Reactive poly(phosphoester)s (PPEs) have been prepared via the acyclic diene metathesis polymerization of the monomers di(buten-3-yl) chlorophosphate and di(undecen-10-yl) chlorophosphate. Molecular weights can be adjusted from 3000 to ca. 50 000 g/mol and have been prepared and characterized in detail. This is the first report on olefin metathesis polymerization of highly electrophilic phosphochlorides, which were postmodified with different nucleophiles, i.e., alcohols, amines, and water, thus allowing the synthesis of side chain polyphosphoamidates, poly(phosphoester)s, and free acids from the same starting polymer. High side-chain functionality was found in all cases.