Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate Anion Catalyzed by Systematically Quaternized Poly(4-vinylpyridines). “Average Side-Chain Length” as a Useful Index for Relative Hydrophobicity of the Polymer Domain

In an attempt to find a relation between the structure of water-soluble polymers and their catalytic efficiencies, decarboxylation (30 °C, aqueous medium) of the title carboxylate ion (1) was used as a kinetic probe. The polymers employed were poly(vinylpyridines) quaternized by octyl (C₈; 3-15 mol %), dodecyl (C₁₂,22-33 mol %), octadecyl (C₈; 3-15 mol %), and docosyl (C₁₂, 3-10 mol %) groups. The majority of the remaining pyridine unit was quaternized by an ethyl group. The augmentation of the decarboxylation rate was associated with formation of the hydrophobic domain by side-chain aggregation as inferred from viscosity reduction and shifts of λof methyl orange. A new parameter “average side-chain length (L)” was introduced as an index of the relative contribution of pendant alkyl groups to the polymer hydrophobicity. The optimal balance of water solubility and hydrophobicity was achieved by the octyl-substituted polymer; L = 5.0. The logarithmic rate constant linearly correlated with this parameter. The transition from polyelectrolyte (extended polymer chain) to polymer micelle (shrunken polymer chain) was shown to occur at nL ~ 40. Poly(vinylpyridines) of different molecular weight were prepared and quaternized by dodecyl (ca. 30 mol %) and ethyl groups. The catalytic efficiency of these polysoaps agreed within the experimental error. The decarboxylation data were compared with the acyl-transfer data between a long-chain hydroxamate and p-nitrophenyl acetate. The local hydrophobicity was influential for the latter reaction, in contrast with the dominant influence of the overall hydrophobicity (L) in the decarboxylation.