The synthesis and characterization of barbiturate- and thiobarbiturate-functionalized polystyrene from polystyrene homopolymer by polymer-modification reactions is discussed. Polystyrene homopolymer quantitatively functionalized at the para position with diethyl oxomalonate functionality was subjected to a condensation reaction with urea and thiourea in the presence of sodium methoxide in methanol. This reaction proceeded essentially to quantitative conversion to the barbiturate- (BAPS) and thiobarbiturate-functionalized polystyrenes (TBAPS) as estimated by 1 NMR, UV, and IR spectroscopies. Thus, several copolymers of styrene with barbiturate- and thiobarbiturate-functionalized styrene were synthesized. The detailed characterizations of quantitatively functionalized polystyrene using gel permeation chromatographic, IR, UV, and 1H NMR spectroscopic techniques as well as thermogravimetric analysis are discussed. An application of the newly synthesized polymer in removing Cu(II) ions from aqueous solution is demonstrated. This is the first report on the synthesis of BAPS and TBAPS by the polymer-modification route or otherwise.