With unsubstituted phenol, formaldehyde, and primary aromatic amines as the starting materials, a series of monofunctional benzoxazine resins with low viscosities at room temperature were developed. The polymerization behavior of these resins into thermosetting materials was monitored by differential scanning calorimetry. Stress relaxation of the obtained polybenzoxazines revealed these polymers to be chemically crosslinked networks.