Synthesis and Characterization of Dimethyladamantyl Unit-Containing Semi-Alicyclic Polybenzoxazole

3,5-Dimethyladamantyl unit-containing semi-alicyclic poly(o-hydroxy amide), precursor polymer of the polybenzoxazole, of high molecular weight was synthesized by the low-temperature solution polycondensation of 5,7-dimethyladamantane-1,3-dicarbonyl chloride with N,N',O,O'-tetrakis(trimethylsilyl)-substituted 4,4'-diamino-3,3'-dihydroxybiphenyl. Subsequent thermal cyclodehydration of the precursor polymer at 300°C successfully afforded the polybenzoxazole with high molecular weight. The poly(o-hydroxy amide) dissolved in polar aprotic solvents, whereas the polybenzoxazole was insoluble in any organic medium tested. The polybenzoxazole was stable up to around 400°C under nitrogen and had a low dielectric constant of 2.866. The mechanical properties of the polybenzoxazole film were excellent, its tensile strength, elongation at break, and tensile modulus were 110 MPa, 6%, and 3.3 GPa, respectively.