a decrease of ELUMO Thus, the most reactivecompound is the pyromellitic dianhydride, because of the strong activation produced bythe presence of the second anhydride group Next in reactivity is the dianhydride with thesulfonyl group, and the lower reactivity corresponds to monomers with a long separationbetween 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 ofdiamines, mainly because of the strong deviation of the linear behaviour observed in thecase of the pyromellitic dianhydride

Table 1 Electronic parameters and kinetic data for several diamines and their correspondingmonobenzamides

QNbamide

EHOMOamide

log Kacylation

This must be attributed to the effect produced on the reactivity of the secondanhydride group by the formation of the amide in the first one Also in this case, theoccurrence of the first reaction causes a decrease in the reactivity of the second anhydride(an increase of ELUMO), 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 fordiamines 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 correspondingmonoamides

Dianhydrides QCðC¼OÞa ELUMO QCðC¼OÞb amide ELUMOamide log K acylation

dianhydrides are not symmetrical there are two possibilities of ring opening, one with theamide group in meta to the substituent and one with the amide in para) Although thereare small differences between both, they are not significant and consequently the values of

ELUMOshown inTable 2are the mean of both possibilities) The reaction of one group inpyromellitic dianhydride increases ELUMOin 0.68 eV (maximum change for diamines was0.14 eV), but in the case of the dianhydride with the aliphatic chain and the ether groupsbetween both rings, only an increase of 0.07 eV is observed

This means that the reactivity for the global acylation does not depend on thereactivity 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 ofthese species is controlled by the energy of the LUMO A representation of ELUMO(monoamide) versus log K is shown inFigure 2.The correlation in this case is very good,thus confirming the usefulness of the electronic parameters to predict the reactivity, even

in a semiquantitative way

Thus, the value of ELUMOcan be used to predict the reactivity of dianhydrides, when

no kinetic data are available InTable 3are shown the ELUMOvalues of several importantdianhydrides, for which kinetic data are not available

All these dianhydrides should have a very high reactivity, because of the lower values

of ELUMOfor both the dianhydride and the monoamide In fact, hexafluoroisopropyliden4,40-diphthalic anhydride should be only slightly less reactive than benzophenonetetracarboxylic dianhydride, and 2,3,6,7-naphthalene tetracarboxylic dianhydride should

be very similar to biphenyl dianhydride But if the reaction is controlled by themonoamide, as we have postulated, the most reactive dianhydride should be 1,4,5,8-naphthalene tetracarboxylic dianhydride, because ELUMO is almost the same than forpyromellitic dianhydride, but ELUMOmonoamide is lower than ELUMOmonoamide of thepyromellitic ( 2.33 versus  2.18 eV)

To conclude, it can be said that the reactivity of diamines and dianhydrides togive polyamic acids, and consequently polyimides, is controlled by the energy of thefrontier orbitals of both types of molecules Although the charges could also play a role inFigure 1 Correlation between EHOMOof the diamines and log K

the control of reactivity, the differences between them are very small and, in addition, inthe case of diamines it is very difficult to determine the real value of charge on the nitrogenbecause the amino group is not in the same plane that the aromatic ring.

For the theoretical study of reactivities, selected diamines and dianhydrides havebeen 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 atlaboratory scale

Table 3 Electronic parameters for dianhydrides from which there are no kinetic data available

For some specific applications, particularly for microelectronics, the purification ofthese monomers is sometimes so critical that the isolation of suitable reactants requiressophisticated purification methods For instance, miniaturization and tougher processingrequirements for advanced microelectronics have forced researchers to attain ultrapurepoly(amic acid)s from monomers purified by zone refining, and dianhydrides isolated insolid ingot form [35].

