Iranian Polymer Journal / Volume I I Number 4 (2002)

ta2612012002

Polyimides Derived from Diisocyanates

Mehdi Barikani 'Department of Polyurethane & Special Substances, Iran Polymer and Petrochemical Institute,

P .O . Box : 14965-115, Tehran, I .R. Iran

Received 15 August 2001 ; accepted 31 October 2001

ABSTRACT

Polyimides are a sophisticated family of materials, which have applications in

highly technical end uses from aerospace to microelectronics . Their diversityis such that it leads them into applications as fibres, films, moulding powders,

coatings and composite prepregs where their major advantage, a highresistance to heat, places them in a niche other polymers cannot enter . The

objective of this review is to tie together some of the early with the more

recent literature data concerning the synthetic aspects of diisocyanatederived polyimides, their solution behaviour, and imidization characteristics.

Its aim is to provide an up-to-date picture of the important factors involved inpolyimide preparation and leave more detailed information to be gathered by

the readers from the many key references cited if they desire. This will

hopefully provide the readers with a more complete as well as correct pictureof the rudiments of diisocyanate derived polyimides in addition to hopefully

simulating research interests in those areas which are less completely

understood . This large and complex family Of polymers will continue to bedeveloped, even though certain products have been sold for more than 35

years. Future developments will include blends and alloys to optimizeprocessing, physical, thermal or other attributes . This `high tech" family will

need many of the high and low temperature requirements of the automotive,aerospace, electrical/electronics and medical industries as they develop

further in the 21 st century.

INTRODUCTION

been published on polyimides during 1975-2001 and at

least 18,000 patents in the last five years. This represents

Polyimides are one of the Most important classes of high

an average increase of 35% per annum. Many major

'temperature polymers available today . They find wide-

multinational chemical and manufacturing companies are

spread application due to the wide range of chemistry and

actively involved in polyimides either as suppliers or end-

properties accessible . Their commercial importance is

users or both . These include Du Pont, IBM, Shell, BP, ICI,

illustrated by the fact that more than 37,000 patents have

General Electric, Ciba-Geigy, Hitachi, Rolls-Royce,

(0) To whom correspondence should be addressed . Email : M . Barikani(Oipi.ac.ir

215

Potyi,mdes Derived from Diisceyanaus

BASF, Upjohn, Dow Chemical, Bayer etc . Joint venturesare becoming increasingly important, not only to spreadcosts but also to make use of existing distribution andmarketing networks.

Major application areas for polyamides includeaerospace, automotive, microelectronic/electrical andgeneral engineering . In aerospace, polyimides are used instructural composites and as high temperature adhesives.They also find use in load bearing structures in theautomotive industry . General engineering applicationsinclude high temperature bearings and seals . Polyimidesfind widespread use in microelectronics and electricalapplications, including dielectrics for integrated circuits(ICs) and electrical insulations . Although the initial goal ofthese materials was high thermo-oxidative stability alongwith excellent mechanical properties, more recentapplications have focused on processability, planarization[1], low thermal expansion coefficient [2,3], improvedadhesion [4] and lower dielectric constants [5,6] . Ofcourse, such properties are desired without sacrificingthermal and mechanical properties.

Researchers have developed high temperature gasseparation membranes based on polyimides [7,8] . Controlof the polymer morphology by copolymerization canprovide improved selectivity and permeability rates of gasseparation membranes [9) . Some researchers areattempting to synthesize recoverable polyimides [10].Success in this area would perhaps allow reprocessing andwould lessen the detrimental environmental impact of suchintractable materials. Information addressing structure-property relationships of a wide spectrum of polyimidescan be found in the literature [11-171.

Synthesis of PolyimidesBy far the most common route to polyimides is the reactionof a dianhydride with a diamine either in solution or in themelt, but using diisocyanates instead of diamines wouldresult in the preparation of the same polymers with similarproperties and easier process.

Polymerization of Diisocyanates and Di-anhydridesIn the year 1854 Wurtz [18] discovered that aceticanhydride and ethyl isocyanate react to form ethylamine-diacetimide (Scheme I) .

This method for synthesis of imides remainuninvestigated for over 100 years until in 1959 Hurd andPrapas [19] obtained N-phenylphthalimide from phthalicanhydride and phenyl isocyanate (Scheme II).

In 1968 Farrissey et al . [20] examined the samereaction as a model for the preparation of polyimides . Thereaction did not take place in purified dry pyridine . Hightemperatures were required in non-polar solvents such asdiglyme and dioctyl phthalate . A further importantobservation was that the reaction proceeded in dipolaraprotic solvents at moderate temperatures and that proticbases such as alcohols and water, as well as tertiary amineswere effective catalysts in these solvents . Alcohols reactwith isocyanates to form urethanes, which slowly reactwith anhydrides to form imides and carbon dioxide, at thesame time regeneration of the alcohol (Scheme III).

