No derivatives of DG with respect to the composition variables are required. This method is particularly useful in cases where the composition dependence of .delta.
G is very complex and no analytical representation of the derivatives can be given.
The method is applied to a ternary mixture of two homopolymers with a copolymer consisting of the same monomers. The sequence distribution of the copolymer is kept constant between random and purely alternating, and phase diagrams are calculated for different chemical compositions of the copolymer. B.A. Wolf, Macromol. G with no need of its derivatives with respect to the composition variables was extended to determine the critical conditions and the condition of the stability of the critical points. The method is applied to a ternary mixture of two homopolymers with a copolymer consisting of the same monomers.