Emulsion polymerization of styrene with sodium borohydride (NaBH4) in an aqueous sodium dodecyl sulfate (SDS) solution was successfully accomplished for the first time. Polystyrene with a high molecular weight (M-w > 2000000) and a broad molecular weight distribution (MWD approximate to 3.5) was obtained in a conversion of less than 30%. Several pieces of evidence that the polymerization proceeded through radical intermediates were observed. Variations in the concentration of NaBH4 showed a critical range in said concentration, i.e., a borderline that determined whether the main reaction was directed to either a polymerization or a competed reaction with variations in the NaBH4 level. Kinetic studies on the emulsion polymerization of styrene with NaBH4 performed at 50, 55, and 60 degreesC showed that the initiator had an approximately 50-min induction period. A plot of -ln(1 - X), where X is the fractional conversion, as a function of time resulted in a linear relationship, showing that the present initiator system followed first-order kinetics with respect to monomer concentration. The Arrhenius plot between ln k vs 1/T gave a good linear relation, and the overall activation energy was observed to be about 37.5 kcal/mol. The employment of CH3I with NaBH4 significantly increased conversion (> 95%) and provided polystyrene with a well-controlled M. and MWD (< 2.3).