While the performance of polymer-polymer bulk heterojunction organic photovoltaics (OPVs) is poor compared with polymer-fullerene QPVs, reducing or eliminating micrometer-scale phase separation in all-polymer OPVs may dramatically improve performance. Herein, we demonstrate that 2-ureido-4[1H]-pyrimidinone (UPy) quadruple hydrogen bonding interactions can be used to prevent micrometer-scale phase separation at temperatures and processing conditions typically used to prepare bulk heterojunction OPVs. UPy-terminated polymers are synthesized by coupling hydroxyl or primary amine terminated polymers to a reactive isocyanate-UPy group in a one-step reaction. Polymer blend films are subsequently prepared by solution blending, casting onto a surface, and thermal and/or solvent annealing. Film microstructure including the presence of phase-separated domains and polymer crystallinity is analyzed by optical microscopy, atomic force microscopy (AFM), and grazing-incidence wide-angle X-ray scattering (GIWAXS). In contrast to unmodified polymer blends, blends of UPy-terminated polymers do not exhibit micrometer-scale phase separation after extended thermal annealing. AFM reveals the presence of crystalline nanofibers and, in some cases, 100-300 nm phase-separated domains in UPy-mediated polymer blends. Fluorescence measurements indicate that UPy modification increases fluorescence quenching in solutions of donor and acceptor polymers, due to hydrogen-bonding associations which reduce the average distance for energy and/or electron transfer. These results show that UPy-mediated interactions can suppress micrometer-scale phase separation in bulk heterojunction polymer blends at temperatures and processing conditions typically used to prepare bulk-heterojunction OPVs. As a result, UPy functionalization may be a powerful route for improving the performance of all-polymer OPVs.