Isoimides

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). 

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

Acetylene terminated polyimide prepolymers have many advantages over conventional polyimides in the areas of processing and solvent resistance. In addition, the presence of the isoimide structure further extends the the utility of these systems by modification of the solubility properties and glass transition temperature. 

This work discusses the thermal crosslinking and isomerization reactions occurring in the acetylene terminated isoimide prepolymer Thermid IP600. The techniques of Fourier Transform Infrared Spectrometry and Differential Scanning Calorimetry are used to determine the contribution of these two reactions during the thermal cure including their kinetics at 183° C. 

Since the utility of these materials is improved by the incorporation of these reactive functionalities without severely decreasing other favorable properties such as thermooxidative stability and solvent resistance the chemistry of the isoimide isomerization and acetylene crosslinking reactions is of considerable interest. Previous work in our laboratory has shown that these materials, when loaded with metal powders, provide a convenient and effective method of optimizing the electrical conductance and thermal stability of aluminum conductor joints. 

Structure of Thermid IP-600 Acetylene Terminated Isoimide Oligomer. 

Figure 5. Infrared Spectrum of Uncured IP-600 Isoimide Oligomer.

Figure 6. Isoimide-Imide Region of Infrared Spectrum of IP-600 as a Function of Time at 183°C.

Figures 8 and 9 show the first order kinetic plots for the isomerization and crosslinking reactions, respectively. In the data analysis the area of the isoimide peak was measured between consistent limits chosen to exclude any contribution from the 1775 cm imide band. These data were generated by measuring the area of the appropriate peak in a baseline corrected spectrum and ratioing this area to that of a reference peak (which was invarient during the experiment) in the same spectrum. This concentration indicative number was then ratioed to the concentration ratio observed on the initial scan. Plots of the log of the ratio of the concentration of the functionality at time "t" to the concentration of the functionality at t = 0 were then constructed. In order to insure that the trends in the data were not artifacts of this procedure or of the baseline correction routine, we also plotted the data in terms of peak intensity in absorbance units and observed the same trends but with more scatter in the data.

Acetylene-terminated oligomers, incorporation of isoimide functionality, 462 Acid functionalities, block copolymers, 258... 

Bis acylhydrazones or bis aroylhydrazones of a-diketones also give derivatives of 1-aminotriazoles on mild oxidation (Scheme 21).i n The product was originally assigned a dihydrotetrazine structure, and other possibilities were considered,but Curtin and Alexandrou proposed the isoimide structure (8) which was, for the product obtained from biacetyl bis(benzoylhydrazone), confirmed by X-ray crystallography. The mechanism given in Scheme 21 has been put forward for its formation ... 

Based on the rate data, it is possible to rank the relative importance of these reactions. Therefore, it follows that reactions 1 and 3 are of key importance, followed by reaction 2 and possibly reactions 4, 5 and 6. Unfortunately, little or no data exists for the latter set of reactions under the conditions employed for poly(amic acid) preparation. However, if isoimide formation were to occur to any significant degree, then reactions 5 and 6 are possible with reaction 6 being the more dominant pathway, since amine concentration due to end-groups or depolymerization would be quite low [21, 22],... 

Poly(amic dialkyl amides), which represent the other type of derivatized polyfamic acid) have been prepared by derivatization of poly(isoimide) [57] as well as monomer derivatization and subsequent polymerization [60]. Whereas the poly(isoimide) derivatization route has a pronounced tendency to produce poly(amic amides) with significant levels of imidization, the monomer derivatization and polymerization route reported in the literature is also not amenable to preparing well-defined polyfamic amides). The use of thionyl chloride to... 

Since the second solvent pair fall within the poor hydrogen bonding group of solvents, increased basicity of the organic base in these solvents would be consistent with the observed behavior. Based on the model compound studies, indications are that the base-catalyzed imidization process may involve a two-step mechanism, Jee Scheme 23. The first step corresponds to the complete or partial proton abstraction from the amide group with the formation of an iminolate anion. Since this iminolate anion has two possible tautomers, the reaction can proceed in a split reaction path to either an isoimide- or imide-type intermediate. Although isoimide model reactions indicate an extremely fast isomerization to the imide under the conditions employed for base-catalysis, all indications to date are that it is not an intermediate in the base-catalyzed imidization of amic alkyl esters. 

It is well documented that the isoimide is the kinetically favoured product and that isomerization yields the thermodynamically stable imide when sodium acetate is used as the catalyst. High catalyst concentrations provide maleimides with low isoimide impurity. The mechanism by which the chemical imidization is thought to occur is shown in Fig. 3. The first step in the dehydration reaction may be formation of the acetic acid-maleamic acid mixed anhydride. This species could lose acetic acid in one of the two ways. Path A involves participation by the neighboring amide carbonyl oxygen to eject acetate ion with simultaneous or subsequent loss of proton on nitrogen to form the isoimide. Path B involves loss of acetate ion assisted by the attack of nitrogen with simultaneous or subsequent loss of the proton on nitrogen to form the imide. If the cyclodehydration is run in acetic anhydride in the absence of the base catalyst, isoimide is the main reaction product. 

The ethynyl terminated imide oligomers are very attractive because their cured polymers are thermally stable (131). However, improvements are required in processability. An interesting approach to this problem was the synthesis and use of ethynyl-terminated isoimide (132). If the cyclodehydration of the amide acid intermediate is performed chemically with dicyclohexylcarbodiimide, isoimide is formed in almost quantitative yield. (Fig. 46). It is claimed that the isoimide provides better flow and solubility compared with the corresponding imide. At elevated temperatures, during cure, the isoimide rearranges into the... 

The well known thermally induced isomerization of an isoimide to an imide was the chemistry selected to test the concept. A series of high molecular weight polyisoimides was prepared based on PMDA and pendent aromatic diamines that on thermal treatment would exhibit the required geometry for reinforcement. Polymerizations of the diamines with PMDA were carried out in DMAC (10% by weight) at room temperature in a dry nitrogen atmosphere. Subsequent cyclodehydration of the polyamic acid to the corresponding polyisoimide was... 

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