Thermosensitive imidazole-containing polymers as catalysts in hydrolytic decomposition of p-nitrophenyl acetate

Poly(N-vinylcaprolactam-co-1-vinylimidazole) (PVCL-Vim) and poly(N-isopropylacrylamide-co-1-vinylimidazole) (PNIPA-Vim) are thermosensitive in water and water-2-propanol solutions. These copolymers are soluble at room temperature; however, they undergo phase transition at higher temperatures (T-transition < 35 degreesC). Catalytic properties of PVCL-Vim and PNIPA-Vim as well as of 1-methylimidazole and poly(1-vinylimidazole) (PVim) in hydrolysis of p-nitrophenyl acetate were studied. Variation of the reaction rate with temperature was investigated at constant concentrations of substrate and catalyst (imidazole groups). It was found that the reaction catalyzed by the copolymers does not follow Arrhenius-type behaviour: as temperature is increased over the aggregation temperature, the reaction rate increases faster than it could be predicted from the Arrhenius law. This was explained via the assumption that the substrate having an affinity to polymer/solvent interface can be adsorbed at the surfaces of polymer aggregates. This leads to higher concentration of substrate around the catalytically active groups. Theoretical consideration supports such a possibility.