Described herein are platinum-based brush-arm star polymers (Pt-BASPs), or a pharmaceutical composition thereof, for delivery of platinum-based agents, such as cisplatin. Also provided are methods and kits involving the Pt-BASPs, or a pharmaceutical composition thereof, for treating proliferative diseases such as cancers (e.g., lung cancer, head-and-neck cancer, esophagus cancer, stomach cancer, breast cancer, pancreas cancer, liver cancer, kidney cancer, or prostate cancer) in a subject.