Catalytic properties of dialkylaminopyridine functionalized, self-organized polyvinylpyridinium ions

The major focus of this study is the investigation of the reactivity and specificity of 4-(dialkylamino)pyridine (DAAP) polymer catalyst systems in the hydrolysis of p-nitrophenyl esters in aqueous solution. The catalyst systems are based on a hydrophobically associating water-soluble polymer and DAAP functional groups. The polymer catalyst systems include an N-ethylated and N-dodecylated-4-vinylpyridinium copolymer that dissolves in water and organizes into hydrophobic regions solubilized by the N-ethylated moieties. The hydrophobic regions serve as binding sites for organic lipophiles while catalytic activity is provided by DAAP functional groups that are bound to the polymeric system by electrostatic, hydrophobic or covalent means. The systems are designed to mimic enzymes. Substrate and catalytic unit are expected to dissolve into the binding domain prior to the reaction of substrate with the catalytic unit. This simplified enzyme mechanism does not account for all of the results observed in the hydrolysis of p-nitrophenyl esters when using the various polymer catalyst systems examined in this study. The key to understanding the mechanism of the polymer catalyst systems is the ability of the copolymer to solubilize p-nitrophenyl esters in its shell or interfacial regions. The polysoap is extremely compact and dissolution of p-nitrophenyl ester and the hydrophobically assembled DAAP functional group into the interior of the polymer core is very slow relative to the hydrolysis rate. The results of the kinetic studies indicate that a change in the structural organization of the polymer-ester complex occurs with increasing concentration of ester. The hydrolysis reaction can occur in three different phases: (1) p-nitrophenyl ester dissolved by the copolymer, (2) aggregated p-nitrophenyl ester dissolved by the copolymer and (3) aggregated p-nitrophenyl ester that is not dissolved by the copolymer. Four important points are required to understand substrate specificity for the polymer catalyst systems. They are (1) the greater capacity of the copolymer for the more hydrophobic p-nitrophenyl esters, (2) the location of p-nitrophenyl ester in the heterogeneous copolymer surface (3) the three phases where reaction occurs and (4) the inability of the copolymer to solubilize aggregates of p-nitrophenyl esters with alkanoate chain lengths $\geq$14 carbons