Muller and Merten [21] obtained oligomeric amide-imides with isocyanate end groups by treating excessamounts_ of diisocyanates with trimellitic acid anhydride.The isocyanate end groups could be masked by reactionwith cresol. The resulting urethane end groups couldregenerate isocyanate groups on further heating at highertemperatures (Scheme IV).

Meyers [22] obtained low molecular weightpolyimides from pyramellitic dianhydride (PMDA) anddiphenylmethane diisoayanate (MDI) in DMF, whileFarrissey et al . [23,24] obtained higher molecular weightpolyimides in DMAc and DMSO. Dipolar solvents usuallycontain trace amounts of protic bases such as water andamines, and the latter are thought to catalyze the reaction.Isocyanates rapidly react with water to form urea, whichreact more slowly with anhydrides to form imides andcarbon dioxide, with regeneration of water (Scheme V).

Alvino and Edelman [25,26] investigated polymer-ization of PMDA and MDI in dipolar solvents in thepresence of a tertiary amine as catalyst . High molecularweight polymers were obtained only when a part of thedianhydride was hydrolyzed by the addition of a controlledamount of water before the addition of MDI.

Onder [27] studied the preparation of polyimidefrom organic diisocyanate with alkali metal salt of alcoholor phenol as catalyst . The polymers are obtained in solu-tion at low reaction temperatures avoiding side-reactionswhich otherwise short reaction times thereby would bedetrimental to polymer molecular weight and ultimate

216

Iranian Polymer Jou rnal / Volume 11 Number 4 (2002)

8arikaui M-

Ironidn "'Mim:erJoterbid % Volume ! 1 Number 4 /2002j

2J7

Polyimides Derived from Diisocyanates

polymer properties . High molecular weight polyimides[27] and polyamideimides [31] were obtained in thepresence of less than 0 .5% of the catalysts . He alsodemonstrated that diisocyanate tetracarboxylic acid/dian-hydride reaction in dipolar aprotic solvents were veryeffectively catalyzed in the presence of alkali metallactamate . This certain catalyst provides an improvedprocess for the preparation of soluble polyimides poly-amides and polyamide-imides [32,33].

The method for making cross-linked resin foamsfrom an ethylenic unsaturated dicarboxylic acid anhydrideand a polyisocyanate have been studied by Corbett et al.

[28] . Interestingly, when diisocyanates were treated withtwo moles of trimellitic acid anhydride, bis(imideacid)swere obtained instead of bis(amide 140 anhydride)s [28](Scheme VI).

The reaction proceeded, albeit slowly, even in non-polar solvents such as Ortho-dichlorobenzene . Becausephthalic anhydride does not readily react with isocyanatesin the absence of dipolar solvents, this seemed to suggestthat the reaction was catalyzed by the presence ofcarboxylic acids . Indeed, Merten and Muller [29] showedthat the reaction was also catalyzed by other acids such aszinc chloride and p-toluenesulphonic acid . Althoughvarious side reactions can be avoided through the absenceof dipolar solvents, the acid catalysts are generally not veryeffective. Takekoshi and Manello [30] synthesized highmolecular weight polyetherimides from diisocyanates andbis(ether anhydride)s by either melt polymerization orhigh-temperature solution polymerization in non-polarsolvents (Scheme VII).

Under these conditions, the complex side reactionsdescribed above where apparently absent, However, therate of polycondensation was slow without the use of acatalyst even at temparature above 200°C . Basic catalystssuch as tertiary amines were not efficient, and are unstableat high temperatures. However, the reaction proceededrapidly in the presence of a small amount of alkali metalcarbonate at temperature above 200°C . Polyetherimides aregenerally melt fusible and soluble in some non-polarsolvents and at high molecular weight polyetherimideswere obtained in 1 h in the presence of less than 0 .5% by

weight of the catalyst.Under basic conditions, such as the presence of the

bases or of dipolar aprotic solvents, isocyanates are known

to undergo dimerization and cyclotrimerization of cyanur-ates form, and polymerization to form nylon-1 [34].Isocyanates also react with dipolar aprotic solvents to formvarious by-products [35] . D'Alelio studied chain extendingpolyimides with aromatic polyisocyanates [36] . Farrisseyet al . [37] prepared a polyimide/CO 2 foam from BTDAand MDI in the presence of polyols and DMSO (SchemeVIII) .

Various polyimides of cellular structures are alsodescribed in other patents and literature [37,38,28].Method for making cross-linked resin foams from adicarboxylic acid, ethylenically unsaturated dicarboxylicacid anhydride and a polyiaocyanate by subsequently add-ing catalyst, surfactant and water was shown by Baker [39]and catalysts for polyimide foam from aromaticisocyanates and aromatic dianhydrides which have greatlyimproved bum-through and flame-spread resistance areprepared by Riccitiello et al. [40] . Flame and temperatureresistant polyimide foams prepared from pyromelliticdianhydride with an aromatic polyisocyanate were alsoreported by Frosch et al . [41] . Ghatge and Mulink [42]synthesized soluble polyimides of moderate molecularweights from BTDA and various 4,4'-diisocyanatophenyl-alkanes in DMAc. Similar results were also described byKhune [43] (Scheme IX).

