Considering the similarity between toluene and polystyrene, I would expect toluene to be the best possible solvent.  What is the molecular weight of the PS?  What concentration solution do you want?  I have personally worked with 50% solutions of PS in toluene (but was atactic).  Try warming the mixture, reducing the molecular weight and contenting yourself with more dilute solution.  Any of that helpful?

Thanks for your reply. I have been working on atactic PS before, it is feasible for toluene as the solvent. However, it dose not dissolve the syndiotactic PS. BTW, the Molecule weight is 250k.  Anyway, thanks.