Recent studies have shown that diethyl vinylphosphonate can be converted into high-molecular-weight polymers by rare earth metal-initiated group transfer polymerization. Here we report on the use of tris(cyclopentadienyl)lanthanide complexes (Cp(3)Ln, Ln = Gd to Lu) for the polymerization of dialkyl vinylphosphonates (alkyl: methyl, ethyl, isopropyl) yielding polymers with precise molecular weight and low polydispersity. Additionally, the thermosensitive behavior of poly(diethyl vinylphosphonate) was characterized, and methods for a conversion of the obtained high-molecular-weight poly(vinylphosphonate)s (M(n) > 250 kg mol(-1)) to poly(vinylphosphonic acid) by both thermal treatment and a mild hydrolysis were established. A series of independently performed reactions showed high activities and initiator efficiencies for the Cp(3)Ln complexes for the homopolymerization of the applied monomers. Poly(vinylphosphonate)s of high molecular weight with a previously unknown low polydispersity index (PDI < 1.05) have been determined by GPC-MALS (multiangle light scattering) methods. The reaction shows a linear M(w) vs consumption plot, thus proving a living type polymerization. The initiation of the reaction has been investigated by end-group analysis with MALDI-ToF and ESI mass spectrometric analysis. A new and interesting:chain-end functionalization of the achieved polymers has been detected over the course of the MS analytical studies. The so far unreported LCST (lower critical solution temperature) of poly(diethyl vinylphosphonate) in water has been evaluated, and the correlation between the molecular weight of the material with this temperature has been determined.