Pauze and Zamek [44,45] reported hot-meltelectrical coating compositions comprise high solidscontent polyesterimides prepared to an acid number of lessthan 6-7 in the presence of a monoether or monoester ofglycol or polyglycol and reacted hot with a blockedpolyisocyanate . Synthesis and characterization of somepolyimides drived from pyromellitic dianhydride and thein-situ generated aliphatic diisocyanates are reported byDurairaj and Venkataroa [46] . Lee investigated the pro-cess for preparing polyimides end-capped with anhydrideor isocyanate groups [47] by using aromatic dianhydridesand aromatic diisocyanates in the presence of a metalacetylacetonate. Polyamideimides produced from eitherlactams or polyamide with isocyanates and anhydrides asthermoplastics are reported by Zecher et al . [48] . Linearpolyurethaneimide elastomers were obtained from poly-ether or polyester diols, diphenylmethane diisocyanate, bi-tolylene diisocyanate and pyromellitic acid dianhydride[49,50] . McGrath et al . [51] reported the polymerization ofBTDA and PMDA with MDI and 4,4'-dicyclohexyl-

218

Iranian Polymer Journal / Volume / / Number 4 (2002)

Ban'kani M.

Scheme IX

0

-0

0 - OC

~ .-NCO--

0

ON

(i)

Scheme X

Iranian Polymer Journal / Volume ! 1 Vwnher 4 (_'UO2j

219

Polyimides Derived from Diisocyanates

methane diisocyanate in dipolar solvents . High molecularweight polymers could not be obtained under theseconditions . Overall, the above polymerization does notseem to be quite quantitative enough to prepare linear highmolecular weight polyimide without carefull control of thereaction conditions and catalyst systems . However, thereaction is useful for applications more tolerant to lessstrict stoichiometry, such as cross-linking systems.

Zecher et al . [52] reported a process for theproduction of polyamideimides from polyisocyanates,polycarboxylic acid anhydrides and lactams of polyamidein which the reaction products are precipitated with a non-solvent. They also reported a patent on the use of poly-amideimide as thermoplastics [53] . Miscible blends ofpolybenzimidazole and diisocyanate-based polyimide werestudied by MacKnight et al . [54] . The results suggest thatthis system displays lower critical solution type phasebehaviour reaction of an aromatic dianhydride with apolycyclic aromatic primery diamine at a controlledreaction rate yields a diamic acid dianhydride oligomer.

The oligomer may be a precursor for an imide foamwhich forms at low temperature, has outstanding physicalcharacteristics, and it is extremley heat resistant [55] . Imaiet al . [56] prepared a polyimide by high temperaturesolution polymerization from PMDA and a diisocyanatewith bulky substituents, 2,5-bis(4-isocyanatophenyl)-3,4-diphenyl-thiophene(Scheme X) .

High molecular weight products were obtained inrelatively non-polar solvents such as benzonitrile, anisoleand nitrobenzene, while only a moderate molecular weightwas achieved in NMP.

Cross-linkable polymers such as polyimide, poly-amide and polyurea bearing maleimide, nadimide oritaconimide pendant group were prepared and character-ized by Nelissaris et at . [57] . The polymers were preparedby reacting 2,4,6-tris(4-aminophenoxy)-S-triazine with anequimolar amount of maleic/nadic/itaconic anhydridefollowed by reaction of the resulting monoamic acid withpyromellitic dianhydride (PMDA) and tolylene diiso-cyanate (TDI). Synthesis and characterization of newcomb-like aromatic polyamide, polyimide and polyureascontaining 1,3,5-triazine ring with two long alkyl groups intheir side chains are reported by Lin et al . [58] . Thesecomb-like polymers were prepared by the reaction of m-phenylene diamine derivative (2,4-bis(N-octadecyl-

anilino)-6-(3,5-diamino-phenyl)-1,3,5-triazine(DA18) witharomatic diacid chlorides, pyromellitic dianhydride anddiisocyanates. They also reported synthesis andcharacterization of new comb-like aromatic polymersbased on 2,4-bis(amino-N-octadecylanilino)-6-substituted-1,3,5-triazines [59] and synthesis and characterization ofnew comb-like polyguanamines containing three long alkylside chain in repeating unit [60] . Imai and Kakimoto [61]investigated soluble high-temperature polymers containinga tetraphenylthiophene unit . In this investigation aromaticpolyimides and copolyimides were synthesized by thereactions of

tetraphenylthiophene diamine withtetracarboxylic dianhydrides, tetraphenyl-thiophenediisocyanate with tetracarboxylic dianhydrides andtetraphenylthiophene diamine with tetracarboxylicdithioanhydrides.