As to the molecular weights of poly(amic acid)s and polyimides, they had been onlyvery seldom measured and reported Thus, the usual criterion for molecular size inpoly(amic acid)s and soluble polyimides had traditionally been the inherent viscosity (inh)until the size exclusion chromatography techniques (GPC) were refined and implemented inlast years The development of many new soluble thermoplastic polyimides has moved alsofor a growing interest in knowing the molecular weights, and for an improvement of theanalytical technique for the determination of Mn’s and Mw’s GPC columns, that can workwith aggressive solvents like DMF, DMA or m-cresol at temperatures up to 70–80
C, areavailable 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 aromaticpolyimides 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 thepresence of a dehydrating agent, such as acetic anhydride/base, the polyimide is attained infew hours Logically, the first approach received much more attention in the early years,because the research effort was mainly focussed to insoluble polyimides based onpyromellitic dianhydride, although the chemical imidization of poly(amic acid) films inthe solid state has been the subject of several studies [38–40] Thermal imidization associated

to classical aromatic polyimides actually needs temperatures of about 300
C to ensure totalrings closing, and that is far from being an optimal approach in many instances becauseelimination of solvent and water at high temperatures can approach about surfaceirregularities, microvoids and even polymer degradation Furthermore, high temperatureshelp for cross-linking side reactions, for example (Scheme 6):

1 NH2end groups with the imide rings of the chains:

2 Thermal imidization by means of non-cyclized ortho-carboxyamides:

ð6Þ

3 Amidation of NH2free groups and ortho-carboxyamides:

These reactions do help for a faster immobilization of the chain and, consequently,for additional difficulties to get 100% cyclodehydration The strong interactions betweenthe poly(amic acid) and the solvent also greatly interferes with the intramolecularcyclization, and its presence does not certainly aid a quantitative conversion It hasbeen demonstrated that solvents and poly(amic acid)s 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 anddianhydrides 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] Averagemolecular weights (Mw) exceeding 150,000 g/mol have been reported for poly(amic acid)sattained by this method from oxydianiline and pyromellitic anhydride [45] Moreover,thermal imidization seems to be more easily achievable on replacing classical high boilingamide solvents such as DMA or NMP by the easy to evaporate THF and methanolmixtures [46]

Chemical imidization is normally promoted by acetic anhydride, in combinationwith organic bases, for instance pyridine or triethylamine, but other dehydrating agentscan be used, such as propionic anhydride, trifluoroacetic anhydride, N,N-dicylo-hexylcarbodiimide and the like Although it can be performed on polyimide films,chemical imidization is mostly carried out in solution, with the final polyimide beingcollected as a precipitate, but most conveniently remaining dissolved all over the process

A premature precipitation of the polymer does not ensure total imidization at all,

as partially imidized species can be insoluble in the organic medium Temperatures andreaction times amply vary depending on the polymer and the cyclization system Thus, ifthe reaction is conducted at room temperature 24 to 48 h are needed for total imidization,while some few hours are enough if the chemical cyclodehydration reaction proceeds at

100
C

The imidization process, either thermally or chemically induced, may be followed

by a variety of means It has been traditionally studied on poly(amic acid)s, as well

as with molecular models, by IR and NMR spectroscopy [47,48] But many otheranalytical methods have been used, for instance: TGA [41,49,50], DSC [42,51], polarizingmicroscopy [41], gas chromatography [52,53], microdielectrometry [54], or torsionalbraid analysis [55] From the numerous contributions on this topic some conclusionscan be drawn Among other features, we remark that a rate reduction of the imidationand the rate constant occurs as the conversion increases, so that it can not be considered

as a classical first order reaction This phenomenon has been explained by consideringentropic factors [56] Since the kinetic data could not be unequivocally assimilated

to a determined reaction order, they were interpreted as if the imidization reactioncould be divided into rapid and slow first order cyclization steps The retardation in

the ring closure reaction has also been explained by the existence of various amide-acidgroups with different reactivities and by the mobility reduction when the linear polymer isconverted into a cyclic chain [38,57].

