Acids to Imides

Fig. 2. Cyclization of amic acid to imides or isoimides via (12). Formation of the mixed anhydride intermediate (12) is shown in text.

R—C—N-C-R II II II OHO Imide CH3CO—NH—OCCH2CH3 Acetyl propionyl imide -ic or -oic acid to -imide... 

Studies of conversion of poly amic acid)s to poly imide)s As mentioned above, IR spectroscopy has often been used to study the conversion of amic acids to imides. Despite the popularity of this technique, important questions have been raised concerning the quantitative aspects of the results, especially at high conversion [38]. NMR spectroscopy, in particular solution-state NMR, has the advantage of greater chemical information, and when care is taken, provides quantitative measurements of the extent of conversion of amic acid to imide. The situation is less clear with insoluble poly(imide)s, since increased linewidth [39] in solid-state NMR spectro-... 

The concept of low-molecular-weight imide prepolymers can be viewed as an alternative route to enhanced processability. The development of such systems has been conducted on the basis of three fundamental requirements. First, the prepolymers should be of low molecular weight, allowing for the possibility of a low melting point and low viscosity. Second, imide groups should be present in the prepolymer so as to remove the particularly troublesome polyamic acid to imide conversion process mentioned previously. Third, the prepolymers should have reactive terminal groups capable of reaction by an addition mechanism so as to convert the molten prepolymer to a cross-linked polymer without the harmful evolution of volatiles. 

When poly(hydrazide amic acid) was heated, the conversion of amic acid to imide was essentially complete before the hydrazide linkages were converted into oxadiazole in any substantial extension. In the last step, conversion from polyhydrazide to polyoxadiazole was accompanied by decomposition, so that, for these copolyimides a better method would be through the oxadiazole-containing monomers [221]. 

Nadicimide end-capped oligomers were usually prepared in two steps namely preparation of amine terminated nadicimide and chain extension with a dianhydride. Appropriate quantities of nadic or methyl nadic anhydride and diamine in glacial acetic acid were refluxed for several hours and the amine terminated nadicimide was recovered by precipitation. Then chain extension with 6FDA was carried out in acetone at 60 C. Chemical imidization of the amic acid to imide was carried out using sodium acetate and acetic anhydride [130,131]. Instead of diamines, triamines can also be used [132-134]. Chemical structure of nadicimide end-capped resins prepared from di and triamine (Figure 23) is given below. 

The reactions of primary amines and maleic anhydride yield amic acids that can be dehydrated to imides, polyimides (qv), or isoimides depending on the reaction conditions (35—37). However, these products require multistep processes. Pathways with favorable economics are difficult to achieve. Amines and pyridines decompose maleic anhydride, often ia a violent reaction. Carbon dioxide [124-38-9] is a typical end product for this exothermic reaction (38). 

AijAT-dicyclohexylcarhodiimide (DCC) also leads to essentially quantitative conversion of amic acids to isoimides, rather than imides (30,31). Combinations of trifluoroacetic anhydride—triethjlarnine and ethyl chi oroform a te—triethyl amine also result in high yields of isoimides (30). A kinetic study on model compounds has revealed that isoimides and imides are formed via a mixed anhydride intermediate (12) that is formed by the acylation of the carboxylic group of amic acid (8). 

After about 12 hours, the ammonium salt of the imide which has precipitated is removed by filtration (Note 3), washed on a Buchner funnel with about 200 ml. of ether and air-dried. It is dissolved in the minimum amount (about 800 ml.) of boding water, and the solution is made acid to Congo red paper with concentrated hydrochloric acid. The free imide precipitates immediately and forms a wlrite slurry which is cooled in an ice bath. The imide is collected on a Buchner funnel and dried at 100° in an oven, or in a vacuum desiccator. The yield is 266-287 g. (65-70%), m.p. 187-191° (Note 4). 

The amidines 10, prepared by condensation of the corresponding imidates with aminoacetal-dehyde dimethyl acetal, undergo cyclization with a variety of acids to l//-3-benzazcpin-2-amines 11 45 Method A has proved to be successful on an industrial scale. 

Bidret, 1 -PHENY L-2-THIO-, 42, 87 Brominadon, nuclear, aluminum chloride as catalyst for, 40, 7 of cyclopropanecarboxylic acid to bromocvclopropane, 43, 9 of phthalide using N-bromosuccin-imide, 42, 26... 

The reaction of EPM-g-MA with primary aliphatic amines results in amide acids, which at elevated temperature convert to imides [12] (Figure 13.6). The reaction of EPM-g-MA with alcohols yields... 

Amination of Free Carboxylic Acids to Amides and Imides... 

Thus amides are found to be only very weakly basic in water [pKa for ethanamide(acetamide) is =0-5], and if two C=0 groups are present the resultant imides, far from being basic, are often sufficiently acidic to form alkali metal salts, e.g. benzene-1,2-dicarboximide (phthalimide, 8) ... 

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