IR and 13 C-CP-Mass-NMR spectra of pyromell-itimide and its precursor based on 4,4-diaminodiphenyl-methane was studied by Schneider et al. [62].

A number of thermostable poly(imide-urea) co-polymers have been synthesized by the reaction of diiso-cyanate and diamine-capped oligomers [63] (Scheme XI).

The polyimide oligomers were prepared by reacting2,2-bis[4-(p-aminophenoxyl) phenyl] propane (PDAP)with pyromellitic dianhydride, benzophenone tetracarbox-ylic acid dianhydride (BTDA), 4,4-oxydiphthalic anhyd-ride (ODA), hydroquinone diphthalic anhydride, (HQDA)or 4,4'-(4,4-biphenylenedioxy) diphthalic anhydride(BPDA).

Polyurethane-imide membrane and their use for theseparation of aromatics from non-aromatics was studied byKoenitzer [64] . The membrane is a polyurethaneimidemade using polyurethane-imide copolymers which aremade by end capping a polyol with a polyisocyanate andthen chain extending by reaction with an anhydride.

Loy et al. [65] studied a process for producing flameretardant high temperature resistant polyimide foamshaving maximum weight loss between 550-650°C by usingdianhydrides and diisocyanates.

Synthesis and characterization of formal groupcontaining diisocyanate and polyimides reported byAvadhani et al . [66] by using bis(3-isocyanatophenoxy)methane, bis(4-isocyanatophenoxy) methane, bis[2-(3-isocyanatophenoxy) ethyl] formal and bis[2-(4-isocyanato-phenoxy) ethyl] formal through Curtius rearrangement

220

Bankani M.

2 0 2N +CH3

Na2CO3

CH3

0 2 N

CH3Fe

HzN

O

CH 3

CH3

p

CH3

NH 2

HzNNH2

/C\ ,/C\2 H2 N-R-NH2+0 P. O

- H 2 N-R-NH-~\l~-Nll P. NHZ + OC ® HZ

CO

\\C~ v0 17

0 QNH-R-NH-C\ /C-N q R NH- 1V

HO-CJ\C-OHO 6

NH-R-N.1\

\ N R NH~-N

HO- " -0H

R.

Poly(imide-urea)

CH3

CH3

Scheme Xi

from the corresponding dicarboxylic acids. The ,diisocyan-ates were polycondensed with pyromellitic dianhydrideand benzophenone tetracarboxylic dianhydride in DMActo obtain formal-group containing polyimides.

Polyimides containing S-triazine rings in the mainchain were reported by Sarwade et al . [67] . In their efforteight S-triazine-containing polyimides were synthesized bysolution polycondensation of diisocyanates having an S-triazine ring with pyromellitic dianhydride in dimethyl-acetamide. Polyimides derived from meta-orientated diiso-

Iranian Polymer Journal / Volume 1 I Number 4 (2002)

cyanates were amorphous and soluble in dipolar aproticsolvents while those based on para-orientated diiso-cyanates were senucrystalline and insoluble in organicsolvents . However, all the polyimides were soluble inconcentrated sulphuric acid. Recently condensation poly-imides, synthesis, solution behaviour and imidizationcharacteristics were reviewed by Volksen [68j.

Wang and Chen [69] reported chemical modifica-tion of epoxy resin by reaction with anhydride-terminatedpolyurethane-imide . This type of polymer synthesized

221

Polymddes Derived from Diisocyanates

(Step 1)

2 OCN-(MDI)-NCO + HO-(macroglycol)-OH OCN-(MDI)- NHr -(macroglycol)- Or -(MDI)- NCOPU Oligomer

Anhydride-terminated PUprepolymer (An . TPU)

Scheme XII

(Step 2)

OCOCN-(PU Oligomer)-NCO + 2 OOC

CO\

,OCN -(PU Oligomer)-N

O + CO2CO'

\oc

Cd

+ nHOOC-PMDA-COOHNCO

- 2CO2 (90-110°C)

IO

III

IO

III

j\ N+A-NHCO-PMDA-COHNy

O O

CHn OCN

SO2

CH2 (

Scheme XIII

from polyether (PTMG) or polyester (PBA) diol, diiso-cyanate (MDI), and dianhydride (BTDA) (Scheme XII).

Thermostable Nomex copoly(amide-imide) withinherent viscosity of 0 .72-1 .31 dL/g was synthesized byreacting diacid-terminated Nomex prepolymer with var-ious diisocyanates terminated polyimide prepolymers [70](Scheme XIII).

The polyimide prepolymer was prepared by using4,4'-diphenylmethane diisocyanate to react with 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, pyromelli-tic dianhydride or 3,3',4,4'-sulphonyldiphthalic anhydrideusing a direct one-pot method in order to improve theirsolubility.