The formation of isoimide as an intermediate step to imide has been confirmedalso in many instances (Scheme 7) Isoimides are less stable than imides, so thatisoimide formation does not seem to play any significant role when conversion intoimide is forced by thermal treatment, but it can affect the imidization process whenrings closure is performed by chemical treatment at moderate temperatures.Furthermore, it has been proved that some solvent/anhydride/base combinationsclearly favour the formation of isoimide [58], what can in turn, offer some advantagefrom a practical view point as polyisoimides are much more soluble than polyimides[14,59,60]

Chemical imidization is less attractive for commercial and experimental polyimidesthat are tested and used in the form of films, but chemical imidation has been the preferredmethod concerning experimental polyimides that are soluble in organic solvents in thestate of full imidation At this respect it is worthy to remark that 100% conversion in thering closure step is virtually impossible to achieve, particularly for thermal imidation

at high temperature (about 300
C) in the solid state, due to the complexity of the processand to the inherent molecular regidity of insoluble polyimides However, for solublepolyimides, solution imidization is possible at mild reaction temperatures, for instance150–200
C, with 100% conversion, and avoiding undesirable side reactions which lead toinsolubility and infusibility [61]

ð7Þ

By using monomers other than dianhydrides and diamines, a number of methods hasbeen outlined to synthesize polyimides, for instance from tetracarboxylic acids and theirhalf diesters This method can be successfully applied to the preparation of aliphatic–aromatic polyimides by melt polycondensation of the salt from the diamine and anaromatic tetracarboxylic acid or half-diester (Scheme 8) The reaction achieved someimportance when polyimides appeared in the 1950s as an alternative for uses such as

fiber-forming and injection-molding polymers [2,62].

ð8Þ

The method was believed to be valid only for polyimides with melting pointslow enough to remain molten during the polymerization, and solution methods wereconsidered as not suitable since the ‘polyamic salts’ are not soluble in aprotic organicsolvents, so that only low molecular weight polymeric salts were obtained The methodwas not used for many years, mainly because aliphatic polyimides did not showmuch higher Tm than conventional nylons, and their main advantage, their high Tgvalue, did not mean any useful improvement as their performances under serviceconditions were comparable to the semicrystalline aliphatic polyamides, which are in turnmuch cheaper

However, polyalkylenimides can be prepared from pyromellitic anhydride anda,o-diaminoalkanes in solution of NMP NMP seems to provide a much more convenientmedium for these reaction than other organic solvents, and in this way, high molecularweight poly(amic acid)s and polyimides have been attained [63,64]

A revision of this approach has been made in last years, and aliphatic and aromaticsalt monomers have been studied as precursors of high molecular weight polyimides.Salt monomers have shown to be actually highly reactive, as they can produce directlypolyimides in a very short reaction time, and this feature has recently been observednot only for aliphatic precursors but for aromatic salts as well [65] Moreover, highmolecular weight polyimides can be achieved by combining the salt monomer methodwith high pressure polycondensation, or with microwave induced polycondensation[65,66] Other advantages of the salt monomer method is that polycondensations canprogress at high conversion in solid state, at temperatures substantially lower than themelting point (Tm) of the polymers, and at a lower temperature than the salt monomermelting point Thus, imide ring closure takes place simultaneously to water or alcoholelimination, rendering polyimide in an one-step direct reaction without passingthrough poly(amic acid) In general, better results (higher inherent viscosities) areachieved from half esters than from tetracarboxylic acids, and another feature of thisrecently revised method is that highly crystalline polyimides, both aliphatic and aromatic,can be attained [66]

Half diesters and their derivatives have been extensively used also in the preparation

of aromatic polyimides by the two-step method The initial step involves the preparation

of the modified monomers, which consist of the half esters themselves or of activatedderivatives The activation of the half esters is normally directed to the enhancement of thecarboxylic acids reactivity, by converting them into other highly electrophilic groups such

as acyl chlorides The global synthetic route is depicted in Scheme (9) The high reactivity

of acid chlorides against diamines, makes the solution method at low temperature notonly recommendable but virtually the only possible one if high molecular weights are to beaccomplished

Low to medium molecular sizes can be also obtained from the half esters directlywith specific amidation catalysts [67,68].

