New pH-responsive linear and crosslinked functional copolymers of N-acryloyl-N′-phenyl piperazine with acrylic acid and hydroxyethyl methacrylate: synthesis, reactivity, and effect of steric hindrance on swelling

Abstract

New functional linear copolymers and crosslinked gels based on a disubstituted heterocyclic acrylamide, i.e., N-acryloyl-N′-phenyl piperazine (AcrNPhP) with acrylic acid (AA) and hydroxyethyl methacrylate (HEMA) were prepared by thermal free-radical polymerization. The composition of the linear copolymers was analyzed by infra-red spectral analysis and elemental analysis methods. The reactivity parameters of the monomers were determined by the Finemann-Ross and Kelen–Tudos linearization methods. Using the values obtained from the Kelen–Tudos linearization methods, the reactivity or resonance stabilization (Q) and polarity (e) of AcrNPhP with AA and HEMA were calculated (AcrNPhP–AA system Q = 1.9, e = −0.6, and AcrNPhP–HEMA system Q = 2.5, e = −1.2). Crosslinked copolymer hydrogels of AcrNPhP with AA and HEMA were prepared by bulk and solution polymerization methods. The method of preparation, the type of crosslinker and monomers showed a profound impact on the pH responsive swelling of the gels. The hydrogels were highly responsive to changes in external pH. The gels composed of AcrNPhP and AA swelled more than 1,200 % above the pK a of acrylic acid. Interestingly, no significant swelling (about 1 %) was observed in solution of low pH (acidic) despite the presence of an ionizable tertiary amine function in the polymer network. This is attributed to the presence of a bulky phenyl group at the tertiary amine function of AcrNPhP which sterically hinders the protonation and thereby the final swelling of the gel. The gels composed of AcrNPhP and HEMA did not display a significant pH responsive behavior due to the same effect. The AcrNPhP–HEMA gel-containing 70 % AcrNPhP swelled to only 50 % despite the presence of a large fraction of the ionizable unit.