Novel high-molecular-weight polybenzoxazine precursors, namely AB-type benzoxazine precursors, were synthesized from aminophenols and formaldehyde. Both H-1 NMR and IR confirmed the structure of the precursors, indicating the presence of a cyclic benzoxazine structure in the backbone of the precursors. The weight-average molecular weight was estimated by size exclusion chromatography to be to in the range of 1300-4500. The precursors gave self-standing thin films when their solutions were cast in dioxane over glass plates and dried, and upon a gradual thermal cure up to 250 degrees C, they afforded polybenzoxazine films. The viscoelastic analyses showed that the glass transition temperatures of the polybenzoxazine films obtained from these novel precursors were as high as 260-300 degrees C. Thermogravimetric analysis results indicated that the onset of decomposition and the char yield of the thermosets derived from these AB-type precursors were higher than those of traditional polybenzoxazine. (c) 2006 Wiley Periodicals, Inc.