The solubility is considered to be related to theircrystallinity. Those copolymers with crystalline structuredisplayed poor solubility. All the Nomex copoly(amide-

imide)s had glass transition temperature in the range of223-352°C and showed 10% weight loss at temperature of438-574°C in air and 441-585°C in nitrogen atmosphere.

Thermostable naphthalene-containing copoly-(amide-imide)s was synthesized by reacting diacid-terminated naphthalene monomer with various diisocyan-ate-terminated polyimide prepolymers [71] (SchemeXlV).

The polyimide prepolymers were prepared byreacting 4,4'-diphenylmethane diisocyanate with one ofthe three mono-or dianhydrides using a d irect one-potmethod.

From imidazole-blocked 2,5-bis[(N-alkyloxy)-methyl]-1,4-benzene diisocyanates and pyromellitic dian-hydride a series of rigid-rod polyintides having linear andflexible (alkoxy) methyl side chains were prepared andcharacterized and their properties were measured and

222

Iranian Polymer Journal / Volume 11 Number 4 (2002)

Bf lu,4 M.

-CH2x-I OCN /\ /\ -2 CO2NCO -.x0 R~

O\ / \ /

n OCN+ n HOOC-R2 - COOH

NCO 2CO2

O QC

\C\/C\

HQ

QM_NRI N MN-C R 2 -C N

\Ci \G/

O O

R2:

Scheme XIV

and discussed with regard to the effect of side chains [72](Scheme XV).

Incorporation of the side chains onto the rigid mainchain greatly enhanced the solubility and fusibility of thepolymers, and the melting point was determined to be277°C .

Yang et al. studied the flexibility improvement of apolyimide resin by using urethane [73] . In order to obtainpolyimides with improved flexibility, imide-urethanecopolymers were prepared by the reaction of MDI withpyromellitic dianhydride and diols such as polypropyleneglycol and butanediol in DMF solvent.

Polypyromellitimide polymers were prepared in atwo-step process by polymerizing pyromellitic dianhyd-ride with different molar ratios of diisocyanates, MDI andTDI in DMF . Intentionally adding water to the reactionmedium to study the effect of adventitious water on the .formation of polymer, it was found that the presence of

water has an important influence upon the molecularweight of the polymer formed [74] (Scheme XVI).

Hay et al . [75] reported a method of preparing apolyimide by reacting together a dianhydride and diiso-cyanate for preparation of colourless and low colourpolyimides. The main approaches to reduce colourationhave been to introduce groups which either disruptextended conjugation along the polymer backbone orwhich eliminate the electronic interactions such as chargetransfer complexation which lead to colour.

Michaud et al. [76] reported a process for obtainingpolyamide-imides (PM) fibres by spining PAI in solutionand to the fibre thus obtained . The fibre obtained by dry orwet spinning into dimethylalkyleneurea followed byremoval .of the .solvent and overdrawing at hightemperature . The yarns and fibres obtained are producedfrom PAI based on tolylene or meta-phenylenediisocyanate, and on an aromatic acid anhydride and/or

223Iranian Polymer Journal / Volume Il Number 4 (2002)

I

.d Iem Diiocyans ee

4a-c

5a-c

(C n H2n+1

OWN ~f \} N O

2a-c

FQ11

1

(DIS-IBDI)

H 2m1 Cn

5a-c

Compounds2a, 4a, 5a, 6a2b; 4b, 5b, 6b2c,4c,5c,6c

-CH2OC0H2,, f1

R=ten~n=8

Cat.4-DMAP/DMPU120°C

l (PMDA) H2a+

2a-c (DIS-IBDI)

Polymer codesC4-PYPIC6-PY-PIC8-PY-PI

-CH2OCnI 2n+1

n-6n=8

H2„+1 Cn

3a-c (C.-Py-PI)

Scheme XVI

224

Iranian Polymer Journal/ Volume 11 Number 4 (2002)

Bantmi M.

an aromatic dianhydride and optionally on one or a number

of diacid compounds.

Polyimides containing polybutadiene blocks wereprepared by copolycondensation of benzophenone-3,3',

4,4'-tetracarboxylic dianhydride, diphenylmethane-4,4'-diisocyanate and isocyanate-end-capped polybutadiene

LBD-3000 [77].

High molecular weight polymers with imide or

parabanic structures were synthesized from 2,2,4,4'-di-

benzyltetramine,4,4'-oxydianiline and pyromelliticid-

anhydride of 2,4'-or 4,4'-dibenzyldiisocyanate [78] . Poly-

amideimides are obtained by reacting 1,5-naphthalene

diisocyanate, aromatic tribasic acid anhydrides and

3,3',4,4'-diphenyl sulphone tetracarboxylic dianhydride in

basic polar solvents [79].

