Phostones, i.e., 2-alkoxy-2-oxo-1,3-oxaphospholanes, are accessible in a one-pot reaction from commercially available 1,3-dibromopropane and alkyl phosphites. These 5-membered cyclic phosphonic acid esters are used for the preparation of linear poly(phosphonate)s via ring-opening polymerization resulting in polymers with a hydrolytically stable P–C bond in the polymer backbone. Phostones have the stable P–C bond within the cycle, which leads to a dramatic increase of the monomer stability toward hydrolysis and long shelf-lives compared to other cyclic phosphoesters, which hydrolyze immediately at contact with water. Two phostone monomers containing ethoxy or butoxy pendant chains were prepared in a single-step synthesis from inexpensive starting materials avoiding the usage of SOCl2 or POCl3. Polymers with ethoxy side chains are water-soluble without a lower critical solution temperature, nontoxic to murine macrophages, and hydrolytically degradable under basic conditions. The polymerization kinetics for different catalyst systems were evaluated for both monomers in order to identify optimal polymerization conditions, resulting in polyphosphonates with molecular weights between 3000 and 25 100 g/mol with reasonable molecular weight dispersities (<1.6). Because of the ease of synthesis and distinct different hydrolysis kinetics compared to side-chain polyphosphonates, we believe that these new polyphostones represent a valuable addition to water-soluble biopolymers for future biomedical applications.