The melting point, solubility and infrared spectra of a number of aromatic imides, mainly disubstituted N.N'-diphenylpyromellitimides, have been determined. The variation in properties observed could be explained satisfactorily by making certain assumptions about the planarity, polarisation and intermolecular interactions of the compounds. ZUSAMMENFASSUNG:Schmelzpunkt, Loslichkeit und Infrarot-Spektren mehrerer aromatischer Imide, hauptsachlich zweifach substituierter N.N'-Diphenylpyromellitimide, wurden gemessen. Die Verschiedenartigkeit der beobachteten Eigenschaften in Abhangigkeit von der Struktur konnte durch gewisse Annahmen iiber Planaritat, Polarisierung und intermolekulare Wechselwirkungen befriedigend erklart werden.

synopsisAromatic polyimides were prepared from pyromellitic dianhydride and a number of aromatic diamines. The effect of certain variables on the polymerization to, and the degradation of, the intermediate polypyromellitamic acids was studied, and a previously unrecorded reaction intermediate was isolated and identified. From these studies the conditions necessary to obtain high molecular weights were defined. Techniques were developed for the fabrication of satisfactory films and moldings. INTRODUCTIONPolyimides are one of a growing number of aromatic and heterocyclic polymers recently developed in the search for structural stability at elevated temperatures Depending on the environment and the exposure time, useful service lives may be obtained in the region of 300-500°C.1The fundamental reactions for synthesis of the imide ring have long been known. In the post-wax period these were applied to give aliphatic polymers via nylon-type condensation routes from aliphatic diamines with both aliphatic and aromatic tetracarboxylic acids and their esters. The polyimides formed were soluble and fusible and were suggested for use in injection-molding processes. These reaction routes proved to be unsatisfactory when attempts were made to prepare fully aromatic polyimides because of the precipitation of low molecular weight "brickdusts" at an early stage in the reaction. As a result of these failures, solution polymerization methods were developed by various ~o r k e r s .~-'~ Related reactions have also been described for the preparation of amide-imide copolymers. lo, l 4In the course of the preparation of polyimides of varying structure for thermal stability determinations, the polymerization reaction was studied in some detail and the formation of a prepolymer complex noted, which has not previously been reported. The conditions necessary for the formation of high molecular weight polymer were confirmed and techniques developed for producing satisfactory films and moldings. EXPERIMENTAL ReagentsPyromellitic Dianhydride (PMDA). This was purified via complex formation with anisoleg to give a pure white material melting at 286°C. This method was found to be preferable to either vacuum sublimation from phosphorus pentoxide or recrystallization from acetic anhydride.Two spot tests were used to detect small degrees of hydrolysis in PMDA. AROMATIC POLYIMIDES 611(1) A few crystals of PMDA were added to dry anisole in a flamed semimicro test tube and the solvent heated to boiling. The formation of turbidity in the hot yellow solution indicated that traces of acid were present.(2) A few crystals of PMDA were added to dry tetrahydrofuran in a flamed test tube and dissolved by shaking. Two drops of pure dry pyridine were then added. The immediate formation of a white precipitate indicated the presence of pyromellitic acid. The dipyridinium salt could be filtered off , dried, and estimated gravimetrically.Diamines. These were purified by recrystallization from benzene or from an alcohol-tetrahydrofuran mixture, followed by prolonged...

Polyimides were synthesised from a number of diamines and dianhydrides and their stability determined by thermogravimetric analysis a t 4OO0C in air. Wherever possible high molecular weight films obtained by thermal condensation of the polyamic acid intermediates were studied. If this was not possible, films obtained from acetic acid anhydride ciclised polymers or thermally cured powders were used.A well defined order of stability was observed for the polyimides prepared from pyromellitic diauhydride and a variety of diamines. The simplest and most symmetrical struetures had the greatest thermal stability. Lowest stabilities were observed when linking groups were electronegative in nature. ZUSAMMENFASSUNG:Polyimide wurden aus einer Vielfalt van Diaminen und Dianhydriden hergestellt. Ihre Stabilitat bei 400 "C in Luft wurde durch thermogravimetrische Analyse bestimmt. Wenn moglich, wurden hochmolekulare Filme verwendet, die durch thermische Kondensation der Poly-Amidsaure-Zwischenprodukte erhalten wurden. Wenn dies nicht moglich war, wurden Filme aus Polymeren, die mit Acetanhydrid cyclisiert waren, oder warmebehandelte Pulver untersucht. Fur die aus Pyromellithsaure-dianhydrid und einer Vielfalt von Diaminen hergestellten Polyimide wurde eine gut definierte Abstufung der Stabilitat beobachtet. Die einfachsten und symmetrischsten Strukturen besaBen die hochste Warmestabilitat ; die schlechtesten Resultate wurden mit elektronegativen Kopplungsgruppen erzielt.

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High molecular weight poly-N,N'-(4,4'-d@henylether)pyromellitim& film was prepared from pyrometlitic dianhydride and 4,4'-diaminodiphenylether and the oxidative degradation at 400"c was studied by weighi loss measurements, infrared spectrometry and saponification experiments. The results indicated the rapid development of crosslinks between the polyimide chains.