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Terpolymer systems which form two azeotropic copolymers frequently give clear terpolymers when the ratio of monomers charged is taken from a line joining the azeotropic copolymer compositions (designated the azeotropic line) as shown when the system is plotted on triangular coordinates. Two terpolymer systems which form three azeotropic copolymers gave clear terpolymers from compositions lying within the area bounded by the three azeotropic lines and in one of the external triangular areas. In the 44 terpolymer systems reported, those containing styrene, various substituted styrenes, acrylic acid, acrylonitrile, methacrylonitrile, methacrylic acid and esters (methyl through butyl), maleic and fumaric esters (methyl and ethyl), vinyl methyl ketone, butadiene, isoprene, and vinyl acetate largely conformed to this behavior. Systems consisting of either two styrene monomers and one methacrylic ester or one styrene monomer and two methacrylic esters are unusual in that clear terpolymers were obtained from all compositions tested. Systems containing acrylic esters, vinylidene chloride, maleic anhydride, and trans‐dichloroethylene did not conform to this behavior. A number of corresponding monomer and polymer compositions, obtained by solution of the terpolymer equations, were plotted on triangular coordinates. The graph showed that, during polymerization, the composition of the polymer species formed and the composition of the residual monomers migrate toward that of one of the pure monomers. Clarity of films cast from solutions containing two or more different polymers was used as an indication of compatibility. High compatibility was found with polymers having compositions on the azeotropic line. Incompatibility was found with polymers having compositions not on the azeotropic line. Incompatibility increased as the distance between these compositions (shown by a triangular graph) increased.

When polymers of two different chemical species are dissolved in a common solvent the resulting solution usually separates into two phases. No obvious relation between phase separation and chemical nature of the monomeric units was observed. Solute molecular weight, temperature, and nature of the solvent were shown to be factors in determining the course of phase separation. A concept of phase separation is suggested which involves low entropy of mixing, greater attraction between like than unlike species of solute, and density differences between solvated polymer molecules.

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