Aspartic acid in a new role: Synthesis and application of a pH-responsive cyclopolymer containing residues of the amino acid

The reaction between diallylamine and dimethyl maleate afforded the Michael addition product dimethyl N,N-diallylaspartate [(CH₂CH-CH₂)₂NCH(CO₂Me)CH₂CO₂Me] I, which upon treatment with dry HCl and ester hydrolysis with aqueous HCl gave its hydrochloride salt [(CH₂CH-CH₂)₂NH⁺CH(CO₂Me)CH₂CO₂Me Cl^-] II and N,N-diallylaspartic acid hydrochloride [(CH₂CH-CH₂)₂NH⁺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 ≈ 100% scale inhibition efficiency for a duration of 50 h at 40 °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.