In-chain poly(phosphonate)s have been prepared via acyclic diene metathesis polymerization of three different monomers. Novel unsaturated phosphonate monomers with asymmetric structure have been developed. The monomers are accessible via a three-step synthesis that can be easily scaled up. This is the first report on poly(phosphonate)s by olefin metathesis where the stable carbon-phosphorus linkage is localized in the polymer backbone. This changes the nature of the degradation products compared to other poly(phosphoester)s. Polymers with molecular weights up to 31000 g mol-1 can be achieved and have been characterized in detail NMR spectroscopy, size exclusion chromatography, thermogravimetry, and differential scanning calorimetry. They have been also compared to structural analogues polyphosphates with respect to crystallization (SAXS, WAXS) and their rheological behavior. Also, solution grown crystals were analyzed rendering some of the herein reported poly(phosphonate)s as interesting defect poly(ethylene)-like structures.