Fractionation of polymers by crystallization from solutions. II

The fractionation of polyethylene and polypropylene is carried out by establishing equilibrium between a polymer solution and the solid polymer. This procedure involves crystallization kinetics, crystal morphology, surface effects, and the formation of solid solutions. If the solution is stirred, the polymer forms thin fibrous crystals which adhere to the stirrer as a loose fabric. The experimental data on the precipitation and extraction of polyethylene in various solvents and of polypropylene indicate that fractionation takes place in the low as well as in the high molecular weight range. The components with a very high molecular weight crystallize first. The method appears to be especially suitable for the separation of branched from linear polymers, atactic from isotactic stereoisomers, and mixtures of polymers which differ considerably in crystallization rate. Thus, 46% pure polypropylene, 7% rubber, 1% pure polyethylene, in addition to block copolymers varying in structure, could be isolated from an ethylene‐propylene block copolymer.