Single-chain polymer nanoparticles (SCNPs) are unique tools for improving catalysis, as they provide handles by which supramolecular effects can be controlled. By using SCNPs and polymers with an organic photoredox catalyst and an electron relay co-catalyst, the efficiency of [2+2] dimerization reactions was improved relative to the analogous small-molecule system. This work centers around characterizing these systems to inform the rational design of SCNPs as catalysts for directed applications in synthesis.