Methods of post-polymerization modification of a polymer are provided herein. The present methods comprise the step of reacting a polymer with at least one nucleophile in a nucleophilic substitution reaction performed without a solvent to produce a functionalized polymer. The nucleophile can be selected from the group consisting of thioacetate, phenoxide, alkoxide, carboxylate, thiolate, thiocarboxylate, dithiocarboxylate, thiourea, thiocarbamate, dithiocarbamate, xanthate, thiocyanate. Nucleophilic substitution reaction can be performed in the presence of a phase transfer catalyst. Nucleophilic substitution reaction can also be performed via a two-step in-situ reactive mixing process with the initial formation of the polymer-amine ionomer (polymer-NR3+Br) which catalyzes the subsequent nucleophilic substitution with a second nucleophile to form a bi-functional polymer.