Postpolymerization modification is a powerful strategy for the rapid generation of functional polymers, but the availability of reactions that enable fast and selective functionalization using only benign materials remains limited. We report the utility of the condensation reaction between beta-ketoesters and primary amines for efficient polymer functionalization at room temperature. Under Bronsted acid catalysis, polymers containing pendent beta-ketoesters could be functionalized with a diverse scope of primary amines containing various polar and nonpolar functional groups. The formed enaminone linkages are robust under ambient conditions but undergo dynamic transamination at elevated temperatures, enabling stimuli-responsive interchange of previously installed functional N-substituents. This protocol can be conducted catalyst-free at high temperature, but the addition of modest amounts of p-toluenesulfonic acid results in rapid substituent exchange, within 10 min in some cases. Furthermore, this equilibrium-controlled reaction was found to be quantitative when the initial substituent (i.e., the liberated byproduct from transamination) is either sterically hindered or electron-deficient. Overall, this postpolymerization modification complements the existing suite of oximeand hydrazone-based postpolymerization modification approaches and could similarly find utility for biologically relevant chemical modifications.