Polyimide dianhydride

Aromatic polyimides are the first example we shall consider of polymers with a rather high degree of backbone ring character. This polymer is exemplified by the condensation product of pyromellitic dianhydride [Vll] and p-amino-aniline [Vlll] ... 

Polyimides for use ia molded products and high temperature films can be produced by the reaction of pyromelHtic dianhydride [89-32-7] and 4,4 -diaminodiphenyl ether [13174-32-8] ia DMAC to form a polyamide that can be converted iato a polyimide (13). DMAC can also be used as a spinning solvent for polyimides. AdditionaUy, polymers containing over 50% vinyHdene chloride are soluble up to 20% at elevated temperatures ia DMAC. Such solutions are useful ia preparing fibers (14). 

Miscellaneous Applications. Ben2otrifluoride derivatives have been incorporated into polymers for different appHcations. 2,4-Dichloroben2otrifluoride or 2,3,5,6-tetrafluoroben2otrifluoride [651-80-9] have been condensed with bisphenol A [80-05-7] to give ben2otrifluoride aryl ether semipermeable gas membranes (336,337). 3,5-Diaminoben2otrifluoride [368-53-6] and aromatic dianhydrides form polyimide resins for high temperature composites (qv) and adhesives (qv), as well as in the electronics industry (338,339). 

Synthesis and Properties. Several methods have been suggested to synthesize polyimides. The predominant one involves a two-step condensation reaction between aromatic diamines and aromatic dianhydrides in polar aprotic solvents (2,3). In the first step, a soluble, linear poly(amic acid) results, which in the second step undergoes cyclodehydration, leading to an insoluble and infusible PL Overall yields are generally only 70—80%. 

The dianhydride of 1,4,5,8-naphthalene tetracarboxyhc acid [81-30-1] has been of research interest for the preparation of high temperature polymers, ie, polyimides. The condensation of the dianhydride with o-phenylenediamines gives vat dyes and pigments of the benzimidazole type. 

Uses. Pyromellitic dianhydride imparts heat stabUity in applications where it is used. Its relatively high price limits its use to these applications. The principal commercial use is as a raw material for polyimide resins (see POLYIMIDES). These polypyromellitimides are condensation polymers of the dianhydride and aromatic diamines such as 4, -oxydianifine ... 

Syntheses. The presence of the ether and imide functionahties provides two general approaches for synthesis. Polyetherimides can be prepared by a nucleophilic displacement polymerkation similar to the haUde displacement inpolysulfone synthesis or by a condensation of dianhydrides and diamines that is similar to normal polyimide synthesis (see POLYIMIDES). 

The two-step poly(amic acid) process is the most commonly practiced procedure. In this process, a dianhydride and a diamine react at ambient temperature in a dipolar aprotic solvent such as /V,/V-dimethy1 acetamide [127-19-5] (DMAc) or /V-methy1pyrro1idinone [872-50-4] (NMP) to form apoly(amic acid), which is then cycHzed into the polyimide product. The reaction of pyromeUitic dianhydride [26265-89-4] (PMDA) and 4,4 -oxydiani1ine [101-80-4] (ODA) proceeds rapidly at room temperature to form a viscous solution of poly(amic acid) (5), which is an ortho-carboxylated aromatic polyamide. 

Diesters of Tetracarboxylic Acids with Diamines. Dianhydrides react with alcohols at a moderate temperature, producing the tetracarboxyhc acid diesters. Ben2ophenonetetracarboxyhc acid diester (BTTA diester) is obtained from B I DA as a mixture of three positional isomers. Partiy because it is an isomeric mixture, the diester of BTTA is soluble in methanol at high concentrations. A state-of-the-art thermoset polyimide,... 

Cycloahphatic diamines react with dicarboxyUc acids or their chlorides, dianhydrides, diisocyanates and di- (or poly-)epoxides as comonomers to form high molecular weight polyamides, polyimides, polyureas, and epoxies. Polymer property dependence on diamine stmcture is greater in the linear amorphous thermoplastic polyamides and elastomeric polyureas than in the highly crosslinked thermo set epoxies (2—4). 

Semicommercial production of 3,3/4,4 -biphenyltetracarboxyhc dianhydride [2420-87-3] in the United States has been announced by Occidental Chemical Corp. (74). This polyimide resin intermediate is prepared by dehalogenative dimerization of 4-chlorophthalate salts (75) or by oxidative coupling of phthalate esters (76). 

Aromatic polyimides are generally produced by the reaction of aromatic dianhydrides with aromatic diamines to yield a material with the general stmcture... 

The polyimide shown is a tme thermosetting resin, but the general reaction procedure, coupling the dianhydride with the diamine, is extremely important throughout polyimide chemistry. The intermediate polyamic acid polymers form the basis for many of the polyimide resins used in advanced composites. 

Often the substitution of fluorine atoms for hydrogen atoms in a polymer chain markedly increases the thermal stabiUty of the base polymer this is tme for polyimides. A typical fluorinated polyimide is prepared from the reaction of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis-(4-amino phenyl)hexafluoropropane according to the following reaction (36) ... 

Nonthermoprocessible Condensation Polyimides. These are obtained from condensation of aromatic dianhydrides with aromatic diamines. They are linear noncross-linked resins but their rigid chain stmcture and strongly hydrogen-bonded character leads to systems which do not melt or soften before decomposition. 

