Polyethylenimine catalysts containing an isolated apolar binding site: Solovolysis of p-nitrophenyl esters

The preparation dodecane-block-poly[ethylenimine-graft-4(5)-methylimidazole] copolymers and related model compounds has been described and such polymers have been described and such polymers have been demonstrated to be efficient catalysts for the hydrolysis of activated phenyl esters in aqueous solutions. Polymeric catalysts that contain isolated apolar blocks exhibited enhanced catalytic activity for the hydrolysis of the p-nitrophenyl esters of acetate and butyrate compared with polymer model compounds. This rate enhancement was atributed to the apolar binding of substrate within the apolar polymer regime. Twenty-to 100-fold increases in the second-order rate constants were observed for the hydrolysis of the longer-chain p-nitrophenyl esters. This is indicative of a significant hydrophobic interaction. The contribution of the apolar block to the hydrophobic interaction was masked in the hydrolysis of the p-nitrophenyl caproate and p-nitrophenyl laurate substrates. In both instances the dominant contribution to the hydrophobic interactions was ascribed to a substrate-imidazole intermediate rather than the apolar block of the catalyst. The pH-rate profiles for the hydrolysis of p-nitrophenyl esters by the various catalysts indicated an absence of any cooperative interactions between imidazole residues or amine groups.