Barikani and Mehdipour [80] investigated the

preparation and properties of polyimides and polyamide-imides from diisocyanates . Different structures of poly-imides were synthesized by reaction of 1,4-phenylene

diisocyanate (PPDI) and 1,5-naphthalene diisocyanate

(NDI) with pyromellitic dianhydride and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (Scheme XVII).

Polyandde-imides were also prepared by reaction ofPPDI and NDI with trimellitic anhydride (Scheme XVIII).

Synthesis and characterization of new soluble and

thermostable polyimides via novel diisocyanates were alsostudied by Barikani et al. [81] . In their study a facile

synthesis of two novel phthalimide diisocyanates contain-

Scheme XVII

PMDA

TDI or MD1 Polyamic acid

0

0 + OCN-Ar-NCO

0

4- 2nCO2

8-

Scheme XVIII

nr -

Iranian PolymerJournal /Volume 1 I Number 412002)

225

Polyuddes Dale' ed from Dtoocyanales

ing 6 or 10 methylene groups is described . Six different

polyimides were synthesized by polycondensation of these

two diisocyanates with pyrotnellitic dianhydride (PMDA),

were obtained from blend solutions of the polyurethaneprepolymer and the polyimide, and then they werethermally treated at various temperatures to give a series of

+ OCN-Ar-NCO

r + 2nCO2

HO 2

n

Ar=

Scheme XIX

3,3',4,4'-benzophenonetetracarboxylic dianhydride(BTDA)and hexafluoroisopropylidene-2,2-bis-(phthalic anhydride)(6FDA) in DMAc (Scheme XIX).

Wang et al . [82] reported synthesis of aromaticpoly(urea-imide) with heterocyclic side chain structure byreacting aryl ether diamine or its polyurea prepolymer withvarious diisocyanates terminated polyimide prepolymers.The aryl ether diamine was obtained by first nucleophilicsubstitution of phenolphthalein with p-chloronitrobenzenein the presence of anhydrous potassium carbonate to forma dinitroaryl ether, and then further hydrogenated todiamine . The polyimide prepolymers were prepared byusing 4,4'-diphenylmethane diisocyanate to react withPMDA and BTDA by using direct one-pot method.Thermostable poly(arnide-imide)s containing para and

meta-benzoic structure were also synthesized by reactingpara and meta-benzoic polyimide prepolymer with variousdiisocyanate terminated polyimide prepolymers [83].

Zuo and Takeichi [84] investigated the preparationand characterization of poly(urethane-imide) films fromreactive polyimides and polyurethane prepolymer.Polyurethane prepolymer was prepared by a reaction of apolyester polyol and 2,4-toluene diisocyanate and thenend-capped with phenol . Soluble polyimide was preparedby the two step synthesis from 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) and 3,3'-diamino-4,4'-dihydroxybiphenyl (AHBP) . The cast films

transparent polyurethane-imide) films . By changing the

ratio of polyurethane and polyimide components poly(urethane-imide) films having various properties fromplastic to elastomer were prepared.

The preparation and properties of porous polyimidefilms were also investigated by Takeichi et al . [85] throughpyrolysis of poly(urethane-imide) films in two steps. Thefirst step was the preparation of poly(urethane-imide) filmsby a reaction between a phenol terminated polyurethaneprepolymer and poly(amide acid) . The second step was thepyrolysis of the poly(urethane-imide) films. Upon thermaltreatment, the thermally less stable urethane domainsdecomposed.

Yeganeh and Barikani [86] prepared two diiso-cyanate monomers containing methylene groups and built-in imide structure from the parent diacide via the Curtius-Weinstock rearrangement. Polyimides were synthesized bysolution polymerization of these diisocyanates with pyro-mellitic dianhydride (PMDA), 3,3,4,4'-benzophenone tetra-carboxylic dianhydride (BTDA) and hexafluoroisopropyl-idene-2,2-bis(phthalic-anhydride), 6FDA (Scheme XX).Introducing methylene linkage into the polymer backboneimproves polymer solubility . New (polyurethane-imide)swere also synthesized by polyaddition reaction of twodifferent diisocyanates with three different imidecontaining diols (87] (Scheme XXI) . Simultaneouspresence of flexible methylene or oxyethylene groups and

226

Iranian Polymer Journal / Volume 11 Number 4 (2002)

o

\ CI

o,w/m~ 'n,H

0

PL.`w(cx,~on,v

~cm02sne,

-~\ n rox`

' Bch ./ enc.

"^"`) 10h

N -(011 2 ) n NCODMA c

HA,!

` _

o

oOCN`,oc

6

a,b a : o= 5b,=0

o

o

o

0n~`__ ~^ ~^^

r

-o-N '/'

0

o

9

*n/*u,w

0 - nxzfo,- omx-'xooc mr=`\

N CH-:1 COOH ~~~~n,~.~/ "mm,

.

o

o

«

»

1 u .h

«

»

o

0

Nx~x1o~~v

s4n~~co~ n""~^

~x

- n/ rco7

` r~,`~»cx

"4,

0

o

u

o

za: 11~5

3a,hb : n=10

o

0

0

v

0

/u)^

o

u

N CH, .')n

''"

"

6o

o

DMA "Ar

0

o

o

A,

o

o

o

o

o

o

~`(o/z~~

N

^.

