Background: Utilizing tumor antigens for the development of patient-specific cancer vaccines has been a promising therapeutic strategy. However, significant challenges remain in delivering subunit vaccine components to elicit antitumor immune responses. To overcome these, we developed rationally designed cancer vaccines using a new nanoplatform called the Protein-Like Polymer (PLP) with unique characteristics that allow for sustained delivery of tumor antigens in conjunction with immunomodulatory compounds. Methods: PLPs containing antigens were synthesized via ring-opening metathesis polymerization (ROMP) and characterized using HPLC, LC-MS, NMR, and GPC. A library of compounds were generated with different sidechain linkage chemistries (amide, ester, or disulfide linkage), DPs, and inclusion of PEG side chains. In vitro assays using gp100-specific T Cells were conducted with fluorescently labeled and non-labeled polymers. In vivo experiments were done using B16 murine melanoma transplanted tumor models. Ability of PLPs to co-deliver immunomodulatory compounds was tested using a small molecule STING agonist 2’3’ cGAMP. Results: Conjugating antigens to the polymer backbone using a cleavable disulfide linkage, designed to be reduced intracellularly, resulted in the best subcellular localization and immune activation (higher levels of immune cell proliferation, cytokine production, and expression of activation markers). Incorporating a diluent amount of PEG side chains increased resistance to enzymatic degradation while improving bioactivity and cell uptake. In vivo studies using PLPs conjugated with melanoma-associated antigen gp100 showed the reductions in tumor size with increased immune cell proliferation and activation in a B16 melanoma model. Additionally, increasing the degree of polymerization, and therefore the density of antigen side chains per unit polymer, improved vaccine efficacy and resistance to proteolysis. Mice treated with STING agonists electrostatically coupled to PLPs showed significantly smaller tumors vs no-treatment at day 14 (0.038g vs 0.76g; p <0.0001) and allowed for subcutaneous administration of the small molecule, which normally must be injected intratumorally. Conclusions: Our work demonstrates the potential of PLPs to overcome inherent difficulties associated with developing cancer vaccines. The modularity of the platform allows for complex nano-architectures including systems capable of delivering challenging compounds, ie small molecule STING agonists, subcutaneously through electrostatic coupling. This technology has the potential to revolutionize cancer vaccinology.