Bioerodible poly(ortho esters) useful as orthopedic implants or vehicles for the sustained delivery of pharmaceutical, cosmetic and agricultural agents from dioxane-based di(ketene acetals). Block copolymers contain these bioerodible poly(ortho esters). These block copolymers have both hydrophilic and hydrophobic blocks. They form micelles in aqueous solution, making them suitable for encapsulation or solubilization of hydrophobic or water-insoluble materials; and they also form bioerodible matrices for the sustained release of active agents.