The controlled functionalization of polystyrene using a process of metalation with a potassium superbase followed by reaction with electrophiles is described. The method is advantageous because metalation is extremely efficient, occurring rapidly even at room temperature, and the degree of functionalization can be controlled over a very broad range. Functionalization with trimethylsilyl groups allows effective monitoring and quantification of the metalation process. Other examples of modification include the introduction of small dendritic fragments by reaction with a dendron having an aldehydic focal point or the introduction of carboxylic acid pendant groups. This method, which has little effect on the polydispersity of the polymer, has broad applicability for the functionalization of a wide spectrum of polymers containing active hydrogens and may also be used to prepare randomly branched or star structures.