Synthesis and characterization of polyimides via a Diels-Alder reaction between novel bisisobenzofurans with bismaleimides and bisacetylenes

As part of a continuing program to develop high performance thermally stable materials, polyimides have been prepared using novel bisisobenzofurans as dienes in Diels-Alder polymerizations. Isobenzofurans are very reactive dienes and have been reported to react with many types of dienophiles including maleimides and acetylenes. Isobenzofuran derivatives are also known to be light and/or air sensitive and thus proper precautions must be taken when working with these compounds.; Novel bisdienes based on 1,3-diphenylisobenzofuran were synthesized and reacted with various bismaleimides via the Diels-Alder reaction. The resulting intermediate polymers were dehydrated using a catalytic amount of p-toluenesulfonic acid (p-TSA) resulting in fully aromatic polyimides. It was found necessary to dehydrate the intermediates concurrent with polymer formation, as isolation of the intermediates proved to be unsuccessful. It appears that the conditions necessary for high molecular weight polymer formation (elevated temperatures) also facilitate a retro Diels-Alder reaction. High molecular weight polyimides, as evidenced by inherent viscosities ranging from 0.76 to 2.66 dL/g, were prepared from 5,5{dollar}spprime{dollar}-oxybis(1,3-diphenylisobenzofuran) and various bismaleimides. The polymers exhibited good thermal stability having glass transition temperatures (T{dollar}sb{lcub}rm g{rcub}{dollar}) ranging from 361 to 390{dollar}spcirc{dollar}C. The temperatures of 10% weight loss as measured by thermogravimetric analysis (TGA) ranged from 486to 550{dollar}spcirc{dollar}C in air. In addition, all polymers were soluble in chloroform and other polar aprotic solvents. The increased solubility is likely due to the incorporation of pendant phenyl groups along the polymer backbone as well as flexible linking groups. Transparent colorless or slightly colored, fingernail creasable films were cast from chloroform. Polymers formed from 1,1{dollar}spprime,3,3spprime{dollar}-tetraphenyl-5,5{dollar}sp prime{dollar}-biisobenzofuran and bismaleimides were insoluble in all organic solvents tested. This indicates that pendant phenyl groups are not solely responsible for the increase insolubility characteristics of the polymers.; Polyimides were also prepared from bisisobenzofurans and two different series of bisacetylenes containing preformed imide rings. One series contained an imide carbonyl para to the dienophilic acetylene group. The other series contained a ketone carbonyl para to the acetylene group. Both carbonyl groups served to activate the otherwise poor acetylene dienophile towards the Diels-Alder reaction. Viscosities of the polymers ranged from 0.12 to 0.94F dL/g. The polyimides formed by this route were also soluble in common organic solvents and fingernail creasable films were cast from chloroform from polymers having viscosities of 0.30 dL/g or higher. The polyimides exhibited b ranging from 251 to 347{dollar}spcirc{dollar}C and showed good thermal stability in both air and helium atmospheres with 10% weight loss ranging from 451 to 482{dollar}spcirc{dollar}C in air.; The initial Diels-Alder adduct between an isobenzofuran and an acetylene has been reported to undergo a rearrangement under catalytic acidic conditions. However, it was observed that this rearrangement occurred thermally without the addition of an acid catalyst. The rearrangement of the adduct along the polymer backbone prevents a retro Diels-Alder reaction from occurring.; Both the bismaleimide approach and the bisacetylene approach lead to soluble, thermally stable polyimides which can be processed in their fully imidized form. In addition, the bisacetylene approach provides thermally stable polyimides without the evolution of volatile by-products and without the addition of reagents other than solvent and monomers.