A multiblock copolymer consisting of main-chain polybenzoxazine and poly(propylene oxide) blocks was synthesized via Mannich polycondensation among 4,4'-dihydroxyldiphenylisopropane, 4,4'-diaminodiphenylmethane, amino-terminated poly(propylene oxide), and paraformaldehyde, which was evidenced by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and gel permeation chromatography. The multiblock copolymer was incorporated into polybenzoxazine to access the nanostructured polybenzoxazine thermosets. The morphology of the thermosets was investigated by means of atomic force microscopy and small angle X-ray scattering. It was judged that the formation of the nanostructures in the thermosetting composites follows the mechanism of reaction-induced microphase separation. Owing to the big difference in thermal stability between polybenzoxazine thermosets and poly(propylene oxide), the nanostructured thermosets were subjected to the pyrolysis at moderate elevated temperatures to remove poly(propylene oxide) microdomains, to access the nanoporous polybenzoxazine thermosets. The nanoporosity of the resulting polybenzoxazine thermosets was investigated by means of Fourier transform infrared spectroscopy and field-emission scanning electronic microscopy. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1148-1159, 2010