The postpolymerization modification of polymers produced by living polymerization is an attractive method to create precision nanomaterials. We describe the living cationic ring-opening polymerization of a 2-alkylthio-2-oxazoline to furnish a polythiocarbamate. The polythiocarbamate is activated toward substitution by N- and S-nucleophiles via oxidation of the S to an SO2. Mild substitution conditions provide broad functional group tolerance, constituting a versatile postpolymerization modification platform with access to a diversity of polyureas and polythiocarbamates. We further demonstrate the utility of this strategy by synthesizing and functionalizing block copolymers.