a process for treating an impure thermoplastic polymer , including raw and recycled polymers , during extrusion to separate low molecular weight compounds therefrom , including : introducing a separating agent into plasticized impure thermoplastic polymer passing through a first high pressure zone , the low molecular weight compounds of the plasticized polymer being at least partially soluble in the separating agent ; extracting at least a part of the low molecular weight compounds from the plasticized polymer into the separation agent during passage of the plasticized polymer through an extraction zone ; removing the separating agent charged with low molecular weight compounds from the extruder as the plasticized polymer passes through a low pressure zone having a residual pressure , p , which is lower the introductory pressure , p e , and for which 1 bar ≧ p ≧ p c ; collecting the charged separating agent as it is removed from the extruder ; thermal vacuum treating the plasticized polymer as it passes through a last low pressure zone to vacuum degas the plasticized polymer at a pressure , p v , for which p v & lt ; 0 . 5 bar and thereby further remove separating agent and low molecular weight compounds from the plasticized polymer ; and passing the plasticized polymer through a last high pressure zone to an extruder head . the combination of solvent extraction in the extruder with subsequent vacuum degassing results in removal of large amounts of low molecular weight compounds with a relatively small expenditure of energy since energy - intensive vacuum degassing is needed only for removal of residuals .