precipitation fractionation

of polymers
A process in which a macromolecules material, consisting of macromolecules differing in some characteristic affecting their solubility, is separated from solution into fractions by successively decreasing the solution power of the solvent, resulting in the repeated formation of a two-phase system in which the less soluble components concentrate in the polymer-rich phase.
Purple Book, 1st ed., p. 68 (