Phosphoester groups were conveniently introduced into benzoxazine monomers by chemical bonding through nucleophilic reaction of a benzoxazine ring with diethyl phosphite (DEP). The reaction process was monitored by phosphorus 31 nuclear magnetic resonance (³¹P-NMR). The characteristic peaks of ³¹P-NMR corresponding to DEP disappeared entirely when the reaction was run at 90 °C for 24 h, which implied the completion of the nucleophilic reaction. The products obtained then underwent direct ring-opening polymerization to produce DEP-modified polybenzoxazines. The introduction of DEP into polybenzoxazines led to a reduction of the onset of the polymerization temperature The adsorption ability for Cd(II) was improved because the phosphoester group provided an efficient chelating site for heavy metal ions.