The partition coefficients of n-alkanes, n-alcohols, volatiles, and typical antioxidants between a low density polyethylene and several alcohols (methanol and ethanol) were predicted without fitting variables Using an off-lattice Flory-Huggins approach. The main advantage of the proposed excluded volume constraint method was to sample at atomistic scale pairwise contact energies and the residual entropic contribution due to possible different conformers and reorientations. The positional entropy was considered both in the polymer and in the liquid as in the original Flory-Huggins theory. Possible biases due to the formulation and conformational sampling were analyzed for molecules different in size, in shape, and in stiffness. The predictions were close to the experimental uncertainty oil the partition coefficients between the amorphous part of the polymer and the tested liquids. The present work confirmed that plastic additives, which are by design highly compatible with the polymer, had also a significant chemical affinity with polar liquids consisting of smaller molecules. Finally, a general predictive model of partition coefficients based oil solubility coefficients was proposed.