A Kabachnik-Fields postpolymerization modification reaction (KF-PMR) was conducted on poly(4-vinyl benzaldehyde) (poly(St-CHO)) with amines and phosphites. Successful KF-PMR was demonstrated by precise structural analysis of the obtained polymers and kinetic investigation of the KF-PMR processes, making it a powerful tool for postpolymerization modifications that lead to α-amino phosphonates as side groups.