Polymeric phosphonate esters have a variety of potential applications. These polymers can be prepared by the polycondensations of dimethyl phosphonate with certain diols. However, this method does not consistently yield high molecular weight polymers. NMR spectroscopy and size exclusion chromatography studies demonstrate that low molecular weights result from methyl group transfer from a methyl phosphonate end group to an alcohol. The inactive phosphonic acid end groups formed by this reaction can be converted into reactive methyl phosphonate end groups by treatment with diazomethane. This allows the preparation of polymers with number average molecular weights greater than 104 Da.