Solution imidization

The films were then thermally cycled at tOO, 200 and 300°C for an hour at each temperature in a forced air convection oven to complete the cyclization. The solution imidization procedure has been further described by Summers, et. al. (42,43). A representative structure for the poly(imide siloxane) copolymers is depicted in Figure 5. 

One of the major goals of this endeavor was to solubilize the normally intractable polyimides by the incorporation of siloxane segments, and, optionally, by solution imidization. Solubilities of a series of siloxane-modified polyimide copolymers were evaluated in a variety of solvents as indicated in Table I. Copolymer solubility was found to be a function of the siloxane oligomer concentration... 

Not only do the chemical structure and the molecular weight affect the processability but also the method of synthesis, in particular the imidation step. Thermally imidized polyimides are always less tractable than solution imidized polyimides. That is because thermally imidized polyimides can undergo cross-linking, and because thermal treatment (about 300 °C) favour chains packing and provide higher molecular order than that achievable by solution imidation. Therefore, solution imidation is always preferable when thermoplastic polyimides are to be developed. 

Arnold CA, Summers JD, Bott RH, Taylor L T, Ward TC, McGrath JE (1987) Structure-property relationship of polyimide-siloxane copolymers prepared by bulk and solution imidization techniques. Polym Preprints, ACS National Meeting 28(2) 217... 

At this time, an even more streamlined approached to oxazinone 1 was investigated. A modified Strecker reaction between 4-fluoro-benzaldehyde and A-benzylethanolamine (NBEA) afforded aminonitrile 16, which was extracted into IPAC (Scheme 7). The subsequent Pinner reaction was effected by addition of HCl (g) to this solution. Imidate hydrochloride 17 HC1 was formed and residual water caused spontaneous hydrolysis to lactone hydrochloride l-HCI. This slurry could be filtered to afford the crystalline lactone hydrochloride (containing ca. 15 wt% NH4CI) or alternatively and preferably it could be treated with aqueous KHCO3 to freebase the amine. The resulting IPAC stream fed directly into the crystallization-induced resolution procedure that had been established. 

Depending on the pH level, imide or carboxylate can be the dominant hydrolysis product. In alkaline solutions carboxylate is preferred. Under neutral conditions, while the carboxylate is still the preferred product, imide formation becomes significant. In acidic solution imide is the dominant hydrolysis product of polyacrylamides. 

The kinetic study of solution imidization under microwave irradiation of polyam-ic acid prepared from 3,3, 4,4 -benzophenonetetracarboxylic acid dianhydride (BTDA) and DDS was performed by Lewis et al. (Fig. 10) [66]. 

The microwave equipment consisted of a microwave generator (85 W) and a tunable cavity operating in the TE mode, while temperature was monitored using a fiber-optic temperature sensor. The samples were maintained in a Teflon vessel of 1.5 cm diameter hole and 1.5 cm deep. During a typical run, 25 W of micro-wave power was required to heat the sample to the desired temperature over 80-200 s. It was demonstrated that microwave irradiation increased the rate of solution imidization over that obtained for conventional treatment by a factor of 20-34, depending on the reaction temperature. The apparent activation energy for this imidization, determined from an Arrhenius analysis, was reduced from 105 to 55 kj/mol when microwave activation was utilized rather than conventional thermal processing. 

The polyimide siloxane copolymers are generally synthesized, as shown in Scheme 1, from a combination of a dianhydride and diamine monomer and an aminopropyl terminated polysiloxane to form the segmented polyamic acid. The polyamic acid is then cyclodehydrated by either thermal or solution imidization to give randomly segmented polyimide siloxane copolymers (4). This is indeed the classical approach of reacting one flexible oligomer with two monomers which form the "hard segment (9). 

Chemical imidization is less attractive for commercial and experimental polyimides that are tested and used in the form of films, but chemical imidation has been the preferred method concerning experimental polyimides that are soluble in organic solvents in the state of full imidation. At this respect it is worthy to remark that 100% conversion in the ring closure step is virtually impossible to achieve, particularly for thermal imidation at high temperature (about 300 °C) in the solid state, due to the complexity of the process and to the inherent molecular regidity of insoluble polyimides. However, for soluble polyimides, solution imidization is possible at mild reaction temperatures, for instance 150-200 °C, with 100% conversion, and avoiding undesirable side reactions which lead to insolubility and infusibility [61]. 

Two methods were used to thesize the polyimides synthesis of the poly(amic acid) with subsequent coating and thermal cure to the polyimide, or direct thesis of the polyimide in one pot by solution imidization (5). These are illustrated in Scheme 1. Representative examples of both these methods are as follows ... 

OPC in DMAC 135 C, others in DMAC mixture at 35 °C ppt — precipitated upon solution imidization... 

Purukawa N, Yuasa M and Kimura Y (1998) Structure analysis of a soluble polysil-oxane- 61ocfc-polyamide and kinetic analysis of the solution imidization of the relevant polyamide acid, J Polym Sci A Polym Chem 36 2237-2245. 

Banerjee et al. [143] synthesized a triamine monomer B3 (1-18, Scheme 9a) with the aim of preparing fluorinated hb-PIs by the A2 + B3 approach using commercially available dianhydrides as A2 monomers. However, during solution imidization of the amide acids that formed by the reaction of the dianhydrides and B3 monomer, an appreciable amount of gelation was observed for aU the molar ratios, except when 6FDA was used as dianhydride for reaction molar ratio of 1 1 and the solution concentration was maintained at 2.7 wt%. Due to the considerable amount of gelation during solution imidization, further characterization was not explored. 

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