Isoimide-imide

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

Using a Rheometrics mechanical spectrometer and powdered polymer samples, the authors compared the rheological behaviour of two polymers with similar chemical compositions but different structures. The rheological profiles of polymers 21 and 22 were determined between 140 and 400°C by increasing the temperature at 10°C min from 140 to 190°C and from 300 to 400°C. In the predominant region of isoimide-imide conversion (190-300°C), the temperatme was raised by 2 or 5°C increments, the dynamic viscosity rj being measured at each temperature step. At 190°C, the viscosity of poly(isoimide) 21 was approximately 5 X 10 Pas and decreased to a minimum value of 10 Pas at 243°C as the polymer softened and melted. Thermal conversion to polyimide 22 concurrently... 

Figure 11 Reaction of 3,3, 4, 4 -benzophenonetetracarboxylic acid dianhydride (BTDA) 18 with 3,3 -carbonylbisbenzeneamine 19 in the presence of dicyclohexylcarbodiimide (DCC) leading to polyisomide (polyiminolactone) 21 via the intermediate polyamic acid 20. Isoimide-imide conversion predominantly occurs by heating polymer 21 at 300°C, followed by a final excursion at 350-400°C.

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.

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.

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... 

Some organic bases like tertiary amines can also catalyze the amide acid cy-clization [6,9], and the nature of the tertiary amine allows the control of the proportions of imide-isoimide-acetamide in the reaction product. For example the reaction of MA with MDA using sodium acetate as catalyst give a BMI with a crystalline melting point at 160 °C. If the condensation is carried out with DAB-CO, an amorphous mixture containing BMI, monomaleimide-mono isoimide, and monomaleimide-monoacetamide is obtained. This amorphous material called Desbimid is more processable than the pure BMI but exhibits similar thermomechanical properties [10]. 

The Nls sianalff i a 41 consists of a main peak at 400.8 eV due to nitrogen imide linking-up the two PMDA and ODA parts. The peak at 399 eV is probably fingerprinting some isoimide content into the material (15.). ... 

In addition to the decrease of the N integral intensity, there is also a change of its chemical nature. In fact the peak at 399 eV (isoimide) increases while the intensity of the imide peak (401.0 eV) fFia 4) decreases during laser treatments (15 to 50 mJ/cm2). The ratio N (399)/N(401) (fig - 71 of these two peaks clearly shows an increase by a factor of 10 for fluences around 50mJ/cm2. We noted that at very high fluence (96 mJ/cm2), the Nls signal completely disappears. 

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