New Cationic Linear Copolymers and Hydrogels of N-Vinyl Caprolactam and N-Acryloyl-N′-ethyl Piperazine: Synthesis, Reactivity, Influence of External Stimuli on the LCST and Swelling Properties

Abstract

New cationic linear copolymers of N-vinyl caprolactam (VCL) and N-acryloyl-N′-ethyl piperazine (AcrNEP) were synthesized by thermal free-radical solution polymerization in dioxane at 75 °C. The chemical composition of the copolymers was determined by 1H NMR spectroscopy. The copolymers were water-soluble at all composition and exhibited lower critical solution temperature (LCST) behavior. The LCST was greatly influenced by the AcrNEP content, changes in pH, temperature, salt and surfactant concentration of the external medium. The copolymers were rich in AcrNEP content due to its higher reactivity over VCL. The reactivity of AcrNEP and VCL were determined by the extended Kelen–Tudos method to be r1AcrNEP = 0.41 and r2VCL = 0.13. The distribution of monomer sequence in the polymer chain was estimated using the terminal copolymerization model and the maximum tendency to alternation (∼70%) was at 45 mol % of AcrNEP in the feed. The effect of polymer concentration in the range 0.02 to 20 wt % on the LCST beha...