Thermally Rearranged Polybenzoxazoles Containing Bulky Adamantyl Groups from Ortho-Substituted Precursor Copolyimides

A new nucleophilic monomer (2,2-bis­(3-amino-4-hydroxyphenyl)­adamantane, ADHAB) having bulky adamantane groups has been synthesized following an efficient synthetic methodology. The main target of this work was to employ a high thermal stable bulky cycloaliphatic moiety as adamantane to obtain aromatic <i>ortho</i>-hydroxypolyimides (poly­(<i>o</i>-hydroxyimide)­s) able to thermally rearrange to give polybenzoxazole (TR-PBO) materials that could be tested as gas separation membranes. Thus, an array of <i>ortho</i>-acetylcopolyimides, <i>o</i>-acetyl PIs) were prepared by reaction of ADHAB and 2,2-bis­(3-amino-4-hydroxyphenyl)­hexafluoropropane (APAF) with 2,2-bis­(3,4-dicarboxy­phenyl)­hexafluoro­propane dianhydride (6FDA) via chemical imidization. Copolyimides and homopolyimides showed inherent viscosities ranging from 0.49 to 0.70 dL/g and provided good-quality dense membranes. Glass transition temperatures of these <i>o</i>-acetyl copolyimides were higher as the amount of ADHAB increased. The thermal stability of the adamantane moiety during the TR process was evaluated by directly synthesizing PBOs, which were made from the reaction, and ulterior thermal cyclization, of 2,2-bis­(4-chlorocarbonylphenyl)-hexafluoro­propane with ADHAB/APAF. TR-PBO membranes made through a thermal treatment at 450 °C for 30 min showed excellent gas separation properties for the CO₂/CH₄ gas pair with values close to the 2008 Robeson limit