Abstract: The invention provides biodegradable, water soluble PEA, PEUR and PEU carrier polymers, which can be used to conjugate, and thereby stabilize and/or solubilize, bioactive agents via polar uncharged or charged groups and activated ester or amino groups contained in the building blocks that make up the backbone of the polymer. The bioactive agents are released at a controlled rate determined by biodegradation of the polymers. The highly versatile Active Polycondensation (APC) method, which is mainly carried out in solution at mild temperatures, readily allows synthesis of such polymers. The invention water soluble polymers can also be used as water solubilizing tethers to attach drugs, and biologics to the surface of such carrier constructs as liposomes, particles and micelles.