Carboxy-terminated polystyrene, poly(styrene-co-acrylonitrile), and polystyrene block-poly(styrene-co-acrylonitrile) with ratios of weight-to number-average molar masses (M) over bar(W)/(M) over bar(n) below 1.3 were synthesized via a controlled radical polymerization mechanism. The polymerizations were initiated viith 4,4'-azobis(4-cyanopentane-carboxylic acid) and 2,2,6,6-tetramethyl-1-piperidyloxyl radical and conducted in bulk at elevated temperatures. The polymerization was monitored by nuclear magnetic resonance, size-exclusion chromatography, end-group titration and differential scanning calorimetry.