The effect of the solvent on the morphology and the particle size of the polymer materials obtained by precipitation polymerization is not yet understood from a physical chemical perspective. Clarifying the effect of the solvent is an important issue to understand the thermodynamic principles of precipitation polymerization and forward the engineering aspects. In this work, we use the scaling theory to deduce the thermodynamic equations that control the phase separation process and particle size in precipitation polymerization at constant T and P. To do so, precipitation polymerization has been undertaken in three steps: (I) coil-to-globule transition, (II) phase separation, and (III) growth stage. The model is then used to analyze the effect of the solvent in precipitation polymerization at constant T and P. To prove the model, we focus on analyzing correlations between the theoretical curves and the experimental curves obtained from precipitation polymerization of a model polymeric system in different solvents and show that we can faithfully reproduce the experimental curves by using the theoretical equations. Finally, we exploit the thermodynamic principles of heterogeneous nucleation on fractal surfaces to develop a novel methodology based on precipitation polymerization in the presence of small concentrations of fractal nanostructures and show how this new approach is able to reduce the particle size up to eight times below the values obtained from conventional precipitation polymerization.