  Complexation in symmetric solutions of oppositely charged polyelectrolytes is
studied theoretically. We include polyion crosslinking due to formation of
thermoreversible ionic pairs. The electrostatic free energy is calculated
within the Random Phase Approximation taking into account the structure of
thermoreversible polyion clusters. The degree of ion association is obtained
self-consistently from a modified law of mass action, which includes long-range
electrostatic contributions. We analyze the relative importance of the three
complexation driving forces: long-range electrostatics, ion association and van
der Waals attraction. The conditions on the parameters of the system that
ensure stability of the complex with addition of salt are determined.
