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RNA ligands of retinoic acid-inducible gene I (RIG-I) hold significant promise as antiviral agents, vaccine adjuvants, and cancer immunotherapeutics, but their efficacy is hindered by inefficient intracellular delivery to the cytosol where RIG-I is localized. Here, we address this challenge through the synthesis and evaluation of a library of polymeric carriers rationally designed to promote the endosomal escape of 5?-triphosphate RNA (3pRNA) RIG-I agonists. We synthesized a series of PEG-block-(DMAEMA-co-AnMA) polymers, where AnMA is an alkyl methacrylate monomer ranging from n = 2–12 carbons, of variable composition, and examined effects of polymer structure on the intracellular delivery of 3pRNA. Through in vitro screening of 30 polymers, we identified four lead carriers (4–50, 6–40, 8–40, and 10–40, where the first number refers to the alkyl chain length and the second number refers to the percentage of hydrophobic monomer) that packaged 3pRNA into ?100-nm-diameter particles and significantly enhanced its immunostimulatory activity in multiple cell types. In doing so, these studies also revealed an interplay between alkyl chain length and monomer composition in balancing RNA loading, pH-responsive properties, and endosomal escape, studies that establish new structure–activity relationships for polymeric delivery of 3pRNA and other nucleic acid therapeutics. Importantly, lead carriers enabled intravenous administration of 3pRNA in mice, resulting in increased RIG-I activation as measured by increased levels of IFN-? in serum and elevated expression of Ifnb1 and Cxcl10 in major clearance organs, effects that were dependent on polymer composition. Collectively, these studies have yielded novel polymeric carriers designed and optimized specifically to enhance the delivery and activity of 3pRNA with potential to advance the clinical development of RIG-I agonists.