0

t.)

0 »

Scheme XX

227

tblymides Derived Soul IRisocyanarcs

1 + excess HOROHHOROOC COOROH

NH2 +2HOO HOO

HZ N

Compound' R-CH2CHz--CH2CH 2OCH2CHr-CH 2CH2 CH2-

(II-IV)„ +n OCN-R NCO-

-E OROOC COORO-CO-NH-a-NH-CO`)„

Compound I R

R"CH3

V (CH2)2

VI (CH2CH2)20

VII (CH2)3

VIII (CH 2 ) 2(CH2)6

IX (CH 2CH2)2O

X (CH2)3

Scheme XXI

also imide structure in PUI, backbone caused improved

solubility in polar aprotic solvents and improved thermalstability up to 360°C, respectively.

Barikani and Mehdipour [88] studied the synthesisand properties of aromatic/cycloaliphatic polyimides andpolyamide-imide from trans-l,4-cyclohexane diisocyanateby condensation reactions of trans-l,4-cyclohexanediisocyanate (CHDI) with pyromellitic dianhydride(PMDA), benzophenone tetracarboxylic dianhydride

(BTDA), and hexafluoroisopropylidene diphthalicanhydride 6FDA, (Scheme XXII) . Also polycondensationof CHDI with trimellitic anhydride led to synthesis ofpolyamide-imide.

A diisocyanate containing flexible oxyethylenemoieties preformed imide rings was carried out and syn-thesized with a Curtius rearrangement of correspondingdiacylazides . The diisocyanate was polycondensed withPMDI, BTDA, and 6FDA to provide polyimides [89].

228

Iranian Polymer Journal / Volume 11 Number 4 (2002)

HO2C -~

f + OCNNCO

N-CO COI

H

aenmmni M.

+ 2n CO2

Ar=

N-Co-O-CH

+ OCN.

NCOHOZC-~

2

,,.^.,^,N-C

H

+ 2n CO2

Scheme XXII

Subramani at al . [90] studied the preparation ofpolyimides by using imidazole-blocked isocyanates withpyromellitic dianhydride in the presence of a basic catalyst.Asai et at . [91] reported preparation of imide containingelastic polymers from elastic polyurea and pyromelliticdianhydride in N-methyl-2-pyrrolidone (NMP) . Synthesisand characterization of poly(urethane-imide)s containing adisperse red type chromophore in the side-chain werereported by Zhu et al . [92] . Four poly(urethaneimide)scontaining a diazo non-linear optical chromophore in theside chain were synthesized by a two step process, whichinclude preparation of diazo chromophore containingdiisocyanates followed by the condensation reaction withdianhydrides.

Yeganeh and Barikani [93] reported the preparationand properties of poly(urethane-imide)s derived from

diisocyanate containing build-in imide rings by polycon-densation reaction of four imides containing diisocyanateswith poly(tetramethylene oxide) glycol (Scheme XXIII).The polymer showed excellent solubility in polar aproticsolvents.

Synthesis and thermal properties of poly(urethane-imide) were also studied by Bibiao et al . [94] . Theirprepared polymers exhibit improved solubility in organicsolvents and also formed flexible and transparent films . Toovercome the shortcoming of the polyamide resin, severalmethods have been devised, (a) method of introducingpolar groups into the backbone or side chains of thepolymer, (b) method of introducing the connecting groupor bulky pendant groups in the polymer and (c) method ofenhancing the flexible of the backbone of the polymer[95] .

Iranian Polymer Journal ,t Volume i I Number 4 (2002) 229

Z-COZHn

1a,b

2 a,b(1)'IMF, Et3NC1CO2EtNaN3(2)reflux

a: n=5

3 a,b

4 a,bb : n=10

Preparation of 4 isocyanates

2)-N-C-0}#CH2)0Innrll~`

NH-C--O-[( CH2)4OLn

a, n=5b, n=10

HO-CH20}-H t OCN(CH2 ; -(CH4-NCO

6 a,b

Scheme XXM

230

Iranian Polymer Journal / Volume 11 Number 4 (2002)

aankmi M.

More recently, Barikani et al . reported synthesisand properties of novel optically active polyimides . In theirstudy a facile synthesis of an optically active diisocyanatecontaining alkylene groups and a preformed imidestructure is described . Polycondensation reactions of the

easiest and most commonly used technique for obtainingand indication of the molecular weight of polyimideprecursors.