ð9Þ

Imidization is achieved by thermal treatment of the poly(amic ester) precursor inthe 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 ofgreat importance in the development of photocurable condensation polyimides [8], and

to study the behaviour of different isomers as starting materials for model polyimides [67]

B One-step Polycondensation Thermoplastic Polyimides

As mentioned before, the first generation of fully aromatic homopolyimides, could be usedonly in a few application because they had to be applied in the form of soluble polyamicacids, what limited the materials to be transformed almost exclusively into films orcoatings They all had to be synthesized by a two-step method

Further improvements in the chemistry of polyimides during the last years have beendirected towards novel, linear species that are soluble in workable organic solvents ormelt-processable while fully imidized Thus, changes had to be introduced in the chemicalstructure to adapt the behaviour and performance of these specialty polymers to thedemands of the new technologies As a consequence, a new generation of condensationpolyimides has appeared, the so-called thermoplastic polyimides

The difficulties to process conventional aromatic polyimides are due to theirinherent molecular features, what is particularly true for the most popular of them:polypyromellitimides Molecular stiffness, high polarity and high intermolecular associa-tion forces (high density of cohesive energy) make these polymers virtually insoluble in anyorganic medium, and shift up the transition temperatures well over the decompositiontemperatures Thus, the strategies to novel processable aromatic polyimides have focussed

on chemical modifications, mainly by preparing new monomers, that provide lessmolecular order, torsional mobility and lower intermolecular bonding

From the various alternatives to design novel processable polyimides some generalapproaches have been universally adopted:

Introduction of flexible linkages, which reduces chain stiffness

Introduction of side substituents, which helps for separation of polymer chainsand hinder molecular packing and crystallization

Use of 1,3-substituted instead of 1,4-substituted monomers, and/or asymmetricmonomers, which lower regularity and molecular ordering

Preparation of co-polyimides from two or more dianhydrides or diamines.Polyimides with flexible linkages have been known from the advent of hightemperature aromatic polyimides In fact, most of the commercial, fully aromaticpolyimides contain ketone or ether linkages in their repeating units [69], and early works

in the field soon demonstrated that dianhydrides having two phthalic anhydride moietiesjoined by bonding groups, gave more tractable polyimides [26,70–73].

Many different linkages have been introduced with these purposes, but the mostpromising are: –O–, C¼O, –S–, –SO2–, –C(CH3)2–, –CH2–, –CHOH–, and –C(CF3)2–.These bonding groups may be located on the dianhydride, on the diamine or on bothmonomers, or they can even be formed during the polycondensation reaction, whensome functional monomers containing preformed phthalimide groups are used [74,75].The presence of flexible linkages has a dramatic effect on the properties of the finalpolymers First, ‘kink’ linkages between aromatic rings or between phthalic anhydridefunctions 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 relative density is actually responsible

of the polymer tractability The suppression of the coplanar structure is maximal whenbulky groups are introduced in the main chain, for instance sulfonyl or hexafluoro-isopropylydene groups, or when the monomers are enlarged by more than one flexiblelinkage Some diamines and dianhydrides with a flexible linkage in their structure havebeen listed in Tables 4 and 5 The combination of those dianhydrides and diamines,and also the combination of some of them with conventional rigid monomers likebenzenediamines, benzidine, pyromellitic dianhydride or biphenyldianhydride, offer amajor possibility of different structures with a wide spectrum of properties, particularlyconcerning solubility and meltability [76–80]

However, very few of the polymers that can be synthesized combining monomers ofTables 3 and 4 have been reported as melt-processable, although many of them are soluble

in highly polar organic solvents All of them show high glass transition temperatures,commonly over 250
C, and, theoretically, they can develop crystallinity upon a suitablethermal treatment, mainly those containing polar connecting groups Thus, depending onthe nature of X and Y in the general formula (Scheme 10), polyimides can be prepared thatshow an acceptable degree of solubility in organic solvents

ð10Þ

Table 6 shows the Tg’s and solubility of some selected polyimides among thoseprepared from monomers of Tables 4 and 5 The combination of non-planar dianhydridesand aromatic diamines containing flexible linkages, provides the structural elementsneeded for solubility and melt processability Some aromatic polyimides marketed asthermoplastic materials are based on these statements [69,81–83]

