Triplet−triplet annihilation upconversion (TTAUC) is an emerging technology in photonics with significant potential impacting a variety of fields (e.g., solar cells, bioimaging, and drug delivery) due to its ability to convert long-wavelength photons to higher photon energies even at low excitation power densities. However, for many practical applications of TTAUC, the transfer of the upconversion system, consisting of a molecular sensitizer and a molecular annihilator, from solution, in which efficient TTAUC systems have been reported, to solid matrices is required. This is a challenge because diffusion facilitates a close contact between molecular components required for triplet−triplet energy transfer and triplet−triplet annihilation. To this end, various approaches to fully integrate a sensitizer and an annihilator into polymers or combining a macromolecular annihilator with monomeric sensitizers have been established. This contribution studies the effect of integrating Ru(dqp)2-inspired molecular sensitizers into the side chains of a PMMA polymer, which -as a macromolecular photosensitizer- is codissolved with 9,10-diphenylanthracene as an annihilator. We study the effect of confining the sensitizers into a comparably small volume on the TTAUC process and compare the results to those of an upconversion system using various concentrations of a monomeric annihilator. We show that our approach of using a macromolecular photosensitizer allows for upconversion at extremely low excitation power densities. Furthermore, the onset of a strong annihilation regime, that is, a regime in which the intensity of the upconverted light scales linearly with the increase of the excitation power, is significantly reduced using the polymeric sensitizer; however, the upconversion intensity sits below those of the monomeric counterparts.