Positron annihilation lifetime and Doppler broadened annihilation radiation have been measured for six different samples of polystyrene at room temperature in vacuum. The polymerization of styrene was carried out as a free-radical emulsion polymerization in the presence of redox pair initiation system, potassium persulphate/sodium bisulphite as an initiator system and dodecyl benzene sodium sulphate as an emulsifier. The effect of the initiator amount on the viscosity-average molecular weight was also studied. It was found that the viscosity-average molecular weight of the polymer samples decreases with increasing the initiator amount. The ortho-positronium (o-Ps) lifetime decreases with increasing the viscosity-average molecular weight up to 8.56 X 10(4) and increases dramatically after that. The intensity of the o-Ps increases with increasing the viscosity-average molecular weight up to 10.43 X 10(4) and decreases slightly after that. A correlation between the S- and W-parameters, which reflect the electron momentum, and the o-Ps intensity reflecting the electron densities in the free volume sites is obtained.