Structural modifications to attain soluble aromatic polyimides have been also carriedout by introducing side substituents, alkyl, aryl or heterocyclic rings One of the firstreferences of this approach described the synthesis of soluble aromatic polyimidescontaining side phthalimide groups [84,85] Since then, many attempts have been made

to prepare new monomers, diamines and dianhydrides, with pendent groups for novelprocessable polyimides Table 7shows some of these monomers

Probably, the most promising species are those containing phenyl pendent groups.The phenyl rest does not introduce any relevant weakness regarding thermal stability, andprovides a factor of molecular irregularity and separation of chains very beneficial in terms

of free volume increasing and lowering of the cohesive energy density [80–91] Fluorene

Table 4 Diamines for polyimides containing flexible linkages.

Table 5 Dianhydrides for polyimides containing flexible linkages.

diamines and the so-called ‘cardo’ monomers, can be considered in this section, and theycan be seen also as valuable alternatives for the preparation of processable polyimides[92–94].

For the preparation of this new generation of aromatic polyimides, syntheticmethods have been outlined which allow to achieve the polymers in their state offull imidization in only one-step However, the classical sequence poly(amicacid) ! polyimide is generally followed somehow, although imidization occurs virtually

at the same time that propagation Amide solvents of high boiling point, as NMP ofN-cyclohexylpyrrolidinone (CHP), nitrobenzene, chloroaromatics, phenols, cresols[36,37,61,95–100], and even carboxylic acids as benzoic acid [101], are solventssuccessfully used for the preparation of processable polyimides Moreover, thebeneficial effect of some basic (isoquinoline, triethylamine, pyridine) and acid (benzoic,hydroxybenzoic, salicylic) catalysts have been observed [80,95] The reaction proceedsusually at low or moderate temperature in the first stage to favour the formation

of a high molecular weight poly(amic acid), while the second part is led at hightemperature to promote the cyclodehydration reaction and to 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 nucleophilicattack of the diamine to the anhydride to form amic acid, and acids catalyse theclosing of the ring with evolution of water Nevertheless, the role of acids inthe formation of six-membered ring imides, like naphthalimides, needs still anexplanation [102]

Table 6 Properties of selected polyimides from monomers containing flexible bridging groups

Table 7 Diamines and dianhydrides used in the preparation of polyimides with bulkyside groups.

(continued )

C Polyimides from Dianhydrides and Diisocyanates

Although the reaction of anhydrides with isocyanates to give imides was very earlyreported [103], it was not until about 1970 that the reaction found application in thesynthesis of polyimides and copolyimides [104–106] (Scheme 11)

ð11ÞTable 7 Continued

The reaction takes different pathways depending on the conditions In the absence ofcatalyst the reaction has been claimed to proceed through a seven-membered polycycleintermediate (Scheme 12) that finally gives rise to polyimide with separation of carbondioxide.

ð12ÞSpectroscopic evidence of the seven-membered rings has been found in thepreparation of polyimides from pyromellitic dianhydride and methylenediphenyl-diisocyanate (MDI) [105] The reaction is conducted in solution of aprotic solvents,with reagents addition at low temperature and a maximum reaction temperature of about

130
C On the other hand, polyimides of very high molecular weight have not beenreported by this method The mechanism is different when the reaction is accelerated bythe action of catalysts Catalytic quantities of water or alcohols facilitate imide formation,and intermediate ureas and carbamates seem to be formed, which then react withanhydrides to yield polyimides [106] Water as catalyst has been used to exemplify themechanism of reaction of phthalic anhydride and phenyl isocyanates, with the conclusionthat the addition of water, until a molecular equivalent, markedly increases the forma-tion of phthalimide [107] (Scheme 13) The first step is actually the hydrolysis of theisocyanates, and it has been claimed that ureas are present in high concentration duringthe intermediate steps of the reaction [107] Other conventional catalysts have been widelyused to accelerate this reaction Thus, tertiary amines, alkali metal alcoholates, metallactames, and even mercury organic salts have been attempted [108]

ð13ÞFor the preparation of polyimides, conventional dianhydrides have been combinedwith aliphatic and aromatic diisocyanates which are well known in the chemistry ofpolyurethanes Other more modern diisocyanates have been also studied [109,110]

Diisocyanates containing an aliphatic sequence with phenylisocyanate end groups [111],and diisocyanates containing preformed imide rings [112], have been recently synthesizedand used as monomers against aromatic dianhydrides.

