The many chain problem of two polymers is studied through the summation of the series of the one-loop diagrams between different polymer chains. The free energy of the polymer blend is found revealing that chain correlations affect the phase separation behavior of polymer chains of finite lengths N. The parameter chi(eff) related with the scattering form factor is found to overestimate the composition and molecular weight dependence of the strength of the interactions of the two different polymers expressed by the generalized Flory-Huggins parameter chi. The composition dependences of both parameters are affected by the proximity to the critical region in accord with recent neutron scattering experiments and in the manner Monte Carlo simulations indicate. These dependences become stronger as the system comes close to the critical point either from the one phase or from the two phase region. The dependence of chi(eff) On the molecular weight is not monotonic and is related with the composition of the system, The critical value of chi is found to present a positive deviation of the order of N--1/2 from the mean field prediction taking the classical value chi(c) = 2/N only at the limit of large molecular weights.