Synthesis and characterization of thermally rearranged (TR) polybenzoxazoles: Influence of isomeric structure on gas transport properties

• Polyhydroxyimides with different meta vs para monomers and their rearranged polybenzoxazoles were synthesized.
• The polyacetylimides were shown to have high molecular weights and narrow distributions by SEC.
• Both meta and para oriented TR polymers (TR400-m-HAB-6FDA-Ac and TR400-p-HAB-6FDA-Ac) surpassed the 1991 CO2/CH4 upper bound.

Isomeric polyhydroxyimides based on 3,3′-diamino-4,4′-dihydroxybiphenyl (m-HAB) or 3,3′-dihydroxy-4,4′-diaminobiphenyl (p-HAB) with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) were prepared via an ester-acid monomer. The polyhydroxyimides were then acetylated using acetic anhydride to change the ortho-functional groups on the polymer chains. These ortho-functional polyimides were used as precursors for thermal rearrangement (TR) to polybenzoxazoles for gas separation membranes. The permeability coefficients of TR polymers significantly improved as the ortho-functional polyimides converted to polybenzoxazoles. The influence of meta and para isomeric monomers on the gas transport properties of the resulting TR polybenzoxazoles were studied. In addition, gas permeation properties show a dependency on the size of the ortho-functionality of the polyimide precursors.

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