Xylenes (or dimethylbenzenes) can dissolve polypropylene"PP"but not at RT. As you know, a polymer takes more time to dissolve in a given liquid solvent than a simple solid compound. The macromolecules are associated by a large number of inter-molecular secondary bonds & the polymeric chains are entangled so dissolution requires disentanglement or separation of chains with a subsequent surrounding of these chains by solvent molecules.

Obviously, polystyrene & PP are both non-polar hydrocarbon polymers with the 1st having phenyl & the 2nd having methyl as pendant groups. Therefore, the PP chains will pack more than the PS chains & this will affect the solubility behavior.

I think that this attached "2 years ago" Research Gate question will help in your research:

https://www.researchgate.net/post/What_solvent_can_be_used_for_the_dissolution_of_Polypropylene

I agree with Nizar Matar. We are speaking of thermoplastic polymers.

Concerning the answer by Sakthidharan C. P. please consider that thermosetting (crosslinked) networks are not soluble by definition. The can only undergo swelling.

Polypropylene and polyethylene are crystalline polymers which depending on the history of the sample (thermal,side groups, mechanical orientation etc) can have circa 40 to 90% crystals in them. The binding forces in the crystals is strong enough to prevent any solvent breaking it down at room temperature. Ideally the temperature should be increased to the melting point to achieve dissolution. However, temperatures near the melting point can be used to achieve solution. So solvents with a boiling point of 150C are useful.

We were early users of GPC to measure molecular weight distributions(MWD) for high density polyethylene(HDPE) and ran the GPC at 135C with ortho-dichloro benzene(ODCB) as solvent. The detector was a differential refractometer and there was sufficient difference in refractive index between ODCB and HDPE to give good results. However the columns did not last too long because of oxidation of the cross linked polystyrene packing. The melting point of 960kg/metre cubed HDPE is about 137C. ).2 melt index polymers were routinely studied.

I hope this information is useful.

Were could I found a paper discussing this?

I observed that many people mentioned 1,2,4 Trichloro Benzene(TCB) to dissolve PP. I tried to make PP solution using 1,2,4-Trichlorobenzene& 0.015 % BHT. But once temperature reduced PP separated from the solution. Any suggestions for a permanent solution?

polypropylene can be dissolved in xylene at high temperature as 80-100 C