Blends of diamidonaphthyridine (Napy) end-functional poly(n-butyl acrylate) (PnBA) and ureidopyrimidinone (UPy) end-functional poly(benzyl methacrylate) (PbnMA) were studied as a function of the component molecular weights to compare with prior theoretical predictions.(1) Macroscopic phase separation was observed to be prevented by the reversible association of end-functional polymers to form supramolecular diblock copolymers, resulting in stabilization of the interface between the polymers. At low molecular weights homogeneous microstructures were observed, in contrast to nonfunctional homopolymer blends of the same molecular lengths, which rapidly phase separate over macroscopic length scales. At higher molecular weights, the blend structure was reminiscent of compatibilized homopolymer blends, with the phase-separated domain size rapidly increasing with temperature. To compare with theoretical phase diagrams, the temperature-dependent Flory-Huggins x parameter was measured, and it was found that PnBA/PbnMA covalent diblock copolymers show unusual lower critical ordering (LCOT) behavior with chi slightly increasing with temperature (chi(T) = 0.036 - 0.56/T).