A free energy density functional theory (DFT) for inhomogeneous polymeric mixtures is developed by treating the polyatomic system as a strongly associating atomic fluid mixture. The theory, derived in terms of segment density, retains the simple form of the DFTs for atomic fluids. Invoking the complete bonding limit of a stoichiometric mixture in the association free energy functional yields a computationally simple and accurate functional for the polyatomic system. Comparisons of theory calculations with molecular simulations are presented for inhomogeneous solutions and blends of linear and branched chains, demonstrating the capability of the theory to accurately capture the entropic and enthalpic effects governing the microstructure.