Thermoplastic Condensation Polyimides. These include General Electric s Ultem Resin and Amoco s Todon polyamideimide although the latter is no longer offered as injection moldable pellets, but as compression moldable powder and in solution. Another resin, P-84, originally developed by Upjohn but now made by Lenzing (Austria), is based on benzoquinone dianhydride and aromatic isocyanates. 

A considerable number of non-cross-linked aromatic and heterocyclic polymers has been produced. These include polyaromatic ketones, aromatic and heterocyclic polyanhydrides, polythiazoles, polypyrazoles, polytriazoles, poly-quinoxalines, polyketoquinolines, polybenzimidazoles, polyhydantoins, and polyimides. Of these the last two have achieved some technical significance, and have already been considered in Chapters 21 and 18 respectively. The most important polyimides are obtained by reacting pyromellitic dianhydride with an aromatic diamine to give a product of general structure (Figure 29.17). 

A number of phosphorus containing polyimides were synthesized by Varma and coworkers and their properties were studied [137-141]. Polyimides have also been synthesized by the reaction of benzophenonetetra-carboxylic dianhydride (BTDA) and bis(3-aminophen-yl)methylphosphine oxide [142]. Copolyimides were... 

Potyimides obtained by reacting pyromellitic dianhydride with aromatic amines can have ladder-like structures, and commercial materials are available which may be used to temperatures in excess of 300°C. They are, however, somewhat difficult to process and modified polymers such as the polyamide-imides are slightly more processable, but with some loss of heat resistance. One disadvantage of polyimides is their limited resistance to hydrolysis, and they may crack in aqueous environments above 100°C. 

Basically, the first approach to correlate the polyimide chain organization to the monomer structure was to take into consideration the electron affinity of the anhydride and the ionization potential of the diamine,10 as shown in Fig. 5.3. The strongest interactions between the polymeric chain are expected when the polyimide is prepared with the dianhydride having the highest electron affinity and die diamine with the lowest ionization potential. The strongest interchain interaction leads to high Tg and low solubility. 

Some diamines carrying very bulky substituents like cardo groups can give colorless polyimides. For example, the bis-9,9-(4-aminophenyl)fluorene (FDA) or brominated and acetylenic FDA derivatives react with 6FDA giving copolymer films62 with low birefringence (low difference between in-plane and out-of-plane refraction index) (Fig. 5.8). A new cardo diamine l,l-bis[4-(4-aminophenoxy)phenyl]cyclododecane (Fig. 5.8) reacts with different aromatic dianhydrides with formation of colorless polyimides.63... 

In addition to the research on fluorinated and cardo polyimides, an important work was devoted to the semiaromatic cycloaliphatic polyimides. Volk-sen points out the potential interest of these materials in electronic industry.64 He reports that the simplest procedure to prepare these materials is to use a cycloaliphatic dianhydride and an aromatic diamine (Fig. 5.9) instead of an aliphatic diamine and an aromatic dianhydride, which leads to formation of gels. 

Figure 5.9 (a) Colorless semialiphatic polyimide (b-f) cycloaliphatic dianhydrides ... 

Different dianhydrides have been syndiesized or are commercially available, and some structures arc shown in Fig. 5.9.64-66 An improved method for preparation of cyclobutanetetracarboxylic dianhydride (CBDA) by photochemical dimerization of die maleic anhydride has been developed by Nissan.67 The polyimide obtained by condensation of CBDA widi oxydianiline gives a transparent and colorless material. The transmittance of 50-pm-thick film is 82% and the UV cutoff is 310 nm. 

Figure 5.12 Rigid heterocyclic diamines and dianhydrides for preparation of high-moduli polyimides.

Photosensitive functions are in many cases also heat sensitive, so the preparation of photosensitive polyimides needs smooth conditions for the condensations and imidization reactions. Some chemical reactants, which can be used for polyamide preparation, have been patented for the synthesis of polyimides and polyimide precursors. For example, chemical imidization takes place at room temperature by using phosphonic derivative of a thiabenzothiazoline.102 A mixture of N -hydroxybenzotriazole and dicyclohexylcarbodiimide allows the room temperature condensation of diacid di(photosensitive) ester with a diamine.103 Dimethyl-2-chloro-imidazolinium chloride (Fig. 5.25) has been patented for the cyclization of a maleamic acid in toluene at 90°C.104 The chemistry of imidazolide has been recently investigated for the synthesis of polyimide precursor.105 As shown in Fig. 5.26, a secondary amine reacts with a dianhydride giving meta- and para-diamide diacid. The carbonyldiimidazole... 

Figure 5.27 Polyimide synthesis with imidazole blocked diisocyanate and dianhydride.

General procedures108 of bodi main solution preparations of polyimides are given below These procedures are representative and can be applied to all types of dianhydrides. 

Synthesis in NMP. The dianhydride (1.35 mmol) is added to a stirred solution of 1.35 mmol of the diamine in NMP (solid content 15% w/w) under N2 at room temperature. After 24 h, 2.97 mmol of pyridine and 2.97 mmol of acetic anhydride are added. After 24 h, the solution is diluted with 8 mL of NMP and then added to 600 mL of ethanol. The polyimide is collected by filtration, washed with ethanol and ether and then dried at 125°C under reduced pressure for 24 h. 

This one step method has been used for the syntheses of rodlike polyimides, for example with 3,3,4,4/-biphenyltetracarboxylic dianhydride (BPDA) and 2,2 -bisBrinuoromcthyll- T-diaminobiphcnyl (PFMB).109... 

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