Chemical imidization

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... 

The thermal imidization of a polyamic acid film (PMDA-ODA or BPDA-ODA) obtained by casting an NMP solution leads to an amorphous polyimide. Two different teams have shown that a polyamic acid solutions in NMP heated at 200°C for a short time (20 min) gives polyimide particles fully cyclized and highly crystalline, as shown by X-ray diffraction and solid 13C NMR spectroscopy.151152 The chemical imidization of the same solution gives only amorphous particles. The difference between the cyclization of a solution and a casted film in the same solvent is intriguing. In the case of the solution, the temperature and the heating time are lower than in the case of the casted film as a consequence, a less organized structure would be expected for the particle. 

Chemical ignition sources, 23 116-117 Chemical imidization, 20 271 Chemical indicator pH determination,... 

Since no free carboxylic acid groups are involved, salt formation is avoided. The polyfamic trimethylsilyl ester) is then readily converted to the final polyimide by thermal or chemical imidization. 

The chemical imidization of poly(amic alkyl esters) was only reported very recently [59], although reports in the literature claim chemical imidization with a traditional acetic anhydride/pyridine mixture [87]. The chemical imidization of poly(amic alkyl esters) is based on the observation that PMDA/ODA based poly(amic ethyl ester) samples, when formulated at low concentrations for size exclusion chromatography, precipitated upon standing overnight [88]. Distillation of the NMP from phosphorus pentoxide to remove low levels of methyl-amine, a known impurity in this particular solvent, eliminated this unusual behavior. The precipitated polymer had significant levels of imidization as evidenced by IR. Apparently, organic bases, such as alkyl amines, were able to catalyze the conversion of amic alkyl esters to the corresponding imide. 

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. 

A series of hexafluoro-isopropylidene-containing polyimides were synthesized using solution polycondensation reactions and then chemically imidized [23-25]. Table 7 shows their chemical compositions Fig. 12 shows the monomer structures. The glass transition temperatures, Mw and Mn, of these polyimides are summarized in Table 8. A Perkin-Elmer DSC was used to determine these Tgs with a heating rate of 20 °C/min. All Tgs were measured at the second heating. 

Thermal imidization presents an interactive array of desired and undesired reactions, which may lead to complete imidization under driving conditions. On the other hand, chemical imidization presents a different set of synthetic challenges on which we will soon publish. 

Polyimides have excellent dielectric strength and a low dielectric constant, but in certain electrolyte solutions they can electrochemically transport electronic and ionic charge. Haushalter and Krause (5) first reported that Kapton polyimide films derived from 1,2,4,5-pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA) undergo reversible reduction/oxidation (redox) reactions in electrolyte solutions. Mazur et al., (6) presented a detailed study of the electrochemical properties of chemically imidized aromatic PMDA- derived polyimides and model compounds in nonaqueous solutions. Thin films of thermally... 

The polyimide powder of 6F + 3,3 -ODA was likewise tested for its solubility limit in DMAc. Uie imide powder was prepared by chemically imidizing the 6F + 3,3 -ODA polyamic acid with pyridine/acetic anhydride, precipitating in distilled water, thoroughly drying at 60°C and heating for 2 hours at 200 C. 

Chemical imidization was used for preparation of the polyimide for initial processing experiments, solely for reasons of convenience. 

The solubility of the polyimide dictates, to a large extent, the synthetic route employed for the copolymerization. The ODPA/FDA and 3FDA/PMDA polyimides are soluble in the fully imidized form and can be prepared via the poly(amic-ac-id) precursor and subsequently imidized either chemically or thermally. The PMDA/ODA and FDA/PMDA polyimides, on the other hand, are not soluble in the imidized form. Consequently, the poly(amic alkyl ester) precursors to these polymers were used followed by thermal imidization [44]. For comparison purposes, 3FDA/PMDA-based copolymers were prepared via both routes. The synthesis of the poly(amic acid) involved the addition of solid PMDA to a solution of the styrene oligomer and diamine to yield the corresponding poly(amic acids) (Scheme 8). The polymerizations were performed in NMP at room temperature for 24 h at a solids content of -10% (w/v). Chemical imidization of the po-ly(amic-acid) solutions was carried out in situ by reaction with excess acetic anhydride and pyridine at 100 °C for 6-8 h. The copolymers were subjected to repeated toluene rinses in order to remove any unreacted styrene homopolymer. 

Figure 5. Polar order stability as determined by thermal ramping (rate 37minute) laser frequency fundamental 1.047 //m. (a) film prepared from the poly(amic acid) PAA-5 heated to a maximum temperature of 280°C in the corona field (b) chemically imidized film of PI-5 poled at 310°C (corona field).

The copolyimides were synthesized via the poly(amic acid)s followed by chemical imidization (Scheme 1). They are soluble in solvents such as THF, chloroform, NMP, tetrachloroethane, DMF and DMSO. The polymer with a relatively low chromophore loading level possesses poorer solubility in THF and chloroform. 

Hyperbranched polyimides can result due to the self-polycondensation reactions of AB2-, A2- and Bs-types. The preparation of hyperbranched polyimides involves chemical imidization of polyamic acid ester synthesized from AB2-monomers, which are carboxylic dianhydrides containing an ether bond and a diamine [6,19,76]. Polyamic acid in combination with a condensation agent is used because it is difficult to separate the synthesized polymer from AB2-type monomers. 

For example, it is possible to prepare hyperbranched polyimides from 3,5-dimethoxyphenol and 4-nitrophthalonitrile in the presence of diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl) phosphonate (DBOP) as a condensation agent at room temperature. Hyperbranched polyimide was obtained through thermal or chemical imidization of the precursor (polyamic acid) (Scheme 1.3) [19]. The obtained hyperbranched polyimide had a relatively great molecular mass (A/ ) of about 190000gmor but low inhinsic viscosity of 0.30 dLg . Therefore, it had a compact configuration and the lack of entanglement of polymer chains. The polymer obtained via chemical imidization was soluble in apiotic polar solvents such as tetrahydrofuran (THF), while the polymer from thermal imidization was insoluble in any solvents. 

In the second step, polyamic acid is cyclo-dehy-drated at elevated temperatures (thermal imidization) or in the presence of a cyclizing agent (chemical imidization). Advantages of this method over one-step polymerization are the use of less toxic solvents and direct processing of soluble polyamic acids to form the final polyimide products in the form of films or fibers by thermal imidization. However, the storage instability of polyamic acid intermediates and the control of thermal imidization are still important issues [28]. A detail description of the thermal and chemical imidization of poly(amic acid) is given below. 

Poly(amic acid)s can also be chemically imidized. This is accomplished by using chemical dehydrating agents combined with basic catalysts [36]. Various reagents have been employed, including... 

Scheme 3.19 Synthetic scheme of polyimides containing TPA-moiety via chemical imidization [183],...

Although the structures of the PBOs were similar, the structures of the precursor polyimides were different [67]. Chemical imidization formed acetate-containing polyimides, whereas thermal and azeotropic imidization introduced hydroxyl-containing polyimides. In thermal rearrangement, acetate-polyimides (derived from chemical imidization) underwent deformation of the acetate domain prior to conversion into PBO via decarboxylation, while... 

Further thermal treatment (k of the polyamide-imide polymer increases the molecular weight and imidization level. In addition to thermal treatment, polyamide-imides can subsequently cyclize through chemical imidization. The chemical treatment is accomplished by using a tertiary amine and an anhydride typically triethylamine and acetic anhydride are common, while other variants have also been used with success [12]. 

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