The cationic monomer, N,N-diallyl-(diethylphosphonato)methylammonium chloride, and zwitterionic monomer, ethyl 3-(N,N-diallylammonio)methanephosphonate, were cyclopolymerized in aqueous solutions using ammonium persulfate or t-butylhydroperoxide as initiators to afford a cationic polyelectrolyte (CPE) and a polyzwitterion ester (PZE), respectively. The CPE and PZE on acidic hydrolysis of the ester functionalities afforded the same polyzwitterionic acid (PZA): poly[3-(N,N-diallylammonio)methanephosphonic acid]. The solution properties of the CPE, pH-responsive PZE, and PZA were studied in detail by potentiometric and viscometric techniques. Basicity constants of the phosphonate (P=O(OEt)O-) and amine groups in the PZE and in the conjugate base of the PZE, respectively, were found to be apparent and as such follow the modified HendersonHasselbalch equation. In contrast to many polycarbobetaines and sulfobetaines, PZE was found to be soluble in salt-free water as well as salt (including Ca2+, Li+)-added solutions, and demonstrated antipolyelectrolyte solution behavior. The PZA, on the other hand, was found to be insoluble in salt-free water, and on treatment with NaOH gave dianionic polyelectrolyte (DAPE) containing trivalent nitrogen and [P=O(O)22-] groups. For the first time, several new phase diagrams of polyethylene glycol-DAPE aqueous two-phase systems (ATPSs) have been constructed in the presence of varying proportions of HCl. The ATPSs may find application in affinity partitioning of metal ions because DAPE is expected to be an effective chelator. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012