A generalized Flory-Huggins parameter chi is utilized to study immiscibility-loop phase behaviour of binary polymer blends. Special emphasis is placed on the temperature dependence of the chi parameter. It turns out that specific interactions, that,gradually become weaker with ascending temperature, lead to miscibility at low temperatures and, eventually, to phase separation at sufficiently high temperatures (lower critical solution temperature behaviour). In addition, if a favourable noncombinatorial entropy contribution to parameter chi becomes more and more dominant with increasing temperature then it leads again to a homogeneous system (upper lower critical solution temperature behaviour). From a molecular point of view, this stabilization of the mixture at high temperatures might be attributed to nonrandom packing effects caused by structural disparities of the constituents. It is also demonstrated that immiscibility-loop phase behaviour can only be observed in blends of constituents having sufficient asymmetry in the polymerization indices.