Conversion of hydroxyl-containing polyimides into polybenzoxazole can be achieved by thermal rearrangement of the aromatic polymer chain with decarboxylation at elevated temperature. Synthetic methods to prepare polyimide precursors arc important for the resulting thermally rearranged (TR) polymer membranes. Here, we report on the effect of several imidization methods on the properties of TR polymer membranes. Thermal and chemical imidizations are the most common routes to prepare polyimides, and solution thermal imidization using an azeotrope is also widely used, especially to obtain soluble polyimide-containing functional groups. We report here on the syntheses of ortho-functional polyimides from 4,4'-hexafluoroisopropylidene diphthalic anhydrides and 2,2'-bis(3-amino-4-hydroxyphenyl)hexafluoropropane by three different imidization methods. Acetate-containing polyimides by chemical imidization and further silylation treatment as well as hydroxyl-containing polyimides by thermal and azeotropic imidization arc characterized using thermogravimetric analysis, density, positron annihilation lifetime spectroscopy, and gas permeation property measurements. Comparison between the precursor polyimides and the resulting thermally rearranged polybenzoxazole (TR-PBO) membranes exhibited significant increase in fractional free volumes and cavity sizes followed by enhanced gas permeation properties.