SELECTIVE CATALYTIC EFFECTS OF STRONGLY IONIZING POLYCATIONS ON ESTER SOLVOLYSIS

The cationic polyion, poly(l-vinyl-3-methylimidazolium iodide) (PVMI), was found to enhance the solvolytic rates of the negatively charged esters 4-acetoxy-3-nitrobenzoic acid and sodium 4-acetoxy-3-nitroben-zenesulfonate in 28.5% ethanol-water solutions. These rates were unaffected by the monomeric analog of the polyion, 1,3-dimethylimidazolium iodide. At high pH values, the enhanced solvolytic rates of the negatively charged esters may be accounted for by the high local concentration of hydroxyl counterions in the vicinity of the polymer chain. At intermediate pH values, the large catalytic effect is attributed to an enhanced suscepttbility of the anionic ester to direct water attack in the vicinity of the polycation. The solvolyses of uncharged esters (p-nitrophenyl acetate and p-nitrophenyl hexanoate) were not accelerated, indicating that hydrophobic forces are insufficient to concentrate the esters in the neighborhood of the polyion. A copolymer of l-vinyl-3-methylimidazolium iodide (VMI) containing 86 mol % vinyl alcohol residues catalyzed the hydrolysis of anionic esters with an efficiency similar to that of the PVMI homopolymer, if the two were compared at equal stoichiometric concentra tions of imidazolium groups. On the other hand, a copolymer of VMI containing 63 mol % p-vinylphenol residues was about twice as effective as PVMI. This is due to the fact that the polycation draws the anionic ester into a region with a high local concentration of the phenoxide nucleophile. © 1968, American Chemical Society. All rights reserved.