Four main-chain type polybenzoxazine precursors were synthesized from the Mannich-type polycondensation of biphenol A, paraformaldehyde, and four typical aromatic diamines: 1,4-phenylenediamine, 4,4′-diaminodiphenyl methane, 4,4′-diaminodiphenyl ether, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane. The solvent effect on the polycondensation was examined. We found that polybenzoxazine precursors prepared in toluene/ethanol exhibit higher purity, higher molecular weight, and lead to better thermal properties after curing than those prepared in chloroform, as recommended by Takeichi's group (T. Takeichi et al., Polymer, 2005, 46, 12172). The cured polybenzoxazine films prepared by these polybenzoxazine precursors display unusual flexibility, extremely high T_g, and low surface energy. They show T_g values as high as 272–353 °C (DMA data), coefficients of thermal expansion as low as 39–49 ppm/°C, contact angles as high as 100–102°, and surface energy as low as 17.7–24.5 mJ m⁻².