Other polyamide-imide chemistries

Other direct one-pot syntheses are known and have been cited in the literature but are not practiced commercially. One example involves the formation of a diacid containing internal di-imide groups from the reaction of TMA and a diamine. The amide bond is then condensed in a second step in the same pot with additional diamine, using pyridine, tri-phenylphosphite, and lithium chloride in NMP [14]. This amide condensation may also be completed by the addition of a thionyl chloride/NMP complex to the diacid in the presence of an acid acceptor [15]. 

Alternatively, other methods use a di-isocyante, such as MDI, to react with the diacid-dimide [16]. This defined architecture allows microstruc-tured polymers to be synthesized which can be useful in membrane and other specialty applications. 

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There are currently two popular commercial methods to synthesize polyamide-imides. One is the acid chloride route, and the other is the isocyanate route. The sjmthesis method used will determine and limit to some extent the applications in which the resultant polymer is used. The chemistry of these two methods is outlined helow. 

The other most popular method of synthesizing commercial PAI uses the so-called isocyanate route. This is the primary route to polyamide-imides which are then utilized as wire enamels or fibers. This PAI chemistry involves the reaction of a di-isocyanate, often -methylenediphenyldi-isocyanate (MDI) (Fig. 12.3) with trimellitic anhydride (TMA) (Fig. 12.4). To overcome the lower reactivity of the carboxylic acid functionality in... 

The chemistry of imidization has been reviewed elsewhere [1-5]. It is briefly outlined below in Eq. (8.1). The key elements are reaction of a diamine and a dianhydride to form an amide-acid polymer via ring opening of the anhydride by nucleophilic attack of the diamine. The polyamide acid is then formed into a polyimide by removal of water. These polymerizations are usually conducted in high-boiling polar solvents to dissolve the monomers and keep the polyamide acid in solution. Heating is frequently used to drive off water and form the polyimide. In some cases, other means of dehydration may be used. 

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