Another approach to polyimides from diisocyanates is based on the reaction ofisocyanates with half esters The isocyanate group readily reacts with the carboxy group

in solution, without catalyst under mild conditions, to yield amic ester with splitting off ofcarbon dioxide (Scheme 14)

ð14Þ

In this way, polyimides have been reported from diisocyanates and half diesters viasoluble poly(amic ester)s, which were converted into the final cyclized polyimides with loss

of alcohol by the classical imidization method at high temperature [113,114]

A related reaction is the condensation of anhydrides with cyanates to imidecarbamates (Scheme 15)

ð15ÞThis reaction has been used to synthesize polyimides (more properly polyimidecarbamates) from dianhydrides and dicyanates [23] The reaction proceeds in nitrobenzene

at high temperature, catalysed by triethylamine The thermal resistance of these polymers

is much lower than that of pure aromatic polyimides, and, therefore, the reaction has notfound practical application

This reaction is accompanied by the separation of hydrogen halide, so that an acidacceptor is needed to catalize the reaction, which is carried out in polar solvents at hightemperature [115] Aromatic dihalides are not suitable reactants for this reaction, unlessactivated dihaloaromatic monomers are used [116].

(b) Aminolysis of Diimides by Diamines Ammonia and amines can readily reactwith cyclic imides to yield ortho-diamides by nucleophilic attack and subsequent ringopening On the basis of this old reaction, polyimides have been synthesized from aromaticdiimides and diamines The reaction has been classified as a migrational polymerization[117] It proceeds in solution through a lineal poly(ortho-diamide), and this intermediate isconverted to the polyimide by heating, with evolution of ammonia, in a similar fashion tothe conversion of poly(amic acid)s into polyimides (Scheme 17)

ð17Þ(c) Transimidization Another possibility is the reaction of diamines with N,N0-disubstituted diimides, by a synthetic route that can be considered as a transimidization,with evolution of monoamine from the intermediate poly(amic amide) In this exchangereaction, the nature of R plays an important role as the residue R–NH2 has to beeliminated to accomplish ring closing, so that short-length R substituents are, in principle,desirable for this approach (Scheme 18) Nonetheless, the chemistry involved in thesereactions has been studied also for the case when R is rather long and constituted byaliphatic aminoacid moieties [118,119]

The global reaction is an equilibrium that moves to the right at relatively hightemperature, and it is necessary to have diamine monomers which are more nucleophilicthan the monoamine, unless specific catalysts as transition metal salts are employed[120,121] An alternative method that uses N,N0-bis-pyridyl- or N,N0-bis-pyrimidylbisphthalimides as monomers has been developed By this transimidation approach,2-aminopyridine or 2-aminopyrimidine are readily displaced by diamines to yield highmolecular weight polyimides [122]

ð18Þ

Polypyromellitimides have been prepared by condensation in solution of NMPfrom aliphatic diamines and N,N0-dialkyloxycarbonyl pyromellitimides The reactioncan be carried out by interfacial polycondensation, as illustrated in Scheme (19)[123,124].

ð19Þ

The same reaction has been studied with aromatic and aliphatic diamines, and theconclusion has been drawn that the procedure is only valid for aliphatic diamines, becausethe low basicity of aromatic diamines does not allow for polymer formation in mildconditions

(d) From Diimides it is also Possible to Attain Polyimides by Reaction withDiisocyanates(Scheme 20)

ð20Þ

However, the molecular weights reported for polyimides prepared by this procedureare comparatively low (inh0.2–0.3 dL/g) [125,126] Moreover, the presence of functionalgrouping consisting of imide plus ureyl linkages makes these polymers thermally unstable.This is also the case of polyimides synthesized from dihydroxyimides and diacyl chlorides[127] or diisocyanates [128] (Scheme 21) The combination in a functional grouping

of imide and carbamates also makes these polyimides unstable and easily attackable

by nucleophiles.

