With phosphorus pentachloride (PCl5) as a cationic initiator, new benzoxazine-based semicrystalline and amorphous thermoplastics with significant molecular weights have been synthesized for the first time. H-1 and C-13 nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and elemental analysis are used to characterize the resulting polymers. Both Mannich base phenoxy-type (Type I) and Mannich base phenolic-type (Type II) polybenzoxazines are obtained. The polymer properties have been studied by differential scanning calorimetry, thermogravimetric analysis, and wide-angle X-ray diffraction.