The invention features a triblock copolymer including a hydrophilic block a hydrophobic block and a positively charged block capable of reversibly complexing a negatively charged molecule, e.g., a nucleic acid, wherein the hydrophobic block is disposed between the hydrophilic block and the positively charged block. Desirably, the triblock copolymer is capable of self-assembling into a supramolecular structure, such as a micelle or vesicle. The invention further features methods of delivering negatively charged molecules and methods of treating a disease or condition using the polymers of the invention.