Solvolysis of p-nitrophenyl esters catalyzed by oligo-4(5)-vinylimidazoles

Abstract

The preparation of low molecular weight poly-4(5)-vinylimidazole is described. Oligomer fractions with molecular weight ranges from a few hundred to two thousand, five hundred were obtained by fractionation through a Sephadex LH-20 column. The apparent dissociation constant of the imidazolium ion in oligo-4(5)-vinylimidazole decreases rapidly as the average molecular weight increases from 390 to 626. At an average molecular weight greater than 626, the apparent dissociation-constant value plateaus. The imidazole group in oligo-4(5)-vinylimidazole with ca. 5 imidazole units per oligomer molecule shows 71% of the solvolytic rate of the imidazole groups which are located in the oligomer with an average molecular weight of 2460 during the solvolysis of p-nitrophenyl acetate. The results indicate that short-range imidazole groups are interacting with each other to catalyze the solvolysis of p-nitrophenyl acetate. Solvolysis of p-nitrophenyl heptanoate catalyzed by oligomer fractions in 21% (v/v) n-propanol-water solutions indicates that the solvolysis rate of p-nitrophenyl heptanoate increases with an increase of oligomer's molecular weight.