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

A structurally ordered polyimide with the main chain having the complete "head-to-tail" bonding sequence (HTPI) or the complete "head-to-head" bonding sequence (HHPI) was prepared. The synthesis was based on the unique reactivity of an unsymmetric spiroalicyclic tetracarboxylic dianhydride, rel-(1R,5S,6R)-spiro[3-oxabicyclo[3.2.1]octane-6,3'-tetrahydrofuran]-2,2',4,5'-tetrone (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 the subsequent methanolysis and reduction gives a monomer for a head-to-tail polyimide, which was successfully converted to HTPI through a polycondensation-dehydration sequence. Additionally, a new dianhydride monomer MHH, which is a 2:1 adduct of DAn and p-phenylenediamine (PPD), was prepared. MHH was subjected to polycondensation with 1 molar amount of PPD and the condensate 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 inherent viscosity in the range of 0.20-0.73 dL/g and were characterized by NMR and IR spectra through a comparison with those of their random isomer RPI. No obvious differences in properties, such as solubility and thermal stability, between the ordered polyimides (HTPI and HHPI) and the random counterpart were found in this study.