An analytically tractable, more realistic extension of random copolymer Flo
ry-Huggins (FH) theory Is developed for A(x)B(1-x)/CyD1-y, A(x)B(1-x)/A(y)B
(1-y), and A/CyD1-y random copolymer binary blends. The theory describes th
e polymer-polymer interactions in terms of the interactions between united
atom groups and includes a temperature-independent contribution chi(s) to t
he effective interaction parameter chi. chi(s) is determined (with no adjus
table parameters) from the lattice cluster theory in the incompressible, at
hermal, fully flexible, long-chain limit. The general, readily applied expr
essions for the interaction parameter chi are illustrated for norbornene-co
-ethylene (NxE1-x/NyE1-y) random copolymer mixtures for which the theory ha
s been successfully used by Delfolie et al. (Macromolecules 1999, 32, 7781)
to explain miscibility data that depart significantly from the predictions
of classic random copolymer FH theory. Further illustrations describe the
influence of chain semiflexibility and sequence dependence on the miscibili
ty of NxE1-x/NyE1-y blends. The theory is then applied to isotopic mixtures
of saturated poly(butadienes) (sPB) whose randomness stems from the random
addition of 1,2 and 1,4 units in the polymerization process. A final appli
cation treats blends of ethylene-co-alpha-alkene random copolymers with sPB
.