Despite the above qualitative analysis, we emphasize that our approach cannot properly identify the critical point because the assumption of no polymer in the supernatant phase breaks down in that region. Furthermore, as shown in Sec. S2.1 in the ESI, we see that in our finite simulation box, the scattering peak at k ≈ 0 gradually emerges as c NaCl is increased. Therefore, we need to introduce an arbitrary discrimination criterion as a limit of validity of our approximations. In our case, we consider the system to be demixing, if the peak at k ≈ 0 is the global maximum of the S(k) curve.