The high molecular weight polystyrenes are synthesized by use of diethyl 2

3-dicyano-2

3 di（p-X substituted phenyl） succinate （X=H

CH3

C1） initiators. The influence of different initiators on the positron annihilation spectra are studied by comparing polystyrectes’ molecular weight and free volume. The results indicate that the decomposition rate of initiator has effect on free volume

but initiator’s density has no great effect on free volume.