The reaction between diallylamine and dimethyl maleate afforded the Michael addition product dimethyl N,N-diallylaspartate [(CH2=CH-CH2)(2)NCH(CO2Me)CH2CO2Me] I, which upon treatment with dry HCl and ester hydrolysis with aqueous HCl gave its hydrochloride salt [(CH2=CH-CH2)(2)NH+CH(CO2Me)CH2CO2Me Cl-] II and N,N-diallylaspartic acid hydrochloride [(CH2=CH-CH2)(2)NH+CH(CO2H)CH2CO2H Cl-] III, respectively. The new monomers II and III underwent cyclopolymerization to give, respectively, cationic polyelectrolytes (CPEs) poly(II) and poly(III). Under the influence of pH, triprotic acid (+) poly(III) was equilibrated to water-insoluble diprotic polyzwitterionic acid (+/-) IV, water-soluble monoprotic poly(zwitterion-anion) (+/- -) V, and its conjugate base polydianion (=) VI. The protonation constants of the carboxyl group and trivalent nitrogen in VI have been determined. A 20-ppm concentration of IV is effective in inhibiting the precipitation of CaSO4 from its supersaturated solution with an approximate to 100% scale inhibition efficiency for a duration of 50 h at 40 degrees C. The aqueous two-phase systems (ATPSs) of VI and polyoxyethylene have been studied. The transformation of water-soluble VI to insoluble IV makes it a recycling ATPS as it can be recycled by precipitation at a lower pH. (C) 2015 Elsevier B.V. All rights reserved.