Polymer-polymer complexation in solvent is studied using an extension of the self-consistent field theory. The model
polymers are capable of forming ladder-like duplex structures. The duplex formation occurs with an abrupt change of entropy, resulting in a first-order transition. Moreover, the complexation can be stabilized by solvent-polymer interactions, instead of the usual specific binding
interactions. Various types of unconventional phase diagrams are predicted. For example, phase separation with decreasing χ-parameter between duplex polymer and solvent can be induced, leading to a lower critical solution temperature (LCST) behavior. Multiphase coexistence points at which two, three, or four phases coexist are also
obtained. Under certain conditions a homogeneous phase becomes unstable when the polymer chain length is decreased, in contrast to the standard Flory-Huggins theory
