Polymeric materials can be used to deliver nucleic acids such as DNA plasmids and siRNA, but often have low efficacy in human cells. To improve gene delivery, an array of over 70 hydrolytically degradable and bioreducible poly(beta-amino ester)s are synthesized and the properties of over 200 nanoparticle formulations fabricated from these biomaterials are evaluated. The effect of different polymer structures on the delivery of nucleic acids of different structures and sizes, including siRNA, linear DNA, and circular DNAs (1.8-26 kb), is evaluated. Significantly, leading hydrolytically degradable polymeric nanoparticles deliver DNA to 90 ± 2% of primary human glioblastoma cells with <10% nonspecific cytotoxicity, better than leading commercially available reagents (p < 0.01). Bioreducible polymeric nanoparticles optimized for siRNA delivery cause up to 85 ± 0.6% knockdown in these cells as well while maintaining high viability. From a single dose, knockdown is higher than for Lipofectamine 2000 (p < 0.01) and persisted for one month. Polymer molecular weight is a driving factor of transfection efficacy for some polymer structures (correlation of r2 = 0.63), but has no influence on transfection for other structures (r2 = 0.01). Polymers with a reducible cystamine functional group dramatically improve siRNA delivery by facilitating quick release while generally decreasing DNA delivery compared with non-reducible counterparts (p < 0.01). Other material properties facilitate DNA delivery compared with siRNA delivery or increase delivery of both DNA and siRNA.