ð21Þ

Another approach is the reaction of diimides with divinyl monomers Examples ofthis route, starting from divilylsulfone and pyromellitic, benzophenonetetracarboxylicand cyclopentanetetracarboxylic diimides have been reported (Scheme 22) The polymer-izations are carried out in solution in the presence of inhibitors of radical polymerization,and the molecular weights achieved are not very high [129] By a similar mechanism,polyimides have been prepared from diallylesters and cycloaliphatic [130] and aromaticdiimides [131]

ð22Þ

2 From Silylated Diamines

The silylation method has received particular attention during last years for thepreparation of a variety of condensation monomers and polymers [132–134] It hasbeen successfully applied to the synthesis of aromatic polyimides, and can be considered as

a recommendable method in some instances, particularly for less reactive diamines because

it has been proved that silylated aromatic diamines are more nucleophilic than freediamines [135,136] By this method, a poly(amic trimethyl silyl ester) is produced in thefirst step (Scheme 23), which can be converted into polyimide by chemical means[91,93,137]

ð23Þ

3 From Dithioanhydrides

The reaction has been described for pyromellitic dithioanhydride and aromatic diamines[138] It is a two-step reaction that involves the formation of a soluble poly(amic thioacid),which is converted into polyimide by cyclocondensation (Scheme 24) The hydrogensulphide that separates can be neutralized by an addition reaction with acrylonitrile Thislatter works as an effective acceptor against SH2 Polyimides with inhhigher than 0.3 dL/ghave not been attained by this method

ð24Þ

4 By a Diels–Alder Reaction

The Diels–Alder reaction of condensation between a diene and a dienophile has beenalso applied to the preparation of polyimides [139] Starting from bismaleimides andbiscyclopentadienones, a soluble poly(hydrophthalimide) of high molecular weight can beattained by solution polycondensation in refluxing chloroaromatic solvents (Scheme 25)[140,141] They are readily converted into polyimides by dehydrogenation (boiling nitricacid) However, thermal dehydrogenation results in a decreasing of the molecular weightsand, furthermore, full aromatisation of poly(hydrogenated phthalimide)s is difficult toaccomplish [142,143]

ð25Þ

Due to the intractability of classical, fully aromatic polyimides, other alternatives weresoon envisioned in order to take profit from the intrinsic high thermal stability of thesepolymers Copolymerization is an obvious procedure, and thus copolyimides have been

developed and have been marketed parallel to aromatic polyimides Chronologically,poly(ester imide)s and poly(amide imide)s were the first and most important copolyimides,but poly(ether imide)s have got great importance since the first processable poly(etherimide)s appeared in the eighties [74,81,120] These three families are the most importantcopolyimides from a practical viewpoint, but other lineal copolyimides have also beendescribed and evaluated, such as poly(anhydride imide)s [144–146].

Many methods of synthesis have been outlined for the preparation of copolyimides,and the most important will be considered here

A Poly(ester imide)s

1 From Monomers Containing Ester Groups

They are generally dianhydrides containing ester groups The polymerization scheme forthese monomers to yield polyimides by reaction with diamines is depicted in Scheme (26)

ð26ÞThe reaction conditions are similar to those described for polyimides from diaminesand aromatic dianhydrides Here also the aliphatic species (R ¼ alkyl) can be made toreact in the melt The aromatic ones have to react in solution of appropriate organicsolvents to yield poly(amic acid ester) in a first step, and poly(ester imide) by cyclo-dehydration 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 bycondensation of diacetylbisphenols (R ¼ arylene) with two moles of trimellitic anhydride(4-carboxyphthalic anhydride), or by condensation of glycols (R ¼ alkylene) with twomoles of alkyltrimellitate by ester interchange in the melt [4,147,148]

The bistrimellitic anhydride esters may also react with diisocyanates to yield poly(esterimide)s in the same way as aromatic dianhydrides and diisocyanates described above.Bisimides containing ester groups have also been used as monomers againstdiamines, to prepare lineal poly(ester imide)s by an aminolysis reaction [149]

