A facile synthesis of functionalized poly [3(4)-methylstyrene] (PMS) latex particles containing aldehyde and carboxylic acid groups was achieved via an emulsion polymerization of 3(4)-methylstyrene in the presence of sodium dodecyl sulfonate, followed by an in-situ oxidation catalyzed by copper(II) chloride and t-butyl hydroperoxide (t-BuOOH) in the presence of t-butyl alcohol (t-BuOH). The structure of the anionic surfactant, metal catalyst, organic solvent, oxidant, and their concentrations strongly affected the rate of oxidation and the stability of the emulsion. The average size of the polymer latex particles was found to increase after oxidation, and the polymer was slightly crosslinked. A free-radical mechanism is proposed involving metal-catalyzed decomposition of t-BuOOH and benzylic oxidation.