  The possibilities to fractionate copolymers with respect to their chemical
composition on a preparative scale by means of the establishment of
liquid/liquid phase equilibria were studied for random copolymers of styrene
and acrylonitrile (san). Experiments with solutions of san in toluene have
shown that fractionation does in this quasi-binary system, where demixing
results from marginal solvent quality, take place with respect to the chain
length of the polymer only. On the other hand, if phase separation is induced
by a second, chemically different polymer one can find conditions under which
fractionation with respect to composition becomes dominant. This opportunity is
documented for the quasi-ternary system dmac/san/polystyrene, where the solvent
dimethyl acetamide is completely miscible with both polymers. The theoretical
reasons for the different fractionation mechanisms are discussed.