Dianhydrides can react with aminoacids or aminoalcohols to yield diacids or dialcoholscontaining preformed imide rings of the following general formulae (Scheme 27):

ð27Þ

Likewise, monomers from trimellitic acid anhydride are of the form (Scheme 28):

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 linking at the moment of application Poly(ester imide) resins are mainly used as electricalinsulating materials and they have been for many years the class of copolyimideswhich have practically deserved most attention, because of their good price-performancebalance, good processability and good properties as high temperature insulating varnishes[160] In fact, the electrical insulators industry undertook a qualitative improvement inthe sixties when poly(ester imide) varnishes irrupted in the market, and they have beenused and improved from then up to now

cross-The chemistry involved in curable poly(ester imide) formulations is well established,and it is still based on trimellitic anhydride, methylene dianiline, and low molecular weightpolyesters of aromatic dicarboxylic acids and glycols, along with a classical heterocyclictriol, 2,4,6-trishydroxyethyl isocyanurate (THEIC), as it is shown in Scheme (29) Thecomposition is formulated in a way that the final polymers, although linear, containfree  OH groups, both as chain ends and as side reactive groups At the moment of

application the resin is cured through the many free hydroxy groups by the action ofdiisocyanates, tetraalkoxytitanates, phenolic resins, or a combination of them, uponheating to eliminate solvents at the same time [160].

ð29Þ

The combination of imide rings and ester linkages in a cured network can bealso accomplished by curing of epoxy-imides with reagents (polyamines or polyacids)containing imide rings Epoxy-imides of very varied composition have been prepared andreported, many of them containing also ester groups [145] Some generic structures areshown in Scheme (30) The thermal properties of these materials are obviously controlled

by the less stable moieties derived from the oxirane ring

ð30Þ

The combination of properties owing to poly(ester imide)s and epoxy resins in anunique material, has been also attempted by using soluble poly(amic acid)s as effectivecuring agents of conventional bisglycidyl ethers [161] The result of these combinations arereal interpenetrated polymer networks

Another approach to poly(imide epoxy) homogeneous materials, is the preparation

of diepoxides from imide-containing reactants (Scheme 31) For instance, the synthesis ofbis(glycidylester) imides from imide-diacids or bis-imides and epichlorhydrine has beenrepeatedly reported [162,163] The combination of them with curing agents (diamines and

dianhydrides) leads to cross-linked polymers of rather complex structure and specialproperties [164–166].

ð31Þ

B Poly(anhydride imide)s

These copolyimides were first described in the 1970s [144,167] as thermally resistantpolymers that can be prepared in high molecular weight from imide-containingmonomers, in the form depicted in Scheme (32) They can be synthesized by meltpolycondensation, in solution at high temperature, or even by solid state polymeriza-tion if the monomers show very high melting temperature The monomers aremixed dianhydrides, which can be readily prepared by reaction of imide-containingdicarboxylic acids with acetyl chloride or acetic anhydride During the polycondensa-tion reaction, acetic anhydride splits off under special conditions of temperature (above

250
C) and low pressure (some mm Hg) thanks to an anhydride interchange reaction.This reaction is the classical pathway for lineal polyanhydrides, described as early as

1932 by Hill and Carothers [168] Half dianhydrides have the advantage thatstoichiometric imbalance is not possible as the reaction consists of the self-polycondensation of a single monomer

C Poly(amide imide)s

Like poly(ester imide)s, poly(amide imide)s were reported in the earliest literature oncondensation polyimides [4,5,173] They constitute a polymer class with average propertiesbetween aromatic polyimides and aromatic polyamides

Aromatic poly(amide imide)s are easier to prepare and to process than botharomatic polyimides and aromatic polyamides, because the copolymers are usuallysoluble in organic solvents Thanks to that, they have found many practicalapplications, and the research effort devoted to poly(amide imide)s has been, and isstill, considerable

The general methods to synthesize poly(amide imide)s are rather simple and includethose monomers and reactions conducting to polyimides or polyamides, consequently,the number and variety of chemical structures reported are countless [174] The mostimportant ones will be mentioned here