Polymer Journal, Vol. 26, No. 3, pp 315-323 (1994) Preparation and Properties of Polyisoimide as a Polyimide-Precursor Amane MOCHIZUKI, Tadashi TERANISHI, and Mitsuru UEDA Department of Materials Science and Engineering, Faculty of Engineering, Yamagata University,Yonezawa 992, Japan (Received August 9, 1993) ABSTRACT: The reactions of N-phenylphthalamic acid with various dehydrating agents have been investigated. N,N'-dicyclohexylcarbodiimide (DCC) and trifluoroacetic anhydride with triethylamine (TEA) were found to be suitable dehydrating agents for the formation of N-phenylphthalisoimide. The isomerization of isoimide to imide was catalyzed by acids or bases, where l,8-diazabicyclo[5,4,0]-7-undecene (DBU) was most effective and a quantitative yield of imide was obtained quickly. Various polyisoimides were prepared by the cyclization of polyamic acids using DCC. These polyisoimides were relatively stable above their glass transition temperature (T 8 ), but easy to convert to corresponding polyimides by high thermal treatment (>250°C). Furthermore, there was a change of 20-70°C in T 8 due to isomeric conversion. KEY WORDS Dehydrating Agent / Isomerization / Polyamic acid / Polyisoimide / Polyimide / High-performance plastics are currently re- ceiving considerable attention for their po- tential uses in aerospace, automotive, electron- ic and related industries. In particular, polyim- ide (Pl) has been widely used as an insulation material for microelectronic devices because of their excellent properties, such as thermal and chemical stability, and low dielectric con- stants.1 Most of them are prepared by the reaction of aromatic dianhydrides with aromatic di- amines. The reaction proceeds through a soluble polyamic acid (PAA) intermediate, which upon further heating undergoes a cyclization reaction leading to imide formation and the evolution of water. The aromatic PI is usually intractable because of its .insolubility to organic solvent and high melting. In most cases, PAA is prepared first, processed into shaped objects and then PI is formed by thermal cyclization reaction. However, this processing method in PAA form has some significant limitations. During the thermal cyclization reaction of PAA, the loss of solvent and the release of water take place. These limitations are associated with improper curing of PAA, which make it a pronounced effect on the electrical and mechanical properties of PI film, such as rising dielectric constant, causing electromi- gration and decreasing tensile strength. On the other hand, a polyisoimide (PII) has a lower glass transition temperature (Tg) than that of corresponding PI and converted easily to PI without elimination of volatile com- pounds. The thermal isomerization of an isoimide to ·imide group is well known process. This process induces resin flow and thereby enhances the processability. Commercial acety- lene-terminated isoimide, IP-600, is an exam- ple of using such an approach to improve processability. 2 Since the first report of the existence of iso- imide was suggested by Hoogewerff and van Dorp in 1892, 3 a number of synthetic route for producing isoimide and wide array of reactions of isoimide intermediate have been described in the literatures. 4 • 5 These aspects are re- 315