Described herein are copolymers, and methods of making and utilizing such copolymers. Such copolymers have at least two blocks: a first block that has at least one unit that is hydrophilic at physiologic pH, and a second block that has hydrophobic groups. This second block further has at least one unit with a group that is anionic at about physiologic pH. The described copolymers are disruptive of a cellular membrane, including an extracellular membrane, an intracellular membrane, a vesicle, an organelle, an endosome, a liposome, or a red blood cell. Preferably, in certain instances, the copolymer disrupts the membrane and enters the intracellular environment. In specific examples, the copolymer is endosomolytic.