Inherent viscosity obtained using Ubbelohde vis-cometer is probably the most commonly cited characteriz-

H 2N-CH-CHy--CO2H +

CO2 H

-

(1) Et 3N, CICO2 EtCH CHy-COzH

CO2 H(2) NaN3

/-CH-CH2-NCO +

/

NCO

Ar-

+ 2nCO2

II

Ill

6 a,ba, n=5b, n= 10

CH-CH2-CON3 BenzenerefluxCON3

-CH-CH2-NCO

NCO

Scheme XXIV

prepared diisocyanate with pyromellitic dianhydride,benzophenone tetracarboxylic dianhydride, and hexa-fluoroisopropylidene diphthalic anhydride resulted in thepreparation of optically active, thermally stable polyimides(96] (Scheme XXIV).

Polyimide CharacterizationA wide range of techniques is used to characterize thephysical and chemical properties of the polyimides,whether in the solid state, the melt or in solution.

Dilute solution viscosity measurements are by far the

Iranian Polymer Journal I Volume I I Number 4 (2002)

ing parameter. This determination is straightforward andrequires no further discussion . The molecular weight andmolecular weight distribution of polyamic acids can bedetermined by gel permeation or size exclusionchromatography and light scattering . The technique can beused in combination with several commercially availablechromatographic columns for molecular size comparisonsrelative to a polystyrene calibration curve, or by theuniversal calibration method [97].

Glass transition temperature (T8) is measured bythermal or dynamic mechanical methods such as differntial

23]

Polyimides Derived from Diisocyanates

scanning calorimetry (DSC), dynamic mechanical thermalanalysis (DMTA), thermomechanical analysis (TMA) andrheometrics dynamic spectoscopy (RDS) . The rheological

behaviour of the polyimides above T . can also be studied

by RDS.The morphology of polyimides specimens can often

be observed by scanning or transmission electronmicroscopy (SEM or TEM).

Ordering or crystallinity in the polymers can bedetected by X-ray diffractometer, DSC, or even UV visible .fluorescence spectroscopy.

Thermal stability or thermo-oxidative stability canbe studied by thermogravimetric analysis (TGA).

Chemical structure of the polyimides can be studiedby FT-NMR or FTIR spectroscopic techniques. Forintractable solid polymers, solid state NMR and pyrolysisGC-MS methods may be applied . DSC has also been usedto monitor polyimide formation from polyamic acid [98].

Other properties which are measured, depending onthe final applications include hydrolytic stability, radiationresistance, electrical properties such as ; dielectric constant,dielectric strength, volume and surface resistivity,dissipation factor, and physical properties such as : tensilestrength, elongation, tensile modulus, compressivestrength, thermal conductivity, creels, tear, hardness, etc.

Conclusion and Future Trends

Polyimides, polyamides and polyamide-imides have beenprepared from the solution reaction of diisocyanates andappropriate comonomers [99] . These polymerizationrequire solvents of sufficient polarity to solubilize mono-mers and products. Yet at the same time, these solventsmust be inert to isocyanates . Solvents including dimethylformamide (DMF), dimethyl acetainride (DMAc) and N-methyl-2-pyrrolidone (NMP) have been used successfullyto prepare polyimides and polyamide-imides . Using diiso-cyanates instead of diamines resulted in the preparation ofthe same polymers with similar properties, but this processavoids both chemical and/or thermal conditions required innormal polyimide synthesis route and polymerization wasperformed at lower temperature and under milderconditions. Formation of the CO2 gas as a by-product was

the main advantage of using diisocyanates instead ofdiamines in preparation of these thermally stable polymers.

Some recent trends in the development of improvedsynthetic routes to polyimides include the use of newazeotrope co-solvents such as N-cyclohexylpyrrolidoneand the use of electromagnetic or microwave curing [100].

Further advances in improving imidization efficien-cy will be required in future . There is a need for newreactive end-caps for thermosetting polyimides to provideeasier processing or increased thermal stability of thecross-links.

Much work has been carried in recent years todevelop new polyimides with improved processingcharacteristics, mechanical and electrical properties . Theuse of polyimides in microelectronics will continue togrow as further improvements are made to the chemistryand properties of the systems . There is no doubt thatfurther applications of polyimides will open up a betterunderstanding of their chemistry and properties obtained.New polyinides will continue to be synthesized to providespecific property improvements in particular applications,such as liquid crystal alignment layer forming agent, fibres,with outstanding thermomechanical behaviour, membranesfor hyperfiltration recovery of aromatic solvents, dialyzatefilter including an asymmetric microporous hollow fibremembrane, and colourless and low colour polyimide whichwill be used in a wide range of applications [101-104].Attempt should be also continued to overcome theshortcoming of the polyimide processing due toinsolubility and non-melting property by producing solublepolyimides. Finally and perhaps most important of them allwould be an increasing driving force to reduce the cost ofthese polymers, for example by synthesis of novel andcheaper monomers.

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Iranian Polymer Journal / Volmoe 11 Number 4 (2002)