We present the synthesis and characterization of the compounds formed in a mixture of vinylphosphonic acid (VPA) and acetic anhydride used for the radical-initiated VPA polymerization. High-molecular-weight PVPA with M, up to 109 000 g.mol(-1) was obtained from the polymerization of a mixture containing VPA, VPAAnh, VPADiAnh and their acetylated derivatives. Relative reactivities of these compounds were estimated. The resulting polymers were characterized by viscosimetry, light scattering and NMR measurements. The complexity of the polymer structure increases with increasing anhydride content in the reaction feed as can be concluded from the (1)H, (13)C and (31)P NMR spectra. This finding is in accordance with a cyclopolymerization mechanism resulting in five- and six-membered anhydride rings within the polymer chain.