First synthesis of both "head-to-head" and "head-to-tail" polyimides usinga common unsymmetric alicyclic tetracarboxylic dianhydride

A structurally ordered polyimide with the main chain having the complete "h ead-to-tail" bonding sequence (HTPI) or the complete "head-to-head" bonding sequence (HHPI) was prepared. The synthesis was based on the unique reacti vity of an unsymmetric spiroalicyclic tetracarboxylic dianhydride, rel-(1R, 5S,6R)-spiro[3-oxabicyclo[3.2.1]octane-6,3'-tetrahydrofuran]-2,2',4,5'-tetr one (DAn), which has one 5-membered and one 6-membered anhydride. Equimolar amount of amine reacts with this compound predominantly at the 6-membered ring. Thus, the reaction of DAn with 1 molar amount of 4-nitroaniline and t he subsequent methanolysis and reduction gives a monomer for a head-to-tail polyimide, which was successfully converted to HTPI through a polycondensa tion-dehydration sequence. Additionally, a new dianhydride monomer MHH, whi ch is a 2:1 adduct of DAn and p-phenylenediamine (PPD), was prepared. MHH w as subjected to polycondensation with 1 molar amount of PPD and the condens ate was dehydrated to give HHPI. Slow addition of PPD into the solution of DAn also afforded a semi head-to-head polyimide. The polyimides had inheren t viscosity in the range of 0.20-0.73 dL/g and were characterized by NMR an d IR spectra through a comparison with those of their random isomer RPI. No obvious differences in properties, such as solubility and thermal stabilit y, between the ordered polyimides (HTPI and HHPI) and the random counterpar t were found in this study.