Polymerization behavior of N-phenylbenzoxazine in bulk was studies in detail to discover new reaction pathways: (1) formation of an intermediary polymer having N,O-acetal-type linkage in the main chain and (2) its rearrangement into Mannich-type linkage in the solid state. Efficient promotion of the main chain rearrangement was achieved by using a dual system composed of imidazole and sulfonic acid.